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EnvironmentalScienceWater Research & Technology

CRITICAL REVIEW

Cite this: Environ. Sci.: Water Res.

Technol., 2016, 2, 800

Received 9th December 2015,Accepted 5th May 2016

DOI: 10.1039/c5ew00289c

rsc.li/es-water

Electrochemical technologies for wastewatertreatment and resource reclamation

Yujie Feng,*a Lisha Yang,a Junfeng Liua and Bruce E. Loganb

Research developments in environmental electrochemistry and their potential to contribute to a cleaner

environment are reviewed here for wastewater treatment applications. Most environmental pollutants can

be successfully eliminated or converted to non-toxic materials by one or more processes, including

electrochemical oxidation, electrochemical reduction, electrocoagulation and electrocoagulation/flotation,

electrodialysis, and electrochemical advanced oxidation processes. Specific examples of applications for

pollutant removal and reclamation of wastewater are given for the different processes, along with research

needs and improvements for commercial application of these electrochemical processes.

1. Introduction

As a consequence of industrial development activities, tech-nological progress and profit have sometimes prevailed overenvironmental concerns, resulting in an increasing numberof detrimental pollutants that have found their way into the

800 | Environ. Sci.: Water Res. Technol., 2016, 2, 800–831 This journal is © The Royal Society of Chemistry 2016

Yujie Feng

Prof. Yujie Feng obtained herPh.D. and M. Phil. from HarbinInstitute of Technology andBachelor's degree from TianjinUniversity. She has been workingat Harbin Institute of Technol-ogy as a Lecturer (1994–1998),Associate Professor (1998–2002)and Professor (since 2002–pres-ent). She is currently the DeputyDirector of the State Key Labora-tory of Urban Water Resourceand Environment (HIT) of theNational Ministry of Science &

Technology. She is also a Visiting Professor at Penn State Univer-sity, USA and Liaoning University, China and a Fellow of Interna-tional Water Association (IWA). Her research is focused on waste-water treatment and energy recovery, and risk evaluation of toxiccompounds or nano-materials in engineering systems.

Lisha Yang

Lisha Yang is a Ph.D. student atHarbin Institute of Technology(2013–present). She received herBachelor's degree and Master'sdegree in Environmental Scienceand Engineering at HeilongjiangUniversity. Her research interestis in the development of newroutes for preparation of nano-sized coating electrode materialsfor applications in electro-chemical technologies for waste-water treatment and resourcereclamation.

a State Key Laboratory of Urban Water Resource & Environment, Harbin Institute

of Technology, Harbin 150090, PR China. E-mail: [email protected];

Fax: +86 451 86287017; Tel: +86 451 86287017, +86 451 86283068bDepartment of Civil and Environmental Engineering, The Pennsylvania State

University, 212 Sackett Building, University Park, PA 16802, USA

Water impact

Electrochemical treatment of pollutants in water can be accomplished in many different ways, for example, by direct oxidation and reduction reactions,through the production of reactive chemical species, or by releasing chemicals that achieve physical removal. These different electrochemical processes arecritically reviewed here, noting specific challenges to advance existing and new technologies for cost-effective water treatment.

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Environ. Sci.: Water Res. Technol., 2016, 2, 800–831 | 801This journal is © The Royal Society of Chemistry 2016

environment. Thus, the removal of environmental pollutantshas become a major issue and a crucial factor for the sustain-able development of modern industrial processes, whichmust comply with regulations to ensure clean environments.

Industrial electrochemistry has undergone developmenttowards cleaner processes and more environmentally friendlyproducts, which is one of the strategies for environmentalprotection. As a consequence, a special research field of envi-ronmental electrochemistry has been developed, which isbased on using electrochemical techniques to remove impuri-ties from gases, liquids, or soils, to prevent or minimize envi-ronmental pollution. The first book devoted to the potentialof electrochemistry for environmental protection appearedfour decades ago by Bockris.1 Since then, several books andreviews have been devoted to this topic,2–20 which have indi-cated the inherent advantages of several different electro-chemical technologies based on their environmental compat-ibility, due to the fact that the main reagent, the electron, isa ‘clean reagent’. Other attractive advantages are related toversatility, high energy efficiency, amenability to automation,and cost-effectiveness.

From the viewpoint of high efficiency and low resource con-sumption, electrochemical technologies can be used either as apretreatment step to increase the biodegradability of a pollutantor as an advanced treatment method further to reduce CODor color in the water to achieve relevant effluent standards.

Based on analyzing the characteristics of the wastewaterquality, two combined electrochemical processes have beenproposed by the research group of Feng and co-workers,21,22

as shown in Fig. 1.In this paper, we reviewed the electrochemistry-based ap-

proaches for removal of pollutants in wastewaters, such aselectrochemical oxidation, electrochemical reduction, electro-coagulation and electrocoagulation/flotation, and electrodial-ysis processes. Emerging technologies such as electro-Fentonand photoelectro-Fenton processes, photoelectrocatalysis,and sonoelectrocatalysis are also discussed along with the rel-

ative advancements and recent achievements. The fundamen-tals of each technology are briefly discussed in order to betterunderstand their advantages and limitations for practical ap-plications in the removal and treatment of environmentalpollutants in water and wastewater.

Junfeng Liu

Dr. Junfeng Liu obtained his Ph.D. and M. Phil. from Harbin In-stitute of Technology. Dr. Liu iscurrently a lecturer at the Schoolof Municipal and EnvironmentalEngineering, Harbin Institute ofTechnology. His research inter-ests are in the fields of environ-mental electrocatalytic mate-rials, photocatalytic materialsand wastewater refractory or-ganic pollutant removal technol-ogy. He has published more than30 papers, including 14 papers

indexed by SCI.

Bruce E. Logan

Dr. Logan earned his B.S. andM.S. in Chemical Engineeringand Environmental Engineering,respectively, at Rensselaer Poly-technic Institute and his Ph.D. inEnvironmental Engineering atUniversity of California, Berke-ley. He started his career as anAssistant Professor at the Univer-sity of Arizona, and became anAssociate Professor, then a Pro-fessor before moving to ThePennsylvania State University.Dr. Logan is currently an Evan

Pugh Professor and the Stan and Flora Kappe Professor of Envi-ronmental Engineering in the Department of Civil and Environ-mental Engineering at The Pennsylvania State University.

Fig. 1 Flow diagram of combined electrochemical processes: (A) two-phase anaerobic (CSTR + EGSB)–aerobic (SBR)–electrocatalytic oxida-tion (1. electroflocculation reactor, 2. electrochemical reactor, 3. CSTR,4. EGSB, 5. SBR, 6. electrochemical reactor); (B) coagulation–electrocat-alytic oxidization–biological contact oxidation combined process (1. ad-justable water tank, 2. electrocoagulation, 3. sedimentation tank, 4. reg-ulating tank, 5. three-dimensional fixed bed electrochemical devicesystem, 6. regulating tank, 7. biological contact oxidization device sys-tem, 8. effluent trough, 9. pump, 10. valve, 11. DC power supply, 12.aeration system, 13. sludge collection system).21,22

Environmental Science: Water Research & Technology Critical review

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2. Electrochemical oxidation

Electrochemical oxidation is considered to be a very powerfultool for breaking up even the most resistant organic com-pounds.23 Anodic oxidation of organic contaminants can beperformed in several different ways, including both directand indirect oxidation.

2.1 Direct anodic oxidation

In direct anodic oxidation (or direct electron transfer to theanode), the pollutants are destroyed after adsorption on theanode surface, without the involvement of any substancesother than the electron. Such oxidation is theoretically possi-ble at more negative potentials than those needed for watersplitting and oxygen evolution. However, this process usuallyresults in electrode fouling due to the formation of polymericlayers on its surface and consequently leads to very poorchemical decontamination.24,25 Gattrell and Kirk investigatedthe electro-oxidation of phenol with platinum andperoxidized platinum anodes using cyclic voltammetry andchronoamperometry. Their studies demonstrated that phenolcan be irreversibly adsorbed on metallic platinum, quicklypassivating the electrode.26

2.2 Indirect anodic oxidation via intermediates of oxygenevolution

To avoid the drawbacks of direct oxidation, the indirect oxi-dation method based on the oxygen evolution region can beused, which has an advantage over direct electrolysis in thatit does not need addition of oxidation catalysts to the solu-tion, and it does not produce by-products.

In this process, the electrochemical reaction leads to par-tial or total decontamination of the electrogenerated speciesat the anode due to physically adsorbed “active oxygen”(adsorbed hydroxyl radicals ˙OH) or chemisorbed “active oxy-gen” (oxygen in the lattice of a metal oxide (MO) anode).27,28

This physisorbed ˙OH is the second strongest oxidant knownafter fluorine, with a high standard potential (E0 = 2.80 V vs.SHE) that ensures the complete combustion of organic com-pounds, and the chemisorbed “active oxygen” participates inthe formation of selective oxidation products.

As has been established in many studies,29,30 the nature ofthe anode material influences not only the efficiency of theprocess, but also the electrode selectivity. For example, “ac-tive anodes” with low oxygen evolution overpotentials such asIrO2, RuO2 or Pt favor the partial and selective oxidation ofpollutants, while “non-active anodes” with high oxygen evolu-tion overpotentials such as SnO2, PbO2 or boron-doped dia-mond (BDD) can facilitate complete combustion, and thusthey are regarded as ideal electrodes for the complete oxida-tion of organics to CO2 in wastewater treatment (Fig. 2).

2.2.1 Platinum electrodes. Platinum anodes have a longhistory of use as electrode materials for the oxidation of or-ganics because of their good conductivity and chemicalstability.

The oxidation of a wide range of biorefractory organiccompounds on platinum anodes has been reported in manystudies.31–38 Platinum electrodes have a relatively low oxygenevolution overpotential (i.e., 1.6 V vs. SHE in 0.5 M H2SO4)which can enable selective conversion of pollutants at a lowcurrent efficiency. In a study by Feng et al.30 on the electro-oxidation of phenol using platinum electrodes, it was foundthat the concentration of phenol rapidly decreased to zero,but residual TOC concentrations suggested that the overalldegradation reactions significantly slowed down due to theformation of intermediate products.

The electro-oxidation of phenol on platinum anodes wasinvestigated in depth by Comninellis and Pulgarin.29,33 Theirexperimental results indicated that aromatic intermediates(hydroquinone, catechol, benzoquinone) initially were formedduring electrolysis, and subsequently the aromatic ring op-ened with the formation of aliphatic acids (e.g., maleic,fumaric, and oxalic acid) which resisted further electro-oxida-tion. Thus, complete TOC removal could not be achieved,and the current efficiency decreased during the electrolysisprocess (Fig. 3).

2.2.2 Ruthenium- and iridium-based oxide electrodes. Di-mensionally stable anodes (DSAs) consist of a titanium basemetal covered by a thin conducting layer of metal oxide ormixed metal-oxide oxides, and were invented by Beer in thelate 1960s.39 RuO2- and IrO2-based anodes have been widelyused due to their mechanical resistance as well as being rela-tively inexpensive (compared to Pt) and successful scale-updemonstrated in some electrochemical industries, such asthe chlor-alkali industry, water electrolysis, and metalelectrowinning.

During the past two decades, DSA-type anodes coated witha layer of RuO2 and IrO2 have begun to be extensivelyemployed in the field of wastewater treatment.30,40–47

However, when these electrodes are used at high currentdensities, organic oxidation can yield low current efficienciesfor complete combustion since they favor the secondary reac-tion of oxygen evolution. Electrochemical destruction of4-chlorophenol in an aqueous medium using a platinum an-ode coated with a RuO2 film has been studied by Johnson

Fig. 2 Scheme of the electrochemical oxidation of organiccompounds on (a) “active” and (b) “non-active” anodes.

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et al.27 They found that this type of electrode was stable andactive when used in a cell with a solid Nafion membranewithout the addition of a soluble supporting electrolyte. How-ever, the time required for complete COD and TOC removalwas too long and the current efficiency was low.

Titanium is also used as an inexpensive coating onelectrodes. The electro-oxidation of 1,4-benzoquinone in water,in which the benzoquinone concentration and intermediateproducts during oxidation with Ti/IrO2 and Ti/SnO2 anodeswere monitored, has been investigated by Pulgarin et al.48 Itwas found that the most important factor was the compositionof the anode. With the Ti/IrO2 anode, the primary oxidationstep was easily achieved (benzene ring rupture), resulting inthe accumulation of carboxylic acids formed as the final non-toxic products. With the Ti/SnO2 anode, carboxylic acids wereformed at a much faster reaction rate and then oxidized, pro-ducing only CO2 as the final product.

Electro-oxidation of Reactive Blue 19 solutions in a three-electrode quartz cell equipped with a Ti/Ru0.3Ti0.7O2 anodewas investigated by Pelegrini et al. They obtained 35% decol-orization efficiency and 9.6% TOC removal after 2 h ofelectrolysis.49

Electrochemical degradation of phenol with five differenttypes of anodes (three RuO2-based electrodes, Ti/PbO2 and Ptelectrodes) was evaluated by Feng et al.30

As shown in Fig. 4, the relative performance for phenoldegradation of the three RuO2 electrodes decreased in theorder: Ti/Sb–Sn–RuO2–Gd > Ti/Sb–Sn–RuO2 > Ti/RuO2. How-ever, these electrodes were less efficient than Pt or Ti/PbO2

electrodes. Aromatic ring opening occurred using all theseelectrodes, but with the three RuO2-based electrodes, phenolwas decomposed into aromatic intermediates, such as benzo-quinone and hydroquinone, or several carboxylic acids, suchas maleic acid, succinic acid, and oxalic acid. Full mineraliza-tion to CO2, or complete TOC removal, only was obtained forthe Ti/PbO2 anode (Fig. 4).

