Direct and indirect electrochemical oxidation of organic pollutants
University Department of Ferrara of Chemistry
Ph.D. in Chemical Sciences Direct and indirect electrochemical oxidation of organic pollutants
Supervisor: Prof. A. De Battisti Ph.D. thesis work by: Dr. Carlos Alberto Martínez Huitle Advisor: Dr. S. Ferro
Ph.D. in Chemical Sciences, Seventeenth Cycle Coordinator: Prof. G. Gilli
“Recuerda que la historia esta repleta de logros increíbles alcanzados por aquellos que fueron bastante locos para creer en sí mismos”
Acknowledgements The present Ph. D thesis was supported by National Council of Science and Technology of México Under the program “Ph. D international scholarships”.
Ph. D. student on Chemistry Department, Laboratory of Electrochemistry Ferrara University, Italy.
I am grateful to Professor Achille De Battisti for giving me the opportunity to work in his research group, for his aid, motivation, comments, advices and friendship. Thanks a lot for many things, that he made for me, but especially, for the opportunity to participate in different international congress meetings, in particularly, in the Brazil and Greece ISE meetings.
I would like to thanks the CONACYT – MEXICO for the financial support during the period of my study and scientific research.
Thanks to Professor Chistos Comninellis for his collaboration and for establishing the contact with Professor Achille De Battisti. It was the first step for starting my Ph. D. in Ferrara, Italy. Also, I am grateful to Professor Comninellis for accepting me in his research group two times in Switzerland, during my Bachelor (2000) and Ph. D. studies (2003). These stages gave me a very important professional and personal experiences.
Thanks to all the members of the jury and thanks to Professor Christos Comninellis, Professor Achille De Battisti, Dr. Sergio Ferro for “reading” my thesis and giving me helpful comments.
Thanks to Beatrice, Ilaria Duo, Gabrielle, Fabio, Flavio, Ricardo, Bahaa, Ivan and Lassinè for the great moments during the time, that we spent together in the EPFL, Switzerland.
I am grateful with my colleges and friends in Department of Chemistry, Ferrara University, during all of these years. Thanks to all of them: Ghiro, Luca, Stefano, Maria, Anna, Letizia, Loretta, Catia, Simone, Gennaro, Nicola, Nikla, Martino, Violetta, Nicola Marcheti, Simona.
Thanks a lot to students and scientists I worked with in the laboratory of the electrochemistry: Tiziano “trop bel”, Sara “esquiava”, Christian “cioccolatino bianco”, Massimo “Max”, Roberto, Chiara “la bella bionda”, Federico “accidenti- accidenti”, Elisa, Costantino “tino”, Morozov “aglio”, Dinara “Dov’é..”, Davide “tarocco dati”, Martina “tina” and Paula “addormentata”. In particularly, to Matteo Maria Borghato for his help with Italian language during my first days in Italy and for his friendship.
Rosa (El Salvador), Claudio (México), Lily (México), Bo (Korea), Rafael (México) and Argentinos, thank you all. You have been my first friends in Italy.
I would like to thank specially to Ilaria Boari and Lulu for great moments in the lab, Spanish, English and Italian class, good conversations and different motivation phrases to me. Non dimenticherò mai tutti i belli momenti fuori e dentro l’università con voi. Grazie mille per la vostra amicizia e incomparabile compagnia.
Grazie Lulu per dare ancora più vita alla comunità messicana nel lab. Y por cada una de las grandes pláticas y discusiones que tuvimos, asimismo los grandes consejos en Grazie Ilaria per la compagnia nel laboratorio; per le infinite conversazioni e tutte le lezioni d’italiano; per i test fati sul computer e per il bello state del 2003. Ed specialmente per la tua simpatia e incomparabile amicizia.
Thanks to Sergio Ferro my great Italian friend: his English corrections, motivation phrases, technical support, precious advice and helpful discussion. Grazie mille per le sue parole: “che pazienza che ho”, “Non ho capito niente”, “Cosa stai facendo”, “sei un porco” e “Finto”. Ti ringrazio per tutte le belle giornate a casa tua, per l’aiuto durante la scrittura degli articoli e le buone conversazioni che abbiamo avuto. Non dimenticherò mai i piccoli regali e le correzioni del mio italiano.
Grazie mille. A todos esos grandes amigos que, desde el otro lado del mar siempre me echaron muchas porras. En especial a Arancha, Edith Villascan y Lucy.
Thank you Elaine. Grazie Mille per i più belli momenti della mia vita. Per il sostegno e il tuo amore. Por los grandes y maravillosos viajes, momentos de espera, risas y la gran comprensión. Você é muito especial em meu coração e em minha vida.
Gracias a mis padres, Martha y Federico por cada momento de apoyo y gran amor. Agradezco infinitamente la ayuda proporcionada cada vez que hablábamos y los grandes consejos que me dieron día con día. Por el “coraje” y “tranquilidad” que me trasmitieron para poder seguir adelante.
Uriel Y Paola, gracias por darme su confianza y ayuda en los momentos difíciles. Por no dejarme solo y darme siempre palabras de aliento.
