Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubility, and Kinetics 9780841204799, 9780841206168, 0-8412-0479-9

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

Chemical Modeling in Aqueous Systems

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility, and Kinetics Everett A . Jenne, EDITOR

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

U.S. Geological Survey

Based on a symposium cosponsored by the Society of Environmental Geochemistry and Health and the A C S Division of Environmental Chemistry at the 176th Meeting of the American Chemical Society, Miami Beach, Florida, September 11-13,

1978.

ACS SYMPOSIUM SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C. 1979

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

93

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

Library of Congress CIP Data Chemical modeling in aqueous systems. (ACS symposium series; 93 ISSN 0097-6156) Includes bibliographies and index. 1. Chemistry—Mathematical models—Congresses. 2. Solution ( Chemistry ) —Congresses. I. Jenne, Everett Α., 1930. II. Society of Envi­ ronmental Geochemistry and Health. III. American Chemical Society. Division of Environmental Chemis­ try. IV. Series: American Chemical Society. A C S sym­ posium series; 93. QD39.3.M3C49 ISBN 0-8412-0479-9

54l'.3422'0184 79-242 ASCMC 8 93 1-914 1979

Copyright © 1979 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective works, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by A C S of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, repro­ duce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATE OF AMERICA

America Chemical

Society Library 1155 16th St. N. W. Washington, D. C. 20036

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

ACS Symposium Series Robert F. Gould, Editor

Advisory Board K e n n e t h B . Bischoff

James P. Lodge

D o n a l d G . Crosby

J o h n L . Margrave

Robert E . Feeney

Leon Petrakis

Jeremiah P. Freeman

F . Sherwood R o w l a n d

E . Desmond Goddard

A l a n C . Sartorelli

Jack H a l p e r n

Raymond B . Seymour

Robert A .

Aaron

Hofstader

James D . Idol, J r .

Wold

Gunter Z w e i g

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.fw001

FOREWORD The A C S S Y M P O S I U M SERIES was founded in 1 9 7 4 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing A D V A N C E S IN C H E M I S T R Y SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. and

Both reviews

reports of research are acceptable since symposia may

embrace both types of presentation.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

PREFACE 'hemical modeling, particularly as related to environmental chemistry, ^

is a rapidly evolving field. However, because of the wide dispersion

of the pertinent literature in scientific publications of many

different

fields and the absence of suitable review articles, investigators are often unaware of recent advances in disciplines other than their own.

There-

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.pr001

fore, it appeared that progress in chemical modeling capability could be significantly increased by a symposium

which would bring together

investigators from the several disciplines. This was facilitated by joint sponsorship of the symposium by both the Society of Environmental Geochemistry and Health and the American Chemical Society. The introduction to the symposium details the current constraints on research in general and chemical modeling in particular. technical papers presented fall naturally into the areas of:

The 3 7 1)

redox

equilibria, 2 ) organic ligand characterization and stability constant estimation, 3 ) adsorption processes, 4 ) evaluation and estimation of thermodynamic and kinetic data, 5 )

interfacing chemical and biological

models, and 6 ) comprehensive chemical models and their applications. By arrangement, certain papers are either partially or totally of a critical review nature.

F o r the benefit of the reader it may be noted

that these review papers deal with thermodynamic data evaluation and estimation or with bioavailability: D . Langmuir reviews and demonstrates available techniques for the estimation and evaluation of thermodynamic data; J. M . Cleveland reviews the solubility and stability constant data for plutonium compounds and complexes, respectively; J. A . Kittrick reviews the problem of how to treat exchangeable

cations in

calculating the solubility of clay (layer silicate) minerals; G . H . N a n collas reviews the thermodynamic data for calcium phosphates;

L. N.

Plummer, T . M . L . Wigley, and D . L . Parkhurst review the kinetics of calcite dissolution—precipitation; J. W . Morse and R. A . Berner review the kinetics and solubility of marine carbonates; S. N . L u o m a reviews the general problem of estimating trace element bioavailability from sediments; F . L . Harrison reviews the extensive studies she and others have made of trace element bioavailability and accumulation by macrobenthic organisms; and finally, D . K. Nordstrom, L . N . Plummer, and others compare the trace element speciation and solubility disequilibrium i n dices obtained with various extant chemical models.

T h e latter authors

xi In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

concluded that differences in the thermodynamic data selected and treatment of redox are the predominant causes of the discordant results obtained. Acknowledgments The diligence, industry, and imagination of two of my associates contributed greatly to the quality of the symposium and the proceedings. Jo Buchard style-edited the review copies, proofed the camera-ready copy of the manuscripts, and made some of the necessary contacts with the authors. Janet W o n g arranged for much of the peer review of manu-

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.pr001

scripts, kept track of manuscript receipt and review status, and made corrections to the various "camera-ready" manuscripts as were deemed necessary. The unfailing support and good humor of these two individuals was a major factor in the success of the symposium and the quality of these "rapid publication" proceedings. Peer reviews were indispensable in maintaining the technical quality of the proceedings. Appreciation is expressed for the assistance of the numerous reviewers who carefully and critically read the papers in this volume. The Editorial Review Committee was of great assistance and made a significant contribution to the quality of these proceedings. This committee consisted of V . C . Kennedy, R. V . James, and R. W . Potter, II, to whom I am indebted. U.S. Geological Survey EVERETT A. JENNE

Menlo Park, California November 14, 1978

xii In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1

Chemical Modeling—Goals, Problems, Approaches, and Priorities

EVERETT A. JENNE

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

U.S. Geological Survey, Water Resources Division, Menlo Park, CA 94025

The b e l i e f that Science can s o l v e s o c i e t a l problems i n acceptable time frames has been discounted by the p u b l i c i n recent years (1, 2). This s i t u a t i o n , according to D. S. Saxton (3), i s a r e s u l t of the "...overreaching expectations that research....would v i s i b l y c o n t r i b u t e t o e a r l y s o l u t i o n s t o d i f f i c u l t s o c i a l problems." Since the t o t a l monetary resources a v a i l a b l e f o r studying such problems are l i m i t e d , the e f f e c t of recent p o l i c y developments and l e g i s l a t i o n has been to increase support f o r a p p l i e d s t u d i e s w i t h a r e s u l t a n t decrease i n support of fundamental research. The country as a whole has turned, and i s t u r n i n g f u r t h e r , toward the use of computational models u t i l i z i n g best guesses and r e a d i l y a v a i l a b l e parameter i n f o r m a t i o n which may have only order-ofmagnitude r e l i a b i l i t y . One r e s u l t of t h i s trend i s an enormous q u a n t i t y of r e p o r t s of a h i g h l y a p p l i e d nature coming out of i n s t i t u t e s , u n i v e r s i t i e s , and governmental agencies. Many of these r e p o r t s are of dubious value (4). The h i g h l y a p p l i e d research approach produces some short-run b e n e f i t s , but i s v i r t u a l l y c e r t a i n to produce long-term l i a b i l i t i e s . One l i a b i l i t y w i l l be that a l a r g e r part of the research community w i l l be t r a i n e d as data gatherers r a t h e r than i n f o r m a t i o n synthes i z e r s w i t h a r e s u l t a n t decrease i n the t r a n s f e r v a l u e of much of the work. I n the f i e l d of chemical modeling, the p r i n c i p a l l o n g term l i a b i l i t y w i l l be that i n s u f f i c e n t fundamental knowledge needed f o r improvement of chemical, b i o l o g i c a l , and hydrologie models w i l l not be a v a i l a b l e w i t h i n the next s e v e r a l years. Although the fundamental problems being addressed h e r e i n are l a r g e p h i l o s p h i c a l i n nature, i t i s a p p r o p r i a t e t o d e f i n e what i s meant by chemical modeling. As used h e r e i n , chemical modeling encompasses the aqueous s p e c i a t i o n of d i s s o l v e d c a t i o n i c elements among organic and i n o r g a n i c a n i o n i c l i g a n d s , of a n i o n i c elements among t h e i r complexes w i t h c a t i o n s , and both c a t i o n i c and a n i o n i c elements among t h e i r redox s t a t e s . Chemical modeling a l s o i n cludes c a l c u l a t i o n of the degree of s a t u r a t i o n of an aqueous media w i t h regard both to metastable and t o e q u i l i b r i u m s o l i d s and c a l c u l a t i o n of s o r p t i o n o r d e s o r p t i o n . A d d i t i o n a l l y , p r e 0-8412-0479-9/79/47-093-003$05.00/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

4

CHEMICAL MODELING IN AQUEOUS SYSTEMS

d i c t i v e chemical modeling must i n c l u d e k i n e t i c s . Chemical modeling can be used t o d e s c r i b e the chemical c h a r a c t e r i s t i c s o f an aqueous system whether i t be a l a k e water or the f l u i d i n a human d i g e s t i v e system, given the r e q u i s i t e input data f o r the system t o be modeled as w e l l as adequate reference data (thermodynamic and k i n e t i c ) , and a p r o p e r l y constructed model. Q u a n t i t a t i v e models are v a l u a b l e because they i n c r e a s e one's a b i l i t y t o d e r i v e i n formation from data, t o p r e d i c t c e r t a i n e f f e c t s of anthropogenic i n p u t s , and t o do s e n s i t i v i t y analyses. The l a t t e r may w e l l be one of the most important and l e a s t recognized uses of such models. For example, i n recent modeling of the i n o r g a n i c s p e c i a t i o n of d i s solved s i l v e r i n San F r a n c i s c o Bay, a s e n s i t i v i t y a n a l y s i s t o t e s t the e f f e c t of metals and l i g a n d s not i n c l u d e d i n the chemical analyses i n d i c a t e d that hydrogen s u l f i d e a t the microgram-perl i t e r l e v e l would complex more s i l v e r than would c h l o r i d e i n the low s a l i n i t y p o r t i o n o f the estuary ( 5 ) . This f i n d i n g then l e d to a p r e l i m i n a r y study of d i s s o l v e d s u l f i d e i n San F r a n c i s c o Bay waters. S e n s i t i v i t y analyses can a l s o be performed to i n d i c a t e needed accuracy and p r e c i s i o n of p a r t i c u l a r analyses and parameters. Chemical models can a l s o be used i n appropriate s i t u a t i o n s to estimate b i o l o g i c a l e f f e c t s on the chemical p a r t of an ecosystem (6) . Goals The u l t i m a t e goal of a l l research i n general and chemical modeling i n p a r t i c u l a r i s a d d i t i o n a l understanding of processes and events which can, under the proper c o n d i t i o n s , f a c i l i t a t e the improvement of human l i f e p h y s i c a l l y , e m o t i o n a l l y , and a e s t h e t i c a l l y . F o r example, w i t h i n the next decade, a p p r o p r i a t e chemicaland b i o l o g i c a l - m o d e l i n g s t u d i e s of s o i l s o l u t i o n s and p l a n t n u t r i e n t uptake can a s s i s t i n i n c r e a s i n g markedly both the quantity and q u a l i t y of t e r r e s t r i a l food p r o d u c t i o n . Chemical modeling can provide i n s i g h t i n t o the b i o a v a i l a b i l i t y and the v a r i a t i o n s i n the b i o a v a i l a b i l i t y of t r a c e elements i n man s d i e t components. Processes such as s u l f i d e complexation, valence r e d u c t i o n , p r e c i p i t a t i o n and organic complexation, may s i g n i f i c a n t l y reduce the a c t i v i t y and hence the b i o a v a i l a b i l i t y of v a r i o u s t r a c e elements. Linkage to b i o l o g i c a l models w i l l c e r t a i n l y i n c r e a s e the amount of i n f o r m a t i o n which can be d e r i v e d . I n a d d i t i o n to the r e s u l t a n t p o t e n t i a l improvement i n human n u t r i t i o n , i n s i g h t i n t o the b i o a v a i l a b i l i t y of t r a c e elements from d i e t a r y components may cont r i b u t e to the c o n t r o l and a l l e v i a t i o n of some human diseases r e l a t e d to t r a c e elements. More immediate goals d i f f e r from one s c i e n t i f i c d i s c i p l i n e to another. One goal of which I am most aware i s the p r e d i c t i o n of the r o l e of aqueous s p e c i a t i o n on the b i o a v a i l a b i l i t y and t o x i c i t y of t r a c e elements t o plankton and e s t u a r i n e d e t r i t u s feeding i n v e r t e b r a t e s . I n the longer range, these s t u d i e s would be extended to i n c l u d e the v a r i o u s a q u a t i c organisms higher i n the 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

1.

JENNE

Goals, Problems, Approaches, and Priorities

5

food c h a i n . The o v e r a l l goal of t h i s symposium i s to promote the science of chemical modeling i n three ways: 1) by promoting the immediate exchange of i n f o r m a t i o n v i a o r a l p r e s e n t a t i o n s by as many workers i n the f i e l d as p r a c t i c a b l e ; 2) by f o s t e r i n g the maximum amount of d i s c u s s i o n of research g o a l s , problems, approaches, and p r i o r i t i e s ; and 3) by preparing a s t a t e - o f - t h e - a r t record i n the form of symposium proceedings. Further goals are to improve the q u a l i t y as w e l l as the a p p l i c a b i l i t y of chemical models i n ensuing years and to help minimize d u p l i c a t i o n of computerized chemical models by i n c r e a s i n g the v i s i b i l i t y of those already a v a i l a b l e . Although new approaches and mathematical frameworks are o b v i o u s l y to be encouraged, i t would appear more p r o f i t a b l e to the s c i e n t i f i c community to upgrade and expand the a v a i l a b l e models r a t h e r than to generate a d d i t i o n a l ones of s i m i l a r type and s t r u c t u r e . This view comes out of our experience that the computer codes, and indeed the models themselves, can be rendered l a r g e l y e r r o r f r e e only by extended use over time by knowledgeable i n v e s t i g a t o r s . I t i s hoped that the formal (7) and i n f o r m a l d i s c u s s i o n s of a t t r i b u t e s of chemical models w i l l i n c r e a s e the r e l i a b i l i t y and usefulness of the next generation of chemical models. Problems Important r e s t r a i n t s to the usefulness of a v a i l a b l e chemical models are inadequacies i n the 1) c a p a b i l i t y to c h a r a c t e r i z e the organic l i g a n d s of n a t u r a l waters; 2) knowledge of redox s t a t u s of waters to permit r e a l i s t i c computation of r e d o x - c o n t r o l l e d s p e c i a t i o n ; 3) a v a i l a b l e thermodynamic data; 4) knowledge of the time dependency of the v a r i o u s processes (such as v a r i a t i o n i n apparent t r a c e element c o n c e n t r a t i o n during a t i d a l c y c l e ) i n general and of k i n e t i c data f o r chemical and b i o l o g i c a l processes i n p a r t i c u l a r ; and 5) e r r o r estimates f o r the preceding r e s t r a i n t s . The f i e l d of chemical modeling i s a l s o c o n s t r a i n e d by the: 6) need f o r comprehensive analyses of the systems to be modeled; 7) l i m i t a t i o n s of the q u a l i t y and scope of published data; 8) s c a r c i t y of adequate l i t e r a t u r e reviews; 9) d i f f i c u l t i e s i n o r g a n i z i n g and conducting i n t e g r a t e d i n t e r d i s c i p l i n a r y s t u d i e s ; and to a l e s s e r e x t e n t , 10) l i m i t a t i o n i n q u a l i t y of graduate training. Organic Ligands. I n v e s t i g a t i o n s of organic compounds d i s solved i n surface waters have l a r g e l y d e a l t w i t h i d e n t i f i c a t i o n of s p e c i f i c compounds or groups (e.g., phenols, polyaromatic hydrocarbons, e t c . ) , determination of complexation c a p a c i t y , o r s e p a r a t i o n — w i t h v a r i a b l e types of c h a r a c t e r i z a t i o n — i n t o f u l v i c and humic-acid f r a c t i o n s . The i n f o r m a t i o n developed i n these s t u d i e s has not been shown to be p a r t i c u l a r l y u s e f u l to q u a n t i t a t i v e chemical modeling. The analyses of s p e c i f i c compounds or

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

6

CHEMICAL MODELING IN AQUEOUS SYSTEMS

groups of compounds have come about because of t a s t e and odor problems i n d r i n k i n g water and subsequently because of t h e p o s s i b l e t o x i c i t y of these compounds t o food c h a i n components and the t o x i c i t y and c a r c i n o g e n i c p o t e n t i a l o f these compounds to man. Complexat i o n - c a p a c i t y values have served to i d e n t i f y and dramatize the p o t e n t i a l l y important r o l e of d i s s o l v e d organic compounds as l i g a n d s capable of promoting the s o l u b i l i z a t i o n and h y d r o l o g i e t r a n s p o r t of t r a c e elements. However, there seems l i t t l e p o i n t i n c o n t i n u i n g these determinations s i n c e t h e i r magnitude i s now known and i t i s not apparent how they can be used i n q u a n t i t a t i v e chemical modeling. The s p e c i a t i o n of c a t i o n s among organic as w e l l as i n o r g a n i c l i g a n d s r e q u i r e s estimates of t h e i r molar q u a n t i t i e s and of t h e i r s t a b i l i t y constants. Two p o s s i b i l i t i e s e x i s t . One i s that there are only a few important n a t u r a l organic l i g a n d s f o r each metal. The other i s that organic complexation i s the r e s u l t of a host of s p e c i f i c organic l i g a n d s , each of which i s present i n too small an amount to be i n d i v i d u a l l y important. I f the former p o s s i b i l i t y proves t o be t r u e , the i n d i v i d u a l l i g a n d s can be i d e n t i f i e d by techniques such as l i q u i d and t h i n - l a y e r chromatography. I f the l a t t e r p o s s i b i l i t y proves t r u e , then an approach such as that of Leenheer and Huffman (8) which f r a c t i o n a t e s s o l u b l e organic compounds i n t o s o l u b i l i t y and f u n c t i o n a l group c l a s s e s w i l l be required. Redox-Dependent S p e c i a t i o n . Valence s t a t e determines the t o x i c i t y of an element. I t g r e a t l y a f f e c t s the complexation s t r e n g t h between metals and v a r i o u s l i g a n d s , hence the degree of s a t u r a t i o n of water w i t h r e s p e c t t o s o l i d phases. There i s almost complete l a c k of agreement w i t h i n the s o i l science and geochemical communities as to the s i g n i f i c a n c e and u s e f u l n e s s of platinume l e c t r o d e e s t i m a t i o n s of redox s t a t u s . The techniques of e s t i mating the c o n c e n t r a t i o n of the valence s t a t e s of i n d i v i d u a l couples a t t h e i r environmental l e v e l s are only now becoming a v a i l a b l e . These techniques o f f e r c o n s i d e r a b l e promise f o r future studies. A v a i l a b l e Thermodynamic Data. There a r e s e v e r a l problems regarding a v a i l a b l e thermodynamic data. The a p p r o p r i a t e thermodynamic data do not e x i s t f o r many s o l i d s of environmental i n t e r e s t . This i s p a r t i c u l a r l y t r u e f o r the s o l i d s which a r e e x t e n s i v e l y s u b s t i t u t e d , such as the c r y p t o c r y s t a l l i n e manganese oxides or the m e t a l l i c hydroxy s u l f a t e s and phosphates. G r e a t l y divergent thermodynamic data e x i s t f o r numerous aqueous complexes and s o l i d s ; some of these values d i f f e r by orders of magnitude. Thus, there i s a great need f o r thorough e v a l u a t i o n and s e l e c t i v e r e d e t e r m i n a t i o n . The e v a l u a t i o n of thermodynamic data i s v e r y time-consuming, r e q u i r i n g e x t e n s i v e l i t e r a t u r e searches and c o n s i d e r a b l e knowledge of a n a l y t i c a l techniques. The e x p e r t i s e and e f f o r t r e q u i r e d to o b t a i n the r e q u i r e d q u a l i t y of r e s u l t s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

1.

JENNE

Goals, Problems, Approaches, and Priorities

7

g e n e r a l l y preclude a comprehensive e v a l u a t i o n of numerous comp l e x e s , s o l i d s , or elements i n the course of a s i n g l e research p r o j e c t or c o n t r a c t . However, s c i e n t i s t s f r e q u e n t l y are comp e l l e d by the nature of t h e i r s t u d i e s to undertake c o m p i l a t i o n and s e l e c t i o n w i t h or without use of e v a l u a t i o n techniques, s i n c e adequately evaluated compilations are not a v a i l a b l e . Hence, the compilations made by s i n g l e i n v e s t i g a t o r s are o f t e n not as thorough and comprehensive as i s d e s i r a b l e . The U.S. N a t i o n a l Bureau of Standards (NBS) i s the governmental agency w i t h primary r e s p o n s i b i l i t y f o r t h i s area although the U.S. G e o l o g i c a l Survey shares r e s p o n s i b l i t y w i t h the NBS i n regard to minerals and the U.S. Department of Energy shares r e s p o n s i b i l i t y w i t h regard to the t r a n s u r a n i c elements. The e v a l u a t i o n and s e l e c t i o n of "best" values i s a job of such magnitude that most of the NBS p u b l i c a t i o n s of the l a s t decade (9, 10, 11, 12, 13) present only s e l e c t e d v a l u e s . The s u b s t a n t i a t i n g references and d i s c u s s i o n f o r these compilations are to f o l l o w i n subsequent NBS p u b l i c a t i o n s . One recent p u b l i c a t i o n (14) on thorium gives the c r i t i c a l e v a l u a t i o n and r e f e r e n c e s . C r i t i c a l e v a l u a t i o n and documentation e f f o r t s need to be g r e a t l y a c c e l e r a ted. As one step i n t h i s d i r e c t i o n , the NBS i s funding two s t u d i e s devoted to the e v a l u a t i o n and s e l e c t i o n of "best s o l u b i l i t y s t u d i e s from the l i t e r a t u r e f o r the carbonate, h a l i d e , phosphate, and s u l f i d e s a l t s of the a l k a l i and a l k a l i n e earths plus V, Cr, Mo, and U (by A l l e n C l i f f o r d of V i r g i n i a P o l y t e c h n i c a l I n s t i t u t e ) and of Cu, Zn, Cd, Sb, Hg, Pb, and As (by Lawrence Clever of Emory U n i v e r s i t y ) . These two reviews are part of Commission V. 6.1 of the I n t e r n a t i o n a l Union of Pure and A p p l i e d Chemistry (IUPAC) program of c o m p i l a t i o n and c r i t i c a l e v a l u a t i o n of the a v a i l a b l e l i t e r a t u r e on s o l u b i l i t y . The thermodynamic data f o r environmentally and geochemically important complexes and s o l i d s ( c h i e f l y , n o n s i l i c a t e ) of 29 elements (Ag, As, Au, Ba, B i , Ca, Cd, Co, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, N i , Pb, Sb, Se, Sn, Sr, Te, Th, T i , U, V, W, and Zn) are being s e m i - c r i t i c a l l y reviewed by Donald Langmuir and Hugh Barnes of Pennsylvannia State U n i v e r s i t y w i t h NBS funding. P o l y s u l f i d e complexes are to be included i n t h e i r review. The s t a t u s of the thermodynamic data on s u l f i d e - and p o l y s u l f i d e - m e t a l complexes w i l l a l s o be discussed i n a r e v i s e d e d i t i o n of "Hydrothermal Ore D e p o s i t s " e d i t e d by Hugh Barnes (15). C r i t i c a l reviews of thermodynamic data published i n the l a s t few years i n c l u d e the four-volume s e r i e s of M a r t e l l and Smith (16, 17, 18, 19), three on organic and one on i n o r g a n i c l i g a n d s ; and the s i n g l e volumes of Christensen et ad. (20) f o r metall i g a n d heats, e t c . , of Baes and Mesmer (21) f o r c a t i o n h y d r o l y s i s , of Langmuir (22) f o r uranium, and of Robie et_ a l . (23) f o r numerous m i n e r a l s . R e l i a b l e a c t i v i t y c o e f f i c i e n t s f o r marine waters or f o r waters of even higher s a l i n i t y continue to be a problem i n c a l c u l a t i o n of aqueous s p e c i a t i o n and computation of m i n e r a l 11

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e q u i l i b r i a . In p a r t i c u l a r , the a p p r o p r i a t e technique to use f o r uncharged i o n p a i r s remains i n d i s p u t e (24). However, the chemical-modeling e r r o r s a t t r i b u t a b l e to u n c e r t a i n t i e s i n a c t i v i t y c o e f f i c i e n t s are g e n e r a l l y l e s s than the u n c e r t a i n t i e s due to other causes such as m i s s i n g or u n r e l i a b l e thermodynamic v a l u e s , m i s s i n g a n a l y t i c a l data, and incomplete model components (5). A d d i t i o n a l l y , a major assessment of a c t i v i t y - c o e f f i c e n t e s t i m a t i o n i s now underway (25). Another p o t e n t i a l problem i n q u a n t i t a t i v e i=iqueou| s p e c i a t i o n i s polymerization. Various elements (Be , Se , Ce , Zn , , V C r * , Mo ]], U F e * , Co , N i * , Ru**, C u \ Z n \ Al , Ga , I n , L n , Pb +, S b , and B i ) are s a i d to e x h i b i t t h i s behavior (26, 21, 27). To the extent that p o l y m e r i z a t i o n occurs and i s not accounted f o r i n the aqueous-speciation submodel, the a c t i v i t y of the v a r i o u s s o l u t e species and s a t u r a t i o n s t a t e of s o l i d forms w i l l be overestimated. For example, the apparent o v e r s a t u r a t i o n of San F r a n c i s c o Bay waters w i t h s i l v e r s u l f i d e was decreased by about 10 percent when the p o l y s u l f i d e complexes of Ag and Cu were included i n the aqueous s p e c i a t i o n submodel (E.A. Jenne and J.W. B a l l , unpublished data, 1978). There i s l i t t l e agreement among the r e l e v a n t s c i e n t i f i c communities as to the p r i o r i t i e s f o r o b t a i n i n g m i s s i n g thermodynamic data. Top p r i o r i t i e s on my own l i s t are f o r data on n a t u r a l organic l i g a n d s , p o l y s u l f i d e complexes (data f o r Ag and Cu e x i s t , 28), low-temperature s o l i d s o l u t i o n of t r a c e elements i n i r o n and i n manganese s u l f i d e ^ and i n c r y p t o c r y s t a l l i n e s u b s t i t u t e d (Ba, L i , Sr) manganese o x i d e s ; and the s o l i d s o l u t i o n s of Sr, Ba, Mn, and Mg i n CaC03, and v a r i o u s t r a n s i t i o n metals p l u s Sr and Ba i n MnC0 . I t should be noted that because of k i n e t i c i n h i b i t i o n s i n the p r e c i p i t a t i o n of v a r i o u s s o l i d s , i t i s h i g h l y d e s i r a b l e i n s o l u b i l i t y and d i s s o l u t i o n s t u d i e s that p h y s i c a l i d e n t i f i c a t i o n of p r e c i p i t a t e s and r e s i d u e s be made by o p t i c a l , X-ray d i f f r a c t i o n , and e l e c t r o n microscopy and d i f f r a c t i o n techniques. 2

H

f

3

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AQUEOUS SYSTEMS

3

5

3

2

6 +

3

2

3

5 +

2

3

4

2

k

2

3 +

3 +

3

Time Dependency. The time dependency of geochemical and environmental-chemistry processes i s a l a r g e l y unrecognized but o f t e n important v a r i a b l e . For example, i t i s not unusual f o r 50 percent or more of the t o t a l annual sediment t r a n s p o r t of a r i v e r to occur i n the course of a s i n g l e storm event (29, p. 487). Marked time v a r i a t i o n s a l s o occur w i t h d i s s o l v e d c o n s t i t u e n t s . G i r v i n et. a l . (30) r e p o r t the c o n c e n t r a t i o n of d i s s o l v e d N i and Zn at a South San Francisco Bay s t a t i o n to vary by f a c t o r s of 1.42 and 1.56, r e s p e c t i v e l y , through a t i d a l c y c l e . S i m i l a r l y , concentrations of d i s s o l v e d t r a c e element v a r i e d i n a complex manner between main-channel and near-shore ( t i d a l f l a t ) s t a t i o n s . D i u r n a l v a r i a t i o n s i n the apparent degree of s a t u r a t i o n of surface waters w i t h respect to c e r t a i n s o l i d s may r e s u l t from b i o l o g i c a l a c t i v i t y and temperature changes. Recent s t u d i e s (V.C. Kennedy, U.S. Geol. Survey, unpublished data, 1978) have shown

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1. J E N N E

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

pronounced d i u r n a l v a r i a t i o n i n phosphate concentrations i n a short reach of a stream c o n t a i n i n g considerable attached algae. The apparent s a t u r a t i o n of ZnSiU3 i n a s m a l l stream has been found to vary d i u r n a l l y (V.C. Kennedy and E.A. Jenne, unpublished data, 1978). These observations and others i n d i c a t e that r e a l i s t i c a p p l i c a t i o n of chemical models must take cognizance of the time s c a l e s of b i o l o g i c , h y d r o l o g i e , and chemical processes. E r r o r Estimates. L i t t l e a t t e n t i o n has been paid to e r r o r s a s s o c i a t e d w i t h e i t h e r the thermodynamic or the a n a l y t i c a l data u t i l i z e d i n chemical models. M.L. Good (31) considers t h i s to be one of the four p r i n c i p a l problems of the day w i t h regard to use and misuse of s c i e n t i f i c data. I t i s e s s e n t i a l that chemical models compute propagated standard d e v i a t i o n s f o r t h e i r a n a l y t i c a l , thermodynamic, and k i n e t i c data. This p r o v i s i o n e x i s t s i n the model of B a l l et a l . ( 3 2 ) . System C h a r a c t e r i z a t i o n . A f u r t h e r l i m i t a t i o n to the adequate t e s t i n g of chemical models and to t h e i r a p p l i c a t i o n t o environmental problems i s the extensive system c h a r a c t e r i z a t i o n ( e s p e c i a l l y chemical analyses) o f t e n r e q u i r e d . Adequate charact e r i z a t i o n o f t e n n e c e s s i t a t e s many d i f f i c u l t and time-consuming analyses, some of which must be done on s i t e , and the c o l l e c t i o n — w i t h adequate p r e s e r v a t i o n — o f numerous subsamples. The appropriate i n t e r f a c i n g of chemical w i t h b i o l o g i c and hydrologie models i s a r a t h e r d i f f i c u l t problem. For example, the p r e d i c t i o n of trace-element bioaccumulation by phytoplankton may r e q u i r e i n some instances that the uptake r a t e s and the compartmentalized l o s s r a t e s f o r v a r i o u s s o l u t e species of the e l e ment present i n the system be known. The e f f e c t , i f any, on compartmentalized l o s s r a t e s of the p a r t i c u l a r s o l u t e species taken up (e.g. HgCH + vs H g ) a l s o needs to be known. The i n t e r a c t i o n e f f e c t of the c o n c e n t r a t i o n of one element upon the uptake and l o s s r a t e s of another element, such as Hg on Se (33, 34, 3 5 ) , a l s o need to be known. I n many i n s t a n c e s , hydrodynamic models may have to be l i n k e d with,or otherwise i n c o r p o r a t e d , i n t o the b i o l o g i c and chemical models to permit p r e d i c t i o n s o f , f o r example, increased trace-element l e v e l s i n o y s t e r s r e s u l t i n g from increased anthropogenic inputs to an estuary. 2+

3

L i m i t a t i o n s of Published Data. The development, t e s t i n g , and s u c c e s s f u l a p p l i c a t i o n of comprehensive chemical models r e q u i r e s at l e a s t moderate amounts of wisdom, experience, and time. I n the f i r s t h a l f of t h i s decade, u n i v e r s i t i e s , i n s t i t u t e s , and government agencies rushed headlong i n t o environmental chemistry. The r e s u l t has been a marked i n c r e a s e i n the number of r e p o r t s of the occurrence and d i s t r i b u t i o n of t r a c e elements, but t h i s great e f f o r t has produced s u r p r i s i n g l y few s t u d i e s of fundamental processes and r e a c t i o n s that would provide an understanding of observed t r a c e element r e d i s t r i b u t i o n . Most of the analyses

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

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reported i n such s t u d i e s have been on p o o r l y c h a r a c t e r i z e d , or even u n c h a r a c t e r i z e d , samples. G e n e r a l l y , trace-element data have been reported f o r sediment, or f o r b i o t a , or f o r water, but only i n f r e q u e n t l y f o r a l l t h r e e . Even more r a r e i s the i n c l u s i o n of u s e f u l hydrodynamic data and atmospheric i n p u t s . A t r i v i a l but i l l u s t r a t i v e example of the r e s u l t of l a c k of experience p l u s l a c k of an adequate l i t e r a t u r e review i s the f e d e r a l l y funded c o n t r a c t study of a few t r a c e elements i n a surface water wherein the c a r e f u l handling of s t e r i l e b o t t l e s — w i t h p l a s t i c gloves ( t o avoid contamination o f the b o t t l e s w i t h t r a c e elements from the f i n g e r s ) — i s noted w i t h obvious p r i d e but no mention i s made of any f i l t r a t i o n o r p r e s e r v a t i o n of the samples or even of the p o s s i b l e need f o r these steps! The l a r g e number of chemical analyses of s o - c a l l e d "whole-water" samples ( i . e . , water samples from which the suspended sediment has not been removed by f i l t r a t i o n or other means) i s an example of the generation of a l a r g e amount of data from which l i t t l e i n f o r m a t i o n can be der i v e d . Proper sampling and sample p r e s e r v a t i o n a r e e s s e n t i a l t o h i g h - q u a l i t y analyses. I t should be noted that "composite" time, d i s c h a r g e , or load a r e g e n e r a l l y not u s e f u l f o r chemical-modeling studies. Adequate L i t e r a t u r e Reviews. I conclude t h a t f a c t o r s other than t o t a l monies a v a i l a b l e f o r research a r e r e s p o n s i b l e f o r l i m i t i n g the s c i e n t i f i c progress i n the f i e l d s of environmental chemistry i n general and of chemical modeling i n p a r t i c u l a r . This c o n c l u s i o n r e s u l t s from the f o l l o w i n g observations. The i n c r e a s e i n the amount of e a r t h - s c i e n c e data published has been tremendous and the r a t e a t which i t i s being published continues to i n c r e a s e . This lament i s of course not new. Barnaby R i c h i s quoted as w r i t i n g i n 1613 that "one of the diseases of t h i s age i s the m u l t i p l i c i t y of books: they doth so overcharge the world that i t i s not able to d i g e s t the abundance of i d l e matter t h a t i s every day hatched and brought f o r t h i n t o the w o r l d " ( 3 6 ) . In the 10 years f o l l o w i n g 1965, the number of papers, patents, and other published r e p o r t s c i t e d by Chemical A b s t r a c t s was a l most as l a r g e as i n the 58 preceding years (37) . The i n c r e a s e i n the absolute amount of published products i s c o r r e l a t e d w i t h the r e l a t i v e l a c k of p r o d u c t i v i t y of an i n c r e a s i n g p r o p o r t i o n of a v a i l a b l e research monies and manpower. Present i n v e s t i g a t o r s are i n c r e a s i n g l y unaware of p r e v i o u s l y published data, p a r t i c u l a r l y the data which have r e c e n t l y become a v a i l a b l e or which have been published i n j o u r n a l s o u t s i d e of the i n v e s t i g a t o r ' s subd i s c i p l i n e . The l a r g e q u a n t i t y of published data and i t s d i s p e r s i o n i n almost innumerable j o u r n a l s and other p u b l i c a t i o n s e r i e s i s , in p a r t , m i t i g a t e d by the p r e p a r a t i o n of review a r t i c l e s . However, the problem s t i l l remains because time, i n the order of years, passes before s y n t h e s i z e r s (reviewers) garner the l a r g e q u a n t i t i e s of s c a t t e r e d new data and d i s t i l l i n f o r m a t i o n from them. A f u r ther problem i s that many of the reviews a r e a c o m p i l a t i o n of

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

"who

found what" i n s t e a d of a c r i t i c a l e v a l u a t i o n and s y n t h e s i s . The r a p i d r a t e of data generation and the s o p h i s t i c a t i o n of techniques and methods o f t e n r e q u i r e d f o r s i g n i f i c a n t research progress can make the products of many s m a l l , one-to-two year grant or c o n t r a c t s t u d i e s of minimal v a l u e . These short-term grants o f t e n do not permit the buildup of e x p e r t i s e necessary to make s i g n i f i c a n t research c o n t r i b u t i o n s . The short-term grants are not conducive to generating i n f o r m a t i o n about mechanisms and processes because observations must be terminated on a r e l a t i v e l y i n f l e x i b l e time schedule during which completion of a f i n a l r e p o r t i s i m p e r a t i v e . The time at which i t i s necessary to terminate observations and measurements o f t e n c o i n c i d e s w i t h the time at which f u r t h e r observations have the g r e a t e s t p o t e n t i a l to i n c r e a s e the i n v e s t i g a t o r s ' c h a r a c t e r i z a t i o n and knowledge of the system. (Obviously, the s m a l l discontinuous grant or cont r a c t may r e s u l t i n a s i g n i f i c a n t c o n t r i b u t i o n where i t complements or adds to an ongoing program.) The funding of long-term i n v e s t i g a t i v e programs at the n a t i o n a l l a b o r a t o r i e s and w i t h i n i n v e s t i g a t i v e governmental agencies has become i n c r e a s i n g l y d i f f i c u l t . I t must a l s o be noted that there has been a s h i f t i n grant and c o n t r a c t funding away from b a s i c research toward applied research. I n t e r d i s c i p l i n a r y Approach. The i n t e g r a t e d i n t e r d i s c i p l i n a r y approach to research i s d i f f i c u l t . That i t should be p o s s i b l e i s shown by, f o r example, the GEOSEX, Manganese Nodule, Deep Sea D r i l l i n g , and RANN (Research A p p l i e d to N a t i o n a l Needs; 38) p r o j e c t s (Pb at the U n i v e r s i t y of I l l i n o i s and M i s s o u r i , Mo at the U n i v e r s i t y of Colorado), funded by the N a t i o n a l Science Found a t i o n . I t would be h i g h l y d e s i r a b l e , and i t should be p o s s i b l e , f o r funding agencies to promote the common use of s e l e c t e d s i t e s and samples by i n v e s t i g a t o r s from the v a r i o u s d i s c i p l i n e s . In many cases, a good deal more i n f o r m a t i o n would be generated by the use of s p l i t s of samples, being c o l l e c t e d i n other s t u d i e s , the use of a r c h i v e d samples, and the use of i n t e r c a l i b r a t i o n and intercomparison samples than by the use of some l o c a l or d i s t a n t set of s p e c i a l samples. Obviously, c o n s i d e r a b l e care must be taken i n the s e l e c t i o n of such samples where unstable c o n s t i t u e n t s or absolute numbers of t r a c e c o n s t i t u e n t s are to be r e p o r t e d . More g e n e r a l l y , use of a l i m i t e d number of samples c o l l e c t e d i n previous or other on-going s t u d i e s would serve to t i e the v a r i o u s s t u d i e s together and to f a c i l i t a t e i n t e r p o l a t i o n and e x t r a p o l a t i o n . That i s , the time has come to concentrate on the generation of i n f o r m a t i o n as opposed to the accumulation of data. The v a r i o u s i n t e r d i s c i p l i n a r y groups d i f f e r s i g n i f i c a n t l y i n at l e a s t two ways. In some, there i s l i t t l e i n t e g r a t i o n of the substudies wherein each i n v e s t i g a t o r c a r r i e s out h i s own i n v e s t i g a t i o n more-or-less independently of the other substudies. In o t h e r s , the emphasis i s on i n t e r r e l a t i o n s h i p and p r i o r i t i e s are set by consensus. Integrated i n t e r d i s c i p l i n a r y groups a l s o d i f f e r i n the way they form. They may come about v i a management

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

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d e c i s i o n s or they may form v o l u n t a r i l y . The l a t t e r type are termed " a s s o c i a t i v e i n t e r d i s c i p l i n a r y groups." Integrated i n t e r d i s c i p l i n a r y groups appear to form only where at l e a s t p a r t of the funding i s i n common. The q u a l i t y of the s c i e n t i f i c products from i n t e g r a t e d i n t e r d i s c i p l i n a r y s t u d i e s i s n e a r l y always above average because of the m u l t i d i s c i p l i n a r y view p o i n t s and wide range of experience of the i n v e s t i g a t o r s t h e i r broad coverage of j o u r n a l s , and t h e i r p r o f e s s i o n a l c o n t a c t s . Such groups c o n s i s t i n g of a h a l f dozen or more p r o f e s s i o n a l members o f t e n have much l e s s elapsed time between a v a i l a b i l i t y of funds and s i g n i f i c a n t progress because a base, i n terms of experi e n c e , and a v a i l a b l e techniques, s u p p l i e s and equipment, i s normally much broader than that of smaller p r o j e c t groups. These l a r g e r groups r e q u i r e l e s s investment i n s m a l l equipment items than i s r e q u i r e d f o r the same number of p r o f e s s i o n a l s organized i n smaller p r o j e c t s and can f r e q u e n t l y a f f o r d more expensive equipment. A s s o c i a t i v e i n t e r d i s c i p l i n a r y groups can be formed by i n v e s t i g a t o r s (without common funds) w i t h overlapping i n t e r e s t s f o r the purpose of sharing r e s e a r c h - l i t e r a t u r e r e s u l t s , l i t e r a t u r e searching i n f o r m a t i o n , experience, equipment, space, support personnel, c o n t i n u i n g peer review, and encouragement. This i s the common academic approach to i n t e r d i s c i p l i n a r y s t u d i e s . D.R. Carson (3) b e l i e v e s that the s h i f t i n g of research emphasis towards the a p p l i e d end of the spectrum w i l l be accompanied by more l a r g e - s c a l e i n t e r d i s c i p l i n a r y research i n u n i v e r s i t i e s . However, i t should be noted that the language and conceptual b a r r i e r s encountered by those undertaking i n t e r d i s c i p l i n a r y research sometimes take months to years to overcome (39). S u c c e s s f u l i n t e r d i s c i p l i n a r y groups r e q u i r e c e r t a i n common attributes: 1. Both i n t e g r a t e d and a s s o c i a t i v e i n t e r d i s c i p l i n a r y groups r e q u i r e at l e a s t one of the f o l l o w i n g — a p r o j e c t d i r e c t o r , lead i n v e s t i g a t o r , c o o r d i n a t o r , or one or more strong i n d i v i duals to e s t a b l i s h and maintain coherence. A s s o c i a t i v e m u l t i d i s c i p l i n a r y groups may f u n c t i o n w i t h a committee (which may be informal) of s e n i o r i n v e s t i g a t o r s when there i s a shared research or i n t e l l e c t u a l commitment. 2. Senior i n v e s t i g a t o r s must get along w e l l or the group w i l l break down and cease to f u n c t i o n . 3. There must be frequent exchange of i n f o r m a t i o n (as opposed to data) v i a a common c o f f e e group and(or) p e r i o d i c seminars on problems, p r e l i m i n a r y f i n d i n g s , and p l a n s . This provides continuous "peer" review and c r i t i q u i n g of approaches and r e s u l t s . Exchanges of recent l i t e r a t u r e searches, d i s c o v e r i e s , and summaries of " i n progress" and unpublished s t u d i e s then occurs n a t u r a l l y . 4. The group, or i n d i v i d u a l p r o j e c t s contained t h e r e i n , should be of a m u l t i y e a r or c o n t i n u i n g nature. In p a r t i c u l a r , key l e a d e r s h i p and a n a l y t i c a l personnel should have a high

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degree of c o n t i n u i t y . One or more of the s e n i o r i n v e s t i g a t o r s must spend a great d e a l of time r e a d i n g , e x t r a c t i n g and s y n t h e s i z i n g i n f o r m a t i o n from l i t e r a t u r e and experimental data. This task can be shared among s e v e r a l of the p a r t i c i p a n t s , as i s g e n e r a l l y the case w i t h graduate students i n a s s o c i a t i v e m u l t i d i s c i p l i n a r y academic groups. 6. E s t a b l i s h i n g time goals f o r s u c c e s s i v e phases of study, and reviewing them p e r i o d i c a l l y , i s e s s e n t i a l to m a i n t a i n i n g the necessary sense of purpose and achievement. However, such time goals should be set c o n s e r v a t i v e l y and should be recognized as being g o a l s , not as a b s o l u t e s . The nature of s c i e n t i f i c i n q u i r y w i l l o f t e n preclude the attainment of such goals w i t h i n the i n i t i a l time frame. T h e i r c h i e f v a l u e i s as a y a r d s t i c k to measure the r e l a t i v e m e r i t of v a r i o u s promising and i n t e r e s t i n g substudies. 7. Formal and i n f o r m a l p o s t - d o c t o r a l and v i s i t i n g - s c i e n t i s t appointments provide new areas of needed e x p e r t i s e and provide accomodation f o r the normal year-to-year a c c o r d i o n - l i k e p a t t e r n of funding l e v e l s of both c o n t r a c t research and f e d e r a l agency programs, 8. The a d m i n i s t r a t i v e c h a i n needs to recognize the e x i s t e n c e of these groups, and to encourage, and, to some e x t e n t , to deal w i t h them as u n i t s . There are c e r t a i n problems p e c u l i a r t o , or at l e a s t more prominent w i t h , these i n t e r d i s c i p l i n a r y groups than w i t h the more conventional i n d i v i d u a l - s c i e n t i s t research s t u d i e s . These i n c l u d e the p r e p u b l i c a t i o n sharing of data as w e l l as s i g n i f i c a n t ideas and concepts, which obscures r i g h t s to authorship of papers and determination of s e n i o r authorship and of s e n i o r versus j u n i o r authorship. These are important matters because they impinge so h e a v i l y on p r o f e s s i o n a l standing and advancement, and on access to research funding. P r o f e s s i o n a l and i n s t i t u t i o n a l r e c o g n i t i o n of the e f f o r t s of the one or more i n v e s t i g a t o r s who c o n t r i b u t e a s i g n i f i c a n t or even a major p o r t i o n of t h e i r time to c o o r d i n a t i o n i s perhaps the most d i f f i c u l t problem, p a r t i c u l a r l y w i t h a s s o c i a t i v e groups, as V.C. Kennedy p o i n t s out (U.S. Geol. Survey, w r i t ten communication, 1978): "Most s c i e n t i s t s do q u a l i t y research f o r one or both of two reasons; f i r s t , they wish to s a t i s f y t h e i r i n t e n s e i n t e r e s t or c u r i o s i t y i n a c e r t a i n f i e l d , or second, they see a f i n a n c i a l or p r o f e s s i o n a l advancement from the s u c c e s s f u l completion of the work. Success i n the second case almost guarantees success i n the f i r s t . In s c i e n c e , success i n the second case commonly i s t i e d to r e c o g n i t i o n of the s c i e n t i s t as a generator of new ideas or techniques as i n d i c a t e d by f i r s t authorship on published t a l k s or papers. The use of "Doe, et a l . " to cover s e v e r a l authors of a paper i s an example of the downgrading of other c o n t r i b u t o r s to a paper. There i s no widely 5.

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recognized means of d i s t i n g u i s h i n g coauthors from j u n i o r authors. The r e s u l t i s that o f t e n each member of a cooperating group gets to p u b l i s h a short paper w i t h himself as s e n i o r author and others as j u n i o r authors. Because numbers of papers a r e important a l s o , we get a r e p e t i t i o n , or near r e p e t i t i o n , of the same ideas spread w i d e l y . S i n g l e , comprehensive r e p o r t s by m u l t i p l e authors of approximately equal s c i e n t i f i c s t a t u s are r a r e except where one i n d i v i d u a l c a r r i e s much of the load or the r e p o r t i s broken down to i d e n t i f y separate p a r t s by authorship. One thought i s to use some other i d e n t i f i c a t i o n of authorship i n s t e a d of one person's name. Perhaps the use of an acronym f o r the authors and an acknowledgements s e c t i o n d e s c r i b i n g each person's c o n t r i b u t i o n would work i n some i n s t a n c e s . " An example of the use of an acronym i s that of FOAM (Friends of Anoxic Mud, 4 0 ) . A s i g n i f i c a n t p a r t of the research e f f o r t of i n t e r d i s c i p l i nary s t u d i e s i s , and w i l l be, done by graduate students and postd o c t o r a l f e l l o w s , A major problem i n the u t i l i z a t i o n of graduate students and p o s t - d o c t o r a l f e l l o w s i s the matter of p r e p a r a t i o n of j o u r n a l a r t i c l e s from the research work. A completed t h e s i s i s i n and of i t s e l f o f t e n a s u f f i c i e n t l y traumatic experience that immediately proceeding to the p r e p a r a t i o n of j o u r n a l a r t i c l e s i s a d i f f i c u l t task f o r most advanced degree r e c i p i e n t s to f a c e . I n deed, t h e s i s completion i n v a r i a b l y continues beyond the a n t i c i p a ted date so that there i s r a r e l y time to work on follow-up j o u r n a l a r t i c l e s . I t i s g e n e r a l l y d i f f i c u l t to get p o s t - d o c t o r a l f e l l o w s to summarize t h e i r research a t r e g u l a r i n t e r v a l s . There are i n e v i t a b l y one o r two more very important things which they f e e l must be i n v e s t i g a t e d before t e r m i n a t i o n of the appointment. I n t h i s way, the b l o c k of time reserved f o r p r e p a r a t i o n of j o u r n a l a r t i c l e s can e a s i l y evaporate. However, t h i s d i f f i c u l t y may be minimized by a " c o n t r a c t " to the e f f e c t that the t h e s i s or postgraduate study i s incomplete u n t i l i t has been reduced to acceptable d r a f t s of j o u r n a l a r t i c l e s . Research s c i e n t i s t s i n v o l v e d i n i n t e r d i s i c p l i n a r y research i n the m i s s i o n - o r i e n t e d F e d e r a l Agencies must face an a d d i t i o n a l c o n f l i c t between the "need" to w r i t e papers and the "need" a t the agency to achieve c e r t a i n o b j e c t i v e s w i t h i n a f i x e d time p e r i o d . Often a complex choice must be made between these two demands on the s c i e n t i s t . On the one hand, i t can be argued that the i n i t i a l phases of a study were a t the taxpayer's expense so they must be reported i n the a r c h i v a l l i t e r a t u r e . On the other hand, p r e s s i n g on to the fundamental g o a l of an adequate understanding of some process o r to the development of some model may i n some instances be considered of s u f f i c i e n t importance t o o v e r r i d e the need f o r and v a l u e of any i n t e r i m s c i e n t i f i c paper(s)*. I t seems c l e a r

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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that i f the p r e l i m i n a r y or i n i t i a l f i n d i n g s are such that they can reasonable be expected to a f f e c t ongoing or i n - t h e - p l a n n i n g stage research elsewhere s i g n i f i c a n t l y then the f i n d i n g should be published w i t h the d i s p a t c h . I f the primary v a l u e w i l l be an a d d i t i o n to some author's b i b l i o g r a p h y and l i s t of p r o j e c t r e p o r t s , then the p u b l i c i n t e r e s t may be e q u a l l y w e l l served by moving as e x p e d i t i o u s l y as p o s s i b l e to the f i n a l o b j e c t i v e . This approach i s hazardous i n the case of a s i n g l e i n v e s t i g a t o r i n that a change of j o b or an untimely death may cause the l o s s of the monies which supported man-years of research. Graduate T r a i n i n g . Another avenue to increased s c i e n t i f i c progress i n c l u d e s the improved academic p r e p a r a t i o n of research s c i e n t i s t s . We should, of course, heed the admonition of Hubbert (41) " — t h a t t h i n k i n g i s p e c u l i a r l y an i n d i v i d u a l e n t e r p r i s e , and that the g r e a t e s t of a l l s c i e n t i f i c achievements — those of the great s y n t h e s i z e r s from G a l i l e o to E i n s t e i n — have, almost without e x c e p t i o n , been the work of individuals." T r a i n i n g i n s c i e n t i f i c research i s presumed to be of a h i g h q u a l i t y i n t h i s country, but i s i n f a c t not i n v a r i a b l y so. Such t r a i n i n g f o r the Ph. D. degree should i n c l u d e : 1) a summer i n f i e l d work (outside of the t h e s i s t o p i c i f necessary) and a summer i n l a b o r a t o r y work; 2) experience i n the p r e p a r a t i o n of c r i t i c a l l i t e r a t u r e reviews; 3) a t l e a s t one t e c h n i c a l paper submitted f o r p u b l i c a t i o n before t h e s i s w r i t i n g i s s t a r t e d ; 4) arrangements f o r adequate t e c h n i c a l s u p e r v i s i o n of t h e s i s research; 5) adequate knowledge p l u s experience i n the b e h a v i o r a l sciences (pyschology, s o c i o l o g y , e t c . ) ; and 6) experience i n one a s s o c i a t i v e i n t e g r a t e d research study. C o n s i d e r a t i o n should be given to dropping nont h e s i s advanced degrees from both the p h y s i c a l and b i o l o g i c a l sciences. Some d i s c u s s i o n of the f i r s t of the above t r a i n i n g items may be i n order. I n t h i s day of s p e c i a l i z a t i o n , i t may seem strange to r e q u i r e a t h e o r e t i c a l chemist to spend a summer i n a f i e l d ecosystem study. This aspect of graduate education i s intended to f a c i l i t a t e i n t e r d i s c i p l i n a r y research and to provide the experience t o a s s i s t i n b r i d g i n g the gap between f i e l d and l a b o r a t o r y research. S i m i l a r l y , the research achievements of many b r i g h t young graduates are l i m i t e d more by t h e i r i n a b i l i t y to work w e l l w i t h others than by l i m i t a t i o n of i n t e l l i g e n c e or w i l l i n g n e s s to work hard. Some focused t r a i n i n g i n b e h a v i o r a l sciences might w e l l increase the i n t r i n s i c value of graduates t r a i n e d i n t h i s way. Approaches and P r i o r i t i e s How, then, can the progress of Science i n general and of chemical modeling i n p a r t i c u l a r be promoted i n the f o l l o w i n g

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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decade? Several avenues e x i s t . I t seems c l e a r that one avenue i s f o r research r e l a t i n g to chemical modeling to be c a r r i e d out w i t h i n the context of i n t e g r a t e d i n t e r d i s c i p l i n a r y , m u l t i s c i e n t i s t s t u d i e s . As R u s s e l l Peterson (42) has r e c e n t l y pointed out, " . . . s p e c i a l i z a t i o n has been necessary f o r s c i e n t i f i c and t e c h n o l o g i c a l advance; and we have learned much more and learned i t q u i c k l y by breaking down phenomena i n t o v a r i o u s compartments and studying them from the standpoints of b i o l o g y , p h y s i c s , chemistry, and so f o r t h . But we must remember that our world does not e x i s t i n compartments; i t comes i n s i n g l e , i n t e r r e l a t e d communities, each part of which a f f e c t s other parts.... i t would do w e l l f o r s c i e n t i s t s to e s t a b l i s h the h a b i t of stepping back o c c a s i o n a l l y from the immediate task to r e f l e c t on what k i n d of world he or she wants f o r h i s or her c h i l d r e n and g r a n d c h i l d r e n and cons i d e r whether the work he or she i s doing i s l e a d i n g i n the r i g h t d i r e c t i o n . I f not, the s c i e n t i s t should have the courage to a l t e r h i s course, even i f i t means some near-term personal s a c r i f i c e . I t r e q u i r e s courage to speak and a c t e f f e c t i v e l y a g a i n s t the s t a t u s quo. But we can b r i n g about the e s s e n t i a l changes. To do so w i l l r e q u i r e that we look at things comprehensively and work toward worldwide goals that provide f o r improving the q u a l i t y of l i f e everywhere." And as E. Odum (43) has s t a t e d so s u c c i n t l y , " I t i s s e l f - e v i d e n t that science should not only be r e d u c t i o n i s t i n the sense of seeking to understand phenomena by d e t a i l e d study of s m a l l e r and smaller components, but a l s o s y n t h e t i c and h o l i s t i c i n the sense of seeking to understand l a r g e components as f u n c t i o n a l wholes,-—Science and technology during the past h a l f century have been so preoccupied w i t h reductionism that s u p r a i n d i v i d u a l systems have s u f f e r e d benign n e g l e c t . We are abysmally ignorant of the ecosystems of which we are dependent p a r t s . — t h e time has come to g i v e equal time, and equal research and development funding, to the higher l e v e l s of b i o l o g i c a l o r g a n i z a t i o n i n the h i e r a r c h i a l sequences. I t i s i n the p r o p e r t i e s of the l a r g e - s c a l e , i n t e g r a t e d systems that h o l d s o l u t i o n s to most of the long-range problems of society." C l e a r l y , i n t e g r a t e d , i n t e r d i s c i p l i n a r y , m u l t i s c i e n t i s t e f f o r t s are r e q u i r e d to achieve the goal of p r e d i c t i v e chemical modeling of aquatic ecosystems and to make progress toward the goal of understanding the b i o a v a i l a b i l i t y of t r a c e elements to the v a r i o u s food chain components w i t h i n the next decade. Funding agencies could w e l l develop ways of p e r m i t t i n g and even of encouraging i n d i v i d u a l i n v e s t i g a t o r s to continue along given broad t o p i c a l or technique pathways to enhance the production

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Goals, Problems, Approaches, and Priorities

17

of s i g n i f i c a n t i n f o r m a t i o n — a s opposed to gathering more data. This c o n t i n u i t y of i n v e s t i g a t i v e area or i n v e s t i g a t i v e techniques appears to be e s s e n t i a l to i n c r e a s e the i n t r i n s i c value of r e search products. The a c c e l e r a t i o n i n r a t e of data generation by the s c i e n t i f i c community and the s o p h i s t i c a t i o n of techniques and methods o f t e n r e q u i r e d f o r s i g n i f i c a n t research progress r e q u i r e the b u i l d u p of e x p e r t i s e and equipment. The r a p i d changes i n s o c i e t a l concerns and governmental programs o f t e n r e s u l t i n p r i n c i p a l i n v e s t i g a t o r s ' t a k i n g on s t u d i e s i n t a n g e n t i a l or even t o t a l l y new areas t h i s d i v e r s i o n of e f f o r t f r e q u e n t l y d e t r a c t s from o r i g i n a l goals and from b u i l d i n g up of e x p e r t i s e i n a given area. The net e f f e c t of these c o n d i t i o n s i s that too many i n v e s t i gators are: 1) i n s u f f i c i e n t l y knowledgeable of published i n f o r mation; 2) unacquainted w i t h the conventional wisdom of the f i e l d ; and 3) unable to secure adequate funding of s u f f i c i e n t d u r a t i o n to maximize the s c i e n t i f i c value of t h e i r t e c h n i c a l products. The l a c k of p e r t i n e n t r e l i a b l e thermodynamic data needed f o r chemical modeling i s a problem which could be r e s o l v e d i n the near f u t u r e f o r waters to which i o n - a s s o c i a t i o n models a r e a p p l i c a b l e . This progress would be f a c i l i t a t e d by a coordinated e f f o r t among the funding agencies, the c o m p i l a t i o n - e v a l u a t i o n agencies, and p r o f e s s i o n a l s o c i e t y groups. S a t i s f a c t o r y modeling of b r i n e s and other concentrated waters w i l l r e q u i r e a major focused e f f o r t . The p r i o r i t y of v a r i o u s n a t i o n a l problems to whose s o l u t i o n chemic a l modeling could c o n t r i b u t e needs to be e s t a b l i s h e d . A s i m i l a r e v a l u a t i o n and p r i o r i t i z a t i o n i s needed f o r k i n e t i c data. Summary Trust i n , and support f o r , s c i e n t i f i c i n v e s t i g a t i o n of fundamental processes has weakened i n the l a s t decade. I n c r e a s i n g emphasis has been given to the "black-box" type of modeling s o l u t i o n s to environmental problems. The u l t i m a t e goal of r e search i n g e n e r a l , and of chemical modeling i n p a r t i c u l a r , i s human betterment. I hope t h i s symposium w i l l c o n t r i b u t e toward t h i s primary o b j e c t i v e . The immediate goals of t h i s symposium are the promoting of the t i m e l y exchange of p e r t i n e n t i n f o r m a t i o n ; the f o s t e r i n g of extensive d i s c u s s i o n of g o a l s , problems, approaches, as w e l l as research p r i o r i t i e s ; and the p r e p a r a t i o n of a symposium proceedings. The p r i n c i p a l r e s t r i c t i o n s on u s e f u l a p p l i c a t i o n of chemical models, to problems ranging from aqueous s p e c i a t i o n to b i o a v a i l a b i l i t y i n the human d i g e s t i v e system, i n c l u d e inadequate knowledge of n a t u r a l organic l i g a n d s ; of redox processes; of thermodynamic, time-dependent and k i n e t i c i n f o r mation and anthropogenic i n p u t s ; and inadequate c h a r a c t e r i z a t i o n of the n a t u r a l systems being modeled. What many observers cons i d e r a low r a t e of progress being made i n s o l v i n g these problems i s due, i n p a r t , to the short term of funding a v a i l a b l e t o many i n v e s t i g a t o r s , to inadequate cognizance and s y n t h e s i s of the a v a i l a b l e t e c h n i c a l l i t e r a t u r e s on the p a r t of some i n v e s t i g a t o r s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

18

CHEMICAL MODELING IN AQUEOUS SYSTEMS

and t o the l a c k of i n t e g r a t e d , i n t e r d i s c i p l i n a r y , s t u d i e s i n many of the systems being modeled.

multiscientist

Acknowledgments P l e a s u r a b l e d i s c u s s i o n s w i t h Ronald James, Jack F e t h , A l a n Jackman, Ralph Cheng, Ray Wildung, and Harvey Drucker, but most e s p e c i a l l y w i t h Vance Kennedy, are acknowledged. Donald Langmuir, Harry L e l a n d , Frank T r a i n e r , and p a r t i c u l a r l y David R i c k e r t provided v e r y h e l p f u l reviews.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch001

Abstract The ultimate goal of research in support of chemical modeling, as in most scientific research, is the improvement of human life physically, emotionally, and aesthetically through the understanding and prediction of processes and events. More immediate objectives include reliable aqueous speciation of trace elements between their valence states and among organic and inorganic ligands, and prediction of sorption equilibria and kinetic parameters as well as solubility controls on trace element solute levels. The objective is to predict toxicity and bioaccumulation in aquatic organisms and ultimately in man. The complex and interactive nature of aqueous speciation (redox states, organic and inorganic complexation) and of sorption-desorption by sediment and biota, and the precipitation-dissolution of solid phases,requires comprehensive models and extensive analyses of an adequate number of appropriately collected and preserved samples. The prediction of chronic toxicity and bioaccumulation of trace elements w i l l require the linking of chemical models to biologic and hydrologic models. This mandates a multiscience, interdisciplinary approach to the development, testing, and application of comprehensive models to assist in solving and preventing environmental problems. Important restraints on the evolution of superior chemical models are the inadequacies in the: 1) capability to characterize the organic ligands of natural waters; 2) knowledge of redox status of waters, which does not permit r e a l i s t i c computation of redox-controlled speciation; 3) available thermodynamic data; 4) knowledge of the time dependency of the various processes in general (such as variation in apparent trace-element concentration during a t i d a l cycle) and kinetic data for chemical and biological processes in particular; and 5) error estimates for the preceding restraints. The f i e l d of chemical modeling is also constrained by the: 6) need for comprehensive analyses of the systems to be modeled; 7) limitations of published data; 8) scarcity of adequate literature reviews; 9) d i f f i c u l t i e s in organizing and conducting integrated interdisciplinary studies; and to a lesser extent, 10) limitation in quality of graduate training.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1.

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Goals, Problems, Approaches, and Priorities

19

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Abelson, P. H. A report from the research community. Science 194, 482 (1976). McElroy, W. D. The global age: Roles of basic and applied research. Science 196, 267-270 (1977). Krieger, J. H. Universities face wide-ranging changes. Chem. Eng. News, Feb. 16, 395 (1976). Schindler, D.W. The impact boondoggle. Science 192,509 (19 76). Jenne, Ε. Α . , Girvin, D. C., B a l l , J . W., and Burchard, J.M. Inorganic speciation of silver in natural waters - fresh to marine, Ch. 4, p. 41-61, (Cum. B i b . , p. 217-270) in Klein, D. Α . , ed., "Environmental Impacts of Nucleating Agents Used in Weather Modification Programs." 270 p. Dowder, Hutchinson & Ross, Stroudsberg, Pennsylvania, 1978. Kester, D . , Byrne, R. H . , Jr., and Liang, Y.-J. Redox reactions and solution complexes of iron in marine systems. Amer. Chem. Soc. Symp. Ser. 8, 56-79 (1975). Nordstrom, D. Κ., Plummer, L . N . , and others. A comparison of computerized chemical models for equilibrium calculations in aqueous systems in Jenne, Ε. Α . , ed., "Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubility, and Kinetics." Amer. Chem. Soc. 1978 (This volume). Leenheer, J . Α . , and Huffman, E . W. D . , J r . Classification of organic solutes in water using macroreticular resins. U. S. Geol. Survey J. Res. 4(6), 737-751 (1976). Parker, V. B . , Wagman, D. D . , and Evans, W. H. Selected values of chemical thermodynamic properties. Tables for the alkaline earth elements (Elements 92 through 97 in the Standard Order of Arrangement). Nat'l Bur. Stand. Tech. Note 270-6, 106 p. (1971). Schumm, R. Η . , Wagman, D. D . , Bailey, S., Evans, W. Η . , and Parker, V. B. Selected values of chemical thermodynamic properties. Tables for the Lanthanide (Rare Earth) elements (Elements 62 through 76 in the Standard Order of Arrangement). Nat'l Bur. Stand. Tech. Note 270-7, 75 p. (1973). Wagman, D. D . , Evans, W. Η., Parker, V. B . , Halow, I . , Bailey, S. M . , and Schumm, R. H. Selected values of chemi­ cal thermodynamic properties. Table for the f i r s t t h i r t y four elements in the Standard Order of Arrangement. Nat'l Bur. Stand. Tech. Note 270-3, 264 p. (1968). Wagman, D. D . , Evans, W. Η . , Parker, V. B . , Halow, I . , Bailey, S. Μ., and Schumm, R. H. Selected values of chemi­ cal thermodynamic properties. Tables for elements 35 through 53 in the Standard Order of Arrangement. Nat'l Bur. Stand. Tech. Note 270-4, 141 p. (1969). Wagman, D. D., Evans, W. Η . , Parker, V. B . , and Schumm, R. H. Chemical thermodynamic properties of compounds of sodium, potassium, and rubidium: An interim tabulation of selected values. Nat'l Bur. Stand.Interagency Report 761034, 76 p. (1976).

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Wagman, D. D., Schumm, R. H., and Parker, V. B. A computer assisted evaluation of the thermochemical data of the com­ pounds of thorium. N a t ' l Bur. Stand. Interagency Report 77-1300, 94 p. (1977). Barnes, H. L., ed., "Geochemistry of Hydrothermal Ore De­ posits." John Wiley, New York (in press). Martell, A. E., and Smith, R. M. " C r i t i c a l Stability Con­ stants, V o l . 1, Amino Acids." 469 p. Plenum Press, New York, 1974. Smith, R. Μ., and Martell, A. E . "Critical Stability Con­ stants, V o l . 2, Amines." 415 p. Plenum Press, New York, 1975. Smith, R. Μ., and Martell, A. E., "Critical Stability Con­ stants, V o l . 4, Inorganic Complexes." 257 p. Plenum Press, New York, 1976. Martell, Α. Ε., and Smith, R. M. " C r i t i c a l Stability Con­ stants, V o l . 3, Other Organic Ligands." 495 p. Plenum Press, New York, 1977. Christensen, J. J., Eatough, D. J., and Izatt, R. M. "Hand­ book of Metal Ligands Heats and Related Thermodynamic Quantities," 2nd ed. 495 p. Marcel Dekker, New York, 1975. Baes, C. F., Jr., and Mesmer, R. E . "The Hydrolysis of Cations," 489 p. John Wiley, New York, 1976. Langmuir, Donald. "Uranium Solution-Mineral Equilibria at Low Temperatures with Applications to Sedimentary Ore De­ posits." 39 p . , Penn. State U n i v . , University Park, Pennsylvania, 1977. Robie, R. Α . , Hemingway, B. S., and Fish, J. R. Thermo­ dynamic properties of minerals and related substances at 298.15K and one bar (10 Pascals) pressure and at higher temperatures. U. S. Geol. Survey B u l l . 1452, 456 p. (1978). Reardon, E . J., and Langmuir, Donald. Thermodynamic pro­ perties of the ion pairs MgCO° and CaCO° from 10 to 50°C. Amer. J. S c i . 274, 599-612 (1974). Pytkowicz, R. Μ., ed. ''Activity Coefficients in Electrolyte Solutions." CRC Press Inc., Palm Spring, Florida (in press). Hunt, J. P. "Metal Ions in Aqueous Solutions." 124 p . , W. A. Benjamin, New York, 1963. Langmuir, Donald. Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Jenne, Ε. Α . , ed., "Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubility, and Kinetics·" Amer. Chem. Soc. 1978 (This volume). Cloke, P. L . The geologic role of polysulfides —Part I I . The solubility of acanthite and covellite in sodium poly­ sulfide solutions. Geochim. Cosmochim. Acta 27, 1299-1319 (1963). Vanoni, V. V., ed. "Sediment Engineering." 745 p. Task Committee, Amer. Soc. C i v i l Eng., New York, 1975. 5

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3

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3

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Goals, Problems, Approaches, and Priorities

21

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G i r v i n , D. C., Hodgson, A . T . , and Jenne, E . A . S p a t i a l and seasonal v a r i a t i o n s of silver, cadmium, copper, nickel, lead, and z i n c in South San F r a n c i s c o Bay water during two consecutive drought y e a r s . Lawrence Berkeley Lab. UCID-4031. U n i v . Calif., B e r k e l e y , California, 1978. 31. Anonymous. Symposium scores misuse of scientific data. Chem. Eng. News, Feb. 10, 17 (1975). 32. Ball, J. W., Jenne, Ε. Α . , and Nordstrom, D. K., WATEQ2—a computerized chemical model f o r t r a c e and major element s p e c i a t i o n and m i n e r a l equilibria of n a t u r a l waters, i n Jenne, Ε. Α . , ed., "Chemical Modeling i n Aqueous Systems, S p e c i a t i o n , S o r p t i o n , Solubility, and Kinetics." Amer. Chem. Soc. 1978 (This volume). 33. Jenne, Ε. Α . , and Luoma, S. N . The forms of t r a c e elements in soils, sediment, and a s s o c i a t e d waters: An overview of their determination and biological availability, p . 110-143, in Wildung, R. E., and Drucker, H., e d . , " B i o l o g i c a l I m p l i ­ c a t i o n s of M e t a l s in the Environment." CONF-750929, NTIS Springfield, Virginia, 1977. 34. K a r i , T a r j a , and Kauranen, Pentti. Mercury and selenium contents of s e a l s from fresh and b r a c k i s h waters in F i n l a n d . Bull. E n v i r o n . Contam. T o x i c o l . 19(3), 273-280 (1978). 35. Freeman, H . C., Shum, G., and Uthe, J. F . The selenium con­ tent in swordfish ( X i p h i a s g l a d i n s ) in relation to t o t a l mercury content. J . E n v i r o n . Sci. H e a l t h A13, 235-240 (1978). 36. Anonymous. Which way now for j o u r n a l s ? Nature 262, 731 (1976) 37. Baker, D. E . Recent trends in growth of chemical literature. Chem. Eng. News, May 10, 23-27 (1976). 38. Lomask, Milton. "A Minor M i r a c l e - An Informal H i s t o r y of the N a t i o n a l Science F o u n d a t i o n . " 286 p . Nat'l Sci. Foundation, Washington, D. C., 1976. 39. Ross, J. E . The Institute f o r Environmental S t u d i e s : U n i v . of Wisconsin-Madison. Chemosphere 5, 185-198 (1973). 40. FOAM (Friends of Anoxic Mud): Goldhaber, M . B., Aller, R. C., Cochran, J. K., Rosenfeld, J. Κ . , Martens, C. S . , and Berner, R. A . S u l f a t e r e d u c t i o n , d i f f u s i o n , and b i o t u r b a t i o n i n Long I s l a n d Sound sediments: Report of the FOAM group. Amer. J. S c i . 277, 193-237 (1977). 41. Hubbert, M. K. Are we r e t r o g r e s s i n g in Science? G e o l . Soc. Amer. Bull. 74, 365-378 (1963). 42. P e t e r s o n , R u s s e l l . S c i e n t i s t s and the holistic approach. Chem. Eng. News, May 15, 3 (1978). 43. Odum, E . P . The emergence of ecology as a new i n t e g r a t i v e discipline. Science 195, 1289-1293 (1977). Disclaimer: The reviews expressed and/ or the products mentioned in this article repre­ sent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2 Sulfur Speciations and Redox Processes in Reducing Environments JACQUES BOULEGUE and GIL MICHARD

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

Laboratoire de Géochimie des Eaux, et Laboratoire de Géochimie et Cosmochime (CNRS LA 196), Université de Paris 7, 2 Jussieu, 75221 Paris Cedex 05, France N a t u r a l w a t e r s a r e g e n e r a l l y i n a dynamic r a t h e r t h a n an e q u i l i b r i u m c o n d i t i o n , and t h e c o n c e p t o f a s i n g l e r e d o x s y s t e m c h a r a c t e r i s t i c o f a g i v e n w a t e r c a n n o t be m a i n t a i n e d . I n t h e most f a v o r a b l e c a s e , measurements o f Eh can be r e l a t e d t o a p a r t i c u l a r r e d o x s y s t e m o r s y s t e m s i n p a r t i a l e q u i l i b r i u m ( 1 ] . The r e d o x s y s t e m must be e l e c t r o c h e m i c a l l y r e v e r s i b l e a t t h e s u r f a c e o f t h e p l a t i n u m e l e c t r o d e a t a r a t e t h a t i s r a p i d compared w i t h t h e e l e c t r o n d r a i n o r s u p p l y by way o f t h e m e a s u r i n g e l e c t r o d e [2]. In n a t u r a l w a t e r s , o n l y t h e Fe ( I I ] / F e ( I I I ) and t h e F ^ S / S ^ systems c o r r e s p o n d t o t h e s e l i m i t a t i o n s ( 3 , 4 , 5 ] . The e l e c t r o c h e m i c a l r e a c t i o n s o f h y d r o g e n s u l f i d e and p o l y s u l f i d e i o n s a r e known t o be r a p i d a t t h e s u r f a c e o f t h e p l a t i n u m e l e c t r o d e (J3). Hence, t h e p o t e n t i a l s o b t a i n e d i n t h e p r e s e n c e o f t h e s e s p e c i e s s h o u l d be e x p l a i n a b l e i n terms of e q u i l i b r i u m of t h e redox c o u p l e H2S/S^~; e v e n t u a l l y t h e s e p o t e n t i a l s m i g h t be u t i l i z e d t o i n f e r r e d o x p r o c e s s e s i n v o l v i n g t h e s u l f u r s p e c i e s . Nore i n f o r m a t i o n i s needed about t h e r e l a t i o n between measured r e d o x p o t e n t i a l s (Eh) and s u l f u r chemistry i n reducing environments. R e d u c i n g e n v i r o n m e n t s a r e f r e q u e n t l y c h a r a c t e r i z e d by t h e p r e s e n c e o f h y d r o g e n s u l f i d e (even i n v e r y low c o n c e n t r a t i o n ) . I n t h e s e e n v i r o n m e n t s s e v e r a l f a c t o r s , s u c h as s l o w d i f f u s i o n o f oxygen, t h e p r e s e n c e o f F e ( I I I ) m i n e r a l s o r o r g a n i c m a t t e r , may r e s u l t i n the incomplete o x i d a t i o n of H2S, which y i e l d s p o l y s u l f i d e i o n s (S^~) and t h i o s u l f a t e ( S 0 § " K -

:

2

2 HS"

+ CU

(1/4)

•>

HS~

ο

+ (n-1)/8

2 S^" "

+

2 HS~

+ 2 G

2

+ 2 0H~

S

Ζ

S

Q

Ζ

+ FLO 2 2

->

H

S G^~ 2 3

+ S~ + 2 n-1

2

0

S 0 " 2

2

+

S2" +

+ H 0 2

H

+

.

The above r e a c t i o n s p r o c e e d e a s i l y and r a p i d l y i n t h e p h y s i c o c h e m i c a l c o n d i t i o n s p r e v a i l i n g i n n a t u r a l w a t e r s ( 5 , 9 ) . Thus,

0-8412-0479-9/79/47-093-025$06.50/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

26

p o l y s u l f i d e i o n s s h o u l d be f o u n d i n r e d u c i n g e n v i r o n m e n t s where the o x i d a t i o n of has been i n c o m p l e t e . P o l y s u l f i d e i o n s can a l s o be p r o d u c e d v i a d i r e c t r e a c t i o n between h y d r o g e n s u l f i d e and b a c t e r i a l l y produced elemental s u l f u r (5,10): HS"

+ (n-1)/8 S

Q

t

SJ*~ + H

+

.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

The d i s t r i b u t i o n o f t h e s u l f u r s p e c i e s between s u l f i d e and p o l y s u l f i d e at e q u i l i b r i u m w i t h elemental c o l l o i d a l s u l f u r i s repres e n t e d i n F i g u r e 1. I n t h i s r e p o r t we examine t h e aqueous s o l u t i o n s a s s o c i a t e d w i t h reducing environments i n r e l a t i o n t o the composition and redox p r o p e r t i e s of s u l f u r s p e c i e s . Experimental The s t u d y o f t h e e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e s u l f i d e / p o l y s u l f i d e r e d o x c o u p l e have been a c h i e v e d by measurements o f pH, Eh ( t h e p o t e n t i a l o f a p l a t i n u m e l e c t r o d e ) and E 2 - ( t h e p o t e n t i a l o f a membrane A g / A g S e l e c t r o d e ) , t h e r e f e r e n c e e l e c t r o d e b e i n g a double j u n c t i o n Ag/AgCl e l e c t r o d e . P o t e n t i a l s g i v e n are r e f e r r e d t o normal hydrogen e l e c t r o d e ( H / H ) p o t e n t i a l . To p r e v e n t any i n t e r f e r e n c e o f oxygen i n t h e c h e m i c a l process e s a l l t h e measurements ( l a b o r a t o r y and f i e l d ) were done i n an e l e c t r o c h e m i c a l c e l l under p u r i f i e d n i t r o g e n atmosphere. D e t a i l s a r e p u b l i s h e d e l s e w h e r e ( 1 1 ) . F i e l d s a m p l i n g was done f r o m a g l o v e box u n d e r n i t r o g e n a t m o s p h e r e . The t e c h n i q u e o f s t u d y o f t h e e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e s u l f u r s p e c i e s was t o measure pH, Eh and Eg2- on aqueous s o l u t i o n s a f t e r i m p o s e d pH v a r i a t i o n s . The pH v a r i a t i o n s were o b t a i n e d by HC1 o r NaQH a d d i t i o n s . Eh, Eg2- and pH were measured when t h e e.m.f. had s t a b i l i z e d . A m o d e r a t e s t i r r i n g was m a i n t a i n e d during a l l t h e measurements. I n a d d i t i o n , t h e c o n c e n t r a t i o n s o f H2S, S^~, S Q , S D § " , SU3~ were measured b e f o r e and a f t e r each e x p e r i m e n t e i t h e r by t h e s t a n d a r d i o d i n e method o r f o l l o w i n g t h e method o f Boulègue e t a l . (1_2) . s

2

+

2

2

Electrochemical

study

o f t h e H^S-H^O and

H?S-Sfl-H D s y s t e m s 7

The r e s u l t s o f pH, Eh and Eg2- measurements a f t e r i m p o s e d pH v a r i a t i o n s c a n be u t i l i z e d t o c h a r a c t e r i z e t h e d i s s o l v e d s u l f u r s p e c i e s . T h i s can be a c h i e v e d by c o n s i d e r i n g s u c c e s s i v e l y t h e p H - E 2 - r e l a t i o n s , t h e Eh-pH r e l a t i o n s and t h e Eh-Eg2- r e l a t i o n s . s

The pH-Ec^2- r e l a t i o n s . The ρΗ-Ε$2- r e l a t i o n s a r e c h a r a c t e ­ r i s t i c of the c o n c e n t r a t i o n of the t o t a l d i s s o l v e d hydrogen s u l ­ f i d e ( Z | H S | ) . In the presence of o n l y d i s s o l v e d hydrogen s u l f i d e one can w r i t e : E 2 - = E§2- - 0 . 0 2 9 5 1 o g ( S " ) 2

2

s

and

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Sulfur Speciations and Redox Processes

BOULEGUE AND M i C H A R D

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

-3i

4

5

6

7

8

9

10 pH

Figure 1. Distribution of sulfur species in the system H S-S (colloid)-H 0— NaCl (0.7M) for %[S] = 10' g-atom/kg.Log (molality) vs. pH at 298 K. &G (Sscoll) = 3.5 kj/mol S (28). 2

8

3

2

298

8

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

28

2

E|H S|/(S -) = 2

+

{1/γ 2-} +

ίίΗ )/Κ ·γ -}

3

Α 2

+

+

Η 3

2

ÎCH ) /K

A r

K

-Y

A 2

H 2 S

}

w i t h ( i ) = a c t i v i t y o f t h e i"th s p e c i e s , = activity coefficient of the s p e c i e s i , K = CHS")·(H )/(H S), K = (S ~)•(H )/(HS"). T h u s , E 2 - i s a s i m p l e f u n c t i o n o f pH a t c o n s t a n t E [ H S J . T a k i n g p K ^ = 7.01 and p K = 14 [5), one c a n w r i t e : +

2

A 1

2

+

A 2

3

2

A

A 2

E 2-

= E§2- - 0 . 0 2 9 5 1 o g ( E [ H s ] )

s

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

+ 0.02951og(4.854

+ 1.455

χ 10

1 4

"

p H

2

+ 8.734 χ

10 °"

2 p H

)

w h i c h i s v a l u a b l e i n t h e H S - H 0 - N a C l ( 0 . 7 NJ s y s t e m . The measure­ ments o f E 2 - and pH i n t h e H S - H 0 - N a C l ( 0 . 7 MJ s y s t e m have g i v e n r e s u l t s i n good a g r e e m e n t w i t h t h e v a l u e s p r e d i c t e d f r o m t h e above e q u a t i o n (see F i g u r e 2 ) , (5,11). I n t h e p r e s e n c e o f h y d r o g e n s u l f i d e , p o l y s u l f i d e i o n s and e l e m e n t a l s u l f u r one c a n w r i t e : 2

2

s

CHS")

2

= Z[S] / ( t 1 / Y S - >

+

2

+

{CH )/K

H

+

A 1

^

H 2

s} +

{ ^

n-K /(H )^ 2-})

n

n

2

(1)

S

2

+

_

w i t h Z [ s ] = Z [ H S ] + E n [ s " ] and K = ( S " ) • ( H ) / ( H S ) . E q u a t i o n 1 c a n be w r i t t e n a l t e r n a t e l y ( H S ) = z [ s ] / f ( p H ) , where f ( p H ) i s t h e pH d e p e n d e n t member i n 1. T a k i n g i n t o a c c o u n t t h e e x p r e s s i o n of K one c a n deduce ( S ) = l [ s ] / g ( p H ) . I n t h e H S - S 8 - H 0 - N a C l (0.7 MJ s y s t e m one c a n o b t a i n ( 5 ) : 2

n

_

2 _

A 2

2

5

gCpH) - 1 0 ·

9 9 9 4

•

1 4 1 0

-

1 B 1

-P

H

•

1Q

2

20.949-2pH

and E 2-

= E|2- - 0 . 0 2 9 5 1 o g ( E [ s J ) + 0 . 0 2 9 5 1 o g ( g C p H ] ) .

s

. (2)

Thus E 2 - c a n be computed i n t h e s y s t e m H S - S 8 - H 0 k n o w i n g Σ [ s ] , w h i c h c a n be done by t i t r a t i o n o f t h e d i s s o l v e d s p e c i e s . I n t h e H S - S - H Q - N a C l ( 0 . 7 CD s y s t e m t h e r e l a t i o n s o b t a i n e d between pH and E 2 - a r e i n good a g r e e m e n t w i t h t h e r e l a t i o n s computed f r o m equation 2 as c a n be s e e n i n F i g u r e 3. A t pH > 9, s l i g h t d e v i a ­ t i o n s were o b s e r v e d o w i n g t o p o l y s u l f i d e i n t e r a c t i o n w i t h t h e Ag/Ag S e l e c t r o d e . Thus t h e p H - E 2 - r e l a t i o n s o b t a i n e d i n t h e H S - H 0 and H SS Q - H 0 s y s t e m s a r e c h a r a c t e r i s t i c o f t h e s e s y s t e m s and t h e y c a n be employed i n n a t u r a l w a t e r s t o c h a r a c t e r i z e t h e p r e s e n c e o r t h e a b s e n c e o f p o l y s u l f i d e i o n s . However, o w i n g t o t h e l a c k o f s e n s i ­ t i v i t y of t h e Ag/Ag S e l e c t r o d e t o s m a l l v a r i a t i o n s of ( S ~ ) t h e pH-Es2- r e l a t i o n s c a n n o t be employed t o d e t e c t s l i g h t d e p a r t u r e s f r o m e q u i l i b r i u m i n t h e above s y s t e m s . s

2

8

2

2

2

3

2

s

2

2

2

2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

2.

Sulfur Speciations and Redox Processes

BOULEGUE AND M i C H A R D

29

Figure 2. Ag/Ag S electrode potentials (E 2-) vs. pH in H S-H 0-NaCl (0.7M) system at 298 K. The curves correspond to calculated pH-E 2relations at con­ stant experimental %[H S]. E^2- are given in mV/(H /H ). %[H S] (mmol/kg): exp. 6 (V; 0.05, exp. 7 0.07, exp. 8 (A) 0.7, exp. 209 (A) 5.8, exp. 203 (Φ) 6.9. 2

s

2

2

H

+

2

1 ^

"Ί

Es*

I

1

1

1

I

2

I

1

l~

• Exp. 3

Ν χ

\

2

\

• Exp. 12

-200-

ν

Exp. 22

v.

-300V". V%.-

-400-500h ι

ι

2 Figure 3. Ag/Ag S (0.7M) system at 298 curves correspond to = 2>[H S] + S[S '] 2

9

2

n

ι

4

6

8

ι

ι

1

1

10 pH

electrode potentials (E 2-) vs. pH in H S-S -H 0-NaCl K. The points correspond to the experimental values and the the calculated values. Ε$2- are given in mV/(H /H ). %[S ~] (mmol/kg): exp. 3 = 0.024, exp. 12 = 4.17, exp. 22 = 12.8. s

2

+

8

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

H

30

CHEMICAL

MODELING

IN AQUEOUS SYSTEMS

The Eh-pH and Eh-Es2- r e l a t i o n s i n t h e H2S-H2O s y s t e m . The Eh v a l u e s i n t h e H2S-H2O s y s t e m were f o u n d t o be s l i g h t l y d e p e n d e n t upon s t i r r i n g (±10 mV]. A t 29B°K t h e p o t e n t i a l s o f t h e p l a t i n u m e l e c t r o d e s were e s t a b l i s h e d q u i t e s l o w l y (1 t o 2 h r ) a t pH > 5. A t pH < 5 i t g e n e r a l l y t a k e s l e s s t h a n one h o u r . The p o t e n t i a l s were i n d e p e n d e n t o f s u c h e l e c t r o d e p r e t r e a t m e n t as c a t h o d i c o r a n o d i c p o l a r i z a t i o n . I n F i g u r e 4 we p r e s e n t t h e Eh-pH r e l a t i o n s o b t a i n e d i n t h e H S - H 0 - N a C l ( 0 . 7 £1) s y s t e m . The Eh v a l u e s a r e t o o h i g h t o be e x p l a i n e d by t h e f o r m a t i o n o f e i t h e r P t S o r P t S 2 - The r e s t p o t e n t i a l s o f t h e p l a t i n u m e l e c ­ t r o d e a r e d e p e n d e n t upon t h e c o n c e n t r a t i o n o f d i s s o l v e d h y d r o g e n s u l f i d e ( Z [ H 2 S ] ) . T h i s c a n be e x p l a i n e d by s e l e c t i v e a d s o r p t i o n o f H S s p e c i e s on t h e p l a t i n u m f o l l o w e d by t h e d i s c h a r g e o f t h e p r o t o n m e d i a t e d v i a c h e m i s o r b e d H S . The c o r r e s p o n d i n g r e a c t i o n s are ( V U : 2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

2

2

H S + PtCe")

Ζ

(e")Pt^2

(e")Pt-H2S

Ζ

P t - H + HS"

2

Pt-H

+ P t - H ->

2 Pt + H

s

2

Pt-HS" + H S

t

P t - H S + HS"

+

t

Pt-H S + H 0 .

2

Pt-HS" + H 0 3

Thus t h e e q u i v a l e n t p r o c e s s electrode i s H Q

+

+ e"

3

2

2

2

c o n t r o l l i n g the potential of thePt

+

Η + HG 2

where Η i s h y d r o g e n a d s o r b e d on t h e p l a t i n u m s u r f a c e , and t h e redox p o t e n t i a l i s Eh = - 0 . 0 5 9 ( p K

H

+ pH + l o g ( H ) ) +

w i t h K|_j = ( H ) / ( e ~ ) " ( H 0 ) , and (H) i s t h e a c t i v i t y o f Η atoms on t h e p l a t i n u m . The s u r f a c e a c t i v i t y o f Η atoms c a n be assumed t o f o l l o w t h e law 3

CH) = k - ( H S )

1 / n

2

as f o u n d i n s o l i d - s o l u t e i n t e r a c t i o n s ( 1 3 ) . I n t h i s i n s t a n c e , - t h e e x p e r i m e n t a l d a t a on t h e Eh - Σ[H2S] r e l a t i o n c o r r e s p o n d t o η -- 2.5 a t pH < 6.5 and η - 1.5 a t pH > 6.5; and t h e b e s t f i t w i t h t h e e x p e r i m e n t a l d a t a g i v e s k - 2.5. T h e s e v a l u e s a r e c o m p a t i b l e w i t h an a d s o r p t i o n p r o c e s s (n =* 1, ( 1 3 ) ) . S i n c e η d e c r e a s e s a s pH i n c r e a s e s f o r c o n s t a n t E [ H 2 S ] , i t i s s u g g e s t e d t h a t H2S, r a t h e r t h a n HS", i s e f f e c t i v e l y m e d i a t i n g t h e d i s c h a r g e o f t h e p r o t o n on t h e p l a t i n u m i n agreement w i t h t h e a b o v e p r o c e s s e s . W i t h pK^j = -3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

2.

BOULEGUE AND M i C H A R D

31

Sulfur Speciations and Redox Processes

Figure 4. Platinum electrode rest potentials Eh (expressed in mV/(H /H ) vs. pH in H S-H 0-NaCl (0.7M) system for different experimental 2[H S] at 298 K. The experimental points corresponding to the same %[H S] have been linked by a continuous curve. The dashed lines correspond to the Eh-pH relationships in the H S-S -H 0 system for different log%[H S] = χ (5). Experimental %[H S] (mmol/kg): exp* 6 (V) 0.05, exp. 7 Ο 0.07, exp,8 (A) 0.7, exp AO (U) - > P11(0) 4.6, exp. 212 (f) 5.7, exp. 209 (A) 5.8, exp. 203 (Φ) 6.9. +

2

2

2

2

2

2

8

2

2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4

ex

32

CHEMICAL MODELING IN AQUEOUS SYSTEMS

[ 1 4 ) , one c a n w r i t e : Eh

= 0.153 - 0.059pH - (0.059/n)·log(H S)

(3)

2

where η i s c h a r a c t e r i s t i c o f t h e a d s o r p t i o n - d i s c h a r g e p r o c e s s o f t h e H S s p e c i e s . E q u a t i o n 3 i s i n good agreement w i t h t h e Eh v a l u e s measured i n t h e H S - H 0 s y s t e m . A l s o t h e r e s u l t i n g E h - E 2 r e l a t i o n s obtained i n t h e H S-H 0 system a r e c h a r a c t e r i s t i c o f t h i s s y s t e m a s c a n be s e e n i n F i g u r e 5. Oxidized s u l f u r species o c c u r r i n g i n n a t u r a l waters ( s u l f a t e , s u l f i t e , t h i o s u l f a t e ) do n o t i n t e r a c t w i t h t h e p l a t i n u m e l e c t r o d e when i n t h e p r e s e n c e o f H S a n d t h e pH-Eh-Es2- r e l a t i o n s f o u n d were s i m i l a r t o t h e above r e l a t i o n s . T h u s , t h e unambiguous r e l a ­ t i o n s f o u n d between pH, Eh and Eg2- i n aqueous s o l u t i o n s o f h y d r o ­ gen s u l f i d e c a n be employed t o c h a r a c t e r i z e s o l u t i o n s and w a t e r s a m p l e s where h y d r o g e n s u l f i d e i s t h e o n l y r e d u c e d s u l f u r s p e c i e s present. An a p p l i c a t i o n o f t h e above r e s u l t s c a n be f o u n d i n t h e s t u d y o f t h e h o t s u l f u r o u s s p r i n g s o f t h e F r e n c h Pyrénées w h i c h i s s u e f r o m m a s s i v e g r a n i t e ( _5, V\]. As s e e n i n T a b l e I , H S , So|~ t o g e t h e r w i t h Na and d i s s o l v e d c a r b o n a t e a r e m a j o r components. P o l y s u l f i d e s p e c i e s were n o t d e t e c t e d (Σ(n-1 ) [sj^"] < 1 0 " g . a t S / k g ) . These s u l f u r o u s s p r i n g s s h o u l d be e q u i v a l e n t t o a H S - H 0 s y s t e m . 2

2

2

s

2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

2

2

B

2

2

T a b l e I . A n a l y s e s o f two s p r i n g s f r o m t h e e a s t e r n Pyrénées: H 18 "Beauté" (Thués) and ϋ 5 " R o s s i g n o l " (Αχ l e s Thermes).The c o n c e n t r a t i o n s a r e given i n 1 0 ~ mol/kg. 4

Spring H 18

Q 5 Spring H 18 • 5

Li 0.128 0.131

Na 28.3 21 . 5

κ 0.659 0.585

n 0.002 0.005

Ca 0.4 0.36

F

CI 2.09 3.68

E[H SJ

S 0 *'

SG " 5.57 1 .38

3.51 1 .41

g

EtCD J 9.1 11.2

2

2

1.99 1.68

2

3

0.05 0.02

T(°C) 66.4 76

pH 8.89 8.74 2

4

Si0 1 .49 1 .47 2

The r e s u l t s o f t h e pH, Eh and Eg2- measurements a f t e r imposed pH v a r i a t i o n s a r e g i v e n i n F i g u r e s 6 and 7 . The agreement o f t h e e l e c t r o c h e m i c a l measurements w i t h l a b o r a t o r y d a t a i s e x c e l l e n t and both s e l e c t e d s p r i n g s a r e e q u i v a l e n t t o a H S-H 0 system. I n t h i s i n s t a n c e t h e c h e m i c a l a n a l y s e s o f t h e s p r i n g s and t h e e l e c t r o c h e ­ m i c a l measurements a r e a l s o i n a g r e e m e n t . 2

2

The Eh-pH and E h - E s 2 - r e l a t i o n s i n t h e H S - S Q - H 0 s y s t e m . Iη the H S - S Q - H D system p o l y s u l f i d e i o n s a r e produced f o l l o w i n g t h e general process 2

2

2

2

(n-1)/8 S

8

+ HS"

t

2

S ~ + H

+

.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(4)

Sulfur Speciations and Redox Processes

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch002

BOULEGUE AND M i C H A R D

Figure 5. Eh-E$2relationships in the H S-H 0-NaCl (0.7M) system for differ­ ent ^[H S]. The experimental points corresponding to the same ^[H S] have been linked by dashed line curves. The Equation 1: Eh = E 2—\- 210mV corre­ sponds to the Eh-E 2relation in the H S-S -H 0 system (5). Eh and E 2- are given in mV/(H /H>). The symbols are the same as in Figure 4. 2

2

2

2

s

s

2

H

3

s

+

-I

I

I

I

1

1

I

^-1

2

3

4

5

6

7

8

9

^1

10 p H

Figure 6. Eh vs. pH in H 18 and Ο 5 springs. Eh is expressed in mV/(H /H ). The experimental points have been linked by a continuous curve. The dashed lines correspond to the Eh-pH relations in the H S-S -HoO system for different log%\H S\ =xat 343 Κ (5). (Ο)Spring Η18, P ë · +

2

2

H

s

rin

0

5

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

L a b o r a t o r y e x p e r i m e n t s (1_5,1_6,1_7) have shown t h a t p o l y s u l f i d e i o n s a r e s t a b l e i n t h e H2S-S8-H2Û s y s t e m even a t v e r y l o w s u l f i d e c o n c e n t r a t i o n s ( a b o u t 1-0" - 1 0 " M, ( 1_7) ). The m a j o r p o l y s u l f i d e i o n s a r e p r o b a b l y S§~, S^" and i n a l a r g e pH r a n g e ÎV7) . The f o l l o w i n g e q u a t i o n s c a n be employed t o compute t h e Eh-pH r e l a t i o n i n t h e H S-S8-H 0 system: 5

2

2

H S

->

S + H

+

K + 2 e

7

= ίο" ·

A 1

α2

=

+ 8 e"

K

5/1

=

ίο" ·

+ 6 e"

K

4/1

=

ίο" ·

+ 5 H

+

4 HS"

-> -«-

S|"

+ 4 H

+

-+ «-

5 S + 2 e"

^5/0=

4 S§" + 2 e"

κ

-> 2

2.21

Κ

S§"

S4 -

1 0

=

· F e ( O H ) > Fe2(OH)4+ > Fe(OH)+ i n order of d e c r e a s i n g abundance. An o b j e c t i o n might be r a i s e d t h a t o r g a n i c complexing p l a y s an important r o l e i n the s p e c i e s d i s t r i b u t i o n of these waters. T h i s p o s s i b i l i t y i s c e r t a i n l y very r e a l , and there i s some evidence that the d i s s o l v e d o r g a n i c carbon i n these waters i s very h i g h , but the good agreement on the Eh c a l c u l a t i o n s and the low pH v a l u e s of these waters suggest that any o r g a n i c s p r e s e n t are p r o b a b l y f u l l y p r o t o n a t e d and have o n l y a minor e f f e c t on complexing. 2 +

Any

chemical

modeling of a c i d mine waters

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

b)

a)

complexes

and s o l i d

2

2 +

,

2

2

3

4 } 2

3

(0H)

2

6

Fe(OH)

3

4

F e r r i c hydroxide, amorphous

2

KFe (S0

2

Jarosite

2

F e ( S 0 ) « 9H 0

6

Coquimbite

4

3

Fe F 3+(S0 ) (OH) ·20H 0 e

F e F

Copiapite

2 +

4

2'

FeF

FeS0 « 7H 0

2

3

f o r computation

Fe (OH)5+ FeSO+, F e ( S 0

considered

Fe(OH)°, Fe(OH)-, Fe (OH)4+,

FeCl+, FeCl°, FeF +,

Melanterite

Solids:

FeCl

FeOH +, Fe(OH)+,

FeSO°

Table I I I phases of i r o n

FeOH+, Fe(OH)°, Fe(OH)~,

Complexes:

Aqueous

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

4

66

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS

r e l i e s h e a v i l y on a c c u r a t e thermodynamic data f o r i r o n s u l f a t e complexes. U n f o r t u n a t e l y , data f o r i o n t r i p l e t s such as FeCSC^)" are not a c c u r a t e l y known and p o s s i b l e complexes such as FeHSO| and Fe(HS04)2 h & been p r o p e r l y i d e n t i f i e d . B e t t e r data are needed f o r these s p e c i e s i n o r d e r to enhance the c a p a b i l i t y of g e n e r a l i z e d c h e m i c a l models. +

v e

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

Water-Mineral

n

o

t

Equilibrium Relationships

A s u i t e of both o x i d i z e d and reduced i r o n m i n e r a l s has been found as e f f l o r e s c e n c e s and p r e c i ­ p i t a t e s i n or near the a c i d mine water of Iron Mountain. The dominant m i n e r a l s tend to be melant e r i t e (or one of i t s d e h y d r a t i o n p r o d u c t s ) , c o p i a p i t e , j a r o s i t e and i r o n h y d r o x i d e . These m i n e r a l s and t h e i r chemical formulae are l i s t e d i n T a b l e I I I from the most f e r r o u s - r i c h at the top to the most f e r r i c - r i c h at the bottom. These m i n e r a l s were c o l l e c t e d i n a i r - t i g h t c o n t a i n e r s and i d e n t i f i e d by X-ray d i f f r a c t o m e t r y . I t was a l s o p o s s i b l e to check the m i n e r a l s a t u r a t i o n i n d i c e s ( l o g ( A P / K ) , where AP = a c t i v i t y product and Κ = s o l u b i l i t y product c o n s t a n t ) o f the mine waters with the f i e l d o c c u r r e n c e s of the same m i n e r a l s . By c o n t i n u a l checking of the s a t u r a t i o n index (S.I.) w i t h a c t u a l minéralogie o c c u r r e n c e s , i n a c c u r a c i e s i n c h e m i c a l models such as WATEQ2 can be d i s c o v e r e d , e v a l u a t e d and c o r r e c t e d (19), p r o v i d e d t h a t these o c c u r r e n c e s can be assumed to be an approach towards e q u i l i b r i u m . We have chosen an e q u i l i b r i u m zone around the S . I . equal to the e s t i m a t e d u n c e r t a i n t y of the s o l u b i l i t y product c o n s t a n t . O u t s i d e of these l i m i t s , the s o l u t i o n i s e i t h e r s u p e r s a t u r a t e d ( p o s i t i v e d i r e c t i o n ) or u n d e r s a t u r a t e d ( n e g a t i v e d i r e c t i o n ) . The S.I. v a l u e s were then p l o t t e d vs. the l o g cond u c t i v i t y to show approaches to m i n e r a l s a t u r a t i o n as an approximate f u n c t i o n of the t o t a l d i s s o l v e d solids. C o n d u c t i v i t y was chosen as a convenient parameter because 1) i t p o i n t s out d i l u t i o n t r e n d s or u n d e r s a t u r a t i o n and corresponds to a s p a t i a l t r e n d f o r downstream s i t e s , 2) i t changes more cons i s t e n t l y than other parameters, and 3) i t covers a l a r g e r range of v a l u e s than Eh, pH or an i n d i v i d u a l ion. A c t i v i t y diagrams (_53J c o u l d have been used but then c e r t a i n assumptions have to be made about the m i n e r a l s t a b i l i t y boundaries such as constant a c t i v i t y of water or of pH or of s u l f a t e . Melanterite. The most c o n c e n t r a t e d a c i d mine i n

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

3.

NORDSTROM E T A L .

Redox Equilibria of Iron

waters, such as those f l o w i n g from the Hornet tunn e l , commonly p r e c i p i t a t e m e l a n t e r i t e , a pure f e r r o u s s u l f a t e , along t h e i r edges. Water which had i n f i l t r a t e d a massive ore body would a l s o p r e c i t i pate m e l a n t e r i t e along the s u r f a c e of an open cut exposed to dry o u t s i d e a i r . But m e l a n t e r i t e was found o n l y on s u l f i d e s u r f a c e s which were r e g u l a r l y washed by r a i n f a l l or running water. In s h e l t e r e d a r e a s , such as the entrance to the Richmond mine, the dominant m i n e r a l o c c u r r i n g on open cuts i s c o p i a p i t e or a mixture of c o p i a p i t e and coquimbite. In F i g u r e 5 we have p l o t t e d S.I. v a l u e s f o r m e l a n t e r i t e i n d i c a t i n g a t r e n d towards s a t u r a t i o n f o r the Hornet e f f l u e n t ( l a b e l e d B). A l l of the o t h e r waters ( c o l l e c t e d at downstream s i t e s ) have been d i l u t e d and o x i d i z e d and t h e r e f o r e appear undersaturated. The r e s u l t s of these c a l c u l a t i o n s compare q u i t e f a v o r a b l y with f i e l d o b s e r v a t i o n s . U n f o r t u n a t e l y , there i s a l a r g e u n c e r t a i n t y assoc i a t e d with the thermodynamic s o l u b i l i t y constant for melanterite. Although i t s s o l u b i l i t y i s w e l l known, the thermodynamic e q u i l i b r i u m constant i s d i f f i c u l t to o b t a i n because the compound i s h i g h l y s o l u b l e and t h e r e f o r e becomes s a t u r a t e d o n l y at high ionic strenths. More i m p o r t a n t l y , i t should be noted that m e l a n t e r i t e commonly forms on rock s u r f a c e s , espec i a l l y along f r a c t u r e s where water can be p u l l e d up from a w a t e r - s a t u r a t e d zone to the s u r f a c e by c a p i l l a r y f o r c e s . Thus, m e l a n t e r i t e s a t u r a t i o n i s o c c u r r i n g i n a microenvironment i n many i n s t a n c e s which makes sampling and i n t e r p r e t a t i o n d i f f i c u l t . Copiapite. T h e o r e t i c a l l y , m e l a n t e r i t e can o x i d i z e to form c o p i a p i t e by the r e a c t i o n : 5FeS0 -7H 0 + 0 4

2 +

2

+ H S0

2

2

4

+

F e F e 3 ( S 0 ) ( O H ) - 2 0 H 0 + 15H 0„ 4

6

2

2

2

As mentioned e a r l i e r , l a r g e amounts of c o p i a p i t e have been found accumulating on ore s u r f a c e s i n areas p r o t e c t e d f o r d i r e c t r a i n f a l l . The mechanism of i t s f o r m a t i o n i s not at a l l c l e a r but from f i e l d o b s e r v a t i o n s i t appears that a c i d mine water i s b e i n g drawn by c a p i l l a r y f o r c e s to an exposed s u r f a c e where i t q u i c k l y evaporates to m e l a n t e r i t e and/or c o p i a p i t e . Coquimbite i s i n t i m a t e l y assoc i a t e d with c o p i a p i t e and these two m i n e r a l s appear t o be v e r y s t a b l e as long as they are p r o t e c t e d from r a i n f a l l or running water. No thermodynamic data

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

68

C H E M I C A L M O D E L I N G I N AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

SATURATION

ZONE

JG

J

tf

UNDERSATURATED

N

Ι'ΛΜ

ΪΊΜΟ

ΓΘΟ

ËTëô

?ΙΘ5

^ôô

N

Ν

N

Jâô

Jâô

«Tiëô

TTëô

5.x

LOG CONDUCTIVITY Figure 5. Saturation indices for melanterite plotted as a function of the log conductivity showing an approach to saturation for the most concentrated ferrousrich waters of the Hornet effluent (B)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

3.

NORDSTROM E T A L .

Redox Equilibria of Iron

are a v a i l a b l e on e i t h e r c o p i a p i t e or coquimbite so t h a t S.I. v a l u e s cannot be c a l c u l a t e d . C o p i a p i t e c o n t a i n s both f e r r o u s and f e r r i c i r o n but a form c a l l e d f e r r i c o p i a p i t e c o n t a i n i n g o n l y i r o n i n the o x i d i z e d f e r r i c s t a t e may a l s o occur. There i s l i t t l e i n f o r m a t i o n on the r e l a t i v e abundance of these m i n e r a l s nor whether c o p i a p i t e i s a n e c e s s a r y p r e c u r s o r to the f o r m a t i o n of f e r r i c o p i a p i t e and coquimbite. A f u r t h e r c o m p l i c a t i o n to the i n t e r p r e ­ t a t i o n of m i n e r a l r e a c t i o n s i s the s o l i d s u b s t i t u ­ t i o n p r o p e r t i e s of c o p i a p i t e . I t not o n l y can have a change i n the f e r r o u s / f e r r i c r a t i o but the d i v a ­ l e n t and t r i v a l e n t s i t e s can accept a l a r g e number of d i v a l e n t and t r i v a l e n t ions such as Mg , C u , Z n , C d + and A l + . Jarosite. The waters which are a s s o c i a t e d with such s o l u b l e s a l t s as m e l a n t e r i t e and c o p i a p i t e com­ monly have pH v a l u e s of 0.8 t o 1.5. As the pH r i s e s to the range of 1.5 t o 2.5, j a r o s i t e w i l l commonly p r e c i p i t a t e i f the d i s s o l v e d i r o n content i s high enough. A f i n e - g r a i n e d y e l l o w p r e c i p i t a t e forms i n B o u l d e r Creek e v e r y summer and f a l l , which when washed and f i l t e r e d , g i v e s a good X-ray p a t t e r n f o r potassium j a r o s i t e . This mineral i s p r e c i p i t a t i n g d i r e c t l y from the water and i t appears to have good crystallinity. Both c o p i a p i t e and j a r o s i t e are a b r i g h t y e l l o w c o l o r and may e a s i l y be confused with sulfur. In F i g u r e 6,the S.I. v a l u e s f o r potassium j a r o ­ s i t e are g i v e n f o r a l l the samples. The values show s u p e r s a t u r a t i o n f o r n e a r l y every sample and prompts us to ask s e v e r a l q u e s t i o n s : (a) are these waters t r u l y s u p e r s a t u r a t e d and i s t h i s a k i n e t i c r e q u i r e ­ ment f o r p r e c i p i t a t i o n ? (b) are o r g a n i c complexes important f o r these waters? (c) are the t h e r ­ modynamic data r e l i a b l e ? Some of these q u e s t i o n s can be answered. F i r s t , the amount of supers a t u r a t i o n i s more than the ί 1.1 log Κ u n i t u n c e r t a i n t y i n the thermodynamic d a t a . Second, the model does not appear to be inadequate f o r the Eh comparison and o t h e r c a l c u l a t i o n s , and i f o r g a n i c complexing were i n t r o d u c i n g a s i g n i f i c a n t e r r o r i t would have shown up i n the c a t i o n - a n i o n balance or other c a l c u l a t i o n s . Downstream i n S p r i n g Creek where no j a r o s i t e has been found the S.I. i s o f t e n more s u p e r s a t u r a t e d than i n Boulder Creek. This s u p e r s a t u r a t i o n suggests t h a t j a r o s i t e s o l u b i l i t y does not c o n t r o l the water c h e m i s t r y of any of the streams. The remaining p o s s i b i l i t y i s that j a r o s i t e i s p r e c i p i t a t i n g o n l y i n microenvironments where the

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

2 +

2 +

2 +

2

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

C H E M I C A L M O D E L I N G IN AQUEOUS

70

Ν

Ν

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

N

L

h 7

K

j

NL

Ν N N N

e J

SYSTEMS

„

\

J

j

4

'

J

6

L

F

SAT U RATION ZON Ε

1.00

1.40

1.80

ë\2Ô

2\60sTÔÔ

^^40

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TTËÔ

7^60

δ". 00

LOG CONDUCTIVITY Figure

6.

Saturation indices for jarosite plotted as a function of the log tivity showing supersaturation for nearly all values

conduc­

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

3.

NORDSTROM E T AL.

Redox Equilibria of Iron

water c h e m i s t r y i s d i f f e r e n t than i n the bulk s o l u ­ tion. The evidence f o r t h i s i s that j a r o s i t e p r e c i p ­ itation i s always found on stream bed s u r f a c e s where b a c t e r i a have been growing, and i s a l s o found i n s i d e of a c i d s l i m e streamers produced by these bacteria. Beyond t h i s , there may be important par­ t i c l e s i z e e f f e c t s (JJ2 ). There c o u l d s t i l l be l o c a l e q u i l i b r i u m c o n d i t i o n s e x i s t i n g here, but they are p r o b a b l y below the r e s o l u t i o n of the sampling t e c h ­ niques. C l e a r l y , more work i s needed before the j a r o s i t e g e o c h e m i s t r y can be a d e q u a t e l y i n t e r p r e t e d . Amorphous f e r r i c h y d r o x i d e . Most of S p r i n g Creek below the Boulder Creek c o n f l u e n c e i s i r o n s t a i n e d and a c t i v e l y p r e c i p i t a t e s i r o n h y d r o x i d e . The pH v a l u e s are commonly i n the range of 2.5 t o 3.5 depending on the season. The i r o n p r e c i p i t a t e s are amorphous to X-rays and appear to be amorphous f e r r i c hydroxide from t h e i r c o l o r and t e x t u r e . S.I. v a l u e s c a l c u l a t e d f o r amorphous f e r r i c hydroxide show a t r e n d towards s a t u r a t i o n with downstream d i l u t i o n and o x i d a t i o n ( F i g u r e 7 ) . We have used the upper s o l u b i l i t y l i m i t of pK = 37.1, where pK i s the apparent s t a b i l i t y c o n s t a n t f o r f e r r i c oxyhydroxides (12) and most of the downstream s i t e s ( l a b e l e d Ν) f a l l n e a t l y i n t o the r e g i o n of pK = 37 t o 39 as would be expected f o r a f r e s h l y p r e c i p i t a t e d material. Very l i t t l e s u p e r s a t u r a t i o n appears i n our computations. A g a i n , the S.I. c a l c u l a t i o n s match v e r y w e l l with the f i e l d o b s e r v a t i o n s and with p r e v i o u s s t u d i e s of f e r r i c hydroxide e q u i l i b r i u m i n n a t u r a l waters ( l ^ l _ 5 f l Z ) . noticeable hiatus o c c u r s at about l o g Κ = 41. Below t h i s value the samples p l o t t e d i n F i g u r e 7 are very high i n f e r r o u s i r o n , low i n pH and show no s i g n of f e r r i c hydroxide precipitation. A

Conclusions We have u t i l i z e d a chemical model i n t h i s i n v e s t i g a t i o n to i n t e r p r e t the e q u i l i b r i u m b e h a v i o r of i r o n i n a c i d mine waters. A successful correla­ t i o n between c a l c u l a t e d and measured Eh v a l u e s has been found, u s i n g WATEQ2, the computerized i o n a s s o c i a t i o n model. This c o r r e l a t i o n supports the b a s i c assumption of homogeneous s o l u t i o n e q u i l i b r i u m i n these waters and s i m u l t a n e o u s l y c o r r o b o r a t e s both the v a l i d i t y of the aqueous model and the q u a n t i t a t i v e i n t e r p r e t a t i o n of Eh measure­ ments i n these waters. T h i s i n t e r p r e t a t i o n makes i t p o s s i b l e to c a l c u l a t e the d i s t r i b u t i o n of i r o n

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

72

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

SUPERSATURATED

p

SYSTEMS

K-37

c u

κ

J

ι'.8θ

e'.eo

LOG

e'.eo

i.oo

M.eO

4.60

CONDUCTIVITY

Figure 7. Saturation indices for amorphous ferric hydroxide plotted as a function of the log conductivity showing an approach to saturation for the more dilute and more oxidized downstream waters

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

3.

NORDSTROM E T

AL.

Redox Equilibria of Iron

73

s p e c i e s based on Eh measurements when f e r r o u s - f e r r i c a n a l y s e s are not a v a i l a b l e . A l t h o u g h the stream waters show e q u i l i b r i u m s a t u r a t i o n with d i s s o l v e d oxygen, there i s no e q u i l i b r i u m between the d i s s o l v e d oxygen and f e r r o u s - f e r r i c redox c o u p l e . M i n e r a l s a t u r a t i o n i n d i c e s f o r m e l a n t e r i t e and amorphous i r o n h y d r o x i d e agree q u i t e w e l l with f i e l d o c c u r r e n c e s of the same m i n e r a l s . J a r o s i t e , how­ e v e r , appears to be s u p e r s a t u r a t e d f o r n e a r l y a l l of the samples r e g a r d l e s s of the presence or absence of the m i n e r a l i n these streams. F i e l d observations i n d i c a t e that j a r o s i t e p r e c i p i t a t i o n occurs i n the microenvironment of b a c t e r i a l c o l o n i e s where the c h e m i c a l c o n d i t i o n s may be q u i t e d i f f e r e n t from the bulk s o l u t i o n . These c o n s i d e r a t i o n s l e a d us to suggest that there i s a k i n e t i c b a r r i e r which h i n ­ ders j a r o s i t e p r e c i p i t a t i o n but does not h i n d e r f e r r i c hydroxide p r e c i p i t a t i o n and that t h i s b a r r i e r i s overcome by the s u r f a c e s of b a c t e r i a l c o l o n i e s (both i r o n - o x i d i z e r s and u n i d e n t i f i e d nonoxidizers). The c h e m i c a l e q u i l i b r i a approach used i n t h i s s t u d y have enabled us to i d e n t i f y which p a r t s of an a c i d mine d r a i n a g e system are i n e q u i l i b r i u m and which p a r t s are not. Our r e s u l t s have p r o v i d e d g r e a t e r i n s i g h t i n t o the chemical p r o c e s s e s of a c i d mine waters i n p a r t i c u l a r and the redox r e l a t i o n s of iron in general. Acknowledgements The r e s e a r c h i n t h i s paper was done i n p a r t i a l f u l f i l l m e n t of a Ph.D. d i s s e r t a t i o n completed by the s e n i o r author. Support from the U.S. Geological Survey, e s p e c i a l l y Bob A v e r e t t , Rich F u l l e r , Jo B u r c h a r d , D o r i s York, Dave V i v i t and the s t a f f of the Redding F i e l d O f f i c e , are g r a t e f u l l y acknowl­ edged. C r i t i c a l comments by G. A. Parks, Κ. B. Krauskopf, J . 0. L e c k i e and B. H a l l e t are g r e a t l y appreciated. We a l s o wish to acknowledge the c a r e ­ f u l review of t h i s manuscript by L. N. Plummer and D. C. T h o r s t e n s o n , both of the U.S. Geological Survey. We a l s o thank Janet B a t t e n f o r t y p i n g the manuscript. Abstract A d e t a i l e d hydrogeochemical i n v e s t i g a t i o n of 14 km of stream waters contaminated by a c i d mine drainage i n Shasta County, California p r o v i d e s i n s i g h t i n t o the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

74

CHEMICAL MODELING IN AQUEOUS SYSTEMS

equilibrium transformations of iron during the oxi­ dative weathering of p y r i t i c ore. Over 60 water analyses covering a range of pH, redox state, total iron concentration, temperature and ionic strength were processed with the computerized chemical model WATEQ2 so that a c t i v i t i e s of free ions and complexes and satutation indices could be determined. The results demonstrate (a) measured Eh values agree quite well with Eh values calculated from the ferrous/ferric a c t i v i t y r a t i o , (b) the dominant complexes in acid mine waters are FeS°4, FeSO+4, Fe(S0 )-2 Fe(OH) , Fe (OH)4+2 and Fe(OH)+2 and (c) acid mine waters low in pH (1 to 2) and high in reduced iron approach saturation with respect to melanterite whereas progressive downstream oxidation and dilution (pH = 2 to 3.5) of these waters forces these waters to become saturated with respect to amorphous iron hydroxide. For these two minerals, the f i e l d observations match the saturation calculations very well. However, a l l of the waters show super­ -saturation with respect to jarosite regardless of the presence or absence of jarosite in the stream waters. These results suggest a strong kinetic hindrance to the precipitation of jarosite. 2+

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

4

2

Literature Cited 1. 2.

3. 4. 5. 6.

Lepp, H . , ed. "Geochemistry of Iron," Benchmark Papers in Geol. 18, Halstead Press, 464 p. (1975). Stumm, W. and Morgan, J . J., "Aquatic Chemistry: An introduction emphasizing chemical equilibrium in natural waters," 583 p . , WileyInterscience, New York, 1970. Neilands, J . Β . , ed. "Microbial Iron Metabolism: A Comprehensive Treatise," 597 p . , Academic Press, New York, 1974. James H. L. Chemistry of the iron-rich sedimen­ tary rocks, U. S. Geol. Survey Prof. Paper 440W, 61 p. (1966). Klemic, Η., James, H. L. and Eberlein, G. D. United States mineral resources, U.S. Geol. Survey Prof. Paper 820, 291-306 (1973). Jenne, E. A. Controls on Mn, Fe, Co, N i , Cu and Zn concentrations in soils and waters: the significant role of hydrous Mn and Fe oxides, p. 337-387, in "Trace Inorganics in Water," Adv. Chem. Ser. 73, 1968.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

3.

NORDSTROM ET AL.

7.

8. 9.

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10. 11. 12.

13.

14.

