Advances in Chemistry Research [1 ed.] 9781619423367, 9781619423275

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Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved. Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved. Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

ADVANCES IN CHEMISTRY RESEARCH

ADVANCES IN CHEMISTRY RESEARCH

Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved.

VOLUME 14

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ADVANCES IN CHEMISTRY RESEARCH

ADVANCES IN CHEMISTRY RESEARCH VOLUME 14

JAMES C. TAYLOR Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved.

EDITOR

Nova Science Publishers, Inc. New York Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

Copyright © 2012 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data

ISSN 1940-0950

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Published by Nova Science Publishers, Inc. † New York Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

CONTENTS Preface Chapter 1

Advances in Oxidative Decomposition of Oxalic Acid in Aqueous Media Abdelkrim Azzouz

Chapter 2

Physico-Chemical Properties of Acrylamide Mohammad Reza Saboktakin

Chapter 3

Acrylamide Formation in Foods: Health Implications and Mitigation Technologies Franco Pedreschi, Salomé Mariotti and Jaime Rozowski

Chapter 4

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vii

Chapter 5

Chapter 6

Chapter 7

Chapter 8

One of the Recent Discoveries in Catalysis: The Phenomenon of Electrochemical Promotion Carmen Jiménez Borja, Antonio de Lucas Consuegra, Jose Luis Valverde, Fernando Dorado, Ángel Caravaca and Jesús González Cobos Physiological Roles for Oxalate Metabolism in Wood-Rotting Basidiomycetes Takefumi Hattori and Mikio Shimada Use of Carbon Monoxide in Slaughtering, Processing and Packaging of Muscle Foods Gry Aletta Bjørlykke, Bjørn Olav Kvamme, Erik Slinde and Oddvin Sørheim The Role of Oxalic Acid in Nanotechnology: Fundamentals and Applications Arturo I. Martinez, Sergio Martinez-Varga and K. I. Camacho Novel Self-oscillating Polymer Chains Yusuke Hara

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1 33

67

99

133

159

181

203

vi Chapter 9

Chapter 10

Contents Oxalic Acid as a Sacrificial Agent in the Photoreduction of Water Contaminants Roberto L. Pozzo, Leandro O. Conte, José L. Giombi, Julio Ferrón and Miguel A. Baltanás Development of High Valuable Astaxanthin Product from Shrimp Waste with Supercritical Fluid Technologies Can Quan and Charlotta Turner

225

237

Chapter 11

Lysophosphatidic Acid James E. East and Timothy L. Macdonald

253

Chapter 12

Thoughts on Evaluation of Heavy Metals Toxicity Simone Morais, Fernando Garcia e Costa and Maria de Lourdes Pereira

273

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Index

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PREFACE This book presents original research results on the leading edge of chemistry research. Each article has been carefully selected in an attempt to present substantial research results across a broad spectrum. Topics discussed include the oxidative decomposition of oxalic acid in aqueous media; the physico-chemical properties of acrylamide; catalysis and the phenomenon of electrochemical promotion; oxalate metabolism in wood-rotting basidomycetes; novel self-oscillating polymer chains; lysophosphatidic acid and heavy metal toxicity. Chapter 1- One of the major environmental issues to be solved is undoubtedly the elimination of organic pollutants from industrial waste-waters, more particularly those produced by food industries and paper pulp manufactures. Physicochemical treatments like coagulation-flocculation and filtration are only profitable to remove high levels of organic pollutants. For lower contents, conventional oxidative decomposition methods are applied. Nonetheless, they turned out to be ineffective, because they generate unavoidably oxalic acid. In other words, oxidative treatments for organic matter decomposition cannot be regarded as being effective without total removal of oxalic acid, the most chemically stable by-product. Conversely, the main criterion for highly efficient oxidative decomposition of organic contaminant resides in the absence of traces of oxalic acid in the final liquid mixture. Chapter 2- Acrylamide appears as a white crystalline solid, is odorless and has high solubility in water (2155 g/L water). Melting point 84.5 °C, boiling point (25 mmHg) 125 °C (192.6°C at atmospheric pressure). Acrylamide is a reactive chemical, which is used as monomer in the synthesis of polyacrylamides used e.g. in purification of water, and in the formulation of grouting agents. Acrylamide is known as a component in tobacco smoke. Acrylamide is primarily reactive through its ethylenic double bond. Polymerisation of acrylamide occurs through radical reactions with the double bond. Acrylamide could also react as an electrophile by 1,4-addition to nucleophiles, e.g. SH- or NH2-groups in biomolecules. Acrylamide is metabolised in the body to glycidamide, a reactive compound formed through epoxidation of the double bond. The toxicological effects of acrylamide have been studied in animal models. Exposure to acrylamide leads to DNA damage and at high doses neurological and reproductive effects have been observed. Carcinogenic action in rodents has been described but carcinogenicity to humans has not been demonstrated in epidemiological studies, although it cannot be excluded. The International Agency for Research on Cancer (IARC) has classified acrylamide as ‖probably carcinogenic to humans‖

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(Group 2A). Neurological effects have been observed in humans exposed to acrylamide. Properties, use and toxic effects of acrylamide are reviewed by IARC (1) and EU (2). Chapter 3- Recent reports of elevated acrylamide levels in some heat processing foods have shocked the food safety world provoking an international health alarm. This hazardous contaminant has been found in various starchy fried and baked foods at significant levels (eg. = 2000µg/kg in potato chips and = 1500 µg/kg in biscuits) considering that acrylamide declared human as low as reasonable achievable intake is 2.60 µg/kg bw/day. Acrylamide is a well-known potential genetic and reproductive toxic compound with mutagenic and carcinogenic properties in experimental mammalians. Recent epidemiological studies have shown that dietary acrylamide was linked to significant increasing in the incidence of breast, endometrial and ovarian cancers in women. This fact reinforces the necessity to reduce dietary acrylamide content and to mitigate adverse in vivo effects of acrylamide after some food intake. Maillard reaction has been shown as the main acrylamide pathway in heat processed foods. Thus, a wide range of methods for acrylamide mitigation that has been reported are based either in: (i) Reducing the precursors of this reaction; or (ii) Operating at process conditions which inhibited Maillard reaction. These technologies have the challenge of being effective not only as an alternative for reducing acrylamide but also for maintaining the desirable sensorial attributes of foods. It is also important to consider the impact of acrylamide mitigation technologies on the daily acrylamide intake in order to elucidate their real effects over the acrylamide bioavailability in the human diet. In this chapter the authors present some of the most relevant issues regarding to acrylamide dietary sources, acrylamide chemical properties and recent findings about its toxicology and metabolism. Chapter 4- In the early 80‘s Stoukides and Vayenas discovered and studied a new phenomenon, called Electrochemical Promotion of Catalysis (EPOC), that in the last decades has had a strong impact in the fields of electrochemistry, heterogeneous catalysis and surface science. It has been recognized as one of the most recent exciting discoveries with great impact on several catalytic and electrocatalytic processes. The EPOC phenomenon is an important electrochemical tool for controlling the catalytic activity and selectivity of a heterogeneous catalyst interfaced with a solid electrolyte via external electrical current or potential application. As a result, electrochemically induced catalytic rate increase up to 200 times larger than the catalytic rate without polarization have been found. Furthermore, the rate increase has been found to be typically 10 to 105 times larger than the electrochemical rate of ion supply from the solid electrolyte to the catalystelectrode given by the Faraday‘s law. Accordingly, this phenomenon was also termed as nonFaradaic electrochemical modification of catalytic activity (NEMCA effect). Since its discovery, several research efforts have been carried out leading to a thorough understanding of the phenomenon which has been applied to more than a hundred catalytic systems including both environmental and energy-related applications. So far the most important achievement in the EPOC field has been the understanding of its origin and mechanism and its contribution to further clarify the effect of electronic promoters in heterogeneous catalysis. The latest progress has been impressive pointing out the technological researches in terms of catalyst-electrodes development and reactor-systems engineering for the industrial point of view.

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This chapter will present some key aspects of the phenomenon of electrochemical promotion such as its origin, mechanism and some experimental details. Also, the history and the evolution of the study of the phenomenon will be commented. In addition, some recent applications of this phenomenon to environmental and energetic reactions will be presented, which have been carried out by the authors‘ group in the Department of Chemical Engineering at University of Castilla-La Mancha (Spain). Finally, the main aims and future trends of EPOC research will be presented. Chapter 5- Oxalic acid is biosynthesized by a wide variety of organisms, and it is transported out of cells. The acid plays an indispensable role in the environment. In forests, the acid fertilizes and detoxifies soils, which accelerates woody plant growth, and this makes a contribution to sustainable global ecosystems. One of the major types of organisms that produce oxalic acid is the wood-rotting basidiomycetes. In these wood-rotting basidiomycetes, biochemical energy for vegetative growth is produced through oxalate biosynthesis, whereby tricarboxylic acid and glyoxylate cycles constitutively and cooperatively work in carbon metabolism; this is in sharp contrast to the situation with other microorganisms. Recent advances relating to terminal enzymes for oxalate biosynthesis, including cytochrome c-dependent glyoxylate dehydrogenase and oxaloacetate acetylhydrolase, in various microorganisms are summarized. A biochemical device to confer oxalate resistance on wood-rotting basidiomycetes is described in relation to the oxalate transporter. Finally, different roles of oxalate metabolism, including catabolism and efflux during the growth of oxalate-degrading or nondegrading fungi, are proposed. Chapter 6- Carbon monoxide (CO) is utilized in the fish and meat industry worldwide, mainly for pretreatment and packaging purposes. Furthermore, CO can be used in slaughter of animals, but the latter application is still mostly on an experimental basis with limited commercial access. Methods and equipment for the sedation and killing of fish and terrestrial animals with gas blends containing CO are described, as well as effects of this early CO exposure on subsequent food quality. Animal welfare aspects are discussed by comparisons with existing methods for sedation and killing. Methods for accurately analyzing the concentration of CO in air, water and foods are highlighted. Studies of the penetration, distribution and dissipation of CO in fish and meat tissues are presented. The use of CO for pretreatment of muscle foods, especially fish, is presented, either as filtered smoke or gas blends with CO. Several packaging systems for CO gas mixtures have been introduced and implemented by the meat industry. An important topic includes the reaction of CO with proteins, in particular heme pigments. CO reacts with the heme protein neuroglobin in the brain and might also take part in biological signaling. The legal aspects involving the use of CO in the slaughtering and processing of muscle foods in different countries are discussed. Aspects from the consumer point of view are described, including both beneficial and undesirable effects of the gas. Chapter 7- Oxalic acid (OA) exhibits interesting chemical properties such as reducing and chelating agent for metal anions. These properties make OA useful for fabrication of new nanostructures, developing of models of how different nanocatalysts work and modification of surface properties of nanostructures. The role of OA on modification of physical and morphological properties of metal oxide nanostructures will be treated extensively here. Additionally, the degradation of OA by high efficient nanocatalyst will be explained in detail. The degradation of OA is an important aspect of nanotechnology because OA is a toxic

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pollutant existing in a range of industrial processes and is an intermediate in the mineralization of some pesticides. The degradation of OA is frequently used as a model pollutant for developing nanocatalyst because it is a simple molecule and can be oxidized to CO2 without formation of any stable intermediate. Lastly, OA has been extensively used for fabrication of self-arranged nanotemplates, modification of surface properties of carbon nanotubes, and fabrication of nanowire arrays. These nanostructures have potential applications in chemical sensors, luminescence and optical modulation devices; all these nanodevices use OA at different stages of their fabrication procedure. Finally, other application of OA in nanotechnology is the controlled and selected dissolution of metal oxides, such as the case of iron oxides where OA is used for purification of specific iron oxide phases. Chapter 8- Intelligent materials in various fields with changing properties and functions have been investigated. Stimuli-responsive polymer systems have been investigated for the purpose of the many types of possible applications. Examples are soft actuators, microfluidics and medical devices, etc. due to their light weight, flexibility and low noise, etc.External devices are needed, however, in order to drive stimuli responsive polymer systems.. All living organisms involve isothermal conversion system of chemical energy into mechanical work. These biological systems are very efficient because the chemical energy is directly converted to the mechanical energy. The authors attempted to develop the autonomous polymer systems like a living organism. This chapterintroduces the novel autonomous polymer sysems that cause self-oscillations which areinduced by the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction is well known for exhibiting temporal and spatiotemporal oscillating phenomena. The BZ reaction has been investigated because it can be treated as a model for the formation of a spatiotemporal structure, such as spiral pattern or target pattern in an unstirred solution, and multistability, periodicity, multiperiodicity in a stirred solution. The process of the BZ reaction is oxidation of an organic substrate by an oxidizing agent in the presence of the catalyst under acidic conditions. The metal catalys of the BZ reaction cause spontaneous redox self-oscillation. Ruthenium tris(2,2‘-bipyridine) that is a metal catalyst of the BZ reaction autonomously change the solubility in the reduced and oxidized state. By utilizing the autonomous solubility change of the Ru catalyst, the self-oscillating polymer systems that include the Ru catalyst into the polymer-main chain are developed. There are two types of polymer systems: One is self-oscillating polymer chain, the other is self-oscillating polymer gel. These polymer chains are mainly composed of poly(N-isopropylacrylamide) [poly(NIPAAm)] covalently bonded to [ruthenium (4-vinyl-4‘-methyl-2,2-bipyridine) bis(2,2‘-bipyridine)bis(hexafluorophosphate), Ru(bpy)3] as a catalyst of the BZ reaction. These polymer systems cause the autonomous and spontaneous aggregation-disaggregation self-oscillation of the polymer chain and the swelling-deswelling self-oscillation of polymer gel under the constant temperature conditions in the presence of the BZ substrates other than the metal catalyst. However, the operating conditions of these autonomous polymer systems are limited to non-biological conditions because the BZ reaction requires the strong acidic conditions and the oxidant. If the autonomous polymer systems drive into the physiological environment, the application filed should be expanded. To extend the driving environment, the authors should modify the molecular design of the self-oscillating polymer systems. As a first step to operate the autonomous polymer system in the biological solution, acrylamide-2methylpropane sulfonic acid (AMPS) was incorporated into the conventional-type selfoscillating polymer chain (poly(NIPAAm-co-Ru(bpy)3) ) as a pH control site. As a result, the