DSA-type anodes coated with a layer of RuO2 or IrO2 canbe used efficiently for organics degradation by indirectelectrolysis by in situ generation of active chlorine throughthe oxidation of chloride ions present in the solution.45,50–54

For example, Kraft55 showed that DSA-type electrodes (IrO2

and IrO2–RuO2) gave higher current efficiencies duringelectrochemical chlorine production than boron-doped dia-mond (BDD) and platinum (Pt) anodes (Fig. 5).

Scialdone et al. investigated the electrochemical oxidationof organics on IrO2–Ta2O5 anodes in the presence and

Fig. 3 Evolution of (1) phenol, (2) aromatic intermediates, (3) aliphaticacids, and (4) CO2 during oxidation of phenol at the Pt anode: i = 50mA cm−2, T = 70 °C.29,33

Fig. 4 Electrochemical degradation of 100 ppm phenol in 60 ml ofelectrolyte as a function of charge passed for different electrodematerials, i = 10 mA cm−2 (–■–) Ti/RuO2; (–▲–) Ti/Sb–Sn–RuO2; (–●–)Ti/Sb–Sn–RuO2–Gd; (–□–) Ti–PbO2; and (–○–) Pt.30

Fig. 5 Dependence of the electrochemical free chlorine productionefficiency on the chloride content of electrolyzed water understandard conditions using four different anode materials (iridium oxide,mixed iridium/ruthenium oxides, platinum, doped diamond).55


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absence of NaCl, in a continuous batch recirculation reactionsystem equipped with a parallel plate undivided electro-chemical cell. Their results showed that in the presence ofNaCl, high current efficiency (CE) was generally obtainedusing DSA anodes at high current densities and low flowrates.56

An electrochemical oxidation approach for the treatmentof a high-salinity reverse osmosis (RO) concentrate was inves-tigated using boron-doped diamond (BDD) and titanium-based dimensionally stable anodes (Ti/IrO2–RuO2). The re-sults showed that both direct oxidation and indirect oxida-tion by active chlorines played a role in the treatment of theRO concentrate, but the contribution was different for the an-odes. The highest COD removal was observed using the BDDelectrode at the same current density as that of the otherelectrode, but the least energy consumption was obtainedusing the Ti/IrO2–RuO2 electrode.

57

2.2.3 Lead dioxide electrodes. PbO2 electrodes have beenused for the production of perchlorate since 1934. In the past30 years, the development of PbO2 anodes for the oxidationof organics has gained great interest in environmental appli-cations because of their good conductivity and largeoverpotential for oxygen evolution in acidic media, which en-able the production of hydroxyl radicals during waterdischarge.58–65 However, practical applications in the oxida-tion of organics with this type of electrode have been limiteddue to its relatively shorter electrode service life, as well asconcerns over the possible release of Pb4+ ions into water.The release of Pb4+ can occur due to the formation of cracksin the coating from increased internal stress generated fromthe electrode position of PbO2.

In order to improve the stability of Pb electrodes, one ap-proach that is being investigated is the incorporation of me-tallic or nonmetallic species such as Fe2+,66–70 Bi3+,71–75

Co2+,76,77 and F− (ref. 69, 76, 78–80) into the PbO2 crystallinematrix.

Andrade et al. showed that incorporation of F, Fe, and Coions into the PbO2 film enhanced its chemical stability com-pared to that of pure PbO2 in the oxidation of simulatedwastewaters containing Blue Reactive 19 dye or phenol.69,76

Another approach for stabilizing lead electrodes is basedon introducing a transition layer between the coating and thesubstrate. Antimony-doped tin oxide has been widely investi-gated as a transition layer for PbO2 elecrodes.81–87 The latticesize of SnO2 is between β-PbO2 and TiO2, and therefore, theSb-doped SnO2 transition layer can enhance the solid solubil-ity and hence reduce the internal stress of the Ti/SnO2/β-PbO2

electrode and improve the binding force between the PbO2

coating and Ti substrate, and it also inhibits the formation ofa TiO2 layer.

Bi et al. explored the electro-deposition of PbO2 on the Tisubstrate with an Sb-doped SnO2 undercoating, using a tradi-tional acidic nitrate solution. They investigated the morphol-ogy and microstructure of the PbO2 coatings by varying theelectro-deposition temperature and time. Their results indi-cated that the electrochemical performance of the deposited

PbO2 was largely a function of the resulting morphologiesand microstructures on the electrodes.82

In order to reveal the mechanism of the enhanced electro-chemical performance of the TiO2-NTs/SnO2–Sb/PbO2

electrode, the interlayer of Sb-doped SnO2 (SnO2–Sb) andTiO2 nanotubes (TiO2-NTs) on Ti were introduced into thePbO2 electrode system.83 This electrode with nanotubes hada more regular and compact morphology than previous Ti/SnO2–Sb/PbO2 electrodes, as well as better oriented crystalswith smaller sizes (Table 1). Kinetic analyses indicated thatthe electrochemical oxidation of nitrobenzene on the PbO2

electrodes followed a pseudo-first-order reaction, and masstransport was enhanced at the constructed electrode.

An alternative approach for stabilizing Pb electrodes wasto position an interlayer of TiO2 nanotubes (NTs) between afluorine resin (FR)-doped PbO2 coating and the Ti sub-strate.88 The improvement in surface properties and micro-structure was investigated by comparison to traditional PbO2

electrodes. This treatment improved the electrochemical re-sistance of the electrode in a Na2SO4 solution to 12.2 Ω withPbO2/TiO2-NTs/Ti, compared to 147 Ω with PbO2/Ti. The ser-vice life of PbO2/TiO2-NTs/Ti was increased to about 335 h,which was 7.1 times that of the PbO2/Ti electrodes. Pb

4+ wasdetected in the electrolyte after a 50 h electrochemical degra-dation test using PbO2/Ti (1.1 × 10−5 M) and PbO2/TiO2-NTs/Ti (3.4 × 10−6 M) electrodes, but none was detected with theFR-PbO2/TiO2-NTs/Ti electrodes. This suggests that firmbonding between PbO2 and the substrate was achieved, withthe PbO2 stably deposited onto TiO2-NTs, and this associa-tion was improved with FR doping. Contact between SO4

2−

and Pb4+ was likely blocked by the FR, inhibiting the anodicdissolution of the PbO2 coating.

To gain better knowledge on the ability of these Pb anodesto eliminate pollutants, many researchers have undertakencomparative studies on the performance of electrochemicaloxidation with boron-doped diamond (BDD) and PbO2 an-odes. A comparative study between PbO2 and BDD anodes forelectrooxidation of cresols (o-, m- and p-cresol) showed thatcomplete electrochemical incineration was achieved at thesame time as the initial pollutant was removed, sinceBDDĲ˙OH) simultaneously destroys all oxidation by-productsformed. The mineralization process of the m-cresol effluent

Table 1 Lattice parameters (a = b and c) and unit cell volume (V) for Sb-doped SnO2 and PbO2 from XRD patterns83

Electrode

Unit cell parameter

a (Å) c (Å) V (Å3)

(Standard SnO2)a 4.738 3.187 71.54

SnO2–Sb in Ti/SnO2–Sb interlayer 4.727 3.185 71.17SnO2–Sb in TiO2-NTs/SnO2–Sb interlayer 4.704 3.173 70.21(Standard PbO2)

b 4.955 3.383 83.06PbO2 in Ti/SnO2–Sb/PbO2 electrode 4.952 3.380 82.88PbO2 in TiO2-NTs/SnO2–Sb/PbO2 electrode 4.950 3.379 82.79

a The lattice parameters of SnO2 (JCPDF 41-1445). b The latticeparameters of PnO2 (JCPDF 65-2826).


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on PbO2 under comparable conditions is much less efficientdue to the lower oxidation ability of PbO2(˙OH). Despite thisfact, a shorter electrolysis time is needed for the total disap-pearance of m-cresol with PbO2 than with BDD.89 Electro-chemical oxidation of ibuprofen (Ibu) using a Ti/Pt/PbO2

electrode and a boron-doped diamond (BDD) electrode wasinvestigated by Ciríaco et al., showing a much higher degra-dation efficiency for the BDD anode of 20 mA cm−2 than thatfor the PbO2-based electrode.90

In contrast, work by Zhao et al.88 showed that a FR-PbO2/TiO2-NTs/Ti anode had similar morphology and surface wet-ting properties, a higher OEP, and better electrochemical per-formance than a boron-doped diamond film (BDD) electrode(Table 2). The improved surface hydrophilic properties ofPbO2 electrodes could produce better conditions leading tothe increased chemical adsorption ability on the surface andsubsequently lower utilization of ˙OH than BDD anodes. Thephysical resistance of the PbO2 electrode was much lowerthan that of BDD, and therefore it had higher conductivity.Hydroxyl radical utilization is significantly enhanced on aPbO2 electrode, which has been shown to produce a higheroxidation rate and higher removal efficiency for 2,4-dichlorophenoxyacetic acid than a BDD electrode.91,92

A comparative study of the electrochemical mineralizationof environmentally persistent long-chain perfluorinated car-boxylic acids (PFCAs) with Ti/SnO2–Sb/Ce–PbO2 and Ti/BDDanodes was carried out using galvanostatic control at roomtemperature.93 The results showed that the performance ofthe PbO2 electrode was comparable with that of a BDDelectrode. After 180 min of electrolysis, the PFNA removal ef-ficiencies on the BDD and PbO2 electrodes were 98.7 ± 0.4%and 97.1 ± 1.0%, respectively, while the corresponding PFDAremoval efficiencies were 96.0 ± 1.4% and 92.2 ± 1.9%.

Other authors have reported similar performance usingPbO2 and BDD anodes during oxalic acid incineration.52,94

There is a strong interaction between this compound and PbĲIV)sites which promotes anodic oxidation, and the rate of oxida-tion was only limited by mass transfer to the electrode surfaceat high current densities and low substrate concentrations.

2.2.4 Tin dioxide electrodes. Pure SnO2 crystal is an n-typesemiconductor, which has a wide band-gap energy (Eg) value(3.5–4.3 eV),95 and its conductivity is too poor to be used asan electrode material. However, the conductivity of SnO2 canbe enhanced by adding some doping elements. In mostcases, antimony is used as a dopant, and new energy bandscan be induced.

Titanium anodes coated with Sb-doped tin oxide havebeen considered as one of the most suitable electrodes in theelectrochemical oxidation of refractory organics because oftheir large overpotential for oxygen evolution (1.9 V vs. SHE)and high yield of hydroxyl radicals.30,96,97

In general, two different mechanisms can be distin-guished for the oxidation of organic pollutants on SnO2 an-odes. One is direct oxidation, and the other is indirect oxida-tion. Direct oxidation can only take place on the surface ofSnO2 anodes. Indirect oxidation can occur via hydroxyl radi-cals, which are generated by oxygen vacancies on the SnO2

anodes.98 Electrochemical oxidation of organic pollutants onSnO2 anodes mainly depends on indirect oxidation.

Research has shown that the electrochemical characteris-tics and service life of SnO2 anodes are influenced by thepreparation method as well as other factors. Ti metal is oftenused as a base material for SnO2 electrodes because of its lowcost and stability. The key aspect of SnO2 anode preparationis achieving a stable catalytic coating on the Ti base, whilealso ensuring that the catalytic coating is stable and well ad-hered to the base material.

The particle size of the SnO2 crystal has a great influenceon the electrochemical characteristics of SnO2 anodes.Smaller particle sizes means larger surface areas, which willimprove the overall electrocatalytic reaction rates. The parti-cle size of SnO2 crystals prepared by electrodeposition andsol–gel methods is on the nanometer scale, which provides avery high specific surface area. Of all preparation methods,the sol–gel method is a relatively simple, effective, and conve-nient way to produce effective SnO2 nanocoatings.

97

The performance of Sb-doped SnO2 anodes has been previ-ously investigated using phenol. It has been shown in the workof Stucki and co-workers that doped SnO2 anodes oxidized awide range of organic compounds with an efficiency about fivetimes higher than that of platinum anodes.34,99 Similar resultswere also obtained by Comninellis and Pulgarin,96 in theirstudies on the electrochemical oxidation of phenol on doped-SnO2 and platinum anodes. They found that aliphatic acidswere rapidly oxidized, with only very small amounts of aromaticintermediates, using a SnO2 anode, but the Pt anode had lowdegradation rates and produced a large amount of intermedi-ates. Comninellis measured a CE of 0.58 using an SnO2–Sb2O5

electrode, and obtained 71% degradation of phenol, comparedto lower CEs of 0.18 (PbO2), 0.17 (IrO2), 0.14 (RuO2) and 0.13(Pt) at a current density of 500 A m−2 (pH = 12.5, initial phenolconcentration = 10 mm, and reaction temperature of 70 °C).100

Table 2 Physicochemical characterization of FR-PbO2/TiO2-NTs/Ti, PbO2/TiO2-NTs/Ti, PbO2/Ti and BDD electrodes88

Electrode Loading capacity (g m−2) ROCPct (Ω) Extrapolated OEP0

a (V vs. SCE) Service lifetime (h)

FR-PbO2/TiO2-NTs/Ti 971.6 12.2 2.50 335PbO2/TiO2-NTs/Ti 950.1 34.8 1.90 170PbO2/Ti 705.9 147 1.80 47BDD — 34 500 2.40 —

a The extrapolated OEP0 at zero current is obtained by using an extrapolation technique from the anodic polarization ( j–E) curve tested in 0.5M H2SO4 aqueous solution, which is equivalent to the minimum decomposition potential for water.