Redox Equilibria of Iron

Jenne, Ε. A. Trace element sorption by sedi­ ments and s o i l s — s i t e s and processes, p. 425-533, in "Symposium on Molybdenum in the Environment, M. Dekker, New York, 1977. Krauskopf, Κ. B. "Introduction to Geochemistry," 721 p. McGraw Hill, New York, 1967. Krauskopf, Κ. B. Geochemistry of micronutrients, p. 7-40 i n "Micronutrients i n A g r i c u l t u r e , " S o i l S c i . Soc. Am., Madison, Wise. 1972. Berner, R. A. Low temperature geochemistry of i r o n , section 26, in "Handbook of Geochemistry," v o l . II-I, Springer-Verlag, B e r l i n . Berner, R. A. "Principles of Chemical Sedimentology," 240 p. McGraw-Hill, New York, 1971. Langmuir, D. and Whittemore, D. O. Variations in the s t a b i l i t y of precipitated f e r r i c oxyhydroxides, p. 209-234, i n Hem, J. D., ed., "Nonequilibrium Systems in Natural Water Chemistry," Adv. Chem. Ser. 106, 1971. Rester, D. R., Byrne, R. H . , J r . and Liang, Y . J. Redox reactions and solution complexes of iron in marine systems, p. 56-79, i n Church, T. M. e d . , "Marine Chemistry in the Coastal Environment," Amer. Chem. Soc. Ser. 18, 1975. Paces, T. Chemical c h a r a c t e r i s t i c s and e q u i l i b r a t i o n in natural water-felsic rockCO system, Geochim. Cosmochim. Acta 36, 217-240 (1972). Jones, B. F., Kennedy, V. C. and Zellweger, G. W. Comparison of observed and calculated con­ centrations of dissolved Al and Fe in stream water, Water Resour. Res. 10, 791-793 (1974). Helz, G. R. and Sinex, S. A. Chemical equilibrium in the thermal spring waters of V i r g i n i a , Geochim. Cosmochim. Acta 38, 18071820 (1974). Gang, M. W. and Langmuir, D. Controls on heavy metals in surface and ground waters affected by coal mine drainage; Clarion River - Redbank Creek watershed, Pennsylvania, p. 39-69, i n F i f t h Symp. Coal Mine Drainage Res., N . C . A . , B . C . R . , L o u i s v i l l e , Kentucky, 1974. B r i c k e r , O. P . , III and Troup, Β. N. Sediment­ -water exchange in Chesapeake Bay, p. 3-27, i n Cronin, L., e d . , "Estuarine Research, v o l . 1. Chemistry, Biology and the Estuarine System," Academic Press, New York, 1975, 2

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CHEMICAL MODELING IN AQUEOUS SYSTEMS Nordstrom, D. K. and Jenne, E. A. F l u o r i t e s o l u b i l i t y e q u i l i b r i a in selected geothermal waters. Geochim. Cosmochim. Acta 41, 175-188 (1977). Sato, M. Oxidation of sulfide ore bodies. I. Oxidation mechanisms of sulfide minerals at 25°C. Econ. Geol. 55, 1202-1231 (1960). G a r r e l s , R. M. and Thompson, M. E. Oxidation of p y r i t e in f e r r i c sulfate s o l u t i o n , Am. J. Sci. 258, 57-67 (1960). Smith, Ε. E . and Shumate, K. S. Sulfide to sulfate reaction mechanism, Fed. Water Qual. Admin. Rept. 14010 FPS 01/70 (1970). Singer, P. C. and Stumm, W. Acid mine drainage: The rate determining step. Science 167, 1121-1123 (1970). Lacey, D. T. and Lawson, F. Kinetics of the liquid-phase oxidation of acid ferrous sulfate by the bacterium T h i o b a c i l l u s ferrooxidans. Biotech. Bioeng. 12, 29-50 (1970). Nordstrom, D. K . , Jenne, E. A. and Averett, R. C. Heavy metal discharges into Shasta Lake and Keswick Reservoirs on the upper Sacramento River, C a l i f o r n i a : a reconnaissance during low flow. U. S. Geol. Survey Open-File Report 76-49, 25 p. (1977). Nordstrom, D. K. "Hydrogeochemical and M i c r o b i o l o g i c a l Factors Affecting the Heavy Metal Chemistry of an Acid Mine Drainage System," Ph.D. Thesis, Stanford U n i v e r s i t y , Stanford, C a l i f o r n i a , 1977. Kennedy, V. C., Jenne, E. A. and Burchard, J. M. Backflushing f i l t e r s for f i e l d processing of water samples prior to trace-element analy­ ses. U. S. Geol. Survey Water Resour. Inv. Open-File Report 76-126, 12 p. (1976). Nordstrom, D. K. Thermochemical redox e q u i l i b r i a of ZoBell's s o l u t i o n . Geochim. Cosmochim. Acta 41, 1835-1841 (1977). Stookey, L . L . Ferrozine — A new spec­ tophotometric reagent for i r o n , Anal. Chem. 42, 779-781 (1970). Gibbs, C. R. Characterization and application of Ferrozine iron reagent as a ferrous iron indicator. Anal. Chem. 48, 1197-1200 (1976). Nelson, Μ. Β., Nordstrom, D. K. and Leckie, J. O. Modifications of the ferrozine method for ferrous iron in the presence of f e r r i c with

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applications to natural waters, (submittted for p u b l i c a t i o n ) . B a l l , J . W., Jenne, Ε. Α . , and Nordstrom, D. K. WATEQ2—a computerized chemical model for trace and major element speciation and mineral e q u i l i b r i a of natural waters, in Jenne, Ε. Α . , ed., "Chemical Modeling in Aqueous Systems— Speciation, Sorption, S o l u b i l i t y , and K i n e t i c s , " Amer. Chem. Soc., 1978 (This volume). Thompson, J. Β . , J r . Local equilibrium in metasomatic processes, p. 427-457, i n Abelson, P. H . , e d . , "Researches in Geochemistry." John Wiley, New York, 1959. Onsager, L . Reciprocal relations in i r r e v e r ­ s i b l e processes. I. Phys. Rev. 37, 405-426 (1931). Helgeson, H. C., Brown, Τ. Η . , N i g r i n i , Α . . and Jones, T. A. Calculations of mass transfer in geochemical processes involving aqueous s o l u ­ tions. Geochim. Cosmochim. Acta 34, 569-592 (1970). Garrels, R. M. and C h r i s t , C. L . "Solutions, Minerals, and E q u i l i b r i a , " 450 p. Harper and Row, New York, 1965. Thorstenson, D. C. Equilibrium d i s t r i b u t i o n of small organic molecules in natural waters. Geochim. Cosmochim. Acta 34, 745-770 (1970). Lamm, C. G. Reduction-oxidation level of soils. Nature 177, 620-621 (1956). Breck, W. G. Redox potentials by e q u i l i b r a ­ tion. J . Mar. Res. 30, 121-139 (1972). ZoBell, C. E. Studies on redox potential of marine sediments. B u l l . Am. Assoc. P e t r o l . Geol. 30, 477-509 (1946). Baas-Becking, L. G. Μ., Kaplan, I. R. and Moore, D. Limits of natural environment in terms of pH and oxidation-reduction poten­ J . Geol. 68, 243-284 (1960). Stumm, W. Redox potential as an environmental parameter; conceptual significance and opera­ t i o n a l l i m i t a t i o n , p. 1-16, i n 3rd Intern. Conf. on Water P o l l u t . Res., Proc. Munich, 1966. Morris, J . C. and Stumm, W. Redox e q u i l i b r i a and measurements of potentials in the aquatic environment, p. 270-285, i n "Equilibrium Concepts in Natural Water Systems," Adv. Chem. Ser. 67, 1967.

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W h i t f i e l d , M. Thermodynamic l i m i t a t i o n s on the use of the platinum electrode in Eh measure­ ments. Limnol. Oceanogr. 19, 857-865 (1974). Doyle, R. W. The o r i g i n of the ferrous ion­ f e r r i c oxide Nernst potential in environments containing dissolved ferrous i r o n . Am. J. S c i . 266, 840-859 (1968). Berner, R. A. Electrode studies in hydrogen s u l f i d e in marine sediments. Geochim. Cosmochim. Acta 27, 563-575, (1963). Kryukov, P. Α . , Zadodnov, S. S. and Goremykin, F. E . Sulphide and carbonate equilibrium and the oxidation-reduction state of sulfur in the mineral regions of the Crimean mineral waters. Dokl. Akad. Nauk. SSSR 142(1) (1962). Skopintsev, Β. Α . , Fomenskaya, Ν. N. and Smirnov, Ε. V. New determinations of the oxidaton-reduction potential in Black Sea water. Oceanology (USSR) 6, 653-659 (1969). W h i t f i e l d , M. Eh as an operational parameter in estuarine studies. Limnol. Oceanogr. 14, 547-558 (1969). Natarajan, K. A. and Iwasaki, I. Eh measure­ ments in hydro-metallurgical systems. Minerals S c i . Eng. 6, 35-44 (1974). B r i c k e r , O. P. Some s t a b i l i t y relations in the system Mn-O -H O at 25°C and one atmos­ phere t o t a l pressure. Am. Mineral. 50, 12961354 (1965). Boulegue, J. E q u i l i b r i a in a sulfide r i c h water from Enghien-les-Bains, France. Geochim. Cosmochim. Acta 4, 1751-1758 (1977). B a l l , J . W., Jenne, E. A. and Burchard, J. M. Sampling and preservation techniques for waters in geysers and hot springs, p. 219-234, Proc. F i r s t Workshop of Sampling Geothermal E f f l u e n t s , October 20-21, 1975, Las Vegas,Nevada, EPA-600/9-76-0111, 1975. Sykes, K. W. The structure and r e a c t i v i t y of the complexes of f e r r i c ions with some simple anions. Chem. Soc. London Spec. Pub. 1, 64-70 (1957). Sapieszko, R. S., P a t e l , R. C. and M a t i j e v i c , E. F e r r i c hydrous oxide s o l s . 2. Thermo­ dynamics of aqueous hydroxo and sulfato f e r r i c complexes. J . Phys. Chem. 81, 10611068 (1977). 2

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79

Helgeson, H. C., Brown, T. H. and Leeper, R. H., "Handbook of Theoretical A c t i v i t y Diagrams Depicting Chemical E q u i l i b r i a i n Geologic Systems Involving an Aqueous Phase at one Atm. and O-300°C." 253 p. Freeman, Cooper and Company, San Francisco, 1969.

Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. November 16, 1978.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

RECEIVED

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4 I n f l u e n c e of R e d o x E n v i r o n m e n t s Arsenic

in

Ground

o n the

Mobility

of

Water

J. GULENS and D. R. CHAMP Atomic Energy of Canada Ltd., Chalk River Nuclear Laboratories, Chalk River, Ontario, CanadaK0J1J0

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch004

R. E. JACKSON Inland Waters Directorate, Fisheries and Environment Canada, Ottawa, Ontario, Canada K1A 0E7 The concept of a sequence of redox r e a c t i o n s i n ground water flow systems has been developed ( 1 ) . This concept i s based on a modified v e r s i o n of the theory presented by Stumm and Morgan ( 2 p. 326-339), and uses the flow system as the h y d r o g e o l o g i c a l framework onwhich a thermodynamically based sequence of redox r e a c t i o n s is superimposed. I n a confined a q u i f e r c o n t a i n i n g excess d i s s o l v e d organic carbon (DOC) and some solid-phase F e ( I I I ) and Mn(IV) m i n e r a l s , redox r e a c t i o n s can occur between the DOC and the o x i d i z e d species present i n the ground water. As the water flows from recharge t o discharge w i t h i n t h i s confined a q u i f e r , the o x i d i z e d species w i l l be reduced i n the f o l l o w i n g sequence: d i s s o l v e d oxygen, n i t r a t e , ( s o l i d ) manganese oxides, ( s o l i d ) f e r r i c hydroxides, s u l f a t e , d i s s o l v e d carbon d i o x i d e and f i n a l l y dissolved nitrogen (1). As a consequence of t h i s concept of redox sequences, i t can be concluded (1) that three s e q u e n t i a l zones or environments may e x i s t i n confined a q u i f e r systems: an o x i d i z i n g zone ( i n the recharge area), a " n e u t r a l " zone ( i n the t r a n s i t i o n area), and a reducing zone ( i n the discharge a r e a ) , Figure 1. The m o b i l i t y and concentration of m u l t i v a l e n t t r a n s i t i o n metals and nonmetals v a r i e s i n each of these zones, a p r i n c i p l e that can be u s e f u l l y a p p l i e d to the abatement and p r e v e n t i o n of ground water p o l l u t i o n . As an example, Fe i s immobilized as the F e ( I I I ) hydrous oxide i n the o x i d i z i n g zone; f u r t h e r downstream i n a l e s s o x i d i z i n g environment, the " n e u t r a l " zone, F e ( I I ) i s s t a b l e and as i t i s a l s o more s o l u b l e than the F e ( I I I ) hydrous oxide, the l a t t e r becomes reduced and m o b i l i z e d as F e ( I I ) . In the reducing zone, s u l f a t e reducing b a c t e r i a may e x i s t and Fe may again be immobilized as the i n s o l u b l e s u l f i d e . Hydrous oxide surfaces of sand immobilize As by a d s o r p t i o n processes ( 3 ) . The r e s u l t s of our s t u d i e s show that the extent of adsorption v a r i e s w i t h the o x i d a t i o n s t a t e of the As, the redox environment and/or the pH of the e l u t i n g water. The i n f l u e n c e of these parameters on the m o b i l i t y of As was s t u d i e d by e l u t i n g As through sand columns: waters of d i f f e r e n t redox 9

0-8412-0479-9/79/47-093-081$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch004

82

Figure

1.

Sequential

redox zones within a confined aquifer: "neutral" (Fe \ Mn ); reducing (S ~) 2

2+

oxidizing

(O , t

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

NO ~); s

GULENS E T A L .

4.

Redox

Environments

and

Arsenic

83

c h a r a c t e r i s t i c s wereused f o r e l u t i o n , and the e l u t i o n behaviour of As(V) was compared to that of A s ( I I I ) . Studies i n buffered s o l u t i o n s showed that an Fe-As complex was formed, the s o l u b i l i t y of which was a l s o dependent on the o x i d a t i o n s t a t e of the As and the s o l u t i o n pH. Experimental E l u t i o n Studies. The e l u t i o n p r o f i l e s of As(V) and A s ( I I I ) through sand columns were obtained using r a d i o a c t i v e a r s e n i c . A s was prepared by i r r a d i a t i n g reagent grade s o l i d A S 2 O 3 and A s 0 i n the Chalk River NRU r e a c t o r . The i r r a d i a t e d s o l i d s were d i s s o l v e d and loaded immediately. *As was obtained from Amersham/Searle Corp. as a s o l u t i o n of a r s e n i c a c i d i n 0.04 M HC1; a p o r t i o n of t h i s so_lution was converted to As ( I I I ) by heating i n the presence of HSO3 (4) (estimated conversion was 70%). The e l u t i o n s t u d i e s were conducted i n p l e x i g l a s columns (2.5 cm I.D. and 28 cm long) packed w i t h sand (60-230 mesh). The Fe and Mn content of the sand was 0.6% and 0.01% (w/w) r e s p e c t ­ i v e l y , determined by atomic absorption spectrometry f o l l o w i n g an HC1 e x t r a c t i o n . The columns were hand packed w i t h the sand, using techniques to minimize d e n s i t y and s i z e segregation of the sands (.5) and thereby m i n i m i z i n g flow inhomogeneities ( 6 ) , and then p r e - e q u i l i b r a t e d by f l u s h i n g w i t h the e l u a t e f o r 3 days p r i o r to l o a d i n g . An a l i q u o t of r a d i o a c t i v e a r s e n i c ( A s or A s ) was loaded onto the base of the column and eluted w i t h upward flow. Eluant f r a c t i o n s were c o l l e c t e d i n an automated f r a c t i o n c o l l e c t o r and analyzed f o r As by γ-ray spectrometry. The As(V) and A s ( I I I ) e l u t i o n p r o f i l e s were obtained from separate columns, e l u t e d i n p a r a l l e l . The r e s u l t s have been corrected f o r the decay r a t e ( A s , t i . = 26.4 h, A s , t j . = 17.7 days). The columns were eluted a t 0.5 mL m i n both i n l a b o r a t o r y and f i e l d s t u d i e s . E l u t i o n s t u d i e s i n the l a b o r a t o r y were conducted at room temperature 298 K, using a i r - s a t u r a t e d d i s t i l l e d water, pH 5.7. E l u t ions i n the f i e l d were c a r r i e d out at ambient temperatures ranging from 278 Κ to 298 K, using ground water a t a temperature of 2 79 K. Ground water was pumped continuously from piezometers by a M a s t e r f l e x p e r i s t a l t i c pump at 50 mL m i n i n t o a 500 mL p l e x i g l a s sampling c e l l where E and pH values were measured; water was then pumped from t h i s c e l l i n t o the columns. The measured p o t e n t i a l of a platinum e l e c t r o d e , Eg, was determined r e l a t i v e to a saturated calomel r e f e r e n c e electrode, but values are quoted r e l a t i v e to a normal hydrogen e l e c t r o d e . 7 6

2

5

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yt

7 6

7 6

7 4

7 4

- 1

- 1

H

Ground Water Geochemistry. Ground water from the lower a q u i f e r i n the lower Perch Lake Basin a t the Chalk R i v e r Nuclear L a b o r a t o r i e s , Figure 2, was used f o r f i e l d e l u t i o n s t u d i e s to provide water of d i f f e r e n t redox c h a r a c t e r i s t i c s . Water from piezometer "0" i n the t r a n s i t i o n area, and from KNEW i n the discharge area was used as being r e p r e s e n t a t i v e of " n e u t r a l " and

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 2. The lower Perch Lake basin, Chalk River Nuclear Laboratories, showing the location of piezometers HA, "O," and KNEW. The screens of these piezometers are 60 cm long and are located at the bottom of the piezometer, adjacent to the numbers on the figure.

Journal of Earth Sciences

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reducing environments r e s p e c t i v e l y . Piezometer HA i n the recharge area represents an o x i d i z i n g environment but i n a c c e s s i b i l i t y to e l e c t r i c i t y precluded i t s use i n the present s i t u a t i o n . Thus a i r saturated d i s t i l l e d water was used i n the l a b o r a t o r y e l u t i o n s t u d i e s to simulate an o x i d i z i n g environment ( i t i s r e a d i l y admitted that the absence of DOC and major and minor elements from the d i s t i l l e d water may a l t e r the e l u t i o n p r o f i l e ) . The geochemical parameters c h a r a c t e r i z i n g the ground water at these l o c a t i o n s are presented i n Table I . I t should be noted that at piezometer "0", w h i l e the d i s s o l v e d Fe c o n c e n t r a t i o n i s low and i s e n t i r e l y due to F e ( I I ) (polarographic d e t e r m i n a t i o n ) , the ground water here a l s o c o n t a i n s l a r g e amounts of suspended hydrated f e r r i c oxide. S o l u t i o n Studies. Reactions between Fe and As i n s o l u t i o n were a l s o s t u d i e d . An amount of FeCl3 s o l u t i o n was added to a buffered s o l u t i o n c o n t a i n i n g As so that the t o t a l a r s e n i c and i r o n c o n c e n t r a t i o n s were equal at 5 χ 10"" M. The s o l u t i o n s were s t i r r e d f o r 16-24 hours, allowed to s e t t l e and a l i q u o t s of the supernatant analyzed p o l a r o g r a p h i c a l l y f o r Fe and As, using a P r i n c e t o n A p p l i e d Research Model 174 Polarograph i n the sampled DC mode (2 mV s " scan r a t e , ΔΕ = 50 mV, 1 second drop t i m e ) . A 0.2 M sodium o x a l a t e (pH 4) s o l u t i o n was used to determine F e ( I I ) and F e ( I I I ) Ç 7 ) , w h i l e As(V) was determined i n a 2 M p e r c h l o r i c a c i d - 0.5 M p y r o g a l l o l s o l u t i o n (8^) and As ( I I I ) i n a 2 M sodium hydroxide - 0.2 M sodium t a r t r a t e s o l u t i o n Ç 7 ) . For heterogeneous s t u d i e s , A s ( I I I ) was adsorbed onto s i l i c a g e l impregnated w i t h f e r r i c hydroxide, according t o the batch method of Yoshida et a l . ( 9 ) . The i n i t i a l supernatant s o l u t i o n , and the supernatant s o l u t i o n r e s u l t i n g from an a c i d r i n s e (1 M HC1) of the s i l i c a g e l were analyzed p o l a r o g r a p h i c a l l y f o r As and Fe. 1

R e s u l t s and D i s c u s s i o n Column E l u t i o n s . The e l u t i o n behaviour of A s ( I I I ) i s s i g n i f i c a n t l y d i f f e r e n t from that of A s ( V ) , i n terms of both i t s time of i n i t i a l appearance and the q u a n t i t y of As e l u t e d . These parameters v a r y f o r each species w i t h the redox c h a r a c t e r i s t i c s of the water used, F i g u r e 3. In an o x i d i z i n g environment, A s ( I I I ) i s detected i n the column e l u a t e 5-6 times sooner than A s ( V ) , and the amount of A s ( I I I ) eluted (^60% of l o a d i n g ) i s ^8 times l a r g e r than that of As(V). In the " n e u t r a l " environment, the r e l a t i v e amounts of both species eluted are unchanged; however, the As(V) moves through the column much more r a p i d l y than before but s t i l l i s retarded w i t h r e s p e c t to the A s ( I I I ) . In the reducing zone, the m o b i l i t y of As (V) i s a c c e l e r a t e d : both species now appear i n the column e f f l u e n t a f t e r l e s s than one column volume i s d i s p l a c e d . Both species are a l s o e l u t e d almost q u a n t i t a t i v e l y (VL00% f o r A s ( I I I ) , ^80% f o r A s ( V ) ) .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

C H E M I C A L M O D E L I N G I N AQUEOUS

TABLE I Ground Water Geochemical D a t a

PH

HA

"0"

5.4

6.9

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580 DO

140

a

KNEW 8.3 75

2.4

13) a r e t o o w e a k l y a c i d i c t o be t i t r a t e d w i t h s o d i u m h y d r o x i d e u n d e r o u r e x p e r i m e n t a l c o n d i t i o n s ( m a x pH 1 1 . 5 ) . As was p r e v i o u s l y d e m o n s t r a t e d , phenolic hydroxyl groups which a r e ortho to carboxyl g r o u p s ( e . g . , o - h y d r o x y b e n z o i c a c i d ) c a n n o t be t i t r a t e d under t h e s e e x p e r i m e n t a l c o n d i t i o n s w h i l e more remote p h e n o l i c hydroxyl groups ( e . g . , p-hydroxybenzoic a c i d ) a r e r e a d i l y t i t r a t e d ( 2 8 ) . While t h e c o n c e n t r a t i o n s and r e l a t i v e abundances o f c a r b o x y l and t i t r a t a b l e p h e n o l i c groups a r e s i m i l a r f o r a l l three samples, t h e t o t a l a c i d i t i e s ( a s d e t e r m i n e d by t h e b a r i u m h y d r o x i d e method) are quite d i f f e r e n t . Since the total t i t r a t a b l e concentration a c i d i c f u n c t i o n a l g r o u p s o b t a i n e d by t i t r a t i o n c a l o r i m e t r y i s

of

l e s s than t h e t o t a l a c i d i t y o f t h e s a m p l e , r i v e r w a t e r humic substances appear to contain a s i g n i f i c a n t c o n c e n t r a t i o n o f weakly a c i d i c phenolic hydroxyl groups (those f u n c t i o n a l groups which were n o t t i t r a t e d i n t h e c a l o r i m e t e r ) . If these a c i d i c p h e n o l i c groups a r e , i n f a c t , ortho to carboxyl

weakly groups,

i t m i g h t be a n t i c i p a t e d t h a t r i v e r w a t e r h u m i c s u b s t a n c e s , having more c h e l a t i n g s i t e s , w o u l d have a l a r g e r m e t a l complexation c a p a c i t y than does s o i l humic a c i d . The a v e r a g e A H a v a l u e s f o r i o n i z a t i o n o f c a r b o x y l g r o u p s and t i t r a t a b l e p h e n o l i c groups and the average pKa o f the t i t r a t a b l e p h e n o l i c groups o f r i v e r w a t e r humic s u b s t a n c e s a r e g i v e n i n Table II. The p r e v i o u s l y r e p o r t e d A H a and p K a v a l u e s f o r s o i l humic a c i d (28) a r e i n c l u d e d f o r c o m p a r i s o n . As p r e v i o u s l y mentioned, the equilibrium constants f o r neutralization of c a r -

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

110

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Table

II.

Thermodynamic acidic

parameters

functional

substances

for

groups

and s o i l

of

humic

ionization river

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch005

Sample

Group

ΔΗ,

(kJ/mole)b'

a

WR-HS

Carboxyl

WR-HS

Phenolic

-11.8

SR-HS

Carboxyl

-

SR-HS

Phenolic

-32.0

+

3.4

3.8

+

3.3

1.7

+

9.2

Soil

HA

Carboxyl

Soil

HA

Phenolic

n.d.

=

a

+0.6

-

the

humic

acid.


2 d i s p e r s i o n i s added, to a c o n c e n t r a t i o n of 42 μπι. C o n s t a n t i o n i c s t r e n g t h (0. 01 M K N O 3 ) , c o n s t a n t t e m p e r a t u r e (25°C) and a f i x e d p H a r e m a i n t a i n e d . N i t r o g e n i s b u b b l e d c o n t i n u o u s l y to r e m o v e a l l c a r b o n a t e s f r o m s o l u t i o n . C o p p e r i s added i n nine steps f r o m 0. 5 to 16 μΜ ; one h o u r e q u i l i b r i u m i s a l l o w e d a f t e r e a c h a d d i t i o n b e f o r e a s u b s a m p l e (30 m l ) i s f i l t e r e d and a c i d i f i e d . T o t a l d i s s o l v e d c o p p e r i s m e a s u r e d i n the f i l t r a t e b y d.p.a.s.v. C a l i b r a t i o n of Μ η θ £ w i t h C u i s c a r r i e d out at the s a m e c o n c e n t r a t i o n , t e m p e r a t u r e , i o n i c s t r e n g t h and p H c o n d i t i o n s . T h e c u p r i c i o n c o n c e n t r a t i o n can then be c a l c u l a t e d u s i n g the L a n g m u i r equation. M a s s b a l a n c e f r o m the m e a s u r e d t o t a l C u and the c u p r i c i o n g i v e s the c o m p l e x e d c o p p e r c o n c e n t r a t i o n , C u L ; the t o t a l l i g a n d c o n c e n t r a t i o n and the c o n d i t i o n a l s t a b i l i t y constant, Κ , f o r the f o r m a t i o n of C u L , C u + L = CuL, +2 (Cu ) a r e then c a l c u l a t e d b y p l o t t i n g ( C u ) v s . , \ and u s i n g +2) 2 (CuLj (Cu 1 (Cu ) __, 1 _ . _ , , , = 7+ — . . The ligand concentration (CuL) K_ ( L . , ) ( L ) * L total total i s o b t a i n e d f r o m the s l o p e and the c o n d i t i o n a l s t a b i l i t y c o n s t a n t is o b t a i n e d f r o m the slope d i v i d e d b y the Y - a x i s i n t e r c e p t . The c o r r e l a t i o n c o e f f i c i e n t i s c a l c u l a t e d f r o m a l e a s t s q u a r e s a n a l y s i s of a l l the data. The m e t h o d has b e e n t e s t e d o n s o m e r e a s o n a b l y w e l l c h a r a c t e r i z e d l i g a n d s (20) and a p p e a r s to w o r k w e l l at l i g a n d c o n c e n t r a t i o n s found i n n a t u r a l w a t e r s (0. 2 μ Μ ). The c o n d i t i o n a l s t a b i l i t y c o n s t a n t s o b t a i n e d f o r n a t u r a l l y o c c u r ­ r i n g l i g a n d s f a l l w i t h i n the r a n g e of s t a b i l i t y constants f o r known and t e s t e d l i g a n d s . The r e s u l t s of t i t r a t i o n s w i t h c o p p e r of a n u m b e r o f l a k e s a n d r i v e r s u s i n g this m e t h o d , a r e g i v e n h e r e . A n a t t e m p t has b e e n m a d e to c o r r e l a t e the data to o t h e r f a c t o r s s u c h a s U V - s p e c t r o p h o t o m e t r i c m e a s u r e m e n t s f o r o r g a n i c content and p H . + 2

+ 2

ir

T

+

v

L

Sample Description S a m p l e s , u s u a l l y two l i t e r s , w e r e t a k e n , f i l t e r e d as soon a s p o s s i b l e (0.45 μπι M i l l i p o r e ) and s t o r e d i n the d a r k u n d e r r e f r i g e r a t i o n . S a m p l e s w e r e c o l l e c t e d f r o m the following fresh water environments:

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

118

- d y s t r o p h i c w a t e r s : low p H ( 4 . 6 ) b r o w n c o l o u r e d w a t e r s , c o n t a i n i n g m u c h o r g a n i c m a t t e r , f l u s h e d out of s o i l s . T h e R i v e r s D i c k i e N o . 5,6 a n d 10, R e d C h a l k N o . 3 and 4 and L a k e D i c k i e b e l o n g to this g r o u p . - m e d i u m a l k a l i n i t y (ΛΙΟ. 5 m e q / l j , low p r o d u c t i v i t y , n o n - p o l ­ luted: Lake Windy - h i g h a l k a l i n i t y (rJ 2 m e q / L ) , n o n - p o l l u t e d : L a k e H u r o n - h i g h a l k a l i n i t y (rj2 m e q / L ) , m e d i u m h i g h p r o d u c t i o n (no bloom): G l o u c e s t e r P o o l , L a k e O n t a r i o , Onaping R i v e r (flows out of L a k e O n a p i n g ) . - r a t h e r h e a v i l y p o l l u t e d w i t h m e t a l s , low a l k a l i n i t y (*g0. 1 m e q / L ) and h a v i n g a m e d i u m h i g h p r o d u c t i o n : W h i t e w a t e r l a k e . - F A : s u p p l i e d to us by S c h n i t z e r i n d r i e d f o r m (15). It has b e e n e x t r a c t e d f r o m the s o i l . R e s u l t s and D i s c u s s i o n C o n d i t i o n a l S t a b i l i t y C o n s t a n t s and C o m p l e x i n g C a p a c i t y . T h e r e s u l t s of c o p p e r t i t r a t i o n s i n p r e s e n c e of M n O ^ of a n u m b e r of n a t u r a l w a t e r s a r e g i v e n i n T a b l e I . Initial analyses done i n contact w i t h a i r a t m o s p h e r e a r e l e s s a c c u r a t e (low c o r r e l a t i o n c o e f f i c i e n t s ) due to the l a r g e a m o u n t of c o p p e r carbonate complexation. C o r r e c t i o n i n v o l v e s the s u b t r a c t i o n of the c a l c u l a t e d C u C O ° c o n c e n t r a t i o n f r o m the m e a s u r e d d i s -

+2 s o l v e d c o p p e r c o n c e n t r a t i o n to o b t a i n C u and C u L . In a l l c a s e s the c o r r e l a t i o n c o e f f i c i e n t s a r e c a l c u l a t e d f o r nine data p o i n t s and a r e thus c o m p a r a b l e a m o n g t h e m s e l v e s . The l o g c o n d i t i o n a l s t a b i l i t y constants of s m a l l , m o n o p r o t i c a c i d s c a n be a s s u m e d to be l i n e a r l y and on a o n e - t o one b a s i s dependent upon the p H , due to c o m p e t i t i o n between

+

+2

Η and C u , u n t i l the p H r e a c h e s the value of the a c i d i t y constant. S i n c e the c o m p l e x a t i o n r e a l l y i s c o m p o s e d of two competitive reactions: 1. Cu + L " = C u L , K_ L + 2

and

2.

HL = H

+

+ L", Κ a

W h e n p H > Κ , equation 1 i s s u f f i c i e n t and K _ i s a L

constant

Ο

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

6.

VAN DEN BERG AND KRAMER

Table I.

I OTIS With

Ligands

119

C o m p l e x i n g c a p a c i t i e s and c o n d i t o n a l s t a b i l i t y con­ stants of n a t u r a l w a t e r s . In m o s t c a s e s only one type of c o m p l e x i n g site p e r l i g a n d m o l e c u l e was found of r e l e v a n c e to the n a t u r a l w a t e r s y s t e m . O r i g i n a l s a m p l e p H shown i n b r a c k e t s , r = c o r r e l . coeff. , o r i g i n a l , undiluted, ligand concentration given i n brackets.

pH Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

Copper

log

K

L

Complexing Capacity

r

* Gloucester Pool

8. 4

9. 3

0. 51 μ Μ

0. 90

*Lake Huron

8. 3

9. 2

0. 20

0. 70

* Whitewater Lake

8. 0

8. 6

0. 68

0. 98

* Onaping River

7. 8

8. 6

0. 38

0. 95

* Windy Lake

6. 6

7. 2

0. 20

0. 57

Lake

Ontario

8. 4

9. 5

0. 33

0. 97

Lake

Ontario

7. 4(8. 4)

8. 6

0. 34

0. 986

D i c k i e No.

5

8. 4(4. 6)

8. 5

5. 35(20)

0. 98

D i c k i e No.

5

7. 6(4. 6)

7. 8

2. 47(20)

0. 987

D i c k i e No.

6

7. 6(4. 6)

7. 8

5. 75

0. 98

D i c k i e No.

10

7. 6(4.9)

7. 8

4.95(10.9)

0.991

Lake Dickie

7. 6(4. 6)

7. 8

2. 19

0. 996

Red Chalk #3

7. 6(6.3)

7. 7

3. 35

0.991

Red

7. 6(4. 7)

7.9(K )

2. 43

0. 992

7-2(K )

5.93

0. 998

7. 8

2. 24

0.986

Chalk#4

L i

L z

Fulvic Acid

7. 6(4. 6)+

*has b e e n d e t e r m i n e d i n p r e s e n c e of c a r b o n a t e , i n a i r a t m o s ­ phere. +pH of F A d i s s o l v e d i n d i s t i l l e d w a t e r .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

120 When p H ^ K

, Κ

a

L

, as u s e d h e r e , b e c o m e s :

[c„-][„ ]'

[,„•] •

L

Now l o g Κ constant,

L Κ

is l i n e a r l y p H dependent.

B u t s i n c e the a c i d i t y

, i s u n k n o w n , the a u t h o r s w e r e unable to c a l c u l a t e

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

Ο

L a r g e poly electrolytic m o l e c u l e s , however, approach colloidal particles in their behaviour. Then Κ determines L the e q u i l i b r i u m s i t u a t i o n for the r e a c t i o n between a m e t a l i o n and c h a r g e s on the c o l l o i d a l p a r t i c l e . So Κ should v a r y with L the d e g r e e of n e u t r a l i z a t i o n and the i o n i c s t r e n g t h to the s a m e extent as the i o n i z a t i o n c o n s t a n t , but i n o p p o s i t e d i r e c t i o n (24). A c c o r d i n g l y it has b e e n found i n s e v e r a l c a s e s that l o g Κ

L v a r i e d w i t h the p H but not w i t h a s l o p e e q u a l to o n e . Takamatsu and Y o s h i d a (25) found that the o v e r a l l constants f o r the f o r m a ­ t i o n of the M L ^ c o m p l e x ( L = H A ) ,

P2

L

M L

2

]

fed y Cu Pb Cd

2+ 2+ 2+

, vvaarryy as f o l l o w s for d i f f e r e n t m e t a l i o n s : 2

:

l o g β..

8. 65 + 0.65

(pH-5)

:

log β

2

8. 35 + 0.30

(pH-5)

:

l o g β..

6. 25 + 0.63

(pH-5)

F o r b e t t e r u n d e r s t a n d i n g of s u c h c o r r e l a t i o n s it i s n e c e s s a r y to know the a c i d i t y constants i n v o l v e d . G e n e r a l l y , these have b e e n d e t e r m i n e d a t h i g h l i g a n d c o n c e n t r a t i o n s (around 10"^ M ) and r a t h e r low p H . The a c i d i t y constants a l s o have b e e n found to be v e r y p H dependent and i o n i c s t r e n g t h dependent, a g a i n fitting the c o m p a r i s o n w i t h c o l l o i d a l p a r t i c l e s . Thus G a m b e (26) found l o g constants a r o u n d 3.6 f o r and 4 . 3 for K.£ a t p H 3 and p H 4 r e s p e c t i v e l y , (27) found l o g

= 5.5

for F A ' s .

C o l e m a n et a l .

for p e a t .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

6.

VAN DEN BERG AND KRAMER

Copper

lOYlS With

Ligdtlds

121

A l l these c o n s i d e r a t i o n s l i m i t the data w i t h w h i c h the r e s u l t s of this study can be c o m p a r e d , s i n c e they w e r e d e t e r ­ m i n e d a t a h i g h e r pH. R e s u l t s of s i m i l a r s t u d i e s a r e s u m ­ m a r i z e d i n T a b l e I I , The d e t e r m i n a t i o n s by B r a n i c a (28) and S h u m a n and W o o d w a r d (11 ) w e r e a c t u a l l y p e r f o r m e d f o r c o n d i t i o n s of s e a w a t e r and l a k e w a t e r , r e s p e c t i v e l y . The o t h e r a u t h o r s p r e c o n c e n t r a t e d t h e i r l i g a n d s . P o l a r o g r a p h i c a l methods have the d i s a d v a n t a g e that the c o m p l e x m a y s p l i t up d u r i n g m e a s u r e m e n t , u n l e s s the m e a s u r e m e n t i s p e r f o r m e d m u c h f a s t e r than the k i n e t i c s of the c o m p l e x d i s s o c i a t i o n . The a p p a r e n t s t a b i l i t y constants thus d e t e r m i n e d m a y w e l l be too s m a l l . M a n t o u r a and R i l e y (30), who w o r k e d at a p H s i m i l a r to the p r e s e n t e x p e r i m e n t s and who a l s o u s e d a s y s t e m w h e r e l i g a n d and m e t a l a r e i n e q u i l i b r i u m w i t h e a c h o t h e r , found constants q u i t e s i m i l a r to those of the p r e s e n t study. The v a l u e d e t e r m i n e d by van D i j k (31 ) i s m e n t i o n e d to show how a c o m p a r a t i v e l y s i m p l e m e t h o d can p r o d u c e the o r d e r of m a g n i t u d e of the s t a b i l i t y constant. He c o m p a r e d the s t r e n g t h of s o m e known c o m p l e x i n g agents w i t h that of HA i n the p r e ­ s e n c e of an i o n exchange r e s i n . E f f e c t of p H on the C o n d i t i o n a l S t a b i l i t y Constant. A n u m b e r of w a t e r s have b e e n a n a l y s e d a t t h e i r o r i g i n a l pH, s o m e have b e e n a d j u s t e d to p H 7.6 f o r i n t e r c o m p a r i s o n , and s o m e have b e e n a n a l y s e d t w i c e at d i f f e r e n t pH's. The r e s u l t s a r e p l o t t e d i n F i g u r e 1. The d y s t r o p h i c w a t e r s a l l g i v e v e r y s i m i l a r v a l u e s at p H 7. 6, and the l o g c o n s t a n t i n c r e a s e s w i t h a s l o p e c l o s e to one w i t h the pH: f o r D i c k i e No. 5, l o g Κ i s 8.5 a t p H 8.4, and 7. 8 at p H 7. 6 The o t h e r s a m p l e s p r o v i d e d g e n e r a l l y h i g h e r c o n s t a n t s . The d e t e r m i n e d f o r L a k e O n t a r i o , i s an o r d e r of m a g n i t u d e h i g h e r than F A . A g a i n , a s h i f t w i t h the p H w i t h a s l o p e c l o s e to one was o b s e r v e d : L a k e O n t a r i o , l o g Κ i s 9. 5 at p H 8.4, and 8.6 at p H 7.4. The a u t h o r s o b s e r v e d that the c o n d i t i o n a l s t a b i l i t y constants s t i l l i n c r e a s e at the h i g h e s t pH's m e a s u r e d (pH 8.6). A p p a r e n t l y the l i g a n d i s at l e a s t s t i l l p a r t i a l l y i o n i z e d so t h e r e has to be a l o g d i s s o c i a t i o n c o n s t a n t l a r g e r than 8.6. By s p e c i f i c a l l y b l o c k i n g a c t i v e g r o u p s on o r g a n i c m a t t e r , S c h n i t z e r and S k i n n e r (32 ) showed that m e t a l s a r e bound by s i m u l t a n e o u s a c t i o n of a c i d i c c a r b o x y l g r o u p s and p h e n o l i c h y d r o x y l g r o u p s . One c o u l d s p e c u l a t e that the h y d r o x y l g r o u p s have a r a t h e r h i g h d i s s o c i a t i o n constant s i n c e they a r e l e s s a c i d i c .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. χ

(Κ )

2

(K )

7. 8

FA

(2) F A e x t r a c t e d f r o m peat.

(1) F A e x t r a c t e d f r o m l a k e w a t e r

7

4.5-5.7

2

7. 16 ( K )

8.51

8.05

8.80 (K )

HA

Lakewater

FA

(2)

F A (1)

5. 1-6.2

7.46

Seawater

HA

Log Constant

Ligand

7.6

6

6.5

8. 0

6.8(?)

8.2

pH

ion

exchange

competition

a. s. v.

gelpermeation

dialysis

d.p.p.

Method

0. 01

0. 1

0. 01

0. 01

0. 7

Ionic Strength

this paper

(31.)

(3 0)

(29)

(28)

Reference

T a b l e I I . C o m p a r i s o n of c o n d i t i o n a l s t a b i l i t y c o n s t a n t s f o r c o m p l e x e s of c o p p e r with naturally occurring organics

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

6.

VAN

DEN

BERG AND

KRAMER

Copper Ions With

123

Ligands

The l i g a n d s that w e r e m e a s u r e d i n the f i r s t g r o u p of ( d y s t r o p h i c ) w a t e r s m a y w e l l be d e r i v e d f r o m s o i l s . F u l v i c a c i d s , b e i n g the s m a l l e s t and m o s t s o l u b l e of the h u m i c c o m ­ pounds, a r e f l u s h e d out m o s t e a s i l y and f o r m a m a j o r p a r t of d i s s o l v e d o r g a n i c s i n n a t u r a l f r e s h w a t e r s (33). The s i m i l a r i t y of the m e a s u r e d s t a b i l i t y constants of F A and dy­ s t r o p h i c w a t e r s s u g g e s t s that s i m i l a r ( c a r b o x y l ) g r o u p s m a y be r e s p o n s i b l e f o r b i n d i n g and that the r e m a i n i n g p a r t of the m o l e c u l e s i s not v e r y m u c h d i f f e r e n t f r o m e a c h o t h e r .

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

A d s o r p t i o n of the M e t a l C o m p l e x on MnCL,.

Adsorption

of the m e t a l - o r g a n i c c o m p l e x , o r p a r t of i t , on the i o n exchange m e d i u m w o u l d a f f e c t the d e t e r m i n a t i o n of the s t a b i l i t y constant i n that the d i s s o l v e d c o p p e r c o n c e n t r a t i o n w i l l be d e c r e a s e d . P r e v i o u s s t u d i e s u s i n g i o n exchange r e s i n s o b s e r v e d no ad­ s o r p t i o n e f f e c t s (15 ) o r do not m e n t i o n i t . The p r e s e n t r e s u l t s w i t h known l i g a n d s (20) show that a d s o r p t i o n , i f any, of the c o m p l e x does not a f f e c t the m e a s u r e m e n t and the r e s u l t s . It was o b s e r v e d , h o w e v e r , that the o r g a n i c m a t t e r has s o m e s o r t of s u r f a c e a c t i v e e f f e c t on the MnO^ d i s p e r s i o n : s u b s a m p l e s , taken d u r i n g the t i t r a t i o n w i t h c o p p e r , of w a t e r s c o n t a i n i n g a h i g h l i g a n d c o n c e n t r a t i o n (10 μΜ ) take m o r e t i m e , up to about t w i c e as long to be f i l t e r e d than s a m p l e s c o n t a i n i n g a v e r y low l i g a n d c o n c e n t r a t i o n (0.3 μΜ ). A l s o , the type o f s a m p l e s e e m s to a f f e c t the f i l t e r speed. A s a m p l e of L a k e O n t a r i o , c o n t a i n i n g 0.5 μΜ l i g a n d s , has r o u g h l y the same e f f e c t as a 5 μιη F A s o l u t i o n . A p p a r e n t l y the s i z e and c o m p l e x i t y of the m o l e c u l e s d e t e r m i n e s i n w h a t way the MnO^ d i s p e r s i o n w i l l be stabilized. F r o m C o u l o m b i c - f o r c e s p o i n t of v i e w one can s p e c u l a t e that t h e r e i s s o m e a d s o r p t i o n at v e r y l o w copper c o n c e n t r a t i o n [^Cu^J < Q ^ ^ J ' o s t of the l i g a n d s a r e c o m p l e x e d and w

n

e

n

m

have a p o s i t i v e c h a r g e , w h i l e the MnO^ s t i l l has i t s n e g a t i v e c h a r g e . A s soon as £ C u r J > {^Lig^^J , b o t h the c o m p l e x and the s u r f a c e of the MnO w i l l have a p o s i t i v e c h a r g e and w i l l r e p e l e a c h o t h e r e l e c t r o s t a t i c a l l y . T h i s w i l l be d i s c u s s e d f u r t h e r in a later publication. M o l e c u l a r W e i g h t o f FA. FA was S c h n i t z e r i n d r i e d form (32). I f one c o m p l e x a t i o n o f one c o p p e r i o n b y one can c a l c u l a t e the m o l e c u l a r w e i g h t o f

s u p p l i e d t o us by assumes the F A - m o l e c u l e , one FA t o be 8 4 1 ,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

124

which compares w e l l to the v a l u e o f 950 as found b y Schnitzer

(34).

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

Determination o f Conditional Stability Constants for M i x e d L i g a n d s and f o r C o m p l e x e s o t h e r than 1:1 . Schubert's (15) study o f c o m p l e x ions by a n i o n exchange technique was c o m m e n t e d upon by I. F e l d m a n , who m e n t i o n e d that the m e t h o d w o r k s o n l y f o r 1:1 c o m p l e x e s . T h i s is i n h e r e n t to any m e t h o d , h o w e v e r , w h i c h does not v a r y the l i g a n d c o n c e n t r a t i o n . In c a s e of a m i x t u r e of l i g a n d s , a n a v e r a g e s t a b i l i t y constant is d e t e r mined (35). T h e d e t e r m i n a t i o n of two s t a b i l i t y constants f o r 8H y d r o x y l - Q u i n o l i n e (20) i n the p r e s e n t study shows that the i o n exchange m e t h o d is c a p a b l e o f a c c u r a t e l y d e t e r m i n i n g two s i t e s o r two l i g a n d s i f t h e s e two s i t e s a r e p r e s e n t i n e q u a l c o n c e n t r a t i o n , and i f the sites r e s u l t i n d i f f e r e n t enough s t a b i l i t y constants r e l a t i v e to d i s c e r n i n g s l o p e c h a n g e s . In o t h e r c a s e s the g r a p h i c a l r e p r e s e n t a t i o n of the t i t r a t i o n w i l l be s l i g h t l y c u r v e d c o n c a v e l y . E x c e p t i n one c a s e , the r e s u l t s produced almost straight lines with high c o r r e l a t i o n coeff i c i e n t s s h o w i n g that one site o r l i g a n d is at l e a s t i n a v e r y d o m i n a n t p o s i t i o n . C o n t r a d i c t o r y r e s u l t s have b e e n p u b l i s h e d e a r l i e r . A r d a k a n i and S t e v e n s o n (19) found o n l y 1:1 c o m p l e x e s of m e t a l s w i t h H A and o b s e r v e d o n l y a s i n g l e c o m p l e x i n g s i t e . S c h n i t z e r and H a n s e n (17) o b s e r v e d the s a m e f o r F A ( f r o m soil). M a n t o u r a a n d R i l e y (30) found two s i t e s o n F A e x t r a c t e d f r o m water. L i g h t A b s o r p t i o n at 260 n m and C o m p l e x i n g C a p a c i t y . T h e c o n c e n t r a t i o n of h u m i c m a t t e r in w a t e r has b e e n r e l a t e d to m e a s u r e m e n t s of l i g h t a b s o r p t i o n at d i f f e r e n t w a v e l e n g t h s at 365 n m and at 250 n m . S c a n n i n g s p e c t r o p h o tome t r i e m e a s u r e m e n t s w e r e m a d e f r o m 220 to 380 n m . T h e p e a k height g e n e r a l l y d e c r e a s e s going to h i g h e r w a v e l e n g t h s and the p e a k was r a t h e r s m a l l to m a k e a c c u r a t e m e a s u r e m e n t s at 365 n m . In m o s t s a m p l e s , h o w e v e r , a p l a t e a u that l a s t e d f r o m 255 to 265 n m , and s o m e t i m e s f r o m 250 to 270 n m , was f o u n d . It was then d e c i d e d to m e a s u r e the a b s o r p t i o n at the height o f this p l a t e a u , at 260 n m . T h e low p H s a m p l e s w e r e b r o u g h t to p H 7 by the a d d i t i o n of s o d i u m b i c a r b o n a t e buffer u n t i l -3 10 M b i c a r b o n a t e was p r e s e n t . The results a r e given in F i g u r e 2, as a b s o r p t i o n v s . c o m p l e x i n g c a p a c i t y . T h e data s c a t t e r w i d e l y w h i c h s u g g e s t s that i t i s not p o s s i b l e to a c c u r a t e l y p r e d i c t the c o m p l e x i n g c a p a c i t y f r o m a n a b s o r p t i o n

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

VAN DEN BERG AND KRAMER

Copper

IOUS With

Ligands

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

log K,

Figure

1.

Conditional mined. (%)=

stability constants vs. pH at which they have been deter­ Dystrophic waters and FA; (X) = other waters.

0

10

20

COMPLEXING CAPACITY (μΜ) Figure 2.

Light absorption

at 260 nm vs. complexing

capacity

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

126

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

m e a s u r e m e n t . One cannot s a y m o r e than that h i g h a b s o r p t i o n might indicate a high complexing capacity. P o s s i b l y only part of the a b s o r p t i o n i s c a u s e d b y c o m p l e x i n g m a t e r i a l . The r e s t i s c a u s e d by o t h e r o r g a n i c m a t t e r . Complexation of C u by some Known Ligands and N a t u r a l l y O c c u r r i n g L i g a n d s at p H 8.3. The e f f e c t i v e n e s s i n l o w e r i n g the f r e e m e t a l i o n c o n c e n t r a t i o n by s o m e known c o m p l e x i n g l i g a n d s and b y F A a n d l i g a n d s as i n L a k e O n t a r i o w a t e r i s s h o w n i n F i g u r e 3. The r e l a t i o n s h i p s have b e e n c a l ­ c u l a t e d at p H 8.3, a s i n s e a w a t e r , a n d f o r 25°C a n d 0. 01 M i o n i c s t r e n g t h . The s t a b i l i t y constants of the k n o w n l i g a n d s have b e e n c o r r e c t e d f o r the p H u s i n g t h e i r a c i d i t y constants a s found i n S i l l en a n d M a r t e l l (36). F o r F A and L a k e O n t a r i o the constants have b e e n t a k e n f r o m T a b l e l a n d a d j u s t e d f o r p H 8. 3. In the c a l c u l a t i o n s , 1 0"^ M c o m p l e x i n g l i g a n d s a n d 10~8 m t o t a l d i s s o l v e d c o p p e r w e r e a s s u m e d . T h e s e c o n c e n ­ t r a t i o n s a r e s i m i l a r to v a l u e s found i n o p e n o c e a n w a t e r w h i c h has about 1 mg. L~*. C (37); i f the o r g a n i c c a r b o n o c ­ c u r r e d as a c o m p o u n d w i t h the Mw. of F A , i t w o u l d be 10"^ M l i g a n d s , but b e c a u s e o f the v e r y l o n g r e s i d e n c e t i m e the m o l e c u l e s p r o b a b l y a r e m o r e c o m p l e x a n d h e a v i e r than F A . H i s t i d i n e , w h i c h has b e e n r e p o r t e d to o c c u r a t l e v e l s o f -8 -7 10 M (38) to 10 M (39), w o u l d b r i n g the f r e e c o p p e r i o n -1 0 8 d o w n to a v e r y l o w l e v e l o f 10 " M; N T A w h i c h m i g h t e n t e r the s e a a s a r e s u l t of p o l l u t i o n , w o u l d b r i n g i t down to -12 10 M , b u t i t i s b i o d e g r a d a b l e a n d i s not p r o d u c e d i n s i t u . Organic matter o c c u r r i n g in Lake Ontario would produce a p [ c u J o f 10. 5. A n d e r s o n and M o r e l (8) c a l c u l a t e d w i t h the R E D E Q L c o m p u t e r m o d e l a ρ £ c u ^ J o f 9. 6 b y i n o r g a n i c c o m ­ plexation. The p r e s e n c e of o n l y 10"^ M o r g a n i c c o m p l e x i n g l i g a n d s m a y w e l l be s u f f i c i e n t to d i m i n i s h the f r e e i o n c o n c e n ­ tration by an o r d e r of magnitude. A n inorganic particulate such as M n O ^ w o u l d not be a s u c c e s s f u l c o m p e t i t o r f o r c u p r i c i o n s + 2

+

under these

circumstances.

C o m p l e x a t i o n o f C u a t V a r y i n g p H . In o r d e r to c o m ­ p a r e the e f f e c t s o f c o m p l e x a t i o n by l a k e o r g a n i c s a t d i f f e r e n t pH's, the f r a c t i o n (a) of t o t a l d i s s o l v e d c o p p e r (Cu^,) w a s c a l c u l a t e d w h i c h i s i n the f o r m o f the f r e e m e t a l i o n

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

6.

Copper Ions with Ligands

V A N DEN BERG AND KRAMER

127

Figure 3. The effect of complexation at pH 8.3 by naturally occurring ligands and some artificial ligands (on Cu ) shown against the conditional stability con­ stant (K ) at this pH. Conditions for the plot are total ligand concentration L = J0" M, total copper concentration Cu = 10~ M, 25° C, μ = 0.01. +2

L

T

7

T

8

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

128

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Figure

4. Model for complexation of copper by lake organics. Ligand tai = log K = 7.8, and log K0n,ert0 = 8.8 at pH 7.6, log K oh = 6.0, log Kcucos = 6.8, pCO, = 10' , = 0.01, 25°C. (Cu ) = . (Cu ), = 1/ (K · [L] + 1), Cu = 2 χ 10 U. 1:L = OH", 2:L = COf, 3:L = FA, 4:L = ligand in Lake Ontario. to

W M, 6

F

Cu

A

0

L

35

T

μ

+2

a

+

T

a

7

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

6.

VAN D E N BERG AND KRAMER

+ 2

(Cu ):

[cu

+

2

Copper

]= . [Cujand 0

IoUS With

α

=K

L

Llgands

129

. £ L ]+ 1'

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

F r o m the r e s u l t s , i n F i g u r e 4, one c a n see that u n t i l about p H 9, o r g a n i c s a r e m u c h m o r e i m p o r t a n t i n c o m p l e x i n g c o p p e r than a r e c a r b o n a t e and h y d r o x y l ion. A t a l o w e r p H of 6, a n d w i t h o r g a n i c s w i t h a s t a b i l i t y c o n s t a n t a s found i n L a k e O n t a r i o , 9 0% o f the copper p r e s e n t i s c o m p l e x e d b y 10" M o r g a n i c s , w h e r e a s a t a p H o f 8, 99. 9% o f the c o p p e r i s complexed. Conclusions C o n d i t i o n a l s t a b i l i t y constants f o r c o m p l e x a t i o n of n a t u r a l o r g a n i c s w i t h c o p p e r have v a l u e s , a t p H 7.6, between 7 8 88 10 , f o r F A , and 10 , f o r ligands i n Lake Ontario. G e n e r a l l y , o n l y one c o m p l e x i n g s i t e p e r m o l e c u l e has been found w i t h a s t a b i l i t y constant of i m p o r t a n c e a t the con­ c e n t r a t i o n s found i n n a t u r a l w a t e r s , f o r F A and o t h e r n a t u r a l ­ ly occurring ligands. Organic matter occurring i n natural waters is a sig­ nificant complexing ligand f o r free metal ions. A cknowledgements T h i s w o r k was s u p p o r t e d by g r a n t s f r o m the N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a and Inland W a t e r s Subvention P r o ­ gram, Environment Canada. Abstract Toxicity to copper in natural waters may be related to the cupric ion concentration. Therefore, a method is developed using MnO as weak ion exchanger, to assess the complexing 2

capacities and conditional stability constants for compounds in natural waters. Constants found are in the range of 10 for fulvic acid to 10 for a ligand in Lake Ontario, at pH 7. 6, 2 5 ° C , and 0. 01 M ionic strength. Calculation of the complexa­ tion of copper by 10 M naturally occurring ligands, at different pH's, shows that the free metal ion concentration is lowered considerably between pH 5 and 9. 7·

8.8

-6

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

8

130

CHEMICAL MODELING IN AQUEOUS SYSTEMS

L i t e r a t u r e Cited 1. 2.

3.

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7. 8. 9.

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11. 12.

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Stumm, W. and Morgan, J . J . "Aquatic Chemistry," 583 p., Wiley-Interscience, New York, 1970. Schnitzer, M. Metal-organic matter interactions i n s o i l s and waters, p. 297-313, i n Faust, S.D. and Hunter, J.V. (Ed.), "Organic Compounds i n Aquatic Environments," Marcel Dekker, New York, 1971. Κamp Nielsen, L. The influence of copper on the photosynthesis and growth of C h l o r e l l a pyrenoidosa, Dan. Tidsskr. Farm. 43, 249-254 (1969). Steeman Nielsen, E. and Wium-Andersen, S. Copper ions as poison i n the sea and i n freshwater, Mar. B i o l . 6, 93-97 (1970). Hutchinson, G.E. "A Treatise on Limnology," v. 1, 1015 p., Wiley & Sons, New York, 1957. Davey, E.W., Morgan, M.J. and Erickson, S.J. A b i o l o g i c a l measurement of the copper complexation capacity of seawater, Limnol. Oceanogr. 18, 993-997 (1973). Sunda, W.G., and G u i l l a r d , R.R. Relationship bet­ ween cupric ion a c t i v i t y and the t o x i c i t y of copper to phytoplankton, J . Mar Res. 34, 511-529 (1976). Anderson, D.M., and Morel, F.M.M. Copper s e n s i t i ­ v i t y of Gonyaulax tamarensis, Limnol. Oceanogr. 23, 283-295 (1978). Jackson, G.A., and Morgan, J . J . Trace metal-chelator interactions and phytoplankton growth i n seawater media: t h e o r e t i c a l analysis and comparison with reported observations, Limnol. Oceanogr. 23, 268282 (1978). Chau, Y.K., Gachter, R., and Lum-Shue-Chan, K. Determination of the apparent complexing capacity of lake waters, J . F i s h . Res. Board Can. 31, 15151519 (1974). Shuman, M.S., and Woodward, G.P., J r . S t a b i l i t y constants of copper-organic chelates i n aquatic samples, Environ. S c i . Technol. 11(8), 809-813 (1977) Brezonik, P.L., Brauner, P.Α., and Stumm, W. Trace metal analysis by anodic s t r i p p i n g voltammetry: e f f e c t of sorption by natural and model organic compounds, Water Res. 10, 605-612 (1976). Günther-Schulze, A. Die ermittlung der Selbstkomplex bildung i n Wässerigen Lösungen von Kupfersalzen mit H i l f e des Permutits, Z. Electrochem. 28, 89-99 (1922). Günther-Schulze, A. Die d i s s o z i a t i o n der chloride zweiwertiger metalle i n Wässerigen Lösung, Ζ. Electrochem 28, 387-389 (1922).

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6.

VAN DEN BERG AND KRAMER

ϋΟΌΌβΐ lOïlS With Ligands

131

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch006

15.

Schubert, J. Ion exchange studies of complex ions as a function of temperature, ionic strength, and presence of formaldehyde, J. Phys. Chem. 56, 113118 (1952). 16. Schubert, J . The use of ion exchangers for the determination of physical-chemial properties of substances, p a r t i c u l a r l y r a d i o t r a c e r s i n s o l u t i o n , I. T h e o r e t i c a l , J. Phys. C o l l . Chem. 52, 340-350 (1948). 17. Schnitzer, Μ., and Hansen, E . H . Organo-metallic interactions i n s o i l s : 8. An evaluation of methods for the determination of s t a b i l i t y constants of m e t a l - f u l v i c acid complexes, S o i l S c i . 109, 333-340 (1970). 18. Gamble, D . S . , Schnitzer, M. and Hoffman, I., C u f u l v i c acid chelation equilibrium i n 0.1 m KC1 at 2 5 . 0 ° C , Can. J. Chem. 48, 3197-3204 (1970). 19. Ardakani, M . S . , and Stevenson, F.J. A modified ion exchange technique for the determination of metal­ - s o i l organic matter complexes, S o i l S c i . Soc. Amer. Proc. 36, 884-890 (1972). 20. van den Berg, C . M . G . , and Kramer, J . R . Determi­ nation of complexing capacities and conditional s t a b i l i t y constants for copper i n natural waters using MnO , Anal. Chim. Acta (in press). 21. Morgan, J.J., and Stumm, W. C o l l o i d chemical properties of manganese dioxide, J . C o l l . S c i . 19, 347-359 (1964). 22. Murray, J.W. The i n t e r a c t i o n of metal ions at the manganese d i o x i d e - s o l u t i o n i n t e r f a c e , Geochim. Cosmochim. Acta 39, 505-519 (1975). 23. Gray, M.J., M a l a t i , M . A . , and Raphael, M.W., The point of zero charge of manganese dioxides, J . Electronanal. Chem., 135-140 (1978). 24. Gregor, H . P . , L u t t i n g e r , L.B., and L o e b l . , E . M . M e t a l - p o l y e l e c t r o l y t e complexes. I . The p o l y a c r y l i c acid-copper complex, J. Phys. Chem. 59 34-39 (1955). 25. Takamatsu, T., and Yoshida, I . Determination of s t a b i l i t y constants of metal-humic acid complexes by potentiometric t i t r a t i o n and i o n - s e l e c t i v e electrodes, S o i l S c i . 125, 377-386 (1978). 26. Gamble, D.S. T i t r a t i o n curves of f u l v i c a c i d : the a n a l y t i c a l chemistry of a weak acid polyelectrolyte, Can. J . Chem. 48, 2662-2669 (1970). 27. Coleman, N . T . , McClung, A.C., and Moore, D.P. Formation constants for Cu(II)-peat complexes, Science 123, 330-331 (1956). 28. Branica, M. personal communication (1978). 2 +

2

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29. Guy, R . D . , and Chakrabarti, C . L . Studies of metal­ -organic interactions i n model systems pertaining to natural waters, Can. J. Chem. 54, 2600-2611 (1976). 30. Mantoura, R.F.C., and R i l e y , J . P . The use of gel­ -filtration in the study of metal binding by humic acids and r e l a t e d compounds, Anal. Chim. Acta 78, 193-200 (1975). 31. van D i j k , H. Cation binding of humic acids, Geoderma 5, 53-67 (1971). 32. Schnitzer, Μ., and Skinner, S . I . M . , Organo-metallic interactions i n s o i l s . 4. Carboxyl and hydroxyl groups i n organic matter and metal r e t e n t i o n , S o i l S c i . 99, 278-284 (1965). 33. DeHaan, H. Limnologische aspecten van humus ver­ bindingen i n het Tjeukemeer, Ph.D. Thesis, Univer­ s i t y of Gröningen, Netherlands, 1975. 34. Hansen, E . H . and Schnitzer, M. Molecular weight measurements of polycarboxylic acids i n water by vapor pressure osmometry, Anal Chim. Acta 46, 247254 (1969). 35. A l l e n , H . E . , Crosser, M . L . , and B r i s b i n , T . D . Metal speciation i n aquatic environment, p. 33-57, in Andrew, R.W., Hodson, P . V . , and Konasewich, D . E . (Ed.), "Toxicity to Biota of Metal Forms i n Natural Water," Proc. of a Workshop held i n Duluth, Minnesota, 1975 , 1976. 36. S i l l e n , L.G., and M a r t e l l , A . E . " S t a b i l i t y Constants of Metal-Ion Complexes," Special P u b l i c a t i o n No. 17, 754 p . , Chem. Soc., London, 1964. 37. Le Williams, P.J. B i o l o g i c a l and chemical aspects of dissolved organic material i n water, p. 301-364, in R i l e y , J.P., and Skirrow, G. ( E d . ) , "Chemical Oceanography," v. 2, Academic Press, London, 1975. 38. Parks, G.A. Adsorption i n the marine environment, p. 241-308, i n R i l e y , J.P., and Skirrow, G. ( E d . ) , "Chemical Oceanography," v. 1, Academic Press, London, 1975. 39. Clark, M.E., Jackson, G . A . , and North, W.J. D i s solved free amino acids i n Southern C a l i f o r n i a coastal waters, Limnol. Oceanogr. 17, 749-758 (1972). RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

7 Ion Exchange on H u m i c Materials—A Regular Solution Approach JOSEPH P. FRIZADO

1

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

Department of Geological Sciences, Northwestern University, Evanston, IL 60201

Humic and f u l v i c acids are a l a r g e p o r t i o n of the organic matter found i n n a t u r a l environments. They are polymeric molecules whose molecular weights range from s e v e r a l hundred to s e v e r a l m i l l i o n (_1). The s t r u c t u r e of these organic molecules i s thought to i n c l u d e a l a r g e number of hydrogen bonds, an aromatic core, and l a r g e numbers of f u n c t i o n a l groups (2^ p.137-198). This very open s t r u c t u r e i s capable of having c a t i o n exchange s i t e s due to the geometry of the d i s t r i b u t i o n of f u n c t i o n a l groups. The a b i l i t y of humic m a t e r i a l s to bind cations has been s t u d i e d i n d e t a i l (2_, p. 203-249*, _3, _4, ,5, 6> 7)· They have been found to be e x c e l l e n t i o n exchangers, w i t h exchange c a p a c i t i e s s i m i l a r to that of smectites ( 4 ) . The conc e n t r a t i o n s of cations a s s o c i a t e d w i t h marine humic m a t e r i a l s have been found to vary from tens of p a r t s per m i l l i o n to s e v e r a l percent f o r any given c a t i o n (8). Sea water contains a much lower c o n c e n t r a t i o n of d i s s o l v e d organic matter than r i v e r water. More than h a l f of t h i s d i s s o l v e d organic load i s of a humic nature ( 9 ) . These d i s s o l v e d organic acids tend to f l o c c u l a t e as the s a l i n i t y increases (10). H a i r and Bassett (11) have observed an increase i n the p a r t i c u l a t e humic a c i d load of an estuary as one approaches the sea. Although no s t u d i e s of the d i s t r i b u t i o n of humic m a t e r i a l s throughout an e s t u a r i n e system have been performed, i t would appear that e s t u a r i e s and t h e i r sediments i n p a r t i c u l a r , act as a major s i n k f o r the d i s s o l v e d and p a r t i c u l a t e humic m a t e r i a l s . Nissenbaum and Kaplan (12) have observed that t e r r e s t r i a l humic m a t e r i a l s are not deposited at great distances from shore i n the marine system. A study of the f l u x of p a r t i c u l a t e carbon through the Chesapeake Bay comes to a s i m i l a r c o n c l u s i o n (13). S h o l k o v i t z (14) has shown that a l a r g e p o r t i o n of the f l o c c u l a t e d m a t e r i a l i s of a humic nature w i t h l a r g e concentrations of 1

Current address: Department of Geography and Earth Sciences, University of North Carolina at Charlotte, UNCC Station, Charlotte, NC 28223.

0-8412-0479-9/79/47-093-133$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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a s s o c i a t e d c a t i o n s . He b e l i e v e s that only a s m a l l f r a c t i o n of the d i s s o l v e d organics are f l o c c u l a t e d w i t h i n the estuary. H i s experiments used f i l t e r e d water samples and neglected any e f f e c t s of c l a y - o r g a n i c i n t e r a c t i o n s . The present study attempts to d e f i n e the i o n exchange prop e r t i e s of humic m a t e r i a l s as a f u n c t i o n of i o n i c s t r e n g t h . Due to the f l o c c u l a t i n g e f f e c t s of a s a l i n i t y g r a d i e n t , the d i s t r i b u t i o n of humic m a t e r i a l s between the d i s s o l v e d and p a r t i c u l a t e loads w i t h i n an estuary are not w e l l known. The humics* i o n exchange parameters have been documented a t low i o n i c strengths and i n marine environments. At the lower i o n i c s t r e n g t h s , some v a r i a t i o n i n the exchange parameters has been observed ( 3 ) . I n order t o e x p l a i n the e f f e c t s of humic m a t e r i a l s on the d i s t r i b u t i o n of t r a c e metals w i t h i n an estuary, one must know the e f f e c t s of t r a n s p o r t through a s a l i n i t y gradient upon the c a t i o n s assoc i a t e d w i t h the organic molecules. Experimental

Methods

The northern p o r t i o n of Chesapeake Bay was s t u d i e d . This upper s e c t i o n of the bay can be c l a s s i f i e d as a c l a s s i c s a l t wedge type estuary (15). The humic m a t e r i a l s used i n t h i s study were e x t r a c t e d from s e v e r a l cores of sediments. Two one-meter cores were taken a t each of two l o c a t i o n s (39°14 N, 76°14 'W; 38°56 N, 76°25 W) w i t h a g r a v i t y c o r e r . The cores of sediments were then d i v i d e d i n t o s e c t i o n s and squeezed under n i t r o g e n t o remove most of the i n t e r s t i t i a l water. The squeezed sediments were then d r i e d , weighed and washed w i t h d i s t i l l e d water. The sediments were then washed w i t h 0.5N KC1 to remove i r o n hydroxi d e s , carbonates and exchangeable c a t i o n s from the c l a y s . Some of the f u l v i c acids are l o s t i n t h i s a c i d wash procedure. The humic m a t e r i a l s were then e x t r a c t e d from the remaining s o l i d s by a 0.1N NaOH s o l u t i o n . A l a r g e volume of concentrated humic m a t e r i a l s o l u t i o n was needed t o perform the i o n exchange experiments. The UV spectra of the d i f f e r e n t humic m a t e r i a l s o l u t i o n s were found to be s i m i l a r . The e x t i n c t i o n c o e f f i c i e n t s , as measured at 270 my, were a l s o found to be i d e n t i c a l . Since the samples seemed t o be of a s i m i l a r nature, a mixture of the samples from the four cores was made. The mixture was then d i a l y z e d against d i s t i l l e d d e i o n i z e d water f o r a p e r i o d of three weeks. The outer s o l u t i o n was r e placed p e r i o d i c a l l y . The molecular weight c u t o f f value f o r the d i a l y s i s membrane (gpectrapor^ 6) was approximately two thousand. The c o n c e n t r a t i o n of the i n t e r i o r humic m a t e r i a l s o l u t i o n was monitored by UV a b s o r p t i o n . Less than f i v e percent of the o r i g i n a l humic m a t e r i a l s o l u t i o n passed through the membrane and was l o s t to the outer s o l u t i o n . The i o n exchange p r o p e r t i e s of the humic m a t e r i a l s were s t u d i e d by p o t e n t i o m e t r i c t i t r a t i o n s . The humic m a t e r i a l s were converted to the a c i d form by a c i d i f i c a t i o n of the s o l u t i o n to a T

f

T

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

!

7.

Ion

FRizADO

on

Humic

135

Materials

pH of l e s s than 2 and d i a l y z e d against 0.01N HC1 s o l u t i o n f o r another week. At t h i s low pH, the f u n c t i o n a l groups i n v o l v e d i n the i o n exchange s i t e s should be i n H~form. I t has been hypothe­ s i z e d that c a r b o x y l i c a c i d and p h e n o l i c hydroxide groups are i n ­ volved i n the s i t e s (2, p.203-249). Both of these groups should be i n Η-form at t h i s pH. The i o n i c s t r e n g t h , I , of the r e a c t i o n s o l u t i o n was maintained by the c o n c e n t r a t i o n of the c h l o r i d e s a l t of the exchangeable c a t i o n being s t u d i e d . The t i t r a n t was a s o l u ­ t i o n of the same metal's hydroxide. In the case of the d i v a l e n t c a t i o n s , the hydroxide s o l u t i o n s were d i l u t e enough to a f f e c t the i o n i c s t r e n g t h of t h e ^ e a c t i o n s o l u t i o n upon a d d i t i o n . The values given f o r Ca ^ and Mg exchanges cover a range of i o n i c strengths rather than a s i n g l e value as w i t h the monovalent cations.. The t i t r a t i o n s were performed under C ^ - f r e e c o n d i t i o n s . A d d i t i o n s of t i t r a n t were made u n t i l the pH was greater than 10. For monovalent c a t i o n s , a constant pH was a t t a i n e d approximately f i f t e e n minutes a f t e r an a d d i t i o n of t i t r a n t . D i v a l e n t c a t i o n s took approximately three times longer to come to e q u i l i b r i u m . +

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

Exchange

+

Results 1-

Some of the t i t r a t i o n curves f o r the Na^irT*" and Κ+ιΉΓ systems are shown i n F i g u r e 1. At l e a s t two d i s t i n c t s i t e s are v i s i b l e throughout the s e r i e s of t i t r a t i o n s . At lower i o n i c s t r e n g t h s , other d e f l e c t i o n s were observed. These d e v i a t i o n s may be due to ion exchange s i t e s w i t h s l i g h t l y d i f f e r e n t c h a r a c t e r i s t i c s than the two major s i t e s . They could a l s o be due to sample inhomog e n e i t i e s , but t h i s i s u n l i k e l y i n that 1) the d e v i a t i o n s only appear at lower i o n i c strengths and 2) the number of d e v i a t i o n s seems to decrease as the i o n i c s t r e n g t h i n c r e a s e s . At i o n i c strengths greater than 0.3 only two s i t e s were observed i n the monovalent s e r i e s of t i t r a t i o n s . Evidence w i l l be discussed elsewhere to suggest that the humic molecules undergo c o n f i g u r a t i o n a l changes as the i o n i c s t r e n g t h changes. This e f f e c t could a l t e r the d i s t r i b u t i o n of exchange s i t e energies and thereby a f f e c t the t i t r a t i o n curves and e x p l a i n the low i o n i c s t r e n g t h deviations. B a c k - t i t r a t i o n s of the r e a c t i o n s o l u t i o n s were a l s o per­ formed. The back t i t r a n t was 0.01N HC1 s o l u t i o n . The shapes of the t i t r a t i o n curves were s i m i l a r f o r both the forward and back t i t r a t i o n s . The number of m i l l i e q u i v a l e n t s added to reach the equivalence p o i n t s was d i f f e r e n t f o r each t i t r a t i o n p a i r . In the back t i t r a t i o n s , the exchange c a p a c i t i e s of the molecules were s l i g h t l y s m a l l e r . The s m a l l e r exchange c a p a c i t i e s can be explained by i r r e v e r s i b l e c o l l o i d formation. These c o l l o i d s would remove some of the exchange s i t e s from contact w i t h the s o l u t i o n . A t i t r a t i o n curve f o r K shows the two s i t e s prevalent i n the N a s e r i e s . The f i r s t s i t e s pK d i f f e r s from i t s N a analog by 0.7 u n i t s . The second s i t e ' s pK i s e s s e n t i a l l y the same as the N a counterpart. This i n d i c a t e s that the lower pK +

+

1

+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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+

s i t e can d i f f e r e n t i a t e between and N a , w h i l e the second s i t e cannot. D i v a l e n t c a t i o n t i t r a t i o n curves are shown i n Figure 2. A l l of these t i t r a t i o n s e x h i b i t e d only one d e f l e c t i o n . None of the lower i o n i c s t r e n g t h t i t r a t i o n curves had any d e v i a t i o n s o r a d d i ­ t i o n a l s i t e s s i m i l a r to that observed i n the monovalent s e r i e s . D e r i v a t i o n of Exchange Constants The t i t r a t i o n data were used to determine the thermodynamic e q u i l i b r i u m exchange constants f o r the r e a c t i o n s i n v o l v e d . To s i m p l i f y matters, we w i l l only discuss the Na :H system. The t i t r a t i o n curves were modeled by i n c l u d i n g a l l of the a c t i v i t i e s of the components i n v o l v e d . The exchange r e a c t i o n can be w r i t t e n i n the f o l l o w i n g forms: +

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

+

+

Hum-H + N a # Hum-Na + Η+

K=

aH+A

a

(1)

N

Hum-Na Hum-Na +x

^

N

Na Hum-H Hum-H

where λ i s the r a t i o n a l a c t i v i t y c o e f f i c i e n t f o r the exchange s i t e occupied by the s u b s c r i p t e d c a t i o n , a i s the a c t i v i t y of the aqueous s p e c i e s , Ν i s the mole f r a c t i o n of the humic m a t e r i a l ex­ change s i t e s that i s occupied by the noted c a t i o n , and Κ i s the e q u i l i b r i u m constant f o r the r e a c t i o n . (Nn^^a» N a * a +)were evaluated from the t i t r a t i o n data. The a c t i v i t y of the N a was h e l d constant by the NaCl s o l u t i o n used t o maintain the i o n i c s t r e n g t h . The a c t i v i t y of the H was measured by e l e c t r o d e . The endpoint f o r the conversion of a s i t e t o the Na-form was found by o b t a i n i n g the p o i n t of maximum slope f o r the p l o t of pH versus base added. The equivalence p o i n t f o r the same s i t e was found a t the minimum slope. The d i f f e r e n c e i n base added between the two p o i n t s i s equal t o one h a l f of the i o n exchange c a p a c i t y of the s i t e i n question. ^Hum-Na ^ l l the amount of base added minus the amount of base needed to i n i t i a t e the exchange r e a c t i o n , d i v i d e d by the t o t a l capacity of the s i t e . The r a t i o n a l a c t i v i t y c o e f f i c i e n t s cannot be evaluated i n any simple manner. F o l l o w i n g the model of T r u e s d e l l and C h r i s t (16), a r e g u l a r s o l u t i o n approach to the problem can l e a d t o expressions f o r the r a t i o n a l a c t i v i t y c o e f f i c i e n t s . I f the exchange s i t e s have the same charge and approximately the same s i z e , then a symmetrical s o l i d s o l u t i o n w i l l be formed where the r a t i o n a l a c t i v i t y c o e f f i c i e n t s f o r the two components are given by: a

+

an(

H

+

+

s

1 0 8

e c

u a

t o

Hum-Na = T 3 B R T

X

(

Vm-H >

%

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

( 3 )

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

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Exchange

on Humic

Materials

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meq base added Figure 1. Titration curves for monovalent cation-hydrogen exchange. (A) Na at 0.11; (B) Na at 0.3 I; (C) Na at 0.5 I; (D) K at 0.1 I. Every third data point is shown. Interpolation is by a cubic spline method. +

+

+

+

meq base added Figure 2. Titration curves for divalent cation-hydrogen exchange. (A) Ca at 0.11; (B) Ca at 0.31; (C) Ca at 0.51; (D) Mg at 0.51. Every second data point is shown. Interpolation is by a cubic spline method. +2

+2

+2

+2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

138

a n d

^ W - H

" — g —

( N

M

a

)

2

(4)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

where ω i s the i n t e r a c t i o n parameter f o r the s o l u t i o n , R i s the gas constant and Τ i s the temperature i n K e l v i n . The i n t e r a c t i o n parameter i s an expression o f the p o t e n t i a l energy due t o the mix­ ture of the two d i f f e r e n t s i t e s . For any p a i r of s i t e s (Hum-Na and Hum-Η i n t h i s c a s e ) , ω should be a constant over an i o n i c s t r e n g t h change. I n these experiments, ω was found t o vary some­ what over the e n t i r e range of i o n i c s t r e n g t h s . By using these ex­ pressions f o r the r a t i o n a l a c t i v i t y c o e f f i c i e n t s , Equation 2 can be transformed i n t o the f o l l o w i n g : N

ρκ'=

pH + l o g a + + l o g Hum-Na 1-NHum-Na Na

2.303RT

2.303RT

H u m

"

N a

(

5 )

i n which the unknowns (pK, ω ) are on the right-hand s i d e of the equation. I f the r e g u l a r s o l u t i o n model holds t r u e , then a p l o t of pK versus Hum-Na w i l l be l i n e a r . P l o t s f o r the Na~^*: H~** (Figure 3) and Ca ^:2H (Figure 4) systems are given. L e a s t squares l i n e s have been drawn i n . The number of p o i n t s used to generate these l i n e s and t h e i r a s s o c i a t e d c o r r e l a t i o n c o e f f i c i e n t s are given i n Table I . The slope and i n t e r c e p t o f these l i n e s can be used to evaluate pK and ω. R e s t r i c t i o n s on the a p p l i c a b i l i t y of the above equations a l l o w the model to be used only i n the c e n t r a l r e g i o n ofHum-Na v a l u e s . The d e v i a t i o n s of those p o i n t s w i t h value of Hum-Na near 0 and 1 are due to these r e s t r i c t i o n s . I t i s i n t e r e s t i n g to note that the e q u i l i b r i u m exchange values f o r the r e a c t i o n s are v a r i a b l e , as shown i n Figure 5. I n the case of Na+:H , as the i o c i c s t r e n g t h i n c r e a s e d , the humic molecules p r e f e r r e d H over Na to a greater extent. Changes i n the d i s t r i ­ b u t i o n of exchange s i t e energies could be a l t e r e d by polymeriza­ t i o n r e a c t i o n s . The exchange c a p a c i t i e s of the humic m a t e r i a l s do not change d r a s t i c a l l y w i t h i o n i c s t r e n g t h as one would expect i f p o l y m e r i z a t i o n r e a c t i o n s were removing exchange s i t e s . Another p o s s i b l e e x p l a n a t i o n i s that the c o n f i g u r a t i o n s of the organic molecules have undergone s l i g h t changes. As the molecular con­ f i g u r a t i o n s are a l t e r e d , the c a t i o n exchange i n v o l v e s a new s o l i d phase. The i n t e r a c t i o n parameter would probably not be a l t e r e d g r e a t l y by such a change i n c o n f i g u r a t i o n . The thermodynamic con­ s t a n t f o r the r e a c t i o n could e a s i l y be a l t e r e d by such a process. N

+

+

N

N

+

+

E f f e c t s of C o n f i g u r a t i o n on Exchange Energies C o n f i g u r a t i o n a l changes i n humic m a t e r i a l s have been docu­ mented only r e c e n t l y . V i s c o s i t y measurements (17), SEM photomi­ crographs (18) and Sephadex g e l f i l t r a t i o n (19) have a l l been used

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

7.

Ion Exchange

FRizADO

0

0.1

on Humic

02

03

04

Ν

Figure 3. log a -log Na

Materials

05

06

139

0.7

0.8

09

10

HUMIC-Να

Plot of pK' vs. N . for the first sodium exchange site. pK! = pH + (N _ /N _ ). (Φ) is for the 0.3 I experiment; (+) is for 0.5 I; Λ) ™ f 0·β I> The least-squares lines have been drawn in. Hum Ka

Hum

Na

Hum

H

or

20

Ν

Figure 4. log a -log Ca

HUMIC - C a

Plot of pK' vs. N . for the calcium exchange site. pK' = 2pH + (N . a/NHum-Hi)(·) 0.3 experiment; Οζ) 0.5 I; (H) 0.6 I. The least-squares lines have been drawn in. Hum Cn

Hum C

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

140

CHEMICAL MODELING IN AQUEOUS SYSTEMS

TABLE I - Exchange Reactions Parameters Cation

I o n i c Exchange No.Pts. C o r r e l . ω* pK** AG Strength Capacity used Coeff. meq/ g kJ/mole kJ/mole r

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

Na

0.6 0.5 0.4 0.3 0.2 0.1

0.35 0.69 0.46 0.71 0.32 0.61 0.25 0.54 0.27 0.80 1.19 0.91

10 7 9 11 12 9 8 11 15 8 19 7

0.93 0.91 0.95 0.94 0.94 0.94 0.96 0.92 0.95 0.96 0.92 0.94

-5.0 -2.4 -4.2 -2.0 -4.0 -3.5 -3.7 -3.2 -5.6 -2.8 -3.0 -2.8

4.3 6.8 4.1 6.8 4.0 6.6 3.9 6.4 3.8 6.3 3.4 6.1

24.5 38.5 23.6 38.5 22.8 37.6 22.1 36.6 21.5 36.0 19.3 34.8

Κ

0.1

0.74 0.95

13 11

0.93 0.94

-5.4 -2.3

2.7 6.2

15.4 35.4

Ca***

0.6 0.5 0.3 0.1

2.58 1.69 2.26 2.10

26 28 21 23

0.96 0.95 0.96 0.93

63.8 71.7 25.2 82.2

10.0 9.1 7.3 8.8

57.0 51.9 41.6 50.2

Mg***

1.0 0.5 0.1

1.52 1.28 1.40

19 22 18

0.83 0.98 0.94

-30.9 4.3 5.8

9.5 9.6 9.4

54.2 54.8 53.7

* ω i s found by t a k i n g the l i n e a r l e a s t - s q u a r e s slope of the pK versus Ν type p l o t , m u l t i p l y i n g by 2.303RT and d i v i d i n g by -2. A l l of the experiments were performed at 298 K e l v i n . ** The monovalent c a t i o n s have two p o s s i b l e s i t e s . The para­ meters of the s i t e s are l i s t e d s e q u e n t i a l l y . *** The n o r m a l i t y of the d i v a l e n t c a t i o n s hydroxide s o l u t i o n s were low, so the i o n i c s t r e n g t h l i s t e d i s the i n i t i a l value. During the course of the t i t r a t i o n , the i o n i c s t r e n g t h was diminished. The e n t i r e range f o r each t i t r a t i o n i s depicted i n Figure 5. 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Ion Exchange on Humic Materials

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

7. FRizADO

10

141

]

9

0.1

02

Ionic

0.3

0.4

0.5

0.6

Strength

Figure 5. Plot of pK vs. ionic strength. (Φ) First Ν a exchange site; (^) second Ν a exchange site; ( ) Ca exchange site. The nature of the divalent cation experiments was such that the ionic strength could only he held within the noted range of values.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

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CHEMICAL MODELING IN

AQUEOUS SYSTEMS

to document changes i n the c o n f i g u r a t i o n of humic a c i d molecules as a f u n c t i o n of pH and i o n i c s t r e n g t h . As the i o n i c s t r e n g t h i n c r e a s e s , the humic molecule i s compressed nonuniformly. The e f f e c t of t h i s compression on the exchange s i t e s can be a p p r o x i mated by using the t h e o r i e s advanced by Eisenman (20) i n h i s work on c a t i o n s e l e c t i v i t y i n i o n exchangers. Eisenman (20) derived a simple model which explained the observed s e l e c t i v i t y sequences f o r monovalent c a t i o n s i n v a r i o u s i o n exchanges. He only considered a coulombic a t t r a c t i o n and the f r e e energy d i f f e r e n c e between the g l a s s and aqueous phases f o r each c a t i o n s t u d i e d . For negative s i n g l y charged s i t e s , the energy of a t t r a c t i o n f o r monovalent c a t i o n s i s greatest when the s i t e s are i n f i n i t e l y spaced. As the distance between exchange s i t e s decreases, r e p u l s i v e forces must be taken i n t o c o n s i d e r a t i o n l e s s e n ing the exchange energy. The other f a c t o r which must be taken i n t o c o n s i d e r a t i o n i s the a n i o n i c f i e l d s t r e n g t h . As the c o n f i g u r a t i o n of an a c t u a l exchange s i t e i s a l t e r e d , the a n i o n i c f i e l d s t r e n g t h i s changed. E i t h e r of these f a c t o r s can cause the v a r i a t i o n of pK w i t h i o n i c s t r e n g t h observed f o r the N a : H system. In the case of d i v a l e n t monovalent exchange, the s i t e spacing becomes more important (21). C a l c u l a t i o n s using the T r u e s d e l l and C h r i s t (21) equations f o r divalent-monovalent c a t i o n exchange have shown that the two processes, compression of the i o n exchange s i t e and the a l t e r a t i o n of the d i s t r i b u t i o n of s i t e s on the molecule can act upon the f r e e energy of exchange i n d i f f e r e n t manners. I t i s a combination of these two opposite e f f e c t s that can cause the minimum value f o r the pK of the Ca 2 exchange. By u t i l i z i n g other m u l t i v a l e n t cations i n a s i m i l a r number, i t i s p o s s i b l e to begin to d i s c e r n q u a l i t a t i v e changes i n the c o n f i g u r a t i o n of the humic molecule. +

+

+

Summary Ion exchange p r o p e r t i e s of humic m a t e r i a l s found w i t h i n n a t u r a l environments must be known to understand the s p e c i a t i o n and d i s t r i b u t i o n of t r a c e metals. Although t h i s paper deals w i t h these p r o p e r t i e s w i t h regards to major c a t i o n s , the e f f e c t s of the humic m a t e r i a l s on trace metals i s f a r greater (8, 22). The e f f e c t s of major-minor c a t i o n exchanges as the humic m a t e r i a l s pass through a s a l i n i t y gradient and during diagenesis must a l s o be s t u d i e d . Due to the changes i n i o n i c s t r e n g t h , both the amounts of a s s o c i a t e d metals and the d i v i s i o n of humic m a t e r i a l s between d i s s o l v e d and p a r t i c u l a t e forms w i t h i n an estuary are extremely v a r i a b l e . I t was found that the humic m a t e r i a l i o n exchange p r o p e r t i e s can be explained by a r e g u l a r s o l u t i o n model s i m i l a r to that of T r u e s d e l l and C h r i s t (16) f o r c l a y s . The thermodynamic constants f o r the exchange r e a c t i o n s studied were found to be d i f f e r e n t f o r each i o n i c s t r e n g t h . Changes i n the c o n f i g u r a t i o n s of the organic molecules could cause the observed v a r i a t i o n s . Other evidence (17,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

7. FRizADO

Ion Exchange

on Humic

Materials

143

18, 19) suggests that the humic molecules undergo some k i n d of compression during an increase of the i o n i c s t r e n g t h of the s o l u ­ t i o n . Using Eisenman s work on c a t i o n s e l e c t i v i t y of e l e c t r o d e s (20), i t can be shown that the observed v a r i a t i o n s of the thermo­ dynamic values can be due to a small compression of the organic molecules. In order to estimate the e f f e c t s of humic m a t e r i a l s upon the d i s t r i b u t i o n and s p e c i a t i o n of metals, the i o n exchange parameters f o r the c a t i o n s i n question must be known, as w e l l as t h e i r v a r i a t i o n s w i t h i o n i c s t r e n g t h . The v a r i a t i o n s of these parameters f o r the major c a t i o n s have been shown to be somewhat s i g n i f i c a n t . These v a r i a t i o n s can a l s o be used to determine the changes i n the spacing and a n i o n i c f i e l d s t r e n g t h of the i o n ex­ change s i t e s on the molecule. With more i n f o r m a t i o n of t h i s type, i t w i l l be p o s s i b l e to p r e d i c t the v a r i a t i o n of the ex­ change parameters f o r other metals, i n c l u d i n g trace metals. E s t u a r i e s are the major pathway of m a t e r i a l s from the r i v e r s to the marine environment. In order to understand how d i s s o l v e d and p a r t i c u l a t e organic matter w i t h i n the estuary a f f e c t the s p e c i a t i o n of c a t i o n s w i t h i n t h i s environment, the i o n exchange parameters as a f u n c t i o n of i o n i c s t r e n g t h must be s t u d i e d . I n a d d i t i o n to the p h y s i c a l t r a n s f e r of m a t e r i a l between the d i s s o l v e d and p a r t i c u l a t e forms, the s a l i n i t y v a r i a t i o n s a l s o a f f e c t the i o n exchange a b i l i t i e s of these organic molecules. These two major processes can a f f e c t the organic m a t e r i a l d i s t r i ­ b u t i o n and a b i l i t y to bind metals, and hence the o v e r a l l d i s t r i ­ b u t i o n of a given trace metal.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

1

Acknowledgments The author would l i k e to thank Dr. Owen P. B r i c k e r of the Maryland G e o l o g i c a l Survey f o r h i s i n v a l u a b l e help i n o b t a i n i n g the samples used i n t h i s study. Support f o r t h i s study was provided by NSF Grant EAR76-12279A01. I would a l s o l i k e to thank Dr. Robert M. G a r r e l s f o r h i s continuing and vigorous c o n t r i b u t i o n s to t h i s study. Abstract Humic and fulvic acids comprise a major portion of naturally occurring organic matter. They are high molecular weight poly­ mers that can act as ion exchangers. Although humic materials have been found in both marine and fresh water environments, evidence suggests that significant amounts of terrigenous humic materials do not reach the marine environment. Humic materials are deposited by the mixing of fresh water and seawater in estuaries. The reactions of these organic molecules with cations were studied by potentiometric titrations. Humic substances were extracted from sediments of Chesapeake Bay. Ion exchange reactions between H and Ca, Mg, Κ and Na were studied in solutions of various fixed ionic strengths. The chloride salt of the H-displacing

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

cation controlled the ionic strength. Analysis of the titration curves demonstrates that the organic molecule ion exchange sites fit a regular solution model. Thermodynamic constants for some of the exchange reactions are given here. The exchange para­ meters varied with ionic strength. These variations apparently are related to changes in configuration of the humic molecules. Literature Cited 1.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch007

2. 3. 4.

5. 6. 7. 8.

9. 10. 11. 12. 13. 14.

Ogura, N. Molecular weight fractionation of dissolved organic matter in coastal sea water by ultrafiltration, Mar. Bio. 24, 305-312 (1974). Schnitzer, M. and Khan, S. "Humic Substances in the En­ vironment," 327 p. Marcel Dekker, New York, 1972. Broadbent, F. and Lewis, T. Soil organic matter-metal Complexes; 4. Nature and properties of exchange sites, Soil Sci. 91, 393-399 (1961). Rashid, M. Role of humic acids of marine origin and their different molecular weight fractions in complexing di- and tri-valent metals, Soil Sci. 111, 298-306, (1971). Gamble, D., Schnitzer, Μ., and Hoffman, I. Cu -fulvic acid chelation equilibrium in 0.1m KC1 at 25.0°C, Can. J. Chem. 48, 3197-3204 (1970). Stevenson, F. Binding of metal ions by humic acids, p. 519-541, in Nriagu, J., ed., "Environmental Biogeochemis­ try," Ann Arbor Sci. Pub., Ann Arbor, Michigan, 1976. Schatz, Α., Schatz, V., Schalscha, Ε . , and Martin, J . Soil organic matter as a natural chelating material, Compost Sci. 4, 25-28 (1964). Nissenbaum, A., and Swaine, D. Organic matter-metal in­ teractions in Recent sediments; The role of humic sub­ stances, Geochim. Cosmochim. Acta 40, 809-816 (1976). Handa, N. Land sources of marine organic matter, Mar. Chem., 341-359 (1977). Ong, Η., and Bisque, R. Coagulation of humic colloids by metal ions, Soil Sci. 106, 220-224, (1968). Hair, M. and Bassett, C. Dissolved and particulate humic acids in an east coast estuary, Estuarine Coastal Mar. Sci. 1, 107-111 (1973). Nissenbaum, A . , and Kaplan, I. Chemical and isotopic evidence for the in-situ origin of marine humic sub­ stances, Limnol. Oceanogr. 17(4), 570-582 (1972). Biggs, R. and Flemer, D. The flux of particulate carbon in an estuary, Mar. Bio. 12, 11-17, (1972). Sholkovitz, E. Flocculation of dissolved organic and in­ organic matter during the mixing of river water and sea water, Geochim. Cosmochim. Acta 40, 831-845 (1976). +2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

7. FRizADO 15.

16. 17.

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18.

19. 20.

21.

22.

Ion Exchange on Humic Materials

Prichard, D. Chemical and physical oceanography of the Chesapeake Bay, p. IVA 1-28, in Schubel, J., ed., "The Estuarine Environment," A.G.I. Short Course Lecture Notes, Washington, D.C., 1971. Truesdell, A . , and Christ, C. Cation exchange in clays interpreted by regular solution theory, Am. J . Sci. 266, 402-412 (1968). Mkuherjee, P. and Lahiri, A. Studies on the rheological properties of humic acid from coal, J . Colloid Sci. 11, 240-243 (1956). Chen, Υ., and Schnitzer, M. Scanning electron microscopy of a humic acid and of a fulvic acid and its metal and clay complexes, Soil Soc. Am. Proc. 40, 682-686, (1976). Reuter, J . Inferred configurational changes of aquatic humic substances, p. 1140, Geol. Soc. Am. Abstr. 9, Seattle National Mtg, 1977. Eisenman, G. On the elementary origin of equilibrium ion specificity, p. 163-179, in Kleinzeller, A. and Kotyk, Α., ed., "Membrane Transport and Metabolism," Academic Press, New York, 1961. Truesdell, A. and Christ, C., Glass electrodes for calcium and other divalent cations, p. 293-322, in Eisenman, G., ed., "Glass Electrodes for Hydrogen and Other Cations," Marcel Dekker, Inc., New York, 1967. Reuter, J . and Perdue, E. Importance of heavy metal­ -organic matter interactions in natural waters, Geochim. Cosmochim. Acta 41, 325-244 (1977).

RECEIVED

November 16,

1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

145

8 Chemical Speciation of Copper i n River Water Effect of Total Copper, p H , Carbonate, and Dissolved Organic Matter

WILLIAM G. SUNDA and PETER J. HANSON

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

National Marine Fisheries Service, Southeast Fisheries Center, Beaufort Laboratory, Beaufort, NC 28516 There i s i n c r e a s i n g evidence that the a v a i l a b i l i t y of a q u e o u s t r a c e m e t a l s t o a number o f o r g a n i s m s i s d e t e r m i n e d by f r e e metal i o n a c t i v i t y r a t h e r than t h e t o t a l c o n c e n t r a t i o n o f metal i n s o l u t i o n (]_-6j. The c h e m i c a l a s s o c i a t i o n s o f t r a c e metals with inorganic and organic ligands a r e major f a c t o r s that c o n t r o l metal i o n a c t i v i t i e s and thus bioavailability. I n t h i s s t u d y , we i n v e s t i g a t e t h e c o m p l e x a t i o n o f c o p p e r b y i n o r g a n i c and o r g a n i c l i g a n d s i n t h e water o f two r i v e r s i n coastal North C a r o l i n a . An i o n - s e l e c t i v e e l e c t r o d e was used t o d i f f e r e n t i a t e between f r e e a n d bound c u p r i c i o n i n t i t r a t i o n s o f r i v e r w a t e r s a n d model s o l u t i o n s . S t a b i l i t y c o n s t a n t s were determined i n chemically defined solutions f o r complexation of c o p p e r by h y d r o x i d e and c a r b o n a t e i o n s , t h e two m a j o r i n o r g a n i c c o p p e r c o m p l e x i n g l i g a n d s i n most n a t u r a l waters ( 7 ) . From t h e s e r e s u l t s , t o t a l c o p p e r i n t h e r i v e r w a t e r was p a r t i t i o n e d into o r g a n i c and i n o r g a n i c forms and s t a b i l i t y constants f o r complexa­ t i o n o f c o p p e r by n a t u r a l o r g a n i c l i g a n d s were c a l c u l a t e d . F i n a l l y , models were c a l c u l a t e d which p r e d i c t t h e v a r i a t i o n s i n chemical s p e c i a t i o n o f copper r e s u l t i n g from changes i n t h e c h e m i ­ cal parameters: pH, carbonate a l k a l i n i t y , concentration of d i s ­ s o l v e d o r g a n i c m a t t e r , and c o n c e n t r a t i o n o f t o t a l d i s s o l v e d copper. The r i v e r s s a m p l e d were t h e N e w p o r t a n d N e u s e . The Newport R i v e r i s a s m a l l c o a s t a l p l a i n r i v e r (mean d i s c h a r g e a p p r o x i m a t e l y 0 . 4 t o 11.2 m 3 s e c " 1 ) w i t h a w a t e r s h e d o f a p p r o x i m a t e l y 3 4 0 k m ? . T h e N e u s e R i v e r , i n c o n t r a s t , i s a l a r g e r r i v e r (mean d i s c h a r g e a p p r o x i m a t e l y 130 m 3 s e c - 1 ) , o r i g i n a t i n g i n t h e piedmont r e g i o n w i t h a d r a i n a g e b a s i n o f a p p r o x i m a t e l y 1.1 χ 10^ k m 2 . The Newport R i v e r w a t e r u s e d i n t h i s i n v e s t i g a t i o n i s c h a r a c t e r i z e d by a h i g h c o n c e n t r a t i o n o f d i s s o l v e d o r g a n i c c a r b o n ( 1 5 mgC l o w pH ( 6 . 0 ) , l o w c a r b o n a t e a l k a l i n i t y ( 0 . 0 6 mM) a n d r e l a t i v e l y l o w c o n ­ c e n t r a t i o n s o f a l k a l i n e e a r t h c a t i o n s T Ï Ï . 1 4 mM C a a n d 0 . 0 3 mM M g ) . The Neuse w a t e r h a d an a p p r e c i a b l y l o w e r c o n c e n t r a t i o n o f d i s s o l v e d o r g a n i c c a r b o n ( 3 . 0 mgC £ - ' ) , h i g h e r pH ( 6 . 8 ) , h i g h e r

American Chemical ^^(M^/EîlifSr^3"14^08'50/0 c

r

s

e

This fr|Pte /^ WJ Çf *Λ^·5· copyright P u b l i s l M P ¥ 9 # Î B n » i c & C h e m i c a l Society

Washington, D. C.

20035

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

148

c a r b o n a t e a l k a l i n i t y ( 0 . 1 5 mM), and a b o u t t h e o f c a l c i u m a n d m a g n e s i u m ( 0 . 1 3 mM C a a n d 0 . 0 4 Materials

and

Water

same c o n c e n t r a t i o n s mM M g ) .

Methods

samples were

c o l l e c t e d from

the

Newport

River

upstream

f r o m N e w p o r t , N o r t h C a r o l i n a and from t h e Neuse R i v e r s e v e r a l m i l e s downstream from K i n s t o n , North C a r o l i n a . Standard procedures f o r h a n d l i n g water samples were adopted to m i n i m i z e changes

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

in the natural s t a t e of the samples during sampling P o r t i o n s o f the samples were f i l t e r e d through g l a s s (Gelman A-E) w i t h i n 24 h r o f c o l l e c t i o n . A portion t e r e d r i v e r w a t e r was e x p o s e d t o h i g h i n t e n s i t y u l t r a t i o n ( L a J o l l a S c i e n t i f i c Model PO-14 Photooxidation

and s t o r a g e . fiber filters of the f i l aviolet radiUnit) to

photooxidize organic matter (8). C a l c i u m and magnesium c o n c e n t r a t i o n s were d e t e r m i n e d i n f i l t e r e d s a m p l e s by f l a m e a t o m i c absorption spectrophotometry (Perkin Elmer 403). D i s s o l v e d o r g a n i c c a r b o n w a s d e t e r m i n e d u s i n g a CHN a n a l y s e r ( F a n d M 1 8 5 ) o n t h e r e s i d u e l e f t a f t e r f r e e z e d r y i n g samples of a c i d i f i e d , filtered r i v e r water. Carbonate a l k a l i n i t y s a m p l e s w a s c a l c u l a t e d f r o m pH a n d librated with a i r . The

chemical

speciation

of

o f f i l t e r e d and U V - t r e a t e d PQQ2 d a t a f o r s a m p l e s e q u i -

copper

in

river

water

and

model

s o l u t i o n s was i n v e s t i g a t e d by a t i t r a t i o n t e c h n i q u e i n w h i c h c u p r i c i o n a c t i v i t i e s w e r e m e a s u r e d a t c o n s t a n t pH a s t h e t o t a l c o p p e r c o n c e n t r a t i o n ( [ C U J Q J ] ) w a s v a r i e d by i n c r e m e n t a l a d d i t i o n s o f CUSO4. p C u ( - l o g c u p r i c i o n a c t i v i t y ) was m e a s u r e d w i t h a c u p r i c i o n - s e l e c t i v e e l e c t r o d e ( O r i o n 9 4 - 2 9 ) a n d pH w i t h a g l a s s e l e c t r o d e (Beckman 39301) b o t h c o u p l e d t o a s i n g l e j u n c t i o n Ag/AgCl reference e l e c t r o d e (Orion 90-01) i n a temperature c o n t r o l l e d (25 + 0 . 5 ° C ) w a t e r b a t h . Total copper concentrations in t h e t i t r a t e d s o l u t i o n s were d e t e r m i n e d d i r e c t l y by a t o m i c a b s o r p tion spectrophotometry ( P e r k i n Elmer 603) u s i n g a g r a p h i t e furnace (Perkin Elmer 2200). Measurement of t o t a l copper c o n c e n t r a t i o n s i s necessary because of a d s o r p t i v e l o s s of copper from s o l u t i o n onto

container

The tions at

and/or

electrode

surfaces.

f o l l o w i n g p r o c e d u r e was f o l l o w e d 2 5 ° C and c o n s t a n t pH. The t h r e e

f o r a l l copper t i t r a e l e c t r o d e s were f i r s t

p r e c o n d i t i o n e d f o r 3 0 m i n i n a s o l u t i o n a t pH 8 c o n t a i n i n g 0 . 1 M T r i s b a s e , 0 . 0 5 M HC1 a n d s u f f i c i e n t C U S O 4 t o a c h i e v e a p C u o f 13.0 to 13.5. The e l e c t r o d e s w e r e t h e n r i n s e d s e v e r a l t i m e s w i t h d i s t i l l e d w a t e r and p l a c e d f o r 30 min i n a p o r t i o n o f t h e s o l u t i o n t o be t i t r a t e d . T h e e l e c t r o d e s w e r e t h e n p l a c e d i n a f r e s h 7 0 m& p o r t i o n o f t h e same s o l u t i o n c o n t a i n e d i n a 1 0 0 m i l b o r o s i l i c a t e g l a s s b e a k e r and t i t r a t e d w i t h CUSO4. S u f f i c i e n t t i m e was a l l o w e d f o r the e l e c t r o d e s to r e a c h steady s t a t e p o t e n t i a l s a f t e r each copper a d d i t i o n . A t no c o p p e r a d d i t i o n , 6 0 m i n w a s a l l o w e d . For c o p p e r c o n c e n t r a t i o n s 2 χ 1 0 " 5 M) i t w a s n e c e s s a r y t o a d d s u f f i c i e n t q u a n t i t i e s o f NaOH t o n e u t r a l i z e h y d r o g e n i o n s r e ­ l e a s e d by t h e c o m p l e x a t i o n o f weak a c i d f u n c t i o n a l groups. T i t r a t i o n s w e r e p e r f o r m e d on u n t r e a t e d , f i l t e r e d , a n d UVt r e a t e d f i l t e r e d r i v e r w a t e r s a m p l e s a t i n s i t u a n d a d j u s t e d pH values. T h e e f f e c t o f pH o n c o p p e r s p e c i a t i o n w a s i n v e s t i g a t e d by t i t r a t i o n o f f i l t e r e d N e w p o r t R i v e r w a t e r a t pH 7 . 0 a n d f i l t e r e d N e w p o r t a n d N e u s e w a t e r s a t pH 8 . 0 . Newport R i v e r water w a s a d j u s t e d t o pH 7 . 0 b y d e c r e a s i n g t h e p a r t i a l p r e s s u r e o f CO? f r o m t h e i n i t i a l a m b i e n t v a l u e o f a b o u t 10 t i m e s t h e a t m o s p h e r i c level. To a d j u s t t h e pH t o 8 . 0 , s o d i u m b i c a r b o n a t e w a s a d d e d t o b r i n g t h e r i v e r w a t e r s a m p l e s t o a c o n c e n t r a t i o n o f 0 . 5 mM w i t h s u b s e q u e n t a d j u s t m e n t o f Ρ(χ)2· T i t r a t i o n s were a l s o c o n d u c t e d a t pH 7 . 0 i n m o d e l s o l u t i o n s c o n s i s t i n g o f 0 . 0 1 M KNO3 a n c l 0 e l !0Îi NaHC03 w i t h a n d w i t h o u t t h e a d d i t i o n o f 0 . 7 5 μΜ h i s t i d i n e t o t e s t e l e c t r o d e b e h a v i o r i n s o l u t i o n s o f known c h e m i s t r y . S t a b i l i t y c o n s t a n t s f o r t h e c o m p l e x a t i o n o f c o p p e r by h y d r o x ­ i d e i o n (Cu h y d r o l y s i s ) were d e t e r m i n e d f r o m m e a s u r e m e n t s o f pCu a n d P L C U J O J ] a s a f u n c t i o n o f pH i n s o l u t i o n s c o n t a i n i n g 0 . 0 1 M KNO3 a n d ! - 0 a n d 2 · 5 μ Μ C U S O 4 . These s o l u t i o n s were f i r s t p u r g e d w i t h n i t r o g e n a t pH < 6 t o r e m o v e CO2 a n d t h e n c l o s e d t o the atmosphere. M e a s u r e m e n t s w e r e made u n d e r a n i t r o g e n a t m o s ­ p h e r e a s t h e s o l u t i o n s w e r e t i t r a t e d w i t h s m a l l q u a n t i t i e s ( < 2 5 \ii per a d d i t i o n ) of a concentrated s o l u t i o n of Κ0Η. For the d e t e r ­ mination of s t a b i l i t y constants f o r carbonate complexes, measure­ a s a m e n t s o f pCu a n d P [ C U J Q T ] *e f u n c t i o n o f pH i n a s o l u t i o n c o n t a i n i n g 0 . 0 1 M NaHC03 a n d 5 μΜ CUSO4. T h e pH a n d t h u s t h e c a r b o n a t e i o n a c t i v i t y was v a r i e d by a d j u s t i n g P c 0 2 W

E

R

E

m(

C u p r i c i o n a c t i v i t i e s and c u p r i c i o n c o n c e n t r a t i o n s were determined using the Nernst equation from the d i f f e r e n c e s i n p o t e n t i a l between t h e t e s t s o l u t i o n s and a s t a n d a r d s o l u t i o n c o n ­ s i s t i n g o f Ι Ο " 5 M CUSO4 a n d 0 . 0 1 M KNO3 a t pH 5 . 4 + 0 . 3 . V a l u e s o f c u p r i c i o n a c t i v i t y i n t e s t s o l u t i o n s w e r e b a s e d on a c u p r i c ion a c t i v i t y c o e f f i c i e n t of 0 . 6 8 in the standard s o l u t i o n as c a l c u l a t e d from the extended Debye-Huckel equation. For measure­ m e n t s i n d e f i n e d s o l u t i o n s c o n t a i n i n g 0 . 0 1 M KNO3 o r 0 . 0 1 M NaHC03 c u p r i c i o n c o n c e n t r a t i o n s c o u l d be d i r e c t l y c o m p u t e d v i a t h e N e r n s t e q u a t i o n b e c a u s e a c t i v i t y c o e f f i c i e n t s w e r e t h e same in both t e s t and s t a n d a r d s o l u t i o n s . Total copper c o n c e n t r a t i o n s i n a l i q u o t samples o f the t i t r a ­ t i o n s o l u t i o n s w e r e m e a s u r e d by e l e c t r o t h e r m a l a t o m i c a b s o r p t i o n u s i n g d i r e c t i n j e c t i o n o f t h e a c i d i f i e d (HNO3) s a m p l e i n t o t h e graphite furnace. S t a n d a r d s w e r e p r e p a r e d a t t h e same a c i d s t r e n g t h a s t h e s a m p l e s and c o p p e r v a l u e s were o b t a i n e d by t h e comparator method. Copper c o n c e n t r a t i o n s i n f i l t e r e d (Gelman A E) N e w p o r t a n d Neuse w a t e r s w e r e 0 . 0 1 1 μΜ and 0 . 0 2 5 μ Μ , r e s p e c ­ t i v e l y , and were w e l l w i t h i n a n a l y t i c a l c a p a b i l i t y . T h u s , no

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

150

s a m p l e p r e c o n c e n t r a t i o n was r e q u i r e d . The p r e c i s i o n a s e s t i m a t e d by r e p l i c a t e a n a l y s e s was a b o u t + 0 . 0 0 0 6 2 μ Μ , ( r e p o r t e d a s one s t a n d a r d d e v i a t i o n ) w h i c h i s e q u i v a l e n t t o a 1% r e l a t i v e standard d e v i a t i o n a t the midrange of the standards. Computation of S t a b i l i t y Constants f o r Hydroxo and Carbonato Complexes. S t a b i l i t y c o n s t a n t s f o r the f o r m a t i o n o f hydroxo com­ p l e x e s w e r e c o m p u t e d by l i n e a r r e g r e s s i o n f r o m t h e f o l l o w i n g equation: [Cu

T 0 T

]

-

[Cu2+] =

0H"

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

a

3

Cu(0H)9

[Cu2+]

a

0H"

+

K

Cu0H+

(

1

)

2

2+ where [CUJOJ] and [Cu ] a r e the measured c o n c e n t r a t i o n s o f t o t a l d i s s o l v e d c o p p e r and c u p r i c i o n r e s p e c t i v e l y and aoH~ i s t h e a c ­ t i v i t y o f h y d r o x i d e i o n ^ s c o m p u t e d ^ f r o m t h e m e a s u r e d pH a n d t h e ion product of water. Kcu0H+ and 3Cu(0H)2 a r e s t a b i l i t y constants f o r t h e f o r m a t i o n o f mono a n d d i h y d r o x o c o m p l e x e s a s d e f i n e d b y the equations: [Cu0H+]

[Cu(0H)2]

(3)

Mass

balance

[Cu

Equation and 4 . The ([Cu a

2 +

0H"-

],

]

T Q T

1

for

total

= [Cu2+]

+

i s derived

left

term

of

[CUTQT]* P h )

soluble

copper

[Cu0H+]

+ [Cu(0H)2]

by a l g e b r a i c equation a

The y - i n t e r c e p t

n

d

t

h

e

i s expressed

by t h e

.

combination

1 was c o m p u t e d

(4)

of

equations

f o r each data

regressed as a l i n e a r

n

and t h e s l o p e

of

equation:

2, set

function

the regression

line

3

of

are

Cu0H+ a n d 3cu(0H)2' respectively. A s i m i l a r l i n e a r r e g r e s s i o n p r o c e d u r e was u s e d f o r t h e determination of carbonato s t a b i l i t y constants. Here t h e e q u a t i o n regressed was: 2 K

([Cu

T 0 T

]

-

([Cu2+]

+ [Οι

2 +

a j

]Σ 1

e

Cu(C03)|-

a

C0§"

+

K

CuC03

H

. β*))/ [Cu2+]

a

C Q

2

-

=

3

.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(

'

8.

1

Ο

The

term

[Cu2+]

a

Σ

151

Copper in River Water

SUNDA A N D HANSON

Q H

* _ $.

represents

the concentration

of

mono

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

p l u s d i h y d r o x o complexes as comptued from s t a b i l i t y c o n s t a n t s ( T a b l e I ) . The a c t i v i t y o f c a r b o n a t e i o n was computed f r o m t h e c o n c e n t r a t i o n o f a d d e d NaHCU3 ( 0 . 0 1 M) a n d c a r b o n a t e a c i d i t y c o n s t a n t s a t i n f i n i t e d i l u t i o n a n d 2 5 ° C r e p o r t e d i n Stumm a n d Morgan ( 9 J . The s t a b i l i t y c o n s t a n t s o b t a i n e d f r o m t h e s l o p e a n d y - i n t e r c e p t of the r e g r e s s i o n of equation 6 a r e d e f i n e d i n terms a n d t h e of t h e c o n c e n t r a t i o n s o f C u 2 + , CUCO3, and Cu(003)2 a c t i v i t y of carbonate i o n . S t a b i l i t y c o n s t a n t s f o r hydroxo and c a r b o n a t o complexes were c o r r e c t e d t o i n f i n i t e d i l u t i o n u s i n g a c t i v i t y c o e f f i c i e n t s c a l c u l a t e d from the extended Debye-Huckel equation. Scatchard plots. S t a b i l i t y c o n s t a n t s f o r t h e b i n d i n g o f Cu by a m o d e l l i g a n d ( h i s t i d i n e ) a n d by n a t u r a l o r g a n i c l i g a n d s i n r i v e r w a t e r were computed u s i n g S c a t c h a r d p l o t diagrams as d e s c r i b e d p r e v i o u s l y by M a n t o u r a a n d R i l e y ( 1 0 ) . The g e n e r a l equation f o r t h i s analysis was: [CuL.] - — — [Cu2+]

=

Κ [L c ι

lui

]

-

Κ [CuL ] c ι

(1

where LCuL-i] and [LJ_JQT] a r e t h e c o n c e n t r a t i o n s o f copper l i g a n d complex and t o t a l l i g a n d . Κς i s a c o n d i t i o n a l s t a b i l i t y c o n s t a n t v a l i d f o r a given set of chemical c o n d i t i o n s of pH, i o n i c strength and c o n c e n t r a t i o n o f c o m p e t i n g metal i o n s ( C a , M g , e t c ) :

K

c

-

^ [Cu

]

([L.]

(7,

+ Σ [H L] + Σ N

[MeL])

Σ [HnL] i s t h e c o n c e n t r a t i o n o f a l l p r o t o n a t e d f o r m s o f a weak a c i d l i g a n d a n d Σ [ M e L ] i s t h e sum o f a l l c o m p l e x e s o f t h e l i g a n d w i t h m e t a l s o t h e r t h a n c o p p e r , p a r t i c u l a r l y Ca a n d M g . The c o n ­ c e n t r a t i o n o f o r g a n i c a l l y bound c o p p e r was c o m p u t e d t o be e q u a l to the t o t a l measured c o n c e n t r a t i o n of copper i n s o l u t i o n minus the computed c o n c e n t r a t i o n s of i n o r g a n i c s p e c i e s : C u 2 + , CuOH+, C u ( 0 H ) 2 , CUCO3 a n d C u ( C 0 3 ) £ ~ . C o n c e n t r a t i o n s o f t h e s e s p e c i e s were c a l c u l a t e d from hydroxo and carbonato s t a b i l i t y c o n s t a n t s determined i n t h i s study (Table I ) . A c t i v i t y c o e f f i c i e n t s used i n t h e s e c a l c u l a t i o n s were computed from t h e extended DebyeHuckel e q u a t i o n u s i n g e s t i m a t e s o f i o n i c s t r e n g t h based on t h e measured c o n c e n t r a t i o n s of C a 2 + , M g 2 + , carbonate a l k a l i n i t y a n d ,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

152

CHEMICAL MODELING IN AQUEOUS SYSTEMS

where of

applicable The

Scatchard

histidine one

is

complex

[CuL] plots

of

plot

straight formed.

should

tration

at

pH

be

linear

histidine

derived

from

8.0)

curves

different

stripping

the

added

sites)

and

a

was

concentration

copper is

the

to

for

-Kc.

river

presence to

likely

stability

a

equal

of

a

to

the

showed

several

estimate groups

total

of

one-to-

function

However,

Here an

of

single

as

water

constants.

adopted

titration

only

[CuL]/[Cu2+]

equal

data

(more

conditional

of

slope

stability

ligands

the there

an x - i n t e r c e p t

indicating

procedure

individual

plot

with

and

for

since

titration

non-linear with

analyses

forward Here a

sites of

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

(titrations

NaHC03.

of

concen­ Scatchard markedly

binding iterative

concentration

ligands

or

binding

constants.

Chemical S p e c i a t i o n Models. Using the s t a b i l i t y constants d e r i v e d by us f o r c o p p e r c o m p l e x e s w i t h h y d r o x o and c a r b o n a t o l i g a n d s ( T a b l e I) and f o r n a t u r a l o r g a n i c l i g a n d s ( T a b l e II), t h e Newport and Neuse R i v e r s were modeled f o r c o p p e r s p e c i a t i o n as a f u n c t i o n o f p H , t o t a l c o p p e r , c a r b o n a t e a l k a l i n i t y and t o t a l dissolved organic the equation: [Cu

T Q T

]

=

matter.

[Cu2+]

+

Speciation

[CuOH+]

[Cu(C03)2~]

+

+

[CuL

models

were

[Cu(0H)2]

T Q T

+

calculated

[CuC03]

+

]

(8)

where

[CuOH+]

[Cu(0H)2]

[CuC03]

a

Cu

2 +

1

a

Cu

2 +

a

0H"

K

CuOH+

a

Cu

2 +

a

0H-

3

Cu(0H)2

a

Cu

2 +

a

C02"

Y

Cu

2 +

K

1

Y

CuC03

1

/

CuOH+

Y

Y

Cu(0H)2

C

U

C

0

3

2

[Cu(C03)2-] a

Cu2+

a

Cu

a

C0

3

2

"

3

Cu(C03)2"

1

Y

Cu(C03)2"

Ν 2 +

I

CLi-T0T]

from

K

c - i '

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

8.

Κ and

3 denote

conditional tion

of

ion

the

for

ith

corrected

their

were

not

valid

153

constants,

under

the

[L-J-T0T] i s

ligand

group

(ligand

by

Scatchard

coefficient. considered

Κς_η·

same

determination,

resolved

activity

stability

constants

organic

ligands

complexes in

as

the

Ν total

single

zero

stability

conditions of

Copper in River Water

SUNDA A N D HANSON

or

the

significant

concentra­

binding

analyses,

Polynuclear

are

chemical site)

and

γ

and m i x e d

and,

thus,

is

the

ligand

not

included

model.