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authors were the first to succeed in causing the self-oscillation under the acid-free condition. They also found that the autonomous polymer system with the pH control site has a high potential for controlling the self-oscillating behavior and increasing displacement of the polymer gel. By utilizing the AMPS-containing polymer system, they observed on-off switching of the transmittance self-oscillation and a viscosity self-oscillation of the high concentration of polymer solution. Byintroducing the AMPS into the polymer network, the displacement of the self-oscillating gel with the AMPS was ten times as large as the conventional-type ofself-oscillating gel without AMPS. Using the significant large displacement of the autonomous polymer gel, the authors were first to succeed in constructing gel actuators such as a self-walking gel and a matter transport gel, bending-stretching type of the polymer gel actuators. The second step, methacrylamidopropyl trimethylammonium chloride (MAPTAC) with a positively charged group was incorporated into conventional-type self-oscillating polymer gel (poly(NIPAAm-co-Ru(bpy)3)) as a release site of an anionic oxidizing agent (bromate ion ). The authors introduced the bromate ion to the MAPTACcontaining polymer system by utilizing the ion-exchange process. As a result, they succeeded in causing the self-oscillation under the oxidant-free condition. Finally, the pH-control and the oxidant supply sites were introduced to the conventional-type self-oscillating polymer system. By adding only the organic acid (malonic acid), they were successful in observing the selfoscillating behavior of the novel self-oscillating polymer system with AMPS and MAPTAC moieties at the same time. The self-oscillation behavior in the biological condition can control the period and waveform by changing the concentration of the malonic acid. Chapter 9- Oxalic acid (OA) as a sacrificial agent (hole scavenger) in the photoreduction of Cr(VI), was evaluated in a radiation efficient photocatalytic fluidized bed (FB) of a TiO2quartz sand catalyst composite vis–à-vis the homogeneous photoreduction process. The potentiality of OA as catalyst activator via its ―intimate‖ contact with TiO2 was also explored. Under anoxic conditions the OA was more efficient in the homogeneous photoreduction of Cr(VI) than in the heterogeneous (photocatalytic) reaction system. However, when dissolved oxygen was present in the solution, the reaction rate of the homogeneous reaction was notoriously diminished, thus indicating that oxygen was competing with Cr(VI) in oxidizing the oxalic acid. The pre-exposure of the photocatalyst to an OA solution for designated dwelling times (thus forming a thin, compact overlayer of titanium oxalate) dramatically enhanced the photocatalytic rate, up to the point of clearly overcoming the homogeneous process. Chapter 10- Astaxanthin (AX) is one of the main pigments included in crustacean shrimp; In addition, astaxanthin is an important antioxidant which has been reported to surpass those of β-carotene or even α-tocopherol. Synthetic astaxanthin is widely used as supplementation of feed by fish farmers and constitutes 10-20% of the feed cost. However, the high cost of synthetic pigments and the growing demand for natural foods have stimulated the research of extracting astaxanthin from natural sources, which is often most time-consuming. Exhaustive methods such as silage consuming large amounts of solvents, which consists of treating shrimp byproducts at the low pH (4-5) to protect them from bacterial decomposition and ease pigment recovery due to partially dissolved calcium salts. Alternative method such as supercritical fluid extraction (SFE) is free of solvents. In recent years, SFE has proved one of the most appealing techniques for solid sample treatment. In fact, supercritical fluids diffuse more readily into matrices than ordinary liquids, thereby improving the extraction yields of analytes from complex matrices.

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Chapter 11- Lysophosphatidic acid (LPA) (Figure 1) is a small glycerophospholipid that is present in blood serum at 1-5 μM and at lower concentrations (100-200 nM) in blood plasma. The first LPA receptor (LPA1) was discovered in 1996 followed by LPA2 and LPA3. Two more divergent LPA receptors (LPA4 and LPA5) have also been described. Tissue distribution of the five LPA receptors is wide and varied (vide infra). Because of the ability of the LPA receptors to stimulate a number of intracellular pathways it is a major player in a number of biological processes. Chapter 12- Heavy metals are ubiquitous environmental contaminants. They have been proved to be toxic to both environmental and human health. The adverse human health effects associated with exposure to heavy metals, even at low concentrations, are diverse and include, but are not limited to, hepatic and renal dysfunction, neurologic disorders, reproductive and developmental failure, and carcinogenic actions. Although some individuals are primarily exposed to these contaminants in the workplace, for most people the main route of exposure to these toxic elements is through the diet (food and water). Speciation, metal interactions and the selection of appropriate biomarkers are key issues in assessing the hazards of heavy metals. Considering the importance of this subject, this communication gives an overview of the remaining challenges faced by the researchers that needed to be intensively explored. Clearly, much remains to be done in this area to fully understand the heavy metals toxicology, and improve the risk assessment analysis for these toxic elements.

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Chapter 1

ADVANCES IN OXIDATIVE DECOMPOSITION OF OXALIC ACID IN AQUEOUS MEDIA Abdelkrim Azzouz Department of Chemistry, University of Quebec at Montreal, Canada

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1. INTRODUCTION One of the major environmental issues to be solved is undoubtedly the elimination of organic pollutants from industrial waste-waters, more particularly those produced by food industries and paper pulp manufactures. Physicochemical treatments like coagulationflocculation and filtration are only profitable to remove high levels of organic pollutants. For lower contents, conventional oxidative decomposition methods are applied. Nonetheless, they turned out to be ineffective, because they generate unavoidably oxalic acid. In other words, oxidative treatments for organic matter decomposition cannot be regarded as being effective without total removal of oxalic acid, the most chemically stable by-product. Conversely, the main criterion for highly efficient oxidative decomposition of organic contaminant resides in the absence of traces of oxalic acid in the final liquid mixture. Oxalic acid is a short chain pollutant recognized as displaying even higher toxicity for human health and more hazardous character on flora and fauna than the very organic contaminant which it arises from. Many organic compounds are potential sources of oxalic acid, which is quite refractory towards oxidation, and accumulates as final product. Thorough mineralization of oxalic acid or other chemically stable intermediates resulting from organic matter oxidation has become a major priority in today‘s wastewater treatment. Photocatalytic degradation of organic pollutants has long been regarded as being the most effective technique [1,2], but the performance still remains unsatisfactory. In the meantime, continuous improvements of conventional oxidative methods gave rise to the so-called AOPs, namely Advanced Oxidation Processes [3], including various ozonation methods in the presence of hydrogen peroxide, UV radiations and/or of catalysts. One of these AOP‘s which have particularly attracted attention is that using ozone at elevated pH, but the treatment of alkaline media does not seem to offer promising prospects for economical and ecological reasons.

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Photoelectrocatalytic decomposition of oxalic acid [4] also showed major drawbacks due to cost limitations for commercial applications. So far, ozonation treatment attempts also failed for diverse objective reasons, more particularly the low reactivity and solubility of ozone. An essential criterion for high ozonation effectiveness resides in a total conversion of oxalic acid into a harmless derivative, namely carbon dioxide, with no toxicity or negative environmental impact. In this regard, catalyzed ozonation turned out to be by far more effective as compared to the non-catalytic routes, in spite of many drawbacks. Significant enhancements of the ozone solubility in aqueous media have been achieved through metalcatalyzed ozonation processes, which, however, have been confronted to water contamination by metals [5,6]. Reportedly, the ozone solubility might be increased by approximately 10 times by using two-phases systems containing water and non-polar fluorinated hydrocarbon solvent, but no complete ozonation was achieved [7]. A more convenient alternative to overcome these shortcomings consists of the use of solid or supported catalysts. Nearly thorough ozonation of oxalic acid was reported in the presence of activated carbon [8-11]. This improvement was explained in terms of simultaneous contributions of both surface phenomena and bulk water reactions between oxidizing species and oxalic acid. Seemingly, the mere presence of dispersed solid surface induced appreciable ozonation enhancement, even when using TiO2, a solid catalyst rather known for its photocatalytic activity [1-2]. Here, adsorption phenomena must be involved, because the catalytic activity was found to be almost linearly proportional to the catalyst surface area. Various solid-supported metal catalysts have been tested in oxalic acid ozonation, and zeolites have particularly focused interest these last decades. Y-type faujasite ion-exchanged with transition metals, more particularly iron, gave appreciable catalytic activity in phenol hydroxylation. Iron combination with cobalt in the Fe-Co-NaY form showed even higher activity than the individual Fe-NaY or Co-NaY catalysts [12]. Here, a possible synergy must involve cations combinations, which opens new prospects in using less pure mono-ionic zeolites. A growing interest has recently been devoted to clay catalysts at the expense of zeolites, owing to their higher availability and their expandable structures devoid of pore size limitations. It appears that the mere presence of clay minerals induces significant improvement in the ozonation efficiency [13] In this regard, hydrotalcite [14,15] and montmorillonite [13,16] ion-exchanged by transition metals already showed interesting catalytic activities in phenol oxidation. Here also, iron confers interesting performances to ion-exchanged clay minerals, as also reported for other oxidative processes of organic wastewaters [17]. Some authors [18] assume that clay minerals presumably generate superoxide radicals, which are very effective in the decomposition of organic compounds, more particularly those exhibiting high chemical stability like oxalic acid. Nevertheless, the role of clay catalysts in ozonation processes still remains to be elucidated. An essential requirement for developing effective elimination of oxalic acid involves a thorough knowledge of the main sources of this contaminant and a better understanding of its physicochemical behavior. Investigations are still in progress in this direction. To provide a clear and comprehensive overview about the achievements made up today in oxalic acid removal, the latest findings in the chemistry of this compound, its toxicity and decomposition processes will be reviewed in this chapter.

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3

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2. ORIGINS, SOURCES AND OCCURRENCE Oxalic acid is a dicarboxylic acid (HOOC-COOH) that usually results from oxidation in nitric acid of various organic compounds, more particularly oligo- and poly-saccharides (starch, cellulose, etc), explaining thereby its occurrence during most of water oxidative treatment processes. Oxalic acid exhibits a strong capacity to combine to metal cations. For this reason, in nature, oxalic acid predominantly occurs as potassium or calcium oxalates, which are assimilated by plants. The presence oxalic acid and oxalates in soils arises from natural processes of biological decomposition of carbohydrates occurring in the very soils [19]. This explains somehow the presence of this compound in the roots and rhizomes of a series of plants (beet, rhubarb, spinach,...), more particularly those belonging to the so-called oxalis plant family which includes sorrel. However, the presence of oxalic acid in soils may also produce advantageous priming effects that favor plant growth, more particularly in forest soils containing organic matter with low biodegradability [20]. Here, oxalic acid and many other organic compounds are supposed to act as major energy sources for the microbial metabolism of soils. Oxalic acid and oxalates exhibit high toxicity to human health, because of their presence in a series of plant-based foods like cocoa, chocolate, nuts, beet, chard, berries, rhubarb, beans, sorrel, spinach and even tea, which are among the oxalate-richest vegetables. For instance, spinach is acclaimed for its health benefits, but it contains appreciable amounts of oxalic acid. That is why excessive consumption of such vegetal foods may be hazardous. Other species (Penicillium and Aspergillus) do not contain oxalic acid, but also display toxicity, because they have the capacity to produce this compound via sugar conversion. Oxalic acid may also form through the so-called ''fusion'' of a series of oxygenated organic compounds with caustic alkalis. In fact, this route consists in converting oxalic acid into alkali cations salts, and has become the main procedure for large-scale manufacture of oxalic acid. The resulting alkaline oxalates are further boiled with a lime aqueous dispersion (milk) and result in the formation of insoluble precipitate consisting of calcium oxalate salts, which is then reacted with sulphuric acid to yield finally oxalic acid. Beside its natural occurrence, oxalic acid is also present as contaminant in many industrial waste-waters, more particularly those resulting from its use as a wood bleaching agent. Indeed, oxalic acid is usually employed in aqueous solutions for dark wood treatment and wood furniture refinishing. Other effluents arise from many industrial activities, where oxalic acid is used as precipitating agent (rare-earth raw mineral processing), as bleaching agent (textile industries and wood pulp bleaching), as rust removing agent (metal treatment), as grinding agent (marble polishing) and as precipitating agent for calcium removal (wastewater treatment).

3. REACTIVITY AND TOXICITY To understand the toxicity mechanisms of oxalic acid, a previous examination of the main chemical properties is necessary. The most important feature of oxalic acid is its acid character. This compound is a bi-acid comprised of two carboxyl groups directly bonded each

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Abdelkrim Azzouz

other, and consequently has two pka values (pKa1 = 1.38 and pKa2 = 4.28) for the two dissociation steps: C2O4H2

→

+ − H + C2O4H

(1)

− C2O4H

→

+ 2− H + C2O4

(2)

Oxalic acid is devoid of any C-H bond. Such bond are supposed to confer hydrophobic characters to the organic compounds. Seemingly, this structure explains why oxalic acid is one of the strongest organic acids, being probably the most hydrophilic organic acid capable of dissociation in water and polar solvents. Its conjugate base, namely the oxalate anion, usually behaves as powerful chelating agent for metal cations. In its dry form, oxalic acid appears a prismatic crystals, which show high solubility in water. These crystals are relatively stable under ordinary conditions, start subliming at a temperature of 150-160°C, and decompose under heating at higher temperature. Carbon dioxide and monoxide, water and formic acid may result from thermal decomposition.