Electrochemical incineration of Tartaric acid 129 Introduction 129 Experimental 131 Cell and electrolyses conditions 131 Techniques and instrumentation 131 Discussion and results 132 Anodic oxidation of Tartaric acid 132 Electrochemical oxidation pathway 142 Conclusions 145 References 147 Chapter 5. Reactivity and Engineering parameters 149 Introduction 149 Experimental part 151 Cell and electrodes 151 Techniques and instrumentation 154 Results and discussion 156 Mass transport coefficient 156 Anodic oxidation of oxalic acid Oxalic acid oxidation and kinetics 160 Conclusions 165 References 167 Chapter 6. Indirect or mediated electrochemical oxidation 171 Anodic oxidation of tartaric and oxalic acids in the presence of halides 171 Introduction 171 Experimental section 175 Cell and electrodes 175 Techniques and instrumentation 175 Discussion and results 176 Electrochemical measurements 176 Anodic oxidation 179 Influence of media 179 Influence of halides 182 Influence of applied current density 190 Variation of pH 190 Representation data as “Volcano Plot” 193 Conclusions 195 References 196
Results and discussion 202 Conclusions 235 References 238 Chapter 8. Hydroxylation 241 Electrochemical and chemical hydroxylation of salicylic acid 241 Introduction 241 Experimental 243 Results and Discussion 245 Chemical hydroxylation 245 Electrochemical hydroxylation 249 Comparison 252 Hydroxylation selectivity 253 Conclusions 256 References 257
Abstract In recent years, the perspectives of application of electrochemistry for environmental pollution abatement have been thoroughly investigated. The feasibility of electro-chemical incineration of organic substrates in wastewater, in particular, has drawn much attention since pioneering papers by Dabrowski in the 70’s, Kirk, Stucki, Kotz, Chettiar and Watkinson in the 80’s, and Comninellis in early 90’s to ours days.
In these works, the influence of the nature of the electrode material on the faradaic efficiency of the anodic mineralization of organics has been considered in detail, showing that optimal conditions for the process in question are met at high-oxygen- overpotential anodes. Basing on these evidences, Comninellis has proposed a general model, which satisfactorily accounts for the different results described in the literature.
More recent results, obtained at conductive diamond electrodes, characterized by very high oxygen overpotential, also fit in the model predictions quite well. Many model organic substrates, like glucose, aldonic and aldaric acids among others, show a complex reactivity toward the anodic mineralization. In fact, in these cases, besides the central role of adsorbed hydroxyl radicals, also the mode of adsorption of the organic species has to be taken into account.
Simpler molecules, like oxalic acid, also support the view that co-electrosorption of hydroxyl radicals and organic species decides of the rate of the anodic mineralization, volcano-plot-approaches of the type applied long since for ethylene electrochemical oxidation being possibly a good interpretative tool. These considerations afford further evidence to the importance of the nature of the electrode material in electrochemical incineration.
Taking into consideration the role of mediators, like Cl-, of particular interest for its common presence in many different types of wastewater, interpretations may be attempted, bringing again the role of the electrode material in a prominent position. Chloride-mediated incineration can be properly accounted for by assumptions on co-adsorbed hydroxy- and chloro-radicals at the electrode surface and on the effect of this on the rate of oxygen evolution reaction. Converting the results of the indirect oxidation, as second alternative in the elimination of the organic pollutants from water.
The aim of this thesis work is emphasizing the role of the electrode material in direct and indirect electrochemical oxidation for wastewater treatment.
List of Symbols List of symbols A electrode surface area (m2) C bulk species concentration (mol m-3) D diffusion coefficient (m2 s-1) de duct equivalent diameter (m), de = 2ws ( w + s ) F faradaic constant (96 487 C mol-1) h a fourth of the disk circumference IL electrolysis limiting current (A) K mass transfer coefficient (m s-1) L electrode length (m) Re Reynolds number s inter-electrode distance (m) Sc Schmidt number Sh Sherwood number, based on cell or channel equivalent diameter w section of cell or channel (m2) z electrons exchanged in electrode reaction v mean fluid velocity in cell or channel (m s-1) Q volumetric flow rate (m3 s-1)
Chapter 1. Introduction
Introduction The intensification of industrial activities, since the second half of XIX century and during all the XX century, has inevitably caused severe environmental pollution with dramatic consequences in atmosphere, waters, and soils. The consequent restrictions imposed by new legislations require effective initiatives for pollution abatement not only in gaseous emissions and industrial aqueous effluents, but also adequate decontamination in soils. Typically, in the latter case, different classes of pollutants may have accumulated during long periods of uncontrolled waste disposal and reclamation may represent a serious technological problem. In all these cases, universal strategies of reclamation can hardly be found, due to the very different Generally, the wastewater treatment is effectuated using primary, secondary or tertiary methods, according to type of pollutants. As far as organic pollutants in wastewaters are concerned, biological abatement may be sometimes impossible, due to the bio-refractory character of substrates. For this reason, physical-chemical methods are applied, but an oxidation with e.g. ozone or chlorine dioxide is not always effective and also transportation and storage of reactants may be a significant An alternative can be the application of electrochemical technologies for wastewater treatment, taking profit of advantages like versatility, environmental compatibility and potential costs effectiveness among others. Accordingly, both the direct and the mediated electrochemical oxidation/incineration can be considered, anyhow representing an interesting subject for different research groups and industries, which are looking for new technologies for the wastewater treatment (Chapter 2). Basically, the electrochemical incineration proceeds under the action of strong oxidants, similarly to the chemical incineration, but the in-situ electro-generation
Chapter 1. Introduction defining optimal process yields, at least in terms of electrode material. Experimental evidences should be of help to understand the interaction between the organic substrate and the electrode surface, thus allowing an optimization of the process conditions and possibly increasing the elimination of water pollutants (Chapter 4). On the basis of the results obtained investigating the above organic substrate, an alternative compound, tartaric acid, has been selected to extend the investigation to more complex molecular frames, possibly involving the formation of intermediates blocking, at different extents, the electroactive sites (again in Chapter 4). In fact, the latter carboxylic acid has a higher complexity, in terms of molecule structure, with respect to oxalic acid. Similarly to what has been done with oxalic acid, tartaric acid has been studied at different electrode materials (Ti/PbO2, BDD and Pt), trying to understand the interactions of this organic substrate and hydroxyl radicals with In consideration of the fact that also the cell design and the way electrodes are prepared (characteristic and methodology of synthesis) can play a role in the electrochemical oxidation process, among other variables, investigations have been carried out at different anode materials (Pb/PbO2, BDD, Ti/Pt and Ti/IrO2-Ta2O5), making use of oxalic acid as a model organic pollutant, with the aim of testing a disk-shaped, parallel-plate electrochemical cell based on a new design, with respect to those reported in the literature. The cell has only one central inlet and several peripheral exit sections, thus allowing the obtainment of a rather complex hydro- dynamics. Measurements of mass transfer coefficient were made using the limiting diffusion current technique based on ferricyanide ion reduction, and overall mass transfer coefficients were correlated to Reynolds numbers ranging from 100 to 800. Thanks to the above features, and comparing the obtained data with those available in the literature, for similar devices, higher mass transfer coefficients could be obtained with the new cell design, thus allowing an improvement for the whole electrochemical process (Chapter 5).