Results

and

Discussion 2+

Hydroxo

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

measured

as

containing Ι.ΟμΜ some o f

0.01

the

be

carbonato

functions

a

PrCu2+j

decrease

accounted

adsorption

of

M KNO3

CUSO4.

could in

and

pH n

in

the by

1).

a

range

approximately

above

this

from

was is

first

plot

separate fold is

(Figure

of

titrations

pH

(Figure

indicative

and

excludes

cupric

of

or

for

the

p[Cu

indicated

the whose

value

the

of

(1θ6·48) (106·

dilution

10

Values 10

1 4

·

in

siderable species error can

constant 11.80) and

U

of

)

at

the

9

natural for

(log

who

of

32

also

of

that

the

range and

constant

at

copper or

pH

was

electrode

found

as

that

the

pH

adsorption

copper

of

of

the

[CU

are

for of

of

the

2 +

species in

a

species in

free

Cu(0H)£

7.7

to

(Cu0H+ Table

monohydroxo

of

10.8 and

I.

Our

complex

previously

published

strength

approaching

ionic

at

]/[CUJOJ]

Analysis

range

given

five­

reduction

complexes. pH

three

to

hydrolysis

observed

the

for

up

concentration

ratio

precipitation in

pH

despite

values infinite

U ) .

T4,

constant

are

V5). as

to

waters.

quite

This the

of

this

11.78)

at

importance and

25°C

reported

used

a cupric data

the

(J4)

of

dihydroxo ranging has

and by

Our

Paulson

dilution (J4)

adsorptive

loss

of

con­

of

procedures

dihydroxo

infinite

1θ"Ό·7 in

dihydroxo

discusses sources

(log

ion-selective electrode

for

complex

from

resulted

copper

computational

variability.

that

his

for

variable

variability

Paulson

with

corrected

curve

hydrolysis

constant

25°C

a function

the

by pH

experimental =

were

mononuclear

constants

stability

much

to pH

containers

indicating

polynuclear

two

literature ]Z_

onto

dissolved

caused

of

13,

9

the

agreement

also

been

as

behavior

formation

within 6 6

due

increasing

dissolved

adsorption

single

P[CUJQT] and

previous

account

close

·

copper

remained

the

decreased

possibility

uncertainty

in

in

6

(1J_,

3

This

stability

Ql_,

reported

measured

stability

falls

to

0

for

a

in

presence

Cu(0H)2)

on

2).

],

but

and

least

1).

formation 2 +

adsorption

then

fall

have

9.0

μΜ

concentration

dissolved

increased with

92% o f

]/[CUJOT])

2 +

the

the

i o n may

(solid) data

to

P([CU

differences

given

to

values

i n c r e a s e d and

reversible A

due

Consistent

At

ion

were

solutions 2.5

in

to

p[Cujpj] of of

cupric

to

and

i n c r e a s i n g pH.

decrease

Up

J

titrations

measured

7.0

range.

solution

KOH

concentrations

Adsorption

the

lost

the

initial

d

values

surfaces.

for

p[Cu

increased with

for

(Figure

constants.

that

stability is

in

32

=

technique

copper.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

154

pH Figure 1. Loss of copper by surface adsorption as a function of pH for solutions containing lOmM KNO ; (M) and (\J) 10 Μ CuSO, , and (Ο) 2.5 Μ CwS0 ; (U) increasing pH and decreasing pH. pH varied by addition of concentrated KOH and HCl. s

μ

t

μ

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 4

s

^

h

)

μ

/[C-TOTJJ

[Cu2

h

Figure 2. Titration data for complexation of copper by hydroxide. All solutions contained lOmM KNO ; (Α) Ι.ΟμΜ CuSO (Ο) 2.5 Μ CuSO (first run), and (•) 2.5μΜ C w S 0 (second run). Curve calculated according to hydroxo constants determined in this work (Table I).

Ρ

Ρ( V

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

156

CHEMICAL MODELING IN AQUEOUS

Table

I.

Stability C032",

Ligand

Ionic

OH" 2C 0

3

Histidine

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

and

constants

for

reaction

h i s t i d i n e at

strength

of

copper

with

SYSTEMS OH",

25°C.

log

log

0

corr.

6.48

11.78

0

corr.

6.74

10.24

0.01

10.45*

0.01

10.55

18.80

References

e2

This

work

II

II

II

II

S i l l en

& Martel 1,

(12)

*

C a l c u l a t e d from a c o n d i t i o n a l s t a b i l i t y constant at 100.21 ( F i g u r e 5) and f i r s t and second hydrogen i o n t i o n c o n s t a n t s of 1 0 9 · 2 0 and 1 0 6 · 0 0 (12.).

pH 7 . 0 0 of associa-

A n a l y s i s o f d a t a f o r pLCu^"1"], P [ C U T Q T ] a n d pH f o r t h e s o l u t i o n c o n t a i n i n g 0 . 0 1 M N a H C Û 3 a n d 5 μ Μ tubOq a r e c o n s i s t a n t w i t h t h e p r e s e n c e o f b o t h mono a n d d i c a r b o n a t o c o m p l e x e s ( F i g u r e 3 ) . C o n s t a n t s f o r t h e s e complexes computed from t i t r a t i o n data a r e g i v e n i n Table I and a r e i n good agreement w i t h p r e v i o u s l y pub­ l i s h e d v a l u e s a s r e p o r t e d i n S i l l e n a n d M a r t e l (21), S u n d a (13) and B i l i n s k i e t a K (7J. A d s o r p t i o n o f c o p p e r i n the 0.01 M b i c a r b o n a t e s o l u t i o n s was n o t a s g r e a t (maximum a d s o r p t i o n 50%) as t h a t w h i c h o c c u r e d i n t h e a b s e n c e o f c a r b o n a t e i o n , s u g g e s t i n g t h a t carbonato complexes of copper a r e not as r e a d i l y adsorbed as c u p r i c ion and/or copper hydroxo s p e c i e s . M o d e l s o l u t i o n s : T i t r a t i o n s a t c o n s t a n t pH a n d a v a r i a b l e p[CuTnï\T D a t a f o r t h e Cu t i t r a t i o n o f a m o d e l s o l u t i o n c o n t a i n i n g 0 . 7 5 μ Μ h i s t i d i n e , 0 . 1 mM N a H C 0 3 a n d 0 . 0 1 M KNO3 a t pH 7.0 ( F i g u r e 4T~was a n a l y z e d by l i n e a r r e g r e s s i o n u s i n g a S c a t c h a r d p l o t diagram (Figure 5). This analysis yielded a s t a b i l i t y con­ s t a n t i n good agreement w i t h p r e v i o u s l y p u b l i s h e d v a l u e s (Table I). A t h e o r e t i c a l c u r v e c a l c u l a t e d f r o m t h i s c o n s t a n t was i n g o o d a g r e e m e n t w i t h t h e m e a s u r e d pCu a n d P [ C U J O T ] d a t a throughout t h e t i t r a t i o n r a n g e o f p [ C u j O T ] ( F i g u r e 4) i n d i c a t i n g good e l e c ­ t r o d e b e h a v i o r e v e n a t c o n c e n t r a t i o n s o f CUJQT a s l o w as 1 0 " 8 · 2 M. pCu a n d PECUJQT] d a t a d i d n o t a g r e e as c l o s e l y w i t h t h e t h e o ­ r e t i c a l l y c a l c u l a t e d curve for a t i t r a t i o n of a s i m i l a r s o l u t i o n c o n t a i n i n g no a d d e d c h e l a t o r . The m e a s u r e d pCu v a l u e s w e r e i n good agreement w i t h v a l u e s c a l c u l a t e d on the b a s i s of i n o r g a n i c c o m p l e x a t i o n f o r t o t a l c o p p e r c o n c e n t r a t i o n s >_ 1 0 " ? M , b u t d e v i ­ ated from p r e d i c t e d values below t h i s c o n c e n t r a t i o n ^ F i g u r e 4 ) . A maximum d i f f e r e n c e b e t w e e n m e a s u r e d a n d c a l c u l a t e d pCu v a l u e s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

II.

Acid

3.0 5.0

8.00

8.00

6.78

8.00

3

3

...

iî

L

&

L

& 2.7 2.9

2.1 1.9 3.0

1.2 1.1 1.2

8.7 8.6 1.6

7.00

&

5.0 1.8 1.0

Ligand

for

E-6

E-7

Copper

8 8 D . (J 8.0 3.3 4.0

E-4* E-3* E-3* E-3* 2.4 2.4

1.1

Schnitzer II

&

II

II

&

II

g

II

of

II

Khan,(18)

II

Riley,(10)

II II

II

II

II II

II

II II

II

II

II

II II

II

II

II

II

II II

II

II

II

II

II

paper

II

This

Mantoura II

Ligands

Reference

Organic

b i n d i n g s i t e s per on d a t a i n ( 1 8 ) ) .

8.3

4.8

11.2

9.5 7.3 4.9

Κ

E-5 E-4

E-5 E-4 E-3

10.9 8.8 6.9

9.7 7.8 5.6

E-5 E-4 E-2 E-5 E-4 E-3

9.0 6.2 4.6

log

Natural

E-5 E-3 E-3

by

8.9 9.6

7.0 6.4 9.9

8.0 7.5 8.2

E-6 E-5 E-4 E-7 E-6 E-5

5.8 5.7 1.0

3.3 1.2 6.7

E-7 E-6 E-4

E-7 E-5 E-4

of

M o l e s o f Cu Binding Sites g organic carbon

Complexation

Ligand Concentration M

Constants

5.95

pH

Stability

Data c o n v e r t e d f r o m number o f b i n d i n g s i t e s p e r g o f o r g a n i c m a t t e r t o o r g a n i c c a r b o n a s s u m i n g n a t u r a l f u l v i c a c i d s c o n t a i n 46% c a r b o n ( b a s e d

Fulvic

Soil

River

Water

*

Water

Conditional

River

Lake

Neuse

Newport

Natural

Table

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

158

CHEMICAL MODELING IN AQUEOUS

1

Τ

SYSTEMS

1

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

/

/ :

/

3

3

or

o /

•

-

1

pH Figure 3. Titration data for complexation of copper by carbonate. Solution con­ tained lOmU NaHCOs and 5μΜ CuSOj,. pH varied by adjusting P Curve calculated according to hydroxo and carbonato stability constants determined in this work (Table I). C O r

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Copper

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

SUNDA AND HANSON

in River

P[

C U

Water

TOT]

Figure 4. Copper titrations of model solutions at 25°C and pH 7.00. (•) Solu­ tion contained 0.01M KN0 and O.lmM NaHC0 in distilled water. (O) Solution contained 0.75μΜ histidine, 0.01M KN0 , and O.lmM NaHC0 . Solid lines through data points are theoretical curves calculated according to constants given in Table I. Dark solid line represents p C w = p[Cu T7· 3

3

3

3

T0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

160

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Figure

5.

Scatchard

plot for histidine model solution. for the data is shown.

The linear regression

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

line

8.

SUNDA AND HANSON

Copper

in River

Water

161

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

T h l s ( 0 . 4 pCu u n i t s ) o c c u r e d a t P [ C U J Q T ] 8 · ° · deviation from i d e a l b e h a v i o r i s a p p a r e n t l y due t o a d i s e q u i l i b r i u m between t h e s o l u t i o n t i t r a t e d a n d t h e membrane s u r f a c e o f t h e c u p r i c i o n selective electrode. Our e x p e r i e n c e h a s shown t h a t 1 0 " ^ M t o t a l copper i s often the lower l i m i t f o r accurate determination of cupric ion activity. However, the exact l i m i t of d e t e c t i o n i s de­ t e r m i n e d by a v a r i e t y o f f a c t o r s : c o m p l e x i n g c h a r a c t e r i s t i c s o f the s o l u t i o n i n c l u d i n g k i n e t i c s of the complexation r e a c t i o n s , p r e c o n d i t i o n i n g o f the e l e c t r o d e s u r f a c e , and time a l l o w e d t o establish equilibrium. The c u p r i c i o n e l e c t r o d e shows b e s t b e ­ h a v i o r i n w e l l b u f f e r e d s o l u t i o n s i n which the k i n e t i c s of com­ p l e x a t i o n r e a c t i o n s are f a s t , such as i n the h i s t i d i n e s o l u t i o n .

Copper t i t r a t i o n s o f r i v e r w a t e r . Copper t i t r a t i o n data i n d i c a t e s t h a t c o p p e r i s h i g h l y bound i n both Newport and Neuse River water (Figures 6 and 7 ) . A g e n e r a l l y c l o s e agreement between t i t r a t i o n c u r v e s o f f i l t e r e d and u n f i l t e r e d samples i s c o n s i s t a n t w i t h m i n o r t o n e g l i g i b l e b i n d i n g by p a r t i c u l a t e m a t t e r r e t a i n e d by g l a s s f i b e r f i l t e r s ( t h e mean r e t e n t i o n s i z e o f g l a s s f i b e r f i l t e r s i s a p p r o x i m a t e l y 0 . 7 t o 0 . 9 ym ( ] J o ) . Close agreement between measurements o f background t o t a l dissolved c o p p e r i n u n t r e a t e d Neuse R i v e r w a t e r ( 0 . 0 2 9 μΜ) and f i l t e r e d Neuse R i v e r w a t e r ( 0 . 0 2 5 μΜ) a l s o i n d i c a t e s t h a t o n l y a m i n o r f r a c t i o n o f c o p p e r was a s s o c i a t e d w i t h p a r t i c u l a t e m a t t e r . Non-ideal behavior of the cupric ion electrode occurs at PECUJOJ] > 7 i n t h e t i t r a t i o n s o f both f i l t e r e d and u n f i l t e r e d N e w p o r t R i v e r w a t e r a t pH 5 . 9 5 a n d f i l t e r e d w a t e r a t pH 7 . 0 a n d 8.0. A t l o w t o t a l c o p p e r c o n c e n t r a t i o n s , m e a s u r e d pCu v a l u e s approach a constant value independent of the t o t a l copper i n solution. S i m i l a r b e h a v i o r was o b s e r v e d f o r f i l t e r e d Neuse River w a t e r a t pH 8 . 0 , b u t n o t a t pH 6 . 7 8 . As i n d i c a t e d by t i t r a t i o n d a t a ( F i g u r e s 6 a n d 7 ) , b i n d i n g o f c o p p e r i n b o t h Neuse a n d Newport R i v e r w a t e r d e c r e a s e s w i t h i n ­ c r e a s i n g t o t a l c o p p e r i n a manner c o n s i s t a n t w i t h a s t e p w i s e t i ­ t r a t i o n o f a number o f d i f f e r e n t l i g a n d s a n d / o r b i n d i n g s i t e s . B i n d i n g o f c o p p e r i n c r e a s e s w i t h i n c r e a s i n g pH c o n s i s t a n t w i t h r e a c t i o n s w i t h p r o t o n a t e d weak a c i d s . C o m p a r i s o n s o f Cu t i t r a t i o n s o f n a t u r a l f i l t e r e d r i v e r w a t e r and U V - i r r a d i a t e d f i l t e r e d r i v e r w a t e r show a l a r g e d e c r e a s e i n binding of copper a f t e r photooxidation of organic matter. UVt r e a t m e n t r e s u l t e d i n > 97% d e s t r u c t i o n o f d i s s o l v e d o r g a n i c m a t t e r i n b o t h Neuse a n d Newport R i v e r w a t e r s , b a s e d on l i g h t a b s o r p t i o n m e a s u r e m e n t s i n t h e w a v e l e n g t h r a n g e 3 0 0 - 5 0 0 nm a n d measurements of d i s s o l v e d o r g a n i c c a r b o n . M e a s u r e d pCu v a l u e s i n n a t u r a l f i l t e r e d N e w p o r t R i v e r w a t e r a t pH 5 . 9 5 a r e a p p r e c i a b l y higher than p r e d i c t e d f o r complexation t o i n o r g a n i c l i g a n d s (CO*}" a n d 0 H _ ) a l o n e b y v a l u e s r a n g i n g f r o m 0 . 7 a t P [ C U J Q T ] 4 . 2 t o 2 . 4 a t P[CUJOT] 6 . 9 . From c a l c u l a t i o n s a t PECUJOT] 6 · 9 ο η 1 Υ 0 · 4 % of t h e copper i n s o l u t i o n i s present as i n o r g a n i c s p e c i e s , p r i ­ m a r i l y C u 2 + , w i t h t h e r e m a i n i n g 99% a p p a r e n t l y bound t o o r g a n i c

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

162

Figure 6. Copper titrations of Neuse River water at 25°C. (Φ) Untreated water at in situ pH 6.78; (\J) glass-fiber filtered water at pH 6.78; (A) glass-fiber filtered water at pH 8.00; (0 ) UV-treated glass-fiber filtered water at 6.78; ( • ) twice filtered UV-treated water at pH 6.78, first filtration by glass-fiber prior to UVirradiation, second filtration by membrane (0.2 ™ nuclepore) after irradiation. Model curves through data points were calculated according to stability constants determined in this work (Tables I and II). Dotted lines indicate limits on data used for calculation of conditional stability constants for organic binding. μ

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Copper in River Water

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

SUNDA AND HANSON

Figure 7. Copper titration of Newport River water at 25°C. (Φ) Untreated water at in situ pH 5.95; ([J) glass-fiber filtered water at pH 5.95; (O) ghss-fiber filtered water at pH 7.00; (A) glass-fiber filtered water at pH 8.00; (0) UV-treated glass-fiber filtered water at pH 5.95. Model curves through data points were calculated according to stability constants determined in this work (Tables I and II). Dotted lines indicate limits on data used for calculation of conditional sta­ bility constants.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

164

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

ligands. The p r e d o m i n a n c e o f b i n d i n g by o r g a n i c l i g a n d s i s c o n f i r m e d by an a l m o s t c o m p l e t e d e s t r u c t i o n o f b i n d i n g c a p a b i l i t y after UV-photooxidation ( F i g u r e 7 ) . In t h e U V - t r e a t e d Newport R i v e r w a t e r a t pH 5 . 9 5 , m e a s u r e d p C u v a l u e s w e r e i n g o o d a g r e e m e n t w i t h those c a l c u l a t e d from i n o r g a n i c complexation i n the range of P [ C U J O T ] 7 t o 4. A s l i g h t i n c r e a s e i n m e a s u r e d p C u o f u p t o 0.4 u n i t s was o b s e r v e d r e l a t i v e t o t h a t p r e d i c t e d f r o m i n o r g a n i c c o m p l e x a t i o n a t P L C U J Q T ] > 7. T h i s d e v i a t i o n f r o m pCu v a l u e s c a l c u l a t e d from c o n s i d e r a t i o n of i n o r g a n i c complexation a t p [ C u j g y ] > 7 i s s i m i l a r to t h a t o b s e r v e d i n t h e model s o l u t i o n c o n t a i n i n g 0.01 M KNO3 a n d 0.1 mM N a H C Û 3 a t pH 7 ( F i g u r e 4 ) a n d t h u s , may a l s o be d u e t o n o n - i d e a l e l e c t r o d e b e h a v i o r a t l o w t o t a l copper concentrations. U V - t r e a t m e n t o f f i l t e r e d Neuse R i v e r w a t e r a l s o c a u s e d a l a r g e d e c r e a s e i n t h e b i n d i n g o f c o p p e r a t pH 6 . 7 8 a n d p [ C u j o j ] 1_ 7 c o n s i s t a n t w i t h p r e d o m i n a n c e o f b i n d i n g o f c o p p e r b y o r g a n i c ligands in the natural f i l t e r e d r i v e r water. However, at values o f PLCUTQJ] > 7 i n t h e p h o t o o x i d i z e d f i l t e r e d Neuse R i v e r w a t e r (pH 6 . 7 8 ; 9 t h e m e a s u r e d pCu v a l u e s b e h a v e a s i f c o p p e r was h i g h l y bound by a s i t e p r e s e n t a t a l o w c o n c e n t r a t i o n 10 7· M) w i t h a r e l a t i v e l y h i g h s t a b i l i t y c o n s t a n t , a l t h o u g h we r e c o g n i z e t h a t t h e c u p r i c i o n e l e c t r o d e may b e e x h i b i t i n g s o m e d e g r e e o f non-ideal behavior. F i l t r a t i o n of the UV-treated r i v e r water t h r o u g h a 0.2 ym f i l t e r ( N u c l e p o r e ) r e d u c e d t h e b a c k g r o u n d c o n c e n t r a t i o n o f c o p p e r i n t h e w a t e r f r o m 0 . 0 2 5 μΜ t o b e l o w t h e d e ­ t e c t i o n l i m i t o f t o t a l c o p p e r a n a l y s i s (0.001 μ Μ ) i n d i c a t i n g t h a t t h e b a c k g r o u n d c o p p e r was e i t h e r a d s o r b e d t o o r i n c o r p o r a t e d into f i l t r a b l e particles. T i t r a t i o n of t h i s r e f i l t e r e d water a l s o g a v e a c u r v e f o r m e a s u r e d pCu v s P E C U J Q T ] Ί η a g r e e m e n t w i t h a reduced level of copper b i n d i n g . The n a t u r e o f t h e p a r t i c l e s t h a t c o p p e r was a p p a r e n t l y a s s o c i a t e d w i t h i s unknown a s i s t h e nature of the chemical a s s o c i a t i o n . T h e p a r t i c l e s may h a v e b e e n hydrous metal oxides or a small f r a c t i o n of organic matter r é s i s t e n t t o p h o t o o x i d a t i o n . We d o n o t k n o w w h e t h e r t h e p a r t i c l e s were i n i t i a l l y p r e s e n t i n the r i v e r w a t e r o r were formed d u r i n g p h o t o o x i d a t i o n , o r w h e t h e r c o p p e r was a d s o r b e d t o t h e s u r f a c e o f the p a r t i c l e s or incorporated i n t o the p a r t i c l e m a t r i x . All of these p o s s i b i l i t i e s are completely c o n s i s t a n t with the observed data. However, s i n c e the apparent degree of a s s o c i a t i o n of copper with the p a r t i c l e s present in UV-treated water i s small at p [ C u j o j ] 1 7, o u r c o n c l u s i o n t h a t c o p p e r i n t h e n a t u r a l river w a t e r i s p r i m a r i l y b o u n d by o r g a n i c m a t t e r a t P E C U J O T ] 7 i s not affected significantly. _

5

Analysis of Organic Binding. Scatchard p l o t s for the binding o f c o p p e r i n f i l t e r e d r i v e r w a t e r showed n o n - l i n e a r i t y i n d i c a t i v e of the presence of binding s i t e s having d i f f e r e n t s t a b i l i t y cons t a n t s ( F i g u r e 8 ) . S i m i l a r n o n - l i n e a r b e h a v i o r has a l s o been o b s e r v e d i n S c a t c h a r d p l o t s f o r c o p p e r b i n d i n g by n a t u r a l organic

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1E-6

E[cuLi] , M

4E-6

u

3

1.27E-5

7 E-6 { 1.67E-5

Figure 8. Scatchard plot for glass-fiber filtered Neuse River water at in situ pH 6.78 and 25°C. Linear segments shown for three resolved binding sites, L L,, and L . Model curve through data points was calculated from resolved stability constants and total concentrations for the three binding sites.

5E2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

CHEMICAL MODELING IN AQUEOUS SYSTEMS

166 ligands (JO). was

from

lake water

Scatchard conducted

plot

and

analysis

soil of

as measured data

for

by

river

gel water

chromatography titrations

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

T h i s was d o n e because o f a p p a r e n t n o n - i d e a l e l e c t r o d e b e h a v i o r a t [CUJOT] < 1 0 " 7 M. Data a t the h i g h e s t c o n c e n t r a t i o n s o f t o t a l copper i n N e w p o r t R i v e r w a t e r a t pH 7 . 0 a n d pH 8 . 0 w e r e a l s o e x c l u d e d b e ­ c a u s e pCu v a l u e s a p p r o a c h t h o s e p r e d i c t e d f o r t h e f o r m a t i o n of s o l i d c o p p e r h y d r o x i d e , i . e . , p C u 6 . 7 a t pH 8 . 0 a n d 4 . 7 a t pH 7.0. I n o u r a n a l y s i s , we m o d e l e d t h e d a t a f o r b i n d i n g o f c o p p e r b y o r g a n i c m a t t e r i n a c c o r d a n c e w i t h t h e f e w e s t number o f binding s i t e s required to account f o r the observed copper t i t r a t i o n data. T h e t i t r a t i o n d a t a f o r e a c h r i v e r a n d pH w e r e a n a l y z e d separately by r e s o l v i n g t h e S c a t c h a r d p l o t s i n t o p o s t u l a t e d o n e , t w o , three and f o u r b i n d i n g s i t e models. The f o u r p o s s i b l e m o d e l s f o r e a c h r i v e r a n d pH w e r e t e s t e d a g a i n s t t h e d a t a u s i n g a r u n s t e s t a n d F - t e s t (17) t o d e t e c t and t e s t t h e s i g n i f i c a n c e o f s y s t e m a t i c d e ­ viations. F r o m t h e s e t e s t s we c o n c l u d e t h a t a t l e a s t t h r e e s e p a r a t e b i n d i n g s i t e s a r e r e q u i r e d t o model t h e Newport t i t r a t i o n d a t a a t pH 5 . 9 5 , 7 . 0 a n d 8 . 0 a n d t h e N e u s e d a t a a t pH 6.78 ( F i g u r e s 6 and 7 ) . Two b i n d i n g s i t e s a r e r e q u i r e d f o r t h e Neuse a t pH 8 . 0 w h i c h i s c o n s i s t e n t w i t h t h e r e s t r i c t e d r a n g e o f the t i t r a t i o n data. T h e a c t u a l n u m b e r o f b i n d i n g s i t e s may b e g r e a t e r t h a n t h e number r e s o l v e d , s i n c e s i t e s h a v i n g s i m i l a r s t a b i l i t y c o n s t a n t s c a n n o t be r e s o l v e d by t h e p r e s e n t t e c h n i q u e . Values for t o t a l c o n c e n t r a t i o n o f b i n d i n g s i t e s and c o n d i t i o n a l stability constants are given in Table II. In g e n e r a l , a c o n s i s t a n t s e t o f c o n d i t i o n a l s t a b i l i t y c o n ­ s t a n t s was o b t a i n e d f o r t h e two r i v e r s i n w h i c h t h e c o n s t a n t s i n c r e a s e w i t h pH a n d d e c r e a s e w i t h t h e r a t i o o f m o l e s o f b i n d i n g s i t e s p e r gram o r g a n i c c a r b o n ( F i g u r e 9 ) . The r e l a t i o n s h i p b e ­ t w e e n s t a b i l i t y c o n s t a n t s a n d pH i s s i m i l a r f o r a l l t h r e e p r o p o s e d b i n d i n g s i t e s i n e a c h r i v e r w i t h v a l u e s o f Δ l o g K c / Δ pH i n t h e t h e range 1.0 to 1 . 3 . M e a n v a l u e s f o r [ί ·_χοτ] " f ° r e a c h ° f i = l , 2 or 3 types of b i n d i n g s i t e s are h i g h e r i n the Newport R i v e r w a t e r r e l a t i v e t o t h o s e f o r t h e Neuse by f a c t o r s r a n g i n g f r o m 4 to 5. T o t a l l i g a n d c o n c e n t r a t i o n Σ [L-J-TOT]» i . e . , t o t a l binding c a p a c i t y , i s h i g h e r i n t h e Newport by a f a c t o r o f 5 w h i c h i s i n agreement w i t h the f i v e f o l d d i f f e r e n c e i n d i s s o l v e d o r g a n i c carbon (DOC): 1 5 mgC i H f o r t h e N e w p o r t a n d 3 . 0 mgC i H f o r the Neuse. T h u s , the g e n e r a l p i c t u r e t h a t emerges i s t h a t the b i n d i n g c h a r a c t e r i s t i c s of the o r g a n i c m a t t e r i n the Newport R i v e r water i s s i m i l a r t o t h a t i n t h e Neuse w i t h t h e m a j o r d i f f e r e n c e b e i n g t h e q u a n t i t y o f b i n d i n g s i t e s p r e s e n t a s i n d i c a t e d by t h e dif­ f e r e n c e i n the q u a n t i t y of d i s s o l v e d o r g a n i c c a r b o n . Ί

It i s l i k e l y t h a t most i f not a l l of the observed b i n d i n g of copper to o r g a n i c matter r e s u l t s from complexation to f u l v i c or humic a c i d s . Newport R i v e r w a t e r has a p r o n o u n c e d y e l l o w i s h brown c o l o r and shows c o n t i n u o u s l y i n c r e a s i n g l i g h t a b s o r p t i o n w i t h d e c r e a s i n g w a v e l e n g t h (1_3) c o n s i s t e n t w i t h t h e p r e s e n c e o f

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

8.

these

substances.

carbon gH

ratios

From u n p u b l i s h e d

(0.02 gN g C ~ ' ) ,

g C " l ) and c a r b o x y l

the to

dissolved those

of

Similarly, organic United able

The binding copper

complex

plain

mixtures

of

functional binding

of

of

to

1.3.

in binding

A

Binding primarily

with

effects

of metals

to

by c h e l a t i o n

an a d j a c e n t composed

(phthalate

of

( 1 3 mmol

consistent indicates

(18). mmol

there

the observed

binding to

the

pH r a n g e

6 to

1.3

copper

of

9),

since

pH v a l u e s to

whereas

Therefore, humic

binding

show

copper

t h e pH a t w h i c h the t i t r a t i o n that

conditional organic

which

acids

these

often

vary

(Δ l o g in

in

which

group

protonated

be m o s t l y for

at

de-

copper

consistent

with

sites. constants

are highly

c a n be

for

dependent

and the

Published

considerably

differences

and

account

reactions

was c o n d u c t e d

was e x a m i n e d .

apparent

to

sites

stability

ligands

less

is

slopes

mechanisms

a r e most

Newport

organic

a phenolic will

with

rings

binding

be p r i m a r i l y

groups

group

and/or

groups

groups

from

chelation

the measurement

curve

for fulvic

reported

that

This

ion from

our data

by n a t u r a l

of

slightly

data).

In a d d i t i o n ,

will

type

only

by c a r b o x y l

the proposed

salicylate

on

constants

of

in

a n d 8.0 a n d

River

for a l l three

carboxyl

η

occur

on a r o m a t i c

Newport

primarily

groups

compounds,

to

results of

these

sites)

pH 7.0

carboxyl

copper.

observed

type

pH 6 . 8 a r e for

to

a carboxyl

capacities for

unpublished

8 may r e s u l t

of has

at

i o n d i s p l a c e s a hydrogen

Our binding

1.0

of

the ob­

the surface

i s thought

gC"l) at

groups of

values

i s that

an i n c r e a s e

or

groups

binding

are sufficient

Δ pH)

pro tona t e d .

of

important.

carboxyl

gC"1)

the binding

Κς/

(equation

copper

indicate

from

(salicylate

Copper

carboxyl

for

of

of

c o n s i s t i n g of

+ 1 mmol

g C " 1 ; Sunda,

with that

of

9)

compounds

sites

two a d j a c e n t

(11+1

the content

copper

compounds

the reaction

molecules

may be

group

matter~Tl0

t h e Neuse

binding

iden­

of

(20) a n d / o r

possibility

pH r e s u l t s

humic

with

phenolic

groups)

organic

neutral

and f u l v i c

For

(Figure

Both

copper

sites

groups

HnL,

second

(21_).

a

binding

(9)

Δ l o g Κ ς / Δ pH

1.0

increase

matter

southeastern indistinguish­

= C u L + nH

colloids

than

humic

(18).

dissolved

the

individual

site

on p o l y e l e c t r o l y t i c

for

of

for

similar

soils

the

pH i s t y p i c a l

charge

River

are

properties

groups.

negative

sites

river

Postulated

i n l o g Κς w i t h

acid

slopes

the range

and

from

that

on p o l y e l e c t r o l y t i c m o l e c u l e s

a protonated

+ HnL

observed

extracted

has c h e m i c a l

acids.

water

to

(0.11

( 1 3 meq g C " ' )

River

reported

nitrogen

ratios

particles.

t o weak

Cu

acids have

that

carbon

ratios

i n Newport

coastal

fulvic

sites

increase

with

served

of

carbon

167

we know

to

1_2> a n d L 3 m a y r e p r e s e n t

within

colloidal

in

i n another

those

different

and f u l v i c

et_ a l _ . ( 1 9 )

data

hydrogen

to

matter

the S a t i l l a ,

as L ] ,

molecules or

humic Beck

matter

from

group

organic

States,

tified

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

Copper in River Water

SUNDA A N D HANSON

portion

stability

a n d Cheam

(22)

reasonably

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

168

Log([Lj. oT]/9C) T

Figure 9. Rehtionship between the log of the conditional stability constant and the log of the binding site concentration per gram dissolved organic carbon for individual binding sites. Newport River pH 8.00; (U) Neuse River pH 8.00; (O) Newport River pH 7.00; (%) Neuse River pH 6.78; (A) Newport River pH 5.95; (£) lake water (10).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

8.

SUNDA AND HANSON

Copper

in River

Water

169

a c c o u n t e d f o r b y d i f f e r e n c e s i n pH a n d t h e r a t i o o f [ C u T o i J / g r a m organic matter. O u r c o n d i t i o n a l c o n s t a n t s a t pH 8 . 0 f o r t h a t p o r t i o n o f t h e b i n d i n g s i t e s d e s i g n e d a s L_2 a r e i n g o o d agreement w i t h s t a b i l i t y c o n s t a n t s m e a s u r e d by M a n t o u r a a n d R i l e y (10) f o r b i n d i n g o f c o p p e r by o r g a n i c m a t t e r f r o m l a k e w a t e r a t t h e same pH a n d s i m i l a r r a t i o o f m o l e s o f b i n d i n g s i t e s p e r g r a m o r g a n i c carbon (Figure 9 ) . M a n t o u r a a n d R i l e y , who u s e d a g e l f i l t r a t i o n t e c h n i q u e to measure s t a b i l i t y c o n s t a n t s , found t h e i r data c o n ­ s i s t e n t w i t h t h e p r e s e n c e o f two s e p a r a t e b i n d i n g s i t e s . To m o d e l o u r t i t r a t i o n d a t a we u s u a l l y h a d t o p o s t u l a t e t h e e x i s ­ t e n c e o f no f e w e r t h a n t h r e e s e p a r a t e o r g a n i c b i n d i n g s i t e s . The a p p a r e n t d i f f e r e n c e b e t w e e n r e s u l t s o f t h e t w o s t u d i e s c a n be r e s o l v e d i f we n o t e t h a t o u r t i t r a t i o n s u s u a l l y c o v e r e d 2 t o 3 o r d e r s o f m a g n i t u d e change i n bound c o p p e r c o n c e n t r a t i o n w h e r e a s , the e x p e r i m e n t s o f Mantoura and R i l e y (10J c o v e r e d o n l y s l i g h t l y g r e a t e r than one o r d e r o f m a g n i t u d e . T h u s , i t was i m p o s s i b l e f o r these i n v e s t i g a t o r s to determine the c h a r a c t e r i s t i c s of copper binding outside of t h e i r experimental range. C h e m i c a l S p e c i a t i o n Models f o r t h e Neuse and Newport R i v e r Waters. Copper s p e c i a t i o n m o d e l s f o r f i l t e r e d Newport a n d Neuse R i v e r waters as a f u n c t i o n o f t o t a l copper c o n c e n t r a t i o n a t i n s i t u pH v a l u e s ( F i g u r e s 1 0 a a n d 1 1 a ) i n d i c a t e t h a t s o l u b l e c o p p e r i s g r e a t e r t h a n 98% bound t o o r g a n i c l i g a n d s i n t h e r a n g e o f t o t a l copper concentration normally encountered i n r i v e r water, i.e., p [ C u j o j ] — 7. At these c o n c e n t r a t i o n s , copper i s p r i m a r i l y c o m p l e x e d by o r g a n i c b i n d i n g s i t e L ] , a s i t e r e p r e s e n t i n g l i g a n d s present a t low concentrations with high s t a b i l i t y constants. S i t e L-| a c c o u n t s f o r a b o u t 1% o f t h e t o t a l b i n d i n g c a p a c i t y b u t >90% o f t h e b o u n d c o p p e r a t P [ C U J O T ] ϋ 7 . Only a t extremely high c o n c e n t r a t i o n s o f t o t a l c o p p e r , i . e . , PECUTQTJ 1 3 . 7 i n t h e Newport and £ 4 . 5 i n t h e Neuse, would i n o r g a n i c s p e c i e s o f copper become d o m i n a n t . I n c r e a s i n g t h e pH o f t h e N e w p o r t f r o m 5 . 9 5 t o 7 . 0 ( F i g u r e s 10a a n d 10b) w i t h o u t c h a n g i n g c a r b o n a t e a l k a l i n i t y does n o t a l t e r t h e o r d e r o f dominance o f copper s p e c i e s . However, in the n a t u r a l range of t o t a l copper c o n c e n t r a t i o n s (i.e., P L C U T Q J ] ^ 7 ) i n c r e a s i n g pH t o 7 . 0 d o e s i n c r e a s e t h e e x t e n t o f t o t a l organic binding r e l a t i v e to inorganic species. This r e ­ s u l t s f r o m t h e s t r o n g pH d e p e n d e n c e o f c o n d i t i o n a l s t a b i l i t y c o n ­ s t a n t s f o r copper complexes w i t h n a t u r a l o r g a n i c l i g a n d s as d i s ­ cussed previously. A t i n s i t u pH a n d i n s i t u c o n c e n t r a t i o n s o f d i s s o l v e d c o p p e r i n t h e Neusë~TÔ.025 μ Μ ) a n d t h e N e w p o r t ( 0 . 0 1 1 μ Μ ) , t h e m o d e l s p r e d i c t s i m i l a r pCu v a l u e s f o r t h e t w o r i v e r s ( 1 0 . 4 a n d 1 0 . 6 f o r t h e Neuse and N e w p o r t ) . Thus i n c o n s i d e r a t i o n o f t h e m a j o r f a c t o r s c o n t r o l l i n g pCu v a l u e s i n t h e two r i v e r s , t h e h i g h e r c o n ­ c e n t r a t i o n o f o r g a n i c b i n d i n g s i t e s and lower t o t a l dissolved c o p p e r i n t h e Newport a r e a l m o s t e x a c t l y c o m p e n s a t e d f o r by t h e h i g h e r pH o f t h e N e u s e . 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 10. Chemical speciation model for dissolved copper in the Newport River at 25°C as a function of total copper concentration, (a) In situ pH 5.95, [Alk] = O.OSmM and I = 0.0005M; (b) pH 1.00, [Alk] = 0.05mU and I = 0.005M; and (c) pH 8.00, [Alk] = 0.55mM and I = 0.001U. D0C^15mgCLK

TOT

p[Cu ]

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

Copper

in River

Water

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

SUNDA AND HANSON

Figure 11. Chemical speciation model for dissolved copper in the Neuse River at 25°C as a function of total copper concentration, (a) In situ pH 6.78, [Alk] = 0.15mM and I = 0.0005M; and (h) pH 8.00, [Alk] = 0.65mM and I = 0.001U. DOC = 3 mgCL . 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

172

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Models f o r copper s p e c i a t i o n as a f u n c t i o n of t o t a l c o n c e n t r a t i o n o f o r g a n i c s i t e s p[Ljoj] ( F i g u r e s 12 a n d 1 3 ) w e r e c a l c u l a t e d a t PCCUJQT] 7 w l t h t h e a s s u m p t i o n t h a t t o t a l o r g a n i c m a t t e r was v a r i e d w i t h o u t a l t e r a t i o n o f t h e r e l a t i v e c o n c e n t r a t i o n s of i n d i v i d u a l binding s i t e s w i t h r e s p e c t to the t o t a l . Copper s p e c i e s a r e a l s o shown v e r s u s d i s s o l v e d o r a a n i c c a r b o n (DOC) u s i n g m e a s u r e d v a l u e s o f 15 a n d 3 . 0 mgC f o r the Newport and Neuse, r e s p e c t i v e l y . A t i n s i t u pH ( F i g u r e s 1 2 a a n d 1 3 a ) , the d o m i n a n c e o f o r g a n i c bound s p e c i e s i s a g a i n d e m o n s t r a t e d by t h e o b s e r v a t i o n t h a t DOC w o u l d h a v e t o d r o p b e l o w a p p r o x i m a t e l y 0.4 a n d 0 . 2 mgC i H f o r t h e Newport and Neuse r e s p e c t i v e l y , before i n o r g a n i c s p e c i e s of copper would dominate. T h e e f f e c t o f pH o n o r g a n i c c o m p l e x a t i o n i s a l s o e v i d e n t a s DOC i n t h e N e w p o r t w o u l d h a v e t o b e r e d u c e d t o a p p r o x i m a t e l y 0 . 1 mgC i H a t pH 7 . 0 a n d 0 . 0 8 mgC i H a t pH 8 . 0 b e f o r e i n o r g a n i c s p e c i e s w o u l d dominate over organic species. S i m i l a r l y , DOC f o r t h e N e u s e w o u l d h a v e t o b e r e d u c e d t o 0 . 1 mgC J T 1 a t pH 8 . 0 . I n a r e c e n t r e v i e w , D u c e a n d D u u r s m a (23) c o n c l u d e t h a t o u r knowledge of d i s s o l v e d o r g a n i c carbon i n the w o r l d ' s r i v e r s i s limited. From a v a i l a b l e d a t a f o r s u c h r i v e r s as t h e A m a z o n , Hudson, M i s s i s s i p p i , M a c K e n z i e and o t h e r s , d i s s o l v e d o r g a n i c c a r b o n w a s f o u n d t o r a n g e f r o m 2 t o 5 mgC £ _ 1 . These v a l u e s are c o m p a r a b l e t o t h a t f o u n d i n t h e N e u s e ( 3 . 0 mgC £ _ 1 ) , b u t l o w e r t h a n i n t h e N e w p o r t ( 1 5 mgC £ _ 1 ) . Comparing t h i s range of DOC c o n c e n t r a t i o n w i t h the r e s u l t s of o u r work and assuming t h a t the complexing c h a r a c t e r i s t i c s of the organic matter remains a p p r o x i m a t e l y t h e s a m e a m o n g w a t e r s h e d s , we w o u l d e x p e c t c o p p e r c h e m i s t r y i n t h e w o r l d ' s r i v e r s t o be d o m i n a t e d b y c o p p e r - o r g a n i c interactions. This c o n c l u s i o n agrees w i t h t h a t of Mantoura et a l . ( 2 4 ) a s b a s e d on t h e i r c o p p e r s p e c i a t i o n model f o r r i v e r water. C o p p e r s p e c i a t i o n i n t h e N e w p o r t and Neuse R i v e r s was c a l c u l a t e d a s a f u n c t i o n o f c a r b o n a t e a l k a l i n i t y a t s e v e r a l pH v a l u e s f o r p [ C u j o j ] 7 and the n a t u r a l s u i t e o f o r g a n i c l i g a n d s d e t e r m i n e d p r e v i o u s l y ( F i g u r e s 14 a n d 1 5 ) . The a l k a l i n i t y r a n g e was s e l e c t e d to r e f l e c t that expected i n the w o r l d ' s r i v e r s . From the data of L i v i n g s t o n e (25), the a l k a l i n i t y of the w o r l d ' s a v e r a g e r i v e r w a s c a l c u l a t e d t o b e a p p r o x i m a t e l y 0 . 9 6 mM. Stumm and Morgan (9J p r e s e n t data t h a t s u g g e s t c a r b o n a t e a l k a l i n i t i e s in the w o r l d ' s freshwater would f a l l i n the approximate range 0.1 t o 8 mM. _1 I n t h e N e w p o r t R i v e r w i t h a DOC o f 15 mgC l~ , o r g a n i c copper s p e c i e s c l e a r l y dominate over the f u l l range of alkalinit i e s ( p [ A l k ] 5 t o 2 ) a t pH 5 . 9 5 , 7 . 0 a n d 8 . 0 . A t P[CU QT] 7 and P[LJQT] 3 . 7 t o 3 . 9 , t h e o r g a n i c b i n d i n g s i t e c o n c e n t r a t i o n s and s t a b i l i t y constants are s u f f i c i e n t l y large for organic matter to s u c c e s s f u l l y outcompete i n o r g a n i c l i g a n d s . Thus, the copper complexes with i n d i v i d u a l f r a c t i o n s of organic binding s i t e s are e s s e n t i a l l y invariant with a l k a l i n i t y (Figures 14a-c). A s pH i s i n c r e a s e d the r e l a t i v e amounts o f i n o r g a n i c s p e c i e s of c o p p e r d e c l i n e e s p e c i a l l y a t low a l k a l i n i t i e s . As a l k a l i n i t y i n c r e a s e s T

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 12. Chemical speciation model for dissolved copper in the Newport River at 25°C as a function of total organic binding site concentration, (a) In situ pH 5.95, [Alk] = O.OSmM and I = 0.0005M; (b) pH 7.00, [Alk] = 0.05m\l and I = 0.0005M; and (c) pH 8.00, [Alk] = 0.55mM and I = 0.001M.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

00

CHEMICAL MODELING IN AQUEOUS SYSTEMS

174

NEUSE RIVER pH 8.00

NEUSE RIVER pH 6.78

Log DOC (mgC-!" )

Log DOC (mgC'!" )

1

1

-1

0

1

2

0

1

2

3

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

-2

Figure 13. Chemical speciation model for dissolved copper in the Neuse River at 25°C as a function of total organic binding site concentration, (a) In situ pH 6.78, [Alk] = 0.15mM and I = 0.0005M; and (b) pH 8.00, [Alk] = 0.65mM and I = 0.001M.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure

14.

T0T

Chemical speciation model for dissolved copper in the Newport a function of carbonate alkalinity at v[Cu ] 7.

p[Alk]

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

River

at 25°C

as

ι—· οι

CHEMICAL MODELING IN AQUEOUS SYSTEMS

176

pH 6.78

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

NEUSE RIVER

NEUSE RIVER

pH 8.00

p[Alk] Figure

15.

Chemical speciation at 25°C as a function

model for dissolved copper in the Neuse of carbonate alkalinity at tp[Cu t] 7 TO

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

River

8.

the

relative

dominate

importance

the

> 10"3·

14b)

and

5.95

(Figure

[A1K]

species at

For general

the

pattern

8.0.

The

lower

allows

stronger

the

situ

competes

the

In

are

associated these

natural

L3

may

ligands

Newport

River

addition

from

at

a

P[CUTQJ]

organicj)

to

data).

ing

ionic

an

(6.0)

and

an

addition

to

a value

by

less

of

-3.42).

same

of of

than

2.7 0.2

the

value

Since

the

calcium (9j,

the our

that as

pH

omission

of

waters. be

total

value

increasing

electrode

slight

log

at

the

of

(Sunda,

the

calcium

initial same a

the

value

of

to

-3.56

of

increase

to

a

had

mM

value

0.94

by

from

mM c a u s e d

only

0.2

units.

have c o n c e n t r a t i o n s ranges

changing

have

0.26

organic]")

conditions,

of

concentration

should

of

Mg(NÛ3)2 addition

concentration waters

contain­

([Cu2+]/[Cu

experimental by

unpub­

concentration

log

an

M

]/[Cu-

p[Cujoj]

values

of

10"1 2 +

water

same

ionic

KNO3

([Cu

River

effect

the

by

concentration

influence

ions

a

backgound

concentration

within

copper

a value

of

to also

copper

water

units

M KNO3)

magnesium

Increasing

M to

not

be

will

of

only

River

is

binding

affect

has

10~3

the

(0.1

for

ion-selective

Newport

c o n c l u s i o n from

copper

is over

matter The

the

an

stability

usually

measurements

matter.

^

terrestrial the

complexes

organic

will and

of

At

out-

assumption

alkalinity

0.3

mM t o

copper.

c a l c i u m and

to

the

and

Neuse

concentrations

only

of

investigated

a minor

of

influence

models.

general

organic

river ing

majority

dissolved

solved

0.04

of

copper

of

(from

or­

6.78

complexing

would

w h i c h may

(LCu2+]/[Cu-organic])

binding

This

0.2

a

pH

for

same

of

CUCO3 c o m p l e x

sample

from

under

and magnesium

The

only

magnesium of

the

conditional

Newport

caused

(7.7),

and magnesium

calcium on

log

7.7

buffer

units

in

strongly

strength

of

mM d e c r e a s e d

Likewise

with

organic

value

Ca(N03)2

background of

that

using

ionic

same

pH

matter ligands

Increased

filtered

by

strength

increasing values

pH

the

at

the

least

Previous

by

initial

In

the

by

of

increase

2)

alkalinity.

water

copper

and

ini

in­

15a).

strength

that

sample

6.0

lished

10~2-8

but

(24).

in

of

inorganic >

compete

binding

of

At

the

5 to

increased a l k a l i n i t y

ionic

strength

(p[Alk]

assumed of

matter.

binding

by

clear

organic

(Figure

organic

the

of

[ΑΙ Κ]

increased

on

b)

increased concentrations

however,

14c). dominant

follows

to

shown,

the

dominance

binding have

is

speciation

have

cations

organic in

(Figure

ion

to

7.0

with

concentrated

because

with two

8.0

pH

copper

independent

valid

pH

increases

t

a

and

and

site

at

complex

> 10"3.2

cupric

9

alkalinities

o u r m o d e l s , we

strictly

result

most

[Alk]

15a

competition 6.78

for

the

concentration

binding

constants

and

pH

177

alkalinities.

River, all

Water

monocarbonato

free

(Figures

species for

organic

14a)

all

Neuse

ganic

in

the

species

(Figure pH

of

in River

inorganic

situ

organic

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

Copper

SUNDA AND HANSON

and

the

principal

copper

to

range

alkalinity

concentration

copper

the

predicted

occur

of

and

concentration.

in

total

values

variables

s p e c i a t i o n models copper,

is

principally pH,

encountered

disin

world

controlling organic

bind-

composition In

rivers

of

organic

c a s e s where

the

matter, toxicity

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

178 of

dissolved

activity, crease

copper

we w o u l d

with

to organisms predict

decreasing

i s determined

toxicity

dissolved

of

by f r e e

dissolved

organic

matter

copper

cupric to

ion

i n -

and d e c r e a s i n g

pH.

Acknowledgements

Lewis

The a u t h o r s wish to thank L i l l i a n A. Flanagan f o r t h e i r able assistance in the laboratory.

a n d J o Ann M. We t h a n k

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

D a v i d R. C o l b y f o r h e l p f u l d i s c u s s i o n s d u r i n g t h e p r e p a r a t i o n of t h i s paper. The r e s e a r c h on w h i c h t h i s m a n u s c r i p t i s b a s e d was s u p p o r t e d by a n i n t e r a g e n c y a g r e e m e n t b e t w e e n t h e D e p a r t m e n t o f Energy and the N a t i o n a l Marine F i s h e r i e s S e r v i c e . This manuscript i s c o n t r i b u t i o n number 7 8 - 5 3 B , S o u t h e a s t F i s h e r i e s C e n t e r , N a t i o n a l M a r i n e F i s h e r i e s S e r v i c e , NOAA, B e a u f o r t , N . C . , USA. R e f e r e n c e t o t r a d e names d o e s n o t i m p l y e n d o r s e m e n t by t h e N a t i o n a l Marine Fisheries Service,

Abstract The complexation of copper by organic and inorganic ligands was investigated in the Neuse and Newport Rivers, two North Carolina rivers that have widely different concentrations of dissolved organic carbon (3 and 15 mgC l ). Potentiometric titrations with a cupric ion electrode were used to measure complexation of copper by organic and inorganic ligands in river waters and chemically defined solutions. Stability constants for complexation of copper by OH and CO , the dominant inorganic ligands for copper in natural waters, were determined. Constants at infinite dilution for CuOH , CUCO , and Cu(CO )2-2 (10 · , 106.74, and 10 · , respectively) agree favorably with previously published values. However, our constant for Cu(OH) (10 · ) is ~ 2 orders of magnitude lower than values that have been widely used in equilibrium calculations of copper speciation in natural waters. As a result many published equilibrium models for copper appear to have overestimated the importance of the copper dihydroxo species resulting in some cases in an appreciable over-estimation of the total level of copper complexation. Copper was highly complexed by dissolved materials in Neuse and Newport river waters. A marked reduction in complexation following UV-phocoxidation of organic matter indicated that copper was bound predominantly to organic ligands. Binding characteristics of the organic matter in the two rivers was similar and a pronounced increase in binding with pH suggested complexation of copper by protonated weak acids. Scatchard plot analysis of the Cu binding data indicated the presence of at least three organic binding sites in each river whose conditional stability constants increased with increasing pH and decreasing ratio of binding site concentration per gram organic carbon. Copper speciation models were computed for the range of pH, organic matter concentration, alkalinity and total copper typically found in rivers. These -1

-

2-

3

+

6 48

3

3

10 24

11 78

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

8.

SUNDA AND HANSON

Copper in River Water

179

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch008

models predicted that the speciation of copper in the world's rivers will be dominated by complexes with natural organic ligands. Binding of copper by organic ligands should have a marked in­ fluence on biological and geochemical reactivity of copper in rivers, affecting important processes and phenomena such as toxicity and nutritional availability to organisms, adsorption onto surfaces, precipitation, and solid solution. Computational models for the chemical speciation of copper in natural waters that ignore organic complexation may inaccurately describe the chemistry of copper in terrestrial waters and perhaps marine waters as well. Literature Cited 1. 2. 3.

4. 5.

6.

7.

8.

9. 10.

Sunda, W.G., and Guillard, R.R.L. The relationship between cupric ion activity and the toxicity of copper to phyto­ plankton. J. Mar. Res. 34, 511-529 (1976). Anderson, D.M. and Morel, F.M.M. Copper sensitivity of Gonyaulax tamarensis. Limnol. Oceanog. 23, 283-295 (1978). Sunda, W.G., Engel, D.W. and Thuotte, R.M. Effect of chemi­ cal speciation on toxicity of cadmium to grass shrimp, Palaemonetes pugio: importance of free cadmium ion. Environ. Sci. Tech. 12, 409-413 (1978). Andrew, R.W., Biesinger, K.E. and Glass, G.E. Effects of inorganic complexing on the toxicity of copper to Daphnia magna. Water Res. 11, 309-315 (1977). Jackson, G.A. and Morgan, J . J . Trace metal-chelator inter­ actions and phytoplankton growth in seawater media: Theoretical analysis and comparison with reported observations. Limnol. Oceanog. 23, 268-282 (1978). Sunda, W.G., and Lewis, J.M. Effect of complexation of natural organic ligands on the toxicity of copper to a unicellular alga, Monochrysis lutheri. Limnol. Oceanogr. (in press). Bilinski, Η., Huston, R. and Stumm, W. Determination of the stability constants of some hydroxo and carbonato complexes of Pb(II), Cu(II), Cd(II) and Zn(II) in dilute solutions by anodic stripping voltammetry and differential pulse polarography. Anal. Chim. Acta 84, 157-164 (1976). Armstrong, F . A . J . , Williams, P.M. and Strickland, J.D.H. Photo-oxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications. Nature 211, 481-483 (1966). Stumm, W. and Morgan, J . J . "Aquatic Chemistry," 583 p. Wiley Interscience, New York. 1970. Mantoura, R.F.C. and Riley, J.P. The use of gel filtration in the study of metal binding by humic acids and related compounds. Anal. Chim. Acta 78, 193-200 (1975).

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15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25.

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Sillen, L.G. and Martell, A.E. "Stability Constants," 754 p. Special Publication No. 17, The Chemical Society of London, 1964. Sillen, L.G. and Martell, A.E. "Stability Constants," 865 p. Special Publication No. 25, The Chemical Society of London. 1971. Sunda, W.G. "Relationship Between Cupric Ion Activity and the Toxicity of Copper to Phytoplankton." Ph.D. thesis, 168 p. Mass. Inst. Tech., Cambridge. 1975. Paulson, A.J. "Potentiometric Studies of Cupric Hydroxide "Complexation." M.S. thesis, 102 p. Univ. Rh.I., Kingston, 1978. Vuceta, J. and Morgan, J . J . Hydrolysis of Cu(II). Limnol. Oceanog. 22, 742-745 (1977). Sheldon, R.W. Size separation of marine seston by membrane and glass-fiber filters. Limnol. Oceanogr. 17, 494-498 (1972). Draper, N.R. and Smith, H. "Applied Regression Analysis," 407 p. John Wiley, New York. 1966. Schnitzer, M. and Khan, S.U. "Humic Substances in the Environment." 327 p. Marcel Dekker, Inc. New York. 1972. Beck, K.C., Reuter, J.H. and Perdue, E.M. Organic and inorganic geochemistry of some coastal plain rivers of the southeastern United States. Geochim. Cosmochim. Acta 38, 341-364 (1974). Gamble, D.S. Titration curves of fulvic acid: the analytical chemistry of a weak acid polyelectrolyte. Can, J. Chem. 48, 2662-2669 (1970). Wilson, D.E. and Kinney, P.J. Effects of polymeric charge variations on the proton-metal equilibria of humic materials. Limnol Oceanog. 22, 281-289 (1977). Cheam, V. Chelation study of copper (II): fulvic acid system. Can. J. Soil Sci. 53, 377-382 (1973). Duce, R.A. and Duursma, E.K. Inputs of organic matter to the oceans. Mar. Chem. 5, 319-339 (1977). Mantoura, R.F.C., Dickson, A. and Riley, J.P. The complexation of metals with humic materials in natural waters. Est. Coast. Mar. Sci. 6, 387-408 (1978). Livingstone, D.A. Chemical composition of rivers and lakes. U.S. Geological Survey Paper 440G, 64 p.(1963).

Disclaimer: The reviews expressed and/or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the National Oceanic and Atmospheric Administration. RECEIVED

November 16,

1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9 N i c k e l Complexes w i t h Soil Microbial MetabolitesM o b i l i t y and Speciation in Soils R. E. WILDUNG, T. R. GARLAND, and H. DRUCKER

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch009

Battelle, Pacific Northwest Laboratories, Richland, WA

99352

I t i s w e l l - e s t a b l i s h e d that i n o r g a n i c physicochemical mech­ anisms play a predominant r o l e i n c o n t r o l l i n g trace element s o l ­ u b i l i t y i n s o i l s and sediments. However, s o l u b l e species of trace elements which hydrolyze i n the n e u t r a l pH range, or tend to form c a t i o n i c i n o r g a n i c species w i t h intermediate to high i o n i c p o t e n t i a l s are often present i n n a t u r a l waters as organic complexes. Less i s known of the form of trace elements i n s o i l and sediment s o l u t i o n s , but on the b a s i s of d e t a i l e d reviews of i n o r g a n i c and organic processes i n f l u e n c i n g trace element c y c l i n g , i t r e c e n t l y has been concluded that d i s s o l v e d organic matter at the sediment water i n t e r f a c e serves to increase the c o n c e n t r a t i o n of trace elements i n waters by decreasing s o r p t i o n r a t e on the s o l i d phase Q, 2). Trace element organic complexes i n s o i l s may be g e n e r a l l y categorized on the b a s i s of t h e i r s o l u b i l i t y Ο), although con­ s i d e r a b l e overlap l i k e l y occurs. The major c l a s s e s of complexes are 1) r e l a t i v e l y high molecular weight humic substances that have a high a f f i n i t y for metals but are l a r g e l y i n s o l u b l e i n s o i l s , and 2) r e l a t i v e l y low molecular weight nonhumic substances derived l a r g e l y from m i c r o b i a l c e l l s and metabolism that e x h i b i t a range i n s o l u b i l i t i e s i n a s s o c i a t i o n w i t h metals. The o r i g i n and p r o p e r t i e s of organic m a t e r i a l s i n both c a t e g o r i e s i n r e l a ­ t i o n to metal complexation i n s o i l s and sediments were r e c e n t l y overviewed (_1, 4 ) . Of the humic substances, the humâtes ( a l k a l i s o l u b l e , a c i d i n s o l u b l e ) and f u l v a t e s ( a l k a l i and a c i d s o l u b l e ) c o n s t i t u t e up to 90% of the s o i l organic matter (.5)· Both f r a c t i o n s e x h i b i t high charge d e n s i t y due p r i n c i p a l l y to a c i d i c f u n c t i o n a l groups that lead to a strong pH dependent a f f i n i t y f o r cations i n s o l u t i o n and strong a s s o c i a t i o n with s o i l minerals and other organic c o n s t i t u e n t s i n s o i l s (6). Although l a r g e l y i n s o l uble i n s o i l s , the f u l v a t e s , thought to be of lower molecular weight than the humâtes, may, i n p a r t i c u l a r , have p o t e n t i a l f o r formation of s o l u b l e complexes w i t h metals ( 7^ . Nonhumate mater i a l s are a l s o l i k e l y to be of importance i n metal s o l u b i l i z a t i o n i n s o i l . These c o n s i s t of components of l i v i n g c e l l s , t h e i r 0-8412-0479-9/79/47-093-181$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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exudates and the e n t i r e spectrum of degradation products which u l t i m a t e l y serve as b u i l d i n g u n i t s f o r the s o i l humic f r a c t i o n . Because of the g e n e r a l l y high turnover r a t e of microorganisms and r e a d i l y decomposable organic matter i n s o i l s , nonhumate m a t e r i a l s , i n c o n t r a s t to humic substances, are i n h e r e n t l y t r a n s i t o r y and the r e l a t i v e q u a n t i t i e s and composition i n s o i l may be expected to vary w i t h carbon sources and environmental c o n d i t i o n s (8, 9 ) . L i m i t e d i n v e s t i g a t i o n s (_7) have a t t r i b u t e d most of the t i t r a t able a c i d i t y i n s o i l s o l u t i o n to the a l i p h a t i c a c i d s (^0) and amino a c i d s ( 1 1 ) . However, a wide range of organic a c i d s and bases of m i c r o b i a l o r i g i n i n c l u d i n g simple a l i p h a t i c a c i d s , c a r b o x y l i c a c i d s d e r i v e d from monosaccharides, products o f the c i t r i c a c i d c y c l e , and aromatic a c i d s are l i k e l y present i n s o i l s o l u t i o n ( J J ) , 12^). Recent evidence i n d i c a t e s that s o i l microorganisms produce water s o l u b l e ligands w i t h a h i g h a f f i n i t y f o r a range of metals (L3, J A ) and m i c r o b i a l a c t i v i t y i n f l u e n c e s Pu s o l u b i l i t y i n s o i l (15). U n f o r t u n a t e l y , although the presence o f organic complexes of Cu, Zn, and Mn i n s o i l s o l u t i o n has been reported (JL6, 17) few i n v e s t i g a t i o n s , summarized by Mortensen ( 18) and Stevenson and Ardakani (12), have i d e n t i f i e d s p e c i f i c water-soluble ligands capable o f metal complexation i n s o i l , and i n t a c t organometal complexes have not been i s o l a t e d and i d e n t i f i e d . Thus, evidence i n support of the presence of s o l u b l e metal complexes i n s o i l i s l a r g e l y c i r c u m s t a n t i a l and there i s a p a u c i t y of knowledge r e g a r d i n g the nature, behavior, and r o l e of organic ligands i n geochemical c y c l i n g . The development of an understanding of the r o l e of microorganisms i n metal complexation has been l i m i t e d by the complexity of s o i l , sediment and m i c r o b i a l systems and d i f f i c u l t i e s i n the experimental s e p a r a t i o n of the e f f e c t s of m i c r o b i a l processes from physicochemical processes i n s o i l s and sediments. To a i d i n understanding the mechanisms of t r a c e metal comp l e x a t i o n by s o i l microorganisms, an experimental approach was developed which e n t a i l e d 1) i s o l a t i o n of organisms from s o i l on the b a s i s of trace metal t o l e r a n c e and C (carbon) requirements, 2) examination of metal transformations and form on growth of i s o l a t e d organisms i n v i t r o , 3) d e t e r m i n a t i o n of the m o b i l i t y and form of s t a b l e metal complexes i n s o i l , and 4) i d e n t i f i c a t i o n and d e t a i l e d study of metal complexes e x h i b i t i n g h i g h metal a f f i n i t i e s and s o i l s o l u b i l i t y as determined from steps 1) through 3 ) . This p r o t o c o l was a p p l i e d to the i s o l a t i o n and examination of metal complexes formed on growth of s o i l b a c t e r i a and f u n g i exposed to metals (Cd, Cr, N i , Pu, T l ) w i t h a range of p r o p e r t i e s and the chemistry of important N i complexes was examined i n detail.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

WILDUNG E T A L .

Nickel

Complexes

with

Soil

Microbial

Metabolites

183

M a t e r i a l s and Methods

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch009

B a c t e r i a and fungi were i s o l a t e d from s o i l on the b a s i s o f metal tolerance and C source, grouped on the b a s i s of morpho­ l o g i c a l , growth and p h y s i o l o g i c a l parameters and examined f o r a b i l i t y to a l t e r the s o l u b i l i t y and form of metals i n e x o c e l l u l a r s o l u t i o n s and a l t e r metal s o l u b i l i t y i n s o i l . S o l u b l e N i com­ plexes were subsequently c h a r a c t e r i z e d i n d e t a i l . I s o l a t i o n of B a c t e r i a and Fungi from S o i l Using Enrichment Techniques. Two enrichment procedures were employed to i s o l a t e microorganisms from s o i l : a t e r t i a r y enrichment, i n which organ­ isms were i s o l a t e d from s o i l a f t e r i n c u b a t i o n to l o g phase i n the presence of added metal, and a two-phase enrichment, i n which organisms were i s o l a t e d from the unincubated s o i l and grown on a number of d i f f e r e n t C sources i n the presence of metals. In the t e r t i a r y enrichment procedure, s o i l ( R i t z v i l l e s i l t loam) was amended w i t h s t a r c h (1.0%), NH4NO3 (0.5%), and Cd,

Cr, N i , T l (1, 10, and 100 μg/g) or Pu (0.05, 5 and 10

μα/g).

The s o i l , w i t h amendments, was brought to 22% moisture and i n c u ­ bated (2£PC) under aerobic c o n d i t i o n s (continuous flow of CO2 free a i r ) . At mid-log growth phase, as determined by CO2 e v o l u ­ t i o n r a t e , an a l i q u o t (1 ml) of a 1:10 incubated s o i l : w a t e r s l u r r y was i n o c u l a t e d i n t o an enriched s o i l e x t r a c t medium c o n t a i n i n g glucose (1%), NH4NO3 (0.5%), K HP0 (0.05%), s o i l e x t r a c t (30%) and Cd, Cr, N i , Pu, or T l added i n s o l u b l e form, to achieve Cd, Cr, Ni and T l c o n c e n t r a t i o n s o f 1, 10, and 100 μg/ml and Pu c o n c e n t r a t i o n s of 0.05, 5 and 10 μ0ΐ/πι1. Con­ t r o l s were i d e n t i c a l except metals were not added. The pH was adjusted to 7.0 f o r aerobic b a c t e r i a and 6.0 f o r f u n g i . S t r e p t o ­ mycin s u l f a t e (0.4%) was added to the fungal enrichment to p r e ­ vent growth of b a c t e r i a . The i n o c u l a t e d c u l t u r e s were incubated (28°C)with shaking (150 rpm) u n t i l maximum c e l l d e n s i t y was obtained and secondary enrichments were i n i t i a t e d by t r a n s f e r r i n g an inoculum of each o f the primary enrichments to f r e s h , enriched s o i l e x t r a c t medium c o n t a i n i n g the metals at the same concentra­ t i o n s . The t e r t i a r y enrichment was then conducted i n a manner s i m i l a r to the secondary enrichment. Pure c u l t u r e s of organisms were i s o l a t e d at the end of each enrichment u s i n g successive pour p l a t e and s t r e a k i n g u n t i l c u l t u r e s d i f f e r i n g i n colony morphology were r e s o l v e d . The pure c u l t u r e s were maintained i n stock on agar s l a n t s i n the presence and absence o f the metals a t the concentra­ t i o n s used i n i s o l a t i o n . The stock c u l t u r e s were passed to f r e s h s l a n t s monthly i n order to assure v i a b i l i t y . A two-phase enrichment procedure was u t i l i z e d to s e l e c t s o i l organisms on the b a s i s of a b i l i t y to metabolize c l a s s e s o f organic compounds i n the f i r s t phase; and on a b i l i t y to grow i n the presence of metals i n the second phase. In the f i r s t phase, a l i q u o t s of a standard m i n e r a l base medium (Table I , glucose used only i n c o n t r o l ) were s e p a r a t e l y amended w i t h 1) aromatic a c i d s , 2

4

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184

C H E M I C A L MODELING IN AQUEOUS SYSTEMS

Table I .

Solution Designation A

Composition o f standard m i n e r a l base medium employed i n m i c r o b i a l s t u d i e s . Volume per l i t e r o f Medium (ml) 100

Composition 685 ml o f KH PO (0.2M), 2

315

2

diluted 10

4

ml o f Na HPO (0.2M)

20.0 g MgS0

4

t o 2000 ml · 7H 0,

4

1.325 g C a C l

2

' 2H 0

2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch009

d i s s o l v e d i n 1000 ml C

1

50 mg ZnSO^ ' 7H 0, 2

50 mg MnSO^ ' H 0, 2

12 mg CuSO^ ' 5H 0, 2

15.9

mg C o ( N 0 ) 3

19.0 mg N a B 0 2

235

4

mg Na Mo0 2

4

?

2

* 6H 0, 2

" 10H 0, 2

* 2H 0, 2

10 ml Fe-EDTA s o l u t i o n (17.9 g Na EDTA-2H 0 and 3.23 g KOH i n 2

186

2

ml added t o 13.7 g F e S 0 « 7 H 0 4

i n 364 ml H o. 2

2

Bubbled f i l t e r e d

a i r through s o l u t i o n o v e r n i g h t and a d j u s t e d pH t o 5.0 w i t h 8M HN0 ) 3

d i s s o l v e d i n 100 ml D

10

10 g NH C1 d i s s o l v e d i n 100 ml 4

Ε ^

100

15 g glucose d i s s o l v e d i n 1000 ml

F

100

2.5 g yeast d i s s o l v e d i n 1000 ml

1/ Glucose was not used i n s t u d i e s o f the e f f e c t

of C sources.

i n c l u d i n g benzoate (0.5%), £ hydroxy benzoate (0.5%), m hydroxy benzoate (0.5%), and tryptophan (0.11%); 2) organic a c i d s , i n c l u d i n g succinate (0.5%), malate (0.5%), l a c t a t e (0.5%), and acetate (0.5%); 3) sugars, i n c l u d i n g glucose (0.5%), sucrose (0.5%), and f r u c t o s e ( 0 . 5 % ) ; and 4) an o r g a n i c a l l y r i c h medium c o n t a i n i n g yeast e x t r a c t and peptone. A l i q u o t s o f these media were i n o c u l a t e d w i t h a s o i l (1 g) and incubated w i t h shaking (200 rpm) f o r 120 hours. From each o f these primary enrichments, a secondary enrichment was performed by i n o c u l a t i n g a l i q u o t s o f s t e r i l e medium c o n t a i n i n g the same C sources and Cd, C r , * N i , T l ( 1 , 10, and 100 μg/ml) o r Pu (0.05, 5 and 10 μΟί/τηΙ). The inocu­ l a t e d m e t a l - c o n t a i n i n g medium was incubated (28°C) w i t h shaking

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

9.

wiLDUNG ET AL.

Nickel Complexes with Soil Microbial Metabolites

185

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch009

(200 rpm) f o r 48 h r . Pure c u l t u r e s were i s o l a t e d from the media e x h i b i t i n g m i c r o b i a l growth ( t u r b i d i t y ) at the h i g h e s t metal c o n c e n t r a t i o n using pour p l a t e and successive s t r e a k i n g t e c h ­ niques. M o r p h o l o g i c a l l y d i f f e r e n t b a c t e r i a l and fungal c u l t u r e s were placed i n t o stock and maintained as p r e v i o u s l y d e s c r i b e d . B a c t e r i a l and fungal enrichments d i f f e r e d only i n that b a c t e r i a l enrichments were conducted w i t h media adjusted to pH 7.0 whereas the fungal enrichments were conducted w i t h media adjusted to pH 6 and contained streptomycin s u l f a t e (0.4%). Morphological and P h y s i o l o g i c a l C h a r a c t e r i s t i c s of Microorganisms I s o l a t e d from S o i l . The 239 b a c t e r i a l i s o l a t e s obtained i n pure c u l t u r e from the two enrichment procedures were c l a s s i f i e d on the b a s i s of morphological, growth and p h y s i o l o g ­ i c a l parameters. These were measured using standard methods (19). A l i q u o t s (2 ml) of stock c u l t u r e s were t r a n s f e r r e d to a standard m i n e r a l base medium (Table I ) , incubated (28°C) on a r o t a r y shaker (150 rpm) f o r 24 hr. M o t i l i t y (wet mount) and gram r e a c t i o n were measured. The c u l t u r e was then used to i n o c u ­ l a t e an agar p l a t e of standard mineral base medium which was c u l t u r e d f o r an a d d i t i o n a l 24 hr and used f o r the c a t a l a s e and oxidase t e s t s . For the gram p o s i t i v e and spore-forming b a c i l l i , a d d i t i o n a l t e s t s were conducted on 24-48 hr c u l t u r e s (standard mineral base agar). These included a c e t o i n production by the Voges-Proskauer t e s t ; a c i d production from the mixed sugars, arabinose, mannose, and x y l o s e ; a c i d production from glucose; and s t a r c h h y d r o l y s i s . Spore l o c a t i o n and morphology were determined i n o l d e r c u l t u r e s . Pigment production on s t a r c h agar was a l s o used to subdivide c e r t a i n of the spore-forming b a c i l l i i n t o major biotypes of B a c i l l u s mycoides. Twenty s t r a i n s considered r e p r e ­ s e n t a t i v e of the c u l t u r e s i n the c o l l e c t i o n on the b a s i s of these t e s t s and chemical c h a r a c t e r i s t i c s of e x c e l l u l a r products (described below) were examined to determine i f e x o c e l l u l a r com­ p l e x a t i o n of N i a l t e r e d N i m o b i l i t y i n s o i l r e l a t i v e to i n o r g a n i c forms. A t o t a l of 250 fungal i s o l a t e s were obtained i n pure c u l t u r e . Colony morphologies of many of the i s o l a t e s were s i m i l a r . On the b a s i s of colony morphology on standard m i n e r a l base medium, organ­ isms were separated i n t o 59 types. These types were r e t a i n e d i n stock to examine metal r e s i s t a n c e c h a r a c t e r i s t i c s and t h e i r a b i l i t y to e x o c e l l u l a r l y modify N i form and s o l u b i l i t y i n s o i l . Further c l a s s i f i c a t i o n to the species l e v e l i s underway. M o d i f i c a t i o n of N i c k e l Form by S o i l M i c r o b i a l I s o l a t e s . To determine the a b i l i t y of the m i c r o b i a l i s o l a t e s to modify the chemical form of N i , standard m i n e r a l base medium (15 l i t e r s ) was prepared (Table I ) and separated i n t o a l i q u o t s (500 m l ) . The a l i q u o t s were frozen r a p i d l y on dry i c e and stored (-26 C) u n t i l used. Immediately p r i o r to use, the a l i q u o t s were r a p i d l y thawed at 37°C i n a water bath, f i l t e r s t e r i l i z e d (0.2 μ) and s e p a r a t e l y P

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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amended w i t h unlabeled N i (as N 1 C I 2 ) and i t s r a d i o i s o t o p e ( 6 3 . 0.3 μ(Σΐ/πι1). In order to develop a b a c t e r i a l inoculum for t h i s medium, stock c u l t u r e s were grown to l o g phase i n s t a n ­ dard m i n e r a l base medium (10 ml) without metal. An a l i q u o t (0.25 ml) of t h i s c u l t u r e was used as an inoculum f o r the growth medium (10 ml) c o n t a i n i n g metal, and the c u l t u r e s were incubated (28°C)with shaking (150 rpm) f o r 48 h r . The fungal inoculum was developed s i m i l a r l y , except f u n g i from stock were streaked on standard m i n e r a l base agar from the stock s o l u t i o n and incubated ( 2 8 C l u n t i l s p o r u l a t i o n was evident. An a l i q u o t (10 ml) of s t e r ­ i l e b u f f e r (Table I , S o l u t i o n A) was added to the surface and the spore suspension p i p e t t e d a s e p t i c a l l y i n t o tubes. An inoculum (0.5 ml) of t h i s spore suspension was added to the growth medium c o n t a i n i n g N i (10 μg/ml). The organisms were incubated (28 C), w i t h shaking (150 rpm) f o r 72 h r . At the end of growth i n the N i - c o n t a i n i n g medium, the c e l l s were separated by f i l t r a t i o n (0.4 μ Nuclepore) and dry weights were determined. An a l i q u o t of the f i l t r a t e (

CHEMICAL MODELING ΓΝ AQUEOUS SYSTEMS

232

The m o l e c u l a r - d i f f u s i o n c o e f f i c i e n t f o r calcium i n d i l u t e aqueous s o l u t i o n s of calcium c h l o r i d e , D ^ i s about 1 χ 10" cm /sec (18,p.700). The molecular-diffusion c o e f f i c i e n t i n the s o i l , D , c a n be estimated by the r e l a t i o n D ~ ^ / T , where Τ i s the t o r t u o s i t y , assumed here t o be about 1.4. The estimated m o l e c u l a r - d i f f u s i o n c o e f f i c i e n t of calcium f o r the s o i l s employed i n t h i s study i s , t h e r e f o r e , about 3 χ 1(Γ cm /sec. Comparing t h i s value w i t h the d i s p e r s i o n c o e f f i c i e n t s i n Table I I , one finds that a t the lowest flow r a t e studied f o r each s o i l the c o e f f i ­ c i e n t s are s i m i l a r , but a t higher f l u x e s they d i f f e r by orders of magnitude. Thus, when the hydrodynamic d i s p e r s i o n c o e f f i c i e n t i s n e a r l y the same as the estimated m o l e c u l a r - d i f f u s i o n c o e f f i c i e n t i n the s o i l , the e q u i l i b r i u m assumption a p p l i e s . When the d i s ­ persion c o e f f i c i e n t i s s i g n i f i c a n t l y l a r g e r , the l o c a l e q u i l i b r i u m assumption does not apply and a k i n e t i c s - b a s e d model presumably i s indicated. In order to f u r t h e r s u b s t a n t i a t e t h i s c o n c l u s i o n , i t i s of i n t e r e s t to compare i t w i t h the p r e d i c t i o n obtained from a simple t h e o r e t i c a l model. Glueckauf's well-known t r a n s p o r t model (19, p. 449-453), supplemented by the more modern concept of hydrodynamic d i s p e r s i o n , i s w e l l s u i t e d f o r t h i s purpose. The model simulates d i s p e r s i o n - a f f e c t e d s o l u t e t r a n s p o r t w i t h i o n exchange for which d i f f u s i o n processes are r a t e l i m i t i n g . I n h i s develop­ ment, Glueckauf assumes: 1) exchange takes place i n porous s p e r i c a l p a r t i c l e s w i t h radius r and p o r o s i t y θ ; 2) w i t h i n these p a r t i c l e s , water and s o l u t e movement occur only by molecu­ l a r d i f f u s i o n ; 3) the r e a c t i o n rate i s determined by the rates of f i l m and i n t r a p a r t i c l e d i f f u s i o n , which are p r o p o r t i o n a l t o the d i f f e r e n c e s between the t h e o r e t i c a l e q u i l i b r i u m and a c t u a l concentrations of s o l u t i o n and adsorbed s o l u t e s , r e s p e c t i v e l y ; and 4) the r e l e v a n t e q u i l i b r i u m r e l a t i o n s are described by Equation 4. Using the modified Glueckauf model described above, and em­ p l o y i n g h i s s i m p l i f y i n g mathematical assumptions (the correctness of which has been confirmed by the authors w i t h the a i d o f numer­ i c a l methods), one obtains 5

m

5

2

D

mg

m

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch011

6

2

ρ

which i s s i m i l a r to Equation 6 w i t h D replaced by the e f f e c t i v e d i s p e r s i o n c o e f f i c i e n t D, which i s given by: D = D + F + F Ρ

1

f

(8)

when Fp and F f describe the c o n t r i b u t i o n s of i n t r a p a r t i c l e and f i l m d i f f u s i o n , r e s p e c t i v e l y , and are expressed by

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

11.

Local

J A M E S AND RUBIN

Equilibrium

Ο.ΙΟρλο^Γ F

233

Assumption

2

ρ ~ D ,θ (6C +ρλ) ρ t

2

(9) 0.133 ρ λ g r.2 F, = f D ^(9C +pX) (l+70qr) 2

2

2

2

m

t

As q decreases, the terms F and F f become small compared to D, and Equation 7 becomes more l i k e Equation 6. This i n d i c a t e s that the l o c a l e q u i l i b r i u m assumption i s a p p l i c a b l e when q i s s u f f i ­ c i e n t l y small. Using the r e l a t i o n s h i p Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch011

p

D = D + ccq ms

(10)

where α i s an e m p i r i c a l constant (jL) , one f i n d s that when the d i s p e r s i o n c o e f f i c i e n t i s n e a r l y equal t o the m o l e c u l a r - d i f f u s i o n c o e f f i c i e n t i n the s o i l , aq i s much smaller than D , g i v i n g mg

D ms

q « M

α

D «Θ πι^υ =— — αΤ

/1 ι \ (11)

I n e q u a l i t y 11 was s u b s t i t u t e d i n t o Equation 8, together w i t h reasonable values o f other parameters and 0.1 cm < α < 0.2 cm as α was found to be i n t h i s study. This leads t o the c o n c l u s i o n t h a t , f o r systems i n which r i s l e s s than 0.1 cm, the l o c a l e q u i l ­ ibrium assumption i s a p p l i c a b l e ( i . e . , q i s s u f f i c i e n t l y small) when D i s n e a r l y equal to D as observed i n the experiments. I n s o i l s i n which the exchanging p a r t i c l e s are not s p h e r i c a l , r would represent approximately the mean d i f f u s i o n path w i t h i n c l a y aggregates or w i t h i n c l a y coatings on coarse p a r t i c l e s . For s o i l s without appreciable c l a y aggregation, the e x p e r i ­ mental r e s u l t s and t h e o r e t i c a l a n a l y s i s described here i n d i c a t e that when d i f f u s i o n i s r a t e - l i m i t i n g , the r a t i o of the hydrody­ namic d i s p e r s i o n c o e f f i c i e n t t o the estimated s o i l s e l f - d i f f u s i o n c o e f f i c i e n t may be a u s e f u l i n d i c a t o r of the a p p l i c a b i l i t y of the l o c a l e q u i l i b r i u m assumption. For r e a c t i n g s o l u t e s i n l a b ­ oratory columns such as those used i n t h i s study, systems w i t h r a t i o s near u n i t y can be modeled using e q u i l i b r i u m chemistry. The experimental r e s u l t s i n d i c a t e that when the r a t i o i s consid­ e r a b l y l a r g e r than one, another r e l a t i o n s h i p , presumably one i n v o l v i n g k i n e t i c s , must be used. m g

Abstract Miscible displacement of calcium-chloride solutions through water-saturated laboratory soil columns was studied for a wide range of constant water-flow rates. Calcium- and chloride-effluent

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch011

234

CHEMICAL MODELING IN AQUEOUS SYSTEMS

histories were obtained. Calcium self-exchange was the only reaction considered. Calcium-effluent histories were compared with predictions from a one-dimensional solute-transport model, assuming local chemical equilibrium. Good agreement between the predictions and the data was obtained for the slowest flow rate studied, but not for the higher fluxes. Thus, the local equilib­ rium assumption applies when the ratio of the hydrodynamic dispersion coefficient to the estimated molecular-diffusion coefficient is near unity. This conclusion is further substan­ tiated by comparison with the results of a theoretical analysis using the relatively simple transport model for solutes affected by ion exchange that has been developed by Glueckauf (19). It is suggested that the data obtained for the higher water fluxes that yield a coefficient ratio much greater than unity cannot be described by assuming local equilibrium, but must be modeled using another relationship, presumably one involving kinetics. Literature Cited 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11.

Perkins, Τ. Κ., and Johnston, O. C . , A review of diffusion and dispersion in porous media, Soc. Petrol. Eng. J., 228 (March), 70-84 (1963). Pfannkuch, H. O., Contribution A l'Etude des Deplacement de Fluides Miscibles dans un Milieu Poreux, Paris Instit. Franc. due Petr. Rev., 18 (2), 215-270 (1963). Nunge, R. J., and G i l l , W. Ν., Mechanisms affecting disper­ sion and miscible displacement, Indust. Eng. Chem., 61 (9), 33-49 (1969). Bear, J., "Dynamics of Fluids in Porous Media", 764 p., American Elsevier Publishing Company, Inc., New York, 1972. Cameron, D. R., and Klute, Α., Convective-dispersive solute transport with a combined equilibrium and kinetic adsorption model, Water Resour. Res., 13 (1), 183-188 (1977). Skopp, J., and Warrick, A. W., A two-phase model for the miscible displacement of reactive solutes in soils, Soil Sci. Soc. Am. Proc., 38 (4), 545-550 (1974). Rubin, J., and James, R. V., Dispersion-affected transport of reacting solutes in saturated porous media: Galerkin method applied to equilibrium-controlled exchange in unidirectional steady water flow, Water Resour. Res., 9 (5), 1332-1336 (1973). Reiniger, P., "Movement and Exchange of Sodium and Calcium in Calcareous and Gypseous Soils", Ph.D. Thesis, Hebrew Univ., Jerusalem, 1970. Lai, S.-H., and Jurinak, J . J., Numerical approximation of cation exchange in miscible displacement through soil columns, Soil Sci. Soc. Am. Proc., 35 (6), 894-899 (1971). Peterson, Ε. Ε . , "Chemical Reaction Analysis", 276 p., Prentice-Hall, Inc., New Jersey, 1965. Cole, R. C., Gardner, R. Α., Koehler, L. F., Anderson, A. C., Bartholomew, O. F . , and Retzer, J . L., Soil Survey of the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

11.

12. 13.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch011

14.

15. 16. 17. 18. 19.

JAMES AND RUBIN

Local Equilibrium Assumption

235

Bakersfield area, California, USDA, Bureau of Plant Industry, Soils, and Agricultural Engineering, Series 1937, Rept. no. 12, 1945. Carpenter, E. J., and Cosby, S. W., Soil survey of Contra Costa County, California, USDA, Bureau of Chemistry and Soils, Series 1933, Rept. no. 26, 1939. Black, C. Α., "Soil-Plant Relationships", 2nd ed., 792 p., John Wiley and Sons, Inc., New York, 1968. Brown, E., Skougstad, M. W., and Fishman, M. J., Methods for collection and analysis of water samples for dissolved minerals and gases in "Techniques of Water-Resources Invest­ igations of the United States Geological Survey", Book 5, Chapter A1, U.S. GPO, Washington, D.C., 1970. Black, C. Α., ed., "Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties", Chapter 60, American Society of Agronomy, Inc., Madison, Wisconsin, 1965. James, R. V., and Rubin, J., Accounting for apparatus-induced dispersion in analyses of miscible displacement experiments, Water Resour. Res., 8 (3), 717-721 (1972). Ripple, C. D., James, R. V., and Rubin, J., Radial particle­ -size segregation during packing of particulates into cylind­ rical containers, Powder Tech., 8, 165-175 (1973). Harned, H. S., and Owen, Β. Β., "The Physical Chemistry of Electrolytic Solutions", 803 p., Reinhold Book Corporation, New York, 1958. Helfferich, F . , "Ion Exchange", 624 p., McGraw-Hill, New York, 1962.

Disclaimer: The reviews expressed and/ or the products mentioned in this article repre­ sent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

12 Adsorption of Lead from Solution on the Quartz- and Feldspar-Containing

Silt Fraction of a Natural

Streambed Sediment DAVID W. BROWN

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

U.S. Geological Survey, Menlo Park, CA 94025 The extensive use of lead antiknock a d d i t i v e s i n gasoline has made lead perhaps the most widely d i s t r i b u t e d t o x i c heavy metal i n the urban environment and has g r e a t l y increased i t s a v a i l a b i l i t y f o r s o l u t i o n i n n a t u r a l waters. I t i s important f o r t h i s reason to know whether i t s i n t r o d u c t i o n i n t o surface and ground waters by r a i n f a l l and runoff w i l l make i t a v a i l a b l e f o r s o l u t i o n or whether chemical processes w i l l place a safe upper l i m i t on i t s s o l u b i l i t y . A model f o r p r e d i c t i n g the concentration o f lead i n surface or ground water must a c c u r a t e l y r e f l e c t the p h y s i c a l and chemical processes that a f f e c t the d i s t r i b u t i o n of lead between the s o l i d and d i s s o l v e d phases. In a n a t u r a l water system the chemical processes that can g r e a t l y vary the concentration of lead are those of complex formation, s o l u t i o n - p r e c i p i t a t i o n , and adsorption. This paper presents a comprehensive method f o r e v a l u a t i n g the surface c h a r a c t e r i s t i c s needed to p r e d i c t the adsorption behavior of lead on the s i l t f r a c t i o n of sediments from a n a t u r a l streambed. Also presented i s a q u a n t i t a t i v e d e s c r i p t i o n of lead adsorption on such m a t e r i a l p r i m a r i l y as a f u n c t i o n of pH between 4 and 8, the range most commonly encountered i n n a t u r a l waters. The p r e d i c t e d pH-dependence i s then compared with e x p e r i mental r e s u l t s at two d i f f e r e n t t o t a l lead concentrations. S p e c i a l emphasis i s placed here on the e f f e c t o f the presence of adsorbed lead and other c a t i o n s on the pH-dependency of t h i s ads o r p t i o n , p a r t i c u l a r l y that which r e s u l t s from the e f f e c t of the adsorbed cations on e l e c t r o s t a t i c p o t e n t i a l and charge d e n s i t y at the s o l u t i o n - s u r f a c e i n t e r f a c e . Chemical Processes A f f e c t i n g Aqueous Lead Concentration Complex Ion Formation. Formation of complexes of lead with the various anions such as c h l o r i d e , f l u o r i d e , carbonate, b i c a r bonate, and hydroxide increases the concentration of lead i n n a t u r a l waters by preventing lead from taking part i n other chemic a l r e a c t i o n s , p r i m a r i l y adsorption, that would lower i t s 0-8412-0479-9/79/47-093-237$06.00/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

238

CHEMICAL MODELING IN AQUEOUS SYSTEMS

c o n c e n t r a t i o n . The important complexes i n most s u r f a c e and ground-water systems are those of hydroxide and carbonate. The known monomeric hydroxy complexes are PbOH+, Pb(OH)? , Pb(OH) and P b ( 0 H ) " . The most important of these i s PbGH*, as the l a t t e r three become s i g n i f i c a n t only above the pH of most n a t u r a l waters, and because Pb0H , a c a t i o n , i s much more r e a d i l y adsorbed to n e g a t i v e l y charged surfaces than are uncharged Pb(0H)2° and the anions Pb(OH) "" and PbCOH)^ "". Thermodynamic data i n d i c a t e t h a t the P b species predominates to about pH 7, f o l l o w e d by PbOH to about pH 10, Pb(0H) ° between pH 10 and 11, then by Pb(0H) "" and P b i O H ) ^ " a t higher pH. Since our i n t e r e s t i s i n the pH range of commonly encountered n a t u r a l waters, we use only the thermodynamic data a p p l i c a b l e to the P b - P b 0 H r e a c t i o n . L i n d (1) gives the formation constant f o r Pb0H ( f o r which the e q u i l i b r i u m e x p r e s s i o n i s [PbOH+HH+UPb "]" ) as 1 0 " · . Hem and Durum (2) g i v e the l e a d hydroxide s o l u b i l i t y product *K ( w r i t t e n [ P b ] [ H * ] " ) as 10 · . 0

3

?

2

4

+

2

3

2 +

+

2

3

2

2+

+

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

+

2-1

1

7

2 3

2 +

2

0

8

1 6

A d s o r p t i o n and C a t i o n Exchange. The r o l e of a d s o r p t i o n of l e a d c a t i o n s onto streambed or a q u i f e r m a t e r i a l must be considered i n any model used to p r e d i c t the aqueous l e a d c o n c e n t r a t i o n i n n a t u r a l waters, p a r t i c u l a r l y i n ground-water systems. Hem (3) and Hem and Durum (2) note that the c o n c e n t r a t i o n s of l e a d i n s u r f a c e and ground waters i n the U n i t e d States are f a r below those which would be p r e d i c t e d from e q u i l i b r i u m w i t h s o l i d m i n e r a l s p e c i e s . I t therefore-appears t h a t p r e c i p i t a t i o n - d i s s o l u t i o n e q u i l i b r i a are not the chemical processes c o n t r o l l i n g l e a d concent r a t i o n s i n n a t u r a l waters. The l a r g e s u r f a c e areas and c a t i o n exchange c a p a c i t i e s of c l a y s and s i l t s provide a s u f f i c i e n t l y l a r g e s i n k f o r l e a d ions such t h a t any accurate e q u i l i b r i u m model used f o r p r e d i c t i n g aqueous l e a d i n n a t u r a l systems would have to consider t h i s e f f e c t . Most n a t u r a l waters come i n t o c o n t a c t w i t h a l a r g e s u r f a c e area of c l a y , s i l t , and sand. At moderate values of pH, the s i l i c a t e s u r f a c e s c o n t a i n a number of n e g a t i v e l y charged adsorpt i o n s i t e s to which c a t i o n s may be t i g h t l y bound; the upper l i m i t to the number of c a t i o n s that may be h e l d to the s u r f a c e a t a given time i s c a l l e d the c a t i o n exchange c a p a c i t y (CEC). Despite the use of t h i s term, we should r e a l i z e t h a t on one hand c a t i o n exchange and a d s o r p t i o n of c a t i o n s could be considered to be two d i f f e r e n t processes, w h i l e on the other hand they may w e l l be two d i f f e r e n t attempts a t e x p l a n a t i o n of the same phenomena. What i s commonly r e f e r r e d to as c a t i o n exchange i s a c t u a l l y i n d i s t i n g u i s h able from simultaneous a d s o r p t i o n of one i o n and d e s o r p t i o n of the same number of e q u i v a l e n t s of another. (This o f t e n happens when a l l a d s o r p t i o n s i t e s are i n i t i a l l y f i l l e d , and f u r t h e r a d s o r p t i o n of other c a t i o n s can occur only i f some of the c u r r e n t l y adsorbed ions are desorbed.) The main d i f f e r e n c e between the c a t i o n exchange and e l e c t r i c a l double-layer a d s o r p t i o n models l i e s mainly i n t h a t the c a t i o n exchange model i s a mass-action approach to the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

12.

BROWN

Natural

Streambed

Sediment

239

competition among d i f f e r e n t c a t i o n s f o r a f i x e d number of nega­ t i v e l y charged s i t e s ; t h i s r e s u l t s i n the i m p o s i t i o n of the requirement of constant charge of s u r f a c e c a t i o n s . The e l e c t r i c a l double-layer approach does not impose t h i s requirement because the magnitude of the negative s u r f a c e charge i s v a r i a b l e and i n f l u e n c e d by s p e c i f i c a d s o r p t i o n of p o t e n t i a l - d e t e r m i n i n g H and OH"" i o n s . I t i s f o r t h i s reason that a v e r s i o n of the double-layer model of c a t i o n a d s o r p t i o n s h a l l be used here i n attempting to r e c o n c i l e ρΗ-dependent a d s o r p t i o n of lead on a f r a c t i o n of a n a t u r a l streambed sediment. The major f e a t u r e of the James and Healy (4) model i s the d e s c r i p t i o n of a d s o r p t i o n by a Langmuir isotherm whose e q u i l i b r i u m constant i s based upon a f r e e energy of a d s o r p t i o n made up of what are r e f e r r e d to as coulombic (AG? ^), s o l v a t i o n ( A G ? ) , and chemical (AG2000 74-2000 4-74

s Ç l j ? f° i be 1.06 Χ 10"" eq m" , K ads i equal to 285, or 10 . given the p H p of 4.3, KQH * by comparison c a l c u l a t e s as 10 ' , more than f i v e orders of magnitude higher than K n . The value of the d i e l e c t r i c constant of the adsorbing s o l i d i s very important i n determining the magnitude of the change i n f r e e energy of s o l v a t i o n undergone by the adsorbed i o n . This term i s used i n both the James and Healy (4) and Levine (5) expressions f o r the s o l v a t i o n free-energy term. As a r e s u l t , the d i e l e c t r i c constant of the s o l i d i s extremely important i n determining the r e l a t i v e values of A G p + and AGpbOH > and hence the r e l a t i v e p r o p o r t i o n s of P b and PbOtF" a d s o r p t i o n . The cyclohexane-adsorbent mixture gave a capacitance c o r r e s ­ ponding to an o v e r a l l d i e l e c t r i c constant of 3.267. Based on the adsorbent volume f r a c t i o n , the Looyenga (10) equation y i e l d e d a value of 16.4 f o r the d i e l e c t r i c constant of the adsorbent. In the s t r i c t sense, the d i e l e c t r i c constant f o r the b u l k p a r t i c l e s as measured here may be i n c o r r e c t as a p p l i e d to a hydrated s u r f a c e environment which has undergone s u b s t a n t i a l changes i n e l e c t r i c a l p r o p e r t i e s upon a d d i t i o n of water. Levine (5), f o r example, uses a value of 10 f o r the d i e l e c t r i c constant of a s o l v a t e d quartz s u r f a c e when the a c t u a l c a p a c i t o r p l a t e value f o r the s o l i d i s only 4.3. The e f f e c t of a v a r i a t i o n i n the value f o r the s o l i d d i e l e c t r i c constant on the pH-dependence of adsorp­ t i o n , however, i s minimal, r e q u i r i n g only a very s m a l l change i n the value of the f i t t i n g parameter A G Ç i n order to produce the same r e s u l t s . For t h i s reason, our use of the measured value of 16.4 f o r the s o l i d should s u f f i c e , producing l i t t l e e r r o r as compared to e s t i m a t i n g a value f o r the s o l v a t e d s o l i d . K

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

a d s

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A d s o r p t i o n of Lead Experimental. In order to study the e f f e c t of pH and t o t a l lead c o n c e n t r a t i o n on a d s o r p t i o n , s l u r r i e s were prepared w i t h pH between 2 and 6 and c o n t a i n i n g t o t a l lead concentrations (ZPb) of 1.0 Χ 10""* and 5.0 X 10 M. These c o n c e n t r a t i o n s , w h i l e l a r g e i n comparison to those normally found i n n a t u r a l waters, were used mainly f o r two reasons. F i r s t , a d s o r p t i o n s i t e s on sediments i n n a t u r a l waters con­ t a i n i n g moderate to l a r g e concentrations of c a l c i u m , magnesium, and potassium w i l l probably be moderately or near-saturated w i t h these adsorbed s p e c i e s , and t h i s l a r g e s u r f a c e c o n c e n t r a t i o n of 1

_if

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

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»l ι ι 1 —I 0 1 2 3 4 NET ADSORPTION DENSITY Γ5„--Γ^ EQUIVALENTS PER SQUARE METER χ ΙΟ 6

Figure

4.

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of linear portion for log K Ns 10

of 0.001M ionic strength determination

ads

H

titration

data

PH Figure

5.

Comparison of lead adsorption data (points and solid connecting with best-fitting James-Healy model predictions (broken lines)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

lines)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

250

CHEMICAL MODELING IN AQUEOUS SYSTEMS

c a t i o n s near one plane may exert a p e r t u r b i n g e f f e c t on the nature of a d s o r p t i o n of other c a t i o n s , lead i n c l u d e d . As an a l t e r n a t i v e to a r i g o r o u s determination of t h i s e f f e c t by a d s o r p t i o n s t u d i e s f o r each of these i o n s , we i n s t e a d use l a r g e t o t a l lead concentra­ t i o n s ; the a d s o r p t i o n of t h i s q u a n t i t y of lead should give r i s e to an a d s o r p t i o n d e n s i t y of charged ions of s u f f i c i e n t magnitude to study such an e f f e c t . Second, a t o t a l lead c o n c e n t r a t i o n , approaching the maximum that can be accommodated by the adsorbent, w i l l cause the r a t e of i n c r e a s e i n a d s o r p t i o n w i t h pH to decrease. The a d s o r p t i o n w i l l r i s e more g r a d u a l l y as pH i s i n c r e a s e d , r a t h e r than suddenly s h i f t i n g from a very low to a very high a d s o r p t i o n ; the l a t t e r i s c h a r a c t e r i s t i c of many s t u d i e s of a d s o r p t i o n of h y d r o l y z a b l e heavy metals. This r i s e i n a d s o r p t i o n w i t h pH i s c a l l e d the " a d s o r p t i o n edge." Adsorption which increases only g r a d u a l l y w i t h an i n c r e a s e i n pH ( i . e . , a gradual a d s o r p t i o n edge) w i l l give r i s e to a l a r g e r amount of u s e f u l data between the two extremes of near-zero and n e a r - t o t a l a d s o r p t i o n , a t which experimental uncer­ t a i n t i e s tend to more g r e a t l y a f f e c t i n t e r p r e t a t i o n of the data. For a d s o r p t i o n which increases extremely r a p i d l y w i t h pH ( i . e . , a steep a d s o r p t i o n edge), a l l that need be done i n f i t t i n g the James and Healy (4) model to the data i s to f i n d the proper value of the chemical free-energy term A G p which w i l l d e s c r i b e the pH at which the sudden i n c r e a s e i n a d s o r p t i o n occurs. For a l a r g e t o t a l lead c o n c e n t r a t i o n , however, we w i l l be able to determine whether the model w i l l s t i l l p r o p e r l y d e s c r i b e the shape of the a d s o r p t i o n edge. Ten separate p o r t i o n s of adsorbent of 0.146 g (5.0 X 10" eq) each were washed to constant c o n d u c t i v i t y w i t h d e i o n i z e d water i n order to remove any adsorbed i o n s . They then were each placed i n 50 mL of s o l u t i o n to give a CEC per u n i t volume of 1.0 X 10" eq L" which contained e i t h e r 1.0 X 10~ or 5.0 Χ 10 M l e a d perc h l o r a t e i n C02~free d e i o n i z e d d i s t i l l e d water. The pH was then adjusted, by the a d d i t i o n of 1.0 normal sodium hydroxide or per­ c h l o r i c a c i d as a p p r o p r i a t e , to values of approximately 2, 3, 4, 5, and 6 f o r both the 5.0 Χ 10"" and 1.0 Χ 10~ M sets of s o l u t i o n s . These s l u r r i e s were then placed i n a C02~free atmosphere a t 25°C f o r 48 hours, during which they were m a g n e t i c a l l y s t i r r e d f o r 5 minutes every h a l f hour. A f t e r 48 hours, the s l u r r i e s were removed from the C02~free atmosphere and the pH measured. A l i q u o t s of 5 mL were then removed and c e n t r i f u g e d at 30 000 G's f o r 5 minutes i n order to remove any suspended p a r t i c u l a t e matter. The supernatants were then analyzed f o r l e a d and sodium by atomic a b s o r p t i o n . c n e m

D

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Results and D i s c u s s i o n . Table I gives the analyses and r e s u l t a n t lead a d s o r p t i o n f o r the 1.0 Χ 10" and 5.0 X 10~ M s l u r r i e s . The percent l e a d adsorbed i s p l o t t e d as a f u n c t i o n of pH i n F i g u r e 5, where the r e s u l t s are a l s o compared w i t h the bestf i t t i n g p r e d i c t i o n s of the James and Healy (4) model. I t can be 4

4

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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v

6.4 10.7 26.2 33.6 90.6

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0.098 0.59 28.4 63.9 99.9

0.087 0.68 28.0 85.5 99.9

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Percent a d s o r p t i o n of l e a d

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Includes analyses f o r sodium present as a r e s u l t of NaOH added i n order to a d j u s t the pH. AG = -7.94 kcal/mol and x = 3.96 Â. AG = -14.05 kcal/mol and d = 2.17 Â.

1

1.85 2.78 3.90 4.38 6.66

1.83 2.86 3.83 4.47 5.73

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[Pb, t o t a l lead

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7.42 16.9 26.5 32.6 95.3

12.6 35.2 72.4 89.2 99.7

VSC-VSP j ι3 model

Table I — A d s o r p t i o n of l e a d from 50 mL s o l u t i o n onto 0.146 gram o f adsorbent as f u n c t i o n o f pH and t o t a l l e a d content, and comparison o f experimental and p r e d i c t e d a d s o r p t i o n of l e a d

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

seen t h a t the r a t e of i n c r e a s e of percent a d s o r p t i o n w i t h i n c r e a s ­ ing pH i s gradual even i n the 1.0 Χ ΙΟ" * M s l u r r i e s . The r i s e i n percent a d s o r p t i o n w i t h pH, f o r t h i s data., i s s t i l l much more gradual than that which would be p r e d i c t e d u s i n g the model of James and Healy (^) . The curves shown i n F i g u r e 5 f o r t h i s model were generated u s i n g a computer program which s e l e c t s the best value of A G p i n order to minimize the d i f f e r e n c e s between p r e d i c t e d and experimental percent a d s o r p t i o n . The value f o r AG obtained i n t h i s manner was -7.94 k c a l mol a result l e a d i n g to an average e r r o r between p r e d i c t e d and experimental data of 15.5 percent a d s o r p t i o n . F i g u r e 5 shows that the optimum agreement o b t a i n a b l e between experiment and p r e d i c t i o n s of t h i s model i s poor. The data a t t o t a l l e a d c o n c e n t r a t i o n of 1.0 Χ 10~ M can by i t s e l f be f i t t e d to the James and Healy (4) model w i t h a minimal average d e v i a t i o n of only 7.7 percent a d s o r p t i o n between theory and experiment. The value of A G p (-12.18 k c a l m o l ) used f o r f i t t i n g t h i s data, however, produces very poor agreement between theory and experiment (an average d e v i a t i o n of 31.4 percent adsorption) f o r the data a t 5.0 Χ ΙΟ"" M t o t a l lead, as i s shown by F i g u r e 6. The James and Healy (4) model t h e r e f o r e cannot r e c o n c i l e the data obtained under these two s e t s of experimental conditions. The f i t t i n g of p r e d i c t e d a d s o r p t i o n and experimental data was performed using L e v i n e s (5) expression f o r the s o l v a t i o n f r e e energy term and h i s suggested value of 30 f o r the i n t e r f a c i a l d i e l e c t r i c constant. Other workers, however, have used values as low as 6.0 f o r t h i s parameter. The e f f e c t of a v a r i a t i o n i n the i n t e r f a c i a l d i e l e c t r i c constant on the f i t of the p r e d i c t e d r e s u l t s i s minimal, the only d i f f e r e n c e being i n the value c a l c u ­ l a t e d f o r the other f i t t i n g parameter, the chemical free-energy term. Use of 6.0 f o r the i n t e r f a c i a l d i e l e c t r i c constant gives almost i d e n t i c a l p r e d i c t i o n s , w i t h an average e r r o r between pre­ d i c t e d and experimental percent a d s o r p t i o n of 16.1 percent. (The chemical free-energy term a r r i v e d a t i n t h i s i n s t a n c e was -9.46 k c a l m o l . ) Experimental lead a d s o r p t i o n data are compared i n F i g u r e 7 w i t h the b e s t - f i t t i n g t h e o r e t i c a l a d s o r p t i o n s , using the optimized parameters A G p = 14.05 k c a l m o l and d = 2.17 A i n the VSC-VSP model. I t i s c l e a r that a much b e t t e r f i t i s obtained u s i n g t h i s model than by u s i n g that of James and Healy ( 4 ) . The average d e v i a t i o n i n percent a d s o r p t i o n between that found e x p e r i ­ mentally and that p r e d i c t e d was found to be a very low 3.4 percent, compared to the r a t h e r poor 15.5 percent average d e v i a ­ t i o n obtained w i t h the b e s t - f i t t i n g James and Healy (4) model. L i k e the James and Healy (4) model, the VSC-VSP model a l s o r e q u i r e s an estimate of the magnitude of chemical i n t e r a c t i o n s , which u s u a l l y must be determined by comparison of experimental data w i t h a d s o r p t i o n p r e d i c t e d at various values of A G p ^ . This model, however, a l s o r e q u i r e s a knowledge of the "average" 1

c n e m

b

c h e m

P b

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

4

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- 1

b

4

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- 1

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

e m

BROWN

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253

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

12.

Figure

7.

Comparison of lead adsorption data (points and solid connecting with best-fitting VSC-VSP model predictions (broken lines)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

lines)

254

CHEMICAL MODELING IN AQUEOUS SYSTEMS

d i s t a n c e of approach d. of an adsorbed c a t i o n to the s u r f a c e . In r e a l i t y , any model i n v o l v i n g only one or two planes of a d s o r p t i o n near the surface w i l l be an o v e r s i m p l i f i c a t i o n of the r e a l s i t u a t i o n , p a r t i c u l a r l y on s u r f a c e s as complex and i r r e g u l a r as oxides and s i l i c a t e s . Such a concept i s nevertheless i n c o r p o r a t e d i n the VSC-VSP model because of i t s a b i l i t y to approximately d e s c r i b e a d s o r p t i o n phenomena. The concept of an "average" d i s t a n c e from the s u r f a c e of an adsorbed c a t i o n does not e a s i l y g i v e r i s e to a method f o r c a l c u l a t i n g such d i s t a n c e , except f o r t h a t by which comparison of experimental data w i t h t h e o r e t i c a l p r e d i c t i o n s of a d s o r p t i o n f o r d i f f e r e n t values of d_ w i l l y i e l d a value which gives the best agreement. What t h i s means, then, i s that the VSC-VSP model as used here has two f i t t i n g parameters, A G p b and d_, w h i l e the James and Healy (4·) model has one, namely, AGp em ( i t should be noted that a l l o w i n g the adsorbed ion's d i s t a n c e of approach x as w e l l as AGp|3 , to vary i n the curvef i t t i n g procedure f o r the James and Healy (40 model does not r e s u l t i n any s i g n i f i c a n t l y b e t t e r f i t between experiment and theory f o r t h i s model. The problems i l l u s t r a t e d i n f i g u r e s 5 and 6 are s t i l l encountered; i n c o n t r a s t to experiment, no s i g n i f i c a n t e f f e c t of t o t a l l e a d c o n c e n t r a t i o n on the a d s o r p t i o n edge p o s i t i o n i s predicted.) Bowden et a_l. (7) use a c u r v e - f i t t i n g approach of t h i s s o r t to estimate the i n n e r - l a y e r capacitance, of which the value of d_ i s a measure. They compare t h e i r experimental data, p l o t t e d as pH versus s u r f a c e charge d e n s i t y , w i t h a f a m i l y of t h e o r e t i c a l curves c a l c u l a t e d using v a r i o u s capacitances, s e l e c t i n g the capacitance g i v i n g the best agreement w i t h experiment. I t seems reasonable to assume that the value of .d should l i e somewhere between the c r y s t a l i o n i c r a d i u s (1.20 Â) of the P b i o n and the c r y s t a l i o n i c r a d i u s plus the diameter of the f i r s t l a y e r of attached water molecules or primary h y d r a t i o n sheath (3.96 Â ) , as i s normally used i n the James and Healy (4) model f o r P b and Pb0H+ i o n s . In i n c o r p o r a t i n g L e v i n e s (5) e x p r e s s i o n f o r the s o l v a t i o n free-energy term i n t o the VSC-VSP model, we were faced w i t h the dilemma of which value to use f o r the i n t e r f a c i a l d i e l e c t r i c constant e-i_ . Levine (5) uses a value of 30 i n h i s s o l v a t i o n free-energy term, w h i l e Bowden et a l . (7) base t h e i r use of 6.0 for t h i s term on work by Hasted et_ a l . (13). Instead of speaking of a d i e l e c t r i c constant which continuously v a r i e s w i t h d i s t a n c e from the s u r f a c e , we should r a t h e r speak of d i e l e c t r i c constants of each of the successive s h e l l s of adsorbed water molecules. The water s h e l l s very near the surface w i l l have low d i e l e c t r i c constants, and the more d i s t a n t s h e l l s w i l l have higher ones. Since the d i e l e c t r i c constant of a d i p o l e i s r e a l l y a measure of the freedom of the d i p o l e to become o r i e n t e d i n response to changes i n e l e c t r o s t a t i c f i e l d s t r e n g t h , i t would seem reasonable that the f i r s t l a y e r of water molecules adsorbed to the charged surface (as the r e s u l t a t l e a s t of Van der Waals and London

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

c n e m

cn

b

e

Cneiri

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2 +

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch012

12.

BROWN

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Streambed

Sediment

255

d i s p e r s i o n f o r c e s ) would be r a t h e r low as a r e s u l t of t h e i r attachment to the surface and r e s u l t i n g i m m o b i l i t y . Such water molecules would a l s o be a f f e c t e d by the e l e c t r i c a l f i e l d s t r e n g t h charge of the adsorbed c a t i o n s , tending to f u r t h e r lower the d i e l e c t r i c constant. B o c k r i s et a l . (14) f o r t h i s reason give 6.0 as the d i e l e c t r i c constant value f o r the f i r s t such l a y e r . I t t h e r e f o r e seems reasonable to say that as a f i r s t a p p r o x i mation, the i n t e r f a c i a l d i e l e c t r i c constant w i t h i n the f i r s t l a y e r of adsorbed water molecules may be taken as the lower value of 6.0. I t i s t h i s lower value which we used i n the Levine Ç5) expression f o r the s o l v a t i o n free-energy term, and i n the surface potential-Gouy p o t e n t i a l r e l a t i o n i n the VSC-VSP model i n order to o b t a i n the p r e d i c t e d a d s o r p t i o n shown i n Figure 7 and t a b u l a t e d i n Table I. Our value f o r d of 2.17 Â l i e s w e l l w i t h i n the diameter (2.76 A) of the f i r s t adsorbed l a y e r of water w i t h i n which the approximation of a 6.0 d i e l e c t r i c constant should be v a l i d . We should a l s o note that t h i s 2.17 A value of d. l i e s w e l l w i t h i n the c r y s t a l i o n i c r a d i u s of P b and the hydrated r a d i u s used by James and Healy (4). As i n the e a r l i e r James and Healy (4) model c a l c u l a t i o n s here, the use of d i f f e r e n t values f o r the i n t e r f a c i a l d i e l e c t r i c cons t a n t d i d not g r e a t l y a f f e c t the f i n a l agreement between theory and experiment, and r e s u l t e d mainly i n a change i n the value of the f i t t i n g parameters. Use of 30, i n s t e a d of 6.0, f o r the i n t e r f a c i a l d i e l e c t r i c constant, gave an average e r r o r between pred i c t e d and experimental data of only 6.1 percent a d s o r p t i o n ; t h i s i s s t i l l b e t t e r agreement w i t h experiment than that p r e d i c t e d by the James and Healy (4) model, but not q u i t e as good as the r e s u l t obtained using 6.0 f o r the i n t e r f a c i a l d i e l e c t r i c constant. ( F i t t i n g parameters here were found to be -11.37 k c a l mol" f o r AG , and 3.00 Â f o r d. ) This r e s u l t appears to lend support to the p r e f e r r e d use here of 6.0 f o r the i n t e r f a c i a l d i e l e c t r i c constant. 2 +

1

c h e m

p b

Conclusion While previous work has o f t e n been conducted under c o n d i t i o n s where only t r a c e q u a n t i t i e s of lead or other heavy metals have been placed i n contact w i t h an adsorbent, very few of these approaches have d e a l t w i t h the problems faced as the adsorbent s i t e s begin to be f i l l e d . The usefulness of the VSC-VSP model i n t a k i n g t h i s i n t o account i s i l l u s t r a t e d here by demonstration of the e f f e c t of charged adsorbed species on the e l e c t r o s t a t i c p o t e n t i a l which acts on the adsorbing i o n s . When a given number of e q u i v a l e n t s of adsorbent are placed i n contact w i t h a comparat i v e l y l a r g e number of moles of c a t i o n s , some of which w i l l a t t a c h to the adsorbent, a d s o r p t i o n w i l l be f u r t h e r opposed i n two ways. F i r s t , of course, the process of a d s o r p t i o n w i l l reduce the number of s i t e s a v a i l a b l e f o r f u r t h e r a d s o r p t i o n . Second, the Gouy potential i s s a i d by Bowden e_t a l . (7_) to decrease from the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

256

CHEMICAL MODELING IN AQUEOUS SYSTEMS

s u r f a c e p o t e n t i a l value i n l i n e a r p r o p o r t i o n to the s u r f a c e charge d e n s i t y o . Since increased c a t i o n adsorption works to make the i n t e r f a c i a l charge d e n s i t y more p o s i t i v e and the opposing surface charge d e n s i t y o more negative, the p o t e n t i a l ψ >

HO HO HO

Si - 0

HO^

H0^"

•Si

Z+ + M'

.-0 Z+ _ + M*

HO HO HO

>

Si - 0 - M

(z-i)+

(4)

(Z-2)+ (5)

\ Q

The f r e e OH i o n i s a powerful l i g a n d . Dugger e_t a l . (31) have pointed out that the l i g a n d p r o p e r t i e s o f the surface OH groups are not g r e a t l y modified by the attached s i l i c o n . I f the S c h i n d l e r model i s c o r r e c t , then the derived r a t e constants should bear a s i g n i f i c a n t r e l a t i o n s h i p to the s t r e n g t h of the i n t e r a c t i o n of the metal i o n w i t h 0H~. I n Table"I and Figures 1 and 2, we show the r a t e constants derived from f i e l d

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

13.

Oceanic

BREWER AND HAO

Elemental

Scavenging

11

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch013

IO" 10

-io

io-

Ni

Cd

9

10 -8 Pb

κ;

'D

Cu

10-

10" 10 -5

1 0 _4

/

/

4"

10 -3 10

1

1

10-1

SCAVENGING

io-2 1 0 - 3 i o - 4 io-5 1 0 - 7 RATE

CONSTANT

(YR ~ ) 1

Figure 1. Plot of the scavenging rate constants derived from advection-diffusionscavenging models against the stability constant for simple hydroxo complexes, * K i

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

266

Cu Ni

Cd

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch013

Pb

10

1

1

1

IO"

SCAVENGING

2

3

4

5

6

10" 10" 1 0 " 10" 10" '

RATE

CONSTANT

(YR' ) 1

Figure 2. Plot of the scavenging rate constants derived from advection-diffusionscavenging models against the stability constant *β for reaction with two hydroxo groups 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

BREWER AND HAo

13.

Oceanic

Elemental

267

Scavenging

measurements compared to the s t a b i l i t y constants f o r hydroxo-complexes "K;L and *Ε>2· c o r r e l a t i o n lends c o n s i d e r a b l e support to the argument that the i n t e r a c t i o n w i t h surface OH-groups i s a dominant c o n t r o l on the i n s i t u a d s o r p t i v e process. The r a t e con­ s t a n t s derived here may be c r i t i c i z e d i n that they c o n t a i n s e v e r a l assumptions; however, the geochemist i s a f f o r d e d some r e l i e f i n an e q u a l l y wide choice of constants. The constants used by S c h i n d l e r (29) are a l s o given i n Table I . The value f o r ' $2 1h Figure 2 f a l l s s i g n i f i c a n t l y away from the other ions r e f l e c t i n g the com­ p a r i s o n of t e t r a v a l e n t and d i v a l e n t ions and the i n t e n s i t y of i n t e r a c t i o n w i t h a second OH-group. T n e

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch013

c

i n

TABLE I Comparison of the adsorption r a t e constants derived from f i e l d s t u d i e s w i t h values f o r * K j and *$2 s e l e c t e d (a) i n t h i s paper and (b) by S c h i n d l e r (29).

Element

Rate Constant (yr" )

Log %

4

-3.7

Th

10°*

Pb

10" ·

Cu

ΙΟ"" ·

Ni

Pu

> PuOt .

3 +

The f i r s t h y d r o l y s i s constant f o r p l u t o n i u m ( I I I ) , the e q u i l i b r i u m constant f o r the r e a c t i o n

Pu

3 +

t

+ H0 2

Pu(OH)

2+

that i s ,

+

+

H,

has been determined from acid-base t i t r a t i o n curves at an i o n i c strength (I) of 0.069 M to be 7.5 X 10~ (2). The s o l u b i l i t y product of the n e u t r a l hydroxide p r e c i p i t a t e , Pu(0H) , i s reported to be 2 X 1 0 (3, p. 299). H y d r o l y s i s of p l u t o n y l (VI) has been i n v e s t i g a t e d e x t e n s i v e l y , but not a l l the data are relevant to environmental concentrations. Potentiometric data at I = 1 M and 25° have been i n t e r p r e t e d to i n d i c a t e the formation of three species, Pu02(0H) , ( P u 0 ) 2 2 5 and (Pu0 )3(OH)5, with r e s p e c t i v e o v e r a l l h y d r o l y s i s constants of 1.1 X 10~ , 3.1 X 10" , and 6.9 X 1 0 ~ ( 4 ) . Another potentiometric study (5), at I = 3 M and 25°, y i e l d e d the f o l l o w i n g o v e r a l l h y d r o l y s i s constants: P u 0 ( 0 H ) , Κ = 5.0 Χ 10" ; ( P u 0 ) ( O H ) , Κ = 5.74 Χ 10" ; 8

3

- 2 0

+

+

2

2

6

9

2 3

+

7

2 +

2

2

( P u 0 ) ( 0 H ) t , Κ = 2.0 2

3

9

2

22

Χ 1 0 " ; (Pu0 )4(OH)^, Κ = 7.68 2

3 0

Χ 10" .