3.1. Thermal Decomposition of Oxalic Acid

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Transition state theory calculations showed that the thermal decomposition of oxalic acid involves both monomolecular and bimolecular mechanisms. According to the reaction temperature, various compounds such as formic acid, carbon dioxide and monoxide, and water are produced [21]. According to this study, calculations of the activation barrier suggests that the thermal decomposition of oxalic acid may proceeds as follows: Monomolecular pathway HO(CO)-(CO)-OH → CO2 + HO-C-OH

(3)

HO-C-OH → H-COOH

(4)

HO(CO)-(CO)-OH → CO2 + CO + H2O

(5)

Bimolecular pathway HO-C-OH + H2O → H-COOH

(6)

Activation barrier calculations indicates that the direct decomposition of oxalic acid into carbon dioxide, carbon monoxide and water (reaction 5) is more probable than the thermal decomposition via the formation of the intermediate dihydroxycarbene (reactions 3 and 4). Reportedly, the direct decomposition of oxalic acid (5) becomes predominant with increasing temperature. The bimolecular pathway must proceed via the formation of dihydroxycarbene, which further reacts with water to generate formic acid (reaction 4). This mechanism pathway is

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supposed to promote the quick production of CO2 and HCOOH by thermal decomposition of gaseous oxalic acid.

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3.2. Chemical Reactivity Oxalic acid displays high reactivity towards alkalis, chlorites, hypochlorites, oxidizing agents, furfuryl alcohol, allyl alcohol, glycerin and silver compounds, more particularly upon heating. That is why the storage of oxalic acid must avoid the contact with such substances. Oxalic acid can also be decomposed when heated in the presence of sulphuric acid. When contacted with powerful oxidizing agents like potassium permanganate, oxalate and oxalic acid totally decomposes into carbon dioxide and water [22]. The reaction is accentuated by the formation of manganese sulphate, which acts as catalyst. One of the most known reaction of oxalic acid is that involving nitric acid. The presence of small amounts of metal cations like Mn2+ induces a catalytic effect that leads to a thorough decomposition of oxalic acid [23]. The decomposition of oxalic acid is a first order reaction and proceeds at temperatures above 80°C; the activation energy of the reaction is of 18.6 kcal/mol. This decomposition method is applicable to the dissolution of an oxalate precipitate. Pure oxalic acid shows high corrosive character and toxicity, and care must taken when handling such a compound, by avoiding direct contact with skin, eyes and mucous. Oxalic acid displays high capacity to bind to metals like calcium, iron, sodium, potassium or magnesium. Consequently, foods with high oxalic acid contents should be consumed moderately, otherwise nutritional deficiencies may result. Indeed, when inhaled, oxalic acid provokes nausea, headaches, esophagus or stomach irritation, blood circulation disorders and kidney damages. Possible treatments against oxalic acid poisoning recommend the ingestion of calcium or magnesium-containing substances (milk of lime, chalk, plaster, …). This ought to inhibit the transfer of oxalic acid in the blood, by favoring the precipitation into the calcium or magnesium oxalate form, inasmuch as only the oxalate salts of alkali metals are soluble in water. Prolonged ingestion of oxalic acid may be hazardous, causing irreversible damages to human health. At high doses, calcium oxalate precipitation may even cause kidney channels clogging by the formation of kidney stones. The latter mainly consists of calcium oxalate. Other combinations with heavier metals precipitate readily. Among them silver oxalate is a hazardous compound that can be prone to explosive decomposition. This precipitation capacity of oxalate should be regarded as advantageous when dealing with the elimination of traces of oxalic acid by coagulation-flocculation, more particularly after incomplete ozonation of organic contaminants. Nonetheless, it appears that the mere presence of oxalic acid and/or of short chain organic compounds exerts a negative effect on the elimination of turbidity and organic matter by coagulation [24]. Clear evidence in this regard was provided by the fact that previously ozonated organic matter is more difficult to coagulate than untreated samples. A possible explanation resides in the fact that heavy organic contaminants can be more readily eliminated by coagulation than lighter ones like oxalic acid. Other actions of oxalic acid on the coagulation of organic matter must also be taken into account.

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4. DECOMPOSITION METHODS Oxalic acid is an oxygenated organic compounds, and displays an appreciable resistance against the actions of oxygen, ozone, or other oxidizing agents. Besides, the two carboxyl groups confer to the chemical structure of oxalic acid two conjugated double bonds on the O=C-C=O, which consolidate the chemical stability of this compound. That is why scientists have focused interest towards powerful physical methods capable of breaking this chain.

4.1. Physical Methods

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Crystallization (not precipitation), settling, filtration, and purely physical adsorption that involves only van der Waals interaction are regarded as being conventional physical methods for removing oxalic acid from aqueous media. Nevertheless, many of apparently physical methods involves physicochemical processes that induce chemical transformation of the reagents implied. One of such physical methods involves the use of ultrasound waves in order to provoke oxalic acid degradation in aqueous media [25]. Ultrasonic decomposition of oxalic acid appeared to be enhanced by the presence of NaCl in the aqueous reaction mixture, but surprisingly addition of hydrogen peroxide was found to hinder the degradation process. Seemingly, attempts to improve such an ultrasonic technique by using bubbling nitrogen or air failed, and did not provide conclusive results. Other attempts were performed via radiolysis of oxalic acid by using alpha and 3H particles [26]. Radiolysis was applied to mixtures of solid oxalic acid and lithium oxalate, but seemingly the oxalic conversion yield was by far lower than when employing gamma radiations. However, such a techniques is not expected to be promised to large-scale implementation because of handling difficulties and high investments costs.

4.2. Chemical Methods In spite of its relatively high chemical stability, oxalic acid exhibits special physical and chemical features that may cause its decomposition. A quick overview of the chemical structure of oxalic acid allow to notice that the relatively strong acidity of oxalic acid even constitutes its Achilles' heel, i.e. its weak point. Hence, oxalic acid exhibits favorable interactions with water, aqueous media and polar solvents, can readily dissolve and ionize in water, and can be eliminated by ion-exchange, adsorption, complexations, coagulation and precipitation processes, with more or less effectiveness. Few decades ago, one of the first methods used for this purpose was performed using ion exchange resins like Permutit SKB to remove oxalic acid from glycol solutions [27]. Promising results were obtained for the extraction of oxalic acid from glycol solutions. The ion-exchange equilibrium was found to fit a Freundlich model. Oxalic acid removal attempts were successfully performed via first order adsorption using flyash as adsorbents [28]. The adsorption obeys both Freundlich's and Langmuir's isotherms and the constants have also been reported for flyash and activated carbon. This is

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an interesting alternative to the use of high cost adsorbents, because the flyash is an industrial waste resulting from large-scale thermal power facilities.

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4.3. Biochemical and Enzymatic Methods Since oxalic acid unavoidably occurs in nature, more particularly in soils, it must undoubtedly be involved in biological, biochemical and enzymatic processes, including biodecomposition reactions. This hypothesis, which has already been demonstrated, has stimulated scientists to test bacteria, fungi and enzymes to achieve oxalic acid degradation. Reportedly, one the first arguments in this regard was provided by the fact that oxalic acid can be eliminated by streptomycetes. [29]. Such actinobacteria belong to the Streptomycetaceae family [30], which grow and develop predominantly in soils and decaying vegetal masses. Oxalic acid may even contribute to the biomass decay. Indeed, during the pine holocellulose decomposition [31], brown rot fungi, and more particularly the Gloeophyllum trabeum fungus showed high effectiveness in the conversion of oxalic acid into CO2. Here, oxalic acid appeared to be involved with iron in the cellulose decay by brown rot fungi. Later, lactic bacteria, isolated from adult human feces and gastric juices, were tested in oxalic acid degradation, and were found to be particularly efficient [32]. Oxalic acid conversion yields of 50% were achieved by lactic acid bacterium, when the latter was previously cultured in a medium already containing 10 mM of oxalic acid. A possible explanation should consist in the fact that oxalic acid is possibly an essential nutrient that significantly contributes to the growth of such bacteria. This is consistent with previous results reported much earlier, according to which, significantly improved decomposition yields of oxalic acid were achieved by using the rumen contents of sheep habituated to the intake of small quantities of this very substance. [33]. However, the presence of an excess of calcium appeared to affect this performance, presumably due the formation of much more chemically stable Ca-oxalate salts. This adaptation to oxalic acid ingestion by animals has recently been confirmed by experimenting stomach rumen of sheep and goats previously fed with oxalic acid [34]. Increasing amounts of oxalic acid administrated produced significant decomposition yield, and this effect was found to be strongly dependent on the animals metabolisms. These results demonstrated the adaptability of the rumen micro-organisms to oxalic acid decomposition. Other bacteria isolated from human feces also showed interesting performance in the degradation of oxalic acid [35]. This finding is of great importance, because it provides clear evidence that oxalic acid may be decomposed in food by such bacteria even in human intestine. Lower performances were obtained with feces produced by stone-forming human adults. This agrees with the fact that presence of a calcium excess appeared to affect the bacteria capacity to decompose oxalic acid [33]. Bacterial activity may also be used to decompose oxalic acid resulting from reactive dye ozonation [36]. In this regard, a certain microorganism (Pandoraea sp. strain EBR-01), isolated from soils, and cultured at pH 7 and 30°C, showed effective capacity to decompose not only the residual non oxidized organic dyes, but also the oxalic acid resulting from their ozonation. This opens new prospects for envisaging new avenues using consecutive treatments by means of ozone and microorganisms for thorough mineralization of organic pollutants.

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The use of this capacity of living organisms to metabolize oxalic acid appears as a new route to explore. Thus, biological method employing rye seedlings or radicles produced almost total decomposition of oxalic, most likely due to the presence of oxalate-specific oxalate oxidase enzyme [37]. Dissolved oxalate was quickly decomposed in the presence of radicles, and almost totally decomposition was achieved by previously frozen spinach. All kind of fresh or heat-dried cereal seedlings or radicles also displayed high effectiveness in oxalate degradation. The enzyme oxalate oxidase may also be isolated from barley roots and introduced into oilseed rape [38]. This allows to envisage new applications for in-vivo oxalate removal in food technologies. Oxalic acid may also be removed via enzymatic decomposition in order to prevent from calcium oxalate precipitation during the brewing process [39]. For this purpose, oxalate decarboxylase enzyme active under the conditions of the brewing medium was introduced during the beginning of the fermentation step. Optimum performances were obtained at pHs of 3-6, and temperatures of 35-75°C.

5. CONVENTIONAL OXIDATIVE DECOMPOSITION TECHNIQUES

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Almost all oxidative decomposition methods for removing organic contaminants produce short chain acids. Decomposition of halogenated organic compounds yields hydrohalogenic acids, while that of oxygenated substrates generates acetic and oxalic acids. Oxalate and oxalic acid display strong reactivity towards strong oxidizing agents such as permanganate anion, and can be subject to autocatalytic oxidation processes [22]. That is why a special interest was devoted to oxalic acid removal via oxidative techniques.

5.1. Fenton Oxidative Techniques Beside coagulation and flocculation, one of the most known methods to eliminate the organic matter, and more particularly oxalic acid, is undoubtedly the so-called Fenton process. which uses a combination of hydrogen peroxide and Fe2+ cations in aqueous media (H2O2/Fe2+). This technique is based on the oxidation of Fe2+ into Fe3+ cations, and was first applied by Fenton to decompose maleic acid [40]: 2+ Fe + H2O2

→

3+ − Fe + OH + OH*

(7)

In this reaction, ferrous cations rapidly oxidize into ferric ions when contacted with hydrogen peroxide. In a second step, the excess amount of hydrogen peroxide decomposes catalytically by Fe(III) and generates hydroxyl radicals according to the reactions: 2+ Fe + H2O2 FeOOH

2+

→

+ 2+ H + FeOOH

→

HO2* + Fe

2+

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Advances in Oxidative Decomposition … 2+ Fe + H2O2

3+ − Fe + OH + OH*

→

9 (10)

Being the main oxidizing agent, the ferric ion can be directly used instead of ferrous cations in the Fenton's reagent (H2O2/Fe3+) [5]. Reportedly, the latter is a powerful catalyst for the chemical decomposition of various organic pollutants in aqueous media such as phenols and derivatives, nitrobenzene, dyes, pesticides and herbicides [41-45]. Nevertheless, care must be taken when using hydrogen peroxide, because the later better reacts with Fe2+ than with Fe3+ ions to yield hydroxyl radicals. The growing interest for this procedure arises mainly from the availability, the absence of toxicity and the lack of environmental impact of the Fenton' reagent. Nevertheless, a major drawback consists in the consumption of significant amounts of iron, inasmuch as the production of each hydroxyl radical needs one Fe2+ cation. In addition, Fenton oxidative treatments often lead to incomplete oxidation of most of organic pollutants, and unavoidably generate oxalic acid, which exhibits even higher toxicity than its starting precursors. Total elimination of oxalic acid requires unavoidably higher oxidation effectiveness. For this purpose, other Fenton-like oxidation procedures such as photo-Fenton processes and other Fenton derivative techniques (H2O2/Fe2+/UV; H2O2/Fe2+/O3) have been developed. A wide literature is available thanks to continuous research in this direction.

