Electrochemical oxidation of organic pollutants for the wastewater treatment:direct and indirect process {{
Carlos A.Martı´nez-Huitle *and Sergio Ferro
Received 21st April 2006
First published as an Advance Article on the web 10th July 2006DOI:10.1039/b517632h
In recent years,there has been increasing interest in finding innovative solutions for the efficient removal of contaminants from water,soil and air.The prent tutorial review summarizes the results of an extensive lection of papers dealing with electrochemical oxidation,which is
propod as an alternative for treating polluted wastes.Both the direct and indirect approaches are considered,and the role of electrode materials is discusd together with that of other experimental parameters.
Apart from discussing the possibility of removing lected contaminants from water using different anodes,efficiency rates for pollutant removal have been collected,the dependence of the rates on operational conditions advantages and disadvantages determining the further full-scale commercial 解除占用
application.
Introduction那些花
银行行长述职报告The intensification of industrial activities,since the latter half of the XIX century and throughout the XX century,has inevitably caud vere environmental pollution with dra-matic conquences in atmosphere,waters,and soils.The conquent restrictions impod by new legislation require effective initiatives for pollution reduction,not only in gaous emissions and industrial aqueous effluents but also adequate decontamination in soils.Typically,in the ca of the latter,
different class of pollutants may have accumulated during long periods of uncontrolled waste disposal and reclamation may reprent a rious technological problem.Due to the extremely diver features of pollution phenomena,universal strategies of reclamation have not been found.1
Generally,wastewater treatment is carried out using pri mary,condary or tertiary methods,depending on the nature of the pollutants.As far as organic pollutants in wastewaters are concerned,biological abatement may sometimes be impos sible,due to the bio-refractory character of the substrates.For this reason,physical-chemical methods are preferably applied,but an oxidation with ozone or chlorine dioxide is not always effective and also transportation and storage of reactant
s may be a significant inconvenience for safe processing.2
An alternative can be the application of electrochemical technologies for wastewater treatment,benefiting from advan-tages such as versatility,environmental compatibility and
Department of Chemistry,University of Ferrara,via L.Borsari 46,I-44100Ferrara,Italy.E-mail:Carlos.Martinez@unimi.it;Fax:+390532240709
{Electronic supplementary information (ESI)available:Tables S1and S2:direct and indirect anodic oxidation of organic compounds (anode materials listed in alphabetical order).See DOI:10.1039/b517632h {Graphical abstract photo by Davide Semeraro (Italy),reproduced with permission.Carlos A.Martinez-Huitle was born in Mexico City,in 1977.He graduated in chemistry at t h e U ni v e r s i d a d d e l as Ame ´ricas—Puebla (2000).After a work experience in Ciba—Specialty Chemicals,he moved to the University of Ferrara,Italy (2002),to work under the supervision of Prof.Achille De Battisti.Here,he earned a PhD in chemical sciences (2005).During the same period,he worked as visiting scientist in the group of Prof.Christos Comninellis
at the EPFL Institute,Switzerland.He is a faculty member at the University of Milan.His rearch interests include electro-chemical oxidation,electrocatalysis and electroanalysis.
Sergio Ferro was born in 1972.He was awarded his ‘‘Laurea’’in Chemistry in 1997,and earned a PhD in Chemical Sciences at the University of Ferrara in 2001,where he is now working as a rearch assistant.Author of about 30papers in international jour-nals,co-author of 2book chap-ters and 3patents,he has received awards from national and international scientific committees.His work deals mostly with electrocatalysis,regarded as an extension of heterogeneous catalysis (its principles and methods)to electrochemical
reactions.
Carlos A.南京是什么城
Martinez-Huitle
Sergio Ferro
CRITICAL REVIEW /csr |Chemical Society Reviews
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2010.09.16
potential cost effectiveness among others described below.2Both direct and mediated electrochemical oxidations can be considered,and have proved to be interesting subjects for different rearch groups and industries eking new technol-ogies for wastewater treatment.3
In recent years,the applications of electrochemistry for environmental pollution abatement have been thoroughly investigated.3,4The feasibility of electrochemical conversion/destruction of organic substrates in wastewater,in particular,has attracted much attention since pioneering studies by Dabrowski in the 70’s,Kirk,Stucki,Kotz,Chettiar and Watkinson in the 80’s,and Comninellis in the early 90’s to the prent day.In the studies,the influence of the nature of the electrode material during the anodic mineralization of organics has been considered in detail,showing that optimal conditions for the process in question are obtained at high-oxygen-overpotential anodes.