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Electrochemical degradation of phenol was investigated byFeng et al. using three different types of anodes: Ti/SnO2–Sb,Ti/RuO2, and Pt. Although phenol was oxidised by all of theanodes at a current density of 20 mA cm−2 or a cell voltage of4.6 V, the intermediate products of phenol degradation, in-cluding benzoquinone and organic acids, were subsequentlyrapidly oxidized by the Ti/SnO2–Sb anode but accumulated insolution using Ti/RuO2 and Pt. The degradation rates of theTi/RuO2 and Pt anodes were considerably slower, as theSnO2–Sb coating improved the catalytic reaction and allowedrapid organic oxidation driven by hydroxyl radicals generatedfrom anodic water electrolysis (Fig. 6).101

The great potential of SnO2 for electrochemical treatment ofmany different organic chemicals has been shown, including:phenol,96,102–108 aliphatic acids,106 dyes,109,110 drugs,111,112

Bisphenol A (BPA),113 nitrophenol,114 2,4-dichlorophenol,115

4-chlorophenol,116 pentachlorophenol,117,118 perfluorooctanoicacid,109,119 perfluorinated carboxylic acids (PFCAs),93 industrialwastewater120 and naphthylamine.121 Nevertheless, the majordrawback that has prevented the practical applications of Ti/SnO2–Sb electrodes is their relatively short service lifetime. Theshort lifetime has been ascribed to the formation of a resistivelayer between the substrate and coating, a passivation layerformed on the outer surface of the coating, and selective lossof the catalyst into the electrolyte solution.

Many methods have been proposed to further improveSnO2 electrodes. For example, doping SnO2 electrodes withrare earth metals can improve the electro-catalytic decompo-sition of organics such as phenol, as shown in several exam-ples summarized in Table 3.122

The addition of Gd can improve the morphology and per-formance of Ti/SnO2–Sb electrodes. Both phenol and interme-diate products (e.g., benzoquinone) were shown to decom-pose more rapidly at 2% doping of Gd for the electrodes,over a doping range of 0–10% (Fig. 7).103

The enhanced performance of Gd-doped Ti/Sb–SnO2

electrodes is due to the increased adsorption capacity for hy-droxyl radicals on the electrode surface, and the lower mobil-ity of oxygen atoms in the SnO2 lattice.

Other new methods to improve the lifetime of SnO2

electrodes include using TiO2 NTs or CNTs serving as carriersfor further loading of the nanocrystal catalyst both to enhancethe performance of the Ti/SnO2 electrode and to improve itsservice lifetime, without reducing its O2 evolution potential.

Another approach to improve the stability based on chang-ing the electrode microstructure was proposed by Zhao et al.,in which a TiO2-NTs/SnO2 electrode that had high oxygen evo-lution potential, excellent electrocatalytic performance, andrelatively long-term stability was constructed by implantingSb-doped SnO2 into highly ordered TiO2 nanotubes (TiO2-NTs) grown in situ on a Ti substrate under controlled condi-tions. The service lifetime of this electrode (TiO2-NTs/SnO2)was 2.4 times higher than that of a traditional Sb-dopedSnO2 (SnO2) electrode. Based on TOC removal rates, the TiO2-NTs/SnO2 electrode also completely mineralized benzoic acid(BA).123

The substrate architecture is a main factor in the im-proved performance of TiO2-NTs/SnO2–Sb anodes for organicpollutant degradation. The pore diameter and length of the

Fig. 6 Electrochemical degradation of phenol (490 mg L−1) on Ti/SnO2–Sb, Ti/RuO2 and Pt anodes at a current density of 20 mA cm−2:(A) phenol degradation, (B) TOC removal, and (C) pH variation.101


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TiO2-NTs substrates of the TiO2-NTs/SnO2–Sb electrode wereshown to be critical factors in enhanced pollutant degrada-tion, with TiO2-NTs electrodes that had an 85 nm pore diam-eter and a 51 m length having the best performance.107

A SnO2–Sb2O4-based anode modified with Cr3C2 and CNTswas examined for phenol oxidation.102 The service lifetimesof the Ti/SnO2–Sb2O4–Cr3C2 and Ti/SnO2–Sb2O4–CNT–Cr3C2

electrodes were 7.4 times and 5.6 times longer than that ofthe Ti/SnO2–Sb2O4 electrode, respectively. The Ti/SnO2–Sb2O4–

CNT–Cr3C2 electrode showed the highest oxygen evolution po-tential, COD removal and current efficiency (CE).

Carbon nanotubes (CNTs) can be incorporated into Ti/SnO2–Sb electrodes using a pulse electrodeposition methodto improve performance.124 An electrode modified with CNTshad a higher specific surface area and smaller lattice size,which provided more active sites for hydroxyl radical genera-tion and contaminant oxidation compared to electrodes with-out CNTs. The oxygen evolution potential of Ti/SnO2–Sb–CNT

is 2.23 V, which is 0.07 V higher than that of an electrodelacking CNTs. As a result, the competitive reaction was weak-ened, the current efficiency was improved, and an acceleratedlifetime test indicated that the service life would be 1816 h ata current density of 50 mA cm−2, which is 4.8 times longerthan that of the Ti/SnO2–Sb electrode without CNTs. Ti/SnO2–

Sb–CNT electrodes have been demonstrated to have a supe-rior electrochemical oxidation and degradation ability usingAcid Red 73 as a model organic pollutant. The CNT-modifiedelectrode had a 1.9× higher kinetic rate constant, 1.3× greaterchemical oxygen demand (COD) and total organic carbon(TOC) removal efficiencies, 1.4× improved mineralization cur-rent efficiency, and a similar (0.98×) specific energy con-sumption compared to Ti/SnO2–Sb electrodes (Fig. 8).

Among the several materials which have been proposed asanodes, synthetic BDD exhibits several technologically impor-tant properties that distinguish it from conventionalelectrodes, such as an inert surface with low adsorption

Table 3 Parameter values of different rare earth metal-doped Ti-based SnO2 electrodes122

Electrode

The optimum molarratio of rare earthmetals to Sn

The time of degradationof phenol from 100 mg L−1

to below 10 mg L−1 (h)

Oxygen evolutionpotential (V (vs.Ag/AgCl))

The sequence of˙OH-producingcapacity

Oxidationmechanisms

Ti–Ce–Sb–SnO2 1 : 50 3.0 2.16 The redox couple ofCe4+/Ce3+

Ti–Eu–Sb–SnO2 1 : 50 2.5 2.21 Adsorbed˙OH speciesTi–Gd–Sb–SnO2 1 : 50 2.5 2.23Ti–Dy–Sb–SnO2 1 : 200 1.6 2.24Ti–Nd–Sb–SnO2 1 : 200 <1.5 2.28

Fig. 7 (a) UV scan curves of electrolytes for phenol and some possible intermediates; (b) UV scan of electrolytes of the 2% Gd-doped Ti/SnO2–Sbanode; (c) UV absorbance at 269 nm and (d) at 290 nm of electrolytes as a function of time for different compositions of Gd-doped Ti/SnO2–Sbelectrodes, i = mA cm−2.103


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properties and a strong tendency to resist deactivation, re-markable corrosion stability even in strongly acidic media,and an extremely high O2 evolution overvoltage.125,126 There-fore, it has received great attention in the past decades as anew electrode material.

2.2.5 Boron-doped diamond (BDD) electrodes. The use ofBDD electrodes has received great attention in recent years forchemical pollutant destruction. The first BDD patent by Careyet al.127 20 years ago claimed that BDD could be used as an an-ode for the oxidation of organics in wastewater. Since then, avery large number of papers and patents have shown that BDDcan be effective in the destruction of organic pollutants, such asphenolic compounds,128–139 synthetic dyes,140–146 pesticidesand drugs,147–153 surfactants154–155 and wastewaters.156–159

The behavior of BDD anodes for the electrochemical oxida-tion of different organic compounds has been investigated indepth by Comninellis and co-workers160–168 and Cañizares andco-workers.128,130–133,141 In a research study on Si/BDD anodesfor a wide range of pollutants by Comninellis et al.,168 it wasfound that independent of the organic pollutant nature, the cur-rent efficiency and the amount of intermediates were affectedby local concentrations of ˙OH relative to organics concentrationon the anode surface. Based on this observation, they proposeda comprehensive kinetic model to relate COD concentrationsand current efficiencies for the electrochemical oxidation of awide range of pollutants with BDD electrodes. The energyconsumption during the process was predicted based on ex-perimental conditions including the applied current inten-sity, organic concentration, and mass-transfer coefficient.

Based on the studies of oxidation of different phenoliccompounds (phenol, chlorophenols, and nitrophenols) and

carboxylic acids on BDD anodes, Cañizares et al. found thatthe organic compounds were completely mineralized regard-less of the characteristics of the wastewater (initial concentra-tion, pH, and supporting media) and operating conditions(temperature and current density) used. They also found thatthe phenolic compounds could be oxidized by the hydroxylradicals on the electrode surface, and also by inorganic oxi-dants electrogenerated on the BDD anodes in the bulk of thesolution, depending on the electrolyte composition, such asperoxodisulfuric acid from sulfuric acid oxidation.132

The rate of oxidation of azo dyes at BDD/Si electrodes canbe tuned indirectly by changing the boron doping level.Bogdanowicz et al. first investigated the influence of the levelof [B]/[C] ratio on the degradation and mineralization of aro-matic pollutants like azo-dyes.144 They found that the me-chanical and chemical stability of the electrodes resultedfrom a microcrystalline layer with a relatively high sp3/sp2

band ratio. The influence of commonly used electrolytes,NaCl and Na2SO4, on the dye removal efficiency was also in-vestigated. They found that the efficiency of the BDD processdepended on the electrode's doping level. Higher amounts ofdopant on the surface of the BDD electrode resulted in thehigher efficiency of dye removal in both electrolytes (Fig. 9).

BDD electrodes show good performance for the electro-chemical oxidation of pesticides and drugs. The degradationof 2,4-D herbicide in a recirculation flow plant has been stud-ied with Pt/air-diffusion and BDD/BDD electrodes by electro-chemical oxidation and electro-Fenton processes. In bothtreatments, the use of a single BDD/BDD cell always achieveda higher degree of degradation, with 59% mineralization and0.42 kW h g−1 TOC specific energy after 300 min of electroly-sis for the electro-Fenton process at 25 mA cm−2.151

The use of BDD anodes has also been widely investigatedfor the removal of surfactants from wastewater. The degrada-tion rates of seven perfluorinated compounds (PFCs) with

Fig. 8 (a) TOC removal efficiency and (b) mineralization currentefficiency (MCE) in 1.0 g L−1 Acid Red 73 and 0.1 M Na2SO4 solution ata current density of 50 mA cm−2.124

Fig. 9 Rubin F-2B concentration as a function of charge passed Q atBDD2 ([B]/[C] = 2000) and BDD10 ([B]/[C] = 10000) in NaCl and Na2SO4

electrolytes. Experimental conditions: [Rubin F-2B]initial = 20 mg L−1,pHinitial = 6.2, [NaCl] = 0.12 M, [Na2SO4] = 0.05 M, T = 20 ± 2 °C.144


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different carbon chain lengths and head groups were com-pared by Zhuo et al. using BDD electrodes.155 Intermediatesof PFCs were detected, with degradation rates showingpseudo-first-order kinetics of perfluoroalkyl carboxylates andsulfonates that increased with carbon chain length. They alsoproposed electrochemical oxidation mechanisms of PFCs ona BDD anode, where PFC decomposition began with a direct,one-electron transfer from a carboxyl or sulfonate group tothe BDD, with the formed PFC radicals decarboxylated or de-sulfonated to yield a perfluoroalkyl radical which permitted adefluorination reaction between a perfluoroalkyl radical anda hydroxyl radical (Fig. 10).

Electrochemical oxidation of a pesticide residue 2,6-dichlorobenzamide (BAM) with Si/BDD and Ti/Pt–Ir anodeswas studied to compare non-active with active anodes.149 Theresults showed that BDD, as a non-active anode, was more ef-ficient than the Pt–Ir electrode, and it produced a loweramount of degradation intermediates due to the non-selective nature of the hydroxyl radicals formed on the anode.The initial degradation pathway was found to be different forthe two cells, where the BDD electrode gave rise to both a ca-thodic and an anodic pathway, compared to the Pt–Ir cellwhich only had an anodic pathway.