Chapter 1. Introduction the spin-trap characteristics of salicylic acid and analysing the obtained metabolites through HPLC measurements. Since hydroxyl radicals have been suggested to play a significant role, especially in the direct electrochemical oxidation, many efforts have been made to confirm their generation, interaction and chemical reactivity at “non- active electrodes” like BDD. This last part of the research was realized at EPFL (Switzerland), in the frame of the scientific collaboration existing between this Finally, in Chapter 9, a general discussion and future perspectives about the application of the electro-chemical oxidation as an alternative to common wastewater treatments are proposed.
Chapter 2. Bibliography Bibliography A thorough analysis of wastewater treatments, as they can be found in the literature, is presented. A distinction was made between these treatments and the so-called “advanced oxidation processes” (AOP). Particular attention has been dedicated to physical- chemical processes, as they are the most significant methods for the treatment of organics dissolved in water effluents, and considering that oxidation is anyway involved In this context, an examination of several AOP is presented: in fact, different methods have been investigated by several research groups for the elimination of organic Concerning the development of new technologies, a mention is deserved by the application of electrochemistry for the protection of the environment, since it offers promising approaches for the prevention of pollution problems in the process industry. The inherent advantage of electrochemical methods has to be found in their environmental compatibility. The fundamental concepts of the method and the different investigations carried out at several electrode materials will be presented, emphasizing On the other hand, a general description of electrode properties will be also given, moving from the nature of electrode reactions (inner and outer sphere) to considerations of electrocatalytic activity for their classification (active or non active).
Chapter 2. Bibliography Wastewater Treatment  The treatment of effluents represents a serious problem, especially for the chemical industry. Over the last twenty-five years, big efforts have been made to limit at the source this type of pollution, by improving processes, recycling products and controlling the treatment of wastes at the production stage. However, considering the large amount of industrial effluents to be treated, for example to retrieve certain solvents, there are inevitably residues requiring a final transformation, which is often delicate. Traditional incineration methods, for their part, pose problems of corrosion and, more seriously, of From the point of view of the industry, the problem must be examined as a whole, since there are no universal and simple methods in this area. The great variety of industrial discharges means that a diversification of techniques must be sought, adapting the treatment to each situation, as much as possible. In spite of the efforts made to develop ever-clean processes, the increasingly severe environmental laws should encourage the research for better-performing treatments, making it possible to obtain environmentally Actually, the processes for the treatment of wastewater may be divided into three main categories of primary, secondary and tertiary treatments, each of which will be discussed separately.
Chapter 2. Bibliography devices shred and grind solids in the sewage. Particle size may be reduced to the extent that the particles can be returned to sewage flow.
Secondary treatment The most obvious harmful effect of biodegradable organic matter in wastewater is BOD, consisting of a Biological Oxygen Demand for dissolved oxygen by microorganisms- mediated degradation of organic matter. Secondary wastewater treatment is designed to remove BOD, usually by taking advantage of the same kind of biological processes that would otherwise consume oxygen in water receiving the wastewater. Secondary treatment by biological processes takes many forms but consists basically of the following: microorganisms provided with added oxygen are allowed to degrade organic material in solution or in suspension, until the BOD of the waste has been reduced to acceptable levels. The waste is biologically oxidized under condition controlled for optimum bacterial growth and at a site where this growth does not influence the environment.
Chapter 2. Bibliography addition, potentially hazardous toxic metals may be found among the dissolved inorganics. The major problem is due to the dissolved organics, because these are the Complete physical-chemical wastewater treatment systems offer both advantages and disadvantages relative to biological treatment. The capital costs of these facilities can be less than those of biological treatment facilities, and they usually require less land. Basically, a physical-chemical treatment process involves [1, 3]: 9 Removal of scum and solid objects 9 Clarification, generally with addition of coagulants, and frequently with the addition of other chemicals 9 Filtration, to remove filterable solids 9 Sedimentation 9 Elimination of heavy metals by precipitation with Ca(OH)2, H2S and/or ionic exchange resins 9 Activated carbon adsorption 9 Inorganic dissolved compounds removal by electrodialysis, ionic exchange and/or inverse osmosis on semi-permeable membrane 9 Disinfection with Cl2, ClO and/or O3 In addition, the wastewater can be treated by a variety of chemical processes, including Therefore, in recent years, combined treatment methods are finding an increasing use, particularly physical-chemical treatment with subsequent biological treatment, and physical-chemical processes employed as tertiary treatment after a biological treatment. Flow sheets of treatment facilities using physical-chemical methods alone are also being developed .