Briefly,electrochemical oxidation is performed by the action of strong oxidants,similar to chemical destruction,but the in-situ electro-generation allows better efficiency of the abatement of the organic substrates.Direct electrochemical destruction has been investigated with particular focus since the end of the eighties,through the testing of different anodic materials for the oxidation of diver organic pollutants dissolved in water.Many organic substrates show a complex reactivity toward the anodic mineralization;in the cas,the central role of adsorbed hydroxyl radicals and the mode of adsorption of the organic species should be taken into account.Results obtained using simple alic acid,support the view that co-electrosorption of hydroxyl radicals and organic species affect the rate of the anodic mineralization,volcano-plot approaches of the type traditionally applied for ethylene electrochemical oxidation being possibly a good interpretative tool.5The considerations lend further evidence to the importance of the nature of the electrode material in electrochemical oxidation.5In this context,a general model mechanism propod by Comninellis 6considers the different stabilization exerted by the electrode material on electrosorbed hydroxyl radicals and satisfactorily accounts for the different results described in the literature.Accordingly,electrodes have been classified as active and non-active,on the basis of their electrocalytic properties.7On the other hand,indirect oxida-tion (also called mediated electrochemical oxidation)is bad
on the activity of strongly oxidant Cl ?,S 2O 2{
8and Ce IV ,and may also reprent an interesting alternative to the aforementioned wastewater treatments.The high acidic con-centration needed to achieve significant concentrations of the above species makes this method more suitable for the treatment of low water-content sludges,the main process being esntially a two-pha one.8–10The oxidative attack of organics in aquatic media does not follow a conceptually different path.We can speak formally of direct and indirect (mediated)process.Into the latter group we can include active-chlorine,ozone-mediated attacks and others,where the main reaction stages take place in the solution bulk.The former group would include tho process who main stages occur at the electrode surface,through adsorption of reactants and intermediates,the strong oxidant being esntially the hydroxyl radical.Regarding the recent mechanism proposal by
the Comninellis group,11which assumed the action of the latter extended to a reaction cage in the vicinity of the electrode surface,rather than limited at the surface itlf,the distinction between direct and mediated oxidation becomes even less stringent.12
The role of mediators such as Cl 2—of particular interest for its common prence in many different t
ypes of wastewater—brings the role of the electrode material to a prominent position.The Cl 2-mediated mineralization has been shown to give good results at low-oxygen-overvoltage electrodes,such as Pt.13–15The addition of chloride ions in the electrolyte allows an increa of the removal efficiency,and a degradation of pollutants can be obtained due to the participation of active chlorine.The results indicate indirect oxidation as cond alternative in the elimination of the organic pollutants from water.Similar to chlorides,bromides can also be effectively ud for the anodic oxidation of organic pollutants,but this anion has scarcely been investigated and only a limited number of examples are reported in the literature.16–20
In investigations of direct and indirect oxidations many organic substrates have been considered,as well as different experimental conditions and anode materials.Nevertheless,in many cas,the electrochemical process leads to the formation of stable carboxylic acids such as maleic,formic,acetic,malonic and oxalic acids.The molecules may reprent the polluting content of industrial in oil manufacturing.21While a thorough optimization of the process can suggest the u of a particular anode material,other motivations,such as the anode material availability and cost,can drive the choice.For the reasons,‘‘mediated electrochemical oxida-tion’’(MEO)was particularly studied by different rearch groups:13–20,22–25both the influence of th
e nature of the anion,and its concentration were analyzed in order to increa the effectiveness of the process.Particular interest has been addresd to chloride mediation due to the ubiquitous character of Cl 2species in wastewaters,and its relatively effective action.Different authors have already published the possible ‘‘direct’’and ‘‘indirect’’roles for the chloride anion in the electrochemical reaction.13–15,22–25
The aim of this work is to prent a thorough analysis of the literature concerning wastewater treatments,emphasizing the u of direct and indirect electrochemical oxidation process as an alternative to other wastewater treatments.