Electrochemical degradation of bisphenol A (BPA) was ex-amined on four different anode materials: Ti/BDD, Ti/Sb–SnO2, Ti/RuO2 and Pt. BPA was readily destroyed on the Ti/Sb–SnO2 and Ti/BDD anodes, while the Pt anode had a mod-erate ability to remove BPA and the Ti/RuO2 anode did not ef-fectively oxidise BPA. Compared to the Pt and Ti/RuO2 an-odes, the Ti/Sb–SnO2 and Ti/BDD anodes were found to havehigher oxygen evolution potentials and higher anodic poten-

tials for BPA electrolysis at the same current densities. Incomparison to the Ti/Sb–SnO2 anode, the Ti/BDD anode withhigh durability and good reactivity for organic oxidationappeared to be the most promising for the effective EC treat-ment of BPA and similar endocrine disrupting chemical(EDC) pollutants.113

Electrochemical oxidation of ibuprofen (Ibu) was exam-ined using Ti/Pt/PbO2 and BDD electrodes in a batch cell atdifferent current densities (10, 20 and 30 mA cm−2) in aNa2SO4 electrolyte. Very good degradation of Ibu wasachieved, with COD removal between 60 and 95%, and TOCremoval from 48 to 92%, in 6 h experiments, with higherrates obtained with the BDD electrode. The combustion effi-ciency (ηC), which can be estimated from the rate of decreaseof TOC compared to that of COD, indicated slightly higher re-moval with the BDD at lower current densities, with 100% re-moval for both types of anodes at 30 mA cm−2.90

BDD and Ti/IrO2–RuO2 electrodes were compared to testtheir effectiveness for electrochemical oxidation of an azo dye(Reactive Red 120) in acidic media (1 M HClO4).

150 Ti/IrO2–

RuO2 exhibited a low oxidation power with high selectivity toorganic intermediates and low TOC removal (10% at 25 °Cand 40% at 80 °C), while the use of the BDD electrodes in-duced total mineralization to CO2. In both cases, thedecoloration of the solution was rapid, but very rapid, nearly100% removal was achieved with the BDD (2 A h L−1) com-pared to a slower rate with Ti/IrO2–RuO2 (25 A h L−1). The ef-fectiveness of these materials was examined based on the in-stantaneous current efficiency (ICE) (%), which wasdetermined from COD measurements using the followingequation:

Fig. 10 Pseudo-first-order kinetic reactions for (a) PFXA and (b) PFXS decomposition; the defluorination ratios for (c) PFXA and (d) PFXS on a BDDelectrode (reaction conditions: [PFXA]0 = [PFXS]0 = 0.114 mM; i = 23.24 mA cm−2; T = 32 °C; electrolyte = 1.4 g L−1 NaClO4).

155


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ICE = (CODt −CODt+Δt)FV/8IΔt (1)

where (COD)t and (COD)t+Δt are the chemical oxygen de-mands at times t and t + Δt (in gO2

dm−3), respectively, I isthe current (A), F is Faraday's constant (96 485 C mol−1), V isthe electrolyte volume (dm3), and 8 is the oxygen equivalentmass (geq.

−1). The ICE was up to 0.13 in the case of Ti/IrO2–

RuO2, and up to 0.45 for the BDD.The electrochemical mineralization of environmentally per-

sistent long-chain perfluorinated carboxylic acids (PFCAs),perfluorononanoic acid (C8F17COOH, PFNA) andperfluorodecanoic acid (C9F19COOH, PFDA). was investigatedover Ti/SnO2–Sb–Ce (SnO2), Ti/SnO2–Sb/Ce–PbO2 (PbO2), andTi/BDD (BDD) anodes. The energy consumption was calculatedbased on the electrical efficiency per log order reduction (EE/O)in the electrochemical oxidation process, as follows:

(2)

where P is the power (W), t is the reaction time (h), V is thetreated wastewater volume at time t (L), and Ci and Cf are theinitial and final concentrations, respectively (mg L−1).

For PFNA, EE/O was 54 W h L−1 for SnO2 and 72 W h L−1

for the PbO2 electrodes. The lowest EE/O value of 42 W h L−1

was achieved with the BDD electrode. The EE/O values forPFDA were EE/OĲSnO2) = 1.4 × EE/O(PbO2) and 1.9 × EE/O(BDD).93

The anodic oxidation of methamidophos (MMD), a highlytoxic pesticide used worldwide, was studied in a sodium sul-fate aqueous solution using Pb/PbO2, Ti/SnO2, or Si/BDDelectrodes at 30 °C. Under galvanostatic conditions, it was ob-served that the performance of the electrode material wasinfluenced by pH and current density, and the MMD degra-dation using Pb/PbO2 in acidic media (pH 2.0 and 5.6) gener-ated formaldehyde as the main product. Under the same con-ditions, Ti/SnO2 had low formaldehyde production comparedto the Pb/PbO2 electrode, while the Si/BDD electrodes did notshow any formaldehyde production. The ATR-FTIR character-istics of MMD in crystalline form and in aqueous solutionwere established, which showed the formation of phosphateas the reaction progressed, suggesting complete MMD miner-alization using the Si/BDD electrode.169

Diamond films are usually deposited on a titanium or sili-con substrate. A diamond layer was deposited onto a 3-D po-rous Pt nano-sheet perpendicular to the BDD hybrid filmusing a simple and facile double template method. Physicaland electrochemical results indicated that the 3-D porous Pt/BDD/Si electrode had a high catalytic ability and was resis-tant to poisoning for methanol electro-oxidation, because ofthe larger electrochemically active area and porous structureand the activity of the BDD substrate.170

BDD electrodes have been fabricated using numerousother materials. Although high-quality BDD films can be de-posited on silicon, tantalum, niobium and tungsten, they areunsuitable for application in wastewater treatment becauseof their poor mechanical strength, the low conductivity of Si,

and the high cost of Ta, Nb and W. Considering the tradeoffsin performance and cost, BDD films synthesized on a tita-nium substrate are preferable due to their good conductivity,high strength, low price, high anticorrosion, and quickrepassivation behavior. However, the poor adhesion strengthof diamond with Ti has been a problem in realizing high-performance BDD/Ti electrodes. For example, it was observedthat an extreme difference in temperature during substratecooling (from 850 °C to ambient temperatures) resulted in alarge thermal residual stress on the formation of a TiC inter-layer, reducing diamond film adhesion to the substrate, anda short service life of the Ti/BDD electrode.171 For this rea-son, various methods have been developed to improve adhe-sion, such as fabrication by the microwave plasma-enhancedchemical vapor deposition (MPCVD) method. A sand-blastedtreaded substrate and the introduced buffer layer were favor-able for producing electrodes with improved adhesion andgood electrochemical properties.172

2.2.6 Carbonaceous electrodes. Building on the idea ofusing three-dimensional electrodes, several types of carbona-ceous anode materials are now being investigated that havehigh specific surface area, good conductivity, excellent ad-sorption capability and better catalytic and electric capabili-ties such as activated carbon fibers,173–178 carbon felt179,181

and carbon nanotubes.182–185

The electrochemical degradation of amaranth, a type of azodye, using an activated carbon fiber (ACF) electrode was investi-gated under potentiostatic or galvanostatic conditions.173,174

With either approach, three different decolorization processesoccurred: adsorption, electroreduction, and electrooxidation.The adsorption was insignificant for the removal of color, CODand TOC. The electrooxidation and electroreduction benefittedcolor, COD and TOC removal, and electroreduction was moreeffective than electrooxidation.

Another azo dye, alizarin red S (ARS), was electro-chemically oxidized using activated carbon fiber (ACF) felt asan anode. The initial pH, current density and the type ofsupporting electrolyte all played an important role in ARSdegradation. The large specific surface area and higher meso-pore percentage of ACF anodes provided effective electro-chemical degradation of the dye, as shown by an increase incolor removal efficiency from 54 to 84% as the specific sur-face area of the ACF anodes was increased from 894 to 1682m2 g−1.175

ACF electrodes have been tested with other materials. Forexample, they were used with TiO2 (TiO2/ACF), with theelectrode prepared using a simple and inexpensive sol–gel-adsorption method.176 Tests using phenol and other organicpollutants showed that intermediates were always produced,demonstrating that the adsorbed ˙OH generated on the TiO2/ACF–graphite anode was the most active species in theelectrochemical oxidation system.

The use of an electrospun ACF electrode modified withCNTs (e-CNT/ACF) was examined for the electrochemical deg-radation of the dye, methyl orange (MO).178 Results showedthat the CNTs in the web-like e-CNT/ACF composites helped


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to improve the pore distribution and conductivity of the com-posite electrode, resulting in ∼90% degradation of MO in 60min, which was much higher than that obtained using onlythe commercial woven ACF under similar conditions.

Two pieces of ACF with CNT packed evenly between themwere used as the anode and cathode to treat wastewater con-taminated with a dye (X-3B) in a system called a seepage car-bon nanotube (SCNT) reactor.179 This reactor was designedto facilitate contaminant mass transfer from bulk solution tothe electrode surface, in order to improve electrochemicalwastewater treatment rates. Comparison of color and COD re-movals showed that the SCNT electrode reactor removed totalcolor by 94.4% and COD by 57.6% in 90 min levels of treat-ment were much higher than 32.8% (color) and 28.0% (COD)obtained using conventional electrochemical reactors (withACF-CNT electrodes positioned vertically at the center of thesame reactor). The research also showed that the current effi-ciency of the SCNT reactor was 340% higher than that of con-ventional reactors, and the energy consumption to mineralizethe same amount of organics was only 16.5% of that for con-ventional reactors.

A novel polyĲaniline-co-o-aminophenol) (PAOA)-modifiedcarbon felt electrode reactor was designed and investigatedfor fluoride removal from aqueous solutions. This reactor de-sign was innovative because it operated under a wider pHrange due to the coating of the electrode with a copolymerPAOA ion exchange film. Contaminant mass transfer frombulk solution to the electrode surface was enhanced by theuse of porous carbon felt as an electron-conducting carriermaterial compared to other reactors.180 The electro-oxidationof CeĲIII) on a carbon felt anode that proceeded with a highcurrent efficiency was studied which showed that at a currentof 2 A, oxidation of cerium had a current efficiency of 92%with the majority of Ce (>80%) oxidized to CeĲIV) within 40min.181

Carbon nanotubes (CNTs) have been used as carbon-based electrodes for chemical degradation or pollutant re-moval. The use of a carbon nanotube filter for electro-chemical water treatment was investigated by Vecitis and co-workers.182–185 They found enhanced performance of thethree-dimensional electrodes due to the high electrode sur-face area and porosity, and therefore an increased number ofelectrochemically active surface sites.

The primary passivation mechanisms and electrode regen-eration methodologies of electrochemical filtration withthree-dimensional carbon nanotube (CNT) networks were in-vestigated using phenol. Polymerization of phenol on theCNT surface resulted in a reduction of current and electro-chemical performance. Polymerization therefore needs to beprevented to stabilize the performance of these electrodes, orthe electrodes must be cleaned. Calcination and redispersionin HCl (pH = 1.7), toluene, and hexanes are effective for re-moval (>97%) of the passivating electropolymer coating.However, prevention is better than post-treatment for dealingwith passivation of CNT, and conducting electrochemical fil-tration at a higher potential could be useful in avoiding the

generation of polymers rather than trying to regenerate theelectrodes after passivation.

In all cases with these sp2-hybridized carbonaceous anodematerials, however, electro-oxidation is generally accompa-nied by surface corrosion. Thus, the application of three-dimensional electrodes will require further investigation intothe electrode passivation mechanisms, electrode regenerationtechniques, and passivation prevention methods.185

2.3 Indirect electro-oxidation via in situ generated chemicaloxidants

To avoid the deactivation of the anode during direct oxida-tion of chemicals, an alternative approach is indirect oxida-tion by destroying pollutants through the electrochemicalgeneration of chemical reactants such as active chlorine, oz-one, persulphate, and hydrogen peroxide.

2.3.1 Electrogeneration of active chlorine. Due to the natu-ral abundance of chloride ions in most waters, and the factthat oxide electrodes are very active for Cl2 evolution, chemi-cal species such as Cl2, HOCl, and OCl−, collectively called ac-tive chlorine, can be electrochemically generated and usedfor electrochemical oxidation of pollutants. Active chlorinespecies are well known to be strong chemical oxidants andare commonly used for organics oxidation, both in model so-lutions and in actual wastewaters.186–193 The production ofactive chlorine in electrolytic cells can be described by thefollowing reaction mechanisms:

2Cl → Cl2 + 2e− (3)

Cl2 + H2O → HClO + H+ + Cl− (4)

HClO ↔ ClO− + H+ (5)

In the presence of active chlorine, oxygen transfer can becarried out by the adsorbed oxychloro species, which are con-sidered intermediates of the chlorine evolution reaction asshown in Fig. 11.188

The predominance of different chemical species in activechlorine is well known to be a function of pH. Cl2 is the pre-dominant stable species at pH <3; when the pH is between 5and 6, active chlorine exists as hypochlorous acid (HClO) andhypochlorous anions (OCl−) are present at pH ≥6; at higherpH values (>7.5), hypochlorous anions (OCl−) are the pre-dominant species. HClO is the most powerful oxidant amongthe active chlorine species for oxidation of organics, and thusreactions are best conducted in acidic rather than alkalinemedia.

