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Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review Carlos A. Martínez-Huitle,† Manuel A. Rodrigo,‡ Ignasi Sirés,§ and Onofrio Scialdone*,∥ †

Instituto de Química, Campus Universitário, Universidade Federal do Rio Grande do Norte, Av. Salgado Filho 3000 Campus Universitário Lagoa-Nova CEP 59078-970 Natal, RN, Brazil ‡ Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies, Ciudad Real, Universidad de Castilla-La Mancha, Ciudad Real 13071, Spain § Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain ∥ Dipartimento di Ingegneria Chimica, Gestionale, Informatica, Meccanica, Università degli Studi di Palermo, Palermo 90128, Italy ABSTRACT: Traditional physicochemical and biological techniques, as well as advanced oxidation processes (AOPs), are often inadequate, ineffective, or expensive for industrial water reclamation. Within this context, the electrochemical technologies have found a niche where they can become dominant in the near future, especially for the abatement of biorefractory substances. In this critical review, some of the most promising electrochemical tools for the treatment of wastewater contaminated by organic pollutants are discussed in detail with the following goals: (1) to present the fundamental aspects of the selected processes; (2) to discuss the effect of both the main operating parameters and the reactor design on their performance; (3) to critically evaluate their advantages and disadvantages; and (4) to forecast the prospect of their utilization on an applicable scale by identifying the key points to be further investigated. The review is focused on the direct electrochemical oxidation, the indirect electrochemical oxidation mediated by electrogenerated active chlorine, and the coupling between anodic and cathodic processes. The last part of the review is devoted to the critical assessment of the reactors that can be used to put these technologies into practice.

CONTENTS 1. Introduction 2. Anodic Oxidation of Organic Pollutants 2.1. Process 2.1.1. Timeline and Fundamentals 2.1.2. Model for Organic Oxidation by Heterogeneous Hydroxyl Radicals 2.2. Figures of Merit 2.3. Influence of Main Operating Parameters on the Performance of Anodic Oxidation 2.3.1. Effect of Anode Material 2.3.2. Effect of Current Density and Fluid Dynamics 2.3.3. Effect of the Nature and Concentration of the Organic Pollutant 2.3.4. Effect of Supporting Electrolyte and Conductivity 2.3.5. Effect of pH 2.3.6. Effect of Temperature 2.3.7. Role of Oxygen 3. Oxidation of Organic Pollutants by Electrogenerated Active Chlorine 3.1. Process

3.1.1. Electrochemical Generation of Active Chlorine 3.1.2. Oxidation of Organics by Electrogenerated Active Chlorine 3.1.3. Formation of Undesired Products 3.2. Influence of Main Operating Parameters 3.2.1. Effect of Anode Material 3.2.2. Effect of pH 3.2.3. Effect of Current Density and Flow Rate 3.2.4. Practical Aspects Related to the Presence of Chlorides 3.3. Advantages, Disadvantages, and Key Aspects to be Addressed and Perspectives 4. Coupling of Anodic and Cathodic Processes 4.1. Cathodic Processes 4.1.1. Direct Reduction 4.1.2. Cathodic Electrogeneration of H2O2 4.2. Coupled Redox Processes 4.2.1. Coupling of Direct Cathodic Reduction and Anodic Oxidation

B B B B C D E E G I K K L M

M M N N O P Q R S T T T V W W

M M Received: June 18, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/acs.chemrev.5b00361 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 4.2.2. Coupling of H2O2 Electrogeneration and Anodic Oxidation: Electro-Fenton-Based Processes 4.3. Advantages, Disadvantages, and Key Aspects to be Addressed and Perspectives 5. Significance of Cell Design and Scale Up 5.1. Flow Pattern and Construction Details 5.2. Combination of Electrochemical Systems with Other Technologies 5.3. Heat Transfer Issues in Electrochemical Cells 5.4. Scaling-Up and Investment Cost of Electrochemical Cells 6. Concluding Remarks Author Information Corresponding Author Notes Biographies Acknowledgments Glossary References