Wastewater treatment
The effective treatment of effluents reprents a rious problem,especially for the chemical industry.Over the last twenty-five years,huge efforts have been made to limit at the source this type of pollution,by improving process,recycling products and controlling the treatment of wastes at the production stage.However,considering the large amounts 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 destruction methods,for their part,po problems of corrosion and,more riously,of emissions,if the treatment conditions are not perfectly controlled.1
From the industry point of view,this problem must be examined as a whole since there are no universal or simple
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methods in this area.The wide 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 clean process,the increasingly vere environmental laws should encourage the rearch for better-performing treatments,making it possible to obtain environmentally compatible effluents.26Actually,the process for the treatment of wastewater may be divided into three main categories:primary,condary and tertiary.Tertiary treatment,also known as advanced wastewater treatment,includes acid/ba neutralization,precipitation,reduction and oxidation process.1
Advanced oxidation process
Different advanced oxidation process have been developed and investigated by veral rearch groups for the elimination of organic pollutants from wastewater,such as Fenton process,photo-as
sisted Fenton process,UV/Fe 3+-oxalate/H 2O 2,photocatalysis,ozone water system,Mn 2+/oxalic acid/ozone,H 2O 2photolysis,O 3/UV and others.27The technol-ogies consist mainly of conventional pha paration techni-ques (adsorption process,stripping techniques)and methods,which destroy the contaminants by chemical oxida-tion and/or reduction.Chemical oxidation aims at the mineralization of the contaminants to carbon dioxide,water and inorganics or,at least,their transformation into harmless products.Obviously,the methods bad on chemical destruc-tion,when properly developed,offer a complete solution to the problem of pollutant abatement,different from tho in which only a pha paration is realized with the conquent problem of the final disposal.It has been frequently obrved 28–31that pollutants not amenable to biological treatments may also be characterized by high chemical stability and/or by a great reluctance to go to complete mineralization.Also,the adoption of the oxidation treatments requires that specific conditions must be considered during the process:the influence of pH,inhibition due to scavenger prence,light wasting,mass transfer limitations,direct ozone attack and appropriate equipment.27In the cas,it is necessary to adopt much more effective reactive systems than tho adopted in conventional purification process.
Electrochemistry and environment
Electrochemistry,as a branch of physical chemistry plays an important role in most areas of science and technology.32Electrochemistry offers promising approaches for the preven-tion of pollution problems in the process industry.The inherent advantage is its environmental compatibility,due to the fact that it us a clean reagent,the electron.The strategies include both the treatment of effluents and waste and the development of new process or products with less harmful effects,often denoted as process-integrated environmental protection.33
The application of electrochemistry for the protection of the environment has been the topic of veral books and reviews.2,3,33–39Besides the process-oriented benefits,electro-chemistry is also playing a key role in nsor technology.
Electroanalytical techniques for monitoring and trace level detection of pollutants in air,water and soil as well as of microorganisms are needed for process automation.Sensors for environmental applications have been already reviewed,34,35while an interesting view on the role of electrocatalysis for electrochemistry and environment has recently been given by Trasatti.40
In a review by Rajeshwar et al.,the promising characteristics of approaches for the prevention and remediation of pollution problems have been explained in detail.2,36Versatility :veral techniques c
an be applied such as direct and/or indirect oxidations and reductions,pha parations,biocide func-tions,concentrations or dilutions;electrochemical methods can deal with many pollutants and treat from microliters to millions of liters.Energy efficiency :the process generally require lower temperature with respect to equivalent non-electrochemical counterparts (e.g.,thermal incineration);the potential can be easily controlled and operational parameters can be designed to minimize power loss.Amenability to automation :the electrical variables ud in the electrochemical process (j ,E )are particularly suited for facilitating data acquisition,process automation and control.Environmental compatibility :the electron is a clean and very effective reagent,who reactivity may be tuned by choosing a suitable electrocatalyst,in order to prevent the production of undesir-able metabolites.Cost effectiveness :the required equipment and operations are generally simple and inexpensive,but diver considerations must be studied for optimal efficiency.For the above reasons,electrochemistry can be considered an alternative for the prevention of pollution problems.Therefore,intensive rearch proceeds with the goal of discovering more efficient techniques,process,materials,technologies and applications of electrochemistry for the remediation and/or prevention of pollution problems.
Electrochemical technologies for wastewater treatment
Electrochemical technologies have gained importance in the world during the past two decades.There are different companies supplying facilities for metal recoveries,the treatment of drinking water as well as process waters resulting from tannery,electroplating,dairy,textile processing,oil and oil-in-water emulsion,etc .3At prent,electrochemical tech-nologies have reached such a state that they are not only comparable with other technologies in terms of cost,but sometimes they are more efficient and compact.The develop-ment,design and application of electrochemical technologies in water and wastewater treatment has been focud on particularly in some technologies such as electrodeposition,electrocoagulation,electrofloculation and electrooxidation.2
Electrochemical oxidation:an alternative in wastewater treatment
Studies on electrochemical oxidation for wastewater treatment go back to the XIX century,2when the electrochemical decomposi-tion of cyanide was investigated.41Extensive investigation of this technology commenced in the 70s,when Nilsson et al.in 1973
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investigated the anodic oxidation of phenolic compounds.
