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Photochemistry of Environmental Aquatic Systems
 9780841210080, 9780841211643, 0-8412-1008-X

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ACS

SYMPOSIUM

SERIES

Photochemistry of Environmental Aquatic Systems Rod G Zika EDITOR University of

William J. Cooper, EDITOR Florida International University

Developed from a symposium sponsored by the Divisions of Geochemistry and Environmental Chemistry at the 189th Meeting of the American Chemical Society, Miami Beach, Florida, April 28-May 3, 1985

American Chemical Society, Washington, DC 1987

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

327

Library of Congress Cataloging-in-Publication Data Photochemistry of environmental aquatic systems. (ACS symposium series, ISSN 0097-6156; 327) "Developed from a symposium sponsored by the Divisions of Geochemistry and Environmental Chemistry at the 189th Meeting of the American Chemical Society, Miami Beach, Florida, April 28May 3,1985." Bibliography: p. Includes index. 1. Environmental chemistry—Congresses 2. Photochemistry—Congresses Congresses. 4. Aquatic ecology—Congresses. I. Cooper, William J. II. Zika, Rodney G., 1940III. American Chemical Society. Division of Geochemistry. IV. American Chemical Society. Division of Environmental Chemistry. V. American Chemical Society. Meeting (189th: 1985: Miami Beach, Fia.) VI. Series. TD193.P48 1987 628.Γ61 86-26489 ISBN0-8412-1008-X

Copyright© 1987 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter 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., 27 Congress Street, Salem, MA 01970, 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 a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. 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 ACS 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 anyrightor permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS Symposium Series M. Joan Comstock, Series Editor Advisory Board Harvey W. Blanch University of California—Berkeley

USDA, Agricultural Research Service

Alan Elzerman Clemson University

W. H. Norton J. T. Baker Chemical Company

John W. Finley Nabisco Brands, Inc.

James C. Randall Exxon Chemical Company

Marye Anne Fox The University of Texas—Austin

W. D. Shults Oak Ridge National Laboratory

Martin L. Gorbaty Exxon Research and Engineering Co.

Geoffrey K. Smith Rohm & Haas Co.

Roland F. Hirsch U.S. Department of Energy

Charles S.Tuesday General Motors Research Laboratory

Rudolph J. Marcus Consultant, Computers & Chemistry Research

Douglas B. Walters National Institute of Environmental Health

Vincent D. McGinniss Battelle Columbus Laboratories

C. Grant Willson IBM Research Department

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Foreword The ACS S Y M P O S I U M S E R I E S was founded in 1974 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 S E R I E S except that in order to save time the papers are not typese 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. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Preface P H O T O C H E M I S T R Y IS N O T A N E W F I E L D , and numerous books exist that cover the basic areas of photochemistry. The study of photochemistry in aqueous solutions, and in particular its application to environmental processes, is relatively new. The field of aquatic photochemistry encompasses a wide diversity of areas within environmental science. Natural waters receiving solar radiation are active photochemica secondary processes are occurring with both living and nonliving particulate matter. Naturally occurring humic substances are relatively efficient initiators of photochemical reactions. Many xenobiotic chemicals in natural waters undergo either direct or indirect photochemical transformations. Within the areas of water and waste water, photochemistry may be the result of open contact chambers or it may be used as a treatment process to control organic compounds or for disinfection. Within atmospheric studies, numerous photochemically induced reactions occur in cloud droplets. Studies that are presently being conducted in the field range from purely phenomenological descriptions of photoproducts in the environment to time-resolved laser spectroscopic studies of primary photoprocesses. This book is a first attempt to compile chapters dealing with this complex and rapidly evolving field of environmental science. ROD G. ZIKA

Rosenstiel School of Marine and Atmospheric Sciences Division of Marine and Atmospheric Chemistry University of Miami Miami, FL 33149 WILLIAM J. COOPER

Drinking Water Research Center Florida International University Tamiami Campus Miami, FL 33199 August 23, 1986

vii

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 1

Introduction and Overview 1

William J.Cooper and Frank L. Herr

2

1

Drinking Water Research Center, Florida International University, Miami, FL 33199 Office of Naval Research, Code 422CB, Arlington, VA 22217 2

Sunlight that arrives at the surface of the earth contains substantial amounts of energy When surface waters which contain either natural or anthropogeni sunlight, light initiate Sunlight induced photochemical reactions in surface waters may broadly be defined as environmental aquatic photochemistry. Within aquatic photochemistry, it is possible to envision reactions involving either inorganic and/or organic molecules. These chemicals could be either natural or anthropogenic and may participate in either homogeneous or heterogeneous reactions. Very often in these environments, a complex array of primary and secondary photoprocesses are occurring simultaneously. Surface waters are diverse in nature. They might be near shore or inland wetland environments or mid-oceanic oligotrophic water. Until recently, sunlight induced photochemistry was not recognized as an important pathway for the transformation of natural and anthropogenic chemicals in surface waters. It is now well established that photochemically mediated processes are important in most, if not all, areas of aqueous phase environmental chemistry. Both direct, primary, and indirect photoprocesses have been documented in natural waters. The f a c t t h a t a l l of these p o s s i b i l i t i e s e x i s t i s e x c i t i n g . There a r e a seemingly endless number o f combinations and p e r m u t a t i o n s t o s t u d y ; h o w e v e r , c a u t i o n s h o u l d be u s e d . The p o t e n t i a l f a c t o r s a f f e c t i n g a n y one s t u d y a r e so c o m p l e x t h a t e x t r e m e care must be taken when i n t e r p r e t i n g d a t a o b t a i n e d from n a t u r a l systems. On t h e o t h e r hand, e x t r a p o l a t i n g d a t a o b t a i n e d i n l a b o r a t o r y s t u d i e s t o n a t u r a l environments g e n e r a l l y r e q u i r e s t h e use o f many assumptions. N u m e r o u s r e v i e w s have been p u b l i s h e d t h a t d e t a i l v a r i o u s aspects o f a q u a t i c photochemistry (1-11). T h i s book b r i n g s t o g e t h e r a group o f papers r e p r e s e n t i n g a number o f t o p i c s , i n o r d e r t o p r o v i d e t h e r e a d e r w i t h an a p p r e c i a t i o n o f t h e c o m p l e x i t y o f the f i e l d and, a t t h e same t i m e , a g l i m p s e o f v a r i o u s areas 0097-6156/87/0327-0001$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS within the r e l a t i v e l y young f i e l d of e n v i r o n m e n t a l a q u a t i c photochemisty. T h i s book i s not an e x h a u s t i v e c o m p i l a t i o n of the f i e l d of e n v i r o n m e n t a l a q u a t i c p h o t o c h e m i s t r y . Direct Photoreactions A wide variety of substances with active chromophores at w a v e l e n g t h s found i n the s u r f a c e s o l a r spectrum occur i n n a t u r a l waters. Some of these substances undergo d i r e c t p h o t o l y s i s , t h a t i s a c h e m i c a l change t h a t r e s u l t s as a d i r e c t consequence of the a b s o r p t i o n of photons by the s u b s t a n c e . C o n c e p t u a l l y , d i r e c t p h o t o r e a c t i o n s are the s i m p l e s t and u s u a l l y the e a s i e s t type o f p r o c e s s to study i n n a t u r a l w a t e r s . S i n c e the r e a c t i o n proceeds r a p i d l y to p r o d u c t s from the p r i m a r y e x c i t e d s t a t e m a n i f o l d , the p h y s i c a l c h a r a c t e r i s t i c s of the r e a c t a n t ' s environment u s u a l l y have o n l y s m a l l e f f e c t s on the r e a c t i o n . Such r e a c t i o n s can o f t e n be s t u d i e d i n pure and/o reactant· C o m p a r a t i v e l y few n a t u r a l m o l e c u l e s f i t i n t o t h i s c a t e g o r y ( 5 ) , and most of these e x h i b i t o n l y weak absorbances a t the h i g h e n e r g y t h r e s h o l d of the s u r f a c e s o l a r spectrum. Most n a t u r a l l y o c c u r r i n g compounds are t h e r e f o r e q u i t e t r a n s p a r e n t to i n c i d e n t s o l a r r a d i a t i o n and r e a c t i o n s of these compounds which proceed v i a d i r e c t p h o t o l y s i s are the e x c e p t i o n r a t h e r than the r u l e . Most examples of d i r e c t p h o t o c h e m i c a l r e a c t i o n s a r e found among the numerous s t u d i e s done on x e n o b i o t i c substances ( 1 , 6 ) , where the e n v i r o n m e n t a l r a t e i s o f t e n d e r i v e d from l a b o r a t o r y measurements which a r e c a r r i e d out i n o r g a n i c s o l v e n t s because of the l i m i t e d s o l u b i l i t y of the compounds. I f the e l e c t r o n i c a b s o r p t i o n s p e c t r a and t h e quantum y i e l d f o r the compound are determined i n water o r i n an o r g a n i c s o l v e n t system which g i v e s a good a p p r o x i m a t i o n t o water, then the c a l c u l a t i o n of an e n v i r o n m e n t a l r a t e i s a r e l a t i v e l y s i m p l e m a t t e r . For more complex m o l e c u l e s , the r e a c t i o n quantum y i e l d i s g e n e r a l l y wavelength independent (12) and the d i r e c t p h o t o l y s i s r a t e c o n s t a n t can be computed f o r a s p e c i f i c l o c a t i o n and t i m e f r o m t h e e l e c t r o n i c a b s o r p t i o n s p e c t r a , t h e quantum y i e l d , and the s o l a r s p e c t r a l i r r a d i a n c e . Indirect Photoreactions The h i g h l i g h t t r a n s p a r e n c i e s of most compounds i n n a t u r a l water to s o l a r r a d i a t i o n d i c t a t e t h a t d i r e c t p h o t o - r e a c t i o n s of these compounds a r e e i t h e r not p o s s i b l e o r r e p r e s e n t o n l y a minor r e a c t i o n pathway. I n d i r e c t photochemical r e a c t i o n s f o r such compounds can o n l y o c c u r e i t h e r through r e a c t i o n w i t h r e a c t i v e m o l e c u l e s i n ground o r e x c i t e d s t a t e s t h a t are themselves p r o d u c t s of p r i m a r y p h o t o c h e m i s t r y , o r through p h o t o s e n s i t i z e d r e a c t i o n s i n w h i c h the e x c i t e d s t a t e s p e c i e s o f some chromophore t r a n s f e r s an e l e c t r o n or energy t o the compound. The importance of b o t h of t h e s e i n d i r e c t r e a c t i o n pathways i n n a t u r a l water systems i s now recognized. A s u b s t a n t i a l amount o f e v i d e n c e e x i s t s i n t h e literature implicating indirect reaction mechanisms in the p h o t o c h e m i s t r y of b o t h x e n o b i o t i c and n a t u r a l compounds (4,5,6,10). The c o m p l e x i t y of natural water systems and the h o s t of

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

COOPER AND HERR

3

Introduction and Overview

s i m u l t a n e o u s p h o t o - r e a c t i o n s w h i c h a r e o c c u r r i n g f r e q u e n t l y make a c l e a r m e c h a n i s t i c i n t e r p r e t a t i o n o f the d a t a d i f f i c u l t . Secondary p h o t o - o x i d a n t s . There are three r e c o g n i z e d primary s o u r c e s o f secondary o x i d a n t s i n n a t u r a l a q u a t i c environments. The f i r s t of these a r i s e s from i n s i t u p r i m a r y p h o t o - r e a c t i o n s w h i c h o f t e n produce f r e e r a d i c a l s and o t h e r r e a c t i v e p r o d u c t s . The range of r e a c t i v i t y f o r f r e e r a d i c a l s v a r i e s from the very strong o x i d i z i n g s p e c i e s l i k e OH t o weakly o x i d i z i n g and more s e l e c t i v e s p e c i e s l i k e CO^ , NO, and 0 · Subsequent r e a c t i o n s o f these r a d i c a l s can l e a d t o o t h e r r e a c t i v e p r o d u c t s . F o r example, OH i n seawater and o t h e r h a l i d e c o n t a i n i n g _ w a t e r s w i l l r a p i d l y c o n v e r t t o d i h a l i d e a n i o n r a d i c a l s l i k e B r ^ and I (5, 13). Hydrogen a b s t r a c t i o n r e a c t i o n s from o r g a n i c compounds by such i n o r g a n i c r a d i c a l s can produce r a d i c a l s w h i c h i n t u r n might r e a c t w i t h oxygen t o f o r m organo-peroxy r a d i c a l s and p o s s i b l y p e r o x i d e s . Reactive n o n - r a d i c a l p r o d u c t s ca example i s b e l i e v e d t which i n turn i s generated primarily from d i s s o l v e d humic substances (HS) ( 1 4 - 1 6 ) . The compexity o f the r e a c t i o n sequence i s shown i n the f o l l o w i n g e q u a t i o n s : 2

2

HS + hr

+

HS*

20 "'

0

2

+

2H

HS *

+0

2

+

+

*

>

HS

>

HS '

>

li]

+

H

2°2

HS0

>

HS0 H

>

20H'

HS0 H

>

HSO*

OH'

+

X"

>

HOX""

HOX"'

+

X"

>

OH"

HS0 '

+

2

+

RH

H 0 2

2

2

V

+

°2

[2] [3J

# +

>

2

+

[4]

2

+

2

R*

+

[5] [6]

+

OH*

17] [8]

+

[9]

V

(where X i s a h a l i d e ) A l t h o u g h t h i s s e t o f r e a c t i o n s does n o t i n d i c a t e a l l of t h e p o s s i b l e s t e p s i n v o l v e d , i t i s i n t e r e s t i n g t o note t h a t the i n i t i a l p r i m a r y p h o t o p r o c e s s e s r e s u l t e d i n 10 s e c o n d a r y p r o d u c t s . In a d d i t i o n , t h e two p e r o x i d e m o l e c u l e s ( i . e . 0 and HSO^H) a r e new chromophores and f u n c t i o n as r e s e r v o i r s f o r further radical production. A second source f o r secondary o x i d a n t s comes from t h e r m a l r e a c t i o n s when r e a c t i v e p h o t o - p r o d u c t s such as H 0 react with o t h e r c o n s t i t u e n t s of the water. Reactions of peroxides w i t h v a r i o u s t r a n s i t i o n m e t a l s , f o r i n s t a n c e , w i l l produce 0 o r OH r a d i c a l s and o f t e n the m e t a l i o n i s c o n v e r t e d t o a new r e a c t i v e oxidation state. T h i s s o r t of p r o c e s s has been demonstrated f o r H

2

2

2

2

2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS the C u ( I I ) / C u ( I ) c o u p l e i n seawater ( 1 7 ) . R e a c t i o n s such as these may be f a r removed from the i n i t i a l p h o t o c h e m i c a l p r o c e s s ; however, they are a consequence of i t and may have the i m p o r t a n t f u n c t i o n of a c t i n g as a redox b u f f e r i n e n v i r o n m e n t a l water systems. The t h i r d s o u r c e o f s e c o n d a r y p h o t o - o x i d a n t s a r i s e s f r o m f l u x e s of a t m o s p h e r i c o x i d a n t s t h r o u g h the s u r f a c e of n a t u r a l water bodies. A l t h o u g h these p r o d u c t s do not o r i g i n a t e from a q u a t i c p h o t o - r e a c t i o n s , they n e v e r t h e l e s s augment the i n s i t u r e a c t i o n s o u r c e s o f s e c o n d a y r e a c t i v e p r o d u c t s and must be t a k e n i n t o a c c o u n t , e s p e c i a l l y when a t t e m p t i n g t o q u a n t i f y r e a c t i o n p r o c e s s e s in the steep g r a d i e n t r e g i o n near the a i r - w a t e r i n t e r f a c e . Thompson and Z a f i r i o u (18) have examined the c h e m i c a l impact on n a t u r a l waters and have c a l c u l a t e d a i r - w a t e r f l u x e s f o r many of the d i v e r s e atmospheric o x i d a n t s . Photosensitized Processes. One o f t h e most s t u d i e d i n d i r e c t p h o t o r e a c t i o n s i s th e n e r g y t r a n s f e r from th chromophores i n environmental waters. T h i s p r o c e s s has been o b s e r v e d i n b o t h s e a w a t e r ( 1 9 ) and f r e s h w a t e r ( 2 0 , 2 1 ) . The e v i d e n c e s u g g e s t s t h a t s i n g l e t o x y g e n i s a common p r o d u c t i n n a t u r a l water systems, but i t s importance r e l a t i v e t o o t h e r s e c o n d a r y p h o t o - p r o d u c t s i n a f f e c t i n g the c h e m i s t r y i s u n c e r t a i n . I t s l i m i t e d s i g n i f i c a n c e i s a f u n c t i o n of i t s s e l e c t i v e r e a c t i v i t y , l o w r a t e c o n s t a n t s and i t s r a p i d r e l a x a t i o n t o the ground s t a t e i n aqueous media. P h o t o s e n s i t i z a t i o n v i a energy t r a n s f e r i n d i l u t e s o l u t i o n s of s e n s i t i z e r and r e c e p t o r i s i n g e n e r a l a low e f f i c i e n c y p r o c e s s . T h i s w i l l p r o b a b l y h o l d t r u e f o r most e n v i r o n m e n t a l a q u a t i c systems. Much h i g h e r e f f i c i e n c i e s m i g h t , however, be o b t a i n e d i n heterogeneous r e a c t i o n environments such as m i c e l l e s , p a r t i c l e s , or o t h e r i n t e r f a c e s where s e n s i t i z e r and r e a c t a n t are c o n c e n t r a t e d . P h o t o s e n s i t i z a t i o n by e l e c t r o n t r a n s f e r , as shown i n e q u a t i o n 2 a b o v e , has a l s o been observed (22,23). The o c c u r r e n c e of 0^ (14,16) i n n a t u r a l waters i s good e v i d e n c e t h a t these p r o c e s s e s a r e o c c u r r i n g , a t l e a s t f o r oxygen r e d u c t i o n . With e l e c t r o n a c c e p t o r s o t h e r than oxygen, t h i s process i s probably s i m i l a r to energy t r a n s f e r i n t h a t t h e r e i s a low p r o b a b i l i t y f o r o t h e r e l e c t r o n a c c e p t o r s t o be i n v o l v e d except i n heterogeneous environments. Heterogeneous

Reactions

S u r f a c e mediated p r o c e s s e s are a l s o an i m p o r t a n t c o n s i d e r a t i o n i n n a t u r a l water p h o t o c h e m i s t r y . I n aqueous media, two d i f f e r e n t surface/interfaces may occur that result in heterogeneous r e a c t i o n s . The two i n t e r f a c e s c o n s i d e r e d here are l i q u i d - s o l i d and liquid-liquid. S u r f a c e p r o c e s s e s i n g e o c h e m i s t r y and a q u a t i c enviroments have been covered i n more d e t a i l i n two r e c e n t books and t h e r e a d e r i s r e f e r r e d t o t h e s e v o l u m e s f o r more d e t a i l s (24,25). N a t u r a l water systems o f t e n c o n t a i n p a r t i c u l a t e m a t t e r . The p a r t i c u l a t e m a t t e r may be e i t h e r l i v i n g o r n o n - l i v i n g . Algae are t h e predominant component of the l i v i n g p a r t i c u l a t e m a t t e r . The

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1. COOPER A N D HERR

Introduction and Overview

5

non-living particulate matter i s primarily inorganic in c o m p o s i t i o n , b u t may h a v e o r g a n i c m a t t e r a s s o c i a t e d w i t h i t . P a r t i c u l a t e - l i q u i d i n t e r f a c e s a r e known t o p a r t i c i p a t e i n t h e r m a l l y and/or p h o t o c h e m i c a l l y mediated r e a c t i o n s . S u r f a c e m i c r o l a y e r s a r e i m p l i c a t e d i n many c h e m i c a l p r o c e s s e s . They a r e exposed t o t h e f u l l s o l a r spectrum o f l i g h t a r r i v i n g a t t h e s u r f a c e , and a r e o f t e n i m p o r t a n t i n p h o t o c h e m i c a l l y mediated r e a c t i o n s . The m i c r o l a y e r , a s s o c i a t e d w i t h most n a t u r a l w a t e r s , i s c o n s i d e r a b l y d i f f e r e n t i n c h e m i c a l c o m p o s i t i o n from t h e u n d e r l y i n g w a t e r column ( 2 6 ) . Hence, the p h o t o c h e m i c a l l y mediated r e a c t i o n s t h a t take p l a c e i n t h i s l a y e r may d i f f e r s u b s t a n t i a l l y from those i n t h e water column. The d i f f e r e n c e s i n t h e r e a c t i o n s may be one of k i n e t i c s ( r a t e s o f t h e r e a c t i o n ) , o r maybe m e c h a n i s t i c i n n a t u r e and t h e r e a c t i o n ( s ) p r o c e e d ( s ) v i a d i f f e r e n t pathways r e s u l t i n g i n d i f f e r e n t reaction products. One extreme case o f the l i q u i d - l i q u i d i n t e r f a c e i s t h e o i l s p i l l ( s l i c k ) problem. r e c e n t l y been reviewed p r o c e s s e s may and do o c c u r s i m u l t a n e o u s l y , and t h a t t h e c h e m i s t r y a s s o c i a t e d w i t h s t u d i e s o f t h i s type a r e e x t r e m e l y complex · N a t u r a l m i c r o l a y e r s form on most water s u r f a c e s . There a r e numerous d i f f i c u l t i e s encountered when s t u d y i n g them. One o f the most p e r p l e x i n g problems c e n t e r s around t h e c o l l e c t i o n o f n a t u r a l samples of m i c r o l a y e r s . Another d i f f i c u l t y i s t h a t p r o c e s s e s o c c u r r i n g i n the m i c r o l a y e r are o f t e n not w e l l c h a r a c t e r i z e d (26). Thus, i n n o v a t i v e approaches a r e r e q u i r e d t o s t u d y p r o c e s s e s s i m i l a r to those i n n a t u r a l w a t e r s . In the case o f p a r t i c u l a t e matter, d i f f e r e n t types of r e a c t i o n s may be i n v o l v e d . R e a c t i o n s t h a t o c c u r on t h e s u r f a c e s o f the non-living particulate matter may involve direct p h o t o c h e m i c a l l y mediated r e a c t i o n s i n s u r f a c e complexes. Another p o s s i b i l i t y i s s u r f a c e semiconductor redox r e a c t i o n s . I n t h e case of t h e l i v i n g a l g a l p a r t i c u l a t e m a t t e r , a t h i r d p r o c e s s would be p h o t o s e n s i t i z e d r e a c t i o n on t h e s u r f a c e o f a l g a e . The p a r t i c u l a t e s u r f a c e heterogeneous r e a c t i o n s may r e s u l t i n p r i m a r y and/or secondary p r o c e s s e s . Most o f t h e examples o f a l g a l a s s o c i a t e d p r o c e s s e s suggest secondary r e a c t i o n s r e s u l t i n g from exudates i n t h e aqueous phase. Quantifying Environmental

Photoprocesses

As i n many a r e a s o f e n v i r o n m e n t a l s c i e n c e , one o f t h e most d i f f i c u l t aspects of environmental photochemistry i s e x t r a p o l a t i n g l a b o r a t o r y based experiments t o t h e n a t u r a l environment. One t o o l t h a t i s b e c o m i n g u s e d more f r e q u e n t l y i s t h a t o f m a t h e m a t i c a l models t o p r e d i c t the d i s t r i b u t i o n of photoproducts i n the environment ( 1 2 ) . M o d e l i n g a q u a t i c p h o t o p r o c e s s e s i s complex, f o r i n order t o d e s c r i b e i n d e t a i l the observed products, i t i s necessary t o understand quantum y i e l d s throughout the s o l a r s p e c t r u m , f o r m a t i o n r a t e s , i n many cases d e c o m p o s i t i o n r a t e s ( t h e p h o t o p r o d u c t s a r e r a r e l y c o n s e r v a t i v e ) , absorbance c h a r a c t e r i s t i c s of t h e a q u a t i c system, and p h y s i c a l m i x i n g o f t h e water masses.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

The s i m p l e s t m o d e l i n g s i t u a t i o n e x i s t s f o r those cases where t h e quantum y i e l d i s a c o n s t a n t or can be assumed t o be a c o n s t a n t o v e r t h e s o l a r spectrum ( 1 2 ) . I n cases where the quantum y i e l d v a r i e s as a f u n c t i o n of w a v e l e n g t h the case i s s u b s t a n t i a l l y more complex. T h i s i s t r u e i n the case o f f o r m a t i o n from humic s u b s t a n c e s (30) and i s p r o b a b l y t r u e f o r many o t h e r e n v i r o n m e n t a l p h o t o p r o c e s s e s where m u l t i p l e chromophores or r e a c t i o n mechanisms are i n v o l v e d . P r o v i d i n g a c c u r a t e model c a l c u l a t i o n s r e q u i r e s t h a t quantum y i e l d s be c a r e f u l l y d e t e r m i n e d . New a p p r o a c h e s a r e n e e d e d i n a c t i n o m e t r y and i n the measurement of the absorbance of a c t i v e chromophores i n d i l u t e s o l u t i o n s t h a t absorb o n l y a s m a l l f r a c t i o n o f the t o t a l i n c i d e n t p o l y c h r o m a t i c r a d i a t i o n . Quantum y i e l d s can be o b t a i n e d t h a t are wavelength-averaged I n the n e a r UV (310 - 410 nm) and these a r e a p p l i c a b l e t o e x i s t i n g models ( 3 1 ) . The use of wavelength-averaged quantum yields i s quite simple and in p a r t i c u l a r , u s e f u l whe possible. Provided tha c a n be o b t a i n e d f o r photoprocesses i n n a t u r a l a q u a t i c systems, i t i s p o s s i b l e t o p r o v i d e a good p h o t o c h e m i c a l model a p p r o x i m a t i o n f o r r e a c t i o n r a t e s i n the environment ( 1 2 ) . To o b t a i n a r e a l i s t i c d i s t r i b u t i o n of p h o t o c h e m i c a l l y d e r i v e d p r o p e r t y , i t i s n e c e s s a r y t o c o n s i d e r p r o p e r t i e s s u c h as d e c a y r a t e s o f t h e s p e c i e s o f i n t e r e s t and p h y s i c a l m i x i n g o f the w a t e r column ( 3 2 ) . Summary I t i s q u i t e apparent from t h i s i n t r o d u c t i o n and the c h a p t e r s t o f o l l o w , t h a t the f i e l d o f e n v i r o n m e n t a l a q u a t i c p h o t o c h e m i s t r y i s q u i t e large. For the most p a r t , i t i s a v e r y young f i e l d and one i n which a c o n s i d e r a b l e e f f o r t remains i n order to o b t a i n a q u a n t i t a t i v e understanding of the s i g n i f i c a n c e to environmental processes. T h i s book i s a f i r s t attempt t o b r i n g t o g e t h e r a s e r i e s of papers d i s c u s s i n g v a r i o u s a s p e c t s of t h i s f i e l d . References

1. Choudhry, G.G. Toxicol. Environ. Chem., 1981, 4, 261-295. 2. Sundstrom, G. ; Ruzo, L.O. In "Aquatic Pollutants: Transformation and Biological Effects"; Hutzinger, O.; van Lelyveld, L.H.; Zoeteman, B.C.J., Eds.; Pergamon Press, N.Y., 1978, 205-222. 3. Zafiriou, O.C. Mar. Chem., 1977, 5, 497-522. 4. Zafiriou, O.C. In "Chemical Oceanography" Vol. 8, 2nd ed.; Riley, J.P.; Chester, R., Eds.; Academic Press, London, 1983, 339-379.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1. COOPER A N D HERR

Introduction and Overview

5. Zafiriou, O.C; Joussot-Dubien, J.; Zepp, R.G.; Zika, R.G. Environ. Sci. Technol., 1984, 18, 358A-371A. 6. Zepp, R.G.; Baughman, G.L. In "Aquatic Pollutants: Transformations and Biological Effects"; Hutzinger, O.; van Lelyveld, L.H.; Zoeteman, B.C.J., Eds.; Pergamon Press, N.Y., 1978, 237-263. 7. Zepp, R.G. In "Dynamics, Exposure and Hazard Assessment of Toxic Chemicals"; Haque, R., Ed.; Ann Arbor Sci. Pub., Ann Arbor, MI, 1980, 69-110. 8. Zepp, R.G. In "The Handbook of Environmental Chemistry. Vol. 2, Part B, Reactions and Processes"; Hutzinger, O., Ed.; Springer-Verlag, N.Y., 1980, 19-41. 9. Zepp, R.G. In "Th Rol f Sola Ultraviole Radiatio i Marine Ecosystems" 1982, 293-307. 10. Zika, R.G. In "Marine Organic Chemistry: Evolution, Composition, Interactions and Chemistry of Organic Matter in Seawater"; Duursma, E.K.; Dawson, R., Eds.; Elsevier Sci. Publ. Co., Amsterdam, 1981, 299-325. 11. Mill, T. In "The Handbook of Environmental Chemistry. Vol. 2. Part A, Reactions and Processes"; Hutzinger, O., Ed., Springer-Verlag, N.Y., 1980, 77-105. 12. Zepp, R.G.; Cline, D.M. Environ. Sci. Technol., 1977, 11, 359-366. 13. Zafiriou, O.C; True, M.B.; Hayon, Ε. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C., this volume. 14. Cooper, W.J.; Zika, R.G. Science, 1983, 220, 711-712. 15. Draper, W.M.; Crosby, D.G. Arch. Environ. Contam. Toxicol., 1983, 12, 121-126. 16. Baxter, R.M.; Carey, J. Nature (London), 1983, 306, 575-576. 17. Moffet, J.W.; Zika, R.G. Mar. Chem., 1983, 13, 239-251. 18. Thompson, A.M.; Zafiriou, O.C. J. Geophys. Res., 1983, 88, 6696-6708. 19. Momzikoff, Α.; Santos, R. Mar. Chem., 1983, 12, 1-14. 20. Hagg, W.R.; Hoigne, J. Environ. Sci. Technol., 1986, 20, 341-348.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1

8

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

21. Zepp, R.G.; Baughman, G.L.; Schlotzhauer, P.F. Chemosphere, 1981, 10, 109-117. 22. Fisher, A.M.; Winterle, J.S.; Mill, T. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C., this volume. 23. Powers, J.F.; Sharma, D.K.; Lanford, C.H.; Bonneau, R.; Joussot-Dubien, J. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds., ACS Symposium Series, Washington, D.C this volume. 24. Davis, J.Α., Ed. "Surface Processes in Aqueous Geochemistry." American Chemical Society, Washington, D.C, 1986. 25. Stumm, W., Ed. "Aquatic Surface Chemistry." WileyInterscience, 1986 26. Williams, P.M.; Carlucci, A.F.; Henrichs, S.M.; Van Vleet, E.S.; Horrigan, S.G.; Reid, F.M.H; Robertson, K.J. Mar. Chem., 1986, 19, 17-98. 27. Payne, J.R.; McNabb, G.D. Mar. Technol. Soc. J., 1984, 18(3), 24-42; Payne, J.R.; Phillips, C.R. Environ. Sci. Technol., 1985, 19, 569-579. 28. Patton, J.S.; Rigter, U.W.; Boehm, P.D.; Feist, D.L. Nature, 1981, 290, 235-238. 29. Larson, R.A., Hunt, L.L.; Ablankenship, D.W. Environ. Sci. Technol., 1977, 11, 492-496. 30. Cooper, W.J.; Zika, R.G.; Petasne, R.G.; Plane, J.M.C. Environ. Sci. Technol., submitted. 31. Draper, W.M. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.G.; Cooper, W.J., Eds. ACS Symposium Series, Washington, D.C, this volume. 32. Plane, J.M.C; Zika, R.G.; Zepp, R.G.; Burns, L.A. In "Photochemistry of Environmental Aquatic Systems"; Zika, R.C; Cooper, W.J. Eds., ACS Symposium Series, Washington D.C, this volume. RECEIVED August 26, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 2

Specific Phototransformation of Xenobiotic Compounds: Chlorobenzenes and Halophenols 1

2

1

1

Pierre Boule , Claude Guyon , Annie Tissot , and Jacques Lemaire 1

Laboratoire de Photochimie Moléculaire et Macromoléculaire, U.A. C.N.R.S. 433, University of Clermont II, B.P. 45, 63170 Aubière, France Rhône Poulenc Santé, Centre de Recherches de Vitry, 13, Quai J. Guesde, 94400 Vitry-sur-Seine, France 2

An original aspect of aqueous photochemistry rela­ ted to the anticipated fact that water would favour pho tochemical processe states, is emphasize halogenophenols. Such photoprocesses would be more spe­ cific than homolytic photodissociation leading to radi­ cals. In chlorobenzenes, a photohydrolysis mechanism is observed. The monochlorobenzene is quantitatively trans­ formed into phenol in aqueous solution, even in acidic media (1 CH -CO-CH -CO-CH

(acetylacetone)

> CH -CO-CO-CH

(biacetyl)

3

2 CH -CO" -

3

3

2

3

3

Acetylacetone i s one of the major products of the photolysis of ace­ tone (20). In the presence of chlorobenzene, the self-quenching r e ­ action competes with the energy transfer and the r a d i c a l s induce the formation of by-products. This self-quenching reaction i s much redu­ ced with hexadeuterated acetone. Up to 0.1 m o l . l " , the major photoprod-uct from CD ~CO-CD i n water i s CD -CO-CO-CD . In the presence of chlorobenzene, the formation of biacetyl-D6 i s i n h i b i t e d through an energy transfer and the photohydrolysis i s s e n s i t i z e d . The active v i b r a t i o n mode associated with CD group favors the non-radiative t r a n s i t i o n T ^ S and shortens the i n t r i n s i c l i f e t i m e of the t r i p l e t state. This accounts f o r the lack of self-quenching reaction. Thus hexadeuterated acetone appears to be a more s p e c i f i c photosensitizer than acetone i n water (19). The s p e c i f i c i t y of the photohydrolysis allows to rule out a mechanism involving an homolytic s c i s s i o n of the C-Cl bond with form­ ation of r a d i c a l s . A d i r e c t photohydrolysis probably occurs from a polarized excited state. I t i s suggested t h i s excited state would be the f i r s t t r i p l e t state to account f o r the s e n s i t i z a t i o n and i n ­ h i b i t i o n experiments ( 2 9 ) · 1

3

3

3

3

Q

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

14

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

cf'

From the s e n s i t i z e d phosphorescence of b i a c e t y l , the quantum y i e l d of inter-system crossing (

8

Table

IV. P h y s i c a l and Chemical P r o p e r t i e s of Skidaway R i v e r W a t e r , Physical

Temp (°C)

1

0

Z99(cm)

c

1983-1985

Chemical P r o p e r t i e s pH

Particulates 31.3 1 17.1 (12.9-68.3)

1

Kicm" )

Properties

21.7 ± 6.3 (7-29)

Suspended (mg L " )

7.7 ± 0.2 (7.4-8.0)

Dissolved organic carbon (mg L " )

4.8 ± 0.3

1

0.22 ( O c t . 0.05 (Feb

21 100

d

(K-Da ) (cm" )

( O c t . 1983) ( F e b . 1984)

P a r t i c u l a t e organic n i t r o g e n (mg I T ) 1

0.15 ( O c t . 1983) 0.01 ( F e b . 1984)

3 3 0

1

C/N

0.20 ± 0.05

4.6 ± 1 . 3

Phosphate a

37

Photolysis of Phenol and Chlorophenols

3. HWANG ET AL.

(yg-atotn/L)

0.7 ± 0.3

0.05 ± 0.02 (0.030-0.096)

3 3 0

Nitrate 1

1

(ug-atom/L)

1.4 ± 0.3

3

K ( l mg" cm' )* 6.5 χ 1 0 " (Oct. 1983) s

0.6 χ Ι Ο "

3

(Feb. 1984) Salinity

0.85

(°/oo)

22.8 ± 2.4 (18-25)

± 0.04 (0.82-0.88)

a

-

E x p r e s s e d as the annual mean ± s t a n d a r d

b

«

Diffuse attenuation

coefficient.

c

*

Z99 - 4.6/K, p h o t i c

zone

d

«

Component o f Κ due t o l i g h t a t t e n u a t i o n by suspended p a r t i c u l a t e s . a o i the absorbance (1-cm p a t h l e n g t h ) o f the p a r t i c u l a t e - f r e e e s t u a r i n e water. D: d i s t r i b u t i o n c o e f f i c i e n t i n e s t u a r i n e water.

deviation

depth. 8

3 3

e
* The concept o f f o r m a t i o n o f a h y d r a t e d e l e c t r o n i n aqueous s o l u t i o n s can be extended t o e n v i r o n m e n t a l s t u d i e s u s i n g s u n l i g h t irradiations. The h y d r a t e d e l e c t r o n s are e a s i l y quenched by molec­ u l a r oxygen, l e a d i n g t o f o r m a t i o n o f s u p e r o x i d e , and s u b s e q u e n t l y , of h y d r o x y l r a d i c a l s . Evidence e x i s t s t h a t a u t o s e n s i t i z e d photoo x i d a t i o n o f i n d o l e s o c c u r s through a s u p e r o x i d e r a d i c a l i o n 0^· · Draper and Crosby (17) r e p o r t the f o r m a t i o n o f 02*· from p h o t o l y s i s of i n d o l e ( a s w e l l as p - c r e s o l and a n i l i n e ) v i a p h o t o i o n i z a t i o n . Superoxide may a t t a c k i n d o l e s d i r e c t l y t o form h y d r o p e r o x i d e s , o r i t may f i r s t r e a c t i n w a t e r t o form hydrogen p e r o x i d e , which may be the p r i m a r y o x i d a n t . Hydrogen p e r o x i d e o x i d a t i o n p r o b a b l y i n v o l v e s f o r m a t i o n o f h y d r o x y l r a d i c a l s from t h e p h o t o l y t i c h o m o l y s i s o f H 0 , w h i c h are I n v o l v e d i n f r e e - r a d i c a l o x i d a t i o n s ( 3 4 ) . 9

IH*

^ i

H

9

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

PICEL ET AL.

55

Photolysis of Indoles and Carbazole

As d i s c u s s e d i n the s e c t i o n on m a t r i x e f f e c t s , i n d o l e s showed 1.3to 5 . 0 - f o l d enhancement ( s e n s i t i z a t i o n ) of t h e i r photod e c o m p o s i t i o n r a t e s i n the aqueous CRM-1 matrix. One r o u t e of s e n s i t i z e d photooxidation of i n d o l e s may be t h r o u g h a s i n g l e t oxygen ( e x c i t e d m o l e c u l a r oxygen) i n t e r m e d i a t e . The e l e c t r o n - r i c h 2,3-double bond of i n d o l e and i t s amino a c i d d e r i v a t i v e t r y p t o p h a n are s u s c e p t i b l e to a t t a c k by s i n g l e t oxygen (19, 35-37). PAH are known s e n s i t i z e r s f o r s i n g l e t oxygen p r o d u c t i o n ?36) and are p r e s e n t at low l e v e l s i n aqueous CRM-1. H e t e r o c y c l i c PAH in s o l u t i o n p o s s i b l y produce s i n g l e t oxygen as w e l l . W i t h r e g a r d to d i r e c t energy t r a n s f e r to i n d o l e s , the y i e l d s of i n d o l e t r i p l e t s i n t r i p l e t acetone quenching were measured by Kasama et a l . ( 18). The y i e l d s o b t a i n e d f o r the t e s t i n d o l e s were 0.2-0.43. The presence of chromophores i n the aqueous CRM-1 s o l u t i o n , t h e r e f o r e , c o u l d s e n s i t i z e the p h o t o l y s i s of indoles d i r e c t l y by energy t r a n s f e r t o the t r i p l e t s t a t e . Because of the comple the m u l t i p l e pathways only speculate as to the e x a c t mechanism or mechanisms of the s e n s i t i z a t i o n process i n t h i s matrix. We found, however> t h a t the photoproduct o b t a i n e d from 3-MI i n d i s t i l l e d w a t e r was the same photoproduct i s o l a t e d i n aqueous CRM-1 s o l u t i o n . This observation i s d i s c u s s e d i n the next s e c t i o n . The p h o t o l y s i s of c a r b a z o l e i n d i s t i l l e d w a t e r and i n aqueous CRM-1 produced k i n e t i c d a t a s i m i l a r t o those o b t a i n e d f o r PAH. O b v i o u s l y , the p h o t o l y t i c b e h a v i o r of c a r b a z o l e i s c l o s e r t o t h a t of PAH than to t h a t of the i n d o l e s s e l e c t e d f o r t h i s s t u d y . Photoproducts. P r o d u c t s produced d u r i n g the exposure of aqueous CRM-1 t o s u n l i g h t were s t u d i e d by a n a l y z i n g c o l o r e d f r a c t i o n s of the e x t r a c t s by GC/MS and s p e c t r o p h o t o m e t r y . A reddish-orange c o l o r appeared p r i m a r i l y i n the a c i d and base f r a c t i o n s , and In the e t h e r s u b f r a c t i o n (S3) of the n e u t r a l f r a c t i o n . The a c i d and base f r a c t i o n s , w h i c h had low c o l o r - t o - m a s s r a t i o s , were examined o n l y spectrophotometrically. The n e u t r a l S3 s u b f r a c t i o n was s t u d i e d i n d e t a i l because of i t s i n t e n s e c o l o r and r e l a t i v e l y s m a l l mass. I s o l a t i o n of the p r o d u c t s r e s i d i n g i n the n e u t r a l S3 subf r a c t i o n r e q u i r e d f u r t h e r s u b f r a c t i o n a t i o n on s i l i c a g e l to o b t a i n enriched colored subfractions (fl-f5) from the bulk of the m a t e r i a l , w h i c h c o n s i s t e d of a l k y l a t e d p h e n o l s . The n e u t r a l S3 s u b f r a c t i o n was p l a c e d on a 10-g, 1-cm-diameter s i l i c a g e l column, and the f o l l o w i n g s u b f r a c t i o n s were c o l l e c t e d : f l , 30 mL of C H o C l ; f 2 , 20 mL of 10% e t h y l e t h e r i n C H C 1 ; f 3 , 20 mL of 20% e t h y l e t h e r i n C H ^ C l ^ f 4 , 20 mL of 50% e t h y l e t h e r i n Ch^CU; and f 5 , 30 mL of 100% e t h y l e t h e r . The chromatograms of the h i g h l y c o l o r e d f4 s u b f r a c t i o n from both the 180-min exposure sample and the dark c o n t r o l are compared i n F i g u r e 3. A major new peak ( l a b e l e d "a," R = 15.7 min) and s e v e r a l minor new peaks appear i n the exposed sample between 13 min and 19 min. Mass s p e c t r a were taken of these product peaks and of p r o d u c t peaks from the i n d i v i d u a l exposures of the t e s t compounds. The mass spectrum and r e t e n t i o n time of the major photoproduct of 3-MI matched those of peak "a" i n F i g u r e 3. I t I s r e a s o n a b l e t h a t the most abundant p r o d u c t s found i n aqueous CRM-1 a r i s e from breakdown 2

2

2

fc

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

1 5

1

1

1

10

15

20

Time (min)

F i g u r e 3.

Chromatograms of t h e n e u t r a l f4 s u b f r a c t i o n s of the s u n l i g h t - e x p o s e d (3 h r ) and unexposed samples. Peak " a " was i d e n t i f i e d as a photoproduct of 3-MI ( s t r u c t u r e V ) .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4. PICEL ET AL.

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Photolysis of Indoles and Carbazole

of 2-MI o r 3-MI because the combined 2-MI and 3-MI peak underwent by f a r the g r e a t e s t degree of d e g r a d a t i o n of any of the component peaks m o n i t o r e d i n aqueous CRM-1. The major p r o d u c t found i n t h e p h o t o l y s i s of 2-MI ( m o l e c u l a r weight = 161) i n d i s t i l l e d water d i d not appear i n t h e S3 s u b f r a c t i o n of the exposed aqueous CRM-1. Beer e t a l . ( 1 5 ) r e p o r t t h a t t h e a u t o x i d a t i o n of s u b s t i t u t e d i n d o l e s ( I ) produces h y d r o p e r o x i d e i n t e r m e d i a t e s ( I I ) , w h i c h decomO-OH -C-R'

Xr

Ό

.

'

^ ^ X ^ NΗ - CΟ -R"

HI

J I Η

-

N-CI I Η Ο

m pose w i t h r i n g c l e a v a g e t o form o-acylamino-ketones ( I I I ) . The h y d r o p e r o x i d e ( I I ) i n t h i s p r o c e s s c a t a l y z e s i t s own p r o d u c t i o n . I t must be noted t h a t product I I I can p r o b a b l y a l s o be produced by s i n g l e t oxygen a t t a c k on t h e 2,3-double bond of s u b s t i t u t e d i n d o l e s (see R e f . 3 6 ) . For 3-MI ( I V ) , s t r u c t u r e V i s proposed as the major p r o d u c t of photooxidation i n water. This structure, o-(N-formyl)aminoacetophenone ( m o l e c u l a r weight = 1 6 3 ) , i s s u p p o r t e d by the mass spectrum of t h e 3-MI p r o d u c t found i n aqueous CRM-1 (120 [ 1 0 0 % ] , 92 [ 5 1 % ] , 135 [ 4 5 % ] , and 163 [ 1 6 % ] ) . S p e c t r a o f the o t h e r p r o d u c t peaks suggest that they a r e analogous products (acylaraino d e r i v a t i v e s ) from t h e p h o t o o x i d a t i o n o f o t h e r a l k y l a t e d i n d o l e s . S p e c t r o p h o t o m e t r i c a n a l y s i s d e t e c t e d product f o r m a t i o n I n a l l t h r e e c o l o r e d f r a c t i o n s , t h a t i s , the a c i d and base f r a c t i o n s , and the n e u t r a l S3 s u b f r a c t i o n . A l l colored fractions exhibited a bathochromic s h i f t w i t h time t o s t r o n g e r absorbance i n the wave­ l e n g t h range o f 300-500 nm. T h i s absorbance i n the b l u e / g r e e n r e g i o n of t h e spectrum e x p l a i n s the complimentary appearance of c o l o r i n the yellow/red r e g i o n . The s h i f t was p o s s i b l y caused by f o r m a t i o n of o x i d a t i o n p r o d u c t s c o n t a i n i n g c a r b o n y l o r h y d r o x y l groups. The acid fraction exhibited a continuous increase i n absorbance w i t h i n c r e a s i n g exposure t i m e . The p r o d u c t s i n t h i s fraction likely contain h y d r o x y l groups as their acidic functionality. Product absorbance i n the base f r a c t i o n appeared p r i m a r i l y a f t e r 90 min of exposure. The b a s i c p r o d u c t s may be t h e r e s u l t of a p r o d u c t - c a t a l y z e d r e a c t i o n , o r they may be secondary p h o t o l y s i s p r o d u c t s from t h e p h o t o l y t i c breakdown of i n i t i a l products. The absorbance i n the n e u t r a l S3 f r a c t i o n was p r o b a b l y

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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due p r i m a r i l y t o f o r m a t i o n o f the o-acylamino-ketones p h o t o l y s i s of i n d o l e s , as d i s c u s s e d above.

from t h e

Summary I n d o l e s and c a r b a z o l e s a r e p h o t o s e n s i t i v e components o f aqueous c o a l o i l . I n s u n l i g h t exposures i n d i s t i l l e d w a t e r , i n d o l e s under­ went a u t o c a t a l y z e d photooxidation, which l i k e l y c o n t r i b u t e d t o t h e i r very high quantum y i e l d s . I n aqueous CRM-1, i n d o l e s a p p a r e n t l y underwent s e n s i t i z e d p h o t o l y s i s because p h o t o l y s i s r a t e s were enhanced, w h i l e a v a i l a b l e r a d i a t i o n was reduced markedly by a b s o r p t i o n by t h e m a t r i x . C a r b a z o l e , on t h e o t h e r hand, underwent d i r e c t p h o t o l y s i s i n b o t h t h e aqueous CRM-1 m a t r i x and i n d i s t i l l e d water. I t e x h i b i t e d a reduced p h o t o l y s i s r a t e i n t h e m a t r i x , s i m i l a r t o t h e b e h a v i o r p r e v i o u s l y observed f o r PAH. P h o t o p r o d u c t s were i s o l a t e d from the n e u t r a l f r a c t i o n I n a highly colored polar subfraction aqueous CRM-1 matched t h a MI i n d i s t i l l e d w a t e r . T h i s p r o d u c t i s thought t o be o-(N forrayl)aminoacetophenone. P h o t o p r o d u c t s were a l s o d e t e c t e d i n t h e a c i d and base f r a c t i o n s o f aqueous CRM-1 by s p e c t r o p h o t o m e t r y . Acknowledgments Work supported by t h e U.S. Department o f Energy, Assistant S e c r e t a r y f o r F o s s i l Energy, under C o n t r a c t W-31-109-Eng-38. Literature Cited

1. Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol. 1985, 19, 74-81. 2. Zepp, R. G.; Baughman, G. L. In "Aquatic Pollutants: Transformation and Biological Effects," O. Hutzinger, I. H. Van Lelyveld, and B. C. J. Zoeteman, Eds., Pergamon Press: Oxford and New York, 1978, pp. 237-263. 3. Zepp, R. G.; Cline, D. M. Environ. Sci. Technol. 1977, 11, 359-366. 4. Zepp, R. G. In "The Handbook of Environmental Chemistry," Vol. 2, Part B, O. Hutzinger, Ed.; Springer-Verlag: Berlin, Heidelberg, and New York, 1982, pp. 19-42. 5. Miller, G. C; Zepp, R. G. Residue Rev. 1983, 85, 89-110. 6. Stamoudis, V. C; Bourne, S.; Haugen, D. Α.; Peak, M. J.; Reilly, C. A. Jr.; Stetter, J.R.; Wilzbach, K. In "Coal Conversion and the Environment: Chemical, Biomedical, and Ecological Considerations," Proc. 20th Hanford Life Sciences Symposium, D. D. Mahlum, R. H. Gray, and W. D. Felix, Eds., U.S. Dept. of Energy Report CONF-80-1039, pp. 67-95, 1981. 7. Smith, J. H.; Mabey, W. R.; Bohonos, N.; Holt, B. R.; Lee, S. S.; Chou, T. W.; Bomberger, D. C.; Mill, T. "Environmental Pathways of Selected Chemicals in Freshwater Systems, Part II: Laboratory Studies," U.S. Environmental Protection Agency Report EPA-600/7-78-074, 1978.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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8. Picel, K. C.; Stamoudis, V. C; Simmons, M. S. "Photolytic and Partitioning Behavior of Polynuclear Aromatic Compounds, Aromatic Amines, and Phenols in Aqueous Coal Oil," U.S. Dept. of Energy Report DOE/MC/49533-1837 (ANL/SER-4), 1985. 9. Zadelis, D.; Simmons, M S . In "Polynuclear Aromatic Hydrocarbons: Formation, Metabolism, and Measurement," W. M. Cooke and A. J. Dennis, Eds., Battelle Press, Columbus, Ohio, 1983, pp. 1279-1290. 10. Mill, T.; Mabey, W.. R.; Lan, Β. Y.; Bareze, A. Chemosphere 1981, 10, 1281-1290. 11. Zepp, R. G.; Schlotzhauer, P. F. In "Polynuclear Aromatic Hydrocarbons: Third International Symposium on Chemistry and Biology — Carcinogenesis and Mutagenesis," P. W. Jones and P. Leber, Eds., Ann Arbor Science: Ann Arbor, Mich, 1979, pp. 141-158. 12. Witcop, B.; Patrick, J. B. J. Am. Chem. Soc. 1951, 73, 713718. 13. Witcop, B.; Patrick 2200. 14. Witcop, B.; Patrick, J. B. J. Am. Chem. Soc. 1952, 74, 38553860. 15. Beer, R. J. S.; Donavanik, T.; Robertson, A. J. Chem. Soc. 1954, 4139-4142. 16. Draper, W. M.; Crosby, D. G. Arch. Environ. Contam. Toxicol. 1983, 12, 121-126. 17. Draper, W. M.; Crosby, D. G. J. Agric. Food Chem. 1983, 31, 734-737. 18. Kasama, K.; Takematsu, Α.; Aral, S. J. Phys. Chem. 1982, 86, 2420-2428. 19. Nilsson, R.; Merkel, P.B.; Kearns, Ό. R. Photοchem. Photobiol. 1972, 16, 117-124. 20. Turro, N. J. "Molecular Photochemistry"; W. A. Benjamin: New York, 1965. 21. Dulin, D.; Mill, T. Environ. Sci. Technol. 1982, 16, 815820. 22. Grossweiner, L. I. Curr. Top, in Radiât. Res. Quart. 1976, 11, 141-199. 23. Santus, R.; Bazin, M.; Aubailly, M. Rev. Chem. Intermed. 1980, 3, 231-283. 23. Creed, D. Photochem. Photobiol. 1984, 39, 537-562. 24. Feitelson, J. Photochem. Photobiol. 1971, 13, 87-96. 25. Hopkins, T. R.; Lurary, R. W. Photochem. Photobiol. 1972, 15, 555-556. 26. Bent, D. V.; Hayon, Ε. J. Am. Chem. Soc. 1975, 91, 26122619. 27. Bryant, F. D.; Santus, R.; Grossweiner, L. I. J. Phys. Chem. 1975, 79, 2711-2716. 28. Grossweiner, L. I.; Brendzel, A. M.; Blum, A. Chem. Phys. 1981, 57, 147-156. 29. Klein, R.; Tatischeff, I.; Bazin, M.; Santus, R. J. Phys. Chem. 1981, 85, 670-677. 30. Zechner, J.; Kohler, G.; Getoff, N.; Tatischeff, I.; Klein, R. Photochem. Photobiol. 1981, 34, 163-168.

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Mialocq, J. C; Amouyal, E.; Bernas, Α.; Grand, D. J. Phys. Chem. 1982, 86, 3173-3177. Hershberger, M. V.; Lumry, R. W.; Verrall, R. Photochem. Photobiol. 1981, 34, 609-617. Hershberger, M. V.; Lumry, R. W. Photochem. Photobiol. 1976, 23, 391-397. Draper, W. M.; Crosby, D. G. J. Agric. Food Chem. 1981, 29, 699-702. Foote, C. S. Acc. Chem. Res. 1968, 1, 104-110. Kearns, D. R. Chem. Rev. 1971, 71, 395-427. Matheson, I. B. C; Lee, J. Photochem. Photobiol. 1979, 29, 879-881.