Review

talysis has also received a great deal of attention.6 The most important advantages of electrochemical methods for the treatment of wastewater are their high efficiency, mild operating conditions, ease of automation, versatility,3−11 and low cost,12 especially when they are powered by renewable energy from wind and solar sources.15 The feasibility of electrochemical technologies has been tested with a wide variety of synthetic wastewater types containing a diversity of target compounds, thus achieving a plethora of results that have allowed appropriate conditions to be set for the quick and complete elimination of organic matter from real effluents.3−11 Despite the above-mentioned advantages, some shortcomings still persist, thus limiting their industrial application. Examples include the short lifetime and/or high cost of some electrode materials and low current efficiency under some conditions, as summarized in Table 1.3,9,16−19 Moreover, some intrinsic drawbacks, such as mass transport limitations, low space-time yield and surface area to volume ratio and gradual temperature increase have not yet been satisfactorily resolved.17 In addition, the effect of pH, pollutant type and concentration, effluent conductivity, electrochemical reactor design, and electrode arrangement, potential or current density distribution and occurrence of undesired reactions on the process performance have not been fully evaluated in all reports.3,4 In conclusion, further studies are necessary to develop these processes on an applicable scale. Hence, in this review, some of the most promising electrochemical tools for the treatment of wastewater contaminated by organic pollutants are discussed in detail with the following goals: (1) to present the fundamental aspects of the selected processes, (2) to discuss the effect of both the main operating parameters and the reactor design on their performance, (3) to critically evaluate their advantages and disadvantages, and (4) to forecast the prospect of their utilization on an applicable scale by identifying the key points to be further investigated, thus avoiding redundant studies that deal with well-established issues. In particular, section 2 is focused on the direct electrochemical oxidation (EO) or anodic oxidation (AO), section 3 discusses the IEO mediated by electrogenerated active chlorine, the more frequently investigated and promising mediated electrochemical oxidation, section 4 describes the use of cathodic processes and their promising coupling with anodic processes, and section 5 is devoted to the critical assessment of the reactors that can be used to put these technologies into practice. Section 6 concludes the review.

X Z AB AB AF AG AG AI AJ AJ AJ AJ AK AK AL

1. INTRODUCTION Environmental conservation, which requires strictly sustainable development to avoid jeopardizing current natural resources, is gradually becoming a matter of major social concern. Every day, increasingly tough legislation is being imposed with regard to effluent discharge. Either traditional physical, chemical, and biological techniques or advanced oxidation processes (AOPs) are currently used for the treatment of wastewater containing inorganic and organic pollutants.1−3 Biological treatments are the most widespread methods among them, but they are timeconsuming, need large operational areas, and do not tend to be completely effective against barely biodegradable and toxic pollutants. Such methods are the state of the art in many urban water treatment facilities and are inadequate in the face of industrial effluents with high concentrations of recalcitrant pollutants, landfill leachates, and effluents with extreme pH.1 Traditional physicochemical processes are often inadequate as well for treating such effluents since they are relatively expensive, ineffective, or a cause of secondary pollution. The utilization of more innovative processes like AOPs, which include wet oxidation, ozonation, heterogeneous photocatalysis, and the Fenton process, among others, requires high energy consumption and/or costly or unstable reagents, and they are not always able to remove all recalcitrant compounds. Consequently, the development of new environmentally friendly technologies that are able quantitatively and quickly to mineralize nonbiodegradable organic matter and eliminate pathogens has become an extremely urgent challenge. Within this context, great progress has been made regarding the electrochemical technologies, especially for the abatement of biorefractory substances. As a matter of fact, the application of these technologies in the fight against environmental pollution has been the topic of several books and authoritative reviews.3−14 The main electrochemical procedures utilized for the remediation of wastewater are electrocoagulation (EC), electroflotation (EFl), electrodialysis (ED), electroreduction (ER), electrochemical oxidation (EO) and indirect electrooxidation (IEO) with active oxidants, so-called mediated electrochemical oxidation.3 Recently, treatment by emerging technologies such as electro-Fenton (EF) and photoassisted systems like photoelectro-Fenton (PEF) and photoelectroca-