Chapter 2. Bibliography special attention given to fundamental chemical aspects. Advanced oxidation processes, although making use of different reacting systems, are all characterized by the same chemical feature: the production of OH radicals. Hydroxyl radicals are extraordinarily reactive species: they attack the most part of organic molecules with rate constants usually in the order of 106–109M?1 s?1 [12, 13]. They are also characterized by a little selectivity of attack, which is a useful attribute for an oxidant used in wastewater treatment and for solving pollution problems. The versatility of AOP is also enhanced by the fact that they offer different possible ways for OH radicals production, thus allowing a better compliance with the specific treatment requirements. A suitable application of AOP to wastewater treatments must consider that they make use of expensive reactants as H2O2 and/or O3 and therefore it is obvious that their application should not replace, whenever possible, more economic treatments as the biological degradation. The potentialities offered by AOP can be exploited to integrate biological treatments by an oxidative degradation of toxic or refractory substances entering or leaving the Another aspect, concerning the opportunity of AOP application, is that referring to the polluting load of wastes normally expressed as COD. Only wastes with relatively small COD contents can be suitably treated by means of these techniques, since higher COD contents would require the consumption of too large amounts of expensive reactants. Wastes with more massive pollutants contents can be more conveniently treated by means of wet oxidation or incineration . Wet oxidation makes use of oxygen or air to achieve pollutant oxidation at high temperatures (130-300 °C) and pressure (0.5-20 MPa). For these wastes, the cost evaluation of fuel consumption will give the selection criteria for the application of AOP or wet oxidation treatment.
Chapter 2. Bibliography Fenton processes The renewed interest of researchers for this classic, old reactive system, discovered by Fenton the last century , is today underlined by a significant number of investigations devoted to its application in wastewater treatments. It has been demonstrated that the Fenton’s reagent is able to destroy toxic compounds in waste- The production of OH radicals by the Fenton reagent  occurs by means of addition of H2O2 to Fe2+ salts: Fe2+ + H O ? Fe3+ + OH- + OH• (1) 22 This is a very simple way of producing OH radicals, neither special reactants nor special apparatus being required. This reactant is an attractive oxidative system for wastewater treatment due to the fact that iron is a very abundant and non toxic element and hydrogen peroxide is easy to handle and environmentally safe. It must be stressed that the behavior of the system cannot be completely explained on the basis of the sole In fact, as it has been pointed out in many recent studies  the adoption of a proper value of pH (2.7-2.8) can result into the reduction of Fe3+ to Fe2+ (Fenton-like) Fe3+ + H O H+ + FeOOH2+ (2) 22
FeOOH2+ ? HO • + Fe2+ (3) 2
Chapter 2. Bibliography process, which takes advantage from UV-VIS light irradiation at wavelength values higher than 300 nm. In these conditions, the photolysis of Fe3+ complexes allows Fe2+ regeneration and the occurrence of Fenton reactions due to the presence of H2O2. Despite the great deal of work devoted by researchers to these processes, scanty indications have been found about their industrial applications. This is not surprising, since Fenton processes application requires strict pH control and sludges can be formed with related disposal problems.
UV/Fe3+-Oxalate/H2O2 An improvement of photo-assisted Fenton processes is the UV-VIS /Ferrioxalate/ H2O2 system, which has been very recently demonstrated  to be more efficient than photo- Fenton for the abatement of organic pollutants: ferrioxalate is the oldest and best-known photo-active example of Fe3+-polycarboxylate complex . Irradiation of ferrioxalate in acidic solution generates carbon dioxide and ferrous ions Fe2+ free or complexed with oxalate, which, in combination with H2O2, provides a continuous source of Fenton’s reagent.
Photocatalysis Photocatalytic processes make use of a semiconductor metal oxide as catalyst and of oxygen as oxidizing agent . Many catalysts have been so far tested, although only TiO2 in the anatase form seems to have the most interesting attributes, such as a high stability and good performance [23, 24]. The initial event in the photocatalytic process is the absorption of the radiation with the formation of electron-hole pairs. The considerable reducing power of formed electrons allows them to reduce some metals and
Chapter 2. Bibliography dissolved oxygen with the formation of the superoxide radical ion O2•?, whereas remaining holes are capable of oxidizing adsorbed H2O or HO? to reactive HO radicals: TiO(h+)+HO?TiO+HO·+H+ (4) 2 2 ad 2 ad
TiO2(h+)+HOad-?TiO2+HOad· (5) These reactions are of great importance in oxidative degradation processes, due to the high concentration of H2O and HO? adsorbed on the particle surface. Unfortunately, a significant part of electron-hole pairs recombine, thus reducing the quantum yield. In the most part of the works devoted to photocatalysis, the possible exploitation of the wavelengths of the solar spectrum is stressed. However, this is only partially true since the overlapping between the absorption spectrum of TiO2 and that of the sun at ground is rather poor . Intensive researches are carried out worldwide to obtain modified (doped) TiO2 with broader absorption spectrum and characterized by a higher quantum yield. Despite the great efforts devoted to the study of photocatalytic processes, no indications have been found in the literature of their application on industrial scale.
Chapter 2. Bibliography enhance the O3 decomposition with formation of OH radicals. The influence of pH is also evident, since in the ozone decomposition mechanism the active species is the conjugate base HO2?, whose concentration is strictly dependent upon pH. The increase of pH and the addition of H2O2 to the aqueous O3 solution will thus result into higher rates of OH radicals production and the attainment of higher steady-state concentrations It must be remarked that the adoption of the H2O2/O3 process does not involve significant changes to the apparatus adopted when only O3 is used, since it is only necessary to add an H2O2 dosing system.
Mn2+/oxalic acid/ozone The system Mn2+/oxalic acid can be conveniently used to enhance ozone decomposition to produce HO radicals. Mn2+ catalyzed ozonation of oxalic acid has been shown to develop according to a radical mechanism at pH > 4.0 at which Mn(III)-dioxalate and Mn(III)-trioxalate are formed. In these conditions, the oxidation process proceeds presumably through the formation of OH radicals as a result of a reaction between manganese complexes and ozone . The system has been demonstrated to be effective for the abatement of refractory pollutants such as pyrazine and pyridine .