RECEIVED May 27, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 5

Quantum Yields of Polychlorodibenzo-p-dioxins In Water-Acetonitrile Mixtures and Their Environmental Phototransformation Rates Ghulam Ghaus Choudhry and G. R. Rarrie Webster Pesticide Research Laboratory, Department of Soil Science, The University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

Photochemistry of four individual isomers of polychlorodibenzo-p-dioxin (PCDDs) namel 1,2,3,4,7 pentachlorodibenzo-2-dioxi 1,2,3,4,7,8-HCDD (2), 1,2,3,4,6,7,8-H (3) OCDD (4) in water-acetonitrile (2:3 v/v) was investigated using laboratory radiations of wavelength 313 nm. The quantum yields for the phototransformation of the dioxins 1-4 in these solvent systems were (9.8 ± 2.4) x 10 , (1.10 ± 0.02) x 10 , (1.53 ± 0.17)x10 and (2.26 ±0.33) x 10 , respectively. These quantum yields and the measured absorption spectra together with solar intensity data available in the literature were utilized to estimate the sunlight (environmental) phototransformation first-order rate constants of the PCDD congeners 1-4 in water under conditions of variable sunlight intensity during various seasons; the corresponding half-lives were also determined. In summer, typical half-lives for the phototransformation of the pollutants 1-4 near the surface of water bodies at 40° north latitude would be 15 ± 4, 6.3 ± 0.1, 47 ±5, and 18 ±3 days, respectively. 6

7

8

-5

-4

-5

-5

Concern has grown r e g a r d i n g e n v i r o n m e n t a l c o n t a m i n a t i o n b y p o l y c h l o r i n a t e d dibenzo-p_-dioxins (PCDDs). PCDDs i n c l u d e 75 congeners. Some o f t h e s e t r i c y c l i c a r o m a t i c s p o s s e s s r e m a r k a b l e t o x i c , t e r a t o g e n i c , mutagenic, and a c n e g e n i c c h a r a c t e r i s i t i c s ( 1 , 2 ) . For i n s t a n c e , t h e LD50 f o r 2 , 3,7 , 8 - t e t r a c h l o r o d i b e n z o - p . - d i o x i n (2,3,7,8-T4CDD) i n female r a t s i s 45 /ig/kg. A l t h o u g h t h e e x t e n t o f t o x i c i t y i s r e l a t e d t o t h e degree o f c h l o r i n a t i o n , t h e r o l e o f p o s i t i o n a l i s o m e r i s a t i o n i s a l s o c r i t i c a l and perhaps t h e dominant one; 2,3,7,8-T4CDD i s a t l e a s t 800 t i m e s more t o x i c t h a n 1,2,3,4T CDD ( 2 ) . R e c e n t l y , i n a d d i t i o n t o s e v e r a l o t h e r o r g a n i c compounds, PCDDs have been d e t e c t e d i n the f l u e gas and f l y a s h e m i t t e d b y some m u n i c i p a l and i n d u s t r i a l i n c i n e r a t o r s l o c a t e d i n Canada, J a p a n , S w i t z e r l a n d , and the N e t h e r l a n d s ( r e f e r e n c e s c i t e d i n Choudhry and 4

0097-6156/87/0327-0061506.00/0 © 1987 American Chemical Society

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H u t z i n g e r (3,4) and Choudhry e t a l . ( 5 ) ) . F o r i n s t a n c e , i n c i n e r a t o r f l y a s h i s known t o c o n t a i n r e l a t i v e l y l a r g e amounts o f o c t a - , h e p t a - , and h e x a - c h l o r o isomers (OgCDD, HyCDDs, and H CDDs) o f d i o x i n b u t o n l y l o w l e v e l s o f 2,3,7,8-T4CDD ( 6 ) . Moreover, Jones ( 7 ) r e p o r t e d t h a t t h e PCDDs r e l e a s e d near t h e Dow Chemical Co., M i d l a n d , M i c h i g a n were > 99.5% HgCDDs, HyCDDs, and OgCDD, r e s p e c t i v e l y and t e t r a c h l o r o isomers o t h e r t h a n 2,3,7,8-T4CDD. I n a d d i t i o n , commercial 2 , 3 , 5 - t r i - , 2 , 4 , 6 - t r i - and p e n t a c h l o r o p h e n o l have been shown t o c o n t a i n 2 , 7 - d i c h l o r o d i b e n z o - p - d i o x i n (2,7-D2CDD) and 1,3,6,8-T4CDD t o g e t h e r w i t h HgCDDs, HyCDDs, and OgCDD i n ppm l e v e l s i n s p i t e o f improved m a n u f a c t u r i n g t e c h n i q u e s ( 7 ) . C h l o r i n a t e d p h e n o l s have w i d e s p r e a d use as f u n g i c i d e s , b a c t e r i c i d e s , s l i m i c i d e s , h e r b i c i d e s , e t c . , and t h e y a r e a l s o used i n t h e p r o d u c t i o n o f phenoxy a c i d s such as 2 , 4 - d i - and 2 , 4 , 5 - t r i c h l o r o p h e n o x y a c e t i c a c i d (2,4-D and 2,4,5-T, r e s p e c t i v e l y ( 8 ) . F o r a l m o s t f o u r decades, 2,4-D and 2,4,5-T and t h e i r d e r i v a t i v e s have r e c e i v e d wide use as h e r b i c i d e s , e s p e c i a l l y f o r the c o n t r o weeds. T e c h n i c a l 2,4-D c o n t a i n 2.7-D2CDD and 1,3,6,8-T4CD g pp ( 9 ) L i k e w i s e , a mean o f more t h a n 1.9 and a maximum o f 47 ppm o f 2,3,7,8T4CDD has been i d e n t i f i e d i n t h e m i l i t a r y d e f o l i a n t Agent Orange ( b u t y l e s t e r s o f 2,4-D and 2,4,5-T i n e q u a l amounts ( 8 ) . Throughout the w o r l d on s e v e r a l o c c a s i o n s , p u b l i c a n x i e t y because o f t h e r e l e a s e o f PCDDs i n t o o u r environment has been seen. F o r example, i n t h e autumn o f 1957, m i l l i o n s o f c h i c k e n s i n t h e E a s t e r n and M i d w e s t e r n U n i t e d S t a t e s d i e d o f a d i s e a s e caused b y a whole s e r i e s o f PCDDs p r e s e n t i n t h e i r f e e d . On J u l y 10, 1976, a wide a r e a near Seveso, I t a l y was c o n t a m i n a t e d b y a n e x t r e m e l y t o x i c f a l l o u t c o n s i s i t i n g o f 2,3,7,8-T4CDD i n a c h e m i c a l m i x t u r e o f o t h e r o r g a n o c h l o r i n e compounds. S i m i l a r l y , a l a r g e q u a n t i t y o f Agent Orange h e a v i l y c o n t a m i n a t e d w i t h 2,3,7,8-T4CDD was a p p l i e d from t h e a i r over j u n g l e s i n South E a s t A s i a ( 8 ) . F i n a l l y , i n 1982, due t o an e l e c t r i c a l transformer ( f l u i d : 65% p o l y c h l o r o b i p h e n y l s ( A r o c l o r 1254) and 35% t r i - and t e t r a c h l o r o b e n z e n e s ) f i r e , an o f f i c e b u i l d i n g i n Binghamton, New Y o r k , U.S.A. was c o n t a m i n a t e d b y numerous d i o x i n congeners c o n t a i n i n g 4-8 c h l o r i n e c o n s t i t u e n t s ( 1 0 ) . 6

The p h o t o c h e m i c a l d e g r a d a t i o n i s an i m p o r t a n t p r o c e s s w i t h a t m o s p h e r i c contaminants and some c h e m i c a l s t h a t r e s i d e on s u r f a c e s such as p e s t i c i d e s on l e a v e s and v e g e t a t i o n o r i n water b o d i e s . A l t h o u g h a good d e a l o f r e s e a r c h work on t h e p h o t o c h e m i c a l f a t e o f PCDDs, b o t h i n s o l u t i o n and i n t h e s o l i d phase has appeared ( 8 , 1 1 ) , t o t h e b e s t o f o u r knowledge no i n v e s t i g a t o r has r e p o r t e d t h e quantum y i e l d (φ) f o r t h e p h o t o l y s i s o f these e n v i r o n m e n t a l p o l l u t a n t s . We have d e t e r m i n e d t h e quantum y i e l d s (^ ) f o r t h e p h o t o c h e m i c a l t r a n s f o r m a t i o n r e a c t i o n s o f f o u r i n d i v i d u a l d i o x i n i s o m e r s , namely 1 , 2 , 3 , 4 , 7 - p e n t a c h l o r o d i b e n z o - p - d i o x i n (1,2,3,4,7-P CDD) ( 1 ) , 1,2,3,4,7,8-H CDD (2), 1,2,3,4,6,7,8-HyCDD (3) and 1,2,3,4,6,7,8,9OgCDD (4) i n w a t e r - a c e t o n i t r i l e (2:3 v / v ) s o l u t i o n u s i n g UV l i g h t o f m a i n l y 313 nm. The p r e s e n t c h a p t e r r e p o r t s t h e v a l u e s o f l a b o r a t o r y φ and t h e e s t i m a t e d s u n l i g h t p h o t o l y s i s h a l f - l i v e s ( ( t w 2 ) ) PCDDs 1-4. r

5

6

o

τ

f

S O

Experimental S u b s t r a t e s and S o l v e n t s .

Sources o f t h e s u b s t r a t e s , v i z . , PCDDs 1-4

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

CHOUDHRY AND WEBSTER

Quantum Yields of Polychlorodibenzo-p-dioxins

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

64

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

(Table I ) , o-nitrobenzaldehyde, o - n i t r o s o b e n z o i c a c i d , p.-nitrotoluene and 1 , 2 , 3 , 4 - t e t r a c h l o r o b e n z e n e and s o l v e n t s were the same as p r e v i o u s l y reported (12). P r e p a r a t i o n o f S o l u t i o n s o f PCDDs 1-4 i n W a t e r - A c e t o n i t r i l e (2:3 v/v). The p r o c e d u r e s f o r the p r e p r a t i o n o f the s o l u t i o n s o f 1,2,3,4,7-P CDD (1) and 1,2,3,4,7,8-H CDD (2) i n H 0-CH CN (2:3 v/v) have been d e s c r i b e d e l s e w h e r e ( 1 2 ) . Techniques f o r the p r e p a r a t i o n o f the s o l u t i o n o f HgCDD 2 i n w a t e r - a c e t o n i t r i l e (2:3 v/v) r e p o r t e d by Choudhry and Webster (12) were f o l l o w e d t o p r e p a r e s i m i l a r s o l u t i o n s o f d i o x i n s 3 and 4. For these i n v e s t i g a t i o n s , the c o n c e n t r a t i o n o f PCDDs 1-4 were 2.805, 3.328, 2.780 and 0.310 μΜ, r e s p e c t i v e l y (Table I I ) . 5

6

2

3

UV A b s o r p t i o n S p e c t r o s c o p y . UV a b s o r p t i o n s p e c t r a l d a t a f o r d i o x i n s 1-4 documented i n T a b l e I were o b t a i n e d u s i n g a s i n g l e beam Bausch and Lomb S p e c t r o n i c 710 S p e c t r o p h o t o m e t e r I r r a d i a t i o n Equipment and E x p e r i m e n t s . The p r e v i o u s l y d e s c r i b e d (11,14) Rayonet P h o t o c h e m i c a l R e a c t o r ( h a v i n g an energy o u t p u t o f 90% between 290 and 310 nm) e q u i p p e d w i t h a merry-go-round a p p a r a t u s was used. The P y r e x p h o t o r e a c t i o n c e l l s ( p a t h l e n g t h ( i ) , 1 cm) and c h e m i c a l f i l t e r s o l u t i o n ( c o n s i s t i n g o f l ^ C r O ^ (0.270 g/L) and Na2C0 (1.000 g/L) i n w a t e r ) u s e d f o r the i s o l a t i o n o f 313 nm l i n e from the R a y o n e t t RPR 3000 A lamps have been d e s c r i b e d ( 1 2 ) . O p t i c a l l y t h i c k s o l u t i o n s of o-nitrobenzaldehyde (10 mM) i n a c e t o n i t r i l e were u t i l i z e d as a c h e m i c a l a c t i n o m e t e r f o r the d e t e r m i n a t i o n o f the i n t e n s i t y (Ι ) o f t h e f i l t e r e d i n c i d e n t l i g h t ( 1 2 J L 5 ) . For t h i s p u r p o s e , the c o n c e n t r a t i o n o f the p h o t o p r o d u c t o - n i t r o s o b e n z o i c a c i d a r i s i n g from the a c t i n o m e t e r was m o n i t o r e d ( 1 2 ) . 3

λ

Analyses. A l l a n a l y s e s were c a r r i e d out by HPLC on a Waters S c i e n t i f i c l i q u i d chromatograph (Model 6000A pump, U6K i n j e c t o r , and an M440 UV absorbance d e t e c t o r ) . S e p a r a t i o n s were made w i t h a 30 cm χ 3.9 mm /iBondapak C^g column ( r e v e r s e phase, RP) ( 1 2 ) . The a n a l y s e s o f the sample s o l u t i o n o f the HyCDD 3 and OgCDD 4 were p e r f o r m e d by t h i s RP-HPLC u s i n g CH OH-H 0 (19:1 v/v) and CH3OH as e l u a n t s , r e s p e c t i v e l y a t a f l o w r a t e o f 1.0 mL/min. I n the c a s e o f 3 and 4, 20 μL o f p - n i t r o t o l u e n e (2.055 mM) and 90 /iL o f 1,2,3,4t e t r a c h l o r o b e n z e n e (0.757 mM) b o t h i n CH3CN, were added r e s p e c t i v e l y t o e a c h 2.0 mL sample s o l u t i o n as i n t e r n a l s t a n d a r d p r i o r t o analyses. A n a l y t i c a l p r o c e d u r e s f o r 1,2,3,4,7-P5CDD ( 1 ) , 1,2,3,4,7,8-H5CDD (2) and a c t i n o m e t r y have been d e s c r i b e d e l s e w h e r e 3

2

(12).

Results T a b l e I r e c o r d s the molar e x t i n c t i o n c o e f f i c i e n t (ε ) a t v a r i o u s w a v e l e n g t h s (λ), o f s o l u t i o n s o f PCDDs 1-4. W a t e r - a c e t o n i t r i l e (2:3 v/v) was u s e d as a s o l v e n t f o r compounds 1-3; whereas, i n the case o f Og-CDD 4, the s o l v e n t was n e a t a c e t o n i t r i l e . These s p e c t r a l d a t a were u s e d i n the p r e d i c t i o n o f the d i r e c t s u n l i g h t p h o t o l y s i s r a t e s , i . e . , k p and c o r r e s p o n d i n g h a l f - l i f e ( t ] y 2 ) s p ) p o l l u t a n t s 1-4 i n a q u a t i c e n v i r o n m e n t s (see b e l o w ) . T y p i c a l f i r s t - o r d e r p l o t s ( E q u a t i o n 1) o f the p h o t o l y s i s d a t e f o r λ

o f

t l i e

S

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. 3

D a t a f o r PCDDs 1-3 were r e c o r d e d u s i n g t h e s o l u t i o n s o f t h e p o l l u t a n t s i n H 0-CH CN (2:3 v / v ) ; w h i l e i n . t h e case o f 4_, t h e s o l v e n t was neat CH-CN. The c o n c e n t r a t i o n s o f PCDDs 1-4 were 47881, 3.328, 2.780 and 6.218 μΜ, r e s p e c t i v e l y . 2

44068 5307 4986 4825 4664 4342 4182 4182 4182 4021 4021 3860 3217 965 483 322 322 161 161 0 23020 3597 3597 3957 3957 3957 4316 4316 4316 4316 4316 3237 2158 360 0

25244 9015 8114 6912 6311 5409 5109 4808 4808 4808 3907 3306 2104 301 0

10434 2660 2660 2660 2660 2455 2455 2455 2250 2250 2250 2046 1227 0

254.0 297.5 300.0 302.5 305.0 307.5 310.0 312.5 313.0 315.0 317.5 320.0 323.1 330.0 340.0 350.0 360.0 370.0 380.0 390.0

a

1,2,3,4,6,7,8,9OgCDD (4)

?

1,2,3,4,6,7,8H CDD (3)

6

) for

1,2,3,4,7,8H CDD (2)

.cm

1,2,3,4,7P CDD (1)

(L.Mol

o f PCDDs

Wavelength, λ (nm)

5

3

Molar e x t i n c t i o n c o e f f i c i e n t s ,

Table I . L i g h t A b s o r p t i o n S p e c t r a

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

b

Y

(1.1±0.02) χ 1 0 " (1.53±0.17)x 1 0 " (2.26±0.33)x 1 0 "

24.5±0.1 191±20 184±24

7.86±0.03 1.02±0.11 1.0610.14

87.2 39.1 52.9

72

72

112

2.780

0.310

5

(9.8±2.4) χ 1 0 "

3

4

1η2 t, = ^ — ρ

3.328

2

C a l c u l a t e d using the f o l l o w i n g r e l a t i o n s h i p : E q u a t i o n 2 was u t i l i z e d .

72

2.805

1

\

45.9±7.4

1

4.31±0.70

(%)

(h)

t

71.2

6

a

Absolute half-life ,

(h)

6

t

Maximum irradiation time, max

I n i t i a l concen­ t r a t i o n of the starting dioxin, Ρ ο (10" M)

Subst­ rate No.

b Quantum y i e l d f o r the photo­ degradation, Φ r

(2:3 v/v) a t 313 nm

Photolysis f i r s t order rate constant k Ρ (10~ sec" )

Average p e r centage d i s ­ appearance o f the s t a r t i n g dioxin after max

Table I I . P h o t o l y s i s o f PCDDs i n W a t e r - A c e t o n i t r i l e

5

5

4

m

Ά

C 25 η c/s ·
Ο

53

ζ

m

ο ζ

< 53

X Ο Η Ο η se m § En H < Ο •η m Ζ

Os ON

CHOUDHRY A N D WEBSTER

5.

Quantum Yields of Polychbrodibenzo-p-dioxins

I n ( P / P ) - kp t Q

67

(1)

t

d i l u t e s o l u t i o n s (absorbance b e i n g 295 nm (see T a b l e I ) . S

S

S

o f

t h e

P

C

D

D

s

1 - 4

a r e

/ 2

Conclusions Our p h o t o c h e m i c a l i n v e s t i g a t i o n s a t 313 nm conducted on the d i l u t e s o l u t i o n s o f 1, 2 , 3 ,4, 7-pentachlorodibenzo-p.-dioxin (1, 2 , 3 ,4, 7-P CDD) (1) (2.805 /iM), 1,2,3,4,7,8-H CDD (2) (3.328 μΜ) , 1,2,3,4,6,7,8-HyCDD (3) (2.780 μΜ) and 1,2,3,4,6,7,8,9-0gCDD (4) (0.310 μΜ) i n watera c e t o n i t r i l e (2:3 v/v) m i x t u r e s l e a d us t o draw s e v e r a l e n v i r o n m e n t a l l y s i g n i f i c a n t c o n c l u s i o n s . The quantum y i e l d s f o r the PCDD congeners 1-4 amounting t o (9.8 ±2.4) χ 1 0 ' . (1.10 ± 0.02) χ 1 0 " , (1.53 ±0.17) χ 1 0 " and (2.26 ± 0.33) χ 1 0 " , r e s p e c t i v e l y , are 5

6

5

4

5

5

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5. CHOUDHRY AND WEBSTER

Quantum Yields of Polychlorodibenzo-p-dioxins 73

r a t h e r low, t h e r e b y r e f l e c t i n g t h a t p o l l u t a n t s such as p o l y c h l o r o d i b e n z o - p - d i o x i n s (PCDDs) p r e s e n t i n t h e a q u a t i c environments w o u l d photodegrade r a t h e r s l o w l y . S i m i l a r c o n c l u s i o n s can be drawn from b o t h a b s o l u t e ( t - ) and s u n l i g h t p h o t o t r a n s f o r m a t i o n ( (t**,^) ) documented i n T a b l e I I and T a b l e I I I , r e s p e c t i v e l y . C o n t r a r y t o t e l p r e v i o u s r e p o r t s on t h e p h o t o d e g r a d a t i o n o f PCDDs i n o r g a n i c s o l v e n t s (2,8, 26) o u r p h o t o l y t i c s t u d i e s u s i n g H2O-CH3CN systems s t r e s s t h a t i n t h e environment PCDD c o n t a m i n a n t s a r e n o t l i k e l y t o p h o t o d i s s i p a t e as r a p i d l y as was c o n c l u d e d b y o t h e r r e s e a r c h e r s . Finally, individual isomers o f c h l o r i n a t e d d i o x i n s p o s s e s s i n g a g r e a t e r number o f l a t e r a l CI s u b s t i t u e n t s a r e a p p a r e n t l y e x p e c t e d t o p h o t o c o n v e r t f a s t e r t h a n those w i t h l a r g e r p e r i C I c o n t e n t s , when such PCDD isomers a r e present i n water bodies. /t)

Acknowledgment We a p p r e c i a t e and acknowledg by W i l d l i f e T o x i c o l o g y D i v i s i o n , Canadian W i l d l i f e S e r v i c e , Ottawa, O n t a r i o , Canada. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

-

Buser, H. R. J. Chromatogr. 1976, 129, 303. Dobbs, A.J.; Grant, C. Nature. 1979, 278, 163. Choudhry, G.G.; Hutzinger, O. Toxicol. Environ. Chem. 1982, 5, 1. Choudhry, G.G., Hutzinger, O. "Mechanistic Aspects of the Thermal Formation of Halogenated Organic Compounds Including Polychlorinated Dibenzo-p_-dioxins", Gordon and Breach Sci. Publ.: New York, 1983. Choudhry, G.G.; Olie, K.; Hutzinger, O. Pergamon Ser. Environ. Sci. 1982, 5, 275. National Research Council of Canada, "Polychlorinated Dibenzo-pdioxins": Criteria for Their Effects on Man and His Environment", NRCC 18574, Ottawa, 1981. Jones, P.A. "Chlorophenols and Their Impurities in the Canadian Environment", Report EPS 3-EC-81-2, Environment Impact Control Directorate, Ottawa, 1981. Choudhry, G.G.; Hutzinger, O. Residue Rev. 1982, 84, 113. Cochrane, W.P.; Singh, J.; Miles, W.; Wakeford, B.; Scott, J. Pergamon Ser. Environ. Sci. 1982, 5, 209. Schecter, A. Chemosphere 1983, 12, 669. Corbet, R.L.; Muir, D.C.G.; Webster, G.R.B. Chemosphere 1983, 12, 523. Choudhry, G.G.; Webster, G.R.B. Chemosphere 1985, 14, 9. Choudhry, G.G.; Roof, A.A.M.; Hutzinger, O. J. Chem. Soc. Perkin Trans. I 1982, 2957. Choudhry, G.G.; van den Broeck, J.A.; Hutzinger, O. Chemosphere 1983, 12, 487. Dulin, D.; Mill, T. Environ. Sci. Technol. 1982, 16, 815. Choudhry, G.G. "Humic Substances: Structural, Photophysical, Photochemical and Free Radical Aspects and Interactions with Environmental Chemicals"; Gordon and Breach Sci. Publ.: New York, 1984.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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17. Choudhry, G.G. Toxicol. Environ. Chem. 1983, 6, 231. 18. Zepp, R.G. "Experimental Approaches to Environmental Photochemistry" in The Handbook of Environmental Chemistry, ed. Hutzinger, O., Vol. 2/Part B, Springer-Verlag: Berlin, West Germany, 1982, 19. 19. Mill, T.; Mabey, W.R.; Lan, Β.Y.; Baraze, A. Chemosphere 1981, 10, 1281. 20. Wolfe, N.L.; Zepp, R.G.; Baugham, G.L.; Fincher, R.C; Gordon, J.A. "Chemical and Photochemical Transformation of Selected Pesticides in Aquatic Systems", U.S. EPA-600/3-76-067, 1976, 12. 21. Zepp, R.G.; Cline, D.M. Environ. Sci. Technol. 1977, 11, 359. 22. Choudhry, G.G. "Interactions of Humic Substances with Environmental Chemicals", in The Handbook of Environmental Chemistry, ed., Hutzinger, 0., Vol. 2/Part B; Springer-Verlag: Berlin, West Germany, 1982, 103. 23. Leighton, P.A. "Photochemistr f Ai Pollution" Academi Press: New York, 1961 24. Koller, L.R. "Ultraviolet Radiation", 2nd ed.; John Wiley: New York, 1965. 25. Roof, A.A.M. "Aquatic Photochemistry" in The Handbook of Environmental Chemistry, ed. Hutzinger, O., Vol. 2/Part B; Springer-Verlag: Berlin, West Germany, 1982, 43. 26. Nestrick, T.J.; Lamparski, L.L.; Townsend, D.I. Anal. Chem. 1980, 52, 1865. 27. Bunce, N.J. Chemosphere 1978, 2, 653. RECEIVED June 24,

1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 6

Mechanism of Photolytic Ozonation 13 ,

2,4

Gary R. Peyton and William H. Glaze 1

SumX Corporation, Austin, TX 78761 Graduate Program in Environmental Sciences, University of Texas, Dallas, TX 75235

2

Photolytic Ozonation has been known for over a decade as a powerful wate treatment fo th destruc tion of organic compounds not been understood. It is shown using kinetic argu ments and data from scavenging experiments that photolysis of aqueous ozone yields hydrogen peroxide, which then reacts with further ozone in a complex series of reactions to yield hydroxyl radical. The proposed mechanism is shown to explain pH behavior of model compound destruction in earlier data. P h o t o l y t i c O z o n a t i o n (Ozone/UV) (1-4) has been i n use f o r over a decade f o r the d e s t r u c t i o n i n water o f compounds which a r e r e f r a c t o r y t o o z o n a t i o n a l o n e . O r i g i n a l l y developed f o r the d e s t r u c t i o n o f m e t a l c y a n i d e complexes (5), i t was soon shown t o be e f f e c t i v e f o r the t r e a t m e n t o f a number o f o r g a n i c compounds as w e l l ( 3 , 6 - 1 0 ) . There has been c o n s i d e r a b l e s p e c u l a t i o n (11,12) c o n c e r n i n g t h e a c t i v e s p e c i e ( s ) r e s p o n s i b l e f o r the d e g r a d a t i v e power o f p h o t o l y t i c o z o n a t i o n , b u t u n t i l r e c e n t l y (4,13,14) almost no e x p e r i m e n t a l d a t a had been o f f e r e d i n s u p p o r t o f t h a t s p e c u l a t i o n . I n t h e p r e s e n t s t u d y , the mechanism o f p h o t o l y t i c o z o n a t i o n was i n v e s t i g a t e d i n the l a b o r a t o r y by measuring ozone, hydrogen peroxi d e , and t o t a l o x i d a n t l e v e l s i n p u r i f i e d water a s a f u n c t i o n o f time i n a t w o - l i t e r s t i r r e d tank r e a c t o r i n which c o n c u r r e n t ozone s p a r g i n g and UV i r r a d i a t i o n were t a k i n g p l a c e . Experiments were r u n i n the presence and absence o f v a r i o u s r a d i c a l scavengers and t h e r e s u l t s i n t e r p r e t e d i n terms o f r e a c t i o n r a t e c o n s t a n t s t a k e n from the l i t e r a t u r e . T h i s paper w i l l d i s c u s s the mechanism o f p h o t o l y t i c o z o n a t i o n , show i t s r e l a t i o n s h i p t o o t h e r o z o n a t i o n p r o c e s s e s , and J

Current address: Aquatic Chemistry Section, Illinois State Water Survey, Department of Energy and Natural Resources, Champaign, IL 61820 'Current address: Department of Environmental Science and Engineering, School of Public Health, University of California, Los Angeles, C A 90024 0097-6156/87/0327-0076$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6. PEYTON AND GLAZE

77

Mechanism of Photolytic Ozonation

i l l u s t r a t e why v a r i a b l e and a p p a r e n t l y c o n f l i c t i n g r e s u l t s f o r p h o t o l y t i c o z o n a t i o n have appeared i n t h e l i t e r a t u r e . Background The p r o d u c t i o n o f h y d r o x y l r a d i c a l by t h e gas phase p h o t o l y s i s o f ozone i n t h e presence o f water vapor i s w e l l documented (J_5), and i s the b a s i s f o r most o f t h e s p e c u l a t i o n c o n c e r n i n g h y d r o x y l r a d i c a l p r o d u c t i o n i n aqueous s o l u t i o n . I n 1957 Taube (J_6) showed t h a t i r r a d i a t i o n o f an aqueous s o l u t i o n o f a c e t i c a c i d t h r o u g h which ozone was b u b b l i n g r e s u l t e d i n an almost s t o i c h i o m e t r i c and l i n e a r ( w i t h t i m e ) p r o d u c t i o n o f hydrogen p e r o x i d e . Through t h e use o f ^ 0 t r a c e r s Taube showed t h a t h a l f o f t h e oxygen c o n t e n t o f t h e p e r o x i d e o r i g i n a t e d i n t h e water w h i l e t h e o t h e r h a l f came from t h e ozone. The p r o d u c t i o n o f hydrogen p e r o x i d e from t h e 2537Â p h o t o l y s i s o f ozone i s c o n s i s t e n t w i t h t h e observed gas-phase p h o t o c h e m i s t r y , where a t w a v e l e n g t h s s h o r t e and 0 ( D ) . I n t h e condense i n t o an 0-H bond o f water o r a b s t r a c t a hydrogen atom, a f t e r which a geminate r e c o m b i n a t i o n o f t h e h y d r o x y l r a d i c a l s c o u l d o c c u r . The c o r r e c t i n t e r p r e t a t i o n o f T a u b e s r e s u l t s was made u n c l e a r , however, by t h e f i n d i n g s o f Baxendale and W i l s o n (j_7), who demonstrated t h a t the p r o d u c t i o n o f h y d r o x y l r a d i c a l s i n t h e presence o f a c e t i c a c i d r e s u l t e d i n t h e g e n e r a t i o n o f hydrogen p e r o x i d e . S i n c e Taube and Bray ( J J 3 ) had a l r e a d y p u b l i s h e d e v i d e n c e t h a t ozone and hydrogen p e r o x i d e r e a c t e d t o produce h y d r o x y l r a d i c a l , i t c a n be seen from the l i t e r a t u r e p r i o r t o 1958 t h a t h y d r o x y l r a d i c a l i s t h e p r i n c i p a l a c t i v e s p e c i e s i n p h o t o l y t i c o z o n a t i o n , r e g a r d l e s s o f whether t h e p r i m a r y s t e p produces hydrogen p e r o x i d e o r h y d r o x y l r a d i c a l . However, t h e a c t u a l r e s u l t s o b t a i n e d i n t h e presence o f o t h e r subs t a n c e s c o u l d v a r y c o n s i d e r a b l y depending on s o l u t i o n c o m p o s i t i o n , r e l a t i v e c o n c e n t r a t i o n s o f i n t e r m e d i a t e p r o d u c t s , and i n p a r t i c u l a r , the n a t u r e o f t h e i n i t i a l p h o t o c h e m i c a l s t e p . 1

Ί

1

1

I n 1982, S t a e h e l i n and Hoigne (j_9) showed t h a t r e a c t i o n o f ozone w i t h peroxy a n i o n ( E q u a t i o n 1 ) , t h e c o n j u g a t e base o f hydrogen p e r o x i d e ( E q u a t i o n 2 ) , was i n many cases f a s t e r than w i t h t h e u n d i s s o c i a t e d p e r o x i d e o r t h e b a s e - c a t a l y z e d d e c o m p o s i t i o n o f ozone, even when o n l y v e r y s m a l l amounts o f p e r o x i d e a r e p r e s e n t . H0 ~ + H 0 2

2

2

O 3 - • H0 H0 " + H

OQ

->

Î

(1) (2)

2

+

2

Recent papers by B u h l e r , S t a e h e l i n , and Hoigne (20) and by Sehested, Holcman, B j e r g b a k k e and Hart (21,22) p r o v i d e d v a l u e s o f r a t e c o n s t a n t s f o r r e a c t i o n s between OH and O3 ( E q u a t i o n 3 ) , H 0 and O3 ( E q u a t i o n 4) t h e f o r w a r d and r e v e r s e r e a c t i o n s i n t h e p r o t o n a t i o n o f o z o n i d e i o n ( E q u a t i o n 5 ) , and t h e d e c o m p o s i t i o n o f HO3 t o y i e l d h y d r o x y l r a d i c a l , E q u a t i o n 6. 2

OH + 0 -> H 0 + 0 H 0 + O3 -> HO + 2 0 H* + O3- t HO3 HO3 HO + 0 3

2

2

2

2

2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(3) (4) (5) (6)

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

In a d d i t i o n , S t a e h e l i n and Hoigne ( 2 3 - 2 5 ) , from i n f o r m a t i o n i n the r a d i o c h e m i c a l l i t e r a t u r e , have p r o v i d e d a c l e a r ( a l t h o u g h i n p r a c t i c e , c o m p l i c a t e d ) p i c t u r e o f the r a d i c a l c y c l e i n the s o - c a l l e d " i n d i r e c t " o z o n a t i o n r e a c t i o n s , whereby s u p e r o x i d e i s a l s o formed from t h e r e a c t i o n o f h y d r o x y l r a d i c a l w i t h o r g a n i c compounds i n t h e presence o f oxygen. In 1 9 8 3 , Peyton and Glaze ( J _ 3 ) p r e s e n t e d e x p e r i m e n t a l e v i d e n c e which showed t h a t ozone p h o t o l y s i s a t 254 nm i n aqueous s o l u t i o n r e s u l t e d d i r e c t l y i n the p r o d u c t i o n o f hydrogen p e r o x i d e . Further s t u d i e s ( 4 , 1 4 ) , and r e e x a m i n a t i o n o f o l d e r d a t a (3) have c o n f i r m e d these c o n c l u s i o n s . The r e s u l t i n g mechanism f o r p h o t o l y t i c o z o n a t i o n i s d i s c u s s e d below. Experimental The apparatus used f o r ozone p h o t o l y s i s s t u d i e s was s i m i l a r t o t h a t described previously (1,4) s t i r r e d tank r e a c t o r wa and bottom heads c u t from 1/4 t h i c k PTFE s h e e t . The r e a c t o r con t a i n e d f o u r b a f f l e s and a s i x - b l a d e p a d d l e - t y p e i m p e l l e r , made from aluminum u s i n g the s t a n d a r d CSTR r e l a t i v e d i m e n s i o n s (6,26,27). A l l t u b i n g and f i t t i n g s were g l a s s or PTFE, so t h a t no f e r r o u s m a t e r i a l s came i n c o n t a c t w i t h ozone o r l i q u i d . Four q u a r t z UV lamp s h e a t h s passed v e r t i c a l l y through the r e a c t o r , e x t e n d i n g through b o t h the top and bottom r e a c t o r heads. The sheaths were s e a l e d i n t o the heads u s i n g v i t o n o - r i n g s . A l l work r e p o r t e d i n t h i s paper was done u s i n g low p r e s s u r e mercury lamps (model no. G8T5, American U l t r a v i o l e t , Chatham, N . J . ) . These lamps a r e r a t e d a t 8 W power consumption w i t h 1.6 W o f 2537Î UV output a t 100 hours o f l i f e , a c c o r d i n g t o the manufacturer. As the lamps had been used i n c o n s i d e r a b l e p r e v i o u s e x p e r i m e n t a t i o n , i n t e n s i t i e s were measured r a d i o m e t r i c a l l y and averaged c y l i n d r i c a l l y around the lamp, g i v i n g a UV power output o f 0 . 4 3 3 W. S t i r r i n g speed was s e t a t 550 rpm u s i n g a phototachometer. T h i s speed was found by Yocum (£6) and P r e n g l e {6) to g i v e good mass t r a n s f e r performance i n l a r g e r r e a c t o r s of t h i s t y p e . Ozone i n the aqueous phase was a n a l y z e d by the i n d i g o method o f Bader and Hoigne (28,29), u s i n g the d i s u l f o n a t e r a t h e r than the t r i s u l f o n a t e as o r i g i n a l l y d e s c r i b e d by those a u t h o r s . T h i s method ( h e r e a f t e r c a l l e d the HBI method) was c a l i b r a t e d i n p u r i f i e d water a g a i n s t the i o d i m e t r i c method o f Flamm (^0) (BKI) and checked by UV absorbance u s i n g the e x t i n c t i o n c o e f f i c i e n t o f Hart e t a l . (31)» The i o d i m e t r i c method was, i t s e l f , c a l i b r a t e d by q u a n t i t a t i v e i o d i n e l i b e r a t i o n u s i n g excess i o d i d e and s t a n d a r d i o d a t e s o l u t i o n , prepared u s i n g d r i e d i o d a t e as a p r i m a r y s t a n d a r d . Ozone i n the gas phase was measured by UV absorbance, c a l i b r a t e d a g a i n s t the wet methods by a b s o r b i n g the gas i n r e a g e n t s o l u t i o n c o n t a i n e d i n the reactor. Hydrogen p e r o x i d e was measured c o l o r i m e t r i c a l l y by c o m p l e x a t i o n w i t h T i ( I V ) ("TI4" method) (32) or by the method o f M a s s c h e l e i n e t a l . (J33) (MDL method). As ozone appears t o i n t e r f e r e n e g a t i v e l y w i t h hydrogen p e r o x i d e measurement u s i n g the TI4 method, ozone was q u i c k l y and v i g o r o u s l y sparged from s o l u t i o n b e f o r e p e r o x i d e measurements were made. The MDL method was not used on o z o n e - c o n t a i n i n g s o l u t i o n s . T o t a l o x i d a n t s were measured i o d i m e t r i c a l l y by the method o f Flamm (^0 ), but w i t h the a d d i t i o n o f a

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6.

PEYTON AND GLAZE

Mechanism of Photolytic Ozonation

79

s m a l l q u a n t i t y o f ammonium molybdate t o c a t a l y z e the r e a c t i o n w i t h p e r o x i d e s (BKI/M method). Water was p u r i f i e d by a M i l l i p o r e or e q u i v a l e n t ion-exchange/ c a r b o n f i l t r a t i o n system, a f t e r which i t was d i s t i l l e d from a l k a l i n e permanganate. T y p i c a l TOC l e v e l s o f the f i n a l p r o d u c t water were 60-100 ug/L as measured by a Dohrman DC-54 l o w - l e v e l carbon analyzer. Other c h e m i c a l s were o f r e a g e n t grade and were used w i t h o u t further purification. Results In a t y p i c a l e x p e r i m e n t , ozone, hydrogen p e r o x i d e and t o t a l o x i d a n t s were measured as a f u n c t i o n o f t i m e , w h i l e ozone was bubbled i n t o the r e a c t o r which c o n t a i n e d the l a m p ( s ) . F i g u r e 2 shows p e r o x i d e a c c u m u l a t i o n f o r these e x p e r i m e n t s as a f u n c t i o n o f pH. Figure 3 compares r e s u l t s o b t a i n e a c i d . I n F i g u r e 4 a r e show where a c e t i c a c i d was r e p l a c e d by sodium b i c a r b o n a t e s o l u t i o n . The r e s u l t s o f s i m i l a r experiments where ozone i s bubbled i n t o d i s t i l l e d water a r e shown i n F i g u r e 5. I n a l l o f t h e r e s u l t s shown, the ozone dose r a t e ( i n l e t stream) was 4 mg/L-min o r 8.3 x 10"5 mol/L-min w h i l e t h e UV f l u x i n t o t h e r e a c t o r was 0.22 W/L o r 2.8 χ 10-5 E/L-min. In a d d i t i o n t o the p h o t o l y t i c o z o n a t i o n e x p e r i m e n t s d e s c r i b e d above, hydrogen p e r o x i d e p h o t o l y s i s e x p e r i m e n t s were performed i n the presence and absence o f h y d r o x y l r a d i c a l s c a v e n g e r s . These e x p e r i m e n t s were r u n u s i n g d i s t i l l e d water (a) as i t came from the l a b r e s e r v o i r , (b) a f t e r o x y g e n a t i o n , and ( c ) a f t e r s p a r g i n g w i t h n i t r o g e n t o remove oxygen. The r e s u l t s a r e shown i n T a b l e I , i n the form o f the o b s e r v e d r a t e c o n s t a n t s .

Table I .

Observed Hydrogen P e r o x i d e P h o t o l y s i s Rate C o n s t a n t s i n t h e Absence and Presence o f Scavengers 1

Solution conditions Deaerated "As i s " * Oxygenated

Apparent r a t e c o n s t a n t , m i n " χ 1p3 No 0.1 M 0.1 M scavenger bicarbonate acetic acid 10.5 9.8 8.2

9.8 10.2 9.6

3.8 2.8 K6

* Meaning n e i t h e r oxygenated nor d e a e r a t e d

Discussion Two cases w i l l be c o n s i d e r e d : I) Ozone p h o t o l y s i s i n aqueous s o l u t i o n r e s u l t s i n the d i r e c t p r o d u c t i o n o f hydrogen p e r o x i d e

O3 + H 0 2

hv —>

0

+ 2

H 0 2

2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(7)

80

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

CSTR idb

0 =O X Y G E N OG = O Z O N E

D = OZONE GENERATOR

DETECTOR

R = REFERENCE

V = VENT

S = SAMPLE

TK = T H E R M A L

OZONE

KILL

UNIT

RV = R O T A R Y

F = ROTAMETER

SIDE

SIDE VALVE

CSTR = STIRRED S = SAMPLE

TANK

REACTOR

POINT

F i g u r e 1. L a b o r a t o r y o z o n a t i o n system.

ol Ο

ι

ι

ι

ι

ι

I0

20

30

40

50

T I M E ,

F i g u r e 2.

min

E f f e c t o f pH on hydrogen p e r o x i d e a c c u m u l a t i o n . T o t a l HOAc/OAc" c o n c e n t r a t i o n i s 0.015 M.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Mechanism of Photolytic Ozonation

PEYTON AND GLAZE

\-

Ο

0.015 M H 0 A c / N a 0 A c , p H 5

Φ

0.015 M H S 0 / N a S 0 4

3

0.015 M H S 0 / N a S 0

2

4

2

.pH 5

2

4

2

4

,pH 3

ol 0

9

18

27

36

45

TIME, min F i g u r e 3.

F i g u r e 4.

Hydrogen p e r o x i d e a c c u m u l a t i o n a c e t i c and s u l f u r i c a c i d s .

i n the presence o f

Ozone p h o t o l y s i s i n t h e presence o f b i c a r b o n a t e .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

82

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

F i g u r e 5.

O x i d a n t c o n c e n t r a t i o n s d u r i n g ozone p h o t o l y s i s i n d i s t i l l e d water.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6.

83

Mechanism of Photolytic Ozonation

PEYTON AND GLAZE

II) Ozone p h o t o l y s i s d i r e c t l y produces two h y d r o x y l r a d i c a l s which a r e c a p a b l e o f a v o i d i n g r e c o m b i n a t i o n and l e a v i n g the s o l v e n t cage. hv O3

+ H0

0

2

(8)

+ 2-OH

2

The case where E q u a t i o n 8 o c c u r s but geminate r e c o m b i n a t i o n i s immediate and complete i s i n d i s t i n g u i s h a b l e from Case I . There may a l s o e x i s t an i n t e r m e d i a t e case where E q u a t i o n 8 o c c u r s , a f t e r which some f r a c t i o n o f t h e h y d r o x y l r a d i c a l s escape w h i l e t h e remainder recombine t o form hydrogen p e r o x i d e . As i n f e r r e d i n an e a r l i e r s e c t i o n , the p e r o x i d e a c c u m u l a t i o n seen i n F i g u r e 2 may be the r e s u l t o f d i r e c t p e r o x i d e p r o d u c t i o n (Case I ) or a product o f h y d r o x y l r a d i c a l (Case I I ) r e a c t i o n w i t h a c e t i c a c i d . Replacement o f a c e t i c a c i d by another a c i d which i s not a r a d i c a l scavenger ( F i g u r a c c u m u l a t i o n , seemingly i n d i c a t i n t e c t e d a c c u m u l a t i n g p e r o x i d e from h y d r o x y l r a d i c a l (produced by E q u a t i o n s 1 , 2, 5 and 6 ) , t h e r e b y p r e v e n t i n g E q u a t i o n 9. •OH

+ H 0 2

2

H0

(9)

+ Η0 ·

2

2

A l t h o u g h i n the Case I I v e r s i o n , one mechanism f o r p e r o x i d e produc­ t i o n ( r e a c t i o n o f h y d r o x y l r a d i c a l w i t h a c e t i c a c i d ) i s removed, d i r e c t r e c o m b i n a t i o n i n the b u l k s o l u t i o n i s s t i l l a p o s s i b i l i t y 2-0H

H 0 2

(10)

2

However, the h i g h c o n c e n t r a t i o n s o f ozone p r e s e n t i n s o l u t i o n (com­ pared t o h y d r o x y l r a d i c a l ) makes i t u n l i k e l y t h a t t h i s o c c u r s t o a v e r y g r e a t e x t e n t , s i n c e the h y d r o x y l r a d i c a l would be scavenged by the swamping c o n c e n t r a t i o n o f ozone. Replacement o f a c e t i c a c i d by another h y d r o x y l r a d i c a l scavenger, b i c a r b o n a t e , r e s u l t e d i n no a c c u m u l a t i o n o f p e r o x i d e ( F i g u r e 4 ) , s e e m i n g l y s u p p o r t i n g Case I I . Yet the s t o i c h i o m e t r y o f the p e r o x i d e p h o t o l y s i s experiments ( T a b l e I ) suggests t h a t the scavenging product, carbonate r a d i c a l anion (Equation 11), •OH

+

HCO3"

co

" 3~

+

H

( 1 Ί



)

r e a c t s w i t h hydrogen p e r o x i d e . T h i s was indeed r e p o r t e d by Weeks and Rabani (34) and the r a t e c o n s t a n t l a t e r r e p o r t e d by Behard, C z a p s k i and Duchovny (35) t o be 8 χ 10^ NT s"" , who u n f o r t u n a t e l y d i d not s p e c i f y t h e p r o d u c t s o f t h a t r e a c t i o n . However, E q u a t i o n 12 i s a l i k e l y p o s s i b i l i t y and g i v e s s t o i c h i o m e t r y c o n s i s t e n t w i t h t h e hydrogen p e r o x i d e p h o t o l y s i s d a t a i n Table I . 1

H 0 2

2

+ -C0 " 3

HCO3-

1

+ Η0 · 2

(12)

In view o f t h i s r e a c t i o n , the l a c k o f p e r o x i d e a c c u m u l a t i o n i n F i g u r e 4 cannot be t a k e n t o r u l e out Case I . The most m e a n i n g f u l d a t a seems t o come from the d i s t i l l e d water e x p e r i m e n t s , F i g u r e 5. I f h y d r o x y l r a d i c a l were produced d i r e c t l y , then a t s t e a d y - s t a t e (about 35-50 minutes i n F i g u r e 5) the r a t i o o f

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

84

h y d r o x y l r a d i c a l r e a c t i o n w i t h ozone, hydrogen p e r o x i d e , h y d r o x y l r a d i c a l r e s p e c t i v e l y , would be k k

3

1 0

[0 ] 3

[OH]

m

k

[H 0 ] . k

9

2

2

kfôTÔH]"

1 0

k

[OH]

1.7

m

x 10~7

[OH]

1 0

[OH]

^ 4.5

and

χ 10"

8 #

"""TÔHÎ 1 2

S i n c e h y d r o x y l r a d i c a l c o n c e n t r a t i o n s a r e more t y p i c a l l y 1 0 " M, r e a c t i o n of h y d r o x y l r a d i c a l w i t h i t s e l f i s seen t o account f o r o n l y 10"5 of the h y d r o x y l r a d i c a l r e a c t i o n s . The product of both E q u a t i o n s 3 and 9 i s Η 0 · , which upon d i s s o c i a t i o n g i v e s s u p e r o x i d e , 2

Η0 · J 2

H

+

+ 0-

,

2

(13)

which, i n t u r n , r e a c t s w i t h ozone ( E q u a t i o n 14) more q u i c k l y than i t d i s p r o p o r t i o n a t e s t o p e r o x i d e ( E q u a t i o n 15) O

H0 2

+ H0

I t thus seems u n l i k e l y t h a t an a p p r e c i a b l e amount o f p e r o x i d e c o u l d be formed i n Case I I i n the presence o f so much ozone and hydrogen peroxide. I t i s t h e r e f o r e concluded t h a t the f i r s t s t e p i n p h o t o l y ­ t i c o z o n a t i o n i s the p r o d u c t i o n o f hydrogen p e r o x i d e ( E q u a t i o n 7 ) , f o l l o w e d by r e a c t i o n o f i t s a n i o n w i t h ozone ( E q u a t i o n s 1 and 2) and ensuing E q u a t i o n s 14, 5, and 6. S i n c e d i s s o c i a t i o n o f the H0 formed i n E q u a t i o n 1 a l s o r e s u l t s i n the p r o d u c t i o n o f s u p e r o x i d e , a second h y d r o x y l r a d i c a l i s a l s o formed from the sequence o f E q u a t i o n s 14, 5, and 6 , w i t h a net consumption o f t h r e e ozone molecules. In the presence o f an o r g a n i c compound (HMH), however, h y d r o x y l r a d i c a l s r e a c t t o g i v e an o r g a n i c r a d i c a l ( E q u a t i o n 16) 2

HMH

+ -OH

~>

H0 2

+ HM-

(16)

which i n the presence o f oxygen can q u i c k l y add oxygen ( E q u a t i o n 17, Swallow, r e f . 3£) then i n many c a s e s , l o s e s u p e r o x i d e ( E q u a t i o n 18) t o y i e l d a n e u t r a l o x i d i z e d o r g a n i c m o l e c u l e , M: HM- +0 ΗΜ0 · ~> 2

2

--> ΗΜ0 · H + M + 0" 2

+

2

(17) (18)

Thus, when HMH i s methanol, M i s formaldehyde. When HMH i s f o r m a l ­ dehyde ( h y d r a t e d t o H0CH 0H), M i s f o r m i c a c i d , which s i m i l a r l y l e a d s t o carbon d i o x i d e . S i n c e t h i s sequence produces s u p e r o x i d e , i t i s not necessary t o p h o t o l y z e another ozone m o l e c u l e t o y i e l d an e l e c t r o n - t r a n s f e r reagent (H0 ") f o r h y d r o x y l r a d i c a l p r o d u c t i o n , and the h y d r o x y l r a d i c a l y i e l d per ozone molecule consumed approaches u n i t y . T h i s c y c l i c scheme has been suggested i n the con­ t e x t o f o t h e r o z o n a t i o n r e a c t i o n s by S t a e h e l i n and Hoigne (23-25) and the f l o w diagram analogous t o t h a t g i v e n by those i n v e s t i g a t o r s i s shown i n F i g u r e 6 f o r the p h o t o l y t i c o z o n a t i o n system, thus u n i ­ f y i n g i t w i t h the body o f knowledge c o n c e r n i n g aqueous ozone chemi­ s t r y which has been e l u c i d a t e d by Hoigne and coworkers. 2

2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PEYTON AND GLAZE

F i g u r e 6.