The polynuclear species would not be expected at environmental plutonium concentrations, at which the most common h y d r o l y s i s product i s Pu02(0H) . The c i t e d h y d r o l y s i s constants f o r t h i s species are i n s a t i s f a c t o r y agreement. Least hydrolyzed of the four o x i d a t i o n states i s p l u t o n y l ( V ) , which has a reported f i r s t h y d r o l y s i s constant of 2 Χ 1 0 ~ at I = 3 Χ ΙΟ M and 25° (2, pp. 478-499). Plutonium (IV) i s the most r e a d i l y hydrolyzed of the four o x i d a t i o n s t a t e s , but only the f i r s t h y d r o l y s i s constant i s known with any confidence. For the r e a c t i o n +

1υ

- 3

Pu

4 +

+ H0 2

t

Pu(0H)

3+

+

H

+

the best value f o r the f i r s t h y d r o l y s i s constant, K j , i s 0.031, as determined p o t e n t i o m e t r i c a l l y at I = 1 M and 25° (6). Step­ wise values f o r to have been c a l c u l a t e d from TTA e x t r a c t i o n

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

324

C H E M I C A L M O D E L I N G IN

AQUEOUS

SYSTEMS

4

y

data at I = 1 M to be 0.35, 0.18, 5.0 Χ 10" , and 5.0 X 10" , r e s p e c t i v e l y (_7), but r e s u l t s are q u e s t i o n a b l e . Successive h y d r o l ­ y s i s constants s u f f e r from such an accumulation of e r r o r s that they become v i r t u a l l y meaningless. I n p a r t i c u l a r , the value f o r Kj^, i f i t r e f e r s to the e q u i l i b r i u m between Pu(0H)"3 and s o l i d Pu(OH) 1+, y i e l d s values f o r the e q u i l i b r i u m c o n c e n t r a t i o n of p l u t o ­ nium (IV) i n n e a r - n e u t r a l s o l u t i o n s that are too high by many orders of magnitude. On the assumption of a r e g u l a r p r o g r e s s i o n of the s t a b i l i t i e s of s u c c e s s i v e h y d r o l y s i s products, o v e r a l l values f o r K i to Ki+ (that i s , f o r the formation of P u ( 0 H ) from Pu(OH)n-i) were estimated to be 0.32, 5.0 Χ 10" , 5.0 Χ 10" , and 3.2 Χ 1 0 " ( 8 ) , but these r e s u l t s cannot be taken s e r i o u s l y i n view of the u n v e r i f i e d assumptions made i n t h e i r c a l c u l a t i o n . The reported s o l u b i l i t y product of Pu(0H)i+, 7 X 10 as measured by pH t i t r a t i o n Ç3, p. 300), i s an exceedingly s m a l l number. I f i t were a t r u e r e p r e s e n t a t i o n of the c o n c e n t r a t i o n of plutonium i n s o l u t i o n , at pH 7 there would be only 7 X 1 0 ~ mole of plutonium per l i t e r ; thus the e q u i l i b r i u m c o n c e n t r a t i o n of p l u t onium i n n e u t r a l water would be about one atom per 2400 l i t e r and there would be no problem w i t h plutonium-contaminated ground water. The s o l u b i l i t y product does not a c c u r a t e l y d e f i n e the concentrat i o n of plutonium i n aqueous s o l u t i o n s because i t merely s t a t e s the c o n c e n t r a t i o n of the P u i o n . At pH values of environmental i n t e r e s t , plutonium w i l l be present not p r i m a r i l y as P u , but as species such as Pu(0H)£ , Pu(OH)"^, u n i o n i z e d PuiOH)^, c o l l o i d a l polymeric forms to be discussed l a t e r , as w e l l as other o x i d a t i o n s t a t e s formed by d i s p r o p o r t i o n a t i o n at low a c i d i t i e s . Thus, the t o t a l plutonium c o n c e n t r a t i o n w i l l be much higher than that d e s c r i b e d by the s o l u b i l i t y product of PuiOH)^. n

3

6

10

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

6

28

4 +

4 +

+

Complexes A b r i e f review of methodology i s i n order before embarking on a d i s c u s s i o n of plutonium complexes. Determination of s t a b i l i t y constants of metal complexes depends on the measurement of the e f f e c t produced by the p a r t i c u l a r l i g a n d on an e q u i l i b r i u m i n v o l v ing the uncomplexed metal i o n . The most common methods are i o n exchange, s o l v e n t e x t r a c t i o n , spectrophotometry, s o l u b i l i t y , potentiometry, and polarography, of which only the l a s t two g i v e values based on a c t i v i t i e s r a t h e r than c o n c e n t r a t i o n s . For t h i s and other reasons, these two methods g e n e r a l l y y i e l d more thermodynamically s i g n i f i c a n t constants. Unfortunately,, they have been r a t h e r i n f r e q u e n t l y used i n studying plutonium complexes. The l e a s t r e l i a b l e method, s o l u b i l i t y , s u f f e r s from the d i f f i c u l t y of a c h i e v i n g e q u i l i b r i u m s o l u b i l i t i e s and from problems i n p r o p e r l y i n t e r p r e t i n g the data. In cases where the s t a b i l i t y constants of plutonium complexes are not known, i t i s p o s s i b l e to o b t a i n a rough estimate by comp a r i s o n w i t h analogous metal ions (that i s , L a for P u , T h f o r Pu " ", NpOj f o r PuO"J, υ θ f o r PuO^ ) . With one e x c e p t i o n , 3 +

4

1

2 +

3 +

+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4 +

16.

CLEVELAND

Plutonium

Equilibria

of

Environmental

Concern

325

t h i s approach i s not a p p l i e d i n t h i s paper because of space l i m i ­ t a t i o n s . Moreover, attempting to e x t r a p o l a t e s t a b i l i t y constants from one element to another i s a s h o r t c u t that can e a s i l y be abused. The analogous ions d i f f e r s i g n i f i c a n t l y from the respec­ t i v e plutonium ions i n such p r o p e r t i e s as i o n i c p o t e n t i a l and o x i d a t i o n - r e d u c t i o n behavior, which can r e s u l t i n d i f f e r e n c e s i n complexation. Two types of s t a b i l i t y constants should be d e f i n e d . For the r e a c t i o n of one or more l i g a n d s , L, w i t h a metal i o n , M, the stepwise s t a b i l i t y constant i s expressed by the r e l a t i o n [MLJ Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

K

n

=

[ML

n - 1

][L] '

whereas the o v e r a l l s t a b i l i t y constant i s d e f i n e d as [ML ] β 11

6

I t follows that Κ

. β

[M][L]

n

n

= — n

s_ .

=

Moreover, s t a b i l i t y constant

equations

η-1

i n v o l v i n g protonated s p e c i e s , such as u n d i s s o c i a t e d or p a r t i a l l y d i s s o c i a t e d a c i d , may be converted to the above expressions by a p p l i c a t i o n of the a p p r o p r i a t e a c i d d i s s o c i a t i o n constant. Standard d e v i a t i o n s are not g i v e n , s i n c e they merely e s t a b l i s h the r e p r o d u c i b i l i t y of a given method and do not n e c e s s a r i l y r e f l e c t the accuracy of the data. Carbonate Complexes. Of the many l i g a n d s which are known to complex plutonium, o n l y those of primary environmental concern, that i s , carbonate, s u l f a t e , f l u o r i d e , c h l o r i d e , n i t r a t e , phos­ phate, c i t r a t e , t r i b u t y l phosphate (TBP), and ethylenediaminetetr a a c e t i c a c i d (EDTA), w i l l be d i s c u s s e d . Of these, none i s more important i n n a t u r a l systems than carbonate, but data on i t s r e a c t i o n s w i t h plutonium are meager, p r i m a r i l y because of competi­ t i v e h y d r o l y s i s at the low a c i d i t i e s that must be used. No s t a b i l i t y constants have been p u b l i s h e d on the carbonate complexes of p l u t o n i u m ( I I I ) and p l u t o n y l ( V ) , and the data f o r t h e ' p l u t o n i ­ um (IV) species are not c r e d i b l e . R e s u l t s from s t u d i e s on the s o l u b i l i t y of plutonium(IV) o x a l a t e i n K2CO3 s o l u t i o n s of v a r i o u s c o n c e n t r a t i o n s have been i n t e r p r e t e d (9) to i n d i c a t e the e x i s t e n c e of complexes as h i g h as Pu(C0 3 ) ^ , a species that i s most u n l i k e l y from both e l e c t r o s t a t i c and s t e r i c c o n s i d e r a t i o n s . From the i n f l u e n c e of K2CO3 c o n c e n t r a t i o n on the s o l u b i l i t y of PuiOH)^ at an i o n i c s t r e n g t h of 10 M, the s t a b i l i t y constant of the complex P u ( C 0 ) " was c a l c u l a t e d (10) to be 9.1 Χ 1 0 at 2 0 ° . This value 2 _

3

2 4

4 6

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

326

CHEMICAL MODELING IN AQUEOUS SYSTEMS

i s based on poor experimental methodology and i n c o r r e c t i n t e r p r e t a ­ t i o n of the data, and i s best ignored. From comparisons w i t h other plutonium complexes and w i t h carbonate complexes of analogous metal i o n s , i t appears to be h i g h by about 30 orders of magnitude. In the case of p l u t o n y l ( V I ) , somewhat more p l a u s i b l e r e s u l t s have been obtained. Data on the s o l u b i l i t y of p l u t o n y l ( V I ) hydrox­ ide i n (NH4)2C03 s o l u t i o n s of v a r y i n g concentrations have been i n t e r p r e t e d (11) to r e v e a l the presence of three complexes, P u 0 ( C 0 ) | " , Pu0 (C0 )(OH)", and Pu0 (C0 )(0H)£~, w i t h s t a b i l i t y constants of 6.7 Χ 1 0 , 4.5 Χ 1 0 , and 2.3 Χ 1 0 , at I = 1 M and 20°. The v a l u e f o r the dicarbonato species i s reasonably c o n s i s t e n t w i t h published constants f o r the analogous u r a n y l ( V I ) s p e c i e s , but the constants f o r the hydroxy species need f u r t h e r verification. 2

3

2

3

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

1 3

3

2 2

2 2

S u l f a t e Complexes. Knowledge of the s u l f a t e complexes of p l u t o n i u m ( I I I ) and (IV) has advanced somewhat i n recent years. Ion exchange data i n d i c a t e d the presence of two complexes of the t r i v a l e n t i o n , Pu(S0i+) and P u ( S 0 ) , w i t h r e s p e c t i v e stepwise s t a b i l i t y constants of 44.7 and 43.7 at I - 2 M and 25° (12). I t is unlikely and K would have such s i m i l a r v a l u e s , and hence K must be regarded w i t h s u s p i c i o n . Because these values were the same i n both 1 M and 2 M HCIO^, i t was concluded that no protonated species were formed. This value of Kj takes precedence over e a r l i e r data (13) i n d i c a t i n g the e x i s t e n c e of two complexes, Pu(S0i ) , w i t h K - 18.1 and PuiHSO^)^, w i t h Κχ = 9.9, at I = 1 M and 28°. In the l a t t e r case a l l determinations were at a constant a c i d i t y of 1 M, and t h e r e f o r e the presence of the protonated complex PuiHSO^)* was not demonstrated by s t u d i e s at v a r y i n g acidity. Numerous determinations have been made of the s t a b i l i t y con­ s t a n t s of s u l f a t e complexes of plutonium(IV), and the r e s u l t s vary by three orders of magnitude. The most p l a u s i b l e values (14) have come from c a r e f u l i o n exchange s t u d i e s at I = 2 M and 25°, which i n d i c a t e the presence of two s p e c i e s , Pu(S0[+) and P u ( S 0 ^ ) , w i t h stepwise s t a b i l i t y constants of 6.6 X 1 0 and 5.8 Χ 1 0 , respec­ t i v e l y . This K i i s i n s a t i s f a c t o r y agreement w i t h the value 4.6 Χ 1 0 obtained p o t e n t i o m e t r i c a l l y at I = 1 M and 25° ( 6 ) . Spectrophotometric and e l e c t r o p h o r e t i c s t u d i e s i n d i c a t e that p l u t o n y l ( V I ) forms complexes c o n t a i n i n g as many as four s u l f a t e groups, w i t h a n i o n i c species predominating at s u l f a t e concentra­ t i o n s above 1 M (15). The monosulfato complex has a reported s t a ­ b i l i t y constant of 14.4 as determined by e x t r a c t i o n s t u d i e s at I = 2 M and 25° (16). +

4

2

2

2

+

+

x

2+

2

3

2

3

F l u o r i d e Complexes. F l u o r i d e i s known to complex plutonium s t r o n g l y , but q u a n t i t a t i v e data on these environmentally important complexes are l i m i t e d . C a t i o n exchange s t u d i e s (17) y i e l d e d values of 4.5 Χ 1 0 a t I = 1 M and 7.9 Χ 1 0 at I = 2 M f o r -the s t a b i l i t y constant of the monofluoro complex of plutonium(IV), which are i n s a t i s f a c t o r y agreement w i t h the value 1.2 Χ 1 0 obtained from 7

7

8

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e x t r a c t i o n data a t I = 2 M and 25° (18). The l a t t e r study a l s o gave 3.4 Χ 1 0 as the o v e r a l l s t a b i l i t y constant, $2> of PuF^ under the same c o n d i t i o n s . (The values c i t e d i n references 19 and 20 have been c o r r e c t e d f o r the i o n i z a t i o n constant of HF to convert them to s t a b i l i t y constants as d e f i n e d i n t h i s d i s c u s s i o n . ) Four f l u o r o complexes of p l u t o n y l ( V I ) have been i d e n t i f i e d from c a t i o n exchange s t u d i e s (19) a t I = 1 M, w i t h o v e r a l l s t a b i l i t y constants as f o l l o w s : Pu0 î ", 3i = 130; P u 0 F , 3 = 1-4 u 0 F 3 , Β3 +

1 4

H

x

2

2

2

p

2

2

= 1.2 Χ 1 0 ; Pu0 F^"", 3i+ = 2 . 0 Χ 1 0 . These r e s u l t s are somewhat clouded by the a u t h o r s f a i l u r e to demonstrate the absence of p l u t o n i u m ( I V ) , a p o t e n t i a l source of e r r o r i n t h i s system. A lower v a l u e , 3i = 12, has been reported from e x t r a c t i o n data a t I = 2 M and 25° (16). 6

6

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

1

C h l o r i d e Complexes. Because of marine d i s p o s a l or storage of a c t i n i d e wastes i n s a l t formations, the r e l a t i v e l y weak c h l o r o complexes of plutonium could become important over extended time p e r i o d s . Two p l u t o n i u m ( I I I ) complexes, P u C l and P u C l J , have been reported (20), the former o c c u r r i n g a t c h l o r i d e c o n c e n t r a t i o n s above 2 M and the l a t t e r i n the presence of 8 M or greater c h l o r i d e i o n c o n c e n t r a t i o n . No s t a b i l i t y data were given. Plutonium(IV) forms complexes c o n t a i n i n g from one to s i x c h l o r i d e l i g a n d s . Q u a n t i t a t i v e values e x i s t f o r the three lower complexes i n 4 M HCIO^ a t 20° as determined by c a t i o n exchange: P u C l , 2 +

3 +

= 1.4; PuCl£ , K = 1.2; PuCl^, K = 0 . 1 (21). Other values f o r K i agree s a t i s f a c t o r i l y w i t h the above: 1.38 by potentiometry (22) , and 1.42 by an e x t r a c t i o n technique a t I = 2 M and 25° (23) ; thus the v a l u e f o r K]_ seems w e l l e s t a b l i s h e d . Values f o r K show more v a r i a t i o n : 0.49 by potentiometry (22) and 0.16 by e x t r a c t i o n (23). Agreement between the i o n exchange and p o t e n t i o ­ m e t r i c values i s s u f f i c i e n t l y good to suggest that K i s somewhere i n the range 0,5 t o 1.2. P o t e n t i o m e t r i c s t u d i e s Ç3, pp. 312-13) a t u n s p e c i f i e d tempera t u r e and i o n i c s t r e n g t h have i n d i c a t e d K j f o r the p l u t o n y l ( V ) complex PUO2CI t o be 0.67, a v a l u e that should be accepted w i t h c a u t i o n i n view of the experimental vagueness. P l u t o n y l ( V I ) forms complexes c o n t a i n i n g up to three or f o u r c h l o r i d e i o n s , but q u a n t i t a t i v e data have been reported f o r only the mono and d i c h l o r o s p e c i e s . The f o l l o w i n g values have been found, a l l by spectrophotometry, f o r the s t a b i l i t y constant of P u 0 2 C l : 1.25 at I = 2 M and 25° (24); 0.73 a t I = 1 M and 23° (3, pp. 312-13); 0.56 a t I = 1 M and 20° (25). Again, agreement i s reasonably good, although i t i s r e g r e t t a b l e t h a t a l l values were obtained by the same technique. The s i n g l e reported s t a b i l i t y constant, K 2 , f o r P u 0 C l , a l s o by spectrophotometry, i s 0.35 at I = 2 M and 25° ( 2 4 ) . +

2

3

2

2

+

2

2

N i t r a t e Complexes. Although there i s spectrophotometric evidence f o r the e x i s t e n c e of n i t r a t e complexes of p l u t o n i u m ( I I I ) ,

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

they are unstable because of o x i d a t i o n to plutonium(IV). The l a t t e r forms complexes c o n t a i n i n g as many as s i x n i t r a t e groups, and they are important i n the chemical p r o c e s s i n g of plutonium by i o n exchange and s o l v e n t e x t r a c t i o n . Values f o r Κχ are i n good agreement: 5.5 by c a t i o n exchange at I = 4 M and 20° (21); 5.3 by e x t r a c t i o n at I = 6 M (26); 4.4 by e x t r a c t i o n at I = 2 M and 25° (23). Strong disagreement e x i s t s among the values f o r K : 23.5 by i o n exchange at I = 4 M and 20° (21); 3.0 by e x t r a c t i o n at I = 2 M and 25° (23); 0.96 at I = 6 M, a l s o by e x t r a c t i o n (26). The i o n exchange v a l u e i s probably the l e a s t r e l i a b l e because of compounding of e r r o r s ; on t h i s b a s i s the most p l a u s i ­ b l e K i s i n the range 1 t o 3. Values f o r K 3 become even l e s s r e l i a b l e ; of the two r e p o r t e d , 15 by i o n exchange at I = 4 M and 20° (21) and 0.33 by e x t r a c t i o n at I • 6 M (26), n e i t h e r can be accepted w i t h assurance. Mono-, d i - , and t r i n i t r a t o complexes of p l u t o n y l ( V I ) have been i d e n t i f i e d , but they are weak. Of the two p l a u s i b l e v a l u e s r e p o r t e d f o r K i , t h a t i s , 0.93 by e x t r a c t i o n at I = 4.1 M (27) and 0.25, a l s o by e x t r a c t i o n , at I = 4.6 M and 25° (28), the l a t t e r i s more r e l i a b l e because of more c a r e f u l experimental c o n t r o l s and the more v a l i d assumptions made i n i t s calculation. 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

2

Phosphate Complexes. Of a l l the systems d e s c r i b e d i n t h i s paper, the most d i f f i c u l t to evaluate i s t h a t of the phosphate complexes of plutonium. Many of the s t a b i l i t y constants were c a l c u l a t e d from s o l u b i l i t y measurements, which o f t e n y i e l d data of dubious accuracy. The values t h a t have been obtained are reported i n Table I I , and many appear to be too h i g h , a common e r r o r i n s o l u b i l i t y determinations. The i o n exchange values f o r the a c i d phosphate complexes of p l u t o n i u m ( I I I ) are the most p l a u s i b l e . The data f o r plutonium(IV) and p l u t o n y l ( V I ) should be used w i t h c a u t i o n . C i t r a t e Complexes. Because of t h e i r a b i l i t y to form s t e r i c a l l y favored c h e l a t e s t r u c t u r e s , many organic l i g a n d s complex plutonium more s t r o n g l y than i n o r g a n i c anions. One of the s t r o n g ­ est n a t u r a l l y o c c u r r i n g c h e l a t i n g agents i s c i t r a t e , and i t forms a number of very s t a b l e complexes w i t h plutonium. Three p l u t o nium(III) complexes have been reported (32), P u ^ B ^ O y ) , Pu(H C H 0 )"£, and P u ( H C H 0 ) 3 , w i t h r e s p e c t i v e s t a b i l i t y 2

6

5

7

2

8

6

5

7

6

1 0

constants of 7.3 X 1 0 , 4.0 X 1 0 , and 1.0 X 1 0 , but these v a l u e s are suspect because they d i f f e r by s e v e r a l orders of magnitude from constants f o r c i t r a t e complexes of other a c t i n i d e s and l a n t h a n i d e s . The apparent e r r o r i s p r i m a r i l y a r e s u l t of f a i l u r e t o a l l o w f o r p l u t o n i u m ( I I I ) h y d r o l y s i s and complexing by s u l f i t e formed from the sodium formaldehyde s u l f o x y l a t e used as a reducing agent, and by improper i n t e r p r e t a t i o n of the data. More r e l i a b l e data are a v a i l a b l e f o r the plutonium(IV) complexes, which are present at c i t r a t e c o n c e n t r a t i o n s as low as 1 0 ~ M (33). S t a b i l i t y constants of the unprotonated species ΡυζΟβΗ^Ογ)" " 15

1

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Table I I Phosphate Complexes of Plutonium

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

Complex

S t a b i l i t y Constant

Method

Reference

Pu ( I I I ) 1 9

Pu(POiJ 2 +

Pu(H P0 ) 2

Ki

4

Ρη(Η Ρ0 )$

K

2

Pu(H P0 )

K

3

2

4

2

4

3

Pu(H P0 )^ 2

Ki+

4

Pu(IV) PuCHPOiO Pu(HP0 )2_ PuCHPO^)^ Pu(HP0 )Î_ PuCHPO^)^ 2

Ki K K Kit K

4

2

3

4

5

Pu(VI) Pu0 (HP0O

Pu0 (H P0 ) 2

3

= = = = =

8.3 6.7 4.8 6.3 6.3

Χ Χ Χ Χ Χ

10 10 10 10 10

Κχ = 1 . 5 X 1 0

2

2

1.5 Χ 1 0 at I = 0.5 30.2 at I = 1 M 5.25 at I = 1 M 5.01 at I = 1 M 3.98 at I = 1 M

Ki

+

Solubility

(29)

C a t i o n exchange

(29)

C a t i o n exchange

(29)

C a t i o n exchange

(29)

C a t i o n exchange

(29)

Solubility Solubility Solubility Solubility Solubility

(30) (30) (30) (30) (30)

Unspecified; presumably solubility Solubility

(31)

M and 20° and 20° and 20° and 20° and 20°

1 2

1 0

9

9

8

at 1 = 0

Κχ = 200 a t I = 0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

330

and PuCCgHsOy)!" have been determined by three separate methods w i t h g e n e r a l l y good agreement, as shown i n Table I I I . Table I I I

S t a b i l i t y Constants of C i t r a t e Complexes

of Plutonium(IV) at I = 0.5 M and Method

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

Spectrophotometric Potentiometric pH t i t r a t i o n

3χ

β lk

5.3 X 10 3.5 Χ 1 0 6.9 Χ 1 0

Reference

2

1.6 Χ 1 0 1.0 Χ 1 0 1.0 Χ 1 0

1 5

1 5

25°

3 0

3 0

2 9

(33) (34) (34)

ΤBP Complexes. T r i b u t y l phosphate (TBP) does not normally occur i n the environment, but i t s use i n nuclear f u e l r e p r o c e s s i n g makes i t a probable c o n s t i t u e n t of r a d i o a c t i v e wastes and hence a p o s s i b l e ground water contaminant. TBP r e a c t s w i t h n i t r a t e com­ plexes of plutonium(IV) and (VI) to form the species Pu(NO3)4·2TBP and Pu02(N03)2*2TBP, which although not e x c e p t i o n a l l y s t a b l e , are u s e f u l i n separations chemistry because of t h e i r s o l u b i l i t y i n organic s o l v e n t s and i n s o l u b i l i t y i n aqueous media. By c o n t r a s t , the p l u t o n i u m ( I I I ) complex, Pu(NO3)3·3TBP, i s p o o r l y e x t r a c t e d by organic s o l v e n t s . The s t a b i l i t y constant of the neptunium(IV) complex, d e f i n e d as [Np(Ν0 ) ·2ΤΒΡ] 3

4

Κ =

, 4

[Νρ][Ν0 ] [ΤΒΡ]

2

3

i s reported to be 130 at I = 2 M (35), and the v a l u e f o r the p l u tonium(IV) complex should be s i m i l a r . Although these complexes and t h e i r s o l v e n t are w a t e r - i n s o l u b l e , they could e x i s t i n ground water i n an e m u l s i f i e d form. EDTA Complexes. E t h y l e n e d i a m i n e t e t r a a c e t i c a c i d (EDTA) and i t s homologues form the most s t a b l e known complexes of plutonium. This d i s c u s s i o n w i l l be l i m i t e d to EDTA, which i s most l i k e l y to be found i n the environment as a r e s u l t of i t s use as a medium f o r the a d d i t i o n of s o l u b l e i r o n to s o i l s . The e q u i l i b r i u m constant f o r formation of the 1:1 c h e l a t e of p l u t o n i u m ( I I I ) , as given by the expression [PuY-J[H+]

2

Κ = 3+

2

[Pu ][H Y ""] 2

(where Ηι+Y would represent the u n i o n i z e d EDTA molecule) , was found i n 0.1 Ν KC1 s o l u t i o n to be 3.9 Χ 1 0 at pH 1.5 by spec­ trophotometry (36) and 1.32 Χ 1 0 at pH 3.3 and 20° by i o n 1 8

1 8

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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exchange (37). Another i o n exchange study, at I = 1 M and pH values from 1.2 to 3.4, y i e l d e d a s t a b i l i t y constant f o r PuY~ of 2.28 Χ 1 0 and i n d i c a t e d the presence at pH 1.5 to 2.0 of another s p e c i e s , PuHY, w i t h a s t a b i l i t y constant of 1.61 Χ 1 0 (38). A s t a b i l i t y constant of 1.6 X 10* f o r PuY~ formation from Pu * and Υ^~ at I = 0.1 M and 20° has been c a l c u l a t e d from spectrophoto­ m e t r y and p o l a r o g r a p h i c data (39). Determination of the s t a b i T T t y constants f o r EDTA c h e l a t e s of plutonium(IV) i s rendered d i f f i c u l t by h y d r o l y s i s of the metal i o n at f a i r l y low pH values and by p r o t o n a t i o n of the EDTA molecule i n more s t r o n g l y a c i d s o l u t i o n . Thus attempts to d e t e r ­ mine the e q u i l i b r i u m constant i n the expression 1 7

9

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

2

34

[PuY][H+]

2

Κ = +

2

[Ρ^ ][Η Υ -] 2

1 7

at pH 3.3 i n 0.1 Ν KC1 s o l u t i o n gave values of 4.57 Χ 1 0 by i o n exchange (37) and 1.26 Χ 1 0 by spectrophotometry (36), but these numbers d e s c r i b e the complexing of a p a r t i a l l y hydrolyzed p l u t o ­ nium s p e c i e s , r a t h e r than the Pu " i o n . In 1 M HNO3 a s p e c t r o 1 7

44

2Lf

p h o t o m e t r i c a l l y determined v a l u e of 1.59 X 1 0 was reported (36), but t h i s , too, i s suspect because i t f a i l s to a l l o w f o r the apparent p r o t o n a t i o n of EDTA(Hi+Y) to form species such as H5Y " and H g Y (40). The d i s s o c i a t i o n constants of these species were determined and combined w i t h other i o n i z a t i o n constants of EDTA to c a l c u l a t e a more accurate v a l u e f o r the c o n c e n t r a t i o n of Y ~" ions i n 1 M HNO3 and from t h i s the s t a b i l i t y constant of PuY at I = 1 M and 25° was c a l c u l a t e d to be 5.7 X 1 0 (41). A more recent v a l u e of 4.0 X 1 0 at I = 0.1 M determined by s p e c t r o ­ photometry and polarography (39) i s i n e x c e l l e n t agreement. Even p l u t o n y l ( V ) i s s t r o n g l y complexed by EDTA. The s t a b i l ­ i t y constant of P u 0 Y has been determined by three methods: 7.7 X 1 0 by spectrophotometry at 20° (42); 8 X 1 0 by p o t e n t i o ­ metry i n 0.1 M KC1 at room temperature (43); 1.5 X 1 0 by i o n exchange at I = 0.05 M (44). The f i r s t two v a l u e s , by v i r t u e of t h e i r c l o s e agreement, take precedence. The s t a b i l i t y constant of the p l u t o n y l ( V I ) c h e l a t e , P u 0 Y ~ , i s somewhat higher than that f o r p l u t o n y l ( V ) . Two d i f f e r e n t spectrophotometric s t u d i e s y i e l d e d values of 1.07 Χ 1 0 at pH 4.0 (36) and 4.0 X 1 0 C42), both a t 20°, whereas an i o n exchange-derived value of 2.46 Χ 1 0 at pH 3.3 and 20° has been reported (37). Although these values i n d i c a t e a high degree of s t a b i l i t y f o r the complex, the metal i o n i s unstable toward r e d u c t i o n (36, 42). EDTA reduces p l u t o n y l ( V I ) to p l u t o n y l ( V ) , or plutonium(IV) i f an excess of l i g a n d i s present (45). However, because a l l o x i d a t i o n s t a t e s are s t r o n g l y complexed by EDTA, the r e d u c t i o n does not r e s u l t i n the r e l e a s e of uncomplexed plutonium i o n s . 4

2+

4

2 5

2 5

3

2

1 2

1 2

1 0

2

2

1 6

1 6

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Polymerization The e q u i l i b r i a discussed above are u s e f u l i n c h a r a c t e r i z i n g plutonium and p r e d i c t i n g i t s behavior i n aqueous systems, but o n l y i f i t i s i n t r u e s o l u t i o n . This i s f r e q u e n t l y the case f o r p l u t o n y l ( V ) and ( V I ) , but much l e s s common w i t h p l u t o n i u m ( I I I ) and ( I V ) . I n p a r t i c u l a r , plutonium(IV) i s very s u s c e p t i b l e t o h y d r o l y s i s and p o l y m e r i z a t i o n a t pH values above 1 (46). I n 0.009 M plutonium(IV) s o l u t i o n s a t 25°, p o l y m e r i z a t i o n increased from e s s e n t i a l l y zero i n 0.1 M HNO3 (pH 1) t o 98 percent i n 0.04 M HNO3 (pH 1.4). Although p o l y m e r i z a t i o n would not occur a t such low pH v a l u e s w i t h the c o n c e n t r a t i o n s of plutonium found i n n a t u r a l waters, i t would be expected a t the higher pH values most common i n the environment. Whether the r e s u l t i n g polymer remains d i s p e r s e d i n the aqueous medium or p r e c i p i t a t e s depends on a number o f f a c t o r s , i n c l u d i n g i t s molecular weight, pH, temperat u r e , and the type and c o n c e n t r a t i o n of anions i n s o l u t i o n ; i n g e n e r a l , t h e presence of polymer w i l l r e s u l t i n a higher concent r a t i o n o f plutonium i n the aqueous phase than would otherwise be the case. I t i s not i n t r u e s o l u t i o n , however, but i s present i n c o l l o i d a l form. Values f o r i t s molecular weight range from 4000 to 1 0 (47), depending on c o n d i t i o n s of formation. Formation of the polymer i s r e l a t i v e l y r a p i d , g e n e r a l l y o c c u r r i n g w i t h i n a matter o f minutes (47); d e p o l y m e r i z a t i o n , on the other hand, r e q u i r e s hours t o days, depending on a c i d i t y and temperature (48), and i s even slower f o r aged polymers (47). Because of t h i s , polymer formation i s o f t e n considered t o be i r r e v e r s i b l e . The presence o f a l a r g e molar excess of c i t r a t e r e t a r d s polymerizat i o n and enhances d e p o l y m e r i z a t i o n (49), doubtless because of complex formation. The s i z e s of the c o l l o i d a l p a r t i c l e s v a r y , but g e n e r a l l y i n c r e a s e w i t h aging, increased i o n i c s t r e n g t h , and the presence o f bicarbonate a t environmental c o n c e n t r a t i o n s (50). 1 0

The nature of the polymer has not been f i r m l y e s t a b l i s h e d . I t has g e n e r a l l y been considered an i n t e r m e d i a t e h y d r o l y s i s product i n which p a r t i a l l y hydrolyzed plutonium species a r e l i n k e d by hydroxide o r oxide b r i d g e s i n t o long c h a i n s . I n t h i s view, the e f f e c t of aging i s t o i n c r e a s e the s i z e of the polymer u n i t s . A more recent c o n c l u s i o n (51), supported by spectrophotom e t r y , x - r a y d i f f r a c t i o n , e l e c t r o n d i f f r a c t i o n , and e l e c t r o n microscopy data, i s t h a t the polymer c o n s i s t s of s m a l l , d i s c r e t e primary p a r t i c l e s ( s i z e 0.5 to 2.0 nm), which may be amorphous o r c r y s t a l l i n e , and secondary p a r t i c l e s which a r e aggregates of the primary p a r t i c l e s . C o l l o i d a l s o l s would thus c o n s i s t of d i s p e r sions of the primary p a r t i c l e s i n aqueous media. I n t h i s concept the slower d e p o l y m e r i z a t i o n of aged polymer i s the r e s u l t of the conversion o f amorphous p a r t i c l e s t o c r y s t a l l i n e p a r t i c l e s on standing. X-ray d i f f r a c t i o n p a t t e r n s of the c r y s t a l l i n e p a r t i c l e s correspond t o that f o r Pu02« A s i m i l a r p a t t e r n i s found f o r p r e c i p i t a t e d PuiOH)^ (47), l e a d i n g t o the suggestion that the primary p a r t i c l e s , which a r e presumably hydrated Pu02> a r e a l s o the f i r s t p a r t i c l e s t o form during p r e c i p i t a t i o n (51).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Concern

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Dispersed polymer can be adsorbed from aqueous media onto surfaces w i t h which i t comes i n contact. Among s o l i d s whose surfaces can adsorb plutonium are numerous m i n e r a l s , g l a s s , and s t a i n l e s s s t e e l . Glass r e p o r t e d l y can adsorb 1.6 Ug c o l l o i d / c m of s u r f a c e , and the a d s o r p t i o n by s t e e l i s much higher (47). A d s o r p t i o n i s h i g h l y v a r i a b l e , depending on c o l l o i d c o n c e n t r a t i o n , pH, age of polymer, nature of presence of anions and complexing l i g a n d s , and nature of the s u r f a c e . I t i s important to bear i n mind t h a t t h i s a d s o r p t i o n i s s t r i c t l y a surface phenomenon and i s g e n e r a l l y i r r e v e r s i b l e ; the r a t e of d e s o r p t i o n i s dependent on depolymerization and s o l u b i l i t y c o n s i d e r a t i o n s . Because of the nature of the process and i t s i r r e v e r s i b i l i t y , i t cannot be d e s c r i b e d by e q u i l i b r i u m i o n exchange parameters, such as d i s t r i b u t i o n c o e f f i c i e n t s [ K ^ ] . The two processes are completely Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

2

u n r e l a t e d ; i n t e r e s t i n g l y enough, the polymer i s r e p o r t e d l y not adsorbed by i o n exchange r e s i n s (46) . A c c o r d i n g l y , i t i s important f o r researchers to e s t a b l i s h the absence of polymeric species i n a given s o l u t i o n before u s i n g i t to determine e q u i l i b r i u m d i s t r i b u t i o n values. Areas Needing Further Research With the exception of a few areas such as p o l y m e r i z a t i o n , the aqueous chemistry of plutonium has r e c e i v e d much study. However, most of the current knowledge a p p l i e s to macro concent r a t i o n s of the element i n strong a c i d s o l u t i o n s , and i t s a p p l i c a t i o n to the picogram l e v e l s and n e u t r a l pH values of environmental systems r e q u i r e s utmost c a u t i o n . Since h y d r o l y s i s i s r e l a t i v e l y more important, i t i s p o s s i b l e that the complex species formed are not simple m e t a l - l i g a n d m o i e t i e s , but r a t h e r i n v o l v e hydroxylated metal ions i n complex species not p r e v i o u s l y i d e n t i f i e d . The present s t a t e of knowledge of plutonium chemistry under environmental c o n d i t i o n s does not appear adequate to permit chemical modeling w i t h any degree of confidence. To reach t h i s d e s i r e d o b j e c t i v e , a d d i t i o n a l data, determined under simulated environmental c o n d i t i o n s — n o t e x t r a p o l a t e d from g r o s s l y d i f f e r e n t c o n d i t i o n s — a r e needed. Several areas of research seem to merit top p r i o r i t y : 1. attempt to v e r i f y published s t a b i l i t y constants of e n v i r o n mental i n t e r e s t at lower metal concentrations and higher pH; 2. determine s t a b i l i t y constants that are not c u r r e n t l y known, the prime example being the plutonium-carbonate system; 3. assess the i n t e r p l a y of complexation, h y d r o l y s i s , and polyme r i z a t i o n a t environmental pH v a l u e s , as these f a c t o r s are important but not w e l l understood under n e u t r a l c o n d i t i o n s ; 4. study the complex chemistry of p l u t o n y l ( V ) , which some workers b e l i e v e to be an important species i n ground waters; 5. attempt to e l u c i d a t e the nature and behavior of polymeric species w i t h the u l t i m a t e o b j e c t i v e of developing q u a n t i t a t i v e , r e p r o d u c i b l e expressions f o r d i s p e r s i o n , p r e c i p i t a t i o n ,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

a d s o r p t i o n , and d e s o r p t i o n of the polymer. This i s a b i g order, and may appear u n r e a l i s t i c to some. I n at l e a s t one man's opinion, however, progress i n these f i v e areas i s d e s i r a b l e , not only f o r meaningful modeling, but more i m p o r t a n t l y , f o r a true under­ standing of the behavior of plutonium i n the environment.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

Abstract The behavior of plutonium in aqueous environmental systems i s governed by the complex interrelationships of three types of chemical equilibria - oxidation state, hydrolysis, and complexa­ tion - and by polymerization, which i s a non-equilibrium process; and our a b i l i t y to understand i t s environmental behavior w i l l de­ pend on our knowledge of these fundamental chemical reactions. Oxidation potentials are known with satisfactory precision. Hydrolysis constants and hydroxide solubility products are of varying but generally dubious r e l i a b i l i t y . The situation with regard to complex s t a b i l i t y constants i s somewhat variable. Com­ plexes with strong chelating legands such as c i t r i c acid, EDTA, and DTPA have been quantitatively described with reasonable accuracy, but data on plutonium complexes with such environmentally-important ions as carbonate and phosphate are meager and questionable. Least understood of all are the irreversible polymerization reac­ tions of plutonium which play so large a part in i t s aqueous chemistry. Data for all of these reactions w i l l be reviewed and evaluated, accompanied when necessary by a critique of the methodo­ logy. The environmental consequences of this information w i l l be assessed; for example the fallacies of attempting to use solubility products to calculate plutonium concentrations, or of employing equilibrium ion exchange concepts to describe non-equilibrium sur­ face adsorption of polymer w i l l be emphasized. Moreover, since most of the literature refers to macro concentrations of plutonium and frequently to relatively strong acid solution, great caution is necessary in extrapolating to environmental conditions. In conclusion, suggestions w i l l be made of areas of plutonium chemis­ try that most urgently require further study. Literature Cited 1. 2.

3. 4.

Cleveland, J. M. "The Chemistry of Plutonium," 653 p. Gordon and Breach Science Publishers, Inc., New York, 1970. Kraus, Κ. Α., and Dam, J. R., p. 466-477, in Seaborg, G. T., Katz, J. J., and Manning, W. Μ., eds., "The Transuranium Elements," McGraw-Hill Book Co., New York, 1949. Katz, J. J., and Seaborg, G. T. "The Chemistry of the Actinide Elements," John Wiley and Sons, Inc., New York, 1957. Cassol, Α., Magon, L., Portanova, R., and Tondello, E. Hydrolysis of plutonium(VI): acidity measurements in perchlorate solutions, Radiochim. Acta, 17, 28 (1972).

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Equilibria

of

Environmental

Concern

CLEVELAND

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Schedin, U. Hydrolysis of metal ions. 62. Plutonyl ion i n sodium perchlorate medium, Acta Chem. Scand., A29, 333 (1975). Rabideau, S. W., and Lemons, J . F. The potential of the Pu(III)-Pu(IV) couple and the equilibrium constants for some complex ions of Pu(IV), J . Am. Chem. Soc., 73, 2895 (1951). Metivier, H., and Guillaumont, R. Hydrolysis of pluto­ nium (IV), Radiochem. Radioanal. Lett., 10, 27 (1972). Baes, C. F., Jr., and Mesmer, R. E. "The Hydrolysis of Cations," 458 p., John Wiley and Sons, Inc., New York, 1976. Gel'man, A. D., and Zaitsev, L. M. Carbonate and oxalato­ -carbonate complex compounds of plutonium(IV), Russ. J . Inorg. Chem., 3, 47 (1958). Moskvin, A. I., and Gel'man, A. D. Determination of the composition and i n s t a b i l i t y constants of oxalate and carbonate complexes of plutonium(IV), Russ. J . Inorg. Chem., 3, 198 (1958). Gel'man, A. D., Moskvin, A. I., and Zaitseva, V. P. Carbonate compounds of plutonyl, Soviet Radiochem., 4, 138 (1962). Fardy, J . J., and Buchanan, J. M. An ion exchange study of the sulfate complexes of plutonium(III), J . Inorg. Nucl. Chem., 38, 579 (1976). Nair, G. Μ., Rao, C. L., and Welch, G. A. Plutonium(III)sulfate complexes, Radiochim. Acta, 7, 77 (1967). Fardy, J. J., and Pearson, J. M. Ion exchange study of the sulfate complexes of plutonium(IV), J . Inorg. Nucl. Chem., 36, 671 (1974). Pozharskii, V. G. Behavior of plutonyl i n hydrochloric and sulfuric acid solutions, Soviet Radiochem., 8, 259 (1966). P a t i l , S. K., and Ramakrishna, V. V. Sulfate and fluoride complexing of uranium(VI), neptunium(VI), and plutonium(VI), J. Inorg. Nucl. Chem., 38, 1075 (1976). Krylov, V. N., and Komarov, E. Investigation of the complex formation of Pu(IV) with the fluoride ion i n solutions of HClO by the ion-exchange method, Soviet Radiochem., 11, 94 (1969). Bagawde, S. V., Ramakrishna, V. V., and P a t i l , S. K. Aqueous TTA complexing of neptunium(IV) and plutonium(IV), J. Inorg. Nucl. Chem., 38, 2085 (1976). Krylov, V. N., Komarov, Ε. V., and Puchlenkov, M. F. Complex formation of Pu(VI) with the fluoride ion i n solutions of HClO , Soviet Radiochem., 10, 705 (1968). Marcus, Y. Anion exchange of metal complexes. XV. Anion exchange and amine extraction of lanthanides and trivalent actinides from chloride solutions, J . Inorg. Nucl. Chem., 28, 209 (1966). Grenthe, I., and Noren, B. On the s t a b i l i t y of nitrate and chloride complexes of plutonium(IV), Acta Chem. Scand., 14, 2216 (1960).

6.

7. 8. 9.

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Plutonium

16.

11. 12.

13. 14.

15. 16.

17.

4

18.

19.

4

20.

21.

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Rabideau, S. W., Asprey, L. B., Keenan, T. K., and Newton, T. W., p. 361-73, "Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958," Vol. 28, United Nations, Geneva, 1958. 23. Bagawde, S. V., Ramakrishna, V. V., and P a t i l , S. K. Complex­ ing of tetravalent plutonium i n aqueous solutions, J . Inorg. Nucl. Chem., 38, 1339 (1976). 24. Newton, T. W., and Baker, F. B. Chloride complex ions of Pu(VI), J . Phys. Chem., 61, 934 (1957). 25. Rabideau, S. W., and Masters, B. J . Kinetics of the reaction between Pu(VI) and Sn(II) i n chloride-perchlorate solution, Phys. Chem., 65, 1256 (1961). 26. Zebroski, E. L., and Neumann, F. K. data quoted i n C. G. Suits and Κ. H. Kingdon, "Progress Report No. 33, April 1-30, 1949," USAEC report KAPL-184, May 20, 1949. 27. Ghosh Mazumdar, A. S., and Swaramakrishnan, C. K. A study of the nitrate and the chloride complexes of plutonium(VI) by solvent extraction technique using TTA as the chelating agent, J. Inorg. Nucl. Chem., 27, 2423 (1965). 28. Heisig, D. L., and Hicks, T. E. The distribution of Pu(VI) and Pu(III) i n thenoyltrifluoroacetone-benzene-nitric acid mixtures, USAEC Report UCRL-1664, Feb. 2, 1952. 29. Moskvin, A. I. Investigation of the complex formation of trivalent plutonium, americium, and curium i n phosphate solutions, Soviet Radiochem., 13, 688 (1971). 30. Denotkina, R. G., Moskvin, A. I., and Shevchenko, V. Β The composition and dissociation constants of phosphate complexes of plutonium(TV) determined by the solubility method, Russ. J . Inorg. Chem., 5, 731 (1960). 31. Moskvin, A. I. Complex formation of the actinides with anions of acids i n aqueous solutions, Soviet Radiochem., 11, 447 (1969). 32. Moskvin, A. I. Zaitseva, V. P., and Gel'man, A. D., Investi­ gation of the complex formation of trivalent plutonium with anions of acetic, c i t r i c , and tartaric acids by the ion exchange method, Soviet Radiochem., 6, 206 (1964). 33. Nebel, D. Spectrophotometric studies of equilibrium of pluto­ nium(IV)-citrate in aqueous solution, Z. Physik. Chem. Leipzig, 232, 161 (1966). 34. Nebel, D. Potentiometric studies of the equilibrium of pluto­ nium citrate i n an aqueous solution, Z. Physik. Chem. (Leipzig), 232, 368 (1966). 35. Moskvin, A. I. Complex formation by neptunium(IV) and pluto­ nium(IV) in nitrate solutions, Russ. J . Inorg. Chem., 16, 405 (1971). 36. Foreman, J . K., and Smith, T. D. The nature and s t a b i l i t y of the complex ions formed by ter-, quadri-, and sexa-valent plutonium ions with ethylenediaminetetraacetic acid (EDTA). Part I I . Spectrophotometric studies, J . Chem. Soc., (Pt. 2), 1758 (1957).

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22.

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16.

CLEVELAND

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of

Environmental

Concern

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37.

337

Foreman, J . K., and Smith, T. D. The nature and s t a b i l i t y of the complex ions formed by ter-, quadri-. and sexa-valent plutonium ions with ethylenediaminetetraacetic acid. Part I. pH titrations and ion exchange studies, J . Chem. Soc., (Pt. 2), 1752 (1957). 38. Gel'man, A. D., Moskvin, A. I., and Artuykhin, P. I. Compo­ s i t i o n and dissociation constants of Pu(V) and Pu(III) com­ plexes with ethylenediaminetetraacetic acid, Soviet J. At. Energy, 7, 667 (1961). 39. Cauchetier, P., and Guichard, C. Electrochemical and spectro­ photometric study of the complexes of plutonium ions with EDTA. I. Plutonium(III) and (IV), Radiochim. Acta, 19, 137 (1973). 40. Klygin, A. E., Smirnova, I. D., and Nikol'skaya, N. A. The solubility of ethylenediaminetetraacetic acid i n ammonia and hydrochloric acid and i t s reaction with uranium(IV) and plutonium(IV), Russ. J . Inorg. Chem., 4, 1279 (1959). 41. Krot, N. N., Ermolaev, N. P., and Gel'man, A. D. The behavior of ethylenediaminetetraacetic acid i n acid solutions and i t s reaction with uranium(IV), Russ. J . Inorg. Chem., 7, 1062 (1962). 42. Cauchetier, P., and Guichard, C. Electrochemical and spectro­ photometric study of plutonium complexes with EDTA. Pluto­ nium(V) and (VI), J . Inorg. Nucl. Chem., 317, 1771 (1975). 43. Kabanova, O. L. Plutonium(V) complexes with ethylenediamine­ tetraacetic acid, Russ. J. Inorg. Chem., 6, 401 (1961). 44. Gel'man, A. D., Artyukhin, P. I., and Moskvin, A. I. A study of complex formation by pentavalent plutonium i n ethylene­ diaminetetraacetate solutions by means of ion exchange, Russ. J. Inorg. Chem., 4, 599 (1959). 45. Kabanova, O. L., Danuschenkova, Μ. Α., and Paley, P. N. The reactions of plutonium ions with ethylenediaminetetraacetic acid, Anal. Chim. Acta, 22, 66 (1960). 46. Costanzo, D. Α., Biggers, R. E., and Bell, J . T. Plutonium polymerization. I. Spectrophotometric study of the polymer­ ization of plutonium(IV), J. Inorg. Nucl. Chem., 35, 609 (1973). 47. Ockenden, D. W., and Welch, G. A. The preparation and properties of some plutonium compounds. Part V. Colloidal quadrivalent plutonium, J. Chem. Soc., 3358 (1956). 48. Brunstad, A. Polymerization and precipitation of pluto­ nium(IV) in n i t r i c acid, Ind. Eng. Chem., 51, 38 (1959). 49. Lindenbaum, Α., and Westfall, W., Colloidal properties of plutonium i n dilute aqueous solution, Int. J. Appl. Radiat. Isotop., 16, 545 (1965). 50. Andelman, J . B., and Rozzell, T. C. Plutonium i n the water environment. I. Characteristics of aqueous plutonium, p. 118-137 in "Radionuclides in the Environment," Adv. Chem. Ser. 93, 1970.

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Lloyd, M. H., and Haire, R. G. Studies on the chemical and colloidal nature of Pu(IV) polymer, in "Proceedings of the XXIVth IUPAC Congress, Hamburg, September 1973;" CONF 730927.2.

Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. November 16,

1978.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch016

RECEIVED

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

17 Estimated Free Energies of Formation, Water Solubilities, and Stability Fields for Schuetteite ( H g O S O ) and 3

Corderoite ( H g S C l ) at 298 3

2

2

2

4

Κ

GEORGE A. PARKS and DARRELL KIRK NORDSTROM

1

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch017

Department of Applied Earth Sciences, Stanford University, Stanford, CA 94305 Mercury forms many double s a l t s of which a few occur natu­ r a l l y ; these i n c l u d e e g l e s t o n i t e , Hg40Cl and t e r l i n g u a i t e , H g 0 C l ( 1 ) , s c h u e t t e i t e , Hg 02S04(_2), and c o r d e r o i t e , Hg3S2Cl2 O). S o l u b i l i t y and thermodynamic data u s e f u l i n a s s e s s i n g the impor­ tance of these compounds i n c o n t r o l l i n g the mercury content of n a t u r a l waters are l a c k i n g . Only an enthalpy of formation f o r s c h u e t t e i t e i s r e p o r t e d . We have estimated the standard f r e e energies of formation of s c h u e t t e i t e and c o r d e r o i t e by f i r s t e s t i ­ mating t h e i r absolute e n t r o p i e s and the m i s s i n g enthalpy of form­ a t i o n of c o r d e r o i t e . Independent estimates were d e r i v e d from s o l u b i l i t y data f o r s c h u e t t e i t e and from vapor-phase s y n t h e s i s data f o r c o r d e r o i t e . The two s e t s of estimates are compared and the r e s u l t s were used to determine the s t a b i l i t i e s of these miner­ a l s r e l a t i v e to the more common mercury m i n e r a l s . The s t a b i l i t y f i e l d diagrams and s o l u b i l i t i e s are used to comment b r i e f l y on c o n d i t i o n s r e q u i r e d f o r f i e l d occurrence. 2

2

3

E s t i m a t i o n of E n t r o p i e s The Debye theory of the heat c a p a c i t i e s of s o l i d elements y i e l d s an expression f o r t h e i r e n t r o p i e s , S = 3R(£nT - K

2

+ Κ )

(4)

(1)

i n which Κχ i s a u n i v e r s a l constant. K i s a constant f o r each element and i s determined by the i n t e r a t o m i c f o r c e constant and atomic mass. Equation 1 assumes that the heat c a p a c i t y and en­ tropy are dominated by v i b r a t i o n a l c o n t r i b u t i o n s . E l e c t r o n i c , d i s o r d e r , r o t a t i o n a l , s t r u c t u r a l , and mixing c o n t r i b u t i o n s are small f c r most monatomic s o l i d s , and are neglected. P o s t u l a t e s by Latimer (5) and Kopp (see P i t z e r and Brewer (4)) make i t pos­ s i b l e to p r e d i c t e n t r o p i e s of polyatomic s o l i d s or compounds. Latimer assumed that the c o n t r i b u t i o n of i n t e r a t o m i c f o r c e 2

1

Current address: Department of Environmental Sciences, University of Virginia, Char­ lottesville, VA 22903. 0-8412-0479-9/79/47-093-339$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

constants to entropy was s m a l l , and p o s t u l a t e d that the entropy of each element i n a s o l i d compound could be approximately accounted f o r by i t s mass alone, F

S = R£nM + S ο

(2)

where SQ has the same v a l u e f o r a l l elements. Kopp proposed that the heat c a p a c i t y of a s o l i d compound could be approximated by the s t o i c h i o m e t r i c sum of the heat c a p a c i t i e s of i t s c o n s t i t u ­ ent elements. On the b a s i s of Kopp s p o s t u l a t e d a d d i t i v i t y of heat c a p a c i t i e s , Latimer suggested t h a t the entropy of a s o l i d compound, M-jA-j , c o u l d be approximated by the s t o i c h i o m e t r i c sum of the c o n t r i b u t i o n s of i t s c o n s t i t u e n t elements, Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch017

1

s

vr a = iS™M + J SA · (3) M. A. ι J Latimer used Equation 2 to o b t a i n entropy c o n t r i b u t i o n s f o r c a t ­ ions and Equation 3, together w i t h experimental e n t r o p i e s of compounds and the c a t i o n c o n t r i b u t i o n s , to estimate a n i o n i c con­ t r i b u t i o n s . Naumov, Ryzhenko and Khodakovsky (6) extended L a t i ­ mer's t a b l e s of c o n s t i t u e n t c o n t r i b u t i o n s . For e s t i m a t i n g the entropy of a p a r t i c u l a r compound w i t h Equation 3, D r o z i n (7) used a n i o n i c c o n t r i b u t i o n s d e r i v e d from compounds of metals i n the same p e r i o d i c subgroup r a t h e r than o v e r a l l averages as Latimer (5) had used. Equations 2 and 3 can be combined to express the entropy of one compound i n terms of another, thus A

S

M.A. 1

S

J

f

+

- N.A. ι 3

( \ 7/

R

£η

W/

,

(4)

N

P i t z e r and Brewer (4) suggest t h i s approach f o r e s t i m a t i o n i f Nj_Aj i s s i m i l a r , i n terms of s t r u c t u r e and p r o p e r t i e s , to M^Aj . This approach should minimize e r r o r a r i s i n g i n n e g l e c t of nonmass-related entropy c o n t r i b u t i o n s . I f the heat c a p a c i t i e s and e n t r o p i e s of s o l i d s are simply the weighted sums of those of t h e i r elemental c o n s t i t u e n t s , then the entropy change should be zero f o r symmetrical r e a c t i o n s such as, 2HgX + P b 0 S 0 2

2

4

= 2PbX + Hg 0S0 2

2

(5)

4

i n which the number of molecules produced i s the same as the num­ ber consumed and the number of atoms i n each product molecule i s the same as the number i n a corresponding r e a c t a n t molecule (7)· X i s a monovalent anion i n Equation 5. Using t h i s p r i n c i p l e , the entropy of Hg^SO^ i s given by S

Hg 0S0 2

= 4

S

Pb 0S0 2

4

"

2 S

PbX

+ 2

2 S

HgX

( 6 ) 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

17.

PARKS AND NORDSTROM

Schuetteite

and

Corderoite

341

and can be estimated i f the other entropies are known. This meth­ od should y i e l d r e s u l t s s i m i l a r to those obtained w i t h Equation 4 but could introduce greater e r r o r s i n c e c l o s e s i m i l a r i t y i n s t r u c ­ ture and p r o p e r t i e s among four compounds of two d i f f e r e n t types i s u n l i k e l y . For t h i s reason Drozin (7) suggests u s i n g compounds of metals i n the same p e r i o d i c subgroup i n comparisons u t i l i z i n g Equation 6.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch017

Tests of E s t i m a t i o n Methods Equation 3. We estimated the e r r o r to be expected i n the use of Equation 3 by comparing e m p i r i c a l e n t r o p i e s w i t h those c a l c u ­ l a t e d u s i n g c o n t r i b u t i o n s t a b u l a t e d by Latimer (5) without m o d i f i ­ c a t i o n . The e m p i r i c a l data were taken from Hepler and Olofsson ( 8 ) , Robie, Hemingway and F i s h e r (9) and the N a t i o n a l Bureau of Stan­ dards T e c h n i c a l Note 270 s e r i e s (11). Data f o r mercury are l i s t e d i n Table I . E m p i r i c a l and c a l c u l a t e d e n t r o p i e s are compared

Table I . Thermodynamic P r o p e r t i e s of Mercury M i n e r a l s and Compounds a t 298.15K, I n c l u d i n g Estimates.

s°, 1

JK m o l

1

+ 76.02

Hg(£)

ΔΗ°,

AG°,

kJ mol ^

kJ mol

0.0

0.0

- 36.23

+170.16

+164.703

montroydite Hg0(c,red,orth. ,) + 70.29 calomel

2+

Hg (aq)

- 90.83

- 58.555

Hg Cl (c)

+191.42

-265.579

-210.773

HgCl (c)

+146.0

-225.9

-180.3

cinnabar

HgS(c,red)

+ 82.4

- 54.0

- 46.4

corderoite

Hg S Cl (c)

[301±17]

2

2

2

3

2

2

[(

u0

n

2

n

2

n

2

11

H C r O " [ 0 , l ] , H Mo0 " [0-2], H S 0 " [ 0 , l ] , H SeO " η 4 η 4 η 4 η 4 [0,11 H W O [ 0 - 2 ] , MnO ", TeO ( O H ) " " [nm = 06, ' η 4 4 n m 15, 24], ( U 0 ) ^ ( 0 H ) " [nm = 10, 11, 22, 35], 2 n ~ 'm W..Ο " 12~39' 2_n

2

2 n

o

y

6

2 n

m

m

v

6

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

n

358

CHEMICAL MODELING IN AQUEOUS SYSTEMS

the complex i s c a l l e d outer sphere o r an i o n p a i r . Bonding i s c h i e f l y l o n g range and e l e c t r o s t a t i c . The i o n p a i r may p e r s i s t through a number o f c o l l i s i o n s w i t h water molecules. Examples of i o n p a i r s are most monovalent and d i v a l e n t oxyanion complexes formed w i t h a l k a l i metals and a l k a l i earths ( f o r example, NaHC03 , SrC03°, CaNÛ3 , and MgHP04°). The l i s t a l s o i n c l u d e s most monov a l e n t and d i v a l e n t metal s u l f a t e and c h l o r i d e complexes. A v a i l able data suggest the f o l l o w i n g l o g K values f o r i o n p a i r s : ML p a i r s ^ 0 - 1 ; ML2 o r M2L p a i r s ^.7 - 1.3, and M2L2 p a i r s 2.3 - 3 . 2 . For i o n p a i r s , K values tend t o be roughly constant f o r a given l i g a n d w i t h metal c a t i o n s o f i d e n t i c a l valence and roughly the same s i z e . For example, the d i v a l e n t metal s u l f a t e i o n p a i r s formed w i t h Ca, Mg, N i , Zn, Cu, Co, Cd, Mn, Fe, and Cu have l o g K values from 2.28 t o 2.35 (see F i g u r e 2 ) . T h i s behavior r e f l e c t s the l a r g e l y e l e c t r o s t a t i c a t t r a c t i o n between the i o n s , n e a r l y independent o f the d e t a i l e d e l e c t r o n c o n f i g u r a t i o n of the c a t i o n ( 1 ) . When complexation i n v o l v e s displacement by the l i g a n d o f water molecules immediately adjacent t o the c a t i o n so t h a t the l i g a n d contacts the c a t i o n , the complex i s c a l l e d i n n e r sphere. Such complexes are g e n e r a l l y more s t a b l e than outer sphere complexes o r i o n p a i r s . A c t u a l l y , there are no pure " i n n e r " o r "outer" sphere complexes. For example, even i n SO4 complexes o f Be, Mg, Zn, N i , Co, and Mn, about 10% of the bonding i s inner and 90% outer sphere. For C r - s u l f a t e , the p r o p o r t i o n s become more than 70% inner and 30% outer sphere ( 1 ) . +

a s s o c

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch018

a s s o c

a s s o c

3 +

Electronegativity P a u l i n g (24) has defined e l e c t r o n e g a t i v i t y (EN) as the power of an atom to a t t r a c t e l e c t r o n s . The concept i s u s e f u l i n comp a r i n g s t a b i l i t i e s of i n n e r sphere complexes when t h e i r bonding i s s i g n i f i c a n t l y c o v a l e n t . The degree o f covalency (as opposed to i o n i c i t y ) o f bonding i n the complex i n c r e a s e s as the d i f f e r ence i n EN (ΔΕΝ) o f the c a t i o n and l i g a n d approaches zero. Based on bonds i n c r y s t a l s , P a u l i n g computes t h a t roughly when ΔΕΝ 1.5. Based on the r e l a t i v e s o l u b i l i t i e s o f t h e i r s a l t s , EN values f o r CO 2~\ -

2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

18.

Aqueous Complexes of Geochemical Interest

LANGMUiR

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch018

7.0

6.0

• so|"

Δ

ε ο ο Ξ

7 /

ο CO* x ΗΡ0|' HCO;

CO'

5.0

ο

V

/

4.0 ΗΡ0

χ/

Δ

3.0 n — •

— —-

u

π

2- •

S0 -Έ-° 4

D

α

π

-

π

—* τ -

2.0 HCO;

i.oh Βα

I Sr

Ca

Mg

111 I

II

in

Be I P b I Cd |UCv>|Co| Ni 'ode Mn Zn VO Fe Cu Hg

I

_J

2.0

I.0 I.5 Electronegativity of Cation

Figure 2. Stabilities of some 1:1 oxyanion complexes plotted against the electro­ negativity of the cation. Literature data have been corrected to I = 0 when necessary. Lines through the data for HCOf and SO ' complexes are for mean values. Curves drawn through the ΗΡΟ/' and CO/' data have no statistical significance. 2

/f

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

359

CHEMICAL MODELING IN AQUEOUS SYSTEMS

360

HP04 ~,

and SO^^ should decrease from near 4 to about 3 i n the order SO^ > HPO4 "" > C O 3 " . Thus, covalency of bonding should be l e a s t f o r the s u l f a t e complexes and greatest f o r the carbonate complexes, which are apparently f o r the most p a r t of inner sphere character f o r c a t i o n EN values above 1.5. For the a l k a l i n e e a r t h s , bonding w i t h a l l four l i g a n d s shown i n Figure 2 i s l a r g e l y i o n i c ( i . e . , independent of EN) be­ cause of t h e i r predominantly outer sphere c h a r a c t e r . The s l i g h t decrease i n s t a b i l i t y of these complexes g e n e r a l l y , from Ba through Be, may r e f l e c t the l i g a n d s c l o s e r approach to the r e l a t i v e l y unhydrated Ba i o n (Ip = 1.5), a p r o x i m i t y not p o s s i b l e i n the case of Mg (Ip = 3 . 0 ) or Be (Ip = 5.7). These s m a l l e r ions s t r o n g l y r e t a i n t h e i r waters of h y d r a t i o n i n complex form­ ation. -

Z

2-

2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch018

1

Schwarzenbach's and Pearson's C l a s s i f i c a t i o n s There are no simple r u l e s i n v o l v i n g addend s i z e , valence, Ip, or EN that can e x p l a i n a l l the observed s t a b i l i t i e s of com­ p l e x e s . However, f u r t h e r i n s i g h t s are provided through approaches suggested by Schwarzenbach ( 2 ) , and Pearson (3) and Ahrland (4). Pearson and Ahrland c l a s s i f y c a t i o n s and l i g a n d s as 'hard or s o f t a c i d s or bases. Soft i n f e r s the s p e c i e s ' e l e c t r o n cloud i s deformable or p o l a r i z a b l e and may enter i n t o e l e c t r o n i c a l l y unique s t a t e s i n complexation. Hard addends are comparatively r i g i d and non-deformable and show an absence of e l e c t r o n i c i n t e r a c t i o n s i n complexation. Schwarzenbach (2) (see a l s o P h i l l i p s and W i l l i a m s ( 3 Q ) ; N a n c o l l a s , (1)) d e f i n e s three c l a s s e s of complexes. Class A i n c l u d e s metal c a t i o n s which have noble gas c o n f i g u r a t i o n s . These are l i s t e d below. 1

!

T

Cation ( i n c r e a s i n g covalency Be +

Na , K', Rb"*

Mg , A l 2+

3 +

Sc , 2 +

3+

cs

3 +

Sr , Y ,

Ti

4 +

Zr

4 +

•HCO cd

ω

u

g

•H

Cs

Ba

Atomic C o n f i g u r a t i o n He

2+

Ca

> )

2+

, La

>·> Ο β CD

Ne

> Ο

Kr

Ar

u

Xe

Class A c a t i o n s have s p h e r i c a l symmetry and low p o l a r i z a b i l i t y and thus are 'hard spheres' (31). Pearson's hard a c i d s i n c l u d e the above, but a l s o M n , U O ^ , V0 , C r , F e , C o , G a , S i , U^ , and T h . These c a t i o n s tend to form l a r g e l y e l e c t r o s t a t i c bonds w i t h l i g a n d s , e s p e c i a l l y when the l i g a n d s are hard (have low 2+

+

2+

3 +

3 +

3 +

3 +

4 +

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

4 +

18.

361

Aqueous Complexes of Geochemical Interest

LANGMUiR

p o l a r i z a b i l i t y ) and the c a t i o n s monovalent or d i v a l e n t . Important complexes are formed with hard l i g a n d s F , H2O, and 0H~. Carbon­ ate and bicarbonate complexes are a l s o important but only f o r the monovalent and e s p e c i a l l y d i v a l e n t c a t i o n s . The s t a b i l i t y of_ complexes i s g e n e r a l l y i n the order OH~>F , C 0 3 ~ > H P 0 4 " » N 0 3 and P 0 ^ 3 " » H P 0 4 ~ > S 0 4 ~ . Complexes are u s u a l l y not formed with S, N, C, CI , Br , or Γ , because these species cannot compete with H2O or OH . Complexes probably increase g e n e r a l l y i n s t a b i l i t y i n the order I 2 (aq)

-217.7

A1(0H)°(aq)

-266.66

A1(0H)~ (aq)

-312.0

-356.1

35

1/

H Si0° (aq)

-312.6

-348.30

45.1

1/

R^SiO^ (aq)

-299.18

-342.18

20.7

(10)

H SiO^"(aq)

-281.31

-330.7

-.7

1/

(£>

Silicon 4

2

Others H0

-56.688

-68.315

16.71

(ID

OH H+

-37.60 0

-54.98 0

-2.56 0

-108.70

-111.58

-32.98

(8) (8) (8)

2

Mg

2+ 1/

Langmuir, Donald, Pennsylvania State Univ. unpublished data,

1978).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

19.

Kaolinite

BASSETT E T A L .

and

Sepiolite

393

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

Thermochemical Studies of K a o l i n i t e .

The e a r l i e s t reported

Gibbs f r e e energy of formation (AG°,298.15) f o r k a o l i n i t e i s -888.1 +0.7 k c a l / m o l , as computed by Barany and K e l l e y (12). Two k a o l i n i t e s were employed i n t h e i r study, one from a deposit near Murfreesboro, Ark., and the second from A l t a Mesa, N. Mex. The free-energy value was obtained by combining the heat of s o l u t i o n measurements from h y d r o f l u o r i c a c i d s o l u t i o n c a l o r i m e t r y of Barany and K e l l e y (12), w i t h heat c a p a c i t y determinations f o r k a o l i n i t e by King and Weller (13). The free-energy value i s much too p o s i t i v e and can be made more r e a l i s t i c by i n c o r p o r a t i n g r e v i s e d and updated thermodynamic data i n t o the r e a c t i o n scheme. In the same study, Barany and K e l l e y a l s o determined the heat of format i o n f o r g i b b s i t e ; Hemingway and others (5) redetermined the enthalpy and heat c a p a c i t y of g i b b s i t e Ç5, 7^) and re-evaluated Barany and K e l l e y s r e s u l t s . Hemingway and others d i s c u s s the experimental work i n d e t a i l and p o i n t out t h a t the heat of s o l u t i o n measurements f o r both g i b b s i t e and k a o l i n i t e appear to be c o r r e c t ; however, the heat of s o l u t i o n of H 0(1) + HF and H 0(1) + HC1 i n HF i s i n e r r o r . In a d d i t i o n , the r e a c t i o n scheme o r i g i n a l l y employed r e q u i r e d that the heat of s o l u t i o n f o r A1C1 *6H 0 (aluminum c h l o r i d e hexahydrate) be known; that value was i n c o r r e c t because of the technique used i n emplacing the sample i n the ampules and l o a d i n g them i n t o the c a l o r i m e t e r . To avoid these sources of e r r o r , Hemingway and Robie (7) used only the h e a t - o f - s o l u t i o n measurement of Barany and K e l l e y and wrote the r e a c t i o n scheme to i n c l u d e g i b b s i t e , which has well-known thermodynamic p r o p e r t i e s . Their r e c a l c u l a t e d value f o r the f r e e energy of formation of k a o l i n i t e at 298.15°K i s -908.07 kcal/mol (7), which i s the most negative f r e e energy reported to date and represents the only c a l o r i m e t r i c value a v a i l a b l e . I t should be noted that the m i n e r a l o g i c a l p u r i t y and c r y s t a l l i n i t y of the two k a o l i n i t e s used by Barany and K e l l e y (12) i s not known, as an X-ray examination was not conducted; however, the chemical a n a l y s i s of the bulk m a t e r i a l i n d i c a t e d that the S i 0 and ΑΙ,^Ο^ content was w i t h i n l h percent of the t h e o r e t i c a l composition of kaolinite. S o l u b i l i t y Studies of K a o l i n i t e . There have been numerous attempts to determine the f r e e energy of formation of k a o l i n i t e from s o l u b i l i t y s t u d i e s (Table I I ) . P o l z e r and Hem (3) reacted an API standard k a o l i n i t e from Lewistown, Mont., f o r 2 years w i t h an a c i d i c s o l u t i o n approaching e q u i l i b r i u m from unders a t u r a t i o n . The sample appears to have been very near to e q u i ­ l i b r i u m , and these authors r e p o r t a AG^ ^ g ^ = -903 kcal/mol. 1

2

2

3

2

2

2

Using SOLMNEQ to recompute the a c t i v i t i e s of the d i s s o l v e d species from t h e i r experimental data, and employing the most recent thermodynamic data, the r e c a l c u l a t e d value i s -907.76 kcal/mol. This i s very c l o s e to -908.1 kcal/mol, which i s the c a l o r i m e t r i c value of Robie and others (8). A d d i t i o n a l confidence may be placed i n t h i s value because the p a r t i c l e s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

394

Table I I . — O r i g i n a l And Recomputed Data For The Free Energy Of Formation Of K a o l i n i t e And S e p i o l i t e Material

Original Size

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

source

AG

Murfreesboro, Ark. A l t a Mesa, N. Mex. Lewiston, Mont. England Georgia 1 Georgia 2 Idaho North C a r o l i n a Georgia 3 South C a r o l i n a Montmorillonite Dry Branch, G a . — Keokuk, Iowa Varied 3

5

Bulk Bulk

f,298.15

Recomputed

Recomputed

log Κ

AG

eq

f,298.15

1

^ β δ δ . ι

2.0-149.0 urn -903.0 (3) Bulk -903.8 (19) Bulk -903.6 (19) Bulk -903.4 (19) Bulk -902.9 (19) Bulk -902.9 (19) Bulk -902.7 (19) Bulk -902.5 (19) .2-5.0 ym -904.2 (20) Bulk -905.8 Bulk -903.6 (4) Bulk -897.5 to -903.6 (4) 4

2

(12) 5.92 6.70 6.88 7.08 7.36 7.33 7.37 7.61 6.20 7.38 7.54

5

Selected value

5.96

3

-908.1

-907.8 -906.7 -906.4 -906.1 -905.7 -905.8 -905.7 -905.4 -907.4 -905.8 -905.5 -901.1 to -907.5 -907.7

Sepiolite Precipitated Balmut, N.Y. Amboseli, K e n y a — Selected v a l u e

0.5-10 ym 63-124 urn

-1101.0 (24) -37.4 -1105.6 (25) -40.2 -1105.6 (26) -40.4 -40.4

3

-1101.0 -1105.4 -1105.6 -1105.6

Average v a l u e from f i v e experimental runs on each c l a y . Accepted v a l u e of Hemingway (7) computed from the heat of s o l u t i o n data of Barany and K e l l e y (12). See t e x t . ^Average value f o r 16 sample runs on same m a t e r i a l (May, Η. Μ., U n i v e r s i t y of Wisconsin, p e r s o n a l communication, 1978). Range of values f o r k a o l i n i t e from 26 l o c a t i o n s . 2

3

5

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

19.

BASSETT E T A L .

Kaolinite

and

Sepiolite

395

l e s s than 2 ym diameter were removed to e l i m i n a t e p a r t i c l e - s i z e effects. Reesman (4) i n v e s t i g a t e d the s o l u b i l i t y of numerous standard c l a y minerals, monitoring the approach to e q u i l i b r i u m from unders a t u r a t i o n f o r s e v e r a l months. The a n a l y t i c a l data have been re-evaluated by us and y i e l d f r e e energies of formation ranging from -900.4 to -907.4 kcal/mol f o r k a o l i n i t e (Table I I ) . Reesman employed a w e l l c r y s t a l l i z e d k a o l i n i t e (Keokuk), which i s g e n e r a l l y a s s o c i a t e d with geodes found i n Iowa. Even though the Keokuk k a o l i n i t e has been proposed as a reference mineral, due to i t s well-ordered and h i g h l y c r y s t a l l i n e nature (14), the samples used by Reesman were bulk samples and probably contained very small p a r t i c l e s which increased the s o l u b i l i t y . An i n d i c a t i o n of t h i s i s the f a c t that he chose to c e n t r i f u g e h i s samples f o r 8 hours to s e t t l e a l l the suspended c o l l o i d a l m a t e r i a l (4). The recomputed value obtained f o r Keokuk k a o l i n i t e i s -905.5 kcal/mol (Table I I ) . This value i s s l i g h t l y greater than 2 kcal/mol more p o s i t i v e than the c a l o r i m e t r i c value, or the f r e e energy of forma­ t i o n determined f o r P o l z e r and Hem*s data, which had the d^ and a shape f a c t o r ( r a t i o of p a r t i c l e surface to p a r t i c l e volume m u l t i p l i e d by d, a value of 14 i s used here). There are no published values f o r the s u r f a c e - f r e e energy f o r c l a y minerals; however, the value f o r hydrated s i l i c a g e l i s approximately 120 ergs/cm (17). Smith and Hem (18) report sur­ face energies f o r the edge and face f o r g i b b s i t e c r y s t a l s as 483 and 140 ergs/cm , r e s p e c t i v e l y . Parks determined that a surface energy of 270 ergs/cm would be a l l that i s r e q u i r e d to e x p l a i n the free-energy discrepancy i n the g i b b s i t e data that he evaluated which was due to 0.010 ym p a r t i c l e s (1). Employing the same reasoning, assuming that 0.015 ym p a r t i c l e s are present i n the bulk samples used by Reesman i n h i s study, then the 2.2 kcal/mol d i f f e r e n c e between the data of Reesman (4) and that of Polzer and Hem (3) would r e q u i r e a s u r f a c e - f r e e energy f o r k a o l i n i t e of 150 ergs/cm . T h i s i s c e r t a i n l y w i t h i n the range of values one would expect f o r a surface composed of hydrated aluminum and silica. Because the exact value f o r the s u r f a c e - f r e e energy and the minimum p a r t i c l e s i z e i n the bulk samples are not known, the p o s i ­ t i o n taken i n t h i s paper i s that a f r e e energy of formation f o r 9

2

2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

396

C H E M I C A L M O D E L I N G IN

AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

1

k a o l i n i t e of -905.5 kcal/mol d e r i v e d from Reesman s data i s the most p o s i t i v e v a l u e one should use f o r w e l l - c r y s t a l l i z e d k a o l i n i t e . There are numerous u n c e r t a i n t i e s i n the s o l u b i l i t i e s determined by Reesman (40 f o r the other k a o l i n i t e s ; many samples contained other c l a y m i n e r a l i m p u r i t i e s ; most were bulk samples; and some were mechanically crushed, which may have d i s t o r t e d the c r y s t a l l i n i t y and increased the s o l u b i l i t y . K i t t r i c k (19, 20) performed two separate s t u d i e s to determine the s t a b i l i t y of k a o l i n i t e . In the f i r s t i n v e s t i g a t i o n , samples of b u l k k a o l i n i t e from seven l o c a l i t i e s were e q u i l i b r a t e d w i t h a d i l u t e a c i d s o l u t i o n f o r 2 years. Recomputed AG^ values f o r the seven k a o l i n i t e s based on h i s f i n a l s o l u t i o n compositions range from -905.4 to -906.7 kcal/mol. In the second study, K i t t r i c k (20) reacted the 0.2 to 5 ym f r a c t i o n s of three m o n t m o r i l l o n i t e c l a y s w i t h low pH (

an e x t r a p o l a t e d value (25). Recent d i s s o l u t i o n experiments at 25°C by S t o e s s e l l (26) using n a t u r a l l y o c c u r r i n g s e p i o l i t e from Amboseli, Kenya, suport the e x t r p o l a t e d value of C h r i s t and others -40.4 (23). S t o e s s e l l obtained a Keq e q u i v a l e n t to 10 * f o r the r e a c t i o n described by expression (4) y i e l d i n g a AG ( s e p i o l i t e ) = -1105.6 kcal/mol. Conclusions The experimental values f o r the f r e e energies of formation of k a o l i n i t e and s e p i o l i t e are given i n Table I I . The value of -907.7 +1.33 kcal/mol recommended f o r k a o l i n i t e , i s the mean of three recomputed f r e e energies of formation weighed e q u a l l y i n the computation, and was obtained from c a l o r i m e t r y , d i s s o l u t i o n , and p r e c i p i t a t i o n data. S e v e r a l values i n the -905 to -906.0 kcal/mol range probably r e f l e c t the more s o l u b l e nature of small p a r t i c l e s t y p i c a l l y present i n bulk samples. A f r e e energy of formation f o r s e p i o l i t e of -1105.4 kcal/mol i s based on e x t r a p o l a t i o n to 25°C of r e s u l t s from measurements made at 51, 70, and 90°C by C h r i s t and others (25). In support of t h i s , S t o e s s e l l (26) has determined a Κ at 25°C which y i e l d s a — eq AG^ at 25°C, only 200 c a l o r i e s more negative than that computed from the r e s u l t s of C h r i s t and o t h e r s . The value recommended here f o r the f r e e energy of formation f o r s e p i o l i t e i s -1105.6 +0.4 kcal/mol.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

398

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

Abstract Clay minerals are present i n almost all surface-water and ground-water systems, and i n many instances may be controlling the concentration of aluminum, s i l i c a , iron, magnesium, or other cations i n solution. The thermodynamic data necessary to evaluate the state of reaction (saturation) are not available for some clay minerals, and for those minerals with published values, the data are i n disagreement by as much as 10 kilocalories per mole for the same clay mineral. A c r i t i c a l review of the available data for kaolinite and sepiolite, incorporating both the most recent thermo­ dynamic data for the components in the reaction schemes and a more complete computation for the solubility data, yields the values of -907.7 ± 1.3 and 1105.6 ± 0.4 kilocalories per mole for the free energy of formation of kaolinite and sepiolite, respectively. Literature Cited 1.

2.

3. 4. 5.

Parks, G. A. Free energies of formation and aqueous solu­ b i l i t i e s of aluminum hydroxides and oxide hydroxides at 25°C. Am. Min. 57, 1163-1189 (1972). Kharaka, Υ. Κ., and Barnes, I. SOLMNEQ: Solution-mineral equilibrium computations. U.S. Geol. Survey Computer Contr. Report PB-215 899, 81 p. (1973). Polzer, W. L., and Hem J. D. The dissolution of kaolinite. J. Geophys. Research 70, 6233-6240 (1965). Reesman, A. L. "A Study of Clay Mineral Dissolution." Ph. D. thesis, Univ. Missouri, 215 p., 1966. Hemingway, B. S., Robie, R. Α., Fisher, J. R., and Wilson, W. H. Heat capacities of gibbsite, Al(OH) , between 13 and 480 Κ and magnesite, MgCO , between 13 and 380 Κ and their standard entropies at 298.15 K, and the heat capacities of calorimetry conference benzoic acid between 12 and 316 K. U.S. Geol. Survey J. Research 5, 797-806 (1977). Hemingway, B. S., and Robie, R. A. The entropy and Gibbs free energy of formation of the aluminum ion. Geochim. Cosmochim. Acta 41, 1402-1404 (1977). Hemingway, B. S., and Robie, R. A. Enthalpies of formation of low albite (NaAlSi O ), gibbsite (Al(OH) ), and NaAlO ; revised values for AH°f,298 and AG°f,29 8 of some aluminosili­ cate minerals. U.S. Geol. Survey J. Research 5, 413-429 (1977). Robie, R. Α., Hemingway, B. S., and Fisher, J. R. Thermo­ dynamic properties of minerals and related substances at 298.15 Κ and 1 bar (10 pascals) pressure and at higher temperatures. U.S. Geol. Survey Bull. 1452, 456 p. (1978). Baes, C. F., and Mesmer, R. E. "The Hydrolysis of Cations." 496 p. Wiley-Interscience, New York, 1976. 3

3

6.

7.

3

8.

8

3

5

9.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

19.

10. silicic

11.

12.

13. Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

cities

14.

15.

16.

17. 18.

19. solubility 20. 1 21. 22.

23. 24.

25. in

BASSETT E T A L .

Kaolinite

and Sepiolite

399

Busey, R. Η., and Mesmer, R. E. I o n i z a t i o n e q u i l i b r i u m s of a c i d and p o l y s i l i c a t e formation i n aqueous sodium c h l o r i d e s o l u t i o n s to 300°C. J. Inorg. Chem. 16, 2444-2446 (1977). Robie, R. Α., and Waldbaum, D. R. Thermodynamic p r o p e r t i e s of minerals and r e l a t e d substances a t 298.15°K (25.0°C) and l atmosphere (1.013 bars) pressure and a t higher tempera­ tures. U.S. Geol. Survey B u l l . 1259, 256 p. (1968). Barany, R., and K e l l e y , Κ. K. Heats and f r e e energies of formation of g i b b s i t e , k a o l i n i t e , h a l l o y s i t e , and d i c k i t e . U.S. Bureau Mines Report Inv. 5825, 13 p. (1961). King, E. G., and Weller, W. W. Low-temperature heat capa­ and entropies a t 298.15°K of diaspore, k a o l i n i t e , d i c k i t e , and h a l l o y s i t e . U.S. Bureau Mines Report Inv. 5810, 6 p. (1961). K e l l e r , W., P i c k e t t , Ε. Ε., and Reesman, A. L. Elevated dehydroxylation temperature of the Keokuk geode kaolinite— a p o s s i b l e reference mineral, in Proceedings I n t e r n a t . Clay Conf. 1, 75-85 (1966). Enüstun, Β. V., and Türkevich, J. S o l u b i l i t y of f i n e p a r t i c l e s of strontium n i t r a t e . J . Am. Chem. Soc. 82, 45024509 (1960). S c h i n d l e r , P. Η., Hofer, F., and Minder, W. L o s l i c h k e i t s ­ -produkte von metalloxiden und hydroxiden 10. L o s l i c h k e i t s ­ produkte von Zinkoxyd, Kupfer Hydroxid und Kupferoxid i n Abhangigkeit von Teilchengrosse und Molarer Oberflache e i n B e i t r a g zur Thermodynamik von G r e n z f l a c h e n f e s t - f l u s s i n g . Helv. Chem. Act. 48, 1201-1215 (1965). Adamson, A. W. " P h y s i c a l Chemistry of Surfaces." 698 p. I n t e r s c i e n c e , New York, 1960. Smith, R. W., and Hem, J . D. E f f e c t of aging on aluminum hydroxide complexes i n d i l u t e aqueous s o l u t i o n s . U.S. Geol. Survey Water-Supply Paper 1827-D, 51 p. (1972). K i t t r i c k , J . A. Free energy of formation of k a o l i n i t e from measurements. Am. Min. 51, 1457-1466 (1966). K i t t r i c k , J . A. P r e c i p i t a t i o n of k a o l i n i t e a t 25°C and atm. Clays Clay Min. 18, 261-266 (1970). Velda, B. "Developments i n Sedimentology." 218 p. E l s e v i e r , Amsterdam, 1977. S i f f e r t , B. Quelques réactions de la silice en s o l u t i o n : La formation des a r g i l e s . Mem. Ser. Carte Geol. A l s a c e L o r r a i n e 21, 86 p. (1962). S i f f e r t , Β., and Wey, R. Synthese d'une s e p i o l i t e á temperature o r d i n a i r e . C. R. Acad. S c i . P a r i s 254, 1460-1462 (1962). Wollast, R., Mackenzie, F. T., and B r i c k e r , O. P. E x p e r i mental p r e c i p i t a t i o n and genesis of s e p i o l i t e a t earth surface c o n d i t i o n s . Am. Min. 53, 1645-1662 (1968). C h r i s t , C. L., H o s t e t l e r , P. B., and S i e b e r t , R. M. Studies the system MgO-SiO -CO -H2O ( I I I ) : the a c t i v i t y - p r o d u c t constant of s e p i o l i t e . Am. J . Sci. 273, 65-83 (1973). 2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

400 26.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Stoessell, R. K. "Geochemical Studies of Two Magnesium Silicates, Sepiolite and Kerolite." Ph. D. thesis, Univ. Calif., Berkeley, 122 p., 1977.

Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. November 16,

1978.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch019

RECEIVED

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

20

Ion Exchange and Mineral Stability: Are the Reactions Linked or Separate?

J. A. KITTRICK

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch020

College of Agriculture Research Center, Washington State University, Pullman, WA 99164

From a general chemical point of view the most important c l a s s of minerals i n a g r i c u l t u r a l s o i l s are those having i o n exchange c a p a c i t y . These same minerals also possess most of the t o t a l inorganic surface area of s o i l s and sediments. They thus have a major i n f l u e n c e on water r e t e n t i o n and on other p h y s i c a l p r o p e r t i e s of soils. They are a l s o important i n the r e t e n t i o n of the many p o l l u t a n t s that f i n d t h e i r way i n t o s o i l s and sediments, i n c l u d i n g Cd (e.g., (1)) and l o n g - l i v e d radiosotopes, i n c l u d i n g iodine-129, neptunium-237, and plutonium-239 (e.g., ( 2 ) ) . Minerals that have i o n exchange c a p a c i t y are a l s o important i n the d i s c o v e r y , recovery and r e f i n i n g of petroleum. In petroleum r e f i n i n g , they are important c h i e f l y as c a t a l y s t s and adsorbents. In petroleum recovery they mainly a f f e c t r e s e r v o i r p e r m e a b i l i t y . In the search f o r petroleum they are c h i e f l y important i n drilling f l u i d s and as marker h o r i z o n s . Much of the p r a c t i c a l importance of minerals that have exchange c a p a c i t y hinges on how they c o n t r o l the composition of waters they contact. Examples have been given f o r the soil s o l u t i o n (3), s p r i n g waters (4), l a k e waters (5,6), and the ocean. A recent trend i n the study of mineral s t a b i l i t y has been toward the use of p r e d i c t i o n s based upon the p r i n c i p l e s of chemic a l thermodynamics. This has been prompted by the i n a b i l i t y of previous e m p i r i c a l approaches to provide q u a n t i t a t i v e p r e d i c t i v e models. Models based upon e q u i l i b r i u m thermodynamics r e q u i r e f r e e energies of formation of minerals and the ions and molecules with which they are i n e q u i l i b r i u m , plus equations and e q u i l i b r i u m constants that d e s c r i b e e q u i l i b r i u m c o n d i t i o n s . With these, models based on e q u i l i b r i u m thermodynamics can provide i n s i g h t i n t o the r e l a t i o n s h i p between v a r i o u s aqueous environments and a s s o c i a t e d minerals that are i n e q u i l i b r i u m with them. Where the aqueous environment and a s s o c i a t e d minerals are not i n e q u i l i b r i um, e q u i l i b r i u m thermodynamics can f u r n i s h a frame of r e f e r e n c e f o r understanding the k i n e t i c s of mineral a l t e r a t i o n and f o r mation. The approach was pioneered by R. M. G a r r e l s and has been

0-8412-0479-9/79/47-093-401$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

C H E M I C A L M O D E L I N G IN

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch020

402

AQUEOUS SYSTEMS

used i n c r e a s i n g l y f o r s t u d i e s i n sedimentary geochemistry (e.g., (8, 9 ) ) . M i n e r a l s that have exchange c a p a c i t y are u s u a l l y r a t h e r complicated, so the determination of t h e i r s t a b i l i t y by c a l o r i m e t r i c or s o l u b i l i t y methods has been d i f f i c u l t . An important problem, unique to these m i n e r a l s , a l s o l i m i t s progress i n the understanding of t h e i r s t a b i l i t y and the s t a b i l i t y of other minerals that compete f o r the same elements. This problem con­ cerns whether e q u i l i b r i a w i t h s t r u c t u r a l ions and e q u i l i b r i a w i t h exchangeable ions are l i n k e d or separate. Almost a l l i n v e s t i g a t o r s of m i n e r a l e q u i l i b r i a t a c i t l y assume that e q u i l i b r i a w i t h s t r u c t u r a l ions and e q u i l i b r i a w i t h exchangeable ions are part of the same r e a c t i o n . Studies of i o n exchange e q u i l i b r i a i n v a r i a b l y assume that the two e q u i l i b r i a are separate. One group must be wrong, and the consequences are f a r from t r i v i a l . The Assumption of Homogeneous E q u i l i b r i u m f o r M o n t m o r i l l o n i t e . M o n t m o r i l l o n i t e w i l l be used as an important example of a m i n e r a l that possesses i o n exchange c a p a c i t y . Consider the e q u i l i b r i u m of N a saturated B e l l e Fourche m o n t m o r i l l o n i t e w i t h water as follows: +

t ( S i

A 1

7.87 0.13 + 12.55

H

+

) ( A 1

F e

) 0

C 0 H )

3.03%.58 0.45 20

= 7.87

H Si0 4

+ 3.16

4

Al

3 +

4

] N a

+

0.56

+ 0.58

Mg

2 +

7 A

S

H

2O

+ 0.45

+

+ 0.56

Na .

Fe

3 +

II]

One cannot t e l l by l o o k i n g at an expression such as equation 1, however, whether the o v e r a l l balanced equation a p p l i e s to a homogeneous e q u i l i b r i u m , or to a heterogeneous e q u i l i b r i u m i n which has been included a second e q u i l i b r i u m , perhaps a s s o c i a t e d w i t h and dependent upon the f i r s t . I f we assume homogeneous e q u i l i b r i u m f o r equation 1 , and i n c l u d e H+ w i t h each c a t i o n to avoid the n e c e s s i t y of a separate H v a r i a b l e , then +

log Κ

=7.87 l o g H S i 0 4

0.58

(log M g

+ 0.56

log

2 +

4

+3.16 +

- 2 log H )

(log A l

3 +

+

- 3 log H )

+0.45 ( l o g F e

3 +

- log

+ +

H) [12]

H where Κ i s the e q u i l i b r i u m constant and the a c t i v i t y of m o n t m o r i l l o n i t e and water are assumed to be u n i t y . The l a s t term has been w r i t t e n as a r a t i o to s i m p l i f y a l a t e r comparison. There i s some question as to whether m o n t m o r i l l o n i t e s can come to e q u i l i b r i u m w i t h a c i d aqueous s o l u t i o n s (10), although there i s recent evidence that they can (11). Equations 1 and 2 are w r i t t e n as i f they apply to a

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch020

20.

403

Ion Exchange and Mineral Stability

KiTTRiCK

homogeneous r e a c t i o n , where K^q depends upon both s t r u c t u r a l and exchangeable i o n s . This K^q i m p l i e s that the s t a b i l i t y of B e l l e Fourche m o n t m o r i l l o n i t e (or of any m i n e r a l having exchangeable ions) depends upon the s o l u t i o n a c t i v i t y of whatever exchangeable ions i t c o n t a i n s . Since n a t u r a l exchangers u s u a l l y c o n t a i n s e v e r a l exchangeable i o n s , understanding the s t a b i l i t y of these minerals i n d e t a i l could become v e r y complicated. To s i m p l i f y the s i t u a t i o n , a s i n g l e exchangeable i o n i s u s u a l l y assumed (e.g., (12)). I n v e s t i g a t o r s of i o n exchange e q u i l i b r i a a r e u s u a l l y i n t e r e s t e d i n the d e t a i l s of exchangeable i o n composition, so they seldom assume the presence of a s i n g l e exchangeable i o n . Furthermore, there do not seem to have been any i o n exchange i n v e s t i g a t i o n s that even suggest the p o s s i b i l i t y of l i n k e d e q u i ­ l i b r i a , such as depicted i n equation 2. I f equation 2 were c o r r e c t (as assumed by most i n v e s t i g a t o r s of m i n e r a l e q u i l i b r i a ) then even a s m a l l change i n the a c t i v i t y of the n e u t r a l molecule H4Si04 under the r i g h t circumstances could make a l a r g e change i n the a c t i v i t y of exchangeable N a . I t i s c l e a r t h a t , i f equation 1 i s c o r r e c t , i o n exchange e q u i l i b r i a cannot be understood without reference to equation 2. +

The Assumption of Heterogeneous E q u i l i b r i u m f o r M o n t m o r i l l o n i t e . I f the d i s s o l u t i o n of m o n t m o r i l l o n i t e i s heterogeneous as assumed by i n v e s t i g a t o r s of i o n exchange e q u i l i b r i a , then i t might be more s u i t a b l e to w r i t e equation 1 as r ( S i

7.87

12.55 H

+

A 1

}

+

0.13 ^

= 7.87 H S i 0 4

4

+ 3.16 A l

3 +

+ 0.58 M g

2 +

7

4 8

·

H

2°

+

3 +

+ 0.45 F e . [3]

The corresponding expression f o r the e q u i l i b r i u m constant i s log Κ

=7.87 l o g H S i 0 4

+0.58 ( l o g M g +

2 +

4

+ 3.16 ( l o g A l +

3 +

+

- 3 log H )

- 2 l o g H ) +0.45 ( l o g F e

3 +

+

- 3 l o g H ) - 0.56 l o g H .

[4]

In equation 3 the m o n t m o r i l l o n i t e i s considered t o e x i s t as a charged s i l i c a t e , but the exchangeable i o n i s omitted. K q i n equation 4 i s then independent of exchangeable i o n composition. Equation 3 assumes that the exchange r e a c t i o n c o e x i s t s w i t h the s i l i c a t e e q u i l i b r i u m , but that the e q u i l i b r i u m constants of the two r e a c t i o n s a r e independent. I f t h i s i s t r u e , then some­ t h i n g i s amiss w i t h respect to current methods f o r approximating the standard f r e e energy of formation of m i n e r a l s having ex­ changeable i o n s , because a l l such methods r e q u i r e i n c l u s i o n of the exchangeable i o n i n the c a l c u l a t i o n (13, 14, 15). For a e

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

C H E M I C A L M O D E L I N G IN

404

AQUEOUS SYSTEMS

consequence that may be e a s i e r to v i s u a l i z e , consider a p o r t i o n of a s t a b i l i t y diagram from Helgeson, Brown and Leeper (16), as shown i n F i g u r e 1. N o t i c e that the s t a b i l i t y f i e l d of Na montmoril­ l o n i t e i n c r e a s e s r e l a t i v e to k a o l i n i t e , as l o g Na"*"/!*"" i n c r e a s e s , i n accord w i t h equation 2. That i s , m o n t m o r i l l o n i t e becomes more s t a b l e as the a c t i v i t y of i t s exchangeable i o n i n c r e a s e s . I f equation 4 were c o r r e c t , the k a o l i n i t e - m o n t m o r i l l o n i t e j o i n would be v e r t i c a l on such a diagram, because the s t a b i l i t y of both minerals would be independent of N a a c t i v i t y . 1

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch020

+

Homogeneous v s . Heterogeneous E q u i l i b r i u m . Why Don't We Know Which i s Right? Ion exchange e q u i l i b r i a are u s u a l l y so much f a s t e r than mineral e q u i l i b r i a that i t i s easy to assume the two are not l i n k e d . However, m i n e r a l e q u i l i b r i a can be r a p i d i f s m a l l amounts of e q u i l i b r a t i n g l i q u i d are used (e.g., (17). Many i n v e s t i g a t o r s may not r e a l i z e t h a t t h e i r equations i n v o l v e such an assumption. Perhaps the main reason the question of homogeneous v s . heterogeneous e q u i l i b r i u m i s not s e t t l e d , f o r minerals having i o n exchange c a p a c i t y , i s that many i n v e s t i g a t o r s wish to apply mineral e q u i l i b r i a and i o n exchange e q u i l i b r i a to immediate p r a c t i c a l problems. They are not i n a p o s i t i o n to perform the necessary b a s i c r e s e a r c h , so merely making an assumption i s an a t t r a c t i v e way to proceed. I f equation 1 represents a hetero­ geneous r e a c t i o n , t h a t f a c t can probably not be determined c a l o r i m e t r i c a l l y , s i n c e i t does not appear to be p o s s i b l e to evaluate mineral e q u i l i b r i a and i o n exchange e q u i l i b r i a separate­ l y w i t h that method. Considering m i n e r a l s w i t h l a r g e exchange c a p a c i t i e s , i t was not u n t i l 1968 that Reesman and K e l l e r published the f i r s t experimental s o l u b i l i t y work on m o n t m o r i l l o n i t e s t a b i l i t y (18) . Since that time, s o l u b i l i t y work on montmorillonite (19-26,10) and on v e r m i c u l i t e (27, 28) has not s p e c i f i c a l l y addressed the question of homogeneous v s . heterogeneous e q u i l i b r i u m . Such a determina­ t i o n was not even p o s s i b l e w i t h the systems used by these i n v e s t i g a t o r s , though i t should be p o s s i b l e using a proposed experimental system to be o u t l i n e d l a t e r i n t h i s paper. The System M o n t m o r i l l o n i t e - S o l u t i o n From equation 2 i t i s evident that i n order to c a l c u l a t e f o r a r e a c t i o n assumed to be homogeneous, i t w i l l be neces­ sary to determine the e q u i l i b r i u m s o l u t i o n a c t i v i t y of H4S1O4, A l ^ , H , M g , F e ^ , and Na . However, i n order f o r the system to c o n t a i n measureable amounts of A l ^ and F e ^ , and f o r the A l i o n species to be known w i t h reasonable c e r t a i n t y , the pH of the system w i l l probably have to be 4.0 or l e s s (20). At t h i s pH the dominant exchangeable ions w i l l be A l ^ (and perhaps Fe^ ) not N a (e.g., (29), page 284). R e w r i t i n g equation 1 f o r ex­ changeable A l ^ and homogeneous e q u i l i b r i u m we have: K^

+

+

2+

+

+

+

+

+

+

+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

+

20.

Ion

KiTTRiCK

[ ( S i

7.87

+ 12.55 H

A 1

+

Exchange and

0.13

) (

= 7.87

405

Mineral Stability

+

^

H Si0 4

+ 3.35 A l

4

3 +

7

'

4 8

H

2+

2°

3

+ 0.58 M g + 0 . 4 5 Fe "t [5]

w i t h the corresponding e q u i l i b r i u m constant expression log Κ

= 7.87

(log M g

2 +

log H S i 0 4

+

- 2 log H )

+ 3.35

4

3 +

(log A l

+0.45 ( l o g F e

3 +

+

- 3 log H )

+

0.58

+

- 3 log H ).

[6] +

Comparing equation 6 w i t h equation 2 we f i n d t h a t , s i n c e N a i s no longer the exchangeable i o n , Κ i s no longer a f u n c t i o n of l o g Na /H . Furthermore, s i n c e Al3+ i s now the exchangeable i o n , the c o e f f i c i e n t of ( l o g Al^+ - 3 l o g H ) i n equation 6 i s g r e a t e r than i n equation 2 because the amount of A l ^ i n equation 5 i s greater than i n equation 1. By r e a r r a n g i n g equation 6 and d i v i d ­ ing through by 3.35, we f i n d that +

+

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch020

+

+

3pH - p A l - 0.13

3 +

= 2.35pH Si0 4

2+

4

- 0.17(2pH - pMg )

3 +

[7] eq where ρ i n d i c a t e s the negative l o g a r i t h m of the r e s p e c t i v e terms. Since heterogeneous e q u i l i b r i u m , on the other hand, i s independent of the exchangeable i o n , we may s t a r t w i t h equation 3 . Equation 4 can then be rearranged as was equation 6 to g i v e 3pH-pAl

3+

(3pH - p F e ) + 0.30 pK

2+

3+

= 2.49pH Si0 + 0.18(3pH-pMg ) + 0.14(3pH-pFe ) 4

4

-0.32pK

- 0.18pH. [8] eq Comparing equation 8 w i t h equation 7 we see that the two l a r g e s t d i f f e r e n c e s i n c o e f f i c i e n t s are 2.35 v s . 2.49 f o r pH^SiO^ and zero and 0.18 f o r pH. I f equation 7 can be d i s t i n g u i s h e d from equation 8 e x p e r i m e n t a l l y , i t w i l l have to be through use of these c o e f f i c i e n t s . In F i g u r e 2, 3pH

ο

I

ο

>

g η

S

i—»

00

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch021

21.

HURD E T A L .

Silica and Silica-Containing Minerals

419

wherein: the v a l u e s i n the 5 t h column r e f e r t o t h e h a l f o f t h e s u s p e n s i o n w h i c h was p o u r e d o f f a f t e r s i x weeks, and t h e 6 t h column r e f e r s t o the r e m a i n i n g s o l i d whose a r e a was d e t e r m i n e d a f t e r a n a d d i t i o n a l 4-6 w e e k s . The r e m a i n i n g numbers r e f e r t o s a m p l e s w h i c h had r e a c t e d w i t h the seawater s o l u t i o n s f o r a t l e a s t s i x weeks, and w h i c h were washed b r i e f l y w i t h d i s t i l l e d w a t e r , d r i e d and a n a l y z e d . T h e r e a r e many t r o u b l i n g i n c o n s i s t e n c i e s i n t h e numbers i n T a b l e I . The o b s e r v a n t r e a d e r w i l l n o t e t h a t i n many c a s e s , the a v e r a g e o f the columns 5 and 6 d a t a f o r a g i v e n m i n e r a l may be v e r y c l o s e t o t h e c o l u m n 1 v a l u e , e v e n t h o u g h t h e numbers i n c o l u m n s 2 a n d 3 a r e q u i t e d i f ­ f e r e n t from each o t h e r . This prompted an experiment which produced the d a t a i n T a b l e I I . I n t h i s e x p e r i ­ m e n t , two grams o f f r e s h l y g r o u n d m i n e r a l w e r e p l a c e d i n a s m a l l p l a s t i c b o t t l e w i t h 75 cm^ o f s e a w a t e r ; t h e b o t t l e a n d c o n t e n t s v i g o r o u s l y s h a k e n f o r c a . 60 seconds and a p p r o x i m a t e l y h a l f o f the s u s p e n s i o n poured o f f as r a p i d l y as p o s s i b l e . Then t h e s p e c i f i c s u r f a c e a r e a o f e a c h p o r t i o n was d e t e r m i n e d . The d a t a i n Table I I suggest that there i s a great d e a l of inhomogeneity i n f r e s h l y ground samples h a v i n g spe­ c i f i c s u r f a c e a r e a s o f c a . < 1 m^/gm. L a r g e r s u r f a c e a r e a s ( i . e . c a . > 20 m^/gm) a r e s e e m i n g l y much l e s s a f f e c t e d probably because of t h e longer s e t t l i n g times of t h e s m a l l e r p a r t i c l e s . These r e s u l t s suggest t h a t u n l e s s a g i v e n sample has been p r e v i o u s l y s i z e s o r t e d , s a m p l e s o f t h e s u s p e n s i o n s h o u l d n o t be p e r i o d i c a l l y removed by p o u r i n g o f f s m a l l volumes b e c a u s e the s u r ­ f a c e a r e a o f s o l i d r v o l u m e o f s o l u t i o n may c h a n g e i n a n u n p r e d i c t a b l e manner. Y e t a n o t h e r d i f f i c u l t y a r i s e s when t h e d a t a i n c o l u m n s 2-4, 7 a n d 8 a r e c o m p a r e d w i t h c o l u m n 1 a n d t h e a v e r a g e s o f c o l u m n s 5 a n d 6. D i f f e r e n c e s among t h e f o r m e r c o l u m n s o f 100% a r e common a l t h o u g h a num­ b e r o f t h e m i n e r a l s show f a i r l y s m a l l s c a t t e r . This s u g g e s t s t h a t t h e v a r i a b i l i t y f r o m one s a m p l e t o t h e n e x t f o r t h e same m i n e r a l i s i m p o r t a n t , e s p e c i a l l y i n v i e w o f t h e f a c t t h a t no a t t e m p t t o s i z e s e p a r a t e t h e f i n e s t p a r t i c l e s f r o m o u r f r e s h l y g r o u n d s a m p l e s was made. We a l s o do n o t a p p e a r t o h a v e e n o u g h i n f o r m a ­ t i o n t o c o n c l u s i v e l y s a y w h e t h e r t h e r e s h o u l d be c o n ­ s i s t e n t increases or decreases i ns p e c i f i c surface a r e a d e p e n d i n g on w h e t h e r d i s s o l u t i o n o r p r e c i p i t a t i o n of d i s s o l v e d s i l i c a o c c u r s . I ti s l i k e l y that inters a m p l e s p e c i f i c s u r f a c e a r e a v a r i a b i l i t y swamps most o f t h e e f f e c t s o f c h e m i c a l r e a c t i o n d u r i n g o u r sam­ pling periods.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

420

CHEMICAL MODELING IN AQUEOUS

* *

•Ν

β eu

ο •Η

4-ι

cd

•Η Ο ω α,

ω u

cd CD

• CO ω

ο

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch021

cd m î-l

ο

CN

ω Β

ο cd m

Ο

m a±r and i s defined as

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

1

D

N a

C

D

Na+,H+

=

D

Na+ *

Na+

+

V C

D

Na+ Na+

+

H

°H+

5

(7) W

where I> + and are the constant s e l f - d i f f u s i o n c o e f f i c i e n t s of sodium and hydrogen i o n s , and v j + and C + are the c o n c e n t r a t i o n of d i f f u s i b l e sodium and hydrogen i o n s . As i n d i c a t e d by H e l f f e r i c h (32), the i n t e r d i f f u s i o n c o e f f i c i e n t i s g e n e r a l l y not a constant but v a r i e s continuously w i t h changing concentrations of the i n t e r d i f f u s i n g species. Equation 7 i s s t r i c t l y true only i f sodium i s the s o l e species d i f f u s i n g from the s i l i c a t e . In the case of the v i t r i c t u f f p r e v i o u s l y modeled, the d i f f u s i o n of sodium i s a l s o r e l a t e d to other c a t i o n s d i f f u s i n g from the glass by the common f l u x of hydrogen i o n s . The use of such an i n t e r d i f f u s i o n c o e f f i ­ c i e n t r e s u l t s i n an exceedingly complex s o l u t i o n to equation 5. A s i m p l i f i e d s o l u t i o n was e s t a b l i s h e d which permits the apparent sodium d i f f u s i o n c o e f f i c i e n t to be an e m p i r i c a l f u n c t i o n of pH. Figure 10 shows four experiments conducted at an average pH of 7.13 and 25 C . Table I I I l i s t s the experimental data. The s o l i d l i n e s represent the l i n e a r r e g r e s s i o n f i t to the data. Although the surface-to-volume r a t i o s are i d e n t i c a l to data i n Figure 3 (pH 4.75), the r e l a t i v e slopes are much g e n t l e r , i n d i c a t i n g a decrease i n the p a r a b o l i c r a t e s w i t h i n c r e a s i n g pH. The two v a r i a b l e s i n the numerical s o l u t i o n which define the rates of mass t r a n s f e r i n experiments of equal surface-to-volume r a t i o s are the s u r f a c e adsorption isotherm and the apparent d i f f u s i o n c o e f f i c i e n t . Exchange experiments were conducted i n which glasses reacted w i t h aqueous s o l u t i o n s at pH 6.0 and 7.0 f o r s e v e r a l weeks were r a p i d l y Na

C

a

R

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

WHITE AND CLAASSEN

Dissolution Kinetics of Silicate Rocks

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

22.

Figure

9.

Effect of pH on the parabolic reaction coefficients for species of different valency during dissolution of a rhyolitic tuff at 25°C

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

467

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

468

Figure 10. Effect of surface-to-volume ratios on inhibition of sodium mass transfer from a rhyolitic tuff at an average pH of 7.13 and 25°C. Solid lines are statistical fits to data points. Dashed lines predict inhibition based on diffusion model.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

*

I

1.05 1.09 1.09 1.15

1.57 1.62 1.64 1.79

Na

0.59 .60 .64 * * .83 .85 .88 .88 .88 * .96 1.01 1.01

Q

6

0.0078 .030 .052 .124 .292 .407 .502 .715 .785 .926 1.10 1.18 1.32 1.41

t

(0.38 X 1 0

Data not a v a i l a b l e .

Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Solid surface area - s o l u t i o n volume r a t i o

Experiment

1 0

6.79 * 6.85 6.89

6.35 6.62 6.49 6.77 6.80 6.70 6.69 6.73 6.70 6.60 6.61 6.80 6.70 6.70

PH

2

cm /L) 6

2

0.024 .051 .094 .130 .291 .420 .526 .732 .879 .939 1.06 1.17 1.22 1.32 1.39 1.44 1.53 1.71 1.74

t Na

0.43 .49 .60 .55 .63 .67 Λ .72 .77 .84 .81 .81 .85 .85 .84 .87 .93 .89 .92

Q

6.52 6.67 * 6.63 6.69 6.74 6.75 6.80 7.01 7.10 6.91 7.00 7.07 6.91 6.91 6.91 6.82 6.95 6.96

PH

(0.75 Χ ΙΟ cm /L)

J

0.025 .052 .088 .120 .286 .416 .523 .730 .887 .937 1.06 1.17 1.22 1.32 1.38 1.44 1.53 1.71 1.74

t

0.25 .35 .37 .41 .47 * .52 .57 .60 .60 .60 .63 .65 .66 .65 .66 .68 .69 .70

%a

(1.5 Χ 1 0

Κ 6

6.85 6.77 6.85 6.79 7.01 7.00 6.98 7.15 7.20 7.22 7.20 7.18 7.20 7.25 7.12 7.11 7.06 7.13 7.06

PH

2

cm /L)

]

L 5

2

.109 .281 .411 .523 .730 .877 .939 1.07 1.17 1.22 1.32 1.38 1.44 1.53 1.71 1.74

.025 .052

t

.48 .50 .54 .55 .57 .57 .58 .57 .57 .59 .61 .62 .63

•k

Na

0.26 .29 * .39 .46

Q

7.42 7.50 7.28 * 7.00 7.29 7.28

6.95 7.16 6.90 7.32 7.48 7.44 7.42 7.54

7.08 7.00

PH

(3.0 Χ ΙΟ ιcm /L)

(mol/cm Χ 1 0 ) , And pH, For Experiments Of D i f f e r e n t T o t a l Surface Areas

6

Table I I I . — E x p e r i m e n t a l Data L i s t e d As Time, t ( s ^ . H T ) , Mass Transfer Of Sodium, Q

3

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

470

CHEMICAL MODELING IN

AQUEOUS SYSTEMS

t i t r a t e d to pH 4.0. The l a c k of sodium r e l e a s e i n t o s o l u t i o n s t r o n g l y suggested that the surface sodium concentration was unaf­ f e c t e d by aqueous hydrogen concentrations. A match to the data (Figure 10) was sought assuming an i d e n t i c a l isotherm to that which defined the surface sodium concentrations at pH 4.75. A new pH-dependent apparent d i f f u s i o n was then c a l c u l a t e d . The dashed l i n e s i n F i g u r e 10 are the s o l u t i o n s generated by the model. Com­ p a r i s o n i s good between r a t e constants based on the l i n e a r regres­ s i o n f i t of the data and those based on the numerical model, using 22

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

an apparent d i f f u s i o n c o e f f i c i e n t of 1.7 χ 1 0 ~ cm /s. As ex­ pected f o r an i n t e r d i f f u s i o n process i n v o l v i n g a v a i l a b l e hydrogen i o n s , the apparent sodium d i f f u s i o n c o e f f i c i e n t decreases w i t h i n ­ c r e a s i n g pH. Conclusions Although simple r a t e expressions can be u t i l i z e d to e x p l a i n s h o r t term, l a b o r a t o r y - c o n t r o l l e d d i s s o l u t i o n of many s i l i c a t e phases, t h e i r a p p l i c a t i o n to modeling n a t u r a l weathering processes i s much more d i f f i c u l t . As i n d i c a t e d , the form of the r a t e ex­ p r e s s i o n , whether l i n e a r or p a r a b o l i c , can depend on the type of s i l i c a t e s t r u c t u r e , the pH of r e a c t i o n , and the time span of the weathering process i t s e l f . Perhaps the best method of deter­ mining the type of r a t e expression a p p l i c a b l e to a s p e c i f i c n a t u r a l s i t u a t i o n i s to compare n a t u r a l water compositions to those p r e d i c t e d by l a b o r a t o r y - d e r i v e d r a t e constants. In the case of long-term p a r a b o l i c k i n e t i c s , the r a t i o of d i s s o l v e d c o n s t i t ­ uents should be d i r e c t l y p r o p o r t i o n a l to the r a t i o s of the para­ b o l i c r a t e constants. In the case of l i n e a r k i n e t i c s , the r a t i o of c o n s t i t u e n t s should be s t o i c h i o m e t r i c a l l y equal to the compo­ s i t i o n of the d i s s o l v i n g phase. In the case of a system i n which more than one s i l i c a t e phase i s d i s s o l v i n g , the r a t i o of the con­ s t i t u e n t s w i l l r e f l e c t the r e l a t i v e r e a c t i o n r a t e s of i n d i v i d u a l phases, whether they are l i n e a r or p a r a b o l i c . The above scheme a p p l i e s only to those c o n s t i t u e n t s which are s u b s t i t u t i o n a l compo­ nents of major d i s s o l v i n g mineral phases, and which are conserved i n the aqueous s o l u t i o n . In a d d i t i o n to the form of the r a t e expression, the magnitude of the r a t e constant must be a s c e r t a i n e d . As i n d i c a t e d i n the d i s c u s s i o n , the composition of the aqueous media can exert a s t r o n g c o n t r o l on the r a t e constant. Modeling of k i n e t i c r a t e s , t h e r e f o r e , can be considerably complicated, as i n closed ground­ water recharge systems where the aqueous composition would be expected to c o n t i n u a l l y vary along the flow path. In the case of the simple R a i n i e r Mesa a q u i f e r system, a q u a n t i t a t i v e model can be derived to e x p l a i n such v a r i a t i o n s . As i n d i c a t e d i n the case of sodium, the r a t e of mass t r a n s f e r can, at any p o i n t i n the r e a c t i o n path, be r e l a t e d to the aqueous sodium concentration by use of an adsorption isotherm. This

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

22.

WHITE AND CLAASSEN

Dissolution Kinetics of Silicate Rocks

471

isotherm appears to be a separable v a r i a b l e independent of other aqueous c o n s t i t u e n t s over the range of s o l u t i o n composition i n v e s t i g a t e d by the authors. The r a t e of mass t r a n s f e r can a l s o be independently r e l a t e d to the aqueous pH by use of an apparent s o d i u m - d i f f u s i o n c o e f f i c i e n t which i s e m p i r i c a l l y determined and which allows f o r the c o d i f f u s i o n of hydrogen i o n s . Thus, the r e a c t i o n r a t e f o r sodium d i s s o l u t i o n f o r the v i t r i c t u f f can be modeled i n a c l o s e d system w i t h a v a r i a b l e pH and d i s s o l v e d Na aqueous composition, by summing the r e s u l t s of s m a l l r e a c t i o n increments, assuming constant source-rock composition. An example of t h i s method i s presented elsewhere i n t h i s volume.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch022

Abstract Experimentally determined dissolution kinetics are applicable to natural weathering processes of s i l i c a t e rocks. Mass transfer from the mineral to the aqueous phase was determined to be incon­ gruent under a range of experimental conditions. Transfer rates of individual species (Q) at times (t) can usually be described by one of two rate expressions; Q = Q + k t or Q = Qo + o

1

1/2

kt p

where k i s a linear rate constant, k i s a parabolic rate constant, and Q i s the mass transferred during an i n i t i a l surface exchange with hydrogen ions. The linear rate constant, k , represents continued surface exchange coupled to incongruent dissolution of the s i l i c a t e framework whereas the parabolic rate constant, k , represents cation diffusion through a relatively undisturbed s i l i c a t e framework. Detailed investigation of dissolution of a v i t r i c tuff indicates that the rate of mass transfer of a species is described by a parabolic expression and i s inversely dependent on the concentration of that species in aqueous solution. A numerical solution to the one-dimensional diffusion equation i s presented using a Freundlich isotherm to relate the aqueous ion concentration and the ion density on the surfaces of the v i t r i c tuff. This ion density on the surface in turn determines the con­ centration gradient within the glass. Results indicate that the apparent diffusion coefficient for sodium decreases from 1.3 x 10 at pH 4.8 to 1.7 x 10 at pH 7.0 and that the surface isotherm remains constant. 1

p

o

1

p

-21

-22

Literature Cited 1. 2.

Garrels, R. Μ., and Mackenzie, F. T. "Evolution of Sedimen­ tary Rocks," 397 p. W. A. Norton and Co., New York, 1971. Daubreé, A. Experiences sus des les decomposition chimiques provoquies par les action mecaniques dans divers mineraux tels que le feldspart, Compt. Rend., 339-345 (1867).

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472

3.

4.

5.

6.

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CHEMICAL

MODELING IN AQUEOUS

Rana, Μ. Α., and Douglas, R. W. The reaction between glass and water Part 2. Discussion of the results, Phys. Chem. Glasses 2, 196-205 (1961). Luce, R. W., Bartlett, R. W., and Parks, G. A. Dissolution kinetics of magnesium s i l i c a t e s , Geochim. Cosmochim. Acta 30, 35-50 (1972). Tamm, Olof Expermentelle studien über die verwitterung und tonbildung von feldspäten, Chemie der Erde 4, 420-430 (1930). Garrels, R. M. and Howard, P. Reactions of feldspar and mica with water at low temperature and pressure, Clays Clay Min. 6, 68-88 (1957). Lagache, Μ., Wyart, J., and Sabatier, G. Dissolution des feldspaths alcalins dan l'eau pure ou chargee de CO a 200ºC, C. R. Acad. Sci. Paris, 253, 2019 (1961). Lagache, M. Contribution à l'etude de l'altération des feldspaths den l'eau 100 et 200°C sous diverses pressions de CO et application à l a synthese des minéraux argileax, Bull. Soc. Fr. Minerai. Cust. 88, 223-253 (1965). Aagaard, P., and Helgeson, H. C. Thermodynamic and kinetic constraints on the dissolution of feldspars, Geol. Soc. Amer. Abstr. 9 (7), 873, 1977. Marshall, C. E. Reactions of feldspars and micas with aqueous solutions, Econ. Geol. 57, 1219-1227 (1962). Paces, T. Steady-state kinetics and equilibrium between ground water and granitic rock, Geochim. Cosmochim. Acta 37, 2641-2663 (1973). Boksay, Ζ., Bouquet, G., and Dobos, S. Diffusion processes in the surface layer of glass, Phys. Chem. Glasses 8, 140-144 (1967). Boksay, Ζ., and Bouquet, G. On the reaction of water molecules with the s i l i c a t e network in the glass phase, Phys. Chem. Glasses 16, 81-83 (1975). Correns, C. W., and van Engelhardt, W. Neue untersuchungen über die verwitterung des kalifeldspates, Chem. Erde 12, 1-22 (1938). Helgeson, H. C. Kinetics of mass transfer among s i l i c a t e s and aqueous solutions, Geochim. Cosmochim. Acta 35, 421-469 (1971). Crank, J. "The Mathematics of Diffusion," 347 p. Oxford Press, London, 1955. Correns, C. The experimental chemical weathering of s i l i c a t e s , Clay Mineral. Bull. 4, 249-265 (1961). Csakvari, Β., Boksay, Ζ., and Bouquet, G. Investigation of surface layers on electrode glass for pH measurement, Anal. Chim. Acta 56, 279-284 (1971). Baucke, F. Investigation of surface layers, formed on glass electrode membranes in aqueous solutions, by means of an ion sputtering method, Jour. Non-Crystalline Solids 14, 13-31 (1974). 2

8.