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5.2. Photochemical Techniques Completion of organic pollutants oxidation often requires additional treatments. For instance, exposure of the previously oxidized waste-waters to UV radiations of less than 254 nm can produce effective ozone photolysis. At 200–300 nm wavelengths, most organic pollutants acquire reactivity towards oxidizing agents like ozone, hydrogen peroxide and others. Here powerful UV generators are needed, because the photochemical process effectiveness is almost linearly proportional to the number of photons of the radiation beam [46]. High energy consumption must be involved for effective and thorough organic contaminant removal without generating oxalic acid as the final product. This is why combined treatment procedures like photo-Fenton and Fenton-like methods have been developed. In the presence of UV radiations, a mixture of Fe3+ ions and H2O2 acquires even higher reactivity towards organic contaminants, more particularly around pH 3. Here, the presence of a proton excess is expected to favor the formation of Fe(OH)2+ complex. The latter is supposed to be the main precursor of hydroxyl radicals when exposed to UV radiations: Fe3+ + H2O → Fe(OH)2+ + H+ +

Fe(OH)2

→Fe3+ + OH

−

+ Fe(OH)2 + hυ→Fe2+ + OH* + Fe2 + H2O2

− →Fe3+ + OH + OH*

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Seemingly, the UV/Fe(III)/H2O2 technique is a powerful procedure for complete removal of organic contaminants, inasmuch as some authors assert that chloro-phenol [47], pesticides and herbicides [48] are thoroughly eliminated from waste-waters. This effectiveness improvement must be due to the production of Fe2+ ions (reaction 18), which generates additional hydroxyl radicals, and regenerates ferric ions (Fe3+) via hydrogen peroxide decomposition (reaction 21). Nonetheless, taking into account the relatively high refractory character of oxalic acid, one strongly believes that total mineralization must require exposure to powerful UV, and consequently a high energy consumption. Here, the presence of oxalic acid should be rather advantageous, inasmuch as oxalate ions are expected to combine with iron to yield ferrioxalate ions, which are well known to act as photo-active catalyst in such processes [49-51]. When irradiated by UV, oxalic acid ought to give rise to Fe(II) and carbon dioxide: + H2C2O4 → 2H + C2O42–

(15)

Fe3+ + 3C2O42– → [Fe(C2O4)3]3–

(16)

[Fe(C2O4)3]3– + hν → [Fe(C2O4)2]2- + (C2O4–)*

(17)

[Fe(C2O4)2]2– → Fe2+ + 2C2O42-

(18)

(C2O4–)* + [Fe(C2O4)3]3–→ [[Fe(C2O4)2]2- + C2O42- + 2CO2

(19)

(C2O4–)* + O2→ (O2–)* + 2CO2

(20)

Fe2+ + H2O2

(21)

→Fe3+ + OH- + OH*

Ferrioxalate exposure to UV radiations yields ferrooxalate ions, and then ferrous cation, which reacts as usual with hydrogen peroxide as a Fenton‘s reagent to produce hydroxyl radicals [51-52]. Here, in the UV/visible region (250–450 nm), Fe(II) forms in a quantum yield of almost 100-120%, which provides clear evidence of the occurrence of a favorable radical propagation step.

5.3. Photocatalytic Oxidation Techniques (UV/Tio2) Such techniques involve the use of solid catalysts that activate under UV exposure. Attempts in the photocatalytic decomposition of oxalic acid in water have already been performed using fullerenes and their mixtures as photo-catalysts [53]. Nonetheless, the most effective catalysts generally used in photocatalytic oxidative methods are those displaying a semiconductor character that allows to generate conduction band electrons and valence band holes via the absorption of electromagnetic radiations. Holes of electric charges are known to exert a very strong oxidizing capacity towards any kind of organic compounds. This effectiveness is supposed to be so powerful that, according to certain authors [54-56], no

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oxygen or air bubbling is required in the reaction mixture, since oxygen absorption is not the rate-limiting step of the oxidation process. One the most commonly studied catalyst in this regard is titanium oxide (TiO2) in both the anatase and rutile forms [57]. In theory, TiO2 must be effective for the elimination of a wide variety of harmful and hazardous organic contaminants like halogenated hydrocarbons, aromatic species, chlorinated phenols, biphenols, dyes, phthalates, DDT, surfactants, dioxins and others. Nonetheless, in many cases, the structure of the surface of TiO2 must play a key role in the oxidation effectiveness, and is expected to induce different performances for the rutile and anatase forms. In all cases, the combined actions of both ozone and photo-activated TiO2 seems to exert a synergistic interaction that ought to enhance the production of OH* radicals [58]. Reportedly, in the presence of UV radiations, TiO2 decompose oxalic acid according to firstorder reaction kinetics, while the initial rate of oxalic acid decomposition obeys a Langmuir– Hinshelwood kinetic model [59]. In the photocatalytic ozonation of oxalic acid, TiO2 may be used in various forms (dispersed powder, pellets, fixed bed, films, others). When used in a thin film form, TiO2 gave appreciable effectiveness in the photoelectrocatalytic or photoelectrochemical decomposition of oxalic acid and 4-chlorophenol when exposed to a monochromatic UV radiation (365 nm). [60].

5.4. Ozonation

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Oxidation of organic species by ozone can occur via various mechanisms. In water, the main pathways involve selectively either molecular ozone at acidic pH (direct ozonation) or OH* radicals (indirect route) in alkaline reaction media [13], as illustrated by scheme 1.

Scheme 1. Main oxidation routes during ozonation.

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In a complete ozonation of oxalic acid or of any oxygenated compound, both mechanisms are supposed to lead to the same final products, namely carbon dioxide and water, unless halogenated or other combined chemical species were present in the starting reaction mixture. Nonetheless, the amount of ozone required to achieve total conversion of oxalic acid must be different from a pathway to the other. Deeper insights in this regard will certainly provide valuable information on the way to achieve economical and green ozonation of oxalic acid.

5.4.1. Molecular Ozone Attack Given the resonance structures of the ozone molecule, the latter may act as a dipole, an electrophile or a nucleophile [61]. These reactions are known to be very selective, inasmuch as only certain chemical structures can be attacked by molecular ozone. In the case of oxalic acid, ozonation most likely takes place according to similar mechanisms pathways (Scheme 2).

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Scheme 2. Possible scheme of an attack of molecular ozone on a C-C bond.

The hypothetic final ozonide generated usually shows low chemical stability and short lifetime. This is why it is supposed to quickly decompose into ozonation products [62]. The electrophilic attack of molecular ozone is particularly localized on sites with high electron density like aromatic ring bearing electron-donors groups (-OH,-NH2,-OCH3,...) or C=C compounds. The latter induce significant charge density on the nearest carbon atoms and should promote the electrophilic attack. In contrast, molecular ozone is expected to show lower reactivity towards electron-acceptor groups such as (-COOH) [62]. Therefore, at least in theory, the electrophilic attack of molecular ozone is less probable on oxalic acid. An attack of molecular ozone on oxalic acid, if any, must rather occur via a nucleophilic route, due the presence of electron-acceptor groups, as already reported for C=N compounds [63]. However, unless improvements are made, for instance by using catalysts or other promoting agents, ozonation of oxygenated organic substrates like oxalic acid by molecular ozone must be very slow [64].

5.4.2. Radical Attack The second major pathway involves the attack of radical species arising from ozone decomposition in aqueous media. The decomposition of ozone in aqueous phase can be favored by different techniques [62], like: Increasing pH beyond neutral; addition of hydrogen peroxide; Ozone exposure to UV radiations;

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use of ferrous ion as catalysts or organic anions (formate), glyoxylic acid or humic substances as promoting agents.

Ozone decomposition is expected to generate hydroxyl radicals (OH*), which should trigger a series of chain reactions having low selectivities but high rate constants. Hydroxyl radicals are the main precursors of other organic radicals. In such a process, even ozone may induce its own decomposition. Humic substances may also promote radical mechanisms, provided that OH* are not consumed in other undesirable side-reactions [65]. The reactivity of OH* radicals is extremely fast as compared to molecular ozone, with reaction rates usually ranging from 106 to 1010 L.mol-1.s-1. At pH levels higher than neutral, ozone reacts with hydroxyl anions, and gives rise to HO2* and (O2-)* radicals. The super-oxide anion radical ((O2-)*) further reacts with ozone, generating the ozonide anion radical ((O3-)*), which decomposes into OH* radicals. This accounts for a conversion of three ozone molecules into two OH* radicals, according to the following reaction scheme:

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− 3O3 + OH + H+ →2OH* + 4O2

(22)

In such reactions, carbonate and bicarbonate behave as OH* scavengers, inasmuch as they react with OH* radicals to yield non-reactive carbonate or bicarbonate radicals. In alkaline media, both molecular ozone and OH* radicals are involved, but the action of the latter is by far faster than that of molecular ozone. Decreasing pH ought to induce to the reverse phenomenon, by enhancing the action of molecular ozone at the expense of that of OH* radicals. Nevertheless, oxidation with ozone alone has seldom been satisfactory, because the main drawback resides in the ozone solubility in the reaction mixture. A possible improvement should consist in increasing the amount of ozone injected in the liquid medium, but this requires higher electric energy consumptions [66]. Such energy consumptions account for serious constraints for large-scale implementation of ozonation treatments. Seemingly, the use of ozone alone for total elimination of organic contaminants including oxalic acid does not offer promising prospects, unless significant improvements are made, e.g. by resorting to the actions of catalysts and/or UV exposure.

5.5. Correlation between pH on OH* Radical In such oxidative treatment processes, pH is expected to play a key-role, by influencing not only the catalyst effectiveness, but also the chemical behavior of the chemical species dispersed in the liquid media. Low levels around pH 3 showed favorable effect [45], while excessively acidic media with pH below 2 were found to be detrimental, as observed in phenol photocatalytic oxidation [67]. No radical action of ozone and no formation of free OH* radicals is supposed to be involved below neutral pH [8,68]. Alkaline pHs beyond 11 are rather known to favor radical oxidation pathway by means of hydroxyl radicals, which do not seem to be involved at all in acidic media [69,70]. However,

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the effect of pH must strongly correlate to the chemical features of the organic substrate [71,72]. Notwithstanding that OH* radicals are powerful oxidizing species, high production of OH* can even be detrimental, because OH* recombine, becoming unaccessible to the oxidation process. Saturated molecules like alkanes are more difficult to oxidize, and often convert into short chain compounds like oxalic acid. AOPs can often achieve oxidative destruction of compounds refractory to conventional ozonation or oxidation by H2O2. However, the knowledge about the mechanism route of AOPs still remains to be enriched. OH* radicals may be generated by various procedures, both with and without exposure to photons. The main techniques are the following: Ozonation in alkaline media ( pH higher than 8.5); Ozonation in the presence of hydrogen peroxide (O3/H2O2); Ozonation in the presence of catalyst (O3/Cat); Ozonation under UV exposure (O3/UV); Ozonation in the presence of hydrogen peroxide under UV exposure (O3/H2O2/UV) Oxidation using hydrogen peroxide and ferrous ions as catalyst (Fenton processes, H2O2/Fe2+); 7. Photo-Fenton process and other Fenton derivative techniques (H2O2/Fe2+/UV; H2O2/Fe2+/O3); 8. Oxidation using hydrogen peroxide under UV exposure (H2O2/UV); 9. Photocatalytic oxidation (UV/TiO2).

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1. 2. 3. 4. 5. 6.

Any combination of the main factors involved in an oxidation process such as hydrogen peroxide or other oxidizing agents, ozone, oxygen, catalysts, along with UV and visible radiations could produce synergy for thorough decomposition of oxalic acid in optimal operating conditions, more particularly regarding the pH of the reaction mixture. Nevertheless, it must be reminded that OH* radicals are not necessarily the main precursors of the oxidation process, because such chemical species seem to be effective only in suitable alkalinity of the reaction mixture. Anyway, so far ozonation at alkaline pH levels was only of limited use, and large scale implementation still rise a series of issues to be solved. Other pH levels should favor different reaction pathways, inasmuch as oxidation at low pH values proceeds essentially through reactions that involve molecular ozone. In spite of these valuable findings, and because of the very nature of oxalic acid, the influence of pH still remains to be thoroughly elucidated. That is why more advanced oxidative treatments are subject to continuous and intensive research throughout the world.

6. ADVANCED OXIDATIVE TECHNIQUES A common feature of conventional oxidative treatments in aqueous media resides in their low effectiveness, because they unavoidably generate short chain compounds, like oxalic acid or halogen-containing by-products, hazardous compounds for health and nature. Oxalic acid is regarded as being a carcinogenic contaminant, which displays high chemical stability against oxidation in aqueous media [5,73-75]. Oxalic acid accumulates as final product, and is

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still present in the reaction mixture even after 8 hours of ozonation of dissolved carbohydrates [76]. During ozonation, many other organic compounds also constitute possible sources of oxalic acid. The chemical behavior of oxalic acid causes scaling problems in some waste waters treatment plants [77,78]. Much more effective removal of organic pollutants may be achieved via advanced oxidation processes (AOPs) [3,79]. The latter, defined as water treatment procedures that yield significant amounts of OH* radicals at almost ambient temperature and pressure. AOPs appear as sustainable alternatives to other techniques like coagulation-flocculation which generates polluted sludges to be treated, or waste incineration which yields hazardous gas pollutants and flyash. There exist two kinds of AOPs: i. those occurring at nearly ambient conditions of temperature and pressure; and ii. those using powerful oxidizing agents like UV radiations, hydrogen peroxide, ozone or their combinations. Both of them are supposed to resort to the formation of hydroxyl radicals for the decomposition of not only the organic pollutants, but also of oxalic acid, which usually show higher chemical stability towards oxidation. Hydroxyl radicals show strong oxidizing capacity, and are supposed to be effective in the oxidation of all types of organic pollutants. They can produce oxidation chain reactions, by acting in two different pathways: i. by extracting a hydrogen atom from hydrogenated chemical species like water, alkanes or alcohols, ii. by combining to the very organic substrate, as well illustrated by the following reaction scheme: RH + OH* → 2OH*

→

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R* + H2O2 → R* + O2

→

ROO* + RH →

H2O + R*

(23)

H2O2

(24)

ROH + OH*

(25)

ROO*

(26)

ROOH + R*

(27)

Oxalic acid should be prone to similar decomposition pathways, but its acid character must strongly correlate to the pH of the reaction mixture. In other words, complete decomposition of oxalic acid necessarily requires optimum pH levels, regardless to the oxidative process and mechanisms involved. However, such advanced oxidative routes display severe shortcomings, more particularly when applied to the treatment of aqueous effluents having high contents in organic contaminant exceeding 50 ppm or containing high oxalic acid concentrations. Such treatments involve necessarily high consumption levels of both oxidizing agent and energy. It was clearly demonstrated that the UV/H2O2 combination produces by far more effective oxidation than the mere ozonation, oxidation by hydrogen peroxide or exposure to UV radiations alone [80]. When applied to polyaromatics, such a combined oxidative treatment technique (UV/H2O2) appears to involve various types of reactive species like OH* and carbonate radicals. Attempts to correlate the oxidation effectiveness to the type of organic substrate and to the amount of OH* radicals generated during the process [81] failed. Indeed, for instance,

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more complex oxidative methods using simultaneously ozone, hydrogen peroxide and UV exposure (O3/H2O2/UV) exhibited surprisingly low effective for similar organic substrate, suggesting a weak and even negligible contribution of OH* radicals [82,83]. Most of advanced ozonation processes are characterized by a visible decrease in the ozone consumption, more particularly at acidic pH [84]. Combined actions of ozone and hydrogen peroxide (O3/H2O2) is well known to generate appreciable amounts of OH* radicals, and turns out to be relatively a low cost process for thorough oxidation, by reducing the ozone consumption. Ozonation under UV exposure (O3/UV) still encounters major shortcomings that do not favor a large-scale implementation, being restricted by high operating costs due to ozone consumption.