The choice of the anode material utilized is important forin situ generation of active chlorine, and DSA-type anodescoated with a layer of RuO2 or IrO2 are particularly effectivedue to their good electrocatalytic properties for chlorine evo-lution, as well as their long-term mechanical and chemicalstability.194–197

Other non-active electrodes, including BDD, SnO2 andPbO2, are not useful for this approach under some conditions


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because the high anodic potential typical of these electrodescan lead to oxidation of Cl− to form chlorate (ClO3

−) andperchlorate (ClO4

−), which do not have any oxidizing capac-ity.198 For example, Palmas et al. showed that chlorates andperchlorates could easily be formed from further oxidation ofhypochlorite on BDD anodes.199 Similar results were reportedby Lacasa et al. who also showed that the anode material sig-nificantly influenced the speciation of chlorine, with the for-mation of perchlorate obtained using BDD electrodes.200

Thus, these electrodes are not useful for treatment of drink-ing water due to the hazardous risks associated with the for-mation of these chemical species. Chlorate and perchlorateformation is minimal on DSA-type and Pt electrodes.201,202

Active chlorine production is suitable for the treatment ofsome types of wastewaters, such as olive oil, textile, and tan-nery effluents.203–229 Due to an abundance of chloride inthese wastewaters, there is usually no need for addition ofchloride salts for effective treatment. For example, Turroet al. investigated the behavior of a Ti/IrO2–RuO2 anode forthe electrochemical oxidation of landfill leachate under dif-ferent concentrations of the supporting electrolyte. Theyfound that the addition of 20 mM NaCl gave results similarto those with no salt addition. In this case, the leachatecontained 175 mM chlorides, and therefore increasing theCl− concentration by ∼10% had a marginal effect on indirectoxidation.230

Under active chlorine mediation, the risk of formation ofchlorinated organic compounds during electrolysis resultshas increased wastewater toxicity, which ultimately couldlimit the wide application of this approach to wastewatertreatment. For example, analysis of the reaction productsduring the oxidation of phenol in the presence of NaCl withDSA-type Ti/SnO2 and Ti/IrO2 anodes showed thatorganochlorinated intermediates were formed. Althoughthese compounds were then mineralized to CO2 or oxidizedto volatile chlorinated compounds (i.e., chloroform), the tox-icity of the solution remained above desirable limits.187 Inanother study, increasing the concentration of NaCl from 20mM to 100 mM resulted in higher COD removal, but the en-

hanced production of organochlorinated compounds resultedin a solution with high ecotoxicity.229 Future work using sa-line solutions must therefore focus on developing both effec-tive electrodes and experimental conditions which do not re-sult in the formation of organochlorinated intermediates thatwill limit the overall reduction in toxic chemical species inwater.

2.3.2 Electro-Fenton method. Indirect electro-oxidationmethods based on the cathodic electrogeneration of hydro-gen peroxide are being developed for the treatment of certainwastewaters, such as acidic wastewaters containing toxic andrefractory organic pollutants.231–236 The direct remediation ofwastewaters using this approach is limited by the low oxida-tion potential of H2O2. The electro-Fenton process is a moreeffective method of wastewater treatment, which is based onusing H2O2 in acidic effluents with Fe2+ ions as catalysts(Fenton's reagent) to give homogeneous ˙OH as a strong oxi-dant of organics according to the reaction:237–239

Fe2+ + H2O2 → Fe3+ + ˙OH + OH− (6)

Electro-Fenton methods have become very attractive be-cause of the much higher degradation rates of organic pollut-ants than those using traditional Fenton approaches due tothe continuous regeneration of Fe2+ at the cathode via:

Fe3+ + e− → Fe2+ (7)

This electrochemical approach thus avoids Fe3+ accumula-tion in the medium, thereby eliminating the production ofiron sludge. The electro-Fenton process has been successfullyused to treat non-biodegradable or refractory organic com-pounds such as phenolic compounds,240–246 dyes,247–249 pesti-cides and herbicides,250–254 leachates,255–257 drugs,258–260 andreverse osmosis concentrates.261

BDD electrodes are particularly effective for the electro-Fenton process. The mineralization process and decay kinet-ics of atrazine and cyanuric acid were examined by means ofan electro-Fenton process with Pt or BDD anodes using anundivided cell with a carbon-felt cathode under galvanostaticconditions. The electro-Fenton process was more effectivewith the BDD for the degradation of both compounds. Therewas nearly total mineralization of atrazine based on 97% to-tal organic carbon (TOC) removal. Efficient removal was dueto rapid oxidation by ˙OH formed at the BDD compared tothose in the bulk solution in a conventional Fenton reaction.Cyanuric acid was more slowly mineralized, mainly via ˙OHproduced at the BDD surface, but it was not degraded usinga Pt anode. These results highlight that electrochemical pro-cesses using a BDD anode are more powerful than the classi-cal electro-Fenton process with Pt or PbO2 anodes.

262

The main limitation of Fenton-based approaches is the re-quirement of low pH conditions. Fe3+ precipitation from so-lution at pH ≥3.5 can lead to the termination of this reac-tion. The electro-Fenton system also requires a low solutionpH, typically within the pH range 2–4, and consequently, it is

Fig. 11 Reaction scheme of chlorine-mediated electrochemical oxida-tion of organics.


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impractical if large amounts of chemicals are needed to acid-ify the water prior to treatment. Therefore, the inherent draw-back of the need for a low pH limits the wide application ofFenton-based approaches, although they are useful when thesolutions are initially quite acidic.

One approach proposed to overcome this low pH require-ment was an electro-Fenton-like (EFL) system that used aKeggin-type iron-substituted heteropolytungstate anionPW11O39Fe(III)(H2O)4

− to substitute for Fe3+ in the conven-tional electro-Fenton system. Experimental results using 0.1M dimethyl phthalate (DMP) showed complete degradationin <80 min at pH = 6.86 at a potential of −0.5 V, with aera-tion using an O2 flow rate of 60 mL min−1. A TOC removal of56% was achieved within 120 min. A comparison with a con-ventional electro-Fenton oxidation treatment of DMB showedthat this approach had a higher efficiency of chemical degra-dation, suggesting its potential use in treatment of water andwastewater with a more neutral pH.263

Further development of electro-Fenton methods is pro-ceeding based on coupling of the process with other AOPswith the aim of obtaining a synergetic effect for water andwastewater treatment. The most developed integrated processis the photoelectro-Fenton approach, where the contaminatedsolution treated under electro-Fenton conditions is exposedto UV illumination favouring the generation of homogeneous˙OH and the photodegradation of complexes of FeĲIII) withorganics:238

Fe(OH)2 + Fe2+ + ˙OH (8)

The mineralization of flumequine, an antimicrobial agentbelonging to the first generation of synthetic fluoro-quinolones which is detected in natural waters, was studiedby using the electro-Fenton and photoelectro-Fenton ap-proaches with UVA light. The photoelectro-Fenton processwas more powerful, resulting in almost total mineralizationwith 94–96% total organic carbon removal.264

Solar photoelectro-Fenton processes (SPEF) are also beinginvestigated, where the electrical energy needed by theelectro-Fenton reactor is produced from solar energy.

The degradation of the industrial textile dye Disperse Blue3, examined using the solar photoelectro-Fenton process,showed a positive effect of sunlight and Fe2+, with completedecolorization and mineralization of solutions in relativelyshort time periods.265 Decolorization and mineralization ofanother dye, Sunset Yellow FCF (SY) azo dye, were examinedby using several processes: anodic oxidation coupled withelectrogenerated H2O2 (AO-H2O2), electro-Fenton, UVA-illuminated photoelectro-Fenton, and SPEF.266 The resultsshowed that the most powerful method was solar photo-electro-Fenton, achieving almost total mineralization andhigher degradation compared to UVA-illuminated photo-electro-Fenton due to the higher UV intensity of sunlight,which quickly photolyzed Fe(III)–carboxylate complexes thatcould not be destroyed by ˙OH in traditional electro-Fentonprocesses.

The mineralization of the antibiotic chloramphenicol in asynthetic sulfate solution at pH = 3 was studied by anodic ox-idation with electrogenerated H2O2 (AO-H2O2), electro-Fenton,UVA photoelectro-Fenton, and SPEF processes. The resultsdemonstrated that SPEF with a BDD anode was the bestmethod among these approaches examined for chemicalmineralization.267

Another integrated process is the sonoelectro-Fenton ap-proach, where the solution in the electrochemical reactor issimultaneously irradiated with ultrasound.268 Compared tothe conventional electro-Fenton approach, a significant syner-getic effect was obtained by this coupled reaction due to theadditional effect of enhancement of the mass transfer rate tothe electrode. For example, during the sonoelectrolytic pro-cess, despite the existence of some degassing, the high yieldof hydrogen peroxide produced at the anode significantly en-hanced the rate of mass transfer of oxygen toward the cath-ode due to the sonication.268

Another electro-Fenton-based approach, called the peroxi-electrocoagulation method, was successfully used to degradechlorophenoxy and chlorobenzoic herbicides, and 2,4,5-trichlorophenoxyacetic acid. In this process, the soluble Fe2+

ions that react with cathodically generated H2O2 are continu-ously supplied to the solution from the sacrificial oxidationof an iron anode. Fenton's reaction occurs with this iron andproduces Fe3+ ions, inducing coagulation. Thus, the pollut-ants are removed by the combined action of degradation by˙OH generated by Fenton's reaction and their coagulationwith Fe(OH)3 precipitates formed from the anodic corro-sion.269 Treatment of sodium dodecyl sulfate (SDS) surfactantwastewater by the peroxi-electrocoagulation process showedthat SDS in the aqueous phase was effectively removed with amean energy consumption of 1.63 kW h kgSDS

−1, with the SDSremoval efficiency reaching 82%.270 The removal of phenolfrom water using this peroxi-electrocoagulation method wasexamined using a mild steel anode and a graphite cathode ina pilot-scale reactor. Ferric hydroxide and hydroxyl radicalsgenerated in the cell removed phenol from the water, makingit drinkable, demonstrating the feasibility of the process.271

3. Electrochemical reduction3.1 Electrodeposition

Aqueous effluents containing metal contaminants from someelectrochemical industries, such as the metallurgical andelectroplating industries, printed circuit boards, and batterymanufacturing require special treatment to remove toxicmetal ions or recycling of valuable materials. The electro-chemical recovery of metals from water has been practiced inthe form of electrometallurgy for a long time.272 The firstrecorded example was in the mid-17th century in Europewhich involved the electrochemical recovery of copper fromcupriferous mine waters.273

The electrochemical mechanism for metal recovery isbased on cathodic deposition, which provides an efficientway to remove heavy/toxic metals or recover precious metals


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from water and wastewater without leaving any residues dur-ing metal separation.274

One example of the selective electrochemical removal ofmetals is the recovery of gold-rich alloys using a filter-press-type electrochemical flow reactor with highly polished vitre-ous carbon (VC) and titanium (Ti) flat cathodes.275 Vitreouscarbon and titanium were determined to be outstandingcathode materials for gold recovery from gold plating waste-water. Both materials are easy to polish and chemically resis-tant to a wide range of chemicals. Cathodic efficiencies werehigher on the Ti cathode in which 23% was observed for agold-rich alloy recovered at 1.0 V, and 15% when only goldwas considered (Table 4).

Metals can also be recovered from aqueous solutionscontaining chelating agents such as EDTA, nitrilotriaceticacid and citrate. Using a two-chamber cell containing a com-mercial cation exchange membrane, a minimum of 40% ofthe metal was recovered with the greatest amount of 90%obtained for copper.276 Electrodeposition can be integratedwith ultrasound to increase metal recovery. It was found thatcopper removal increased to 95.6%, and EDTA was also re-moved (84% COD removal) from the wastewater.277

The fluid mechanical environment is also important in re-actor efficiency. A hydrodynamic study of a bench-scaleelectrochemical reactor using parallel plates with an inert flu-idized bed (glass beads) showed the importance of fluid mo-

tion on the removal of cadmium and lead ions from an aque-ous synthetic wastewater (Fig. 12).278

An electrochemical reactor with a rotating cylinderelectrode (RCE) and a pH controller were utilized to optimizethe electrochemical recovery of nickel from a synthetic nickelplating wastewater. Control of the pH to ∼4 was crucial forrecovering high-purity nickel, while preventing the precipita-tion of hydroxides and oxides (Fig. 13).279

Table 4 Current efficiency (CE) for gold-rich alloys and for gold recoveryon VC and on Ti for the potentials shown275

Cathode VC Ti

E vs. SCE (V) Alloy/% Au/% Alloy/% Au/%

−1.0 — — 23 15−1.1 11 5 19 11−1.2 11 5 17 8−1.4 18 9 — —−1.5 21 7 — —−1.6 16 5 — —

Fig. 12 Schematic representation of the complete apparatus developed and employed in the experimental procedure: (1) electrochemical reactor;(2) hydraulic system; (3) centrifuge pump; (4) Łrecirculating batch flow system; (5) pressure gauge; (6) a dispositive to facilitate the injection of atracer substance.278

Fig. 13 Sketch of the electrochemical reactor with a rotating cylinderelectrode utilized to perform the electrochemical recovery of nickel: (a)rotating cylinder electrode, (b) rotating motor, (c) pH control system.279


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3.2 Cathodic electrochemical dechlorination

Chlorinated organic compounds (COCs) are prevalent inmany wastewaters, particularly those from industries that usesolvents, as well as chemical industries that produce herbi-cides, fungicides, and pesticides. The presence of these COCsin the environment poses threats to human health due totheir toxicity and high stability. Many methods have beenused for cleaning wastewaters containing COCs. Compared toconventional physico-chemical, biological and chemicaldehalogenation techniques, electrochemical reductive dechlo-rination has emerged as an attractive technique to destroyCOCs due to its mild reaction conditions and the avoidanceof possible secondary pollutants.280–285

3.2.1 Electrode materials. The choice of cathode materialscan have a major impact on the efficiency of electrochemicaldechlorination processes, as the material can govern the reac-tion pathway and the selectivity for pollutant destruction.286

During the past few years, silver-based cathodes287–296 havebeen shown to have extraordinary electrocatalytic activity to-wards the reduction of COCs, showing dechlorination reduc-tion peaks at less negative potentials than those needed byglassy carbon, leading to up to 1 V gains in energy effi-ciency.286 Studies suggest that the reason for the excellent ac-tivity of Ag is the formation of a bridge-like R⋯X⋯Agadsorbed intermediate.297,298

Another electrode widely used for the dechlorination ofCOCs is a palladium-based electrode.289–304 The use of a Pdcathode leads to the preferential production of totally satu-rated products, as a consequence of their catalytic activity forhydrogen evolution and hydrogenation reactions. The abilityof palladium to adsorb hydrogen helps to promote indirecthydrodehalogenation.305

3.2.2 Electroreduction of chlorinated organic compounds.Over the years, electroreduction methods have been provento be highly effective for the dechlorination of a wide rangeof COCs such as chlorinated volatile organic compounds(VOCs),306–315 polychlorophenols,316–323 and polychlorinatedhydrocarbons.324,325 The efficiency and extent of chemical de-struction vary for different chemicals and anodes.