2

9.

10. 11.

12.

13.

14.

15.

16. 17. 18.

19.

SYSTEMS

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

22.

20.

21.

22.

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23.

24.

25.

26.

WHITE AND CLAASSEN

Dissolution Kinetics of Silicate Rocks

473

Tchoubar, C., and Oberlin, A. Altération de l'albite par action d'eau. Etude en microscopie et microdiffraction électroniques de l a precipitation et de l'evolution des fibres de boehmite formée, Jour. Microsc. 2, 415-432 (1963). Tchoubar, C. Formation de l a kaolinite à partier d'albite por l'eau à 200ºC. Etude en microscopie et diffraction électroniques, Bull. Soc. Fr. Mineral. Crist. 88, 483-518 (1965). Parham, W. E. Formation of halloysite from feldspar: low temperature a r t i f i c i a l weathering versus natural weathering, Clays Clay Min. 17, 13-22 (1969). LaIglesia, A., Martin-Caballero, J. L., and Martin-Vivaldi, J. L. Formation de kaolinite par precipitation hologene à temperature ambiante, Emploi de feldspaths potassiques, Compt. Rend. D279, 1143-1145 (1974). Petrović, R. Rate control in feldspar dissolution-II. The protective effect of precipitate, Geochim. Cosmochim. Acta 40, 1509-1521 (1976). Petrović, R., Berner, R. Α., and Goldhaber, M. B. Rate control i n dissolution of a l k a l i feldspars I. Study of residual grains by X-ray photoelectron spectroscopy, Geochim. Cosmochim. Acta 40, 537-548 (1976). Busenberg, Ε., and Clemency, C. V. The dissolution kinetics of feldspars at 25ºC and 1 atm-C0 partial pressure, Geochim. Cosmochim. Acta 40, 41-49 (1976). Loughnan, F. "Chemical Weathering of Silicate Minerals", 155 p. Elsevier, Amsterdam, 1969. Wollast, R. Kinetics of the alteration of Κ feldspar i n buffered solutions at low temperature, Geochim. Cosmochim. Acta 31, 635-648 (1967). White, A. F., and Claassen, H. C. Kinetic model for the dissolution of a rhyolitic glass, Geol. Soc. Am. Abst. 9, 1223 (1977). Halsey, G., and Taylor, H. S. The adsorption of hydrogen on tungsten powder, Jour. Chem. Physics 15, 624-630 (1947). Crank, J., and Nicolson, P. A practical method for numerical evaluation of solutions of partial differential equations of the heat type, Proc. Cambridge Phil. Soc. 43, 50-67 (1947). Helfferich, F. "Ion Exchange", 327 p. McGraw-Hill, New York, 1962. 2

27. 28.

29.

30. 31.

32.

Disclaimer: The reviews expressed and/ or the products mentioned in this article repre­ sent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED November16,1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23

C a l c i u m Phosphates—Speciation, Solubility, a n d

Kinetic

Considerations

G. H. NANCOLLAS, Z. AMJAD, and P. KOUTSOUKOS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14214

The calcium phosphates are widely dispersed in the environment. They occur as constituents in a l l classes of rock and the apatites also occur naturally in pegmatites and other veins of hydro-thermal origin. In more recent times, the increase i n phosphate concentrations in lakes and rivers near heavily populated areas has been a major factor i n the resurgence of interest in physico-chemico processes such as precipitation and dissolution of phosphate salts. Despite the continuous entry of phosphate ions into a lake, the phosphate concentration does not increase proportionally (_1, 2) indicating that at least some phosphate i s removed by precipitation. Since the calcium concentration i s also relatively high, perhaps through the use of lime additions for the removal of phosphates from sewage,calcium phosphate precipitation may be of particular importance. In the lime addition process, an induction period precedes the formation of precipitate and, typically, 85-90% of the phosphates can be removed in a recycling reactor at pH 8.0 (3). In addition to i t s precipitation i n natural water systems, calcium phosphate deposits, normally attributed to hydroxyapatite (Ca^CPO^^OH hereafter HAP), are also formed as deposits in boilers. One of the earliest studies of this phenomenon was made by Clark and Gerrard (_4) who examined a number of other deposits where phosphate control was practiced. They observed a wide variation in chemical composition but concluded that HAP was the most likely calcium phosphate phase to be formed under the boiler conditions. Recently, the formation of calcium orthophosphates in industrial cooling water systems has become increasingly important. Higher phosphate levels are being encountered in cooling waters due to increased water reuse, use of lower quality make-up water such as tertiary sewage treatment plant effluent and the use of organic phosphonate scale and corrosion inhibitors which are degraded to orthophosphate. The increased phosphate levels combined with alkaline operating conditions can lead to the formation of highly insoluble calcium phosphate scale deposits (_5) . Despite the importance of the precipitation of calcium 0-8412-0479-9/79/47-093-475$05.75/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

476

CHEMICAL MODELING

IN AQUEOUS

SYSTEMS

phosphates, there i s s t i l l considerable uncertainty as to the nature of the phases formed i n the early stages of the precipitation reactions as a function of supersaturation, pH, and temperature. In the environment, complicating factors such as the presence of ions other than calcium and phosphate, and the heterogeneous nucleation of calcium phosphate phases on foreign substances have to be taken into account. Thus i t has been shown 05, 8, 9) that the nature of the calcium phosphate phase which forms during precipitation from aqueous solution i s markedly dependent upon the calcium and phosphate concentrations, pH, and the nature and extent of the surface phase on which precipitation i s occurring. In this paper we discuss the chemistry of aqueous calcium phosphate systems from the point of view of both equilibrium and kinetic considerations. It w i l l be shown that the chemical composition of the calcium phosphate precipitated under any given set of conditions, may be determined kinetically rather than simply on the basis of thermodynamic driving forces. Equilibrium Considerations In discussions of the precipitation of calcium phosphate, the phase which i s usually emphasized i s the thermodynamically most stable, HAP. However, most calcium phosphate solutions in which precipitation experiments are made, are i n i t i a l l y supersaturated with respect to four additional phases. The solubility isotherms are shown in Figure 1 as a function of pH. Thus, at a pH of 7.0, i n order of increasing s o l u b i l i t i e s , i t i s necessary to take into account tricalcium phosphate (Ca3(PO^) hereafter TCP), octacalcium phosphate (Ca H(P0 ) .2 1/2 H 0, hereafter OCP), anhydrous dicalcium phosphate (CaHP0 , hereafter DCPA), and dicalcium phosphate dihydrate (CaHPO^.2H 0, hereafter DCPD). The corresponding thermodynamic solubility products, K , at 25°, are:-

2

4

4

3

2

4

2

so

2 +

5

[Ρθ|-]

2 +

3

[P0 ~] y y

2 +

4

[P0 ~] [H+ly^y^ =1.25

for HAP,

[Ca ]

for TCP,

[Ca ]

for OCP,

[Ca ] 2+

for DCPA, [Ca ] and for DCPDi,[Ca ] 2+

3

3

2

3

[ O H j y ^ y ^ 4.7 χ 10~ 3

2

=1.20

3

2

[HP0 "]y 2

[HP0 "]y

2

2

=1.26

59

_1

9

(mol 1 ) (1C»

29

1

47

(mol Γ )

5

χ ΙΟ" (mol l " ) ^ ! ! ) χ 1θ"

χ 10" 7

=2.49 χ 10""

7

1

8

(12)

1

2

(mol l " ) ( 1 3 ) 1

2

(mol 1" ) (14)

In these expressions, the square brackets represent the molar concentrations of the species indicated, and y i s the activity coefficient of a Z-valent species. In order to calculate the free ionic concentrations in these expressions, i t i s necessary to take into account ion-pair and complex formation. The equilibria in pure calcium phosphate solutions are:z

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

NANCOLLAS E T A L .

23.

H

+

H

+

H

+

+ Η Ρ0 2

Η Ρ 0 , Κ = 164.1

4

3

+ HPO2~

2

+ PO "

1

7

2

1

lmpl*" , (.15.)

4

% HPO ", Κ = 2.33 χ Ι Ο

2+

477

lmol- , (_15) ,

2

Ca + H P0

12

1

lmol- , Ç16)

1

CaH P0+K=31.9 lmol" , (17)

4

2

2

1

Ca + HPO "

CaHP0 Κ = 681 lmol"" , ill) 4

2+

Ca + Ρ O "

CaPO"

2+

+

Ca + OH"

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

4

Η Ρ 0 , Κ = 1.58 χ ΙΟ

3

2+

Calcium Phosphates

* CaOH

Κ = 2.9 χ 10

6

1

lrnol" ,

(17)

1

Κ =32.4 lmol" , (18)

Values of K, the themodynamic association constants are given at 25°. The concentrations of ionic species i n the solutions at any time can be determined from mass balance, electroneutrality, and the appropriate equilibrium constants as described previously (19, p. 85-92) by successive approximations for the ionic strength. The activity coefficients of Z-valent ionic species may be calcu­ lated from an extended form of the Debye-Huckel equation such as that proposed by Davies (20, p. 34-53). -log y

2

z

= Az [I

1 / 2

/(l + I

1 / 2

)-0.3l],

(1)

which has been shown to represent the activity coefficients of multiple charge ions up to ionic strengths of about 0.2 mol 1""1 to within + 1%. This and other activity coefficient equations have been discussed elsewhere (20, . 34-53; 21, p. 242-279; 22, p. 229-252). In some cases, i t is possible to study a system at a series of constant ionic strengths and to determine the best values of the parameters to be used in equations for the calcula­ tion of activity coefficients (23). In typical environmental situations, the liquid phase may contain a number of ions which can form complexes with the multiple charge cations present. In order to be able to calculate the thermodynamic driving force for the precipitation of a particular solid phase, i t is necessary to determine the concentrations of free l a t t i c e ions in the solutions. The commonly used procedures for the calculations of solute components in a homogeneous solution are the "equilibrium constant" and the "free energy minimization" methods. The former u t i l i z e s an approach wherein f i r s t , a "basis" i s selected from the species, usually that having the highest concentration at equilibrium. The other solution species are formed from this "basis" by a series of chemical reactions, and their concentrations can be expressed through the use of equilibrium constants, in terms of the concentration of the chosen "basis". The resulting set of nonlinear simultaneous equations consists of as many unknowns as there are elements and can be solved by conventional numerical methods. The "free energy minimization" method u t i l i z e s only free energy c r i t e r i a for chemical equilibria making no distinction among the constituent species and i s essentially a constraint non-linear minimization problem. A number of search methods have p

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

478

CHEMICAL MODELING IN AQUEOUS SYSTEMS

been proposed in order to effect the desired minimization (.24, 25, 26) . The efficiency of the "equilibrium constant" computational method i s c r i t i c a l l y dependent upon the choice of the "basis" set and the i n i t i a l estimate of the equilibrium concentrations of the components. In contrast, the "free energy minimization" method can be used for chemical systems containing a large number of elements or species without the need for a large number of memory locations i n computer calculations. The method always converges to the solution, no matter how poor the starting estimates, but considerable time may be required to reach a solution, making i t unsuitable for routine equilibrium calculations. The solution of the non-linear simultaneous equations are usually based upon techniques that change a many-variable iteration problem into a set of single-variable problems, treating the iterations as a nested set, each of which iterates on one variable only. Sillen (27) used a simple search technique whereas the program of Perrin and Sayce (_28) used iteration formulae involving the ratios of the observed and calculated total concentrations of metal and ligand. Although these methods do not require e x p l i c i t matrix inversion, they suffer from a rather slow convergence rate. The widely used Newton-Raphson method was employed in the development of EQUIL in order to effect rapid quadratic conversion for the calculation of the equilibrium concentrations of species i n mixed electrolytes (29). The procedure, based on the " s t a b i l i t y constant" method states the problem with simplicity and generality using a numerical technique based on a modified Newton-Raphson method in order to solve the non-linear simultaneous equations. The modifications made i n the Newton-Raphson procedure included scaling the matrix, eigenvector analysis, development of an iteration matrix, and adopting a convergence forcer. In typical environmental situations, i n the presence of relatively large concentrations of background electrolytes which are assumed not to form complexes with the reacting species, the activity coefficients can be assumed to remain constant. The EQUIL computer program provides a much, faster convergence for the calculations of ionic species, and overcomes numerical shortcomings such as the i l l - c o n d i t i o n of the matrix and overcorrections to the estimates for each iteration. It can be seen i n Figure 1 C2) that although HAP i s the most stable salt under many conditions, i t becomes less stable than DCPA and DCPD i f the solution i s sufficiently acid. The position of the singular points in Figure 1 at which two solid phases are in equilibrium with the aqueous solution, i s clearly dependent upon pH and upon the ionic strength of the solution. Where multiple charge ions are involved, the importance of introducing activity coefficient corrections becomes increasingly apparent. Thus, at ionic strengths of about 0.1 mol l " " , HAP ionic products may be i n error by as much as a factor of 10 i f activity coefficients are neglected (6). A calcium phosphate solid phase 1

6

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

23. NANCOLLAS E T A L .

Calcium Phosphates

479

10-5] 3 0

4 0

50

60

p H

70

&0 Wiley and Sons

Figure 1. Calcium concentrations and pH values of solutions saturated with respect to various calcium phosphate phases in the ternary system. Ca(OH) H PO,-H 0 at 25°C (2). 2

3

2

16 - a

mMCa DCPD

1-2

///////m////// - b

\

OCP #///#////////

\

c

TCP

0-4

//////////////

d

HAP #//////////// 0

200

Time (min)

400

600

Figure 2. Growth of HAP seed crystals. Plots of total calcium as a function of time. Approximate saturation levels with respect to the various calcium phosphate phases also are shown.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

480

CHEMICAL MODELING IN AQUEOUS SYSTEMS

which i s exposed to a solution more acid than the corresponding singular point may be expected to become covered by a surface coating of a more acid calcium phosphate. The apparent solubility behaviour w i l l then be quite different from that of the phase i t s e l f , and measurement of solution concentrations might lead to apparent high s o l u b i l i t i e s (2). Since calcium phosphate solutions are usually very dilute and s o l u b i l i t i e s low, the surface phases may persist for long periods of time. As pointed out by Brown (2), this may have led many workers to the conclusion that the properties of calcium phosphates in general, and HAP in particular, are rather variable (30, 31, 32).Clearly when working with a diagram such as that in Figure 1, i t is essential to establish that the systems are at equilibrium in order to be able to use thermodynamic solubility products which refer to the free energy of a single solid phase. It i s now quite well established that kinetic factors may be considerably more important in determining the nature of the solid phase present than are considerations based solely upon equilibrium solubility data. Kinetic

Considerations

Numerous kinetic studies have been made of the spontaneous precipitation of calcium phosphates from solutions containing concentrations of l a t t i c e ions considerably in excess of the solubility values (33, 34). Although attempts are usually made to control the mixing of reagent solutions, i t i s d i f f i c u l t to obtain reproducible results from such experiments since chance nucleation of solid phases may take place on foreign particles in the solution. Many of these d i f f i c u l t i e s can be avoided by studying the crystal growth of well-character!zed seed crystals in metastable supersaturated solutions of calcium phosphate. Such solutions are stable for periods of days and the kinetics of growth of the added seed material may then be studied by following the concentration changes as a function of time. Not only are such methods capable of yielding highly reproducible results, but this model may be much closer to the environmental situations in which precipitations may take place preferentially on solid phases already present. During the precipitation reaction the possible formation of a number of calcium phosphate phases may be readily seen from Figure 1. Indeed, one might expect, following the Ostwald-Lussac empirical rule that the least stable phase would precipitate f i r s t , and experience has shown that the two hydrated salts DCPD and OCP are the ones that precipitate most easily, particularly at ambient temperatures (2). The eventual transformation of precursor phases to the thermodynamically stable HAP i s also a problem which i s l i t t l e understood. The stoichiometry of the i n i t i a l l y precipitated solid i s invariably less than the 1.67 for the calcium:phosphate (Ca;P) molar ratio required for HAP (34, 35) . However, the errors associated with

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23.

NANCOLLAS E T A L .

Calcium Phosphates

481

these ratios, calculated from small differences in the measured total molar concentrations of calcium T and phosphate Tp, are very large unless special precautions are taken to increase the precision of the analytical methods; ΐ 0.1% precision i s routinely required for stoichiometry determinations to 1%. In many experimental studies, the Ca:P ratio of the precipitating phases has been found to be 1.45 t 0.05. This has led to the proposal of TCP as the precursor phase which i s converted to the thermodynamically stable HAP by an autocatalytic mechanism (36). The conversion process has been studied as a function of solution environment (37), the presence of stabilizing agents such as pyrophosphate and organic phosphonates (38, 39, 40) and inorganic ions such as magnesium (41, 42). Recent studies of the maturation of freshly precipitated amorphous calcium phosphate at pH 7.4 (43) have shown that the ripening of the crystals i s accompanied by inflexions in the time profiles of calcium and phosphate concentration in the solutions. These were attributed to phase changes during crystal maturation probably through an intermediate OCP-like, crystalline phase. The crystallographic similarity between OCP and HAP had already prompted Brown (44) to propose the phase as a precursor in the precipitation of apatite. The problems associated with the irreproducibility of the results of spontaneous precipitation studies were overcome with the development of seeded growth techniques (45, 46) which enable the effects of factors such as temperature,supersaturation, and ionic strength to be studied quantitatively _47, 48) . The further development of a pH-stat method (49) enables studies to be made of calcium phosphate crystal growth on well characterized seed material under conditions of constant hydrogen ion activity. The simplest calcium phosphate phase for such studies i s DCPD and, as can be seen in Figure 1, at pH values below about 6.0, DCPD, DCPA, and HAP are the only thermodynamically stable calcium phosphate phases. The kinetics of crystal growth of wellcharacterized DCPD seed crystals in metastable supersaturated solutions of calcium phosphate under these conditions has been studied and a typical growth curve following the addition of seed crystals i s shown in Figure 3. In Figure 3 the volume of base required for constant pH, controlled by the pH-stat, i s also plotted. Crystal growth takes place immediately upon inoculation with seed crystals and the rate curves are consistent with the kinetic equation dT Ca 2 Rate of crystal growth = — — — =-ksN . (2) In equation 2, k i s the specific rate constant for growth, s i s a term proportional to the number of DCPD growth sites and Ν i s the number of moles of DCPD to be precipitated before equilibrium i s reached (50). The applicability of equation 2 in describing DCPD crystal growth i s demonstrated in Figure 4 in which the

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

C a

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

482

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

6

5 I0 T 8

CQ

4

ml

Time (min)

Figure 3. Crystal growth of DCPD. DCPD seeds (Q) initial T = 4.37 mM, T = 10.50 mM, pH = 5.60, 25°C; (A) uptake of 0.05M potassium hydroxide. HAP seed (Q)T = 6.346 mM, Ί = 6.317 mM, pH 5.59,37°C. Ca

p

Ca

ρ

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23.

NANCOLLAS E T A L .

Calcium Phosphates

integrated form i s plotted. The observed independence of the rate of crystallization upon the f l u i d dynamics and an activation energy of 48.9 * 4 kJ mol" , obtained from the temperature coefficient of the rate constant, point to a surface-controlled process. The crystallization of a large number of other 2-2 symmetrical electrolytes has also been shown to follow the same kinetic equation (4(5, £7, 4:8) . As the pH i s increased (see figure 1), other calcium phosphate phases may be stabilized as precursors to the formation of apatite and the growth of HAP seed crystals in stable supersaturated solutions i s much more complex (6^, 7.' 49) . Measurements of the specific surface area, SSA, of the products grown at various times indicate that the i n i t i a l formation of a macrocrystalline or amorphous precursor leads to a rapid increase in SSA. The development of these phases i s also observed by scanning electron microscopy, and dissolution kinetic studies of the grown material have indicated the formation of OCP as a precursor phase (6_ 1) . The overall precipitation reaction appears to involve, therefore, not only the formation of different calcium phosphate phases, but also the concomitant dissolution of the thermodynamically unstable OCP formed rapidly in the i n i t i a l stages of the reaction. In the presence of magnesium ion the overall rate of crystallization i s reduced and lower Ca:P ratios are observed for the f i r s t formed phases (51). It appears that the magnesium ion stabilizes a precursor phase, the stoichiometry of which appears to correspond to a mixture of DCPD and OCP. The p o s s i b i l i t y of DCPD, the simplest calcium phosphate phase, as a precursor to apatite precipitation was proposed by Francis (52). Although the seeded crystallization experiments y i e l d highly reproducible results and, i n the case of the pH-stat method, the hydrogen ion activity could be held constant during the reaction, they suffer from the disadvantage that the calcium and phosphate ionic concentrations decrease appreciably as the reaction proceeds towards solubility equilibrium. At pH values above about 6.0 (see Figure 1), at each stage the supersaturated solutions are metastable with respect to various calcium phosphate phases which can form and subsequently dissolve as the concentrations i n the supersaturated solutions decrease. Since the concentration changes become very small as the reaction proceeds, a relative analytical error of only a few percent in the total calcium or total phosphate concentration can preclude differentiation between the possible crystalline phases. To overcome the problems associated with the changing of solution concentrations during precipitation, a new method has been developed (53) in which the chemical potentials of the solution species are maintained constant during the reaction. Following the addition of well-characterized seed material to stable supersaturated solutions of calcium phosphate at the required pH, the concentrations of l a t t i c e ions are maintained 1

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

483

r

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

484

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

constant by the simultaneous addition of the reagent solutions containing calcium and phosphate ions, controlled by a glass electrode probe. The compositions of the reagent solutions are calculated from the results of exploratory measurements and, during the course of the reactions, their constancy i s verified by analysis. Typical data are shown in Table I at a supersaturation corresponding to level c i n Figure 2. At 37° and pH = 7.40, i t can be seen that the use of reagent solution concentrations with a Ca:P ratio, R, of 1.45 results in the formation of a phase on the surface of the HAP seed crystals i n which a l l solution parameters remain constant to within t 0.3%. In addition, i t i s found that the rate of reaction i s directly proportional to the inoculating seed concentration, thus confirming that the growth of the crystals takes place without interference from secondary nucleation. To have obtained a kinetic precipitation stoichiometry to this precision by techniques previously used would have required concentration analysis to at least t 0.03%. Although, as mentioned previously, i t had been assumed that the ratio, R, was close to that for TCP, thereby invoking TCP as the precursor, i t i s clear from the results in Table I that the ratio i s significantly lower than the Ca:P = 1.50 required for TCP. Results of a typical experiment at higher supersaturation are shown in Table II in which the i n i t i a l supersaturation i s above that for OCP (Figure 2, level b) but s t i l l below DCPD. I t can be seen i n Table II that the choice of reagent solutions with Ca:P of 1.33 yielded the required constancy of l a t t i c e ion concentrations for at least the f i r s t 10 min of reaction following the addition of HAP seed crystals. The results clearly indicate the formation of OCP i n the early stages of the reaction and this phase was confirmed by x-ray and infra-red analysis. At longer times (15-20 min) the expected hydrolysis to a more basic phase takes place with a Ca:P ratio of approximately 1.45, the value observed i n so many calcium phosphate precipitation studies. I t i s significant that, using the constant composition method, more than twice the original seed material could be grown as OCP i n the early stages of the reaction. The results of these studies support a model for calcium phosphate precipitation in which OCP, formed as a precursor phase, hydrolyzes either p a r t i a l l y or completely to HAP depending upon the rate of precipitation. The normally observed higher Ca:P values can be accounted for by assuming that only a fraction of the OCP molecules transforms to HAP. I f the overall rate of reaction i s such that only one i n every three molecules i s capable of transforming into HAP, the observed stoichiometry of the precipitating phase would be 1.44, or within 0.01 of the normally observed 1.45 - 0.05. The hydrolysis probably takes place a layer at a time as has been proposed from unit c e l l x-ray analysis of the phases (54) . The results of a preliminary experiment at even higher supersaturâtion corresponding to level a in Figure 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

NANCOLLAS E T A L .

23.

Calcium Phosphates

485

(T = 1.20 mM, Tp = 1.20 mM, [KC1] = 30 mM, pH = 7.40)indicate that reagent solutions with a 1:1 calcium to phosphate ratio, corresponding to DCPD, produce an i n i t i a l transient phase for approximately 10 min of reaction. Infrared analysis indicates the presence of DCPD and OCP i n the grown material and this i s the f i r s t time that DCPD has been grown on HAP seed material at pH levels above 7.0. The results suggest that DCPD i s the ultimate apatite precipitation precursor. As w i l l be shown later, the epitaxial growth of DCPD on HAP and other calcium phosphate at pH levels below 6.0 follows the characteristic DCPD growth kinetics. Ca

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

Table I Constant composition seeded growth of HAP crystals at pH 7.40, 37°C* Time (min)

Ο 20 34 48 60 75 90 *

T (mM) C a

T (mM)

R**

1.160 1.161 1.160 1.156 1.157 1.170 1.161

1.450 1.457 1.457 1.450 1.457 1.462 1.457

p

0.800 0.797 0.796 0.797 0.794 0.800 0.797

I n i t i a l solution: 150 ml of 0.800 mM CaCl , 0.552 mM KH P0 , 0.457 mM KOH, 8.40 mM KC1 and 5.0 mg of HAP seed. Titrant solutions: 10.00 mM CaCl and 6.90 mM KH P0 with 11.91 mM KOH. 2

2

4

2

2

**

4

R i s the molar ratio of calcium to phosphate in solution at each time.

One of the advantages of the constant composition method i s the a b i l i t y to study growth rates, even at very low supersaturation, with a precision hitherto unobtainable. Experiments made at a concentration corresponding to level d in Figure 2 provide the opportunity for the formation of only a single phase, HAP. Typical data for an experiment in which HAP seed was added to such supersaturated solutions are shown i n Table III from which i t can be seen that HAP i s grown by direct precipitation at pH = 7.40 and 37° (55). The stoichiometry of the precipitated phase i s constant, 1.66 - 0.01 for more than 6 hours of reaction and HAP was confirmed by infrared and x-ray diffraction studies. Experiments at a higher pH of 8.50, of significance i n cooling tower applications, also indicate the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING

486

IN AQUEOUS

SYSTEMS

Table II

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

Constant composition seeded growth of HAP crystals at pH 7.40, 37°C*. Time (min)

T^ (mM)

Ο 1 3 5 7 10

1.200 1.197 1.208 1.214 1.234 1.248

*

T_

R

(SM)

1.596 1.580 1,595 1.602 1.616 1.622

1.33 1.32 1.32 1.32 1.31 1.30

I n i t i a l Solution: 150 ml of 0.800 mM CaCl , 0.575 mM KH P0 , 0.854 mM KOH and 5.0 mg of HAP seed. Titrant solutions: 10.00 mM CaCl and 7.19 mM KH P0 with 12.81 mM KQH. 2

2

4

2

2

4

exclusive formation of HAP at low supersaturation. Typical data are shown i n Table IV from which i s can be seen that more than 100% of the original seed material i s grown directly at HAP. The results confirm previous suggestions based upon approximate stoichiometry determinations using conventional precipitation experiments that at sufficiently low supersaturation HAP can precipitate without the need for precursor formation both at the surface seed crystals (6)and spontaneously from solution (56). Epitaxial Considerations It i s clear from the foregoing that the epitaxial growth of one calcium phosphate phase either upon another phase or upon a foreign substrate may constitute an important step in the precipitation of apatites. Crystal growth studies of the influence of different calcium phosphate seed material (90 have shown that a l l calcium phosphate phases are good nucleators for DCPD under solution conditions (see Figure 1, pH = 5.60) i n which DCPD and HAP are the l i k e l y calcium phosphate phases. At higher pH (> than about 6) only OCP would readily induce growth i n metastable supersaturated solutions (9). A more detailed study was made (8) in which HAP seed crystals were added to metastable supersaturated solutions at pH values between 4.5 and 5.1. As shown i n Figure 3, the growth curve, following the addition of a relatively low concentration of HAP seed crystals (24 mg HAP l ) i s characterized by a brief induction period (13-15 min) followed by the growth of DCPD with the required molar Ca:P ratio of 1.01 ± 0.01. The entire growth phase consists of DCPD which was - 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23.

NANCOLLAS

Calcium Phosphates

ET AL.

487

Table III

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

Constant composition seeded growth of HAP crystals at pH 7.40, 37°C*. Time (min)

T (mM)

T (mM)

R

Ο 60 120 180 240 360

0.300 0.303 0.300 0.298 0.295 0.300

0.180 0.178 0.180 0.176 0.176 0.180

1.67 1.69 1.67 1.69 1.67 1.67

*

C a

p

Newly formed HAP as % of original seed 0,0 8.0 15.0 21.0 27..0 37.0

I n i t i a l solution: 500 ml 0.300 mM C a C l 0,180 mM KH P0 2.000 mM KC1 and 20.0 mg of HAP seed. Titrant solutions; 3.900 mM CaCl , 2.340 mM KH P0 and 4.390 mM KOH. 2/

2

2

2

4

4

confirmed by x-ray analysis and which appeared to grow epitaxially from the surface of the HAP seed material. The formation of DCPD on the surface of the added HAP seed crystals involves a nucleation step and the subsequent crystal growth of these DCPD nuclei. I t can be seen i n Figure 4 that, following the induction period the reaction follows the same kinetics as that observed for the growth of DCPD crystals themselves. No evidence was found for the formation of a more basic calcium phosphate phase despite the fact that the solutions were supersaturated with respect to both HAP and DCPD. It i s clear that the rate of nucleation, which i s proportional to (In T / T o ) " where T o is the solubility value, i s markedly dependent upon supersatur­ ation, sharply decreasing as the supersaturation decreases. Estimations based upon the relative rate of heterogeneous nucleation (8)indicate that heterogeneous nucleation of DCPD on the added HAP seed of low concentration i s essentially complete within the induction period f a l l i n g to zero at the commencement of normal DCPD growth. At higher seed concentrations (about 230 mg HAP l"" ) a more basic calcium phosphate with Ca:P = 1.52 ± 0.04 crystallizes on the growth sites of the HAP seed material and no evidence i s found for the presence of DCPD. The dependence of the growth phase on solid/solution ratio i s of particular importance not only for the interpretation of the results of biological precipitation studies but also for the formation of calcium phosphate i n environmental systems. I t may be explained by the competition between heterogeneous nucleation of DCPD and the growth of the active sites already present on the seed substrate surface. The latter process occurs more extensively i n the i n i t i a l stages of reaction when the seed concentration i s C a

C a

2

C a

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

20

40

60

80 Time

100

120

140

160

(min)

Figure 4. Plots of the integrated form of rate Equation 2. N* and N are the values of Ν corresponding to the instant of seed addition and equilibrium respec­ tively. (A) T = 9.314 mM, T = 3.935 mM, pH 5.62, 37°C; (O) T = 10.20 mM, T = 5.861 mM, pH 5.62, 37°C. 0

Ca

p

Ca

p

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23.

NANCOLLAS

Calcium Phosphates

ET AL.

489

Table IV Constant composition seeded growth of HAP crystals at pH 8.50, 37°C*.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

Time (min) Ο 25 55 95 140 165 240

Newly formed HAP as % of original seed

R (mM) Ο. 300 0.300 0.297 0. 308 0.307 Ο. 303 0.306

0.180 0.178 0.178 0.182 0.182 0.180 0.180

1.67 1.68 1.64 1.69 1.68 1.68 1.69

0.0 13.0 27.0 43,0 61,0 74.0 104,0

* I n i t i a l solution: 500 ml of 0.300 mM CaCl , 0.180 mM KH P0 , 2.000 mM KC1 and 20.0 mg HAP seed. Titrant solutions: 3.900 mM CaCl , 2.340 mM KH P0 , 4.39 mM KOH. 2

2

2

2

4

relatively high. In these circumstances, the calcium and phosphate concentrations in the solution are rapidly lowered below that required for effective DCPD nucleation. The results of these studies confirm the suggestions made earlier that the nature of the calcium phosphate phase which forms i n the precipitation process i s dependent not only upon the supersaturation, ionic strength, and pH, but also upon the rate at which reactions proceed. Influence of Other Substances As might be expected for a reaction which i s controlled largely by a surface process, the presence of other components i n the solution may have a marked influence upon the rate of precipitation. Studies have been made of the precipitation of calcium phosphate in the presence of a number of tricarboxylic acids of the Kreb's Cycle, c i t r i c acid, i s o c i t r i c acid, cis-aconitic acid, trans-aconitic acid and t r i c a r b a l l y l i c acid (57). I t was shown that those acids having an hydroxy1 group, ( c i t r i c and iso-citric) are effective precipitation inhibitors. Moreover, by computing the a c t i v i t i e s of calcium citrate complexes and following the uptake of citrate using carbon-14 labelled material, i t was shown that the inhibiting effect of c i t r i c acid cannot be explained simply i n terms of the formation of solution calcium complexes of the ligands. A relatively small degree of surface adsorption i s sufficient to markedly retard the crystal growth reaction. Thus, i n a recent study of the crystallization of barium sulfate (58), i t was shown that the inhibition by an organic phosphonate additive,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

490

Time

(Min)

Figure 5. Constant composition growth of HAP seed at low supersaturation (T = 0.30 mM, T = 0.18 mM, [KCl] = 2 mM, 37°C, pH 8.50). Plots of titrant added as a function of time. Experiments in the presence of magnesium and strontium. Ca

p

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

23.

NANCOLLAS E T A L .

491

Calcium Phosphates

nitrilotri(methylenephosphonic acid), was sufficiently large that only about 4% of the surface of the barium sulfate crystals needed to be covered by inhibitor molecules for complete inhibition of crystal growth. This provides striking confirmation of the suggestion that precipitation may proceed through the development of a relatively small number of active sites. The inhibition of calcium phosphate crystal growth ( i n i t i a l conditions T = 1.66 mM, T = 1.00 mM, pH = 7.0, 25°) by typical growth inhibitors such as the organic phosphonate, hydroxyethylidene di(phosphonic acid), HEDP, has also been demonstrated (57). Precipitation i s completely inhibited at a concentration of additive of 10" M and the results of adsorption experiments indicate that a relatively small fraction (10-30%) of the seed surface must be covered for complete inhibition of precipitation. The rate i s markedly reduced even when only 1-10% of the surface i s covered by HEDP. At pH values below about 6.0 the influence of HEDP and fluoride, an important environmental constituent i s particularly interesting. I t has been shown that the nature of the precipitating calcium phosphate phase can be controlled not only by the concentration of inoculating HAP seed (SObut also by the presence of additives. Thus fluoride ion accelerates the crystallization of calcium phosphate probably through the formation of fluorapatite (Ca (P0 ) F, FAP) (59, 60). Under the conditions of the experiments, ( T = 6.2 75 mM, T = 6.275 mM, 37°, pH = 5.59) the addition of HEDP, which retards the rate of mineralization, increases the nucleation of DCPD and the amount of this phase formed. Fluoride ions, on the other hand, i n increasing the rate of mineralization, rapidly reduce the concentration levels of calcium and phosphate below those required for effective nucleation of DCPD. The resulting growth phase then consists of a more basic calcium phosphate with no evidence for the presence of DCPD (61). These results may have significant implications i n discussions of the influence of other environmental components on the precipitation of calcium phosphates. The design of model experiments aimed at elucidating the mechanism of calcium phosphate precipitation under specific natural water conditions therefore requires very careful planning together with a knowledge of the specific influences of the individual components. It i s interesting to note that at pH values of about 7.0, there i s a marked influence upon the crystallization of HAP seed crystals by changing the nature of the alkaline metal cation in the background electrolyte (6). The greatest differences are found between potassium and sodium chlorides, of particular importance i n the environment. The p o s s i b i l i t y that sodium ion can replace calcium in the calcium phosphate solid i s well known and i t has been found that from 5.5 to 6.0 mol % of sodium i s incorporated in the growing HAP crystals i n the presence of sodium chloride but almost no potassium i s observed i n the C a

p

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

5

5

4

3

C a

p

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

492

IN AQUEOUS SYSTEMS

presence of potassium chloride (6). The p o s s i b i l i t y of studying the influence of traces of metal ions using the new constant composition approach i s particularly interesting. Studies have recently been made at concentrations corresponding to level d in Figure 2 at a higher pH of 8.5. Exclusive formation of HAP without the need for precursor phase was confirmed (P. Koutsoukos and G. H. Nancollas, unpublished data, 1968), the precipitating phase having a Ca:P molar ratio of 1.67 + 0.02. A typical plot of the rate of mixed titrant addition, reflecting the rate of crystallization, i s shown in Figure 5 and i t i s seen that low concentrations of magnesium ions have a marked inhibiting effect upon the HAP crystallization. It i s interesting to note that the magnesium ion i s not incorporated into the HAP crystals during precipitation. The inhibiting influence of magnesium ion upon the crystallization of calcium phosphate at supersaturation levels corresponding to level a in Figure 2, has been attributed to i t s adsorption at the surface of the crystals (51). The presence of magnesium ion also appears to change the stoichiometry of the first-formed surface phase as discussed earlier. It can be seen in Figure 5 that the presence of strontium ion also markedly retards the rate of HAP crystallization. In this case, however, an observed increase in calcium concentration as the reaction proceeds, indicates some substitution of S r for C a ions. This has been reported by other workers (41, 62, 63, p. 2225) and may be ascribed to the similarity of ionic r a d i i . The ions. This has been reported by other workers (4JL, 63^, 64) and may be ascribed to the similarity of ionic r a d i i . The constant composition technique enables more detailed studies to be made of the influence of magnesium, strontium, and other cations on the precipitation of calcium phosphates. As far as anions, typical of the environment, are concerned, the effect of carbonate i s of particular significance (64, p. 514-525). Our preliminary studies show that low levels of carbonate ion (^ 1 mg L"i) significantly reduce the rate of calcium phosphate precipitation (P. Koutsoukos and G. H- Nancollas, unpublished data, 1968) while phosphate ions have a similar effect upon the constant composition growth of calcium carbonate crystals (T. Kazmierczak and G. H. Nancollas, unpublished data, 1978). Studies of these reactions under conditions in which the a c t i v i t i e s of a l l ion species are maintained constant are continuing. 2 +

2 +

Acknowledgements We wish to acknowledge p a r t i a l support of this work through research grants from the National Institutes of Health, National Institutes of Dental Research, (Grant # DE03223) and the National Science Foundation (Grant # ENG74,15486).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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NANCOLLAS E T A L .

Calcium Phosphates

493

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Abstract The calcium phosphate phases which precipitate in solutions containing calcium and phosphate ions are markedly dependent upon factors such as pH, supersaturation, ionic strength, temperature and the nature and extent of the solid phases already present. Although thermodynamic considerations w i l l yield the driving forces for precipitation of particular phases, their formation under a specific set of conditions may be determined much more by kinetic factors. Metastable phases may persist for considerable periods in the supersaturated solutions. In making the thermo­ dynamic calculations, i t is essential to take into account both activity coefficient terms and the formation of ion-pairs and complexes i n the solutions. Methods are described for the highly reproducible kinetic study of precipitation reactions in the presence of seed materials. A recently developed constant composition techniques throws new light on the mechanism of c a l ­ cium phosphate precipitation and enables solid stoichiometries and rates of reaction to be determined with a precision hitherto unobtainable even at the very low supersaturations of significance in the environment. Literature Cited 1.

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"Phosphates i n Detergents and the Eutrophication of America's Waters", 23rd Report, Comm. on Govt. Operations, US Printing Office, Washington, p. 27 (1970). Brown, W.E. Solubilities of phosphates and other sparingly soluble compounds, p. 203-239, i n G r i f f i t h , E.J., Beeton, Α., Spencer, J.M., and Mitchell, D.T., ed., "Environmental Phos­ phorus Handbook," Wiley and Sons, New York, 1973. Ferguson, J.F., Jenkins, D., and Eastman, J. Calcium phos­ phate precipitation, in 44th Ann. Conf. Water Pollution Con­ t r o l Fed., Ser. 5 (2) Oct. 4, (1971). Clark, L.M. and Gerrard, W.E. The precipitation of hydroxyapatite, 3[Ca (P0 )2], Ca(0H) Part I, J. Soc. Chem. Ind. 57, 295-297 (1938). Varsanik, R.G. The nature and control of calcium orthophosphate depositions in cooling water systems. Paper 142, "Corrosion '75," Nat. Assoc. Corr. Eng., Toronto, Can., 1975. Nancollas, G. H. and Tomazic, B. Growth of calcium phosphate on hydroxyapatite crystals. Effect of supersaturation and ionic medium. J. Phys. Chem. 78, 2218-2225 (1974). Tomazic, B. and Nancollas, G.H. The seeded growth of calcium phosphates. Surface characterization and the effect of seed material. J. Coll. Interface Sci. 50, 451-461 (1975). Barone, J.P. and Nancollas, G.H. The seeded growth of c a l ­ cium phosphates. The effect of solid/solution ratio in con­ trolling the nature of the growth phase. J . C o l l . Interface Sci. 62, 421-431 (1977). 3

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch023

RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24

Chemistry of Calcium Carbonate in the Deep Oceans

JOHN W. MORSE Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

ROBERT C. BERNER Department of Geology and Geophysics, Yale University, New Haven, CT 06520 Recent p e l a g i c sediments c o n t a i n i n g over 30% calcium carbon­ a t e , by dry weight, cover a q u a r t e r o f the s u r f a c e o f the e a r t h (see F i g u r e 1 ) . These sediments make up a v a s t and c h e m i c a l l y r e a c t i v e carbonate r e s e r v o i r which has a major i n f l u e n c e on the chemistry o f the oceans and atmosphere. I n order to have a pre­ d i c t i v e understanding of the n a t u r a l carbon d i o x i d e system and the i n f l u e n c e o f man on i t , the chemical dynamics o f c a l c i u m c a r ­ bonate d e p o s i t i o n i n the deep ocean b a s i n s must be known i n detail. The dominant source of c a l c i u m carbonate f o r deep sea s e d i ­ ments i s b i o g e n i c , being d e r i v e d from the remains of s m a l l p e l a g ­ i c organisms l i v i n g near the ocean s u r f a c e . Only a s m a l l p o r t i o n of the calcium carbonate produced by these organisms i s preserved i n deep sea sediments. Current estimates i n d i c a t e that between 75 and 95 percent of the calcium carbonate being produced i n the open ocean i s subsequently d i s s o l v e d Ç2, _3). Most of the d i s s o ­ l u t i o n i s thought to occur a t o r very near the sediment-water i n t e r f a c e , r a t h e r than i n the water column d u r i n g s e t t l i n g ( 4 ) . The balance between c a l c i u m carbonate p r o d u c t i o n and d i s s o ­ l u t i o n i s the major pH b u f f e r i n g mechanism of seawater over periods of time a t l e a s t on the order of thousands of years Ç5). The atmospheric carbon d i o x i d e r e s e r v o i r i s l e s s than 2 percent the s i z e of the seawater r e s e r v o i r (6) and there i s a c t i v e ex­ change between these two r e s e r v o i r s across the a i r - w a t e r i n t e r ­ face. Consequently, the carbon d i o x i d e content o f the atmosphere and accumulation of calcium carbonate i n the deep oceans a r e c l o s e l y coupled. In order to understand the chemistry of c a l c i u m carbonate accumulation i n the deep oceans, the sources of c a l c i u m carbonate, i t s d i s t r i b u t i o n i n recent p e l a g i c sediments, the s a t u r a t i o n s t a t e of seawater o v e r l y i n g deep-ocean sediments w i t h r e s p e c t to c a l c i t e and a r a g o n i t e , and the r e l a t i o n between s a t u r a t i o n s t a t e and d i s s o l u t i o n r a t e must be known. These aspects of calcium carbonate chemistry are examined i n t h i s paper.

0-8412-0479-9/79/47-093-499$09.25/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 1. Distribution of recent pelagic sediments containing greater than 30 wt % calcium carbonate by dry weight (after Ref. I).

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

O

E

3

W

8

o

g

24.

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

Sources and D e p o s i t i o n a l Patterns of Calcium Carbonate i n Deep Sea Sediments Biogenic calcium carbonate, d e r i v e d from the t e s t s ( s h e l l s ) of p e l a g i c organisms, i s the dominant source f o r calcium carbonate p r e s e n t l y being deposited i n deep ocean b a s i n s . F o r a m i n i f e r a (animals) and coccolithophores ( p l a n t s ) are the most important sources of c a l c i t i c calcium carbonate. Pteropods (animals) a r e the most important source f o r a r a g o n i t i c calcium carbonate. Photomicrographs o f t y p i c a l r e p r e s e n t a t i v e s of these organisms are presented i n Figure 2. The r e l a t i v e abundance of these major sources and species d i s t r i b u t i o n s , w i t h i n a given type of organ­ ism, are dependent on the environmental c o n d i t i o n s a t a given l o ­ c a t i o n . Consequently, the calcium carbonate a r r i v i n g a t the sediment-water i n t e r f a c e i s g e o g r a p h i c a l l y and, perhaps, seasonal­ l y v a r i a b l e . Current estimates of the average p e l a g i c aragonite to c a l c i t e production r a t i o range from 1 (7) t o 0.25 ( 8 ) . The r e l a t i v e abundance o f aragonite to c a l c i t e and the s i z e d i s t r i b u ­ t i o n of the b i o g e n i c c a l c i t e a r r i v i n g a t the sediment-water i n t e r f a c e have a strong i n f l u e n c e on d i s s o l u t i o n k i n e t i c s and are important f a c t o r s i n calcium carbonate p r e s e r v a t i o n . The general p a t t e r n of calcium carbonate accumulation i n deep ocean basins has been known f o r approximately one hundred years. In most areas the abundance of calcium carbonate i n s e d i ­ ments i s found to decrease w i t h i n c r e a s i n g water depth (see Figure 3 ) . From t h i s general p a t t e r n two important sediment marker l e v e l s have been i d e n t i f i e d . The f i r s t i s the calcium carbonate compensation depth (CCD). Below the CCD calcium c a r ­ bonate becomes a minor ( l e s s than 10%) component o f the sediment. At the CCD c l o s e to 100 percent of the calcium carbonate i s being d i s s o l v e d and t h a t the r a t e of i n p u t and d i s s o l u t i o n c l o s e l y b a l ­ ance. The second i s the aragonite compensation depth (ACD), which f r e q u e n t l y occurs a t depths on the order of 3 km shallower than the CCD. Below the ACD aragonite i s not preserved i n the sediments. These e a r l y f i n d i n g s suggested that temperature and pressure are important c o n t r o l s on, calcium carbonate d e p o s i t i o n and, hence, that calcium carbonate accumulation i n the deep oceans i s chemically c o n t r o l l e d . Berger (10) defined an important sediment marker l e v e l which occurs above the CCD c a l l e d the f o r a m i n i f e r a l l y s o c l i n e ( F L ) . I t i s the depth a t which there i s a maximum change i n the r a t i o of " e a s i l y d i s s o l v e d " to " r e s i s t a n t " p e l a g i c f o r a m i n i f e r a . The c l a s s i f i c a t i o n of p e l a g i c f o r a m i n i f e r a i n t o these c a t e g o r i e s i s based on observations of f o r a m i n i f e r a l assemblages suspended a t d i f f e r e n t depths i n the P a c i f i c Ocean and the changes i n f o r a m i n i f e r a l assemblages i n sediments w i t h i n c r e a s i n g water depth. At present, there i s considerable controversy over how much d i s ­ s o l u t i o n of calcium carbonate must occur to produce the FL. Adelseck (11) has experimentally determined that approximately 80 percent of the c a l c i t i c f r a c t i o n must be d i s s o l v e d . This roughly

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

502

Figure 2. Examples of typical calcium carbonate tests which are deposited in pehgic sediments. (1) and (2) are foraminifera (calcite); (3) is a coccolithophore (calcite); (4) and (5) are pteropods (aragonite). Note the difference in size as shown on scales.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24.

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agrees w i t h Berger's (12) minimum l o s s v a l u e , based on d i r e c t ob­ s e r v a t i o n s , of approximately 50 percent. The d i f f e r e n c e i n depth between the FL and the CCD v a r i e s c o n s i d e r a b l y . Under c e n t r a l s u b t r o p i c a l water masses i t i s f r e q u e n t l y about 800 m. Berger (12) has i d e n t i f i e d another sediment marker l e v e l i n the sediments. This i s the R l e v e l a t which the p r o p o r t i o n of " r e s i s t a n t " p l a n k t o n i c f o r a m i n i f e r a show the f i r s t s i g n i f i c a n t i n c r e a s e . Berger's estimate of minimum d i s s o l u t i o n l o s s a t the R l e v e l i s 10 percent. Adelseck's (11) experimentally d e t e r ­ mined amount of d i s s o l u t i o n necessary to produce the R l e v e l i s approximately 50 percent d i s s o l u t i o n . Q

Q

Q

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

C a l c u l a t i o n o f Calcium Carbonate S a t u r a t i o n State General C o n s i d e r a t i o n s . I n order t o f a c i l i t a t e the d i s c u s ­ s i o n of methods f o r c a l c u l a t i n g the s a t u r a t i o n s t a t e of seawater w i t h respect t o calcium carbonate, i n i t i a l c o n s i d e r a t i o n w i l l be given t o pure calcium carbonate phases. The method most f r e ­ quently used expressing the s a t u r a t i o n s t a t e of a s o l u t i o n w i t h respect t o s o l i d phase i s as the r a t i o (Ω) of the i o n a c t i v i t y (a) product to the thermodynamic s o l u b i l i t y constant (K). F o r the calcium carbonate phase c a l c i t e , the expression f o r the s a t ­ u r a t i o n s t a t e i s defined as (e.g., 13): Ca ++ aCO. (1)

Where the s u b s c r i p t " c " denotes c a l c i t e . A s u b s c r i p t e d "a" w i l l be used i n the same manner to denote a r a g o n i t e . I f Ω i s g r e a t e r than 1 the s o l u t i o n i s supersaturated, l e s s than 1 undersaturated, and equal to 1 i n e q u i l i b r i u m w i t h respect to c a l c i t e . Because of d i f f i c u l t i e s i n p r e c i s e l y c a l c u l a t i n g the t o t a l i o n a c t i v i t y c o e f f i c i e n t (γ) of calcium and carbonate ions i n seawater, and the e f f e c t s of temperature and pressure on the ac­ t i v i t y c o e f f i c i e n t s , a s e m i - e m p i r i c a l approach has been g e n e r a l l y adopted by chemical oceanographers f o r c a l c u l a t i n g s a t u r a t i o n s t a t e s . This approach u t i l i z e s the apparent ( s t o i c h i o m e t r i c ) s o l u b i l i t y constant ( K ) , which i s the e q u i l i b r i u m i o n molal (m) product. Values of K are d i r e c t l y determined i n seawater (as i o n i c medium) a t v a r i o u s temperatures, pressures and s a l i n i t i e s . In t h i s approach: f

T

Ca

CO. (2)

1

The r a t i o of Κ t o Κ i s then: c c

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

C a l c u l a t i o n of Calcium and Carbonate T o t a l M o l a l i t i e s . B e ­ cause of the constancy of composition of seawater, the t o t a l i o n calcium c o n c e n t r a t i o n i n seawater can be c a l c u l a t e d , i n "open" ocean seawater samples, d i r e c t l y from h i g h p r e c i s i o n s a l i n i t y measurements, u s i n g the r e l a t i o n s h i p (14):

where S i s s a l i n i t y i n p a r t s per thousand. This r e l a t i o n s h i p can be expected to be constant under "open" ocean c o n d i t i o n s to w i t h ­ i n a t l e a s t 0 . 3 % and i s independent of temperature and p r e s s u r e . The c a l c u l a t i o n of the t o t a l i o n m o l a l carbonate i o n concen­ t r a t i o n i s more complex than f o r c a l c i u m because i t i s p a r t of the carbonic a c i d system. The f o l l o w i n g r e a c t i o n s take place between CC>2 and carbonic a c i d i n seawater:

G e n e r a l l y only one e q u i l i b r i u m constant i s used to d e s c r i b e the r e a c t i o n between gaseous carbon d i o x i d e and H 2 C O 3 . The thermody­ namic and apparent e q u i l i b r i u m constants f o r the above r e a c t i o n s are:

where a and a are the Henry's law c o e f f i c i e n t s f o r i n f i n i t e d i ­ l u t i o n and seawater, r e s p e c t i v e l y , and Ρ i s the p a r t i a l p r e s ­ sure of C 0 gas, and: 2 Q

s

2

where 2 ^ 3 ^ H

C

e r e

represents the sum of d i s s o l v e d CO^ and t r u e

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24. H

MORSE AND BERNER

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C

2 V

Using e i t h e r the above thermodynamic or apparent ( s t o i c h i o ­ m e t r i c ) equations, i t i s p o s s i b l e to c a l c u l a t e m — i f any two of 3 the four p o s s i b l e measurable q u a n t i t i e s f o r the c a r b o n i c a c i d s y ­ stem are known. These four measurable q u a n t i t i e s are: 1. T o t a l C 0 defined as: 2

2

C

+

° 2 " \C0

3

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

2.

= C

·

4.

m

( 9 )

Carbonate a l k a l i n i t y defined as: A

3

+

"Ήοο" C0--

'"HCO;

+

2 m

co7

(10)

p

co

2

pH

Carbonate a l k a l i n i t y i s not a d i r e c t l y analyzable q u a n t i t y , but i t can be d e r i v e d from the t i t r a t i o n a l k a l i n i t y (A^_) by a r e ­ l a t i o n s h i p which i s temperature and pressure dependent. pH i s a l s o v a r i a b l e w i t h pressure and temperature. I n a d d i t i o n , pH can be defined s e v e r a l ways depending on which b u f f e r system i s used, whether l i q u i d j u n c t i o n p o t e n t i a l s are considered, and what d e f i ­ n i t i o n of i o n i c s t r e n g t h i s used. The values of the apparent d i s ­ s o c i a t i o n constants and calcium carbonate s o l u b i l i t y constants are dependent on e x a c t l y what d e f i n i t i o n of seawater pH i s used and what s t a n d a r d i z a t i o n technique i s used (15, 16, 17). Although every combination of the four c a r b o n i c a c i d systemanalyzable variables has been used, the one p a i r f r e q u e n t l y used because of i t s r e l a t i v e ease from both an a n a l y t i c a l and c a l c u l a ­ t i o n standpoint i s pH and a l k a l i n i t y . When t h i s p a i r i s used, the equation r e l a t i n g t o t a l carbonate m o l a l i t y to pH and carbonate alkalinity i s :

-co;- •

• K

2

The i n s i t u pH can be c a l c u l a t e d by making the appropriate temperature and pressure c o r r e c t i o n s on the measured pH. Tempera­ ture c o r r e c t i o n s are made u s i n g the equation of Harvey (18) pH

= pH 2

+ x(t

- t ).

(12)

1

For the general case where the pH i s measured a t 25°C (t-^) and the

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

506

CHEMICAL MODELING IN AQUEOUS SYSTEMS

i n s i t u temperature ( t = t ) i s l e s s than 20°C, the values of χ given by Harvey (18) y i e l d Equation 13. 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

p H

=

t

p H

i s

25 " ° ·

0 4 8 5

+

°-

0 1 0 5

"

2 0 )

( 1 3 )

·

Ben-Yaakov (19) has presented a more p r e c i s e and complex method f o r c a l c u l a t i n g the change i n pH of seawater w i t h temperature, based on the changes i n the apparent d i s s o c i a t i o n constants of b o r i c and c a r b o n i c a c i d i n seawater w i t h temperature, p r e s s u r e , and s a l i n i t y . He found that the temperature c o e f f i c i e n t depended on pH and s a l i n i t y . However, the r e s u l t s obtained by h i s method and Equation 13 are i n c l o s e agreement when the s a l i n i t y i s near 35°/oo and the pH i s close to 8. Once the pH at the i n s i t u temperature has been c a l c u l a t e d , the a c t u a l i n s i t u pH can be c a l c u l a t e d by c o r r e c t i n g the pH f o r the e f f e c t of pressure by use of Equation 14 which was d e r i v e d from the data presented by Culberson and Pytkowicz (20, Table 4) pH.

= pH

s

4

- [(P-l) χ

t

10~ ]

is [4.28 - 0.04

t.

- 0.4

(pH

- 7.8)]

(14)

m

is where Ρ i s the absolute pressure i n atmospheres. In order to c a l c u l a t e the i n s i t u carbonate a l k a l i n i t y , i t i s f i r s t necessary to c a l c u l a t e the i n s i t u a l k a l i n i t y c o n t r i b u t i o n of b o r i c a c i d to the analyzed q u a n t i t y , t i t r a t i o n a l k a l i n i t y . This i s done by f i r s t c a l c u l a t i n g the c o n c e n t r a t i o n of b o r i c a c i d v i a the boron to c h l o r i n i t y r a t i o of C u l k i n (21) and a g e n e r a l s a l i n i t y - m o l e c u l a r weight conversion formula 5

(1.21 χ 10" )S \

1000

(15)

/

The next step i s the c a l c u l a t i o n of the f i r s t apparent d i s ­ s o c i a t i o n constant of b o r i c a c i d i n seawater (Kg ) under i n s i t u conditions. K can be c a l c u l a t e d a t the i n s i t u temperature using the equation of Edmond (22) which i s based on the data of Lyman (23) 1

f

B

l o

" S

V

- (t

2 9 1

9

; 273)

+

°-

0 1 7 5 6

(

t

+

is

2 7 3 )

'is -3.3850

°-

3 2 0 5 1

(ObW)

·

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

-

A

R (

;, T273) ·

( 2 9 )

From the above c o n s i d e r a t i o n s a l l c a l c u l a t i o n s necessary to determine the jLn s i t u s a t u r a t i o n s t a t e of seawater can be c a r r i e d out. A sample c a l c u l a t i o n using the methods described i s pre­ sented i n Table I I . E s t i m a t i o n of U n c e r t a i n t y i n Ω. The determination of the t o t a l i o n molal c o n c e n t r a t i o n of calcium from s a l i n i t y measurement i s r e l a t i v e l y p r e c i s e w i t h a probable e r r o r of l e s s than 0.3% under open ocean c o n d i t i o n s . Dickson and R i l e y (37) have r e c e n t l y discussed the e f f e c t of a n a l y t i c a l e r r o r s on the e v a l u a t i o n of the components of the a q u a t i c carbon-dioxide system f o r seawater at 25°C and 1 atmosphere t o t a l pressure. T h e i r conclusions i n d i c a t e that i f a l k a l i n i t y and t o t a l carbon d i o x i d e are the measured par­ ameters a probable combined u n c e r t a i n t y i n the t o t a l carbonate i o n molal c o n c e n t r a t i o n from 3 to 6 percent r e s u l t s , depending on PcOo' I f pH and a l k a l i n i t y are the measured parameters the u n c e r t a i n t y i s approximately 4 percent. In a d d i t i o n to the probable e r r o r introduced by a n a l y t i c a l p r e c i s i o n , the absolute accuracy of the measurements introduces an e r r o r which i s d i f f i c u l t to evaluate. The r e s u l t s of the GEOSECS i n t e r c a l i b r a t i o n study (38) were i n d i ­ c a t i v e of t h i s problem. A c o n s e r v a t i v e guess i s that accuracy introduces at l e a s t a one percent f u r t h e r u n c e r t a i n t y . I t i s also d i f f i c u l t to determine e x a c t l y what e r r o r i s introduced through temperature and pressure c o r r e c t i o n s to i j i s i t u c o n d i t i o n s . For the deep sea t h i s may introduce a f u r t h e r u n c e r t a i n t y of at l e a s t

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Calcium Carbonate in the Deep Oceans

Table I I Sample C a l c u l a t i o n o f Calcium Carbonate S a t u r a t i o n State i n Seawater ( A f t e r Morse et a l . (35)) Step 1. Data: The pH a t 25°C i s 7.85. The t i t r a t i o n a l k a l i n i t y at 25°C i s 2.38 meq/kg. The sample was c o l l e c t e d a t 3400 m water depth where the i n s i t u temperature was 2.0°C. The s a l i n i t y i s 34.6°/oo. Step 2. Pressure = (3400 m)/(10 m/atm) = 341 atm.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

Step 3. Using Equation 13 the pH a t 2°C i s c a l c u l a t e d to be 8.09. Step 4.

An i n s i t u pH = 7.95 i s c a l c u l a t e d by use of Equation 14.

Step 5.

The m o l a l i t y of b o r i c a c i d c a l c u l a t e d by Equation 15 i s 4.35 χ ΙΟ" . 4

Step 6.

The value of

c a l c u l a t e d from Equations 16, 17, and is

8

73

18 i s Ι Ο " * . Step 7.

The i n s i t u carbonate a l k a l i n i t y determined by use of Equation 19 i s 2.32 meq/kg.

Step 8.

The i n s i t u value of the second apparent d i s s o c i a t i o n constant of carbonic a c i d i n seawater c a l c u l a t e d by Equations 20, 21, and 22 i s Η Γ · . 9

Step 9.

3 3

The t o t a l carbonate i o n m o l a l c o n c e n t r a t i o n can now be c a l c u l a t e d v i a Equation 11 to be 9.04 χ 10"^.

Step 10. The t o t a l calcium i o n molal c o n c e n t r a t i o n can be c a l c u l a t e d from Equation 4 to be 1.015 χ ΙΟ " . -

2

Step 11. From s t e p s 9 and 10 the calcium carbonate t o t a l m o l a l product i s 9.18 χ Κ Γ . 7

!

Step 12. The value of K at t i s c a l c u l a t e d using Equation 23 to be 4.69 χ 10" and the value K at t j _ i s calculated using Equation 24 t o be 9.67 χ 1 0 " . c

i

s

7

?

a

s

7

Step 13. The values of A V and ΔΚ a r e c o r r e c t e d to t by use of Equations 26 and 27. Using these values i n Equation 25 y i e l d s K* = 9.87 χ 10" . AV i s c a l c u l a t e d from . -7 Equation 28. Then by Equation 29, Κ' = 15.92 χ 10 . is Step 14. From Equation 2, Ω = 0.93 and Ω = 0.58. C

i

s

7

0

i s

1

d

ε

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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one percent. Therefore, the t o t a l u n c e r t a i n t y i n IMP i n the deep oceans i s at l e a s t 6 percent. Since the change i n s a t u r a t i o n s t a t e w i t h depth i n the deep oceans i s g e n e r a l l y about 20 percent/km t h i s u n c e r t a i n t y i n the IMP would r e s u l t i n an un­ c e r t a i n t y of about 300 m i n the depth of a given s a t u r a t i o n s t a t e . I n F i g u r e 4 the u n c e r t a i n t y i n the d i s s o l u t i o n r a t e of b i o g e n i c deep sea c a l c i t e from the P a c i f i c Ocean (30) which r e s u l t s from the u n c e r t a i n t y i n the s a t u r a t i o n s t a t e (Ω) i s presented. The u n c e r t a i n t y i n the IMP r e s u l t s i n an u n c e r t a i n t y i n the d i s s o l u ­ t i o n r a t e of approximately 40 percent. At t h i s time i t i s not p o s s i b l e to make an accurate estimate of the absolute u n c e r t a i n t y i n the i n s i t u values of the apparent s o l u b i l i t y constants f o r c a l c i t e and a r a g o n i t e . An approximate maximum u n c e r t a i n t y can be determined f o r K at 5000 m water depth and 2°C by u s i n g two d i f f e r e n t data s e t s . The maximum value f o r K i s obtained by using K value and temperature co­ e f f i c i e n t c a l c u l a t e d by Berner (26) and the e f f e c t of pressure of Ingle (32). The value f o r K obtained by t h i s method i s 19.8 χ 10" m o l e k g " . The minimum value i s obtained by using the K value and temperature c o e f f i c i e n t of I n g l e et a l . (25), and the AV at 2°C of Hawley and Pytkowicz (36) c o r r e c t e d f o r the d i f f e r e n c e i n molar volumes between c a l c i t e and a r a g o n i t e (-35.8 cm /mole). The value obtained by t h i s method i s 10.7 χ 10"" mole k g " . There i s approximately a f a c t o r of 2 d i f ­ ference i n the value of K obtained by these methods. Even i f the u n c e r t a i n t y i s a tenth of t h i s v a l u e , i t r e s u l t s i n an approx­ imate u n c e r t a i n t y of a f a c t o r of 2 i n c a l c u l a t e d d i s s o l u t i o n r a t e s , and i n d i c a t e s that the u n c e r t a i n t i e s i n the i n s i t u values f o r the apparent s o l u b i l i t y constants of c a l c i t e and a r a g o n i t e are much l a r g e r than i n the _in s i t u values f o r the calcium c a r ­ bonate i o n m o l a l product.

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!

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c

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The D i s t r i b u t i o n of Calcium Carbonate S a t u r a t i o n States and T h e i r R e l a t i o n to Sediment Marker Levels The Geochemical Ocean S e c t i o n Program (GEOSECS) has produced data from which i t i s p o s s i b l e to p r o f i l e the s a t u r a t i o n s t a t e of seawater w i t h r e s p e c t to c a l c i t e and a r a g o n i t e i n the A t l a n t i c and P a c i f i c oceans, Representative north-south c a l c i t e s a t u r a ­ t i o n p r o f i l e s f o r the Western A t l a n t i c and C e n t r a l P a c i f i c oceans are presented i n F i g u r e s 5 and 6 (based on_39). I t was observed t h a t the s a t u r a t i o n s t a t e of seawater w i t h respect to c a l c i t e a t the CCD was c l o s e to constant (Ω = 0.70 1" 0.05) except i n the southern extremes (39). Broecker and Takahashi (31) have r e ­ c e n t l y found that the carbonate i o n c o n c e n t r a t i o n i s c l o s e to constant at the FL, when appropriate c o r r e c t i o n s are made f o r pressure. The s a t u r a t i o n s t a t e of seawater at the FL, c a l c u l a t e d by the method presented i n t h i s paper, i s 0.80 ± 0.05. Berger (40) has presented p r o f i l e s f o r RQ, FL, CCD and CSL ( c a l c i t e s a t u r a t i o n l e v e l ) i n the eastern and western A t l a n t i c ocean (see

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

24. MORSE AND BERNER

Calcium Carbonate in the Deep Oceans

%

Uncertainty

in

515

Ω

Figure 4. Relationship between the uncertainty in saturation state (il) and the resultant uncertainty in dissolution rate

Figure 5.

Distribution of calcite saturation states in the Western Atlantic Ocean (after Ref. 39)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

Figure 7 ) . His r e s u l t s i n d i c a t e that R and CSL are c l o s e to co­ i n c i d e n t w i t h the probable u n c e r t a i n t y of t h e i r d e t e r m i n a t i o n . A d e t a i l e d p r o f i l e showing the r e l a t i o n s among the sediment marker l e v e l s and the s a t u r a t i o n s t a t e of seawater w i t h respect to c a l c i t e and aragonite i n the Northwest A t l a n t i c Ocean i s presented i n F i g u r e 8. An approximate r e l a t i o n s h i p between the degree of undersatu r a t i o n of seawater w i t h r e s p e c t to c a l c i t e and the extent of d i s s o l u t i o n can be e s t a b l i s h e d by comparing the s a t u r a t i o n s t a t e at the v a r i o u s sediment marker l e v e l s w i t h estimates of the amount of d i s s o l u t i o n r e q u i r e d to produce these l e v e l s . I n F i g u r e 9 the " d i s t a n c e " from e q u i l i b r i u m ( 1 - Ω) has been p l o t t e d a g a i n s t the estimated percent d i s s o l u t i o n of the c a l c i t i c sediment f r a c t i o n . W i t h i n the l a r g e u n c e r t a i n t i e s that e x i s t i n the amount of d i s ­ s o l u t i o n r e q u i r e d to produce the FL and R l e v e l s , a l i n e a r r e ­ l a t i o n between the degree of u n d e r s a t u r a t i o n and extent of d i s ­ s o l u t i o n can be e s t a b l i s h e d . The i n t e r c e p t of the l i n e a r p l o t w i t h the FL and RQ l e v e l s i n d i c a t e s that approximately 1 5 percent more m a t e r i a l has been l o s t than B e r g e r s ( 1 2 ) minimum l o s s estimate of 5 0 % and 1 0 % , r e s p e c t i v e l y .

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The D i s s o l u t i o n K i n e t i c s of Calcium Carbonate i n Seawater Calcium carbonate i s accumulating i n deep ocean sediments, i n which the o v e r l y i n g water i s undersaturated w i t h r e s p e c t to both aragonite and c a l c i t e , and sediment marker l e v e l s c l o s e l y c o r r e ­ spond to unique s a t u r a t i o n s t a t e s . This i n d i c a t e s that d i s s o l u ­ t i o n k i n e t i c s play an important r o l e i n determining the r e l a t i o n between seawater chemistry and c a l c i u m carbonate accumulation i n deep ocean b a s i n s . I t i s , t h e r e f o r e , necessary to have knowledge of the d i s s o l u t i o n k i n e t i c s of c a l c i u m carbonate i n seawater i f the accumulation of c a l c i u m carbonate i s to be understood. Two experimental approaches have been used to determine c a l ­ cium carbonate d i s s o l u t i o n k i n e t i c s i n seawater. The f i r s t i s suspension of d i f f e r e n t carbonates i n the ocean a t v a r i o u s depths. A f t e r a g i v e n p e r i o d of time, the samples are recovered and the r a t e of d i s s o l u t i o n determined by weight l o s s . The second exper­ imental approach i s the d e t e r m i n a t i o n of d i s s o l u t i o n k i n e t i c s i n the l a b o r a t o r y a t d i f f e r e n t u n d e r s a t u r a t i o n s . A d e t a i l e d d i s c u s ­ s i o n of the f i n d i n g s of these s t u d i e s i s presented i n t h i s section. Open Ocean D i s s o l u t i o n Experiments. The f i r s t d i r e c t s t u d i e s of c a l c i u m carbonate d i s s o l u t i o n i n deep seawater were made by Peterson ( 4 1 ) and Berger ( 4 2 ) . Peterson suspended spheres of I c e l a n d spar c a l c i t e , h e l d i n pronged p l a s t i c c o n t a i n e r s , a t v a r i o u s depths i n the C e n t r a l P a c i f i c Ocean f o r four months. The amount of d i s s o l u t i o n was determined by weight l o s s , which was s m a l l r e l a t i v e to the t o t a l weight of the spheres. On the same mooring Berger suspended sample chambers, which c o n s i s t e d of

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

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Figure 6. Distribution of calcite saturation states in the Central Pacific Ocean (after Ref. 39)

2|-|

Ν 40°

40° S

Figure 7. The depth distribution of the R and calcite saturation levels, the foraminifera! lysocline and the calcium carbonate compensation depth in the Western and Eastern Atlantic Ocean (after Ref. 40) 0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Figure 8. A detailed profile of cahite and aragonite saturation states and sediment marker depth in the Northwestern Atlantic Ocean. (Percentages are estimates of the amount of calcite dissolution which must occur to produce a given marker level.)

Figure 9. Rehtionship between the distance from equilibrium (1 - Q) of seawater with respect to calcite and the amount of dissolution which is estimated to have occurred. (A) Adeheck's (11) experimentally determined amount of dissolution; (B) Berger's (12) minimum loss estimate for the amount of dissolution at the R level and foraminiferal lysocline. 0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

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c y l i n d e r s whose ends were covered w i t h f i n e n e t t i n g , c o n t a i n i n g an assortment of f o r a m i n i f e r a and pteropods. Both f r e s h l y c o l ­ l e c t e d and H2O2- t r e a t e d speciments were used. Berger determined the r a t e of d i s s o l u t i o n f o r c a l c i t e and aragonite by the same method used by Peterson. The r e l a t i v e s u s c e p t i b i l i t y of d i f f e r e n t species of f o r a m i n i f e r a to d i s s o l u t i o n was a l s o determined. Both Peterson (41) and Berger (42) found that d i s s o l u t i o n s t a r t e d at approximately 0.5 km water depth and the r a t e of d i s ­ s o l u t i o n increased slowly w i t h i n c r e a s i n g water depth u n t i l a depth of approximately 3.8 km was reached. Below t h i s depth the r a t e of d i s s o l u t i o n r a p i d l y increased w i t h i n c r e a s i n g water depth. The change i n the s a t u r a t i o n s t a t e of seawater, w i t h respect to c a l c i t e , i n the deep water of t h i s region i s c l o s e to l i n e a r w i t h depth (43). Consequently, the r e s u l t s of these experiments i n ­ d i c a t e d that the r a t e of d i s s o l u t i o n was not simply r e l a t e d to s a t u r a t i o n s t a t e . Edmond (44) proposed that the r a p i d increase i n d i s s o l u t i o n r a t e could be a t t r i b u t e d to a change i n water v e l o c i t y . Morse and Berner (45) pointed out that t h i s could be true only i f the r a t e of d i s s o l u t i o n was t r a n s p o r t c o n t r o l l e d . Their c a l c u l a t i o n s i n d i c a t e d that the r a t e of d i s s o l u t i o n measured by Peterson (41) was over 20 times too slow f o r d i f f u s i o n con­ t r o l l e d d i s s o l u t i o n , t h i s being the slowest transport process. I t was, t h e r e f o r e , suggested that the r a t e of d i s s o l u t i o n was sur­ face c o n t r o l l e d and the change i n the r e l a t i o n s h i p between i n ­ creasing water depth and d i s s o l u t i o n r a t e could be a t t r i b u t e d to a change i n s u r f a c e - c o n t r o l l e d r e a c t i o n mechanisms. Berger (10) found that the d i f f e r e n t species of f o r a m i n i f e r a used i n the water column d i s s o l u t i o n experiment d i s s o l v e d at d i f f e r e n t r a t e s . Based on these f i n d i n g s , he constructed a s o l u b i l i t y index f o r the r e l a t i v e r e s i s t a n c e of d i f f e r e n t species of f o r a m i n i f e r a to d i s s o l u t i o n . From t h i s index the d i f f e r e n t species of f o r a m i n i f e r a were placed i n the general categories of e a s i l y s o l u b l e and r e s i s t a n t . When the r a t i o of s o l u b l e - t o r e s i s t a n t s p e c i e s , o c c u r r i n g i n surface sediments, i s p l o t t e d against water depth, a d i s t i n c t maximum i n the r a t e of change i n the r a t i o , w i t h i n c r e a s i n g water depth, i s found. The depth at which t h i s occurs i s c a l l e d the f o r a m i n i f e r a l l y s o c l i n e . In the area of the P a c i f i c Ocean where the water column d i s s o l u t i o n experiment was performed, the depth of the r a p i d increase i n d i s ­ s o l u t i o n r a t e and the f o r a m i n i f e r a l l y s o c l i n e are c l o s e to coincident. M i l l i m a n (46) has r e c e n t l y conducted an experiment i n the Sargasso sea (NW A t l a n t i c Ocean) s i m i l a r i n design to the Peterson (41) and Berger (42) experiments. In h i s experiment, preweighed samples of biogenic c a l c i - t e , magnesium c a l c i t e and aragonite were suspended at d i f f e r e n t depths i n sacks made of nylon panty hose. The r e s u l t s which he obtained f o r b i o g e n i c c a l c i t e were s i m i l a r to those of Peterson and Berger, i n that a gradual increase i n d i s s o l u t i o n r a t e w i t h i n c r e a s i n g water depth was found above a c r i t i c a l depth. Below t h i s depth the r a t e of

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

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d i s s o l u t i o n increased r a p i d l y w i t h i n c r e a s i n g water depth. The c r i t i c a l depth was found to c l o s e l y correspond w i t h the depth of the FL i n the area of the experiments. The same general p a t t e r n of d i s s o l u t i o n r a t e versus water depth was found f o r the b i o g e n i c aragonite and magnesian c a l c i t e . An i n t e r e s t i n g f i n d i n g f o r the aragonite was that i t s c r i t i c a l depth was approximately 700 m below the ACD and c l o s e to the c a l c i t e RQ l e v e l i n the area of the experiments. M i l l i m a n ' s r e s u l t s f o r c a l c i t e and aragonite are summarized i n F i g u r e 10. Honjo and Erez (47) have attempted to overcome the problem of nonuniform water c i r c u l a t i o n through sample chambers by p l a c i n g samples i n chambers through which water i s pumped a t a constant r a t e . T h e i r r e s u l t s from 5500 m i n the Sargasso sea y i e l d d i s ­ s o l u t i o n r a t e s f o r b i o g e n i c c a l c i t e 3 to 4 times f a s t e r than ob­ served by M i l l i m a n (46) and approximately 4 times greater f o r b i o g e n i c a r a g o n i t e . However, even t h e i r r a t e s may represent minimum values as the samples tended to "pond" i n the bottom of the sample chambers and were thus not uniformly exposed to sea­ water of constant u n d e r s a t u r a t i o n (Honjo, S., Woods Hole Oceanogr. I n s t . , personal communication, 1978). The d i s s o l u t i o n p a t t e r n w i t h i n c r e a s i n g water depth found i n a l l of the water column experiments are i n general agreement, i n ­ d i c a t i n g that the d i s s o l u t i o n k i n e t i c s of both s y n t h e t i c and b i o ­ genic c a l c i t e and aragonite are not simply p r o p o r t i o n a l to the degree of u n d e r s a t u r a t i o n . The r e s u l t s a l s o i n d i c a t e that the nature of the assemblage of b i o g e n i c t e s t s should have a s i g n i f i ­ cant e f f e c t on the o v e r a l l d i s s o l u t i o n r a t e of a given calcium carbonate phase. Laboratory D i s s o l u t i o n Experiments. The development of a high p r e c i s i o n pH-stat method (48) f o r the determination of calcium carbonate r e a c t i o n k i n e t i c s has made accurate l a b o r a t o r y measurement of n e a r - e q u i l i b r i u m c a l c i t e and aragonite d i s s o l u t i o n rates p o s s i b l e . The b a s i c concept i n the pH-stat technique i s the maintenance of a constant degree of d i s e q u i l i b r i u m . The carbonate m o l a l i t y i s kept constant by bubbling a gas. of constant P^Q through the s o l u t i o n and m a i n t a i n i n g a constant pH w i t h a pH-stat. The r a t e a t which a c i d must be added to transform the carbonate produced by d i s s o l u t i o n to carbon d i o x i d e and water i s p r o p o r t i o n a l to the r a t e of calicum carbonate d i s s o l u t i o n . The c o n c e n t r a t i o n of calcium i n seawater i s approximately 1000 times that of carbonate. I f only a s m a l l amount of calcium carbonate i s allowed to d i s s o l v e i n a r e l a t i v e l y l a r g e volume of seawater, the change i n calcium c o n c e n t r a t i o n i s n e g l i g i b l e . I t i s p r e s e n t l y p o s s i b l e to make accurate d i s s o l u t i o n r a t e determinations at s a t u r a t i o n s t a t e s that are constant to w i t h i n b e t t e r than 2%. Using the pH-stat technique, Morse and Berner (45) found that the d i s s o l u t i o n r a t e of s y n t h e t i c and p e l a g i c b i o g e n i c c a l c i t e did not i n c r e a s e l i n e a r l y w i t h i n c r e a s i n g u n d e r s a t u r a t i o n . Beyond a c r i t i c a l u n d e r s a t u r a t i o n a very r a p i d i n c r e a s e i n the r a t e of 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

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d i s s o l u t i o n w i t h i n c r e a s i n g u n d e r s a t u r a t i o n was found. This change was a t t r i b u t e d to a change i n s u r f a c e - c o n t r o l l e d r e a c t i o n mechanisms. By comparing the undersaturations used i n the l a b ­ oratory w i t h those found i n the water column a t the s i t e of the Peterson (41) and Berger (42) experiments, i t was p o s s i b l e t o c o n s t r u c t a curve which c o r r e c t l y p r e d i c t e d the observed r e l a t i o n between water depth and d i s s o l u t i o n r a t e . This i s shown i n F i g ­ ure 11, along w i t h r e s u l t s of the Peterson and Berger experiments. A major problem encountered i n comparing the r a t e s of d i s ­ s o l u t i o n measured i n the l a b o r a t o r y w i t h r a t e s measured i n the water column was that the r a t e s measured i n the l a b o r a t o r y were 10 t o 100 times f a s t e r . I t has subsequently been shown (30) that the d i f f e r e n c e i n r a t e s f o r the b i o g e n i c c a l c i t e can be l a r g e l y a t t r i b u t e d t o the f a c t that Berger (42) used only l a r g e foramin­ i f e r a , w h i l e Morse and Berner (45) used a t o t a l carbonate sediment sample. The reason f o r the d i f f e r e n c e i n r a t e s f o r the Iceland spar and s y n t h e t i c c a l c i t e s i s l e s s c e r t a i n . One p o s s i b i l i t y i s that surfaces exposed i n seawater experience b i o f o u l i n g . Peterson (41) observed macro-biofouling (e.g. barnacle attachment) on some of h i s spheres. U n f o r t u n a t e l y , no scanning e l e c t r o n microscope examinations of the sphere surfaces were c a r r i e d out. I t i s very l i k e l y that m i c r o - b i o f o u l i n g occurred which may have s u b s t a n t i a l l y reduced the surface area a v a i l a b l e f o r d i s s o l u t i o n . A l s o , Peterson (41) p r e t r e a t e d the spheres w i t h HC1 which may have reduced the surface c o n c e n t r a t i o n of a c t i v e s i t e s . Berner and Morse (50) determined the d i s s o l u t i o n r a t e of s y n t h e t i c c a l c i t e i n a r t i f i c i a l seawater and NaCl-CaCl2 s o l u t i o n s over a wide range of s a t u r a t i o n s t a t e s and d i f f e r e n t Pc02 l * Their i n t e r p r e t a t i o n of the sudden i n c r e a s e i n d i s s o l u t i o n r a t e w i t h i n c r e a s i n g u n d e r s a t u r a t i o n i n the n e a r - e q u i l i b r i u m r e g i o n of concern i n the oceans was that i t i s necessary t o reach a c r i t i ­ c a l u n d e r s a t u r a t i o n f o r the r e t r e a t of monomolecular steps on the c a l c i t e s u r f a c e . The degree of u n d e r s a t u r a t i o n necessary f o r the supposed change i n r e a c t i o n mechanism was found to depend on the c o n c e n t r a t i o n of orthophosphate i n the s o l u t i o n . Recent s t u d i e s of non-biogenic c a l c i t e (51) and b i o g e n i c magnesian c a l c i t e s (52) have confirmed the importance of phosphate as an i n h i b i t o r of c a l c i t e d i s s o l u t i o n . The e f f e c t of phosphate was explained by Berner and Morse (50) as a change i n c r i t i c a l undersaturation necessary t o enable monomolecular steps to penetrate, v i a curved embayments, between s t r o n g l y adsorbed phosphate i o n s . The measured adsorbed phosphate concentrations and those p r e d i c t e d by theory agreed w e l l . (A general d i s c u s s i o n of c a l c i t e d i s s o l u t i o n k i n e t i c s i s presented elsewhere i n t h i s volume (53)). A c l a s s i c a l approach to r e a c t i o n k i n e t i c s i s to determine the e m p i r i c a l order of a r e a c t i o n and i t s r a t e constant. The equation f o r t h i s i s : v a

R = kd-Ω)

11

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

u e s

(30)

CHEMICAL MODELING IN AQUEOUS

522

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

Figure 10. Rate of calcite and aragonite dissolution as a function of depth as determined by Milliman (46) in water column experiments in the Sargasso sea

1

SPHERES (mg crn^yf ) 0

0Λ

0.1

o

1

I 0

ι

02

FORAMINIFERA 0.2 0.3

1

I 10

0.3 1

(7„ d a y ) 0.4

1

1

0.5

1

1 20

Γ

1

J 30

1

SEDIMENT (% day" )

Figure 11. Plot of the Peterson (41) and Berger (42) results for their watercolumn dissolution experiments in the Central Pacific Ocean, and the Morse and Berner (45) laboratory experiments as a function of equivalent depth. The depth of the lysocline was calculated from the data of Bramlette (49) (after Ref. 45).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24.

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523

where R i s the d i s s o l u t i o n r a t e , k the r a t e constant, and η the e m p i r i c a l r e a c t i o n order. I n l o g a r i t h m i c form t h i s equation i s : log

R = l o g k + η logU-Ω)

(31)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

The value of the r a t e constant and e m p i r i c a l r e a c t i o n order can be determined by the i n t e r c e p t and s l o p e , r e s p e c t i v e l y , of a p l o t of log R versus l o g ( l - f i ) , i f the p l o t i s l i n e a r . The data f o r s y n t h e t i c c a l c i t e d i s s o l u t i o n i n seawater con­ t a i n i n g d i f f e r e n t phosphate concentrations (50) has been p l o t t e d i n t h i s manner i n Figure 12. For a given phosphate c o n c e n t r a t i o n , the p l o t s are l i n e a r over the range of undersaturations s t u d i e d . This i n d i c a t e s that i t i s u n l i k e l y that a change i n r e a c t i o n mechanism occurs i n the range of undersaturations of concern i n the oceans. The concept of a " c r i t i c a l " undersaturation at which a n e a r - e q u i l i b r i u m change i n r e a c t i o n mechanism occurs does not, t h e r e f o r e , appear to be s t r i c t l y v a l i d . The concept, which o r i g ­ i n a t e d i n the open ocean experiments (41, 42) and was f u r t h e r e d by e a r l y l a b o r a t o r y experiments (45, 5Q\ i s most probably an a r t i f a c t of the manner i n which the data were p l o t t e d . However, the con­ cept i s s t i l l q u i t e u s e f u l (see below). The l i n e a r p l o t s i n Figure 12 steepen w i t h i n c r e a s i n g phos­ phate c o n c e n t r a t i o n . I n Figure 13 the l o g of the r a t e constant and the e m p i r i c a l r e a c t i o n order have been p l o t t e d against the d i s s o l v e d phosphate c o n c e n t r a t i o n . Both the e m p i r i c a l r e a c t i o n order and the l o g of the r a t e constant change i n a l i n e a r manner w i t h phosphate c o n c e n t r a t i o n . de Kanel and Morse (54) made a d e t a i l e d study of the uptake k i n e t i c s of orthophosphate on the surfaces of c a l c i t e and arag­ o n i t e i n seawater. T h e i r r e s u l t s i n d i c a t e that the data are best f i t to the E l o v i c h equation f o r chemisorption (see 55_, 56). This i n d i c a t e s that the uptake k i n e t i c s can be explained by an ex­ p o n e n t i a l decrease during r e a c t i o n i n a v a i l a b l e surface r e a c t i o n s i t e s and(or) a l i n e a r increase i n the a c t i v a t i o n energy a s s o c i a ­ ted w i t h the chemisorption process. Their conclusions s t r o n g l y imply that carbonate surfaces are e n e r g e t i c a l l y heterogeneous. The Berner and Morse (50) model f o r the i n f l u e n c e of phos­ phate ions on d i s s o l u t i o n was i m p l i c i t l y f o r a s u r f a c e w i t h a homogeneous d i s t r i b u t i o n of a d s o r p t i o n s i t e s or k i n k s . I f e n e r g e t i c heterogeneity i s accompanied by a s p a c i a l l y hetero­ geneous d i s t r i b u t i o n of adsorbed ions on the surface of c a l c i t e , then the f i n d i n g s of de Kanel and Morse (54) suggest that a t any given phosphate c o n c e n t r a t i o n there w i l l be a range of " c r i t i c a l " u n d e r s a t u r a t i o n s , each corresponding t o a d i f f e r e n t p o r t i o n of the c a l c i t e s u r f a c e . Consequently, i t i s expected that as the surface phosphate c o n c e n t r a t i o n increases and s u r f a c e s i t e s are f i l l e d up, there would be both a decrease i n the t o t a l d i s s o l u t i o n r a t e and an i n c r e a s e i n the s e n s i t i v i t y of the d i s s o l u t i o n r a t e to the degree of u n d e r s a t u r a t i o n . U l t i m a t e l y , a l i m i t would be

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

524

SYSTEMS

log(i-n) Figure 12. Log of the rate of dissolution of synthetic calcite in seawater contain­ ing different phosphate concentrations vs. the log of 1 — Ω. Based on the data of Berner and Morse (50). The two lowest dissolution rates measured have been excluded as they are highly anomalous. 20 ι

—ι

Ρ(μΜ) Figure 13. Empirical reaction order and log of the rate constant for synthetic calcite dissolution in seawater as a function of phosphate concentration based on the data presented in Figure 12

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

24.

MORSE AND BERNER

Calcium Carbonate in the Deep Oceans

525

approached whereby a l l p o r t i o n s of the surface are e q u a l l y covered by adsorbed phosphate ions and a true d i s c o n t i n u i t y i n raté as p r e d i c t e d by the Berner and Morse (50) model, a t t a i n e d . Because of the present l a c k of knowledge of the d i s t r i b u t i o n of surface s i t e s and t h e i r energies, and of the dependence of and d i s s o l u t i o n r a t e upon s i t e energy and spacing, i t i s not p o s s i b l e to construct a q u a n t i t a t i v e model f o r the r e l a t i o n s h i p between phosphate adsorpt i o n and d i s s o l u t i o n r a t e . Morse (30) c a r r i e d out an examination of the n e a r - e q u i l i b r i u m d i s s o l u t i o n k i n e t i c s of calcium carbonate-rich deep sea sediments. His r e s u l t s are summarized i n Figure 14. The sediment samples from d i f f e r e n t ocean basins have d i s t i n c t l y d i f f e r e n t r e a c t i o n orders and e m p i r i c a l r a t e constants. The d i s s o l u t i o n r a t e equations f o r the d i f f e r e n t sediment samples are: R ( % / d a )

3

5

2

(32) 0

y>Atlantic = l O d V "

5

p a c l f i c

7

2

3

R(%/day)

R ( % / d a

4

I n d i a n " 1° · (1-Ω) ·

= 10 · (1-Ω) ·

(33)

I t was suggested that d i f f e r e n t p a r t i c l e s i z e d i s t r i b u t i o n s combined to give d i f f e r e n t f u n c t i o n a l i t i e s t o the t o t a l r a t e o r that d i f f e r e n t surface h i s t o r i e s e x i s t e d f o r the samples from d i f ­ f e r e n t l o c a t i o n s . Although the samples were taken from d i f f e r e n t ocean basins, i t was Morsels (30) o p i n i o n that they should not be taken as r e p r e s e n t a t i v e of these b a s i n s , as v a r i a b i l i t y w i t h i n a given b a s i n could probably be a t l e a s t as l a r g e as that found i n samples from d i f f e r e n t ocean b a s i n s . Morse (30) found that the r a t e of d i s s o l u t i o n per gram of calcium carbonate decreased w i t h extent of d i s s o l u t i o n i n an i r r e g u l a r manner (see Figure 15). I t was a l s o found that the d i s ­ s o l u t i o n r a t e s i n deep sea sediment pore waters and seawater were the same a t equivalent phosphate c o n c e n t r a t i o n s . This i n d i c a t e d that no important unknown d i s s o l u t i o n i n h i b i t o r was present i n the pore waters. The r a t e of d i s s o l u t i o n of s y n t h e t i c and deep sea b i o g e n i c (pteropods) aragonite i n seawater have a l s o been determined i n the l a b o r a t o r y by the pH-stat method (57). The r e s u l t s of the ex­ periments to determine the change i n the r a t e of d i s s o l u t i o n as a f u n c t i o n of u n d e r s a t u r a t i o n are presented i n F i g u r e 16. The pteropods were found to d i s s o l v e at .only about 3% the r a t e , per u n i t s u r f a c e area, of the s y n t h e t i c aragonite. The r e s u l t s a l s o i n d i c a t e a change i n the e m p i r i c a l r e a c t i o n order from 2.92 t o 7.37 a t Ω = 0.44. The r a t e equations f o r pteropod d i s s o l u t i o n are: 2

4

2

R(%/day) = 1 0 · ° ( 1 - Ω ) ·

92

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(35)

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

526

-log

(Ι-β)

Figure 14. Log of the rate of dissolution in percent per day vs. the log of (1 — Ω ) . ( Ο JWhole Indian Ocean sediment dissolved in Atlantic deep-seawater, (•) whole Indian Ocean sediment dissolved in deep-sea sediment pore water; (Δ) the greater than 62 μτη size fraction of the Indian Ocean sediment dissolved in Afantic deepseawater; (Φ) whole Pacific Ocean sediment dissolved in Atlantic deep-seawater; (A.) the 125 —500 μύτη size fraction of Pacific Ocean sediment dissolved in Atlantic deep-seawater; f | ) whole Atlantic Ocean sediment dissolved in Long Island Sound seawater (46); () 150 to 500 ^m foraminifera dissolved in the Pacific Ocean water column. Error bars are based on an uncertainty in pH of ±0.01 (after Ref. 30).

1.0

ι —

1 .

1

1

__>62_/im

1.0. S.

i.o. s X .

0

0

1

20

1

1

40 60 % Dissolved

-J

80

100

Figure 15. The measured rate of dissolution (R ) V gram of calcium carbonate, divided by the initial rate of dissolution (R ) vs. the percent of the calcium car­ bonate which has dissolved. Runs were carried at Ω = 0.40.1.O.S. = Indian Ocean sediment, P.O.S. = Pacific Ocean sediment (after Ref. 30). M

er

r

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24.

MORSE AND BERNER

Calcium Carbonate in the Deep Oceans

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

R(%/day) = 10

3.14

7.37

(1-Ω)

527

(36)

Equation 35 a p p l i e s over the undersaturation range Ω = 1 to 0.44. At greater undersaturations Equation 36 a p p l i e s . In Figure 17 the l o g the the d i s s o l u t i o n r a t e o f s y n t h e t i c aragonite, pteropods, t o t a l c a l c i t i c P a c i f i c Ocean deep sea s e d i ­ ment, and the 125 to 500 ym s i z e f r a c t i o n of the same sediment have been p l o t t e d against the seawater t o t a l carbonate i o n con­ c e n t r a t i o n . I t i s i n t e r e s t i n g that a t approximately 50% s a t u r a ­ t i o n , w i t h respect to c a l c i t e C~22pM CO3"), the pteropod d i s ­ s o l u t i o n r a t e i s not much greater than the t o t a l c a l c i t i c sediment d i s s o l u t i o n r a t e . This i s probably the r e s u l t of the pteropods being much l a r g e r than the average b i o g e n i c c a l c i t e component and having a l e s s r e a c t i v e s u r f a c e . Morse et a l . (57) found two major d i f f e r e n c e s between the behavior of aragonite and c a l c i t e d i s s o l u t i o n k i n e t i c s i n sea­ water. The f i r s t was that the r a t e of pteropod d i s s o l u t i o n per u n i t mass increased w i t h i n c r e a s i n g extent of d i s s o l u t i o n (see Figure 18). The second was that phosphate, i n s t e a d of i n h i b i t i n g d i s s o l u t i o n , caused an i n i t i a l increase i n d i s s o l u t i o n r a t e and then had almost no e f f e c t (see Figure 19). The s i z e of the s h o r t term c a t a l y t i c e f f e c t increased w i t h i n c r e a s i n g phosphate concen­ t r a t i o n and decreasing undersaturation (see Figure 20). Comparison of Laboratory and Water Column D i s s o l u t i o n Rates. There are s e v e r a l problems i n comparing calcium carbonate d i s s o l u ­ t i o n r a t e s measured i n the water column and l a b o r a t o r y experiments. These problems can be b r i e f l y summarized as: 1) samples of d i f ­ f e r e n t o r i g i n and surface h i s t o r y have been s t u d i e d , 2) d i f f e r e n t s i z e f r a c t i o n s have g e n e r a l l y been used, 3) i n order to compare r e s u l t s i t i s necessary to be able t o a c c u r a t e l y c a l c u l a t e Ω of the water a t the l o c a t i o n s and depths where the water column ex­ periments were conducted, and 4) d i s s o l u t i o n r a t e s f o r the water column experiments have g e n e r a l l y been i n c o r r e c t l y reported as average d i s s o l u t i o n rates (percent weight l o s s of o r i g i n a l sample weight d i v i d e d by time of exposure). Morse eX _al. (57) suggested two p o s s i b l e approaches to these problems. One was to make an approximate c a l c u l a t i o n of the Ω i n the water column experiment chambers. They used the f o l l o w i n g assumptions: 1) the d i s s o l u t i o n k i n e t i c s , f o r a given sample type and s i z e f r a c t i o n , are the same i n the l a b o r a t o r y and water c o l ­ umn, 2) the Ω i n the sample chambers was c l o s e to constant during the experiments and 3) that there was l i t t l e change i n the r a t e of d i s s o l u t i o n per u n i t mass w i t h extent of d i s s o l u t i o n . The approx­ imate s a t u r a t i o n s t a t e i n the sample chambers can then be compared w i t h the s a t u r a t i o n s t a t e of water a t the sample chamber s i t e c a l c u l a t e d by the methods presented i n t h i s paper. I n order to c a l c u l a t e the s a t u r a t i o n s t a t e i n the chambers, i t i s necessary to i n t e g r a t e Equation 30 and solve f o r Ω. The r e s u l t i n g equation i s :

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

528

Figure 16.

Log of the dissolution rate vs. the log of (1 — n) for synthetic arago­ nite and pteropods (after Ref. 57)

-ι—ι—ι—ι—ι 80

70

60

1 ι ι ι ι ι ι ι

CO3"

50

40

30

(μηηοΙβε

kg" )

20

ι ι

10

1

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125—500 pm size fraction. (A) indicates the aragonite equilibrium total carbonate ion concentration at 25° C, 1 atm (26). (C) indicates the calcite equi­ librium total carbonate ion concentration at 25°C,1 atm (25).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

MORSE AND BERNER

Calcium Carbonate in the Deep Oceans

0.4 h- Calcitic Pacific Ocean Sediment

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

20

%

40 60 Dissolved

80

529

H

100

Figure 18. Ratio of measured rates of dissolution per gram to the initial dissolu­ tion for pteropods and calcite Pacific Ocean sediment (57)

m

2 Time

4

-1-9.7—1

P 0 4

(hr)

χ 106

Figure 19. Change in the rate of syn­ thetic aragonite dissolution, relative to dissolution in very low phosphate sea­ water, as a function of time (57)

3

Figure 20. Rate of dissolution relative to the rate of dissolution in very low phos­ phate seawater (Rate*) after a half hour of reaction vs. surface phosphate concen­ tration at Ω = 0.8 and Ω = 0.5 (57)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

530

(37)

Ω =

The second approach was to c a l c u l a t e the r a t e constant (k) f o r the water column experiments. The major assumptions f o r t h i s approach were: 1) that Ω i n the sample chamber was the same as that of the surrounding water, 2) the e m p i r i c a l r e a c t i o n order (n) i s the same i n the water column and l a b o r a t o r y and 3) that there was l i t t l e change i n the r a t e of d i s s o l u t i o n per u n i t mass w i t h extent of d i s s o l u t i o n . I n order to c a l c u l a t e k, Equation 37 must be solved for k Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

k =

100 (1-Ω) 1η t Π

(38)

where f i s the weight percent d i s s o l v e d ( f o r d e r i v a t i o n see 57). In comparing t h e i r l a b o r a t o r y d i s s o l u t i o n r a t e s f o r pteropods w i t h water column r e s u l t s , Morse et a l . (57) found t h a t the major problem was the d i f f e r e n c e s i n the samples. Berger (43) used pteropods both f r e s h l y c o l l e c t e d from net tows and cleaned w i t h H2O2 to remove o r g a n i c matter. Neither sample was t r u e l y r e p r e ­ s e n t a t i v e of pteropods a r r i v i n g a t the sediment-water i n t e r f a c e . Berger i s of the o p i n i o n that the H2O2 t r e a t e d samples are more r e p r e s e n t a t i v e of pteropods a r r i v i n g at the sediment-water i n t e r f a c e based on observations of pteropod t e s t s from deep water net tows and box cores. Honjo and Erez (47) used only pteropods g r e a t e r than 831 ym i n s i z e . This i s a much l a r g e r s i z e f r a c t i o n than used i n the l a b o r a t o r y experiments (greater than 125 ym). Based on experience w i t h f o r a m i n i f e r a (Morse, 30) i t i s reasonable to expect that the r e s u l t s of Honjo and Erez (47) should g i v e s i g n i f i c a n t l y slower d i s s o l u t i o n r a t e s than found i n the l a b o r a ­ t o r y . M i l l i m a n (46) used ooids (roughly s p h e r i c a l f i n e - g r a i n e d aragonite aggregates) making any intercomparison w i t h other measurements v i r t u a l l y meaningless. Another problem i n comparing Berger's (42) r e s u l t s to those of other experiments i s t h a t a l l H2O2 t r e a t e d pteropod samples suspended below the a r a g o n i t e s a t u r a t i o n depth completely d i s ­ s o l v e d . Morse e_t a l . (57) found that i f the slowest p o s s i b l e d i s s o l u t i o n r a t e f o r Berger's sample from immediately below the aragonite s a t u r a t i o n l e v e l was used to c a l c u l a t e the s a t u r a t i o n s t a t e , i n the sample chamber, that the c a l c u l a t e d v a l u e of Ω i n the chamber was only 0.08 h i g h e r than that of the surrounding water. This i s e q u i v a l e n t to a d i f f e r e n c e of a f a c t o r of 2 i n k. Since the assumption maximizes the d i f f e r e n c e , t h i s r e s u l t was considered to be very good agreement between t h e i r l a b o r a t o r y measurements and B e r g e r s water column measurements. Morse et a l . (57) found that the c a l c u l a t e d Ω i n the sample chambers used by Honjo and Erez (47) was always h i g h e r than t h a t of the surrounding water w i t h the d i f f e r e n c e s being 0.22 a t 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

24.

MORSE AND BERNER

Calcium Carbonate in the Deep Oceans

531

3600 m, and 0*25 a t 4800 and 5500 τη or f a c t o r s of approximately 100, 12 and 9 i n k, r e s p e c t i v e l y . The d i f f e r e n c e s were p r i m a r i l y a t t r i b u t e d to the f a c t that Honjo and Erez (47) used a l a r g e r s i z e f r a c t i o n of ptetopods than Morse ejt _ a l . , (57). When the average amount of aragonite d i s s o l u t i o n a t 5500 m water depth i n each of M i l l i m a n s (46) four experiments was c a l c u l a t e d , the Ω i n the chambers was higher than that of the surrounding water by 0.36, 0.36, 0.35, and 0.33 or f a c t o r s of approximately 55, 55, 37 and 28 i n k, r e s p e c t i v e l y . Again the c o n s i s t a n t d i f f e r e n c e was p r i m a r i l y a t t r i b u t e d (57) to the d i f f e r e n t m a t e r i a l used i n the water column and l a b o r a t o r y experiments. We have taken the f i r s t approach i n comparing f o r a m i n i f e r a d i s s o l u t i o n r a t e s . Because sample d i f f e r e n c e s are l e s s severe, t h i s should provide a b e t t e r comparison of l a b o r a t o r y and water column r e s u l t s . The Ω i n B e r g e r s (42) deepest chambers i s c a l ­ c u l a t e d to be h i g h by only 0.01 and 0.09, a t 5000 m and 6000 m depth, r e s p e c t i v e l y . The Ω i n the Honjo and Erez (47) sample chamber i s only 0.02 lower than the surrounding water. The Ω i n M i l l i m a n s (46) sample chambers gives a l a r g e r disagreement w i t h the Ω of the water column, w i t h average c a l c u l a t e d Ω being 0.18 higher than the surrounding water. This may be due to r e s t r i c t e d flow of water through h i s chambers.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

T

T

!

Conclusions The accumulation of calcium carbonate i n deep ocean sediments i s a complex process. I t i s p r i m a r i l y governed by the i n t e r p l a y between b i o l o g i c a l p r o d u c t i o n of calcium carbonate i n the nearsurface ocean and the chemistry of deep ocean waters. A f t e r over 100 years of study, the major problem of determining the s a t u r a ­ t i o n s t a t e of deep ocean water remains l a r g e l y unresolved. I t i s c u r r e n t l y p o s s i b l e , using recent l a b o r a t o r y measurements, to a r r i v e a t s a t u r a t i o n s t a t e s that d i f f e r by as much as a f a c t o r of 2. Both l a b o r a t o r y and water column experiments i n d i c a t e that calcium carbonate d i s s o l u t i o n k i n e t i c s are not simply r e l a t e d to s a t u r a t i o n s t a t e . I t i s our o p i n i o n that the s a t u r a t i o n s t a t e problem must be r e s o l v e d and c o n s i d e r a b l y more d e t a i l added to our present knowledge of calcium carbonate d i s s o l u t i o n k i n e t i c s and accumulation patterns before attempts to model the accumulation of calcium carbonate i n deep ocean sediments can be t r u l y successful. Acknowledgements The authors wish to thank Drs. Frank J . M i l l e r o and John R. Southam, f o r t h e i r extremely h e l p f u l d i s c u s s i o n s and a d v i c e , and Catherine T u t t l e f o r her patience and perseverance i n preparing the f i g u r e s and typing t h i s paper. Support was provided by N a t i o n a l Science Foundation grants (Marine Chemistry Program) OCE77-08134, J.W. Morse, P.I. and OCE77-03473, R.A, Berner, P . I .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

532

SYSTEMS

Abstract The balance between calcium carbonate i n f a l l and dissolution in deep ocean basins i s the main control of both atmospheric CO and the pH of seawater. Calcium carbonate i s being deposited at depths 1 to 2 km below the depth at which seawater becomes under­ -saturated with respect to calcite. This indicates that the p r i ­ mary control of calcium carbonate deposition i n the deep oceans is the kinetics of dissolution. The dissolution kinetics of pelagic biogenic calcite and aragonite i n seawater are complex. The empirical reaction order and rate constant are dependent on the source of the calcium carbonate. The rate of dissolution changes i n a nonlinear manner with extent of dissolution. Only 3 to 5% of the total surface area i s available for reaction. Phos­ phate i s a strong inhibitor of calcite dissolution, but not of aragonite dissolution. Modelling calcium carbonate deposition is d i f f i c u l t due to the nonlinear character of the depositional processes.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

2

Literature Cited 1.

Sverdrup, H.U., Johnson, M.W., and Fleming, R.H. "The Oceans" 1087 p., Prentice Hall, Englewood C l i f f s , New Jersey, 1942. 2. Pytkowicz, R.M. The carbon dioxide-carbonate system at high pressure i n the oceans, p. 83-135, i n Barnes, H., ed. "Oceanogr. Mar. B i o l . Ann. Rev. 6," George Allen and Unwin., London (1968). 3. Broecker, W.S. "Chemical Oceanography," 214 p. Harcourt Brace Jovanovich, New York, 1974. 4. Adelseck, C.G., J r . and Berger, W.H. On the dissolution of planktonic formaminifera and associated microfossils during settling and on the sea floor, p. 70-81, i n S l i t e r , W.V., Be, A.W.H., and Berger, W.H., eds., Cushman Found. Foram. Res. Spec. Publ. No. 13, 1975. 5. Pytkowicz, R.M. Carbonate cycle and the buffer mechanism of recent oceans, Geochim. Cosmochim. Acta 31, 63-73 (1967). 6. Woodwell, G.M., Whittaker, R.H., Reiners, W.A., Likens, G.E., Delwiche, C.C., and Botkin, D.B. The biota and the world carbon budget, Science 199, 141-145 (1978). 7. Berner, R.A. Sedimentation and dissolution of pteropods i n the ocean, p. 243-260, i n Andersen, N.R. and Malahoff, Α., eds., "The Fate of Fossel Fuel CO i n the Oceans," Plenum Press, New York, 1977. 8. Berger, W.H. Deep-sea carbonate: pteropod distribution and the aragonite compensation depth, Deep Sea Res, (in press). 9. Murray, J . and Hjort, J. "The Depths of the Oceans," 831 p. Macmillan, London, 1912. 10. Berger, W.H. Planktonic foraminifera: selective solution and the lysocline, Mar. Geol. 8, 111-138 (1970). 2

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Berner, R.A. The role of magnesium i n the crystal growth of calcite and aragonite from sea water, Geochim. Cosmochim. Acta 39, 489-504 (1975). Morse, J.W. Dissolution kinetics of calcium carbonate i n sea water: VI. The near equilibrium dissolution kinetics of calcium carbonate-rich deep sea sediments, Amer. Jour. Sci. 278, 344-353 (1978). Broecker, W.S. and Takashashi, T. The relationship between lysocline depth and the i n situ carbonate ion concentration, Deep Sea Res. 25, 65-95 (1978). Ingle, S.E. Solubility of calcite i n the ocean, Mar. Chem. 3, 301-319 (1975). Millero, F.J. Effects of pressure and temperature on activity coefficients, Ch. 13, i n Pytkowicz, R.M., ed. "Activity Coefficients i n Electrolyte Solutions," CRC Press, W. Palm Beach, Florida (in press). Pytkowicz, R.M. Activity coefficients of bicarbonates and carbonates i n seawater, Limnol. Oceanogr. 20, 971-975 (1975). Morse, J.W., de Kanel, J., and Craig, H.L., J r . A literature review of the saturation state of seawater with respect to calcium carbonate and i t s possible significance for scale formation on OTEC heat exchangers, Ocean Engineering (in press). Hawley, J.E. and Pytkowicz, R.M. Solubility of calcium carbonate i n seawater at high pressures and 2°C, Geochim. Cosmochim. Acta 33, 1557-1561 (1969). Dickson, A.G. and Riley, J.P. The effect of analytical error on the evaluation of the components of the aquatic carbondioxide system, Mar. Chem. 6, 77-85 (1978). Takahashi, T., Weiss, R.F., Culberson,C.H.,Edmond, J.M., Hammond, D.E., Wong, C.S., L i , Y.H., and Bainbridge, A.E. A carbonate chemistry profile at the 1969 GEOSECS intercalibration station in the Eastern Pacific Ocean, Jour. Geophys. Res. 75, 7648-7666 (1969). Takahashi, T. Carbonate chemistry of seawater and the calcite compensation depth i n the oceans, p. 11-26, i n S l i t e r , W.V., Be, A.W.H., and Berger, W.H., eds., Cushman Found. Foram. Res. Spec. Publ. No. 13, 1975. Berger, W.H. Carbon dioxide excursions i n the deep sea record: Aspects of the problem, p. 502-542, i n Andersen, N.R. and Malahoff, Α., eds., "The Fate of Fossil Fuel CO i n the Oceans," Plenum Press, New York, 1977. Peterson, M.N.A. Calcite: rates of dissolution i n a v e r t i ­ cal profile i n the Central Pacific, Science 154, 1542-1544 (1966). Berger, W.H. Foraminiferal ooze: solution at depths, Science 156, 383-385, (1967). Berner, R.A. and Wilde, P. Dissolution kinetics of calcium carbonate i n seawater: I. Saturation state parameters for kinetic calculations, Amer. Jour. S c i . 272, 826-839 (1972). 2

41.

42. 43.

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Edmond, J.M. An i n t e r p r e t a t i o n of the c a l c i t e spheres experiment [ a b s t . ] , Amer. Geophys. Union 52, 256 (1971). Morse, J.W. and Berner, R.A. D i s s o l u t i o n k i n e t i c s of calcium carbonate i n sea water: I I . A k i n e t i c o r i g i n f o r the l y s o c l i n e , Amer. Jour. S c i . 272, 840-851 (1972). M i l l i m a n , J.D. D i s s o l u t i o n of calcium carbonate i n the Sargasso sea (Northwest A t l a n t i c ) , p. 641-654, i n Andersen, N.R. and Malahoff, Α., eds., "The Fate of F o s s e l F u e l CO i n the Oceans,", Plenum Press, New York, 1977. Honjo, S. and Erez, J . D i s s o l u t i o n r a t e s of calcium carbon­ ate i n the deep ocean: an in s i t u experiment i n the North A t l a n t i c Ocean, Earth Planet Sci. Lett. ( i n p r e s s ) . Morse, J.W. D i s s o l u t i o n k i n e t i c s of calcium carbonate i n sea water: I I I . E f f e c t s of n a t u r a l i n h i b i t o r s and the p o s i t i o n of the chemical l y s o c l i n e , Amer. Jour. S c i . 274, 638-647 (1974). Bramlette, M.N. P e l a g i c sediments, p. 345-366, in Sears, Μ., ed., "Oceanography," Am. Assoc. Adv. S c i . Washington, D.C., 1961. Berner, R.A. and Morse, J.W. D i s s o l u t i o n k i n e t i c s of calcium carbonate i n seawater IV. Theory of c a l c i t e d i s s o l u t i o n , Amer. Jour. S c i . 274, 108-134 (1974). Sjoberg, E.L. K i n e t i c s and mechanism of c a l c i t e d i s s o l u t i o n i n aqueous s o l u t i o n s at low temperatures, A c t a Univ. Stockholmiensis. Stockholm C o n t r i b . Geol. 32, 92 p. (1978). Walter, L.M. and Hanor, J.S. E f f e c t of orthosphosphate on the d i s s o l u t i o n of b i o g e n i c magnesian c a l c i t e s , Geochim. Cosmochim. Acta ( i n p r e s s ) . Plummer. L.N. and Wigley, T.N.L. Critical review of the k i n e t i c s of c a l c i t e d i s s o l u t i o n and p r e c i p i t a t i o n , i n Jenne, E.A., ed., "Chemical Modeling. S p e c i a t i o n , S o r p t i o n , S o l u b i l i t y and K i n e t i c s i n Aqueous Systems," Amer. Chem. Soc. Symp. S e r i e s , Washington, D.C. ( t h i s volume). de Kanel, J . and Morse, J.W. The chemistry of orthophosphate uptake from seawater onto c a l c i t e and a r a g o n i t e , Geochim. Cosmochim. A c t a 42, 1335-1340 (1978). Aharoni, C. and Ungarish, M. K i n e t i c s of a c t i v a t e d chemi­ s o r p t i o n Part 1 - The n o n - E l o v i c h i a n part of the Isotherm, Faraday Trans. I 72, 400-408 (1976). Aharoni, C. and Ungarish, M. K i n e t i c s of a c t i v a t e d chemi­ s o r p t i o n Part 2 - T h e o r e t i c a l models, Faraday Trans. I 73, 456-464 (1977). Morse, J.W., de Kanel, J . , and H a r r i s , K. The d i s s o l u t i o n k i n e t i c s of calcium carbonate i n seawater V I I . The d i s s o l u ­ t i o n k i n e t i c s of s y n t h e t i c a r a g o n i t e and pteropods, Amer. Jour. S c i . ( i n p r e s s ) . 2

47.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch024

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25 Critical Review of the Kinetics of Calcite Dissolution and Precipitation L. N. PLUMMER and D. L. PARKHURST U.S. Geological Survey, Reston, VA 22092 T. M. L. WIGLEY

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

University of East Anglia, Norwich NR47TJ, England

The r a t e of c a l c i t e d i s s o l u t i o n i s known to depend on the hydrodynamic c o n d i t i o n s o f the environment and on the rate of heterogeneous r e a c t i o n at the mineral s u r f a c e . Numerous l a b o r a ­ t o r y s t u d i e s demonstrate t r a n s p o r t and s u r f a c e - c o n t r o l l e d aspects of c a l c i t e r e a c t i o n s i n aqueous s o l u t i o n s , but u n t i l r e c e n t l y , no study has been comprehensive enough to enable comparison of k i n e t ­ i c r e s u l t s among d i f f e r i n g hydro-chemical systems. We have s t u d i e d the d i s s o l u t i o n k i n e t i c s of c a l c i t e i n s t i r r ­ ed C 0 - water systems a t C 0 p a r t i a l pressures between 0.0003 and 0.97 atm and between 5° and 6 0 ° C , using pH-stat and f r e e d r i f t methods 0 1 ) · Our r e s u l t s suggest a mechanistic model f o r reac­ t i o n s at the calcite-aqueous s o l u t i o n i n t e r f a c e that has broad i m p l i c a t i o n s t o the c o n t r o l s on c a l c i t e d i s s o l u t i o n and p r e c i p i t a ­ t i o n under d i v e r s e chemical and hydrodynamic c o n d i t i o n s . This paper reviews the subject of the k i n e t i c s of c a l c i t e d i s s o l u t i o n and p r e c i p i t a t i o n by comparing p r e d i c t i o n s made by our mechanistic model w i t h published l a b o r a t o r y r e s u l t s . 2

2

Summary of the R e s u l t s of Plummer et a l . ( l ) Experimental. We s t u d i e d the d i s s o l u t i o n of semi-opticâl grade c r y s t a l s of Iceland spar (44.5 cm g"" and 96.5 c m ^ " ) i n d i l u t e s o l u t i o n s as a f u n c t i o n of pH, PCO2 and temperature. The "pH-stat" method was used to i d e n t i f y forward r e a c t i o n s f a r from e q u i l i b r i u m ( i n the near absence of backward r e a c t i o n ) . The "free d r i f t " method was used to study the r e a c t i o n near e q u i l i b r i u m where both forward and backward r a t e s must be considered. Details of the experimental procedures are given elsewhere (_1)» Figure 1 summarizes our pH-stat r e s u l t s f o r three C 0 p a r t i a l pressures at 25°C as a f u n c t i o n of pH. In g e n e r a l , the p a t t e r n shown i n Figure 1 demonstrates three c o n t r o l s on the forward r a t e . 1) At low pH, d i s s o l u t i o n r a t e shows l i t t l e dependence on P C 0 . The slope o f the l o g r a t e vs pH p l o t i s approximately - 1 . 0 and r a t e i s p r o p o r t i o n a l t o the bulk f l u i d a c t i v i t y of H . 2) At intermediate pH, forward r a t e depends on PC0 and pH and the slope 2

1

1

2

2

+

2

0-8412-0479-9/79/47-093-537$09.25/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

538

-3.0

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

25°C

-4.0

Γ

SX

-5.0!

ο Ε

~

-6.0!




χ

-3(e)

+ HCO'

— 3 3 ( e ) 2

3

CaCO

H+ + CO " (s) 3(s) +

H

ts)

( 4 b )

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25.

PLUMMER ET

|Ca

2 +

Calcite Dissolution and Precipitation

AL.

+

- HC(T| , , 3 (o)

+ HCO"

, 3(s)

+

CaCO

545

+ H C0° , 2 3(s)

3

(5b)

and ka

- HCO~| , + 0H~ (o) (s)

2 +

+

s

3

+

x

CaCO„ + H O 3

, 2 (s)

(6b)

x

where HCO3 on the surface maintains e q u i l i b r i u m w i t h the surface species and the n o t a t i o n | C a - H C O 3 | (o) p o i n t s to our uncer­ t a i n t y as to the p h y s i c a l nature of the C a - HC0 association during backward r e a c t i o n . R e c a l l i n g our assumption that the surface species are i n e q u i l i b r i u m w i t h c a l c i t e , forward and backward r a t e terms f o r surface species have a net r a t e of zero, and thus c a n c e l . The r a t e s of r e a c t i o n s 4 , 5 , 6 are then g i v e n by: 2 +

2 +

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

3

1

R

4

Rt; "5

=

k

1

+

2+ — " 4 Ca(o)- HCO^( )

a

l H

a

k

( o )

a

/, *2 άH πC ρ0 ( ) " k*4

— _

ko

a

Λ

Λ Λ / 3 o

2

u pCn0^3/( )\ C a, ( N) -» Hu Cr n0 o3/( o \) ' H

a 3-n„

x

6

=

k

3 H 0( )

"

a

2

o

k

4

,a

aa

a

s

o

1 , 1 R

( 7 )

0

a

2+ — — Ca(o) HC03(o)* 0H( ) e a

a

>

s

( 9 )

and the net r a t e of d i s s o l u t i o n i s given by R = R

+ R

4

(10)

+ R.