6.1. Ozonation under UV Exposure In aqueous media, ozone shows a maximum absorption of UV radiations at 253.7 nm, with a molar extinction coefficient of 3300 M-1.cm-1, and reacts with water molecules. This gives rise to hydrogen peroxide, which then decomposes into hydroxyl radicals (OH*) [49]: O3 + H2O + hν→ O2 + H2O2

(28)

This reaction is supposed to take place in two consecutive steps, namely the ozone photolysis (29) and formation of hydrogen peroxide (30). The latter is known to take place faster than photolysis, and must be predominant in normal conditions:

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O3+ hν

→O2 +O*

O* + H2O → H2O2→ 2OH*

(29) (30)

Without ozone, UV irradiation of hydrogen peroxide produces direct photolysis into hydroxyl radicals, as illustrated by reaction 31 [5]: H2O2 + hν → 2OH*

(31)

This reaction usually generates less hydroxyl radicals than ozone photolysis, because of the excessively low extinction coefficient of H2O2 at 254 nm (18.6 M-1.cm-1). Higher absorptivity of H2O2 may improve the effectiveness of hydrogen peroxide photolysis, but this occurs only at higher radiation frequency, and requires thus higher energy consumption. This makes that ozone photolysis turns out to be a more convenient route for organic pollutant removal, in spite of high operating costs. However, ozone photolysis is often attenuated by the presence, in the liquid media, of organic contaminants having high absorptivity of UV radiations, more particularly those bearing aromatic rings. The latter are well known to be strong chromophoric groups for UV photons, which act as competitive species for ozone in UV photons absorption [85,86]. That is why phenolic contaminants such as phenol, p-cresol, 2,3-xylenol, 3,4-xylenol cannot be totally decomposed, and often result in the formation of oxalic acid, which turns out to be quite refractory to oxidation in most treatment techniques.

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6.2. Ozonation under UV Exposure in the Presence of Hydrogen Peroxide The mere addition of hydrogen peroxide to the ozone/UV process causes an appreciable acceleration of the ozone decomposition, an increase in the production of OH* radicals and, thereby, higher ozonation effectiveness [49,50]. In such processes, the decomposition of two ozone molecules generates two OH* radicals: 2O3 + H2O2 → 2OH* + 3O2

(32)

According to some data [87,88], the best performance was achieved when H2O2 was added after the oxidation of highly reactive substances with ozone alone. The implementation of a radical system makes possible the oxidation of refractory molecules: it allows getting full advantage of selective reactions of molecular ozone, before the process converts into nonselective attack by free radical. This opens new prospects for thorough mineralization of refractory organic contaminants, including acetic and oxalic acids, because hydrogen peroxide is a low-cost and available oxidizing agent.

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6.3. Catalytic Ozonation Non-catalytic ozonation was found to give much lower performances than in the presence of catalysts [13]. Generally, the use of catalysts induces more or less significant enhancement of the ozonation process as compared to ozonation at elevated pH [89,90]. Nevertheless, only certain catalysts can produce almost total oxidation of the organic pollutants, because in most cases the reaction mixture still contains at least traces of non converted oxalic acid. Ozonation can be improved by using catalysts, such as dissolved metal ions for homogenous ozonation, or pure or mixed oxides like Fe2O3, MnO2, Ru/CeO2, alumina, metal-doped Al2O3, TiO2, graphite, zeolites and clay minerals for heterogeneous processes. To understand the catalyst role on the ozonation enhancement, a quick overview of the main catalytic ozonation processes performed so far would be useful.

7. CATALYSTS FOR OZONATION 7.1. Dissolved Metal Cation Catalysts In homogeneous ozonation processes, various dissolved metal cations like Ti2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+,...) have been tested so far [91]. Among all these catalysts, Fe2+ or Mn2+ cations appear to produce more effective ozonation than even with Fe3+or at elevated pH. In almost all cases, iron displays interesting performances in both dispersed and supported forms, as reported for other oxidative processes [17].

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7.2. Heterogeneous Catalysts

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Solid catalysts exhibited even higher effectiveness, inasmuch as Al2O3, TiO2 (anatase) and clay-supported metals produced complete ozonation of the organic contaminants into carbon dioxide [92]. The contribution of the solid catalyst to the ozonation enhancement was provided by attempts performed using ceramic honeycomb [93]. Indeed, oxalic acid was eliminated at room temperature in a proportion of ca. seventeen times higher than without catalyst. Nevertheless, the results provided by the authors appear to be contradictory, because both partial and total mineralization of oxalic acid were reported. Improved ozonation of many organic pollutants was also reported for alumina alone [94] or containing CuO [95]. It appears that the mere presence of dispersed solid surface enhances significantly ozonation, presumably by improving the ozone retention in water [13]. Heterogeneous catalysis of oxalic acid ozonation in aqueous solution is also possible on other metal oxides like MnO2 alone showed interesting activity in the pH range 3.2–7.0 [96]. On such a catalyst, ozonation appeared to take place through the formation of a surface Mn oxalic acid complex. As a confirmation of previous statements, pH greatly influences the catalyst activity and reaction mechanism pathway. Kinetics study of such a process [97] showed that pH fluctuations modify both the chemical structure of the active sites and dissociation equilibrium of oxalic acid. Partial deactivation of the catalyst active sites was observed during ozonation, likely due to metal leaching. The adsorption of oxalic acid turned out to be a kinetic determining step, and dissolved manganese was also involved in the catalytic ozonation of oxalic acid. The catalyst particle size played a key-role, and for values smaller than 10 μm, the kinetic model fit well the experimental data. Larger particles seems to cause mass-transfer limitations, due to internal and external diffusion hindrance for both reactants to the solid surface of MnO2.

7.3. TiO2 Catalysts and Derivatives TiO2 is well-known as being a photoactive catalyst for oxidation processes, but its use in ozonation is a new option to be explored, inasmuch as total decomposition of oxalic acid was reported in the presence of TiO2 [1,2,98,99]. Catalytic ozonation using TiO2 gave high oxalic acid decomposition yield, more particularly at increased ozone flow rate [99]. This indicates that the ozonation efficiency is still determined by the ozone solubility. The TiO2 surface contributes to the ozonation improvement, but does not display sufficient adsorption capacity for ozone. The highest oxalic acid removal yield obtained at pH 2 suggests that ozonation on TiO2 proceeds mainly via molecular ozone. Combination of many factors that promote oxalic acid ozonation may produce advantageous synergy. That is why photocatalytic ozonation of oxalic acid must undoubtedly be a more effective decomposition technique, which can be applied for total mineralization of organic contaminants. Indeed, the simultaneous presence of TiO2, UV radiation and ozone induces significant increase in the oxidation rate of oxalic acid, as compared to photocatalysis alone or ozonation alone, more particularly in acidic media (pH 3) [100]. Successful attempts using other titanium combinations like strontium titanate (SrTiO3) have been achieved in the catalytic ozonation of oxalic acid [101]. Other catalysts such as

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MO2/TiO2 and Rh/TiO2 produced thorough removal of oxalic acid at pH 2.5 [102]. Here also, the highest performances were obtained in acidic media.

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7.4. Carbon-Based Catalysts Carbon-based catalysts turn out to be quite effective in the elimination of oxalic acid via ozonation. Indeed, granulated activated carbon (GAC) was used in ozonation, with continuous regeneration by removing adsorbed organic species by the very ozonated liquid media [9-11]. Reportedly, almost thorough mineralization of oxalic acid is possible in the presence of activated carbon [8-11]. Here, the mechanism pathway is assumed to involve both surface phenomena and bulk water reactions. The oxidizing species may react with both adsorbed and dispersed oxalic acid. Nevertheless, these performances, more particularly those related to the removal of refractory by-products like oxalic acid are subject to controversy. Investigations have to be pursued in this regard. Platinum deposition on carbon nanotube (Pt-CNT) produced improved ozonation of oxalic acid, and this performance was explained in terms of an involvement of a Ptred/Ptox redox couple [103]. Pt-CNT catalyst did not undergo Pt leaching, and showed higher stability than those obtained via platinum deposition on activated carbon. The method used for the metal deposition was found to negatively influence the catalytic activity, inasmuch as calcination induced oxygen chemosorption on Pt clusters, affecting thereby the catalyst effectiveness. On such a catalyst, ozonation is supposed to proceed via ozone decomposition and hydrogen peroxide formation. Nearly thorough ozonation of oxalic acid (99.3%) was obtained on solid catalysts obtained through Pt deposition on graphite [104]. The amount of oxalic acid converted over graphite alone is ca. half of that obtained over Pt/graphite catalyst, but approximately sixteen times higher than that attained with ozone alone, suggesting a favorable synergy of Pt and graphite.

7.5. Aluminosilicate-Based Catalysts Total mineralization of rice husk ash (RHA) containing small amounts of MgO, P2O5, SO3, K2O, CaO, MnO2, Fe2O3, CuO, ZnO and only traces of carbon gave an effective catalyst for oxalic acid ozonation at pH 3 [105]. This catalyst produced more higher effectiveness that non catalytic ozonation at acidic, neutral and even alkaline pH. Nonetheless, no complete removal of oxalic acid was achieved. This must be due to the low ozone solubility, which still remains to be improved, as well supported by the ozonation enhancement by increasing the ozone throughput. Here, silica combinations with other metal oxides must play a key-role in the ozonation improvement. Since the RHA catalyst was found to be as effective as TiO2based catalyst, but more active than pure silica or alumina, solid catalysts based on mixed oxides such as aluminosilicates are also expected to exhibit higher performances. That is why zeolites and clay minerals have particularly drawn attention these last decades. Indeed, Y-type faujasite ion-exchanged with iron and cobalt (Fe-Co-NaY) gave high ozonation yields [12]. The fact that the individual Fe-NaY or Co-NaY catalysts showed lower activities suggests a possible synergy between both cations. Unlike zeolites, clay catalysts are more available and

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have expandable structures devoid of pore size constraints, which makes them to be regarded as promising catalysts. Indeed, effective catalytic oxidation of phenol was already reported for hydrotalcite [14,15] and montmorillonite [13,16] ion-exchanged by transition metals. Notwithstanding that clays minerals are supposed to generate highly reactive superoxide radicals [18], one strongly believes that the catalytic effect of clay minerals in ozonation still remains to be elucidated. In all cases, on solid catalysts, simultaneous contributions of both surface phenomena and bulk water reactions between oxidizing species and oxalic acid must be involved, but the contribution of the surface reaction still remains predominant [68]. Advanced catalytic ozonation requires less ozone, and is supposed to be effective in both acidic and alkaline media.

8. OZONATION OVER CLAY-BASED CATALYST

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8.1. Effect of the Solid Surface As a common feature, various solid like TiO2 [2], alumina alone [94], CuO-Alumina [95] or Fe2O3-Alumina [106], carbon-based materials, oxides in both pure and combined forms and others gave higher ozonation effectiveness as compared to homogeneous processes. Their increased activity was found to involve a surface reaction between non-adsorbed ozone and adsorbed oxalic acid. In almost all cases, this ozonation improvement was explained in terms of reagent adsorption, inasmuch as a direct proportionality appears to occur between the catalyst activity and specific surface area. However, the structure of the catalyst surface must also be taken into account, inasmuch as various catalysts showed different activity in oxalic acid ozonation. For instance, as already stated, Y-type zeolites containing both iron and cobalt displayed even higher activity than the mono-ionic forms [12], indicating a possible synergy between both cations. Here, iron seems to play a key-role, which still remains to be elucidated. Clay catalysts such as hydrotalcite [14,15] and montmorillonite ion-exchanged by transition metals, displayed appreciable activity in phenol oxidation [16]. Here also, the mere presence of clay minerals lamellae dispersed in the liquid media seems to improve the ozonation efficiency, mainly by reducing the ozone consumption. Similar phenomenon was noticed for alumina-catalyzed ozonation, where the ozone consumption was ca. two times lower than in the non catalytic route [94]. Because of the low solubility of ozone, this ozonation enhancement must be explained only in terms of surface reaction involving at least one adsorbed reagent. On clay-based catalysts, one of the possible interactions of ozone with the solid surface may proceed via adsorption followed by the production of oxygen molecules according to the following scheme: O3 + Clay →[O3 - Clay]

(33)

[O3 - Clay] →[O - Clay] + O2

(34)

O3 + [O – Clay] →2O2 + Clay

(35)

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On clay mineral surface, ozone is probably attracted by the framework oxygen atoms, especially those surrounding the ion-exchange sites, which display electronegativity and behave as electron-rich sites. A previous adsorption of oxalic acid is also possible, but this process must strongly depends on pH, and exerts direct influence on the further decomposition of the adsorbed ozone [107]. Oxalate anions (C2O42-), instead, are supposed to be attracted by the positive charges of the exchangeable cations, giving rise to oxalate–cation complexes, in agreement with other results [6]. On clay minerals, Oxalate-Meta-Clay complexes (OMC) may form through the bridging effect of bivalent or multivalent cations (M2+), which is expected to occur between oxalate anion and the negatively charged surface of the clay mineral. In this case, two or many exchangeable sites must be involved according to the reactions 36 and 37. − − [M2+-(O-Clay )2] → [M+-(O-Clay )2]+

(36)

− − C2O42- + [M+-(O-Clay )2]+ → [C2O4 ---M---(O-Clay)2]

(37)

− − [C2O4 ---M---(O-Clay)2]-→[C2O42—M2+]Solid + (O-Clay )2

(38)