Mechanism of Photolytic Ozonation

C h a i n r e a c t i o n i n i t i a t e d by p h o t o l y t i c

ozonation.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

86

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

Attempts t o use f u l l k i n e t i c c a l c u l a t i o n s t o more q u a n t i t a t i v e l y c o n f i r m t h e i n i t i a t i o n s t e p i n p h o t o l y t i c o z o n a t i o n encounter d i f f i c u l t y because o f t h e s e n s i t i v i t y o f the r e s u l t s t o t h e twenty or s o l i t e r a t u r e v a l u e s o f r a t e c o n s t a n t s which must be used i n t h e c a l c u l a t i o n . A d d i t i o n a l c o n f i r m a t i o n was thus sought from o t h e r e x perimental r e s u l t s . P r e v i o u s l y p u b l i s h e d (3) d a t a on t h e d e s t r u c t i o n o f c h l o r o f o r m , t e t r a c h l o r o e t h y l e n e and bromodichloromethane by ozone and p h o t o l y t i c o z o n a t i o n show t h a t a l l t h r e e compounds a r e d e s t r o y e d more r a p i d l y by ozone a t h i g h e r pH, i n agreement w i t h t h e a c c e p t e d base c a t a l y z e d ozone d e c o m p o s i t i o n mechanism (JM9). Photol y t i c o z o n a t i o n o f a l l t h r e e compounds, however, i s c o n s i d e r a b l y f a s t e r a t lower pH. T h i s would n o t be expected i f E q u a t i o n 8 were t h e i n i t i a t i o n s t e p . Furthermore, t h e d i s s o c i a t i o n o f both H 0 t o H0 ~ and H 0 t o 0 ~ , f a v o r e d a t h i g h e r pH, would appear t o enhance the r a t e o f p h o t o l y t i c o z o n a t i o n , s u g g e s t i n g t h a t t h e p r o t o n a t i o n o f o z o n i d e i o n ( E q u a t i o n 5 ) , and thus OH p r o d u c t i o n i s being' f a c i l i tated i n acidic solution and pH b e h a v i o r f o r c h l o r o f o r m chloromethane a r e c o n s i s t e n t w i t h t h e v a l u e o f 6.15 f o r t h e d i s s o c i a t i o n c o n s t a n t o f HOo, r e p o r t e d by B u h l e r , S t a e h e l i n , and Hoigne (20). 2

2

2

2

2

Conclusions

1 ) The photolysis of ozone in aqueous solution results in the direct production of hydrogen peroxide. 2) These findings unify photolytic ozonation with the existing body of knowledge concerning aqueous ozonation, as elucidated by Hoigne and coworkers. 3) After initiation by peroxide production, the reaction may proceed with considerably lowered UV input, due to the chain reaction which is carried by superoxide (Figure 6). 4) Results equivalent to those from photolytic ozonation should be obtainable using hydrogen peroxide addition, provided that direct excitation of substrate or intermediates is not important for the substrate in question. Acknowledgments This work was funded in part by cooperative agreement CR808825 with the United States Environmental Protection Agency, Drinking Water Research Division, Water Engineering Research Laboratory, Cincinnati, OH. The understanding and cooperation of the project officer, Mr. J. Keith Carswell, was greatly appreciated. Literature Cited 1. Peyton, G. R.; Huang, F. Y.; Burleson, J. L.; Glaze, W. H. Environ. Sci. Technol. 1982, 16, 448. 2. Glaze, W. H.; Peyton, G. R.; Lin, S.; Huang, F. Y.; Burleson, J. L. Environ. Sci. Technol. 1982, 16, 454. 3. Glaze, W. H.; Peyton, G. R.; Huang, F. Y.; Burleson, J. L.; Jones, P. C. "Oxidation of Water Supply Refractory Species by Ozone with Ultraviolet Radiation." Final Report, EPA-600/2-80-110, August, 1980.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

6. PEYTON AND GLAZE Mechanism of Photolytic Ozonation

87

4. Glaze, W. H.; Peyton, G. R.; Sohm, B.; Meldrum, D. A. "Pilot Scale Evaluation of Photolytic Ozonation for Trihalomethane Precursor Removal." Final Report to USEPA/DWRD/MERL, Cincinnati, OH, Cooperative Agreement #CR-808825, J. Keith Carswell, Project Officer. 5. Garrison, R. L.; Mauk, C. E.; Prengle, H. W., Jr. Proc. 1st International Symposium on Ozone for Water and Wastewater Treatment; Rice, R. G.; Browning, Μ. Ε., editors; International Ozone Institute, Syracuse, N.Y., 1975. 6. Prengle, H. W., Jr.; Hewes, C. G. III; Mauk, C. E. Proc. 2nd International Symposium on Ozone Technology; Rice, R. G.; Richet, P.; Vincent, Μ. Α., Eds.; International Ozone Institute, Syracuse, NY, 1975; p. 211. 7. Prengle, H. W., Jr.; Mauk, C. E.; Payne, J. E. Proc. Interna­ tional Ozone Institute Forum on Ozone Disinfection, Chicago, Illinois, June 2-4, 1976. 8. Fochtman, E. G.; Huff International Symposiu Ozone Technology, 2nd 9. Lee, M. K.; See, G. G.; Wynveen, R. A. Proc. Symposium on Advanced Ozone Technology, Toronto, Ontario, Nov. 16-18, 1977. 10. Kuo, P. P. K.; Chian, E. S. K.; Cheng, B. J. Environ., Sci. Technol. 1977, 11, 1177. 11. McCarthy, J. J.; Cowen, W. F.; Chian, E. S. K. Proc. 32nd Industrial Waste Conference, Purdue University, 1977, p. 310. 12. Prengle, H. W., Jr. Environ. Sci. Technol. 1983, 17, 743. 13. Peyton, G. R.; Glaze, W. H. Presented at the Sixth World Ozone Congress, held by the International Ozone Association, Washington, D.C., May 23-26, 1983. 14. Peyton, G. R.; Glaze, W. H. Presented at the Symposium on Aquatic Photochemistry, 189th Annual Meeting of the American Chemical Society, April-1985. 15. Okabe, H. "Photochemistry of Small Molecules"; Wiley-Interscience: New York, 1978, p. 244. 16. Taube, H. Trans. Farad. Soc. 1957, 53, 656. 17. Baxendale, J. H.; Wilson, J. A. Trans. Farad. Soc. 1957, 53, 344. 18. Taube, H.; Bray, W. C. J. Am. Chem. Soc. 1940, 62, 3357. 19. Staehelin, J.; Hoigne, J. Environ. Sci. Technol. 1982, 16, 676. 20. Buhler, R. F.; Staehelin, J.; Hoigne, J. J. Phys. Chem. 1984, 88, 2560. 21. Sehested, K.; Holcman, J.; Bjergbakke, Ε.; Hart, E. J. J. Phys. Chem. 1984, 88, 4144. 22. Sehested, K.; Holcman, J.; Bjergbakke, E.; Hart, E. J. J. Phys. Chem. 1984, 88, 269. 23. Hoigne, J.; Staehelin, J.; Buhler, R. "Rates of Reactions and Decomposition of Ozone in Water: A Few Reasons for Many Miscon­ ceptions," presented at the Sixth World Ozone Conference, held by the International Ozone Association, Washington, D.C. May 23-26, 1983. 24. Staehelin, J.; Hoigne, J. Vom Wasser, 1983, 61, 337. 25. Staehelin, J.; Hoigne, J. Environ. Sci. Technol. 1985, 19, 1206. 26. Yocum, F. H. Paper presented to 86th National Meeting ©f American Institute of Chemical Engineers, April 1-5, 1979.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

88

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

27. Horak, J., and J. Pasek. "Design of Industrial Chemical Reactors from Laboratory Data"; Heyden and Sons, Inc., Philadelphia, 1978, p. 358. 28. Bader, H.; Hoigne, J. Water Res. 1981, 15, 449. 29. Bader, H.; Hoigne, J. Ozone: Science and Engineering 1982, 4, 169. 30. Flamm, D. L. Environ. Sci. Technol. 1977, 11, 978. 31. Hart, E.; Sehested, K.; Holcman, J. Anal. Chem. 1983, 55, 46. 32. Parker, G. A. In "Colorimetric Determination of Nonmetals"; Boltz, D. R.; Howell, J. Α., Eds.; Wiley, 1928, p. 301. 33. Masschelein, W. J.; Davis, M.; Ledent, R. Water and Sewage Works, August 1977, 69. 34. Weeks, J. L.; Rabani, J. J. Phys. Chem. 1966, 70, 21. 35. Behard, D.; Czapski, G.; Duchovny, J. J. Phys. Chem. 1970, 74, 226. 36. Swallow, A. J. Prog. Reaction Kinetics, 1978, 9, 195. RECEIVED May 27, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 7

Consequences of OH Radical Reaction in Sea Water: Formation and Decay of Br2 Ion Radical -

1

1

2

Oliver C. Zafiriou , Mary B. True , and E. Hayon 1

Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 Chemistry Department, Queens College of the City University of New York, Flushing, NY 11367

2

The interaction of OH radical with seawater yields oxidized bromine dized bromine specie subsequen have been studied by extensive flash photolysis experiments and also by pulse radiolysis. OH appears to give a nearly quantitative yield of the dibromide ion-radical in seawater. This radical decays by parallel first- and second-order reactions, the latter process being well-known but irrelevant in nature. The former involves reaction with some components of the carbonate system in seawater, resulting in decay rates for the dibromide ion-radical of roughly 20003000 s in seawater at pH 8 (halftimes of ca 300 microseconds). -1

The concept that free radicals are important intermediates in photochemical and other redox interactions of oxygen, organic compounds and heavy metals i n natural waters has received considerable support recently ((1-3) and references therein; this volume). Some of the major primary radicals expected are: hydroxyl (OH), superoxide (02~). and various organic moieties (R, RO, R00). Of these, OH i s of interest because of i t s extremely high r e a c t i v i t y , s i g n i f i c a n t formation rate from a known source ( n i t r i t e photolysis, among others) and the analogy of i t s known key role i n tropospheric chemistry. The purpose of this study i s to explore the fate of OH radicals and the identity and chemistry of their progeny i n seawater. This paper presents some of the experimental evidence concerning r a d i c a l formation and behavior i n seawater and a r t i f i c i a l seawater obtained by the fast-reaction kinetics technique of f l a s h photolysis-kinetic spectrophotometry (4) supplemented by pulse r a d i o l y s i s (5). The companion paper which follows presents results on related reactions and rates observed in media simpler than seawater and applies them to p a r t i a l l y explain the data reported here using a simple reactionmechanistic model. Known OH reaction rate constants and the major-ion composition of seawater combine to imply that OH should react i n seawater almost exclusively with bromide ion, with a small amount of side-reaction 0097-6156/87/0327-0089S06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

90

with the carbonate system and dissolved organic matter (DOM) (6). Table I estimates OH r e a c t i v i t y towards seawater components; similar conclusions were reached by Hoigné (7), although he took the f i r s t product to be B r " rather than BrOH". 2

TABLE I.

Expected Reactions of OH with Constituents of Seawater (pH 8.1, S = 35"/··)

Reactant

Concentration (M)

Br~ a

Total C0§(9.31 Free) Total HC0 (72.3X Free) 3

a

Rate Constant M-l s-1

8xl0"

4

d

3xl0"

4

e

τ of Total OH Reaction 97.7

Initial Products BrOH-

1.06xl0

10

4.2

xlO

8

1.5

CO3 + OH-

*5

xlO*

0.2

N0

2.02xl0" b

N0 -

4 x l 0

-6

2

Dissolved Organic Matter

c

2xl0" 0.55

ci-

5

610*

0.2

n

0.01

6 hours assuming t h i s i s the only sink. The r e s u l t s o f these s t u d i e s i n d i c a t e t h a t Cu(I) o x i d a t i o n by 0« i s t h e major o x i d a t i v e pathway but t h a t o x i d a t i o n by H^O^ i s also significant. Other o x i d a n t s may be i n v o l v e d , but no concentration or k i n e t i c data i s a v a i l a b l e . I n p a r t i c u l a r , 0^ may be i m p o r t a n t as an o x i d a n t here i n a d d i t i o n t o i t s r o l e as a reductant o u t l i n e d previously. These r e s u l t s i n d i c a t e t h a t a v a r i e t y o f r e a c t i o n s may l e a d t o Cu(I)/Cu(II) interconversions i n seawater. The r e s u l t s a r e summarised s c h e m a t i c a l l y i n F i g u r e 3 t o demonstrate how copper redox c y c l i n g probably occurs. The b a s i c f e a t u r e s a r e a dynamic redox c y c l e i n v o l v i n g th s p e c i e s which r e a c t w i t previously. They a r e p r o b a b l y r e a c t i v e w i t h o h t e r p h o t o c h e m i c a l l y generated oxidants and r e d u c t a n t s , such as 0^ and organic radicals. C u C l r e d u c t i o n may a l s o be i m p o r t a n t d e s p i t e i t s n e g l i g i b l e r e a c t i o n w i t h H^O^ as i t i s r a p i d l y reduced by o t h e r r e d u c t a n t s such as f e r r o u s cytochrome C ( 2 4 ) . The major Cu(I) and C u ( I I ) s p e c i e s , i n d i c a t e d i n the boxes, do not p a r t i c i p a t e i n these r e a c t i o n s but do undergo p r i m a r y p h o t o l y s i s . The above s t u d i e s i n d i c a t e that the p h o t o l y s i s o f l-3 CuCO^ a r e not s i g n i f i c a n t but p h o t o l y s i s o f C u ( I I ) o r g a n i c complexes may be, based on the model l i g a n d s . T h i s scheme i s h i g h l y q u a l i t a t i v e a t the p r e s e n t t i m e . The s p e c i a t i o n o f C u ( I ) i s p r o b a b l y d o m i n a t e d by c h l o r i d e e v e n a t n a t u r a l l e v e l s so e x t r a p o l a t i o n o f t h e l a b o r a t o r y d a t a i s v a l i d . However, f o r C u ( I I ) there i s s t i l l widespread disagreement about t h e n a t u r 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 c o n s t a n t s o f n a t u r a l o r g a n i c C u Q l ) c h e l a t o r s , r e s u l t i n g i n estimates f o r the f r a c t i o n o f f r e e Cu ( i . e . [Cu(II)] /[Cu(II)] ) which v a r y by two o r d e r s of magnitude (12,25, fg). T h i s matces i t d i f f i c u l t t o e s t i m a t e i n s i t u r a t e s o f secondary r e d u c t i o n r e a c t i o n s i n v o l v i n g minor C u ( I I ) s p e c i e s which a r e p r o p o r t i o n a l t o t h i s f r a c t i o n . And t h e r e i s no d i r e c t evidence t h a t LMCT r e a c t i o n s i n v e s t i g a t e d u s i n g model l i g a n d s a c t u a l l y o c c u r i n seawater. A further complication i s t h a t steady s t a t e c o n c e n t r a t i o n and r a t e d a t a a r e u n a v a i l a b l e f o r many p o t e n t i a l l y i m p o r t a n t r e a c t i v e s p e c i e s i n v o l v e d i n copper redox c y c l i n g , such as 0 . I n o r d e r t o determine the o v e r a l l e f f e c t t h a t these p r o c e s s e s m i g h t have w i t h o u t h a v i n g q u a n t i t a t i v e d a t a f o r e v e r y p o t e n t i a l l y i m p o r t a n t r e a c t i o n i n v o l v e d procedures were developed t o measure two i m p o r t a n t parameters i n the copper system a t n a t u r a l l e v e l s ; Cu(I) and [ C u ( I I ) ] /[Cu(II)] . ?

C u C

a n d

r

f r e e

t o t a l

Measurement o f C u ( I ) i n Seawater The f o r m a t i o n o f Cu(I) i n c o a s t a l waters exposed t o s u n l i g h t was investigated. Seawater c o l l e c t e d from B i s c a y n e Bay was s t o r e d i n t h e d a r k f o r 2 weeks and exposed t o s u n l i g h t i n round bottomed

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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quartz f l a s k s . The r e s u l t s , shown i n F i g u r e 4, i n d i c a t e t h a t photochemical f o r m a t i o n o f C u ( I ) o c c u r s and c o r r e l a t e s w i t h t h e photochemical p r o d u c t i o n o f h^C^» I n a second experiment, a l l f l a s k s were exposed t o 5 hours o f s u n l i g h t and t h e decay i n t h e Cu(I) s i g n a l monitored ( F i g u r e 5 ) . C u ( I ) f o r m a t i o n observed here i s p r o b a b l y t h e r e s u l t o f a c o m b i n a t i o n o f p r i m a r y and secondary p r o c e s s e s . Primary p r o c e s s e s may be i m p o r t a n t i n d e s t r o y i n g C u ( I I ) complexes w h i c h are i n e r t t o s e c o n d a r y r e a c t i o n s , thereby making secondary p r o c e s s e s more important. The decay o f C u ( I ) a f t e r i r r a d i a t i o n i s much slower t h a n t h e e x p e c t e d , s i n c e i t s h a l f l i f e i n t h e presence o f 0~ i s under 6 m i n u t e s . T h e r e f o r e , the presence o f r e l a t i v e l y l o n g l i v e d r e d u c t a n t s p e c i e s such as must be i n v o k e d . Artificial seawater c o n t a i n i n g u l t r a f i l t r a t e a l s o produced C u ( I ) upon s o l a r I r r a d i a t i o n , but c h l o r i d e f r e e media d i d not ( 4 ) , i n agreement w i t h k i n e t i c e x p e c t a t i o n s based on t h e e x t r e m e l y r a p i d o x i d a t i o n o f Cu(I) i n c h l o r i d e f r e e media C u ( I ) was measured i the SOLARS I I I c r u i s e . R e s u l t s from a c a s t taken a t a s t a t i o n west o f Tampa a r e shown i n F i g u r e 6. The c a s t was made a t 1400 on a sunny, f l a t calm day; o p t i m a l c o n d i t i o n s f o r photochemical activity. Hydrographie data, obtained during a CTD c a s t i m m e d i a t e l y a f t e r w a r d s i n d i c a t e d a w e l l s t r a t i f i e d water column. The p r o f i l e i n d i c a t e s t h a t Cu(I) i s p r e s e n t a t the s u r f a c e where i t c o n s t i t u t e s a b o u t 15% o f t h e t o t a l c o p p e r . The c o n c e n t r a t i o n d e c l i n e s w i t h depth and i s below t h e l i m i t o f d e t e c t i o n a t 90m. The H 0 p r o f i l e shows s i m i l a r c h a r a c t e r i s t i c s , w i t h a s u r f a c e maximum. A s t u d y o f t h e d i u r n a l v a r i a b i l i t y o f Cu(I) was made a t a c o a s t a l s i t e c o n t a i n i n g h i g h DOC, o f f t h e F l o r i d a Everglades (Station 1). The r e s u l t s i n d i c a t e v i r t u a l l y no C u ( I ) above t h e l i m i t o f d e t e c t i o n d e s p i t e h i g h daytime p e r o x i d e l e v e l s shown i n F i g u r e 7. T h i s may be a r e s u l t o f more e x t e n s i v e C u ( I I ) c h e l a t i o n by o r g a n i c matter i n t h i s h i g h l y p r o d u c t i v e c o a s t a l s i t e . Or Cu(I) l i f e t i m e s may be e x t r e m e l y s h o r t because o f a h i g h photochemical production of short l i v e d r a d i c a l oxidants. The r e s u l t s i n d i c a t e t h a t , a p a r t from t h e c o a s t a l s i t e , t h e r e i s a g e n e r a l c o r r e l a t i o n between C u ( I ) l e v e l s and H 0 c o n c e n t r a t i o n . T h i s agreement may be c o i n c i d e n t a l because o t h e r r e d u c t a n t s o f photochemical o r i g i n may have a s i m i l a r d i s t r i b u t i o n t o p e r o x i d e . 2

2

2

2

Measurement o f [ C u ( I I ) K /[Cu(II)l _ tree total Ί

i

n

Measurements o f [CixC XI) ] f /1CuC IX) ] i seawater, were c a r r i e d o u t t o c a l c u l a t e i n s i t u r a t e s o f secondary r e a c t i o n s such as t h e r e d u c t i o n o f C u ( I I ) by H 0 and t o l o o k f o r evidence o f primary photoreactions involving Cu(II) organic complexes. The a c e t y l a c e t o n e l i q u i d - l i q u i d p a r t i t i o n procedure was used t o measure [Cu(II)] /[Cu(II)] as a f u n c t i o n o f depth o f f the c o a s t o f F l o r i d a a P s t a t i o n 2 Surlng the SOLARS I I I c r u i s e . Data are shown i n F i g u r e 6, a l o n g w i t h p e r o x i d e and Cu(I) d a t a . P r o f i l e s o f the t h r e e parameters have s i m i l a r c h a r a c t e r i s t i c s . ^Cu(II)]^ / [Cu(II)] _ p r o f i l e s a r e c h a r a c t e r i s e d by a s u r f a c e maxima and d e c r e a s i n g v a l u e s w i t h depth t o r e l a t i v e l y u n i f o r m low v a l u e s from ree

2

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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Photochemistry of Copper Complexes

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F i g u r e 4. P h o t o c h e m i c a l f o r m a t i o n o f C u ( I ) i n seawater exposed t o s u n l i g h t i n February,Ο y and A p r i l , # , 1985. L i g h t f l u x v a l u e s i n b r a c k e t s a r e f o r F e b r u a r y , 1985.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

6

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TIME(HOURS)

F i g u r e 5. Dark decay of C u ( I ) s i g n a l i n seawater exposed t o 5 hours s u n l i g h t i r r a d i a t i o n .

1

7

4

[ H ^ m o l L " χ 10 ) ICudlMludl^ixlO ) 0 (fe 10 15 0 2 4 6 8 0

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F i g u r e 6. Depth p r o f i l e o f C u ( I ) , [ C u ( I I ) ] 7[Cu(II)] and . H 0„ w i t h accompanying h y d r o g r a p h i e S i f t , s t a t i o n-^ September 1985. 9

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

t o t

^

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50m t o the deepest depths sampled. There was about a t w e n t y f o l d i n c r e a s e b e t w e e n t h e deep v a l u e s a n c ^ t h e s u r f a c e v a l u e s . To d e t e r m i n e i f t h e i n c r e a s e i n f r e e Cu towards the s u r f a c e i s p h o t o c h e m i c a l l y i n d u c e d , samples o f seawater c o l l e c t e d i n Biscayne Bay were exposed t o s u n l i g h t i n q u a r t z f l a s k s m a i n t a i n e d a t 25 i n a water bath and [Cu(II)]. /[Cu(II)l ^ measured with i r r a d i a t i o n time. The r e s u l t s , i n F i g u r e 8 , d e m o n s t r a t e t h a t s u n l i g h t d o e s l e a d t o an i n c r e a s e , p r o b a b l y as a r e s u l t o f p h o t o - d e s t r u c t i o n of the l i g a n d and/or copper complex, p o s s i b l y i n v o l v i n g LMCT r e a c t i o n s . The minimum was l o c a t e d a t the base o f t h e mixed l a y e r and c o i n c i d e d w i t h what i s g e n e r a l l y the zone of maximum p r o d u c t i v i t y . An e x p l a n a t i o n f o r the observed s p e c i a t i o n t r e n d s i s the b i o l o g i c a l p r o d u c t i o n o f c h e l a t o r s i n the zone of maximum p r o d u c t i v i t y ( i . e . lower p h o t i c zone) which a r e degraded p h o t o - c h e m i c a l l y as they mix upward. Further i n v e s t i g a t i o n i s necessary to confirm t h i s hypothesis. An i n s i t u r a t e o f r o m t h i s d a t a and th o f t h e C u ( I ) m e a s u r e d c a n be a c c o u n t e d f o r by t h i s s o u r c e . T h e r e f o r e , o t h e r pathways not c o n s i d e r e d i n the s i m p l e model must be i m p o r t a n t . +

Ί

Conclusions A v a r i e t y of reactions are involved i n C u ( I ) / C u ( I I ) i n t e r c o n v e r s i o n i n seawater. S e v e r a l important r e a c t i o n s have been s t u d i e d and r a t e c o n s t a n t s determined a t e l e v a t e d copper c o n c e n t r a t i o n s . The s y s t e m cannot be c o m p l e t e l y c h a r a c t e r i z e d by t h i s d a t a base a t p r e s e n t because the nature and steady s t a t e c o n c e n t r a t i o n s o f many p o t e n t i a l l y i m p o r t a n t s p e c i e s such as 0 have not been determined. N e v e r t h e l e s s , measurement o f C u ( I ) ana [ C u ( I I ) ] /[Cu(II)l at n a t u r a l copper l e v e l s i n d i c a t e s the net e f f e c t o f t h e s e sunlîgnt i n d u c e d p r o c e s s e s has c o n s i d e r a b l e i n f l u e n c e on copper s p e c i a t i o n i n seawater. Measurement o f C u ( I ) i n d i c a t e s up t o 15% of copper i s p r e s e n t as C u ( I ) i n s u r f a c e seawater under optimum c o n d i t i o n s . D e p t h p r o f i l e c h a r a c t e r i s t i c s i n d i c a t e a s u r f a c e maxima which i s c o n s i s t e n t w i t h a p h o t o c h e m i c a l r e d u c t i o n mechanism. This i s supported by t h e measurement of C u ( I ) f o r m a t i o n i n seawater exposed to sunlight. Profiles of [ C u ( I I ) ] /[Cu(II)] i n d i c a t e that sunlight influences Cu(II) organic I n t e r a c t i o n s . ° T h i s work has i m p o r t a n t i m p l i c a t i o n s f o r the r o l e of copper i n seawater. The uptake c h a r a c t e r i s t i c s o f Cu(I) by organisms may be very d i f f e r e n t than those f o r C u ( I I ) which may a f f e c t 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 copper i n seawater. The d e t e c t i o n of a twenty f o l d i n c r e a s e i n f r e e Cu between 50m and the s u r f a c e i s i m p o r t a n t as numerous i n v e s t i g a t i o n s have e s t a b l i s h e d t h a t t o x i c i t y i s r e l a t e d t o f r e e Cu and n o t t o t a l . This study demonstrates t h a t 2 2 ' g e n e r a l l y c o n s i d e r e d as an o x i d a n t , a l s o f u n c t i o n s as a r e d u c t a n t i n some systems, such as t h i s one. Hydrogen p e r o x i d e i s g e n e r a l l y u n r e a c t i v e w i t h o r g a n i c compounds u n l e s s c a t a l y s e d by t r a n s i t i o n metals and t h e r e f o r e r e a c t i o n w i t h t r a n s i t i o n metals c o u l d be an i m p o r t a n t d e g r a d a t i v e pathway. The l o w ambient c o n c e n t r a t i o n s of t r a n s i t i o n metals i n seawater could e x p l a i n the long l i f e t i m e of Η 0 i n seawater. 9

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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

f

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However, r e d u c t i o n o f C u ( I I ) i s u n l i k e l y t o be i m p o r t a n t i n H , ^ d e g r a d a t i o n because o x i d a t i o n of the r e s u l t a n t C u ( I ) l e a d s back to 2°2 °2 m o The o x i d a t i o n o f C u ( I ) by H^O i s p r o b a b l y not i m p o r t a n t e i t h e r because o f the low Cu(I) c o n c e n t r a ­ t i o n s i n seawater; c a l c u l a t i o n s i n d i c a t e t h a t would have a h a l f l i f e o f a t l e a s t 100 days i f t h i s was t h e s o l e d e g r a d a t i v e pathway. T h i s i s c o n s i d e r a b l y l o n g e r than p r e s e n t e s t i m a t e s . H

v

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n

Acknowledgment T h i s work was s u p p o r t e d by t h e O f f i c e o f N a v a l c o n t r a c t N00014-85C-0020.

Research

under

Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Sunda, W.G.; Guillard, R.R.L. J. Mar. Res. 1976, 34, 511-9. Sunda, W.G. and Ferguson R.F I "Trac Metal i Seawater" Wong, C.S. Ed.; Plenum Moffett, J.W.; Zika, R.G.; Petasne, R.G. Anal. Chim. Acta. 1985, 175, 171-9. Moffett, J.W. and R.G. Zika. Determination of Cu(I) at subnanomolar levels in seawater. (submitted) Moffett, J.W., and R.G. Zika. Solvent extraction of copper acetylacetonate in studies of copper speciation in seawater. (submitted)· Balzani, V.; Carassiti, V. "Photochemistry of Coordination Compounds"; Academic: London, 1970. 430pp. Ferraudi, G.; Muralidharan, S. Coord. Chem. Rev. 1981, 36, 45-88. Langford, C.H.; Wingham, M.; Sastri, V.S. Environ. Sci. Tech. 1973, 7, 820-2. Mantoura, R.F.C.; Dickson, Α.; Riley, J.P. Estuar. Coast. Mar. Sci. 1978, 6, 387-408. Mills, G.L.; Hanson, A.K.; Quinn, J.G.; Lammela, W.R.; Chasteen, N.D. Mar. Chem. 1982, 11, 3-55-78. Gamble, D.S.; Underdown, A.W.; Langford, C.H. Anal. Chem. 1980, 52, 1901-8. Zuehlke, R.W.; Kester, D.R. In "Trace Metals in Seawater"; Wong, C.S., Ed.; Plenum: New York, 1983; pp. 773-88. Zika, R.G. In "Marine Organic Chemistry"; Duursma, E.K.; Dawson, R. Ed.; Elsevier: Amsterdam, 1981; pp. 299-325. Moffett, J.W.; Zika, R.G. Mar. Chem. 1983, 13, 239-251. Zika, R.G.; Moffett, J.W.; Cooper, W.J.; Petasne R.G.; Saltzman, E.S. Geochim. Cosmochim. Acta. 1985, 49, 1173-84. Gray, R.D. J. Amer. Chem. Soc. 1969, 91, 56-62. Sigel, H.; Flierl, C; Griesser, R. J. Amer. Chem. Soc. 1969, 91, 1061-4. Davies, G.; Higgins R.; Loose, D.J. Inorg. Chem. 1976, 15, 700-3. Otto, M.; Lerchner, J.; Pap, T.; Zuanziger, H.;Hoyer, E.; Inczedy, J.; Werner, G. J. Inorg. Nucl. Chem. 1981, 43, 1101-5. Moffett, J.W.; Zika, R.G. Reaction of hydrogen peroxide with copper and iron in seawater. (submitted)

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21. 22. 23. 24. 25. 26.

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Klug-Roth, D.; Rabani, J. J. Phys. Chem. 1976, 80, 588-91. Zafiriou, O.C.; Joussot-Dubien, J.; Zepp, R.G.; Zika, R.G. Environ, Sci. Tech. 1984, 18, 358A-71A. Zepp, R.G.; Cline, D.M. Environ. Sci. Tech. 1977, 11, 359-66. Yandell, J.K. Aust. J. Chem. 1981, 34, 99-106. Van den Berg, C.M.G. Mar. Chem. 1984, 15, 1-18. Waite, T.D.; Morel, F.M.M. Anal. Chem. 1983, 55, 1268-74.

RECEIVED June 24,

1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 10

Time-Resolved Fluorescence Measurements on Dissolved Marine Organic Matter P. J. Milne, D. S. Odum, and Rod G. Zika Rosenstiel School of Marine and Atmospheric Sciences, Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL 33149

The fluorescenc organic matter origi been measured. The lifetimes were all closely similar (2-2.4 ns) despite their widely different origins and histories. Implications of this similarity to other physical and chemical properties of humic/fulvic substances is discussed as is the interpretation of the deactivation pathways of the excited states of marine chromophores. Q u a n t i t a t i v e measurement o f t h e p h o t o p h y s i c s o f t h e d i s s o l v e d o r g a n i c chromophores found i n n a t u r a l water i s needed t o b e t t e r u n d e r s t a n d the r a t e s o f energy t r a n s f e r from t h e i r e x c i t e d s t a t e s and t o p r o v i d e i n f o r m a t i o n as t o t h e k i n e t i c s o f e x c i t e d s t a t e r e a c t i o n s they may undergo. S p e c i f i c areas o f i n t e r e s t i n n a t u r a l w a t e r c h e m i s t r y a r e t h e k i n e t i c s o f quenching i n t e r a c t i o n s and energy t r a n s f e r p r o c e s s e s t o o t h e r c h e m i c a l s p e c i e s ; t h e e f f e c t o f such i n t e r a c t i o n s on t h e i n i t i a t i o n o f secondary photochemical r e a c t i o n s i s p a r t i c u l a r l y important ( I ) . These measurements may a l s o a i d i n t h e c h a r a c t e r i z a t i o n o f t h e p h y s i c a l and c h e m i c a l p r o p e r t i e s o f t h e humic and f u l v i c a c i d f r a c t i o n s o f d i s s o l v e d o r g a n i c m a t t e r . The p r e s e n t study d e s c r i b e s an e x p e r i m e n t a l setup f o r d e t e r m i n a t i o n o f luminescence l i f e t i m e s and p r e s e n t s v a l u e s o b t a i n e d f o r some n a t u r a l o r g a n i c m a t e r i a l s . F l u o r e s c e n c e Decay Rates Time r e s o l v e d f l u o r e s c e n c e measurements o f s i m p l e o r g a n i c m o l e c u l e s a r e r e g u l a r l y used t o a n a l y z e a wide range o f c h e m i c a l and b i o l o g i c a l processes (_2). T h i s i s p o s s i b l e by v i r t u e o f the ways i n w h i c h a number o f phenomena c a n i n f l u e n c e t h e r a t e a t w h i c h a m o l e c u l e f l u o r e s c e s , t h a t i s undergoes an e l e c t r i c d i p o l e t r a n s i t i o n f r o m an e x c i t e d e l e c t r o n i c s t a t e t o a l o w e r , u s u a l l y t h e g r o u n d , s t a t e of t h e same m u l t i p l i c i t y . The k i n e t i c parameter t h a t i s measured r e f l e c t s t h e sum o f the r a t e s o f p r o c e s s e s d e p o p u l a t i n g

0097-6156/87/0327-0132$06.00/0 © 1987 American Chemical Society

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the e x c i t e d s t a t e of the fluorophore b e i n g m o n i t o r e d . On a m o l e c u l a r l e v e l , the i n t r i n s i c r a t e of d e - e x c i t a t i o n of a g i v e n f l u o r o p h o r e i s s u b j e c t to a number of f a c t o r s i n c l u d i n g symmetry r e q u i r e m e n t s , the f r e q u e n c y of the f l u o r e s c e n t r a d i a t i o n , the i n t e g r a t e d a r e a under the a b s o r p t i o n curve as w e l l as on s p a t i a l r e s t r i c t i o n s imposed by the n e c e s s i t y of m o l e c u l a r o r b i t a l o v e r l a p of the e x c i t e d and ground s t a t e s . T y p i c a l l y , y^alues ^ f J^he r a t e c o n s t a n t f o r f l u o r e s c e n c e are i n the range of 1 0 - 1 0 s (3)· The i n t e n s i t y of f l u o r e s c e n c e from an e x c i t e d m o l e c u l e w i l l be dependent on the f l u o r e s c e n t decay r a t e and o t h e r n o n - r a d i a t i v e p r o c e s s e s s u c h as i n t e r s y s t e m c r o s s i n g t o t h e t r i p l e t s t a t e , i n t e r n a l conversion to the ground s t a t e , and any p h o t o c h e m i c a l r e a c t i o n s . In condensed media, v i b r a t i o n a l r e l a x a t i o n ^from^upper v i b r a t i o n a l l e v e l s of e x c i t e d s t a t e s i s u l t r a f a s t ( >10 s ) so t h a t v i b r o n i c r e l a x a t i o n i s complete b e f o r e e l e c t r o n i c r e l a x a t i o n can t a k e p l a c e . I n t e r n a l c o n v e r s i o n i s a p r o c e s s t h a t i s a l s o v e r y f a s t ( p i c o s e c o n d s ) at h i g lower l y i n g l e v e l s of th the main competing p r o c e s s to f l u o r e s c e n c e i s o f t e n intersystem c r o s s i n g to the t r i p l e t m a n i f o l d of energy l e v e l s . W r i t i n g e q u a t i o n s f o r these p r o c e s s e s a l l o w s d e f i n i t i o n of the f l u o r e s c e n c e quantum y i e l d , 0, and the f l u o r e s c e n c e decay t i m e , t , i n terms of sums of the v a r i o u s f i r s t o r d e r r a t e c o n s t a n t s (nb. o t h e r p o s s i b l e d e a c t i v a t i o n pathways a l s o e x i s t ) . f

Pathway

Rate Constant

M + hv — > *M*

—>

1 * M —> l

M*

J

X

M*

I

M + hv

k

3 * M

k isc

f

—>

Μ + Δ

k

—>

(products)

k^

ic

1 * M

U s i n g the steady s t a t e a p p r o x i m a t i o n l e a d s 0_ = k. / ( k . + k, + k. + k ) f f f isc ic ρ

to:

and t. f

- ( k- + k, + k, + k f isc ic ρ

F r o m t h i s f r a m e w o r k i t ^ can be s e e n t h a t t h e fluorescence i n t e n s i t y o f a m o l e c u l e i s d e p e n d e n t upon t h e m a g n i t u d e o f k^ r e l a t i v e to the sum of a l l o t h e r d e a c t i v a t i o n path r a t e s . I t i s a l s o c l e a r t h a t l i f e t i m e measurements themselves do not g i v e the i n d i v i d u a l r a t e c o n s t a n t s f o r the p r o c e s s e s of i n t e r e s t , however i n c o n j u n c t i o n w i t h the e m i s s i o n and product quantum y i e l d s , they do p r o v i d e a way i n which t h i s can be c a l c u l a t e d .

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

E x p e r i m e n t a l Methods The e x p e r i m e n t a l setup employed i n t h i s work used c o n v e n t i o n a l r i g h t angled v i e w i n g o f the time course o f the f l u o r e s c e n c e i n t e n s i t y of a sample c o n t a i n e d i n a s m a l l volume (0.2 ml) f l o w cell. The f l u o r e s c e n c e f l o w c e l l was f i l l e d f r o m a s e p a r a t e r e s e r v o i r v i a a p e r i s t a l t i c pump. S o l u t i o n parameters such as pH c o u l d be monitored d i r e c t l y from t h i s l a r g e r r e s e r v o i r , as c o u l d t h e oxygen p a r t i a l p r e s s u r e by b l a n k e t i n g o r b u b b l i n g t h e c e l l c o n t e n t s w i t h n i t r o g e n o r oxygen. The e x c i t a t i o n s o u r c e was a l a s e r (PAR 2100 D y e s c a n , m o d i f i e d t o a l l o w use of the primary l i n e ) a t a w a v e l e n g t h o f 337.lnm. The f l u o r e s c e n t e m i s s i o n was d e t e c t e d by a m o d i f i e d side-on p h o t o m u l t i p l i e r (PMT Hammamatsu R928), a f t e r passage through a monochromator (ISA HR 320). P a r t i c u l a r a t t e n t i o n was p a i d t o t h e w i r i n g o f t h e v o l t a g e d i v i d e r network o f the anodes o f t h e PMT t o ensure f a s t e s t p o s s i b l f l u o r e s c e n t photon f l u x t h r o u g h a h i g h - s p e e d , wide-band ( t o 0.5GHz) s i n g l e t r a c e a m p l i f i e r ( T e k t r o n i x 7A29) and monitored on a t r a n s i e n t waveform d i g i t i z e r ( T e k t r o n i x 7912AD). The d i g i t i z e r was i n t e r f a c e d t o a l a b o r a t o r y computer ( T e k t r o n i x 4502) f o r d a t a a c q u s i t i o n , c o n t r o l and p r o c e s s i n g of the e x p e r i m e n t a l r e s u l t s . A f a s t photodiode (EG&G E l e c t r o - o p t i c s FND-100Q) monitored the f i r i n g o f the l a s e r p u l s e ( u s u a l l y a t a r e p e t i t i o n r a t e of 2Hz) and p r o v i d e d an e x t e r n a l t r i g g e r s i g n a l f o r the d i g i t i z e r . Deconvolution o f the observed decay p u l s e was n e c e s s a r y because o f the f i n i t e w i d t h (approx. 1.9ns FWHM) o f the e x c i t a t i o n p u l s e . T h i s was c a r r i e d out u s i n g a F o u r i e r t r a n s f o r m method ( 4 ) . D e s p i t e the n o n - t r i v i a l c o m p u t a t i o n a l requirements and t h e need t o f i l t e r out the h i g h frequency components of the F o u r i e r d i v i s i o n b e f o r e t a k i n g t h e i n v e r s e t r a n s f o r m ( 5 ) i t was f e l t t h a t t h i s method was t h e most d i r e c t way o f g e n e r a t i n g t h e t r u e i m p u l s e r e s p o n s e f u n c t i o n o f t h e system. The r e s u l t i n g f l u o r e s c e n c e decay curves were f i t t o a s i n g l e ( o r double) e x p o n e n t i a l . The phase p l a n e p l o t method o f Demas ( 6 ) and o t h e r s ( 2 ) , was a l s o used f o r d e c o n v o l u t i o n but w i t h l e s s s u c c e s s . Two t e s t s of the r e l i a b i l t y o f the d e c o n v o l u t i o n procedure u s e d i n t h i s s t u d y were c a r r i e d o u t . I n t h e f i r s t o f t h e s e , a n a l y t i c f u n c t i o n s o f the form * (-b*t) y=a*exp were generated and c o n v o l v e d , by complex m u l t i p l i c a t i o n of the two F o u r i e r t r a n s f o r m s i n the frequency domain, w i t h e x p e r i m e n t a l l y determined i n s t r u m e n t response c u r v e s . The i n s t r u m e n t response c u r v e s were approximated as the time course o f l a s e r p u l s e f l a s h e s r e c o r d e d from the same c e l l and c e l l geometry, a t an i n t e n s e Raman s c a t t e r i n g frequency of the s o l v e n t , which f o r water a t our e x c i t a t i o n frequency was 380.6 nm. T h i s a p p r o x i m a t i o n was based on c o n s i d e r a t i o n o f t h e time course o f t h e o f f - r e s o n a n c e Raman s c a t t e r as b e i n g t h e same as t h e e x c i t i n g l i g h t p u l s e on a t i m e s c a l e s i g n i f i c a n t l y s h o r t e r than the f l u o r e s c e n c e time s c a l e under investigation (7)· The resulting synthetic decay curves,

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Time-Resolved Fluorescence Measurements

c o r r e s p o n d i n g to a c o n v o l u t i o n of the i n s t r u m e n t response f u n c t i o n and exact f l u o r e s c e n t decays of v a r i o u s known l i f e t i m e s , were then d e c o n v o l v e d . For known l i f e t i m e s down t o 500ps, the c a l c u l a t e d v a l u e s were w i t h i n 5%, the e r r o r growing to some 30% f o r known l i f e t i m e s of lOOps. These e s t i m a t e s then, are the lower l i m i t s of measurement a t t a i n a b l e w i t h our o b s e r v a t i o n system. The main l i m i t a t i o n of the system i s seen to be the p u l s e w i d t h of the l a s e r flash. I n t h e s e c o n d t e s t , a number o f f l u o r e s c e n t compounds o f r e l a t i v e l y w e l l known l i f e t i m e s i n the nanosecond time range (8,9) were used as standards, allowing evaluation of both the i n s t r u m e n t a l and c o m p u t a t i o n a l a s p e c t s of the measurement. Table I shows the v a l u e s o b t a i n e d f o r 2 , 5 - d i p h e n y l o x a z o l e (PPO), anthracene and q u i n i n e b i s u l p h a t e . A l l chemicals were a n a l y t i c a l grade and not f u r t h e r p u r i f i e d b e f o r e use. Anthracene and PPO were d i s s o l v e d i n c y c l o h e x a n e , q u i n i n e i n 0.1N H^SO^; s o l v e n t s were not degassed. The c a s e of q u i n i n e i s o standard for fluorescenc k i n e t i c s ( 10) . I n a g r e e m e n t w i t h p r e v i o u s w o r k ( 1 0 ) we f o u n d s a t i s f a c t o r y f i t s of our deconvolved d a t a to a b i e x p o n e n t i a l r a t h e r than a s i n g l e e x p o n e n t i a l model. TABLE I .

Fluorescence

decays of s t a n d a r d

substances

Compound

l(nm)

t(ns)

solvent

reference

PPO

440 440 400

1.2 1.3 1.4

cyclohex. cyclohex. ethanol

( t h i s work) (Lampert et a l , 8 ) (Zuker et a l , 9 )

Anthracene

420 410 415 380

4.1 4.1 4.0 5.3

cyclohex. cyclohex. cyclohex. ethanol

( t h i s work) (Lampert et a l , 8 ) (Rayner et a l , 1 1 ) (Zuker e t al,9)

Quinine*

l(nm)

t (ns)

t (ns)

reference

400 450 500 550

5.0 5.0 5.2 5.4

17.2 19.7 21.2 21.9

( t h i s work)

400 450 500 550

2.6 3.6 10.3

18.2 19.1 19.7

(O'Connor et

* s o l v e n t used was

L

0.1N

2

al,10)

-

H„S0, 2 4

Sample P r e p a r a t i o n A number of samples of d i s s o l v e d o r g a n i c m a t e r i a l were used i n t h i s i n v e s t i g a t i o n . The f i r s t of these was a commercial p r e p a r a t i o n of "humic a c i d " ( A l d r i c h ) which was d i s s o l v e d (lmg/lOOml) i n d i l u t e

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

b a s e (0.01M NaOH). T h i s s u b s t a n c e i s d e r i v e d f r o m a p e a t l i k e m a t e r i a l . These s o l u t i o n s were u l t r a s o n i c a t e d (30sec) t o speed d i s s o l u t i o n and f i l t e r e d through 0.2um p o l y c a r b o n a t e (Nuclepore) membrane f i l t e r s t o ensure homogeneity o f the s o l u t i o n . Two other samples were marine material isolated by u l t r a f i l t r a t i o n o f n a t u r a l seawater samples through a Nuclepore 500 MWCO membrane, which has a nominal r e t e n t i o n o f a l l compounds o f m o l e c u l a r weight g r e a t e r than 500 D a l t o n s (see however Staub e t a l , 12). U l t r a f i l t r a t i o n was c a r r i e d out i n an T e f l o n coated u l t r a f i l t r a t i o n c e l l (Amicon 2000B) under a n i t r o g e n p r e s s u r e o f 5 0 p s i . The two samples were chosen t o r e p r e s e n t two q u i t e d i f f e r e n t l i g h t regimes o f t h e ocean. One sample was taken o f f s h o r e from Whitewater Bay ( 1 3 ) and r e p r e s e n t s a near s h o r e , terrestrially i n f l u e n c e d s u r f a c e water. T h i s sample was c o n c e n t r a t e d from an i n i t i a l volume o f a p p r o x i m a t e l y 17 l i t e r s t o a f i n a l volume o f 1000ml ( x l 7 ) . The second sample was taken from deep (1500m) w a t e r , w e l l below the p h o t i c zone Bahamas and r e p r e s e n t s a n o t been exposed t o s u n l i g h t f o r some t i m e . T h i s sample, which was kept i n t h e dark throughout sampling and p r e p a r a t i o n , was c o n c e n t r a t e d from a p p r o x i m a t e l y 17 l i t e r s t o 900ml ( x l 9 ) . F u r t h e r c o n c e n t r a t i o n o f these samples was d e l i b e r a t e l y avoided t o guard a g a i n s t any a g g r e g a t i o n o f the o r g a n i c m a t t e r . The f i n a l two s a m p l e s i n v e s t i g a t e d c o n s i s t e d o f a m a r i n e f u l v i c a c i d i s o l a t e d by XAD-2 r e s i n a b s o r p t i o n ( 1 4 ) o f a b u l k s a m p l e o f near s u r f a c e seawater c o l l e c t e d from a s i t e i n the G u l f o f M e x i c o ( 1 5 ) and a l s o a s o i l f u l v i c a c i d e x t r a c t e d f r o m an h o r i z o n Β podsol ( 1 6 ) . These samples were d i s s o l v e d i n f i l t e r e d G u l f Stream seawater and d e i o n i z e d ( M i l l i Q ) water r e s p e c t i v e l y , w i t h the a d d i t i o n o f s u f f i c i e n t 0.1M NaOH t o e f f e c t d i s s o l u t i o n o f the s o l i d s o i l f u l v i c a c i d . Results S i n g l e e x p o n e n t i a l f l u o r e s c e n c e decay times f o r t h e d i f f e r e n t d i s s o l v e d o r g a n i c chromophores are t a b u l a t e d i n Table I I . R e p l i c a t e d e t e r m i n a t i o n s o f t h e l i f e t i m e s o f these samples was g e n e r a l l y b e t t e r than 5%. D e s p i t e t h e d i v e r s i t y o f t h e sources o f these m a t e r i a l s , t h e i r f l u o r e s c e n t l i f e t i m e s are s i m i l a r . I t was not found necessary t o model any o f the decay f u n c t i o n s w i t h more t h a n a s i n g l e e x p o n e n t i a l , p e r h a p s i n d i c a t i n g some u n d e r l y i n g s i m i l a r i t y i n the nature o f the f l u o r o p h o r e s t o be found i n t h e s e m o l e c u l e s . T h i s i s i n disagreement w i t h the work o f Lapen and S e i t z (17) who suggested t h a t t h e l i f e t i m e o f a s o i l f u l v i c a c i d they i n v e s t i g a t e d was best f i t by a double e x p o n e n t i a l , ( w i t h components l i f e t i m e s o f 1.0 and 6.0ns) r e f l e c t i n g the h e t e r o g e n e i t y of a complex m i x t u r e . Recent work ( L a n g f o r d p e r s o n a l communication) on t h e f l u o r e s c e n c e l i f e t i m e s o f a w e l l c h a r a c t e r i z e d Armedale h o r i z o n Β s o i l f u l v i c a c i d m a t e r i a l has suggested t h a t m a t e r i a l t o have t h r e e components c o n t r i b u t i n g t o t h e o v e r a l l f l u o r e s c e n c e o f the sample. While i t i s u n c e r t a i n , w i t h the time r e s o l u t i o n a t t a i n a b l e w i t h o u r measurement system, t h a t a component o f t h e o r d e r o f lOOps c o u l d be adequately time r e s o l v e d , t h e non-

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

MILNE ET AL. TABLE I I .

137

Time-Resolved Fluorescence Measurements F l u o r e s c e n c e l i f e t i m e s of d i s s o l v e d o r g a n i c m a t t e r (seawater samples) and humic a c i d ( A l d r i c h ) measured under d i f f e r e n t s o l u t i o n c o n d i t i o n s

Sample

l(nm)

Whitewater Bay

460

2.4

pH 8.0 S= 33.62

Tongue of the Ocean

460

2.3

pH 8.2 S= 34.65

Fulvic

460

2.4

alk.

Yucatan (XAD-2)

460

2.1

alk. S=35

Humic a c i d

460

Humic a c i d

460

2.3

pH

11

Humic a c i d

460

1.8

pH

1

Humic a c i d

460

2.0

pH 8 latm

acid

Humic a c i d

460

lifetime(ns)

s o i n . cond.

0

2

pH 8 0.1M KI

1.9

o b s e r v a t i o n of a l o n g e r (6-7 ns) decay i n the samples measured i n t h i s work i s of i n t e r e s t . Work to f u r t h e r e l u c i d a t e the number of i d e n t i f i a b l e components i n the same samples i s c l e a r l y i n d i c a t e d . I n a n o t h e r r e p o r t of the measurement of l i f e t i m e s of humic m a t e r i a l s (18) a s i m i l a r constancy of t f o r samples of apparently w i d e l y d i f f e r e n t s o u r c e s was a l s o n o t e d . The effects of some important solution variables on f l u o r e s c e n c e l i f e t i m e s of ( A l d r i c h ) humic a c i d were a l s o b r i e f l y i n v e s t i g a t e d . V a l u e s o b t a i n e d as a f u n c t i o n of pH i n d i c a t e d t h a t the l i f e t i m e was somewhat l o n g e r at pH 11 than at e i t h e r pH 8 or pH 1. I t i s p e r h a p s i n t e r e s t i n g to s p e c u l a t e on the t h e possible i n v o l v e m e n t of i o n i z a b l e f u n c t i o n a l g r o u p s , which i o n i z e at h i g h pH, b e i n g p a r t of the chromophore. Phenols f o r i n s t a n c e , i o n i z e at h i g h e r pH v a l u e s and a r e known t o be p r e s e n t as p a r t o f t h e complex s t r u c t u r e of these m o l e c u l e s . Temperature v a r i a t i o n s may have an e f f e c t on the l i f e t i m e s and yields of luminescence p r o c e s s e s . Measurements made here were c a r r i e d out at room temperature (22+ 2C) f o r the most p a r t . Some l i m i t e d temperature runs over the range of 5-60 C, gave l i n e a r Arrhenius plots of relative fluorescence intensity versus temperature. f

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS Oxygen

M o l e c u l a r oxygen i s an e f f i c i e n t quencher of the f l u o r e s c e n c e of a r o m a t i c hydrocarbons (19) e s p e c i a l l y i n non-aqueous s o l v e n t s where the solubility of oxygen and i t s diffusion coefficient are p r o p o r t i o n a t e l y h i g h e r than i n aqueous s o l u t i o n . In these systems quenching by oxygen approaches a d i f f u s i o n c o n t r o l l e d p r o c e s s , w h e r e i n e a c h e n c o u n t e r o f an o x y g e n m o l e c u l e w i t h an e x c i t e d f l u o r o p h o r e r e s u l t s i n quenching. I t i s known t o quench b o t h the s i n g l e t and t r i p l e t e x c i t e d s t a t e s , and q u e n c h i n g c a n be b o t h c h e m i c a l or p h y s i c a l i n n a t u r e (3)· The p h o t o c h e m i c a l p r o d u c t i o n of s i n g l e t oxygen s p e c i e s from t e r r e s t r i a l humic m a t e r i a l s has been noted (20,2J.,22). Measurement of the decay times f o r s o l u t i o n s of humic a c i d t h a t were s a t u r a t e d ( l a t m ) w i t h oxygen d i d not show any r e d u c t i o n o v e r those at e q u i l i b r i u m w i t h a i r o r s a t u r a t e d w i t h n i t r o g e n . G i v e n t h a t the maximu u n d e r 760mm 0^ i s o e f f i c i e n c y of quenching at a d i f f u s i o n c o n t r o l l e d l i m i t of 10 s , then the product of the d i f f u s i o n c o e f f i c h e n t ^ a n d the quencher c o n c e n t r a t i o n w i l l be l e s s than or e q u a l t o 10 s , so t h a t e i t h e r s i n g l e t or t r i p l e t s t a t e s having lifetimes of the o r d e r of nanoseconds w i l l at b e s t o n l y be s l i g h t l y quenched i n even oxygen saturated s o l u t i o n s . I t appears then that there is little i n t e r a c t i o n , e i t h e r p h y s i c a l or c h e m i c a l , of oxygen w i t h the e x c i t e d s i n g l e t s t a t e s produced by i r r a d i a t i o n of o r g a n i c m a t t e r , at l e a s t a t t h i s e x c i t a t i o n wavelength. Work on oxygen quenching i n p r o t e i n systems (23) has shown t h a t f l u o r o p h o r e s a t t a c h e d to p r o t e i n m o l e c u l e s may be i n a c c e s s i b l e to t h e d i f f u s i o n o f a s m a l l , u n c h a r g e d and n o t p a r t i c u l a r l y h y d r o p h i l i c m o l e c u l e such as oxygen. These workers used e l e v a t e d pressures (up to lOOatm) of oxygen to achieve significant q u e n c h i n g , and p o i n t e d out t h a t s i n c e oxygen does not form any complexes w i t h the m o l e c u l e s they s t u d i e d , quenching would r e q u i r e a c t u a l short-range i n t e r a c t i o n w i t h the f l u o r o p h o r e . Quenchers which do form complexes w i t h the f l u o r o p h o r e , or o t h e r p a r t s of the m o l e c u l e , s h o u l d be expected to have a w i d e r r a d i u s of a c t i o n f o r energy t r a n s f e r . The o n l y o t h e r p o t e n t i a l quencher b e s i d e s oxygen measured i n t h i s study was i o d i d e i o n , which a t a s o l u t i o n c o n c e n t r a t i o n of 0.1M d i d show a s m a l l r e d u c t i o n i n fluorescent l i f e t i m e . Further work on the s t a t i c and dynamic quenching of these m o l e c u l e s i s under i n v e s t i g a t i o n . Conclusion I n i t i a l measurement of the f l u o r e s c e n c e l i f e t i m e s of samples of m a r i n e o r g a n i c m a t t e r and a t e r r e s t r i a l humic a c i d have suggested an u n d e r l y i n g s i m i l a r i t y i n t h i s p h y s i c a l p r o p e r t y of a l l of these m o l e c u l e s . T h i s i s i n accord w i t h the apparent homogeneity of certain other of the spectral properties (fluorescence and a b s o r p t i o n s p e c t r a , p h o t o s e n s i t i z i n g c h a r a c t e r i s t i c s e t c ) observed f o r a number of n a t u r a l water chromophores. At the same time t h i s i s at v a r i a n c e w i t h o b s e r v a t i o n s of o t h e r p r o p e r t i e s , such as m e t a l

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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i o n c o m p l e x a t i o n , s t r u c t u r a l d e t e r m i n a t i o n s based on d e g r a d a t i v e s t u d i e s e t c which seem t o i m p l y much g r e a t e r h e t e r o g e n e i t y a t a m o l e c u l a r l e v e l (24)· The q u e s t i o n o f how s e n s i t i v e a parameter the time r e s o l v e d f l u o r e s c e n c e p r o p e r t i e s o f t h e s e c h r o m o p h o r e s c a n be t o b o t h s t r u c t u r a l d i f f e r e n c e s and photochemical r e a c t i v i t y o f these m o l e c u l a r assemblages remains u n c l e a r . The o b s e r v a t i o n t h a t oxygen, at n a t u r a l l e v e l s , i s not l i k e l y t o be a s i g n i f i c a n t quenching agent f o r t h e e x c i t e d s i n g l e t s t a t e s o f these chromophores i s o f i n t e r e s t to the i n t e r p r e t a t i o n o f the p h o t o s e n s i t i z i n g a b i l i t y and d i r e c t p h o t o r e a c t i v i t y of t h i s important c l a s s of m a t e r i a l s . Ac knowledgmen t s T h i s work was c a r r i e d out w i t h t h e support o f the O f f i c e o f N a v a l R e s e a r c h u n d e r Navy G r a n t N00014-85C-0020. We t h a n k D r . G.R.Harvey f o r k i n d l y s u p p l y i n thank Dr. C.H.Langfor a v a i l a b l e t o us and f o r h i s h e l p f u l comments on t h e m a n u s c r i p t . Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15 16.