Nanostructured Pd thin films prepared by a cyclicvoltammetric deposition method were shown to be effectivein the electrochemical reductive dechlorination of carbon tet-rachloride (CT). Electrochemical characterization and CT re-moval using gas chromatography showed that adsorbed hy-drogen was important for removing CT from acidic solutionsthrough a surface reaction with chemisorbed CT molecules,providing a good mechanistic reason for the efficiency ofelectrochemical dechlorination.312

Electrochemical reduction of several polychloroethanes hasbeen investigated using electrodes containing different transi-tion metals, including PCEs (1,1-dichloroethane (1,1-DCA), 1,2-dichloroethane (1,2-DCA), 1,1,1-trichloroethane (1,1,1-TCA),1,1,2-trichloroethane (1,1,2-TCA), 1,1,1,2-tetrachloroethane(1,1,1,2-TeCA), 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), hexa-chloroethane (HCA), 1,1-dichloroethylene (1,1-DCE), 1,2-

dichloroethylene (1,2-DCE), tetrachloroethylene (TCE) andchloroethylene (CE)). The electrocatalytic activity of PCE wasfound to be affected by the specific structure, with the effi-ciency of the electrocatalytic degradation increasing with thenumber of Cl atoms bound to the same carbon center. Thenumber of Cl atoms bound to a second C atom had differenteffects: there was an activation enhancement due to polar ef-fects. For the most active electrodes examined, the order of in-creasing electrocatalytic reactivity was: 1,1-DCA < 1,1,1-TCA <

1,2-DCA < HCA < 1,1,2,2-TeCA < 1,1,2-TCA < 1,1,1,2-TeCA.Two distinct reduction mechanisms for the reductive cleavageof PCEs were observed. Geminal PCEs showed sequentialhydrodechlorination where one chlorine atom was lost in eachreduction step, until completely dechlorinated ethane wasobtained. Alternatively, reduction of vicinal PCEs involved re-moval of two chlorine atoms in an overall 2e− process resultingin the formation of the corresponding (chloro)ethylene, whichcould be further reduced by using more negative potentials.301

The size of the metal particles on the electrode is impor-tant. Electrospun polyacrylic acid (PAA)/polyvinyl alcohol(PVA) polymer nanofibers were used to immobilize Fe/Pd bi-metallic nanoparticles to treat trichloroethylene (TCE)-con-taminated groundwater. This bi-metal nanostructure that wasimmobilized within the polymer nanofibers was much moreeffective in remediation of TCE, especially at high initial con-centrations, than colloidal-sized Fe/Pd nanoparticles.313

Other materials have been used for successful electro-chemical dechlorination of chemicals. TCE degradation wasinvestigated in a recirculated solution of an electrolysis sys-tem containing a cast iron anode and a copper foam cathode.The cast iron anode generated a reducing electrolyte thatprevented the electron and proton combination withdissolved oxygen, thus the reduction of TCE on the copperfoam cathode was enhanced. The conductivity of the electro-lyte was an important factor for both the final elimination ef-ficiency (FEE) of TCE and specific energy consumption. Un-der optimal conditions, FEE reached up to 98%, at an energyconsumption of 6.49 kW h kg−1. This electrolysis system wasproposed to remediate groundwater contaminated by chlori-nated organic solvents or wastewater contaminated with chlo-rinated compounds.314

Several types of electrodes have been examined for degrada-tion of 2,4-dichlorophenoxyacetic acid (2,4-D). Palladium/nickelfoam (Pd/Ni foam) electrodes resulted in 87% removal of 2,4-Dwithin 4 h, with generation of phenoxyacetic acid (PA),o-chlorophenoxyacetic acid (o-CPA) and p-chlorophenoxyaceticacid (p-CPA) as intermediates. The palladium loading and theNaCl concentration impacted the dechlorination kinetics of2,4-D.300 With Pd/PPy–SLS/foam–Ni electrodes, 2,4-D wascompletely dechlorinated. The stability of the electrode was goodas the dechlorination efficiency was maintained at 100% even af-ter being reused 8 times.321

Roughened silver–palladium cathodes (Pd/Ag(r)) have alsobeen tested for electrocatalytic hydrogenolysis (ECH) dechlo-rination of 2,4-D in an aqueous solution.321 The mechanismof ECH dechlorination of 2,4-D on the Pd/Ag(r) cathode can


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be described as occurring in several steps. First, 2,4-D isadsorbed to Ag followed by the generation of chemisorbedhydrogen on Pd. Hydrogenolysis of 2,4-D then proceeds as incatalytic hydrogenation by the reaction of the adsorbed 2,4-Dwith chemisorbed hydrogen, followed by desorption of theproduct. The effects of OH−, Cl−, and ethanol were discussed,and the results showed that Cl− was detrimental to ECH de-chlorination, whereas OH− was able to promote the dechlori-nation of 2,4-D. Ethanol deactivated the Pd/Ag(r) cathode ifan aqueous alkaline solution was used for the reaction. Un-der optimal conditions, 25 mM 2,4-D was selectivelydechlorinated to phenoxyacetic acid with 85% removal at aCE of 66% at 298 K after 6 h. The only products generated be-sides phenoxyacetic acid were 2-chlorophenoxyacetic acid and4-chlorophenoxyacetic acid.

A multifunctional Pd/C gas-diffusion cathode was examinedfor phenol degradation in successive environments of hydrogengas and then air. Hydrogen gas was initially fed for 5 min at arate of 25 mL s−1 to reach saturation. Hydrogen gas was thenfed into the gas compartment for the first 60 min (electrolysistime) of the experiment, followed by air. The electrochemical re-duction and oxidation was examined using three different chlo-rinated phenols [4-chlorophenol (4-CP), 2,4-dichlorophenol(2,4-DCP) and pentachlorophenol (PCP)].322 The Pd/C gas-diffusion cathode reductively dechlorinated the phenols whenhydrogen gas was present, and then it accelerated the two-electron reduction of O2 to H2O2 by feeding air for oxidationof the chemicals. The combined process of reduction and oxi-dation improved the chlorinated phenol degradation effi-ciency with the removal efficiency of chlorinated phenolsreaching nearly 100%. The degradation sequence was thebest for 4-CP, followed by 2,4-DCP, and then PCP.

1,2,3-Trichlorobenzene (1,2,3-TCB) was used as a model forthe degradation of a persistent organic pollutant (POP) underan inert gas atmosphere, as summarized in Table 5.323 Traceamounts of dichlorobenzene were observed with differentamounts depending on the electrode material. Electrodeswith Ru and Pd were selective mainly for meta-position de-chlorination, while those with Pt groups were selectivemainly for ortho-position (o-position) dechlorination. A PdOsintered electrode had an especially high selectivity formeta-position (m-position) dechlorination.

The electrochemical dechlorination of chlorobenzenes wasstudied in the presence of various arene mediators such asnaphthalene, biphenyl, phenanthrene, anthracene, and

pyrene.324 As the amount of naphthalene was reduced to 0.01equivalents, there was complete dechlorination of mono-chlorobenzene with 77 × 104 C mol−2 of electric quantity. Thissame amount of total charge was then used in the presence offour different types of arene mediators: biphenyl, phenan-threne, anthracene, and pyrene. Complete dechlorination wasachieved with all mediators except anthracene. Similar resultswere also obtained when this reaction was applied to 1,3-di-and 1,2,4-trichlorobenzene, with phenanthrene appearing tobe the most effective mediator among those examined.

3.3 Cathodic electrochemical denitrification

Electrochemical reduction of nitrate and nitrite ions hasgained more attention in the past several years,325–329 particu-larly for the treatment of nitrate-containing ground waters. Asfor many other chemicals, nitrate reduction products dependon the nature of the electrode surface,330–334 making this reac-tion very interesting from a mechanistic point of view.

Electrochemical nitrate reduction was investigated oncoinage (copper, silver and gold) and transition-metalelectrodes (platinum, palladium, rhodium, ruthenium, irid-ium) at 0.1 M nitrate ions in acid solutions.333 The conclu-sion was that the rate-determining step on most of theseelectrodes was the reduction of nitrate to nitrite, based onthe values of the Tafel slope, the kinetic order, and the effectof co-adsorbing anions. Cyclic voltammetry showed that cur-rent densities at given applied voltages for nitrate reductiondepended strongly on the nature of the electrode. The activi-ties for the electroreduction of nitrate decreased in the orderRh > Ru > Ir > Pd and Pt for the transition-metal electrodes,and in the order Cu > Ag > Au for the coinage metals. On-line mass spectrometry measurements for Pt and Rh showedno formation of gaseous products such as nitric oxide (NO),nitrous oxide (N2O) or nitrogen (N2), suggesting that ammo-nia and hydroxylamine were the main products usingtransition-metal electrodes. This conclusion was in agree-ment with the known mechanism for NO reduction, whichforms N2O or N2 only if NO is in solution. For Cu, measure-ments showed the production of gaseous NO, which could beexplained by the weaker binding of NO to Cu as compared totransition metals.

Different cathode materials were examined for nitrate re-duction in an electrocatalytic reactor consisting of a solidpolymer electrolyte/Pt electrode assembly. The reactivity andselectivity of electrocatalytic reduction of NO3

−/NO2− in the

membrane electrode assembly (MEA) reactors were largely de-pendent on the metallic composition of the cathode. Pt alonewas relatively inactive, but rates were significantly improvedby the deposition of Ni, Cu, Ag and/or Rh onto the Ptelectrode. Although the activity was enhanced mostly by Cuor Ag, their reaction mechanisms were quite different. Cata-lytic hydrogenation of nitrate (H2–NO3

− reaction) occurred onthe Cu–Pt cathode, compared to electrochemical nitrate re-duction using a Ag–Pt cathode.334

These results suggest that synergistic positive electro-chemical impacts can be obtained when using two or more

Table 5 The dechlorination rates with different noble metals323

Cathode electrode Dechlorination rate (%)

Sintered RuO2 (major)/Pt/PdO 91Pt (major)/IrO2/RuO2 81Sintered RuO2 59Sintered PdO 96Sintered Pt 53Sintered PdO/Pt 97Sintered Pd/Pt 82Plain Pd plate 70


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metallic species on the surface of the workingelectrodes.335–338 Anastasopoulos et al. synthesized composi-tional gradient thin films of PdCu alloys and found that alow concentration of Pd in copper resulted in the normally ir-reversible copper surface redox becoming reversible. Thischange led to a sharp increase in the rate of both nitrate andnitrite reduction. In both cases, these effects were thought tobe associated with the ability of Pd to activate water.339

Sn modification of polycrystalline palladium or platinumfilms on gold electrodes enhanced nitrate reduction. Modifica-tion with Sn was the key to these rates as pure Pd did not havea measurable impact on nitrate reduction. The dominant vol-atile products identified were N2O and smaller amounts ofN2, while NH2OH was the dominant non-volatile product.340

The activity for electrocatalytic reduction of nitrate on theamorphous Pd33Ni60P7 electrode was tested with cyclicvoltammetric in a neutral 0.1 mol L−1 KClO4 solution with orwithout KNO3. The Pd33Ni60P7 alloy enhanced nitrate reduc-tion in comparison with the electrodeposited films of Pd, Niand Pd–Ni, likely attributed to its amorphous structure. Thereduction reaction of NO3

− on the electrode was found to bea totally irreversible process.341

The use of carbon as a substrate material for the prepara-tion of the working electrode has major advantages over othermaterials, due to its good conductivity, high surface area, goodfluid permeability, relatively high overpotential for hydrogenevolution, and high chemical stability over a wide range of pHvalues. Hybrid electrode materials, with metals dispersed onactivated carbon fiber (ACF) and surface-functionalized carbonnanotubes (CNTs), have therefore been pursued for nitrate re-duction. Pd/Sn-modified activated carbon fiber (ACF)electrodes were fabricated by the impregnation of Pd2+ andSn2+ ions onto ACF by Wang et al. Electrocatalytic reductionof nitrate was shown, by this Pd/Sn–ACF electrode, with thePd/SnĲ4/1)-modified ACF electrode producing the highestrates of nitrate reduction over a pH range of 5.3–7.6.342