H2O2 photolysis This process is realized by irradiating the pollutant solution containing H2O2 with UV light having wavelengths smaller than 280 nm. This causes the homolytic cleavage of H2O2 : ??? · H2O2 hv 2OH (6)
Chapter 2. Bibliography Since H2O2 itself is attacked by OH radicals, the overall quantum yield becomes one. The rate of photolysis of aqueous H2O2 has been found to be pH dependent and to increase when more alkaline conditions are used . This may be primarily due to the higher molar absorption coefficient of the peroxide anion HO2?, which is 240M?1 cm?1, at 254 nm.
O3/UV O3/UV process is an advanced water treatment method for the effective oxidation and destruction of toxic and refractory organics in water. Basically, aqueous systems saturated with ozone are irradiated with UV light of 254 nm in a reactor convenient for such heterogeneous media. The extinction coefficient of O3 at 254 nm is 3600M?1 cm?1, much higher than that of H2O2. The O3/UV oxidation process is more complex than the other ones, since OH radicals are produced through different reaction pathways; under these conditions, the system has the chemical behavior of both O3/H2O2 and H2O2/UV systems [5, 10]. From the photochemical point of view, the absorption spectrum of ozone provides a much higher absorption cross section than H2O2, and inner filter effects by, e.g., aromatics are less problematic .
Electrochemistry for the Environment An increase world population with growing industrial demands has lead to a situation where the protection of the environment has become a major issue and crucial factor for the future development of industrial process, which will have to meet the requirements of sustainable development.
Chapter 2. Bibliography Electrochemistry offers promising approaches for the prevention of pollution problems in the process industry. The inherent advantage is its environmental compatibility, due to the fact that the main reagent, the electron, is a clean reagent. The strategies include both the treatment of effluents and waste and the development of new processes or products with less harmful effects, often denoted as process-integrated environmental protection The application of the electrochemistry for the protection of the environmental has been the topic of several books and reviews [31-38]. Besides the process-oriented benefits, electrochemistry is also playing a key role in the sensor technology. Electroanalytical techniques for monitoring and trace level detection of pollutants in the air, water and soil as well as of microorganisms are needed for process automation. Sensors for environmental applications have been already reviewed [32, 34], while an interesting view on the role of electrocatalysis for electrochemistry and environment has recently The electrochemical processes offer several promising approaches for the prevention and remediation of pollution problems. Among the main characteristics that are attractive, the following can be chosen : Versatility: direct/indirect oxidations and reductions, phase separations, biocide functions, concentrations or dilutions; can deal with many pollutants: gases, liquids Energy efficiency: these processes generally have lower temperature requirements than those of equivalent non-electrochemical counterparts (e. g., thermal incineration); the potentials can be controlled and electrodes and cells can be designed to minimize power losses due to poor current distribution, voltage drops and side reactions.
Chapter 2. Bibliography Amenability to automation: the electrical variables used in the electrochemical processes (I, E) are particularly suited for facilitating data acquisition, process Environmental compatibility: the electron is a clean reagent, and the high selectivity of many of these processes can be used to prevent the production of unwanted side- Cost effectiveness: the required equipment and operations are generally simple and, if properly designed, are also inexpensive.
Therefore, intensive research continues with the goal of discovering more efficient techniques, processes, materials, technologies and applications of the electrochemistry for the remediation and/or prevention of pollution problems.
Electrochemical technologies for wastewater treatment Electrochemical technologies have recuperated their importance in the world during the past two decades. There are different companies supplying facilities for metal recoveries, treating drinking water or process water, treating various wastewaters resulting from tannery, electroplating, dairy, textile processing, oil and oil-in-water emulsion, etc. At the present time, electrochemical technologies have reached such a state that they are not only comparable with other technologies in terms of cost, but also more efficient and compact. The development, design and application of electrochemical technologies in water and wastewater treatment have been focused particularly to some technologies such as electrodeposition, electrocoagulation, electrofloculation and electrooxidation. These techniques will be explained in brief, being the electrochemical oxidation ours central point.
Chapter 2. Bibliography Electrodeposition The electrochemical recovery of metals has been used in form of electrometallurgy since long time ago . The first recorded example of electrometallurgy was in mid-17th century in Europe . It involved the recovery of copper from cupriferous mine water electrochemically. During the past two and half centuries, electrochemical technologies have grown into areas as energy storage, chemical synthesis, metal production, surface The electrochemical recovery of metals can be used in the metal surface finishing industry. It has to bear in mind that it is unable to provide a complete solution to the industry’s waste management problems because it cannot treat all the metals either technically or economically. The electrolytic recovery of metals involves two steps: collection of heavy metals and stripping of the collected metals. The first step involves plating and the stripping can be accomplished chemically or electrochemically. Actually, metals powders can be formed on the surface of carbon electrodes and a physical Another application is in the printed circuit board manufacturing industry: for treating dilute effluent, an ion-exchange can be used, while high concentration streams can be treated directly, using a recovery system as in metal surface finishing industry.
Chapter 2. Bibliography , wastewater from coke-production , mining wastewater , groundwater , food processing wastewater , fat-containing solutions , restaurant wastewater , or food industry effluents , dairy wastewater , urban sewage , pit waters , colloidal particles , heavy metals containing effluents [86-89], gold and silver recover from cyanide solution  and many other water and wastewaters [71, 91, 92].