Zika, R.G. In "Marine Organic Chemsitry"; Duursma, E.K. and Dawson, R., Eds.; Elsevier: Amsterdam, 1981. Demas, J.N. "Excited state lifetime measurements"; Academic Press: New York, 1983. Turro, N.J. "Modern Molecular Photochemistry"; Benjamin Cummings: Menlo Park, CA., 1978. Andre, J.C.; Vincent, L.M.; O'Connor, D.; Ware, W.R. J. Phys. Chem. 1979, 83, 2285. Wild, U.; Holzworth, Α.; Good, H.P. Rev. Sci. Instrum. 1977, 48, 1621. Demas, J.N.; Adamson, A.W. J. Phys. Chem. 1971, 75, 2463. Kinoshita, S.; Kushida, T. Rev. Sci. Instrum. 1982, 52, 469. Lampert, R.A.; Chewter, L.A.; Phillips, D.; 0Connor, D.; Roberts, A.J.; Meech, S.R. Anal. Chem. 1983, 55, 68. Zuker, M.; Szabo, A.G.; Bramall, L.; Krajcarski, D.T.; Selinger, B.K. Rev. Sci. Instrum. 1985, 56, 14. O'Connor, D.V.; Meech, S.R.; Phillips' D. Chem. Phys. Lett. 1982, 88, 22. Rayner, D.M.; McKinnon, A.E.; Szabo, A.G.; Hackett, P.A. Can. J. Chem. 1976, 54, 3246. Staub, C; Buffle, J.; Haerdi, W. Anal. Chem. 1984, 56, 2843. Moffett, J.W.; Zika, R.G. 1986, (this volume). Stuermer, D.H.; Harvey, G.R. Deep Sea Res. 1977, 24, 303. Harvey, G.R.; Boran, D.A.; Chesal, L.A.; Tokar, J.M. Marine Chemistry. 1983, 12, 119. Gamble, D.S.; Schnitzer, M. In "Trace Metal and Metal Organic Interactions in Natural Water"; Singer, P.S., Ed.; Ann Arbor Science: Ann Arbor, 1973. 17. Lappen, A.J.; Seitz, W.R. Anal. Chim. Acta. 1982, 134, 31. 18. Fischer, A.M. Ph.D. Thesis, University of California, Santa Cruz, 1985. T

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19. 20. 21. 22. 23. 24.

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Berlman, I.B. "Handbook of Fluorescence Spectra of Aromatic Molecules"; Academic Press: New York, 1965, p. 35. Zepp, R.G.; Wolfe, N.L.; Baughman, G.L.; Hollis, R.C. Nature 1977, 267, 421. Woolf, C.J.M.; Halmans, M.T.H.; van der Hiejde, H.B. Chemosphere 1981, 10, 59. Haag, W.R.; Hoigne, J.; Gassman, E.; Braun, A. Chemosphere 1984, 13, 641. Lakowicz, J.R.; Weber, G. Biochemistry 1973, 12, 4161. Aiken, G.R.; McKnight, D.M.; MacCarthy, P. "Humic Substances in Soil, Sediment, and Water"; Wiley-Interscience: New York, 1985.

RECEIVED June 24, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 11

Primary Photochemical Processes in Photolysis Mediated by Humic Substances 1,4

2

Anne M. Fischer , John S. Winterle , and Theodore Mill

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1

Department of Chemistry, University of California, Santa Cruz, CA 95064 Syntex Research, Institute of Pharmaceutical Sciences, Palo Alto, CA 95064 Department of Physical Organic Chemistry, SRI International, Menlo Park, CA 94025 2

3

Natural water sample were probed for phototransien flash kinetic spectroscopy was used to study two transients common to most samples. One transient with a maximum around 720 nm was quenched by decreasing pH and nitrous oxide. It was present in all waters with DOC and had a spectrum which resembled that of a solvated electron. The signal was linear with laser power. The quantum yield for this transient was measured. Samples with higher ground state absorbance yielded a transient with a maximum at 475 nm that was quenched by oxygen. This transient seemed to be a photophysical hybrid with triplet and radical cation character. Additional work done to characterize these transients and predict their environmental fates is discussed. A q u a t i c p h o t o c h e m i s t r y has become a d i s c i p l i n e t h a t now i n t e r e s t s more t h a n j u s t a h a n d f u l o f s c i e n t s t s . Recent r e v i e w s (1-3) have c h r o n i c l e d a t r e n d toward an i n t e r e s t i n k i n e t i c s and mechanisms from i n i t i a l work which r e l i e d on the a n a l y t i c a l c h e m i s t r y o f f i n a l products of aquatic photoreactions. Some o f t h e b e t t e r understood a q u a t i c p h o t o p r o c e s s e s have i n v o l v e d r e a c t i v e oxygen s p e c i e s such as hydrogen p e r o x i d e (4,5) s i n g l e t oxygen (6,7) p e r o x y l r a d i c a l s (JO, and s u p e r o x i d e r a d i c a l a n i o n (9)· Humic substances have been i m p l i c a t e d as the p r e c u r s o r or i n i t i a t o r o f the photoprocesses i n a l l o f these s t u d i e s . Humic substances a r e polymeric oxidation products of biological d e t r i t a l materials. They a r e v a r i a b l e i n s t r u c t u r e depending on t h e environment o f t h e i r o r i g i n and r e s i d e n c e . Humic s u b s t a n c e s a r e p r o b a b l y t h e most v a r i a b l e s u b s t a n c e t o have a g e n e r i c c h e m i c a l name. They have been c h a r a c t e r i z e d t o v a r y from 'Current address: Rosenstiel School of Marine and Atmospheric Sciences, Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL 33149 0097-6156/87/0327-0141 $06.00/0 © 1987 American Chemical Society

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500 t o 200,000 i n m o l e c u l a r weight and c o n t a i n c o n j u g a t e d o l e f i n i c and a r o m a t i c f u n c t i o n a l g r o u p s , as w e l l as c a r b o x y l and h y d r o x y l and a s m a l l percentage o f n i t r o g e n o u s f u n c t i o n a l groups. S e v e r a l monographs have been p u b l i s h e d v e r y r e c e n t l y which d e s c r i b e t h e c h a r a c t e r z a t i o n o f t e r r e s t i a l and marine humic substances (10,11). One u n i f y i n g f e a t u r e o f humic s u b s t a n c e s i s t h a t t h a t they have a f e a t u r e l e s s ground s t a t e a b s o r p t i o n w i t h a d r a m a t i c a l l y i n c r e a s i n g o p t i c a l d e n s i t y below 375 nm. Thus humic substances can absorb s u n l i g h t and cause i n d i r e c t p h o t o r e a c t i o n s w i t h o r g a n i c compounds t h a t do not absorb s o l a r r a d i a t i o n . V a r i o u s s t u d i e s have been undertaken i n an attempt t o elucidate t h e p r i m a r y p r o c e s s e s and p h o t o c h e m i c a l transients g e n e r a t e d immediately a f t e r l i g h t i s absorbed by humic substance s o l u t i o n s and n a t u r a l w a t e r s . E a r l i e r work (12-14,6) used steady s t a t e s t u d i e s i n phosphoresence, i l l u m i n a t e d ESR, and a k i n e t i c a n a l y s i s o f p h o t o p r o d u c t s t o probe humic substance p h o t o c h e m i s t r y and p h o t o p h y s i c s . T h i p h o t o t r a n s i e n t i s an e x c i t e c o n t a i n i n g o x y g e n and n i t r o g e n and may be a t r i p l e t . These s u g g e s t i o n s have i n t e r e s t e d s e v e r a l groups (15-18) i n u s i n g e x c i t e d state laser absorption spectroscopy to d i r e c t l y observe and c h a r a c t e r i z e the humic s u b s t a n c e . The o b j e c t i v e s of t h e study p r e s e n t e d i n t h i s paper were t o observe and c h a r a c t e r i z e p h o t o t r a n s i e n t s produced by laser e x c i t a t i o n o f n a t u r a l waters and humic substance (HS) s o l u t i o n s . The photosysterns were s t u d i e d on two s c a l e s . On the l a b o r a t o r y s c a l e p u l s e d l a s e r f l a s h p h o t o l y s i s was used t o study t h e time r e s o l v e d and s p e c t r a l b e h a v i o r o f t h e p h o t o c h e m i c a l t r a n s i e n t s . S t u d i e s t o i d e n t i f y and q u a n t i f y the t r a n s i e n t s i n c l u d e d adding energy and e l e c t r o n a c c e p t o r s and model compounds t o t h e s o l u t i o n and v a r y i n g parameters such as pH, m e t a l , and oxygen c o n t e n t . On the f i e l d s c a l e l a b o r a t o r y d a t a taken a t s o l a r a c t i n i c wavelengths i s e x t r a p o l a t e d u s i n g p u b l i s h e d s o l a r photon f l u x e s t o p r e d i c t e n v i r o n m e n t a l e f f e c t s o f the p h o t o t r a n s i e n t s s t u d i e d i n t h i s work. T h i s paper thus c o n t a i n s an o v e r v i e w o f many experiments performed over f o u r y e a r s ( 1 6 ) . Methods and Apparatus Three types o f samples were examined. N a t u r a l waters (NW) were sampled from s e v e r a l f r e s h w a t e r l a k e s , an a q u i f e r , and a c o a s t a l seawater sample from a k e l p b e d . NW s a m p l e s were f i l t e r e d and s t e r i l i z e d w i t h a 0.22 m i c r o n f i l t e r w h i c h was p r e w a s h e d ( b y r i n s i n g w i t h 500 ml d o u b l e d i l t i l e d d e i o n i z e d (DDI) w a t e r and s t o r e d a t 4 C and s u b j e c t e d t o p h o t o c h e m i c a l a n a l y s i s w i t h i n 24-48 hours a f t e r sampling. M i n i m a l s t o r a g e t i m e was n e c e s s a r y t o m i n i m i z e p h o t o l y i s and a d s o r p t i o n o f t h e low 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 o r g a n i c compounds (DOC) w h i c h were f o u n d t o be t h e p r e c u r s o r s t o the p h o t o t r a n s i e n t s . The second k i n d o f samples were c o n c e n t r a t e d n a t u r a l waters (CNW). These samples were c o n c e n t r a t e d by r o t a r y e v a p o r a t i o n t o g i v e a ground s t a t e O.D. g r e a t e r than 0.1 a t 355 nm, t h e l a s e r e x c i t a t o n wavelength i n the s o l a r a c t i n i c range. A l l o t h e r handing of CNW samples was i d e n t i c a l t o t h a t d e s c r i b e d f o r NW samples.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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The t h i r d k i n d of sample was a humic substance e x t r a c t (HSX) i n t o DDI. The h u m i c s u b s t a n c e u s u a l l y was an u n t r e a t e d s o i l p r o v i d e d by A l d r i c h as "Humic A c i d " , a peat s o i l mined i n Germany and packaged w i t h o u t t r e a t m e n t . O t h e r h u m i c s u b s t a n c e s were homogenized u n t r e a t e d p e a t , l e o n a r d i t e , and t o p s o i l purchased from t h e I n t e r n a t i o n a l Humic Substance S o c i e t y . These samples were p r e p a r e d by n e u t r a l water e x t r a c t i o n w i t h r o l l i n g o r s t i r r i n g f o r 24-60 h o u r s . The p r o p o r t i o n o f b u l k t o water was 2 0 - 4 0 g / l . Double d i s t i l l e d a c i d s and b a s e s were u s e d t o a d j u s t t h e pH d u r i n g p r e p a r a t i o n and a n a l y s i s . A f t e r e x t r a c t i o n the humic substance e x t r a c t samples were c e n t r i f u g e d a t 15,000 rpm f o r 20 minutes and f i l t e r e d through a c l e a n 0.22 micron f i l t e r . Another type o f humic s u b s t a n c e e x t r a c t a n a l y z e d f o r p h o t o t r a n s i e n t s was s o l u t i o n s of p u r i f i e d humic and f u l v i c a c i d s . These s o l i d s had been s t r i p p e d o f m e t a l s and s e p a r a t e d i n t o humic and f u l v i c f r a c t i o n s by t h e methods of (19). The s a m p l e s most t h o r o u g h l y a n a l y z e d were t h e HSX p r e p a r e d from A l d r i c S e a r s v i l l e Lake, S t a n f o r d D u r i n g sample p r e p a r a t i o n r e a g e n t s such as t h e e x t r a c t i o n w a t e r were e x a m i n e d f o r c o n t a m i n a t i o n by U V - V i s , f l u o r e s c e n c e s p e c t r o s c o p y , and e x c i t e d s t a t e a b s o r p t i o n a f t e r l a s e r e x c i t a t i o n . A l s o t h e samples were checked f o r changes i n the above s p e c t r a l c h a r a c t e r i s t i c b e f o r e s t o r a g e and i m m e d i a t e l y p r i o r t o a n a l y s i s . I f t h e r e were s i g n i f i c a n t c h a n g e s t h e s a m p l e was d i s g a r d e d a s u n s u i t a b l e f o r f u t h e r photochemical a n a l y s i s . The f l a s h p h o t o l y s i s apparatus was a Quanta Ray DCR-1 Nd:YAG p u l s e d l a s e r e x c i t i n g source (FWHM = 7 n s , frequency t r i p l e d t o 355 nm o r q u a d r u p l e d t o 266 nm) and a f l a s h lamp m o n i t o r i n g l i g h t source ( E . G. & G. h i g h p r e s s u r e xenon a r c lamp FWHM = 50 u s ) . The l a s e r power was u s u a l l y 5 mj/pulse u n l e s s t h e e f f e c t o f l a s e r power v a r i a t i o n was under s t u d y . The l a s e r power was m o n i t o r e d and kept as s t a b l e as p o s s i b l e by measurement w i t h a l a s e r power m e t e r b e t w e e n e v e r y s e r i e s of a n a l y z i n g p u l s e s . The l a s e r i t s e l f had a p u l s e t o p u l s e v a r i a t i o n o f +5% and a s i n g l e shot n o i s e l e v e l o f 0.05 AD. However most of the d a t a had l e s s e r r o r and a lower d e t e c t i o n l i m i t d e c r e a s i n g by the square r o o t o f t h e number o f p u l s e s i n each measurement. Thus the s i g n a l t o n o i s e r a t i o was enhanced g r e a t l y by s i g n a l a v e r a g i n g 30 t o 150 p u l s e s and r e p e a t i n g e a c h o f these measurements a t l e a s t t h r e e t i m e s . The l a s e r was f o c u s e d t o a b o u t 4 mm o n t o a 1 cm c u v e t t e o r f l o w c e l l w i t h a r e p e t i t i o n r a t e of 5 Hz. The apparatus was e l e c t r o n i c a l l y tuned so t h e l a s e r would f i r e a t t h e maximum o f t h e f l a s h lamp p u l s e . See F i g u r e 1 f o r a schematic of the f l a s h p h o t o l s i s a p p a r a t u s . A f t e r p a s s i n g through t h e sample t h e xenon a r c probe beam was d i r e c t e d through a 0.45 m monochromator and d e t e c t e d w i t h an EMI D279 o r EMI 9876B p h o t o m u l t i p l i e r tube s e t t o 1200-1500 V. The s i g n a l was then d i g i t i z e d by a B i o m a t i o n 6500 t r a n s i e n t r e c o r d e r (2 o r 5 o r 10 ns p e r p o i n t ) and s e n t t o a Z-80 m i c r o c o m p u t e r f o r s i g n a l a v e r a g i n g and a n a l y s i s . The apparatus and d a t a a n a l y s i s routines are f u r t h e r described i n the l i t e r a t u r e (20). The d a t a a n a l y s i s p r o v i d e d f o r the p l o t t i n g o f time v e r s u s t r a n s m i t t e d f l a s h l a m p i n t e n s i t y o r time v e r s u s e x c i t e d state a b s o r p t i o n as AD a t a g i v e n e x c i t a t i o n and m o n i t o r i n g wavelength. To t a k e a AD e x c i t e d s t a t e a b s o r p t i o n spectrum measurements were

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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made s e p a r a t e l y f o r each m o n i t o r i n g w a v e l e n g t h . P h o t o l y s i s of the sample from p r o l o n g e d exposure to l a s e r p u l s e s was m o n i t o r e d by checking AD a t 475 nm o r 720 nm a f t e r e v e r y few measurements. I f t r a n s i e n t absorbance d e c r e a s e d , f r e s h sample was used. A f l o w c e l l system, F i g u r e 2 was developed to make i t more convenient t o study d i l u t e samples o r take s p e c t r a w i t h a minimum of p h o t o l y s i s . The sample volumes c i r c u l a t e d were l a r g e , from 200 t o 1000 m l . The f l o w c e l l had a f i l t e r f l a s k as t h e s a m p l e reservoir. The p u r g i n g gas e n t e r e d through the vacuum o u t l e t . I n s e r t e d i n the mouth of the c e l l was the e l e c t r o d e of a pH meter and the i n l e t t u b i n g l e a d i n g t o t h e f l o w c e l l , and t h e o u t l e t t u b i n g l e a d i n g from the f l o w c e l l . The sample r e s e r v o i r was kept homogenous by s t i r r i n g w i t h a magnetic s t i r r e r as w e l l as p u r g i n g w i t h the gas. The f l o w was m a i n t a i n e d by a p e r i s t a l t i c pump which c i r c u l a t e d 1-5 ml/min through 3mm O.D. tygon t u b i n g . The t u b i n g was r i n s e d w i t h water and the washings were t e s t e d f o r o r g a n i c contaminants by UV-Vis spectroscopy. The washing t r a n s i e n t absorbance s i g n a l . Thus t h e t u b i n g c o n t r i b u t e d no p h o t o c h e m i c a l l y a c t i v e o r d e t e c t i v e contaminants to the samples. Quenching r a t e c o n s t a n t s were measured by examining the e x t e n t and r a t e of decay of the p h o t o t r a n s i e n t under study a t s e v e r a l c o n c e n t r a t i o n s of the quencher. A Stern-Volmer a n a l y s i s of the change of l i f e t i m e of the t r a n s i e n t w i t h changing quencher c o n c e n t r a t i o n y i e l d e d a dynamic quenching r a t e c o n s t a n t k . A Stern-Volmer a n a l y s i s of the change i n e x c i t e d s t a t e a b s o r p t i o n w i t h i n c r e a s i n g quencher c o n c e n t r a t i o n was used when t h e r e was a c o m b i n a t i o n of l i f e t i m e and c o n c e n t r a t i o n quenching. I f the e x t e n t o f d y n a m i c v s . s t a t i c q u e n c h i n g was n o t known, t h e n t h e r a t e c o n s t a n t c a l c u l a t e d was an e m p i r i c a l o r apparent r a t e c o n s t a n t k app D e t e c t i o n and C h a r a c t e r i z a t i o n of a 720 nm Absorbing (T720) P h o t o t r a n s i e n t i n D i l u t e Waters A t r a n s i e n t w i t h an e x c i t e d s t a t e a b s o r p t i o n AD maximum a t 720 nm was d e t e c t e d i n d i l u t e n a t u r a l w a t e r s ( a d j u s t e d t o a pH o f 7) i r r a d i a t e d w i t h 266 nm l a s e r l i g h t . The AD s p e c t r u m p l o t t e d 400-450 ns a f t e r the l a s e r f l a s h resembled t h a t of the s o l v a t e d e l e c t r o n i n water (21). Nitrous oxide i s known t o react s p e c i f i c a l l y w i t h the aquated e l e c t r o n , R e a c t i o n 1. N0 2

+ e~ + H 0 2

> N

2

+ θ"

(1)

i t r o u s o x i d e was found to a t t e n u a t e the T720 s i g n a l i n d i l u t e NW. The r a t e c o n s t a n t ^ > r _ j n i t x o u s o x i d e quenching of T720 was measured to be k = 5 χ 10 M s . The measured r a t e c o n s t a n t agrees w e l l w i t h thaï i n the l i t e r a t u r e k = 7.5 χ 10 M s . The T720 s i g n a l was l i n e a r w i t h i n c r e a s i n g îaser power from 0.5 m j / p u l s e t o 10 m j / p u l s e p a s s i n g through the o r i g i n . These d a t a suggested t h a t T720 was an aquated e l e c t r o n , formed from a s i n g l e photon p r o c e s s i n waters c o n t a i n i n g d i s s o l v e d o r g a n i c m a t t e r . DDI water produced no AD s i g n a l when i r r a d i a t e d under the c o n d i t i o n s employed i n t h i s study. D i l u t i o n s of NW and c o n c e n t r a t e d NW showed the e x c i t e d

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FISCHER ET AL.

Pnmary Photochemical Processes in Photolysis

SAMPLE

v

V

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BIOMATION 6500 DIGITIZER F i g u r e 1.

COMPUTER

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f l a s h apparatus

magnetic stirrer F i g u r e 2.

Flow c e l l a p p a r a t u s .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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s t a t e a b s o r p t i o n s i g n a l t o be p r o p o r t i o n a l t o c o n c e n t r a t i o n o f o r g a n i c m a t t e r measured as l i g h t absorbed i n t h e ground s t a t e a t the l a s e r e x c i t a t i o n wavelength which i m p l i e d t h a t the e l e c t r o n was generated from the DOM. F i g u r e 3 shows the AD spectrum o f d i l u t e n a t u r a l waters as a t h r e e d i m e n s i o n a l s u r f a c e o f wavelength v s . time v s . AD. F i g u r e s 3 and 4 were prepared as composites o f much e x p e r i m e n t a l data. F i g u r e 3 shows the decay o f the t r a n s i e n t i n NW t o be e x p o n e n t i a l . The l i f e t i m e o f T720 v a r i e d d i r e c t l y w i t h i n c r e a s i n g pH. T720 r e a c t i o n w i t h v a r i o u s c o n c e n t r a t i o n s o f oxygen was a l s o measured. The ^que^nclying r a t e c o n s t a n t was e x p e r i m e n t a l l y determined t o be 7 χ 10+ M s , t h i s was a l s o i n f a i r agreement w i t h t h e l i t e r a t u r e v a l u e o f t h e r a t e constant O^Q the j j e a c t i o n o f aquated e l e c t r o n s w i t h oxygen, k = 1.8 χ 10 M s . The quenching r e a c t i o n s d i s c u s s e d here were dynamic, i . e . the l i f e t i m e o f the e x c i t e d s t a t e d e c r e a s e d w i t h i n c r e a s i n g a c i d i t y , oxygen c o n t e n t , and n i t r o u s oxide content. Copper a l s and an a p p a r e n t r a t e cal^Ujlat^ed^from the d a t a . The l i t e r a t u r e v a l u e , k = 3.5 χ 10 M s , was c l o s e t o t h e m e a s u r e d v a l u e . However t h i s p r o b a b l y was f o r t u i t o u s s i n c e t h e v a l u e i n t h e l i t e r a t u r e was r e p o r t e d as a dynamic quenching r a t e c o n s t a n t , i n c o n t r a s t t o t h e e m p i r i c a l data. The l a s e r d a t a taken i n t h i s study on copper and o t h e r m e t a l s suggested t h a t t h e mechanism o f copper quenching o f T7 20 was s t a t i c , i . e . t h e amplitude o f t h e D s i g n a l decreased w i t h i n c r e a s i n g copper c o n c e n t r a t i o n , b u t the l i f e t i m e was n o t affected. A l l l i t e r a t u r e v a l u e s o f quenching r a t e c o n s t a n t s w i t h aquated e l e c t r o n s are from ( 2 2 ) . D e t e c t i o n and C h a r a c t e r i z a t i o n o f a 475 nm Absorbing T r a n s i e n t i n Humic Substance E x t r a c t s (HSX) and Concentrated N a t u r a l Waters (CNW). The e x c i t e d s t a t e absorbance s p e c t r a o f HSX and CNW were monitored f r o m 350 t o 800 nm 400-450 ns a f t e r t h e 355 nm l a s e r p u l s e . A l l HSX and CMW samples measured produced e x c i t e d s t a t e s p e c t r a which h a d b r o a d maxima a t a b o u t 475 nm. F i g u r e 4 shows a t h r e e d i m e n s i o n a l s u r f a c e o f time v s . wavelength v s . e x c i t e d s t a t e a b s o r p t i o n t y p i c a l o f d a t a taken on HSX and CNW samples. The t h r e e dimensional p r o f i l e o f T475 i n F i g u r e 4 i l l u s t r a t e s several c o n t r a s t s between T475 and T720, F i g u r e 3. T475 had a g r e a t e r e x c i t e d s t a t e absorbance than T720 o v e r a l l by a t l e a s t 2-3x. Also T720 showed an e x p o n e n t i a l d e c a y i n t i m e , w h i l e T475 h a s more complex k i n e t i c s . A d d i t i o n a l l y T475 had a s h o r t l i v e d decay below 600 nm w h i c h f o l l o w e d t h e l a s e r f l a s h . Because o f i t s s h o r t l i f e t i m e a s i n g l e t assignment was s u g g e s t e d . These same HSX and CNW s o l u t i o n s a l l were measured t o have a f l u o r e s c e n t l i f e t i m e o f a b o u t 3 ns ( 16 ) . These d a t a c o u l d be due t o t h e e m i s s i o n and a b s o r p t i o n o f l i g h t by t h e same t r a n s i e n t . Above 600 nm t h e k i n e t i c s o f T475 become e x p o n e n t i a l a g a i n . Below 600 nm the decay appears t o be b i - e x p o n e n t i a l . Experiments t o c h a r a c t e r i z e T475 showed a dynamic quenching by oxygen. The (^enc^hing r a t e c o n s t a n t c a l c u l a t e d from these d a t a was k = 7 x l 0 M s . A survey o f quenching o f T475 w i t h water

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Primary Photochemical Processes in Photolysis

11. FISCHER ET AL.

147

au

0.4

GROUND STATE SPECTRUM

|_o.2

PROFILE OF TRANSIENT SPECTRUM OF CONCENTRATED SAMPLE 200

300

time (ns)

nm

λ( ) F i g u r e 3. Time v s . wavelength v s . e x c i t e d s t a t e a b s o r p t i o n o f S e a r s v i l l e Lake water m o n i t o r e d from 400 nm t o 800 nm from t = 0 to 300 ns a f t e r 266nm p u l s e d l a s e r e x c i t a t i o n , 5 m j / p u l s e .

PROFILE OF TRANSIENT SPECTRUM OF AN UNCONCENTRATED ENVIRONMENTAL SAMPLE

X(nm)

F i g u r e 4. Time v s . wavelength v s . e x c i t e d s t a t e a b s o r p t i o n o f A l d r i c h humic substance e x t r a c t e x c i t e d a t 355 nm and m o n i t o r e d f r o m 350 nm t o 700 nm from the time o f t h e l a s e r f l a s h t o 300 ns.

American Chemical Society Library 1155 16th St., N.W.

Washington, D.C 20039 In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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s o l u b l e t r i p l e t energy a c c e p t o r s w i t h v a r i o u s E from 30 t o 70 k c a l / m o l e gave complex r e s u l t s which were d i f f i c u l t t o i n t e r p r e t (16). No s e n s i t i z e r except oxygen gave dynamic quenching of the T475 time t r a c e . The quenchers w i t h E^ above 57 k c a l / m o l e showed a c o m b i n a t i o n o f s t a t i c and dynamic quenching. These r e s u l t s s u g g e s t e d t h a t T475 has p a r t i a l t r i p l e t c h a r a c t e r , but t h a t t h e r e a r e o t h e r a d d i t i o n a l p r o c e s s e s . These p r o c e s s e s p r o b a b l y i n v o l v e e l e c t r o n and p r o t o n t r a n s f e r t h a t are mediated by T475. A s u r v e y a s e t of t r i p l e t s e n s i t i z e r s (23) i n HSX s o l u t i o n s w i t h experiments i n s u n l i g h t suggested an o p e r a t i o n a l ( b l a c k box mechanism) t r i p l e t e n e r g y l e v e l of 57 k c a l / m o l e . T h i s agreed w i t h the HS t r i p l e t energy measured e a r l i e r by phosphorescence ( 1 2 ) . This suggests that p o l l u t a n t s ( d i s s o l v e d i n s u n l i t waters c o n t a i n i n g humic s u b s t a n c e s ) w i t h t r i p l e t e n e r g y l e v e l s b e l o w 57 k c a l / m o l e a r e l i k e l y t o undergo secondary p h o t o y s i s by an energy transfer mechanism. Most of the experiment argon s a t u r a t i o n t o e l i m i n a t samples was u t i l i z e d d u r i n g experiments to determine the e f f e c t of oxygen on the t r a n s i e n t , and n i t r o u s o x i d e was used to c h a r a c t e r i z e T720 i n d i l u t e s o l u t i o n s and t o r e s o l v e T720 f r o m T475 i n HSX s o l u t i o n s e x c i t e d by 355 nm l a s e r p u l s e s . T

R e s o l u t i o n of T720 from under T475 S i g n a l i n HSX N i t r o u s Oxide S a t u r a t i o n

U s i n g Argon

and

S p e c t r a of n i t r o u s o x i d e s a t u r a t e d samples of A l d r i c h HSX h a v i n g a g r o u n d s t a t e absorbance of 1.0 a t 355 nm showed an i n c r e a s e i n a b s o r b a n c e a t 475 nm and a d e c r e a s e i n a b s o r b a n c e a t 720 nm compared to argon s a t u r a t e d samples, see F i g u r e 5. Each p o i n t i n t h e f i g u r e i s a r e s u l t of a t l e a s t 5 measurements of 50 p u l s e s so t h e s i g n a l t o n o i s e i n F i g u r e 5 i s a b o u t 0.01 D i n this experiment. S u b t r a c t i o n of the n i t r o u s o x i d e spectrum from the a r g o n spectrum i n F i g u r e 5 showed a n e g a t i v e band a t 475 nm and a p o s i t i v e band a t 720 nm. This consistently r e p e a t e d experiment with Aldrich HSX suggested t h a t the 720 nm a b s o r b i n g t r a n s i e n t e x i s t e d i n a l l t y p e s of samples s t u d i e d . T h i s experiment a l s o showed t h a t T720 c o u l d be produced by l a s e r e x c i t a t i o n a t 355 nm, a s o l a r a c t i n i c wavelength as w e l l as by UV r a d i a t i o n a t 266 nm. The quenching of the l o n g wavelength t a i l o f the spectrum i n d i c a t e d t h a t the spectrum w i t h an a p p a r e n t s i n g l e maximum a t 475 nm (T475) a l s o had a component due to the absorbance of T720 proposed to be the aquated e l e c t r o n . The i n c r e a s e i n i n t e n s i t y under n i t r o u s o x i d e s a t u r a t i o n of the 475 nm s i g n a l p r o b a b l y was due t o the f a c t t h a t argon and oxygen have about the same s o l u b i l i t y , and n i t r o u s o x i d e i s t h i r t y times more s o l u b l e than e i t h e r ( 2 4 ) . N i t r o u s o x i d e would be more e f f e c t i v e i n r i d d i n g an i n i t i a l l y a i r s a t u r a t e d sample of r e s i d u a l oxygen which quenches the 475 nm t r a n s i e n t , while argon would be l e s s e f f e c t i v e . A n o t h e r e x p l a n a t i o n would be t h a t n i t r o u s o x i d e i n h i b i t s the back r e a c t i o n between the s o l v a t e d e l e c t r o n and i t s p a r t n e r r a d i c a l c a t i o n by s c a v e n g i n g the back r e a c t i o n of the r a d i c a l c a t i o n w i t h the s o l v a t e d e l e c t r o n . Then the s o l v a t e d e l e c t r o n absorbance would be quenched and the absorbance of the r a d i c a l c a t i o n would appear

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g u r e 5. E x c i t e d s t a t e s p e c t r a of A l d r i c h s o i l e x t r a c t taken The difference u n d e r n i t r o u s o x i d e and a r g o n s a t u r a t i o n . was b e t w e e n t h e two s i g n a l s shows a band a t 720 nm w h i c h quenched by n i t r o u s o x i d e .

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t o i n c r e a s e s i n c e i t was l e s s l i k e l y t o r e a c t w i t h an a q u a t e d electron. Furthermore i f the r a d i c a l c a t i o n were a component o f t h e l o n g l i v e d absorbance a t 475 nm f o u n d i n HSX and CNW t h e a b s o r b a n c e o f t h e 475 nm t r a n s i e n t w o u l d be l a r g e r . A s i m p l e r e a c t i o n scheme t h a t q u a l i t a t i v e l y e x p l a i n s the e f f e c t shown i n F i g u r e 4 i s shown below i n R e a c t i o n 2 and R e a c t i o n 3. T h i s scheme d o e s n o t i n c l u d e o r g a n i c r a d i c a l s and a n i o n s , w h i c h were a l s o suspected t o be p r e s e n t i n the system. HS + hv

+

HS ' + e~

+

N„0 + H S ' + e~ ν 2 (aq) Quantum Y i e l d f o r P r o d u c t i o n Extrapolation to F i e l d S i t u a t i o n

(2)

N

(aq) +

> HS " + N„ + 0~ o r 0H~ 2 o f Red 720 nm T r a n s i e n t

(3) and

A f t e r T720 was t e n t a t i v e l w i t h n i t r o u s o x i d e and f o r the p r o d u c t i o n o f the aquated e l e c t r o n i n d i l u t e n a t u r a l waters c o n t a i n i n g d i s s o l v e d o r g a n i c m a t t e r and i n humic substance e x t r a c t was m e a s u r e d by t h e c o m p a r i s i o n m e t h o d . I n t h i s method t h e l i t e r a t u r e v a l u e f o r the e x t i n c t i o n c o e f f i c i e n t o f the s o l v a t e d e l e c t r o n a t 720 nm was used ( 2 1 ) . The e x c i t e d s t a t e a b s o r p t i o n o f T720 was m e a s u r e d . These were u s e d i n a l g e b r a i c r a t i o t o an a c t i n o m i t e r f o r which a quantum y i e l d had a l r e a d y been t h o r o u g h l y determined. A l l t r a n s - r e t i n a l was used as an a c t i n o m e t e r t o c a l c u l a t e the number of photons reaching the humic substance chromophore p r o d u c i n g the s o l v a t e d e l e c t r o n . The ground s t a t e absorbance o f a l l - t r a n s r e t i n a l i n methanol was a d j u s t e d t o t h a t o f t h e NW sample a t t h e e x c i t i n g wavelength 266 nm, and the HSX sample a t 355 nm. The v a l u e s f o r the r e t i n a l t r i p l e t quantum y i e d s and e x t i n c t i o n c o e f f i c i e n t s i n methanol e x c i t e d by 266 nm and 355 nm l a s e r p u l s e s and monitored a t 450 nm were those determined p r e v i o u s l y ( 2 5 ) . The e x c i t e d s t a t e a b s o r p t i o n o f the a c t i n o m e t e r and humic substance s o l u t i o n were measured i m m e d i a t l y a f t e r p h o t o l y s i s by a v e r a g i n g t h e f i r s t t e n nanosecnds of D a f t e r t h e peak of the l a s e r f l a s h . Each time the experiment was r u n t h r e e e x c i t e d s t a t e o p t i c a l densities D were meaured: ( 1 ) A l l - t r a n s r e t i n a l i n methanol, argon saturated, (2) Humic substance s o l u t i o n sample, argon saturated , and ( 3 ) Humic substance s o l u t i o n , n i t r o u s oxide saturated. S i n c e n i t r o u s o x i d e i s an e l e c t r o n s c a v e n g e r , t h e s i g n a l w i t h n i t r o u s o x i d e s a t u r a t i o n was thought t o be caused by t r a n s i e n t s o t h e r than aquated e l e c t r o n s . The D under n i t r o u s o x i d e s t u r a t i o n was s u b t r a c t e d from the D under argon s a t u r a t i o n to c a l c u l a t e a s i g n a l g e n e r a t e d by t h e s o l v a t e d e l e c t r o n . This c a l c u l a t i o n assumes t h a t n i t r o u s o x i d e r e a c t e d w i t h n e a r l y a l l o f the s o l v a t e d e l e c t r o n s a t the time o f measurement, t h i s c a l u l a t i o n and more d e t a i l s of the e x p e r i m e n t a l c o n d i t i o n s a r e g i v e n i n d e t a i l elsewhere ( 1 6 ) . The quantum y i e l d f o r the p r o d u c t i o n of aquated e l e c t r o n s by 355 nm l a s e r e x c i t i o n of A l d r i c h HSX ( w i t h ground s t a t e absorbance o f 1.0 a t 355 nm) was determined t o be 0.014. The quantum y i e l d f o r the p r o d u c t i o n o f aquated e l e c t r o n s by 266 nm e x c i t a t i o n o f t

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S e a r s v i l l e Lake NW ( w i t h ground s t a t e absorbance 0-125 a t 266 nm) was determined to be 0-07 by the same method. The quantum y i e l d f o r A l d r i c h HSX was s i g n i f i c a n t l y lower than t h a t f o r S e a r s v i l l e Lake. The HXS quantum y i e l d was lower i n p a r t b e c a u s e o t h e r p h o t o t r a n s i e n t s , such as T475 were simultaneously produced i n the more c o l o r e d humic substance e x t r a c t samples. Thus a g i v e n amount of absorbed photons c r e a t e d s e v e r a l t r a n s i e n t s from the absorbed photons measured by the a c t i n o m e t e r . I t would be u s e f u l to have an e s t i m a t e d e x t i n c t i o n c o e f f i c i e n t f o r T475 to b e t t e r u n d e r s t a n d the f a t e of photons absorbed by HSX s o l u t i o n s w i t h ground s t a t e o p t i c a l d e n s i t i e s near 1.0 a t 355 nm. Then the v a l u e c a l c u l a t e d f o r the aquated e l e c t r o n quantum y i e l d would a l s o be a b e t t e r e s t i m a t e . Waters from S e a r s v i l l e Lake had fewer i n t e r f e r e n c e s w i t h T720. The n i t r o u s o x i d e s a t u r a t e d b l a n k s o l u t i o n s gave a s i g n a l a t n o i s e l e v e l , and n i t r o u s o x i d e quenched the NW s i g n a l to j u s t above n o i s e l e v e l i n d i c a t i n g few o aquated e l e c t r o n f o r th for the p r o d u c t i o n of aquated electrons may be wavelength dependent. Quantum y i e l d s t u d i e s w i t h melanins and s i m i l a r model humâtes have quantum y i e l d v a l u e s i n the same range w i t h wavelength dependence, i n c r e a s i n g w i t h photon energy. S p i k i n g Experiments w i t h M e l a n i n F u n g a l m e l a n i n samples p r o v i d e d as a g i f t from Dr. James M a r t i n of t h e U n i v e r s i t y o f C a l i f o r n i a a t R i v e r s i d e were n e u t r a l w a t e r e x t r a c t e d by the same method as the humic substance e x t r a c t s . An A l d r i c h HSX was prepared i n p a r a l l e l . Both s o l u t i o n s and a 1:1 c o m b i n a t i o n were a d j u s t e d to a ground s t a t e o p t i c a l d e n s i t y of 1.0 a t 355 nm t h e l a s e r e x c i t a t i o n w a v e l e n g t h . Ground s t a t e and f l u o r e c e n c e s p e c t r a taken of m e l a n i n and HS s o l u t i o n s were s i m i l a r . B o t h s a m p l e s had f u o r e s c e n c e e m i s s i o n maxima n e a r 460 nm w i t h e x c i t a t i o n at 355 nm. E x c i t e d s t a t e a b s o r p t i o n s p e c t r a of the three s o l u t i o n s taken u n d e r e x p e r i m e n t a l c o n d i t i o n s as d e s c r i b e d i n p r e v i o u s s e c t i o n s gave e x c i t e d s t a t e s p e c t r a t h a t were p r o p o r t i o n a l to the spectrum shown i n F i g u r e 4, the time v s . wavelength v s . AD f o r HSX and CNW solutions. The e x c i t e d s t a t e s p e c t r a were a l s o s i m i l a r i n shape and w i t h a broad AD maximum around 475 nm as shown i n F i g u r e 6. The r e s u l t s of t h i s work tend to v a l i d a t e p r e v i o u s work showing t h a t m e l a n i n s have s t r u c t u r a l as w e l l as f u n c t i o n a l s i m i l a r i t i e s , as a l s o shown by NMR, IR, ESR, and t r a n s i e n t ESR (11,26-28). Most s i g n i f i c a n t y m e l a n i n i s a l s o î .iown t o g e n e r a t e a s o l v a t e d e l e c t i o n s w i t h a quantum y i e l d c l o s e t o t h a t f o u n d f o r humic s u b s t a n c e s (29-30). These s t u d i e s have a l s o f o u n d m e l a n i n t o g e n e r a t e s u p e r o x i d e a n i o n s and hydrogen atoms u s i n g t r a n s i e n t ESR and s p i n traps. A l t h o u g h e x t r a p o l a t i o n of m e l a n i n work to humic substances seems v a l i d , the s t r u c t u r e s found i n m e l a n i n s are as d i v e r s e as those found i n humic s u b s t a n c e s , so t h i s model g i v e s us l i t t l e more i n s i g h t i n t o the a c t u a l s t r u c t u r e of humic substance chromophores.

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Reduction of Paraqat (Methyl Viologen) with HSX A 10 M s o l u t i o n o f methyl v i o l o g e n (MV ) d i d n o t have s i g n i f i c a n t ^ g r o u n d o r e x c i t e d s t a t e absorbance from 350 nm t o 750 nm. A 10 M s o l u t i o n o f methyl v i o l o g e n i n A l d r i c h HSX had ground s t a t e s p e c t r a c h a r a c t e r i s t i c o f humic substances b e f o r e and a f t e r laser irradiation. A t r a n s i e n t a b s o r p t i o n spectrum o f t h e above s o l u t i o n measured under t h e s t a n d a r d experimental condition^ d e s c r i b e d p r e v i o u s l y showed a spectrum s i m i l a r t o t h a t o f MV r e p o r t e d i n the l i t e r a t u r e (31). , F i g u r e 7. These d a t a show evidence o f i n t e r a c t i o n between MV and the humic substance v i a a photoprocess. The e x c i t e d s t a t e a b s o r p t i o n s p e c t r a show t h a t HS e x c i t e d s t a t e a b s o r p t i o n a t 475 nm and 720 nm (thought t o be i n p a r t due t o a s o l v a t e d e l e c t r o n ) , a r e s i g n i f i c a n t l y quenched. This leads to t h e c o n c l u s i o n tha^t an e l e c t r o n t r a n s f e r r e a c t i o n has taken p l a c e g e n e r a t i n g an MV io trace of D monitore a t t r i b u t e d t o MV c o n c e n t r a t i o n i n c r e a s i n g i n t h e f i r s t few hundred nanoseconds a f t e r t h e l a s e r f l a s h . Simultaneous quenching of p o r t i o n s o f the humic substance spectrum are c o n s i s t e n t w i t h the humic substance d o n a t i n g an e l e c t r o n and forming a c h a r g e - t r a n s f e r complex w i t h humic s u b s t a n c e s . T h i s i s t h e f i r s t known o b s e r v a t i o n o f such a h e r b i c i d e - h u m i c s u b s t a n c e charge t r a n s f e r i n t e r a c t i o n w i t h f l a s h p h o t o l y s i s . The r e s u l t s shown i n t h i s s e c t i o n and t h e n i t r o u s o x i d e quenching o f T720 seem t o be good evidence f o r the generation o f HS charge-transfer complexes i n v o l v i n g an aquated e l e c t r o n upon i r r a d i a t i o n o f HS s o l u t i o n s . S e e

2

Summary and C o n c l u s i o n s : Phototransients

Environmental

S i g n i f i c a n c e o f Observed

The quantum y i e l d c a l c u l a t e d from l a s e r d a t a f o r s o l v a t e d e l e c t r o n p r o d u c t i o n by A l d r i c h HSX e x c i t e d a t 355 nm was e x t r a p o l a t e d t o a day averaged s u r f a c e steady state concentration of s o l v a t e d e l e c t r o n ^ j n s u n l i t waters. I n d i r e c t s u n l i g h t an average r a t e o f 1.62 χ 10 photons per second i m p i n g i n g on a square c e n t i m e t e r o f s u r f a c e water was c a l c u l a t e d from s o l a r photon f l u x e s (32-33). U s i n g a b s o r p t i v i t i e s o f HS from 299 t o 800 nm ( w i t h an O.D. o f 0.1 a t 355 nm) and a s o l v a t e d e l e c t ^ g n p r o d u c t i o n quantum y i e l d o f 0.014 ( 1 6 ) , a r a t e o f 3.50 χ 10 moles of s o l v a t e d e l e c t r o n s p r o d u c e d p e r second was c a l c u l a t e d . Based on a l i f e t i m e o f 0.25 microseconds f o r s o l v a t e d e l e c t r o n s i n a i r s a t u r a t e d n a t u r a l w a t e r s , t h e s u r f a c e steady s t a t e c o n c e n t r a t i o n ^ ^ t h e t o p c o b i c c e n t i m e t e r o f water was c a l c u l a t e d t o be about 10 M. T h i s c o n c e n t r a t i o n wass w i t h i n t h e range o f c a l c u l a t e d f o r r e a c t i v e oxy o r g a n i c r a d i c a l s i n n a t u r a l waters by ( 2 3 , 8 ) . The d e t a i l s o f t h i s and f u r t h e r c a l c u a t i o n s which e x t r a p o l a t e d t h e l a s e r d a t a t o f i e l d c o n d i t i o n s were d e s c r i b e d i n d e t a i l elsewhere (16). To gauge t h e s i g n i f i c a n c e o f t h e t r a n s i e n t s observed i n t h i s l a s e r study i n s u r f a c e w a t e r s , the s p e c i a t i o n o f t h e v a r i o u s oxy anions and c a t i o n s , and o f t h e r e l a t i v e p o p u l a t i o n s o f the v a r i o u s

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400

500

600

700

153

800

WAVELENGTH (nm) F i g u r e 6. E x c i t e d s t a t e a b s o r p t i o n was measured f o r m e l a n i n , 1:1 HS:raelanin, and a c o n t r o l HS s o l u t i o n , and m o n i t o r e d a t 400-450 ns a f t e r the 355 nm l a s e r f l a s h .

0 040

%m

WAVELENGTH (nm) F i g u r e 7. E x c i t e d s t a t e spectra^ o f an A l d r i c h humic c o n t r o l s o l u t i o n s p i k e d w i t h 10 M m e t h y l v i o l o g e n .

substance

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o x i d a t i o n s t a t e s of m e t a l s must be t a k e n i n t o c o n s i d e r a t i o n . For i n s t a n c e r e c e n t e v i d e n c e f o r a mechanism f o r the p r o d u c t i o n of hydrogen p e r o x i d e from the d i s m u t a t i o n of s u p e r o x i d e i o n s and f o r the e x i s t e n c e of reduced m e t a l s p e c i e s i n s u r f a c e waters has been p a r t i a l l y e x p l a i n e d by the s o l a r f l u x and the quantum y i e l d f o r the solvated electron. I f the o p e r a t i o n a l t r i p l e t energy l e v e l f o r e x c i t e d s t a t e s of humic substances i s indeed around 57 k c a l / m o l e , then p o l l u t a n t s w h i c h have t r i p l e t energy l e v e l s below t h i s v a l u e are l i k e l y to p h o t o r e a c t by an e n e r g y t r a n s f e r m e c h a n i s m m e d i a t e d by h u m i c substances or DOM. I f a redox p o t e n t i a l of an environmentally r e a c t i v e t r a n s i e n t produced by humic s u b s t a n c e s i s h i g h e r than t h a t o f a x e n o b i o t i c , then the x e n o b i o t i c may be o x i d i z e d , e t c . i f the reaction i s also k i n e t i c a l l y favorable. J u s t as t h e b i c a r b o n a t e b u f f e r s y s t e m s t a b i l i z e s n a t u r a l w a t e r s to a s i g n i f i c a n t e x t e n t a g a i n s t r a d i c a l changes i n pH, i t i s p o s s i b l e t h a t humic substance n a r r o w range of redox p o t e n t i a l p h o t o l y t i c p r o c e s s e s i f t h e y have an below t h a t of humic s u b s t a n c e s . T h i s c o n j e c t u r e i s s u p p o r t e d by p h o t o c h e m i c a l s t u d i e s of o i l s p i l l s c o n t a i n i n g p o l y a r o m a t i c h y d r o c a r b o n s , which are d e g r a d a b l e m a i n l y by p h o t o l y t i c p r o c e s s e s i n f i e l d s t u d i e s ( 3 4 ) . Work done here documunting a p h o t o c h e m i c a l l y induced e l e c t r o n t r a n s f e r from HS to m e t h y l v i o l o g e n was p r o m i s i n g . Studies using f l a s h p h o t o l y s i s as an a i d i n u n d e r s t a n d i n g t h e m e c h a n i s m s o f e l e c t r o n d o n a t i n g h e r b i c i d e s are underway. Also thiocarbamate p e s t i c i d e s a r e known t o be d e g r a d e d by an i n d i r e c t p h o t o l y t i c mechanism mediated by HS ( 3 5 ) . Thus HS p h o t o t r a n s i e n t s t h a t a c t as e n e r g y d o n o r s and e l e c t r o n a c c e p t o r s have b e e n shown t o have e n v i r o n m e n t a l s i g n i f i c a n c e i n f i e l d s t u d i e s by o t h e r workers as w e l l as by e x t r a p o l a t i o n s of l a s e r d a t a t a k e n i n t h i s s t u d y . A l l of the humic substance s o l u t i o n s and n a t u r a l water samples d i s c u s s e d i n the m a t e r i a l s and methods s e c t i o n of t h i s paper were shown to have common p h o t o t r a n s i e n t s , T475 and T720. T h i s i m p l i e s a g l o b a l common energy s i n k i n n a t u r a l waters t h a t contain d i s s o l v e d o r g a n i c m a t t e r , w i t h q u a n t i f i a b l e energy and e l e c t r o n t r a n s f e r c a p a b i l i t i e s . I t i s expected t h a t c o n t i n u e d l a s e r s t u d i e s of t h i s system backed up by complementary f i e l d studes w i l l enable us to b e t t e r u n d e r s t a n d the human e c o l o g y of n a t u r a l w a t e r s when solar f l u x and HS p h o t o c h e m i s t r y i s taken i n t o perspective consideration. Acknowledgment s A. F i s c h e r wishes to thank J . W i n t e r l e , J . M o f f e t t , P. M i l n e , R.G. Z i k a , and W.J. Cooper f o r t h e i r v a l u a b l e comments and a s s i s t a n c e i n the t a s k of t h i s paper. Literature Cited

1. Zafiriou, O. C; Joussot-Dubien, J.; Zepp, R. G.; Zika R.; Environ. Sci. Technol. 1984, 18, 358-371. 2. Zafiriou, O. C; In "Chemical Oceanography"; Riley, J. P.; Chester, R., Eds.; Academic: London, 1983; Vol. 8, p. 339.

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3. Zika, R. G. In "Marine Organic Chemistry: Evolution, Composition, Interactions, and Chemistry of Organic Matter in Seawater"; Duursma, Ε. K.; Dawson, R., Eds.; Elsevier: Amsterdam, 1981; p. 299. 4. Cooper, W. J.; Zika R. G. Science 1983, 220, 711. 5. Draper, W. M.; Crosby, D. G. Arch. Environ. Contam. Toxicol. 1983, 12, 121. 6. Zepp, R. G.; Baughman, G. L.; Schlotzhauer, P. F. Chemosphere 1981, 10, 119. 7. Haag, W. R.; Hoigne, R. J.; Gassman, E.; Braun, A. Chemosphere 1984, 13, 641. 8. Hill, T.; Hendry, D. G.; Richardson, H. Science 1980, 207, 886-887. 9. Baxter, R. M.; Carey, J. H. Freshwater Biol. 1982, 12, 285. 10. Rashid, M. A. "Geochemistry of Marine Humic Compounds"; Springer-Verlag: New York, 1985. 11. Christman, R. F. Gjessing Ε T "Aquati d Terrestrial Humic Materials" 12. Ziechman, W. Z. Pflanzeneraehr. Bodenkd. 1977, 140, 133-57. 13. Slawinska, D.; Slawinski, J.; Sarna, T. J. Soil Sci. 1975, 26, 93. 14. Rabek, J. F.; Ranbey, B.; Polymer Eng. Sci. 1975, 15, 40. 15. Mill, T.; Davenport, J. E.; Winterle, J. S.; Fischer, Α.; Maybe, W. H.; Tse, D.; Baraze, Α.; Conference on Gas- Liquid Chemistry of Natural Waters, Brookhaven National Laboratories, April 1984. BNL 51757, Vol 1 and 2. p. 25-1. 16. Fischer, A. Ph.D. Thesis, University of California, Santa Cruz, 1985. 17. Power, J. F.; Sharma, D. K.; Langford, C. H.; Bonneau, R.; Joussot-Dubien, J.; In Press. 18. Braun, Α., Personal Communication. 19. MacCarthy, P.; Peterson, M. J.; Malcolm, R. L.; Thurman, Ε. M. Anal. Chem. 1979, 51, 2041. 20. Horwitz, J. Ph.D. Thesis, University of California, Santa Cruz, 1983. 21. "Optical Spectra of Nonmetallic Inorganic Transient Species in Aqueous Solution," U. S. National Bureau of Standards Reference Data Series 69, 1973. 22. "Selected Specific Rates of Reactions of Transients from Water in Aqueous Soluton. 1. Hydrated Electron," U.S. National Bureau of Standards Reference Data Series 49, 1973. 23. Zepp, R. G.; Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol. 1985, 19, 462. 24. Green, D. W. "Perry's Engineering Handbook"; McGraw Hill: New York, 1984. 25. Bensasson, R. V.; Land, E. J.; Truscott, T. G. "Flash Photolysis and Pulse Radiolysis"; Pergamon: Oxford, 1983. 26. Rabek, J. F.; Ranbey, B. Photochem. Photobiol. 1978, 28, 557. 27. Slawinski, J.; Puzyna, W.; Slawinska, D. Photochem. Photobiol. 1978, 28, 459. 28. Slawinski, J.; Puzyna, W.; Slawinska, D. Photochem. Photobiol. 1978, 28, 459. 29. Kalyanaraman, B.; Felix, C. C.; Sealy, R. C. Photochem. Photobiol., 1982, 36, 5.

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30. 31. 32. 33. 34. 35.