Other approaches to enhance nitrate reduction include de-position onto traditional glassy carbon electrodes to study thereaction kinetics. A GC/MWCNT–Rh electrode was prepared byelectrodepositing multi-walled carbon nanotubes modifiedwith rhodium particles on a glassy carbon electrode. It is inter-esting to observe that during prolonged electrochemical reduc-tion of nitrite, the catalytic activity of this hybrid electroderemained almost unchanged demonstrating good temporalstability. This suggests the substantial absence of poisoningdue to irreversible and strong adsorption of reactants, interme-diates, and/or reaction products on the active catalytic sites.343

Nitrate concentrations can also impact rates, showing thatthey do not proceed according to zero-order kinetics.344–349

Katsounaros et al. showed that the impact of nitrate con-centration on the rate of nitrate reduction could be ade-quately described by a simple Langmuir–Hinshelwood kineticmodel. The selectivity to nitrogen increased from 70 to 83%as the concentration of nitrate increased from 100 to 1500mg L−1, and then it remained almost constant at higher ni-trate concentrations. Ammonia exhibited the opposite trend,

with a decrease from 25 to 11%. Faradaic efficiency (% FE)increased with the increase of nitrate concentration from25% at 0.1 M to 78% at 1 M, with 95% of the nitrate reducedin both cases. At high concentrations of nitrate, hyponitriteand hydroxylamine were detected as intermediates of thereduction.350

To develop a theoretical model of the electrochemical re-duction of nitrate ions, a powerful analytical method, calledthe Homotopy Analysis Method (HAM), was used. This ap-proach, which provides a convenient way to control and ad-just the convergence region and rate of approximation serieswhen necessary, was used to obtain approximate analyticalsolutions to a nonlinear ordinary differential equation. Theobtained analytical expressions of concentrations and currentwere found to provide satisfactory agreement with numericalsolutions.351

4. Electrocoagulation/electrocoagulation–flotation

Electrocoagulation (EC) is a process which causes the in situelectrochemical production of coagulated species and metalhydroxides that destabilize and aggregate particles or precipi-tate and adsorb the dissolved contaminants.352–354

Compared to the conventional coagulation process, theelectrocoagulation process has been proven to be very effec-tive for contaminant removal from water with two outstand-ing characteristics. On the one hand, it provides better re-moval capabilities for the same species than chemicalcoagulation without addition of chemicals. On the otherhand, it produces less sludge, thus lowering the sludge dis-posal cost.355–359

In practice, an electrocoagulation (EC) process will be oftenfollowed by an electroflotation (EF) process that is a simpleprocess where pollutants can float to the surface of a waterbody via tiny bubbles of hydrogen and oxygen gases generatedfrom water electrolysis. This combined system can be consid-ered as an electrocoagulation–flotation (ECF) process.360,361

4.1 Effects of various operating parameters on EC

It can be noticed from the literature that the efficiency of ECprocesses significantly depends on the operating parameters.Solution pH and current density are the variables that havebeen studied widely.362–365

4.1.1 Effect of solution pH on performance. The influentpH has been established to be an important operating factorinfluencing the performance of electrochemical processes.For the EC process, the maximum pollutant removal effi-ciency is obtained at an optimum solution pH depending onthe nature of the pollutants. The pollutant removal efficiencydecreases by either increasing or decreasing the pH of the so-lution from the optimum pH. It should be noted that one ofthe advantages of this process is that the effluent pH afterelectrocoagulation treatment would increase for acidic influ-ents but decrease for alkaline influents.


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A study of the influence of the pH of waste in the coagula-tion with aluminum by conventional and electrochemical dos-ing showed that a simple change in the pH of the waste couldresult in a significant change in the efficiency of the coagula-tion process, and that if the same pH conditions were found atthe end of the treatment, the efficiencies of the solution dosingand electrochemical dosing technologies were very similar.366

The effect of initial pH on the batch removal of syntheti-cally prepared wastewater having high concentration of hu-mic substances has been investigated by the electro-coagulation method using plate electrodes. The effects ofinitial pH on an electrocoagulation system may be twofold.These are distribution of the aluminum hydrolysis product,transformation of the humic substance related to the initialpH and finally the effects of the gel layer especially at highhumic substance concentrations and high initial pH formedon the anode surface. They observed that the initial humicsubstance concentration and initial pH have great effects onthe removal rate and efficiency. For example, in the range of200–500 mg L−1, a decrease in the removal efficiency was ob-served due to gel layer formation on the surface of the anode.In order to prevent this gel formation, the initial pH of thewastewater was adjusted to 5.0 and high removal efficiencieswere observed.367

The removal of hexavalent chromium from synthetic solu-tions with different pH values using the electrocoagulationmethod was studied. The results showed that the pH of thesolution has a significant effect on the CrĲVI) removal effi-ciency and the maximum chromium removal efficiency wasobtained at pH = 4. They further reported that the pH of thesynthetic solution after the EC process increased with an in-crease in the electrolysis time due to the generation of OH˙ inthe EC process.368

4.1.2 Effect of applied current density on EC. The currentdensity is an important experimental parameter for control-ling the electrochemical reaction rate. In the case of theelectrocoagulation process, the current density determinesthe amount of Al3+ or Fe2+ ions released from the respectiveelectrodes and thus affects the electrocoagulant dosage ratein the electrochemical cell.369

It should be noted that the selection should be fixed withrespect to other operating parameters like solution pH, tem-perature and flow rate.

The effect of the current density and flow rate of acontinuous-flow electrocoagulation–flotation reactor on theremoval efficiency for direct red 81 dye was investigated bySalim Zodi et al. The results indicated that the current effi-ciency (Faradaic yield) was strongly dependent on flow ratesand current densities. For example, the efficiency of this spe-cific reactor produced a considerable DR 81 removal from71.5% at 100 A m−2 to 90.2% at 200 A m−2 for the same flowrate of 10 l h−1. When the flow rate was increased to 28 l h−1,the dye removal efficiency increased from 61.5% at 100 A m−2

to 76.8% at 200 A m−2. Hence, to achieve higher DR 81 re-moval efficiency, the current density needs to be higher whenthe flow rate is increased.370

It is a known fact that the operating current density inECF processes directly determines the coagulant dosage andthe rate of bubble generation, which influence both mixingand mass transfer in the reactor.

For example, the work by Holt et al. has reported that atlow operating currents in which settling dominates, the pol-lutant fraction that is removed by flotation increases as thecurrent increases because at higher operating current densi-ties, bubble densities increase, resulting in a greater upwardmomentum flux and thus faster removal of pollutants andcoagulants by flotation from the active reactor volume to thesurface.371

On the contrary, the obtained results from the studies byMohora show that the increase in operating current densitycaused a decrease in reactor DOC removal efficiency. Theyconcluded that for higher operating current densities, morealuminum was available per unit of time in the ECF reactorvolume but its residence time in the active reactor volumewas shorter, which caused the decline in NOM removalefficiency.372

4.2 Application of EC and ECF processes

Electrocoagulation was first proposed in the nineteenth cen-tury and the first treatment plant was erected in 1889 in Lon-don for the treatment of sewage wastewater. EC or ECF pro-cesses are now used widely in the treatment of many types ofwastewaters including dye and textile wastewater,373–377 re-fractory oily wastewater,378–380 municipal wastewater,381–385

manufacturing wastewater,386–388 and wastewaters with phe-nol,389 toxic metals390–393 and inorganic metals.394–398 Benchand pilot-scale research studies on using EC and ECF tech-nologies to remove pollutants from many types of water andwastewaters were recently reviewed by Emamjomeha et al.,399

while Khandegar et al. provided a more focused review on re-moval of dyes from textile effluents.400

The performance of EC/ECF technologies can be enhancedby integrating the process into a process train with othertechnologies. For example, Farhadi et al. compared electro-coagulation with photoelectrocoagulation, peroxi-electro-coagulation, and peroxi-photoelectrocoagulation for the re-moval of COD from pharmaceutical wastewater. Underoptimum operating conditions for each process, the COD re-moval efficiency was in the order of peroxi-electrocoagulation> peroxi-photoelectrocoagulation > photoelectrocoagulation> electrocoagulation.401 Cotillas et al. looked into coupling ofelectrocoagulation with iron electrodes and UV irradiation(photoelectrocoagulation) for the simultaneous removal ofturbidity and bacteria (Escherichia coli) from treated munici-pal wastewaters. Coupling UV irradiation to electro-coagulation was shown to improve the process performancecompared to electrocoagulation alone. An examination of theeffect of current density on process performance showed thatthere was a synergistic interaction of both technologies atlow current densities (1.44 A m−2), but an antagonistic effectat high current densities (7.20 A m−2). This antagonistic effectwas caused by the less efficient transmission of UV


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irradiation to the bulk solution due to the increase in theconcentration of solids.402

Grey water treatment using electrocoagulation (EC) was ex-amined in an integrated process with a submerged mem-brane bioreactor (SMBR). The combined EC–SMBR processresulted in a 13% reduction in membrane fouling. COD andcolor removal increased from 86% and 91.2%, respectively,using only the submersed membrane bioreactor, to 88.6%and 93.7% with EC–SMBR.403

Treatment using EC was enhanced when combined withozone treatment or hydrogen peroxide treatment. Song et al.showed that the color removal efficiencies of the azo dye (C.I.Reactive Black 5), reached 10% by treatment with only ozoneand 83% with only EC, but the combined process achieved94% color removal.404 A combined electrocoagulation processfollowed by electrogenerated hydrogen peroxide treatmentwas evaluated for copper ion removal from an industrialwastewater.405 The maximum COD removal was 80%, whichwas about 20% greater in terms of COD removal than that ofcopper electrocoagulation alone. This additional organics re-duction was due to a Fenton-like reaction between CuĲII) ionsthat remained in solution and the peroxide, generating OH˙

which oxidized the organics that were not adsorbed by theelectrocoagulation treatment.

5. Electrodialysis

An electrodialysis (ED) process is an electrochemical separa-tion process where ions are moved across polymeric anion-and cation-exchange membranes in a potential field (voltage).When an electrical potential difference is applied across analternating series of cation- and anion-exchange membranesbetween two electrodes, positive ions migrate to the cathode(negative electrode) and negative ions migrate towards theanode. The presence of the ion exchange membranes trapsthem in alternating compartments, resulting in streams of di-lute and concentrated ions.406

Development of ED processes was initiated in the 1950sfor production of table salt, when polymeric IEMs weremanufactured on a commercial scale. Since then, ED hasbeen a useful process for separation of ions and salts usingIEMs. The development of a bipolar membrane, which is acomposite membrane consisting of a cation-exchange layerand an anion-exchange layer pressed together, realized thesplitting of solvents into H+ and OH−/CH3O

− at the interfaceunder a reverse potential bias. The use of this bipolar mem-brane has resulted in new technologies categorized as bipolarmembrane electrodialysis (BMED).407 This solvent splittingtechnique has been used in more applications than otherconventional ED techniques (CED) in chemical or bio-chemical synthesis, food processing, and pollution control(Fig. 14).

BMED was combined with an organic extraction processand an ion exchange process to develop an improved methodfor Cu2+ recovery (Fig. 15).408 This integrated process re-quired three steps: first, extraction of copper ions from the

mixture using an organic extractant HR (a complexing agent,where R = OH− or CH3O

−) at a slightly alkaline pH, which wasmaintained by the OH− ions supplied by a bipolar mem-brane; second, movement of the bound Cu2+ ions across thecation-exchange resins to decrease the ohmic resistance ofthe compartment filled with HR to the other compartmentalong with the organic solution; third, release of the boundCu2+ ions by substitution with the H+ ions from water split-ting, which regenerates the HR. Tests showed that this ap-proach, along with the use of ion exchange resins as a cham-ber spacer, improved current densities and substantiallyincreased performance. An average current efficiency of 90%was achieved at a current density below 5 mA cm−2 with a gelion-exchange resin, compared to a porous cation resin.409

A BMED process as a tool for the reclamation of NaOHfrom glyphosate neutralization liquor was investigated as wellas its subsequent use as an absorbent for CO2 capture.410 Abench-scale BMED process was also used to examine the in-fluence of operating conditions on the recovery of HCl andNaOH from seawater RO concentrates. The results showedthat this technology was a technically feasible option for theproduction of 1.0 M or higher acid and base solutions thatcould potentially be used within the treatment plant, withcurrent efficiencies in the 60–90% range (Fig. 16).411

ED has also been integrated with processes used in watertreatment and reuse, including pressure-driven membraneprocesses, such as microfiltration (MF), ultrafiltration (UF),nanofiltration (NF), and reverse osmosis (RO). A pilot-scalewastewater treatment and reuse system was developed usingED and MF, which indicated that the treated water usingsuch integration could meet the standards of water reuse andits quality remained stable for more than 6 months.412

UF was used as a pre-treatment process prior to ED (UF–ED) to treat river water and use it to add to surface waters tobalance the depletion of surface water caused by excessive ex-ploitation.413 The results showed acceptable reductions inconcentrations for the following ions: Cl−, SO4

2−, NO3−,

HCO3−, Na+, Mg2+, K+ and Ca2+. The use of NF, rather than

UF, resulted in insufficient removal of these ions, especiallythe monovalent ions.