2. ANODIC OXIDATION OF ORGANIC POLLUTANTS 2.1. Process

2.1.1. Timeline and Fundamentals. Anodic oxidation, AO, or so-called EO, is the most popular electrochemical procedure among the electrochemical advanced oxidation processes (EAOPs) for removing organic pollutants from wastewater.3−6 Also, EAOPs are essentially based on the EO approach. Going back to the beginning,4,5 pioneering studies by Dabrowski in the 1970s, followed by Kirk, Stucki, Kotz, Chettiar, and Watkinson’s work in the 1980s, and Johnson, De Battisti, and Comninellis’s work in the 1990s allow the elucidation of the fundamentals of EO regarding its application as a promising alternative for treating wastewater (Figure 1). Later on, several research groups studied the feasibility of EO for wastewater decontamination, decisively contributing to B


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Table 1. Principal Factors Determining Electrolysis Performance3,9,16−19 factor electrode potential and current density values potential and current distribution mass transport regime electrochemical cell design electrolysis medium electrode materials current input

remarks they control both the reactivity and reaction rates, eventually determining the efficiency of the process they determine the spatial distribution of the reactant consumption and hence they should be as homogeneous as possible a high mass transport coefficient leads to a greater uniformity of pollutant concentration in the reaction layer near the electrode surface, usually resulting in higher efficiency the cell dimensions, the presence or absence of a separator, the geometry of the electrodes, and the interelectrode distance as well as the operation mode (batch or flow) are some of the crucial parameters that affect the figures of merit of electrochemical treatment the supporting electrolyte nature and concentration, pH, temperature, and matrix composition (i.e., presence of radical scavengers, chelating substances, and natural organic matter) ideally, the electrode material should be cheap, exhibiting physicochemical and mechanical stability in the electrolysis medium and high activity toward oxidation of organics and low activity toward side reactions (e.g., O2 evolution) continuous, pulsed, or alternate current to favor current and decontamination efficiency

different nature and concentration. This is also correlated to the • OH reactivity, whose action on organic pollutants may take place in the vicinity of the anode surface or in the bulk solution.19,22−24 2.1.2. Model for Organic Oxidation by Heterogeneous Hydroxyl Radicals. Early mechanistic studies by Feng and Johnson suggested that oxidation of organic pollutants involves O-transfer reactions at high anodic potential, via the production of adsorbed •OH generated from water discharge reactions (eqs 1 and 2), where S[ ] represents the surface sites where the • OH species can be adsorbed:25−30

determination of the operating conditions for the treatment of actual effluents, some going from laboratory-scale to pilot-scale systems. As stated by several experts, EO consists in the oxidation of pollutants in an electrochemical cell by3,18 (1) direct anodic oxidation (i.e., direct electron transfer to the anode), which yields very poor decontamination; (2) indirect or mediated oxidation via chemical reaction with electrogenerated species from water discharge at the anode surface,18 such as physically adsorbed “active oxygen” [so-called physisorbed hydroxyl radical (•OH)] or chemisorbed active oxygen [i.e., oxygen in the lattice of a metal oxide anode (MO)].18 The action of these oxidizing species, whose formation depends on the electrocatalytic activity of the anode, leads to total or partial decontamination, respectively. Nevertheless, the efficiency of decontamination (poor, partial, or total) by both processes is affected by several conditions (electrode passivation, electrode nature, pollutant structure, and temperature, among others), as discussed in the next sections. Given the existence of different heterogeneous species formed from water discharge, two main approaches have been proposed for wastewater pollution abatement by EO:20 (1) electrochemical conversion, in which refractory organics are selectively transformed into biodegradable compounds under the action of chemisorbed active oxygen and (2) electrochemical combustion (i.e., electrochemical incineration), whereby organics are completely mineralized to H2O, CO2, and inorganic ions by physisorbed •OH. Both approaches require the application of high anode potentials, thus causing competition with the oxygen evolution reaction (OER).21 Conversely, direct EO is theoretically possible at low potential values, which are set before O2 evolution, but the reaction rate is usually slow and depends strongly on the electrocatalytic activity of the anode. The main problem of this approach is the progressive decrease in the catalytic activity, commonly called the poisoning effect, owed to the formation of a polymer layer on the anode surface.19,22 This deactivation, which depends on the adsorption properties of the anode surface, the absence of oxygen in solution, the concentration, and the nature of the organic compounds, is particularly favored in the presence of aromatic organic substrates. At this point, oxidation of organic pollutants before or in the potential region of O2 evolution seems to be a key parameter for promoting both the selectivity and the efficiency of the treatment, but it strongly depends on the specific features of the anode material, which determine its ability to produce •OH of