The formation of metal-oxalate precipitate is also possible (reaction 38), depending on the pH level in the reaction mixture. The latter is expected to strongly influence not only the coagulation-flocculation capacity of oxalic acid, but also the net charge and zeta potential of the clay mineral, along with the cation exchange process. Decreasing pH should induce an increase of the density of positive charges on the clay surface, and thereby an enhancement of oxalate anions adsorption. Reportedly, titania catalyst, which has an isoelectric point close to that of clays, promotes similar oxalate adsorption [108]. Besides, other interactions may also occur between the ozonation reagents and the clay mineral surface. For instance, molecular oxalic acid may also interact via coulombic attraction with the anionic surface, while the hydrocarbon chain of oxalic acid may merely adsorb through van der Waals interactions

8.2. Role of Ph on the Clay Behavior and Reagent-Surface Interactions Oxalic acid is an acidic species, and the catalyst surfaces often display acid-base properties. Consequently, pH is expected to play a key-role in the interactions occurring between the dispersed reagents and catalyst surface. The catalyst activity in ozonation strongly depends on the pH of the reaction mixture [94]. Regardless to the catalyst type, ozone is capable of oxidizing organic compounds via two different reaction mechanisms according to the pH level. Below neutral pH, ozonation is supposed to involve only molecular O3. Higher pHs, instead, are expected to generate OH‖ hydroxyl radicals via ozone decomposition [109,110]: − 3O3 + OH +H+ →2OH* + 4O2

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Reportedly, the photocatalytic decomposition of organic substrates in the presence of dispersed ZnO [111] and ozonation of chlorinated compounds [7] proceed through similar reaction pathways. Both molecular ozone and radical mechanisms are expected to occur simultaneously during oxalic acid ozonation, and the pH level appears to favor one pathway at the expense of the other. Similar observations have been reported in the presence of clay catalysts [13]. pH was found to determine the net charge and zeta potential on the clay lamellae surface, and consequently the levels of exfoliation and dispersion of the clay mineral. This ought to control the accessible surface area and the possible adsorption and other surface processes. Hydroxyl ions behave as OH* radical precursors, and a direct proportionality was found between the alkalinity of the reaction mixture and the production of OH* radicals [109]. The latter are well known to be much more reactive towards the organic substrate than molecular ozone [3]. Thus, in agreement with other results reported for phenol ozonation [110], increasing pH will undoubtedly influence ozonation. Hydroxyl ions may also react with ozone [112], yielding other reactive species (O2*– and HO2*) [3,107]. Nevertheless, the low solubility of ozone causes this reaction to be a determining step for ozonation kinetics [110]. The wide literature available in this regard often provides contradictory data related to the improvement of catalytic ozonation of oxalic acid with increasing pH. Now, it is well established that different initial pH‘s should result in different mechanism pathways that produce different ozonation products. This is why, it is strongly believed that pH changes within large variation ranges must be subject to controversy, because the effectiveness of the ozonation cannot be compared for different reaction pathways. In all cases, a better understanding of the rôle of pH requires a judicious and deep analysis of ozonation in acid media separately of that achieved at elevated pHs.

8.2.1. Ozonation at Low pH on Clay-Based Catalysts Even below neutral pH, the role of increasing pH on oxalic acid ozonation is still unclear. With activated carbon catalysts [8], no radical action of ozone and no formation of free OH* radicals were involved in acidic media [8,68]. This indicates that ozonation of oxalic acid proceeds predominantly through molecular ozone [112]. This is also consistent with other results reported for the ozonation of oxalic acid with cobalt(II) [6] or Fe(III) catalysts [5]. Oxalic acid was almost totally decomposed in the presence of CuO/Al2O3 [95] or Fe2O3/Al2O3 [106] in water, more particularly at low initial pH values. On the surface of clay minerals, low pHs provide high proton amounts that ought to shift equilibrium towards proton adsorption, and the clay particles become positively charged. This should induce an increase of the net charge on the clay surface, enhancing thereby the electrostatic repulsion and dispersion of the clay mineral lamellae, and improving the accessible surface area [13]. If adsorption is involved, as long as pH is moderately low, for instance around pH 4, adsorption is favored and oxidation via molecular ozone is improved. Even with a different structure, TiO2 produced signification conversion yield during ozonation of oxalic acid, more particularly at acidic pH, the highest level being obtained at pH [99]. This provides evidence that TiO2 induces a catalytic effect, presumably by improving ozone retention and/or oxalic acid adsorption. By analogy to clay minerals, ozonation on TiO2 must also proceed via molecular ozone, and increasing pH should alter the ozonation effectiveness.

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In the presence of montmorillonite exchanged with transition metal cations [13], the general tendency is that increasing pH up to neutral appeared to attenuate the ozonation of oxalic acid. Similar observations were made for the nitrobenzene ozonation with MnOcontaining catalysts [107]. This suggests the occurrence of a threshold pH around pH 6-7, beyond which change in the reaction pathways should take place after thorough suppression of the direct action of molecular ozone. However, a dramatical decay of the ozonation effectiveness takes place at excessively low pHs, as reported for oxalic acid ozonation on clay-based catalysts at pHs below 2.5 – 3.0 [13]. Such low pH levels are supposed to induce changes in the compositions of both the catalyst and liquid media via ion-exchange, protonation and possibly even through an autotransformation of the clay catalysts. Thus, it results that low pH must favor the direct attack of oxalic acid by molecular ozone, as long as the acidity of the reaction mixture does not alter the structure of the clay catalyst. However, for montmorillonite-based catalysts, pH changes unavoidably take place during the ozonation of oxalic acid, explaining thereby the decay in time of the ozone efficiency. The use of buffers could be a convenient route to overcome this drawback [7], provided that the buffer addition does not hinder the ozonation process [95]. Deeper insights into the buffer influence are necessary in this regard.

8.2.2. Ozonation at Elevated pH on Clay-Based Catalysts As already stated, both molecular ozone and radical mechanisms are expected to occur simultaneously during oxalic acid ozonation. Therefore, regardless to the type of catalysts, increasing pH is expected to favor specifically the radical pathway, by generating hydroxyl radicals via ozone decomposition [7,109-111]. The amount of OH* radicals in the ozonation process appears to vary almost linearly with the alkalinity of the reaction mixture [109]. Hydroxyl ions are regarded as being precursors of OH* radicals, which are supposed to be more reactive than ozone towards the organic substrate. The rate of OH* radicals attack was estimated to be 106 - 109 times faster than that of molecular ozone [3]. Hence, one must expect that increasing pH till neutral and beyond will influence ozonation, at least in terms of the decomposition rate of dissolved ozone. This finding was confirmed later by the significant increase of the phenol ozonation rate when pH is raised from 2 to 12 [110]. Furthermore, hydroxyl ions is also supposed to react readily with ozone [110], giving rise to very reactive species like super-oxide anion O2*–, hydroperoxyl HO2* and OH* radicals [3]. This is consistent with other results [107], although slight discrepancies arise in the ozonation effectiveness due to specific catalytic pathways. In spite of the apparent favorable effect of hydroxyl ions, their reaction with ozone seems to be a kinetics-determining step [110], most likely due to the low solubility of ozone. Nonetheless, increasing pH beyond 4 or 5 induces cation exchange, and appears to promote precipitation of the free cations into hydroxides [113]. As already stated, this should lead unavoidably to changes in the compositions of both the liquid media and clay catalysts, and these changes are expected to evolve in time with the progress of the ozonation process. The subsequent decrease in the net charge and zeta potential ought to promote coagulationflocculation of the clay mineral, and reduce the number of clay lamellae with accessible surface, affecting thereby the adsorption process and the ozonation efficiency. Here, the coagulation capacity of the clay mineral is strongly dependent on the charge compensating

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cation. As a general feature, Na-exchanged montmorillonite (NaMt) showed much lower catalytic activity in the ozonation of oxalic acid than when exchanged with transition metal (TM-Mt) [13]. This may be explained in terms of high coagulation-flocculation capacity of NaMt as compared to TM-Mt catalysts. This is consistent with similar observations already reported for other clay-based catalysts [114]. In addition, ozonation seems to be difficult to achieve at high pH levels without inducing modifications in the compositions of both the liquid media and catalyst. Cations belonging to the catalyst surface can be prone to possible release and precipitation processes [113,115]. Detrimental effect was also reported for the photocatalytic activity of ZnO, due to an alteration of the catalyst structure [111]. Increasing amounts of carbonates are produced by the progressive formation of carbon dioxide, inducing continuous cation release and modification of the ionic force in the liquid solution. This may affect the surface area and activity of the clay catalyst, unless the clay buffer capacity is high enough to resist pH change, at least within a narrow variation range. That is why, in spite of the wide literature available, so far it is still not clear whether excessively increased pH‘s have favorable influence upon ozonation. However, unless oxalic acid is totally decomposed into carbon dioxide, radical ozonation of organic substrate at pH beyond neutral cannot be applied to any water treatment, because radical species are non selective and may produce undesired toxic by-products. In addition, the difficulty in controlling the alkalinity of the reaction mixture may cause technology limitations, more particularly in drinking water production. That is why oxidative treatments in alkaline media do not seem to be economical and ecological alternatives.

8.2.3. Adsorption and OH* Radical Contributions The ozonation mechanisms through heterogeneous catalysis are still not clearly elucidated, and many scientists assume the involvement of an adsorption of organic substrates and/or ozone on the catalyst surface. For instance, phenol ozonation on alumina [116] is assumed to take place via the adsorption of polar organic intermediates arising from direct ozonation of phenol and the formation of OH* and HO2* radicals. In the presence of TiO2 catalyst [98], ozonation of oxalic acid is supposed to involve not only OH* radicals, but also an adsorption of oxalic acid on the catalyst surface, followed by ozone attack. As a general feature, ozonation on solid catalysts must occur through: 1. Organic substrate adsorption or concentration at the catalyst surface; 2. Oxidation of the organic substrate adsorbed or concentrated in the vicinity of the catalyst surface by molecular ozone; 3. Ozone decomposition on the catalytic sites, if any, to generate more reactive radicals. More specifically on TiO2 [117], ozonation may involve various but simultaneous processes, namely: 1. a mere ozonation reaction between dispersed species in the bulk aqueous phase; 2. adsorption of polar organic substrates on the catalyst surface; 3. oxidation of the adsorbed substrate by the supported metal catalyst, if any, or by OH* radicals released in the reaction.

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Regardless to the ozonation mechanism pathway implied, the attack by OH* radicals, if any, should occur much faster than that of molecular ozone on oxalic acid in both adsorbed and dispersed forms. The rate constant of the reaction between hydroxyl radical and oxalic acid is of ca.107 L.mol-1.s-1.[117]. This value is in the same magnitude as that of acetic acid (1.6 107) and to a lesser extent of formic acid (1.3 108), but is much lower than those of many other organic substrates like alcohols, aldehydes, heavier carboxylic acids, carbohydrates, aromatics and derivatives. This confirms once again that short chain acids like oxalic acids are relatively difficult to oxidize, and thorough mineralization requires more effective oxidative routes.

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8.3. Role of the Exchangeable Cation The specific activity of the charge-compensating cations must strongly influence the ozonation catalytic pathway. However, the rôle of the exchangeable cations cannot be dissociated from that of pH, and there exists a strong interdependence between both parameters. At high pH, the exchangeable cations are supposed to yield reactive superoxide radicals [18], but at acidic pH, they presumably combine to oxalate anion adsorbed on the clay mineral surface [13]. The formation of cation(II)-oxalate complex in the vicinity of the clay surface is well supported by the high chelating capacity of oxalate for metal cations [6]. Such a chelating process must strongly depend on the mobility and availability of the exchangeable cation. According to the pH level, clay minerals, more particularly in the mono-ionic form, may be prone to ion-exchange, releasing the charge-compensating cations in equilibrium with the liquid reaction mixture [118]. Slight change in the reaction mixture composition may displace equilibrium towards a release or an uptake of the exchangeable cations. For instance, low amounts of clay minerals in the ozonation mixture ought to favor cation release in the liquid media [115]. Therefore, it clearly appears that at acidic pH, the exchangeable metal cations always display a certain mobility within a critical layer in the vicinity of the clay surface. For a given pH and a given clay mineral, various exchangeable cations are expected to behave differently, being more or less retained on the clay surface [113,115]. Consequently, the attraction strength of the clay mineral towards each cation must strongly depend on the pH level of the ozonation mixture [113]. Conversely, increasing pH ought to reduce the cation mobility in the neighborhood of the clay surface, and less free cations may be prone to the formation of the cation-oxalate complex [113,115]. Even at a given pH, each metal cation exhibits a specific catalytic activity in the ozonation of oxalic acid. Therefore, the various cation(II)-oxalate complexes generated will differ in terms of both number and reactivity towards ozone on the clay surface. As a result, both the amount and nature of the exchangeable cations must play key-roles in the oxalic acid ozonation efficiency, because for each exchangeable cation there must exist a specifical equilibrium concentration. However, the role of clay-based catalysts in the ozonation of oxalic acid must necessarily be correlated to the influences of both the exchangeable cation and pH, because strong synergy are possible. These parameter interactions still remain to be elucidated and investigations must be pursued in this regard.