Electrochemical Oxidation: An alternative in wastewater treatment Studies on the electrochemical oxidation for wastewater treatment go back to the 19th century, when the electrochemical decomposition of cyanide was investigated . Extensive investigation of this technology commenced since late of 1970s. During the last two decades, research works have been focused on the efficiency in oxidizing various pollutants on different electrodes, improvement of the electrocatalytic activity and electrochemical stability of the electrode materials, investigation of factors affecting the process performance and the exploration of the mechanisms and kinetics of the pollutant degradation. Experimental investigations, focused mostly on the behavior of anodic materials, have been realized by different research groups. Attempts for an electrochemical oxidation/incineration treatment for waste or wastewater can be subdivided in two important categories : 9 Direct oxidation at the anode 9 Indirect oxidation using appropriate, anodically-formed oxidants
Chapter 2. Bibliography Direct and Indirect Electrochemical Oxidation Direct anodic oxidation Electrochemical oxidation of pollutants can occur directly at anodes through the generation of physically adsorbed “active oxygen” (adsorbed hydroxyl radicals, •OH) or chemisorbed “active oxygen” (oxygen in the oxide lattice, MOx+1) . This process is usually called “anodic oxidation” or “direct oxidation” and the course for the anodic oxidation has been described by Comninellis ; the complete combustion of the organic substrate or its selective conversion into oxidation products is schematically represented in Figure 1. The electrochemical conversion transforms only the toxic non- biocompatible pollutants into biocompatible organics, so that a biological treatment is still required after the electrochemical oxidation. In contrast, the electrochemical combustion yields water and CO2 and no further purification is necessary. Nevertheless, the feasibility of this process depends on three parameters: (1) the generation of chemically or physically adsorbed hydroxyl radicals, (2) the nature of the anodic material and (3) the process competition with the oxygen evolution reaction. The anodic oxidation does not require the addition of large amounts of chemicals to wastewater or to feed O2 to cathodes; moreover, there is no tendency of producing secondary pollution and fewer accessories are required. These advantages make the anodic oxidation more attractive than the other oxidation processes. As previously commented, the most important parameter in this process is obviously the anode material. Among the investigated anode materials, the following can be mentioned: glassy carbon , Ti/RuO2, Ti/Pt-Ir [96, 97], carbon fibers , MnO2 [99, 100], Pt- carbon black [101, 102], porous carbon felt , stainless steel  and reticulated vitreous carbon [104, 105]. Unfortunately, none of them has sufficient activity and, at
Chapter 2. Bibliography the same time, satisfactory stability. Pt, PbO2, IrO2, SnO2, and conductive diamond films Table 1 presents a comparison among the different anodes for the degradation of some important pollutants, under different conditions. Also, it is worth mentioning that this table contains the more relevant researches in the frame of the direct electrochemical oxidation from the beginning of the application of this method to nowadays. Different parameters have been resumed; nevertheless, the current density and the current efficiency (CE)  are of particular interest, as well as the possible intermediates. The current efficiency  is a measure of the process effectiveness; it can be alternatively expressed by means of the Electrochemical Oxidation Index (EOI), the Apparent Current Phenol and derivates are among the mostly investigated examples in electrochemical studies. Dabrowski et al. in 1975  have also tried the use of electrochemical oxidation for the destruction of phenolic waste on a pilot-scale plant; Chettiar and Watkinson used synthetic wastewater solutions in 1981  and 1983 .
Chapter 2. Bibliography Anode Pollutant Current Efficiency Removal Comments Metabolites Ref. density, (ICE, EOI, efficiency intensity ACE, %) or potential 3D carbon felt nitroaromatic 500- – 10-50% COD H2SO4 from 50- 2-methyl  waste 1000A/m2 removal 96%, T=25- quinoxaline, 2,3- 40°C dimethyl quinoxaline, quinoxaline Au benzoquinone 10mA/cm2 46% (after 48 Vol=50 ml Hydroquinone,  hrs) [BQ]= 50ml resorcinol, p- soln. stock, benzoquinone, [sol. pyrocatechol, stock]=100mg/ formic, maleic, L succinic, malonic, fumaric, acetic acids Au Pentachlorophenol Polymerization 
Chapter 2. Bibliography Anode Pollutant Current Efficiency Removal Comments Metabolites Ref. density, (ICE, EOI, efficiency intensity ACE, %) or potential Packed bed reactor Phenol I=3.0V 16% (after 1.5hrs.) Phenol removal [Phenol]=1.4×10- Phenol,  of PbO2 pellets 72-100%, 2M, 1M H2SO4, benzoquinone, depending First Electro- maleic acid and CO2 [phenol]; ?J-? % chemical oxidation removal, ? studies, Innovative [phenol]- ? % reactor, Study at removal, ? different [phenol], [H2SO4]- ? % applied current removal, ?T – ? density, [H2SO4], [benzoquinone] T=25-50°C, conversion and ? dissolved oxygen [CO2] conversion after 1. 5h and considerable [CO2] achieved PbO2 Indoles I=0.5-500mA 32% 0.05 Na2SO4 Polymerization 
PbO2 1-Methyl indol I=0.5-500mA 99% 0.05 Na2SO4 Polymerization  PbO2 2-Methyl-indol I=0.5-500mA 58% 0.05 Na2SO4 Polymerization 
PbO2 3-Methyl-indol I=0.5-500mA 27% 90% (60mA) and 0.05 Na2SO4 Polymerization  99% (100mA) PbO2 Tryptophan I=0.5-500mA 37% 0.05 Na2SO4 Polymerization 