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Kalyanaraman, B.; Felix, C. C.; Sealy, R. C. J. Am. Chem. Soc. 1984, 106, 7327. Kosower, Ε. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524. Mill, T.; Hendry, D. G.; and Richardson, H. Science 1980, 207, 886. Davenport, J. E.; Winterle, J.; Mabey, W. R.; Mill T. USEPA Report, Contract #68-03-2981. Payne, J. R.; Phillips, C. R. Environ. Sci. Technol. 1985, 19, 7. Crosby, D. G. In "Pesticide Chemistry: Human Welfare and Environment", Miyamoto, J.; Kearney, P. C., Eds.; Pergamon: Oxford, 1983, Vol. 2., p. 339.

RECEIVED July 1,

1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 12

Laser Flash Photolytic Studies of a Well-Characterized Soil Humic Substance 1

1

1

2

Joan F. Power , Devendra K. Sharma , Cooper H. Langford , Roland Bonneau , and Jacques Joussot-Dubien 2

1

2

Department of Chemistry and Canadian Center for Picosecond Flash Photolysis, Concordia University, Montreal, Quebec, Canada CNRS Laboratoire de Chimie Physique A, Universite de Bordeaux I, 33405 Talence Cedex, France

Laser flash photolysi characterised soil humic substance, Armadale Fulvic Acid (P.E.I., Canada), have been carried out with excitation at 355 nm on picosecond and nanosecond time scales. Three principal transient absorption signals have been observed in aqueous solution: a component with a maximum absorption at 675 nm and a lifetime of about 1 microsecond (at pH = 7.0), a second component with a maximum absorption at 475 nm and a lifetime of 110 microseconds, and a third component with a broad, featureless transient absorption spectrum and a lifetime in excess of 100 microseconds. The 675 nm signal is believed to be a solvated electron on the basis of lifetime and quenching data, and is observed 20 ps after excitation. The 475 nm signal is believed to be a radical cation on the basis of its concurrent appearance with the electron at 20 ps. The third feature­ less component emerges nanoseconds after excitation and is believed to correspond to the triplet states of the humic material. The r o l e o f d i s s o l v e d h u m i c m a t e r i a l s i n initiating the photochemical transformations o f o r g a n i c compounds i n natural waters h a s been a subject o f i n c r e a s i n g study over t h e l a s t decade. T h i s s u b j e c t shows g r e a t p r o m i s e of p r o v i d i n g a d e t a i l e d understanding o f t h e m u l t i p l e

0097-6156/87/0327-0157$06.00/0 © 1987 American Chemical Society

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mechanisms by w h i c h many o r g a n i c s u b s t a n c e s a r e d e g r a d e d i n n a t u r a l s y s t e m s under t h e a c t i o n o f s u n l i g h t . A number o f s t u d i e s have y i e l d e d evidence f o r the p a r t i c i p a t i o n o f a wide v a r i e t y o f t r a n s i e n t s p e c i e s i n t h e p h o t o l y s i s i n i t i a t e d by a b s o r p t i o n o f l i g h t by humic substances. These i n c l u d e Ό Η r a d i c a l s (1), 02 ( 2 ) , R00( 3 ) , and O 2 " ( 4 ) . Their presence i n natural systems suggests multiple photopathways i n c l u d i n g O 2 oxidations, O2" reductions, light i n i t i a t e d free radical d e c o m p o s i t i o n s , and H 2 O 2 c h e m i s t r y . S u n l i g h t may a l s o i n f l u e n c e the s p e c i a t i o n o f metal ions i n s u r f a c e waters t h r o u g h p h o t o r e d o x r e a c t i o n s o f t h e humic l i g a n d s ( 5 ) . The need f o r photophysical s t u d i e s i n t h i s area i s indicated by the d i v e r s i t y o f the photochemistry observed. Flash photolysis experiments i n the e a r l y t i m e domain may i d e n t i f y precursors to l a t e r transient species, as w e l l a mechanisms. One l a s e r p h o t o p h y s i c a l s t u d y (6) has a l r e a d y been r e p o r t e d by F i s c h e r e t a l , i n w h i c h a v a r i e t y o f n a t u r a l w a t e r s a m p l e s and s o l u t i o n s o f humic s u b s t a n c e s t a n d a r d s were e x c i t e d a t 355 nm ( n e a r t h e a b s o r p t i o n maximum f o r e x c i t a t i o n o f humic s u b s t a n c e f l u o r e s c e n c e ) . Extensions of t h e i r work a p p e a r i n t h i s symposium. They r e s o l v e d two components o f t r a n s i e n t a b s o r p t i o n common t o a l l t h e samples s t u d i e d : a component w i t h maximum a b s o r b a n c e a t 475 nm w i t h a l i f e t i m e of several microseconds and a signal a t 700 nm which was a t t r i b u t e d t o a s o l v a t e d e l e c t r o n on t h e b a s i s o f i s i t s s e n s i t i v i t y t o N 2 O and on i t s a b s o r p t i o n s p e c t r u m . In this work, we have carried out l a s e r f l a s h p h o t o l y s i s s t u d i e s on a w e l l characterized sample o f fulvic acid (Armadale, P.E.I., Canada), f o r which t h e physicochemical data are well established ( 7 , 8, 9 ) . Our s t u d i e s have e x t e n d e d from p i c o s e c o n d t o m i c r o s e c o n d domains and i n c l u d e s t u d i e s o f pH e f f e c t s , O2 and m e t a l ion quenching. Our o b j e c t i v e has been t o l i n k t h e p h o t o p h y s i c a l events o f t h e nanosecond time s c a l e where t r a n s i e n t s d e c a y , w i t h t h o s e o f t h e e a r l i e r s c a l e , where t h e y a p p e a r , and t o s t u d y t h e e v o l u t i o n o f t h e o b s e r v e d transient species over t h e e n t i r e time s c a l e . On t h i s b a s i s we hope t o i d e n t i f y t h e m u l t i p l e pathways o f the p h o t o p h y s i c s o f t h i s complex n a t u r a l m i x t u r e which can account f o r the d i v e r s i t y o f consequent photochemistry. l

1

Experimental

Section

Sample. The w e l l c h a r a c t e r i z e d f u l v i c a c i d u s e d i n our s t u d i e s i s t h e A r m a d a l e f u l v i c a c i d d e s c r i b e d by Hansen and S c h n i t z e r (7) and Gamble ( 8 ) . I t was e x t r a c t e d from the Bh h o r i z o n o f a P.E.I, p o d z o l and i t s p h y s i c o c h e m i ­ c a l p r o p e r t i e s have been w e l l summarized by Gamble and Schnitzer (9) and L a n g f o r d e t a l ( 1 0 ) . The number average molecular weight of this material has been

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determined a t 951 g/mol ( 7 , .10). F u n c t i o n a l group analysis o f the m a t e r i a l has y i e l d e d the f o l l o w i n g composition basis: 3.3 meq/g phenolics, 7.7 meq/g carboxylate, 3.6 meq/g aliphatic alcohol, 2.5 meq/g k e t o n e , and 0.6 meq/tf q u i n o i d ( 9 ) . Préparât i o n . F u i v i c a c i d s t o c k s o l u t i o n s were p r e p a r e d i n M i l l i p o r e u l t r a p u r e water or glass d i s t i l l e d water w h i c h had been p r e v i o u s l y a d j u s t e d t o pH 2.0. Samples were p r e p a r e d by d i l u t i o n o f s t o c k s o l u t i o n s , and pH was a d j u s t e d by a d d i t i o n o f 0.1 M HC1 o r NaOH ( p r e c i s i o n = + 0.15 u n i t s ) . Sample c o n c e n t r a t i o n was 200-250 mg/1 o f FA. The o p t i c a l d e n s i t y o f t h e samples was d e t e r m i n e d i n a 1 cm o r 1 mm q u a r t z c e l l a t 355 nm on a Beckman UVVis or Perkin Elmer absorption spectrophotometer. M i l l i m o l a r stock s o l u t i o n s of metal ions were p r e p a r e d d i r e c t l y from a v a i l a b l e m e t a l c h l o r i d e s ( r e a g e n t g r a d e ) . Picosecond Δ A w h i l e nanosecond s p e c t r s o l u t i o n s e x c e p t as o t h e r w i s e i n d i c a t e d . Flash Photolysis Spectrometers. Picosecond timescale measurements were c a r r i e d o u t a t 355 nm ( 3 r d H a r m o n i c , Nd:YAG) w i t h 20 ps h a l f w i d t h p u l s e s from t h e p a s s i v e l y mode l o c k e d l a s e r system at the Canadian Center f o r Picosecond Laser Flash Photolysis (.11.) . The p u l s e e n e r g y was 3 mj and t h e w a v e l e n g t h r a n g e o f t h e p r o b e p u l s e used f o r o b s e r v a t i o n o f s p e c t r a e x t e n d e d from 400 t o 700 nm. Nanosecond t i m e s c a l e measurements were r e c o r d e d on a mode l o c k e d Nd:YAG l a s e r system l o c a t e d i n t h e CNRS L a b o r a t o i r e de C h i m i e P h y s i q u e A, U n i v e r s i t é de Bordeaux I, T a l e n c e , France. The p u l s e e n e r g i e s r a n g e d f r o m 1040 mj a t 355 nm. P u i s e d u r a t i o n was 300 p s . D e t a i l s o f the system d e s i g n a r e p r o v i d e d elsewhere ( 1 2 ) . Results

and

ks_signments

K i n e t i c Overview. The o b s e r v a t i o n s may b e s t be i n t r o d u c e d i n summary by t h e k i n e t i c map d e p i c t e d i n FIGURE 1. The k i n e t i c b e h a v i o r o f t h e sample may be r e s o l v e d i n t o four d i s t i n c t regions, temporally. Two ( I I , I I I ) have been s p e c t r a l l y c h a r a c t e r i z e d and match t h o s e observed by F i s c h e r e t a l ( 6 ) . Third, there emerges (IV) a featureless "black background transient absorbance, w h i c h may be t r a c e d from i t s o r i g i n i n e a r l y n a n o s e c o n d t o i t s d e c a y i n t h e l a t e r m i c r o s e c o n d domain. The f i n a l component ( I ) r e m a i n e d i n a c c e s s i b l e t o d e t a i l e d s t u d y on both f l a s h p h o t o l y s i s systems u s e d owing t o i t s a p p e a r a n c e i n an awkward t i m e domain. Time r e s o l v e d s p e c t r a f o r components ( I I ) and ( I I I ) are reported a t pH 4.0 and 7.0 i n t h e p r e s e n c e and absence o f N 2 O , a w i d e l y used e l e c t r o n s c a v e n g e r (See FIGURE 2). As e x p e c t e d (6, 1 6 ) , t h e 675 nm peaked s p e c t r u m i s quenched i n t h e p r e s e n c e of N 2 O , consistent with the behavior of a solvated electron. Quenching 1 1

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Delta Abe.

(IV)

Black

R e e l due

C > iQp

ys)

Time

FIGURE J. Schematic summary o f l i f e t i m e components observed f o r the photolysis o f Armadale fulvic acid i n the nanosecond time domain. E x c i t a t i o n a t 3 5 5 nm.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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A

Wave 1 e n g t h

(mn)

F I G U R E 2. Nanosecond transient absorption spectrum of FA a t 2 0 0 n s a f t e r e x c i t a t i o n b v a 300 ps puise i n the p r e s e n c e o f NgO: (A) pH = 4 . 0 , (B) pH = 7.0. Spectra are corrected f o r ground s t a t e a b s o r p t i o n . Pulse energy i s 30 m.j .

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results in the p r e s e n c e o f C u * and O2 f u r t h e r c o n f i r m t h i s assignment. The 475 nm t r a n s i e n t reported in t h e e a r l i e r work (6) a p p e a r s i n these s p e c t r a as w e l l , and i n a l l ca.4es i s superimposed on a "black , f e a t u r e l e s s background (component IV of our kinetic scheme). The 475 nm t r a n s i e n t decays with a l i f e t i m e of approximately 1-10 microseconds. Picosecond excited state a b s o r p t i o n s p e c t r a have been r e c o r d e d f o r samples a t pH 2.0, 4.0 and 7.0, and i n all cases, the presence of the 675 nm t r a n s i e n t i s e s t a b l i s h e d at a probe p u l s e delay of only 20 ps a f t e r excitation, which is as early as i t i s p o s s i b l e to measure w i t h an e x c i t a t i o n p u l s e o f 20-30 ps h a l f width (See FIGURE 3). Measurements made by R e n t z e p i s (14.) on the s o l v a t i o n time of electrons in aqueous media, i n d i c a t e t h e emergenc s p e c t r u m a t 650-700 Thus, our r e s u l t i s c o n s i s t e n t w i t h prompt g e n e r a t i o n o f the e l e c t r o n . The 475 nm absorbing species i s also present at 20 ps, s u g g e s t i n g t h a t i t i s formed concurr e n t l y w i t h the s o l v a t e d e l e c t r o n . " P i c o s e c o n d Domain" s t u d i e s a l s o show t h e growth o f a broad featureless transient spectrum i n the e a r l y n a n o s e c o n d domain (See FIGURE 4A); t h i s f e a t u r e matches the characteristics of t h e " b l a c k " r e s i d u e (component IV) o f our " n a n o s e c o n d " k i n e t i c summary. This species i s not s e e n u n t i l a f t e r a p p r o x i m a t e l y 2 n s . I t i s not a "primary" species l i k e the o t h e r two. Its absorption spectrum grows in with a r i s e t i m e o f a b o u t 2 ns and a c h i e v e s an a p p r o x i m a t e s t e a d y s t a t e at 6-8 ns f o l l o w i n g e x c i t a t i o n (See FIGURE 4 B ) . The featureless "black" spectral signature is c l e a r l y s e e n when t h e i n i t i a l s p e c t r u m ( a t 20 ps) o f t h e promptly formed 475 and 675 nm t r a n s i e n t s i s s u b t r a c t e d from s p e c t r a r e c o r d e d at later times (See FIGURE 4 ) . This result i n d i c a t e s complete f o r m a t i o n o f t h e 475 nm t r a n s i e n t w i t h i n the f i r s t 20 ps following excitation, and f u r t h e r s u g g e s t s i t s f o r m a t i o n c o n c u r r e n t l y w i t h t h e appearance of the solvated electron, since i t , too, r e m a i n s unchanged, i n spectrum p a s t 20 p s . Therefore, i t a p p e a r s l i k e l y t h a t t h e 475 nm t r a n s i e n t is in fact, the radical cation, R-* , associated with the site g e n e r a t i n g the s o l v a t e d e l e c t r o n . Because p h o t o i o n i z a tion of phenols i s known to occur i n water, h i g h l y s u b s t i t u t e d p h e n o l i c s t r u c t u r e s i n the f u l v i c a c i d c o u l d be good candidates to account f o r the g e n e r a t i n g s i t e s of the s o l v a t e d e l e c t r o n s . T h e r e i s some c o m p l i c a t i o n of the sample b e h a v i o r at pH 7.0 i n t h e e a r l y t i m e domain (See FIGURE 4 B ) . An e a r l y component o f d e c a y i s p r e s e n t c e n t e r e d at 475 nm with a d e c a y t i m e o f about 100 ps. Results obtained in our l a b o r a t o r i e s on p i c o s e c o n d e m i s s i o n o f the Armadale sample ( 1_5_) , u s i n g a s t r e a k camera, i n d i c a t e a 2

1 1

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.

FIGURE 3. Transient a b s o r p t i o n s p e c t r u m o f FA a t pH = 2.0, 4.0, and 7.0. S p e c t r a were r e c o r d e d 20 ps after a 20 ps e x c i t a t i o n pulse are corrected f o r ground s t a t e absorption. P u l s e e n e r g y was 3 m j .

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS A .180—

D •

1

t

Β Signal

38. 2 - -

Intensity ( Arb. Units ) 34.2--

F I G U R E 4. (A) F o r m a t i o n o f t h e " b l a c k r e s i d u e " c o m p o n e n t on t h e e a r l y n a n o s e c o n d time s c a l e . Spectra recorded a r e d i f f e r e n t i a l , t h e 20 p s S p e c t r u m has been s u b s t r a c ted. Pulse energy w a s 3 m.j : (B) P i c o s e c o n d t o e a r l y n a n o s e c o n d k i n e t i c p r o f i l e f o r FA a t pH = 7 . 0 . A b s o r p ­ tion signal was i n t e g r a t e d o v e r t h e o b s e r v a t i o n wave­ l e n g t h r a n g e o f 4 7 5 - 5 2 0 nm. Pulse energy 3 mj.

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s i g n i f i c a n t component of emission, which decays w i t h a time c o n s t a n t o f 100 ps e m i s s i o n ( X> 400 nm) . The e a r l y decay i n a b s o r p t i o n a t pH 7.0 seems t o c o r r e s p o n d t o t h i s e m i s s i o n component i n d u r a t i o n . Quenching S t u d i e s . The solvated electron component a t 675 nm was s t u d i e d under c o n d i t i o n s o f oxygen and c o p p e r quenching (FIGURE 5). I t appears that i n our (concentrated) solutions of fulvic acid (200 mg/1), and a t e l e v a t e d l a s e r power (3-30 m j ) , i t i s formed by a one p h o t o n process. Conditions f o r o b s e r v a t i o n of these s p e c t r a w i t h e x c i t a t i o n a t 355 nm and on t h e nanosecond s c a l e match t h o s e c i t e d by p r e v i o u s a u t h o r s ( 6 ) . M o r e o v e r , we were a b l e t o o b s e r v e t h e s o l v a t e d e l e c t r o n f o r m a t i o n at reduced power (2-3 mj) on the picosecond scale in ( c o n c e n t r a t e d ) s o l u t i o n (200 mg/1), s u g g e s t i n g , a t l e a s t f o r our m a t e r i a l , that t h e r e i s no i n t e n s i t y t h r e s h o l d f o r f o r m a t i o n o f e~ While both the s e n s i t i v i t y to N 2 O s t r o n g l y s u g g e s t i t s i d e n t i t y as a s o l v a t e d e l e c t r o n , we a r e a b l e t o offer further confir­ mation. In the presence of d i s s o l v e d O2 t h e s o l v a t e d e l e c t r o n i s scavenged a c c o r d i n g to the e q u a t i o n : O2

+ e~

=02"

w i t h an o b s e r v e d r a t e constant of 2 χ Ι Ο M~ s" a c c o r d i n g to pulse r a d i o l y s i s s t u d i e s (1.6) . A SternVolmer p l o t f o r q u e n c h i n g by oxygen y i e l d e d a quenching rate constant o f 1.2 ± 0.1 χ Ι Ο M~ s " , which i s i n r e a s o n a b l e agreement w i t h t h e r a d i o l y t i c measurements. Measurements c a r r i e d out f o r q u e n c h i n g o f e ~ < a q > by Cu* y i e l d e d a q u e n c h i n g r a t e c o n s t a n t , k q , o f 1.6 + 0.2 χ ΙΟ M" s , which i s i n s a t i s f a c t o r y agreement w i t h t h e r a d i o l y t i c a l l y measured r a t e c o n s t a n t ( 1 6 ) : 1

1

1

0

1

0

1

1

2

1

1

0

Cu*

2

- 1

+ e-

= Cu*

kobs

= 3.3

χ

ΙΟ

1

0

M"

1

s"

1

Both s e t s of r e s u l t s c l e a r l y i n d i c a t e t h a t the s o l v a t e d e l e c t r o n i s quenched by f r e e metal i o n and uncomplexed oxygen. We have a l s o examined t h e s t a t i c q u e n c h i n g b e h a v i o r for Cu* and O2 by c o m p a r i n g the values of Δ A o , the initial transient absorption at t = 0, at v a r i o u s c o n c e n t r a t i o n s of quencher. In b o t h c a s e s , we o b s e r v e d a p p a r e n t s t a t i c q u e n c h i n g e f f e c t s , a l t h o u g h t h e s m a l l O2 e f f e c t s s h o u l d be r e g a r d e d w i t h some c a u t i o n . Both s e t s of results suggest quenching by s p e c i e s bound t o t h e f u l v i c a c i d c l o s e t o the g e n e r a t i n g s i t e o f t h e s o l v a t e d electron. pH measurements have p r o v i d e d f u r t h e r c o n f i r m a t i o n of the s o l v a t e d e l e c t r o n ' s identity. Reaction o f H* w i t h e~ s o l v . y i e l d s h y d r o g e n a c c o r d i n g t o t h e r e a c t i o n : 2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS H*

+ e2H

= H = H

kobs

= 2.1

χ

10

1 ϋ

M"

1

s"

1

2

The l i f e t i m e i s 480 ns a t pH 4.0, yielding a k v a l u e o f 1.2 ± 0.2 χ 10 M s , i n approximate agreement w i t h the r a d i o l y s i s v a l u e quoted above ( 1 6 ) . The v a l u e o f kq f o r H q u e n c h i n g was e s t i m a t e d from the S t e r n - V o l m e r l a w a n d l i f e t i m e s o f 4 8 0 n s ( a t pH 4.0) and 1 1 3 0 n s ( a t pH 7 . 0 ) . A comparison of v a l u e s of Δ Α ο from m e a s u r e m e n t s made i n d i c a t e s s t a t i c q u e n c h i n g on v a r i o u s t i m e s c a l e s b y a f a c t o r o f 1.5 - 2 o n g o i n g f r o m pH 7.0 t o pH 4.0. I n s p e c t i o n o f t h e s p e c t r a a t 20 p s d i r e c t l y show t h e s t a t i c q u e n c h i n g e f f e c t ( S e e FIGURE 3 ) . C o n s i s t e n t l y , we h a v e f o u n d t h a t t h e q u e n c h i n g r a t e constants evaluated for the solvated electron are a p p r o x i m a t e l y h a l f the literature values observed f o r quenching by fre aggregate nature o a significant fraction of the s o l v a t e d e l e c t r o n s are generated in the interior of the humic particles. D i f f u s i o n of the quenchers i n t o the i n t e r i o r i s l i m i t e d , reducing the overall effectiveness of the quenching p r o c e s s e s , and hence, t h e o b s e r v e d k q ' s . S t u d i e s were c a r r i e d o u t on t h e s a m p l e a t s p e c i f i c l o n g t i m e s a f t e r e x c i t a t i o n i n an attempt to study the 4 7 5 nm and " b l a c k r e s i d u e " s i g n a l q u e n c h i n g . Quenching of the 475 nm signal has remained inaccessible to quantitative analysis due t o i t s low i n t e n s i t y , and t o the f a c t that i t i s superimposed on the more i n t e n s e background l e v e l o f " b l a c k r e s i d u e " s i g n a l (See FIGURES 2A a n d 2 B ) . The "black residue" signal persists for hundreds of microseconds while the v e r y weak c a t i o n radical component decays with a lifetime of a few microseconds. The value i s quite pH s e n s i t i v e a n d generally matches the range of values reported in p r e v i o u s work (6). Accurate determination of Τ for e i t h e r s p e c i e s was n o t f e a s i b l e . We h a v e v e r i f i e d t h e l i n e a r i t y o f f o r m a t i o n o f b o t h s i g n a l s y_s energy. L i n e a r i t y (except f o r s a t u r a t i o n of the R s i g n a l a t t h e h i g h e s t e n e r g i e s ) s u g g e s t s monophot o n i c f o r m a t i o n p r o c e s s e s , a t e n e r g i e s up t o 30 m j . Quenching studies of the "black residue" were c o n d u c t e d by comparison of signal amplitudes at the s i n g l e time o f 100 m i c r o s e c o n d s a f t e r e x c i t a t i o n . From t h i s c o m p a r i s o n , we c o m p u t e t h e r a t i o Q: q

l u

1

_ i

+

+

Q

= ΔΑI

Ml

χ

100

ΔΑο

where ΔΑίΜΪ i s the transient absorption observed f o r concentration, I M I, of quencher, w h i l e Δ Ao i s t h e s i g n a l i n t e n s i t y observed in the absence of quencher (See FIGURE 6 ) . 2+

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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POWER ET AL.

Laser Flash Photolytic Studies of a Humic Substance

167

. 140-4-

FIGURE 5. P l o t o f t r a n s i e n t a b s o r b a n c e vs pulse for the s o l v a t e d electron transient absorption n a n o s e c o n d t i m e s c a l e a t 675 nm.

energy on t h e

FIGURE 6. Plot of "black residue" signal intensity q u e n c h i n g vs concentration of metal i o n quencher at a t i m e o f 100 m i c r o s e c o n d s . Sample pH = 6.0. A l l signals c o r r e c t e d f o r ground s t a t e absorption.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

Q u e n c h i n g o f t h i s l o n g t r a n s i e n t w a s s t u d i e d a t pH 6.0 i n t h e p r e s e n c e of millimolar quantities of C u Z n , and N i . M e t a l i o n q u e n c h i n g was f o u n d t o f o l l o w the order C u > Ni* Zn ^. T h e maximum value of Q was c o m p u t e d and p l o t t e d y s t h e two e l e c t r o n r e d u c t i o n p o t e n t i a l (18) f o r each metal i o n s p e c i e s (I.U.P.A.C. v a l u e s (18 ) : + 2

t

+ 2

f

2

f 2

2

M

f

f 2

+ 2e

= M+

I n t e r e s t i n g l y , t h i s y i e l d s a l i n e a r r e l a t i o n s h i p between Q and Ε although the slope (value) i s small. Moreover, we have been able t o add oxygen quenching r e s u l t s t o these l i n e s . These results may suggest an e l e c t r o n donating character to the f u l v i c acid interaction with the quenchers, although the small slope o f the curve seems to eliminat e l e c t r o n t r a n s f e r (redox In v i e w o f r e c e n t work by Zepp e t a l (.19) i n determining t h e energv o f t r i p l e t s t a t e s formed i n t h e irradiation o f humic waters in sunlight, a set of triplet states i s implied i n o u r p h o t o p h y s i c a l scheme. T r i p l e t s t a t e e n e r g i e s w e r e d e t e r m i n e d (1J9) b y m e a n s o f the c i s - t r a n s i s o m e r i s a t i o n o f 1,3-pentadiene, a process which occurs v i a a t r i p l e t pathway. Approximately 40% of the t r i p l e t s t a t e s produced i n the samples s t u d i e d were c a p a b l e o f s e n s i t i z i n g t h i s i s o m e r i s a t i o n w h i l e t h e remaining 60* were of lower e n e r g i e s and s t u d i e d by s e n s i t i z e d formation of s i n g l e t oxygen. T h e mean e n e r g y of the higher energy triplet states was 250 k J / m o l ( c o r r e s p o n d i n g t o a n a b s o r p t i o n maximum o f 4 6 5 n m ) . The b r o a d r a n g e o f t r i p l e t s t a t e energies reported is consistent with character of the "black residue" s i g n a l we o b s e r v e d i n o u r e x p e r i m e n t s . In considering the s u p e r p o s i t i o n o f t h e w e a k c a t i o n r a d i c a l s i g n a l on the underlying residue, our r e s u l t s predict a dual character f o r the transient a b s o r p t i o n o b s e r v e d a t 475 nm. This proposal i s consistent with the results of F i s c h e r e t a l (6) whose work demonstrated a composite nature t o t h e 475 nm transient that they observed several microseconds after excitation. Their signal intensity was partially quenched by triplet energy a c c e p t o r s and c o r r e l a t e d d i r e c t l y with the photooxidat i o n r a t e o f t h e probe molecules para-isopropy1 phenol and p a r a - m e t h o x y p h e n o l . Conclus ionsConcerning Band A s s i g n m e n t s . Two a s p e c t s o f the photophysics are clear: electrons and r a d i c a l cations a r e formed promptly. The t h i r d w e l l d e f i n e d component, the "black residue", arises with a time constant similar t o the decay o f the f l u o r e s c e n c e o f f u l v i c a c i d and has a range of energies, plus lifetime and q u e n c h i n g patterns, that suggest the t r i p l e t s that s e n s i t i z a t i o n e x p e r i m e n t s show t o be p r e s e n t . We c a n

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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POWER ET AL.

Laser Flash Photolytic Studies of a Humic Substance

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+

add t h a t t h e main t r a n s i e n t s f o r e r > and R c a n n o t be e x c i t e d w i t h t h e 5 3 0 nm g r e e n l i n e o f t h e N d r Y A G l a s e r . a q

Discussion A P r o p o s a l f o r a M o d e l o f FA S p e c t r a and P h o t o p h y s i c s . In the study o f humic substance photophysics the inherent complexity of the materials presents a considerable challenge to the i n t e r p r e t a t i o n of the results. The r o l e o f q u i n o i d chromophores has f r e q u e n t l y been invoked i n explanations of the photochemical properties o f h u m i c w a t e r s ( 4 , 5, 6 ) . We f e e l , h o w e v e r , t h a t s i n c e the dominant fraction o f our sample f u n c t i o n a l i t y i s phenol carboxylate, i t s c o n t r i b u t i o n t o the photochemic a l b e h a v i o u r o f humic s u b s t a n c e s s h o u l d n o t be underemphasized. We wish t o propose a model from a more general s t a r t i n g point A general proble a model. The main p h o t o a c t i v e g r o u p s o f f u l v i c a c i d s be they phenolic, phenol carboxylate o r q u i n o i d , pose a difficulty f o r the explanation o f humic substance spectra. In a l l cases, t h e dominant fraction of a b s o r p t i o n o f t h e s e s p e c i e s l i e s w e l l i n t o UV b e l o w 300 nm. One w o u l d t h e r e f o r e e x p e c t a maximum f l u o r e s c e n c e emission f o r excitation i n t h e 250-300 nm w a v e l e n g t h range s i n c e this corresponds to a p a t h w a y f o r maximum population of singlet fluorescing states of organic chromophores o f these s t r u c t u r e s . In fact, humic substance fluorescence i s most e f f i c i e n t l y e x c i t e d a t 350-400 nm rather than at the shorter wavelengths (_13! ) . The e m i s s i o n i s b r o a d band and e x t e n d s f r o m 400-500 nm. While the fluorescence properties of a number of phenol c a r b o x y l a t e model compounds have been r e p o r t e d and compared with those of fulvic acid, i t has been p o i n t e d out by B u f f l e et a l (13) t h a t few o f t h e s e compounds a p p r o a c h t h e e x c i t a t i o n and e m i s s i o n wavelengths of f u l v i c acid. Furthermore, the r a t i o o f f l u o r e s c e n c e t o c o n c e n t r a t i o n of dissolved o r g a n i c c a r b o n was m u c h h i g h e r f o r t h e s e m o d e l c o m p o u n d s that f o rf u l v i c acids. A t h i g h DOC l e v e l s (> 1 0 0 m g / 1 ) , the wavelengths o f f l u o r e s c e n c e e x c i t a t i o n and emission shifted significantly to the r e d as concentration increased. This indicated that the fluorescence properties of the dissolved humic materials were s t r o n g l y a f f e c t e d by a g g r e g a t i o n phenomena. An explanation of the observed properties may p e r h a p s be found i n the l i t e r a t u r e on p h o t o p h y s i c s o f synthetic polymers (2JS). A common process i nthe photophysics of s y n t h e t i c polymers (in solution and i n the s o l i d phase) which c o n t a i n aromatic s u b s t i t u e n t s i s the formation o f excimers and e x c i p l e x e s , a c h i e v e d by the face to face alignment of aromatic r i n g s t r u c t u r e s , with a typical spacing of 4 X. The fluorescence of excimers i s typically broadband, f e a t u r e l e s s and

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

s i g n i f i c a n t l y r e d s h i f t e d from t h e u n a s s o c i a t e d (monomeric) fluorescence. Some typical examples from t h e literature collected i n TABLE I indicate excimer e m i s s i o n from many common aromatic centers (See TABLE I) . TABLE I Compound

A b s o r p t i o n Maximum ex (nm) Monomer Dimer ( Excimer) OC2H4

OOCH-CH(Reference

383

470

280

340

405

460

21) ( S o l u t i o n ;

HC-^H CH CUff (Reference

E m i s s i o n Maximum ex (nm) Monomer Dimer ( Excimer)

25) ( S o l u t i o n )

Bisphenol A d i g l y c i d y i ether resin

275

(Reference

26)

(Solution)

iReference

24)

(Solution)

330 350

Where ground s t a t e a l i g n m e n t o f a r o m a t i c chromopho­ r e s i s optimum f o r e x c i m e r ( o r e x c i p l e x ) f o r m a t i o n , t h e e x c i m e r forms upon t h e a b s o r p t i o n o f l i g h t r a t h e r t h a n by e n e r g y t r a n s f e r from an e x c i t e d singlet state to a ground s t a t e chromophore. The term " e x c i m e r " i n t h e s e c a s e s i s l o o s e l y a p p l i e d , and has been r e f e r r e d t o more p r e c i s e l y as a " c o n t a c t p a i r , preformed i n the ground s t a t e " o r an " e x c i t e d p a i r complex" (21). The case o f excimer formation i n a series of p h e n o l s has been r e p o r t e d (2Λ) and may be r e l e v a n t t o fulvic acid photophysics. Excimer f o r m a t i o n was observed i n concentrated solutions of p-cresol f o r e x c i t a t i o n a t 280 nm but not f o r ο or m c r e s o l s . However, the absorption spectra of concentrated solu­ t i o n s o f t h e ο and m i s o m e r s c o n t a i n e d a l o n g w a v e l e n g t h tail owing t o ground s t a t e a s s o c i a t i o n s , b e l i e v e d to arise from hydrogen bonding. This i s especially suggestive since hydrogen bonding i s thought t o be f

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Laser Flash Photolytic Studies of a Humic Substance

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crucial to formation and a g g r e g a t i o n o f FA's ( 2 7 ) . Fluorescence e x c i t e d i n t h i s longer wavelength region of the absorption spectrum f o r o, m cresoi, had t h e character of excimer fluorescence. This p a r t i c u l a r case o f the c r e s o l s i l l u s t r a t e s the point that, i n some c a s e s , e x c i m e r f l u o r e s c e n c e may n o t be excited at the shorter absorption wavelengths. Emission from t h e e x c i m e r s t a t e s may r e q u i r e a b s o r p t i o n by ground state conformations close to the excimer geometry; t h i s i s e s p e c i a l l y t r u e where t h e l i f e t i m e o f t h e monomer i s s h o r t c o m p a r e d w i t h t h e t i m e r e q u i r e d f o r d i f f u s i o n a l encounter. I t seems l i k e l y t h a t t h e humic substances, because they are aggregates, p r o v i d e many possibilities f o r a c h i e v i n g these conformations, since chromophore c o n c e n t r a t i o n i s locally high within the aggregates. In the case o chromophores a r e d i s s i m i l a r sed r e d s h i f t i n t h e ground s t a t e a b s o r p t i o n due t o a greater charge t r a n s f e r character to the transitions. It seems plausible then, t h a t excimers and e x c i p l e x e s a c c o u n t f o r t h e 3 5 0 nm excitation maximum f o r humic substance fluorescence, s i n c e they match t h e c h a r a c t e r of the f l u o r e s c e n c e emission a n d may account f o r the observed e x c i t a t i o n wavelengths. The s o l u t i o n behavior o f e x c i p l e x e s i s e v e n more complex, and a c c o u n t s f o r the formation of radicals, t r i p l e t e x c i p l e x e s and o r d i n a r y t r i p l e t s t a t e s (2j0, 2 2 ) . The general kinetic scheme f o r the photophysics of e x c i p l e x e s h a s b e e n s u m m a r i z e d b y M a t t e s a n d F a r i d (20.) and i s diagrammed b e l o w . Some o f o u r r e s u l t s may b e r e c o n c i l e d w i t h t h e p r o c e s s e s s u m m a r i z e d by t h i s scheme:

* [Pr/Dt]~ [AVDtb-DÀ

IA D ,m

D,A

A,D+ products

products

( A - acceptor s 0= donor ) The f o r m a t i o n o f t h e s o l v a t e d e l e c t r o n and r a d i c a l c a t i o n may r e s u l t f r o m t h e l o s s o f an e l e c t r o n f r o m an e x c i t e d EDA c o m p l e x t o y i e l d a r a d i c a l c a t i o n a n d g r o u n d state acceptor molecule ( 2 0 ) . This is a s u i t a b l y low e n e r g y p r o c e s s t o be m o n o p h o t o n i c :

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

[A D/H 0]—* A +D + H 0 , , l

T

2

t

2

The results of Fischer et a l (6) i n d i c a t e a c o n n e c t i o n b e t w e e n t h e 4 7 5 nm a n d 7 0 0 nm transients i n samples with a high TOC content. This connection supports our hypothesis because o f the i n c r e a s e i nthe number o f a r o m a t i c c e n t e r s a n d a g g r e g a t i o n (27.) o f t h e sample t h a t o c c u r s a t h i g h e r humic s u b s t a n c e concentrat ion. References

1. W.M. Draper, D.G. Crosby, J. Agric. Food Chem., 1984, 32, 231-237. 2. R.G. Zepp, N.L Wolfe G.L Baughmon R.C Hoilis Nature, 1977, 267 3. T. Mill, D.G. Hendry, H. Richardson, Science, 1980, 207, 886-887. 4. R.M. Baxter, J.H. Carev, Nature, 1983, 306, 575576. 5. T.M. Waite, F.M.M. Morel, Environ. Sci. Technol., 1984, 18, 860-868. 6. A.M. Fischer, Τ. Mill, D.S. Kliger, D. Tse, IHSS Symposium Proceedings. Birmingham, UK, July 23-28, 1984. 7. E.H. Hansen, M. Schnitzer, Anal. Chim. Acta, 1969, 46, 247-254. 8. D.S. Gamble, Can. J. Chem., 1970, 48, 2662-2669. 9. D.S. Gamble, M. Schnitzer, In "Trace Metal and Metal Organic Interactions in Natural Water", P.S. Singer Editor, Ann Arbor MI, Ann Arbor Science Publishers Inc., pp. 265-302. 10. C.H. Langford, D.S. Gamble, A.W. Underdown, S. Lee, In "Aquatic and Terrestrial Humic Materials", R.F. Christman and E.T. Gjessing Editors, Ann Arbor MI, Ann Arbor Science Publishers Inc., Chapter 11, pp. 219-237. 11. Ν. Serpone, D.K. Sharma, To Be Published. 12. R. Bonneau, J. Photochem.. 1979, 10, 439-449. 13. J. Buffle, P. Deladoey, J. Zumstein, W. Haerdi, Schweiz Z. Hydrol.. 1982, 44, 326-361. 14. P. Rentzepis, R.P. Jones, J. Chem. Phys., 1973, 59, 766-773. 15. C.H. Langford, D.K. Sharma, R. Lesage, J.F. Power, Environ. Tech. Lett.. In Press. 16. M.S. Matheson, In "The Solvated Electron", A.C.S. Symposium, E.J. Hart, Chairman, Advances in Chemistry Series, Washington, 1965, p. 51. 17. K. Kryszewski, B. Nadolski, In "Singlet OxygenReactions with Organic Compounds and Polymers", B. Ranby and J.K. Rabek Editors, Wiley, New York, 1978, pp. 244-253.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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18.

C.R.C. Handbook of Chemistry and Physics. Two electron reduction potentials were examined so that a comparison could be made between the redox properties of Cu , Zn ,O + , and N i , since Ni does not exist as a stable species. See 63rd Edition, C.R.C. Press, Boca Raton, Florida, USA, 1983, pp. D162 167. 19. R.G. Zepp, P.T. Schlotzhauer, R.M. Sink, EnyironmenUl Sci. Tech., 1985, 19, 74-81. 20. S.L. Mattes, S. Farid, Science, 1984, 226, 917-921. 21. M. Graley, A. Reiser, A.J. Roberts, D. Phillips, In "Photophysics of Synthetic Polymers", Science Reviews, Middlesex, England, 1982, pp. 26-38. 22. J. Birks, In "Photophysics of Aromatic Molecules", Wiley Interscience, London, England, 1970, Chapter 9. 23. D. Phillips, A.J Roberts "Photophysic f Synthetic Polymers" England, 1982. 24. A. Harriman, B.W. Rockett, J. Photochem.,1974, 2, 405-408. 25. G. Rumbles, In "Photophysics of Synthetic Polymers", D. Phillips and A.J. Roberts Editors, Science Reviews, Middlesex, England, 1982, pp. 525. 26. N.S. Allen, J.P. Binkley, B.J. Parsons, G.D. Phillips, N.X. Tennent, In "Photophysics of Synthetic Polymers", D. Phillips and A.J. Roberts Editors, Science Reviews, Middlesex, England, 1982, pp. 128 135. 27. A. Underdown, CH. Langford, D.S. Gamble, Environ. Sci. Tech., 1985, 19, 132-136. +2

+2

2

+2

2

RECEIVED June 24,

1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

+

Chapter 13

Natural Photosensitizers in Sea Water: Riboflavin and Its Breakdown Products Kenneth Mopper and Rod G. Zika Rosenstiel School of Marine and Atmospheric Sciences, Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL 33149

Photosensitized reactions in surface seawater are implicated in a the photogeneratio specie hydroge peroxide to the photo-oxygenation of natural and anthropogenic compounds. Previous work has concentrated on the excited state properties of humic substances, but our preliminary work has shown that dissolved flavins, although present at low concentrations (usually < 1 nM), are an important class of compounds contributing to marine photochemical processes, and, as a group, undergo dynamic photochemical transformations in the sea. In this paper we present evidence for and discuss the implications of photoreactions of flavins in seawater, including their photosensitizing capacity, their contribution to the photoproduction of hydrated electrons, superoxide anion and hydrogen peroxide, the role of the excited state reactions of flavins on the initiation of free radical reactions, and the role of flavins in photosensitized production of low molecular weight organics, such as carbonyl compounds. We also present the first detailed data on temporal and depth distribution of flavins in coastal and open ocean environments. In environmental photochemistry, r e s e a r c h on p h o t o s e n s i t i z e d i n i t i a t e d r e a c t i o n s i n n a t u r a l waters has f o c u s s e d m a i n l y on humic m a t e r i a l s because they a r e one o f t h e most abundant and a l s o most s t r o n g l y a b s o r b i n g c o n s t i t u e n t s i n these waters. However, o t h e r potentially important biogenically produced and p h o t o r e a c t i v e compounds e x i s t i n seawater. One such c l a s s o f compounds, which has received little attention either i n terms of their concentration and distribution or their photochemical c h a r a c t e r i z a t i o n i n seawater, i s t h e f l a v i n s ( F L ) . F l a v i n s a r e o f s p e c i a l i n t e r e s t because they a r e among t h e few n a t u r a l l y o c c u r r i n g o r g a n i c compounds i n seawater w i t h l a r g e c r o s s s e c t i o n s a t i n c i d e n t 0097-6156/87/0327-0174$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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s u n l i g h t f r e q u e n c i e s (1). In a d d i t i o n , e f f i c i e n c i e s of i n t e r s y s t e m c r o s s i n g from s i n g l e t to t r i p l e t s t a t e s are h i g h Ç2). Figure 1 g i v e s the s t r u c t u r e s and the s y s t e m a t i c names of r i b o f l a v i n and i t s dominant photochemical breakdown p r o d u c t s . V i r t u a l l y n o t h i n g i s known a b o u t t h e s p a t i a l o r t e m p o r a l d i s t r i b u t i o n of f l a v i n s i n the marine environment. In f a c t , o n l y two a n a l y s e s of f l a v i n s i n seawater have been p r e v i o u s l y ^reported, one from the M e d i t e r r a n e a n Ç3) and one from an A u s t r a l i a n c o r a l reef (4). Past s t u d i e s have l a r g e l y been b i o l o g i c a l i n scope, c o n c e r n e d p r i m a r i l y w i t h the r e l e a s e of v i t a m i n s by marine organisms (4-7) and w i t h the e c o l o g i c a l r o l e t h a t v i t a m i n s may have i n t h e s e a ( 8 ) . A s e r i o u s l i m i t a t i o n to such s t u d i e s has been the l a c k o f s e n s i t i v e ( p g / L ) and s e l e c t i v e a n a l y t i c a l methods t o determine low n a t u r a l l e v e l s of these compound. T r a d i t i o n a l l y t h i s has been done by the use of b i o a s s a y t e c h n i q u e s . R e c e n t l y , the development of modern h i g h performance l i q u i d chromatographic (HPLC) t e c h n i q u e s , couple f a c i l i t a t e d the measuremen p r o d u c t s i n marine waters ( 4 ) . I n t h i s ' p a p e r we p r e s e n t the f i r s t d e t a i l e d measurements of f l a v i n c o n c e n t r a t i o n s i n open ocean and c o a s t a l e n v i r o n m e n t s , and p r e l i m i n a r y experiments d e a l i n g w i t h the marine p h o t o c h e m i s t r y and photosensitizing p r o p e r t i e s of riboflavin and i t s breakdown products. Experimental The d i u r n a l and d e p t h d i s t r i b u t i o n s o f d i s s o l v e d f l a v i n s were d e t e r m i n e d a t two s t a t i o n s o f f F l o r i d a : F l o r i d a Bay ( c o a s t a l ; 25 19.4 N, 81° 09.8'W) and Tongue of the Ocean ( o c e a n i c ; 25° 2 4 N , 78° 7»W) d u r i n g the SOLARS I c r u i s e ( A p r i l 1-7, 1985; R/V Calanus) and t h e SOLARS I I c r u i s e ( J u n e 12 - 18, 1985; R/V Cape F l o r i d a ) , r e s p e c t i v e l y . Samples were taken w i t h a g l a s s b o t t l e (opened below the surface) or with teflon-lined Go-Flo bottles (General Oceanics). S a m p l e s f o r d i s s o l v e d f l a v i n s were e x t r a c t e d and a n a l y z e d onboard by HPLC, u s u a l l y w i t h i n 10 min of s a m p l i n g . The r e v e r s e d phase HPLC s e p a r a t i o n s were done on a 5 urn H y p e r s i l C-18 c o l u m n u s i n g a m o b i l e p h a s e o f 30% m e t h a n o l / 7 0 % 20 mM s o d i u m a c e t a t e (pH 5.6) i n w a t e r a t a f l o w r a t e of 1.5 m l / m i n . The f l a v i n s were d e t e c t e d by f l u o r e s c e n c e u s i n g an e x c i t a t i o n band of 320-390 nm and an e m i s s i o n band of 440-520 nm. D e t a i l s of the e x t r a c t i o n and chromatography are g i v e n elsewhere (9) ; t y p i c a l chromatograms of f l a v i n s i n n a t u r a l seawater samples are g i v e n i n F i g u r e 2. In a d d i t i o n to field measurements, s e v e r a l p r e l i m i n a r y l a b o r a t o r y experiments were run t o d e l i n e a t e the photochemical d e c o m p o s i t i o n pathways of f l a v i n s and t o e l u c i d a t e t h e i r r o l e i n i n i t i a t i n g secondary p h o t o - r e a c t i o n s i n seawater. For c l a r i t y , d e t a i l s of these experiments are p r e s e n t e d under the a p p r o p r i a t e sections following. f

f

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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FORMYLMETHYLFLAVIN

LUMI F L A V I N

LUMICHROME

F i g u r e 1. S t r u c t u r e s and names o f f l a v i n s .

0

8

mm

F i g u r e 2. Re versed-phase HPLC s e p a r a t i o n s o f a 12.5 nM standard mixture of f l a v i n s (0.25 pmol o f each compound i n j e c t e d ) and a n a t u r a l seawater sample from B i s c a y n e Bay, F L . - approximate c o n c e n t r a t i o n s a r e 190 pM r i b o f l a v i n ( R F ) , 22 pM l u m i f l a v i n (LF) and 250 pM lumichrome ( L C ) . The l a r g e i n i t i a l peak may be humic s u b s t a n c e s .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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R e s u l t s and

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Discussion

P h o t o s e n s i t i z a t i o n by R i b o f l a v i n and Lumichrome. The p h o t o s e n s i ­ t i z i n g p r o p e r t i e s of r i b o f l a v i n i n s e a w a t e r , s p e c i f i c a l l y w i t h r e s p e c t to s i n g l e t oxygen f o r m a t i o n , have been i n v e s t i g a t e d by Momzikoff e t a l . ( 1 0 ) . T h e i r work demonstrated t h a t r i b o f l a v i n was a p a r t i c u l a r l y good producer of s i n g l e t oxygen and they suggested t h a t e or H* t r a n s f e r r e a c t i o n s would not be i m p o r t a n t , a l t h o u g h t h e y d i d not e x p e r i m e n t a l l y demonstrate t h i s . Their conclusion i s based on the f a c t t h a t processes competing f o r the e x c i t e d t r i p l e t state of the flavin ( F l ) are dominated by photophysical d e a c t i v a t i o n p r o c e s s e s (e.g. i n t e r a c t i o n w i t h ground s t a t e oxygen, fluorescence and vibrational relaxation). Given the high c o n c e n t r a t i o n of d i s s o l v e d oxygen r e l a t i v e to o r g a n i c s u b s t r a t e s i n seawater, the r a t i o of i n t e r m o l e c u l a r e o r H* t r a n s f e r to energy t r a n s f e r r e a c t i o n s i n n a t u r a l seawater would be expected t o be small. Nevertheless, thi r e a c t i o n s c o u l d be a s i g n i f i c a n t source of f r e e r a d i c a l s i n the environment. Futhermore, f l a v i n s , because of t h e i r b i o l o g i c a l source and limited water solubility, could function more e f f i c i e n t l y v i a e o r H* t r a n s f e r r e a c t i o n s i n s u b s t r a t e e n r i c h e d m i c r o - e n v i r o n m e n t s such as on p a r t i c l e s , micro-organisms o r i n the sea s u r f a c e m i c r o l a y e r . I n order to t e s t whether e n v i r o n m e n t a l l y s i g n i f i c a n t l e v e l s of n a t u r a l l y o c c u r r i n g f l a v i n s do indeed undergo e or H* t r a n s f e r reactions, the photosensitized degradation of methionine by r i b o f l a v i n was examined. M e t h i o n i n e was chosen as a c o n v e n i e n t s u b s t r a t e to work w i t h f o r a n a l y t i c a l reasons and because d i f f e r e n t p r o d u c t s r e s u l t from i t s r e a c t i o n w i t h Δ 0^ or v i a o x i d a t i o n by t r i p l e t s e n s i t i z e r s (11). F i g u r e 3 show^s t h e e f f e c t o f added r i b o f l a v i n upon t h e p h o t o i n d u c e d d e g r a d a t i o n o f m e t h i o n i n e i n seawater. D u r i n g the r e a c t i o n , the f l u o r e s c e n c e spectrum of the s o l u t i o n r a p i d l y changed. The c h a n g e was i n t e r p r e t e d as t h e c o n v e r s i o n of r i b o f l a v i n to lumichrome (see below). Methionine was m e a s u r e d by c o n v e r t i n g i t t o t h e d a n s y l d e r i v a t i v e and t h e n q u a n t i f y i n g i t by HPLC a g a i n s t an i n t e r n a l s t a n d a r d ( 12 ). No m e t h i o n i n e s u l f o x i d e , which i s the product of the r e a c t i o n of m e t h i o n i n e w i t h s i n g l e t oxygen ( 1 1 ^ wa^s observed. The product of the r e a c t i o n of m e t h i o n i n e w i t h FL s h o u l d be m e t h i o n a l , t h e presence of w h i c h , a l t h o u g h not a n a l y z e d f o r , was suggested by i t s characteristic odor i n the head space over the irradiated solutions. I n subsequent r e l a t e d experiments m e t h i o n a l was indeed i d e n t i f i e d as a major product u s i n g the HPLC method of Mopper and Stahovec (13) w h i c h i s based on r e a c t i o n of c a r b o n y l s i n seawater with 2,4-dinitrophenylhydrazine. I t i s apparent from F i g u r e 3 t h a t sub-nanomolar c o n c e n t r a t i o n s of f l a v i n s can f u n c t i o n as e f f i c i e n t p h o t o s e n s i t i z e r s In n a t u r a l seawater. Figure 3 a l s o shows a comparison between the relative efficiency of riboflavin and two other natural aquatic p h o t o s e n s i t i z e r s , namely a s o i l f u l v i c a c i d and p h l o r g l u c i n o l based p o l y m e r w h i c h i s b e l i e v e d t o be an exudate of a b e n t h i c macrophyte ( 1 4 ) , to o x i d i z e methionine. B a s e d on t h e c o n c e n t r a t i o n o f s e n s i t i z e r n e e d e d t o decompose m e t h i o n i n e a t a r a t e o f 0.05

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g u r e 3. Comparison of t h e c o n c e n t r a t i o n dependence of added p h o t o s e n s i t i z e r on t h e p h o t o d e g r a d a t i o n r a t e o f m e t h i o n i n e . F o r e a c h case t h e s o l u t i o n s were 5 μΜ i n m e t h i o n i n e and were i r r a d i a t e d f o r 2 hours u s i n g o n l y wavelengths > 300 nm i s o l a t e d from a mercury medium p r e s s u r e lamp. A l l samples were prepared i n a i r s a t u r a t e d a r t i f i c i a l seawater o f i o n i c s t r e n g t h 0.7 and w i t h pH o f 8.1.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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179

u m o l / L / h r , t h e r e l a t i v e e f f i c i e n c i e s a r e 280:100:1 f o r r i b o f l a v i n , p h l o r g l u c i n o l polymer and f u l v i c a c i d , r e s p e c t i v e l y . It i s difficult to extrapolate t h i s r e l a t i v e comparison t o n a t u r a l s e a w a t e r because o f t h e u n c e r t a i n t y i n v o l v e d i n d e t e r m i n i n g humic s u b s t a n c e c o n c e n t r a t i o n and absorbance and quantum y i e l d v e r s u s wavelength dependence. I f i t i s assumed t h a t marine humic s u b s t a n c e s have c h a r a c t e r i s t i c s s i m i l a r t o t h e model f u l v i c a c i d u s e d i n t h i s s t u d y , t h a t t h e i r c o n c e n t r a t i o n i s i n the range o f 150-800 ug/L ( 1 5 ) , and t h a t t h e f l a v i n c o n c e n t r a t i o n i s 1 nM (.376 u g / L ) , then t h e f l a v i n s p o t e n t i a l l y c o n s t i t u t e a s i g n i f i c a n t f r a c t i o n ( i . e . 13-70%) of the combined p h o t o s e n s i t i z e r a c t i v i t y i n seawater. Thus, even low c o n c e n t r a t i o n s of f l a v i n s i n n a t u r a l s e a w a t e r c o u l d be c o m p e t i t i v e w i t h much h i g h e r l e v e l s o f o t h e r naturally occurring photosensitizers. I f f l a v i n s are highly v a r i a b l e i n t h e i r d i s t r i b u t i o n , as might be a n t i c i p a t e d g i v e n t h e i r b i o l o g i c a l o r i g i n , then a c o n s i d e r a b l e range o f p h o t o s e n s i t i z e r a c t i v i t y from t h e s e compound R o l e o f F l a v i n s i n Hydrogen P e r o x i d e Formation. F l a v i n s may f u n c t i o n as p h o t o s e n s i t i z e r s through p h o t o e l e c t r o n e j e c t i o n ( 1 6 ) . I f t h i s process_ i s o c c u r r i n g i n s e a w a t e r , t h e o n l y s i g n i f i c a n t reaction o f e ( a q ) would be w i t h 0^ t o produce 0~ · D i s p r o p o r t i o n a t i o n o f 0^ would g e n e r a t e Η 0^. The o t h e r r e a c t i v e product would p r o b a b l y be the f l a v i n r a d i c a l c a t i o n . Reducing agents such as EDTA, m e t h i o n i n e and othej: amino a c i d s h a v e been shown t o e n h a n c e t h e g e n e r a t i o n o f 0~ In aerobic s o l u t i o n s o f p h o t o s e n s i t i z e r s i n c l u d i n g f l a v i n s ( Γ7-18). These r e s u l t s a r e c o n s i s t e n t w i t h a p i c t u r e of a l i g h t d r i v e n system of f l a v i n (LC,RF) a c t i n g as a p h o t o s e n s i t i z e r , m o l e c u l a r oxygen as an e l e c t r o n acceptor and an u n i d e n t i f i e d substance (RH), p o s s i b l y h u m i c / f u l v i c a c i d o r some component t h e r e o f , as an e l e c t r o n donor w h i c h i n c o n c e r t produce hydrogen p e r o x i d e i n seawater. These r e a c t i o n s can proceed as shown i n e q u a t i o n s 1 through 4:

3

F1*

+

RH

3 * Fl

+

0^

>

+ Fl

+

0

HF1.