ED was also tested in conjunction with a membrane biore-actor (MBR).414,415 A CED process was used to concentratethe feed before it was pumped into MBRs to increase the so-lution conductivity, and thus provide a more stable voltage in

Fig. 14 Schematic of BMED (BP, bipolar membrane; A: anionicmembrane; C: cationic membrane; M+: cation; X−: anion; H+: hydrogenion; OH−: hydroxide ion; CH3O

−: methoxide ion).407


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the ED stack. Water with high nitrate content was purified byusing this integrated process. Denitrification by the MBR ofthe concentrate produced by CED resulted in the treated wa-ter with a nitrate concentration below the acceptable value of50 mg L−1.

6. Photoelectrochemical andsonoelectrochemical approaches

In recent years, the integration of electrochemistry with photo-catalysis and sonochemistry methods has led to a new and inter-esting possibility for treatment of pollutants from wastewater.

6.1 Photo-assisted electrochemical methods

In the recent years, there has been growing interest in theintegration of photocatalysis and electrocatalysis for the treat-

ment of toxic and/or recalcitrant organic compounds. Conse-quently, a number of studies have shown that DSA-type oxideelectrodes can be utilized in photo-assisted electrochemicaldegradation processes in which the limitation of generationof highly reactive oxidants can be overcome through UV lightirradiation. In addition, these anodes can generate chloro ox-idant species (Cl2, HOCl, and OCl−), when electrolysis is car-ried out using a solution with a high chloride concentrationunder certain pH conditions. Thus, the combination of thechloro oxidant species generation and UV irradiation can re-sult in the formation of highly reactive species.416–424

The photo-assisted electrochemical (PAE) removal of di-methyl phthalate ester (DMP) was examined using a one-compartment filter press flow cell and a commercial DSA an-ode.424 Removal rates were similar to those reported usingelectrooxidation and a BDD anode. The highest rates of DMP

Fig. 15 Concentration of copper from mixture solutions.408

Fig. 16 Bipolar membrane electrodialysis scheme to obtain acid and base (BP: bipolar membrane; C: cationic membrane; A: anionicmembrane).411


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and TOC removal were attained at high NaCl concentrationsand current densities, due to the generation of high concen-trations of reactive oxidants, such as ˙OH radicals, as well asother oxidants (h+, O2˙

−, and Cl˙), in the PAE method. Thus,the PAE method improved the production of these chemicalsand the rate of pollutant transport and degradation.

6.2 Sonoelectrolysis (SECT)

In an electrochemical oxidation process for the treatment ofpollutants, the current efficiency usually gradually decreasesduring treatment. Thus, the pollutants and their intermedi-ates are often adsorbed onto the electrode surface during oxi-dation and reduce the active sites on the electrode surface,resulting in partial or complete poisoning of the electrode.Treatment efficiency requires that consistently higher oxida-tion efficiency be maintained. In addition, an electrochemicalprocess is often limited by the mass transport in the system,which also decreases the current efficiency.

Recently, sonochemical technologies have been proposedin order to activate the electrode surface and enhance masstransfer efficiency. This hybrid process has been applied todegradation of several organic pollutants, including textiledyes,425,426 aromatics,427,428 nitro compounds,429 and chlori-nated compounds.430,431 The efficient conductive diamondelectrochemical oxidation (CDEO) coupled with ultrasound(US) was recently used for the degradation of progesterone inwastewater. Synergistic effects were clearly observed on theoxidation rates due to the improvement of mass transfer tothe conductive-diamond surface of the electrode.432

7. Combination of electrochemicalprocesses for resource reclamationfrom wastewaters

In recent years, various electrochemical approaches havebeen used to degrade organic matter in aqueous solutions,but wastewater treatment facilities consume a large amountof energy. Therefore, it is useful to develop novel hybrid tech-nologies that can purify water and save or even generate elec-trical power.

The possibility for both electrochemical degradation ofwastewater organics and simultaneous production of hydro-gen fuel has been recently explored. The electrochemical oxi-dation process with an option of microfiltration for real mu-nicipal wastewater treatment was investigated in terms oforganic matter removal and hydrogen generation.433 Hydro-gen production was enhanced with municipal wastewaterdue to the presence of specific organics and by adding NaClto increase conductivity. The average current efficiency andthe energy efficiency increased by approximately 10% whenwastewater organics were present, whereas they increased by>20% when 50 mM NaCl was added to wastewater.

Other research studies have shown that it is possible tocarry out wastewater treatment, hydrogen production, andwaste heat recovery in a single electrochemical processing de-

vice. A theoretical analysis showed that converting hydrogenenergy to electricity and utilizing waste heat recovery for ad-sorption chiller or heat pump applications resulted in goodenergy savings.434

Hydrogen gas production with simultaneous COD removalwas achieved by electrochemical treatment of landfill leach-ate. The rates and yields of hydrogen gas were investigatedusing different applied voltages with aluminum electrodesand a DC power supply. Hydrogen gas was concluded to bemainly produced by ED of the leachate organics based onnegligible H2 gas production using water in control experi-ments. The highest COD removal (77%) was obtained withan applied voltage of 4 V.435

The recovery of heavy metals from industrial aqueous so-lutions has received great attention in recent years, mainlydue to more stringent laws for the protection of the environ-ment. Conventional techniques for metal ion abatement,such as hydroxide precipitation or direct electroreduction, donot result in sufficient removal. One alternative technique iselectrodialysis with the advantage that the low concentrationof heavy metals can be concentrated, and the remaining ef-fluent water can be diluted for reuse. However, the disadvan-tage of this approach is that it does not work well at highmetal concentrations due to membrane fouling. Therefore, afuture research direction could be some hybrid method to en-hance electrodialysis processes through methods that reducemembrane fouling. For example, for copper recovery and wa-ter reuse from copper electroplating wastewater, a laboratory-scale process was developed that combined electrolysis (EI)and ED. The results showed that this combined processcould achieve high recovery of both copper and water usingwastewater with high or low concentrations of copper. Almost99.5% of copper and 100% of water could be recovered.436

Compared to copper, the recovery of nickel is more chal-lenging because the electrodeposition of nickel on the cath-ode is difficult due to the hydrogen evolution reaction, andtherefore the recovery efficiency is low.437 The feasibility ofnickel recovery and water reuse was investigated using threeelectrochemical processes (EI, ED and electrodeionization(EDI)) for both high and low nickel concentrations in waste-water. Almost 99.8% of nickel could be recovered, with a pu-rity of 93.9%, and nearly 100% of water could be reused.438

ED has been combined with membrane processes, such asmembrane filtration (MF), nanofiltration (NF), and reverse os-mosis (RO) processes along with precipitation–neutralizationprocesses to treat recycled water and sulfuric acid rinses fromlead battery production lines.439 On average, 88 wt% sulfuricacid and 25 wt% rinse water were recovered. This treatmentresulted in savings due to both water and acid recovery, alongwith additional savings due to a reduction in the chemicalsneeded for neutralization and the costs of sludge disposal.

An integrated ED–electrochlorination process was alsoused to treat wastewater. The process reduced the wastewaterconductivity and TOC concentrations, and produced a valu-able hypochlorite solution in the electrode rinse compart-ment (Fig. 17).440


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8. Conclusions

Environmental electrochemistry can be used to achieve acleaner environment, as evidenced by the increasing numberof new technologies and successes in treatment of pollutedwaters that were provided here in this review. Some of themost promising aspects are based on the use of different oxi-dation strategies and combinations of different technologies.

Electrochemical oxidation is considered to be a very pow-erful tool able to mineralize completely non-biodegradableorganic matter. Anodic oxidation of organic contaminantscan be performed in several different ways including directand indirect oxidation. Compared with direct anodic oxida-tion that leads to very poor decontamination, indirect anodicoxidation via intermediates of oxygen evolution can avoidelectrode fouling, in which the nature of the electrode mate-rial strongly affects both process selectivity and efficiency.

Other strategies of indirect oxidation pathways for organicoxidation, such as active chlorine mediation and E-Fenton pro-cesses, are powerful and effective approaches for wastewatertreatment. For the interaction of active chlorine with organics,further research directions should focus on developing novelelectrode materials that can suppress the side formation oforganochlorinated intermediates. Results from the literaturecited here show that the electro-Fenton process with a BDDanode was the more powerful treatment for organic pollutantswith a higher amount of reactive ˙OH than is expected to beformed on a BDD alone. Further development of the electro-

Fenton process seems to be towards integrated processes, suchas photoelectro-Fenton, sonoelectro-Fenton and peroxi-electrocoagulation methods, with the aim of obtaining a syner-getic effect for water and wastewater treatment.

Although laboratory and pilot tests have been successful,industrial applications of these electrochemical oxidationmethods are still limited, due to the relatively high energyconsumption needed to treat low concentrations of chemicalsin wastewater.

Future development of electrochemical oxidation tech-niques will require development of anode materials with spe-cific characteristics that can make the process economicallycompetitive with other conventional technologies. Energyconsumption could be reduced using so-called “advancedelectrochemical oxidation processes”, based on the combina-tion of anodic and cathodic electrogeneration of highly oxi-dizing hydroxyl radicals.

Three electrochemical reduction processes, electrodeposi-tion, cathodic electrochemical dechlorination, and electro-chemical denitrification, were presented as the proposedtechnologies for the removal and reclamation of pollutantsfrom wastewater.

For electrodeposition, the main advantage is that the de-posited metals can be easily recycled by electrometallurgicalprocesses. However, the surface of the cathode is modifiedduring electrodeposition and it may need additional treat-ment. The electrodeposition of some heavy metals, likenickel, on the cathode is difficult due to the hydrogen

Fig. 17 Experimental set-up.440


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evolution reaction which can greatly reduce the recovery effi-ciency of the metal.

For cathodic electrochemical dechlorination, the choice ofthe cathode materials has been found to have a major effecton the efficiency of this process, as it governs the reactionpathway and the selectivity. One of the main issues in theelectrochemical reductive dechlorination of organic chloridesis the energy consumption associated with the process. Thecost of the electricity needed for driving the electrolysis is toohigh to enable commercial success of this approach, andtherefore suitable electrocatalysts are needed to lower the cellpotential and thus reduce energy demands.

Electrochemical denitrification is receiving increased at-tention because of its advantages as an environmentallyfriendly, safe, selective, and cost-effective technique. From anenvironmental viewpoint, it is advantageous in that nochemicals are needed for the process, and nitrogen com-pounds formed by the electrocatalytic reduction of nitratecan have value. The efficiency of electrode reactions dependsstrictly upon the chemical and physical nature of the workingelectrodes. Synergistic electrochemical effects, in terms ofcatalytic activities, can be obtained when two or more metal-lic species are mixed together on the surface of the workingelectrodes. Although the reaction mechanisms on binary orternary electrodes have been extensively examined, many sub-jects including the specific role of the foreign metal, the opti-mal surface composition, the surface morphology, and otherfactors are still under investigation and evaluation.

Electrocoagulation (EC) and electrocoagulation/flotation(ECF) processes have become effective technologies to re-move pollutants from many types of water and wastewaters,and the performance of EC/ECF technologies can be en-hanced by integrating the process into a process train withother technologies. The amount of chemicals required issmall, and the amount of sludge produced is less than thatrequired compared to conventional coagulation. However,this method has disadvantages such as anode passivationand sludge deposition on the electrodes that can inhibit con-tinuous operation of the process. In addition, high concentra-tions of iron and aluminum ions can be released into the ef-fluent that would have to be removed. Among these variablesin the electrocoagulation (EC) and electrocoagulation/flota-tion (ECF) processes, solution pH and current density are twokey operating parameters which significantly affect the effi-ciency of the electrocoagulation (EC) processes.

ED is one of the most recent technologies that has been usedto separate plating chemicals from rinse water. The advantagesof ED are that a low concentration of heavy metals can be con-centrated, and that the treated effluent water can be diluted forreuse. However, ED does not work well for high concentrationsof metals due to membrane fouling. The integration of ED withother pressure-driven membrane processes, such as micro-filtration (MF), ultrafiltration (UF), nanofiltration (NF), and re-verse osmosis (RO), will help further development of this electro-chemical technology to obtain better performance of thesesystems for water treatment and reuse.

The integration of electrochemistry with photocatalysisand sonochemistry methods has led to a new and interestingpossibility for the treatment of pollutants in wastewater.Using a photo-assisted process, it is possible to promote thedirect generation of highly reactive species, such as ˙OH radi-cals, and also a series of other oxidants (h+, O2˙

−, and Cl˙) inthe bulk solution that would be absent in a purely electro-chemical approach. Sonochemical technology shows promiseas a method to improve the electrochemical process by acti-vating the electrode surface and enhancing mass transfer effi-ciency. Presently, greater effort is needed to fully understandthe degradation process that occurs with hybrid electro-chemical technologies.

The electrochemical processing of wastewaters can have highenergy demands, and therefore developing novel hybrid technol-ogies that can purify water and generate or save energy is ur-gently needed to advance the applications of these technologies.The possibility for both electrochemical degradation of wastewa-ter organics and simultaneous resource reclamation of materialsin wastewater during treatment could make many of these pro-cesses useful compared to existing technologies.

Acknowledgements

This work was supported by the National Natural Science Fundfor Distinguished Young Scholars (Grant No. 51125033) andthe State Key Laboratory of Urban Water Resource and Envi-ronment (Harbin Institute of Technology) (No. 2015DX05).

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