Mieluch et al.studied for the first time the electrochemical oxidation of phenol compounds in aqueous solutions.22In 1975,Dabrowski et al.studied the electrochemical purification of phenol-containing wastes in a pilot plant,43while Papouchado et al.investigated the pathways of phenolic compounds anodic oxidation.44Later,in 1979,Koile and Jonhson examined the electrochemical removal of phenolic films from platinum anodes;45in the same period,Smith de Sucre obtained relevant results in phenol electro-oxidation during wastewater treat-ment,46and in the 80s the studies were continued in collaboration with Chettiar.71,72
During the last two decades,rearch work has focud on the efficiency in oxidizing various pollutants at different electrodes,on the improvement of the electrocatalytic activity and electrochemical stability of the electrode materials,the investigation of factors affecting the process performance and the exploration of mechanisms and kinetics of pollutant degradation.3Experimental investigations,focusing on the behaviour of different anodic materials,have been carried out by different rearch groups,the results of which warrant a detailed description.Attempts for an electrochemical oxida-tion/destruction treatment for waste or wastewater can be subdivided into two
important categories:direct oxidation at the anode,and indirect oxidation using appropriate anodi-cally-formed oxidants.4
Direct and indirect electrochemical oxidation
Electrochemical oxidation mechanism
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,MO x +1).6This process is usually called ‘‘anodic oxidation’’or ‘‘direct oxidation’’and the cour for the anodic oxidation was described by Comninellis;6the complete destruction of the organic substrate or its lective conversion into oxidation products is schematically reprented in Fig.1.
When a toxic,non-biocompatible pollutant is treated,the electrochemical conversion transforms the organic substrate into a variety of metabolites;often,biocompatible organics are generated,and biological treatment is still required after the electrochemical oxidation.In contrast,electrochemical degra-dation yields water and CO 2,no further purification being 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.
A mechanism for the electrochemical oxidation of organics,bad on intermediates of oxygen evolution reaction in aqueous media,was formerly propod by Johnson.47–52The process involves anodic oxygen transfer from H 2O to organics via hydroxyl radicals formed by water electrolysis.
The electrochemical oxidation of some organics in aqueous media may take place without any loss in electrode activity,except at high potentials,and with concomitant evolution of oxygen.24,53–55Furthermore,it has been described that the nature
of the electrode material strongly influences both the lectivity and the efficiency of the process.6,55–57To interpret the obrvations,a comprehensive model for the anodic oxidation of organics in acidic medium,including the competition with the oxygen evolution reaction,has been propod.6,55–57More recent results,obtained at conductive diamond electrodes 7(which are characterized by a very high oxygen overpotential),fit the model predictions quite well.Bad on the results,Comninellis explained the differences considering two limiting he so-called ‘‘active’’and ‘‘non-active’’anodes.7
In both cas,the first reaction (eqn (a))is the oxidation of water molecules leading to the formation of adsorbed hydroxyl radicals:
高迁M +H 2O A M(HO ?)+H ++e 2
(a)
Both the electrochemical and chemical reactivities of adsorbed hydroxyl radicals depend strongly on the nature of the ud electrode material.
With active electrodes there is a strong interaction between the electrode (M)and the hydroxyl radical (OH ?).Adsorbed hydroxyl radicals may interact with the anode,forming a so-called higher oxide MO (eqn (b)).This may be the ca when higher oxidation states are available,for the electrode material,above the thermo-dynamic potential for the oxygen evolution (1.23V vs .SHE).6
M(HO ?)A MO +H ++e 2
(b)
With active electrodes,the redox couple MO/M acts as a mediator in the oxidation of organics (eqn (c)).This reaction is
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in competition with the side reaction of oxygen evolution,which is due to the chemical decomposition of the higher oxide (eqn (d)):
MO +R A M +RO (c)MO ?M z 1
2
O 2
(d)
The oxidative reaction via the surface redox couple MO/M (eqn (c))may be much more lective than the reaction involving hydroxyl radicals (eqn (e)).A typical example of an active electrode is the ca of IrO 2.6
With a non-active electrode ,weak interactions exist between the hydroxyl radical and the electrode surface.In this ca,the oxidation of organics is mediated by hydroxyl radicals (eqn (e))and may result in fully oxidized reaction products such as CO 2.