+

0„

->

Fl

+

H0 .

+

RH

>

Fl

+

RH

Fl

+

—> x

3

2

HF1.

+

R.

(1)

(2) n

2

+

(3)

(4)

*

ljy c i t h e r r e d u c t i o n o f t h e FL by t h e donor o r by o x i d a t i o n of FL by oxygen f o l l o w e d by subsequent r e g e n e r a t i o n of the s e n s i t i z e r by t h e donor. Other i n t e r p r e t a t i o n s can be made, such as direct photoionization to form hydrated electrons; however, i n f o r m a t i o n as t o t h e p h o t o c h e m i s t r y o f f l a v i n s i n seawater, t h e i r excited state reactions with m o l e c u l a r oxygen and p l a u s i b l e e l e c t r o n donors i s needed i n order t o a s s e s s whether f l a v i n s p l a y an a p p r e c i a b l e r o l e i n t h e p r o d u c t i o n o f h y d r o g e n p e r o x i d e i n

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

marine systems ( 1 9 ) . A p r e l i m i n a r y e v a l u a t i o n of f l a v i n s as a p o t e n t i a l p h o t o c h e m i c a l source of H^O^ and o x i d a n t of e o r H" donor s u b s t r a t e s was made by comparing r a t e s of p r o d u c t i o n of H«0« d u r i n g s u n l i g h t exposure o f ( 1 ) G u l f Stream seawater (GSS), ( 2 ) GSS w i t h 5 χ 10 M m e t h i o n i n e , (3) GSS w i t h 1 χ 10 M r i b o f l a v i n and ( 4 ) GSS w i t h 1 χ 10 M r i b o f l a v i n and 5 χ 10 M methionine. I t i s e v i d e n t from F i g u r e 4 t h a t r i b o f l a v i n , even when added a t o n l y 1 nM, a c o n c e n t r a t i o n t y p i c a l of t o t a l d i s s o l v e d f l a v i n s i n c o a s t a l waters (see b e l o w ) , i s a s i g n i f i c a n t source of l ^ O - (and perhaps 0^ ). In a d d i t i o n , hydrogen p e r o x i d e p r o d u c t i o n can De enhanced by t h e a d d i t i o n of an e o r H" donor such as m e t h i o n i n e . That enhancement i s a l s o observed when o n l y the donor i s added i n d i c a t e s t h a t r e a c t i o n mechanisms s i m i l a r to those f u n c t i o n i n g f o r f l a v i n s m i g h t be o c c u r r i n g f o r the n a t u r a l p h o t o s e n s i t i z i n g c o n s i t u e n t s i n seawater. P r o d u c t i o n of Low M o l e c u l a Weigh (LMW) C a r b o n y l Compound fro F l a v i n Induced P h o t o c h e m i c a r e a c t i o n s are importan i n i t i a t e f r e e r a d i c a l r e a c t i o n s i n seawater. I n p a r t i c u l a r , the generation of f l a v i n radicals from e l e c t r o n o r H* transfer r e a c t i o n s may p l a y an i m p o r t a n t r o l e i n the o x i d a 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 seawater. At the low c o n c e n t r a t i o n s of f l a v i n s i n s e a w a t e r , H" a b s t r a c t i o n w i l l p r o b a b l y be the major r e a c t i o n pathway of the f l a v i n r a d i c a l p r o v i d e d s u f f i c i e n t c o n c e n t r a t i o n s o f e l e c t r o n s o r hydrogen atom donors a r e p r e s e n t . Other s i g n i f i c a n t p a t h w a y s are p r o b a b l y a d d i t i o n r e a c t i o n s of HF1* t o 0 or to l o n g l i v e d r a d i c a l s such as those found i n humic m a t e r i a l s . I n seawater t h e r a d i c a l , R*, g e n e r a t e d i n t h e a b o v e r e a c t i o n , w o u l d r e a c t p r e d o m i n a n t l y w i t h ground s t a t e 0 t o form the organoperoxy r a d i c a l (20) which can then undergo s e v e r a l r e a c t i o n s t o y i e l d LMW c a r b o n y l compounds as w e l l as o t h e r o x i d i z e d p r o d u c t s ( 2 1 - 2 2 ) t h r o u g h pathways such as the f o l l o w i n g . 2

2

R. R0 . 2

RO .

f

+ R 0 . 2

+ +

0

>

2

!

RH >

>

R0 .

(5)

2

ROOH + R\

(6) 1

V a r i o u s p r o d u c t s ( i . e . ROOR , R CH0, RCHO, R CH OH, R 0., RO., 0 ) f

f

(7)

f

2

Low m o l e c u l a r weight o r g a n i c f r a g m e n t s , such as s i m p l e o r g a n i c a c i d s and c a r b o n y l compounds, a r e common p r o d u c t s f o r m e d f r o m p h o t o s e n s i t i z e d o x i d a t i o n of more complex o r g a n i c s u b s t r a t e s by f l a v i n s ( 2 j 23-24). Thus, by m o n i t o r i n g the p h o t o p r o d u c t i o n of t h e s e " f r a g m e n t s " , one may be a b l e t o s t u d y v a r i o u s p h o t o o x i d a t i v e p r o c e s s e s even i n complex media, such as seawater and o t h e r n a t u r a l w a t e r s , where the n a t u r e o f the o r g a n i c s u b s t r a t e s b e i n g o x i d i z e d i s not known. Experiments were performed t o determine whether the p r o d u c t i o n of c a r b o n y l compounds c o u l d be used to s t u d y f l a v i n - s e n s i t i z e d p h o t o o x i d a t i o n of o r g a n i c m a t t e r i n seawater. D u p l i c a t e s o l u t i o n s (0.5 - 1 umol/L) of r i b o f l a v i n , lumichrome + r i b o s e , l u m a z i n e

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

MOPPER AND ZIKA

Natural Photosensitizers in Sea Water

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F i g u r e 4. P h o t o c h e m i c a l p r o d u c t i o n r a t e s of H«0^ i n midday s u n l i g h t : (0) i n G u l f Stream seawater (GSSVO, ( O ) i n GSSW + 1 χ 10 M r i b o f l a v i n , (·) i n GSSW + 5 χ _ψ M methionine, ( + ) i n GSSW + 5 χ 10 M m e t h i o n i n e + 1 χ 10 M riboflavin.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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[2 ,4 ( 1H, 3H)-pteridinedionçI, and marine humic a c i d ( i s o l a t e d by G.R. Harvey; 2 5 ) i n f i l t e r - s t e r i l i z e d , a e r a t e d seawater ( B i s c a y n e B a y , F L ) and o r g a n i c - f r e e d i s t i l l e d w a t e r were i r r a d i a t e d i n s e a l e d , s t e r i l e q u a r t z g l a s s v e s s e l s w i t h a sunlamp (Westinghouse M o d e l ΜΗ 400/E; lower c u t - o f f 320 nm) f o r 4-6 h. A l i q u o t s o f i r r a d i a t e d samples and c o n t r o l s were removed f o r c a r b o n y l a n a l y s i s by HPLC of t h e 2,4 d i n i t r o p h e n y l - h y d r a z o n e s . C o n t a m i n a t i o n from sample workup and p r e c o n c e n t r a t i o n s t e p s was a v o i d e d by i n j e c t i n g d e r i v a t i z e d seawater samples d i r e c t l y i n t o the l i q u i d chromatograph (13). Peak i d e n t i f i c a t i o n was a c h i e v e d by c o - i n j e c t i o n w i t h a u t h e n t i c s u b s t a n c e s . A n a l y s i s o f u n d e r i v a t i z e d samples showed no peaks i n the r e g i o n o f t h e chromatograms where c a r b o n y l hydrazones elute. The addition of p h o t o s e n s i t i z e r s to natural seawater s i g n i f i c a n t l y e n h a n c e s c a r b o n y l p r o d u c t i o n r a t e s , T a b l e I and F i g u r e 5, p r e s u m a b l y a s a r e s u l t o f e n h a n c e d p h o t o s e n s i t i z e d o x i d a t i o n rates of d i s s o l v e i n t e r p e t a t i o n i s not s t r a i g h t f o r w a r a r e s i g n i f i c a n t s o u r c e s f o r LMW c a r b o n y l s a s i n d i c a t e d by t h e p r o d u c t i o n o f t h e s e compounds i n o r g a n i c - f r e e d i s t i l l e d w a t e r c o n t a i n i n g o n l y these p h o t o s e n s i t i z e r s , Table I . I n t h e case o f r i b o f l a v i n , t h e absorbed energy i s t r a n s f e r r e d i n t r a m o l e c u l a r l y t o the r i b i t y l group f o l l o w e d by o x i d a t i v e f r a g m e n t a t i o n (26-27). I r r a d i a t i o n o f an e q u i m o l a r s o l u t i o n o f lumichrome and r i b o s e gave a s i g n i f i c a n t l y lower c a r b o n y l p r o d u c t i o n r a t e then r i b o f l a v i n a l o n e , T a b l e I I , s u p p o r t i n g t h e i d e a t h a t i n t r a m o l e c u l a r energy t r a n s f e r s a r e i m p o r t a n t i n r i b o f l a v i n d e c o m p o s i t i o n and c a r b o n y l production. Isoalloxazine-Containing P r o d u c t s from t h e P h o t o d e g r a d a t i o n o f Riboflavin. I r r a d i a t i o n o f r i b o f l a v i n i n deoxygenated aqueous s o l u t i o n l e a d s t o a s e r i e s of r e a c t i o n s whereby t h e r i b i t y l s i d e c h a i n i s ^ c l ^ a v e d and the i s o a l l o x a z i n e r i n g i s reduced ( 2 8 ) . Both t r i p l e t ( RF ) and s i n g l e t ( RF ) e x c i t e d s t a t e s a r e p h o t o r e a c t i v e . The three main i s o a l l o x a z i n e p r o d u c t s o f t h e r e a c t i o n a r e lumichrome, l u m i f l a v i n and f o r m y l m e t h y l f l a v i n ( s e e F i g u r e 6 a ) . Song and M e t z l e r (28) suggested tl^at ^lumichrome a r i s e s p r i n c i p a l l y from photocleavage of the RF excited state, whereas ^ o r j p y l m e t h y l f l a v i n and l u m i f l a v i n a r i s e from the c l e a v a g e o f t h e RF state. The p h o t o d e c o m p o s i t i o n pathways and p r o d u c t s o f r i b o f l a v i n i n seawater have n o t been studied i n detail, although some o b s e r v a t i o n s have been made. Z i k a (12) found lumichrome as t h e o n l y major i s o a l l o x a z i n e p r o d u c t w h i l e Mopper and Z i k a (29) a l s o d e t e c t e d minor amounts of l u m i f l a v i n i n a d d i t i o n t o lumichrome p r o d u c t i o n , upon i r r a d i a t i o n ( s u n l i g h t o r p o l y c h r o m a t i c artificial l i g h t ) o f seawater s o l u t i o n s o f r i b o f l a v i n . Dunlap and S u s i e ( 4 ) a l s o observed t h e p r o d u c t i o n o f minor amounts of l u m i f l a v i n from t h e p h o t o d e g r a d a t i o n o f FMN ( r i b o f l a v i n - 5 - p h o s p h a t e ) i n seawater, t h e major product being lumichrome. The absence o f t h e expected f o r m y l m e t h y l f l a v i n ( i t s e l f q u i t e p h o t o l a b i l e ) and t h e o n l y minor p r o d u c t i o n o f l u m i f l a v i n a r e p o s s i b l y due t o e f f i c i e n t quenching o f the t r i p l e t s t a t e . D i s s o l v e d oxygen i s t h e most l i k e l y c a n d i d a t e as a quencher, due t o i t s h i g h c o n c e n t r a t i o n i n s e a w a t e r , a l t h o u g h 1

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

13.

MOPPER AND ZIKA

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Natural Photosensitizers in Sea Water

T a b l e I . E f f e c t of P h o t o s e n s i t i z e r s on the P h o t o p r o d u c t i o n LMW Aldehydes i n S t e r i l e Seawater and D i s t i l l e d Water 2 Photosensitizer

Seawa

C

2

GXL

(nmol/ 1/h)

Riboflavin

of

193 17

7

C

C

l

2

GXL

C

l

(nmol/l/h)

107 25

C

GXL

2

(%)

0

55

100

0

Lumazin

20

2

5

0

0

0

0

0

0

Humic A c i d

27 12

4

5

9

0

19

75

0

1 P h o t o p r o d u c t i o n i s taken as net p r o d u c t i o n ( p r o d u c t i o n i n i r r a d i a t e d sample minus dark c o n t r o l and minus p r o d u c t i o n i n I r r a d i a t e d seawater a l o n e ) . 2

[ R i b o f l a v i n ] = 1 uM;

[Lumazin] = 1 uM;

[Humic A c i d ] = 10 mg

1

-1

.

3 C

1

= Formaldehyde;

= A c e t a l d e h y d e ; GXL

= Glyoxal.

4 P e r c e n t aldehyde p r o d u c t i o n i n seawater due to p h o t o c h e m i c a l d e g r a d a t i o n of the p h o t o s e n s i t i z e r ; i . e . : [ ( c a r b o n y l p r o d u c t i o n i n d i s t i l l e d H 0 ) / ( p r o d u c t i o n i n s e a w a t e r ) ] X 100%. 9

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

F i g u r e 5· HPLC chromatograms o f c a r b o n y l compounds formed d u r i n g t h e i r r a d i a t i o n o f f i l t e r - s t e r i l i z e d seawater c o n t a i n i n g added p h o t o s e n s i t i z e r s : A. 1 μΜ o f r i b o f l a v i n , B. 10 mg/L of h u m i c a c i d a n d , C. no a d d i t i o n . C. - f o r m a l d e h y d e , C^ ~ a c e t a l d e h y d e , GXL - g l y o x a l , LC - lumichrome, R - excess EfNPH reagent.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

13.

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Natural Photosensitizers in Sea Water

MOPPER AND ZIKA

Table I I . P r o d u c t i o n o f C a r b o n y l Compounds from I r r a d i a t e d S o l u t i o n s o f R i b o f l a v i n , Lumichrome & R i b o s e , and Ribose

Sample

Rate o f (nmol/l/h)

1

t o LMW 2

2

Formaldehyde

Acetaldehyde

PU^

PU^

Ï8

Ï8

Ô

Ô

Riboflavin ( 1 uM)in distilled H 0

103

30

75

19

Ribose ( 1 0 uM)

< 1

1

0

0

Lumichrome & Ribose ( 1 uM each) in d i s t i l l e d

Aldehydes

2.9%

H 0 2

28%

2

< 0.1%

^ C o r r e c t e d f o r dark c o n t r o l s and p r o d u c t i o n i n i r r a d i a t e d alone.

water

2

Rib: Ribose o r R i b i t y l ; PU. & Ρϋ^: Unknown c a r b o n y l compounds (see F i g u r e 5 ) ; P U may be g l y c o l a l d e h y d e . 2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

186

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

low c o n c e n t r a t i o n s o| both p h e n o l , bromide and i o d i d e i o n have been shown t o quench RF (28, 30). Figure 6B o u t l i n e s t h e de-activation pathways a v a i l a b l e f o r e x c i t e d s t a t e flavins, i n c l u d i n g i n t e r n a l conversion (generation of heat), fluorescence, phosphorescence and r e a c t i o n with quenchers and r e a c t i v e substrates. Our p r e l i m i n a r y s t u d i e s i n d i c a t e t h a t RF and FMF have s h o r t h a l f l i v e s i n s u n l i g h t i r r a d i a t e d seawater (on the o r d e r o f a few minutes o r l e s s ) , whereas LC and LF a r e s i g n i f i c a n t l y more s t a b l e ( w i t h h a l f l i v e s on t h e o r d e r o f hours f o r LF t o days f o r LC). Evidence f o r P h o t o c h e m i c a l C o n t r o l Over the D i s t r i b u t i o n o f F l a v i n s i n t h e Sea. Based on t h e r e l a t i v e p h o t o l a b i l i t y o f f l a v i n s i n seawater (see a b o v e ) , one would expect t o f i n d s t r o n g d i u r n a l and depth variations i n flavin concentrations i n the sea, i f p h o t o c h e m i c a l processes are o f f i r s t o r d e r importance. To examine t h i s , we determined th s i t e o f f F l o r i d a and th (Tongue o f the Ocean). The r e s u l t s f o r the d i u r n a l study are shown i n F i g u r e 7. As p r e d i c t e d from the h a l f l i v e s , RF and FMF appear to be q u i t e p h o t o l a b i l e , b e i n g present at r e l a t i v e l y high c o n c e n t r a t i o n s o n l y a t n i g h t . The s t a b l e p h o t o p r o d u c t s , LC and L F , on the o t h e r hand, are present throughout the e n t i r e d i u r n a l c y c l e . N o t e t h a t the c o n c e n t r a t i o n o f lumichrome i s more than an o r d e r o f magnitude g r e a t e r than the o t h e r f l a v i n s , s u g g e s t i n g t h a t i t t u r n s o v e r a t a slower r a t e . R e s u l t s from o c e a n i c depth p r o f i l e measurements o f f l a v i n s i n the Tongue o f Ocean, F i g u r e 8, i n d i c a t e t h a t d u r i n g the a f t e r n o o n , RF i s a t r e l a t i v e l y low c o n c e n t r a t i o n s i n the upper 25 meters, w h i c h i s i n agreement w i t h the d i u r n a l s t u d y . I n the deeper water (> 300 m), the o n l y d e t e c t a b l e f l a v i n i s RF s u g g e s t i n g t h a t LC and LF a r e s o l e l y d e r i v e d from p h o t o c h e m i c a l p r o c e s s e s , n o t m i c r o b i a l processes. I n a d d i t i o n , t h e p r o f i l e s a l s o suggest t h a t lumichrome i s more s t a b l e than l u m i f l a v i n i n the water column, as lumichrome i s advected t o greater depths, which supports our p r e l i m i n a r y e x p e r i m e n t a l r e s u l t s on f l a v i n s t a b i l i t i e s . From these f i r s t f i e l d s t u d i e s , we conclude t h a t f l a v i n s as a group undergo dynamic p h o t o c h e m i c a l t r a n s f o r m a t i o n s i n the sea. I n a d d i t i o n , as a r e s u l t o f t h e r a p i d breakdown of RF, most o f t h e photosensitizing i s actually done by LC, t h e major stable p h o t o p r o d u c t and t h e f l a v i n d e t e c t e d a t t h e h i g h e s t l e v e l s i n seawater. F i n a l l y , the 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 f l a v i n s i n n a t u r a l seawater, 0 . 1 - 2 nM, i s s u f f i c i e n t l y h i g h t h a t f l a v i n s s h o u l d be c o n s i d e r e d as p o t e n t i a l l y important p h o t o s e n s i t i z e r s i n the sea. Acknowledgments T h i s r e s e a r c h was supported by t h e N a t i o n a l S c i e n c e F o u n d a t i o n on g r a n t OCE-8517041. The o p p o r t u n i t y t o p a r t i c i p a t e i n c r u i s e s as p a r t o f t h e SOLARS program was p r o v i d e d i n p a r t by t h e O f f i c e o f Naval Research on g r a n t N00014-85C-0020. We a l s o w i s h t o thank Susan Vastano, W i l l i a m Stahovec, P e t e r M i l n e and Cindy Moore f o r their excellent assistance.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1

Natural Photosensitizers in Sea Water

MOPPER AND ZIKA

FL * Δ

FL* Δ FL* Q

FMF

/

hydrolysis?^

/ RF

"

*

3 a

RF

^ Μ

. ι

F

FMF

L 0

LC

1 ·

- ^

F

f y isc

L

~ λ

c-r

3 '« ' F

L

^ λ

F

L

u* +

H

*

" e

FLH-S

F i g u r e 6 . A, Pathways f o r p h o t o d e g r a d a t i o n o f r i b o f l a v i n . B. Pathways f o r d e - a c t i v a t i o n o f f l a v i n s ( F L ) i n aqueous s o l u t i o n i n c l u d i n g r e a c t i o n w i t h quenchers (Q) and s u b s t r a t e s (SH) and e n e r g y ( Δ ) l o s s e s through i n t e r n a l c o n v e r s i o n and e m i s s i o n steps.

:

oJ 0600

1000

ίο

1400 LOCAL1800 TIME 2200 0200 0600

F i g u r e 7. T i m e o f d a y v a r i a t i o n s various f l a v i n s .

i n the concentrations of

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

CONCENTRATION 0

5

10

15

F i g u r e 8. Depth p r o f i l e s the Ocean.

0

1

2

(picomolar) 3

0

20

40

60

80

100

f o r LC, LF and RF i n the Tongue o f

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

13.

MOPPER AND ZIKA

Natural Photosensitizers in Sea Water

189

Literature Cited

1. Zika, R.G. In "Marine Organic Chemistry"; Duursma, E.K.; Dawson, R., Eds.; Elsevier: Amsterdam, 1981; p. 299. 2. Heelis, P.F. Chem. Soc. Rev. 1982, 15-39. 3. Momzikoff, A. Cah. Biol. Mar. 1969, 10, 221-230. 4. Dunlap, W.C.; Susie, M. Mar. Chem. 1985, 17(3), 185-198. 5. Aaronson, S.; De Angelis, B.; Frank, O.; Baker, H. J. Phycol. 1971, 7, 15-218. 6. Momzikoff, A. Cah. Biol. Mar. 1973, 14, 323-328. 7. Gaill, F.; Momzikoff A Biol 1975 29 315-319 8. Provasoli, L.; Carlucci, A.F. In "Algal Physiology and Biochemistry"; Stewart, W.D.P., Ed.; University of California Press: Berkely, 1974; Chap. 27. 9. Vastano, S.; Mopper, K.; Zika, R.G. Mar. Chem. 1986, in preparation. 10. Momzikoff, A.; Santus, R.; Giraud, M. Mar. Chem. 1983, 12, 1-14. 11. Sysak, P.S.; Foote, C.S.; Ching, T. Photochem. Photobiol. 1977, 26, 19-27. 12. Zika, R.G. Ph.D. Dissertation, Dalhousie University, 1978. 13. Mopper, K.; Stahovec, W.L. Mar. Chem. 1986, in press. 14. Ragan, M.A.; Craigie, J.S. Can J. Biochem. 1976, 54, 66-73. 15. Harvey, G.R.; Boran, D.A. In "Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization"; Aiken, G.R.; McKnight, D.M.; Wershaw, R.L.; MacCarthy, P., Eds.; John Wiley and Sons: New York, N.Y., 1985; pp. 233-247. 16. Getoff, N.; Solar, S.; McCormick, D.B. Science 1978, 201, 616-618. 17. Frisell, W.R.; Chung, C.W.; MacKenzie, C.G. J. Biol. Chem. 1959, 24, 1297. 18. Massey, V.; Palmer, G.; Ballow, D. In "Flavins and Flavoproteins"; Kamin, H., Ed.; University Park Press, 1971; pp. 349-361.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

190

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

19. Zika, R.G.; Moffett, J.W.; Petasne, R.G.; Cooper, W.J.; Saltzman, E.S. Geochim. Cosmochim. Acta 1985, 49, 1173-1184. 20. Mill, T. In "Handbook of Environmental Chemistry"; Hutzinger, O., Ed., Springer-Verlag: New York, 1980; Vol. 2A, pp. 77-105. 21. Graedel, T.; Weschler, C.J. Rev. Geophys. Space Phys. 1981, 19, 505-539. 22. Cox, R.A.; Patrick, K.F.; Chant, S.A. Environ. Sci. Techn. 1981, 15, 587-592. 23. Enns, K.; Burgess, W.H. J. Amer. Chem. Soc. 1965, 87(24), 5766-5770. 24. Armstrong, J.S.; Hemmerich, P.; Traber, R. Photochem. Photobiol. 1982, 35 25. Harvey, G.R.; Boran, D.A.; Chesal, L.A.; Toker, J.M. Mar. Chem. 1983, 12, 119-132. 26. Smith, E.C.; Metzler, D.E. J. Amer. Chem. Soc. 1963, 85, 3285-3288. 27. Song, P.S.; Smith, E.S.; Metzler, D.E. J. Amer. Chem. Soc. 1965, 87, 4181-4184. 28. Song, P.S.; Metzler, D.E. Photochem. Photobiol. 1967, 6, 691-701. 29. Mopper, K.; Zika, R.G., unpublished data. 30. Lasser, Ν.; Feitelson, J. Photochem. Photobiol. 1975, 25, 249-254. RECEIVED July 1, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 14

Do Polycyclic Aromatic Hydrocarbons, Acting as Photosensitizers, Participate in the Toxic Effects of Acid Rain? Jacques Kagan, Edgard D. Kagan, Isabelle A. Kagan, and Peggy A. Kagan Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60680

The light-dependent toxicity of non-carcinogenic polycyclic aromatic hydrocarbons (PAH) such as naphthalene, fluorene, phenanthrene chrysene anthracene 9 methylanthracene examined in Daphni magna Aedes aegypti and Rana pipiens, and in fish (Pimephales promelas). Data presented were obtained in the laboratory, except for the tadpoles and fish, which were exposed to sunlight. The light-dependent toxicity of the carcinogenic benzo[a]pyrene in mosquito larvae is shown for comparison. Although the mechanism of light-dependent toxicity has not been elucidated, the effects are probably too rapid to involve modifications of the genetic material. PAH's are generated in the combustion processes held responsible for acid rain. Our results suggest that one must now question whether the death of aquatic organisms in natural environments should be ascribed solely to an increase in acidity. Polycyclic aromatic hydrocarbons (PAH's) are found i n the atmosphere, soil, and i n the food chain U ) ; they have received considerable attention, p a r t i c u l a r l y because of their mutagenic and/or carcinogenic properties, which are thought to depend upon metabolic activation of the hydrocarbons into oxygenated products (1). A relationship between the carcinogenicity of PAH's and t h e i r high photodynamic activity toward protozoa such as Coleps and Paramecium species was f i r s t noted by Mottram and Doniach (2) and Doniach (_3). It was further investigated by o s t e i n et al. using R_ caudatum ( 4 ) , and by Small et a l . (_5) and Bauer and Graef {6) using Tetrahymena pyriformis. In the interval, Matoltsy and Fabian demonstrated a photodynamic effect of carcinogenic PAH s i n third-instar larvae of Drosophila melanogaster (7-8.)· Few papers have described photodynamic effects produced by PAH's i n larger aquatic organisms. Morgan and Warshawsky compared the rates of photosensitized immobilization of nauplii of the brine shrimp Artemia salina induced by nineteen carcinogenic and twenty0097-6156/87/0327-0191 $06.00/0 © 1987 American Chemical Society

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two non-carcinogenic PAH s (9). They reported a correlation between photodynamic a c t i v i t y and carcinogenicity, except for compounds having six or more rings, which showed no photosensitizing ability regardless of carcinogenicity. They even suggested that carcinogenesis by PAH s might result from sub-lethal photodynamic effects. Interestingly, they found benz[a]acridine, a non-carcinogenic compound used as reference, to be almost twice as photoactive as benz[a]pyrene, a highly carcinogenic molecule. As a consequence of the early research on the photobiology of PAH's which correlated carcinogenicity and light-dependent toxicity, one would have expected non-carcinogenic PAH s to be non-phototoxic. The results of Morgan and Warshawsky [9) already suggested that this premise was not firmly supported by experimental facts. Anthracene, a small non-carcinogenic PAH, provides another example invalidating the suggested general relationship. The acute t o x i c i t y of t h i s compound to bluegill sunfish (Lepomis macxochirus) i n the presence of sunlight was discovere (10). Oris et al. also f i s h treated with d i l u t e solutions of anthracene ill) . Spacie et a l . analyzed the uptake, depuration, and biotransformation of anthracene and benzo[a]pyrene i n bluegill sunfish (12). The lightdependent toxicity of anthracene to Daphnia pulex and third-instar larvae of A^ aegypti was reported by Allred and Giesy (13), and that in immature Rana pipiens by Kagan et al. (14). After investigating the toxicity of photosensitizing molecules to several organisms, particularly larvae and eggs of insects (1519), we evaluated the light-dependent t o x i c i t y of representative non-carcinogenic PAH's i n aquatic organisms, such as the larvae of the mosquito Aedes aegypti, brine shrimps (A^_ salina), water fleas Daphnia magna, embryonic forms of Rana pipiens, and f i s h (fathead minnows, Pimephales promelas).

Materials and Methods The l i g h t source was a bank of 8 tubes No. RPR-3500A from the Southern New England Ultraviolet Co, Hamden, CT, mounted 5 cm apart. They emit between 320 and 400 nm, with a maximum at 350 nm. The organisms were irradiated between 7 and 9 cm from the sources, where the light intensity, measured with a Yellow Springs Radiometer YSI 65A and probe YSI 6551, was 13 W.m . The chemicals were sensitizers from Fisher Chemicals, used without further purification. Each stock solution contained 1 g/1 i n ethanol or dimethyl sulfoxide. Serial dilutions were made by mixing 0.100 ml with 0.500 ml of solvent, repeating the process as often as needed. One complete series of experiments with a l l the organisms and a l l the sensitizers was conducted i n a dark room, under a dim amber light. Dark controls tested the effect of the solvent alone, a previously irradiated sensitizer solution (at the highest concentration used), and the various sensitizer solutions. The L C Q values were obtained graphically, from the plots expressing the percent survival of the organisms as a function of the sensitizer concentrations (on a logarithmic scale). Brine shrimp eggs (0.5 g, from Patco, Fort Atkinson, Wisconsin) in 500 ml of salt solution (9) were kept over5

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Effects of Polycyclic Aromaùc Hydrocarbons on Acid Rain

night i n a water bath with vigorously bubbling air at 28°C- A brine shrimp suspension (0.5 ml) was transferred into a 7 ml vial, and 2.5 ml of aged water and 20 pi of the desired sensitizer solution were added. A l l the organisms were s t i l l alive after 2 h of incubation in the dark. The irradiation was then conducted for 30 m in, the results recorded, and the organisms irradiated for another 30 min. Typical experiments involved 150-200 organisms per vial. At least 6 series of determinations were performed with each sensitizer. Daphnia magna obtained from the same supply house were maintained i n the laboratory and fed commercial yeast. About 10 mature organisms were transferred into each glass v i a l , and the t o t a l volume of water was adjusted to 3 ml. The sensitizer (20 ul) was added, and the open v i a l s were irradiated after 1 h of incubation. Sensitizer concentrations ranged from 6.7 to 0.0009 ppm. After 30 min of irradiation, the number of survivors i n each vial was recorded and the irradiation resumed for another 30 min. The death of the immobilized organisms wa and movements of the thoraci sionally, young organisms were born during the course of the experi ments. Their fate was not included i n the results. The mosquito larvae came from a stock of Aedes aegypti (Rock) o r i g i n a l l y obtained from Prof. G. Craig, Jr., University of Notre Dame, and maintained i n our laboratory. In the morning egg sheets were placed i n a pan containing water and some liver powder and were incubated at 32 °C for about 6 h. The f i r s t - i n s t a r larvae were transferred into clean aged tap water, and 12-20 larvae were transferred into each vial. The volume was adjusted to 3 ml, and 20 ul of sensitizer solution was added. After overnight incubation i n the dark, the vials were f i r s t examined for light-independent toxic effects and then irradiated by UV lamps for 30 min. The surviving larvae were counted under a microscope and then irradiated f o r another 30 min. The L C values were determined as above, using the results from 4 to 6 series of experiments. Dead larvae were found f l o a t i n g on top of the water or lying at the bottom; they did not respond to mechanical or light probing, and microscopic examination revealed that their mouthparts were immobile. Tadpoles of Rana pipiens (embryonic stages, Schumway 24 to 28) were collected at the F i f t y - f i f t h Street Pond i n Downers Grove, I l l i n o i s on June 5, 1983 and on June 8-10, 1984, as well as i n Devil's Lake State Park, Wisconsin, on June 15, 1984. In each experiment 10 to 20 tadpoles were placed i n a 100 ml beaker containing 50 ml of aged tap water. After addition of sensitizer, the organisms were kept i n the dark f o r 1 h, exposed to sunlight f o r 30 min, returned to the darkroom, and their survival determined after 24 h. The average from 4 experiments was used for determining the L C ^ Q values. The fathead minnows (Pimephales promelas), purchased from a local fishing supply house, were about 5 cm long and weighed about 0.8 g. They were kept for at least 24 h i n aged tap water prior to irradiation. Five fish were placed In a beaker containing 400 ml of sensitizer solution for 30 min before exposure to natural sunlight for 1 h. Each series of experiments was conducted i n duplicate. 5Q

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Results and Discussion The solubility properties of PAH s severely limit the amounts which can be utilized i n experiments i n aqueous media. The maximum nominal sensitizer concentration used was 6.67 ppm, which exceeds the published solubility values i n pure water (20) for most of the compounds tested (Figure 1). The small amount of solvent used to introduce the sensitizers into water must slightly increase this solubility, but the new values were not measured. Usually, the PAH's studied here caused l i t t l e t o x i c i t y when animals were maintained i n the dark. When the toxicity was enhanced by treatment with long wavelength ultraviolet light, solutions of sensitizers previously irradiated i n the absence of organisms showed no enhanced t o x i c i t y i n the dark. The toxic reactions therefore depended on the simultaneous presence of the organisms, the sensitizer, and the light. The magnitude of the effects depended upon the species used, t h e i r stag conditions selected f o establish absolute comparisons, and t h i s report i s meant only to demonstrate the effects which can be observed, rather than to produce detailed quantitative values. Although we originally intended to observe the sunlight-dependent properties of PAH's, the variab i l i t y of "climatic conditions made these studies irreproducible. Consequently, light sources emitting principally i n the long-wavelength range of the u l t r a v i o l e t spectrum were also u t i l i z e d . The emission characteristics of the lamps used i n this work, as provided by the manufacturer, are shown i n Fig 2. Naphthalene, anthracene, phenanthrene, 9-methylanthracene, fluorene, chrysene, fluoranthene, and pyrene were studied. In most cases, sunlight-dependent toxicity was demonstrated but, except for tadpoles and f i s h , only the results obtained indoors using the UV light sources are reported here. Daphnia. The PAH's were not toxic to Daphnia i n the dark. The presence of ultraviolet light dramatically affected the activity of some PAH's, k i l l i n g the organisms with L C Q ' S as low as a few parts per billions. The results, obtained by averaging the data obtained i n four to s i x experiments, are recorded i n Table I. The L C values, with standard deviations i n parentheses, were obtained immediately after irradiation of organisms which had been i n contact for 1 h with the sensitizers i n the dark. Neither naphthalene nor f luorene displayed any effects, and chrysene showed only minimal light-dependent toxicity, even at the highest concentration selected for this study, 6.7 ppm. The L C C Q of 9-methylanthracene was one-fifth that of phenanthrene after 30 min treatment,, about one tenth after 1 h. Anthracene, i n turn, had an L C about 7 times smaller than 9-methylanthracene after 30 min, 3 times smaller after 1 h. Fluoranthene and pyrene were s t i l l more active than anthracene, by a factor of 5 after 1 h of irradiation. The experiments involving 30 min and 1 h of irradiation showed that ligfit-dependent toxicity levels were not d i r e c t l y proportional to the exposure times. The largest change (a 5-fold increase i n L C C Q for a doubling of the irradiation time) was observed for 9-methylanthracene and for pyrene. 5

5 0

50

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PHENANTHRENE 1.29

Effects of Polycyclic Aromatic Hydrocarbons on Acid Rain

CHRYSENE 0.002

FLUORANTHENE 0.26

PYRENE 0.135

Figure 1 . Structure and aqueous solubility of PAH's i n m g A g (Data taken from ref. 20)

Figure 2. Emission characteristics for the ultraviolet lamps used in this work (RPR-3500A from the Southern New England Ultraviolet Co, Hamden, CT)

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Table I. Survival of D. magna

SENSITIZER

LCt-Q after irradiation for 1 h 30 min

Naphthalene

N.R.

N.R.

Fluorene

N.R.

N.R.

Chrysene

N.R.

1.9 (0.85)

Phenanthrene

1.0 (1.2)

0.45 (0.3)

0.03 (0.04)

0.02 (0.03)

0.011 (0.004)

0.004 (0.005)

0.02 (0.02)

0.004 (0.004)

9-Methylanthracen Anthracene Fluoranthene Pyrene

The effect of the incubation time on the level of light-dependent t o x i c i t y was tested more s p e c i f i c a l l y with fluoranthene. The incubation time ranged from 5 min to 120 min, with a 30-min irradiation i n a l l cases. The LCcn value was 0.032 after 5 min, 0.015 after 120 min, and 0.012 ppm after overnight incubation. Variations i n i r r a d i a t i o n time, therefore, produce a greater effect than v a r i a tions i n incubation time. Oris et al. (11) and Allred and Giesy (13) described the lightdependent t o x i c i t y of anthracene against D. pulex. They u t i l i z e d fixed concentrations of sensitizers, measuring the time required for immobilizing the organisms with sunlight. Therefore, their results are not directly comparable to ours, which were obtained by varying the concentrations and keeping the irradiation times fixed. However, their active concentrations were 0.03 to 0.003 ppm, within the range used i n our work, where we recorded the death of the organisms rather than their immobilization. Mosquito larvae. Three strains of the mosquito Aedes aegypti were used i n our qualitative studies: the wild type (Rock), Trinidad (DDT resistant), and DLS^ (Dieldrin resistant). Many of our early surveys, with these sensitizers as well as with others (17-18, and unpublished results), established that the three strains showed exactly the same response toward the photosensitized treatments. In other words, resistance toward DDT or Dieldrin gave A^ aegypti no resistance toward photoactive insecticides. Consequently, the determinations summarized i n Table II were made with the Rock s t r a i n only.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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The results (with standard deviations i n parentheses) are for the irradiation of larvae which had been i n contact overnight with the sensitizer i n the dark. No dark t o x i c i t y was observed, except with f luorene, 9-methylanthracene, and pyrene, which had L C ^ Q values of 3.0, 2.8, and 3.0 ppm respectively. These are i n excess of t h e i r aqueous solubility. The results observed with mosquito larvae are generally s i m i l a r to those obtained with magna. Here again naphthalene, fluorene and chrysene showed l i t t l e or no light-dependent toxicity. Pyrene and fluoranthene were clearly the most active, the l a t t e r giving a L C Q value down to 0.012 ppm after 1 h of irradiation. 5

Table II.

L C

5

Q

values for A. aegypti

SENSITIZER 30 min

1 h

N.R.

N.R.

Fluorene

2.7 (0)

2.7 (0)

Chrysene

2.7 (0)

1.7 (1.5)

Phenanthrene

0.5 (0.1)

0.5 (0.1)

9-Methylanthracene

1.5 (1.4)

0.4 (0.3)

Naphthalene

Anthracene

0.15 (0.04)

0.15 (0.04)

Fluoranthene

0.05 (0.05)

0.012 (0.001)

Pyrene

0.02 (0.03)

0.02 (0.03)

Brine shrimps. The brine shrimps (Artemia salina) used were one day old, younger than those used by Morgan and Warshawsky {9). For convenience we recorded the toxic effects i n terms of short-term immobilization rather than death of the organisms. Table III summarizes the light-dependent t o x i c i t y , following incubation of the organisms for 2 h i n the dark. The relative a c t i vity of the sensitizers resembled that found above but the range of L C Q values was smaller. Our results cannot be directly compared to those of Morgan and Warshawsky ( 9· ), who studied three of our compounds in older organisms without systematically varying the concentration of the sensitizers. In that work, r e l a t i v e photodynamic activities (with Benz[c]acridine = 1) were 0.72 for pyrene, 0.20 for chrysene, and 0.15 for fluoranthene, a ranking similar to that found i n Table 3, but at the concentrations used (between 2 and 40 ppm) 5

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many compounds were certainly not totally dissolved. Also, i t i s not clear from the article whether a l l the measurements were made at a single concentration. The combination of the two changes, age of the organisms and concentrations of sensitizers, could account for the quantitative differences observed i n the two studies.

Table

III.

LC Q 5

values for A . salina after irradiation for 1 h 30 min

LCCQ

SENSITIZER

Naphthalene

N.R.

N.R.

Fluorene

N.R.

3

Chrysene

N.R.

3

Phenanthrene

N.R.

N.R.

9-Methylanthracene

0.25

0.25

Anthracene

0.04

0.02

Fluoranthene

0.04

0.04

Pyrene

0.008

0.008

Tadpoles. Some of the PAH's were tested for sunlight-dependent t o x i c i t y to tadpoles i n order to compare them to anthracene (14). After incubation with the test compounds for 1 h, the organisms were exposed to sunlight for 30 min either i n the morning or i n late afternoon. No toxicity was detected at that point; only the following day was any mortality apparent. Table IV summarizes the results for the 24-h survival of the late embryonic stages of Rana pipiens (Schumway 24 to 28) irradiated for 1 h after 30 min of incubation i n the dark. Dark controls displayed no evidence of t o x i c i t y . The large number of tadpoles at the same developmental stage required to repeat t h i s study with u l t r a v i o l e t l i g h t sources was no longer available to us at the time.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Table

IV.

L C

5

0

values for immature R. pipiens.

SENSITIZER

^50

Naphthalene

N.R.

Chrysene

N.R.

Phenanthrene

N.R.

9-Methylanthracene

N.R.

Anthracene Fluoranthene

0.09 (0.02)

Pyrene

0.14 (0.08)

Minnows. Only preliminary results were available with pyrene and fluoranthene. Pyrene induced no immediate acute toxicity, but a 24-h L C C Q value of 0.2 ppm was observed after only 30 min of incubation and 1 h of irradiation with sunlight (Fig 3). In contrast, fluoranthene produced immediate light-dependent toxicity (Fig 4). The L C Q values reported here are higher than those observed i n b l u e g i l l sunfish exposed to anthracene for 48 h p r i o r to i r r a d i a tion, which took place over 3 days Q0). I t i s l i k e l y that longer treatments of our f i s h with the other PAH's would also result i n increased light-dependent toxicity . 5

Concluding Comments Mechanism. The high light-dependent toxicity observed in this work may be largely accounted for by a good match between the emission of the lamps, which peaked at 350 nm (Fig 2), and the main absorption bands of fluoranthene (357 nm, e 8400), anthracene (355 nm, 7770) and, to a lesser extent, pyrene (334 nm, 29,400). Conversely, the i n a c t i v i t y of naphthalene (221 nm, 10,600), fluorene (301 nm, 10,000), or chrysene (319 nm, 12,200) i s certainly due i n large part to the very low emission of our l i g h t source into the absorption region of the molecules. Sunlight, which provides very l i t t l e energy below 350 nm, also f a i l e d to induce any appreciable t o x i c i t y with the last group of sensitizers. The light-dependent toxicity data reported i n this paper result from the analysis of restricted interactions between selected organisms, PAH s, and light. An important limitation of this work concerns the relatively short exposure of the organisms to the chemicals (the longest incubation was overnight with the mosquito

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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O.OOI

0-01

0-1

CONCENTRATION

I

(ppm)

Figure 3 . Fate of I\_ promelas treated with pyrene for 30 min and exposed to sunlight for 1 h. The survival of the organisms was recorded 2 4 h after the irradiations. The L C Q i s 0.2 ppm. 5

0.001

0.01

0.1

I

MM

C O N C Ε Ν Τ R ATI 0 Ν

Figure 4 . Fate of R_ promelas treated with fluoranthene for 30 min and exposed to sunlight for 1 h. The survival of the orga­ nisms was recorded 1 h ( L C = 0.5 ppm) and 2 4 h ( L C Q = 0.1 ppm) after the irradiations. 5

0

5

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Effects of Polycyclic Aromatic Hydrocarbons on Acid Rain

larvae), and to the l i g h t (no more than 1 h). Also, the lamps did not provide any emission i n the UVB region, and therefore did nopt fully simulate solar radiation. We intend to perform a more comprehensive study l a t e r and, p a r t i c u l a r l y , to determine the l i g h t dependent t o x i c i t y of PAH's singly and i n combinations, when the sensitizers are already present i n the water surrounding the organisms at b i r t h as well as when they are introduced at d i f f e r e n t developmental stages. Another l i m i t a t i o n of t h i s work concerns the toxic effects observed. Only acute t o x i c i t y data were reported, and more subtle effects at sub-lethal concentrations were neglected. More recently, however, we also observed the delayed light-dependent t o x i c i t y of pyrene toward first-instar mosquito larvae. Figure 5 describes the survival of A^ aegypti larvae i n the presence of 0 . 0 0 5 ppm of pyrene, i n the dark (solid) and treated with UV light (open). One day later, 58% of the larvae were a l i v e (compared to 98% of the controls), and 38% survive The survival curve for i n the controls, and p r a c t i c a l l y a l l of these pupae successfully produced adults. At the next higher concentration, 0 . 0 3 ppm, a l l the larvae died upon irradiation. At the next lower concentration, 0.0009 ppm, there was no significant difference between the fate of the organisms which had been irradiated and that of the dark cont r o l s (Fig 6 ) . The LC50 value for pyrene based on the delayed effects was 0 . 0 0 4 5 ppm, compared to the acute t o x i c i t y value of about 0.01 ppm obtained i n these particular series of experiments. The mechanism of light-dependent toxicity for the PAH s at the cellular level i s unknown. The rapid death of the organisms suggests that modifications of t h e i r genetic material are probably not of primary importance. Indeed, experiments with benzo [a ]pyrene dramatically illustrate that for mosquito larvae carcinogenicity i s a very minor risk compared to light-dependent toxicity (Figure 7 ) . Damage of cellular membranes, perhaps through singlet oxygen-mediated l i p i d peroxidation, would be a reasonable path. Oris et al. (11) reported that bluegill sunfish treated with anthracene and ultraviolet light displayed damage to t h e i r g i l l epithelium, as well as increased respiration rate. They suggested that the respiration apparatus was one potential s i t e of toxic action. Our f i s h appeared to respond similarly, but no detailed morphological examinations have yet been conducted on any of the organisms treated i n this study. There has also been much interest i n the effects of PAH's on human skin, particularly concerning the treatment of psoriasis ( 2 1 2 2 ) . Whether or not a single mechanistic scheme w i l l account for a l l the photobiological properties of these molecules remains to be established. The acid-rain connection. This report demonstrates the l i g h t dependent toxicity of representative PAH's i n common aquatic organisms. Naphthalene and fluorene were essentially inactive under our experimental conditions. Others were extremely toxic, particularly fluoranthene and pyrene, but only i n the presence of u l t r a v i o l e t light. Typically, PAH's are present i n coal and petroleum products, and they are also produced by incomplete combustion of organic matter (2J3). More than 2 0 0 PAH's have been found i n the environment. Their production i n combustion processes i s favored by an oxygen-deficient flame, temperatures i n the range 6 5 0 - 9 0 0 °C, and fuels which are not highly oxidized ( 2 4 ) . PAH's and oxides of In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Figure 5. Fate of lst-instar A^ aegypti larvae treated with 0.005 ppm of pyrene i n the dark (solid circles), and irradiated (open circles) for 30 min . The survival of the larvae, pupae, and adults i s shown with solid lines, broken lines, and dotted lines respectively. Reproduced with permission from Ref. 30. Copyright 1986, Pergamon Press.

0

1

2

3

4

5

6

7

8

9

10 II

DAYS

Figure 6. Fate of lst-instar aegypti larvae treated with 0.0009 ppm of pyrene i n the dark (solid circles), and irradiated (open circles) for 30 min. The survival of the larvae, pupae, and adults i s shown with solid lines, broken lines, and dotted lines respectively.

0

1

*

3

4

5

6

7

8

0

0

Π

DAYS

Figure 7. Fate of first-instar larvae of aegypti treated with 0.005 ppm of benzo[a]pyrene i n the dark (solid circles) and i r r a ­ diated for 30 min (open circles). The survival of the larvae, pupae, and adults i s shown with solid lines, broken lines, and dotted lines respectively. Reproduced with permission from Ref. 30. Copyright 1986, Pergamon Press. In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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203

suirur and nitrogen are generated simultaneously during the combustion of organic fuels. Both types of pollutants can be transported far from their points of formation, and while the inorganic components' role i n the generation of acid rain i s well recognized, i t i s conceivable that some of the ecological damage usually assigned to acid rain might instead be due to photodynamic reactions, particularly i n aquatic environments. It has been estimated that nearly 230,000 metric tons enter the world's oceans and surface waters every year (25). The ecological consequences of these findings deserve scrutiny. Oris et a l . observed that current environmental concentrations of anthracene were less than the measured acute t o x i c i t y threshholds f o r the aquatic organisms which they studied (11). However, they warned that small increases of PAH concentrations i n surface waters could cause dramatic impact on aquatic ecosystems. Pyrene and fluoranthene, which are more phototoxic than anthracene, may be among the most abundant PAH's found i example, these two togethe hydrocarbons (26.). Though the concentration of PAH's i n natural waters varies, i t can be very high (27). Of course, the presence of sensitizers i n aquatic environments does not necessarily induce light-dependent toxicity toward a l l the organisms, since light attenuation through natural waters takes place rather rapidly, depending upon the nature of the impurities (28-29). The intensity, fluence, and spectral distribution of the light to which a given organism i s exposed, as well as the rate of photodegradation of the sensitizers, w i l l ultimately determine the photobiological response. Sensitized organisms which remain near the surface during the daytime should be the most severely affected.

Acknowledgments. We are indebted to Prof. G. Craig, J r , Notre Dame University, for the o r i g i n a l mosquitoes, to Prof. R. L. Willey, University of I l l i n o i s at Chicago, f o r the i d e n t i f i c a t i o n of the Daphnia magna, and to Prof. H. E. Buhse, Jr., who assisted with the collection of tadpoles and their identification. We are also grateful for the generous advice which they provided. Initial funding for this research was received from the National Institutes of Health (GM 24144). J. K. was later indebted to many review panels, particul a r l y at the National Institutes of Health, the National Science Foundation, and the Environmental Protection Agency, f o r generous comments, and to the Research Board of the University of I l l i n o i s for financial support. Literature cited

1. Yang, S. K.; Deutsch, J.; Gelboin, H. V. In "Polycyclic Hydrocarbons and Cancer", Gelboin, H. V.; Ts'O, P. 0. P. Eds.; Academic: New York, 1978; Vol. 1, pp. 205-231. 2. Mottram, J.C.;Doniach, I. Nature 1938, 140, 933-4. 3. Doniach, I. Brit. J. Exp. Pathol„ 1939, 20, 227-35. 4. Epstein, S. S.; Small, M..; Falk, H. L.; Mantel, N. Cancer Res. 1964, 24, 855-62. 5. Small, M.; Mantel, N.; Epstein, S. S. Exp. Cell Res. 1967, 45, 206-17. In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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6. Bauer, L.; Graef, W. Zentralbl. Bakteriol. Parasitendkd, Infektionskr. Hyg, Erste Abt. Prig. Reihe Β Hyg. Praev. Med. 1976 161, 304-16. 7. Matoltsy, A. G.; Fabian, G. Nature 1946, 149, 877. 8. Matoltsy A. G.; Fabian G. Arch. Biol. Hungarica 1947, 17, 16570. 9. Morgan, D. D.; Warshawsky, D. Photochem. Photobiol 1977, 25, 39-46. 10. Bowling, J. W.; Laversee, G. J.; Landrum, P. F.; Giesy, J. P. Aquatic Toxicol. 1983, 3, 79-90. 11. Oris, J. T.; Giesy, J. P.; Allred, P. M.; Grant, D. F.; landrum, P. F. in The Biosphere: Problems and Solutions, T. N. Veziroglu Ed.; Elsevier: Amsterdam, 1984; pp 636-658. 12. Spacie, Α.; Landrum, P. F.; Lever see, G. J. Ecotoxicol. Environ. Saf. 1983, 7, 330-41. 13. Allred, P. M.; Giesy, J. P. Environ. Toxicol. Chem. 1985, 4, 219-26. 14. Kagan, J.; Kagan, 1984, 10, 1115-22. 15. Kagan, J.; Chan, G. Experientia 1983, 39, 402-3. 16. Kagan, J.; Beny, J.-P.; Chan, G.; Dhawan, S. N.; Arora, S. K.; Prakash, I. J. Nat. Prod. 1983, 46, 646-50. 17. Kagan, J.; Beny, J.-P.; Chan, G.; Dhawan, S. N.; Jaworski, J. Α.; Kagan, E. D.; Kassner, P. D.; Murphy, M.; Rogers, J. A. Insect Sci. Application 1983, 4, 377-81. 18. Kagan, J.; Kolyvas, C. P.; Jaworski, J. Α.; Kagan, E. D.; Kagan, I. Α.; L.-H. Zang, L.-H. Photochem. Photobiol. 1984, 40, 479-84. 19. Kagan, J.; Kolyvas, C. P.; Lam, J. Experientia 1984, 40, 13967. 20. Mackay, D.; Shiu, W. Y. J. Chem. Eng. Data 1977, 22, 399-402. 21. Kochevar, I. E.; Armstrong, R. B.; Einbinder, J.; Walther, R. R.; Harber, L. C. Photochem. Photobiol. 1982, 36, 65-9. 22. Kaidbey, K. H.; S. Nonaka, S. Photochem. Photobiol. 1984, 39, 375-8. 23. Guerin, M. R. in "Polycyclic Hydrocarbons and Cancer"; Η. V. Gelboin; P.O.P. Ts'O, Eds.; Academic: New York, 1978; Vol. 1, pp. 3-42. 24. Baum, E. J. in "Polycyclic Hydrocarbons and Cancers"; Η. V. Gelboin; P. O. P. Ts'O, Eds.; Academic: New York, 1978; Vol 1, pp. 45-70. 25. Neff, J. M. "Polycyclic Aromatic Hydrocarbons in the Aquatic Environment"; Applied Science Publishers, Barking, U.K., 1979; 266 pp. 26. Bjørseth, Α.; Sortland Olufsen, B. in "Handbook of Polycyclic Aromatic Hydrocarbons"; A. Bjørseth, Ed.; Marcel Dekker: New York, 1983; pp. 507-24. 27. Borneff, J.; Kunte, H. in "Handbook of Polycyclic Aromatic Hydrocarbons"; A. Bjørseth, Ed.; Marcel Dekker: New York, 1983; pp. 629-652. 28. Smith, R.; Baker, K. S. Photochem. Photobiol. 1979, 29, 311-23. 29. Calkins, J. 1982. in "The Role of Solar Ultraviolet Radiation in Marine Ecosystem"; J. Calkins, Ed.; Plenum: New York; pp. 247-262. 30. Kagan, J.; Kagan, E. D. Chemosphere 1986, 15, 243-51. f

RECEIVED May 27, 1986 In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 15

Photochemistry in Aqueous Surface Layers: 1-Naphthol 1,2

1

Richard A. Larson and Stewart A. Rounds 1

Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30613

2

i-Naphthol was reactiv d direc photolysi i buffered aqueou sunlight was about 90 min) and in cyclohexane (half­ -life about 15 min). The reaction rate in water increased with pH. The mechanisms of the principal photolysis pathways in both solvents did not involve singlet oxygen. In aqueous solution, the major ether-extractable product, lawsone, may have been formed by a radical process involving 1,2-naphthoquinone and superoxide as intermediates. In cyclo­ hexane, most of the photolysis products observed were very polar. Several oxygen-containing compounds derived from the solvent were also observed. In a two-phase system including cyclohexane and pH 7 buffer, the photolysis appeared to proceed by independent mechanisms in both phases, but intermediate polar products formed in the organic layer diffused into the aqueous layer. In a system containing a surface-active material, SDS, the rate of photolysis decreased; the photoproducts were typical of those observed in cyclohexane rather than water.