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CONCLUSION The data reviewed herein allow to conclude that total oxidation of oxalic acid is often difficult to achieve, due to the relatively high chemical stability of this compound, as compared to other organic substrates. The wide literature examined show clearly that attempts through photochemical and photocatalytic methods failed due to their low performances and high cost limitations. More powerful oxidative techniques such as combined ozonation methods using hydrogen peroxide, UV radiations and/or catalysts, at different pH levels showed promising prospects, but still display major shortcomings mainly due the low solubility and reactivity of ozone. Hence, improvements of the mass transfer of ozone and/or oxalic acid towards the reaction interface has become an essential requirement to achieve thorough mineralization of oxalic acid into carbon dioxide. In this regard, dissolved cations showed interesting catalytic activity, but may cause water contamination by metals. Reportedly, certain water mixtures with non-polar fluorinated hydrocarbons gave ozone solubility almost ten times higher than in water alone, but oxalic acid was not totally converted. Solid catalysts such zeolites and clay minerals have particularly drawn attention, by displaying even higher activities in the ozonation of oxalic acid in water at room temperature. This effectiveness improvement was explained in terms of reagent adsorption. In the presence of solid catalysts, ozonation may involve not only a reaction between bubbled ozone and dissolved oxalic acid in the bulk solution, but also a surface reaction between adsorbed and dispersed reagents. The presence of transition exchangeable cations, more particularly iron, was found to induce significant enhancement of the ozonation process. Various exchangeable cations showed different catalytic activities, and Fe (II)-ion exchanged montmorillonite gave the highest oxalic acid conversion yields. pH appears to strongly influence the ozonation of oxalic acid, by modifying the chemical compositions of both the liquid media and catalyst. At acidic pH, a clay-ozone synergy must be involved, due not only to ozone adsorption on the catalyst surface, but also to the interactions occurring between the metal cation and oxalate anion in the vicinity of the clay surface. pHs around 3-4 are regarded to be optimal for a total ozonation of oxalic acid, presumably due to a series of phenomena, such as: i. enhancements in the clay exfoliation and adsorption process; ii. increases in the density and reactivity of the hypothetical cationoxalate catalytic sites. Given the relatively high chemical stability of oxalic acid, moderately effective methods for oxalic acid ozonation can be used for total decomposition of other less stable organic substrates. Clay minerals, even in the native form, also produced appreciable oxalic acid conversion, without any purification or other previous treatments. This provides a proof-of-concept of a synergic effect of the clay-ozone system on the total removal of oxalic acid. Clay-based catalysts are supposed to generate highly reactive species, but the mechanism pathway still remains to be deeply investigated. Research is still in progress in this direction. The high availability of clay minerals, their low operating costs and negligible environmental impact are favorable arguments to envisage a development and implementation of economical and ecological technologies for total decomposition of organic pollutants.

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REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

D. Chatterjee, S. Dasgupta, R.S. Dhodapkar and N.N. Raob, J. Mol. Catal. A: Chem. 260, 264 (2006). F.J. Beltran and F.J. Rivas, R. Montero-de-Espinosa, Appl. Catal. B: Environmental 39 (3), 221 (2002). R. Munter, Proc. Estonian Acad. Sci. Chem. 50(2), 59 (2001). A. Taicheng, L. Guiying, Z. Xihai, F. Jiamo, S. Guoying and K. Zhu, Appl. Catal. A: General 279(1-2), 247 (2005). X.F. Zhu and X.H. Xu, Journal of Zhejiang University Science 5(12),1543 (2004). D.S. Pines and D.A Reckhow, Environ. Sci. Technol. 36(19), 4046 (2002). D. Bhattacharyya, T.F. Van Dierdonck, S.D. West and A.R. Freshour, J. Haz. Mat. 41(1), 73 (1995). F.J. Beltran, F.J. Rivas, L.A. Fernandez, P.M. Alvarez and R. Montero-de-Espinosa, Ind. Eng. Chem. Res. 41, 6510 (2002). B. Pinker, and W.D. Henderson, Proc. Reg. Conf. Ozone, UV-light, AOPs Water Treatm., Sept 1996, (Amsterdam, Netherland), pp. 307–318. J.P. Kaptijn, Ozone: Sci. Eng. 19, 297 (1997). U. Jans and J. Hoigne, Ozone: Sci. Eng. 20, 67–90 (1998). J.N. Park, J. Wang, K.Y Choi, D.Y. Dong, S.I. Hong and C.W. Lee, J. Mol. Catal. A: Chemical 247, 73 (2006). A. Azzouz, A. Kotbi, P. Niquette, A.V. Ursu, F. Monette and R. Hausler, Reaction Kinetics, Mechanisms and Catalysis 99(2), 289-302 (2010). K.Z. Zhu, C.B. Liu, X.K. Ye and Y. Wu, Appl. Catal. A: Gen. 168(2), 365 (1998). A. Dubey, V. Rives and S. Kannan, J. Mol. Catal. A: Chem. 181, 151 (2002). V. Ramaswamy, M.S. Krishnan and A.V. Ramaswamy, J.Mol. Catal. A: Chem. 181(1), 81 (2002). S. Perathoner and G. Centi, Topics in Catalysis 33 (1-4), 207-224 (2005). H.H. Chung, J. Jung, J.H. Yoon and M.J. Lee, Catalysis Letters 78 (1-4), 77 (2002). W. Riemenschneider and M. Tanifuji, Oxalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim (Germany), 2002. U. Hamer and B. Marschner, Soil Biology and Biochemistry 37(3), 445 (2005). J. Higgins, X. Zhou, R. Liu and T.S.T. Huang, J. Phys. Chem. A 101 (14), 2702 (1997). K.A. Kovacs, P. Grof, L. Burai and M. Riedel, J. Phys. Chem. A 108, 11026 (2004). M. Kubota, J. Radioanal. Nucl. Chem. 75 (1-2), 39 (1982). W.C. Becker and C.R. O'Melia, Ozone: Sci. Eng. 18 (4), 311 (1996). M. Dükkanci and G. Gündüz, Ultrasonics sonochemistry 13(6), 517 (2006). O.S. Gal and B.B. Radak, Intern. J. Radiat. Phys. Chem. 7(4) 584 (1975). C. Tien and G. Thodos, Chem. Eng. Sci. 13(3), 120 (1961). K.K. Jain, G. Prasad and V.N. Singh, J. Chem. Technol. Biotech. 29(1), 36 (1979). B. Nitsch and H. J. Kutzner, Zeitschrift für allgemeine Mikrobiologie 9(8), 613 (1969). P. Kämpfer, The Family Streptomycetaceae, Part I: Taxonomy". The prokaryotes: a handbook on the biology of bacteria, Springer (Dworkin, M et al., eds.). Berlin (2006). E. Espejo and E. Agosin, Appl. Environ Microbiol. 57(7), 1980 (1991). D. Ochi, Japan Patent No. JP2009/066881, April 8, 2010.

Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

28 [33] [34] [35] [36] [37] [38] [39] [40] [41]

[42]

[43] [44]

Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved.

[45] [46]

[47] [48] [49]

[50] [51] [52] [53]

[54] [55] [56] [57] [58]

Abdelkrim Azzouz P.S. Watts, Australian J. Agr. Res. 8(3), 266 (1957). A.J. Duncan, P. Frutos and S.A. Young, Animal Science 65, 451 (1997). H. Ito, T. Kotake and M. Masai, Intern. J. Urolog. 3(3), 207 (1996). A. Kurosumi, E. Kaneko and Y. Nakamura, Earth Env. Sci. Biodegrad. 19(4), 489 (2008). T. Betsche and B. Fretzdorff, J. Agr. Food Chem. 53(25), 9751 (2005). C. Thompson, J.M. Dunwell, C.E. Johnstone, V. Lay, J. Ray, M. Schmitt, H. Watson and G. Nisbet, Euphytica 85(1-3), 169 (1995). W.R. Hiatt and J.L. Owades, United States Patent No. 4652452, march 3, 1987. H.J. Fenton, J. Chem. Soc. 65, 889 (1884). S. Esplugas, A. Marco and E. Chamarro, Use of Fenton reagent to improve the biodegradability of effluents, Proc. Int. Reg. Conf. Ozonation and AOPs in Water Treatm., Sept. 1998 (Poitiers, France) pp. 20-1–20-4. G.F. Ijpelaar, R.T. Meijers, R. Hopman and J.C. Kruithof, Oxidationof herbicidesin groundwater by the Fenton process: A realistic alternative for O3/H2O2 treatment, Proc. Int. Reg. Conf. Ozonation and AOPs in Water Treatm., Sept. 1998 (Poitiers, France) pp. 19-1–20-1. M. Trapido, Y. Veressinina and R. Munter, J. Environ. Eng. 124, 690 (1998). M. Trapido and A. Goi, Degradation of nitrophenols with the Fenton reagent. Proc. Estonian Acad. Sci. Chem. 48, 163 (1999). R.J. Watts, M.D. Udell and R.M. Monsen, Water Environ. Res. 65, 839 (1993). D. Fassler, U. Franke and K. Guenther, Advanced techniques in UV-oxidation, Proc. Eur. Workshop Water Air Treatm. AOT, Oct. 11–14, 1998 (Lausanne, Switzerland) pp. 26–27. [47] G. Ruppert, R. Bauer, G. Heisler and S. Novalic, Chemosphere 27, 1339 (1993). Y. Sun and J.J. Pignatello, Environ. Sci. Technol. 27(2), 304 (1993). C. Gottschalk, J.A. Libra and A. Saupe, Ozonation of Water and Waste Water. WileyVCH, Weinhein, Germany (2000). Hoigne, J. Mechanisms, rates and selectivities of oxidations of organic compounds initiated by ozonation of water, In Handbook of Ozone Technology and Applications. Ann Arbor Science Publ., Ann Arbor, MI, 1982. R. Zepp, B. Faust and J. Hoigne, Environ. Sci. Technol. 26, 313 (1992). A. Safarzadeh-Amiri, J. Bolton and S. Cater, Water Res. 31, 787 (1997). S. Muthu, P. Maruthamuthu and P.R. Vasudeva Rao, Fullerenes, Nanotubes and Carbon Nanostructures 1(4), 481 (1993). J.F. Klausner and D.Y. Goswami, Solar detoxification of wastewater using nonconcentrating reactors, AlChE Symp. Ser., Heat Transfer, 89, 1995 (Atlanta, USA) pp.445–451. P. Wyness, J.F. Klausner, D.Y. Goswami and K.S. Schanze, J. Sol. Energy Eng. 116, 12 (1994). J. Bedford, J.F. Klausner, D.Y Goswami and K.S. Schanze, J. Sol. Energy Eng. 116, 8 (1994). R.W Matthews, Water Res. 20, 569 (1986). M. Mehhjouei, S. Müller, D. Möller, J. Photochem. Photobiol. A: Chem. 217(2-3), 417 (2011). M.M. Kosani. J. Photochem. Photobiol. A: Chem. 119(2), 119 (1998).

Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

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Advances in Oxidative Decomposition …

29

[59] G. Waldner, M. Pourmodjib, R. Bauer and M. Neumann-Spallart, Chemosphere 50(8), 989 (2003). [60] P.S. Bailey, Ozonation in Organic Chemistry, Vol. 1, Academic Press Inc., New York (1978). [61] M. Doré, Chimie des oxydants et traitement des eaux, Lavoisier, Paris (1989). [62] A.H. Riebel, R.E. Erickson, C.J. Abshire and P.S. Bailey, J. Amer. Chem. Soc. 82, 1801 (1960). [63] F. Biasotto, Étude du mécanisme d’action d’un catalyseur solide lors de l’ozonation de la matière organique dans le traitement des eaux usées industrielles, PhD Thesis, University of Quebec at Montreal, 2002, pp 5 – 27. [64] J. Hoigné and H. Bader, Vom Wasser 48, 283 (1977). [65] J.H. Carey, Water Pollut. Res. J. Can. 27, 1 (1992). [66] T.Y. Way and C.C. Wan, Ind. Eng. Chem. Res. 30, 1293 (1991). [67] F.P. Logemann and J.H.J. Annee, Water Sci. Technol. 35(4), 353 (1997). [68] S. Preis, M. Krichevskaya, Y. Terentyeva, A. Moiseev and J. Kallas, J. Adv. Oxid. Technol. 5, 1 (2000). [69] M. Krichevskaya, T. Malygina, S. Preis and J. Kallas, Photocatalytical oxidation of deicing agents in aqueous solutions and aqueous extract of jet fuel, Proc. 2nd Int. Conf. Oxidation Technologies for Water and Wastewater Treatment, May 2000 (ClausthalZellerfeld, Germany) pp. 6–12. [70] S. Preis, Y. Terentyeva and A. Rozkov, Water Sci. Technol. 35, 165 (1997). [71] S. Preis, M. Krichevskaya and A. Kharchenko, Water Sci. Technol. 35, 265 (1997). [72] M. Koch, A. Yediler, S. Lienert, G. Insel and A. Kettrup, Chemosphere 46(1),109 (2002). [73] N. Nilvebrant and A. Reimann, Xylan as a source for oxalic acid during ozone bleaching, 4th European Workshop on Lignocellulosics and Pulp, Sept. 1996 (Stresa, Italy) pp. 485-491. [74] M. Ristolainen and R. Alén, Characterization of effluents from TCF bleaching of hardwood kraft pulp, Intl. Pulp Bleaching Conf. April 1998 (Washington, USA), Book 2, pp 523-525. [75] A.K. Holen, P.J. Kleppe and S.T. Moe, Reaction products from ozonation of dissolved carbohydrates, Proc. 1998 Int. Pulp Bleaching Conference, 1998 (Helsinki, Finland). [76] G. Annergren and P. Sandström, A bleach plant designed for closure, Congrès Zellcheming-Hauptversammlung 91, 1996 (Baden-Baden, Germany) 1996, 50 (10A), pp. V12-V16. [77] P. Ulmgren, Nordic. Pulp. Paper Res. J. 12(1), 32 (1997). [78] W.H. Glaze, J.W. Kang and S.S. Ziegler, Treatment of hazardous waste chemicals using AOPs, Proc. 10th Ozone World Congr., March 1991 (Monaco, France) pp. 261–279. [79] T. Tuhkanen, Oxidation of Organic Compounds in Water and Waste Water with the Combination of Hydrogen Peroxide and UV Radiation. Ph.D. Thesis, University of Kuopio, 1994. [80] W.H. Glaze, J.W. Kang and D.H. Chapin, Ozone: Sci. Eng. 9, 335 (1987). [81] M. Trapido, Y. Veressinina and R. Munter, Proc. Estonian Acad. Sci. Chem. 43, 61 (1994). [82] M. Trapido, Y. Veressinina and R. Munter, Environ. Technol. 16, 729 (1995).