M(HO ?)+R A M +m CO 2+n H 2O +H ++e 2
(e)
In the above schematic equation,R is a fraction of an organic compound containing no heteroatoms,which needs one oxygen atom to be fully transformed into CO 2.7This reaction competes with the side reaction of hydroxyl radicals (direct or indirect consumption,through the formation of hydrogen peroxide as intermediate)to oxygen (eqn (f))without any participation of the anode surface:
M HO .ðÞ?M z 1
O 2z H z z e {
(f)思念作文500字
A non-active electrode does not participate in the anodic reaction and does not provide any catalytic active site for the adsorption of reactants and/or products from the aqueous medium.In this ca,the anode rves only as an inert substrate,which can act as a sink for the removal of electrons.In principle,only outer-sphere reactions and water oxidation are possible with this kind of anode.Interm
ediates produced by the water oxidation are subquently involved in the oxidation of organics in aqueous medium.7
The electrochemical activity (which may be related to the overpotential for oxygen evolution)and chemical reactivity (rate of the organics oxidation with electrogenerated hydroxyl radicals)of adsorbed OH ?are strongly linked to the strength of the M–OH ?interaction.As a general rule,the weaker the interaction,the higher the anode reactivity for organics oxidation (fast chemical reaction);boron-doped diamond electrodes (BDD)are typical non-active electrodes,character-ized by high stability and acceptable conductivity.This model assumes that the electrochemical oxidation is mediated by hydroxyl radicals,either adsorbed at the surface (in the ca of active electrodes)or free,in the ca of the non-active ones.7Direct anodic oxidation
The anodic oxidation does not require the addition of large amounts of chemicals to wastewater or the feeding of O 2to cathodes,as in Fenton process;27moreover,there is no tendency to produce condary pollution and fewer accessories are required.The advantages make the anodic oxidation more attractive than other oxidation process.3As previously commented,the most important parameter in this process is
the anode material.Among the investigated anode materials,the following can be mentioned:stainless steel,58glassy carbon,59Ti/RuO 2,Ti/Pt–Ir,60,61carbon fibers,62MnO 2,63,64Pt–carbon black,65,66porous carbon felt 67and reticulated vitreous carbon.68,69Unfortunately,none of them have either sufficient activity or satisfactory stability.Pt,PbO 2,IrO 2,SnO 2,and conductive diamond films are the most extensively studied anodes.
Bad on the literature,a collection of data obtained at different anodes for the degradation of some important pollutants,under different conditions,can be found in Table S1of the electronic supplementary information {.It is also worth mentioning that this table contains the most relevant rearch in the frame of the direct electrochemical oxidation from the beginning of its application to the prent time.On the other hand,different parameters have been resumed:the current density,the current efficiency (CE)53and other efficiency parameters are of particular interest,as well as the possible intermediates and/or final metabolites.The current efficiency 53,56is a measure of the process effectiveness;it can alternatively be expresd by means of the Electrochemical Oxidation Index (EOI),the Apparent Current Efficiency (ACE),and/or the Instantaneous Current Efficiency (ICE),as described in Appendix 1.
Phenol and derivates are among the most investigated examples in electrochemical studies.Initial re
arch was carried out by Nilsson et al.in 1973;42Mieluch et al.,22and Dabrowski et al.also tried the u of electrochemical oxidation for the destruction of phenolic waste on a pilot-scale plant,in 1975,43while Smith de Sucre,Chettiar and Watkinson ud synthetic wastewater solutions in early 80’s.70–72
Smith de Sucre and Watkinson investigated the oxidation of phenol for wastewater treatment applications at packed lead dioxide anodes.They operated in both divided and undivided cells,obtaining analogous oxidation rates.The percentages of oxidized phenol ranged from 53%to 77%,depending on the type of cell and operational conditions (current intensity from 10to 20A,and variable pH values).70
Considering ref.72,the process has not found a commercial u becau of the low reaction rate and/or low efficiency.One reason for the low reaction rate,in the electrochemical oxidation of some organic compounds,is electrode he blocking of the electrode surface by reaction products.In the ca of phenol,polymerization products are obtained.Under the conditions,a 96%destruction of phenol was obtained,with a 50%reduction in biological oxygen demand but a reduction in total organic carbon of only 22%.
From an examination of data in Table S1of the ESI {,Pt and PbO 2are the most widely investigated anode materials for electrooxidation.PbO 2is one of the classic high-oxygen-overpotential materials and it is expected to perform quite well in electrochemical mineralization of organics (e.g.,for the oxidation of gluco at different anodic materials,Fig.2).At this electrode material,Kirk realized the oxidation of aniline,obtaining good removal efficiencies:the current efficiency ranged from 15%to 40%for the complete oxidation of the organic substrate to CO 2.73The operating current density was also reasonably high:80–160A m 22.The performance of this electrode material was further evaluated in terms of faradaic
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