The f a c t t h a t t h e a i r - w a t e r i n t e r f a c e i s a n a r e a of u n u s u a l p r o p e r t i e s r e l a t i v e t o t h e u n d e r l y i n g w a t e r column has been recognized f o r many y e a r s . The s u r f a c e t e n s i o n o f p u r e w a t e r . f o r example, i s much g r e a t e r t h a n t h a t o f n a t u r a l w a t e r s c o n t a i n i n g o r g a n i c m a t t e r , even t h o u g h t h e bulk c o n c e n t r a t i o n of such m a t e r i a l s may be v e r y low (1-10 mg/1). I n f a c t t h e s u r f a c e t e n s i o n of many n a t u r a l w a t e r s i s v e r y close t o t h a t o f weakly a s s o c i a t e d l i q u i d s such as benzene. T h i s suggests t h a t much o f t h e o r g a n i c m a t t e r n o m i n a l l y d i s s o l v e d i n w a t e r must h a v e c o n s i d e r a b l e s u r f a c e - a c t i v e p r o p e r t i e s . M o l e c u l e s showing such a c t i v i t y a r e those where t h e r e i s a s t r o n g s e p a r a t i o n between p o l a r a n d n o n p o l a r r e g i o n s w i t h i n t h e molecular s t r u c t u r e ; a 0097-6156/87/0327-0206$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

15.

LARSON AND ROUNDS

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207

simple example would be a long-chain f a t t y a c i d or a d e t e r g e n t s u l f o n a t e . Two t y p e s of s u r f a c e - a c t i v e molecules, wet and dry s u r f a c t a n t s , h a v e been d i s t i n g u i s h e d (1). V e t s u r f a c t a n t s are t h o s e i n w h i c h a l a r g e p o r t i o n of t h e molecule i s p h y s i c a l l y l o c a t e d i n t h e s u b s u r f a c e l a y e r and only a small f r a c t i o n of t h e s u r f a c t a n t p r o j e c t s above t h e s u r f a c e i n t o t h e a i r . Such molecules a r e n o r m a l l y of q u i t e l a r g e size and i n c l u d e p r o t e i n s , w h i c h t y p i c a l l y have a few h y d r o p h o b i c c h a i n s and a l a r g e r e g i o n s of h y d r o p h i l i c s t r u c t u r e . D r y s u r f a c t a n t s a r e those such as d e t e r g e n t s , i n w h i c h most of t h e molecule p r o j e c t s out of t h e w a t e r and only a small p o r t i o n i s a s s o c i a t e d w i t h i t . A l t h o u g h i n e a r l y s t u d i e s of s u r f a c e m i c r o l a y e r s , most a t t e n t i o n was g i v e n t o t h e d e t e r m i n a t i o n of d r y s u r f a c t a n t s , r e c e n t work has c o n f i r m e d t h a t most of t h e m a t e r i a l c o n s i s t s of polymeric, wet surfact a n t s . F o r example, r e c e n t work on s u r f a c e - a c t i v e m a t e r i a l s i n f r e s h w a t e r s i n d i c a t e s t h a t humic s u b s t a n c e s were t h e p r e d o m i n a n t surface-active material e x c e p t i o n a l cases was th p r e s e n c e of d e t e r g e n t s (2). A n o t h e r e v e n t w h i c h w i l l lead t o a s u r f a c e f i l m on w a t e r i s a s p i l l of a h y d r o p h o b i c m a t e r i a l s u c h as petroleum. In t h i s case, i t is likely, at least i n a f r e s h spill, t h a t the surface layer w i l l be dominated by i n s o l u b l e m a t e r i a l ( a l i p h a t i c and a r o m a t i c hydrocarbons) r a t h e r t h a n s u r f a c e - a c t i v e material. M a n y h y d r o p h o b i c p o l l u t a n t s may t e n d t o become more concentrated m n a t u r a l l y o c c u r r i n g m i c r o l a y e r s , b u t because t h e y a r e t y p i c a l l y t h i n (




Products

H0 2

II.

Algae + Light H 0 2

F i g u r e 6. anilines.

2

+ P-Algae

—£—> L l

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H 0 2

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P o s s i b l e pathways f o r t h e a l g a l - i n d u c e d

oxidation of

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

ZEPP ET AL.

Table I I I .

Decay and Formation of Hydrogen Peroxide

Comparison o f Rate C o n s t a n t s and K Substituted Anilines

Compound

log K

Aniline m-Toluidine £-n-Butylani1ine

0.98 1.40 2.6

f o w

223

s for Alkyl-

I n Algae Suspension log k b

a

r e l

Q W

-0.73 -0.21 0.91

c

a

R e f . 15; ^ R a t i o o f r a t e c o n s t a n t i n s u n l i g h t t o r a t e c o n s t a n t f o r £-nitroanisole a c t i n o m e t e r ( c o n d i t i o n s 1 M a n i l i n e ; 240 mg c h l a/m^ Chlamydomonas sp.; pH 7.0 i n phosphate b u f f e r ; computed v a l u e . c

t h i s concept, a n i l i n e s t r o n g l by Chlamydomonas sp. ( F i g u r o x i d e o r a p r e c u r s o r t h e r e o f was d i v e r t e d by r e a c t i o n w i t h t h e s u b s t r a t e on t h e c e l l . The i d e a was supported f u r t h e r by e a r l i e r s t u d i e s (14) t h a t showed t h a t a n i l i n e s a r e p a r t i c u l a r l y s u s c e p t i b l e t o o x i d a t i o n s c a t a l y z e d by p e r o x i d a s e s . When H2O2 was added r e p e t i t i v e l y t o dark a l g a l s u s p e n s i o n s (Chlamydomonas, N o s t o c , Anabaena) c o n t a i n i n g a n i l i n e s , however, no a n i l i n e o x i d a t i o n o c c u r r e d when the p e r o x i d e decomposed. A n i l i n e was r a p i d l y o x i d i z e d under t h e same c o n d i t i o n s w i t h added p e r o x i d a s e . These r e s u l t s i n d i c a t e t h a t o t h e r s u b s t a n c e s , p r o b a b l y c a t a l a s e enzymes ( 1 6 ) , were r e s p o n s i b l e f o r t h e d e c o m p o s i t i o n o f H2O2 by t h e a l g a e i n c l u d e d i n t h e p r e s e n t s t u d y . The i n h i b i t i o n o f p h o t o p r o d u c t i o n o f H2O2 by a n i l i n e must be a t t r i b u t e d t o p h y s i c a l o r c h e m i c a l quenching o f a p r e c u r s o r t o the H2O2 o r p o s s i b l y t o a l i g h t - i n d u c e d r e a c t i o n between H2O2 and a n i l i n e on t h e s u r f a c e o f t h e a l g a l c e l l ( F i g u r e 6 ) . A d e t a i l e d e x a m i n a t i o n o f t h e n a t u r e o f t h i s o x i d a t i o n was beyond t h e scope o f t h i s study. Conclusions (1) Green and b l u e - g r e e n a l g a e a r e c a p a b l e o f e f f i c i e n t l y c a t a l y z i n g the removal o f t r a c e hydrogen p e r o x i d e from water. A second-order r a t e e x p r e s s i o n d e s c r i b e s t h e removal. The median second-order r a t e c o n s t a n t f o r n i n e a l g a e was about 4xl0"^m^ (mg/chl a)"^h"^ · (2) Exposure o f a l g a e t o s u n l i g h t r e s u l t s i n p h o t o g e n e r a t i o n o f hydrogen p e r o x i d e . T h i s f i n d i n g i n d i c a t e s t h a t the m i c r o b i o t a cont r i b u t e t o the photoproduction of peroxide i n n a t u r a l waters, although further research i s required t o e s t a b l i s h the g e n e r a l i t y of t h i s p h o t o p r o c e s s . (3) A green a l g a e p h o t o c a t a l y z e d t h e o x i d a t i o n o f a n i l i n e s i n a r e a c t i o n t h a t e x h i b i t e d a pronounced hydrophobic e f f e c t . Anilines a l s o i n h i b i t e d t h e p h o t o p r o d u c t i o n o f H2O2 by Chlamydomonas i n d i c a t i n g t h a t p h o t o c h e m i c a l l y produced p e r o x i d e o r a p r e c u r s o r t h e r e o f p l a y s a key r o l e i n t h e o x i d a t i o n .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

224 Acknowledgment s

A p o r t i o n o f t h i s work was performed under t h e B i l a t e r a l Agreement of E n v i r o n m e n t a l P r o t e c t i o n , P r o j e c t 02.03-31 "Forms and Mechanisms by Which P e s t i c i d e s and Chemicals a r e T r a n s p o r t e d " , between t h e U n i t e d S t a t e s and t h e Union o f S o v i e t S o c i a l i s t R e p u b l i c s . Tke t e c h n i c a l a s s i s t a n c e o f J . Mims and C. Bakke i s acknowledged as i s the p r e p a r a t i o n o f t h e f i g u r e s by B. B a r t e l l , R. Moon and P. W i n t e r . Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Penkett, S.A.; Jones, B.M.R.; Brice, K.A.; Eggleton, A.E.J. Atmos. Environ. 1979, 13, 123. Cooper, W.J.; Zika, R.G. Science 1983, 220, 711. Draper, W.; Crosby, D.G. Arch. Environ. Cont. Toxicol. 1983, 12, 121. Bielski, B.H.J.; Cabelli D.E. Arudi R.L. Ross A.B J. Phys. Chem. Ref Leung, P-S.K.; Hoffmann, M.R. J. Phys. Chem. 1985, 89, 5267. Dunford, H.B.; Stillman, J.S. Coord. Chem. Rev. 1976, 19, 187-251. Draper, W.M.; Crosby, D.G. J. Agr. Food Chem. 1984, 32, 231. Starr, R.C. J. Phycol. 1978, 14, 47-100. Zepp, R.G.; Schlotzhauer, P.F. Environ. Sci. Technol. 1983, 17, 462-468. Kratz, W.A.; Myers, J. Am. J. Bot. 1955, 42, 282-287. van Baalen, C; Marier, J.E. Nature 1966, 211, 951. "Standard Methods for the Examination of Water and Wastewater"; 13th Ed.; American Public Health Association: New York, 1971; pp. 746-747. Dulin, D.; Mill, T. Environ. Sci. Technol. 1982, 16, 815. Job, D.; Dunford, H.B. Eur. J. Photochem. 1976, 66, 607. Leo, Α.; Hansch, C.; Elkins, D. Chem; Rev. 1971, 71, 525. Saunders, B.C.; Holmes-Seidle, A.G.; Stark, Β.Ρ. "Peroxidase"; Butterworths: London, 1964.

RECEIVED July 8, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 17

Photocatalysis by Inorganic Components of Natural Water Systems Cooper H. Langford and John H. Carey Department of Chemistry, Concordia University, Montreal, Quebec, Canada, and National Water Research Laboratory, Canada Center for Inland Waters, Burlington, Ontario, Canada

A brief inventor f aquati component potentially activ includes colloida y and sulfides, metal complexes of organic ligands, and certain metal ions in clays. The reactions are to be understood in terms of the redox reactions following ligand to metal charge transfer and its extension to solid lattices, reactions of photogenerated electron hole pairs according to the "microcorrosion cell" model. Studies, many of which were directed to development of waste treatment techniques, have suggested probable pathways. Inventory

o f Chromophores

The f i r s t i n o r g a n i c s p e c i e s t o b e c o n s i d e r e d a s c a n d i d a tes f o r photoreactions important i n natural waters were c a r b o x y l i c a c i d complexes o f Fe+ andC u [ e . g . t h e NTA c o m p l e x e s I (1, 2). In these complexes, photooxidation of the organic ligand i s i n i t i a t e d b y an e x c i t a t i o n process i n which a Jigand l o c a l i z e d e l e c t r o n i s promoted into the partly f i l l e d d s h e l l o f the metal ion. This results i n reduction o f t h e metal ion. Since the reduced metal i o n i s o f t e n r e a d i l y r e o x i d i z e d by oxygen, the r e a c t i o n s can be rendered c a t a l y t i c . The i m p l i c a tions o f t h e model studies have been v a l i d a t e d by results obtained f o r both marine (3) and fresh (4) waters. In considering the extensions o f these preced e n t s , we m u s t consider t h e range o f p o s s i b l e metal organic complexes s a t i s f y i n g the c o n d i t i o n s f o r charge transfer within t h e wavelength range available i n i l l u m i n a t e d waters, r e d u c i b i l i t y o f the metal ion, and an i r r e v e r s i b l e f i r s t step f o r the oxidation of the 3

+ 2

0097-6156/87/0327-0225$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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o r g a n i c l i g a n d t h a t can e f f e c t i v e l y b l o c k back r e a c t i o n . The m e t a l ions are few [ n o t a b l y F e ( I I I ) , ΜηίΙΙΙ, I V ) , perhaps U0+ , a n d C u ( I I ) i n t h e uv l i m i t ) . The organic ligands that give i r r e v e r s i b l e o x i d a t i o n are predomina­ t e l y t h o s e w h i c h c o n t a i n a c a r b o x y l a t e and can decompose r a p i d l y a f t e r one e l e c t r o n o x i d a t i o n t o r e l e a s e CO2 and escape back r e a c t i o n or those containing f u n c t i o n a l i t y l i k e phenol where the r e s u l t of one e l e c t r o n o x i d a t i o n is a relatively stable r a d i c a l w h i c h may survive. The o v e r a l l impression i s that the range of r e a c t i o n s w i l l be l i m i t e d and t h a t the evidence to date i s c o n s i s t e n t with such a notion. Indeed, since a major source of organic ligands i n f r e s h w a t e r s i s humic m a t e r i a l which has s t r o n g e r l i g h t a b s o r p t i o n at long wavelength than most m e t a l c o m p l e x e s and has p h o t o i n i t i a t i o n p a t h w a y s o f i t s own ( 5 , 6) w h i c h may be metal ion quenched, the i n t e r a c t i o n of meta ion wit organi ligand much t o quench p h o t o c h e m i s t r initiate i t ! A more promising p o s s i b i l i t y f o r r e a c t i o n s of ge­ n e r a l s i g n i f i c a n c e began t o emerge w i t h the r e c o g n i t i o n of the i m p l i c a t i o n s f o r n a t u r a l water photochemistry of the d e v e l o p i n g subject of initiation of photoreaction f o l l o w i n g b a n d gap i r r a d i a t i o n o f s e m i c o n d u c t o r s . It is i n t e r e s t i n g to r e f l e c t , i n the face of current torrent of papers on t h i s subject (7), t h a t t h e i d e a has been a r o u n d f o r a r a t h e r l o n g t i m e (8) and t h a t some of the earliest thoughts on its utilization for solution r e a c t i o n s were connected to waste water treatment. A report from the Robert A. T a f t C e n t e r a p p e a r e d i n 1969 ( 9 ) a n d CCIW w o r k e r s r e p o r t e d t h e d e c h l o r i n a t i o n o f P C B s u s i n g a n a t a s e i n 1975 (.10) . There are i m p o r t a n t p a r a l l e l s b e t w e e n t h e way that i l l u m i n a t i o n of a semiconductor can produce r e a c t i v e species in s o l u t i o n and the r e d u c t i o n of the metal ion and o x i d a t i o n o f t h e l i g a n d c h a r a c t e r i z i n g t h e l i g a n d t o metal charge transfer photochemistry described above. But, aspects of the behaviour of the s e m i c o n d u c t o r solution interface can overcome s e v e r a l of the limita­ t i o n s mentioned f o r the homogeneous c o m p l e x e s . When a semiconductor is illuminated with light of energy greater than i t s optical band gap, an electron is t r a n s f e r r e d to the c o n d u c t i o n band l e a v i n g a h o l e i n the v a l e n c e band. At the i n t e r f a c e , e l e c t r o n t r a n s f e r can o c c u r e i t h e r f r o m t h e c o n d u c t i o n b a n d t o an a c c e p t o r , A, i n s o l u t i o n or from a donor, D, in solution to the v a l e n c e band. These processes are s h o w n i n F I G U R E 1. They compete with the fruitless recombination of electron and hole to produce thermal energy. In the case of the metal complexes above, the r e c o m b i n a t i o n c o u l d o n l y be a v o i d e d b y p r o d u c t i o n o f " s t a b l e " c h e m i c a l intermediates. In this case, there are two other p o s s i b i l i t i e s for r e s i s t a n c e to the back r e a c t i o n . The 2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

LANGFORD AND CAREY

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first is essentially thermodynamic and t h e s e c o n d i s essentially kinetic. When a p a r t i c l e o f s u f f i c i e n t s i z e e l e c t r o c h e m i c a l l y e q u i l i b r a t e s w i t h t h e Eh o f t h e s u r r o u n d i n g s o l u t i o n , the Fermi l e v e l i n the semiconductor b u l k comes to the p o t e n t i a l o f t h e e n v i r o n m e n t a l Eh. This usually results i n band b e n d i n g at the i n t e r f a c e and a depletion layer at the surface of the semiconductor w i t h a p o t e n t i a l g r a d i e n t (1_1) f a v o u r i n g the s e p a r a t i o n of the photogenerated charge c a r r i e r s as i s shown i n FIGURE 2. This means t h a t t h e b a n d b e n d i n g can promote charge s e p a r a tion in a way u n a v a i l a b l e to a molecular system. The e x a c t e n e r g e t i c s w i l l d e p e n d on t h e r e l a t i o n b e t w e e n t h e v a l e n c e and c o n d u c t i o n band e n e r g i e s a n d t h e Eh o f t h e solution. One o f the e l e c t r o n t r a n s f e r processes, the one r e s u l t i n g from the transfer of the c a r r i e r toward the particle solutio r i z e d as a photoreaction from the t r a n s f e r o bending toward the i n t e r i o r of the p a r t i c l e , i s thought of c o r r e s p o n d i n g l y as a t h e r m a l r e a c t i o n . But, the rate of each r e a c t i o n w i l l n o r m a l l y be g o v e r n e d by t h e r u l e s of i n t e r f a c i a l e l e c t r o n t r a n s f e r . The two most i m p o r tant factors are the degree of e x e r g o n i c i t y of the i n t e r f a c i a l e l e c t r o n t r a n s f e r and t h e e x t e n t of overlap of the d i s t r i b u t i o n of energy l e v e l s of s o l u t e species a n d t h e b a n d e d g e s (.LL) . In n a t u r a l w a t e r s , many interesting semiconducting p a r t i c l e s , e s p e c i a l l y those of the w e l l hydrated hydrous o x i d e s which have l a r g e "interior" surface, w i l l have particle sizes much s m a l l e r that the t y p i c a l thickness of t h e d e p l e t i o n layer at the solution semiconductor interface. I t might be a s s u m e d t h a t s u c h s m a l l p a r t i c l e s would c a r r y no advantage over molecular excited states in the c o m p e t i t i o n of separation of charge c a r r i e r s ys b a c k r e a c t i o n . This i s not e n t i r e l y the case. E v e n a s m a l l p a r t i c l e may h a v e many a t o m s a n d t h e m o b i l i t y of a c a r r i e r , once g e n e r a t e d , is sufficiently large to carry i t away from the s i t e of g e n e r a t i o n . Moreover, a t y p i c a l semiconductor will be capable of b e i n g "doped" with localized s i t e s of one t y p e o r t h e other. These l o c a l i z e d s i t e s a r e t r a p s t h a t can c a p t u r e photogenerated carriers in times of the order of a n a n o s e c o n d o r two. E v i d e n c e f o r t h i s t r a p p i n g mechanism has r e c e n t l y been d e v e l o p e d i n a s t u d y o f t h e p i c o s e c o n d t r a n s f e r f r o m an e x c i t e d d y e t o T 1 O 2 w h e r e the trapping p r o c e s s can be r e s o l v e d and t h e back r e a c t i o n i s slow (12). The p i c o s e c o n d s p e c t r a s h o w t h a t t h e t i m e c o n s t a n t s for trapping o f an excess c a r r i e r from a semiconductor band to the l o c a l i z e d s i t e s which correspond to doping is a process i n the nanosecond time domain. However, the trapped site spectra d i d not appear under a l l c i r c u m s t a n c e s when l i g h t was a b s o r b e d a t t h e

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F I G U R E 1. The Photocatalyti " C o r r o s i o n " C e l l I l l u s t r a t e d by T i 0 . Redox C o u p l e s may n e e d t o b e A d s o r b e d to Compete w i t h R e c o m b i n a t i o n . 2

Note that t h e i fReaction i s

SEMICONDUCTOR

SOLUTION

Fermi Level

F I G U R E 2. Energetics of Oxidation tor Electrolyte Interface.

at the

Semiconduc-

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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semiconductor surface. Analysis of the circumstances indicated that the c r u c i a l factor was the competition between recombination of holes and e l e c t r o n s a t t h e s u r f a c e and the escape o f c a r r i e r s i n t o t h e i n t e r i o r . A key f a c t o r f o r the e f f i c i e n t escape o f a c a r r i e r from the s u r f a c e to the i n t e r i o r was the presence of a reactant on the surface that could react with either electrons or holes. At high light intensity, these initial steps must be i n t h e p i c o s e c o n d time domain. Therefore, efficient photochemistry will depend very s t r o n g l y on the presence of adsorbed reactants at the surface. With adsorbed reactants, even the small p a r t i c l e c a n c o n s i d e r a b l y f a v o u r c h a r g e s e p a r a t i o n b y an e s s e n t i a l l y k i n e t i c mechanism. We s h o u l d a d d p a r e n t h e t i c a l l y t h a t t h e o b s e r v a t i o n s just described may provide the r e s o l u t i o n o f a minor controversy. There of l i g h t absorptio complexes. The g r e a t e r a b s o r p t i v i t y o f t h e s e m i c o n d u c tor suggests that light i s absorbed by the solid. However, the requirement of a surface scavenger t o prevent rapid recombination will give the overall kinetics the characteristics indicating a surface complex. C o l l o i d a l p h o t o c a t a l y s t s can form i n n a t u r a l waters i n a v a r i e t y o f ways. In general, precipitation of a solution species initiates. The p r o c e s s e s include at least the following p o s s i b i l i t i e s : i. Iron and manganese oxides a r e p r e c i p i t a t e d by oxidation of l o w Eh s o l u t i o n s i n e n v i r o n m e n t s such as those which occur when ground water emerges at the s u r f a c e o r r e d u c i n g water flows from a wetland. i i . U, V, Cu, Se, a n d Ag a r e p r e c i p i t a t e d as low valency oxides (or metals) when w a t e r s at high Eh mix with reducing waters or encounter organic reducing matter. i i i . F e , C u , A g , Z n , P b , Hg, N i , Co, A s , a n d Mo a r e p r e c i p i t a t e d as s u l f i d e s by r e d u c t i o n o f s u l f a t e w a t e r s , u s u a l l y by s u l f a t e r e d u c i n g b a c t e r i a . The f i n a l category o f chromophore that we w i l l consider here i s the coloured inorganic i o n adsorbed ( o r i o n e x c h a n g e d i n t o ) a s i t e on a c l a y p a r t i c l e . In such cases, the light absorption i s l o c a l i z e d and charge separation does not occur because of migration of carriers i n the solid. Rather, moderately e f f i c i e n t r e a c t i o n c a n a r i s e when t a r g e t m o l e c u l e s c a n b e c o a d s o r bed and t h e i n i t i a t i n g chromophore r e a c t s e f f i c i e n t l y with the coadsorbed molecule. An a p p r o p r i a t e e x a m p l e i s the uranyl exchanged c l a y p h o t o c a t a l y s t which accomplis h e s t h e o x i d a t i o n o f a l c o h o l s t o k e t o n e s (13.) .

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230 Photocatalysts

i n the

Naj^j~al

Setting

Especially in the c a s e o f the p a r t i c u l a t e s e m i c o n d u c t o r s , t h e r e i s now an e x t e n s i v e l a b o r a t o r y chemistry of the catalytic reactions which may be accomplished. However, t h e s i t u a t i o n i n a n a t u r a l s e t t i n g i s much more complex t h a n i s c h a r a c t e r i s t i c o f model s t u d i e s . Before we b e g i n a r e v i e w o f i m p o r t a n t r e a c t i o n s now known, i t i s u s e f u l t o s e t out some comments on the f a c t o r s , w h i c h differentiate the laboratory study from the n a t u r a l system. F i r s t , we must r e m a i n aware t h a t n a t u r a l w a t e r s do not receive extensive illumination at wavelengths s h o r t e r than about 350 nm. Thus, some o f t h e p r o c e s s e s involving high band gap oxide photocatalysts which attract a great deal of l a b o r a t o r y a t t e n t i o n do not figure importantly i a t t e n t i o n should b longer wavelengths , shallow waters, models of the i l l u m i n a t i o n a v a i l a b l e have p r o v e d a d e q u a t e t o a c c o u n t f o r the o v e r a l l r a t e s o f photoreaction (14.) so t h a t this factor i s w e l l understood. On the other hand, we do not require large quantum yields to produce significant effects. Half l i v e s of d i r e c t p h o t o l y s i s of organic c h r o m o p h o r e s have been e s t i m a t e d between a few h o u r s and t h i r t y days ( 1 4 ) . I f the shortest reflect quantum y i e l d s n e a r one, then values near 0.001 are still i n t e r e s t i n g on t h e t i m e s c a l e o f one month. With c o l l o i d a l p h o t o c a t a l y s t s , i n c r e a s e d d i f f u s i v e ness of l i g h t c a u s e d by s c a t t e r i n g i n t u r b i d w a t e r s can l e a d t o enhanced p h o t o l y s i s r a t e s . For example, a s t u d y of the e f f e c t s of c l a y s on the r a t e o f a uv p h o t o l y s i s showed an i n i t i a l i n c r e a s e in the photolysis r a t e on a d d i t i o n of clay followed by a d e c r e a s e when the c l a y c o n c e n t r a t i o n f i n a l l y produced o f f s e t t i n g l i g h t attenuation (15)· The light available for photochemistry s h o u l d be measured by t h e r m a l l e n s i n g and photoacoustic measurements. T h i s has been d e m o n s t r a t e d t o be e f f e c t i v e f o r w a t e r s c o n t a i n i n g humic c o l l o i d s and suspended f i n e sands (UB). A very important difference between laboratory photocatalysts and corresponding materials in the natural setting is relative purity. In t h e l a b o r a t o r y , most work has been c o n d u c t e d u s i n g pure m i n e r a l phases. In n a t u r a l w a t e r s , many m i n e r a l s have c o l o u r s character i s t i c of e x c i t a t i o n of e l e c t r o n s from impurity l e v e l s within the bandgap. In t h i s c a s e , the h o l e energies w i l l be l e s s f a v o r a b l e for oxidative p r o c e s s e s than i n the case of excitation at w a v e l e n g t h s h o r t enough t o promote e l e c t r o n s from the d e e p e r l y i n g c o n d u c t i o n band. This has been well documented by l a b o r a t o r y s t u d i e s d i r e c t e d toward the shifting of the water o x i d a t i o n reaction catalyzed by anatase toward t h e v i s i b l e by

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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doping the titanium dioxide l a t t i c e with metal ions with partly filled d shells. The c a s e o f d o p i n g w i t h C r ( I I I ) is an e s p e c i a l l y interesting e x a m p l e (1_7). Hydrogen e v o l u t i o n f r o m w a t e r was o b s e r v e d a t C r ( I I I ) d o p e d T 1 O 2 irradiated with light passed t h r o u g h a 4 1 5 nm c u t o f f filter. The e l e c t r o n promoted t o t h e c o n d u c t i o n band has t h e r e d u c i n g power o f t h e c o n d u c t i o n band edge. I n t h i s case, the Cr(IV) p r o d u c e d has t h e o x i d i z i n g power (locally) necessary t oo x i d i z e water. I t appeared that the C r i s l o c a t e d i n the s u r f a c e r e g i o n o fthe particle which was p r o b a b l y necessary t o the oxidation of a solution species. A typical coloured internal ion i na m i n e r a l might not be as s t r o n g l y o x i d i z i n g as C r ( I V ) b u t i f i twere, t h e r e would be a d i f f i c u l t y i n i t s m i g r a t i o n to t h e s u r f a c e t oo x i d i z e a s o l u t e . An additional relevant p r o b l e m was i d e n t i f i e d i n t h i s study o f C r dopin cut o f f a ta dopin to the s o l u b i l i t y l i m i t i n anatase. The a u t h o r s b e l i e v e that a chromic oxide surface layer forms which b l o c k s reaction. T h i s c a l l s a t t e n t i o n t o a common s t a g e i n t h e aqueous o x i d a t i v e weathering o f mineral p a r t i c l e s ,t h e f o r m a t i o n on t h e s u r f a c e o f a f i l m o f an o x i d e i n the higher oxidation state of the ion. This i s seen, especially, i n t h e i n c r e a s e o f Fe2Û3 i n the oxide analysis o f the products of weathering. Such s u r f a c e f i l m s may i n t e r d i c t t h e e x p e c t e d p h o t o r e a c t i v i t y o f the u n d e r l y i n g pure m i n e r a l a t , as t h e example s u g g e s t s , l o w total levels. On t h e o t h e r h a n d , s u c h f i l m s c a n p r o d u c e oxide semiconductors on u n d e r l y i n g m i n e r a l s t h a t might o t h e r w i s e seem u n p r o m i s i n g . Another aspect o fthe impurity doping which w i l l be common i n m i n e r a l p a r t i c l e s i s i l l u s t r a t e d b y t h e e f f e c t o f Nb(V) on a n a t a s e . This i o n substitutes isomorphically f o rTi(IV) and as an η dopant c r e a t e s a S h o t t k y barrier atthe interface. This assists i n j e c t i o n o f an e l e c t r o n from a r e d u c i n g s o l u t e i n t o t h e c o n d u c t i o n band (13.)The f l a t b a n d i s a l s o s h i f t e d cathodically. I t has r e c e n t l y been c l a i m e d t h a t t h e r e l a t i v e inefficiency of haematite as a photoanode i s a function o f low mobility of carriers and that this problem may b e overcome by doping w i t h S i ( I V ) ( 1 9 ) . I n t h i s we s e e t h e last effect o f doping, the modification of carrier mobi1ity. Having r a i s e d the question o f surface f i l m s on mineral particles, i t i s t e m p t i n g t o a l s o comment h e r e on t h e t e n d e n c y o f p a r t i c l e s t o acquire organic coa­ tings. Again, t h e photochemical consequences a r e complex. The o r g a n i c m a t t e r may a b s o r b l i g h t . These excited states may " s e n s i t i z e " the underlying minerals by e l e c t r o n o r h o l e i n j e c t i o n . Some e v i d e n c e f o r s u c h a process h a s been obtained i n studies o ff u l v i c acid a d s o r b e d o n a n a t a s e (20.). On t h e o t h e r hand, hydrous f e r r i c oxides have r e c e n t l y been o b s e r v e d t oquench t h e

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photochemistry i n i t i a t e d by t h e f u l v i c a c i d . As a t h i r d e f f e c t , t h e o r g a n i c f i l m may be t h e r e d o x species which r e a c t s w i t h t h e phot©generated c a r r i e r s f r o m t h e m i n e r a l part icle. Εathways o f

reaction -

reduction

As m e n t i o n e d a b o v e , b o t h electrons and holes m u s t be e f f i c i e n t l y separated and b o t h must r e a c h the p a r t i c l e surface. E v e n when both reach the s u r f a c e , recombina­ t i o n may s t i l l predominate i f b o t h are not consumed i n appropriate fast reactions. It i s worth r e i t e r a t i n g that the requirements f o r f a s t r e a c t i o n s are t w o f o l d . The f i r s t is favorable energetics. The r e d o x c o u p l e s m u s t be i n c l u d e d w i t h i n t h e b a n d gap as shown i n FIGURE 2. But, i t i s a l s o necessary to maximize the r a t e of interfacial electro standard problem o such, one for whic precedents Above, we mentioned the general theoretical rules. H e r e , we w i l l g i v e more a t t e n t i o n t o s p e c i f i c e x a m p l e s . In most c a s e s , e x p e r i m e n t a l r e s u l t s can p r o v i d e precedents. Since the main p o i n t w i l l be t o c o m p a r e t h e r a t e s of competing processes, i t is useful to d i v i d e the d i s c u s s i o n i n t o r e d u c t i o n and o x i d a t i o n r e a c t i o n s . In o r d e r f o r an oxidant to react r a p i d l y , i t w i l l be m o s t a d v a n t a g e o u s f o r i t to be p r e s e n t adsorbed on the s u r f a c e so t h a t mass t r a n s p o r t w i l l not l i m i t the rate. T h i s means that we should first c o n s i d e r the solvent which i s always at the i n t e r f a c e . The possibi­ l i t y o f w a t e r a c t i n g a s an a c c e p t o r t o produce hydrogen has a t t r a c t e d a great deal of a t t e n t i o n because of the p o s s i b i l i t y of s t o r i n g s o l a r energy i n a chemical fuel by t h i s process. U n f o r t u n a t e l y , water i s not t h a t easy to reduce. In every case where t h e r e has been s u c c e s s , a known catalyst for hydrogen e v o l u t i o n has been required. Naturally occurring materials do not o f f e r many o f t h e known catalysts. The o n l y e x a m p l e u s i n g a m a t e r i a l which might occur i n n a t u r a l waters i s t h e use o f ZnS as a catalyst for H2 e v o l u t i o n (21). Here the special property of ZnS to photodegrade partially leaving a deposit of Zn m e t a l on t h e s u r f a c e l i n k s t h e example to other cases. Hydrogen e v o l u t i o n most o f t e n is achieved by the mediation of a metal catalystcommonly t h e noble metals known f o r t h e i r lower overvoltages for hydrogen. T h i s " u n r e a c t i v i t y " can prove b e n e f i c i a l i n some e n v i r o n m e n t a l c o n t e x t s . I f water i s not r a p i d l y reduced there is a better opportunity for direct reaction with a solute (including xenobiotics). The n e x t most likely acceptor to occur at the surface of a mineral p a r t i c l e i s the oxygen molecule which w i l l generally be present at reasonable levels throughout the p h o t i c zone. Oxygen i s s t r o n g l y adsorbed

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on a number o f s u r f a c e s and e s p e c i a l l y on o x i d e s u r f a ces. The g e n e r a t i o n o f the superoxide ion, O 2 " , from one e l e c t r o n r e d u c t i o n o f the oxygen m o l e c u l e i s a w e l l known r e a c t i o n t h a t has r e c e n t l y been shown t o occur in humic w a t e r s (22.). Many s t u d i e s of p h o t o c a t a l y s i s at semiconductor surfaces i n c l u d i n g e a r l y work in one o f our l a b o r a t o r i e s on t h e p h o t o l y s i s of PCBs i n a n a t a s e s l u r r i e s (9) have n o t e d t h a t no r e a c t i o n occurs i n the absence of oxygen. Superoxide i s a good n u c l e o p h i l e . A l s o , a r e a c t i o n o f the f o l l o w i n g t y p e i s known:

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cl

Cr —

° 0.1. In t h e s o l u t i o n p h a s e p h o t o l y s i s o f f e r r i c p e r c h l o rate i n the presence of tertiary alcohols w i t h more carbons than t - b u t a n o l , we observed that H a b s t r a c t i o n b y OH radicals produced some radicals that d i d not reduce F e ( I I I ) to Fe(II). The a d d i t i o n o f C u ( I I ) i o n s caused these r a d i c a l s to be oxidized and resulted in additional ferrous ion production. For example, the r a t i o of Fe(II) quantum yields for 3-ethy1-3-pentanol w i t h o u t and w i t h 0.0050M a d d e d C u ( I I ) was 0 . 8 4 4 . When the anatase dispersio c o n d i t i o n s , the r a t i fairly directly tha a n a t a s e s u r f a c e i s k i n e t i c a l l y e q u i v a l e n t t o OH r a d i c a l . Several s t u d i e s of the hydroxylated anatase surface are a v a i l a b l e i n the l i t e r a t u r e . Two t y p e s of surface OH g r o u p s h a v e b e e n d i s t i n g u i s h e d , t h o s e b o u n d t o o n e T i and t h o s e w h i c h b r i d g e two. The s i n g l y bound h y d r o x y l s are thought to be r e a d i l y exchangeable, the b r i d g i n g ones n o t . F o r most c r y s t a l f a c e s , t h e s e two t y p e s o c c u r i n a l t e r n a t e rows and a r e t h u s a v a i l a b l e i n a p p r o x i m a t e l y e q u a l numbers. A p l a u s i b l e e x p l a n a t i o n f o r the above results i s that either type o f s u r f a c e h y d r o x y l can f u n c t i o n as a h o l e t r a p . I f the hole i s trapped at a singly bound hydroxyl, a species similar t o t h e OH r a d i c a l i s produced. But, i f the hole i s trapped at a bridging group, a weaker oxidant capable only of the d i r e c t o x i d a t i o n r e a c t i o n pathway, i s produced. This model has p r e c e d e n t i n t h e homogeneous s o l u t i o n c h e m i s try of Fe(III) where the bridged dimer i s inactive t o w a r d p h o t o p r o d u c t i o n o f OH r a d i c a l . FIGURE 4 shows t h a t there i s a drop i n the F e ( I I ) y i e l d as a c i d c o n c e n t r a t i o n i s i n c r e a s e d . This probably r e f l e c t s the increase of p o s i t i v e charge at the surface a n d an e l e c t r o s t a t i c b a r r i e r t o h o l e t r a n s f e r a c r o s s t h e surface. The s u r f a c e a l s o w i l l c h a n g e i t s p r o c l i v i t y t o adsorb v a r i o u s potential reactants as a function of surface charge. T h i s w i l l be a g e n e r a l p r o b l e m . It is important t o keep track of the relation between the point of z e r o c h a r g e o f t h e p a r t i c l e s a n d t h e pH o f t h e s o l u t i o n i n c o n s i d e r i n g any s p e c i f i c r e a c t a n t . Anatase i s not a unique system. Many of the features of anatase were, a f t e r a l l , " a n t i c i p a t e d " by t h e homogeneous c h e m i s t r y o f F e ( I I I ) . This leads to the suggestion that the mechanistic studies of anatase should provide c o n s i d e r a b l e guidance to the behaviour of other oxide systems. For t h i s reason, i t i s i n t e r e s t i n g to r e c o r d h e r e a number of reactions that have been

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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r e p o r t e d u s i n g l i g h t a b s o r p t i o n by a n a t a s e as initiating step. Many o f t h e s e were explored for their possible u t i l i t y i n photochemical waste water treatment. Donors of a p p r o p r i a t e redox p o t e n t i a l to r e a c t w i t h holes at the anatase surface include organic acids, carbohydrates, fats, CN~, and halides (24). (The c y a n i d e r e a c t i o n has been studied for i t s u t i l i t y in treatment of the waste streams from gold mining operations in the Canadian Northwest Territories.) More immediately r e l e v a n t to n a t u r a l water i s the observation t h a t an a n a t a s e s l u r r y c o u l d e f f e c t the d e c o l o r a t i o n of a chlorinated bleach plant e f f l u e n t . A s a m p l e o f amber c o l o u r , pH = 1.8, a n d l o w r e s i d u a l c h l o r i n e was irradiated in the presence o f 0.5% (wt) a n a t a s e w i t h l i g h t o f 350 nm f o r p e r i o d s up t o 18 h r . The o p t i c a l absorbance decreased by half in 1080 min. Small amounts of c h l o r i d e and formaldehyd r e a c t i o n may provid r e l a t i o n between p h o t o b l e a c h i n g metal ions. If s o , we are brought to the question of the r e a c t i v i t y of c o l l o i d a l i r o n oxides. As m e n t i o n e d a b o v e , some laboratory s t u d i e s have indicated that anatase i s s u p e r i o r to a l l other oxides tested with reference reactions such as the a l c o h o l o x i d a t i o n s (2_5) land hydrogen e v o l u t i o n as w e l l ( 2 4 ) ] . H o w e v e r , r e c e n t work has a l s o s u g g e s t e d t h a t a l l t h a t i s required to increase the efficiency of f e r r i c oxide photoanodes i s s u i t a b l e doping w h i c h may occur r e a d i l y in nature. Recently two i m p o r t a n t s t u d i e s have given s p e c i f i c evidence for the photooxidative a c t i v i t y of f e r r i c oxides. The first is the report of Faust and H o f f m a n (2JS) t h a t h a e m a t i t e c a n c a t a l y z e t h e photooxidation of S0z to sulfate. The s c a v e n g i n g c o m p o u n d was present in large concentration in these e x p e r i m e n t s so t h a t t h e r e w e r e no s e r i o u s m a s s t r a n s p o r t l i m i t a t i o n s on the d e l i v e r y of the substrate directly to the s u r f a c e . The s e c o n d i s the r e p o r t of the o x i d a t i o n of o x a l a t e at a hydrous f e r r i c oxide surface (27). There has been some c o n c e r n that t h i s l a s t r e a c t i o n i s the r e a c t i o n of a specific molecular species since the homogeneous ferric oxalate complex does undergo p h o t o o x i d a t i o n at w a v e l e n g t h s b e l o w 520 nm. The distinction is not too important. I f the i r o n l i g a n d centered hole reaches the s u r f a c e f r o m t h e i n t e r i o r i t m u s t be t r a n s f e r r e d to the ligand. If i t i s g e n e r a t e d by l i g h t a b s o r p t i o n a t t h e s u r f a c e , t h e same local structure will result. The difference will be that the s e c o n d c a s e w i l l show a small quantum yield which we will characterize by a t t r i b u t i n g i t t o t h e c o n s e q u e n c e s o f low h o l e m o b i l i t y in the i n t e r i o r . I f that p r o b l e m can be o v e r c o m e , t h e mechanisms merge. The p r e l i m i n a r y i n d i c a t i o n i s t h a t the r e a c t i o n i s e f f i c i e n t enough to suggest the semicond u c t o r mechanism. Thus, we s e e t h a t some e v i d e n c e f o r f e r r i c oxides reacting with organic acids i s i n .

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

Relationships

to B i o l o g i c a l

Activity

One l a s t p o i n t i s i m p o r t a n t . The pathways i n i t i a t e d by i n o r g a n i c p h o t o c a t a l y s t s may n o t t e r m i n a t e w i t h t h e immediate r e a c t i o n s t h e m s e l v e s . The e f f e c t o f p h o t o d e c h l o r i n a t i o n o f PCBs r e n d e r s t h e o r g a n i c p r o d u c t s a c c e s s i b l e t o biodégradation. A s i m i l a r r e s u l t was o b t a i n e d i n a s t u d y o f a l i g n i n model compound, v e r a t r y l a l c o h o l (2i> ) . I r r a d i a t i o n o f an a n a t a s e d i s p e r s i o n o f a s o l u t i o n gave v a n i l l y l a l c o h o l w h i c h i s b i o d e g r a d e d by a c o l o n y o f o r g a n i s m s c a p a b l e o f d e g r a d i n g a r o m a t i c s more than t h r e e times f a s t e r . I t i s q u i t e p o s s i b l e t h a t one o f t h e most i m p o r t a n t a r e a s f o r f u t u r e i n v e s t i g a t i o n w i l l be t h e r e l a t i o n o f i n o r g a n i c p h o t o c h e m i s t r y t o t h e increase of biodegradabi1ity of refractory organic matter. References

1. T. Trott, R.W. Henwood, C.H. Langford, 1972, Envir. Sci. Technol., 6, 367. 2. C.H. Langford, M. Wingham, V.S. Sastri, 1973, Envir. Sci. Technol., 7, 820. 3. R.H. Collienne, 1983, Limnol. Oceanog., 28, 83. 4. C.T, Miles, P.L. Brezonik, 1981, Envir. Sci. Technol., 15, 1089. 5. R.G. Zepp, P.F. Scholtzauer, R.M. Sink, 1985, Envir. Sci. Technol., 19, 74. 6. J.F. Power, C.H. Langford, D.K. Sharma, R. Bonneau, J. Joussot-Dubien, 1985, this symposium. 7. M. Graetzel, ed., 1983, "Energy Resources Through Photochemistry and Catalysis", Academic Press, New York. 8. For a Review of Earlier Work See: 1974, "Photoeffects in Adsorbed Species", Discuss. Faraday Soc., 58. 9. L.C. Kinney, V.R. Ivanuski, 1969, Robert A. Taft Research Center Report Number TWRC-13 Cincinnati, Ohio. 10. J.H. Carey, J. Lawrence, H.M. Tosine, 1976, Bull. Envir. Contam. Toxicol., 16, 697. 11. H. Gerischer, 1979, Topics Appl. Phys., 31, 115. 12. A.D. Kirk, C.H. Langford, C. St-Joly, D.K. Sharma, R. Lesage, 1984, JCS Chem. Communs. 13. S.L. Suib, K.A. Carrado, 1985, Inorg. Chem., 24, 863. 14. T. Mill, et al, 1981, Chemosphere, 10, 1281. 15. G.C. Miller, R.G. Zepp, 1979, Envir. Sci.Technol., 13, 453. 16. J.F. Power, C.H. Langford, Unpublished. 17. E. Borgarello, J. Kiwi, M. Graetzel, E. Pellizetti, M. Visca, 1982, J. Am. Chem. Soc., 104, 2996. 18. P. Salvador, 1980, Solar Energy Material's, 2, 413. 19. C. Leygraf, M. Hendewerk, G.H. Somorjai, 1982, J. Catal., 78, 341.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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AND

CAREY

Photocatalysis

by

Inorganic

Components

20.

239

C.H. Langford, 1982, Proc. ENERGEX, 1982, Solar Energy Society of Canada, Regina, Saskatchewan, August 1982, p. 928. 21. N. Zeug, J. Buchler, H. Kisch, 1985, J. Am. Chem. Soc., 107, 1459. 22. R.M. Baxter, J.H. Carey, 1984, Nature, 206, 575. 23. C.H. Langford, J.H. Carey, 1973, Can. J. Chem., 51. 24. T. Sakata, T. Kawai, 1983, Reference 7, Chapter 10. 25. J.H. Carey, B.G. Oliver, 1980, Water Poll. Res. J. of Canada, 15, 157. 26. B.C. Faust, M.R. Hoffman, Abstracts Environmental Division, 186th ACS National Meeting, Washington, DC, August 28th, 1983 - September 2nd, 1983. 27. M.C. Goldberg, K.M. Cunningham, Abstracts Environmental Division, 185th ACS National Meeting, Seattle, Washington, March 27th, 1983 - April 1st, 1983. RECEIVED July

15, 1986

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 18

Catalyzed Photodegradation of the Herbicides Molinate and Thiobencarb 1

2

R. Barton Draper and Donald G. Crosby

Department of Environmental Toxicology, University of California, Davis, CA 95616

A survey of oxidizin f supplementin the natural oxidant rice herbicides molinate (I) and thiobencarb (VIII) were degraded rapidly in sunlight-irradiated suspensions of TiO and ZnO. ZnO served both as a semiconductor photooxidant and as the Zn(II) fertilizer normally applied for plant nutrition. In a flooded rice field, isolated basins were treated with Ordram 10G (molinate); after 3 days, an aqueous ZnO suspension, stable in field water at pH 8 to 9, was applied. The resulting immediate decrease in molinate half-life from 60 h to 1.5 h indicates that applying ZnO before releasing agricultural wastewater may provide an economical means of intentionally degrading persistent chemical residues. 2

Herbicides are applied extensively to protect C a l i f o r n i a ' s valuable r i c e crop. During 1983, for example, flooded r i c e f i e l d s i n the Sacramento Valley received a e r i a l applications of 422 metric tons of molinate (_S_-ethyl hexahydro-1 j£-azepine-1 -carbothioate) ( I ) and 100 metric tons of thiobencarb [^-(4-chlorophenyl)methyl-N,N^ diethylcarbamothioate](VIII) as the 10% (a.i.) granular formulations Ordram 10G and Bolero 10G, respectively. After holding i r r i g a t i o n flood water for the four to eight days required for residue d i s s i p a t i o n and e f f e c t i v e h e r b i c i d a l a c t i v i t y , farmers release i t into a g r i c u l t u r a l drains which empty into the Sacramento River. Seasonal f i s h k i l l s i n these a g r i c u l t u r a l drains are attributed to the r i c e herbicides. The molinate concentration i n the Colusa Basin Drain has approached 350 Pg/L during early June when herbicide use i s heaviest. Farther downstream i n the Sacramento River, near Sacramento's drinking water treatment f a c i l i t y , concentrations of molinate and thiobencarb have exceeded 20 ug/L 'Current address: Department of Chemistry, University of Texas, Austin, TX 78712 Correspondence should be addressed to this author. 0097-6156/87/0327-0240$06.00/0 © 1987 American Chemical Society

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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and 6 ug/L, respectively. Removal of these herbicides from i r r i g a t i o n waste-water by natural v o l a t i l i z a t i o n , hydrolysis, photolysis, and microbial breakdown proceeds so slowly that adequate d i s s i p a t i o n does not occur before w i l d l i f e and human drinking water supplies are threatened. Our investigation evaluated the f e a s i b i l i t y of enhancing the natural oxidizing a c t i v i t y of r i c e f i e l d water i n order to i n t e n t i o n a l l y degrade r i c e herbicides. Expérimenta Herbicide solutions were prepared by d i l u t i o n of a concentrated aqueous stock solution with d i s t i l l e d deionized water or r i c e f i e l d water. Solutions or suspensions i n gas-tight Pyrex vessels were s t i r r e d and irradiated i n a photoreactor (_2), a 150 χ 36 cm chromelined cylinder f i t t e d with s i x F40BL fluorescent lamps. A 4nitroanisole/pyridine actinometer (_3) had a h a l f - l i f e of 50 min i n the reactor, equivalen Rate Measurements. Suspensions of titanium dioxide ( T i 0 ) or zinc oxide (ZnO) i n d i s t i l l e d water or r i c e f i e l d water containing the herbicide were subjected to i r r a d i a t i o n for 2 h i n the photoreactor either (1) wrapped i n f o i l to exclude l i g h t , (2) exposed to l i g h t with no semiconductor, or (3) exposed to l i g h t with semiconductor added. Samples were p e r i o d i c a l l y withdrawn, extracted onto bondedphase extraction cartridges, and analyzed by gas chromatography (glc). 2

Ti02"Induced Photoproducts of Molinate. A 100 mg/L suspension of T i 0 i n 6.4 χ 10~^ j4 aqueous molinate, irradiated 40 min i n the photoreactor, provided a mixture of photoproducts. Molinate sulfoxide and hexamethyleneimine were extracted, derivatized, and detected according to a procedure described by Soderquist, et a l . (4_) · A second irradiated suspension, extracted onto a C-8 bondedphase cartridge and eluted with methanol, provided a concentrated solution of photoproducts suitable for analysis by high-performance l i q u i d chromatography (HPLC), g l c , and glc with a mass spectrometer detector (GC-MS). 2

ZnO-Induced Photoproducts of Thiobencarb. A 100 mg/L suspension of ZnO i n 4.7 χ 10" JM aqueous thiobencarb, irradiated 15 min i n the photoreactor, provided a mixture of photoproducts. After a c i d i f i c a t i o n , extraction with a bonded-phase cartridge, and e l u t i o n , eluates were dried over anhyd sodium sulfate and concentrated under Ν . The C-8 cartridge eluate (methanol) was treated with diazomethane and the cyclohexyl cartridge eluate (ethyl acetate) was treated with ^-methyl-N-(Jt-butyldimethylsilyl) trifluoroacetamide (MTBSTFA) for glc (FID detector) and GC-MS analysis. F i e l d Application of ZnO. F i e l d plots consisted of four 2.4 m diameter aluminum rings placed i n a flooded r i c e f i e l d at the UCD Rice Research F a c i l i t y . In August, 1984, the r i c e plants were removed from these isolated basins, and granular molinate (Ordram

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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10G) was applied at an a g r i c u l t u r a l rate equivalent to 6.3 kg(a.i.)/ha. Water samples were collected from each basin at six to nine h intervals and stored i n glass at -4°C u n t i l analyzed. pH, dissolved oxygen (DO), a i r temperature, and water temperature were intensively monitored. After 3 d, two basins were treated with an aqueous suspension of ZnO (1 g/L) at an application rate equivalent to 14 kg(Zn)/ha, giving a 8.3 mg(ZnO)/L suspension i n the basins. Water samples from both treated and control basins were taken at 0, 5, 15, 45, 120, 240, 411, and 1300 min after the ZnO application and analyzed for molinate and 4-ketomolinate. In a similar experiment, performed several days l a t e r , an application of 6.7 kg(Zn)/ha provided a 4 mg(ZnO)/L suspension i n the basins. Results and Discussion, In our investigation of thiobencarb proved to b herbicide was degraded appreciably i n sunlight by primary photolysis, and both broke down only slowly when exposed to natural photosensitizers (£, 5_, 6_, T_, ) · In addition to their satisfactory physical-chemical properties, molinate and thiobencarb are key pesticides for protection of a r i c e crop with a potential annual market value over 500 m i l l i o n d o l l a r s . The herbicides, used on 500,000 acres of r i c e i n C a l i f o r n i a , control Echinochloa c r u s - g a l l i (barnyard grass) and Leptochloa f a s c i c u l a r i s (sprangletop), contributing to a r i c e y i e l d that i s three times the world average {S)· However, molinate and thiobencarb also contribute to serious l o c a l water p o l l u t i o n problems. For the intentional destruction of these herbicide residues, the i d e a l agent would be inexpensive, rapid acting, d i s s i p a t i v e or controllable, non-polluting, and added d i r e c t l y to the rice f i e l d or the adjacent drains. From a survey of a variety of reagents, the semiconductor photocatalysts titanium dioxide and zinc oxide emerged as a t t r a c t i v e candidates. Photooxidation by Titanium Dioxide and Zinc Oxide. Titanium dioxide and zinc oxide have the a b i l i t y to photo-induce the oxidation of organic compounds (9_, 10, 11 , ) · The redox reaction involves the absorption of l i g h t by the semiconductor s o l i d which results i n promotion of an electron from the valence band to the conduction band, leaving an electron-deficiency or "hole". In such n-type semiconductors (those made electron-rich by doping with an electron donor), the photogenerated hole migrates towards the solution interface where oxidations occur. The electron moves away from the interface into the bulk of the semiconductor p a r t i c l e . Ultimately, a negatively charged aggregate i s formed which can act as a reducing center and e f f e c t solution-phase reductions, such as that of oxygen to superoxide. Surface-sensitized oxidations should occur with adsorbed organic molecules having oxidation potentials less positive than that of the semiconductor valence band. The valence band positions of T i 0 *d ZnO, measured i n water at pH 1, are reported to be +3.1 and +3.0 V ar

2

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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vs. SCE respectively ( 11 ), making the photoexcited catalysts potential oxidants of a wide variety of organic compounds (Table I ) . In water, reactions may proceed by surface-sensitized oxidation, i n i t i a l oxidation of water to hydroxyl r a d i c a l (12), or possibly by reduction of oxygen with subsequent disproportionation to hydrogen peroxide and subsequent photolysis to hydroxyl r a d i c a l s . f

Table I . Anodic oxidation potentials of some environmental pollutants (19). E

p nV

Compound

2-Chlorophenol 2,4-Dichlorophenol 3-Chlorophenol 4,4*-Dimethoxybiphenyl 4-Hydroxybiphenyl 2-Hydroxybiphenyl 3-Chloro-4-hydroxybiphenyl 3-Hydroxybiphenyl Biphenyl 4,4'-Dichlorobiphenyl 2,5,2 ,5'-Tetrachlorobiphenyl Decachlorobiphenyl Benzene Arochlor 1232 Arochlor 1242 Arochlor 1254 Arochlor 1260 1

+ + + + + + + + + + + + + + + + +

a/2

.V vs. SCE

0.63 0.65 0.73 1.18 1 .24 1.27 1 .30 1.47 1 .82 1 .88 2.38 2.63 2.32 2.23 2.39 2.42 2.43

In our experiments, T i 0 and ZnO did not perform equivalently; a 100 mg/L suspension of TiO^ provided the same molinate degradation rate as d i d a 5 mg/L suspension of ZnO. A further advantage of ZnO as an intentional additive was i t s i n s t a b i l i t y i n l i g h t and water (13, 14); as i t slowly dissolves, Zn(II) would become available f o r r i c e plant n u t r i t i o n . The pH dependence of ZnO s t a b i l i t y could, however, l i m i t the u t i l i t y of this catalyst; a t pH

NCSC H 2

5

/==\ ? I

N(C S C H 2

^ > Γ

245

5

IV

ν p0 NCSC H 2

5

Γ

NH

Molinate I VI Figure 1.