5

6

F l u x c a l c u l a t i o n s (4) show that boundary l a y e r a c t i v i t i e s are approximately equal to bulk f l u i d a c t i v i t i e s f o r a l l species other than HT; thus k aH C0°(o) i s replaced by k a H C 0 ( B ) and k3aH 0(o) becomes k a H 0 ( B ) . Because the forward r a t e dependence of r e a c t i o n 4 i s t r a n s p o r t - c o n t r o l l e d , 2

2

2

R

4,f

=

k

( a

2

2

3

2

3

a

l H(B) " H(o)^

k

l

a

H(B)

at low pH. At higher pH, the forward r a t e of 4 i s more d i f f i ­ c u l t to describe but i s always small r e l a t i v e to the other terms i n 10 and can be neglected. Equation 10 i s then i d e n t i c a l w i t h the e m p i r i c a l equation 3 , and defines ki+ as I f I III k = k + k a + k a . (12) 4

4

4

H C 0

( s )

4

0 H ( g )

From the p r i n c i p l e of microscopic r e v e r s i b i l i t y , forward

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

and

546

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

backward r a t e s of a l l r e a c t i o n s must balance a t e q u i l i b r i u m , and thus, equations 7 - 9 g i v e t k

ι 4

k1K2 >

=

K

1 1 k

k2K2 , and k

4 = K

c

1 1 1

k3K2

4

K

K

c l

K

c w

where K K , Κ , and K are e q u i l i b r i u m const; tants f o r H C0? = H + H C O 3 , HCOI = H + C O 3 " , CaCO3 ( c a l c i t e ) = C a + CO , and H 0 = Η + OH . S u b s t i t u t i o n i n t o equation 12 ao l9

2

w

+

2

+

2 +

k

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

2-

UCX-LUCO

2

4

7

=

Wl

K

+

1

c

I"

k

- T ~ H ) a

H 0( )l

a a

2 H C03(s)

+

k

2

3

2

( s

s

'

J

4

(13)

>

By r e p l a c i n g k j i n 13 w i t h the e x p e r i m e n t a l l y determined values of k , we can compare the t h e o r e t i c a l dependence of k4 on PC0 and temperature w i t h the observed PC0 and temperature de­ pendence ( F i g u r e 4 ) . Because k j > k , the computed values of k should show a systematic b i a s ( w i t h the computed values below the experimental v a l u e s ) . However, the PC0 dependence should remain u n a l t e r e d by u s i n g ki i n s t e a d of k j , and, i f the k j term i s r e l a ­ t i v e l y s m a l l , the temperature dependence w i l l a l s o be l a r g e l y unchanged. F i g u r e 4 shows that the PC0 and temperature trends i n com­ puted and observed values of k are s i m i l a r . Computed values are below the experimental values as expected, i m p l y i n g a k value some ten t o twenty times k . This q u a l i t a t i v e and q u a n t i t a t i v e agreement between theory and experiment gives f u r t h e r support f o r our mechanistic model. In terms of the s a t u r a t i o n r a t i o , Ω (Ω = IAP/K , where IAP i s the i o n a c t i v i t y product of c a l c i t e i n s o l u t i o n ; Ω i s 1 during p r e c i p i t a t i o n , and Ω = 1 a t e q u i l i b r i u m ) , equations 3 and 13 define r a t e as x

2

2

x

4

2

2

4

x

x

C

a

H

+

a

H (s)

R = ot+e-(a+

g ) Ω

(14)

+

+

where α = k i a H , and 3 = k aH C03 + k a H 0 . At low PC0 (

'

( 1 5 )

H

I t i s apparent that i n order to use our r a t e model, a thermo­ dynamic e v a l u a t i o n of the bulk f l u i d and surface s p e c i a t i o n i s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25.

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ET AL.

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r e q u i r e d . Thermodynamic s p e c i a t i o n i n the bulk f l u i d can be c a l c u l a t e d through the use of thermodynamic models of the aqueous phase, such as the computer program WATEQF ( 5 ) . A parameter more d i f f i c u l t to assess i n our r a t e model i s the surface a c t i v i t y of H . At r e l a t i v e l y high C 0 p a r t i a l pressures, as i n our e x p e r i ­ ments (1), we have concluded that surface PC0 and aH 0 are near the bulk f l u i d values and surface pH i s then determined by c a l ­ c i t e e q u i l i b r i u m owing t o the r a p i d r e a c t i o n w i t h H . But as shown i n a l a t e r s e c t i o n of t h i s paper, t h i s c o n c l u s i o n ( t h a t surface PC0 = bulk f l u i d PC0 ) may be only approximately true i n our own experiments, and i t becomes c r u c i a l to any a n a l y s i s of c a l c i t e k i n e t i c s a t low C 0 p a r t i a l pressures. +

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2

Comparison With Other Studies For purposes of d i s c u s s i o n , r e l e v a n t s t u d i e s i n the pub­ l i s h e d l i t e r a t u r e can be grouped under four t o p i c s : 1) d i s s o l u ­ t i o n i n a c i d s , 2) C02-dependence, 3) e f f e c t of i m p u r i t i e s and 4) p r e c i p i t a t i o n . These t o p i c s cover a diverse l i t e r a t u r e i n methods and c o n d i t i o n s of experimental study, and no s i n g l e set of r a t e measurements i s complete enough f o r d i r e c t comparison w i t h a l l other s t u d i e s . In a d d i t i o n , r a t e equations derived from experiments are u s u a l l y of l i m i t e d a p p l i c a b i l i t y beyond the experimental r e s u l t s . To date, only the t h e o r e t i c a l model of Plummer et_ a l . (1) described above i s comprehensive enough t o a l l o w p r e d i c t i o n s covering the range of experimental r e s u l t s i n the l i t e r a t u r e . In some cases we are unable to make d i r e c t com­ parisons of observed r a t e s . But where s u f f i c i e n t data are not a v a i l a b l e f o r d i r e c t comparison, we have e i t h e r attempted to cast the r e s u l t s of others i n t o the framework of our model, or we have compared observations w i t h p r e d i c t i o n s based on our model. Our a n a l y s i s of the l i t e r a t u r e i s not exhaustive and i s i n part l i m i t e d to r e p o r t s g i v i n g s u f f i c i e n t data to enable d i r e c t or i n d i r e c t comparison. In order t o cast r e s u l t s of other s t u d i e s i n terms of our model, some t r a n s f o r m a t i o n of data has been necessary. In some cases we have had to estimate a c t i v i t y c o e f f i c i e n t s i n s o l u t i o n s , and t h i s has been done v i a the Davies equation ( 6 ) , log γ. =

-AZ 1

2

[ \ 1+

-0.31 ) 1*

(16)

J

where A i s a constant dependent on temperature, z i s i o n charge, and I i s i o n i c s t r e n g t h ( I = 1/2 Ζ m±z±> where mi i s m o l a l i t y o f the ith i o n ) . I n other cases we nave r e l i e d on an aqueous model and our r a t e model t o p r e d i c t rates under experimental c o n d i ­ t i o n s , or we have used our r a t e equation t o p r e d i c t concentra­ t i o n - t i m e curves f o r comparison w i t h o b s e r v a t i o n s . The aqueous model used t o make p r e d i c t i o n s i s s i m i l a r t o that of MIX2 (7) f o r the chemical system Ca0-Mg0-K 0-Na 0-HCl-H S0i -H C0 -H 0. The 2

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

aqueous model i n c l u d e s 23 i o n p a i r s i n a d d i t i o n to the major species and uses the c o n s t r a i n t s of mass a c t i o n , mass balance and charge balance i n s o l u t i o n . Using a computer program, RATECALC, r e a c t i o n progress and r a t e are f o l l o w e d as a f u n c t i o n of time by way of equations 3 and 13. D i s s o l u t i o n i n A c i d s . Most k i n e t i c work w i t h c a l c i t e has dealt w i t h d i s s o l u t i o n i n a c i d s . King and L i u (8) measured the i n i t i a l r a t e of s o l u t i o n of r o t a t e d marble c y l i n d e r s i n d i l u t e a c i d s between 15° and 35°C. They found that r a t e of d i s s o l u t i o n increased w i t h i n c r e a s e d a c i d c o n c e n t r a t i o n , increased speed of r o t a t i o n and i n c r e a s e d temperature. Rate was i n v e r s e l y r e l a t e d to v i s c o s i t y , supporting t h e i r c o n c l u s i o n of a t r a n s p o r t - c o n ­ t r o l l e d r e a c t i o n . Figure 5 shows some of t h e i r r e s u l t s i n d i l u t e HC1 s o l u t i o n s at 4000 rpm. King and L i u s r a t e s can be described by a l i n e a r r e l a t i o n i n the a c t i v i t y of H (Figure 5), where the slope i s .022, .032, and .042 at 15°, 25°, and 35°C, r e s p e c t i v e l y , and comparable to our k , which i s 0.051 at 25°C 01) f o r suspended p a r t i c l e s s t i r r e d at 1800-2300 rpm. The data of King and L i u (8) give an a c t i v a t i o n energy of 5.6 k c a l / m o l , which i s n e a r l y three times our value (2.0 k c a l / m o l ) . We do not expect c l o s e agree­ ment i n experimental values of k j , owing to l a r g e d i f f e r e n c e s i n hydrodynamic c o n d i t i o n s between experimental methods. F i g u r e 6 compares l o g r a t e as a f u n c t i o n of pH from v a r i o u s sources. The slope of a best f i t l i n e between pH 2 and 5.5 i s -0.95 c o n f i r m i n g the f i r s t order r e a c t i o n i n air". The data of King and L i u (8) and Plummer et a l . CO i n d i l u t e HC1 s o l u t i o n s , and Berner and Morse (3) i n a r t i f i c i a l sea water s o l u t i o n s at low pH show c l o s e agreement. Table I summarizes experimental methods used by v a r i o u s workers l e a d i n g to the r a t e s given i n Figure 6, and compares our estimates of k i from these data w i t h the temper­ ature dependence of ki· C l e a r l y , there are r e a l d i f f e r e n c e s i n k i between e x p e r i ­ ments. The highest value of k i i s estimated from the data of Weyl (9), who d i r e c t e d a j e t of C O 2 - s a t u r a t e d water (pH -3.9) onto the surface of c a l c i t e . Weyl found that the r a t e of s o l u t i o n v a r i e d w i t h the j e t v e l o c i t y . His r a t e s imply that k j v a r i e s from 0.11 to 0.23 when v e l o c i t y of the j e t increases from 18 to 35 m sec · The s m a l l e s t value of k (.0073) i s derived from the data of Tominaga et a l . (10). These authors r o t a t e d a d i s k of marble i n HC1 s o l u t i o n s (0.1750 - 0.5317N) at 485 rpm. Rate of d i s s o l u t i o n was f o l l o w e d by the volume of C0 evolved. A f t e r an i n i t i a l p e r i o d f o r s a t u r a t i o n of the a c i d w i t h C0 , r a t e of gas evolved becomes l i n e a r i n the cumulative amount of C0 produced. Because the a c i d c o n c e n t r a t i o n decreased as c a l c i t e d i s s o l v e d , we e x t r a ­ p o l a t e d the observed l i n e a r r e l a t i o n i n C0 production back to the i n i t i a l c o n d i t i o n to estimate i n i t i a l r a t e s under known a c i d c o n c e n t r a t i o n s . C o r r e c t i o n to pH v i a the Davies equation leads to the r a t e s shown f o r these authors i n Figure 6. Most of the v a r i a t i o n i n k i (Table I ) i s probably due to f

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

25.

PLUMMER E T AL.

Calcite Dissolution and Precipitation

D.O

0.002

0.004 0.006 0.008 0.010 0.012 MOLARITY OF HCI

0.0

0.002

0.004 0.006 0.008 ACTIVITY OF H

549

0.010 0.012

+

Figure 5. Dissolution of rotated marble cylinders in dilute HCI solutions be­ tween 15° and 35°C at 4000 rpm. Rate is shown as a function of concentration (upper) and activity of H (lower), k, is defined by slope on plots of rate vs. aH\ +

ο King and Liu (fi) ® Tominaga et aj. (10) I Weyl (9) ο Wentzler (39) ° Berner and Morse (3) Lund et al. (13) —

Barton and Vatanatham (38) Δ Plummer et aj. (1)

No

3

0 Sjoberg (14)

PH

4

Figure 6. Summary of log rate as function of pH in acids at 25°C. All data are far from equilibrium and independent of Pco . The slope is —0.95 and rate may be assumed, within the uncertainties of the data, to befirstorder in aH . %

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

) o|

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

Pry

SYSTEMS

T3 s C CO CO Xi

4J CO 05 TJ

3 -H g CJ CO — iI

0) 4-1

4J h CO 3 T3 4J CO

χ; α;

* i* 4-1 0)

Ο ι—I • υ Ο PC

525

u

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

25.

PLUMMER E T A L .

Calcite Dissolution and Precipitation

551

d i f f e r e n c e s i n hydrodynamic c o n d i t i o n s of the experiment, being the lowest under the approximately laminar boundary l a y e r c o n d i ­ t i o n s a t the surface of a slowly r o t a t i n g disk (10), and highest at the impact of a high v e l o c i t y j e t on the c a l c i t e surface (_9). Transport dependence of the r e a c t i o n i n HCI s o l u t i o n s i s a l s o documented by the work o f Kaye (LI) and Nierode e t a l . (12)« The temperature dependence of k j shows considerable v a r i a ­ t i o n (2.0 - 5.6 kcal/mole), and may a l s o be a f u n c t i o n of the hydrodynamic c o n d i t i o n s of the experiment. Lund e t a l . (13) obtained an a c t i v a t i o n energy of 15 kcal/mol between -15.6 and +1.0°C under c o n d i t i o n s where they concluded both surface reac­ t i o n and transport processes i n f l u e n c e d the r a t e . They found r a t e p r o p o r t i o n a l t o the 0.63 power of rather than the f i r s t order r e a c t i o n observed f o r c o n d i t i o n s of transport c o n t r o l . The data shown i n Figure 6 give the v a r i a t i o n i n l o g r a t e as a f u n c t i o n of pH f a r from e q u i l i b r i u m . We have shown that be­ low pH 4, C0 p a r t i a l pressures between 0 and 1.0 atm do not i n f l u e n c e the rate s i g n i f i c a n t l y . Above pH 4, a l l data shown i n Figure 6 are a t C 0 p a r t i a l pressures l e s s than 10"" atm and can thus be compared. Only the data o f Plummer et_ a l . (1) and Sjoberg (14) are a t pH greater than 5.5 f a r from e q u i l i b r i u m , and both confirm a plateau i n rate as a f u n c t i o n of pH (Figures 1 and 6). We have concluded that t h i s plateau defines the rate of the forward r e a c t i o n i n the near absence of both C 0 and H and i s c o n t r o l l e d by r e a c t i o n with H 0 (1). I t i s not understood, how­ ever, why S j o b e r g s r a t e s are approximately 2.8 times greater than our rates f a r from e q u i l i b r i u m i n the near absence of H and C0 , and are s i g n i f i c a n t l y lower than most observed rates at pH l e s s than 5.0 (Figure 6). The rates f a r from e q u i l i b r i u m i n Figure 6 were obtained i n a v a r i e t y of aqueous s o l u t i o n s i n c l u d i n g HCI, H N O 3 , H S0if, NaClC a C l and KC1 t o 0.7 molar a t 25°C, and suggest that a t l e a s t f o r these c o n d i t i o n s , the forward rate i s not s i g n i f i c a n t l y dependent upon the presence of these c o n s t i t u e n t s i n s o l u t i o n . 2

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C0 -dependence. There are r e l a t i v e l y few i n v e s t i g a t i o n s that deal w i t h the e f f e c t s of C0 -dependence on r a t e of d i s s o l u ­ t i o n . Erga and Terjesen (15) followed c a l c i t e d i s s o l u t i o n by the f r e e d r i f t method t o near e q u i l i b r i u m i n C0 -water s o l u t i o n s a t 25°C by measurement of d i s s o l v e d calcium as a f u n c t i o n of time. PC0 was maintained constant by bubbling gas mixtures. The c a l ­ c i t e used was of " n a t u r a l o r i g i n " and surface area estimated from p a r t i c l e s i z e was 125 cm g~ . Experiments a t .95 - .97 atm C0 showed that r a t e was d i r e c t l y p r o p o r t i o n a l t o surface area of the p a r t i c l e s , but independent of gas v e l o c i t y , l e a d i n g t o t h e i r c o n c l u s i o n that the r e a c t i o n rate i s independent of 1) the t r a n s ­ f e r of carbon d i o x i d e from the gas to l i q u i d phase, and 2) chemi­ c a l r e a c t i o n i n the bulk f l u i d . Rate of d i s s o l u t i o n was propor­ t i o n a l t o s t i r r i n g rate t o the power ( s t i r r i n g c o e f f i c i e n t ) 0.22 (between 280 and 555 rpm). Because s t i r r i n g c o e f f i c i e n t s of .5 t o 2

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552

CHEMICAL MODELING IN AQUEOUS SYSTEMS

1.0 are expected i n t r a n s p o r t - c o n t r o l l e d r e a c t i o n s ( 1 6 ) , the low s t i r r i n g c o e f f i c i e n t of Erga and Terjesen (15) f u r t h e r supports t h e i r c o n c l u s i o n t h a t f o r t h e i r experimental c o n d i t i o n s , "the r a t e determining steps are l o c a l i z e d a t the s o l i d - l i q u i d i n t e r ­ face or i n the l i q u i d f i l m i n immediate contact w i t h i t " . Erga and Terjesen (15) p u b l i s h e d two s e t s of data that we can use f o r comparison w i t h our r a t e model. The f i r s t i s a p l o t of d i s s o l v e d Ca vs time i n a f r e e d r i f t run a t 0.954 atm C 0 , and the second i s a p l o t of d i s s o l u t i o n r a t e vs t o t a l calcium i n the s o l u t i o n f o r f o u r f r e e d r i f t runs a t C0 p a r t i a l pressures of 0.135, 0.392, 0.664 and 0.952 atm ( 1 5 ) . Figure 7 compares two concentration-time curves c a l c u l a t e d from equation 3 ( u s i n g the program RATECALC mentioned e a r l i e r ) w i t h that observed by Erga and Terjesen. The dashed curve ( F i g ­ ure 7^ i s c a l c u l a t e d u s i n g the r e p o r t e d surface area (1250. cm 1" ) and the s o l i d curve i s a s i m u l a t i o n of t h e i r experiment assuming t h e i r s u r f a c e area was 1/2 the r e p o r t e d v a l u e . F i g u r e 7 shows that the r e s u l t s of Erga and Terjesen (15) d i f f e r from ours by a constant f a c t o r of two. That i s , the form of our r a t e equa­ t i o n p r e d i c t s the observed shape o f the concentration-time curve w i t h an u n c e r t a i n t y of a f a c t o r of two i n the r a t e c o n s t a n t s . This u n c e r t a i n t y i s s i m i l a r t o the u n c e r t a i n t y i n the measurement of k.3(_l). Surface area adjustments w i t h i n a f a c t o r of two may a l s o r e f l e c t u n c e r t a i n t i e s i n e s t i m a t i o n of area based on p a r t i ­ c l e s i z e , and ( o r ) d i f f e r e n c e s i n r e a c t i o n s i t e d e n s i t y among d i f f e r i n g m a t e r i a l . What i s important i s that the shape of the f r e e d r i f t data of Erga and Terjesen (15) i s c l o s e l y matched by the form of our r a t e equation ( F i g u r e 7 ) . The match may be even c l o s e r i f one c o n s i d e r s the expected ( s m a l l ) decrease i n surface area during r e a c t i o n , which was not accounted f o r i n the computer s i m u l a t i o n . Erga and Terjesen used 100g o f c a l c i t e i n 10 l i t e r s of d i s t i l l e d water, so that a t 0.95 atm C 0 , n e a r l y 10 percent of the weight of t h e i r s t a r t i n g m a t e r i a l was d i s s o l v e d by the end of t h e i r run. Using the surface area c o r r e c t i o n of 1/2, we a l s o c l o s e l y match concentration-time curves f o r s i m i l a r experiments r e p o r t e d by these authors i n subsequent p u b l i c a t i o n s (17, 18) as shown f o r the r e s u l t s o f Terjesen et al - (17) i n F i g u r e 8. Figure 9 shows that i f we use the surface area c o r r e c t i o n of 1/2, our r a t e equation a l s o c l o s e l y simulates the observed r a t e s of Erga and Terjesen (15) a t other CO^ p a r t i a l pressures. Be­ cause i t i s necessary t o consider both forward and backward r e a c ­ t i o n i n a n a l y s i s of the r e s u l t s of Erga and Terjesen ( 1 5 ) , we have made the c a l c u l a t i o n s of Figure 9 assuming s u r f a c e PC0 i s equal t o the bulk f l u i d value and the surface pH i s determined by c a l c i t e e q u i l i b r i u m , i d e n t i c a l t o the c a l c u l a t i o n s o f Plummer et_ a l . ( 1 ) . As b e f o r e , we have not c o r r e c t e d f o r expected decreases i n s u r f a c e area during t h e i r experiments. But even without t h i s c o r r e c t i o n the match i n computed and observed r a t e s s t r o n g l y sup­ ports the PC0 dependence of forward and backward r a t e p r e d i c t e d 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25.

PLUMMER

Calcite Dissolution and Precipitation

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Ε £

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150 200 250 TIME (Minutes)

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Figure 7. Comparison of observed calcium in solution (15) during free drift experiment at 0.954 atm C0 and 25°C with simulated reaction. The dashed line is calculated using the reported surface area^and the solid line is calculated assuming the area is half the reported value.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

2

TIME (Minutes)

Figure 8. Comparison of simulated and observed calcium-time curve of Terjesen et al. (17) in a free drift run at 0.97 atm C0 and 25°C. The solid line represents the simulated reaction using the same surface area correction as found for the free drift experiments of Erga and Terjesen (15). 2

2.5^

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DISSOLVED CALCIUM (mmoles liter" ) 1

Figure 9. Comparison of observed and simulated rate for four C0 partial pres­ sures at 25°C in the free drift experiments of Erga and Terjesen (15). The varia­ tion in rate as a function of Pco is closely predicted by the rate model of Plummer et al. (1). 2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

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by Plummer e_t a l . Q ) . Figure 10 compares the C0 and H dependence of d i s s o l u t i o n r a t e observed by Berner and Morse (3) f a r from e q u i l i b r i u m i n pseudo-sea water (a CaCl2 - NaCl s o l u t i o n of the i o n i c s t r e n g t h and t o t a l calcium content of sea water) w i t h the PC0 - a H de­ pendence given by equation 1. We have found that the r a t e of d i s s o l u t i o n f a r from e q u i l i b r i u m becomes independent of PC0 be­ low approximately 1 0 ' atm. Most of Berner and Morse's r a t e s f a r from e q u i l i b r i u m are at C0 p a r t i a l pressures equal to or below 1 0 ~ atm, and as expected, show l i t t l e or no C0 depend­ ence f a r from e q u i l i b r i u m (Figure 10)· According to equation 1, the d i f f e r e n c e i n forward r a t e at constant pH between 0 and 1 atm C0 should be approximately 1.2 χ 10" mmol cm" s e c " , but the one value of Berner and Morse (3) a t 1 atm C0 that can be p l o t ­ ted on Figure 10 shows no C0 dependence. Although most of the r a t e s of Berner and Morse (_3) shown i n Figure 10 are w i t h i n a f a c t o r of two of our r a t e s , the expected r e l a t i o n of increased r a t e with increased PC0 i s even reversed i n some of t h e i r low PCO2 data (Figure 10). Because most of the r a t e s of Berner and Morse (_3) f a r from e q u i l i b r i u m are a t low PC0 , t h e i r data do not provide an adequate t e s t of PC0 dependence. Almost a l l of the data of Sjoberg (14) are f o r very low C0 p a r t i a l pressures. However, two r a t e s f a r from e q u i l i b r i u m a t 0.97 atm CO2 ( i n 0.7M KC1 a t pH4.4 and 5.0) are approximately twice the measured values at 1 0 " atm C0 ( 1 4 ) . Below pH 4, S j o b e r g s r a t e s f a r from e q u i l i b r i u m are dominated by r e a c t i o n w i t h H and become independent of PC0 , as we have found (1)· Although there are s i g n i f i c a n t d i f f e r e n c e s between the a b s o l u t e values of S j o b e r g s r a t e s and ours (Figure 6 ) , the PC0 depend­ ence observed by Sjoberg agrees reasonably w e l l w i t h that observ­ ed i n our experiments. +

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1

2

E f f e c t of I m p u r i t i e s . In the previous s e c t i o n s we have con­ s i d e r e d c a l c i t e d i s s o l u t i o n 1) f a r from e q u i l i b r i u m i n v a r i o u s s o l u t i o n s , and 2) both f a r from and near e q u i l i b r i u m but only i n pure C0 -water s o l u t i o n s . We now i n v e s t i g a t e c a l c i t e d i s s o l u t i o n i n s o l u t i o n s where backward r e a c t i o n must be considered i n the presence of i m p u r i t i e s . I m p u r i t i e s can be defined as c o n s t i t u e n t s present i n the aqueous phase that are not part of the o r i g i n a l s t o i c h i o m e t r i c composition of the reactant ( 2 ) . The presence of i m p u r i t i e s can have a profound e f f e c t on the r a t e of d i s s o l u t i o n or p r e c i p i t a t i o n , b u t , u n f o r t u n a t e l y , there i s no simple way of p r e d i c t i n g , a p r i o r i , the t o t a l e f f e c t that an impurity or com­ b i n a t i o n of i m p u r i t i e s can have on the r a t e of r e a c t i o n . The e f f e c t of i m p u r i t i e s can be p a r t i a l l y evaluated through thermodynamic a n a l y s i s . C e r t a i n l y , there are many other ways that i m p u r i t i e s may a f f e c t the r a t e of r e a c t i o n ( 2 ) , and by a c ­ counting f o r the thermodynamic e f f e c t of i m p u r i t i e s , other e f ­ f e c t s may be i d e n t i f i e d . Two c l a s s e s of thermodynamic problems are evident. 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

25.

PLUMMER E T AL.

Calcite Dissolution and Precipitation

555

The f i r s t i s concerned w i t h what thermodynamic changes w i l l take place i n a heterogeneous system upon a d d i t i o n of an impu­ r i t y . This problem can be described i n terms of changes i n mineral s o l u b i l i t y . I f an impurity contains a c o n s t i t u e n t that i s present i n the reactant (common i o n e f f e c t ) , the s o l u b i l i t y of the reactant i s u s u a l l y decreased and p r e c i p i t a t i o n may occur. Impurities may a l s o cause changes i n a c t i v i t y c o e f f i c i e n t s . De­ c r e a s i n g a c t i v i t y c o e f f i c i e n t s cause s o l u b i l i t y t o increase ( s a l t i n g - i n ) . Increasing a c t i v i t y c o e f f i c i e n t s can cause p r e c i p ­ i t a t i o n ( s a l t i n g - o u t ) . These and other thermodynamic e f f e c t s of i m p u r i t i e s on s o l u b i l i t y are discussed elsewhere (_2, 19_, 20). The second type of thermodynamic problem i s concerned w i t h comparing p a r a l l e l r e a c t i o n s i n pure and impure systems. In the case of c a l c i t e d i s s o l u t i o n we ask, how w i l l r a t e change a t con­ stant pH and PCO2 owing to the presence or absence of an impu­ r i t y ? Equation 14 shows that thermodynamic e f f e c t s of i m p u r i t i e s on the r a t e of c a l c i t e d i s s o l u t i o n are accounted f o r by c a l c u l a ­ t i o n of the bulk f l u i d s a t u r a t i o n (Ω) and the e q u i l i b r i u m a c t i v ­ i t y of H i n the a d s o r p t i o n l a y e r ( a H ( s ) ) . The f o l l o w i n g ex­ ample demonstrates one p o s s i b i l i t y . Consider the d i s s o l u t i o n of c a l c i t e a t 25°C and 0.95 atm C 0 i n separate s t a r t i n g s o l u t i o n s of pure water and 0.2 M C a C l s o l u t i o n . Thermodynamic c a l c u l a t i o n s i n d i c a t e that the surface e q u i l i b r i u m pH (assuming surface PC0 i s equal t o the bulk f l u i d value) i s lower i n the C a C l s o l u t i o n (5.51) than i n the pure s o l u t i o n (6.02). A f t e r some i n i t i a l d i s s o l u t i o n , pH 5.36 i s reached, a t d i f f e r e n t times, i n both s o l u t i o n s . In the pure s o l ­ u t i o n Ω i s lower (0.025) and the r a t i o , a H / a H ( s ) , i s higher (4.57) than comparable values i n the 0.2 M C a C l s o l u t i o n , 0.479 and 1.41, r e s p e c t i v e l y . The increase i n Ω from the pure s o l u ­ t i o n t o the C a C l 2 s o l u t i o n i s nearly 6 times l a r g e r than the accompanying decrease i n a H / a H ( s ) r a t i o . Equation 14 then shows that r a t e of c a l c i t e d i s s o l u t i o n a t constant pH and PCO2 should be slower i n the CaCl2 s o l u t i o n s than i n pure s o l u t i o n s . These thermodynamic e f f e c t s are complex and almost every impurity i s a s p e c i a l case. R e s o l u t i o n of the thermodynamic e f ­ f e c t s of i m p u r i t i e s u s u a l l y r e q u i r e s a computer program, such as RATECALC mentioned e a r l i e r . One can only properly address the question as to what other e f f e c t s i m p u r i t i e s may have on rate of r e a c t i o n , a f t e r the thermodynamic e f f e c t s of i m p u r i t i e s have been evaluated. This i s the approach we have f o l l o w e d i n our e v a l u a ­ t i o n of the data o f Berner and Morse ( 3 ) , Morse (21) and Sjoberg (14) i n v a r i o u s s a l t s o l u t i o n s of the i o n i c s t r e n g t h of sea water ( I = 0.7). No attempt has been made to t r e a t rate data of Berner and Morse (3) or others f o r which trace i m p u r i t i e s (such as PO^) cause l a r g e changes i n r a t e without s i g n i f i c a n t l y a l t e r i n g the thermodynamic p r o p e r t i e s of the system. C l e a r l y , other mecha­ nisms must be considered i n i n t e r p r e t i n g these r e s u l t s . Table I I summarizes our c a l c u l a t i o n s of d i s s o l u t i o n r a t e i n pseudo-sea water and sea water f o r comparison w i t h the pH-stat +

+

2

2

2

2

+

+

2

+

+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING

556

IN AQUEOUS

SYSTEMS

measurements o f Berner and Morse (3)· Berner and Morse t a b u l a t e measured pH, PC0 and r a t e . Because many of these data depend s i g n i f i c a n t l y on backward r e a c t i o n , our c a l c u l a t i o n s of the r a t e s expected i n t h e i r experiments have employed a thermodynamic model f o r t h e i r aqueous s o l u t i o n s . We have had t o c o r r e c t t h e i r meas­ ured pH values f o r the e f f e c t s o f r e s i d u a l l i q u i d j u n c t i o n po­ t e n t i a l . The r e s i d u a l l i q u i d j u n c t i o n p o t e n t i a l i n sea water has been estimated by Hawley and Pytkowicz (22) to be -3.2 mv, or +.054 pH (added t o the measured v a l u e ) . As Berner and Morse r e ­ port measured pH t o two decimal p l a c e s , we have added 0.05 pH t o t h e i r measured values i n making our thermodynamic c a l c u l a t i o n s . For comparison, dual c a l c u l a t i o n s are presented i n Table I I f o r uncorrected and c o r r e c t e d pH. I n order t o o b t a i n d i f f e r e n t pH values i n pseudo-sea water and a r t i f i c i a l sea water a t constant PC0 , Berner and Morse added various amounts of HCI o r NaOH t o t h e i r s t a r t i n g s o l u t i o n s . Because the amounts of these a d d i t i o n s are not given i n t h e i r paper, we have c a l c u l a t e d them using the reported pH, PC0 , composition of the i n i t i a l s o l u t i o n (pseudosea water or a r t i f i c i a l sea w a t e r ) , and the charge balance c r i t e ­ r i a f o r the aqueous model i n RATECALC. Our estimates of the amounts of these a d d i t i o n s of HCI and NaOH have been considered i n c a l c u l a t i o n of r a t e and are shown i n Table I I . Table I I a l s o gives the c a l c u l a t e d surface e q u i l i b r i u m pH (assuming surface PC0 = bulk f l u i d P C 0 ) , bulk f l u i d s a t u r a t i o n index ( S I = l o g Ω) and p r e d i c t e d r a t e . Far from e q u i l i b r i u m the c o r r e c t i o n f o r r e s i d u a l l i q u i d j u n c t i o n p o t e n t i a l i s not s i g n i f i c a n t , but the r e s i d u a l l i q u i d j u n c t i o n p o t e n t i a l becomes extremely important c l o s e t o e q u i l i b ­ rium ( s p e c i f i c a l l y f o r r a t e s l e s s than approximately 100 mg cm y r " ) . Figure 11 shows a c l o s e c o r r e l a t i o n between c a l c u l a t e d r a t e s based on the j u n c t i o n p o t e n t i a l c o r r e c t e d pH and the ob­ served r a t e a t various C 0 p a r t i a l pressures. Most of our c a l c u ­ l a t e d r a t e s are w i t h i n a f a c t o r of two of the observed. Some of the data (Figure 11) show a trend w i t h the computed rates l a r g e r than the observed as r a t e decreases. This trend i n departure of computed and observed r a t e s i s not understood. Sources of un­ c e r t a i n t y i n c l u d e the f o l l o w i n g : 1) u n c e r t a i n t y i n the (thermo­ dynamic) pH of the bulk f l u i d , 2) u n c e r t a i n t y i n the surface e q u i l i b r i u m pH due t o some u n c e r t a i n t y i n surface PC0 , 3) p o s s i ­ b l e i n h i b i t i n g e f f e c t s i n t h e i r s o l u t i o n s , and 4) one must be aware o f the p o s s i b i l i t y o f other r e a c t i o n mechanisms o c c u r r i n g which are p r e s e n t l y u n i d e n t i f i e d . I f higher pH values are not due e n t i r e l y t o the a d d i t i o n of NaOH, but due, i n p a r t , t o the d i s s o l u t i o n of small amounts of CaC03, the bulk f l u i d s a t u r a t i o n w i t h respect t o c a l c i t e would be higher than that given i n Table I I and the c a l c u l a t e d r a t e would be slower. One r a t e near e q u i l i b ­ rium a t one atmosphere C 0 (pH 6.00, Table I I ) i s c a l c u l a t e d t o be 15 times f a s t e r than the observed value. This d i f f e r e n c e could be accounted f o r by accompanying d i s s o l u t i o n of a small amount of CaC0 .

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

2

2

2

2

2

2

1

2

2

2

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25.

P L U M M E R E T AL.

Calcite Dissolution and Precipitation

557

Figure 10. Dissolution rates from pHstat experiments of Berner and Morse (3) in pseudo-sea water at 25°C. No Pco dependence is observed at C0 partial pressures < 10' - atm, as expected. Range of rates observed by Plummer et al. (1) in dilute solutions shows that most rates at low Pco far from equilibrium agree with our rate model. One rate measurement of Berner and Morse (3) at 1 atm C0 shows little or no Pco dependence. 2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

1

5

2

2

2

Figure 11. Comparison of observed rates of Berner and Morse (3) in pseudo-sea water and sea water at 25°C with the predicted rates. Most rates are within a factor of two of the observed.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

-0 18 -0 18 -0 18 -0 18 -0 18 -1 5 -1 5 -1 5 -1 5 -1 5 -1 5 -2 0 -2 0 -2 .0 -2 0 -2 0

6.00 5.96 5.93 5.87 5.79 6.80 6.71 6.70 6.68 6.42 6.12 7.11 7.09 7.04 6.99 6.95

2

018 5 5 5 5 5 5 5 5 5 5 5

C0

-0 -1 -1 -2 -2 -2 -2 -2 -2 -3 -3 -3

ρ

Log

5.08 3.92 4.42 3.94 4.55 4.80 4.82 4.95 5.23 4.44 4.80 5.19

PH

OBSERVED

75. 318. 920. 1240. 1280. 18.8 96. 360. 430. 740. 950. 0.85 2.9 49. 71. 220.

1479. 12023. 5623. 35481. 3311. 1238. 1738. 1479. 1072. 6026. 1380. 759.

Rate -2 -1 mg cm yr

-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-

-00.153 0.028 0.152 0.034 0.016 0.013 0.007 -00.048 0.021 0.007

HCI

]

25 23 22 19 15 5 4 4 4 2 1 3 3 2 2 2

957 661 074 214 969 409 390 289 095 244 122 518 357 987 659 422

3 095 0•0000000.007 000-

NaOH

Computed adjust­ ments to i n i t i a l solution i n mmole/kg IL^O

6 091 6 079 6 071 6 058 6 042 6 867 6 858 6 857 6 856 6 841 6 832 7 123 7 .122 7 .118 7 .115 7 .113

bea Water 11 5 987 6 823 91 6 824 91 87 7 353 65 7 354 7 354 15 05 7 354 85 7 355 7 355 29 87 7 866 15 7 866 37 7 866

-0 29 -0 37 -0 42 -0 54 -0 70 -0 15 -0 33 -0 35 -0 39 -0 91 -1 51 -0 03 -0 .07 -0 .17 -0 .27 -0 .35

-2 -5 -4 -6 -5 -5 -5 -4 -4 -6 -6 -5 j

Log Ω

Equil­ ibrium pH

COMPUTED USING UNCORRECTED pH

1513. 1807. 2005. 2354. 2743. 93. 183. 191. 209. 381. 526. 15. 35. 80. 121. 150.

5031. 19892. 6621. 18914. 4930. 2937. 2658. 2189. 1327 . 6230. 2927 . 1410.

Rate -2 -1 mg cm yr

-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-

-00.135 0.020 0.135 0.030 0.013 0.010 0.005 -00.043 0.018 0.006

HCI

j

29.144 26.565 24.782 21.569 17.926 6.075 4.930 4.817 4.598 2.519 1.259 3.954 3.773 2.597 3.357 2.987

3.475 -0-0-0-0-0-0-00.009 -0-0-0-

NaOH

-0.19 -0.27 -0.33 -0.44 -0.60 -0.05 -0.23 -0.25 -0.29 -0.81 -1.41 +0.07 +0.03 -0.07 -0.17 -0.25

-2.01 -5.81 -4.81 -6.77 -5.55 -5.05 -4.95 -4.75 -4.19 -6.77 -6.05 -5.27

Log Ω

6 108 6 094 6 085 6.069 6 051 6 872 6 863 6 862 6 860 6.843 6.833 7.127 7 .126 7.115 7.122 7 .118

5 989 6.823 6.824 7.353 7.354 7.354 7.354 7.355 7.355 7.866 7.866 7 866

Equil­ ibrium pH

COMPUTED USING CORRECTED pH

Computed adjust­ ments to i n i t i a l solution i n mmole/kg H2O

Table II. Calculation of the Rates of Berner and Morse (3_)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

m

2

1089. 1439. 1665. 2067. 2509. 34. 135. 145. 164. 353. 503. -39. -17. 35. 80. 113.

4854. 17780. 5952. 16897. 4434. 2658. 2410. 1992. 1224. 5592. 2649. 1297.

cm" -1 yr

Rate

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

-2. -2. -2, -2, -2. -2, -2, -2, -2, -2, -2

7.37 7.32 7.28 7.24 7.22 7.16 7.08 6.96 6.71 6.65 6.51

.6 .6 .6 .6 .6 .6 .6 .6 .6 .6 .6

-2.0 -2.0 -2.0 -2.0 -2.0 -2.0 -2.5 -2.5 -2.5 -2.5

6.93 6.35 6.82 6.57 6.45 6.19 7.30 7.22 6.87 6.58

6.7 27. 51. 68. 76. 176. 242. 336. 530. 610. 675.

245. 360. 380. 600. 710. 950. 20. 135. 540. 840.

-0-0-0-0-0-0-0-0-0-0-0-

-0-0-0-0-0-0-0-0-00.156

1. .801 1. .600 1. .456 1. .325 1. .264 1. .098 0, .910 0. .687 0. .381 0. .334 0, .242

2.312 1.920 1.791 1.003 0.760 0.416 1.665 1.439 0.145 -0-

-0.16 -0.26 -0.34 -0.42 -0.46 -0.57 -0.73 -0.97 -1.48 -1.59 -1.87

-Sea

-0.39 -0.55 -0.61 -1.11 -1..35 -1.87 -0.15 -0.31 -1.01 -1.59

164. 213. 229. 337. 378. 463. 64. 121. 283. 362.

7,.444 7,.442 7,.441 7,.440 7,.439 7 .438 7 .436 7 .434 7 .433 7 .431 7 .430 67. 103. 130. 154. 165. 196. 231. 275. 342. 356. 386.

W a t e r , Near E q u i l i b r i u m

7..112 7,.109 7..108 7..101 7,.099 7,.096 7,.371 7,.368 7..360 7,.357

-0-0-0-0-0-0-0-0-0-0-0-

-0-0-0-0-0-0-0-0-00.116

2. .029 1. .801 1. .635 1, .491 1. .422 1, .234 1, .023 0, .773 0, .432 0, .376 0, .271

2.721 2.156 2.011 1.126 0.853 0.468 1.618 1.880 0.222 -0-

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

-0. -0, -0, -0, -0. -0, -0, -0, -1, -1, -1

-0, -0. -0, -1, -1, -1, -0, -0. -0, -1,

.06 .16 .24 .32 .36 .48 .63 .87 .37 .49 .77

.29 .45 .51 .01 .25 .77 .21 .05 .91 .49

7,.447 7,.444 7..443 7 .441 , 7,.441 7 .439 , 7 .437 , 7,.435 7,.432 7..431 7 .430

7,.116 7..111 7,.110 7,.102 7,.100 7,.096 7..369 7,.373 7,.361 7,.358

25 67 96 123 136 170 210 258 330 335 375

183 201 319 362 446 86 22 266 350

128

ZD

οι οι

ο

"2.

ο!.

S"

OK

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to

οι

CHEMICAL MODELING IN AQUEOUS

560

SYSTEMS

F i g u r e 11 shows that our r a t e equation, w i t h a p p r o p r i a t e thermodynamic c o r r e c t i o n s , s a t i s f a c t o r i l y p r e d i c t s observed r a t e i n sea water over a wide range of pH and PC0 f o r values of Ω between 0.0 and 0.6. Nearer e q u i l i b r i u m , u n c e r t a i n t i e s i n the c a l c u l a t i o n s and the experimental data (3) are too l a r g e f o r a r e l i a b l e t e s t of our model. We have a great deal more d i f f i c u l t y i n i n t e r p r e t i n g the r e s u l t s of Sjoberg (23, 1 4 ) . Most of our problems stem from S j o b e r g s experimental design which i n v o l v e d d i s s o l u t i o n i n 0.7 M KC1 s o l u t i o n u s i n g both pH-stat and f r e e d r i f t methods. Except f o r a few pH-stat measurements at 0.97 atm, most measurements were made a t very low PCO2. U n f o r t u n a t e l y , PCO2 was not a c t u a l l y c o n t r o l l e d and we only know that PCO2 was low owing t o bubbling of "C02-free" n i t r o g e n . Although there was probably very l i t t l e t r a n s f e r of C 0 from the r e a c t i o n system, and thus the r e a c t i o n was e s s e n t i a l l y c l o s e d t o C 0 as Sjoberg assumed, the a c t u a l PC0 i n s o l u t i o n depends on a number of f a c t o r s i n c l u d i n g bubbling r a t e and d i s s o l u t i o n r a t e . Our k i n e t i c r e s u l t s show that s u r ­ face PC0 i s extremely important i n c o n t r o l l i n g r a t e because C 0 e q u i l i b r i u m w i t h c a l c i t e determines surface pH. Although Sjoberg was c o r r e c t i n assuming a r e a c t i o n system c l o s e d to C 0 and i n assuming PC0 was very low, s u r f a c e PC0 may have v a r i e d s i g n i f i ­ c a n t l y . Our c a l c u l a t i o n s show that i f surface PC0 v a r i e d by an order of magnitude i n S j o b e r g s f r e e d r i f t experiments, surface e q u i l i b r i u m pH would vary by 0.66 pH. Sjoberg found that r a t e i n 0.7 M KC1 s o l u t i o n s was described 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

1

2

2

2

2

2

2

2

2

2

1

(17) where the brackets denote c o n c e n t r a t i o n , k i s a r a t e constant, A i s surface area, and C i s the value of f C a ] [CO ] at e q u i l ­ i b r i u m . Taking values of k and C from Sjoberg (23), equation 17 becomes, a t 20°C, 2+

R = 1.60 χ 1 0 "

6

(1 -

1/2)

2

- 1

where R i s i n mmol cm" s e c . At low equation (equation 14) reduces t o

PCO2

2-

(18) and h i g h pH, our r a t e

a+ R

R = 1.14 χ 1 0 "

7

(1

) a

(19)

+

H (s)

at 20°C. I g n o r i n g , f o r the moment, the obvious d i f f e r e n c e i n r a t e constants, we f i r s t compare the form of equations 18 and 19 c o r r e l a t i n g çfc w i t h ( a H / a H ( s ) ) . Using RATECALC, we reconstructed a f r e e d r i f t d i s s o l u t i o n run i n 0.7 M KC1 i n a c l o s e d system a t 25°C. Figure 12 shows that these two terms behave s i m i l a r l y , but that 1/2 i s s l i g h t l y l a r g e r than the term +

+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

25.

PLUMMER +

ET AL.

561

Calcite Dissolution and Precipitation

+

( a H / a H ( s ) )Ω . I n our c a l c u l a t i o n , though, we assumed a c l o s e d system and that surface PCO2 was equal t o the bulk f l u i d v a l u e , which i s c a l c u l a t e d near 1 0 ~ * atm. Bubbling of pure N gas w i l l lower t h i s value i n the bulk f l u i d and thus lower^ the s u r ­ face PC0 . For example, r e f e r r i n g t o Figure 12, a t Ω^ = 0.5, our c a l c u l a t i o n s show that Ω^ and our term (aH /aH (s))Ω are i d e n t i c a l i f surface PC0 i s 10"" rather than 1 0 " . Thus, c o n s i d e r i n g the u n c e r t a i n t i e s i n S j o b e r g s PC0 and the problems a s s o c i a t e d with determining low values of surface PCO2, there may be no fundamental d i f f e r e n c e between the form of S j o b e r g s r a t e equation and ours under comparable experimental c o n d i t i o n s . Comparing r a t e constants i n equations 18 and 19 shows that S j o b e r g s i n i t i a l rates i n f r e e d r i f t experiments are 14 times f a s t e r than ours. There may be considerable u n c e r t a i n t y i n S j o b e r g s i n i t i a l free d r i f t rates as they are based on slopes of calcium vs time curves c a l c u l a t e d from the measured pH. We found i n our free d r i f t experiments (1) that rate was u n r e l i a b l e f o r the f i r s t s e v e r a l hours of r e a c t i o n a t a PC0 of 1 0 ~ ' owing t o non-equilibrium of the C0 -water system. Furthermore, S j o b e r g s f r e e d r i f t r a t e f a r from e q u i l i b r i u m i s 3.2 times f a s t e r than h i s i n i t i a l rate determined by pH-stat. Sjoberg (23, 14) a l s o shows that i n i t i a l r a t e decreases i n a supposedly C 0 - f r e e system by a d d i t i o n of C a C l . According t o our r a t e equation, added calcium should have l i t t l e e f f e c t on r a t e f o r a C 0 - f r e e system. The decrease i n rate observed by Sjoberg f o r a d d i t i o n of calcium may point t o the presence of small amounts of C0 i n h i s s t a r t i n g s o l u t i o n s . Despite problems i n i n t e r p r e t i n g S j o b e r g s r e s u l t s , there are s e v e r a l points of general agreement. Sjoberg's pH and PC0 dependence f a r from e q u i l i b r i u m i s s i m i l a r t o ours, as shown e a r l i e r . Forward rate i n the near absence of C 0 and H appears constant (Figure 6) which a l s o agrees w i t h the form of our f o r ­ ward r a t e . We a l s o f i n d a s i m i l a r temperature dependence i n acids (Table I ) , and a t low PC0 and high pH, Sjoberg s (14·) r a t e constant has a temperature dependence of 6.8-8.4 kcal/mol which compares w i t h 7.9 kcal/mol measured by us. Because of our un­ c e r t a i n t i e s i n e s t i m a t i n g PC0 i n Sjoberg's experiments, f u r t h e r s p e c u l a t i o n regarding the c o m p a t i b i l i t y of h i s r e s u l t s and ours i s not warranted. Morse (21) r e c e n t l y reported n e a r - e q u i l i b r i u m pH-stat rates i n sea water a t 25°C and 1 0 ~ * atm C 0 . His r a t e s are a l l l e s s than 8 mg c m y r " . Table I I I compares our c a l c u l a t i o n s (using equation 15 and assuming surface PC0 i s equal t o the bulk f l u i d value) of a s e t of Morse's rates near e q u i l i b r i u m , and shows gen­ e r a l l y poor agreement. In c a l c u l a t i o n of Ω and aH ( s ) , no c o r ­ r e c t i o n f o r l i q u i d j u n c t i o n p o t e n t i a l e r r o r i n measured pH was necessary, as i n our e a r l i e r c a l c u l a t i o n s f o r pseudo-sea water. In sea water one can use the apparent constant approach t o solve thermodynamic problems i n the carbonate system d i r e c t l y . We have c a l c u l a t e d surface e q u i l i b r i u m pH using the C 0 s o l u b i l i t y data 6

2

2

2

+

+

6 m h

6 , 2

2

1

2

1

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1

1

2

5

2

1

2

2

2

2

2

1

2

+

2

1

2

2

2

5 3

2

-2

1

2

+

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

7.9 6.8 4.7 3.7 1.4 0.79 0.12 0.049 0.017

Rate mg cm y r

1

1

4

2

7.418 7.419 7.419 7.419 7.420 7.420 7.421 7.422 7.422

81.5 74.3 67.4 60.4 52.8 45.6 35.2 24.8 17.0

_ 1

7.518 7.510 7.502 7.493 7.486 7.477 7.465 7.452 7.442

Implied pH(s)

Calculated pH(s) Rate (open system) mg c m " y r

3

5

7.509 7.501 7.493 7.484 7.476 7.468 7.457 7.447 7.439

PCO2

(open system) .

— T h e o r e t i c a l c a l c i t e e q u i l i b r i u m pH i n a sea water system c l o s e d t o CO2.

6/

— Value o f pH(s) i m p l i e d by the observed r a t e and equation 15.

5/

— C a l c u l a t e d u s i n g equation 15 and the open system e q u i l i b r i u m pH.

— T h e o r e t i c a l c a l c i t e e q u i l i b r i u m pH a t b u l k f l u i d

3 /

6

.

pH(s) ( c l o s e d system)

— R e c a l c u l a t e d t o show t h i r d s i g n i f i c a n t f i g u r e using the apparent constants o f Weiss (24), Lyman (25), I n g l e et a l . ( 2 6 ) , and the measured pH and reported PCO2

2/

^ M o r s e (21).

0.620 0.649 0.680 0.712 0.745 0.780 0.828 0.876 0.917

7.32 7.33 7.34 7.35 7.36 7.37 7.383 7.395 7.405

2

Ω

PH

Observed

Table I I I .

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25.

PLUMMER

E T AL.

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563

of Weiss (24), the apparent constants of Lyman (25), and the ap­ parent c a l c i t e s o l u b i l i t y constant o f Ingle e t a l . (26). Also shown i n Table I I I are values of surface pH necessary f o r our r a t e equation t o reproduce Morse's r a t e s . The i m p l i e d surface pH i s 0.10 t o 0.02 higher than the c a l c u l a t e d e q u i l i b r i u m value at 1 0 - · atm C 0 ( open system e q u i l i b r i u m ) . This d i f f e r e n c e i n pH i m p l i e s a surface PCO2 s l i g h t l y lower than the bulk f l u i d v a l u e , i . e . , 10"" · t o 1 0 " · during Morse's pH-stat rate meas­ urements. The surface e q u i l i b r i u m pH values i m p l i e d by our rate equa­ t i o n and Morse's rates are a l l w i t h i n 0.01 pH of the t h e o r e t i c a l pH f o r c a l c i t e e q u i l i b r i u m i n sea water closed t o CO2 (Table I I I ) . In a c l o s e d system, c a l c i t e e q u i l i b r i u m determines both surface pH and PCO2, and r a t e depends, i n p a r t , on the f l u x of CO2 t o the s u r f a c e . Sjoberg (2^3) noted a s t i r r i n g dependence of rate a t pH 8 and very low C 0 p a r t i a l pressures, where c a l c i t e d i s s o l u t i o n has p r e v i o u s l y been a t t r i b u t e d t o surface r e a c t i o n a l o n e . The m a t e r i a l used by Morse was "whole Indian Ocean sediment" which i s l a r g e l y of biogenic o r i g i n . I t i s expected that the r e a c t i n g surface area i s considerably l e s s than the BET surface area used by Morse t o normalize r a t e . This may a l s o e x p l a i n some of the d i s p a r i t y between c a l c u l a t e d and observed rates (Table III). C a l c u l a t i o n of d i s s o l u t i o n r a t e using equations 14 and 15 becomes extremely s e n s i t i v e t o values of Ω and pH as e q u i l i b r i u m i s approached. C a l c u l a t e d rates l e s s than 100 mg c m y r " must be open t o considerable u n c e r t a i n t y owing t o the d i f f i c u l t y i n e s t i ­ mating Ω and surface pH with s u f f i c i e n t accuracy. 2

5 3

2

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2

68

2

5 6

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-2

1

P r e c i p i t a t i o n . The rate model o f Plummer eit_ a l . (_1 ), a l ­ though derived from d i s s o l u t i o n experiments, accounts f o r both forward and backward r e a c t i o n . I f no other mechanisms occur, t h i s r a t e model should a l s o describe the k i n e t i c s of c r y s t a l growth of c a l c i t e . There are s e v e r a l studies of the c r y s t a l growth of c a l c i t e (27 - J34 ), a l l using the seeded growth t e c h ­ nique of Reddy and Nancollas (_27, 28). By t h i s method, w e l l c h a r a c t e r i z e d seed c r y s t a l s are introduced i n t o a s t a b l e super­ saturated s o l u t i o n o f NaHC0 -CaCl2. C r y s t a l growth begins imme­ d i a t e l y and i s f o l l o w e d by measurement of pH and t o t a l d i s s o l v e d calcium. The p a r t i a l pressure of CO2 i s not c o n t r o l l e d and tends to increase i n s o l u t i o n as c r y s t a l l i z a t i o n proceeds. Our c a l c u ­ l a t i o n s show that some CO2 i s a l s o outgassed from s o l u t i o n during c r y s t a l growth. Most c r y s t a l growth experiments have been con­ ducted between pH 8 and 10, and a t CO2 p a r t i a l pressures between 10~ and 10"~ atm, the only exception being the work of Nancollas et a l . (34) which i s near pH 6 and 1.7 atm CO2. At low PC0 , Reddy and Nancollas (_27, .28) found that the r a t e of c r y s t a l growth was described by an equation of the form 3

3

5

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

564

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Figure 12. Correlation of Ω with the term aH /aH+(s) Ω showing similarity of the terms, indicating that the form of Sjoberg's rate equation (14, 23) is similar to ours +

1/2

Figure 13. Typical variations in cal­ cium, pH, log Pco , and calcite satura­ tion (Ω) during crystal growth experi­ ments of Reddy (30, 35) in dilute NaHC0 -CaCl solutions

Ω '

2

3

2

0, Ό

1

20

*

α

Equilibrium

!

1

40 60 80 TIME (Minutes)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

100

25.

PLUMMER E T AL.

R

Calcite Dissolution and Precipitation

k

565

}

= ~G Karoos" ~

( 2 0 )

2

Y±

where 1 · H+(s) +

(

2 2

)

+

C a l c u l a t i o n of the term a H / a H ( s ) i n s e v e r a l low PC0 c r y s t a l growth experiments gives values near 0.93 i n i t i a l l y and i n c r e a s ­ ing t o 0.97 a t the t e r m i n a t i o n of the growth experiments. Thus the_ form of equations 21 and 22 i s s i m i l a r because the term i s n e a r l y constant and near u n i t y . More r e c e n t l y a rate equation which i s second order i n the instantaneous calcium y i e l d has been proposed (29) and a p p l i e d t o low PC0 c r y s t a l growth data (30, 32, 33.)· These s t u d i e s de­ s c r i b e r a t e of c r y s t a l growth by an equation of the form 2

2

R = k (C - C )

2

(23)

g

where R i s r a t e of c r y s t a l growth, k i s a c r y s t a l growth r a t e constant, C i s the c o n c e n t r a t i o n of d i s s o l v e d calcium i n s o l u t i o n as a f u n c t i o n of time and C i s the e q u i l i b r i u m calcium concen­ t r a t i o n a t that time. As a means of t e s t i n g our r a t e model during c r y s t a l growth, we examined d e t a i l s of t o t a l calcium and pH during four comparable c r y s t a l growth experiments (30, 35). Figure 13 compares t o t a l c a l c i u m , pH, l o g PC0 and c a l c i t e s a t u r a t i o n (Ω) i n the bulk f l u i d during a t y p i c a l run. Rate of c r y s t a l growth was c a l c u ­ l a t e d using the d i f f e r e n c e i n successive calcium measurements, and v a r i e s from about 80 t o 2 χ 10~ mmol cm" s e c i n the four s e l e c t e d c r y s t a l growth experiments. When we assume that surface PC0 i s equal t o that c a l c u l a t e d i n the bulk f l u i d , and assume that c a l c i t e e q u i l i b r i u m determines surface pH, the c a l c u l a t e d r a t e s of c r y s t a l growth vary from 4 to 68 times f a s t e r than the observed v a l u e s , and g e n e r a l l y average ten to twenty times f a s t e r than the observed. These d i f f e r e n c e s between computed and observed rates are 2

9

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

s i g n i f i c a n t and i n d i c a t e e i t h e r that our model i s wrong, or that surface PCO2 values d i f f e r from those i n the bulk f l u i d . We have used these d i f f e r e n c e s to p r e d i c t the chemical p r o p e r t i e s of the surface during Reddy s c r y s t a l growth experiments (30, 35) assum­ ing our r e a c t i o n mechanism model describes the r e a c t i o n . Figure 14 shows the r a t i o of the i m p l i e d surface PC0 to the bulk f l u i d value as a f u n c t i o n of r e a c t i o n progress (Ω). E a r l y i n the r e ­ a c t i o n , when c r y s t a l growth i s most r a p i d , our r a t e model and the data of Reddy (30, 35) imply that surface PCO2 i s approximately 5 times l a r g e r than the bulk f l u i d v a l u e . This r a t i o decreases as the r a t e of c r y s t a l growth decreases, w i t h the i m p l i e d surface PC0 n e a r l y twice the bulk f l u i d value a t the t e r m i n a t i o n of the run (which i s s t i l l q u i t e f a r from e q u i l i b r i u m ) . In Figure 15 we compare the t h e o r e t i c a l open system e q u i l ­ i b r i u m surface pH (assuming surface PC0 = bulk f l u i d PC0 ) w i t h the surface pH i m p l i e d by the PC0 imbalance shown i n Figure 14. If our r e a c t i o n mechanisms can be a p p l i e d to c r y s t a l growth, our r a t e model p r e d i c t s that surface pH i s i n i t i a l l y 0.6 pH l e s s than the t h e o r e t i c a l open system e q u i l i b r i u m value and the d i f f e r e n c e decreases to about 0.3 pH a t t e r m i n a t i o n of the experiment. These d i f f e r e n c e s d i m i n i s h as the r e a c t i o n approaches e q u i l i b r i u m . These c a l c u l a t i o n s n e i t h e r prove nor disprove our mechanism model, s i n c e the i m p l i e d PCO2 (and pH) d i f f e r e n c e s are q u a l i t a ­ t i v e l y c o n s i s t e n t . That i s , during the c r y s t a l growth e x p e r i ­ ments, CO2 produced by CaC03 p r e c i p i t a t i o n increases i n the bulk f l u i d and some i s l o s t to the atmosphere. This net f l u x of CO2 i s c o n s i s t e n t w i t h the c a l c u l a t e d higher surface CO2 p a r t i a l pressures. We have made a p a r a l l e l c a l c u l a t i o n , s i m i l a r t o that given above f o r c r y s t a l growth a t low PC0 , using c r y s t a l growth data (34) near pH 6 and 1.7 atm. C0 . During t h i s experiment, of 110 minutes d u r a t i o n , most of the c r y s t a l growth occurred i n the f i r s t 40 minutes, during which Ω v a r i e d from 17.3 ( i n i t i a l l y ) t o 7.4 ( a t 40 minutes), w i t h the f i n a l value of Ω near 5.8 i n the bulk f l u i d . C a l c u l a t e d r a t e s (assuming surface PC0 i s equal t o the bulk f l u i d v a l u e s , and that c a l c i t e e q u i l i b r i u m then d e t e r ­ mines surface pH) a r e a l l somewhat f a s t e r than the observed r a t e s , but during the f i r s t 40 minutes of r e a c t i o n , computed and observed r a t e s d i f f e r by only a f a c t o r of two or l e s s . A f t e r 40 minutes, the observed r a t e s decrease f a s t e r than the c a l c u l a t e d r a t e s ; the f i n a l c a l c u l a t e d r a t e being approximately 12.4 times f a s t e r than the observed. I t i s apparent from our c a l c u l a t i o n s of the r a t e of c a l c i t e c r y s t a l growth that the agreement i n computed and observed r a t e i s f a r more s a t i s f a c t o r y at higher PC0 than low PC0 . 1

2

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Summary and D i s c u s s i o n This review has been l i m i t e d to s t u d i e s i n which s o l i d s u r ­ face areas were reported. We have not considered s p e c i f i c exper-

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

P L U M M E R E T AL.

Calcite Dissolution and Precipitation

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Reddy (30)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch025

S1 Reddy (35)

/Equilibrium 0

1

2

3

Ω

4

5

6

7

Figure 14. Ratio of surface"Pco to bulk fluid Pco during crystal growth ex­ periments of Reddy (30, 35), as implied by the observed rates and the rate model of Plummer et al. (1) 2

2

9.0 8.8

Open System Equilibrium pH

8.6 h 8.4

Implied Surface pH Δ

On

Λ Λ

^

Ο •

8.2 8.0 0

3

4 Ω

Figure 15. Comparison of calculated pH in equilibrium with calcite at the bulk fluid Pco during crystal growth experiments of Reddy (30, 35) with the surface pH implied by our rate model and the observed rate as a function of calcite satu­ ration (Ω ) 2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

iments i n v o l v i n g i n h i b i t i o n . From the remaining k i n e t i c s t u d i e s w i t h c a l c i t e , r a t e s range from -1 χ 10" mmol cm" s e c " ( p r e c i p ­ i t a t i o n ) to about +1 χ 10"~ mmol cm s e c , while the r e s u l t s of v a r i o u s workers represent a range i n pH of about 0 to 10, PCO2 from 0.000001 to 1.7 atm, and temperature between -15 and 60°C. S o l u t i o n compositions vary from the r e l a t i v e l y simple CO2 - H 0 system to the C0 - sea water system. Hydrodynamic c o n d i t i o n s range from s t i r r e d batch experiments to experiments w i t h r o t a t i n g d i s k s and c y l i n d e r s and s o l u t i o n " d r i l l i n g " experiments w i t h h i g h velocity jets. Most r a t e s p r e d i c t e d using equations 3 and 13, or 14 and 15 are w i t h i n a f a c t o r of 20 of the observed and many are w i t h i n a f a c t o r of 2 or b e t t e r . The form of the forward r a t e as a func­ t i o n of PC0 and pH seems to agree w i t h that observed by us. In a d d i t i o n , the shape of c o n c e n t r a t i o n - time curves p r e d i c t e d by equations 3 and 13 are s i m i l a r to those observed i n f r e e d r i f t d i s s o l u t i o n experiments. U n c e r t a i n t i e s greater than a f a c t o r of 2 are probably s i g n i f i c a n t and point to three problem areas: 1) At low pH, r a t e depends s i g n i f i c a n t l y on the hydrodynamic t r a n s p o r t constant f o r H which i s not w e l l d e f i n e d . For example, at 25°C, our c a l c u l a t i o n s from observed r a t e s show that k j may vary from ^0.007 cm s e c under approximately laminar boundary l a y e r c o n d i t i o n s at the end of a r o t a t i n g d i s k (10) to about 0.23 cm s e c " at the impact of a j e t (at ^35 m sec ) on the c a l c i t e surface (9). Under the t u r b u l e n t c o n d i t i o n s of the s t i r r e d batch experiments of Plummer e_t a_l. ( 1 ) , k i i s near 0.05 cm s e c " . 2) Accurate comparison of r e s u l t s r e q u i r e s knowledge of r e a c t i o n s i t e d e n s i t y per u n i t surface area. C a l c i t e m a t e r i a l s used f o r k i n e t i c study have i n c l u d e d n a t u r a l marbles, limestones, hydrothermal c r y s t a l s of Iceland spar, t e s t s of calcareous organisms and l a b o r a t o r y and commercial p r e c i p i t a t e s . Surface areas, es­ timated by BET methods and g r a p h i c a l methods (based on p a r t i c l e s i z e d i s t r i b u t i o n ) range from about 0.005 to 2 m g . There are apparent d i s c r e p a n c i e s between g r a p h i c a l and BET surface areas and the question i s r a i s e d as to which type of surface area es­ timate i s most r e p r e s e n t a t i v e of the r e a c t i n g surface area. One could argue that BET surface areas overestimate the r e ­ a c t i n g surface area. The r e a c t i n g surface area may c o i n c i d e w i t h a smoothed aqueous f i l m on the surface which can have much l e s s surface area than that determined by BET methods. The surface area of i n t e r s t i c e s may be e s s e n t i a l l y non-reactive owing to con­ t a c t w i t h s o l u t i o n s which are nearer to e q u i l i b r i u m . The uncer­ t a i n t y i n r e a c t i n g surface area i s most s i g n i f i c a n t i n r e a c t i o n s w i t h biogenic m a t e r i a l , (36, 21). Even i f the r e a c t i n g surface area can be d e f i n e d , i t i s s t i l l necessary to determine the num­ ber of r e a c t i o n s i t e s per u n i t r e a c t i n g area. The number of r e ­ a c t i o n s i t e s may depend on the number of imperfections i n the c r y s t a l s t r u c t u r e , so that l e s s p e r f e c t c r y s t a l s may d i s s o l v e f a s t e r than more p e r f e c t c r y s t a l s under otherwise i d e n t i c a l con­ d i t i o n s . C l e a r l y , an i n t e r c a l i b r a t i o n i s r e q u i r e d where r a t e s 5

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In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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PLUMMER E T AL.

Calcite Dissolution and Precipitation

are measured f o r d i f f e r i n g m a t e r i a l of known BET surface area under i d e n t i c a l hydrochemical c o n d i t i o n s . 3) The t h i r d general area of u n c e r t a i n t y concerns the c o n t r o l s of surface pH. We have shown (1) that a t r e l a t i v e l y high bulk f l u i d PC0 (>0.03 atm), surface PC0 and aH 0 are near the bulk f l u i d values and surface pH i s c o n t r o l l e d by c a l c i t e e q u i l i b r i u m at the bulk f l u i d PC0 and aH 0. C a l c i t e e q u i l i b r i u m a t the bulk f l u i d PC0 c o n t r o l s surface pH throughout most of our d i s s o l u t i o n experiments, and accounts f o r our c l o s e agreement w i t h the r e ­ s u l t s of Erga and Terjesen (15) (PC0 = 0.135 - 0.952) and Terjesen (17) (PC0 = 0.97 atm). When we assume surface PCO2 i s equal t o bulk f l u i d PC0 during c a l c i t e d i s s o l u t i o n i n sea water, c a l c u l a t e d r a t e s greater than 100 mg/cm /yr are w i t h i n a f a c t o r of two of those observed by Berner and Morse ( 3 ) . Even c a l c u ­ l a t e d r a t e s of c a l c i t e c r y s t a l growth a t high PCO2 are w i t h i n a f a c t o r of two o f the observed ( 3 4 ) . At low PCO2 and nearer e q u i l i b r i u m , however, d i s c r e p a n c i e s i n c a l c u l a t e d r a t e s of d i s s o l u t i o n and p r e c i p i t a t i o n become s i g ­ n i f i c a n t , w i t h c a l c u l a t e d r a t e s , based on the assumption of s u r ­ face PC0 = bulk f l u i d PC0 , g e n e r a l l y f a s t e r than the observed r a t e . C a l c u l a t i o n s show that near e q u i l i b r i u m , c a l c u l a t e d r a t e s of d i s s o l u t i o n and p r e c i p i t a t i o n are extremely s e n s i t i v e to s u r ­ face pH. I t seems l i k e l y that our e a r l i e r p r e d i c t i o n that s u r ­ face PC0 i s equal t o the bulk f l u i d value (1) i s only v a l i d a t r e l a t i v e l y high PC0 (PC0 >0.03 atm). Surface PC0 depends i n part on the f l u x of CO2 t o the s u r ­ face during d i s s o l u t i o n and from the surface during p r e c i p i t a t i o n . P o s s i b l e k i n e t i c problems r e l a t e d t o the CO2 f l u x are discussed elsewhere (37, 4·). During d i s s o l u t i o n , surface PCO2 i s expected to be s l i g h t l y l e s s than the bulk f l u i d v a l u e , and during p r e c i p i ­ t a t i o n , surface PC0 should be greater than the bulk f l u i d v a l u e . This e f f e c t i s most n o t i c a b l e when PC0 of the bulk f l u i d i s low (0.1*6 (San F r a n c i s c o B a y ) .

a

Ammonium acetate

Hydroxylamine

Ammonium oxalate

acid

nitric

Acetic

Con

Zinc extractant

Table IV C o r r e l a t i o n c o e f f i c i e n t s showing t h e r e l a t i o n o f Zn t o Fe, Mn, organic carbon and p a r t i c l e s i z e i n sediments e x t r a c t e d w i t h e i t h e r a c e t i c a c i d , hydroxylamine hydro­ c h l o r i d e i n 0 . 0 1 Ν n i t r i c a c i d , ammonium o x a l a t e , ammonium a c e t a t e , o r concentrated n i t r i c a c i d . E n g l i s h data were l o g transformed f o r s t a t i s t i c a l a n a l y s i s .

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

26.

LUOMA

AND BRYAN

591

Trace Metal Bioavailability

t o overwhelm any c o n t r o l o f organics and p a r t i c l e s i z e on Zn concentrations i n t h e sediments.

Biological Availability. D i r e c t E x t r a c t i o n o f A v a i l a b l e Form, Concentrations of Zn i n S c r o b i c u l a r i a c o r r e l a t e d at a high l e v e l o f s i g n i f ­ icance w i t h t h e wide range o f e x t r a c t a b l e Zn concentrations i n the sediments o f t h e E n g l i s h e s t u a r i e s (Table V). W i t h i n a s i n g l e estuary (San F r a n c i s c o Bay) there was no s i g n i f i c a n t

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

Table V C o r r e l a t i o n c o e f f i c i e n t s d e s c r i b i n g t h e r e l a t i o n s h i p between t h e c o n c e n t r a t i o n o f Zn e x t r a c t e d from sediments and t h e concentra­ t i o n o f Zn observed i n Macoma (San F r a n c i s c o Bay) and S c r o b i c u l a r i a (Southwest England).

Extractant Con n i t r i c acid Hydrochloric a c i d Ammonium o x a l a t e Hydroxylamine hydrochloride Ammonium acetate Ammonia

C o r r e l a t i o n w i t h Zn concentrations i n clam San F r a n c i s c o Bay Southwest England 0.09 0.10 nd

0.U8** 0Λ8** nd

0.33 0.20 nd

0.37*

0.62**

** p0.39- For San F r a n c i s c o Bay (n=28) r > 0.k6. * p< 0.05. For E n g l i s h data r >0.30; f o r San Fran­ c i s c o Bay r >0.36. nd not determined. c o r r e l a t i o n between Zn e x t r a c t e d from t h e sediments and Zn i n t h e animals. The strongest c o r r e l a t i o n s w i t h t h e animals were ob­ served f o r t h e ammonium a c e t a t e - s o l u b l e f r a c t i o n o f Zn i n t h e sediments ( E n g l i s h data) and f o r t h e two e x t r a c t a n t s which d i s ­ solve amorphic oxides o f i r o n and manganese (oxalate i n England and hydroxylamine h y d r o c h l o r i d e i n San F r a n c i s c o Bay). The d i r e c t e x t r a c t i o n methods e x p l a i n l e s s than 12 percent o f t h e v a r i a n c e i n t h e Zn concentrations o f Macoma, and, d e s p i t e t h e i r s t a t i s t i ­ c a l s i g n i f i c a n c e , l e s s than h-0 percent o f t h e v a r i a n c e i n Scrobicularia. Influence o f Substrate Concentrations. San F r a n c i s c o Bay. To assess t h e i n f l u e n c e o f Zn p a r t ­ i t i o n i n g i n t h e sediments on uptake, r e g r e s s i o n s were c a l c u l a t e d between t h e r a t i o d e s c r i b i n g Zn b i o a v a i l a b i l i t y (Equation l ) and

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

592

CHEMICAL

MODELING

IN

AQUEOUS

SYSTEMS

concentrations of t o t a l organic carbon, i r o n oxides and manganese oxides (the l a t t e r two substrates as e x t r a c t e d by v a r i o u s t e c h n i q u e s ) . Among a l l combinations o f v a r i a b l e s , the strongest c o r r e l a t i o n was observed f o r the r e g r e s s i o n y where y and

x

= a + b Χ

(2)

λ

= the b i o a v a i l a b i l i t y r a t i o (Equation

l),

Fe

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

X

1

=

Hydam

^AmAc where Fen^dam t h e c o n c e n t r a t i o n o f Fe removed by hydroxylamine h y d r o c h l o r i d e , and Mn/^c i s t h e c o n c e n t r a t i o n o f Mn removed by ammonium acetate (Figure 3 ) . The c o r r e l a t i o n c o e f f i c i e n t was highly s e n s i t i v e t o the forms o f Mn and Fe employed i n t h e r e g r e s s i o n . Hydroxylamine-soluble Mn and a c e t i c a c i d - s o l u b l e forms o f both Fe and Mn showed a weak r e l a t i o n s h i p t o the b i o a v a i l a b i l i t y of Zn. The v a r i a b l e Χχ i n c r e a s e d as the f r a c t i o n o f Mn e x t r a c t e d by ammonium acetate d e c l i n e d toward the head of the estuary. The concentrations o f Mn removed by other e x t r a c t a n t s d i d not d e c l i n e r a p i d l y upstream. At the upstream s t a t i o n s the concentrations o f Zn i n Macoma r e l a t i v e t o t h a t i n the sediments was a l s o high. Although t h i s suggested t h e v a r i a t i o n s i n X± may be r e l a t e d t o s a l i n i t y , no s i g n i f i c a n t c o r r e l a t i o n was found between b i o a v a i l ­ a b i l i t y and s a l i n i t y . Moreover, both X j and b i o a v a i l a b i l i t y were highest at a s t a t i o n o u t s i d e t h e mouth of the e s t u a r y , where the s a l i n i t y was near t h a t of seawater. The i n v e r s e c o r r e l a t i o n between t h e b i o a v a i l a b i l i t y o f Zn and Μη/^_ was r e l a t i v e l y weak (r=0.hl). The p o s i t i v e r e s i d u a l s of t h i s r e l a t i o n s h i p were l a r g e l y data c o l l e c t e d during the w i n t e r . The w i n t e r i n f l u x o f f r e s h water i n t o San F r a n c i s c o Bay was accompanied by an i n c r e a s e i n t h e sediments o f hydroxylaminee x t r a c t a b l e Fe (presumably, f r e s h l y p r e c i p i t a t e d Fe) and humic substances (27). We have data from too few San F r a n c i s c o Bay s t a t i o n s t o i n c l u d e the humics i n r e g r e s s i o n c a l c u l a t i o n s . How­ ever, the i n c r e a s e i n hydroxylamine-Fe g e n e r a l l y c o i n c i d e d w i t h increases i n Zn concentrations i n Macoma; thus, t h e combined Fe/Mn r a t i o i n Equation 2 explained 60 percent of the temporal and s p a t i a l v a r i a n c e i n the Zn concentrations o f the b i v a l v e when a l l the data were considered. The l a r g e s t negative r e s i d u a l s i n F i g u r e 3 represented s t a ­ t i o n s w i t h high concentrations of t o t a l organic carbon, suggesting Zn concentrations i n Macoma were lower than expected from the Fe/Mn r a t i o where t o t a l organic carbon concentrations were e l e v a t e d . Organic carbon concentrations were i n c l u d e d i n t h e independent v a r i a b l e o f the r e g r e s s i o n . i s

0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

LUOMA

AND BRYAN

Trace Metal Bioavailability

593

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

26.

Figure 3. The bioavailability of zinc (Zn /Zn t) to Macoma balthica as related to the ratio of hydroxylamine-soluble iron/ammonium acetate-soluble manganese in oxidized sediments of San Francisco Bay (r = 0.77). rlnm

SC(Mmen

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

594

CHEMICAL

y

±

= a + b X

MODELING

IN

AQUEOUS

SYSTEMS

(3)

2

where y^ = t h e b i o a v a i l a b i l i t y r a t i o (Equation l ) , and

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

X

2

=

^dar/^AmAc)

X

carbon).

The c o r r e l a t i o n c o e f f i c i e n t o f t h i s r e g r e s s i o n was s l i g h t l y higher ( r = 0 . 8 3 vs r = 0 . 7 T ) than when Fe/Mn alone was employed as the v a r i a b l e . The r e s i d u a l s o f the r e g r e s s i o n suggested an overestimâtion o f Zn b i o a v a i l a b i l i t y at the lowest carbon concen­ t r a t i o n s - i . e . > the negative e f f e c t o f carbon on b i o a v a i l a b i l i t y appeared t o occur only a t high l e v e l s o f carbon. The e f f e c t o f e x c l u d i n g low organic carbon concentrations from the r e l a t i o n ­ s h i p was t e s t e d by comparing c o r r e l a t i o n c o e f f i c i e n t s when carbon values below 0 . 6 , 1 . 0 , 1 . 5 , or 3 . 8 percent were removed from c a l c u ­ l a t i o n of the X^ v a r i a b l e . This was accomplished by n o r m a l i z i n g a l l carbon concentrations t o one o f the above and assuming con­ c e n t r a t i o n s lower than t h e chosen value were t o equal one ( i . e . a l l carbon values lower than t h e n o r m a l i z i n g number had no e f f e c t on the value o f the v a r i a b l e ) . The maximum c o r r e l a t i o n c o e f f i ­ c i e n t was observed when organic carbon was normalized t o 1 . 0 percent ( F i g u r e h). Normalizing t o carbon l e v e l s lower than 1 . 0 percent p r o g r e s s i v e l y d e t r a c t e d from t h e r e l a t i o n s h i p , w h i l e f a i l u r e t o i n c l u d e the higher carbon l e v e l s a l s o r e s u l t e d i n a less-tnan-optimum c o r r e l a t i o n . When organic carbon was normal­ i z e d t o 1 . 0 percent the c o r r e l a t i o n c o e f f i c i e n t ( r = 0 . 9 6 ) suggest­ ed 92 percent o f the v a r i a n c e i n the b i o l o g i c a l a v a i l a b i l i t y o f Zn t o Macoma c o u l d be a t t r i b u t e d t o v a r i a t i o n s i n concentrations of Fe, Mn and organic carbon i n the sediments (Figure 5 ) . The c o r r e l a t i o n between the denominator o f t h e b i o a v a i l a b i l i t y r a t i o (^ ^i ^ ) 3- v a r i a b l e X^ was weak ( r . = 0 . 5 l ) ; t h u s , most o f the v a r i a n c e i n the Zn c o n c e n t r a t i o n s o f M. b a l t h i c a appeared t o be due t o v a r i a t i o n s i n the r a t i o o f i r o n , manganese and organic carbon. I f t h i s r a t i o i s i n d i c a t i v e of the p a r t i t i o n i n g of Zn i n sediments,then n e a r l y a l l the v a r i a t i o n i n Zn concentrations i n Macoma may be e x p l a i n e d by the form o f Zn i n the sediments. Southwest England. The e f f e c t s o f sedimentary v a r i a b l e s on Zn concentrations i n S c r o b i c u l a r i a were i n i t i a l l y t e s t e d by m u l t i p l e r e g r e s s i o n . Values o f the F - s t a t i s t i c were c a l c u l a t e d to determine the l e v e l o f s i g n i f i c a n c e at which sedimentary v a r i a b l e s e x p l a i n e d Zn l e v e l s i n the clam (Table V I ) . S e v e r a l d i f f e r e n t r e g r e s s i o n equations were c a l c u l a t e d t o compare the d i f f e r e n t methods o f e x t r a c t i n g Zn, Mn and Fe. The m u l t i p l e r e g r e s s i o n analyses employed only the 29 s t a t i o n s at which seaweed was found. In the f i r s t run o f the m u l t i p l e r e g r e s s i o n , o n l y the t h r e e v a r i a b l e (Fe, Mn, organic carbon) i n c l u d e d i n the San F r a n c i s c o Bay study were analyzed (.Table V I ) . The c o n c e n t r a t i o n of Zn, Mn and organic carbon i n the sediments explained kg percent o f the n

an(

5 e

m e ] Q

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

LUOMA

AND BRYAN

Trace Metal Bioavailability

595

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

1.00-1

0.85H

080-j

?5

?0

70 NORMALIZING

VALUE

?5

?0

?5

(MINIMUM % EFFECTIVE CARION)

Figure 4. Correlation coefficients for the relationship between the bioavailability of zinc to M. balthica and the sedimentary variable, X = (Fe m/Mn ) (1/ organic carbon), graphed as a function of the value to which organic carbon concentrations were normalized. All values less than the normalized value were assumed to equal one in the calculation. 2

Hyda

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

AmAc

CHEMICAL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

596

CF

Ε Hydom

/

ΜΝ

AmAc 3

χ

MODELING IN AQUEOUS

CI/CO

SYSTEMS

I N SEDIMENT

Figure 5. The bioavaihbility of zinc to Macoma balthica (Zn i /Zn nt) as related to the ratio in San Francisco Bay sediments of (Fe/Mn) χ (1/C ) where C signifies only organic carbon concentrations of greater than 1 % are included in the calculation (r = 0.96). c am

sedime

N

N

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

11.2** 0.01 6.77**

1+.68*

i+.00*

5.UU**

5.65**

6.09**

5.0U*

2.73

2.76

1Λ6

1.58

3.13*

(-)

Carbonate carbon

3.7U*

1+.69*

6.92**

8.39**

9.22**

(+)

Humic substances

6.76**

7.50**

2.kh 2.78

5.03*

U.U2*

5.80**

(+)

Solute Zn

0.67

0.91

2.21

(+)

Extractable Cd

a

* F^values >3.08= p< 0 . 0 5 * ** F-value >5-29= p < 0 . 0 1 * * (+) and (-) i n d i c a t e the s i g n of the slope taken by t h e v a r i a b l e . Only the v a r i a b l e s used i n San F r a n c i s c o Bay study were i n c l u d e d i n t h i s m u l t i p l e r e g r e s ­ s i o n r u n t o f a c i l i t a t e comparison between t h e study areas.

U9-0

1.59

62.2

0.07

0.00

1.6l

6U.2

a

0.03 0 . 2 0

66.6

o.Uo 6 . 0 0 * *

3.66*

0.05

+

Ο

7.10**

cd -P ο EH

ι—I

6.53**

ω -Ρ cd cd M ο

66.9

Η αϊ -Ρ Ο ΕΗ

71.1

CD -Ρ cd Η

9.01**

-Ρ cd Η cd Ο T+T

Iron

Γ (-)

J

, ο !±| cd ~+~

Zinc

0.01

2

CD •Η -Ρ cd Ο -Ρ CD Ο cd ΓίΤ" (+)

r

F-value s Organic Manganese carbon

Table VI Values o f the F - s t a t i s t i c c a l c u l a t e d from a m u l t i p l e r e g r e s s i o n o f Zn c o n c e n t r a t i o n i n S c r o b i c u l a r i a p l a n a versus t h e combinations o f v a r i a b l e s l i s t e d . Percent v a r i a n c e ( r ) e x p l a i n e d by the r e g r e s s i o n i s a l s o sho-wn.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

CHEMICAL

598

MODELING

IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

variance i n S c r o b i c u l a r i a . More o f t h e v a r i a n c e i n S c r o h i c u l a r i a •was explained by ammonium acetate than when other e x t r a c t a n t s were employed (Table V I ) . T o t a l organic carbon had a s i g n i f i c a n t neg­ a t i v e e f f e c t , t h e c o n c e n t r a t i o n o f humic substances a p o s i t i v e e f f e c t , and s o l u t e concentrations o f Zn a p o s i t i v e e f f e c t . Oxala t e - s o l u b l e Mn had a weak p o s i t i v e e f f e c t , although n e i t h e r t o t a l Mn nor ammonium a c e t a t e - s o l u b l e Mn s i g n i f i c a n t l y a f f e c t e d t h e r e ­ g r e s s i o n . Carbonates had a weakly s i g n i f i c a n t e f f e c t ( n e g a t i v e ) , i n one combination o f v a r i a b l e s . N e i t h e r Cd nor Fe showed any e f f e c t detectable by m u l t i p l e r e g r e s s i o n . The r e g r e s s i o n equa­ t i o n y i e l d i n g t h e highest c o r r e l a t i o n c o e f f i c i e n t (r=0.8U3) was

log

Zn +

g c r o b e

°-

= 0 . 0 3 + 0.30 ( l o g H) - 0 . 5 9 ( l o g C)

1 9

Z

l

+ 0.17 (log M n

'

+

W

(h)

^solute)

λ 22

) - 0 . 0 8 ( l o g CO )

Q x a l

where Η = t h e c o n c e n t r a t i o n o f humic substances (absorbance/ g sediment) and C = t o t a l organic carbon (percent). Using data from a l l ho s t a t i o n s , simple r e g r e s s i o n s were a l s o c a l c u l a t e d f o r the equation log y where

2

(5)

= l o g a + (b) ( l o g X ) 3

y^ = t h e c o n c e n t r a t i o n o f Zn i n S c r o b i c u l a r i a and X

3

=

Zn

^ sediment^

(

X

S

u

"

b

s

t

r

a

t

e

1

/

S

u

"

b

s

t

r

a

t

e

2)

f o r a l l combinations o f t h e Fe, Mh, organic carbon, carbonate and humic m a t e r i a l s u b s t r a t e s . These c a l c u l a t i o n s employed a l l 36 data p o i n t s . The r e s u l t s were s i m i l a r t o those c a l c u l a t e d by m u l t i p l e r e g r e s s i o n (Table V I I ) . The c o r r e l a t i o n was strongest when t h e f r a c t i o n o f Zn e x t r a c t e d by ammonium acetate was employed (Figure 6 a ) . When t h e product o f ammonium a c e t a t e s o l u b l e Zn and t h e r a t i o o f humic substance concentrations t o t o t a l organic carbon was employed as X i . e . 9

X

3

=

x

( H / C )

( 6 )

6 7 . I percent ( r = 0 . 8 l 9 ) o f t h e v a r i a n c e i n S c r o b i c u l a r i a was e x p l a i n e d ( F i g u r e 6 b ) . When t h e r a t i o s Mn/C and MnC0 were included with i n X t h e c o r r e l a t i o n was a l s o improved over t h a t between Z n ^ ^ a n d ΖΠ i n the b i v a l v e (Table V I I ) . I n c l u d i n g Mh w i t h t h e organic r a t i o i n a more complex independent v a r i a b l e 3

Z n

A m A c

x

* •

(

Z

l

W

x

(H/C)

x

i^w*

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(7)

LUOMA

26.

Trace Metal Bioavailability

A N D BRYAN

599

Table V I I C o r r e l a t i o n c o e f f i c i e n t s showing r e l a t i o n o f Zn con­ c e n t r a t i o n s i n S c r o b i c u l a r i a t o combinations o f s e d i ­ mentary v a r i a b l e s , i n t h e equation l o g y = l o g a + (b) ( l o g X ) where y = Zn c o n c e n t r a t i o n s i n S c r o b i c u l a r i a and XQ = (Zn ) (substrate / s u b s t r a t e ). X w ζ Headings are f a c t o r s which make up X3 v a r i a b l e . 3

Ό

Substrate w Substrate ζ

Zn

0.5^

0, .36

0.39

0. .37

3

O.kk

0. .22

0.25

0, .25

6

0.58

0, ,36

0Λ2

0, .32

Fe^/Carbon

0.6l

0, Λ 6

0.50*

0, Λ6

Fe^/Carbon

0.62

0, Λ8

0.53*

0, .kk

Mn /Carbon

0.59

0. M

0.51*

0, Λ3

Mn^/Carbon

0.66*

0, . 5 5 *

0.59*

0, . 5 2 *

Mn^/Carbon

Ο.62

0, .51

0.55*

0, Λ 8

Fe /C0

3

0.6l

0. Λ 8

0.51*

0, Λ 5

Fe /C0

3

0.60

0, Λ 6

0.Λ9*

0, .kk

Mn /C0

k

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

Zn^

Zn,

Fe /m

i

Fe /Mn i|

Fe /Mn 6

1

u

6

Zn

1

0

3

0

3

0.6U*

0. .52

Ο.56*

0, . 5 0 *

Mn /C0

3

0.68*

0. . 5 5 *

0.58*

0, . 5 ^ *

Mn /C0

3

0.6k*

0, .52

0.55*

0, . 5 1 *

humi c s/organi c carbon

0.82*

0, . 6 0 *

0.68*

0, . 5 0 *

humics/carbonate

0.55

0, ,ko

O.kk

0, .37

1

u

6

C o e f f i c i e n t s exceeding those between S c r o b i c u l a r i a and sedimentary Zn alone (from Table I I ) . S u b s c r i p t i d e n t i f i c a t i o n numbers a r e : l=ammonium a c e t a t e ; 3=hydrochloric a c i d ; U=oxalate; 6=concentrated n i t r i c acid. d i d not a f f e c t t h e c o r r e l a t i o n c o e f f i c i e n t . Concentrations o f Zn i n S c r o b i c u l a r i a were higher than p r e d i c t e d by t h e independent v a r i a b l e i n Equation 6 at s e v e r a l s i t e s w i t h v e r y h i g h Mn concen­ t r a t i o n s . A s l i g h t improvement i n the c o r r e l a t i o n c o e f f i c i e n t ( r = 0 . 8 U 2 ; r = 0 . 7 0 9 ; F i g u r e 6 c ) was observed when only the h i g h Mn values were i n s e r t e d i n Equation 5 by n o r m a l i z i n g Mn concen­ t r a t i o n s t o 350 yg/g t o exclude the lower values (assuming a l l concentrations l e s s than 350 yg/g took t h e value o f 1 i n v a r i a b l e X 4 , Equation 7 ) .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

600

MODELING

IN AQUEOUS

SYSTEMS

10000, 0> \

D


350 pg/g (r == 0.84). AmAc

oxal

oxal

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

LUOMA

AND BRYAN

Trace Metal Bioavailability

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

26.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

601

602

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

The r e s i d u a l s o f t h e r e l a t i o n s h i p i n Figure 6c suggested s a l i n i t y a l s o had some e f f e c t on Zn concentrations i n S c r o b i ­ c u l a r i a . S t a t i o n s from near t h e heads o f t h e e s t u a r i e s (with one exception) had high p o s i t i v e r e s i d u a l s , w h i l e those s t a t i o n s from near t h e mouths o f t h e e s t u a r i e s g e n e r a l l y had negative r e s i d u a l s . Because o f evaporation and t h e t r a n s i e n t nature o f s a l i n i t i e s i n the s m a l l E n g l i s h e s t u a r i e s , i t i s d i f f i c u l t t o r e l i a b l y determine t h e s a l i n i t y S c r o b i c u l a r i a was exposed t o on the mudflat. Therefore, t h e s a l i n i t y e f f e c t w i l l r e q u i r e further investigation.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

Discussion To date, attempts t o d e f i n e t h e b i o l o g i c a l l y a v a i l a b l e f r a c t i o n o f p a r t i c u l a t e - b o u n d metals have l a r g e l y bypassed any d i r e c t c o n s i d e r a t i o n o f metal p a r t i t i o n i n g among physicochemical forms. I n s t e a d , e x t r a c t a n t s have been sought t h a t simulate b i o ­ l o g i c a l processes, d i r e c t l y removing t h e b i o l o g i c a l l y a v a i l a b l e f r a c t i o n o f metals. Some c o r r e l a t i o n between metal concentra­ t i o n s e x t r a c t e d from s o i l s and concentrations i n p l a n t s have been observed w i t h i n s o i l s e r i e s (e.g. ammonium acetate e x t r a c ­ t i o n o f Zn; DTPA e x t r a c t i o n o f Cu (30 j )?.but among d i f f e r e n t types of s o i l s these r e l a t i o n s h i p s break down. The search f o r a u n i ­ v e r s a l extractant f o r the b i o l o g i c a l l y a v a i l a b l e f r a c t i o n of metals has r e c e n t l y been extended t o r a t i o n a l i z e t h e development of complex e x t r a c t i o n schemes f o r aquatic sediments (31 32)· Luoma and Jenne (2k) used l e s s i n v o l v e d methodology t o show some c o r r e l a t i o n between e x t r a c t a b l e n u c l i d e concentrations and t h e uptake by Macoma o f Z n , C d , C o and A g from l a b o r a t o r y prepared sediments. A c l o s e c o r r e l a t i o n between ammonium ace­ t a t e - s o l u b i l i t y and Z n uptake was observed. I n nature, ammonium acetate a l s o r e f l e c t e d the b i o a v a i l a b i l i t y o f Zn t o S c r o b i c u l a r i a i n the e s t u a r i e s of southwest England b e t t e r than d i d other e x t r a c t a n t s . The c o r r e l a t i o n was not s u f f i c i e n t l y strong t o have p r e d i c t i v e v a l u e , however, and w i t h i n the narrow­ er range o f Zn concentrations i n San F r a n c i s c o Bay no c o r r e l a t i o n was observed. The e x c h a n g e a b i l i t y o f Zn, i m p l i e d by ammonium acetate e x t r a c t i o n , may i n f l u e n c e t h e a v a i l a b i l i t y o f t h e m e t a l , but i t i s not the only f a c t o r determining uptake. To d i r e c t l y assess t h e e f f e c t o f Zn p a r t i t i o n i n g i n s e d i ­ ments on uptake, we have assumed p a r t i t i o n i n g , and, i n response, a v a i l a b i l i t y w i l l change w i t h changes i n t h e concentrations o f substrates which b i n d Zn. Despite t h i s obvious o v e r s i m p l i f i c a ­ t i o n we have found r a t i o s o f sedimentary substrate concentra­ t i o n s t h a t e x p l a i n a very l a r g e p r o p o r t i o n o f t h e v a r i a n c e i n e i t h e r the b i o a v a i l a b i l i t y or t h e concentration o f Zn i n two species o f b i v a l v e s . We have reported s i m i l a r r e s u l t s f o r l e a d (.33). Although t h e r a t i o s o f substrates which p r e d i c t e d Zn uptake d i f f e r e d between southwest England and San F r a n c i s c o Bay, most o f those d i f f e r e n c e s were c o n s i s t e n t w i t h chemical 5

6 5

1 0 9

6 0

l l o m

6 5

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch026

26.

LUOMA

AND

BRYAN

Trace Metal Bioavailability

603

d i f f e r e n c e s between the areas. Substrate concentrations suggested i n o r g a n i c oxides were more dominant than organic m a t e r i a l s i n the sediments of San F r a n c i s c o Bay, w h i l e organic m a t e r i a l s and carbonates occurred i n h i g h concentrations i n the E n g l i s h e s t u a r i e s . D i s t r i b u t i o n of two oxide forms i n the s e d i ­ ments e x p l a i n e d most changes i n the a v a i l a b i l i t y of Zn t o Macoma i n the former e s t u a r y , w i t h a negative e f f e c t on a v a i l a b i l i t y o c c u r r i n g only at higher concentrations (we w i l l consider the mathematical models that have been developed and the model parameters required to predict concentration changes in the animals. In addition, experimental data w i l l be provided that indicate the sensitivity of model parameters to differences i n physicochemical form of the elements in the water and to differences in metabolic responses among species. These kinds of 0-8412-0479-9/79/47-093-611$06.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

612

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

i n f o r m a t i o n w i l l p r o v i d e an i n d i c a t i o n o f t h e r e l i a b i l i t y o f m o d e l p r e d i c t i o n s and o f the a r e a s i n w h i c h a d d i t i o n a l d a t a a r e needed.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

M a t h e m a t i c a l M o d e l s and M o d e l

Parameters

Mathematical Models. The a c c u m u l a t i o n o f an e l e m e n t by any pathway c a n i n v o l v e a number o f d i f f e r e n t p r o c e s s e s . I f the r a t e - d e t e r m i n i n g p r o c e s s can be d e s c r i b e d m a t h e m a t i c a l l y , a model c a n b e d e v e l o p e d to p r e d i c t c h a n g e s i n c o n c e n t r a t i o n w i t h t i m e and l o c a t i o n . A c o n s i d e r a b l e e f f o r t h a s b e e n made to d e v e l o p m o d e l s to p r e d i c t the d i s t r i b u t i o n o f r a d i o n u c l i d e s r e l e a s e d i n t o the e n v i r o n m e n t (_15). The t y p e s o f m o d e l s d e v e l o p e d to p r e d i c t c o n c e n t r a t i o n s o f r a d i o n u c l i d e s i n a q u a t i c organisms i n c l u d e e q u i l i b r i u m (lj>, 17_, 18) and dynamic m o d e l s ( 1 £ > 2 0 ) . The t y p e o f m o d e l to b e u s e d i n a g i v e n s i t u a t i o n d e p e n d s on the n a t u r e o f the r e l e a s e and on the p r o p e r t i e s o f t h e ecosystem. When r e l e a s e s o f m e t a l s or r a d i o n u c l i d e s are c o n t i n u o u s and s t e a d y - s t a t e c o n d i t i o n s a r e p r e s e n t , an e q u i l i b r i u m m o d e l s u c h as a c o n c e n t r a t i o n f a c t o r m o d e l c a n b e used. I n t h i s c a s e , t h e i m p o r t a n t p a r a m e t e r needed f o r the m o d e l i s the c o n c e n t r a t i o n f a c t o r , t h e r a t i o o f the c o n c e n t r a t i o n i n t h e a n i m a l to t h a t i n t h e w a t e r . The c o n c e n t r a t i o n i n t h e a n i m a l i s d e t e r m i n e d t h e n by m u l t i p l y i n g the c o n c e n t r a t i o n i n the w a t e r by t h e c o n c e n t r a t i o n f a c t o r . I n the c a s e o f a c c i d e n t a l e p i s o d i c r e l e a s e s , a dynamic s i t u a t i o n e x i s t s i n which organisms a c c u m u l a t e the m a t e r i a l f o r a r e l a t i v e l y s h o r t p e r i o d and t h e n lose i t with a c h a r a c t e r i s t i c time c o n s t a n t . In this case,the i m p o r t a n t p a r a m e t e r s needed f o r the m o d e l a r e b i o l o g i c a l t u r n o v e r r a t e c o n s t a n t s and c o n c e n t r a t i o n factors. Model Parameters. C o n c e n t r a t i o n f a c t o r s were d e t e r m i n e d f o r l a r g e numbers o f b i v a l v e m o l l u s c s . V a l u e s were o b t a i n e d by d e t e r m i n i n g the c o n c e n t r a t i o n o f s t a b l e a n d / o r r a d i o a c t i v e n u c l i d e s i n a n i m a l s and w a t e r t h a t were c o l l e c t e d d i r e c t l y f r o m t h e e n v i r o n m e n t and t h a t were s a m p l e d i n l a b o r a t o r y e x p e r i m e n t s . T h e s e d a t a were c o m p i l e d f o r u s e i n m o d e l s to p r e d i c t r a d i o n u c l i d e c o n c e n t r a t i o n s i n whole organisms or t h e i r t i s s u e s . In g e n e r a l , f o r b i v a l v e m o l l u s c s a s i n g l e v a l u e i s g i v e n f o r each element. The t u r n o v e r o f t r a c e m e t a l s i n o r g a n i s m s d e p e n d s on d y n a m i c p r o c e s s e s o f exchange w i t h elements i n the e n v i r o n m e n t . Compartments o f e l e m e n t s a r e i d e n t i f i e d f r o m a m a t h e m a t i c a l a n a l y s i s o f the changes i n c o n c e n t r a t i o n d u r i n g a c c u m u l a t i o n or loss. The r e s o l u t i o n o f c o m p a r t m e n t s i s l i m i t e d by e x p e r i m e n t a l e r r o r , and the c o m p a r t m e n t s t h a t c a n be i d e n t i f i e d a r e t h o s e whose c o n c e n t r a t i o n s d i f f e r s i g n i f i c a n t l y i n t h e i r e x p o n e n t i a l change. T h e s e c o m p a r t m e n t s may be p h y s i o l o g i c a l , s t r u c t u r a l , o r c h e m i c a l e n t i t i e s , and t h e i r m e t a b o l i c r o l e s may n o t b e known. The t u r n o v e r t i m e o f an e l e m e n t i n an o r g a n i s m depends upon t h e o r g a n i s m and e l e m e n t . In small organisms w i t h a l a r g e

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

Physiochemical Form of Trace Metals

HARRISON

613

surface-to-volume r a t i o , the turnover time o f monovalent elements such as Na or Cs may be minutes, whereas i n large organisms and m u l t i v a l e n t elements, months may be r e q u i r e d . The turnover i s a l s o a f u n c t i o n o f the metabolism o f the element by the organism. The q u a n t i t i e s accumulated by organisms when the concentrations are increased i n the water d i f f e r g r e a t l y f o r those elements that are and are not under homeostatic c o n t r o l . Turnover r a t e s were determined by f o l l o w i n g the accumulation or l o s s o f r a d i o n u c l i d e s from animals i n the f i e l d or i n the l a b o r a t o r y . They were determined also by f o l l o w i n g the increased q u a n t i t i e s o f trace metals i n animals that were exposed to increased q u a n t i t i e s o f the element i n the water. The change i n concentration o f a trace element i n an organism at any time may be described by: dC/dt = k£W(t) - k C ( t ) ,

(1)

0

where W(t) C k£ = k = Q

= the concentration i n the water at time, t , = the concentration i n the organism, the b i o l o g i c a l accumulation rate constant, the b i o l o g i c a l loss rate constant.

At steady-state c o n d i t i o n s , dC/dt = 0, and k W = k C(s), £

(2)

Q

where C(s) = the concentration conditions.

i n the organism a t steady state

Assuming f i r s t order k i n e t i c s and a constant concentration i n the water, equation 1 upon i n t e g r a t i o n becomes: k.W -k t C(t) = [1 - e ° ] . (3) ο S u b s t i t u t i n g C(s) = k|W/k i n t o equation 3 , we have: Q

C(t) = C(s) [1 - e " * ^ ] .

(4)

In those s i t u a t i o n s where the c o n c e n t r a t i o n i n the water i s known, concentration f a c t o r s (CF) can be s u b s t i t u t e d f o r concentrations i n the animal to g i v e : -k t CF(t) = CF(s) [1 - e ° ] (5) (

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

614

CHEMICAL

MODELING

IN

AQUEOUS

SYSTEMS

where CF(t) = the c o n c e n t r a t i o n f a c t o r i n the organism a t time, t , CF(s) = the c o n c e n t r a t i o n f a c t o r i n the organism a t steady-state c o n d i t i o n s . The l o s s of stable or r a d i o a c t i v e n u c l i d e s may be described by: C(t) = C ( i ) [ e "

k o t

],

(6)

where

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

C(i)

= the i n i t i a l c o n c e n t r a t i o n i n the organism.

From k , the b i o l o g i c a l ha I f - l i f e (T%) o f a trace element i n an organism may be determined from the r e l a t i o n s h i p : 0

k ο

V a r i a t i o n s i n Model Parameters R e s u l t i n g from Species D i f f e r e n c e s Whole Body Radionuclide Concentration Factors and Turnover Rates. A s e r i e s o f experiments were performed to determine the turnover r a t e s and c o n c e n t r a t i o n f a c t o r s o f Co, Cs, Mn, and Zn i n the marine clam Mya a r e n a r i a and the oyster Crassostrea gigas. The r e s u l t s published p r e v i o u s l y (21, 2_2) are included f o r comparison. Radionuclide accumulation was followed i n l a b o r a t o r y systems i n which the concentrations of s t a b l e elements were kept constant (220. Changes i n concentrations were followed i n c r i t i c a l t i s s u e s as w e l l as the e n t i r e body. The loss of r a d i o n u c l i d e s was followed i n animals that had accumulated the r a d i o n u c l i d e s for 48 d i n the l a b o r a t o r y . A f t e r exposure to the s t a b l e and r a d i o a c t i v e n u c l i d e s , they were t r a n s f e r r e d to n o n r a d i o a c t i v e , u n f i l t e r e d , c i r c u l a t i n g seawater(at the Marine Laboratory o f C a l i f o r n i a State U n i v e r s i t y , Humbolt) i n which b i o l o g i c a l loss of the r a d i o n u c l i d e s was followed. In the oyster C. g i g a s , l a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s and turnover rates o f Co, Cs, Mn, and Zn were found i n the s o f t t i s s u e s (Table I ) . The highest c o n c e n t r a t i o n f a c t o r measured was f o r Zn and the lowest f o r Cs. In the clam M. a r e n a r i a , large d i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s and turnover rates of these elements were a l s o found (Table I ) . Cobalt had the highest c o n c e n t r a t i o n f a c t o r and Cs had the lowest. In both animals, two compartments were i d e n t i f i e d f o r most elements. D i f f e r e n c e s i n the percentages of the element i n each compartment were found for those elements which had two i d e n t i f i a b l e compartments. Comparison of the c o n c e n t r a t i o n f a c t o r s f o r Mn and Co shows more than an order of magnitude

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

27.

HARRISON

615

Physiochemical Form of Trace Metals

d i f f e r e n c e between the values for oysters and clams. Greater s i m i l a r i t y was found i n turnover r a t e s than c o n c e n t r a t i o n f a c t o r s ; Co, Mn, and Zn had turnover times o f 2 to 3 mo i n both TABLE I C o n c e n t r a t i o n F a c t o r s and H a l f - L i v e s i n S o f t Tissues

Concentration Factor

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Element Zinc Cobalt (_22) Manganese Cesium (22)

1200 50 35 10

Cobalt (21) Manganese Zinc Cesium (21)

790 590 320 5

Half-life,d Compartment Compartment 2 1 A. Oysters 98 ( i o o ) 130 (62) 6 (38) 98 (76) 4 (24) 6 (60) 2 (37) Β. Clams 120 (100) 70 (70) 8 (30) 110 (67) 30 (33) 60 (25) 4 (75) a

Value i n p a r e n t h e s i s i s the percent of the t o t a l body element i n t h e compartment.

s p e c i e s , whereas f o r Cs i t was < 1 wk. These data on c o n c e n t r a t i o n f a c t o r s , turnover r a t e s , and compartment s i z e s suggest d i f f e r e n t metabolic pathways o f these elements i n both an ima 1 s. Tissue Turnover o f R a d i o n u c l i d e s . Oysters and clams that had accumulated e i t h e r 54Mn and 65Zn or 60Co and 137 for about lh mo i n the l a b o r a t o r y were t r a n s f e r r e d to u n f i l t e r e d oceanic water, and changes i n concentrations o f the r a d i o n u c l i d e s i n the t i s s u e s were followed for about 5 mo. Large d i f f e r e n c e s were found i n the r a t e s o f r a d i o n u c l i d e s loss from the t i s s u e s o f both the o y s t e r s (Figure l)and the clams (Figure 2 ) . Three p a t t e r n s o f loss during the experimental p e r i o d were observed. From many t i s s u e s , the l o s s was monophasic, from others i t was b i p h a s i c , but i n some there was an increase or a period o f l i t t l e change before the loss occurred. The l a s t p a t t e r n was seen for 65Zn i n some t i s s u e s o f oysters and clams and i n d i c a t e s that z i n c may be m o b i l i z e d from some t i s s u e s f o r accumulation i n others. The accumulation o f M n , C o , Z n , and C s , was followed i n M. a r e n a r i a under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s . The changes i n c o n c e n t r a t i o n f a c t o r s o f the r a d i o n u c l i d e s during accumulation d i f f e r e d i n each t i s s u e , and, i n a given t i s s u e , w i t h each r a d i o n u c l i d e (Figure 3). Only f o r 1 3 7 were steady-state c o n d i t i o n s approached i n a l l the t i s s u e s , even though the accumulation was followed for more than 5 mo. The s c a t t e r i n the data during the sampling p e r i o d was g r e a t e s t i n Cs

54

6 0

6 5

1 3 7

C 8

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Crassostrea gigas Gills

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Muscle MO [\ I I I

I

k

i Λ

10 1 -

ioo U

Ε Ε & i

l

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"

J

J

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0.1

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l

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I I L

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J

I L

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1

1

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1

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I L 150 250

50

150 250

Time, days Figure 1. Percent of initial radioactivity in tissues of Crassastrea gigas during loss period

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

27.

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Physiochemical Form of Trace Metals

HARRISON

Mya arenaria Gills

Muscle

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

100

prn—ι—

10

Ί

\

-

_

1 0.1 1000

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-\

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50 100 150 0

50 100 150 0

J

L

50 100 150 0

J 50

L 100 150 0

50 100 150

Time, days

Figure 2. Percent of initial radioactivity in tissues of Mya arenaria during loss period

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

618

Figure 3.

Concentration factors in tissues of My a arenaria during accumulation period

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

HARRISON

Thysiochemical Form of Trace Metals

619

the d i g e s t i v e gland and stomach and the v i s c e r a . Also i n these t i s s u e s higher f r a c t i o n a l standard d e v i a t i o n s i n the concentrations were found i n the e i g h t animals s a c r i f i c e d a t each sampling time. The turnover r a t e s o f r a d i o n u c l i d e s were measured i n b i v a l v e molluscs both i n the f i e l d and i n the laboratory (21-30). Seymour (27) obtained a b i o l o g i c a l h a l f - l i f e o f 300 d for ^Zn i n C. gigas and Wolfe (7) an e c o l o g i c a l h a I f - l i f e o f 347 d i n C. v i r g i n i c a . F r a z i e r (_10) c a l c u l a t e d a h a l f - l i f e on the order o f 40 to 50 d f o r Zn i n v i r g i n i c a , a value that i s c l o s e r to the 100 d reported here for gigas. Some r e s u l t s i n d i c a t e that the rates obtained i n the f i e l d vary s e a s o n a l l y ; such v a r i a t i o n i s suggested by the turnover r a t e s o f ^Zn i n C. gigas (26) and o f Zn and Cu i n virginica ( 11). F r a z i e r (K)) found that the uptake of metals by oysters t r a n s f e r r e d to contaminated environments depends upon the season. Uptake by Cj_ v i r g i n i c a was r a p i d i n the summer and f a l l , but was low i n the e a r l y s p r i n g . Rates o f accumulation and l o s s may d i f f e r s i g n i f i c a n t l y . George and Coombs (31) followed the accumulation and loss o f Cd i n M y t i l u s e d u l i s . The rate o f Cd uptake was 18 times f a s t e r than that o f e x c r e t i o n . They concluded that the slower e l i m i n a t i o n i s a consequence o f a need to d e t o x i f y and store Cd by an i m m o b i l i z a t i o n mechanism. Whole Body Stable Element Concentration F a c t o r s . Concentrations o f some s t a b l e elements i n populations o f oyster C. gigas and the clam Saxidomus n u t t a l l i were monitored to determine the normal seasonal changes i n concentrations. For each sample, the soft t i s s u e s were separated from the s h e l l , r i n s e d i n seawater, and pooled to give a composite sample. Ten to twelve clams and 50 to 100 oysters were d i s s e c t e d a t each sampling. The wet t i s s u e s were weighed and d r i e d i n an oven a t 103°C f o r at l e a s t 48 h. Samples were weighed a f t e r ashing to constant weight a t 450 °C i n a muffle furnace. Elemental analyses were performed by neutron a c t i v a t i o n (32). The concentration i n the s o f t t i s s u e s o f oysters on an ash weight b a s i s was Zn - Fe > Mn > Co (Table I I ) . The c o n c e n t r a t i o n f a c t o r s o f the s t a b l e elements, Co, Mn, and Zn were higher than those determined by r a d i o n u c l i d e studies i n the laboratory ( c f . Tables I and I I ) . The d i f f e r e n c e s between the r a d i o a c t i v e and s t a b l e n u c l i d e c o n c e n t r a t i o n f a c t o r s may be due to the presence i n the organism of slowly exchangeable or nonexchangeable compartments. Steady-state c o n d i t i o n s o f stable and r a d i o a c t i v e isotopes would not be reached i n such compartments during the period o f most laboratory experiments, and the c o n t r i b u t i o n of these compartments to the concentration f a c t o r would be overlooked. Because most stable element analyses do not d i s t i n g u i s h compartments but give the t o t a l amount i n a l l compartments, the concentration f a c t o r o f a stable isotope would be l a r g e r than

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

620

MODELING IN AQUEOUS

TABLE I I T r a c e M e t a l C o n c e n t r a t i o n s and C o n c e n t r a t i o n i n the Oyster Crassostrea gigas

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Concentration Element

( y g / g wet w e i g h t ) Range Mean

Zinc

100

Manganese Cobalt Iron

10 0.06 100

73-140 3.1-21. 0.008-0.21 38.

-380

Seawater (yg/i) 6 1.2 0.1 3

SYSTEMS

Factors

Concentration Factor 17,000. 8,300. 600. 33,000.

t h a t o f the r a d i o i s o t o p e . O p h e l (_33) i n d i c a t e d t h a t i n some b i o t a s u c h a s i t u a t i o n e x i s t s f o r S r and p e r h a p s f o r C o . D i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s b e t w e e n s t a b l e and r a d i o a c t i v e i s o t o p e s may r e s u l t a l s o f r o m o t h e r f a c t o r s . I f more t h a n one c h e m i c a l f o r m o f t h e e l e m e n t a r e p r e s e n t i n t h e e n v i r o n m e n t , t h e y may b e s u b j e c t to d i f f e r e n t t u r n o v e r and concentration processes. Seasonal effects could r e s u l t i f d i f f e r e n t p h y s i c o c h e m i c a l forms dominate d u r i n g p a r t s o f the y e a r . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n s o f C o , F e , M n , and Z n d u r i n g t h e s a m p l i n g p e r i o d were f o u n d i n o y s t e r s c o l l e c t e d a t n e a r - m o n t h l y i n t e r v a l s o v e r a number o f s e a s o n s ( F i g . 4 ) . C o n c e n t r a t i o n s o f Z n r a n g e d f r o m 3300 t o 9 0 0 0 , o f Fe f r o m 2000 t o 9 1 0 0 , o f Mn f r o m 270 to 7 8 0 , and o f Co f r o m 2 . 5 to 5 . 5 y g / g a s h w e i g h t d u r i n g t h e 17 mo p e r i o d . Seasonal f l u c t u a t i o n s i n c o n c e n t r a t i o n s o f these metals i s c l e a r l y e v i d e n t . The p a t t e r n o f changes i n F e , Z n , and Co were s i m i l a r ; c o n c e n t r a t i o n s were h i g h i n t h e s p r i n g and l a t e f a l l , and low i n t h e summer and p a r t of winter. The f a l l p e a k was s e e n i n b o t h 1972 and 1973. The p a t t e r n o f c h a n g e o f Mn was o p p o s i t e f r o m t h e o t h e r t h r e e , i . e . , i t s c o n c e n t r a t i o n was low when t h e o t h e r s were h i g h . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n b e t w e e n some o f t h e s a m p l e s were m e a s u r e d . The c o n c e n t r a t i o n o f Z n on O c t o b e r

1,1973,

was 4100 and on November 10, 1973, i t was 9000 y g / g a s h weight. T h i s approximate d o u b l i n g o f the c o n c e n t r a t i o n i n about 6 wks i n d i c a t e s a v e r y r a p i d r a t e o f c h a n g e . The d i f f e r e n c e s i n r a t e s o f change i n c o n c e n t r a t i o n s u g g e s t t h a t t h e d y n a m i c s o f a c c u m u l a t i o n and l o s s may d i f f e r d u r i n g t h e y e a r . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n s o f C o , F e , and Zn w i t h t i m e were o b s e r v e d i n c l a m s a l s o ( F i g u r e 5 ) . C o n c e n t r a t i o n s o f Fe r a n g e d f r o m 600 to 1 5 , 0 0 0 , o f Z n f r o m 250 t o 500, and o f Co f r o m 6 . 5 to 15.5 u g / g a s h w e i g h t . S e a s o n a l t r e n d s were n o t as o b v i o u s i n c l a m s as i n o y s t e r s , b u t c y c l i c c h a n g e s d u r i n g t h e 18-mo i n t e r v a l were a p p a r e n t . ?