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Abdelkrim Azzouz

[83] M. Trapido, Y. Veressinina and J. Kallas, Degradation of nitrophenols by ozone combined with UV-radiation and hydrogen peroxide, Proc. Int. Conf. on Application of Ozone and also on UV and Related Ozone Technologies at Wasser, October 2000 (Berlin, Germany) pp. 421– 435.]. [84] R. Munter, J. Kallas, S. Preis, S. Kamenev, M. Trapido and Y. Veressinina, Comparative studies of AOP for aromatic and PAH destruction, Proc. 12th World Ozone Congr., May 1995 (Lille, France) pp 395–406. [85] M. Trapido and J. Kallas, Environ. Technol. 21, 799 (2000). [86] H. Paillard, R. Brunet and M. Doré, Water Res. 22, 91 (1988). [87] J.P. Duguet, E. Brodard, B. Dussert and J. Mallevialle, Ozone: Sci. Eng. 7, 241 (1985). [88] S. Cortes, J. Sarasa, P. Ormad, R. Gracia and L. Ovelleiro, Comparative efficiency of the systems O3/high pH and O3/CAT for the oxidation of chlorobenzenes in water, Proc. Int. Reg. Conf. Ozonation and AOPs in Water Treatm., Sept. 1998 (Poitiers, France) pp14-1–15-1. [89] N. Karpel Vel Leitner, F. Delanoe, B. Acedo, F. Papillault and B. Legube, Catalytic ozonation of succinic acid in aqueous solution: A kinetic approach, Proc. Int. Reg. Conf. Ozonation and AOPs in Water Treatm., Sept. 1998 (Poitiers, France) pp.15-1–161. [90] R. Garcia, J.L. Aragues and J.L. Ovelleiro, Ozone: Sci. Eng. 18(3), 195 (1996). [91] B. Legube, B. Delouane, N. Karpel Vel Leitner and F. Luck, Catalytic ozonation of salicylic acid in aqueous solution: Efficiency and mechanisms, Proc. Reg. Conf. Ozone, UV-light, AOPs Water Treatm., Sept.1996 (Amsterdam, Netherlands) pp.509– 514. [92] L. Zhao, Z.Z. Sun and J. Ma, Huan Jing Ke Xue 28(11), 2533 (2007). PMID: 18290478 [Abstract]. http://www.ncbi.nlm.nih.gov/pubmed/18290478. [93] C.H. Ni and J.N. Chen, Water Science and Technology 43(2), 213 (2001). [94] P. Yunzheng, M. Ernst and J-C. Schrotter, Effect of phosphate buffer upon CuO/Al2O3 and Cu (II) catalyzed ozonation of oxalic acid solution, Proc. International Ozone Association 25(5), 393-397 (2003). World Congress No16, Sept. 2003 (Las Vegas, USA) 2003. [95] R. Andreozzi, A. Insola, V. Caprio, R. Marotta and V. Tufano, Appl. Catal. A: General 138 (1), 75 (1996). [96] R. Andreozzi, V. Caprio, A. Insola, R. Marotta and V. Tufano, Ind. Eng. Chem. Res. 36 (11), 4774 (1997). [97] H. Paillard, M. Doré and M. Bourbigot, Prospect concerning applications of catalytic ozonation in drinking water treatment, Proc. 10th Ozone World Congr., March 1991 (Monaco, France), Vol. 1, pp. 313–331. [98] H. Guo, Z.Y. Bao and Y.M. Dong, Technology of Water Treatment 2008-03. [Abstract]. http://en.cnki.com.cn/Article_en/CJFDTOTAL-SCLJ200803018.htm [99] G. Marcì, E. García-López and L. Palmisano, J. Appl. Electrochem. 38(7), 1029 (2008). [100] J.J. Wu, M. Muruganandham, L.T. Chang, G.J. Lee, V.N. Batalova and G.M. Mokrousov, Ozone: Sci. Eng. 33 (1), 74 (2011). [101] C. Quispe, J. Villasenor, G. Pecchi and P. Reyes, J. Chil. Chi. Soc. 51(4), 1049 (2006). [102] Z.Q. Liu, J. Ma and Y.H. Cui, Carbon 46 (6), 890 (2008). [103] Z.Q. Liu, J. Ma, and L. Zhao, Huan jing ke xue Huanjing kexue bian ji Zhongguo ke xue yuan huan jing ke xue wei yuan hui Huan jing ke xue bian ji wei yuan hui 28(6),

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Advances in Oxidative Decomposition …

[104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116]

1258 (2007). [Abstract]. PubMed ID: 17674732. http://en.cnki.com. cn/Article_en/CJFDTOTAL-SCLJ200803018.htm. J.J. Wu, S.H. Chen and M. Muruganandham, Ind. Eng. Chem. Res. 47(9), 2919 (2008). F.J. Beltran, F.J. Rivas and R. Montero-De-Espinosa, Water research 39(15), 3553 (2005). J. Ma, M. Sui, T. Zhang and C. Guan, Water Research 39(5), 779 (2005). J. Bangun and A.A. Adesina, Appl. Catal. A: General 175(1), 221 (1998). E.E. Chang, P.C. Chiang and I. Shu Li, Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management 11(1), 20 (2007). T. Poznyak, R. Tapia, J. Vivero and I. Chairez, J. Mex. Chem. Soc. 50(1), 28 (2006). C. Hariharan, Appl. Catal. A: General 304, 55 (2006). C.D. Adams, and S. Gorg, J. Envir. Eng. 128(3), 293 (2002). R.N. Yong and Y. Phadungchewit, Can. Geotech. J. 30(5), 821 (1993). M. Chorom and P. Rengasamy, European Journal of Soil Science 46(4), 657 (1995). P. Moss, Plant and Soil 18(1), 124 (1963). N. Alhayek, B. Legube and M. Doré, Environmental Letters 10, 415 (1989). B. Delouane, Contribution à l’étude de l’oxydation de la matière organique, PhD Thesis, University of Poitiers, France, 1994. P. Rengasamy, European Journal of Soil Science 34(4), 723 (1983).

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Copyright © 2012. Nova Science Publishers, Incorporated. All rights reserved. Advances in Chemistry Research, Nova Science Publishers, Incorporated, 2012. ProQuest Ebook Central,

In: Advances in Chemistry Research. Volume 14 Editor: James C. Taylor

ISBN: 978-1-61942-327-5 © 2012 Nova Science Publishers, Inc.

Chapter 2

PHYSICO-CHEMICAL PROPERTIES OF ACRYLAMIDE Mohammad Reza Saboktakin Ph.D of Organic & Physical Chemistry Islamic Republic of Iran

1. INTRODUCTION

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Synonyms: 2-propenamide, ethylene carboxamide, acrylic acid amide, vinyl amide, propenoic acid amide. CAS No.: 79-06-1. Molecular mass 71.09.

Acrylamide appears as a white crystalline solid, is odorless and has high solubility in water (2155 g/L water). Melting point 84.5 °C, boiling point (25 mmHg) 125 °C (192.6°C at atmospheric pressure). Acrylamide is a reactive chemical, which is used as monomer in the synthesis of polyacrylamides used e.g. in purification of water, and in the formulation of grouting agents. Acrylamide is known as a component in tobacco smoke. Acrylamide is primarily reactive through its ethylenic double bond. Polymerisation of acrylamide occurs through radical reactions with the double bond. Acrylamide could also react as an electrophile by 1,4-addition to nucleophiles, e.g. SH- or NH2-groups in biomolecules. Acrylamide is metabolised in the body to glycidamide, a reactive compound formed through epoxidation of the double bond. The toxicological effects of acrylamide have been studied in animal models. Exposure to acrylamide leads to DNA damage and at high doses neurological and reproductive effects have been observed. Carcinogenic action in rodents has been described but carcinogenicity to humans has not been demonstrated in epidemiological studies, although it cannot be excluded. The International Agency for Research on Cancer (IARC) has classified acrylamide as ‖probably carcinogenic to humans‖ (Group 2A). Neurological effects have been observed in humans exposed to acrylamide. Properties, use and toxic effects of acrylamide are reviewed by IARC (1) and EU (2).

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Mohammad Reza Saboktakin

Acrylamide (AA) is produced at higher temperatures when preparing foods containing both proteins and carbohydrates. In addition, tobacco smoke also contains AA. In animal experiments, AA was found to be clearly carcinogenic, producing a large number of different cancer localisations. For reasons of health prophylaxis and as a result of the most recent investigations on human metabolism, it can be assumed that AA is also carcinogenic in humans. These facts make acrylamide one of the most important environmental carcinogens and are the reason for its relevance in environmental medicine. It is possible to establish the current body burden of humans via various exposure routes by determining the acrylamidehaemoglobin adduct (N-2- carbamoylethylvaline: AAVal) in the blood. In this opinion, the Human Biomonitoring Commission describes the background exposure of the non-smoking general population in Germany based on the AAVal contents in the blood. Compared with these background values, individual, cause-related human biomonitoring results can be evaluated.

2. PHYSICO-CHEMICAL PROPERTIES Acrylamide (AA) is a white, crystalline solid at room temperature. Its melting point is 84°C. The physico-chemical properties of acrylamide are shown in Table 1. Table 1. Physico-chemical properties of acrylamide

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Acrylamide IUPAC CAS Number Stum formular Molecular Weight [g/mol] Properties Density 30ºC Melting Point Boiling Point Vapour Pressure Iog POW Solubility in Water MAK Commission Classifiction

Acrylamide 79-06-1 C3H5NO 71.078 white, crystalline solid at RT 1.127 g/cm3 84 - 84.5 ºC 125 C at 3.3 Pa 0.9 Pa at 25 ºC -0.67 to 1.65 2.155 g/l at 30 ºC Skin Absorption H Carcinogen Category 2 Germ Cell Mutagen Category 2

Acrylamide Data Sheet [7].

3. PRODUCTION AND USE Nowadays, acrylamide is produced on a major technical scale through catalytic hydration of acrylonitrile. By far the greatest part (99.9%) of the monomeric acrylamide produced in the

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Physico- Chemical Properties of Acrylamide

35

European Union (EU) is used in the production of polyacrylamides. Among other purposes, polyacrylamides are used as dispersants and flocculants in drinking water treatment. In addition, high-molecular polyacrylamides can be modified chemically by the introduction of non-ionic, anionic or cationic groups for various purposes and subsequently used as ion exchanger, thickener or as processing aid in the paper industry. Besides, acrylamide is also used as a co-polymer in the synthesis of dyestuffs and for various plastics. Other uses for acrylamide polymers are found in the crude oil industry (drill hole cement), the building(additive to hydraulically binders), the paper (to improve tear resistance), the mining (clarification of circulation water) and the textile industries, in which polyacrylamides are used as colouring aid as well as for binding textile fibres. In research, acrylamide is used in the production of polyacrylamide gels for electrophoresis. The annual production of acrylamide in the EU is estimated at 80,000 to 100,000 tonnes. When using polyacrylamides, their content of monomeric acrylamide must not exceed 0.1 percent by weight, as they otherwise have to be classified as Class 2 carcinogens. In polyacrylamides destined for drinking water treatment, the residual content of monomeric acrylamide may only be 0.025%.

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4. EMISSION INTO THE ENVIRONMENT Acrylamide enters the environment during its production and when using its polymers (residual content of monomers). Due to its high water solubility, acrylamide is mainly released into water and water-containing compartments. The European Risk Assessment Report calculates the total continental release of acrylamide into water from all conceivable sources at a maximum of 280 kg/day. On the other hand, due to the low vapour pressure of acrylamide, its release into the atmosphere is negligibly low at 0.38 kg/day. In the environmental media, particularly water, soil and air, acrylamide is degraded within a few days. This is either via a bacterial process or due to the reaction with hydroxyl radicals. For this reason, no accumulation of acrylamide occurs either in the environment or in the food chain.

5. EXPOSURE OF HUMANS The intake of acrylamide by the non-smoking general population is almost exclusively via food. Acrylamide is produced at elevated temperatures when foods containing both proteins and carbohydrates are prepared. This means that acrylamide is formed during processes such as baking, roasting or frying etc. Compared with this, intake of acrylamide with tobacco smoke is far greater than that with food.

6. INTAKE WITH FOOD In 2002, evidence was produced for the first time by a Swedish working group that acrylamide is produced by the heating of foods. In this case, those foods are involved which –

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like potatoes – contain both carbohydrates and proteins, and are prepared at elevated temperatures. In further studies, it was possible to show that reducing sugars such as glucose for example, and the amino acid asparagine, play a crucial role in the production of acrylamide. The mechanism of this reaction, during which acrylamide is formed temperatureand time-dependently, is accepted as having been clarified in the meantime. The fact that acrylamide is not formed until reaching temperatures above 120°C is of great practical importance. A sudden rise in acrylamide formation occurs at temperatures between 170 and 180°C. Against the background of the carcinogenicity of acrylamide as incontestably confirmed by animal experiments, the realization that the substance occurs in a great number of foods has triggered many activities on a worldwide basis. Already in 2002, the BfR (Bundesinstitut for Risikobewertung, [German] Federal Institute for Risk Assessment) published a list with the acrylamide concentrations in various foods (Table 2).

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7. INTAKE WITH TOBACCO SMOKE According to this, the highest acrylamide concentrations were found in potato products such as potato chips, French fries etc. (up to 4000 μg/kg), although bakery products such as bread, gingerbread, butter cookies and also coffee contained appreciable amounts of acrylamide (up to 1000 μg/kg). In Germany, a number of measures have been taken to reduce the acrylamide content in foods. A comparison of the AA contents of foods published by the BfR in 2002 with the corresponding values by the Bavarian State Agency for Health and Food Safety (Bayerisches Landesamt für Gesundheit und Lebensmittelsicherheit; LGL Bavaria) (Tables 2 and 3) show tendencies to a reduction of the AA concentrations in foods. AA is also produced when cooking food at home, which means that the measures taken above naturally have no influence; consequently, preventive health measures can only be undertaken by informing the general public. Table 2.

Product Potato chips French fries, cooked Potato sticks Fried potatoes, cooked Cracker bread Bread Bread rolls Breakfast cereals Cornflakes Butter cookies Gingerbread Pretzel sticks Powdered coffee

# of Investiagted Samples 221 54 26 6 95 52 12 39 9 8 17 7 35

Acrylamide concentrations in various food categories in Germany [13].

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Acrylamide [µg/kg] Median Range 750 130 - 3680 250 20 - 3920 1430 630 - 2870 240 n.n - 280 170 n.n - 2840