F i g u r e 2.

Major products of the TiC^-catalyzed photooxidation of molinate.

Major products of the ZnO-catalyzed photooxidation of thiobencarb.

1

«

Γ

χ

TIME (days) F i g u r e 3.

Diurnal variation of pH i n r i c e f i e l d basins.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Figure 5 ·

E f f e c t of ZnO (14 kg/ha Zn) on molinate concentration i n f i e l d water (•) when applied on Day 3 (j) compared to an untreated control (#) ·

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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17. 18. 19.

Draper, W.M.; Crosby, D.G. Arch, Environ, Contarn, T o x i c o l , 1983, 12, 121-126. Crosby, D . G . ; Wong, A . S . J. Agr. Food Chem. 1973, 21, 10521054. Dulin, D . ; Mill, T. Environ. S c i . Technol. 1982, 16, 815-820. Soderquist, C.J.; Bowers, J.B.; Crosby, D . G . J. A g r i c . Food Chem. 1977, 25, 940-945. Draper, W.M.; Crosby, D.G. J. Agr. Food Chem. 1981, 29, 699702. Draper, W.M.; Crosby, D.G. J. A g r i c . Food Chem. 1984, 32, 231237. Ross, R . D . ; Crosby, D.G. Environ. T o x i c o l . Chem. 1985, 4, 773778. Rutger, J.N.; Brandon, D.M. S c i . Am. 1981, 244, 43-51. Carey, J.H.; Lawrence Toxicol. 1976, 16 Fox, M.A. Acc. Chem. Res. 1983, 16, 314-321. Fox, M.A.; Chen, C.-C; Younathan, J.N.N. J. Org. Chem. 1984, 49, 1969-1974. Jaeger, C.D.; Bard, A.J. J. Phys. Chem. 1979, 83, 3146-3152. Blok, L.; de Bruyn, P . L . J. C o l l o i d Interface S c i . 1970, 32, 518. Gerischer, H. J. E l e c t r o a n a l . Chem. 1977, 82, 133-143. DeBaun, J.R.; Bova, D . L . ; Tseng, C . K . ; Menn, J.J. J. Agr. Food Chem 1978, 26, 1098-1104. Lay, M.-M.; Niland, A . M . ; DeBaun, J.R.; Menn, J.J. In "Pesticide and Xenobiotic Metabolism i n Aquatic Organisms"; Khan, M.A.Q.; Lech, J.J.; Menn, J.J., eds; ACS SYMPOSIUM SERIES No. 99, American Chemical Society: Washington, D.C.,1979; pp. 95-119. Crosby, D . G . ; Bowers, J.B. J. Agr. Food Chem. 1968, 16, 839843. Mikkelson, D . S . ; De Datta, S . K . ; Obsemea, W.N. S o i l S c i . Soc. Am. J. 1978, 42, 775-730. Fenn, R.J.; Krantz, K.W.; Stuart, J.D. J. Electrochem. Soc. 1976, 123, 1643-1647.

R E C E I V E D June 2 7 , 1 9 8 6

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 19

Photochemical Modeling Applied to Natural Waters 1

1

2

2

John M. C. Plane , Rod G. Zika , Richard G. Zepp , and Lawrence A. Burns 1

Rosenstiel School of Marine and Atmospheric Sciences, Division of Marine and Atmospheric Chemistry, University of Miami, Miami, FL 33149 Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30613 2

In this presentation an attempt i s made to examine the application of modeling photochemical processes i n natural water systems. For many photochemical reactions occurring i n natural waters a simple photochemical model describing reaction rate as a function of intensity, radiation attenuation, reactant absorptivity and quantum yield is insufficient. Other factors governing the species distribution must be considered. These factors are divided into processes that cause production, decay, and mixing and transport. Three different photochemically active compounds are used as examples to illustrate how these various factors affect their distribution. Hydrogen peroxide i s used as an example of a compound which i s produced in situ, while trifluralin and pentachloro-phenol are compounds that have very different absorption spectra and are introduced from external sources. The photochemically mediated distribution of these example compounds is evaluated by taking into consideration the effects of l i g h t attenuation, equilibrium partitioning of the compound into nonaqueous phases (e.g. sediments), and physical mixing in the water column. Many c h e m i c a l s found i n n a t u r a l w a t e r s a r e p h o t o s e n s i t i v e t o sunlight. F o r i n s t a n c e , some t r a c e o r g a n i c s p e c i e s , o f e i t h e r t e r r e s t r i a l or i n s i t u b i o l o g i c a l o r i g i n , photoreact to_form r e a c t i v e t r a n s i e n t s s u c h as h y d r o x y l r a d i c a l s , H 0 , 0 , o r s i n g l e t oxygen. These p r o d u c t s i n t u r n t y p i c a l l y e x h i b i t f u r t h e r c h e m i s t r y ( 1 ) . M o r e o v e r , some t r a n s i t i o n m e t a l c o m p l e x e s and i n o r g a n i c s p e c i e s such as NO and N0~ can be p h o t o l y z e d d i r e c t l y 9

9

0097-6156/87/0327-0250$06.00/0 © 1987 American Chemical Society

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by s u n l i g h t , changing t h e i r o x i d a t i o n s t a t e and p r o d u c i n g r e a c t i v e p r o d u c t s t h a t a r e i n v o l v e d i n f u r t h e r r e a c t i o n s (1_). The a b i l i t y t o model the c h e m i s t r y o f p h o t o r e a c t i v e s p e c i e s i n n a t u r a l waters i s of great i n t e r e s t f o r four reasons. First, e n v i r o n m e n t a l p h o t o c h e m i c a l modeling i s n e c e s s a r y t o e x t r a p o l a t e l a b o r a t o r y r e s u l t s t o n a t u r a l water systems. M o d e l i n g can be used t o t e s t h y p o t h e s e s c o n c e r n i n g t h e k i n e t i c s and m e c h a n i s m s o f p h o t o r e a c t i o n s i n the environment and t o examine t h e c o n t r i b u t i o n of competing p r o c e s s e s t h a t a l t e r o r form a c h e m i c a l . Second, many p h o t o r e a c t i v e s p e c i e s a r e coupled t o b i o l o g i c a l c y c l e s ; some a r e p r o d u c e d by organisms, w h i l e o t h e r s may e x e r t e f f e c t s on them. I n p a r t i c u l a r , knowledge o f t h e e f f e c t s o f l i g h t on s p e c i a t i o n o f t o x i c o r g a n i c c h e m i c a l s o r heavy m e t a l s can be o f s p e c i a l importance. T h i r d , recent r e s e a r c h i n the chemistry of the t r o p o s p h e r e has shown t h a t the oceans a r e p r o b a b l y an i m p o r t a n t source o f s e v e r a l t r a c e s p e c i e s found i n the atmosphere. Because most biological an v a r i a t i o n s , t h e r e may the a i r - s e a exchange o f p h o t o c h e m i c a l l y and p h o t o b i o l o g i c a l l y s u p p l i e d , and r e a c t i v e g a s e s . C u r r e n t a i r - s e a exchange r a t e c a l c u l a t i o n s , however, g e n e r a l l y do n o t take d i e l l i g h t v a r i a t i o n s i n t o account. Gases f o r which n a t u r a l waters a r e a s o u r c e , such as COS, C H I , CH SCH , H S and CO, s h o u l d v a r y d i e l l y because t h e i r s u r f a c e c o n c e n t r a t i o n s s h o u l d be r e s p o n s i v e t o l i g h t i n t e n s i t y and t o mixed t u r b u l e n t d i l u t i o n . F i n a l l y , photochemical modelling i n c o n j u n c t i o n w i t h measurements of s p a t i o t e m p o r a l d i s t r i b u t i o n s o f t r a n s i e n t p h o t o p r o d u c t s can be a p p l i e d t o e s t i m a t e t h e age, o r i g i n , and m i x i n g r a t e s o f water masses. T h i s use o f p h o t o c h e m i c a l l y derived products t o complement other tracer studies i s a p o t e n t i a l l y p o w e r f u l t o o l because t h e p r o d u c t s o f l i g h t - i n i t i a t e d r e a c t i o n s a r e o f t e n u n i q u e , a r e o n l y generated i n t h e i l l u m i n a t e d u p p e r l a y e r s of deep water b o d i e s , and because t h e p h o t o r e a c t i o n rates can be d e s c r i b e d through standard chemical kinetic treatments. 3

3

3

2

I n t h i s c h a p t e r , we d i s c u s s some o f the c h e m i c a l , p h y s i c a l , and b i o l o g i c a l p r o c e s s e s t h a t a r e o f t e n s i g n i f i c a n t i n g o v e r n i n g the d i s t r i b u t i o n o f p h o t o c h e m i c a l r e a c t a n t s and t r a n s i e n t s i n a w a t e r column. We a r e n o t c o n c e r n e d h e r e w i t h q u e s t i o n s o f horizontal t r a n s p o r t , and our d i s c u s s i o n s a r e r e s t r i c t e d t o o n e - d i m e n s i o n a l , m i x e d - l a y e r , p h o t o c h e m i c a l models. Examples o f such models a r e p r e s e n t e d where a p p r o p r i a t e . Our d i s c u s s i o n o f the f a c t o r s g o v e r n i n g the d i s t r i b u t i o n o f c h e m i c a l s p e c i e s i n n a t u r a l waters i s d i v i d e d i n t o f i v e p a r t s — i n _ s i t u p h o t o c h e m i c a l f o r m a t i o n , e x t e r n a l sources, i n s i t u photochemical decomposition, e x t e r n a l l o s s , and t u r b u l e n t m i x i n g and t r a n s p o r t . An attempt a l s o w i l l be made t o i n d i c a t e where l a b o r a t o r y measurements can be used as i n p u t i n t o these models. F a c t o r s Governing

Species D i s t r i b u t i o n

In situ formation. The c h e m i c a l and b i o l o g i c a l processes g e n e r a t i n g a c h e m i c a l s p e c i e s (T) i n n a t u r a l s u r f a c e waters may be s y m b o l i c a l l y expressed by the r a t e e q u a t i o n

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS d [ T ] / d t = Rf + Rf l a

+ Rf b

(1)

where R f ^ i s t h e r a t e of p h o t o c h e m i c a l r e a c t i o n s t h a t form T, Rf i s t h e r a t e of thermal r e a c t i o n s forming T, and R f ^ i s the r a t e or b i o l o g i c a l r e a c t i o n s forming T. F o r some compounds, R f ^ o r Rf, a r e important terms and may dominate t h e f o r m a t i o n of T. An example of t h i s i s C 0 , which has l a r g e b i o l o g i c a l sources and p r o b a b l y v e r y small photochemical sources through reactions such as the d e c a r b o x y l a t i o n o f o r g a n i c a c i d s . F o r some cases such as ^ ( ^ , i t i s d i f f i c u l t t o o b t a i n any d i r e c t i n f o r m a t i o n about R f ^ and Rf^» G e n e r a t i n g a model i n terms o f e x c l u s i v e p h o t o c h e m i c a l f o r m a t i o n o f T, and comparing model p r e d i c t i o n s w i t h observed p r o f i l e s o f Τ i n a water column, can o f t e n p o i n t t o the r e l a t i v e c o n t r i b u t i o n o f these d i f f e r e n t t r a n s i e n t f o r m a t i o n terms. Rf i s a f u n c t i o n o f t h e quantum y i e l d f o r t h e p h o t o c h e m i c a l f o r m a t i o n p r o c e s s , as w e l l as t h e r a t e o f l i g h t a b s o r p t i o n by t h e photoreactive species. v a r i e t y of f a c t o r s i n c l u d i n of the p h o t o r e a c t i v e s p e c i e s , and l i g h t i n t e n s i t y . L i g h t i n t e n s i t y i n t u r n depends on l a t i t u d e , s e a s o n , time of day, and c l o u d cover (2) · A l s o , l i g h t i n t e n s i t y d e c r e a s e s w i t h i n c r e a s i n g depth because of a t t e n u a t i o n by s c a t t e r i n g and a b s o r p t i o n . It i s often possible to obtain some i n f o r m a t i o n from l a b o r a t o r y measurements on quantum y i e l d s f o r a p a r t i c u l a r p r o c e s s . An example i s a r e c e n t study on t h e f o r m a t i o n quantum y i e l d s of H 0« i n s e v e r a l d i f f e r e n t n a t u r a l waters (3,4). These quantum y i e l d v e r s u s wavelength p l o t s f o r a l l o f the waters s t u d i e d a r e s u r p r i s i n g l y s i m i l a r w i t h the r a t e of production of being a linear function of sample absorbance at the irradiation w a v e l e n g t h s . Recent s t u d i e s (5,6) i n d i c a t e t h a t p h o t o r e a c t i o n s o f organic p r e c u r s o r s , which a r e l a r g e l y r e s p o n s i b l e for light a t t e n u a t i o n i n n a t u r a l w a t e r s , make an important c o n t r i b u t i o n t o the p r o d u c t i o n of H^C^ i n seawater. The o r g a n i c p r e c u r s o r s a r e m i x t u r e s w i t h p o o r l y c h a r a c t e r i z e d c h r o m o p h o r e s , so i t I s n o t p o s s i b l e t o d e f i n e p r i m a r y quantum y i e l d s f o r photoproduction from the i n d i v i d u a l r e a c t i v e species. The approach f o l l o w e d by Z i k a e t a l . ( 4 ) i n v o l v e d d e f i n i n g an apparent monochromatic quantum y i e l d , C^ , f o r t h e o v e r a l l p r o d u c t i o n p r o c e s s as t h e number of moles o f Ho?2 P °duced P E i n s t e i n o f photons absorbed a t wavelength The monochromatic a c c u m u l a t i o n r a t e i s then t h e r a t e of photons absorbed per u n i t volume of water, m u l t i p l i e d by Cy · The q u a n t i t y C-^ can be measured over the range o f s o l a r wavelengths i n samples o f seawater i n t h e l a b o r a t o r y . A depth p r o f i l e o f t h e d i f f e r e n t i a l s p e c t r a l i n t e n s i t y o f sunlight can e i t h e r be measured u s i n g a m u l t i p l e bandwidth u n d e r w a t e r l i g h t d e t e c t o r and i n p u t d i r e c t l y i n t o a p h o t o c h e m i c a l model, o r be c a l c u l a t e d u s i n g a s u i t a b l e l i g h t a t t e n u a t i o n model. F o r e x a m p l e , t h e m o d e l o f Zepp and C l i n e (2) c a l c u l a t e d t h e a t t e n u a t i o n of l i g h t i n the water column due t o a b s o r p t i o n and scattering. The absorbance o f t h e p a r t i c u l a r body of water o f i n t e r e s t must be s p e c i f i e d f r o m o b s e r v a t i o n . The m o d e l t h e n s p e c i f i e s s o l a r f l u x e s a t s e a l e v e l as a f u n c t i o n o f time o f day, t o t a l ozone column d e n s i t y , l a t i t u d e , and season, p a r t i a l l y based on a n a l y s i s o f c l i m a t o l o g i c a l d a t a . T h i s type o f model can then be 9

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a d a p t e d [ e . g . , s e e Z i k a e t a l . , (_4)] t o c a l c u l a t e t h e r a t e o f photons absorbed a t a s e l e c t e d d e p t h . F i n a l l y , t h e monochromatic accumulation rate i s integrated over the s o l a r spectrum t o o b t a i n the t o t a l a c c u m u l a t i o n r a t e . The r e s u l t s of t h i s procedure a r e i l l u s t r a t e d i n F i g u r e 1, w h i c h shows the v a r i a t i o n w i t h depth o f the p h o t o c h e m i c a l r a t e of p r o d u c t i o n o f 1^02 a t m i d d a y a t 40 Ν f o r summer and w i n t e r i n a f r e s h w a t e r sample. A r a p i d decrease of the rate with depth i s apparent, particularly i n this dark colored f r e s h w a t e r sample which a t t e n u a t e s most o f the n e a r - u l t r a v i o l e t r a d i a t i o n i n the upper 20 cm. F i g u r e 2 shows how t h e c a l c u l a t e d r a t e o f p h o t o c h e m i c a l p r o d u c t i o n v a r i e s w i t h time of day and e x t e n t o f c l o u d cover a t a chosen l a t i t u d e and season. E x t e r n a l A d d i t i o n o f a Chemical S p e c i e s Two o t h e r s o u r c e term p o s s i b i l i t y of dry or w a t e r s u r f a c e f r o m t h e a t m o s p h e r e . The r a t e o f d r y d e p o s i t i o n d e p e n d s on t h e c o n c e n t r a t i o n of the s p e c i e s i n the gas phase, the c o n c e n t r a t i o n i n s u r f a c e w a t e r , the Henry's Law c o n s t a n t , and the r e a c t i v i t y o f t h e s p e c i e s w i t h seawater c o n s t i t u e n t s , as w e l l as t h e wind speed and the a e r o s o l i n the a i r a d j a c e n t t o t h e water surface. The r a t e o f wet d e p o s i t i o n depends on the c o n c e n t r a t i o n of the s p e c i e s i n the p r e c i p i t a t i o n and the f r e q u e n c y and d u r a t i o n of p r e c i p i t a t i o n e v e n t s . L i t t l e i n f o r m a t i o n i s c u r r e n t l y a v a i l a b l e a b o u t t h e s e p r o c e s s e s , a l t h o u g h t h e r e i s some e v i d e n c e t h a t a r a i n s t o r m can i n j e c t s i g n i f i c a n t q u a n t i t i e s of t ^ ? * PP meter o f the water column. This e f f e c t i s usually s h o r t - l i v e d , however, due t o r a p i d m i x i n g ( 4 ) . Second, many n a t u r a l and a n t h r o p o g e n i c c h e m i c a l s p e c i e s a r e i n j e c t e d i n t o n a t u r a l waters from t e r r e s t r i a l r u n - o f f o r o t h e r environmental events. 0

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In S i t u D e c o m p o s i t i o n o f a Chemical S p e c i e s The d e c o m p o s i t i o n of a c h e m i c a l s p e c i e s w i t h i n the water column may be d e s c r i b e d by: - d [ T ] / d t = R d + Rd,. + Rd,, + R d d l i Id b J

u

(2)

where Rd, r e p r e s e n t s r a t e s o f t h e r m a l r e a c t i o n s removing T, Rd^. and R d d e s c r i b e the r a t e o f removal o f Τ as a r e s u l t o f i n d i r e c t and direct photochemical r e a c t i o n s and Rd^ r e p r e s e n t s the b i o l o g i c a l removal r a t e . Rd^ depends on f a c t o r s such as t h e i o n i c s t r e n g t h , the c o m p o s i t i o n and temperature of the w a t e r , and the c o n c e n t r a t i o n s of Τ and the m a t e r i a l s w i t h which i t r e a c t s . S u f f i c i e n t information f o r p r e d i c t i v e estimates of rates of t h e r m a l and b i o l o g i c a l decay o f t e n a r e n o t a v a i l a b l e . Empirical r a t e s determined by s t o r i n g samples i n the dark f o r p e r i o d s o f time c a n be u s e d , but t h i s procedure i s prone t o e r r o r s such as w a l l e f f e c t s (_7). The r a t e o f i n d i r e c t p h o t o c h e m i c a l r e m o v a l , R d ^ , e x p e c t e d t o be dependent on the same f a c t o r s as Rd^ as w e l l as the i n t e n s i t y of s o l a r r a d i a t i o n i n the water column. This solar 1

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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g u r e 1. P h o t o c h e m i c a l model c a l c u l a t e d a c c u m u l a t i o n r a t e s o f 2^2 depth f ° h i g h o r g a n i c m a t e r i a l c o n t e n t f r e s h w a t e r sample from South F l o r i d a a q u i f e r . M i d - d a y v a l u e s a t 40 l a t i t u d e f o r two seasons a r e shown. H

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i n t e n s i t y d e t e r m i n e s the s t e a d y - s t a t e c o n c e n t r a t i o n of the s p e c i e s w i t h which Τ r e a c t s . The r a t e of d i r e c t p h o t o c h e m i c a l removal i s dependent on the e x t i n c t i o n c o e f f i c i e n t and t h e quantum y i e l d f o r t h e p r i m a r y process. As an example of t h i s p r o c e s s , c o n s i d e r the case of two compounds w i t h v e r y d i f f e r e n t a b s o r p t i o n s p e c t r a i l l u s t r a t e d i n F i g u r e 3. The e f f e c t of t h e i r r a t h e r d i f f e r e n t e x t i n c t i o n c o e f f i c i e n t s on the r a t e of d i r e c t p h o t o c h e m i c a l d e c o m p o s i t i o n i s demonstrated i n F i g u r e 4. These p l o t s show the d e p t h dependence of t h e d e c a y c o n s t a n t s i n two w a t e r b o d i e s w i t h d i f f e r e n t l i g h t transmission characteristics. I n t h e more o p a q u e , m o d e r a t e l y p r o d u c t i v e w a t e r , the r a t e c o e f f i c i e n t s decrease markedly w i t h d e p t h . In g e n e r a l , depends on the same f a c t o r s as the r a t e of photochemical production, R « f l

E x t e r n a l Removal of a Chemical

Species

Two types of p h y s i c a e x t e r n a l removal of a c h e m i c a l s p e c i e s . F i r s t , the s p e c i e s o f i n t e r e s t may be l o s t to the atmosphere through w a t e r - a i r exchange. T h i s p r o c e s s depends on the H e n r y s Law c o n s t a n t f o r the c h e m i c a l s p e c i e s , as w e l l as the a t m o s p h e r i c c o n c e n t r a t i o n and the s t r u c t u r e o f t h e s u r f a c e m i c r o l a y e r s (_§_)· Wind s t r e s s and t u r b u l e n c e of the water body s u r f a c e have a pronounced e f f e c t , e s p e c i a l l y f o r s u r f a c e - a c t i v e m a t e r i a l s f o r w h i c h bubble scavenging and s u r f a c e f i l m e j e c t i o n as a e r o s o l t a k e s p l a c e . T r a n s f e r r a t e s at the a i r - w a t e r i n t e r f a c e are complex problems i n themselves and are not d e a l t w i t h i n t h i s Chapter. The second important p r o c e s s t o c o n s i d e r i s p h y s i c a l l o s s of a t r a n s i e n t by p a r t i t i o n i n g i n t o sediments on the bottom or suspended p a r t i c l e s i n the water column. The p o t e n t i a l s i g n i f i c a n c e of t h i s l o s s term i s demonstrated i n F i g u r e 5, a p l o t of the l o g of the p a r t i t i o n c o e f f i c i e n t s between o c t a n o l and water ( l o g Ρ) f o r almost 20,000 i n d u s t r i a l compounds. The narrow d i s t r i b u t i o n of these c o e f f i c i e n t s i s s t r i k i n g , the more so i n the c o n t e x t of F i g u r e 6. F i g u r e 6 i l l u s t r a t e s p s e u d o - f i r s t - o r d e r system halflives, e s t i m a t e d by the EXAMS model ( 9 ) , f o r p h o t o l y s i s of a s e r i e s of generic organic substrates with d i f f e r i n g K 's ( P « K ) and s e d i m e n t / w a t e r p a r t i t i o n c o e f f i c i e n t s , (K ) ^ Y i t w i t h trie same p h o t o l y s i s r a t e c o n s t a n t s i n the water column^of a f r e s h w a t e r pond. As the p a r t i t i o n c o e f f i c i e n t s i n c r e a s e , the f r a c t i o n of s u b s t r a t e i n t h e i l l u m i n a t e d water column, and thus the system l o s s r a t e , d e c r e a s e s due to i n c r e a s e d s o r p t i o n i n t o bottom s e d i m e n t s . It is n o t e w o r t h y t h a t t h i s s i m u l a t i o n , when compared w i t h F i g u r e 5, i n d i c a t e s t h a t s o r p t i o n i n t o bottom sediments has l i t t l e or no e f f e c t on the t o t a l r a t e of p h o t o c h e m i c a l l o s s f o r a preponderance of i n d u s t r i a l c h e m i c a l s . f

T u r b u l e n t M i x i n g W i t h i n and Below the P h o t i c Zone Any m o d e l o f p h o t o c h e m i s t r y i n n a t u r a l w a t e r s must t a k e i n t o a c c o u n t the e f f e c t of t u r b u l e n t m i x i n g on the c o n c e n t r a t i o n s of t r a n s i e n t s p e c i e s and t h e i r d i e l and s e a s o n a l v a r i a t i o n s . For the o c e a n s , t h e p a r t of the water column of p a r t i c u l a r i n t e r e s t i s the

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

256

PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS ZENITH ANGLE

F i g u r e 2. P h o t o c h e m i c a l model c a l c u l a t e d H^O^ a c c u m u l a t i o n r a t e s of H O v e r s u s time of day i n summer a t 4θ l a t i t u d e and 10 cm d e p t h t o r d i f f e r e n t e x t e n t s of c l o u d c o v e r . The water used was f r o m a S o u t h F l o r i d a a q u i f e r and c o n t a i n e d a h i g h l e v e l o f organic m a t e r i a l .

C

37\ S 37 H

C

H

N

300

350

400

λ

450

500

550

(nm)

F i g u r e 3 . E l e c t r o n i c a b s o r p t i o n s p e c t r a of p e n t a c h l o r o p h e n o l t r i f l u r a l i n i n water.

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

and

19.

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257

Moderately Productive Water

Clearest Natural Water

t

Trifluralin

16

20

24

28

Depth (m)

Depth (m)

F i g u r e 4. Computed depth dependence f o r the d i r e c t p h o t o l y s i s o f trifluralin and p e n t a c h l o r o p h e n o l i n d i f f e r e n t water types (average r a t e c o n s t a n t s p a r e n t h e s i z e d ) .

400-i

T3

Log Ρ F i g u r e 5. D i s t r i b u t i o n o f o c t a n o l - w a t e r p a r t i t i o n (P) computed f o r 19,965 i n d u s t r i a l c h e m i c a l ( 1 7 ) .

coefficients

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PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS

log K,

log K

p

F i g u r e 6. Computed e f t e c t s o f s o r p t i o n t o bottom sediments on t h e p s e u d o - f i r s t - o r d e r system h a l f l i v e s ( l o g m a s s / f l u x ) f o r a s e r i e s o f g e n e r i c p h o t o r e a c t i v e s u b s t r a t e s , each having t h e same photolysis rate but differing octanol-water partition c o e f f i c i e n t s (K ) and s o r p t i o n p a r t i t i o n c o e f f i c i e n t s (K ) .

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259

u p p e r 100 m. P h o t o c h e m i s t r y l a r g e l y takes p a r t i n about t h e f i r s t 30 m ( t h e p h o t i c z o n e ) , a l t h o u g h t h i s d e p t h depends somewhat on t h e l i g h t transmission c h a r a c t e r i s t i c s of the water. T u r b u l e n t m i x i n g near t h e s u r f a c e i s i n many cases dominated by s u r f a c e wind s t r e s s . Heat t r a n s p o r t may a l s o l e a d t o v e r t i c a l mixing. These m i x i n g p r o c e s s e s r e s u l t i n an homogeneous l a y e r b e l o w t h e s u r f a c e known as t h e mixed l a y e r , the depth o f which i s l a r g e l y a f u n c t i o n of w i n d - s t i r r i n g . This l a y e r g e n e r a l l y v a r i e s between 20 and 40 m over much o f t h e ocean. The s i t u a t i o n i n l a k e s i s s i m i l a r e x c e p t t h a t t h e mixed l a y e r i s u s u a l l y much s h a l l o w e r . The aim o f a t u r b u l e n t m i x i n g model i s t o a s s e s s t h e i m p o r t a n c e o f m i x i n g w i t h i n t h e p h o t i c zone, and t h e way i n which t u r b u l e n c e can mix water c o n t a i n i n g s p e c i e s t h a t have e i t h e r been p h o t o c h e m i c a l l y produced o r d e p l e t e d , from t h e p h o t i c zone down t o depths below t h e mixed l a y e r . As i n d i c a t e d i n t h e i n t r o d u c t i o n , the aim of t h i s Chapter i s n o t t o d i s c u s s the consequences o f horizontal transport. u s u a l l y not necessar t y p i c a l l y a t a s c a l e of s e v e r a l hundred k i l o m e t e r s , t h e v e l o c i t y o f h o r i z o n t a l t r a n s p o r t , and t h e l i f e t i m e s o f many p h o t o c h e m i c a l s p e c i e s o f a few hours t o a few days (1)· When s e t t i n g up a s t u d y , an i n v e s t i g a t o r has s e v e r a l types o f o n e - d i m e n s i o n a l m i x i n g l a y e r models from which t o choose. These models a r e d i s t i n g u i s h e d by t h e ways i n which they attempt t o s o l v e t h e o n e - d i m e n s i o n a l c o n s e r v a t i o n e q u a t i o n s of momentum, h e a t , and s a l i n i t y t h a t d e s c r i b e t h e system. For instance, the turbulence c l o s u r e model o f M e l l o r and D u r b i n (10) has been used t o model t h e p h o t o c h e m i s t r y o f ^ C L i n the ocean ( 4 ) . The r e s u l t s o f t h i s work will be used i n tne d i s c u s s i o n s below. This model uses second-order closure assumptions t o reduce the conservation e q u a t i o n s t o two e q u a t i o n s — one f o r t h e mean s p e c i f i c t u r b u l e n c e k i n e t i c e n e r g y a n d one f o r t h e v e r t i c a l t r a n s p o r t o f s p e c i f i c h o r i z o n t a l momentum. T h i s m o d e l h a s been c r i t i c i z e d f o r i t s t e n d e n c y t o u n d e r p r e d i c t t h e d e p t h o f t h e mixed l a y e r and the s i z e o f t h e s t e p change i n t h e temperature p r o f i l e a t t h e bottom o f t h e l a y e r , when compared w i t h measurements. A n o t h e r approach i n v o l v e s the usage o f eddy c o e f f i c i e n t s and mixing lengths. T h i s method t r e a t s a l l t r a n s p o r t by analogy t o m o l e c u l a r d i f f u s i o n , assuming t h a t t h e t u r b u l e n t f l u x e s can be e x p r e s s e d by t h e g r a d i e n t o f t h e t r a n s p o r t e d q u a n t i t y m u l t i p l i e d by an a p p r o p r i a t e eddy d i f f u s i o n c o e f f i c i e n t . A r e c e n t example of t h i s t y p e o f m o d e l i s t h e work o f Denman and G a r g e t t ( 1 1 ) on phytoplankton i n t h e upper ocean. T h i s model i s used f o r c a l c u l a t i o n s presented l a t e r i n t h i s Chapter. F i n a l l y , bulk, m i x e d - l a y e r models a r e a v a i l a b l e w h i c h assume t h a t the t o p l a y e r o f the ocean i s mixed t h o r o u g h l y and t h a t t h e v e r t i c a l d i s t r i b u t i o n o f t e m p e r a t u r e , s a l i n i t y , and h o r i z o n t a l v e l o c i t y w i t h i n t h i s mixed l a y e r i s , i f not u n i f o r m , then a t l e a s t v e r y much s m a l l e r than t h e v a r i a t i o n s a c r o s s the l a y e r b o u n d a r i e s o r the v a r i a t i o n s w i t h i n t h e thermocline. The a s s u m p t i o n o f a u n i f o r m m i x e d l a y e r p e r m i t s v e r t i c a l i n t e g r a t i o n o f t h e c o n s e r v a t i o n e q u a t i o n s and y i e l d s e x p r e s s i o n s f o r the t u r b u l e n t t r a n s p o r t s i n terms o f t h e mean q u a n t i t i e s and the e x t e r n a l i n p u t s . The t u r b u l e n c e energy e q u a t i o n i s used t o

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o b t a i n an e x p r e s s i o n f o r the e v o l u t i o n o f the l a y e r depth which i s n e e d e d f o r c l o s u r e . Examples o f models o f t h i s type a r e those o f N i l i e r (12) and Garwood ( 1 3 ) . The p r e d i c t e d temperature p r o f i l e s o f these models have g e n e r a l l y compared w e l l w i t h measurements (14). D e t a i l e d comparisons between these d i f f e r e n t types o f o n e - d i m e n s i o n a l models, however, a r e n o t the purpose o f the p r e s e n t d i s c u s s i o n , and the reader i s r e f e r r e d t o the r e f e r e n c e d papers and the r e v i e w by N i i l e r and K r a u s ( 1 5 ) . Once a model has been s e l e c t e d , an a d d i t i o n a l c o n s e r v a t i o n o r c o n t i n u i t y e q u a t i o n i s r e q u i r e d t o d e s c r i b e the t r a n s i e n t s p e c i e s of i n t e r e s t . The eddy c o e f f i c i e n t s t h a t govern the t r a n s p o r t of t h e t r a n s i e n t have been taken t o be t h e same as those f o r heat t r a n s p o r t [see e.g., (4)]· Thus, t h e temporal development o f the t r a n s i e n t can be modeled a l o n g w i t h the development and d i u r n a l v a r i a t i o n o f the s t a b i l i t y o f the water column. I n t h i s manner, t h e e f f e c t s o f w i n d - s t i r r i n g and s u r f a c e heat f l u x as w e l l as o f photochemical processe be i n v e s t i g a t e d . A r e c e n t example o f such a model i s t h a t d e s c r i b i n g t h e d i s t r i b u t i o n o f H^O^ i n the upper l a y e r o f the ocean ( 4 ) , u s i n g t h e M e l l o r and Durbin model ( 1 0 ) . B e f o r e c o n s i d e r i n g the e f f e c t of t u r b u l e n t m i x i n g on t r a n s i e n t d i s t r i b u t i o n , r e f e r t o F i g u r e 7, w h i c h shows the c a l c u l a t e d time-depth p r o f i l e s f o r ^ 0 2 d u r i n g w i n t e r i n the G u l f o f Mexico over a 2-day p e r i o d b e g i n n i n g w i t h s u n r i s e , assuming the e f f e c t s o f p h o t o c h e m i c a l p r o c e s s e s alone and a complete absence of t u r b u l e n t m i x i n g . The r a t e s of p h o t o c h e m i c a l p r o d u c t i o n t h a t were i n p u t i n t o the model a r e taken from l a b o r a t o r y quantum y i e l d measurements, d i s c u s s e d e a r l i e r (see s e c t i o n " I n s i t u f o r m a t i o n " ) . These a r e i n c o r p o r a t e d i n t o a l i g h t a t t e n u a t i o n model t h a t employs t h e e x t i n c t i o n c o e f f i c i e n t of l i g h t i n water w i t h the c l a r i t y o f the G u l f o f M e x i c o . The dark decay l i f e t i m e o f H^O^ was chosen t o be 4 d a y s , and c o n s t a n t w i t h d e p t h . The d a r k decay l i f e t i m e v a r i e s from h o u r s i n c o a s t a l e n v i r o n m e n t s t o o v e r a week i n o l i g o t r o p h i c w a t e r s , but 4 days i s commonly observed f o r o f f s h o r e s u r f a c e water ( 7 ) . I t i s c l e a r from F i g u r e 7 t h a t , i n the absence o f any m i x i n g p r o c e s s e s , ^ 0 2 b u i l d s up t o h i g h c o n c e n t r a t i o n s w i t h l i t t l e d i e l v a r i a t i o n because the dark decay l i f e - t i m e i s much l o n g e r than one day. As e x p e c t e d , ^ 0 2 i s not produced a t a l l below the p h o t i c zone. Measurements o f ^ 0 2 i n the G u l f o f Mexico ( 1 6 ) , however, reveal much lower surface concentrations and significant c o n c e n t r a t i o n s down t o about 70 m, and t u r b u l e n t m i x i n g has been invoked t o account f o r these o b s e r v a t i o n s . The t u r b u l e n t m i x i n g model was s e t up i n t h i s case w i t h an i n i t i a l temperature p r o f i l e o f n e u t r a l s t r a t i f i c a t i o n between the s u r f a c e and 80 m, and then a l l o w e d t o v a r y a c c o r d i n g t o s u r f a c e w i n d s t r e s s and heat f l u x . The i n i t i a l d e v i a t i o n o f the c u r r e n t s f r o m g e o s t r o p h y was assumed t o be z e r o . The s u r f a c e heat f l u x was a l l o w e d t o v a r y d i e l l y i n a s i n u s o i d a l manner w i t h a s m a l l c o n s t a n t h e a t f l u x s u p e r i m p o s e d , s i m u l a t i n g n e t h e a t i n g i n summer and cooling i n winter. F i g u r e 8 shows the c a l c u l a t e d time-depth s e c t i o n o f over a 2-day p e r i o d . To e m u l a t e h e a t f l u x c o n d i t i o n s t y p i c a l o f autumn/winter, a s u r f a c e c o o l i n g was superimposed on the d i e l l y

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

19.

ο

50.

261

Photochemical Modeling Applied to Natural Waters

PLANE ET AL.

h

TIME AFTER SUNRISE

ÎH0URS)

the of F i g u r e 7. C a l c u l a t e d time-depth section c o n c e n t r a t i o n due t o p h o t o c h e m i c a l p r o d u c t i o n and d e s t r u c t i o n a l o n e , over a two-day p e r i o d b e g i n n i n g 0600 h r s l o c a l t i m e . C o n d i t i o n s : m i d - w i n t e r , l a t i t u d e = 30°N, G u l f o f M e x i c o .

12

18.

24.

30.

36.

42. -8

χ TO M TIME RFTER SUNRISE

(H0URS) H

F i g u r e 8. C a l c u l a t e d time-depth section of the 2^2 c o n c e n t r a t i o n , over a two-day p e r i o d b e g i n n i n g 0600 h r s l o c a l t i m e . C o n d i t i o n s : m i d - w i n t e r , l a t i t u d e = 30°N,,-Gulf_^f M e x i c o , wind s t r e s s = 1 dyne cm , s u r f a c e c o o l i n g = 10 Kms In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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v a r y i n g heat f l u x , which has a noon-time maximum of 10 Km s The model was i n t e g r a t e d f o r 10 days p r i o r t o the 2 d a y s _ J h a t are shown. The wind s t r e s s was s e t a t a v a l u e of 1 dyne cm , which r o u g h l y corresponds t o a 10-knot w i n d . A c o m p a r i s o n between F i g u r e s 7 and 8 shows the s i g n i f i c a n t e f f e c t t h a t t u r b u l e n t m i x i n g can have on the v e r t i c a l p r o f i l e of H^O^ i n the water column. D u r i n g the day, s u r f a c e h e a t i n g e s t a b l i s h e s a s t a b l e l a y e r i n the upper 20 m t h a t tends t o t r a p most o f the H^O^ produced i n the s u r f a c e w a t e r . The peak s u r f a c e c o n c e n t r a t i o n s o c c u r about 2 hours b e f o r e sunset r a t h e r than a t noon, when the maximum s o l a r f l u x o c c u r s , because ^ 0 2 c o n t i n u e s to a c c u m u l a t e d u r i n g the a f t e r n o o n , even though the l i g h t f l u x i s decreasing. A f t e r sunset there i s a g r a d u a l e r o s i o n of the s u r f a c e thermocline brought about both by nocturnal cooling and w i n d - s t i r r i n g , which c o n t i n u e throughout the n i g h t and e s t a b l i s h a deepening n e u t r a l l y s t r a t i f i e s h o r t l y before sunrise between the near s u r f a c e n e u t r a l l y s t r a t i f i e d l a y e r and the p r e e x i s t i n g n e u t r a l l y s t r a t i f i e d l a y e r below 20 m, any a d d i t i o n a l s u r f a c e c o o l i n g w i l l a l l o w r a p i d m i x i n g of the water down to the next t h e r m o c l i n e a t 70 m. The c a l c u l a t e d time-depth s e c t i o n of the 1-hour changes i n ^2®7 due t o combined photochemical and t r a n s p o r t p r o c e s s e s i s shown i n F i g u r e 9. T h i s i l l u s t r a t e s more c l e a r l y t h a t the i n t e n s e m i x i n g i s c o n f i n e d t o r e l a t i v l e y b r i e f p e r i o d s from s h o r t l y b e f o r e s u n r i s e to midmorning at g r e a t e r depth. The i n t e n s e m i x i n g to 70 m depth i s due t o s u r f a c e c o o l i n g superimposed on the d i u r n a l l y v a r y i n g heat f l u x as the c o o l s u r f a c e water breaks through the i n v e r s i o n e s t a b l i s h e d d u r i n g the p r e v i o u s day. The i n c r e a s e i n i s c o n f i n e d to the s u r f a c e s t a b l e l a y e r d u r i n g the day and becomes u n i f o r m w i t h i n the upper 20 m d u r i n g the night. Once the 20 m t h e r m o c l i n e i s b r o k e n , t h i s l a y e r i s r a p i d l y flushed. E s s e n t i a l l y a l l o f t h e H 0 b e t w e e n 20 and 70 m i s r a p i d l y i n j e c t e d w i t h i n a few hours around s u n r i s e . The s i z e of t h i s e f f e c t , of c o u r s e , depends upon the magnitude of s u r f a c e c o o l i n g d u r i n g the course of the n i g h t . By c o n t r a s t , i n the summer months when t h e r e i s a net p o s i t i v e h e a t i n g of the s u r f a c e w a t e r , a t h e r m o c l i n e develops t h a t t r a p s the ^ 0 2 a t the d e p t h where i t i s produced i n the p h o t i c zone, except f o r some w i n d - s t i r r i n g near the surface. T h i s i s shown i n F i g u r e 10. W i n d - s t i r r i n g produces a mixed l a y e r deeper than the p h o t i c zone ( c f . F i g u r e s 7 and 10), and the c o n c e n t r a t i o n s i n the mixed l a y e r are s i g n i f i c a n t l y h i g h e r than i n w i n t e r because of the i n c r e a s e d r a t e of p r o d u c t i o n d u r i n g summer· The e f f e c t s of v a r y i n g the wind s t r e s s on the water s u r f a c e a l s o s h o u l d be c o n s i d e r e d . For the purpose of i l l u s t r a t i o n , the EXAMS I I c o m p u t e r m o d e l ( 9 ) u s i n g eddy d i f f u s i o n c o e f f i c i e n t s p r o v i d e d by Denman and G a r g e t t ( 1 1 ) , i s u s e d t o compare t h e p h o t o c h e m i c a l d e s t r u c t i o n of p e n t a c h l o r o p h e n o l and t r i f l u r a l i n i n n a t u r a l waters. As was d e s c r i b e d e a r l i e r i n F i g u r e 3, t h e s e compounds have v e r y d i f f e r e n t a b s o r p t i o n s p e c t r a i n the range of s o l a r w a v e l e n g t h s , and hence d i f f e r e n t r a t e s of p h o t o c h e m i c a l decay (see F i g u r e s 5 and 6 ) . 2

2

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263

Photochemical Modeling Applied to Natural Waters 1 HOUR CHANGE HjOg

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F i g u r e 9. C a l c u l a t e d time-depth s e c t i o n of the one-hour changes i n H^C^ o v e r a two-day p e r i o d b e g i n n i n g a t 0600 h r s l o c a l t i m e . C o n d i t i o n s : m i d - w i i ^ t e r , l a t i t u d e = 30 N, _ _ ^ i l f M e x i c o , wind s t r e s s = 1 dyne cm , s u r f a c e c o o l i n g = 10 Kms ·

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F i g u r e 10. C a l c u l a t e d time-depth section of the 2°? c o n c e n t r a t i o n , o v e r a two-day p e r i o d b e g i n n i n g 0600 h r s l o c a l time. C o n d i t i o n s :_^nid-summer, w i n d s t r e s s =• 1 dyne cm s u r f a c e h e a t i n g = 10 Kms In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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The r e s u l t s of the model c a l c u l a t i o n s are shown i n F i g u r e 11. In a l l c a s e s , the depth p r o f i l e s t h a t would have r e s u l t e d w i t h c o m p l e t e r a p i d m i x i n g o f t h e e n t i r e w a t e r c o l u m n i s shown f o r comparison. A u n i f o r m v e r t i c a l d i s t r i b u t i o n o f s u b s t r a t e was assumed as the i n i t i a l c o n d i t i o n . In F i g u r e 11a, the v e r t i c a l p r o f i l e s o f the two compounds are compared over a 12-hour p e r i o d , under c o n d i t i o n s of low wind s t i r r i n g and i n m o d e r a t e l y p r o d u c t i v e n a t u r a l water. T r i f l u r a l i n absorbs l i g h t up t o about 540 nm and shows s i g n i f i c a n t p h o t o c h e m i c a l removal down t o depths of about 30 m. By c o n t r a s t , p e n t a c h l o r o p h e n o l i s p h o t o l y z e d r a p i d l y o n l y a t depths l e s s than 10 m. I n F i g u r e l i b , the same comparison i s made, b u t u n d e r h i g h wind c o n d i t i o n s . For b o t h compounds i t i s c l e a r t h a t t h e i r c o n c e n t r a t i o n p r o f i l e s f o l l o w the w i n d - s t i r r e d m i x i n g l a y e r t h a t d e v e l o p s a f t e r a few hours and produces a w e l l - d e f i n e d b a s e a t about 25 m. T h i s w i l l be the case i f the w i n d - s t i r r i n g i s s t r o n g enough, when t u r b u l e n t m i x i n g w i l l become f a s t e r than the r a t e of p h o t o c h e m i c a l d e s t r u c t i o w i l l r e s u l t w i t h i n the F i g u r e s 11c and l i d demonstrate the e f f e c t o f i n c r e a s i n g the rate of photochemical d e c o m p o s i t i o n by r e p e a t i n g the model c a l c u l a t i o n s i n F i g u r e 11a and l i b , but now f o r the case of the c l e a r e s t n a t u r a l water. In F i g u r e 11c, which i l l u s t r a t e s the r e s u l t s f o r low w i n d s , t r i f l u r a l i n i s almost c o m p l e t e l y removed a f t e r 12 hours as s o l a r wavelengths now p e n e t r a t e down t o 40 m. P e n t a c h l o r o p h e n o l i s now s i g n i f i c a n t l y removed near the s u r f a c e , but the r a t e of removal below 30 m i s s t i l l s l o w (on the t i m e - s c a l e of 12 h o u r s ) . F i g u r e l i d i s the case of h i g h wind s t i r r i n g . The p r o f i l e of p e n t a c h l o r o p h e n o l shows s t r o n g m i x i n g and homogeneity w i t h i n the mixed l a y e r and f o l l o w s the base of the mixed l a y e r a t 25 m. By c o n t r a s t , the p r o f i l e s of t r i f l u r a l i n show o n l y s l i g h t d i s c o n t i n u i t i e s due t o m i x i n g , the r e s u l t of the i n c r e a s e d p h o t o l y s i s r a t e s i n the c l e a r water competing w i t h the w i n d - d r i v e n turbulent mixing. Conclusions In t h i s C h a p t e r , we h a v e d i s c u s s e d i n some d e t a i l t h e f a c t o r s affecting the c o n c e n t r a t i o n of a p h o t o c h e m i c a l r e a c t a n t or t r a n s i e n t p r o d u c t i n the water column. These f a c t o r s have been d i v i d e d i n t o p r o c e s s e s t h a t cause p r o d u c t i o n , decay, and m i x i n g and t r a n s p o r t . In any model of p h o t o c h e m i s t r y i n n a t u r a l w a t e r , a l l of these p r o c e s s e s may be coupled t o g e t h e r and a l l must be c o n s i d e r e d . I t c e r t a i n l y s h o u l d be c l e a r from the examples i n the l a s t s e c t i o n t h a t t u r b u l e n t m i x i n g p l a y s a s u b s t a n t i a l r o l e i n d e t e r m i n i n g the p r o f i l e o f t h e r e a c t a n t o r t r a n s i e n t p r o d u c t i n and b e l o w t h e p h o t i c zone. The c o u p l i n g of p h o t o c h e m i s t r y and p h y s i c a l m i x i n g i n a model i s a c h a l l e n g i n g t a s k f o r t h r e e r e a s o n s . F i r s t , the p h o t o c h e m i c a l p r o d u c t i o n and d e c o m p o s i t i o n r a t e s , as w e l l as the d a r k decay r a t e s of i n t e r e s t , are o f t e n poor e s t i m a t e s . T h i s i s b o t h because of the l a c k of good l a b o r a t o r y k i n e t i c measurements and because the l i g h t a t t e n u a t i o n models i n t o which such measurements are i n c o r p o r a t e d are subject to additional uncertainties. Second, the one-dimensional mixing models make certain assumptions and

In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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a p p r o x i m a t i o n s i n o r d e r t o s o l v e t h e momentum and heat c o n t i n u i t y e q u a t i o n s ( 1 5 ) . The l a c k o f p r e c i s i o n o f these m i x i n g models must be b o r n e i n m i n d . T h i r d , there i s the question of o b t a i n i n g s u i t a b l e f i e l d measurements o f r e a c t a n t o r t r a n s i e n t product p r o f i l e s i n order to v a l i d a t e the p r e d i c t i o n of a model. As a n a l y t i c a l t e c h n i q u e s improve, p r o f i l e s c a n c e r t a i n l y be measured with higher a c c u r a c i e s . When making a comparison w i t h a model, however, t h e r e i s t h e i m p o r t a n t q u e s t i o n o f t h e h i s t o r y o f t h e water column i n t h e few hours o r days b e f o r e a p r o f i l e i s measured. F o r example, i f a p r o f i l e o f a t r a n s i e n t was measured d u r i n g a p e r i o d o f l i g h t winds, the model would p r e d i c t a s h a l l o w m i x i n g l a y e r w i t h t h e t r a n s i e n t trapped near t h e s u r f a c e , whereas a p e r i o d o f h i g h winds s h o r t l y b e f o r e might have mixed t h e t r a n s i e n t w e l l below t h e p h o t i c zone. Therefore, t h e model i n this case would appear t o have u n d e r e s t i m a t e d the depth o f t h e m i x i n g l a y e r , depending o f course on t h e l i f e t i m e o f t h problem i s t o i n i t i a t e and then have t h e model work f o r w a r d i n real time being c o n t i n u o u s l y updated w i t h r e l e v a n t d a t a such as t h e wind speed, c l o u d cover and s u r f a c e heat f l u x , and p e r i o d i c a l l y compared w i t h measured p r o f i l e s . I n s p i t e o f the d i f f i c u l t i e s t h a t a r e p r e s e n t i n d e v e l o p i n g m o d e l s f o r a q u a t i c p h o t o c h e m i s t r y , much p r o g r e s s has a l r e a d y been made. Given the importance of being able t o p r e d i c t the d i s t r i b u t i o n and l i f e t i m e s o f many t r a n s i e n t s i n t h e upper oceans a n d i n f r e s h w a t e r , i t i s t o be e x p e c t e d t h a t a g r e a t d e a l o f r e s e a r c h w i l l be done i n t h i s a r e a i n t h e f u t u r e .

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