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

HARRISON

Physiochemical Form of Trace Metals

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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CHEMICAL

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SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Saxidomus nuttalli

J

F

M

A

M J J A Time months

S

O

N

D

f

Figure 5. Concentration of metals in soft tissues of the clam Saxidomus nuttalli

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

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Seasonal f l u c t u a t i o n s i n metal c o n c e n t r a t i o n s were described for the oyster C. v i r g i n i c a (JL, ICO ; elevated l e v e l s i n the summer are followed by decreased l e v e l s i n the w i n t e r . F r a z i e r (10) found that Mn concentrations i n C. v i r g i n i c a were p o s i t i v e l y c o r r e l a t e d only w i t h those of Fe; Fe concentrations c o r r e l a t e d with Cd and Zn; and Cd, Cu, Fe, and Zn concentrations were h i g h l y c o r r e l a t e d w i t h each other. We found Co, Fe, and Zn to behave s i m i l a r l y i n C. gigas and Mn to vary i n v e r s e l y . F r a z i e r (JJO), i n reviewing the hypotheses suggested f o r the c o n t r o l o f metal uptake, categorized these hypotheses i n t o four groups r e l a t e d t o : 1) s h e l l growth and metabolism, 2) feeding, 3) spawning, and 4) p h y s i c a l chemistry o f the metal e i t h e r i n the water or a t the water-membrane i n t e r f a c e . He i d e n t i f i e d two fundamentally d i f f e r e n t patterns o f behavior i n C^ v i r g i n i c a e x e m p l i f i e d by Mn and Zn. The concentrations o f Mn i n s o f t t i s s u e are c o r r e l a t e d s i g n i f i c a n t l y w i t h s h e l l growth, whereas Zn concentrations d r a m a t i c a l l y decrease before the period o f maximum s h e l l growth. The changes i n Zn seem to be c l o s e l y r e l a t e d to gonadal development and spawning. D i f f e r e n c e s i n the p a t t e r n s o f changes i n Mn and Zn were a l s o evident i n the r e s u l t s reported here on C. gigas. However, no data are a v a i l a b l e that r e l a t e the changes d i r e c t l y to s h e l l growth or to reproductive changes. Seasonal f l u c t u a t i o n s i n metal c o n c e n t r a t i o n s were reported for the mussel M y t i l u s e d u l i s (34). Mussels from the Elbe/Cuxhaven s i t e i n Germany have highest concentrations normally i n l a t e winter and s p r i n g , and the lowest i n l a t e summer and autumn. Bryan (_35) reported annual f l u c t u a t i o n s i n the content o f Co, Cu, Fe, Mn, N i , Pb, and Zn i n s c a l l o p s . In g e n e r a l , highest c o n c e n t r a t i o n s were found i n the winter and lowest i n the summer and autumn. The data a v a i l a b l e demonstrate seasonal changes i n element c o n c e n t r a t i o n i n a number o f b i v a l v e molluscs. The maximum and minimum concentrations do not occur a t the same time o f the year i n a l l ecosystems. This i s expected because seasonal changes are not concurrent i n a l l ecosystems. I d e n t i f i c a t i o n has not been made o f the r e l a t i v e c o n t r i b u t i o n to these changes o f v a r i a t i o n s i n the amounts and physicochemical forms o f the elements i n the water and i n the metabolic processes i n the organisms. However, seasonal changes can be s i g n i f i c a n t and should be taken i n t o c o n s i d e r a t i o n i n model development. E f f e c t o f P h y s i c a l and Chemical F a c t o r s Presence and Absence o f P a r t i c l e s . Turnover o f Co and Cs i n Mya a r e n a r i a were measured by f o l l o w i n g both the accumulation and loss o f r a d i o n u c l i d e s . However, the c o n d i t i o n s under which the accumulation were measured d i f f e r e d from those o f l o s s . During the accumulation p e r i o d , the animals were maintained i n f i l t e r e d water i n a closed system, whereas during the loss p e r i o d , they

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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M O D E L I N G IN

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SYSTEMS

were maintained i n once-through, u n f i l t e r e d seawater that contained the food organisms and p a r t i c u l a t e m a t e r i a l to which they are exposed normally. Fluxes of Cs and Co i n the clam during the accumulation and loss periods were c a l c u l a t e d (Table I I I ) . The f l u x of an element i n the t i s s u e s i s the product of TABLE I I I Fluxes i n Mya a r e n a r i a Determined During and Loss Experiments (21)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Body p a r t

Cesium , yg/d/g (1x10*) Accumulation Loss

Mantle G i l l s and l a b i a l palps Muscle D i g e s t i v e gland and stomach Viscer a

Accumulation

Cobalt , yg/d/g (1x10*) Accumulation Loss

8.1

8.8

5.5

1.2

14. 3.0

14. 5.1

3.5 3.7

14. 0.4

16. 3.4

9.3 7.1

47. 4.4

100. 21.

the loss rate constant and the c o n c e n t r a t i o n i n the animal. R e l a t i v e l y good agreement for l ^ C s between the c a l c u l a t e d values of f l u x e s from l a b o r a t o r y accumulation data and those from f i e l d loss data were obtained; the r a t e constants determined by the two methods were s i m i l a r . These r e s u l t s suggest that the chemical form of the r a d i o n u c l i d e was s i m i l a r and that the major source o f the l ^ C s £ ^ i £ f m the water; no s i g n i f i c a n t d i f f e r e n c e s were detected between the two s i t u a t i o n s . Greater d i f f e r e n c e s were found i n the f l u x of Co during the two periods than i n that of Cs. Much l a r g e r f l u x e s of Co were measured during loss than during up t a k e , i n the g i l l s , the d i g e s t i v e gland and stomach, and the v i s c e r a . These d i f f e r e n c e s may r e f l e c t the a v a i l a b i l i t y of food or may i n d i c a t e that c o b a l t f o l l o w s a d i f f e r e n t metabolic pathway during accumulation and l o s s . Thus, c o b a l t may be deposited more r a p i d l y i n a compartment than i t can be m o b i l i z e d from i t . We i n v e s t i g a t e d the e f f e c t s of p a r t i c u l a t e m a t e r i a l i n another s e r i e s of experiments (_25). Oysters were introduced i n t o a discharge canal o f a nuclear p l a n t before scheduled r e l e a s e s of r a d i o a c t i v i t y . The q u a n t i t i e s of the r a d i o n u c l i d e s accumulated during the r e l e a s e were measured during a p e r i o d of normally high ( J u l y ) and a p e r i o d o f normally low b i o l o g i c a l p r o d u c t i v i t y (December). During each p e r i o d , the animals were d i v i d e d i n t o two groups: one received untreated seawater, and the other f i l t e r e d seawater. During the J u l y experiment, the oysters were placed i n the canal at 1700 h the evening before the r e l e a s e . Some oysters o r

t

e

c

a m s

s

r 0

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

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Physiochemical Form of Trace Metals

HARRISON

were removed from the water a t 0850 h ( p r e r e l e a s e ) ; the r e l e a s e occurred at 0900 h and terminated a t 1100 h. Only ^Zn was detected i n the p r e r e l e a s e animals maintained i n n o n f i l t e r e d seawater (Table I V ) . The r e l a t i v e l y higher concentrations i n the p r e r e l e a s e oysters maintained i n f i l t e r e d water compared to those i n n o n f i l t e r e d water may be because o f increased a v a i l a b i l i t y r e s u l t i n g from the leaching o f p a r t i c l e s that had been deposited on the f i l t e r during the 8-h i n t e r v a l during which the p a r t i c l e s were c o l l e c t e d . Desorption o f r a d i o n u c l i d e s from the p a r t i c l e s on the f i l t e r would occur any time the water flowing through the f i l t e r had a lower s p e c i f i c a c t i v i t y than that i n which the p a r t i c l e s were o r i g i n a l l y suspended. In the animals sampled at 1100 h ( p o s t r e l e a s e ) , concentrations o f a l l r a d i o n u c l i d e s were elevated. Concentrations i n p o s t r e l e a s e oysters i n n o n f i l t e r e d seawater were higher than those i n f i l t e r e d seawater f o r a l l r a d i o n u c l i d e s . The amounts accumulated per hour i n animals i n f i l t e r e d water compared to those i n n o n f i l t e r e d water were lower by 95% f o r C o , by 78% f o r Mn, by 52% f o r Z n , and by 40% f o r > C s . On December 4, 1973, the p r e r e l e a s e oysters were removed a t 0900 h, and the p o s t r e l e a s e were removed a t 1230 h; animals were placed i n the discharge canal water a t 1530 h on December 3. Compared w i t h the expected concentrations i n the water, the amounts o f most r a d i o n u c l i d e s accumulated per hour during the release were lower i n December than i n J u l y (Table I V ) . The d i f f e r e n c e s between the q u a n t i t i e s accumulated i n oysters held i n f i l t e r e d vs. n o n f i l t e r e d water suggest that p a r t i c l e s p l a y an important r o l e i n the accumulation^ o f elements. P a r t i c l e s i n the water may be l i v i n g microorganisms, organic d e t r i t u s , i n o r g a n i c m a t e r i a l , or any combination o f the three and may vary i n q u a n t i t y , both w i t h time and l o c a t i o n . The r o l e o f p a r t i c l e s i n r a d i o n u c l i d e accumulation d i f f e r e d w i t h each r a d i o n u c l i d e . I n oysters, ^ C o p p to be accumulated p r i m a r i l y from the suspended p a r t i c u l a t e f r a c t i o n , whereas 134,137ç appears to be accumulated p r i m a r i l y from the soluble f r a c t i o n . The d i f f e r e n c e s between the amounts o f r a d i o n u c l i d e s accumulated i n J u l y and December probably r e s u l t e d p r i m a r i l y from d i f f e r e n c e s i n the composition o f the suspended p a r t i c l e s ; p a r t i c l e loads were 25 and 15 mg/1 dry weight, r e s p e c t i v e l y (25). Lack o f c o r r e l a t i o n was reported i n the l e v e l s o f Cu and Zn i n f i l t e r e d water and those i n oysters (_36). This poor degree o f c o r r e l a t i o n may be r e l a t e d to the i n g e s t i o n o f suspended p a r t i c u l a t e m a t e r i a l i n the seawater (2_, _37_). Because the p a r t i c l e s may vary i n q u a n t i t y and composition both w i t h time and l o c a t i o n i n an environment, the q u a n t i t i e s o f those elements w i t h high a f f i n i t i e s f o r p a r t i c l e s that are accumulated by organisms should vary correspondingly. E f f e c t o f Physicochemical Form. The physicochemical form o f the element may a f f e c t the q u a n t i t i e s accumulated. To assess the 6 0

1 3 4

54

6 5

1 3 7

a

e a r s

s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

detected.

6.28

80 ± 7 157 ± 11 26

70 ± 10 60 ± 12 0

9.31

ND ND 0

30 ± 8 40 ± 8 3

ND 498 ± 20 166

ND 310 ± 12 103 349.

60 ± 25 200 ± 30 14 2.97

467.

ND 500 ± 22 162

ND 324 ± 15 108

130 ± 30 200 ± 40 23

423.

258.

9.05

12.8

4.85

Amount accumulated per hour during the r e l e a s e

C

8.0 ± 2.7 210 ± 6.3 101

1.0 ± 1.0 99 ± 4.0 49

Cs

200 ± 20 490 ± 20 145

1 3 7

1.0 ± 1.0 14 ± 3.8 7

Cs

46 ± 5.1 72 ± 3.6 13

1 3 4

ND 340 ± 6.7 170

Zn

ND 160 ± 4.8 80

6 5

150 ± 25 750 ± 30 300

Co

ND 260 ± 7.7 130

a

6 0

ND 120 ± 4.7 60

Mn

i n Oysters (25)

'Radionuclide c o n c e n t r a t i o n was c a l c u l a t e d from the known d i l u t i o n of the source m a t e r i a l .

)

ND, not

December 4, 1973 N o n f i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) F i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) Seawater c o n c e n t r a t i o n (pCi/1)

D

J u l y 31, 1973 N o n f i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h ) F i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) Seawater concentration ( p C i / l )

54

TABLE IV R a d i o n u c l i d e Concentrations

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

27.

HARRISON

627

Physiochemical Form of Trace Metals

e f f e c t s o f the physicochemical form o f Cu and Zn on accumulation, groups o f twelve oysters were held i n a flow-through system i n seawater c o n t a i n i n g ^ Cu, ^^Zn, and e i t h e r g l y c i n e (1 χ 10" M\ ethylenediaminetetraacetate (EDTA, 1 χ 10~ M), "yellow s t u f f " (2 mg/1), or c l a y (5 mg k a o l i n / 1 ) . Animals were removed from the test s o l u t i o n s , d i s s e c t e d , weighed, and the r a d i o n u c l i d e s q u a n t i f i e d . A f t e r a 24 h exposure, the c o n c e n t r a t i o n f a c t o r s i n the oysters d i f f e r e d both w i t h the t e s t m a t e r i a l and w i t h the element (Table V ) . In the t i s s u e s , c o n c e n t r a t i o n f a c t o r s o f both elements g e n e r a l l y were high i n the g i l l s and the d i g e s t i v e gland and stomach and low i n the muscle and the blood. The data on physicochemical forms suggest that t h i s i s an important f a c t o r i n the accumulation o f elements. Concentration f a c t o r s both increased and decreased. George and Coombs (31) reported that complexation to EDTA, humic or a l g i n i c a c i d s , or p e c t i n doubled the r a t e s o f accumulation and f i n a l t i s s u e concentrations i n e d u l i s and e l i m i n a t e d a lag p e r i o d . Whether the changes i n the q u a n t i t i e s accumulated are due to d i f f e r e n c e s i n rates of transport of the d i f f e r e n t forms o f metals across b i o l o g i c a l membranes, to changes i n feeding r a t e s stimulated by the presence o f organic m a t e r i a l i n the water, or to i n t e r a c t i o n s of metals and l i g a n d s i n the water that a l t e r the q u a n t i t i e s o f the b i o l o g i c a l l y a v a i l a b l e forms o f the metal i s not known. However, d i f f e r e n c e s i n turnover r a t e s from laboratory and f i e l d studies may be explained i n p a r t by t h i s f ac tor. E f f e c t o f Concentration. To p r e d i c t the amounts o f r a d i o a c t i v e or stable n u c l i d e s o f an element that may be accumulated when increased q u a n t i t i e s are present i n the water, we must know i f the c o n c e n t r a t i o n o f the stable n u c l i d e i s under metabolic c o n t r o l . To t e s t for a homeostatic mechanism for Zn and Mn i n oysters and f o r Co and Cs i n clams, groups o f oysters and clams were exposed to increased concentrations o f these elements i n an attempt to saturate any r e g u l a t o r y mechanisms. The r e s u l t s reported e a r l i e r for Co and Cs on clams (21) are included for comparison to those for Mn and Zn i n o y s t e r s . The accumulation o f ^ Mn and 65Zn f m the water by oysters was followed i n groups o f animals held i n d i f f e r e n t concentrations of Mn and Zn i n the water (2, 5, or 10 Pg/1) and the same c o n c e n t r a t i o n o f 65Zn j 5 4 ^ Concentration f a c t o r s of both ^Kn j 6 5 ^ were lower i n animals maintained i n water c o n t a i n i n g 5 or 10 g/1 (Figure 6 ) . This r e d u c t i o n was not l i n e a r , however, as a f i v e - f o l d increase i n c o n c e n t r a t i o n d i d not r e s u l t i n a f i v e - f o l d decrease i n concentration factor. The accumulation o f ^Co and ^ C s from the water by clams was followed at concentrations o f 0.5, 2.5, and 12.5 ug/1 o f s t a b l e Co and Cs (21). Clams do not seem to regulate Cs i n the range o f concentrations t e s t e d , but do seem to 4

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

6

6

4

r 0

a n c

a n c

η#

n

3 7

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

628

MODELING IN AQUEOUS

SYSTEMS

TABLE V

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

C o p p e r - 6 4 and Z i n c - 6 5 C o n c e n t r a t i o n F a c t o r s i n O y s t e r S o f t T i s s u e s A f t e r 2 4 - h E x p o s u r e to T e s t M a t e r i a l s Body Part

None (control)

Mantle G i l l s and l a b i a l palps S h e l l muscles D i g e s t i v e gland and stomach Viscera Body f l u i d Remains Whole body Mantle G i l l s and l a b i a l palps S h e l l muscles D i g e s t i v e gland and s t o m a c h Viscera Body f l u i d Remains Whole

body

Glycine

Clay

Yellow Stuff

EDTA

100

A. 160

Copper 120

120

30

210 60

340 60

280 30

170 20

90 7

160 100 10 30 80

230 220 10 30

180 80 5 30 80

120 90 4 14 60

50 30 2 6 20

82

60

1.1

1

1

0

65 Zinc

16

B. 23

31 4

37 4

170 23

153 37

3.0 0.1

10 9 0.8 2

13 12 0.8 2

82 64

1.0 0.9

4 24

86 61 4 11

10

12

57

49

0.1

D 3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

0.1 0.1

27.

HARRISON

Physiochemical Form of Trace Metals

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Crassostrea gigas

629

Mya arenaria

Time, days Figure 6. Concentration of metals in soft tissues of Crassostrea gigas and Mya arenaria exposed to different levels of metals in the water. Manganese and zinc: (A) 2; (M) «5; and (%) 10 ng/L. Cobalt and cesium:(%) 0.5; (f) 2.5; and i | j 12.5

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

630

CHEMICAL

MODELING IN AQUEOUS SYSTEMS

regulate Co a t the higher c o n c e n t r a t i o n s ; the Co c o n c e n t r a t i o n f a c t o r s were lower (Figure 6 ) . Shuster and P r i n g l e (_5) exposed C^_ v i r g i n i c a to increased l e v e l s o f metals i n the water under c o n t r o l l e d l a b o r a t o r y experiments. They reported an approximate doubling o f metal concentrations i n the t i s s u e s upon doubling the metal i o n c o n c e n t r a t i o n i n the water. The uptake o f Cd by e d u l i s at low concentrations was d i r e c t l y p r o p o r t i o n a l to the concentrations i n the seawater w i t h a maximum concentration f a c t o r o f 167 a t 0.7 mg Cd/1 ( 3 1 ) . At higher c o n c e n t r a t i o n s , the c o n c e n t r a t i o n f a c t o r decreased, which was considered to i n d i c a t e a s a t u r a t i o n o f the a v a i l a b l e b i n d i n g s i te s. Boyden (38) reports that Cd c o n c e n t r a t i o n f a c t o r s o f P a t e l l a were 10 times g r e a t e r i n a Cd-contaminated environment than could be expected from a p r o p o r t i o n a l increase r e l a t e d to environmental c o n c e n t r a t i o n s . On the other hand, Cu c o n c e n t r a t i o n f a c t o r s o f Ostrea e d u l i s , C. gigas, and e d u l i s were g r e a t l y decreased i n a g r o s s l y Cu-contaminated environment. Although r e g u l a t i o n o f turnover o f some elements at elevated concentrations i s suggested from the data, the mechanisms are not c l e a r l y e s t a b l i s h e d , and the response i s species dependent. The changes i n c o n c e n t r a t i o n f a c t o r s may not be a c e l l u l a r but instead may be an organismic response to the t o x i c e f f e c t s o f the increased l e v e l s o f metals. Oysters decreased the time the s h e l l was opened at increased concentrations o f Cu (_13). A c l o s u r e response was a l s o found i n mussels ( F . L . H a r r i s o n and D.W. R i c e , unpublished data, 1978). The t o x i c responses may i n c l u d e s h e l l c l o s u r e , changes i n pumping and feeding r a t e s , and changes i n respiration. Conclusion The turnover o f s t a b l e and r a d i o a c t i v e n u c l i d e s o f trace metals d i f f e r s g r e a t l y w i t h animal s p e c i e s , element, time, and physicochemical form o f the metal i n the water. Concentration f a c t o r s and turnover rates o f a given element can range s e v e r a l orders o f magnitude i n value i n d i f f e r e n t b i v a l v e molluscs. The r e l i a b i l i t y o f models developed to p r e d i c t c o n c e n t r a t i o n change i s dependent on the s e l e c t i o n o f input parameters appropriate f o r the s i t u a t i o n under c o n s i d e r a t i o n . E q u i l i b r i u m models are used to assess the environmental impact o f power p l a n t s i t i n g . The use o f a s i n g l e , maximum c o n c e n t r a t i o n f a c t o r for b i v a l v e molluscs as input i n t o the model i n t h i s s i t u a t i o n i s appropriate f o r screening purposes, i . e . , to determine whether the maximum c r e d i b l e value would impact the environment. However, when more r e a l i s t i c estimates are r e q u i r e d , s e l e c t i o n o f c o n c e n t r a t i o n f a c t o r s a p p l i c a b l e to the s i t e , species, and s i t u a t i o n i s necessary. Dynamic models are used i n the case o f a c c i d e n t a l r e l e a s e s where time i s an important c o n s i d e r a t i o n . The use o f appropriate

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

27.

HARRISON

Physiochemical Form of Trace Metals

631

rate constants and concentration f a c t o r s even i n s i m p l i f i e d dynamic models can r e s u l t i n b e t t e r approximations of the c o n c e n t r a t i o n maxima and c o n c e n t r a t i o n changes that can be expected than the use o f e q u i l i b i u m models. Acknowledgements

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

The author thanks John W. Dawson and David W. R i c e , J r . f o r their technical assistance. Work performed under the auspcies o f the U.S. Department of Energy by the Lawrence Livermore Laboratory under c o n t r a c t number W-7405-ENG-48. Abstract Bivalve molluscs effectively concentrate numberous elements. To maintain healthy populations of molluscs and to minimize adverse effects on man from their consumption, we must be able to predict changes in concentration after stable and radioactive nuclides are released into their environment. Both equilibrium and dynamic predictive models have been developed. Model parameters needed include concentration factors and turnover rate constants. Concentration factors and rate constants determined experimentally in the oyster Crassostrea gigas and the clam Mya arenaria differ widely with species and element. The physical form of the element in the water affects turnover also; accumulation of radionuclides of Co, Cs, Mn, and Zn is greater in water containing suspended particles. The chemical form of the element in the water affects its accumulation. When glycine, ethylenediaminetetraacetate (EDTA), "yellow stuff", or clay are added to seawater, the accumulation of Cu and Zn by the oyster differs with each test material; glycine increases and EDTA decreases the accumulation of both elements. The concentrations of stable Co, Fe, Mn, and Zn in oysters and of Co, Fe, and Zn in clams fluctuate. In oysters, seasonal changes were evident. Concentrations of Co, Fe, and Zn seem to vary together, whereas that of Mn varied inversely. Before accurate predictions can be made of the accumulation of stable and radioactive nuclides of elements by bivalve molluscs, we need concentration factors, rate constants, and information on the effects of metabolic and environmental factors on these parameters for each animal species of interest. Literature Cited 1. 2.

Galtsoff, P.S. The American oyster, Crassostrea virginica gemlin. U.S. Fish. Wildl. Serv. Fish Bull. 64, 480 p. (1964). Brooks, R.R. and Rumsby, M.G. The biogeochemistry of trace element uptake by some New Zealand bivalves. Limnol. Oceanogr.

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3.

4.

5.

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6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

10, 521-527 (1965). Pringle, B.H., Hisson, D.E., Katz, E.L., and Mulawka, S.T. Trace metal accumulation by estuarine mollusks. J . Sanit. Eng. Div. Amer. Soc. C i v i l Eng. 94, 455-475 (1968). Kopler, F.C. and Mayer, J . Studies on trace metals i n shell f i s h , p. 67-80, i n Proc. Gulf and South Atlantic Shellfish Sanit. Res. Conf., March 21-22, 1967. U.S. Public Health Serv. Publ. No. 999 UIH-9, 1969. Shuster, C.N., J r . and Pringle, B.H. Trace metal accumulation by the American Eastern oyster Crassostrea v i r g i n i c a . Proc. Nat. Shellfish Assoc. 59, 91-103 (1969). Roosenburg, W.H. Greening and copper accumulation i n the American oyster, Crassostrea virginica, i n the v i c i n i t y of a steam electric generating station. Chesapeake Sci. 10, 241-252 (1969). Wolfe, D.A. Levels of stable Zn and Zn i n Crassostrea virginica from North Carolina. J . Fish. Res. Bd. Canada 27, 47-57 (1970). Huggett, R.J., Bender, M.E., and Stone, H.D. U t i l i z i n g metal concentration relationships i n the Eastern oysters (Crassostrea virginica) to detect heavy metal pollution. Water Research 7, 451-460 (1973). Windom, H.L. and Smith, R.G. Distribution of iron, magnesium, copper, zinc, and silver i n oysters along the Georgia coast. J. Fish Res. Bd. Canada 29, 450-452 (1972). Frazier, J.M. The dynamics of metals i n the American oyster, Crassostrea virginica. I. Seasonal effects. Chesapeake S c i . 16, 162-171 (1975). Frazier, J.M. The dynamics of metals i n the American oyster, Crassostrea virginica. I I . Environmental effects. Chesapeake Sci. 17, 188-197 (1976). Okazaki, R.K. Copper toxicity i n the Pacific oyster, Crassostrea gigas. Bull. Environ. Contam. Toxicol. 16, 658-664 (1976). Harrison, F.L. and Rice, D.W., J r . Copper sensitivity of adult Pacific oysters, i n Thorp, J.H., III, and Gibbons, J.W., ed., "Symp. Energy and Environmental Stress i n Aquatic Ecosystems," (in press). P h i l l i p s , D.J. The use of biological indicator organisms to monitor trace metal pollutants i n the marine and estuarine environments. Environ. Pollut. 13, 281-317 (1977). Hoffman, F.O. Chm, "The Evaluation of Models Used for the Environmental Assessment of Radionuclide Releases," 131 p., Proc. Workshop, Oak Ridge National Laboratory, Oak Ridge, Tennessee, CONF-770901, 1978. Ng, Y.C., Burton, C.A., Thompson, S.E., Tandy, R., Kretner, H.K., and Pratt, M. "Prediction of the Maximum Dosage to Man from the Fallout of Nuclear Devices. IV. Handbook for Estimating the Maximum Internal Dose to Man from Radionuclides Released to the Biosphere." Lawrence Livermore Laboratory 65

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Report UCRL-50163 (Part IV), 1968. Soldat, J.Κ., Baker, D.A., and Corley, J.P. Application of a general computation model for composite environmental radia­ tion doses, p. 483-498, in "Environmental Behavior of Radio­ nuclides Released i n the Nuclear Industry," IAEA, Vienna, 1973. 18. Lyon, R.B. "RAMM: A System of Computer Programs for Radio­ nuclide Pathway Analysis Calculations," Atomic Energy of Canada Limited, AECL-5527, 1976. 19. Hijama, Y. and Shimizu, M. Uptake of radioactive nuclides by aquatic organisms; the application of the exponential model, p. 463-476, in "Environmental Contamination by Radio­ active Materials," Proc. Seminar, IAEA, Vienna, 1969. 20. Cutshall, N. Turnover of Zinc-65 in oysters. Health Phys. 26, 327-333 (1974). 21. Harrison, F.L. Accumulation and loss of cobalt and caesium by the marine clam Mya arenaria, under laboratory and f i e l d conditions, in Symp. Interaction of Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, Washington, July 10-14, 1972. IAEA, Vienna, 1973. 22. Cranmore, G. and Harrison, F.L. Loss of Cs and Co from the oyster, Crassostrea gigas. Health Phys. 28, 319-333 (1975). 23. Harrison, F.L. Accumulation and distribution of Mn and Zn in freshwater clams, p. 198-220, i n Proc. Second Nat'l. Symp. on Radioecology, Ann Arbor, Michigan, May 15-17, 1967. AEC Symp. Series, CONF-670503, 1969. 24. Salo, E.O. and Leet, W.L. The concentration of Zn by oysters maintained i n the discharge canal of a nuclear power plant, p. 363-371, in Proc. Second Nat'l. Symp. on Radioecology, Ann Arbor, Michigan, May 15-17, 1967. AEC Symp. Series, CONF-670503, 1969. 25. Harrison, F.L., Wong, K.M., and Heft, R.E. Role of water and particulates in radionuclide accumulation in the oyster Crassostrea gigas, p. 9-20, in Cushing, C.E., J r . , ed., "Radioecology and Energy Resources," Dowden, Hutchinson, and Ross, Stroudsberg, Pennsylvania, 1976. 26. Seymour, A.H. and Nelson, V.A. Biological half-lives for zinc and mercury in the Pacific oyster, Crassostrea gigas, p. 849856, in Proc. Third Nat'l. Symp. on Radioecology, Oak Ridge, Tennessee, May 10-12, 1971. CONF-710501, 1972. 27. Seymour. A.H. and Nelson, V.A. Decline of Zn in marine mussels following the shutdown of Hanford reactors in Symp. Interaction of Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, Washington, July 10-14, 1972. IAEA, Vienna, 1973. 28. Pentreath, R.J. The accumulation from water of Zn, Mn, Co, Fe by the mussel, Mytilus edulis. J. Mar. B i o l . Ass. U.K. 53, 127-143 (1973). Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

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137

60

54

63

65

65

65

58

54

59

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634 29.

30.

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31.

32.

33.

34.

35.

36.

37.

38.

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

Pentreath, R.J. The roles of food and water i n the accumulation of radionuclides by marine teleost and elasmobranch fish, p. 421-436, i n Symp. Interaction of Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, Washington, July 10-14, 1972. IAEA, Vienna, 1973. Van Weers, A.W. Uptake and loss of Zn and Co by the mussel Mytilus edulis L., p. 385-401, i n Symp. Interaction of Radioactive Contaminants with the Constituents of the Marine Environment, Seattle, Washington, July 10-14, 1972. IAEA, Vienna, 1973. George, S.G. and Coombs, T.L. The effects of chelating agents on the uptake and accumulation of cadmium by Mytilus edulis, Mar. B i o l . 39, 261-268 (1977). Heft, R.E. Absolute instrumental neutron-activation analysis at Lawrence Livermore Laboratory i n Third Intern'l Conf. on Nuclear Methods i n Environmental and Energy Research, October 10-13, 1977. Ophel, I.L., Fraser, C.C., and Judd, J.M. Strontium concentration factors in biota and bottom sediments of a fresh water lake, p. 509-530, i n Intern'l Symp. Radioecology Applied to the Protection of Man and His Environment, Vol. 1/2, Rome, Italy, Sept. 7, 1971, Eur-4800, 1972. Karbe, L., Schnier, C.H., and Siewers, H.O. Trace elements in mussels (Mytilus edulis) from coastal areas of the North Sea on the Baltic. Multielement analyses using instrumental neutron activation analysis. J . Radioanal. Chem. 37, 927943 (1977). Bryan, G.W. The occurrence and seasonal variation of trace metals i n the scallop Pecten maximus (L.) and Chlamys opercularis (L.). J . Mar. B i o l . Ass. U.K. 53, 145-166 (1973). Cronin, L.L., Pritchard, D.W., Schubel, J.R., and Sherk, J.A. "Metals i n Baltimore Harbor and Upper Chesapeake Bay and their Accumulation by Oysters," Chesapeake Bay Institute, The Johns Hopkins University, Univ. Maryland, 72 p., 1974. Kopler, F.C. and Mayer, J . Concentrations of 5 trace metals in the waters and oyster (Crassostrea virginica) of Mobile Bay, Alabama. Proc. Nat. Shellfish Assoc. 63, 27-34 (1973). Boyden, C.R. Effect of size upon metal content of s h e l l f i s h . J. Mar. B i o l . Ass. U.K. 57, 675-714 (1977). 65

60

RECEIVEDNovember16,1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

28 Relationships of Activities ofMetal-LigandSpecies to Aquatic Toxicity V. R. MAGNUSON, D. K. HARRISS, M. S. SUN, and D. K. TAYLOR Department of Chemistry, University of Minnesota, Duluth, Duluth, MN 55812

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

G. E. GLASS U.S. Environmental Protection Agency, Environmental Research Laboratory, 6201 Congdon Boulevard, Duluth, MN 55804 Much has been p u b l i s h e d concerning t o x i c i t y of metals t o aquatic l i f e although o n l y during the past decade have there been i n t e r p r e t a t i o n s o f the t o x i c i t y data i n terms of the r e l a t i v e t o x i c i t y o f p a r t i c u l a r s p e c i a t i o n forms, e.g., C u , CuOH , CuCCL 3 (l-7). The s p e c i f i c o b j e c t i v e of t h i s paper i s t o i l l u s t r a t e the use o f Factor A n a l y s i s i n d i s c r i m i n a t i n g between t o x i c and nont o x i c s p e c i e s . The procedure t o be f o l l o w e d i s : determination of e q u i l i b r i u m aqueous s p e c i a t i o n , c a l c u l a t i o n of a p p r o p r i a t e factors, correlation of t o x i c i t y with these f a c t o r s , and i n t e r p r e t a t i o n of the c o r r e l a t i o n a n a l y s i s i n terms o f p a r t i c u l a r species activities. The a n a l y s i s o f t o x i c i t y data i n terms of s p e c i a t i o n products i s a d i f f i c u l t task s i n c e the v a r i a b l e s , species a c t i v i t i e s , u s u a l l y are numerous and o f t e n are i n t e r r e l a t e d . F a c t o r A n a l y s i s a l l o w s one t o determine a s m a l l number o f s t a t i s t i c a l l y independent, l i n e a r combinations o f a c t i v i t i e s ( f a c t o r s ) . C o r r e l a t i o n of these combinations ( f a c t o r s ) w i t h t o x i c i t y allows d i s c r i m i n a t i o n to be made between t o x i c and non-toxic s p e c i e s . Conclusions drawn from a s t a t i s t i c a l study o f t h i s type are o n l y as v a l i d as the data upon which the study i s based. The p u b l i s h e d t o x i c i t y study (Andrew, B i e s i n g e r , and Glass (2)) which we use f o r i l l u s t r a t i o n i s s c i e n t i f i c a l l y sound, however, the number o f experimental p o i n t s and l i m i t e d ranges o f some o f the experimental v a r i a b l e s do r e s t r i c t the a b i l i t y t o d i s c r i m i n a t e between s p e c i e s . Andrew et al. (2) s t u d i e d the e f f e c t s of carbonate, o r t h o phosphate, and pyrophosphate on the t o x i c i t y o f c o p p e r ( I I ) t o Vapkvwa magna a t constant pH and t o t a l hardness. They r e p o r t e d m o r t a l i t y r a t e s and r e c i p r o c a l s u r v i v a l times t o be d i r e c t l y c o r r e l a t e d w i t h c u p r i c and copper-hydroxo i o n a c t i v i t i e s as determined by e q u i l i b r i u m c a l c u l a t i o n s . They a l s o found t o x i c i t y t o be n e g a t i v e l y r e l a t e d t o a c t i v i t i e s o f s o l u b l e copper carbonate (CuCCL), and independent o f t o t a l o r d i s s o l v e d copper concentrat i o n . Data f o r t h e i r s e t o f experiments i n v o l v i n g a d d i t i o n o f orthophosphate a r e i n c l u d e d i n Table I but were not used i n some 2 +

0-8412-0479-9/79/47-093-635$05.50/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

o

CHEMICAL

636

M O D E L I N G IN

AQUEOUS

SYSTEMS

of our l a t e r c a l c u l a t i o n s due t o u n c e r t a i n t y as t o the s t a b i l i t y constants f o r phosphate complexes. C a l c u l a t i o n s when a l l p o i n t s were used are r e f e r r e d t o as 30 p o i n t and those i n which the orthophosphate p o i n t s are excluded are r e f e r r e d t o as 26 p o i n t . The c a l c u l a t i o n s described below are of two types; f i r s t , r e c a l c u ­ l a t i o n of m e t a l - l i g a n d s p e c i a t i o n and comparison w i t h previous r e s u l t s , and secondly, i n t e r p r e t a t i o n of the t o x i c i t y data i n terms of s p e c i a t i o n u s i n g f a c t o r a n a l y s i s and m u l t i p l e r e g r e s s i o n . The s p e c i a t i o n c a l c u l a t i o n s were performed u s i n g REDEQL2 (8_) and SPSS (9) was used f o r f a c t o r a n a l y s i s and m u l t i p l e r e g r e s s i o n .

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

Sources of Data and L i m i t a t i o n s S t a b i l i t y Constants. The choice of e q u i l i b r i a to i n c l u d e and the accuracy of the r e l a t e d s t a b i l i t y constants have, of course, a major e f f e c t upon the p r e d i c t e d s p e c i a t i o n and upon i n f e r e n c e s drawn from c o r r e l a t i o n s of a c t i v i t i e s of the species w i t h t o x i c i t y . A measure of the e f f e c t s o f l i m i t e d accuracy i n the s t a b i l i t y constants on p r e d i c t e d s p e c i a t i o n concentrations can be obtained through r e p e t i t i v e c a l c u l a t i o n s on a system w h i l e a l l o w i n g f o r s m a l l , random v a r i a t i o n s i n the s t a b i l i t y constants. Such c a l c u ­ l a t i o n s have been c a r r i e d out using a three-metal, t h r e e - l i g a n d model ( 31 complexes ) w i t h concentrations ranging from 1 n i t o 0.01 uM. Random v a r i a t i o n s of 0.05 t o -0.05 u n i t s i n the mantis­ sas of the p K s l e d t o c o n c e n t r a t i o n changes ranging from 6% t o 30$ w i t h a mean change of 14?. I n g e n e r a l , the percentage changes were g r e a t e s t f o r the species whose concentrations were s m a l l f r a c t i o n s of the t o t a l c o n c e n t r a t i o n . Use of s m a l l e r random v a r i a t i o n s of 0.01 t o -0.01 u n i t s produced p r o p o r t i o n a t e l y s m a l l e r changes, about 1/5 as g r e a t . T

T o x i c i t y S t u d i e s . I t i s d i f f i c u l t to f i n d p u b l i s h e d t o x i c i t y s t u d i e s which are w e l l documented and which a l s o c o n t a i n a s u f f i ­ c i e n t number of data p o i n t s t o a l l o w meaningful s t a t i s t i c a l analyses. Minimal necessary documentation r e q u i r e s pH, a l k a l i n i t y , measured concentrations of the p a r t i c u l a r metals or l i g a n d s under study, and a complete a n a l y t i c a l background a n a l y s i s . This pre­ sumes of course t h a t : e q u i l i b r i a i n v o l v i n g the background species are important and w i l l be considered; t o t a l i n o r g a n i c carbon w i l l be d e r i v e d from a l k a l i n i t y and knowledge of other a c i d s present; and the nominal added metal or l i g a n d does not n e c e s s a r i l y equal the measured d i s s o l v e d metal or l i g a n d . With regard t o i n c l u d i n g a l l e q u i l i b r i a , f o r the e x p e r i m e n t r e f e r r e d t o i n l i n e 7 of Table I , the c a l c u l a t e d a c t i v i t y of CuSO^ i s twice t h a t of C u ^ O H ^ a n d f i v e times t h a t of Cu(0H)". The r e l a t i v e magnitudes of these a c t i v i t i e s l e a d one t o b e l i e v e t h a t the amount of copper combined w i t h s u l f a t e and other l i g a n d s must be taken i n t o account i f c a l c u l a t e d a c t i v i t i e s of species such as — 2+ Cu(0H) and C u ( 0 H ) are to be meaningful. q

p

9

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

28.

MAGNUSON

ET

AL.

M etal-Ligand Species and

Aquatic Toxicity

637

With regard t o c a l c u l a t i o n of t o t a l i n o r g a n i c carbon ( T I C ) , knowledge of the a n a l y t i c a l background and the a l k a l i n i t y a l l o w the c a l c u l a t i o n of i n o r g a n i c carbon from ( f o r a l k a l i n i t y expressed as mg C CaCO^)

T I C

(1+2x10

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

I 30^00 " ^

-10.33+pH )

1 0

1 0

J '

when other a c i d s are not present, w i t h a p p r o p r i a t e adjustments when other a c i d s are present. E m p i r i c a l equations expressing TIC i n terms of pH and hardness have been used but seem not t o be s a t i s f a c t o r y over a wide range of pH ( 1 0 ) . With regard t o c a l c u l a t e d versus measured amounts of metals , i n the work of Andrew, B i e s i n g e r and Glass (2), the measured d i s s o l v e d copper v a r i e d between 53 and 100? of the nominal copper added t o the system (see Table I ) and t h e i r data i s r e p r e s e n t a t i v e of s e v e r a l other p u b l i s h e d r e p o r t s when l i k e comparisons are made. A n a l y s i s of Data The i s s u e t o be addressed i n a subsequent s e c t i o n i s the a t t r i b u t i o n of the t o x i c i t y of copper i n aquatic systems t g . p a r t i ­ c u l a r s p e c i a t i o n forms, e.g., Cu , Cu^OH)^ " , Cu.(CO^)^ A n a l y s i s of p u b l i s h e d experimental data i n t h i s area normally i s d i f f i c u l t f o r s e v e r a l reasons: s t a t i s t i c a l l y s m a l l numbers of data p o i n t s i n r e l a t i o n t o the number of v a r i a b l e s , l a c k of i n d e ­ pendence of the v a r i a b l e s w i t h c o r r e l a t i o n c o e f f i c i e n t s o f t e n of the order of 0.8 or 0.9 (see Table I I ) , and s m a l l ranges and s c a t t e r of p o i n t s due t o the u s u a l experimental p r a c t i c e of v a r y ­ i n g as few parameters as p o s s i b l e i n a p a r t i c u l a r run w i t h the i n t e n t of determining b i v a r i a t e r e l a t i o n s h i p s . In the f i r s t set of experiments of Andrew 2X at. (2), for example, pH, hardness and t o t a l a l k a l i n i t y are h e l d constant w h i l e copper i s added t o s o l u t i o n s w i t h f i x e d added amounts of NaHCO^, or Na^HPO,, or Na^P^O^, i n a d d i t i o n t o copper added to the back­ ground, w i t h no subset of experiments i n c l u d i n g more than s i x cases. In our s p e c i a t i o n c a l c u l a t i o n s r e l a t e d t o t h i s set of n i n e t e e n experiments, some t w e n t y - f i v e complexes of copper i n c l u d ­ i n g three w i t h carbonate, two w i t h phosphate, s i x w i t h hydroxide, and s i x w i t h pyrophosphate are i n v o l v e d . I t i s p o s s i b l e t o c r e a t e new v a r i a b l e s , e.g., as l i n e a r combinations of the a c t i v i t i e s , intended t o c h a r a c t e r i z e a p a r t i c ­ u l a r aspect of the system and then i n t e r p r e t the data i n terms of t h i s smaller set of new v a r i a b l e s . One might, f o r example, use 2 +

+

1

the sum of the a c t i v i t i e s of C u , Cu0H and Cu^OH)^" " as a v a r i a b l e r a t h e r than the three species s e p a r a t e l y (10)· In t h i s way a l a r g e number of b i v a r i a t e c o r r e l a t i o n s can be made, but

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

1

Measured Dissolved Cu(uM) "•0.02 0.20 0.33 0.41 0.61 1.02 1.54 ~0.02 0.60 0.98 1.45 ~0.02 0.54 0.76 1.07

Nominal Added Cu(pM) Control 0.20 0.32 0.51 0.80 1.27 2.01 Control 0.80 1.27 2.01 Control 0.80 1.27 2.01

? +

+

CuOH o

?+ Cu (0H£ * * Cu(0H)° *

Cu(QH)

Ρ Cu(OH),

10.06 9.00 8.78 8.69 8.51 8.29 8.11

o

o

Lake Superior Water 0nly,pH 7.85-8.05 9.80 14.57 7.85 12.99 17.84 8.72 12.41 6.75 11.87 16.71 8.50 11.97 6.54 11.65 16.49 16.40 8.41 11.79 6.44 11.56 16.22 8.24 11.44 6.27 11.39 8.01 10.99 6.05 11.16 16.00 7.83 10.64 5.87 10.98 15.82 Lake S u p e r i o r Water + 48 μΜ NaHC0 ,pH 7.85--8.05 10.11 9.83 14.62 7.85 12.97 17.80 8.50 16.26 8.24 11.44 6.28 11.41 8.27 8.02 11.00 6.07 11.21 16.07 8.08 15.92 7.83 10.64 5.90 11.05 Lake Superior Water + 210 μΜ Na HP0.,pH 7.85--8.05 10.12 17.82 9.84 14.64 7.86 * 12.98 8.58 8.30 11.57 6.34 11.46 16.31 8.42 16.16 8.15 11.27 6.19 11.32 8.27 8.00 10.97 6.04 11.17 16.03

Cu

8.71 7.18 7.03 6.88

8.77 7.19 6.97 6.78

8.77 7.69 7.48 7.38 7.21 6.99 6.81

π CuC0°

TABLE I C a l c u l a t e d E q u i l i b r i u m Values f o r A c t i v i t i e s of S e l e c t e d Species Involved i n the Andrew s Values shown are the negative logarithms of the a c t i v i t i e s

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

10.88 9.35 9.20 9.06

11.01 9.44 9.23 9.06

11.06 9.96 9.74 9.64 9.47 9.25 9.07

p Cu(C0„)f"

Experiments.

C/J

9

en

ιΟ

1s

>

Ο

C

M

§

M

η

GO 00

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Cu0H

+ 2

Cu (0H)2

+

Cu( OH )°

Cu(0H) 3

Cu( OH )

6.46

11.86

8.45

8.74

8.55 6.87 6.67

16.06 12.69 12.28

10.85 10.54 9.16 8 . 8 6 8.95 8.65

7.62 7.41 3

9.86

7.07 7.12 7.17 7.29 7.52

9.01 9.21 9.31 9.56 10.02

11.56 11.78 6.12

5.89

11.44

11.35 11.40

5.67 5.72 5.77

16.76 16.80 16.85 16.97 17.20 7

7.89

9.75

XXX XXX

XXX

6.75 6.90 7.03 7.24

xxx xxx xxx 6.96 7.09 7.21 7..40 8.04 9.91 9.76 11.05 14.78

9.39

8.88 9.15

8.38

6.50

5.86

5.59 5.70

5.47

11.07 11.18 11.27 11.42 12.04 13.92

16.48 16.56 16.69 17.30 19.21

7.00 8.80

6.36

5.93

6.06 6.18

16.39

2

9.06 9.67 11.41

8.89

8.67 8.79

7.23

5.47

8.86

8.29 7.99 7.67

6.03 5.78 5.66 5.57

Lake S u p e r i o r Water + (0,2.5,5.0,10.0,50.0,500 μΜ) added N a ^ P 0 , pH 7.4-7.6

6.88 7.01 7.24

6.77 6.83

3.38

3.82 4.13 4.21 4.37

9.66

10.06

7.83

16.78 16.58

11.97 11.76 11.57

16.39

11.74

3

Cu(C0 )

9.51

CuC0°

18.45

?

2

13.64

2

Lake S u p e r i o r Water + 110 μΜ Na P 0 , p H 7.85-8.05

2 +

*** Values3 Assumed To Be 5.

5

5

5 5 5 5 5

~0.02 0.77 1.24 1.95

Control 0.80 1.27 2.01

Cu

TABLE I Cont'd.

Lake S u p e r i o r Water + (0,1*2,4,10 mM) added NaHC0 ,pH 7.3-7.5

Measured Dissolved Cu(yM)

Nominal Added Cu(yM)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

2

CD

δ*

ο

8. S"

a,

CO

CTQ

>

M H

Ο

d

ο

I

to

9°

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 2 +

+

0.22

Cu(C0 )^3

0.65

-0.21

Cu(OH)^" CuCO°

0.34

Cu(OH)~

0.94 0.89

+

0.99

2 +

Cu(OH)°

2

Cu (0H)^

CuOH

Cu

Cu

0.17

0.60

-0.13

0.45

0.95

0.96

0.99

Cu0H

+

0.05

0.46

-0.14

0.41

0.91

0.96

0.94

2

Cu (0H)

2 +

0.07

0.47

0.18

0.70

0.91

0.95

0.89

Cu(0H) 2

-0.12

0.06

0.83

0.70

0.41

0.45

0.34

Cu(0H)~

2

-0.21

-0.26

0.83

0.18

-0.14

-0.12

-0.21

Cu(0H ) ~ 4

C o r r e l a t i o n C o e f f i c i e n t s Between the A c t i v i t i e s of Copper Species i n t h e 26 p o i n t C a l c u l a t i o n s

TABLE I I

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

0.84

-0.26

0.06

0.47

0.46

0.60

0.65

CuC0°

0.84

-0.21

-0.12

0.07

0.05

0.17

0.22

3

2Cu(C0 ) 2

CO H M CO

*


Ε ο

I

>

Ω

g

ο

Ο

4^

as

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

28.

MAGNUSON

ET

Metal-Ligand Species and

AL.

Aquatic Toxicity

641

d i r e c t comparisons between the new v a r i a b l e s are d i f f i c u l t s i n c e i n t e r r e l a t i o n s h i p s of the new v a r i a b l e s are not c l e a r and they may be c o r r e l a t e d w i t h each other. What i s d e s i r e d i s a l o g i c a l , systematic procedure t o gener­ ate a s m a l l set of u n c o r r e l a t e d v a r i a b l e s which a l l o w d i r e c t com­ p a r i s o n , make chemical sense, and are amenable t o easy i n t e r p r e ­ t a t i o n . A mathematical procedure which meets these c r i t e r i a i s the use of f a c t o r a n a l y s i s ( 9 , 11, 12, 13) t o generate s t a t i s t i ­ c a l l y independent new variabTes"XfacTors7 f o l l o w e d by c o r r e l a t i o n of t o x i c i t y w i t h the generated f a c t o r s . Factor a n a l y s i s has two general uses; t o see whether under­ l y i n g p a t t e r n s o f r e l a t i o n s h i p s e x i s t among s e t s of v a r i a b l e s , and t o rearrange or reduce the data ( v a r i a b l e s ) t o a smaller s e t o f f a c t o r s or components t h a t may be taken as primary or source v a r i a b l e s . One o f t e n i s cautioned about the f i r s t use, employment of f a c t o r a n a l y t i c methods t o determine r e l a t i o n s h i p s among the v a r i a b l e s , i n t h a t spurious p a t t e r n s may be i n d i c a t e d . I n our a p p l i c a t i o n s , however, the d e s i r e d p a t t e r n s , i . e . combinations of a c t i v i t i e s , normally w i l l be known or can be p r e d i c t e d and the use of f a c t o r a n a l y s i s w i l l be t o generate a reduced set of s t a t i s t i ­ c a l l y independent components ( f a c t o r s ) , e.g. three f a c t o r s r e ­ p l a c i n g e i g h t i n d i v i d u a l a c t i v i t i e s . I n cases where the p a t t e r n s are not c l e a r , one r e t a i n s the c r i t e r i o n t h a t the r e s u l t s must be c h e m i c a l l y r a t i o n a l and may r e j e c t those t h a t f a i l t o meet t h i s c r i t e r i o n . These f a c t o r s then are used i n m u l t i p l e r e g r e s s i o n analyses t o attempt t o i d e n t i f y t o x i c s p e c i e s . T o x i c i t y of Copper t o Aquatic Forms General. A v a r i e t y of c o n c l u s i o n s have been p u b l i s h e d recent years on the r e l a t i v e t o x i c i t y of f r e e copper i o n (Cu ), hydroxo copper complexes, (CuOH , Cu(0H)°, Cu(0H) , Cu(OH)77 2\ ο 2Cu^OH)^;, and carbanato copper complexes (CuCO^, CuiCO^)^ )· o

The statements have been, i n g e n e r a l , q u a l i t a t i v e i n nature r a t h e r than q u a n t i t a t i v e . Shaw and Brown ( 1 4 ) concluded t h a t CuCO^ i s as 2+ t o x i c as Cu ; Pagenkopf at at. (15) f i n d the major t o x i c species 2+ + to be Cu w i t h a p o s s i b l e c o n t r i b u t i o n from CuOH : Andrew (it at. (2)

s t a t e t h a t copper t o x i c i t y i s d i r e c t l y r e l a t e d o n l y t o the 2 +

+

a c t i v i t i e s of C u , CuOH , and C u ^ O H ) ^ ; Chakoumakos 2 +

at.

+

(16)

r e p o r t C u , CuOH , Cu(0H)° and C u ^ O H ^ t o be t o x i c forms and found CuHCO* CuC0° and Cu(CO ) " n o t t o x i c under t h e i r c o n d i t i o n s ; Hawarth and Sprague (10) found the smoothest response s u r f a c e , of t h o s e t r i e d , f o r conper t o x i c i t y t o be generated by [Cu ] + [CuOH ] + [Cu^OH) ]and, on the b a s i s of t h i s , c l a i m these three t o be the t o x i c forms and, on the b a s i s on non-smooth response s u r f a c e s , t h a t Cu(0H) and the carbonato copper species are not toxic. 2

+

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

642

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

Most o f t h e experiments r e p o r t e d upon i n the above referenced papers i n v o l v e d systems w i t h pH i n range 6.5-8.5, hardness^up t o 35Qmg JT as CaCCL, i n i t i a l a l k a l i n i t y as h i g h as 30Qmg t as CaCCL, and t o t a l copper up t o 5mg t~ . In the f o l l o w i n g s e c t i o n s , f a c t o r a n a l y s i s and m u l t i p l e r e ­ g r e s s i o n are used i n an attempt t o determine the r e l a t i v e t o x i c i t y of t h e copper species discussed above. The a c t i v i t i e s o f e i g h t 2 +

+

2 +

2

species ( C u , CuOH , C u ( 0 H ) , Cu(0H)°, Cu(0H)~, C u ( 0 H ) J CuCO°, 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

Cu(CO^)

2

) a r e i n c l u d e d i n the c a l c u l a t i o n s .

F a c t o r Determination. One i s s t r o n g l y tempted t o generate " u n i v e r s a l " f a c t o r s f o r the species o f i n t e r e s t , v a l i d over t y p i c a l ranges o f the parameters i n v o l v e d . The problem i s t h a t the p o i n t s f o r a g i v e n experiment are u n l i k e l y t o be r e p r e s e n t a ­ t i v e o f the p o i n t s used t o generate t h e " u n i v e r s a l " f a c t o r s and the f a c t o r s l o s e t h e i r s t a t i s t i c a l independence. One has l i t t l e choice then, when u s i n g p u b l i s h e d data, but t o use t h e experimen­ t a l p o i n t s d i r e c t l y i n the determination o f the f a c t o r s as w e l l as i n the c o r r e l a t i o n s and we have done so i n the systems r e p o r t e d here although we l a t e r d i s c u s s t h e s t r u c t u r e o f t h e experimental design which would be most u s e f u l t o t h i s type of a n a l y s i s . The q u a l i t y o f species s e p a r a t i o n obtained through use o f p r e v i o u s l y p u b l i s h e d experimental p o i n t s i s good however, s i n c e the parameter s e l e c t i o n s made by the experimenters o f t e n were f o r the purpose o f studying t o x i c i t y as a f u n c t i o n o f s p e c i a t i o n . A comparison o f f a c t o r s determined w i t h 100 random p o i n t s and w i t h 30 and 26 experimental p o i n t s can be made from t h e data shown i n Table I I I . A short d e s c r i p t i o n of t h e data may be u s e f u l t o those u n f a m i l i a r w i t h f a c t o r a n a l y s i s . The values shown a r e " f a c t o r l o a d i n g s " f o r the species on the f a c t o r s , e.g. the t o p l i n e under t h e 100 random p o i n t heading s t a t e s t h a t a c t i v i t y ( C u ) = 0.88xFactor 1 - 0.19xFactor 2 + 0.27xFactor 3 ( 2 ) 2 +

2 +

and the a c t i v i t y o f C u can be c a l c u l a t e d from values o f t h e f a c t o r s . The " f a c t o r l o a d i n g s " a r e c o r r e l a t i o n c o e f f i c i e n t s and the r e l a t i v e importance o f the f a c t o r s i n determining t h e a c t i v i t y of Cu i s given as t h e square o f the c o e f f i c i e n t s , i m p l y i n g t h a t F a c t o r 1 i s s u f f i c i e n t t o e x p l a i n 77$ o f the v a r i a n c e i n t h e a c t i v i t y o f Cu , F a c t o r 2 e x p l a i n s and F a c t o r 3 e x p l a i n s 7%. The f a c t t h a t these add up t o 0.88 r a t h e r than 1.00 s t a t e s t h a t these three f a c t o r s a r e s u f f i c i e n t t o e x p l a i n o n l y 88% o f t h e v a r i a t i o n i n the a c t i v i t y o f Cu i n the 100 p o i n t s . I n compari­ son, three f a c t o r s a r g s u f f i c i e n t t o e x p l a i n 98% of the v a r i a t i o n of t h e a c t i v i t y o f Cu i n Andrew s 30 p o i n t and 26 p o i n t cases. The a c t u a l a c t i v i t i e s o f t h e species are not used i n t h e determination o f f a c t o r s , b u t i n s t e a d K a i s e r normalized values of the a c t i v i t i e s a r e used, +

f

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2+

0.04

:

I

*** Value l e s s than p r e c i s i o n shown

39.0 Percent o f 48.6 12.4 o v e r a l l variance i n the p o i n t s accounted f o r by the 3 f a c t o r s

ι 0.56

27.5

-0.08

14.9

0.95

; 0.70

2

-0.18

Cu(C0 )

3

0.87

-0.07

0.88

0.04

0.38

CuCO

3

-0.13

-0.04

0.92 0.97

0.11

0.36

0.02

0.05

***

' 0.95

0.20

0.24

Factor 3

0.05

0.05

Factor 2

-0.12

-0.01

1.00

Factor 1

Andrew's 30 P o i n t s

2Cu(OH) 4

3

Cu(OH)

-0.03 0.08

0.04

0.96

0.22

0.27

Factor 3

0.73

0.06

0.59_

2 +

-0.19

Factor 2

0.97

0.88

Factor 1

100 Random P o i n t s

Cu(oH)°

2

Cu (0H)

+1 CuOH

Cu

Species

oJi at.

Experiments

I 0.96 |

Factor 1

27.5

-0.08

-0.09

14.9

0.95

0.87

-0.13

-0.04 0.91 0.97

0.11

0.04

0.02 0.35

0.19

0.24

Factor 3

0.05

-0.05

Factor 2

Andrew's 26 P o i n t s

Varimax Rotated F a c t o r M a t r i c e s A f t e r R o t a t i o n w i t h K a i s e r N o r m a l i z a t i o n

Three-Factor Case f o r Random P o i n t s and t h e Andrew

TABLE I I I

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

644

C H E M I C A L MODELING IN AQUEOUS SYSTEMS

T

where X i s the experimental v a l u e , X i s t h e normalized v a l u e , X the mean of the X and σ the standard d e v i a t i o n of the X, K a i s e r normalized v a r i a b l e s have nominal means o f zero and standard de­ v i a t i o n s of one. Equation 2 should be w r i t t e n i n s t e a d as T

- 0.19 F

2 +

A ( C u ) = 0.88 ^

+ 0.27

2

(2')

2+ 2+ where A ( C u ) i s the K a i s e r normalized a c t i v i t y of Cu and the f a c t o r s are K a i s e r normalized v a r i a b l e s . From known values of a c t i v i t i e s , one can c a l c u l a t e the value of the f a c t o r ( f a c t o r s c o r e ) VÂJOL f a c t o r score c o e f f i c i e n t s ,

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

f

F

l

=

C

l l ï X

+

C

12

X ,

2 "-+

+ C

m 'n X

i k )

where t h e c are the f a c t o r score c o e f f i c i e n t s and the X* a r e the normalized ? i l u e s o f the activités. The means and standard d e v i ­ a t i o n s of the a c t i v i t i e s of the s p e c i e s , p l u s the f a c t o r score c o e f f i c i e n t s f o r the 26 p o i n t case are g i v e n i n Table IV. The second f a c t o r would be w r i t t e n F

T

2

2 +

f

+

= -1.09 A ( C u ) + 7.19 A ( C u 0 H ) + ...

(.5)

TABLE IV Means, Standard D e v i a t i o n s , and F a c t o r Score C o e f f i c i e n t s f o r Andrew s 26 P o i n t s w i t h Three F a c t o r s . (Means and Standard Deviation i n Molarities) 1

Species Cu

Mean Activity

2 +

CuOH

+

Cu (0H)

2 +

2

4.28xl0~

8

2.65xl0~

8

1.91xl0"

Standard Deviation

F a c t o r Score C o e f f i c i e n t s Factor 1 Factor 2 Factor 3

6.10x10

-19.55

-1.09

3.11

48.73

7.19

-7.89

0.23

0.29

0.06

3.33xlO" 10

8

3.36xlO"

10

Cu(0H)°

9.39xl0"

-38.18

-14.37

4.95

Cu( OH)"

3.62xl0~

12

2.85xl0~

12

16.76

14.23

-1.05

Cu(OH)^"

3.62xl0"

17

3.8lxl0~

1 7

-5.61

-7.48

-0.09

CuC0°

5.75xl0~

7

9.26xl0"

7

-1.34

-0.01

1.14

4.14xl0"

9

1.20xl0"

8

0.49

0.05

0.18

2

Cu(C0 ) " 3

7

9.03xl0"

7

Using m u l t i p l e r e g r e s s i o n , b i o l o g i c a l response (BR) can then be c o r r e l a t e d w i t h the f a c t o r s ( f a c t o r s c o r e s ) as BR = aF^^ + b F

2

+ c F + ... 3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(6)

28.

MAGNUSON

Metal-Ligand Species and Aquatic Toxicity

ET AL.

645

thereby generating a r e l a t i o n s h i p between b i o l o g i c a l response and a c t i v i t i e s , e.g. Equations 7 and 8. The f a c t o r score c o e f f i c i e n t s ( c ) are not c o r r e l a t i o n c o e f f i c i e n t s f o r independent v a r i a b l e S ^ a n d the concept o f r e l a t i v e weight o f the a c t i v i t i e s o f the species does not h o l d , i . e . t h e squares o f the c o e f f i c i e n t s do not g i v e the r e l a t i v e importance of the a c t i v i t i e s i n the f a c t o r . The q u a l i t y o f species s e p a r a t i o n appears t o be b e t t e r i n the 30 and 26 experimental p o i n t cases, t h a t i s t o say t h a t the l o a d ­ ings g e n e r a l l y are more r e s t r i c t e d t o s i n g l e f a c t o r s , e s p e c i a l l y 2

f o r Cu(0H)° and C u ( C 0 ) ~ although Cu(OH)" and CuC0° are somewhat l e s s w e l l separated (Table I I I ) . The boxes drawn are t o i n d i c a t e the p r i n c i p a l species dominating a f a c t o r , e.g. under Andrew's 30 Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

3

2 +

+

2 +

p o i n t case F a c t o r 1 I s p r i m a r i l y ( C u , CuOH , C u ( 0 H ) , Cu(0H)°) 2

2

w i t h F a c t o r 2 represented w e l l by (Cu(COH)", Cu(OH) ") and F a c t o r 3 by (CuCO^, C u ( C 0 ) " ) . 3

2

Note i n the 100 random p o i n t case t h a t

Cu(0H)° d i s t r i b u t e s 35% on F a c t o r 1 and 53% on F a c t o r 2 and Cu(C0 ) d i s t r i b u t e s A9% on F a c t o r 2 w i t h 31% on Factor 3, r e l a ­ t i v e l y poor s e p a r a t i o n s . An o f t e n used rule-of-thumb i s t h a t l o a d i n g s g r e a t e r than 0.3 should be considered s i g n i f i c a n t (12, p.10). As w i l l be evident from the d i s c u s s i o n below on s e p a r a b i l ­ i t y o f s p e c i e s , the 100 random-point-factors are a more accurate r e p r e s e n t a t i o n o f the behavior o f the hydroxo complexes over a wide pH range than are the 30 and 26 p o i n t f a c t o r s . The e x p e r i ­ ments o f Andrew at at. are r e s t r i c t e d t o three pH values (7.4> 7.5, 7.95) thereby l i m i t i n g the r e l a t i v e v a r i a t i o n o f hydroxo complex a c t i v i t i e s , c r e a t i n g a set o f data more e a s i l y represented by three f a c t o r s and p o s s i b l y g i v i n g the f a l s e impression t h a t t h e separations achieved are b e t t e r . Further a t t e n t i o n has not been given t o f a c t o r s generated from random p o i n t s due t o problems o f s t a t i s t i c a l independence e a r l i e r d i s c u s s e d . Separation o f species by f a c t o r a n a l y s i s i s l i m i t e d i n s e v e r a l ways. I n order o f decreasing importance the s e p a r a b i l i t y i s c o n t r o l l e d by: s p e c i a t i o n behavior over the ranges o f para­ meters used, experimental p o i n t d i s t r i b u t i o n over the ranges o f parameters, number o f species i n v o l v e d i n the c a l c u l a t i o n , and number o f f a c t o r s determined. With regard t o s p e c i a t i o n behavior: i n F i g u r e 1 are d i s ­ played the s p e c i a t i o n curves f o r the a c t i v i t i e s o f f r e e copper i o n and the hydroxo-complexes o f copper over a range o f pH. Inspec­ t i o n o f the curves r e v e a l s t h a t CuOH and C u ( 0 H ) have the same p a t t e r n o f behavior^ over the e n t i r e range ana cannot be separated, Cu(0H)^ and Cu(0H) ~ are s i m i l a r enough t h a t s e p a r a t i o n i s not t o be expected, and over the range o f pH values used by Andrew oX at. 7.4-8.0, one should not expect s e p a r a t i o n s other than those ob­ served. One might, upon f i r s t glance, f e e l t h a t the forms of t h e f a c t o r s are completely described by the c o r r e l a t i o n c o e f f i c i e n t s 2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

646

Figure 1.

MODELING IN AQUEOUS

SYSTEMS

Speciation curves for the activities of free copper ion and the hydroxo complexes of copper vs. pH

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

28.

MAGNUSON E T A L .

Metal-Ligand Species and

Aquatic Toxicity

647

given i n Table I I . The boxes drawn around sets of c o r r e l a t i o n c o e f f i c i e n t s correspond t o the boxes i n Table I I I and indeed, the l a r g e s t c o r r e l a t i o n c o e f f i c i e n t s l i e w i t h i n the boxes although q u i t e s u b s t a n t i a l values do l i e without. No mathematical manipul a t i o n can o b v i a t e the data i n Table I I and of course the f a c t o r s are d e r i v e d from the c o r r e l a t i o n c o e f f i c i e n t s . The u s e f u l n e s s of f a c t o r a n a l y s i s l i e s i n i t s a b i l i t y to determine a s m a l l set of s t a t i s t i c a l l y independant v a r i a b l e s which r e t a i n as much informat i o n as p o s s i b l e about the system. With regard t o p o i n t d i s t r i b u t i o n : i t should be c l e a r from Figure 1 t h a t the pH values must be r e p r e s e n t a t i v e of the range 6.0-8.0 f o r the above d i s c u s s e d s e p a r a t i o n t o be achieved. With regard to the number of s p e c i e s i n v o l v e d : the number of f a c t o r s i s r e s t r i c t e d t o no more than one-half the number of v a r i ables i f convergent, unique values are t o be obtained f o r communa l i t i e s of the v a r i a b l e s (11, p.200), (the communality, of a v a r i a b l e i s the amount of the v a r i a n c e of t h a t v a r i a b l e accounted f o r by the common f a c t o r s ) . Therefore, t o use a f i v e f a c t o r decomposition of a set of species one must have at l e a s t 10 species i n c l u d e d i n the c a l c u l a t i o n . With regard t o the number of f a c t o r s : i t should be obvious t h a t t o o b t a i n a s e p a r a t i o n of the f o u r groups mentioned l a t e r , at l e a s t f o u r f a c t o r s must be used. I f other species which have q u i t e d i f f e r e n t p a t t e r n s of behavior over the set of p o i n t s are i n c l u d e d i n the c a l c u l a t i o n , s u f f i c i e n t a d d i t i o n a l f a c t o r s must be i n c l u d e d to account f o r each s p e c i a t i o n p a t t e r n or the p a t t e r n s w i l l smear over the f a c t o r s . A d d i t i o n of more species or f a c t o r s f o r Andrew s data would not have proven f r u i t f u l due t o the l i m i t e d pH range i n v o l v e d i n the experiment. F i n a l l y , a f t e r a l l of the above have been taken i n t o account, i t i s p o s s i b l e t o " r o t a t e " a set of f a c t o r s w i t h d i f f e r e n t types of r o t a t i o n y i e l d i n g somewhat d i f f e r e n t species l o a d i n g ( 9 ) . I f the s p e c i a t i o n p a t t e r n s f o r the p o i n t s are not d i s t i n c t however, r o t a t i o n of the f a c t o r s cannot achieve a s e p a r a t i o n . We have found Varimax r o t a t i o n , which maximizes the squared l o a d i n g s of v a r i a b l e s i n each f a c t o r w h i l e r e t a i n i n g the o r t h o g o n a l i t y of the f a c t o r s t o be most u s e f u l . 1

Copper T o x i c i t y t o Daphnia Magna Lake S u p e r i o r water was used as the d i l u t i o n water f o r a l l experiments by Andrew (2). A copy of the current a n a l y t i c a l background f o r Lake S u p e r i o r water, Table V, was obtained from the Environmental Research Laboratory-Duluth and was used i n our s p e c i a t i o n c a l c u l a t i o n s . The water has a t o t a l hardness o f -45mg l as CaCO , a l k a l i n i t y of H2mg l as CaCO , and a pH from 7.4 t o 8.2. -1

3

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

648

CHEMICAL

MODELING IN

AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

TABLE V Concentrations used t o Simulate the A n a l y t i c a l Background of Lake S u p e r i o r Water ( M o l a r i t i e s ) Calcium Magnesium Sodium Potassium Cadmium Chromium Copper Iron Mercury Methyl Mercury Manganese Nickel Lead

3.26x10"^" 1.10x10 \ 5.60x10 \I 1.29x10 1.80x10 Q 3.80x10"^ 1.60x10 * 5.40x10 5.01x10 \

L

5.01X10"Q

5.50x10"^ 3.40xl0"\f 4.84xl0"

n

lu

Zinc Cobalt Barium Strontium Silver

1.10x10 -9 3.40x10 7 1.00x10' 7 1.80x10 -10 1.00x10'

Carbonate Chloride Sulfate Silicate Phosphate Nitrate

8.81x10r 4 3.61x10" 3.65x10^ -5 4.47x10' 3.24x10" 1.66x10"

The pH was adjusted i n each o f t h e i r experiments by r e g u l a t ­ ing C0p-air mixtures bubbled through the s o l u t i o n , e.g. Na^P^O^ was added t o the s o l u t i o n f o l l o w e d by b u b b l i n g s u f f i c i e n t CO^ through the s o l u t i o n t o b r i n g the pH back t o 7.95. This process i n c r e a s e s the t o t a l i n o r g a n i c carbon. REDEQL2 does a l l o w one t o determine the amount o f 2H + CO ~ necessary t o add t o b r i n g the pH back t o 7 . 9 5 . The t o t a l carbonate c o n c e n t r a t i o n s used i n our c a l c u l a t i o n s are l i s t e d i n Table V I . TABLE V I T o t a l Carbonate Values used i n R e c a l c u l a t i o n of Andrew's Data T o t a l Carbonate Andrew's Table 2

3

4

Experiments 1-7 8-11 12 - 15 16 - 19 1 2 3 4 5 1-4 5 6

0.881 0.929 1.114 0.889 0.935 1.936 2.938 4.932 10.940 0.920 0.931 1.119

S p e c i a t i o n C a l c u l a t i o n s . Although REDEQL2 was used t o determine s p e c i a t i o n i n Andrew's work as w e l l as ours, c o n s i d e r -

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

28.

MAGNUSON

Metal-Ligand Species and Aquatic Toxicity

ET AL.

649

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

able d i f f e r e n c e s e x i s t i n p r e d i c t e d e q u i l i b r i u m a c t i v i t i e s due t o m o d i f i c a t i o n s which have been made t o the program and the a s s o c i ­ ated data base. The major c o n t r i b u t i o n t o the d i f f e r e n c e s i s the i n c l u s i o n o f more complexes, e.g. Cu(0HL. Complexation by unc h a r a c t e r i z e d o r g a n i c l i g a n d s and a b s o r p t i o n o f s p e c i e s o r sus­ pended matter were not considered i n these s p e c i a t i o n c a l c u l a ­ t i o n s . (Both organic complexation and a b s o r p t i o n should be l e s s s e r i o u s problems i n t h i s Lake S u p e r i o r study as compared t o a non-laboratory system. ) I n Table V I I some comparative data a r e l i s t e d and i n Table I are shown the c a l c u l a t e d a c t i v i t i e s f o r the f r e e copper i o n p l u s the hydroxo and carbonato complexes. TABLE V I I Comparisons o f Some C a l c u l a t e d Copper Species A c t i v i t i e s from t h i s work (M-G) w i t h t h a t o f Andrew, at at (ABG ) C a l c u l a t e d A c t i v i t y o f Copper Species (μΜ) Measured 2+ Cu' dissolved copper (μΜ) ABG M-G ~0.02 0.20 0.33 0.41 0.61 1.02 1.54

0.001 0.011 0.016 0.020 0.031 0.034 0.076

***

CuOH AbG M-G 0.001 0.009 0.015 0.019 0.028 0.047 0.071

CuCO Cu(QH)° M-G ABG* M-G ABG

***

02 18 30 36 54 89 35

0.014 0.178 0.288 0.363 0.537 0.891 1.349

0.002 0.001 0.003 0.002 0.004 0.002 0.006 0.003 0.005 0.010 0.008 0.015 * Cu(0H)p was not considered i n ABG c a l c u l a t i o n s *** Value l e s s than p r e c i s i o n shown

0.002 0.020 0.033 0.042 0.062 0.102 0.155

S i n g l e Ion C o r r e l a t i o n s w i t h T o x i c i t y . The data g i v e n i n the f i r s t f i f t e e n cases, e x c l u d i n g those w i t h added Na^HPO,, were used t o determine c o r r e l a t i o n s o f ^ j n v e r s e median s u r v i v a l time w i t h species a c t i v i t y f o r f r e e Cu and the seven l i s t e d com­ p l e x e s . The values f o r r ranged from 0.97 t o ^ 9 8 w i t h s i g n i f i ­ cance 0.00001 f o r a l l species but one, C u ( 0 H ) , which e x h i b i t e d an r o f 0.95. F i v e of the p o i n t s ( 1 , 8, 9, 10, 12 excluding Na HP0. p o i n t s ) are " z e r o - p o i n t s " , i . e . median s u r v i v a l time exceeded the l i f e o f the experiment, and a b e t t e r t e s t o f the hypothesis o f t o x i c i t y due t o copper species r e s u l t s from d i s c a r d i n g the z e r o - p o i n t s . When t h i s i s done, the c o r r e l a t i o n c o e f f i c i e n t s decrease^by 0.01 or l e s s and the l e v e l o f s i g n i f i c a n c e remains a t the 10 level. In F i g u r e 2 a r e ^own the i n v e r s e median s u r v i v a l time versus a c t i v i t y p l o t s f o r Cu and CuC0~, both f o r the 15 p o i n t cases and the 10 p o i n t cases. +

p

p

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

650

Figure 2. Reciprocal median survival time vs. activity for Cu : (A) 10 point case, (C) 15 point case; for CuC0 °: (B) 10 point case, (D) 15 point case 2+

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

28.

MAGNUSON

ET

651

Metal-Ligand Species and Aquatic Toxicity

AL.

F a c t o r A n a l y s i s and M u l t i p l e Regression. F a c t o r scores f o r the 26 and 30 p o i n t cases, generated u s i n g the f a c t o r score c o e f f i c i e n t s i n Table IV, were c o r r e l a t e d w i t h i n v e r s e median s u r v i v a l time, t " . The 26-point case y i e l d e d the f o l l o w i n g r e l a t i o n s h i p i = 0.0046 F

1

+ 0.0027 ?

2

- 0.00013 F' + 0.0061

(7)

2

- 0.00011 F' + 0.0055

(8)

w h i l e the 3 0 - p o i n t case y i e l d e d

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch028

i = 0.0051 F

x

+ 0.0024 F

Standard e r r o r s , 95$ confidence i n t e r v a l s , and l e v e l s of s i g n i f i ­ cance are t a b u l a t e d i n Table V I I I f o r the c o e f f i c i e n t s i n Equa­ t i o n s Τ and 8.

TABLE V I I I Standard E r r o r s , 96% Confidence I n t e r v a l s , L e v e l s o f S i g n i f i c a n c e and Changes i n R f o r the 26-Point and 30-Point Regression Equations 26-Point C o e f f i c i e n t Case χ 10" 4.6

2.7 F Constant

-0.13 6.1

Standard

0

E r r o r χ 10 0.57 0.60 0.62 0.60

3

95% Confidenc I n t e r v a l χ 10" (3.4,5.8) (1.4,3.9) (-1.4,1.2) (4.9,7.4)

L e v e l of Change, Significance i n R

· t)

where t i s t h e temperature i n °C.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(45) ( 4 6 )

30.

V A N LuiK A N D juRiNAK

Heavy Metals in Electrolyte Solution

699

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

Experimental V a l i d a t i o n of the model i n v o l v e d a p p l i c a t i o n of the model t o 1) measurements of n a t u r a l b r i n e macro- and micro-component con­ c e n t r a t i o n s t o determine whether p r e d i c t e d s a t u r a t e d phases c o r r e s ­ ponded w i t h n a t u r a l l y o c c u r r i n g p r e c i p i t a t e d phases, and 2) mea­ surement of e q u i l i b r i u m s a t u r a t i o n concentrations of Cu, Pb, Cd, and Zn i n 2, 4, and 6 molal a r t i f i c i a l b r i n e s o f known composition to v a l i d a t e the p r e d i c t i v e a b i l i t y of the micro-component p o r t i o n of the model. In these s t u d i e s , thermodynamic e q u i l i b r i u m c o n d i t i o n s were assumed. Constant pH monitoring was used as an estimator of t h e e q u i l i b r a t i o n times i n v o l v e d , s i n c e a s t a b l e pH i s an o v e r a l l i n ­ d i c a t o r of the i n t e r n a l chemical s t a b i l i t y of pH dependent pro­ cesses. I n order t o a l l o w the assumptions of atmospheric O2 and CO2 p a r t i a l p r e s s u r e s , water scrubbed compressed a i r was pumped i n t o the r e a c t i o n v e s s e l s under constant temperature and constant vigorous mixing c o n d i t i o n s . The a i r was introduced by f r i t t e d g l a s s bubblers which had been t e f l o n coated w h i l e a i r flowed through them i n order to minimize the g l a s s - s o l u t i o n i n t e r f a c e . Sampling from the r e a c t i o n v e s s e l s was done using a 50 ml Class A p i p e t . With r e a c t i o n v e s s e l s c o n t a i n i n g i n i t i a l l y 4000 ml of s o l u t i o n , t h i s represented a withdrawal of 1.25 percent of t h e t o t a l s o l u t i o n per sampling, a l l o w i n g the assumption t h a t the s o l u t i o n was not s i g n i f i c a n t l y a f f e c t e d by any given sampling w i t h regard to volume or composition. Samples were immediately f i l t e r e d through Whatman #42 f i l t e r paper. Macro-component c a t i o n analyses were done by d i s t i l l e d water d i l u t i o n and subsequent flame emission or flame atomic a d s o r p t i o n . C h l o r i d e s were determined by p o t e n t i o m e t r i c t i t r a t i o n s per­ formed on d i s t i l l e d water d i l u t i o n s . Carbonates and bicarbonates were e i t h e r determined by t i t r a t i o n or by c a l c u l a t i o n from pH measurements. S u l f a t e s were determined by the g r a v i m e t r i c method u t i l i z i n g BaS04 p r e c i p i t a t i o n . Trace metal analyses were performed using sample a c i d i f i c a ­ t i o n , complex formation using ammonium dithiocarbamate (APDC), e x t r a c t i o n w i t h methylisobutylketone (MIBK), and a n a l y s i s of ex­ t r a c t s by flame atomic a d s o r p t i o n . D e t a i l s of the method are given by Brooks, P r e s l e y , and Kaplan (29). R e s u l t s and D i s c u s s i o n V a l i d a t i o n of the Osmotic C o e f f i c i e n t C a l c u l a t i o n . The r e ­ s u l t s f o r the osmotic c o e f f i c i e n t s c a l c u l a t e d f o r a s e r i e s of b r i n e s r e p r e s e n t i n g an evaporative c o n c e n t r a t i o n sequence of sea water b r i n e s are given i n F i g u r e 1 and Table I . I n F i g u r e 1, t h e agreement between the measured (.10) and c a l c u l a t e d osmotic c o e f f i c i e n t s are compared. Table I a l s o i n c l u d e s the c o e f f i c i e n t of v a r i a t i o n between the c a l c u l a t e d and measured v a l u e s . The maximum c o e f f i c i e n t of v a r i a t i o n encountered was 0.90 percent and

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

700

M O D E L I N G IN

AQUEOUS

SYSTEMS

the average c o e f f i c i e n t of v a r i a t i o n over the range considered was 0.26 percent.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

The Mean M o l a l A c t i v i t y C o e f f i c i e n t C a l c u l a t i o n f o r the Macro-Component S a l t s . The mean m o l a l a c t i v i t y c o e f f i c i e n t s f o r the macro-component c h l o r i d e and s u l f a t e s a l t s are given i n F i g u r e s 2a and 2b, r e s p e c t i v e l y . Because of the u n a v a i l a b i l i t y of measured a c t i v i t y c o e f f i c i e n t data i n b r i n e s , v a l i d a t i o n of the r e s u l t s presented i n F i g u r e s 2a and 2b was not p o s s i b l e .

The Solvent A c t i v i t y C a l c u l a t i o n . A s e r i e s of measurements on a Great S a l t Lake (GSL) b r i n e c o n c e n t r a t i o n sequence showed that the s a t u r a t i o n vapor pressure lowering over a GSL b r i n e at or near the l a b i l e s a t u r a t i o n p o i n t should be about 9.5 m i l l i b a r s at 25°C. A p p l i c a t i o n of the model to comparable GSL b r i n e r e s u l t e d

Osmotic C o e f f i c i e n t f o r Sea Water Concentrates at 25°C

Table I .

I

(molal)

ή.

φ cale.

Coeff. of + v a r i a t i o n (%)

φ meas. 0.953

0.896

0.962

0.970

0.586

2.535

0.989

0.995

0.427

3.013

1.023

1.023

0.000

3.586

1.065

1.065

0.000

4.275

1.117

1.115

0.127

5.107

1.181

1.180

0.059

6.115

1.260

1.260

0.000

1.800

0.941

2.135

C a l c u l a t e d from the model, using molar input data. Molar compo­ s i t i o n r a t i o s normalized w i t h respect to sodium: Na+ = 1, K+ = .021, M g = .136, C a = .0001, C I " = 1.174, So| = -.056. 2 +

Interpolated Wood (10).

2 +

g r a p h i c a l l y from data presented by Robinson

χ C o e f f i c i e n t of v a r i a t i o n = — * X

100.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

and

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

30. V A N LuiK A N D J U R I N Α κ

Heavy Metals in Electrolyte Solution

701

Figure 1. Comparison of osmotic coefficients calculated in the model with mea­ sured osmotic coefficients for sea water concentrates. ( ) NaCl, measured; ( ) sea water concentrates, measured; (A) sea water concentrates, calculated using the model.

Molal

Ionic Strength

Figure 2. Mean molal activity coefficients of macrocomponent salts as a function of ionic strength for sea water concentrates. Normalized molar composition ratios: Na = 1.00; K = 0.021; Mg = 0.136; Ca , Cd , Cu , Pb , Zn = 0.001; Cl{ =1.174; S O / ' = 0.056. (O) Computation points. +

+

2+

2+

2+

2+

2+

2+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

702

CHEMICAL

M O D E L I N G IN

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SYSTEMS

i n a s o l v e n t a c t i v i t y v a l u e e q u i v a l e n t to a s a t u r a t i o n vapor p r e s ­ sure lowering of 8.2 m i l l i b a r s ( c o e f f i c i e n t of v a r i a t i o n 10.39%). To assess the meaning of such a discrepancy, f i e l d measurements by Turk (31) were compared to the l a b o r a t o r y r e s u l t s of Dickson et a l . (3"UJ, f o r two b r i n e s s i m i l a r i n composition to GSL b r i n e , at a s p e c i f i c g r a v i t y of 1.20 k g / l i t e r . Turk (31) found t h a t the s a t u r a t i o n vapor pressure lowering f o r t h i s b r i n e was 9.5 m i l l i ­ bars w h i l e the comparable v a l u e measured by Dickson _e_t a l . (30) was nearer 7.6 m i l l i b a r s ( c o e f f i c i e n t of v a r i a t i o n 15.7%T. Saturated vapor pressure lowering values f o r sea water concen­ t r a t e s presented by Turk (31) allowed f u r t h e r comparisons between the vapor pressure lowering as c a l c u l a t e d by the model and e x p e r i ­ mental v a l u e s . The data are shown i n Table I I . Since the s o l v e n t a c t i v i t y i s s t r o n g l y dependent on the s o l u t e composition, the agreement demonstrated i n Table I I i s considered reasonable.

Table I I . S a t u r a t i o n Vapor Pressure Lowerings f o r a S e r i e s of Sea Water Concentrates: C a l c u l a t e d Vs. Measured Values at 25°C 4-

(1-a )e w s millibars v

s s millibars

Coeff. of v a r i a t i o n (%)

I molal

P* g/ml

a * w

1.80

1.064

.9522

1.51

1.17

17.94

2.14

1.075

.9424

1.83

1.68

6.04

2.54

1.087

.9301

2.22

1.90

10.98

3.01

1.103

.9148

2.70

2.28

11.93

3.59

1.120

.8955

3.31

2.60

16.99

4.73

1.142

.8834

3.70

3.80

1.89

*

5.11

1.166

.8400

5.07

5.39

4.33

6.11

1.196

.8004

6.33

6.97

6.81

As c a l c u l a t e d from molar i o n c o n c e n t r a t i o n data u s i n g the model i n t h i s study, e i s the s a t u r a t e d vapor pressure i n m i l l i b a r s at 25°C. s

Measured values i n t e r p o l a t e d from g r a p h i c a l l y presented data g i v i n g vapor pressure lowering at 25°C as a f u n c t i o n of b r i n e d e n s i t y (31), e i s the s a t u r a t e d vapor pressure of the b r i n e i n m i l l i b a r s at 25°C. f

s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

30.

V A N LuiK A N D J U R I N Α κ

Heavy Metals in Electrolyte Solution

703

S i n g l e Ion A c t i v i t y P r e d i c t i o n s . I n l i e u of an attempt t o t e s t the carbonate system p r e d i c t i v e c a p a b i l i t y of the model, use was made of the f a c t that i n most a e r o b i c n a t u r a l a q u a t i c systems the s o l u t i o n c o n c e n t r a t i o n of heavy metal c a t i o n s i n the absence of i n t e r f a c i a l phenomena i s o f t e n c o n t r o l l e d by the carbonate, b a s i c carbonate or oxide (hydrous) s o l i d phase form of the metal (22, 32, 33, 34). Thus, the a b i l i t y of the model t o p r e d i c t the chemistry of heavy metals i n b r i n e i n a sense was used t o t e s t the v a l i d i t y of the carbonate subroutine. The g e n e r a l procedure was t o assume that the t r a c e metal s o l u b i l i t y i n b r i n e was c o n t r o l l e d by e i t h e r the carbonate, b a s i c carbonate or hydrous oxide form of the metal. The heavy metal and carbonate i o n a c t i v i t i e s were determined by the model. The r e s u l t a n t c a l c u l a t e d s o l u b i l i t y of the heavy metals i n b r i n e was then compared w i t h e x p e r i m e n t a l l y determined values. I t should be noted that the complexity and i l l d e f i n i t i o n of carbonate s a l t s i n h i g h l y s a l i n e s o l u t i o n s i s documented i n l i t e r ­ ature by the complete absence of s i n g l e s a l t osmotic and a c t i v i t y c o e f f i c i e n t data. This s i t u a t i o n r e q u i r e s the use of t h i s l e s s than s a t i s f y i n g technique t o d e f i n e heavy metal chemistry i n brines. Considerable e f f o r t was expended i n attempts to v a l i d a t e the a b i l i t y of the model t o p r e d i c t t r a c e element s o l u b i l i t i e s i n both n a t u r a l and a r t i f i c i a l b r i n e s of v a r i o u s compositions and i o n i c strengths i n the l a b o r a t o r y , but a f t e r n i n e long term exper­ iments i t became apparent that the k i n e t i c s of t r a c e metal carbo­ nate p r e c i p i t a t i o n would make t h i s type of l a b o r a t o r y v a l i d a t i o n of the model extremely d i f f i c u l t . A search of the l i t e r a t u r e showed that metal carbonate p r e c i p i t a t i o n k i n e t i c s were g e n e r a l l y very slow, e s p e c i a l l y i n complex s o l u t i o n s (35). I n p a r t i c u l a r , the slowness of the k i n e t i c s of cadmium carbonate p r e c i p i t a t i o n were d e s c r i b e d as n o t o r i o u s and as p r e s e n t i n g a serious problem i n wastewater treatment and c o r r o s i o n c o n t r o l systems aimed a t cadmium removal from s o l u t i o n (33, 34). Of the f o u r metals being s t u d i e d , copper was the only t r a c e metal which was p r e c i p i t a t e d r a p i d l y enough t o be e s s e n t i a l l y a t e q u i l i b r i u m w i t h the a r t i f i ­ c i a l b r i n e s a f t e r a p e r i o d of t e n days. C h a r a c t e r i s t i c a l l y , i n both the n a t u r a l and a r t i f i c i a l b r i n e s , the r e d u c t i o n of copper i n s o l u t i o n was accompanied by the establishment of a l g a l growths and subsequent severe f r o t h i n g i n the s t o r e d , aerated, constant temperature r e a c t i o n v e s s e l s . The attempt t o v a l i d a t e the model, t h e r e f o r e , s h i f t e d from the a d d i t i o n of metals t o b r i n e t o measuring metals i n n a t u r a l b r i n e s which could be assumed to be near e q u i l i b r i u m w i t h t h e i r c o n s t i t u e n t t r a c e metals. North Arm Great S a l t Lake, Utah (GSL) b r i n e was chosen because i t s composition r e l a t i v e t o South Arm b r i n e s i s f a i r l y constant (36, 37), a l l o w i n g data from other workers t o be m e a n i n g f u l l y compared w i t h t h i s present study. Cu, Pb, Cd, and Zn were determined i n the GSL North Arm B r i n e s and

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch030

704

M O D E L I N G IN

AQUEOUS

SYSTEMS

compared w i t h data on GSL North Arm b r i n e s given by Taylor et a l . (38). The f i l t e r e d b r i n e r e s u l t s f o r the two s t u d i e s are i n essen­ t i a l agreement, as shown i n Table I I I . The disagreement between the u n f i l t e r e d b r i n e r e s u l t s of the two s t u d i e s r e f l e c t s sampling location differences. Table IV shows the p r e d i c t e d and measured c o n c e n t r a t i o n of Cu, Pb, Cd, and Zn i n the GSL North Arm b r i n e . The measured values are compared w i t h the p r e d i c t e d s o l u b i l i t i e s c a l c u l a t e d f o r the b a s i c carbonate, carbonate, and hydroxide phases. The lead s u l f a t e form was a l s o i n c l u d e d . The measured and c a l c u l a t e d values i n most cases appear w i t h c o e f f i c i e n t s of v a r i a t i o n r e p r e ­ s e n t i n g the s i x b r i n e s used t o provide the average values reported. In g e n e r a l , the agreement between p r e d i c t e d and measured was good f o r a f i r s t generation model. The data show that the con­ c e n t r a t i o n of Cd i n the b r i n e was c o n t r o l l e d by the s o l i d phase o t a v i t e , CdC03. The copper i n the b r i n e was supersaturated about seven times w i t h respect to b a s i c copper carbonate, m a l a c h i t e , Cu2(0H)2C03. Lead i n the b r i n e was found to be supersaturated about fourteen times w i t h regard to e i t h e r PbC03 or Pb(0H)2« Zinc was shown to be s l i g h t l y supersaturated w i t h respect to hydroz i n c i t e , Ζη(0Η)χ 2(C03)Q.4. The p r e d i c t e d values f o r the v a r i o u s s o l i d phases shown i n Table IV were c a l c u l a t e d using the hydroxide and carbonate a c t i ­ v i t y p r i n t o u t of the model. These values were used to c a l c u l a t e the e q u i l i b r i u m t r a c e c a t i o n a c t i v i t i e s i n the b r i n e . The a c t i ­ v i t y values were then d i v i d e d by the p r e d i c t e d s i n g l e i o n a c t i v i t y c o e f f i c i e n t s and thereby converted t o the m o l a l c o n c e n t r a t i o n s c a l e . These m o l a l u n i t s were then converted to molar u n i t s , and then i n t o the m i l l i g r a m s per kilogram of s o l u t i o n u n i t s reported. Considering the assumptions incorporated i n t o the carbonate subroutine and i n using the m o l e - f r a c t i o n s t a t i s t i c a l model f o r the p a r t i t i o n i n g of the mean m o l a l a c t i v i t y c o e f f i c i e n t s i n t o s i n g l e i o n a c t i v i t y c o e f f i c i e n t s , the agreement between p r e d i c t e d and measured t r a c e metal concentrations i n GSL b r i n e was b e t t e r than expected. A n a l y s i s of X-ray d i f f r a c t i o n p a t t e r n s of the p r e c i p i t a t e s r e s u l t i n g from the a d d i t i o n of s a l t s of the four metals to GSL North Arm b r i n e showed t h a t both copper and lead p r e c i p i t a t e d i n the b a s i c carbonate form. Zinc and cadmium compounds were not found i n these X-ray d i f f r a c t i o n t r a c e s , which may be due t o these compounds comprising l e s s than f i v e percent of the s o l i d m a t e r i a l analyzed. β

Summary and

Conclusions

The o b j e c t i v e of t h i s study was to develop a model which would d e s c r i b e the chemistry of t r a c e metals i n h i g h l y concen­ trated e l e c t r o l y t e solutions.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

.0002^

.0007

.0002

.0001

+

+

+

+

.0006

.0049

.0026

.0025

Cu

Zn

Pb

Cd

.0066

.0028

.0060

.0010

.0012 .0015

+

.0009

.0006

+

±

±

Unfiltered

LO

ο ο ο

r—

ο

•

ο

•

• ο r—

LO • r—

CO



ο θ3 C D (/> S- s- c Li_ > "Ο

Οc4-> ο3 ω υ

ο

CDCD "O

+->

03 J Q E S-

Ε

Ο

CD

•ι—

en CO s- -a

C D C 0 3 C D «— OC 4->θ3 ->I — C CD+S STi

+-> •ι-

Ε

+J

ο

LO ο ^t-

CO I —

o

ο

o

OsJ

^·

ο ο ο

ο ο

Ο ο LO

r—

Ε ο •r— 4->

03

Ο

«Ι­

Ο

CO -4-> r—

Ζ5 ce CD C0

Ο

•r—

03

•Γ-

S4-> Ε E

ΟD οθ 3cr C Ο ο

υ

Ι ­

Ε ο

CD S -C D Ο > 03 CD ο θ3 Οί­CD ^- TJ


o ο

o t—

Ο

cr» cr»

Γ -

•Γ—

>>

-M Ε

Ο

ο

^

Ο

4->

CO

Ο

ο

ρ—-

ο

LO

cr»

•Γ-

Ο ο < : -Μ

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

719

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

720

portion of the sediments, was mixed with fresh sediment in approximately a 1:2 sediment:pore water ratio. A 2 ml sample of the sediment slurry was placed in a small glass tube and stoppered with a rubber septum. The sediment transfers were done under an N2 atmosphere to avoid oxidation. A volume of standard arsenic or phosphate solution was injected with a syringe through the septum. The tubes were shaken for approximately 24 hours. The solids were separated by centrlfugation and f i l t r a t i o n through a 0.45 ym f i l t e r , and the pore water analyzed for arsenic or phosphorus. Blanks were run using arsenic and phosphorus in d i s t i l l e d water and pore water to investigate sorption to the glass tubes and precipitation in the pore waters. Both factors were found to be unimportant in removing arsenic or phosphorus from the experimental solution. Results. The experimental results (Figure 4) show significant differences in the loss from solution of the various arsenic species and phosphate. Clearly, some species are removed from solution to a much greater extent than others, with the relative removals being as follows: P0 > AS0 > MMAA > As0 > CA 4

4

most strongly removed

3

least strongly removed

where MMAA = monomethyl arsonic acid and CA = cacodylic acid. Phosphate and arsenate were both removed to a great extent, while cacodylic acid was removed only slightly and in some cases not at a l l . Since the loss from solution occurred only when the sediments were present, either sorption or surface precipitation must be controlling the concentrations. Since adsorption is probably the major process controlling the observed loss from solution, 1t is instructive to f i t the data obtained to a Langmuir isotherm. Adsorption constants for the various species calculated from the data obtained are given in Table IV. Constants for arsenite were not calculated because the data indicated that arsenite loss from solution does not follow an adsorption isotherm. Inspection of the arsenite loss from solution isotherm shown in Figure 4 indicates that the loss Is linearly dependent on concentration over the concentration range used. The constants given In Table IV show the strong adsorption of phosphate and arsenate relative to cacodylic acid, with monomethyl arsonic acid intermediate. Constants for the adsorption of arsenate on amorphous iron and aluminum hydroxide are also included in Table IV to compare the sediments with well-characterized adsorbents. It was assumed that loss from

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 4. Sorption of phosphate and arsenic species by anaerobic Menominee River sediments

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

to

722

CHEMICAL

MODELING

IN AQUEOUS

SYSTEMS

Table IV Sorption of Phosphate and Arsenic Species by Anaerobic Sediments

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

Species PO

4

As0

4

MMAA

Γ

*-ι

ymol g

K a

-i

Ν

vimol 1

b

d

9.1

15

7.7

32

3

3.6

28

11

3

d

Correlation Coefficient? 0.86 0.98 0.38

As0 ·

h

1250

137

~

--

As0

h

1700

51

—

—

4

CA

f

4

g

'

3.6

320

4

e

0.85

?Langmuir c o e f f i c i e n t s . Number of p o i n t s . C o r r e l a t i o n of l i n e a r i z e d form of data to a l e a s t squares ^approximation of the best s t r a i g h t l i n e through the p o i n t s . Ignoring one data p o i n t shown i n Figure 4 due to p o s s i b l e background i n t e r f e r e n c e . Ignoring three points i n which l i t t l e or no adsorption was JC observed. Adsorption on amorphous A l ( 0 H ) g . rAdsorption on amorphous Fe(0H)o. Data from Ferguson e t ajk ( 3 1 j .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

31.

HOLM

Heterogeneous Interactions of Arsenic

ET AL.

723

solution followed a Langmuir isotherm. Since a small number of points were used and there was some scatter in the data, there is some uncertainty in the constants. However the constants are useful for calculations, as will be seen later in this paper. Discussion. Wauchope (4) reported that adsorption of phosphate and various arsenic species on alluvial soils varied as follows: As0 « MMAA > DMAA > P0 Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

4

most strongly adsorbed

4

least strongly adsorbed

The arsenic species adsorbed in the same pattern in both studies, while phosphate changed from being the most strongly adsorbed in this study to the least strongly adsorbed in Wauchope's study. The soils used in Wauchope's study were presumably aerobic, with iron and aluminum hydroxides present. The sorption of phosphate and the arsenic species on the twelve soils used in Wauchope's experiment correlated well with both the clay content and the iron content of the s o i l s . The Menominee River sediments, however, were anaerobic, so iron should have been present as Fe(OH)? rather than the Fe(0H)3 which was, presumably, present in Wauchope's alluvial s o i l s . The solubility of Fe(0H)2 is greater than that of Fe(0H) (30) and its surface properties may d i f f e r from those of Fe(0H)3. Tfius, differences in amount and speciation of iron may have accounted for differences in phosphate sorption. Ferguson and Anderson (31) investigated the adsorption of arsenate and arsenite on Tron and aluminum hydroxides and found that for both adsorbents the adsorption of arsenate followed a Langmuir isotherm while the sorption of arsenite varied linearly with concentration. In the concentration range used in the experiment described above, they found that arsenate was much more strongly adsorbed than arsenite. They did not explain the rather surprising difference in behavior between arsenate and arsenite; however, their results give support to the results found in this experiment. Combining the results of the three studies, i t appears that arsenate is more strongly adsorbed than monomethyl arsonic acid, which is more strongly adsorbed than cacodylic acid, and that a l l three probably follow Langmuir isotherms. Arsenite sorption, on the other hand, varies linearly with concentration over concentration ranges of environmental significance. Phosphate adsorption also follows a Langmuir isotherm, but the strength of adsorption relative to the arsenic species varies depending on the adsorbent. 3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL MODELING IN AQUEOUS SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

724

Adsorption processes are very dependent on the solid surface characteristics and pH, as well as other factors (adsorbate and adsorbent identity and concentration, solution ionic strength, identity of other ions present, e t c . ) . Since neither surface characteristics nor pH were measured or controlled, the numerical values obtained in the experiment need to be interpreted with care, as was discussed previously. The relative trends, however, will probably be valid for sediments similar to those used in the experiment, i . e . , anaerobic river sediments where sediments control the concentration of the species in solution. Wauchope's findings Indicate that the relative trends may not be valid for aerobic sediments or s o i l s . Adsorption in sediments or soils occurs primarily on various clays and on amorphous iron and aluminum hydroxide particles and coatings on other particles. The concentration of the adsorbing species obviously varies greatly in different sediment samples. Thus the constants determined for one sediment sample will apply only to the particular concentrations and composition of adsorbing species found in that sample, i . e . , the constants will be sample specific. As long as the adsorbing species remain the same in different sediment samples, although at different concentrations, the relative adsorbing patterns found for different species will remain the same, and the results from one set of experiments can be used as an aid to understanding the processes occurring in a similar sediment sample. If, however, the adsorbing species change between samples, then the relative adsorptive strengths of the sediments for different anions may change, and the results from experiments with one sediment cannot be applied to the different sediments. Further experiments are planned to better elucidate the relative adsorbing strengths of phosphate and arsenic species on aerobic sediments. Rates of Species Transformations in Anaerobic Sediments Kinetics and Adsorption. If microorganisms only take up dissolved arsenic species, then adsorption can affect rates of species transformations by lowering concentrations of reactants. In this section calculations of changes in concentrations of reactants and products of arsenic species transformations in sediments are presented. Rates of transformation are assumed to be proportional to species concentrations and the rate of another reaction, V, coupled to the transformation. That 1s, the transformation reaction is not assumed to be a source of energy or structural material for the microorganisms. Thus, -

d

(y*0

= kV (Species) .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

31.

HOLM ET AL.

Heterogeneous Interactions of Arsenic

725

If V is constant, for example i f V is described by the Michelis-Menten equation and the substrate concentration is high enough that V approaches Vmaxs then kV is a lumped constant and the transformation rate is described by a f i r s t order equation. Calculating the effects of adsorption on kinetics necessarily involves mixing kinetic and equilibrium data. However, the time scales for adsorption and desorption reactions are much shorter than those for mlcrobially mediated arsenic species transformations. Adsorption and desorption reactions reach equilibrium over a period of 24 hours or less (32, 33). On the other hand, Woolson (2) estimated conversion rates of 0.067 to 0.404 % day" for oxidative metabolism of cacodylic acid to arsenate in model aquatic systems. Assuming f i r s t order kinetics, these conversion rates translate to half lives of 5.8 to 34.7 months. Thus, Woolson's model ecosystems were probably at equilibrium at a l l times with respect to adsorption and desorption of arsenic species.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

1

Examples of the Influence of Adsorption on Kinetics. Three cases of arsenic species transformations influenced by adsorption are considered in this section. The three cases considered are: 1) reactant not adsorbed, product adsorbed (e.g. demethylation of cacodylic acid to arsenate), 2) reactant adsorbed, product not adsorbed (e.g. methylation of arsenate to cacodylic acid), 3) both reactant and product adsorbed and competing for adsorption sites (e.g. demethylation of methanearsonic acid to arsenate). The equations for each case are shown in Tables V through VII, respectively. Most of the symbols in Tables V, VI, and VII have their common meanings. The symbol [B] stands for the concentration of adsorbed species Β and is the product of Γ, the number of moles of Β adsorbed per gram of adsorbent, and U, the number of grams of adsorbent per l i t e r . Thus, [B] has the units moles per l i t e r . An example of each case was calculated using the adsorption constants for anaerobic Menominee River sediments listed in Table IV. The f i r s t order rate constant used was 0.04 day" , corresponding to a half l i f e of 17 days. Initial reactant concentrations of 0.001 M were chosen to approximate conditions in moderately contaminated sediments (16). A sediment solids concentration of 60 g l " (corresponding to 95% water) was used. In the f i r s t two cases, setting up and solving the differential equation was straightforward. In case 2 a closed form solution could not be obtained, so the equation was solved numerically. Setting up the differential equation for case 3 involved solving a cubic equation, so a different approach was taken. For case 3 the system of mass balance and Langmuir equations was solved for [A] and [B]. Then the system was allowed to s

s

1

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

726

MODELING IN AQUEOUS

SYSTEMS

Table V Case 1. F i r s t Order Reaction. Product Adsorbed According to Langmuir Isotherm. Reactant not Adsorbed A

-> Β

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

=JP

- k [A]

C = [A] + [B] U Γ„ IB].

+ [B]

s

[B]

Κ + [B]

S

at t = 0

^[A] = C,

[Β] = 0

-kt [A] = Ce B

=

(independent of adsorption)

-N + (N

2

+ 4KC(l-e" )) k t

where Ν = Ur Example:

1 / 2

+ Κ - C(l-e~ ) kt

m

demethylation of cacodylic acid (weakly adsorbed) to arsenate (strongly adsorbed).

Key to Symbols:

A

reactant, Β product

[A]

aqueous concentration of [A]

U

concentration of adsorbent in g

Κ

constant in Langmuir isotherm adsorption capacity of adsorbent in mol g"l

[B]

c

S

concentration of adsorbed product t^s

=

U r

B

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

31.

H O L M E T AL.

Heterogeneous Interactions of Arsenic

727

Table VI Case 2. F i r s t Order Reaction. Reactant Adsorbed According to Langmuir Isotherm. Product Not Adsorbed. dt

dt

K

l

M

J

C = [A] + [A] + [B]

at t = 0 C = [k]s,o + [A]

s

UF [A]

0

Κ + [A]

s

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

M-

[A]

œ

Q

Ν,

α N

α

+

l

2

(^2-1

n

= "

where N = 2 κ + y-2

k

c°

t+

n s t a n t

α = [A]/C

1

H = 2 + κμ

K

2

=

K

/

C

y = r u/c

ά

œ

Other symbols described in Table V. Solve numerically using Newton iteration. Constant calculated by substituting a

Q

Case 3. A

t Q t

= [A]

B

s

t o t

=

Q

+

roA

[A]

at t = 0

1

s

[A]

2

K + [A] + [B] K /K A

A

- [Β]

+

[B]

B

3

s

[B] - i k i L _ _ S

C

Table VII First Order Reaction. Product and Reactant Adsorbed According to Competitive Langmuir Isotherm.

U r [ A ]

= [A] /

Kg + [B] + [A] K /K B

4 A

Symbols described in Table V. Procedure:

1. 2. 3.

Solve equations 1-4 for [A] and [B]. Compute ΔΑ = [A] ( l - e " ) for a short interval t. (Choose t such that ΔΑ [A]). Subtract ΔΑ from A and add ΔΑ to B . kt

t Q t

t Q t

In Chemical Aqueous Systems;the Jenne,procedure. E.; 4. Go toModeling step 1in and repeat

ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

728

MODELING IN AQUEOUS

SYSTEMS

react as i f there were no solid present and [A] was the total concentration of A. In other words, an amount of Α, ΔΑ, was changed to Β according to the equation ΔΑ = [AlO-e-kt), where t was chosen to be less than 0.1 ti/2. Then A was subtracted from C A and added to C B and [A] and [B] were recalculated. The entire process was repeated until the desired time interval was covered. The method used for the solution of case 3 was tested on systems for which the differential equation could be set up. The approximate solution was within 0.1% of the exact solution in each case. Demethylation of cacodylic acid to arsenate was chosen as an example of case 1, product adsorbing with reactant not adsorbing, because cacodylic a d d is only weakly adsorbed by anaerobic sediments and arsenate is strongly adsorbed. For the same reason, methylation of arsenate to produce cacodylic acid was chosen as an example of case 2, reactant adsorbed and product not adsorbed. Demethylation of methanearsonic acid to arsenate was chosen as an example of case 3, reactant and product both adsorbing. The predicted arsenate vs time curves for demethylation of cacodylic acid are shown Tn Figure 5. Both curves are for f i r s t order decay with a rate constant of 0.04 day"' but with different amounts of adsorbent present. Adsorption of arsenate results in dissolved arsenate concentrations that are lower than total arsenate. An s-shaped curve results because arsenate produced during the f i r s t few days is almost entirely adsorbed. Predicted arsenate and cacodylic acid vs time curves for methylation of arsenate to produce caco*c|yMc acid are plotted in Figures 6 and 7, respectively. Adsorption of arsenate lowers the dissolved arsenate concentration and, therefore, results in a slower rate of methylation. The apparent rate constant for this reaction in sediment containing 60 g of solids per l i t e r is 0.03 d a y compared with 0.04 d a y ' in the absence of sediment. Demethylation of monomethylarsonic acid to produce arsenate with the two species competing for adsorption sites is depicted in Figure 8. Adsorption of MMAA results in slower conversion rates and the apparent rate constant in this case is 0.03 day-1. Since arsenate is adsorbed more strongly than MMAA the final arsenate concentration is less than the i n i t i a l MMAA concentration. The apparent rate of decomposition of adsorbed reactant is quite sensitive to the product of the parameters U and Γ^. For MMAA demethylation using the parameters of case 3, for which Ur^ is 216 μΜ, the apparent rate constant is 0.04 day" . If ΙίΓοο is increased to 500 the apparent rate constant drops to 0.03 day" . Dilution of sediment decreases U and, thus, 1

1

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

31.

HOLM

Heterogeneous Interactions of Arsenic

ET AL.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch031

Ο

ΙΟ

20

30 TIME

40

50

729

60

(days)

Figure 5. Dissolved arsenate from demethylation of cacodylic acid, k = day , = 7.7 ^mol g~\ Κ = 32 mol L . (a) U = 0; (b) U = 60g Is . 1

1

μ

0.04

1


( s i l i c a t e rock) (H ) + M

+

+ HCO3

+

thus, H ions are consumed, i n c r e a s i n g the pH and b i c a r b o n a t e , and metal ions appear i n s o l u t i o n . The r e a c t i o n progress may be modeled as a s e r i e s of s t e p s , the beginning and end of each step defined by a s p e c i f i c pH change, which, i n t u r n , f i x e s concentra­ t i o n s of a l l major carbonate species by e q u i l i b r i u m (equation 1) and mass-balance (equation 2) r e l a t i o n s h i p s : 2.16 X

(2)

3

10" .

For the a n a l y s i s presented here, an approximation to c o n t i n u o u s l y v a r y i n g pH was_made by choosing 0.5-pH i n t e r v a l s , and the boundary v a l u e s f o r HCO3 i n Table I I were c a l c u l a t e d . Changes i n HCO3 w i t h i n each pH i n t e r v a l were then determined, these changes r e s u l t i n g from consumption of hydrogen ions from H2CO3 w i t h an, e q u i v a l e n t p r o d u c t i o n of both HCO3 and c a t i o n s , M * Because 2.08 meq/L H2CO3 i s converted to HCO3, i n a r e a c t i o n which goes from pH = 4.5 to pH = 8.0, 2.08 meq/L c a t i o n s are produced. C a t i o n compositions of s e v e r a l R a i n i e r Mesa ground-water samples are shown on F i g u r e 3, w i t h the c a t i o n composition of a sample having 2.08 meq/L t o t a l c a t i o n s i n d i c a t e d by the arrow and by the numerical values l i s t e d . The s t r a i g h t l i n e s drawn through the data p o i n t s are to f a c i l i t a t e determination of a c a t i o n composi­ t i o n and are not intended to imply composition changes during r e a c t i o n ; t h i s would only be t r u e i f a l l the samples represented p o i n t s along a s i n g l e f l o w path or along s i m i l a r f l o w paths. I n R a i n i e r Mesa, i t i s more probable t h a t the samples represent s i m i l a r l o c a t i o n s along a s e r i e s of d i s s i m i l a r f l o w paths, and t h e r e f o r e , i n theory, any one of the a c t u a l samples could be modeled. Instead, a s i n g l e median value was chosen as t y p i c a l . A s s o c i a t e d w i t h each pH i n t e r v a l t h e n _ i s a midpoint pH, a change i n bicarbonate c o n c e n t r a t i o n ( A J H C 0 3 p , and a set of r a t e c o n s t a n t s . These v a l u e s are presented i n Table I I . White and n+

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

8.0

7.5

7.0

6.5

6.0

5.5

5.0

4.5

pH interval boundary

7.75

7.25

6.75

6.25

5.75

5.25

4.75

Mean pH (reaction pH)

4

5

5

2.08x10"

1.98x10"

1.73x10"

3

3

3

1.2 3x10"

3

4

6.42x1ο-

2.56xl0"

8.80x10"

3.05x10"

3

[HC0 ~] a t boundary mol/L

9.73χ10"

4

5

2.56χ10-

4

4

5.00χ10-

5.87X10"

4

3.86x1ο-

4

5

1.68χ10"

5.75xl0"

3

A[HC0 "] mol/L

14.07

13.98

13.90

13.82

13.73

13.64

13.57

-log k Na +

2

+

2

14.54

14.54

14.54

14.54

14.54

14.54

14.54

mol/cm sec

-log k K

14.91

14.66

14.40

14.15

13.90

13.65

13.40

(25°C)

-log k Ca

Table I I . — B o u n d a r y Values and K i n e t i c Rate Constants Used i n Simulation of R a i n i e r Mesa Ground Water

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

2 +

14.90

14.70

14.49

14.28

14.08

13.88

13.67

-log k Mg

2 +

780

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

Claassen (3) determined k i n e t i c r a t e constants (k . ) f o r r e l e a s e M of c a t i o n s from v i t r i c m a t e r i a l of the R a i n i e r Mesa Member. These r a t e constants are pH-dependent, because the mechanism of the r e a c t i o n i n v o l v e s d i f f u s i o n of hydrogen i o n s i n t o the g l a s s , and a corresponding (and e q u i v a l e n t ) d i f f u s i o n of c a t i o n s out; thus, c o d i f f u s i o n determines the r a t e at which c a t i o n s appear i n solution. The equation which determines the mass of any one species t r a n s f e r r e d during a given pH i n t e r v a l i s :

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

q

= tft^nk

(3)

n +

where Π

q

i s mass of s p e c i e s Μ t r a n s f e r r e d during any pH i n t e r M v a l (at a constant pH equal to the mean pH of the i n t e r v a l ) , equivalents/1., σ i s the a q u i f e r s u r f a c e area i n cm i n contact w i t h 1 L of ground water, t . i s the time t h a t the ground water i s i n contact w i t h the a q u i f e r to produce a 0.5-pH change during r e a c t i o n i n t e r v a l , i , i n seconds, k i s the k i n e t i c r a t e constant of production of species M n+ M at the mean pH of the i n t e r v a l considered, i n moles per square centimeter per second, n+ n i s the formal charge of species M The t o t a l mass of major c a t i o n s t r a n s f e r r e d to s o l u t i o n during reaction interval, i , i s : n +

2

T. = Iq 1

M

T.=

at.


0.99

1.0

α

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

LU

0.0

0.0

l.o Na

+

+ K

+

2.0

+ C a

2

+

Figure 3.

+ Mg

2 +

3.0

4.0

, I N MILLIEQUI V A LE NTS PER LITER

Rainier Mesa groundwater data Ο Na"* φ

1.0 N a

S

+

+ K

+

Na

Composition Of Typical Sample

+

2.0

+ C a

2

+

+ Mg

2 +

,IN

3.0

4.0

M I L L I E Q U I V A L E N T S PER LITER

Figure 4. Change in water composition of typical sample from kinetic data (no montmorillonite precipitation)

Ο £

A l l o w i n g Montmorillonite

Ca

Precipitation

I-

+

, Compos it ion Of Typical Sample

ΡΕ ζ ζ uj LU

Ο Na"* 2

•

Ca Mg

2+




Ο

ο

s

00

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2


>

π

CO

786

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

D i s c u s s i o n of R e s u l t s and Conclusions The imperfect f i t of the k i n e t i c p r e d i c t i o n s to the f i e l d data may have i t s o r i g i n i n any of s e v e r a l areas. The i n t e r v a l s of 0.5 pH u n i t s chosen are probably too l a r g e f o r constant k i n e t i c r a t e constants to be assumed, e s p e c i a l l y f o r C a and M g i n the pH range 4.75 - 6.75. This shortcoming can be e l i m i n a t e d simply by decreasing the i n t e r v a l s t o , f o r example, 0.1 or 0.01 pH u n i t . The authors have developed an expression to modify k i n e t i c r a t e constants f o r s u r f a c e - s o r p t i o n e f f e c t s . C o r r e c t i o n s to r a t e constants have not been made i n the c a l c u l a t i o n s reported here, but the experimental values f o r r a t e constants are w i t h i n a few percent of those which would be a p p r o p r i a t e f o r the R a i n i e r Mesa a q u i f e r . The b a s i c methodology reported here i s b e l i e v e d v a l i d ; only minor changes i n the computed values are expected from more precise calculations. The assumption t h a t m o n t m o r i l l o n i t e of reported composition i s being produced and i s the only a u t h i g e n i c phase i s probably only an approximation. The composition may be i n e r r o r or may vary a r e a l l y w i t h i n R a i n i e r Mesa as a r e s u l t of a r e a l v a r i a t i o n s i n water q u a l i t y . Although no z e o l i t e s or other c l a y m i n e r a l s were reported i n the b u l k of the P a i n t b r u s h T u f f , very s m a l l amounts may remain undetected by e i t h e r x-ray d i f f r a c t i o n a n a l y s i s or t h i n - s e c t i o n petrography and may a f f e c t the aqueous composition by p r e c i p i t a t i o n or by i o n exchange. I t has a l s o been assumed that the d e v i t r i f i e d R a i n i e r Mesa Member has had i n s i g n i f i c a n t e f f e c t on the water chemistry. This assumption i s supported by comparison of the observed water compo­ s i t i o n w i t h compositions resulting·from r e a c t i o n s of both d e v i t ­ r i f i e d and v i t r i c m a t e r i a l w i t h water c o n t a i n i n g d i s s o l v e d carbon d i o x i d e (see F i g u r e 2 ) . I t i s obvious that at l e a s t some of the water r e c h a r g i n g R a i n i e r Mesa must pass through the d e v i t r i f i e d m a t e r i a l and that there w i l l be some i n t e r a c t i o n , but the r e s u l t s of t h i s study i n d i c a t e t h a t the i n t e r a c t i o n i s s m a l l , because s i g n i f i c a n t i n t e r a c t i o n should preclude a reasonable match to f i e l d data.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

2 +

2 +

E f f e c t i v e A q u i f e r Surface Table IV and F i g u r e 5 c o n t a i n the best r e s u l t s o b t a i n a b l e w i t h i n the assumptions and approximations o u t l i n e d above. Included h

i n the t a b l e are a set of at product terms which c o n t a i n the most s i g n i f i c a n t a p p l i c a t i o n of the methodology presented here. As p r e v i o u s l y d i s c u s s e d , one of the purposes of t h i s work i s to determine the e f f e c t i v e s u r f a c e area of a q u i f e r m a t e r i a l i n contact w i t h ground water. Such a v a l u e i s unobtainable through present h y d r a u l i c - t e s t i n g techniques and i s necessary f o r v a l i d p r e d i c t i o n s to be made concerning the a q u i f e r s o r p t i o n p r o p e r t i e s from laboratory-developed s o r p t i o n data. Only the water a c t u a l l y i n contact w i t h a q u i f e r m a t e r i a l , under n a t u r a l c o n d i t i o n s of a,

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

34.

CLAASSEN

AND WHITE

Kinetic Data and Groundwater Systems

787

can r e f l e c t the nature of surface w i t h which the waste products w i l l come i n contact. I f we consider that the match t o the t y p i c a l R a i n i e r Mesa ground-water sample presented on F i g u r e 5 i s adequate, an estimate of t o t a l residence time of the ground water w i t h i n R a i n i e r Mesa w i l l a l l o w determination of σ. Clebsch (9) presented a range f o r residence of R a i n i e r Mesa ground water of 0.8-6 years based on i t s t r i t i u m content. A p l o t of σ v s t was made using each of s e v e r a l values of σ

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

and the f o l l o w i n g r e l a t i o n s h i p : t = —

(see Table I V ) . σ The r e s u l t i n g curve i s presented i n F i g u r e 7. The residence time l i m i t s of 0.8 - 6 year (2.5 χ 1 0 - 1.9 χ 1 0 s) were then i d e n t i f i e d w i t h the corresponding σ values of 6.4 χ 1 0 to 2.3 χ 10 cm /L. Thordarson (1) and Benson (LBL, Berkeley, w r i t t e n commun., 1978) both d e s c r i b e the P a i n t b r u s h Tuff as p a r t i a l l y s a t u r a t e d . Core samples obtained from the lower p a r t o f the P a i n t b r u s h i n the t u n n e l complex were reported by Diment and others (2) t o have s a t u r a t i o n l e v e l s of 55-91 percent, w i t h an average of 77 percent. Benson (LBL, Berkeley, w r i t t e n commun., 1978) reported an average value f o r water s a t u r a t i o n i n the P a i n t b r u s h of 90 percent. I t would t h e r e f o r e be expected that the t o t a l a q u i f e r - p o r e surface would not be i n v o l v e d i n t r a n s p o r t o f water. As a matter of f a c t , assuming that a l l of the saturated-pore space i s e f f e c t i v e i n t r a n s m i t t i n g water, one might p r e d i c t t h a t , on the average, some­ where between 77 and 90 percent of the a q u i f e r surface i s i n contact w i t h p e r c o l a t i n g water. E s t i m a t i o n o f a q u i f e r σ through B.E.T. and p o r o s i t y measure­ ments of cores obtained from the P a i n t b r u s h Tuff i n w i d e l y s c a t t e r e d l o c a t i o n s i n R a i n i e r Mesa y i e l d e d values ranging from 8.0 χ 1 0 t o 2.3 χ 10 cm /L. Even i f the lower of the measured s a t u r a t i o n values i s used t o estimate the f r a c t i o n of surface area a c t u a l l y i n contact w i t h l i q u i d , the σ values (6.2 χ 1 0 t o 1.8 χ 1 0 cm /L) appear t o be much too high when compared w i t h the r e s u l t s of the geochemical k i n e t i c a n a l y s i s . The i m p l i c a t i o n i s t h a t , on the average, only about 3 percent of the t o t a l pore space i s e f f e c t i v e i n t r a n s m i t t i n g water, as determined by the r a t i o o f measured σ t o the σ derived from w a t e r - q u a l i t y k i n e t i c s . S t a t i n g t h i s d i f f e r e n t l y , the r o c k - s u r f a c e area apparently i n contact w i t h u n i t volume of l i q u i d i s only about 3 percent of that which would be p r e d i c t e d on the b a s i s of measurements made on s o - c a l l e d n a t u r a l - s t a t e m a t e r i a l . This discrepancy may be magnified somewhat by the f a c t that we are d e a l i n g w i t h p a r t i a l l y saturated m a t e r i a l , but none of the measurements made have i n d i c a t e d s a t u r a t i o n a t the 3 percent l e v e l . We must conclude that the major w a t e r - t r a n s p o r t i n g pores c o n s t i t u t e only a s m a l l f r a c t i o n of the t o t a l interconnected (and probably saturated) pore space. 7

8

6

6

2

7

2

7

7

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

788

MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

CHEMICAL

Figure 7.

Relation between reaction time (t) and surface to volume ratio (σ) for aqueous reaction of Rainier glass

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

34.