Chapter 2. Bibliography Anode Pollutant Current Efficiency Removal Comments Metabolites Ref. density, (ICE, EOI, efficiency intensity ACE, %) or potential PbO2 glucose 100-900A/m2 EOI=30-60% 100% 1M H2SO4, T=25- 9 derivates of  57°C, Specific glucose interaction between Pb (IV) sites and carboxylic groups PbO2 Chloranilic acid 6.3-50mA/cm2 GCE=0.7-0.2 90% H2SO4, T=25-60°C Hydroquinone,  ketomalonic and oxalic acid PbO2 p-Benzoquinone 10mA/cm2 CE(%)=0.6 [p-BQ]=5×10-4 M muconic acid, maleic  maleic acid assuming one- and oxalic acids electron reaction PbO2 Urine-waste E=1.8 V vs. 95% H2SO4, T=80°C NO2, NO, NO-2 3,  biomass mixtures NHE, around NH4+ (0.2-0.4 mA/cm2)
Chapter 2. Bibliography Anode Pollutant Current Efficiency Removal Comments Metabolites Ref. density, (ICE, EOI, efficiency intensity ACE, %) or potential Pt or Ti/Pt Phenol 300A/m2 30%, TOC pH=12, Stop oxidation with  [phenol]=1000mg/ maleic and oxalic L, Na2SO4, acids competition with oxygen evolution reaction Pt/WOx Formic and oxalic 10mA/cm2 100%, CE aprox. 95% [HCOOH]=8×10- CO2  acids 4M, [(COOH)2]=1.0×10- 3M, 0.5M H2SO4
Chapter 2. Bibliography Anode Pollutant Current Efficiency Removal Comments Metabolites Ref. density, (ICE, EOI, efficiency intensity ACE, %) or potential Ti/Pt/Ir
Chapter 2. Bibliography 50 mg/l at Si/BDD, and to about 300, 650 and 950 mg/l at Ti/SnO2, Ta/PbO2 and Pt, respectively. Another aspect to be taken into consideration is the production of powerful oxidants, like the peroxodisulphate [179, 180]; these species can participate in the As can be observed in Table 1, other anode materials have been employed for the direct oxidation of pollutants: pure metals [127, 144], DSA® electrodes, carbonaceous materials (e.g.: glassy carbon, carbon felt, granular carbon, graphite) [95, 103, 127, 138, Also the column of intermediates is a key-point for the evaluation of data in Table 1: in fact, several metabolites are generally produced during the oxidation of the original organic substrate. Starting from an aromatic compound, hydroxylated derivatives are found as initial intermediates but, in the final stages of the oxidation process, several carboxylic acids are produced, the last being usually oxalic acid. The formation of these acids increases the time process and highlights possible mass transport limitations; interestingly, some anode materials are more efficient than others for their elimination.
Chapter 2. Bibliography application of this technique [96, 184]. As another drawback, if the chloride content in the raw wastewater is low, a large amount of salt must be added to increase the process efficiency . Pollutants can also be degraded by the electrochemically generated hydrogen peroxide [185-190]. The electrically generated ozone is also reported for wastewater treatment [191, 192]. Farmer et al.  proposed another kind of mediated electrooxidation for the treatment of mixed and hazardous wastes; in this case, metal ions, usually called mediators, are oxidized at an anode from a stable, low-valence state to a reactive, high-valence state, which can directly attack organic pollutants. The reaction may also produce free hydroxyl radicals, which are useful for the destruction of the organic pollutants. Subsequently, the mediators are regenerated at the anode, thus forming a closed cycle. Typical mediators are Ag2+, Co3+, Fe3+, Ce4+ and Ni2+ [193-198]. The mediated electrooxidation usually needs to operate in highly acidic media; unfortunately, the resultant pollution from the added heavy metals limits its application. Table 2 summarizes different examples in which a mediated electrochemical oxidation has been adopted for the elimination of organic pollutants.
Chapter 2. Bibliography Anode Pollutant Current Efficiency Efficiency Removal Removal Mediator Notes Metabolites Ref. density, (ICE, EOI, (ICE, EOI, efficiency efficiency intensity %) %) (mediator (mediator or mediator mediator absence) presence) potential absence presence BDD Chloro 30mA/cm2 95% (Before 100%, Hypochlori Na2SO4 Oxalic,  phenols 15 Ah/L) COD te or 5000mg/L, maleic and peroxo- T=30- fumaric disulphate 60°C acids, hydro- quinone, benzo- quinone, trichloro- acetic acid DSA landfill 7.5 A/dm2 29%, Chlorine/  leachate COD; hypochlori 36.1% te Ammoniu m.
Graphit landfill 7.5 A/dm2 21%, Chlorine/  e leachate COD; hypochlori 11% te Ammoniu m.
Chapter 2. Bibliography Anode Pollutant Current Efficiency Efficiency Removal Removal Mediator Notes Metabolites Ref. density, (ICE, EOI, (ICE, EOI, efficiency efficiency intensity %) %) (mediator (mediator or mediator mediator absence) presence) potential absence presence Stainless steel Nitrite 2.00 AM 30% Hypochlorite pH  dependence Ti/ Sn-Pd- Ru-oxide landfill 7.5 A/dm2 30.3%, COD; 67.6%, COD; Chlorine/ at 15A/dm2 chloramines  leachate 37.6% 92%, hypochlorite and 7500mg/l Ammonium. Ammonium NaCl Ti//Pt/Bi- PbO2 phenol 100mA/cm2 24% (after 24 26% (after 79%, COD 98%, COD Active pH = 12 CHCl3  Ah/L) 26Ah/L) (After 24 Ah/L)(After 24 Ah/L)Chlorine [NaCl]=0.1M, [NaCl]=0.1M, 30% (after 99%, COD 20Ah/L) (After 20 Ah/L) [NaCl]=0.5M [NaCl]=0.5M Ti/IrO2 phenol 0.1 A/cm2 EOI=0.65 EOI=0.65 25%, [phenol] 98%, [phenol] Electro- Selective organochlorinate  ; generated oxidation, d compounds ClO- Na2SO4 + and after to NaCl volatile chlorinated compounds Ti/PbO2 landfill 7.5 A/dm2 27.4%, COD; Chlorine/  leachate 33.1% hypochlorite Ammonium.
Chapter 2. Bibliography
Electrode Materials In every new technology, materials play an important role. In this section, some significant parameters, relating to the above subject and pertaining to the application of direct and indirect oxidations, will be briefly commented.
Overpotential for the oxygen evolution  For an anode material, the catalytic activity toward organic oxidation depends on the value of oxygen evolution overpotential. Table 3 gives a comparison of most extensively investigated anode materials .