CLAASSEN

AND WHITE

Kinetic Data and Groundwater Systems

789

An important aspect of t h i s f i n d i n g l i e s i n i t s c o n t r i b u t i o n to understanding t r a n s p o r t of waste m a t e r i a l s i n n a t u r a l systems. Too o f t e n , p r e d i c t i o n s of the waste-sorbing p o t e n t i a l of an a q u i f e r are made on the b a s i s of l a b o r a t o r y experiments u t i l i z i n g crushed a q u i f e r m a t e r i a l which i s intended to reach e q u i l i b r i u m w i t h a waste-containing s o l u t i o n . The amount of waste removed from s o l u t i o n i s measured, and the s o r p t i o n c a p a c i t y o f the a q u i f e r m a t e r i a l i s determined as weight, o r equivalent-weight contaminant, per u n i t weight of a q u i f e r m a t e r i a l . More s o p h i s t i c a t e d e x p e r i ­ ments may measure surface area of crushed m a t e r i a l and attempt t o r e l a t e t o g r a i n s i z e of i n - s i t u m a t e r i a l . Cores are u t i l i z e d i n such experiments only r a r e l y . None of these techniques have the p o t e n t i a l t o p r e d i c t the a q u i f e r surface area e f f e c t i v e i n remov­ ing waste. The techniques used i n t h i s report w i l l be r e f i n e d and f u r t h e r t e s t e d i n t h i s and other systems. Montmorillonite

Supersaturation

The c a l c u l a t i o n of m o n t m o r i l l o n i t e s a t u r a t i o n index present at the end of each 0.5-pH i n t e r v a l from the k i n e t i c a l l y generated s o l u t i o n composition and the e q u i l i b r i u m constant f o r the Aberdeen m o n t m o r i l l o n i t e was presented on Figure 6. A r a p i d increase i n s a t u r a t i o n at lower values of pH slowing a t higher pH values i s i n d i c a t e d . This behavior suggests that the r a t e o f production of s o l u b l e c a t i o n s i s greater than the r a t e at which species r e q u i r e d f o r m o n t m o r i l l o n i t e p r e c i p i t a t i o n are removed from s o l u t i o n . Note that i t has not been s t a t e d that m o n t m o r i l l o n i t e p r e c i p i t a t e s i n the c l a s s i c a l sense; that i s , as a simple c r y s t a l l i n e substance. I t i s more l i k e l y that formation of an amorphous-aluminosilicate m a t e r i a l precedes both the i n c o r p o r a t i o n of the i r o n and magnesium and the subsequent rearrangement t o form the f i n a l phase. F i g u r e 8 shows the changes i n r a t e of production of the sum of a l l c a t i o n s as the r e a c t i o n proceeds. Because the r e a c t i o n r a t e i s dependent on pH, greater r a t e s occur a t lower pH values. I t i s , however, c u r i o u s to observe the time-dependence on removal of Mg + from s o l u t i o n , and, presumably, a l s o the r a t e of p r e c i p i t a t i o n of m o n t m o r i l l o n i t e . As shown on Figure 6, m o n t m o r i l l o n i t e s a t u r a ­ t i o n i s reached a t about 1.9 χ 10 s. P r i o r t o t h i s time, the p r e c i p i t a t i o n r a t e must be zero; t h i s i s i n d i c a t e d on Figure 8 by the v e r t i c a l arrow. Once s a t u r a t i o n has been reached, a small but f i n i t e r a t e of p r e c i p i t a t i o n i s a n t i c i p a t e d and expected t o increase as the s a t u r a t i o n index i n c r e a s e s . The r a t e s of p r e c i p i ­ t a t i o n c a l c u l a t e d from the data of Table IV are i n d i r e c t o p p o s i t i o n to t h i s e x p e c t a t i o n ; r a t e s are i n i t i a l l y high and decrease w i t h time more r a p i d l y than the r a t e at which production of a l l d i s ­ solved species decreases. This would i n d i c a t e that the r a t e c o n t r o l l i n g step i n the formation of m o n t m o r i l l o n i t e i s one that i n v o l v e s a species undergoing s i g n i f i c a n t change w i t h time o r r e a c t i o n progress. The most l i k e l y species i s the hydrogen i o n , or the species whose s o l u t i o n c o n c e n t r a t i o n i s c o n t r o l l e d by 2

5

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

790

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

J

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

*«s "5 *§ Co

•2

1 Si Ο Ο

•2 ο ο ν­ α Ο

1 •Β CO

3? Ο

οό

ε

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

34.

CLAASSEN

AND WHITE

Kinetic

Data and Groundwater 3 +

hydrogen such as AlCOH)^ (10) o r F e (11). r e q u i r e d t o s u b s t a n t i a t e t h i s hypothesis.

Systems

791

Further work i s

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

Summary A technique f o r determination of e f f e c t i v e a q u i f e r surface area i n contact w i t h p e r c o l a t i n g ground water has been presented. The method u t i l i z e s laboratory-determined chemical k i n e t i c d i s s o ­ l u t i o n data w i t h g e o l o g i c , h y d r o l o g i e , and ground-water-quality data t o y i e l d an estimate of e f f e c t i v e a q u i f e r surface area, a parameter not obtained by other techniques. A n a l y s i s of the r e s u l t s obtained f o r the R a i n i e r Mesa ground­ water system by a p p l i c a t i o n of the above method i n d i c a t e d that although measurements of water s a t u r a t i o n on core samples from the P a i n t b r u s h Tuff evidenced a high degree of s a t u r a t i o n (77 to 90 p e r c e n t ) , the major w a t e r - t r a n s m i t t i n g regions of the a q u i f e r c o n s t i t u t e only about 3 percent of the t o t a l , i f the p o r o s i t y i s e q u a l l y d i s t r i b u t e d . I f the regions of major h y d r a u l i c conductiv­ i t y are f r a c t u r e s , measurements of σ on core samples w i l l be meaningless, and the method presented here w i l l provide the only means f o r an estimate of σ. Although the P a i n t b r u s h has been described as a h y d r a u l i c system of primary p o r o s i t y , the c a l c u l a ­ t i o n s of σ suggest that i t i s a system of secondary p o r o s i t y , which provides the major throughput. This a l l o w s f o r h i g h l y water-saturated rock, w i t h l i t t l e water movement, coupled to f r a c t u r e s or other conductive passages through which most of the water f l o w s . The e f f e c t of such a system on the t r a n s p o r t of waste can be s i g n i f i c a n t i f core data are used to p r e d i c t s o r p t i v e behavior; i n the P a i n t b r u s h T u f f , s o r p t i v e behavior i s n e a r l y two orders of magnitude lower than that p r e d i c t e d from core measure­ ments. R e s u l t s of experiments u t i l i z i n g crushed rock and e q u i l i b r a ­ t i o n w i t h waste s o l u t i o n s to determine s o r p t i o n behavior cannot be e x t r a p o l a t e d to a c t u a l a q u i f e r c o n d i t i o n s , even i f B.E.T. surface area i s known. This technique has been commonly used i n the past to assess w a s t e - s t o r a g e - s i t e s a f e t y . As the primary h y d r a u l i c c o n d u c t i v i t y decreases and secondary c o n d u c t i v i t y becomes more prominent, t h i s methodology becomes l e s s and l e s s v i a b l e f o r input to modeling of waste t r a n s p o r t . The method presented i n t h i s r e p o r t should r e s u l t i n more r e a l i s t i c waste-transport modeling.

Abstract Kinetic modeling was used to estimate the effective surface area of aquifer in contact with a unit volume of ground water for a composite saturated-unsaturated groundwater system in southern Nevada. This aquifer property, not obtainable by other means, i s necessary for r e a l i s t i c modeling of solute transport i n ground­ water systems. The results of the kinetic modeling indicate that

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

792

C H E M I C A L MODELING IN AQUEOUS SYSTEMS

only a small part of the total interconnected pore space i s available for transport of water to the water table. The aquifer studied i s composed of both v i t r i c (glassy) and d i v i t r i f i e d (crystalline) volcanic tuff of nearly identical chemical com­ position. Comparison of laboratory and f i e l d data indicated that only the v i t r i c phase has a significant influence on ground-water composition. Laboratory determination of mass-transfer rates from the v i t r i c material to solution as functions of pH allowed a simulation of the natural water's cation composition. Simulated results were improved considerably when the model was modified to take into account precipitation of the clay mineral, montmoril­ lonite. Estimates of surface area per unit volume obtained from the kinetic model are about 3 percent of those obtained indepen­ dently from surface area measurements using the Braunauer, Emmett, and Teller (B.E.T.) equation. Literature Cited 1.

2.

3.

4. 5.

6. 7.

8.

9.

Thordarson, William, Perched ground water i n zeolitized-bedded tuff, Rainier Mesa and v i c i n i t y , Nevada Test Site, Nevada, U.S. Geol. Survey Open-file Rept., TEI-862, 93 p. (1965). Diment, W. H., Wilmarth, V. R., McKeown, F. Α., Dickey, D. D., Hinrichs, Ε. N., Botinelly, T., Roach, C. H., Byers, S. Μ., Jr., Hawley, C. C., Izett, G. Α., Clebsch, Alfred, Jr. Geological Survey investigations in the U12b.03 and U12b.04 tunnels, Nevada Test Site, U.S. Geol. Survey Open-file Rept., TEM-996, 75 p. (1959). White, A. F., and Claassen, H. C. Dissolution kinetics of s i l i c a t e rocks, application to solute modeling, in Jenne, E.A., ed., "Chemical Modeling—Speciation, Sorption, Solubility, and Kinetics in Aqueous Systems," Am. Chem. Soc., 1978 (this volume). Garrels, R. Μ., and Christ, C. L. "Solutions, Minerals and Equilibria", 450 p., Harper & Row, New York, 1965. U.S. Geological Survey, Results of exploration of Baneberry Site, early 1971, U.S. Geol. Survey Rept. USGS-474-245, NTIS, Springfield, VA, 92 p. (1974). Kittrick, J. A. Stability of montmorillonites II, Aberdeen Montmorillonite, Soil Sci. Soc. Amer., 35, 820-823 (1971). White, A. F., and Claassen, H. C. Geochemistry of ground water associated with tuffaceous rocks, Oasis Valley, Nevada, Geol. Soc. Am. Abs. 7 (7), 1316-1317 (1976). White, A. F., and Claassen, H. C. Kinetic model for the dissolution of a r h y o l i t i c glass, Geol. Soc. Am. Abs. 9, (7), 1223 (1977). Clebsch, Alfred, J r . Tritium age of ground water at the Nevada Test Site, Nye County, Nevada, C122-C125, in "Short Papers in the Geologic and Hydrologie Sciences", U.S. Geol. Survey Prof. Paper 424-C, (1961).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

34.

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Kinetic Data and Groundwater Systems

793

10. Hem, J. D., Roberson, C. E., Lind, C. J., and Polzer, W. L. Chemistry of aluminum in natural water, U.S. Geol. Survey Water-Supply Paper 1827-E, 57 p. (1973). 11. Hem, J. D., and Cropper, W. H. Survey of ferrous-ferric equilibria and redox potentials, U.S. Geol. Survey Water-Supply Paper 1827-A, 55 p. (1959). Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch034

RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

35

A

Preliminary Comprehensive

of S u l f i d i c

Marine

Model

for the C h e m i s t r y

Sediments

LEONARD RORERT GARDNER

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

Department of Geology, University of South Carolina, Columbia, SC 29208

Diagenetic reactions i n subaqueous sediments commonly produce systematic v e r t i c a l changes i n the composition o f pore waters. In general these reactions involve the decomposition o f sediment organic matter, the regeneration o f nutrients and the p r e c i p i t a t i o n and d i s s o l u t i o n o f various mineral phases. As a r e s u l t o f the v e r t i c a l concentration gradients that develop i n pore waters, d i f f u s i v e exchange of dissolved substances between the sediment and overlying water can occur. In environments where the overlying water i s oxygenated, decomposition o f sediment organic matter i s c a r r i e d on by aerobic organisms that u t i l i z e dissolved oxygen d i f f u s i n g into the sediment. However, i f the oxygen demand i s greater than that supplied by d i f f u s i o n , the sediment w i l l become anoxic at some depth. At t h i s point the oxidation o f organic matter can be continued by anaerobic organisms that u t i l i z e s u l f a t e o r n i t r a t e as oxidants. In marine sediments the high concentrat i o n o f s u l f a t e i o n i n sea water enables s u l f a t e reduction to dominate the process o f anaerobic decomposition. In recent years various workers 11-7) have succ e s s f u l l y developed models based on the mathematics o f d i f f u s i o n ( 8 ) to describe v e r t i c a l p r o f i l e s o f selected chemical parameters i n marine sediments dominated by s u l f a t e reduction. Several papers ^9, 10) have also proposed models f o r nitrogen diagenesis i n the upper aerobic zone o f such sediments. Most o f these models, however, deal with only one or^two r e l a t i v e l y well behaved parameters, such as SO5 o r CO2$ which do not i n t e r a c t strongly with other components o f the sediment besides organic matter. A t r u l y comprehensive model f o r such sediment should deal simultaneously with a l l of the major chemical parameters of the system and i d e a l l y should be formulated as an i n i t i a l value prob0-8412-0479-9/79/47-093-795$05.00/0 © 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

796

CHEMICAL MODELING IN AQUEOUS SYSTEMS

lem so that the depth at which the sediment becomes anoxic could be f r e e l y predicted rather than assumed as i n the nitrogen diagenesis models previously c i t e d . Formulation of such a model would require consideration of the k i n e t i c s of the many complex reactions that may occur near the oxidized-reduced boundary as a r e s u l t of i n t e r a c t i o n s between downward d i f f u s i n g oxygen with upward d i f f u s i n g reduced substances such as H2S and ΝΗ4· Furthermore the upper portion of most marine sediments are inhabited by burrowing organisms and thus the model would have to take into account the movement of substances r e s u l t i n g from bioturbation i n order to accurately predict the depth to the oxidized-reduced boundary. The author has attempted to formulate such a model but because of the many ad hoc assumptions that were required and because of the d i f f i c u l t i e s encoun­ tered i n obtaining reasonable numerical solutions t h i s work i s not ready f o r presentation. However i f one i s w i l l i n g to ignore the upper oxidized p o r t i o n of the sediment or to assume that the sediment i s anoxic a t the surface, then, as w i l l be shown below, i t i s pos­ s i b l e to develop a reasonably comprehensive and r e a l i s t i c model f o r sediment dominated by s u l f a t e reduction. Although previous models f o r s u l f i d i c marine sediments have s u c c e s s f u l l y simulated p r o f i l e s of s u l f a t e , carbon dioxide and ammonia, development of a model f o r H2S p r o f i l e s has been stymied by the f a c t that H2S may p r e c i p i t a t e as FeS and/or FeS2· This i n a b i l i t y to p r e d i c t H2S p r o f i l e s has i n turn prevented p r e d i c t i o n of pH p r o f i l e s . Only recently have experi­ mental studies of the k i n e t i c s of i r o n s u l f i d e forma­ t i o n ( 1 1 , 12) provided a basis f o r formulating models f o r the diagenesis o f H2S i n the presence o f i r o n oxides. This paper proposes a system of 10 non-linear, simultaneous d i f f e r e n t i a l equations (Table I) which; upon f u r t h e r development and v a l i d a t i o n , may serve as a comprehensive model f o r p r e d i c t i n g steady state, v e r t i c a l p r o f i l e s of chemical parameters i n the s u l f i d e dominated zones o f marine sediments. The major objec­ t i v e o f the model i s to p r e d i c t the v e r t i c a l concentra­ t i o n p r o f i l e s o f H 2 S . h y d r o t r i o l i t e (FeS) and p y r i t e (FeS2). As with any model there are a number of assumptions involved i n i t s construction that may l i m i t i t s a p p l i c a t i o n . In a d d i t i o n to steady state, the major l i m i t i n g assumptions of t h i s model are the assumptions that the sediment i s f r e e of CaC03, that the d i f f u s i o n c o e f f i c i e n t s of a l l dissolved s u l f u r species are equivalent and that dissolved oxygen does not penetrate into the zone o f s u l f a t e reduction.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

35.

GARDNER

Sulfidic

Marine

Sediments

797

Also I t should be emphasized that the model developed here has not been v a l i d a t e d by q u a n t i t a t i v e comparisons with comprehensive f i e l d data because of my lack of access to such data. However, as w i l l be shown, the p r o f i l e s of H 2 S , FeS and FeS generated by the model compare favorably i n a q u a l i t a t i v e fashion with such p r o f i l e s as are a v a i l a b l e i n the l i t e r a t u r e . Thus I f e e l that the work presented here represents a s i g n i ­ f i c a n t step towards the development of a t r u l y compre­ hensive model f o r the chemistry of s u l f i d i c marine sediments. 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

Construction of the Model The formulation of t h i s model i s based on the mathematics of diagenesis developed by Berner (£)· Using the chain r u l e of p a r t i a l d i f f e r e n t i a t i o n (13, p. 3 3 5 ) Berner showed that f o r any property of a sedi­ ment, P, that i s a continuous d i f f e r e n t i a b l e function of depth, x, and time, t , *E = d£ ot dt

s

6£ οχ

(1)

where S(= dx/dt) i s the r a t e of sediment deposition minus the sum of the rates of erosion and compaction. Equation 1 i s known as the general diagenetic equation and i s the basis f o r the system o f equations presented i n Table I . I t should be noted here that horizontal v a r i a t i o n s i n Ρ are assumed n e g l i g i b l e . The t o t a l d e r i v a t i v e dP/dt represents the rates of a l l the dia­ genetic processes a f f e c t i n g P. Omitting the usually n e g l i g i b l e e f f e c t of the flow of water r e s u l t i n g from compaction, dP/dt can be represented i n general by (2)

where D i s the whole sediment d i f f u s i o n c o e f f i c i e n t (8) and CR represents the a d d i t i v e rates of a l l chem­ i c a l reactions a f f e c t i n g P. P o s i t i v e terms f o r CR imply production of Ρ whereas negative terms imply consumption. In the case where Ρ i s a s o l i d substance, D can be assumed to equal zero so that there i s no e f f e c t due to d i f f u s i o n . Also since steady-state con­ d i t i o n s are assumed, δp/δt = 0 and thus the general diagenetic equation reduces to

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

798

C H E M I C A L MODELING

IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

In view of the f a c t that s u l f a t e reduction i s driven by the microbial decomposition o f organic matter [(OC) i n Table i j the diagenetic equation f o r t h i s substance w i l l be derived f i r s t . Although the mechanisms by which s u l f a t e reducing b a c t e r i a decom­ pose sediment organic matter are complex and not f u l l y understood, Berner (2, and others (2) have success­ f u l l y modeled steady-state p r o f i l e s of s u l f a t e on the assumption that the k i n e t i c s of decomposition are f i r s t order and that the organic substrate i s i n s o l i d form. Thus by s u b s t i t u t i n g D == 0 and CR = -K (OC) i n equation 3 the diagenetic equation f o r organic carbon i s found as

2 per mole and i s a f u n c t i o n o f sphere diameter and the density and molecular weight o f goethite. As f o r r e a c t i o n 8 , p y r i t e formation, Rickard reports that the k i n e t i c s o f t h i s process obey the following equation1

lf& = KFS2

di

CH+)(H S)A A 2

h

S

where (FeS ) 2

55

FeS in mol Γ"

1

2

KFS2

= r a t e constant (Table II)

A

= surface area of FeS i n cnr

h

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

U0)

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Rate constant for FeS precipitation

Rate constant for FeS precipitation

Ratio of Ν to C in organic matter

Molar surface area of HFe0

Molar surface area of S°

KFS

KFS2

α

SHM

SOM

1

2

2

cm mol^l

cm mol""-

2

none

- -1 cm6sec-1 mol

mo 1 ~" ^ 1~% cm"*2 s e c

sec"*

1

1

cm sec"

2

cm sec"

Units

- 1

2.29 χ 10

4.53 χ 10

0.155

5

5

5

1.0 χ 10-13

1.5 χ 10-7

5.0 χ io-i°

3.0 χ ΙΟ"

6

4.S χ ίο-*

ValMe

c

c

assumed

b

assumed

a

source

4

C6

C

β

(SHM) (KFS)/S

« -1.5 CSHM)CKFS)/D

K/S 2

3

C » -K/D

7

2

C = UOM) U T M ) U F s 2 ) / s

5

2

C = -0.5 (SOM)VSTM) UFs2)/ϋ

C = S/D

Molar surface area of FeS cm mol~"l 2.67 χ 1U c a. Typical of coastal sediments, southeastern U.S.A. b. Consistent v/ith Berner* s {,22) empirical relation between S and K. c. Based on spherical particles 2 \im i n diameter. DERIVED CONSTANTS t

STM

Rate constant for sulfate reduction

Κ

2

Diffusion coef·

0

2

Sedimentation rate

S

PRIMARY CONSTANTS ι Svmbol DescriDtion

Table II· Primary and derived constants for model equations.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

802

CHEMICAL

A

s s

MODELING IN

AQUEOUS

SYSTEMS

surface area of S ° i n cnr.

Again assuming spherical p a r t i c l e s A^and A expressed as

can be

g

A

h

= STM

(FeS)

Ul)

A

s

=» SOM

(S°)

(12)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

where STM and SOM are the surface areas per mole of FeS and S°, r e s p e c t i v e l y . Thus i n summary the CR expression f o r H2S i s given by KFS(H ) (H S) / SHM(HFe0 ) +

CR - 0.5(OC) - 1.5 -0.5

2

3

2

2

2

KFS2(H+)(H S)STM (FeS) SOM(S°) 2

2

(13)

2

where the c o e f f i c i e n t s , -1.5 and -0.5, are derived from the stoichiometry of reactions 6 and 7. Again i t i s l e f t to the reader to v e r i f y that s u b s t i t u t i o n of the above r e s u l t f o r CR i n equation 3 leads to the d i f f e r e n t i a l equation f o r t o t a l dissolved H2S shown i n Table I . From t h i s d i s c u s s i o n the o r i g i n of the d i f f e r e n t i a l equations f o r H F e 0 , FeS, S° and FeS2 should also be obvious· With regard to the stoichiometry o f reactions 6 and 7 i t should be mentioned that Rickard was unable to a s c e r t a i n the exact mechanisms and reactions underlying h i s k i n e t i c r e s u l t s . I have written r e a c t i o n 6 as an o v e r a l l d e s c r i p t i o n of the process of FeS formation because the c o e f f i c i e n t s of each reactant i n equation 6 matches i t s corresponding exponent i n equation 85. Furthermore r e a c t i o n 6 i s compatible with R i c k a r d s i n t e r p r e t a t i o n of the k i n e t i c mechanisms involved i n t h i s process and with h i s report of the production o f native s u l f u r i n h i s experiments. In the case of reaction 7, however, the achievement of harmony between k i n e t i c s and stoichiometry i s more problematic. Again Rickard was unable to a s c e r t a i n the exact mechanisms and reactions involved i n p y r i t e formation. He d i d not attempt to write an o v e r a l l r e a c t i o n but suggested that the process involves d i s s o l u t i o n o f both FeS and S°, the formation of p o l y s u l f i d e ions and t h e i r subsequent r e a c t i o n with F e * ions to form p y r i t e plus native s u l f u r . As with r e a c t i o n 6 I have chosen to write react i o n 7 i n such a fashion that the stoichiometric c o e f f i c i e n t s of the reactants correspond with t h e i r respective exponents i n equation 10· I f one assumes that R i c k a r d s r e a c t i o n v e s s e l s were devoid, as seems l i k e l y , of strong o x i d i z i n g agents such as f e r r i c i r o n 2

1

2

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

35.

GARDNER

Sulfidic Marine Sediments

803

or 0 and that the N? c a r r i e r gas used i n h i s experi­ ments was not reduced to ammonia, then the constraints of reaction 7 force one to postulate the reduction of H to Η2· This i s a somewhat unattractive requirement i n view of the lack of supporting experimental evidence f o r Ho production but i t i s at l e a s t thermodynamically f e a s i b l e i n that the equilibrium concentration of aqueous H2 would have to be at l e a s t 10^ times greater than the concentration of H2S. Given t y p i c a l concen­ t r a t i o n s of H2S i n anoxic sediments i t i s u n l i k e l y that an equilibrium pressure of H2 could develop at one atmosphere t o t a l pressure. I f one i s w i l l i n g to admit the presence of O2 i n the system, then r e a c t i o n 7 could be modified as follows 1 2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

+

2FeS(s) + S°(s) 2FeS2(s) +

+ H

+

+ HS~(aq) + kOz

-> (14)

H 0. 2

T h i s , however, i s undesirable because O2 does not occur i n the rate equation and because, as assumed, i t i s u n l i k e l y that there i s s u f f i c i e n t O2 present i n sedi­ ments at depths where s u l f a t e reduction and p y r i t e formation are a c t i v e . Furthermore the often c i t e d reaction between FeS and S°, FeS(s) + S°(s) -» FeS2(s)

(15)

i s unappealing because i t bears no obvious r e l a t i o n to the rate equation and because i n conjunction with reac­ t i o n 6 i t would upon completion ( i . e . at a s u f f i c i e n t l y great depth) produce FeS and FeS2 i n at l e a s t a one to one mole r a t i o which contradicts f i e l d studies that commonly show almost t o t a l conversion o f FeS to FeS2 at depth. On the other hand the combination of reactions 6 and 7 allow but, as w i l l be seen, do not demand com­ p l e t e conversion to FeS2» Although other reactions involving s i d e r i t e formation, methane formation, N£ reduction and/or reactions with organic complexes have been proposed f o r p y r i t e formation (14b) none of them have been studied k i n e t i c a l l y and thus siinply cannot be considered at t h i s time. F i n a l l y although Rickard reports that framboidal p y r i t e was not produced during his experiments t h i s does not n e c e s s a r i l y mean that framboidal p y r i t e formation does not obey the s t o i c h i ­ ometry and k i n e t i c s of equations 7 and 10· At t h i s point construction of the model i s com­ p l e t e except f o r incorporation of H* into the system of equations. As discussed above,H i s an important v a r i a b l e i n the k i n e t i c s of i r o n s u l f i d e formation. +

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

804

CHEMICAL

MODELING IN

AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

However because the rate law expressions f o r i r o n s u l f i d e formation (equations 8 and 10) are non-linear the d i f f e r e n t i a l equations f o r H2S and the i r o n s u l ­ f i d e s are not amenable to e x p l i c i t s o l u t i o n . Thus i t i s important to develop an equation f o r ¥r that can be incorporated i n a numerical s o l u t i o n technique such as that of Runge-Kutta (U>)· Fortunately an appropriate d i f f e r e n t i a l equation f o r H* can be developed from charge balance considerations. Here i t i s assumed that dissolved substances other than those l i s t e d i n Table I are not a f f e c t e d by diagenesis. I f t h i s i s true, then a charge balance d i f f e r e n c e equation can be written (16): (NH4)

- (Ν*φ

0

+ (H+)

+

- (H )

0

= (OH")

- (0H")

+ 2(S0|")

- 2(SO^)

Q

+ (HC05) - ( H C 0 5 )

+ 2(C0|")

- 2(C0^")

Q

+ (HS~) - (HS~)

o

o

0

(16)

where the subscripted q u a n t i t i e s represent the concen­ t r a t i o n s o f these substances i n the overlying sea water. Here i t should be pointed out that i n develop­ ing the equations i n Table I i t has been t a c i t l y assumed that pH dependent species of C02t H2S and NH3 (such as HC0§, HS", etc.) have equivalent d i f f u s i o n c o e f f i c i e n t s . In p r i n c i p l e a separate d i f f e r e n t i a l equation could have been w r i t t e n f o r each species. However each of these would incorporate a pH dependent d i s t r i b u t i o n function. The programing complexity that t h i s would generate i s not j u s t i f i e d given the prelim­ inary nature of the model and the other rough assump­ tions involved. In equation 16 each of the carbonate species can be expressed i n terms of t o t a l CO2 and H* by means o f the appropriate carbonate equilibrium con­ stants. The other pH dependent species such as HS* can be treated i n a s i m i l a r fashion. In t h i s manner equa­ t i o n 16 can be expressed i n terms o f the v a r i a b l e s (H S), ( S 0 ) , (CO2), (NH ) and (H+), the i n i t i a l con­ centrations and the various equilibrium constants. Using the chain-rule of p a r t i a l d i f f e r e n t i a t i o n (12, p. 335) the t o t a l d i f f e r e n t i a l o f (H ) can be obtained and i s o l a t e d on the l e f t hand of the equation. D i v i ­ sion o f both sides of the r e s u l t i n g equation by dx y i e l d s a f i r s t order d i f f e r e n t i a l equation o f the form 2

4

4

+

= f[(H S), (S0 ), (CO2), (NH4), 2

4

(H S)\ 2

( S 0 ) , ( C 0 ) \ (NH4) J e

4

1

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(17)

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

35.

GARDNER

Sulfidic Marine Sediments

805

which can be incorporated i n a Runge-Kutta algorithm. Space l i m i t a t i o n s unfortunately do not permit a more e x p l i c i t d e r i v a t i o n of t h i s equation than that sketched here. Development of t h i s model has neglected several substances that i n p r i n c i p l e p o s s i b l y could have been included. The most obvious omissions are dissolved phosphate released by the decomposition of organic matter and the p o s s i b l e d i s s o l u t i o n or p r e c i p i t a t i o n of CaC03. Both of these substances would a f f e c t the pH p r o f i l e . The e f f e c t of phosphate on pH would probably be small because i t s concentration i n organic matter i s low compared to Ν and C. The e f f e c t of CaC03 on pH however could be important and thus a p p l i ­ cation of the model should probably be r e s t r i c t e d to CaC03 free sediments. The Simulation Algorithm The equations of the system presented i n Table I are both non-linear and simultaneous. Thus there i s no obvious way of obtaining e x p l i c i t solutions and one must therefore resort to numerical techniques. Because of i t s e f f i c i e n c y and wide use, I chose a Runge-Kutta method with fourth-order accuracy (1£)· The d e t a i l s of the algorithm developed f o r t h i s problem w i l l eventu­ a l l y be published i n an appropriate j o u r n a l . The algorithm can i n p r i n c i p l e handle the system of equa­ tions presented i n Table I . In p r a c t i c e however prob­ lems a r i s e because some of the equations are second order and thus the algorithm must be supplied with i n i t i a l values ( i . e . at χ = o) f o r both concentration and the f i r s t d e r i v a t i v e . As w i l l be discussed below the algorithm may generate u n r e a l i s t i c concentrations i f inappropriate values are chosen f o r the f i r s t deriv­ a t i v e . I t should be emphasized that t h i s does not r e s u l t from d e f i c i e n c i e s i n the algorithm, such as error buildup, because such e f f e c t s can be minimized by choosing s u f f i c i e n t l y small depth increments. The performance of the algorithm with respect to error buildup was evaluated by having the algorithm compute p r o f i l e s f o r a hypothetical sediment devoid of r e a c t i v e i r o n oxide. For such a case the equations i n Table I e i t h e r vanish or uncouple and become l i n e a r so that e x p l i c i t solutions can be obtained f o r each ^except H" ) · Using e x p l i c i t l y determined i n i t i a l values f o r the required f i r s t derivatives and a step s i z e of 0.05 cm, a l l of the numerically computed values agreed with those computed from t h e i r corresponding e x p l i c i t func­ tions to at l e a s t four d i g i t s to a depth of 100 cm. h

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

806

CHEMICAL

MODELING

IN

AQUEOUS

SYSTEMS

Furthermore at each step the r a t i o of the change i n c a t i o n i c charge to the change i n anionic charge (see equation 16) was 1.0000. Thus i t appears that the c h i e f Limitations of the algorithm are those due to p o s s i b l e d e f i c i e n c i e s i n the conceptual framework of the model and to the appropriateness of the i n i t i a l conditions supplied to the algorithm. The problems associated with c o r r e c t l y s p e c i f y i n g the i n i t i a l f i r s t d e r i v a t i v e values became evident luring e a r l y attempts to simulate p r o f i l e s of H2S and Lron s u l f i d e s . As pointed out e a r l i e r the d i f f e r e n t i a l equation f o r organic carbon has an e x p l i c i t s o l u t i o n . This s o l u t i o n can be substituted f o r (OC) i n the equa­ tions f o r S04, NH4 and CO2 which are l i n e a r and can thus be solved a n a l y t i c a l l y . Thus the e x p l i c i t s o l u ­ tions obtained can be used to c a l c u l a t e the c o r r e c t I n i t i a l f i r s t d e r i v a t i v e s f o r these parameters. The equation f o r H2S, however, i s non-linear and coupled to :hose f o r HFe02, FeS, S° and FeS2« Thus there i s no )bvious a n a l y t i c a l way to c o r r e c t l y s p e c i f y the i n i t i a l i e r i v a t i v e f o r H2S. However since FeS p r e c i p i t a t i o n :an not occur at the zero concentration of H2S assumed it χ o, i t seemed reasonable to assume that at χ ο :he d e r i v a t i v e of H2S should be equal i n magnitude but >pposite i n sign to that of S04» U t i l i z a t i o n of t h i s issumption i n the algorithm r e s u l t e d i n H2S coneentra­ vons that g r e a t l y exceed the concentrations of H2S :hat would be produced i n a system devoid of r e a c t i v e .ron oxide but otherwise s i m i l a r . Furthermore simula:ion runs showed that although the algorithm obeyed lass balance constraints on Fe i t i n v a r i a b l y converts it depth e s s e n t i a l l y a l l of the i n i t i a l l y supplied ^ a c t i v e HFe02 to F e S . In cases where the i n i t i a l concentration of organic carbon i s l e s s than four times :he i n i t i a l concentration o f HFe02, t h i s behavior of :he algorithm i s also u n r e a l i s t i c because according to •eaction 5 the maximum number o f moles o f reduced s u l "ur i n a l l forms (H2S, FeS, FeS2t S°) that can be proluced i s equal to one h a l f of the i n i t i a l number moles >f decomposable carbon. Thus one i s faced with the »roblem of e i t h e r f i n d i n g an i n i t i a l d e r i v a t i v e f o r hS that gives acceptable r e s u l t s or incorporating into he algorithm the appropriate organic carbon substrate ionstraint on i r o n s u l f i d e formation. The f i r s t a l t e r lative was attempted on a t r i a l and e r r o r basis but oon proved f u t i l e . Fortunately however I was able to iodify the algorithm by incorporation o f an appropriate rganic carbon c o n s t r a i n t . The carbon constraint on i r o n s u l f i d e formation i s ased on r e a c t i o n 5 and Berner s (12) demonstration s

3

2

1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

35.

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Sulfidic Marine Sediments

GARDNER

that the t o t a l s u l f a t e reduced during b u r i a l to depth x, SR, i s given by L S where ( 0 C ) = i n i t i a l concentration of organic carbon. Since the s o l u t i o n of the d i f f e r e n t i a l equation f o r (OC) i s o

(OC) = ( O C ) ^ e x p ( ^ * ) ]

U9)

i t follows that SR = U . 5 [ ( 0 C ) - (OC)]* In the case of a sediment devoid of i r o n oxide, such that none of the reduced s u l f a t e i s incorporated i n s o l i d phases such as FeS, the concentration of t o t a l dissolved HoS equals (SO^)^ - (SO4) providing the d i f fusion c o e f f i c i e n t s f o r H S and SO4 are equivalent. Thus i t i s reasonable to assume that i n a system where h a l f of the reduced s u l f a t e has been incorporated i n s o l i d phases as a r e s u l t of reactions with i r o n oxide, the concentration of H S should be equal to one h a l f of H S concentration that would p r e v a i l i n a system devoid 0 1 i r o n oxide but otherwise s i m i l a r . This idea can be expressed mathematically as

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

o

2

2

2

SR - 2(FeS ) - (FeS) - (S°)

(H S 2

(S0)

o

-

2

(SO4)

=

SR

< 2 0 )

*

Thus i f no i r o n s u l f i d e genesis occurs because of the absence of i r o n oxide, then (H S) = (SO^Jp - (SO4)· Otherwise (H S) w i l l be some f r a c t i o n of ( S 0 4 ) - (SO4) depending on the amount of SR that has not been incorporated into s o l i d phases as reduced s u l f u r . This constraint on H S and i r o n s u l f i d e genesis was incorporated into the o r i g i n a l algorithm i n the following manner. At the end of the Runge-Kutta c a l c u l a t i o n s f o r each depth increment the following tolerance r a t i o was computed* 2

o

2

2

SRC

- 2(FeS2)

» [(SR

- (FeS)

-

(S°)

SR(H2S) (S0 ) 4

o

-

(SO4)

J/SR

.

(21)

I f the carbon constraint i s obeyed exactly SRC equals zero. I f SRC l i e s between -0.05 and +0.05 the algorithm accepts the r e s u l t s and proceeds to the next increment. I f SRC l i e s outside t h i s range, the algorithm repeats the c a l c u l a t i o n s f o r the increment with

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

CHEMICAL

808

MODELING

IN

AQUEOUS

SYSTEMS

appropriately adjusted values of (H S)' u n t i l tolerance is satisfied. 2

Simulation of Iron S u l f i d e Genesis In order to i l l u s t r a t e the q u a l i t a t i v e agreement of the carbon constrained model developed here with f i e l d data c i t e d i n the l i t e r a t u r e , several simulations of H2S, FeS, FeS and pH p r o f i l e s are presented i n Figures 1 and 2· Because of t h e i r appearance i n the r a t e equations f o r i r o n s u l f i d e k i n e t i c s these simulations have a l s o been designed to i l l u s t r a t e the poss i b l e e f f e c t s of pH and reactant molar surface areas on i r o n s u l f i d e genesis. In both figures s u l f a t e reduction and i r o n s u l f i d e formation begin about one cm below the sediment surface. The p r o f i l e s of pH above t h i s depth have been simulated on the basis of a model developed (but not discussed here) f o r the o x i dized zone of the sediment. This model incorporates reactions pertaining to the aerobic decay of organic matter, bioturbation and nitrogen diagenesis. Since reactions i n the aerobic zone are assumed to be uncoupled from those i n the zone o f s u l f a t e reduction the model f o r the aerobic zone has been used here simply as a means o f varying the i n i t i a l pH at the top of the zone of s u l f a t e reduction. As can be seen,the H2S p r o f i l e s f o r a l l four simul a t i o n s shown i n Figures 1 and 2 show maximum concent r a t i o n s i n the submillimolar range and subsequent decreases with depth. In sediment hosting a c t i v e s u l f a t e reduction and p y r i t e formation, H2S concentrations a t t a i n maximum values of about 10"3 moles l i t e r * 1 and commonly decrease t h e r e a f t e r with depth (IS, 12). In general incorporation of the carbon constraint i n the algorithm produces simulations that agree with q u a l i t a t i v e predictions of H2S p r o f i l e s by Goldhaber and Kaplan (18, p. 638). The simulations o f FeS shown by jobs 433, 1502 and 1673 are s i m i l a r to published p r o f i l e s (12, 20) i n that they show abrupt peaks j u s t below the oxidized-reduced boundary. Also, as i s commonly the case i n natural sediments, the simulated peak concentrations of FeS are about an order o f magnitude lower than the ultimate concentration of FeS2» Thus i n both magnitude and shape the simulations shown here bear strong resemblance to those r e c e n t l y described i n the l i t e r a t u r e , suggesting that the carbon constrained model has p o t e n t i a l a p p l i c a t i o n to the study of s u l f u r diagenesis i n marine sediments. The simulations shown i n Figure 1 are designed to i l l u s t r a t e the possible e f f e c t of the i n i t i a l pH at the

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Figure 1. Simulations of sediment chemistry showing effects of differences in pH at the top of the zone of sulfate reduction. Values for model constants are listed in Table II unless otherwise on figure. Ρ and ANP are parameters of the aerobic model that control the depth and pH at the oxidized-r educed boundary.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

oo

810

CHEMICAL MODELING IN AQUEOUS

SYSTEMS

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

:1 .si CO

KH

co

Ο Co Qù ^

be β •Si

III S, s * ν. CJ

s-

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fi δ

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

35.

GARDNER

Sulfidic Marine Sediments

811

s t a r t of s u l f a t e reduction on the course of i r o n s u l f i d e genesis. The pH at the oxidized-reduced boundary must be c o n t r o l l e d at l e a s t i n part by reactions i n the oxidized zone, such perhaps as ammonia oxidation. Thus i t i s conceivable that a comprehensive understanding of i r o n s u l f i d e genesis might require an understanding of the factors that control the pH at t h i s boundary. In both simulations the surface area parameters (SHM, STM, SOM) and the i n i t i a l organic carbon concentration are i d e n t i c a l . Thus the only d i f f e r e n c e between the simul a t i o n s as f a r as i r o n s u l f i d e genesis i s concerned i s that i n job 1502 the s t a r t i n g pH i s 6.47 whereas i n job 1673 i t i s 7.75. As can be seen t h i s d i f f e r e n c e i n s t a r t i n g pH has no e f f e c t on the p r o f i l e of FeS and only s l i g h t l y a l t e r s the maximum FeS concentration. Although the r e s u l t s of t h i s simulation experiment are preliminary they suggest that the pH at the oxidizedreduced boundary i s not l i k e l y to be an important factor i n i r o n s u l f i d e genesis. This r e s u l t s from the fact that the rapid depletion of SOç ion j u s t below the oxidized-reduced boundary i s not compensated by an adequate increase i n dissolved H^S. Thus charge balance consideration require a rapid r i s e i n pH which soon d i s s i p a t e s the i n i t i a l k i n e t i c advantage of low pH. The simulations shown i n Figure 2 are designed to i l l u s t r a t e the e f f e c t of surface area. In both, the depth to the zone of s u l f a t e reduction as well as the i n i t i a l conditions at the beginning of s u l f a t e reduct i o n are i d e n t i c a l except f o r the assumed molar surface areas. In job 433 the molar surface areas of S°, SOM, and FeS, STM, are about an order of magnitude l a r g e r than the molar surface area of HFe02» SHM. Although the magnitudes of SOM, STM and SHM chosen f o r job 433 are l a r g e r than f o r jobs 1502 and 1673 (Figure 1) the r a t i o s of SOM and STM to SHM are s i m i l a r i n a l l three jobs. As can be seen by comparing Figures 1 and 2 the p r o f i l e s of FeS and FeS i n job 433 are very s i m i l a r to those f o r jobs 1502 and 1673. On the other hand i n job 230 (Figure 2) the molar area of HFe02 i s about an order of magnitude l a r g e r than the molar areas of S° and FeS so that the r a t i o s of SOM and STM to SHM are about one hundred times smaller than they are i n jobs 1502, 1673 and 433. This dramatic d i f f e r e n c e r e s u l t s i n the p r e c i p i t a t i o n of approximately equal amounts of FeS and FeS2 at depth i n job 230. Thus the s i z e (and r e a c t i v i t y ) of i r o n oxide p a r t i c l e s supplied to the sediment could be an important f a c t o r i n producing FeS enriched sediments such as those discussed by Berner ( 8 ) from the Black Sea. 2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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MODELING IN AQUEOUS

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Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

Concluding Remarks Although the above discussion does not constitute a rigorous v e r i f i c a t i o n o f the model ( £ L ) · the simil a r i t y of the simulations presented here to observed p r o f i l e s o f H 2 S , FeS and FeS 2 i n marine sediments i n d i cates that the model may be o f value i n the study and i n t e r p r e t a t i o n of v e r t i c a l patterns i n s u l f u r diagenes i s · Comprehensive multiparameter analyses o f sediment p r o f i l e s from a v a r i e t y o f s i t e s w i l l be required to v a l i d a t e the model* In t h i s endeavor techniques w i l l have to be devised to a s c e r t a i n the molar surface areas of the various s o l i d phase reactants. Eventually i t may be p o s s i b l e to expand the model presented here to include processes i n the aerobic zone so that the depth to the oxidized-reduced boundary can be predicted as well as the pH p r o f i l e through t h i s boundary. This achievement would c o n s t i t u t e a t r u l y comprehensive model. Acknowledgements The author g r a t e f u l l y acknowledges the assistance of R. Underwood and G. Johnson who guided the author i n developing the Runge-Kutta algorithm. W. £· Sharp, W. S . Moore and M. Goldhaber reviewed the manuscript and provided h e l p f u l suggestions and discussions.

Abstract This paper proposes a system of ten non-linear, simultaneous differential equations which, upon further development and validation, may serve as a comprehensive chemical model for sulfide dominated marine sediments. These equations were developed in accordance with R. A. Berner's mathematics of diagenesis and incorporate D. T. Rickard's work on the kinetics of iron sulfide formation. A Runge-Kutta algorithm has been developed to provide numerical solutions for the equations. When utilized with an organic carbon substrate limitation on the production of reduced sulfur, the algorithm generates profiles of H S, FeS and FeS that agree qualitatively with measured profiles reported in the recent literature. Experiments with the algorithm suggest that the ratio of FeS to FeS at depth depends strongly on the i n i t i a l molar surface area of goethite but that the profiles of FeS, FeS , and H S are not greatly affected by the i n i t i a l pH. 2

2

2

2

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

35.

GARDNER

Sulfidic Marine Sediments

813

Literature Cited 1. 2.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch035

3.

4. 5.

6. 7. 8. 9.

10.

11. 12. 13. 14.

Berner, R.A. Stoichiometric models for nutrient regeneration in anoxic sediments. Limnol. Oceanogr. 22, 781-786 (1977). Berner, R.A. Diagenetic models of dissolved species in the i n t e r s t i t i a l waters of compacting sediments. Amer. Jour. Sci. 275, 88-96 (1975). Berner, R.A. Kinetic models for the early diagenesis of nitrogen, sulfur, phosphorus, and silicon in anoxic marine sediments, p. 427-450, in Goldberg, E.D. (ed.), "The Sea," v o l . 5, John Wiley & Sons, New York, 1974. Berner, R.A. An idealized model of dissolved sulfate distribution in recent sediments. Geochim. Cosmochim. Acta 28, 1497-1503 (1964). Martens, C.S. and Berner, R.A. Interstitial water chemistry of anoxic Long Island sound sediments. l. Dissolved gases. Limnol. Oceanogr. 22, 10-25 (1977). Lasaga, A.C. and Holland, H.D. Mathematic aspects of non-steady-state diagenesis. Geochim. Cosmochim. Acta 40, 257-266 (1976). Toth, D . J . and Lerman, A. Organic matter reactivity and sedimentation rates in the ocean. Amer. Jour. Sci. 277, 465-485 (1977). Berner, R.A. "Principles of Chemical Sedimentology" 204 p. McGraw-Hill Book Co., New York, 1971. Vanderborght, J., Wollast, R., and Billen, G. Kinetic models of diagenesis in disturbed sediments. Part 2. Nitrogen diagenesis. Limnol. Oceanogr. 22, 794-803 (1973). Vanderborght, J. and Billen, G. Vertical d i s t r i bution of nitrate concentration in i n t e r s t i t i a l water of marine sediments with n i t r i f i c a t i o n and denitrification. Limnol. Oceanogr. 20, 953-961 (1975). Rickard, D.T. Kinetics and mechanism of the sulfidation of goethite. Amer. Jour. Sci. 274, 941-952 (1974). Rickard, D.T. Kinetics and mechanism of pyrite formation at low temperature . Amer. Jour. Sci. 275, 636-652 (1975). Hildebrand, F.B. "Advanced Calculus for Applications, " 646 p. Prentice-Hall, Inc., Englewood C l i f f s , New Jersey, 1965. Berner, R.A. Sedimentary pyrite formation. Amer. Jour. Sci. 268, 1-23 (1970).

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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15. 16. 17.

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18. 19. 20.

21.

22.

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

Froberg, C.D. "Introduction to Numerical Analysis," 433 p. Addison-Wesley Publishing Co., Reading, Massachusetts, 1969. Gardner, L.R. Chemical models for sulfate re­ duction in closed anaerobic marine environments. Geochim. Cosmochim. Acta 37, 53-68 (1973). Berner, R.A. Sulfate reduction, pyrite formation, and the oceanic sulfur budget, p. 347-361, in Dryssen, D. and Jagner, D., ed., "The Changing Chemistry of the Oceans," Nobel Symposium 20, Almquist and Wiksell, Stockholm, 1972. Goldhaber, M.B. and Kaplan, I.R. The sulfur cycle, p. 569-655, in Goldberg, E . D . , ed. "The Sea," vol. 5, John Wiley & Sons, New York, 1974. Jorgensen, B.B. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnol. Oceanogr. 22, 814-831 (1977). FOAM (FriencTs of Anoxic Mud): Goldhaber, M.B., Allen, R . C . , Cochran, J . K . , Rosenfeld, J . K . , Martens, C . S . , and Berner, R.A. Sulfate reduction, diffusion, and bioturbation in Long Island sound sediments: Report of the Foam Group. Amer. Jour. Sci. 277, 193-237 (1977). Behrens, J.C., Beyer, J.E., Madsen. O.Β.G. and Thomsen, P.G. Some aspects of modelling the long-term behaviour of aquatic ecosystems. Ecological Modelling 1, 163-198 (1975). Berner, R.A. Sulfate reduction and the rate of deposition of marine sediments. Earth Planet. Sci. Lett. 37, 492-498 (1978).

RECEIVED November16,1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

36

WATEQ2—A and

Computerized Chemical

M a j o r E l e m e n t Speciation a n d

of N a t u r a l

Model

Mineral

for

Trace

Equilibria

Waters

JAMES W. BALL and EVERETT A. JENNE

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

U.S. Geological Survey, Water Resources Division, Menlo Park, CA 94025 DARRELL KIRK NORDSTROM Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903 The p r o t e c t i o n of ecosystems, upon which our h e a l t h and l i v e s depend ( 1 ) , r e q u i r e s that we understand n a t u r a l processes and develop the c a p a b i l i t y to p r e d i c t the e f f e c t of changes, such as the a d d i t i o n of p o l l u t a n t s , on these ecosystems. The p r e d i c t i o n of trace-element behavior i n ecosystems r e q u i r e s a multicomponent model by which one can: 1) c a l c u l a t e aqueous s p e c i a t i o n of the t r a c e elements among both n a t u r a l organic and i n o r g a n i c l i g a n d s ; 2) evaluate s o l u b i l i t y hypotheses: 3) account f o r s o r p t i o n d e s o r p t i o n processes; and 4) i n c o r p o r a t e chemical k i n e t i c s . This paper documents a chemical model that p a r t i a l l y accomplishes the f i r s t two of these four g o a l s . The present model has evolved from WATEQ, the e a r l i e r water-mineral e q u i l i b r i a model w r i t t e n i n P l / 1 by T r u e s d e l l and Jones (2, 3), and from WATEQF, the F o r t r a n v e r s i o n of Plummer et a l . ( 4 ) . These models, i n t u r n , drew on the preceding model of Barnes and C l a r k e ( 5 ) . The r e l a t e d PL/1 model, SOLMNEQ (6) , drew on the models of Barnes and C l a r k e (5) and a p r e p u b l i c a t i o n v e r s i o n of T r u e s d e l l and Jones (2) as w e l l as the thermodynamic data treatment of Helgeson (7) and Helgeson et_ a l . ( & ) . The WATEQ program contains an extensive thermodynamic data base which was c a r e f u l l y s e l e c t e d f o r use w i t h low-temperature n a t u r a l waters (9, 2 ) . A c t i v i t y c o e f f i c i e n t s f o r the major ions are c a l c u l a t e d from a computer f i t of an extended Debye-Huckel equation c o n t a i n i n g two a d j u s t a b l e parameters (2, 3_) . These a c t i v i t y c o e f f i c i e n t s are considered more r e l i a b l e than the standard Debye-Huckel equation or the Davies equation f o r h i g h i o n i c s t r e n g t h s o l u t i o n s (up to 1-3 m o l a l ) . The method of c a l c u l a t i o n i n WATEQ i s b a c k - s u b s t i t u t i o n f o r the c a t i o n s and successive approximation f o r the anions w i t h convergence on mass balance f o r anions. WATEQF changed to the more r a p i d backs u b s t i t u t i o n method f o r anion mass balance convergence. I n a d d i t i o n , manganese s p e c i a t i o n i s included i n WATEQF, and an o p t i o n f o r c a l c u l a t i n g a c t i v i t y c o e f f i c i e n t s by e i t h e r the Debye-Huckel or the Davies equation i s provided. WATEQ2 r e t a i n s most of these f e a t u r e s , and a d d i t i o n a l m o d i f i c a t i o n s are explained below.

0-8412-0479-9/79/47-093-815$05.25/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

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MODELING IN AQUEOUS

SYSTEMS

We have added ten a d d i t i o n a l elements (Ag, As, Cd, Cs, Cu, Mn, N i , Pb, Rb, and Zn), complexes of Br and I , s e v e r a l metastable s o l i d s , some s p a r i n g l y s o l u b l e s a l t s , and s e v e r a l i o n p a i r s of major c o n s t i t u e n t s t o the model. Other changes i n c l u d e r e ­ o r g a n i z a t i o n of the computer code i n t o a s e r i e s o f e x t e r n a l sub­ r o u t i n e s and changing the mode of convergence t o decrease the number o f i t e r a t i o n s r e q u i r e d . Because of the i n t e r a c t i v e nature o f aqueous s o l u t e s p e c i a ­ t i o n c a l c u l a t i o n s , i t would be d e s i r a b l e t o enter a t once i n t o the chemical model the r e a c t i o n s and thermodynamic data f o r a l l elements whose i n c l u s i o n might a f f e c t the computed a c t i v i t y o r e q u i l i b r i u m s o l u b i l i t y o f other s o l u t e s p e c i e s . However, our experience i s that the g r e a t e s t r e l i a b i l i t y i s obtained by adding only the data f o r one element, or f o r one l i g a n d group, a t a time; then t e s t data s e t s and r e a l world water sample analyses are r u n before making f u r t h e r a d d i t i o n s t o or changes i n the model. Various a s s o c i a t e s , whom we have f r e q u e n t l y c a l l e d on f o r s p e c i a l i z e d knowledge and i n f o r m a t i o n , have m a t e r i a l l y a s s i s t e d i n t h i s modeling e f f o r t . C o l l a b o r a t i v e s t u d i e s have o f t e n pro­ vided the impetus t o add some s p e c i f i c element, l i g a n d group, o r group of s o l i d phases t o the model. Apparent oversaturâtion w i t h one or more s o l i d phases o f an element has o f t e n prompted us t o seek out and add data f o r a d d i t i o n a l s o l u t e complexes o r more s o l u b l e s o l i d phases. The p a r t i t i o n i n g of an unexpectedly l a r g e p o r t i o n of an element i n t o a p a r t i c u l a r complex has l e d us to make an expanded c o m p i l a t i o n f o r the complex or t o c o n s u l t w i t h colleagues t o a s s i s t i n s e l e c t i n g best v a l u e s . Colleague c r i t i c i s m s ( c o n s t r u c t i v e and k i n d f o r the most p a r t ) of s t u d i e s i n press and i n p r e p a r a t i o n have prompted us to make s p e c i f i c t e s t s and proceed immediately w i t h some change o r a d d i t i o n , which would otherwise have awaited a "more opportune time." L. N. Plummer provided frequent c o n s u l t a t i o n and s u p p l i e d a p r e p u b l i c a t i o n copy of the r e a c t i o n s and a s s o c i a t e d thermodynamic data f o r the man­ ganese s e c t i o n of the WATEQF chemical model ( 4 ) . B. F. Jones and A. H. T r u e s d e l l have a l s o been p a r t i c u l a r l y h e l p f u l on many occasions. I n our e f f o r t t o c o l l e c t the appropriate data and develop the r e q u i s i t e understanding of geochemical processes, we have developed some adjunct computer programs. These i n c l u d e AACALC (Atomic Absorption and emission spectrometry CALCulation), EQLIST ( E Q u i l i b r i u m computation L I S T i n g ) , and EQPRPLOT ( E Q u i l i b r i u m computation P R i n t i n g and PLOTing). AACALC (FORTRAN) reduces atomic a b s o r p t i o n o r emission spectrometry data t o c o n c e n t r a t i o n s , EQLIST (PL/1) c o n s t r u c t s t a b l e s from the WATEQ2 (input) card f i l e , and EQPRPLOT (FORTRAN) c o n s t r u c t s r a t i o and s c a t t e r p l o t s o f d i s s o l v e d c o n s t i t u e n t s , a c t i v i t y products (AP), o r a c t i v i t y product to s o l u b i l i t y product r a t i o s (AP/K) v i a computer t e r m i n a l p r i n t e r or tape-driven p l o t t e r .

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

BALL

36.

817

Computerized Chemical Model

ET AL.

A d d i t i o n s and M o d i f i c a t i o n s to the Model

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

The thermochemical r e a c t i o n values vary according to the way the r e a c t i o n i s w r i t t e n . Therefore, a l l r e a c t i o n s i n the present model which have been added o r r e v i s e d , together w i t h the s e l e c t e d thermochemical v a l u e s , and o p e r a t i o n a l i n f o r m a t i o n t o f a c i l i t a t e input and output of the data, are a v a i l a b l e i n an adjunct r e p o r t (10). Elements. Rather l a r g e sets of s o l u t e complexes and m i n e r a l phases have been added f o r Ag, As, Cd, Cu, Mn, N i , Pb, and Zn. A d d i t i o n a l l y , Cs and Rb have been added t o the model. The merit of i n c l u d i n g Cs and Rb i n the model, i n s p i t e o f the near absence of s o l u t e complexes o r pure s o l i d s , i s that i n the f u t u r e the a c t i v i t i e s of the uncomplexed a l k a l i metal ions can be used t o compute t h e i r probable s u b s t i t u t i o n i n t o c e r t a i n s i l i c a t e minerals and to examine ion-exchange processes. Alkali metals complex w i t h 0H~, CI , and NO3 only a t such high i o n i c strengths that the b a s i c assumptions of a multicomponent i o n a s s o c i a t i o n model are no longer v a l i d . I n a d d i t i o n , thermodynamic data f o r these complexes are h i g h l y u n c e r t a i n . For these reasons, such complexes have been dropped from the model. S i m i l a r l y , there are data o n a l k a l i metal compounds such as Rb2S which might have been i n c l u d e d . However, the pure a l k a l i metal s u l f i d e s a r e known t o be h i g h l y unstable and/or deliquescent (11) and a r e r a r e l y found as m i n e r a l s . Therefore, they have not been i n c l u d e d i n the model. The manganese s e c t i o n s of WATEQF (4) were h e a v i l y u t i l i z e d i n preparing a s i m i l a r s e c t i o n f o r WATEQ2 (10). The s o l u t e por­ t i o n was u t i l i z e d i n i t s e n t i r e t y , b u t the MnOH"" and Mn(0H) a s s o c i a t i o n r e a c t i o n s were expressed i n terms o f H2O and H*" i n s t e a d o f OH , and the HMn0 complex, a d u p l i c a t i o n o f the Mn(0H)3 complex, was excluded. The f o l l o w i n g subset of the m i n e r a l species was s e l e c t e d : p y r o l u s i t e , b i r n e s s i t e , n s u t i t e , b i x b y i t e , hausmannite, p y r o c h r o i t e , manganite, r h o d o c h r o s i t e , M n C l 2 ' 4 H 2 0 , MnS(green), MnSO^, M n ( S O ) , Μ η ( Ρ 0 ) and MnHP0 . The f o l l o w i n g subset was excluded: MnO, M n ( 0 H ) , M n C l , MnCl2-H 0, Μη01 ·2Η 0, M n 2 S i 0 i and M n S i 0 . To the s e l e c t e d set were added s i x minerals f o r which thermochemical data are unknown, i n order t o o b t a i n l o g AP values of the i n d i v i d u a l minerals f o r d i f f e r e n t waters. The s i x minerals are^_ cryp£omelane (Ko^Mnf^gMn^ O^y), h o l l a n d i t e 1

3

2

2

i+

3

3

4

3

+

2

2

2

2

3

(Bao,78

F e

§.57

M n

6.59

M n

^ ° 1 6 ) > psilomelane

(Ba [ C a * 1 gK 1 ^ ί η Ί Α η ^ 0 2

0

4

2

7 8

0

0 #0

ι6

· 2. 5H 0) , t o d o r o k i t e 2

( 0 . 3 9 3 g 0 . 4 7 3 ? t l 3t+Mn^ 0 ·2Η 0) , l i t h i o p h o r i t e ( L i 2 A l M n i M n ^ 0 5 - 1 4 H O ) , and r a n c i e i t e (Ca^ Mn^ Μηίί 0 -3H 0). Ca

M

Mn

+

12

2

+

8

+

3

2

h

56

9

2

Aqueous Complexes. A l l s o l u t e r e a c t i o n s are w r i t t e n as a s s o c i a t i o n (formation) r e a c t i o n s whereas the s o l i d r e a c t i o n s a r e w r i t t e n as d i s s o c i a t i o n ( d i s s o l u t i o n ) r e a c t i o n s . For mass

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

818

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

b a l a n c i n g purposes, a l l s o l u t e r e a c t i o n s are w r i t t e n i n terms of the f r e e form of the parent s p e c i e s , so t h a t a l l constants are f o r o v e r a l l r a t h e r than stepwise r e a c t i o n s , P o l y s u l f i d e s and S u l f i d e . The p o l y s u l f i d e complexes of Ag and Cu have been added to the model i n an attempt to reduce the apparent o v e r s a t u r a t i o n w i t h Ag2S(s) c a l c u l a t e d f o r San Fran­ c i s c o Bay waters (12). C a l c u l a t i o n of the a c t i v i t y of p o l y s u l f i d e ions r e q u i r e s the assumptions: 1) the q u a n t i t y of Sg ( f r e e _ s u l f u r ) i s not a l i m i t a t i o n on i t s r e a c t i o n w i t h b i s u l f i d e (HS ) to form p o l y s u l f i d e s ; and 2) p o l y s u l f i d e s are i n e q u i l i b r i u m w i t h bisulfide. I n a d d i t i o n to the s u l f i d e complexes of the added t r a c e elements, the Fe(HS)2 and Fe(HS)3 complexes (13) have been i n ­ cluded to i n c r e a s e the r i g o r of the s u l f i d e s p e c i a t i o n . The s u l f i d e r e a c t i o n s have been r e w r i t t e n i n terms of HS r a t h e r than S s i n c e HS i s the dominant s u l f i d e i o n i n most waters. S u l f a t e . V a r i o u s p u b l i s h e d v a l u e s f o r the a s s o c i a t i o n constants and a s s o c i a t i o n e n t h a l p i e s of metal s u l f a t e i o n p a i r s and t r i p l e t s (14, 15, 16, 17) show good agreement _(± 10%) except f o r NaSOi4. Log Κ v a l u e s f o r the formation of NaSOi4 range from the 0.226 v a l u e of Lafon and T r u e s d e l l (18) to the 1.17 v a l u e of Pytkowicz and Rester (19), as c i t e d by F i s h e r (20). I f the one low v a l u e of Lafon and T r u e s d e l l (18) and the h i g h values of F i s h e r and Fox (21) and F i s h e r (20) are dropped, the remaining four v a l u e s average 0.70 + 0.05 (22, 23, 24, 25) which i s i d e n t i ­ c a l to the v a l u e s e l e c t e d by Smith and M a r t e l l (26), who may have used the same e v a l u a t i o n technique. G. M. Lafon (Johns Hopkins U., personal communication, 1978) has suggested t h a t the low v a l u e should be discounted and t h a t the formation of a sodium s u l f a t e i o n t r i p l e t i s u n l i k e l y . Most of the other a s s o c i a t i o n constants were obtained from R. M. S i e b e r t and C. L. C h r i s t ( C o n t i n e n t a l O i l Co., U. S. Geol. Survey, p e r s o n a l communication, 1976) a f t e r comparing t h e i r values w i t h those reported i n the l i t e r a t u r e . Enthalpy v a l u e s were a l s o s e l e c t e d from the p r e l i m i n a r y data of R. M. S i e b e r t and C. L. C h r i s t which had been evaluated by the Fuoss equation (27). C a r e f u l checking w i t h published l i t e r a t u r e values showed no s e r i o u s d i s c r e p a n c i e s and i t was f e l t that u s i n g data from one source would help m a i n t a i n i n t e r n a l c o n s i s t e n c y . The i o n t r i p l e t Fe(S0i )2 was one e x c e p t i o n . The l o g Κ f o r t h i s com­ p l e x i s the average of the r e s u l t s of I z a t t et a l . (25) and Mattoo (28) which d i f f e r by l e s s than 1%. The enthalpy of a s s o c i a t i o n has not been p u b l i s h e d but i t has been estimated by assuming t h a t the d i f f e r e n c e between i t and FeS0"t i s equal to the d i f f e r e n c e between Α 1 ( 8 0 ) and A1S04. Although the r e l i ­ a b i l i t y of t h i s e s t i m a t i o n cannot e a s i l y be determined, i t c e r t a i n l y i s b e t t e r than assuming ΔΗ = 0. F l u o r i d e . For e q u i l i b r i u m c a l c u l a t i o n s i n a c i d s o l u t i o n s , s t a b i l i t y constants are needed f o r HF0 H F 2 , and ( H F ) s p e c i e s . These species become important when the pH drops below 4.5 and f l u o r i d e c o n c e n t r a t i o n r i s e s above 5 χ 10 M. S e v e r a l 2

+

4

2

2

-4

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Computerized Chemical Model

BALL ET AL.

36.

819

measurements have been made on the d i s s o c i a t i o n of HF° a t 298.15°K and the agreement i s e x c e l l e n t (29-37). The weighted mean value of l o g Κ = 3.169±0.010 (1σ, unweighted) given i n B a l l e t a l . (10) was c a l c u l a t e d from these i n v e s t i g a t i o n s a f t e r dropping the high value of P a t e l e t a l . (34) and the low value of V a s i l e v and K o z l o v s k i i (37) which i s necessary i n order to m a i n t a i n con­ s i s t e n c y w i t h the k i n e t i c data of Kresge and Chiang (_38, 39) . For the r e a c t i o n : T

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

HF°

+

F~

t

HF~

CI)

the s t a b i l i t y constant has a l a r g e r u n c e r t a i n t y owing t o the competing HF° a s s o c i a t i o n e q u i l i b r i u m . Reported l o g Κ values range from 0.49 t o 0.70 (29, 30, 32, 33, 35, 36) and the weighted mean value i s 0.58 ± 0.05. Aqueous HF dimers have been shown t o e x i s t by Warren (35) who measured a l o g Κ of 0.43 ± 0.05 f o r the r e a c t i o n : 2HF°

t

(HF)°.

(2)

Enthalpy values f o r the c a l c u l a t i o n o f temperature dependence are not a v a i l a b l e f o r r e a c t i o n 2. The l o g Κ f o r the d i s s o c i a t i o n of HF° and f o r r e a c t i o n 1 have been measured between 0 and 100°C by Broene and DeVries (29), E l l i s (30) and Hamer and Wu (33). V a s i l ' e v and K o z l o v s k i i (37) have a l s o obtained enthalpy data f o r these r e a c t i o n s by c a l o r i m e t r i c t i t r a t i o n . The average ΔΗ = -3.46 ±0.75 k c a l m o l " f o r HF° d i s s o c i a t i o n and ΔΗ = 1.09 ± 0.30 k c a l mol f o r r e a c t i o n 1. S i l i c a t e m i n e r a l s are more s o l u b l e i n n a t u r a l waters having high f l u o r i d e c o n c e n t r a t i o n s and low pH values than i n other waters. High c o n c e n t r a t i o n s of d i s s o l v e d s i l i c a may be maintained by the formation of h e x a f l u o r o s i l i c i c a c i d : 1

1

0

+

S i (OH). + 6F" + 4 H ? S i F ^ " 4 6

+ 4H 0. 2

(3)

o

The e q u i l i b r i u m constant f o r t h i s r e a c t i o n has been measured a t 25°C by Roberson and Barnes (40) and the enthalpy i s estimated from the data of Wagman et_ a l . (41). Reaction 3 i s important i n many a p p l i c a t i o n s : a) chemical processes i n v o l v i n g v o l c a n i c gases and condensates (40), b) chemical r e a c t i o n s i n a c i d , h a l o g e n - r i c h hot s p r i n g s (42, 43), c) waters r e c e i v i n g e f f l u e n t from phosphate processing p l a n t s (44), d) f l u o r i d a t i o n of water s u p p l i e s (45), e) a n a l y t i c a l chemistry, such as i n the d e t e r ­ m i n a t i o n of e i t h e r f l u o r i d e or d i s s o l v e d s i l i c a (46) and f ) the formation and hydrothermal a l t e r a t i o n of ore d e p o s i t s (47., ^8» 49). The l o g Κ and ΔΗ f o r the formation of the aqueous_complexes CaF , F e F , FeF^, F e F , B F ( 0 H ) , B F ( 0 H ) and BF (OH) have been evaluated by Nordstrom and Jenne (50) and are i n good agree­ ment w i t h those s e l e c t e d by Smith and M a r t e l l (26) who used a d i f f e r e n t e v a l u a t i o n procedure. This a d d i t i o n to WATEQ2 +

2 +

3

3

2

2

3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

820

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

makes a f a i r l y complete inventory of aqueous f l u o r i d e complexes. N e u t r a l and Polymeric Aluminum and I r o n . The a s s o c i a t i o n constants and e n t h a l p i e s of aluminum and i r o n hydrox­ ides have been evaluated by comparing the c r i t i c a l l y s e l e c t e d data of Baes and Mesmer (51) w i t h that of R. M. S i e b e r t and C. L. C h r i s t (personal communication, 1976). D i f f e r e n c e s between the two data s e t s are n e g l i g i b l e and the f i n a l s e l e c t i o n was from Baes and Mesmer (51) because data on more complexes are found there. Important new species added to tljie model are the polynuclear complexes Fe2(0H)2 and Fe3(0H)5+4 . Some controversy has a r i s e n over the e x i s t e n c e of Fe(0H)3 and A1(0H)3. Baes and Mesmer (51) have i n d i c a t e d that although the formation constant of A1(0H)§ i s o n l y known from one measurement (52) and has a l a r g e u n c e r t a i n t y , i t i s r e a l , with a log Κ -15.0 f o r the r e a c t i o n

Al

3 +

+ 3H 0

+

A1(0H)

2

+ 3H .

(4)

Baes and Mesmer (51) a l s o suggest that the l o g Κ f o r Fe

3 +

+ 3H 0

Fe(0H)

2

+ 3H

+

(5)

i s l e s s than -12 and t h i s agrees w i t h the g e n e r a l l y accepted value of -13.6 (53). Recently, Byrne (54) and Kester et a l . (55) have presented evidence f o r the e x i s t e n c e of Fe(0H) and r e ­ confirmed the v a l u e of the l o g K. We have t h e r e f o r e included both n e u t r a l species i n the model. Others. E q u i l i b r i u m a s s o c i a t i o n constants c a l c u l a t e d from f r e e energy data (41) f o r two aqueous a r s e n i c f l u o r i d e s p e c i e s , As0 F and HAs0 F , were so h i g h that the two species accounted f o r v i r t u a l l y a l l the A s i n s e v e r a l water samples, p r a c t i c a l l y i r r e s p e c t i v e of the f l u o r i d e c o n c e n t r a t i o n . The E c a l c u l a t e d from the a c t i v i t i e s of A s and As5+ under these c o n d i t i o n s was near -4 v o l t s , i . e . w e l l below that at which water decomposes (0 to -0.83 v o l t s from pH 0 to 14). From the o r i g i n a l data of Dutt and Gupta (56) the l o g Κ = 2.832 f o r 2

3

3

5 +

H

3 +

H As0 3

+ F" = HAs0 F" + H 0 3

(6)

2

and l o g Κ = -3.037 f o r 2

H3AsO4 + F" = A s 0 F " + H

+

+ H20 .

(7)

Thus, there appears to have been a computational e r r o r i n con­ v e r t i n g the s t a b i l i t y data of Dutt and Gupta (56) to standard f r e e energies of formation. E q u i l i b r i u m l o g K£ values c a l c u l a t e d from f r e e energy data (41) f o r two lead hydroxychlorides (PbOHCl, Pb2(0H) Cl) d i d not agree w i t h those of the o r i g i n a l authors (57). However, r e v i s e d AG data (10) from NBS (B. R. S t a p l e s , N a t ' l Bur. Stand., personal 3

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

36.

BALL

ET AL.

Computerized Chemical Model

821

communication, 1978) agree very w e l l w i t h the o r i g i n a l data. Organic Ligands. The model has been expanded to permit s e n s i t i v i t y analyses of n a t u r a l l y o c c u r r i n g organic l i g a n d s . These composite l i g a n d groups are r e f e r r e d to as f u l v a t e and humate. The model d e f a u l t s to molecular weights of 650 and 2000, r e s p e c t i v e l y , f o r these two l i g a n d groups. Reported molecular weights f o r these substances vary w i d e l y (S. A. Jacobs and E. A. Jenne, unpub. data, 1978). Therefore, i f a c o n c e n t r a t i o n f o r e i t h e r substance i s used as input data, without an accompanying a n a l y t i c a l l y determined molecular weight, a warning message i s p r i n t e d , and a l l p e r t i n e n t output data a r e f l a g g e d . Reported e q u i l i b r i u m constants f o r these m e t a l - l i g a n d complexes a l s o vary widely and should t h e r e f o r e be user s u p p l i e d . I n the absence of s u p p l i e d v a l u e s , the model d e f a u l t s t o data from Smith and M a r t e l l (26) f o r o x a l i c a c i d (10). S o l i d Phases. S u l f a t e s . S o l u b i l i t y product constants and f r e e energies of formation f o r the j a r o s i t e m i n e r a l group ( j a r o s i t e , n a t r o j a r o s i t e , and hydronium j a r o s i t e or c a r p h o s i d e r i t e , as the hydrogen form i s termed i n the o l d e r l i t e r a t u r e ) have been compiled by Nordstrom (58). Considerable d i s c r e p a n c i e s occur between d i f f e r e n t i n v e s t i g a t i o n s because the s o l u t i o n e q u i l i b r i a are very complicated: s e v e r a l strong complexes are formed and attempts a r e seldom made to account f o r the e f f e c t of hydroxide and s u l f a t e complexation of the c a t i o n s i n v o l v e d on apparent s o l u b i l i t y . There i s a l s o a l a c k of consistency between values f o r the j a r o s i t e s o l u b i l i t y product constant, p a r t l y because d i f f e r e n t complexes were used. Of the four i n v e s t i g a t i o n s made on j a r o s i t e , Browne r e s u l t s (59, 60) must be discounted because of very l a r g e u n c e r t a i n t i e s i n the r e s u l t s . A mean value of -98.80 + 1.1 f o r the log K has been s e l e c t e d f o r WATEQ2 from the works of Zotov e t a l . (61), Vlek et a l . (62) and Kashkai et a l . (63). I f the d i s s o l u t i o n r e a c t i o n i s w r i t t e n as: s p

6H

+

+

+ KFe (S0 ) ( 0 H ) (s) t K + 3 F e 3

2

3 +

+ 2S0^" + 6^0

(8)

then the l o g Κ i s -14.8 + 1 . 1 . The l o g Κ f o r n a t r o j a r o s i t e , w r i t t e n i n the same manner as r e a c t i o n 8, i s -11.2 + 1 . 0 from the work of G. C l i f t o n ( C o n t i n e n t a l M a t e r i a l Co., personal communication, 1977) and agrees w i t h the value obtained by Kashkai et a l . (63). Only one l o g Κ value i s a v a i l a b l e f o r hydronium j a r o s i t e (63) . The ΔΗ f o r the r e a c t i o n 8 has been estimated to be -31.28 k c a l m o l " by u t i l i z i n g the data of Zotov e t a l . (61) f o r the entropy of j a r o s i t e , the p r e v i o u s l y c i t e d i n v e s t i g a t i o n s f o r the mean AG° value and Wagman e t / a l ^ (41, 64) f o r the e n t h a l ­ pies of the i o n s . The enthalpy f o r n a t r o j a r o s i t e d i s s o l u t i o n i s d e r i v e d from the entropy and f r e e energy values given by G. C l i f t o n (personal communication, 1977) and f o r hydronium j a r o ­ s i t e a l i n e a r c o r r e l a t i o n between f r e e energies and e n t h a l p i e s was assumed f o r the j a r o s i t e group s i n c e no data a r e a v a i l a b l e . 1

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

822

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SYSTEMS

We have observed m e l a n t e r i t e , FeSOi4· 7H 0 , to be one of the common s u l f a t e m i n e r a l s produced by the o x i d a t i o n of p y r i t e during weathering. U n f o r t u n a t e l y , i t s s o l u b i l i t y and r e l a t e d thermo­ dynamic p r o p e r t i e s are not w e l l e s t a b l i s h e d . The l o g Κ f o r m e l a n t e r i t e d i s s o l u t i o n has been d e r i v e d from the f r e e energies of formation of the c o n s t i t u e n t species and the g r e a t e s t source of u n c e r t a i n t y l i e s w i t h the AGf f o r F e . We p r e f e r the v a l u e of -21.8 ± 0.5 (65) which r e s u l t s i n a l o g Κ of -2,47. The enthalpv of d i s s o l u t i o n i s 2.86 k c a l m o l - based on ∆Ηf = -22.1 k c a l mol f o r Fe2+ from Larson and Hepler (65). The l o g Κ and ΔΗ values i n B a l l et a l . (10) f o r the d i s s o l u t i o n of epsomite have been obtained from the f r e e energy and enthalpy data given i n Wagman e t a l . (41) and Parker e t a l . (66). For the l o g Κ and ∆Η o f potassium alum s o l u b i l i t y , values were obtained from the f r e e energies and e n t h a l p i e s of Wagman et a l . (41) and K e l l y e t a l . (67). F l u o r i t e . The s o l u b i l i t y and r e l a t e d thermodynamic p r o p e r t i e s of f l u o r i t e have had l a r g e u n c e r t a i n t i e s , i . e . 2 to 3 orders of magnitude. Nordstrom and Jenne (50) u t i l i z e d s i m u l ­ taneous m u l t i p l e r e g r e s s i o n a n a l y s i s (68) t o evaluate these thermochemical data. The r e v i s e d l o g Κ (10) agrees q u i t e w e l l w i t h the upper l i m i t of f l u o r i t e i o n a c t i v i t y product c a l c u l a t i o n s of many geothermal waters i n the western United S t a t e s . Although a t o t a l u n c e r t a i n t y of ± 0.5 was assigned t o the l o g Κ (to i n ­ clude a n a l y t i c a l and computational e r r o r s ) , more recent i n v e s t i ­ g a t i o n s i n d i c a t e that the l o g Κ f a l l s between -10.5 and -11.0 (69, 70, 71) so that the u n c e r t a i n t y i n the l o g Κ a t 298.15K i s ±0.25. At t h i s l e v e l of d e v i a t i o n the a n a l y t i c a l and computational u n c e r t a i n t i e s inherent i n the c a l c u l a t i o n s of the i o n a c t i v i t y product are l i k e l y to be g r e a t e r than those i n the thermodynamic properties. Other a l k a l i n e e a r t h f l u o r i d e s ( B a F , S r F ) have been added to the model. However, they are l e s s l i k e l y than t h e i r r e s p e c t i v e s u l f a t e s or carbonates t o be s o l u b i l i t y l i m i t i n g phases. Others. Thermochemical data f o r the f e r r o u s c h l o r i t e , g r e e n a l i t e ( F e S i 0 ( 0 H ) ) , and p h l o g o p i t e ( K M g l S i s O i 10(0H) ) d i s s o l u t i o n were taken from Plummer ejt a l . (4) who used the f r e e energy v a l u e s of Eugster and Chow (72) f o r g r e e n a l i t e and B i r d and Anderson (73) f o r p h l o g o p i t e . S o l u b i l i t y c a l c u l a t i o n s were added f o r two allophanes, f o r which the e q u i l i b r i u m constants and formulae are a f u n c t i o n of pH. Paces (74) found c o l d ground waters c o l l e c t e d from s p r i n g s i n g r a n i t i c rocks of the Bohemian M a s s i f of Czechoslovakia t o be supersaturated w i t h respect t o k a o l i n i t e w h i l e being unsaturated w i t h respect to amorphous s i l i c a . He i n t e r p r e t e d t h i s as an i n d i c a t i o n that a metastable a l u m i n o s i l i c a t e more s o l u b l e than k a o l i n i t e was c o n t r o l l i n g the c o n c e n t r a t i o n s of alumina and s i l i c a i n these waters. This a l u m i n o s i l i c a t e was f u r t h e r hypothe­ s i z e d to be of v a r i e d chemical composition, c o n t r o l l e d by the mole 2

2

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

1

2

3

2

5

2

4

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

2

36.

BALL

Computerized Chemical Model

ET AL.

823

f r a c t i o n of s i l i c a and d i s s o l v e d by the r e a c t i o n : [Al(OH) ] 3

( 1 x )

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

(l-x)Al

3 +

[ S i 0 ] ( s ) + 3(l-x)H 2

+

x

+ xH SiO

+ (3-5x)H 0 .

4

(9)

2

In equation 9, x i s the mole f r a c t i o n of s i l i c a and i s equal t o 1.24 - 0.135pH. This expression d e s c r i b e s the l i n e a r v a r i a t i o n between pure amorphous hydrous alumina and s i l i c a as a f u n c t i o n of pH (75). The e q u i l i b r i u m constant f o r t h i s substance was c a l c u l a t e d by combining two endmember constants from the l i t e r a ­ ture and i n c o r p o r a t i n g the pH-dependence equation i n t o the r e s u l t i n g e x p r e s s i o n , y i e l d i n g an expression f o r the e q u i l i b r i u m s o l u b i l i t y (75) of: l o g Κ = -5.7 + 1.68pH.

(10)

Under f i e l d c o n d i t i o n s the s o l u b i l i t y of t h i s m a t e r i a l should be lower due to the l a r g e d i f f e r e n c e i n the speed of c r y s t a l l i z a t i o n of amorphous alumina versus amorphous s i l i c a . In f a c t , a b e s t f i t l i n e to f i e l d samples from the S i e r r a Nevada i s described by the equation (75): log Κ = -5.4 + 1.52pH.

(11)

Copper f e r r i t e s have been i n c l u d e d i n the model, but have as yet not been found to be e q u i l i b r i u m c o n t r o l s on copper or i r o n s o l u b i l i t y . The c a l c u l a t e d a c t i v i t y products f o r the two m i n e r a l s , cuprous f e r r i t e and c u p r i c f e r r i t e , are c h a r a c t e r i s t i c a l l y s e v e r a l orders of magnitude oversaturated when compared to t h e i r respec­ t i v e e q u i l i b r i u m constants i n a wide v a r i e t y of surface waters. Ponnamperuma et_ a l . (76) d e s c r i b e a f e r r o s o f e r r i c hydroxide (Fe3(0H)s), o f f e r i n g evidence that most i r o n ( I I ) i n reduced s o i l s other than a c i d s u l f a t e s o i l s i s present i n t h i s form. Using a l o g Κ of 17.56 from Ponnamperuma et a l . (76) f o r the reaction: 2Fe(0H)(s) + F e J

2 +

+ 20H

_

Fe(0H)(s) J o

(12)

and l o g Κ values f o r the i o n i z a t i o n of water and f e r r i c hydroxide d i s s o l u t i o n , we c a l c u l a t e a l o g Κ of 20.222 f o r the r e a c t i o n : F e ( 0 H ) ( s ) + 8H J o

+

2Fe

3 +

+ Fe

2 +

+ 8H0 . Ζ

(13)

Biedermann and Chow (77) d e s c r i b e a f e r r i c h y d r o x y - c h l o r i d e ( F e ( O H ) 2 . 7 C l 0 3 ) which has been seen to p r e c i p i t a t e from sea water, having a log Κ of 3.04 + 0.05 (51) f o r the r e a c t i o n : Fe(0H)

2

7

C 1 ( s ) + 2.7H 3

+

Fe

3 +

+ 2.7H0 +

0.3Cl-.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(14)

824

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

Chien and Black (78) c a l c u l a t e a l o g Κ of -114.4 f o r the fluorocarbonato a p a t i t e r e a c t i o n : C a

9.5Na 3 Mg i e

6

e l

+ l t

(C0 ) 3

l e 2

+

0.36Na + 0.144Mg

F

2 f l

2+

, (PO )i 8

l t

+ e 8

( s ) t 9.5Ca

2+

+

+ 1.2C0 + 2.48F~ + 4.8P0L*~ . 3

(15)

These r e a c t i o n s have been added to the model. Morey e_t a l . (79) have c a l c u l a t e d an e q u i l i b r i u m constant f o r amorphic s i l i c a which best f i t s t h e i r f i e l d data. The l o g Κ f o r t h i s r e a c t i o n of -2.71 has a l s o been added. Redox Couples. The model c a l c u l a t e s the redox p o t e n t i a l of the couples: H 0 / 0 , H 0 / 0 , F e / F e , N0 /N0 , S "/S0C , and A s / A s , given the r e q u i s i t e c o n c e n t r a t i o n s of the couple mem­ bers. D i s s o l v e d oxygen i s a l l that i s r e q u i r e d f o r c a l c u l a t i o n of both the H 0 / 0 and H 0 / 0 couples. The H 0 / 0 couple i s k i n e t i c a l l y i n h i b i t e d and i s g r o s s l y out of e q u i l i b r i u m except at elevated temperatures (80). Therefore, the o p t i o n of u s i n g pE from d i s s o l v e d oxygen f o r redox s p e c i a t i o n has been dropped from the model. Recent s t u d i e s (81) show that when the f o l l o w i n g three c o n d i t i o n s are f u l f i l l e d , the platinum e l e c t r o d e provides a r e ­ l i a b l e and accurate estimate of the f e r r o u s - f e r r i c redox p o t e n t i a l , Fe /Fe ' drainage waters. The c o n d i t i o n s are: 1) l a r g e volumes of water must f l o w past the e l e c t r o d e s during emf measurement; 2) water samples must be p r o p e r l y f i l t e r e d (30%; 4) r e v i s e d anion mass balance c a l c u l a t i o n , a l l o w i n g f o r f a s t e r 2

+

2

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

BALL

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

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Computerized Chemical Model

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827

convergence; and 5) improved set of headings used i n the p r i n t e d r e s u l t s of the s o l u t e modeling c a l c u l a t i o n s . S p e c i f i c conductance c a l c u l a t e d from input major c o n s t i t u e n t data using the method of Laxen (83) has been added t o the model as a check on a n a l y t i c a l input data. D i f f e r i n g input and c a l c u ­ l a t e d s p e c i f i c conductances i n d i c a t e that one or more e r r o r s may e x i s t i n the a n a l y t i c a l input data. A mass balance s e c t i o n f o r the hydrogen s u l f i d e species was added t o the anion mass balance c a l c u l a t i o n s when we observed that strong HS complexing of some t r a c e metals sometimes rendered c a t i o n mass balance convergence i m p o s s i b l e . A c t i v i t y c o e f f i c i e n t s were o r i g i n a l l y c a l c u l a t e d using the extended Debye-Huckel equation and whenever a new complex was added t o the program i t was necessary to estimate the a parameter. This problem was overcome by s u b s t i t u t i n g the more general Davies equation which has adequate r e l i a b i l i t y a t low i o n i c strengths and i s u s u a l l y more accurate a t high i o n i c strengths (84). Since a c i d mine waters can have i o n i c strengths approaching that of sea water, i t i s d e s i r a b l e to use a theory f o r a c t i v i t y c o e f f i c i e n t s that can reach somewhat above 0.1 m o l a l , the u s u a l upper l i m i t f o r extended Debye-Hiickel c a l c u l a t i o n s . The Davies equation i s considered s a t i s f a c t o r y to 0.5 m o l a l . The extended Debye-Huckel equation w i t h f i t parameters (2, 3) has been r e t a i n e d f o r the major i o n s , Ca, Mg, Na, K, CI and SO^, and the Debye-Huckel equation i s used t o c a l c u l a t e the p o l y s u l f i d e a c t i v i t y c o e f f i c i e n t s , f o r which a parameters have been estimated by Cloke (85). There are i n c o n s i s t e n c i e s i n the model f o r the c a l c u l a t i o n of a c t i v i t y products f o r the " c l a y s . " ^Exchangeable c a t i o n s are disregarded f o r the low exchange c a p a c i t y k a o l i n i t e , h a l l o y s i t e , c h l o r i t e , and moderate c a p a c i t y i l l i t e . For c e r t a i n expansible l a y e r s i l i c a t e s and two z e o l i t e s , the l o g i o a c t i v i t y of s e l e c t e d c a t i o n s i s added i n t o the sum of the a c t i v i t y products. The m i n e r a l phases treated i n t h i s manner, and the s o l u t e c a t i o n s considered as exchangeable c a t i o n s , are b e i d e l l i t e ([A ] i + A + + A +), c l i n o p t i l o l i t e and mordenite (A + + A +), B e l l e Fourche m o n t m o r i l l o n i t e and Aberdeen m o n t m o r i l l o n i t e (A + + A + + A +). Note that the square root of the d i v a l e n t c a t i o n i s used i n the sum i n keeping w i t h the p r a c t i c e i n the i o n exchange l i t e r a t u r e (86) . R e v i s i o n of the c a l c u l a t i o n of exchangeable c a t i o n c o n t r i b u t i o n to the a c t i v i t y product has been delayed pending the p e r t i n e n t reviews of K i t t r i c k (87), as w e l l as that of Bassett elt a l . (88) presented a t t h i s symposium. o f

t

n

e

2 +

M g

H

Na

Na

K

Na

K

K

M o d i f i c a t i o n s i n the Code The PL/1 language computer code has been e x t e n s i v e l y a l t e r e d i n the process of b u i l d i n g i t i n t o WATEQ2; i n f a c t , minor a l t e r a t i o n s are f a r too numerous to mention here. Several e r r o r s i n the o r i g i n a l code were c o r r e c t e d and some major changes, noted below, were made to improve program execution and ease of use

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

828

CHEMICAL

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and to broaden i t s u s e f u l n e s s . The i n p u t and output aspects of program o p e r a t i o n are g i v e n , along w i t h the thermodynamic data base, i n a supplementary r e p o r t (10). Arrays which must be increased i n s i z e when species a r e added are now a u t o m a t i c a l l y a d j u s t a b l e i n dimension merely by supplying a p p r o p r i a t e input data. The r e s u l t s of s o l u t e and m i n e r a l c a l c u l a t i o n s w i l l not appear i f the a c t i v i t y o r a c t i v i t y product, r e s p e c t i v e l y , has not been c a l c u l a t e d . This e l i m i n a t e s extraneous non-information and shortens the l i s t i n g c o n s i d e r a b l y , an advantage e s p e c i a l l y when a simple l a b o r a t o r y s o l u t i o n i s considered and/or a low-speed remote computer t e r m i n a l i s utilized. Some data are entered i n t o the model as " c a r r i e d - o n l y " data, p r i m a r i l y f o r p l o t t i n g u s i n g a subsequent computer program. How­ ever, as the model evolves, some of these c a r r i e d - o n l y data be­ come input to the model i t s e l f o r t o adjunct c a l c u l a t i o n s . S p e c i f i c conductance, which was i n i t i a l l y c a r r i e d - o n l y data, i s now compared t o a computed " s p e c i f i c conductance" as a q u a l i t y o f - a n a l y s i s screening technique. The l i s t i n g o f the r e s u l t s of the m i n e r a l e q u i l i b r i u m c a l c u l a t i o n s has been d r a s t i c a l l y a l t e r e d , w i t h the d e l e t i o n of AG , AG per e q u i v a l e n t c a t i o n , and a l l v a l u e s i n base 10 form. Information now p r i n t e d f o r each species f o r which an a c t i v i t y product i s c a l c u l a t e d i n c l u d e s l o g AP/K, SIGMA ( A n a l y t i c a l ) , SIGMA (Thermodynamic), l o g A P / K ^ n , and l o g ΑΡ/Κ^χ. As discussed p r e v i o u s l y , SIGMA(A) i s the propagated standard d e v i a t i o n i n the a n a l y t i c a l values and SIGMA(T) i s the standard d e v i a t i o n i n the thermodynamic data. The l o g i o K i and logioKmax values have been changed from + 5% of the l o g i o K v a l u e (2) t o e x p e r i m e n t a l l y determined v a l u e s which may represent a l e s s s o l u b l e or more s o l u b l e form o f the s o l i d phase than that s e l e c t e d as the "best" v a l u e . WATEQ2 c o n s i s t s of a main program and 12 subroutines and i s patterned s i m i l a r l y to WATEQF ( 4 ) . WATEQ2 (the main program) uses input data to set the bounds of a l l major a r r a y s and c a l l s most of the other procedures. INTABLE reads the thermodynamic data base and p r i n t s the thermodynamic data and other p e r t i n e n t i n f o r m a t i o n , such as a n a l y t i c a l expressions f o r e f f e c t of tempera­ ture on s e l e c t e d e q u i l i b r i u m constants. PREP reads the a n a l y t i c a l data, converts concentrations t o the r e q u i r e d u n i t s , c a l c u l a t e s temperature-dependent c o e f f i c i e n t s f o r the Debye-Huckel equation, and t e s t s f o r charge balance of the input data. SET i n i t i a l i z e s values of i n d i v i d u a l species f o r the i t e r a t i v e mass action-mass balance c a l c u l a t i o n s , and c a l c u l a t e s the e q u i l i b r i u m constants as a f u n c t i o n of the input temperature. MAJ_EL c a l c u l a t e s the a c t i v i t y c o e f f i c i e n t s and, on the f i r s t i t e r a t i o n o n l y , does a p a r t i a l s p e c i a t i o n of the major anions, and performs mass a c t i o n mass balance c a l c u l a t i o n s on L i , Cs, Rb, Ba, Sr and the major c a t i o n s . TR_EL performs these c a l c u l a t i o n s on the minor c a t i o n s , Mn, Cu, Zn, Cd, Pb, N i , Ag, and As. SUMS performs the anion mass r

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action-mass balance c a l c u l a t i o n s , and t e s t s the r e s u l t s _ a g a i n s t input c o n c e n t r a t i o n values f o r the anions C03~, S0^~, F , P0$ , CI and S , and p r i n t s the r e s u l t s of each set of i t e r a t i v e c a l c u l a t i o n s . MAJ_EL, TR_EL and SUMS are executed r e p e t i t i v e l y u n t i l mass balance to w i t h i n 0.1% of the input c o n c e n t r a t i o n s i s achieved f o r the s i x anions, or u n t i l 40 i t e r a t i o n s have elapsed. I f convergence i s not reached i n 40 i t e r a t i o n s , a warning message i s p r i n t e d and execution continues j u s t as through convergence had been reached. SOLUTES performs computations not r e l a t e d to the mass balance c a l c u l a t i o n s , such as E , s p e c i f i c conductance, pC^and pCH^ c a l c u l a t i o n s , p r i n t s out a l l the s o l u t e d a t a , and performs necessary l o g a r i t h m conversions f o r use i n subsequent c a l c u l a t i o n s . RATIO c a l c u l a t e s and p r i n t s mole r a t i o s c a l c u l a t e d from a n a l y t i c a l m o l a l i t y and l o g a c t i v i t y r a t i o s . APCALC c a l c u ­ l a t e s thermodynamic a c t i v i t y products f o r the v a r i o u s m i n e r a l species considered by WATEQ2. OUTPNCH generates a card deck of a subset of the c a l c u l a t e d a c t i v i t i e s , a c t i v i t y products and i n ­ put c o n c e n t r a t i o n s f o r subsequent use w i t h p l o t t i n g programs. ERRCALC, d i s c u s s e d p r e v i o u s l y , uses i n p u t a n a l y t i c a l standard d e v i a t i o n s to c a l c u l a t e the propagated standard d e v i a t i o n i n the log of the a c t i v i t y products f o r a subset of m i n e r a l s considered. PHASES p r i n t s the r e s u l t s of the a c t i v i t y product and e r r o r c a l c u l a t i o n s , and computes and p r i n t s the s a t u r a t i o n s t a t e of each m i n e r a l w i t h respect to a thermodynamic e q u i l i b r i u m constant f o r each r e a c t i o n considered. 2

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Acknowledgements We are pleased to acknowledge the e f f o r t s of L. N. Plummer and R. W. P o t t e r I I , both of the U. S. G e o l o g i c a l Survey, f o r t h e i r c a r e f u l reviews of t h i s manuscript; to Β. F. Jones, U. S. G e o l o g i c a l Survey, f o r many h e l p f u l d i s c u s s i o n s ; and to J . M. Burchard, whose help i n many aspects of t h i s work i s e s p e c i a l l y appreciated. Abstract The computerized aqueous chemical model of Truesdell and Jones (2, 3), WATEQ, has been greatly revised and expanded to include consideration of ion association and solubility equilibria for several trace metals, Ag, As, Cd, Cu, Mn, Ni, Pb and Zn, solubility equilibria for various metastable and(or) sparingly soluble equilibrium solids, calculation of propagated standard deviation, calculation of redox potential from various couples, polysulfides, and a mass balance section for sulfide solutes. Revisions include expansion and revision of the redox, sulfate, iron, boron, and fluoride solute sections, changes in the possible operations with Fe (II, III, and II + III), and updating the model's thermodynamic data base using c r i t i c a l l y evaluated values (81, 50, 58) and new compilations (51, 26; R. M. Siebert and

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C. L. Christ, unpublished data 1976). Mechanical revisions include numerous operational improvements i n the computer code. Literature Cited 1. 2.

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Paces, Tomas. Steady-state kinetics and equilibrium between ground water and granitic rock: Geochim. Cosmochim. Acta 37, 2641-2663 (1973). 76. Ponnamperuma, F. Ν., Tianco, Ε. M., and Loy, Teresita. Redox equilibria i n flooded s o i l s : I. The iron hydroxide system. Soil Sci. 103, 374-382 (1967). 77. Beidermann, G., and Chow, J . T. Studies on the hydrolysis of metal ions. Part 57. The hydrolysis of iron(III) ion and the solubility product of Fe(OH) Cl0.30in 0.5 M (Na )Cl medium. Acta Chem. Scand. 20, 1376-1388 (1966). 78. Chien, S. H., and Black, C. A. Free energy of formation of carbonate apatites i n some phosphate rocks. Soil Sci. Soc. Amer. J . 40, 234-239 (1976). 79. Morey, G. W., Fournier, R. O., and Rowe, J . J . Solubility of amorphous SiO at 25°C. J. Geophys. Res. 69, 1995-2002 (1964). 80. Sato, Motoaki. Oxidation of sulfide ore bodes, l. Geochemical environments i n terms of Eh and pH. Econ. Geol. 55, 928-961 (1960). 81. Nordstrom, D. Κ., Jenne, Ε. Α., and B a l l , J. W. Redox equili­ bria of iron in acid mine waters, i n Jenne, Ε. Α.,ed.,"Chem­ i c a l Modeling in Aqueous Systems. Speciation, Sorption, Solu­ b i l i t y , and Kinetics," Amer. Chem. Soc., (this volume). 82. Bevington, P. R. "Data Reduction and Error Analysis for the Physical Sciences," 336 p., McGraw-Hill Book Co., New York, 1969. 83. Laxen, D. P. H. A specific conductance method for quality control i n waters. Water Res. 11, 91-94 (1977). 84. Butler, J . N. "Ionic Equilibrium, A Mathematic Approach," 547 p., Addison-Wesley, New York, 1964. 85. Cloke, P. L. The geologic role of polysulfides - Part I I . The solubility of acanthite and covelite i n sodium poly­ sulfide solutions. Geochim. Cosmochim. Acta 27, 1299-1319 (1963). 86. Helffrich, Friederich, "Ion Exchange." 624 p., McGraw-Hill, New York, 1962. 87. K i t t r i c k , J. A. Ion exchange and mineral s t a b i l i t y : Are the reactions linked or separate? i n Jenne, Ε. Α., ed., "Chemical Modeling i n Aqueous Systems, Speciation, Sorption, Solubility, and Kinetics," Amer. Chem. Soc., (this volume). 88. Bassett, R. L., Kharaka, Υ. Κ., and Langmuir, D. C r i t i c a l review of the equilibrium constants for clay minerals, i n Jenne, Ε. Α., ed., "Chemical Modeling i n Aqueous Systems, Speciation, Sorption, Solubility, and Kinetics," Amer. Chem. Soc., (this volume). +

-

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch036

2.70

2

Disclaimer: The reviews expressed and/ or the products mentioned in this article repre­ sent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED November 16, 1978.

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

37

Chemical

Modeling

of T r a c e M e t a l E q u i l i b r i a i n

Contaminated Soil Solutions Program

Using

the Computer

GEOCHEM

SHAS V. MATTIGOD and GARRISON SPOSITO

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521 The study of trace metal chemistry i n s o i l s has assumed added importance r e c e n t l y because of the p o s s i b l e d e l e t e r i o u s e f f e c t s on s o i l organisms, p l a n t s , and human beings that could r e s u l t from the accumulation of these metals t o t o x i c l e v e l s i n s o i l s contaminated, e.g., by the land d i s p o s a l o f sewage sludges or by the seepage of geothermal brines l o s t a c c i d e n t l y from storage lagoons. Although the s o i l s o l u t i o n i s an open, dynamic, n a t u r a l water system whose composition r e f l e c t s many r e a c t i o n s o c c u r r i n g simultaneously among i t s aqueous c o n s t i t u e n t s and between those c o n s t i t u e n t s and the assembly of mineral and o r ­ ganic s o l i d phases present, a knowledge of the general features of s o i l trace metal e q u i l i b r i a i s expected t o be a u s e f u l guide to p r e d i c t i n g what w i l l occur i n nature i f contamination takes place. These general features cannot be assessed conveniently by laboratory experiments because of the complexity of s o i l s o l u t i o n s . A more v i a b l e a l t e r n a t i v e i s assessment by a computer model that i s based on thermodynamic a s s o c i a t i o n and s o l u b i l i t y product constants. The computer program GEOCHEM i s adapted and being developed for s o i l s o l u t i o n s from the REDEQL2 program created o r i g i n a l l y by Morel and Morgan and t h e i r coworkers at Caltech (jL) · The p r i n c i p a l ways i n which GEOCHEM d i f f e r s from REDEQL2 are: 1) i t includes thermodynamic data f o r a few hundred a d d i t i o n a l s o l u b l e complexes and s o l i d s that are p a r t i c u l a r l y relevant t o trace metal e q u i l i b r i a i n s o i l , 2) i t contains a subroutine f o r c a t i o n exchange on constant charge surfaces that i s based on a thermo­ dynamic model, and 3) i t contains a subroutine f o r the e s t i ­ mation of s i n g l e - i o n a c t i v i t y c o e f f i c i e n t s at i o n i c strengths up to 3 M. A number of important t h e o r e t i c a l problems have a r i s e n i n connection w i t h the development of GEOCHEM. These problems may be c l a s s i f i e d broadly i n t o four c a t e g o r i e s : (a) s t a b i l i t y con­ stants f o r trace metal complexes with inorganic l i g a n d s ; (b) s t a b i l i t y constants f o r trace metal complexes with organic l i g a n d s ; (c) s o l u b i l i t y product constants f o r s o i l c l a y m i n e r a l s , and (d) thermodynamic c a t i o n exchange constants and exchanger 0-8412-0479-9/79/47-093-837$05.00/0

© 1979 American Chemical Society

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

838

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

phase a c t i v i t y c o e f f i c i e n t s . Any reasonably accurate computa­ t i o n of trace metal e q u i l i b r i a i n a s o i l s o l u t i o n must be pre­ ceded by a s u c c e s s f u l attack on these four problem areas.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

S t a b i l i t y Constants f o r Soluble Inorganic Complexes of Trace Metals Studies i n v o l v i n g t r a c e metal s p e c i a t i o n i n s o i l s o l u t i o n s r e q u i r e the values of s t a b i l i t y or a s s o c i a t i o n constants of complexes of the trace metals with a number of i n o r g a n i c and organic ligands which e x i s t i n these environments. Much of these data can be obtained r e a d i l y from p u b l i s h e d compilations (_2-7.). However, experimental data on a s s o c i a t i o n constants for complexes of the trace metals w i t h ligands such as CO^ , HC0 ", P0 3~ HPO^ ", t^PO^" are not o f t e n a v a i l a b l e . I n view of t h i s lack o f experimental data, modeling s t u d i e s of chemical e q u i l i b r i u m i n n a t u r a l water systems have t y p i c a l l y followed two approaches. One of the approaches, represented by computer programs such as SOLMNEQ and WATEQ (8^, 9 ) , i s t o i n c l u d e only those complexes f o r which the values of experimental a s s o c i a t i o n constants are a v a i l a b l e . The second approach c o n s i s t s of i n ­ c l u d i n g i n the e q u i l i b r i u m c a l c u l a t i o n s estimates of the values of the a s s o c i a t i o n constants of complexes that are b e l i e v e d t o form, but f o r which experimental data do not yet e x i s t , along w i t h experimentally d e r i v e d values of a s s o c i a t i o n constants (10, 11). The l a t t e r approach seems p r e f e r a b l e because i t tends to i n c l u d e i n the c a l c u l a t i o n s a l l known i n t e r a c t i o n s of metals and ligands through complex formation and thus provides r e s u l t s which are expected t o be q u a l i t a t i v e l y more r e l i a b l e than the r e s u l t s obtained through the former approach. The methods a v a i l a b l e f o r c a l c u l a t i n g i o n a s s o c i a t i o n constants c o n s i s t of t h e o r e t i c a l methods based on e l e c t r o s t a t i c s (12, 13) and e m p i r i c a l methods based on c o r r e l a t i o n s of a s s o c i a ­ t i o n constants w i t h p r o p e r t i e s of ions such as e l e c t r o n e g a t i v i t y , charge, r a d i i , c o o r d i n a t i o n number, e t c . (14, 15, 16, 17, 18). Recently, Mattigod and S p o s i t o (19) estimated the a s s o c i a t i o n constants f o r complexes of a number of cations of the f i r s t t r a n s i t i o n metal s e r i e s w i t h many of the i n o r g a n i c ligands considered to be of importance i n the t r a c e metal chemistry of s o i l s o l u t i o n s . The a s s o c i a t i o n constants c a l c u l a t e d by Mattigod and Sposito (19) w i t h the method of Rester and Pytkowicz (13), f o r carbonate and phosphate complexes appear t o be sys­ t e m a t i c a l l y d i f f e r e n t from those estimated r e c e n t l y w i t h the c o r r e l a t i o n method of Nieboer and McBryde (17). The l a t t e r method has a f i r m e r e m p i r i c a l b a s i s and, t h e r e f o r e , has been p r e f e r r e d . Values of the r e v i s e d estimated and measured assoc­ i a t i o n constants f o r some t r a c e metal-inorganic l i g a n d complexes are l i s t e d i n Table I . These and other estimated values have been incorporated i n t o GEOCHEM. They w i l l be replaced when r e l i a b l e experimental values become a v a i l a b l e . 2

3

4

s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

37.

Computer Program

M A T T I G O D A N D SPOSITO

GEOCHEM

839

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

There i s some experimental evidence which i n d i c a t e s that mixed-ligand complexes of t r a c e metals may be very s t a b l e and dominant i n s o i l s (20). I n modeling the s p e c i a t i o n of elements i n sea water, Dyrssen and Wedborg (10) found that mixed-ligand complexes of c e r t a i n t r a c e metals were s i g n i f i c a n t . At present, experimental data on mixed-ligand complexes are r a t h e r sparse. However, s t a b i l i t y constants can be estimated by a s t a t i s t i c a l method suggested by Dyrssen, Jagner and Wengelin (21). C u r r e n t l y , GEOCHEM i n c l u d e s estimated s t a b i l i t y constants of hydroxy-chloro complexes of a few trace metals. Lack of data on many mixed-ligand complexes i s c l e a r l y one of the s i g n i f i c a n t shortcomings i n modeling s t u d i e s at present.

Table I . Estimated and measured common logarithms of a s s o c i a t i o n constants f o r some t r a c e m e t a l - i n o r g a n i c l i g a n d i o n p a i r s at 25°C, 1 atm

Co

.2+ Ni

Cu

5.31

5.53

5.78

6.73

2.72

2.89

3.08

4.29

2.79

7.93

8.13

8.37

9.85

8.02

2+ Mn

_ 2+ Fe

co ~

4.52

HCO3-

1.95

Ligand 3

2

3

7.19 HP0

4 4

2

3.58 1.35

H P0 " 2

4

(22),

^ S c h i n d l e r et a l 3

Morgan

(24),

4

2

+

3.60

5

3.04

2.93

2.70

5

1.49

1.53

6

η

Zn

4.76

1

3.20

2+

6

1.76

3.30 1.60

2

7

7

2 , Huston and Stumm (23) Bilinski

.

Smith and A l b e r t y (25),

S i g e l et a l (27_) ,

Stability Metals

r,

Nriagu

Nriagu (26),

(28)

Constants f o r Soluble Organic Complexes of Trace

The s o l u b l e , metal-complexing, organic f r a c t i o n of sewage sludge i s a heterogeneous assembly of molecules that i s charac­ t e r i z e d by wide ranges of chemical composition, molecular

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

840

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

weight, and f u n c t i o n a l group a c i d i t y (29). This complexity poses a d i f f i c u l t problem f o r the chemical modeling of s o i l s o l u t i o n s contaminated by the i n c o r p o r a t i o n of trace metalbearing sludges. A s u c c e s f u l model w i l l r e q u i r e , i n p a r t i c u l a r , information about the many p o s s i b l e r e a c t i o n s of trace metals w i t h the f u l v i c a c i d f r a c t i o n of sludges. The d i s s o c i a t i o n of protons from f u l v i c a c i d (FA) e x t r a c t e d from a n a e r o b i c a l l y - d i g e s t e d sewage sludge has been i n v e s t i g a t e d by p o t e n t i o m e t r i c t i t r a t i o n (29). I n that study, a number of important f a c t s was brought to l i g h t which suggest that the behavior of f u l v i c m a t e r i a l i n a s o i l s o l u t i o n w i l l indeed be complex. According to Sposito and Holtzclaw (29), there appear to be four separate c l a s s e s of d i s s o c i a b l e f u n c t i o n a l groups that range i n a c i d i t y from very strong ( i o n i z e d at pH < 2) to very weak ( i o n i z e d at pH > 10). (These c l a s s e s are j u s t four out of a continuum of c l a s s e s of f u n c t i o n a l group a c i d i t y i n sludge-derived FA, since no a c i d or a l k a l i n e f i n a l end p o i n t s appear i n the t i t r a t i o n curve between pH 1 and 11.) Based on t h i s evidence, Sposito et a l . (30), suggested that the a c i d i c f u n c t i o n a l group c l a s s e s i n sludge-derived FA be designated I, I I , I I I and IV f o r those groups that t i t r a t e approximately i n the pH ranges < 3, 3-5, 5-8, > 8, r e s p e c t i v e l y . The r e a c t i o n s between sludge-derived f u l v i c m a t e r i a l and t r a c e metals are poorly understood and, at present, no r e l i a b l e thermodynamic s t a b i l i t y constants are a v a i l a b l e . A p r o v i s i o n a l approach to r e s o l v i n g t h i s d i f f i c u l t y , which has been employed o f t e n i n modeling s t u d i e s , i s to i d e n t i f y c a r e f u l l y c e r t a i n c l a s s e s of known organic acids whose proton d i s s o c i a t i o n con­ s t a n t s f a l l i n t o the ranges observed f o r the s o i l s o l u t i o n organics and which can be expected to be present or to simulate c l o s e l y the organic acids present (whether " n a t u r a l " or from contamination) i n a s o i l s o l u t i o n . Table I I l i s t s a set of aromatic, a l i p h a t i c , and amino acids that are f r e q u e n t l y observed i n s o i l leachates, together w i t h the r e l a t i v e concentrations of each i n a mixture of model water-soluble s o i l organics that provides the t o t a l of 2.2 meq of H found per gram of sludged e r i v e d FA (30). I t has been observed that the f u n c t i o n a l groups i n these acids are s i m i l a r to the f u n c t i o n a l groups present i n sludge f u l v i c m a t e r i a l . The measured s t a b i l i t y constants f o r trace metal complexes w i t h these model organic acids are assumed to be good approximations to the unknown s t a b i l i t y constants f o r the assembly of s o i l s o l u t i o n organics, i n the sense that a mixture of the model organic acids whose proton t i t r a t i o n curve s e m i q u a n t i t a t i v e l y simulates that of sludge-derived FA (Figure 1) w i l l a l s o produce a comparable d i s t r i b u t i o n of various trace metals among organic and i n o r g a n i c species i n a s o i l s o l u t i o n . The s e v e r a l obvious l i m i t a t i o n s and dangers i n t h i s p r o v i s i o n a l approach, p r e s e n t l y used i n GEOCHEM, are not considered to be as serious as completely n e g l e c t i n g the organic s p e c i a t i o n of trace metals i n a s o i l

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

M A T T I G O D A N D SPOSITO

Computer Program GEOCHEM

841

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

I.OOp

-11.00-10.00 -9.00 -8.00 -7.00 -6.00 -5.00 -4.00 -3.00 -2.00

log [H ] +

Figure 1. Comparison between the potentiometric titration curve for a sewagesludge-derived fulvic acid ( ) and the simulated titration curve for a model fulvic acid ( )

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

842

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

s o l u t i o n whose organic c o n s t i t u e n t s may approach 10 M i n t o t a l c o n c e n t r a t i o n (e.g., a s o i l s o l u t i o n a f f e c t e d by sewage sludge application)·

Table I I . Organic acids used as a model f o r sludge-derived f u l v i c a c i d

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

Acid

Benzenesulfonic Salicylic Phthalic Citric Maleic Ornithine Lysine Valine Arginine

Concentration (pM)

4.27 4.27 3.97 4.14 3.97 4.36 4.36 4.36 4.49

S o l u b i l i t y Product Constants f o r S o i l Clay M i n e r a l s S o l i d phases comprise the dominant f r a c t i o n of the s o i l volume and, as such, the d i s s o l u t i o n , p r e c i p i t a t i o n and i o n exchange p r o p e r t i e s of the s o i l minerals have a profound i n f l u e n c e on the composition of s o i l s o l u t i o n s . Therefore, i t i s e s s e n t i a l to i n c l u d e the r e a c t i o n s these s o l i d s may undergo i n s o i l s i n s i m u l a t i o n s t u d i e s of chemical e q u i l i b r i u m . R e l i a b l e thermodynamic data on the s t a b i l i t y of many of the important oxides, hydroxides, carbonates, phosphates, and s i l i ­ cates are a v a i l a b l e and can be i n c o r p o r a t e d i n t o a computer program such as GEOCHEM. I n many s o i l s , p h y l l o s i l i c a t e s dominate the clay-s'ize f r a c t i o n . Smectites as a group of the p h y l l o s i l i c a t e s are ubiquitous i n s o i l s and sediments. Because of t h e i r large surface areas and high c a t i o n exchange c a p a c i t i e s , smectites, when present, play a s i g n i f i c a n t part i n i n f l u e n c i n g the composition of s o i l s o l u t i o n s . U n l i k e other c l a y m i n e r a l s , smectites a l s o e x h i b i t a broad range of chemical compositions and, t h e r e f o r e , i t i s u n l i k e l y that the standard f r e e energy of formation of every n a t u r a l l y - o c c u r r i n g smectite w i l l be determined e x p e r i m e n t a l l y . This d i f f i c u l t y has prompted a

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

37.

MATTIGOD AND

SPOSITO

Computer Program

GEOCHEM

843

number of attempts t o estimate AGf 298 15 for s i l i c a t e s i n general (31, 32, 33) and smectites i n p a r t i c u l a r (34, 35). These methods are based on simple p h y s i c a l models which are r e l a t e d p r i m a r i l y to c l a s s i c a l e l e c t r o s t a t i c s . The success of these methods i s based on the observation that the short-range c o o r d i n a t i o n environment of a c a t i o n that i s found i n s i l i c a t e minerals does not tend to vary much from one mineral t o another. Since covalency plays i t s most important r o l e i n nearest neigh­ bor i n t e r a c t i o n s , i t f o l l o w s that i t s c o n t r i b u t i o n t o the s t r u c ­ t u r a l energy of a mineral w i l l be about the same among s i l i c a t e s and, t h e r e f o r e , that the r e l a t i v e s t a b i l i t i e s of the minerals l a r g e l y w i l l be determined by longer-ranged i o n i c i n t e r a c t i o n s . Nriagu (34), i n comparing h i s method w i t h the method of Tardy and G a r r e l s (35), concluded that the accuracy of estima­ t i o n of Gf 298.15 values of p h y l l o s i l i c a t e s i n e i t h e r method was about the same. But, each of these methods contains a c r i t i c a l , ad hoc assumption that i s d i f f i c u l t t o j u s t i f y on geochemical grounds. Tardy and G a r r e l s (35) had t o assume that the hydroxyl ions i n Mg-bearing l a y e r s i l i c a t e s are a s s o c i a t e d only w i t h magnesium i o n s , i n s o f a r as thermochemical c a l c u l a t i o n s are concerned. Nriagu (34) was forced t o decrease the values of AGf 298.15 for solid KOH and NaOH by about 15 percent from the measured values i n order t o o b t a i n agreement between the r e s u l t s of h i s method and experimental data f o r Na- and K-containing c l a y m i n e r a l s . These problems w i t h the methods of Tardy and G a r r e l s (35) and Nriagu (34) prompted the development of a method by Mattigod and Sposito (36) f o r e s t i m a t i n g the AGf 298 15 values of smectites which not only shows a s l i g h t improvement i n p r e d i c t i o n , but a l s o i s free of the a r b i t r a r y assumptions i n ­ herent i n some of the other methods. This method employs the concept of i o n i c bonding as a p p l i e d t o c l a y m i n e r a l s , mentioned e a r l i e r , along w i t h a s t a t i s t i c a l equation t o r e l a t e the charge on a smectite due t o isomorphous s u b s t i t u t i o n and the valence and radius of the i n t e r l a y e r c a t i o n t o the free energy changes brought about by changes i n c o o r d i n a t i o n environment. These changes i n free energy occur i n t r a n s f e r r i n g a c a t i o n from r e l a t i v e l y h i g h - p o t e n t i a l s i t e s i n a hydroxide t o a r e l a t i v e l y l o w - p o t e n t i a l i n t e r l a y e r s i t e i n a smectite. The r e l a t i o n s h i p developed by Mattigod and Sposito (36) i s : AG

and

= Σn. AG

In δ

(n.) - (Σn.Ζ.-12) AG H 0 2

( 1 )

-

δ

= 1.9283 C + 0.3501 R - 0.2819 Ζ + 3.5427

where, AGf = standard free energy of formation of a smectite n i = r e a c t i o n c o e f f i c i e n t of the i hydroxide Zi = charge on the i c a t i o n ( i n c l u d i n g S i ) th

th

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

(1)

844

CHEMICAL

AG^ C

R Ζ

MODELING IN AQUEOUS

SYSTEMS

= s t a n d a r d f r e e energy of f o r m a t i o n of the i h y d r o x i d e component = c a t i o n e x c h a n g e c a p a c i t y (due t o i s o m o r p h o u s s u b s t i t u t i o n ) o f the s m e c t i t e per formula u n i t , i n eq/fw ο = P a u l i n g r a d i u s (A) o f t h e e x c h a n g e a b l e (interlayer) cation = v a l e n c e of the exchangeable c a t i o n

E q u a t i o n 1 p r o v i d e d estimates of A G ^ 298 15 ^ smectites w h i c h showed b e t t e r agreement w i t h t h e e x p e r i m e n t a l l y d e r i v e d v a l u e s t h a n t h e o t h e r two methods ( 3 4 , 3 5 ) . A c o m p a r i s o n among t h e methods i s g i v e n i n T a b l e I I I .

Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

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The t a s k o f c a l c u l a t i n g t h e c o m p o s i t i o n o f a s o i l e x c h a n g e r p h a s e i n e q u i l i b r i u m w i t h a s o i l s o l u t i o n h a s two d i s t i n c t p a r t s : t h e t h e r m o d y n a m i c e x c h a n g e e q u i l i b r i u m c o n s t a n t s must be d e t e r ­ m i n e d and t h e a c t i v i t y c o e f f i c i e n t s o f t h e components o f t h e e x c h a n g e r p h a s e must be e s t i m a t e d . The f i r s t p a r t o f t h e p r o b l e m , t h a t o f o b t a i n i n g exchange e q u i l i b r i u m c o n s t a n t s , is n o t p a r t i c u l a r l y d i f f i c u l t s i n c e a number o f measurements o f AG f o r t h e common c l a y m i n e r a l s ( e . g . , s m e c t i t e s , v e r m i c u l i t e s , and k a o l i n i t e s ) h a s b e e n p u b l i s h e d . A l t e r n a t i v e l y , a semie m p i r i c a l a p p r o a c h s u c h as t h e J a m e s - H e a l y m o d e l i n c l u d e d i n REDEQL2 (I) may be e m p l o y e d t o e s t i m a t e a s t a n d a r d f r e e e n e r g y of adsorption for a c a t i o n i c species. e x

S m e c t i t e s a r e one o f t h e most i m p o r t a n t s o i l c l a y m i n e r a l s as r e g a r d s c a t i o n e x c h a n g e . The r e v e r s i b l e e x c h a n g e r e a c t i o n s o f t h e s e m i n e r a l s w i t h m e t a l c a t i o n s may be p i c t u r e d as k i n e t i c a l l y favored ( i . e . , rapid) p r e c i p i t a t i o n - d i s s o l u t i o n reactions (44). From t h i s p o i n t of v i e w , i t i s m e a n i n g f u l to w r i t e the "exchange half-reaction": MX ( s , a q ) m

= M^Uq)

+ mX" (s,aq) 1

(2)

where X r e f e r s t o one e q u i v a l e n t o f t h e a n i o n i c p o r t i o n o f a s m e c t i t e e x c h a n g e r and M*™ i s an e x c h a n g e a b l e c a t i o n . The t y p i c a l e x c h a n g e r e a c t i o n t h e n i s p i c t u r e d as a s e t o f p a i r s o f r e a c t i o n s s u c h as E q . 2, w i t h e a c h r e a c t i o n p a i r i n v o l v i n g two d i f f e r e n t metal c a t i o n s . The a n a l o g y b e t w e e n E q . 2 and t h e dissolution reaction for a solid is evident. H o w e v e r , t h e r e a r e some i m p o r t a n t d i f f e r e n c e s b e t w e e n t h e e q u i l i b r i u m c o n s t a n t f o r E q . 2 and t h e u s u a l K o · First, the compound on t h e l e f t - h a n d s i d e o f E q . 2 i s n o t a d r y s o l i d a t s t a n d a r d t e m p e r a t u r e and p r e s s u r e , b u t i n s t e a d i s a h o m o i o n i c s m e c t i t e i n c o n t a c t w i t h an aqueous s o l u t i o n . The s t a n d a r d s t a t e f o r M X ( s , a q ) a c c o r d i n g l y i s the homoionic c l a y m i n e r a l at s t a n d a r d t e m p e r a t u r e and p r e s s u r e i n e q u i l i b r i u m w i t h an i n f i n S

m

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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