Anode Value Conditions Pt 1.3 0.5M H2SO4 Pt 1.6 0.5M H2SO4 IrO2 1.6 0.5M H2SO4 Graphite 1.7 0.5M H2SO4 PbO2 1.9 1M HClO4 SnO2 1.9 0.5M H2SO4 Pb–Sn (93:7) 2.5 0.5M H2SO4 Ebonex® (titanium oxides) 2.2 1M H2SO4 Si/BDD 2.3 0.5M H2SO4 Ti/BDD 2.7 0.5M H2SO4 DiaChem 2.8 0.5M H2SO4
Chapter 2. Bibliography density or in the presence of a high concentration of chlorides or metallic mediators. When the current density is high, significant decrease of the current efficiency is expected, due to the production of oxygen.
Oxidants Formation potential H2O/·OH (hydroxyl radical) 2.8 O2/O3 (ozone) 2.07 SO42?/S2O82? (peroxodisulfate) 2.01 MnO2/MnO42? (permanganate ion) 1.77 H2O/H2O2 (hydrogen peroxide) 1.77 Cl?/ClO2? (chlorine dioxide) 1.57 Ag+/Ag2+ (silver(II) ion) 1.5 Cl?/Cl2 (chlorine) 1.36 Cr3+/Cr2O72? (dichromate) 1.23 H2O/O2 (oxygen) 1.23 Table 4. Formation potential for some chemical reactants .
The boron-doped diamond (BDD) films deposited on titanium substrate  or other valve metals, as in DiaChem electrodes , give the highest value of oxygen evolution overpotential (Table 3). As a result, the anodic oxidation can take place, at this electrode surface, at significantly high current density with minimal contributions from the oxygen evolution side reaction. An effective process is thus expected, and indeed this material is the most active anode for the oxidation of various pollutants; more details will be given in the following sections.
Chapter 2. Bibliography homogeneous or heterogeneous chemical reactions occur at the electrode surface, in the region associated to surface phenomena. The electronic transfer at the electrode surface follows these reactions. Finally, the mass transfer from the electrode surface to the bulk The electron transfer reaction is influenced by the nature and the structure of the reacting species, the potential, the solvent, the electrode material and the adsorbed layers on the electrode. In order to understand these influences (interactions between reactant and electrode surface), microscopic theories have been developed, on the basis of two main concepts, which are known as inner-sphere and outer-sphere electron transfer reactions.
Chapter 2. Bibliography
Outer-sphere electron transfer reaction The term outer-sphere is used to describe a reaction, in which the activated complex maintains the coordination sphere originally presented in the reactant species (Figure 2A).
Chapter 2. Bibliography
Inner-sphere electron transfer reaction A reaction is described in terms of inner-sphere when the reactants share a ligand in the activated complex (Figure 2B). Therefore, both the reactant and the products species, as well as the activated complex, are involved in very strong interactions with the electrode surface (specific adsorption). This kind of reaction implies multistep electron transfer reactions . The benzoquinone/hydroquinone redox equilibrium is a typical inner- sphere reaction that involves complex electron and proton transfer mechanisms.
Chapter 2. Bibliography Frequently, the electrochemical oxidation of some organics in aqueous media takes place, without any loss in electrode activity, only at high potentials and with concomitant evolution of oxygen [106, 129, 199, 221]. Furthermore, it has been found that the nature of the electrode material influences strongly both the selectivity and the efficiency of the process [94, 221]. To interpret these observations, a comprehensive model for the anodic oxidation of organics in acidic medium, including the competition with the oxygen evolution reaction, has been proposed by Comninellis . The model (Figure 3) permits to illustrate the differences between two limiting cases, i.e. the so-called “active” and “non-active” anodes.
Figure 3. Scheme of the electrochemical oxidation of organic compounds on active (reactions a, b, c, d) and non active anodes (reactions a, e, f). M designates an active site at the anode.
Chapter 2. Bibliography In both cases, the first reaction (Equation `a’ in the figure) is the oxidation of water molecules leading to the formation of adsorbed hydroxyl radicals: M+HO?M(HO•)+H++e? (a) 2
Both the electrochemical and chemical reactivity of adsorbed hydroxyl radicals depends At active electrodes, there is a strong interaction between the electrode (M) and the hydroxyl radical (OH•). In this case, adsorbed hydroxyl radicals may interact with the anode, forming a so-called higher oxide MO (Equation `b’). This may be the case when higher oxidation states are available, for the electrode material, above the thermo- dynamic potential for the oxygen evolution (1.23 V vs. SHE).
M(HO•)?MO+H++e? (b) We can speculate that, at active electrodes, the redox couple MO/M acts as a mediator in the oxidation of organics (Equation `c’). This reaction is in competition with the side reaction of oxygen evolution, which is due to the chemical decomposition of the higher oxide (Equation `d’): MO + R ? M + RO (c) 1 MO ? M + O2 (d) 2 The oxidative reaction via the surface redox couple MO/M (Equation `c’) may be much more selective than the reaction involving hydroxyl radicals (Equation `e’). A typical example of an active electrode is the case of IrO2.
Chapter 2. Bibliography This model assumes that the electrochemical oxidation is mediated by hydroxyl radicals, either adsorbed at the surface (in the case of active electrodes) or free, in the case of the non-active ones.
Chapter 2. Bibliography
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Chapter 3. Global Parameters
Global parameters Total organic carbon (TOC) The total organic carbon (TOC) is amount of organic carbon in the sample, expressed in concentration of carbon (mol C/L or mg/L). The persulphate combined with irradiation UV (is the technique of analyzer TOC Dohrmann DC-80). The sample reacts with ion, associated at the same time to irradiation UV. The oxidation rate is remarkably increased respect to the method with single UV or only persulphate, due to the simultaneous ionization of dissolved organics; production of free sulphate radicals and hydroxyls highly reactive. After the CO2 produced is transported with a air or nitrogen, oxygen flow until the detector.