S[ ] + H 2O → S[•OH] + H+ + e−

(1)

S[•OH] + R → S[ ] + RO + H+ + e−

(2)

The undesirable OER occurs concomitantly as follows: S[•OH] + H 2O → S[ ] + O2 + 3H+ + 3e−

(3)

In 1994, a simplified mechanism for the selective oxidation or combustion of organics with simultaneous O2 evolution was proposed by Comninellis.20 This model was based on both the analysis of •OH radicals formed from water discharge at different anodes (Pt, Ti/lrO2, and Ti/SnO2) using N,Ndimethyl-p-nitrosoaniline (RNO) as a spin trap and the results obtained for the electrolysis of phenol and salicylic acid. The indirect technique for the detection and identification of •OH radicals by means of RNO is based on an addition reaction (spin trap) to produce a more stable radical (•OH + spin trap → spin aduct).20 This reaction is very selective with a high rate for •OH radicals (k = 1.2 × 1010 M−1 cm−1). In the case of Pt, Ti/lrO2, and Ti/SnO2, RNO was electrochemically inactive on their surfaces, allowing the real detection of •OH radicals formed from water electrolysis. In accordance with the results, selective oxidation of organics occurs with electrodes forming the so-called higher oxide, MOx+1 (i.e., chemisorbed active oxygen), whereas combustion occurs with electrodes that allow the accumulation of •OH radicals (i.e., physisorbed active oxygen) on their surface. Thus, while Pt and IrO2 anodes favor selective oxidation because the •OH concentration is almost zero, SnO2 promotes complete combustion because of the significant accumulation of •OH radicals at its surface.20,31,32 Subsequently, the assumptions proposed by Comninellis in the first EO mechanism were confirmed by the results obtained at conductive boron-doped diamond (BDD) film electrodes, fitting the model predictions quite well.21,22,33 The evidence for the formation of •OH radicals on BDD was demonstrated by using the electron spin resonance (ESR) technique during the electrolysis of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) C


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Figure 1. Timeline of EO from its first application to the present. Based on the existing literature and authoritative reviews.3−6,8−11,18

solution with BDD,21 confirming the formation of •OH during anodic polarization of diamond electrodes. Similarly to RNO and DMPO, other spin trap species have been used to demonstrate the formation of •OH radicals;3,21,22 however, more studies must be performed in order to understand the behavior of electrogeneration and concentration of these oxidants on different anode surfaces. On the basis of these outcomes, Comninellis explained the different behavior of electrodes in EO by considering two limiting cases: the so-called “active” and “nonactive” anodes.3,34,35 The proposed model20,21 assumes that the initial reaction in both kinds of anodes (generically denoted as M) corresponds to the oxidation of water molecules leading to the formation of physisorbed hydroxyl radical [M + H2O → M(•OH) + H+ + e−]. At active anodes, the surface interacts strongly with •OH and then a so-called higher oxide or superoxide (MO) may be formed [M(•OH) → MO + H+ + e−]. This may occur when higher oxidation states are available for a metal oxide anode, above the standard potential for O2 evolution. The MO/M redox couple acts as a mediator in the oxidation of organics (MO + R → M + RO) and competes with the side OER via chemical decomposition of the higher oxide species (MO → M + 1/2O2).19,21,22 At nonactive electrodes, where the formation of a higher oxide is excluded, hydroxyl radicals, called physisorbed active oxygen [M(•OH)], allow the nonselective oxidation of organics, which may result in complete combustion to CO2:22,32

chemical reactivity (which is related to the rate of organics oxidation) of physisorbed M(•OH) are highly dependent on the strength of the M−•OH interaction.19,34 Thus, while anodes with low O2 evolution overpotential (i.e., anodes that are good catalysts for the OER) lead to the partial oxidation of organics, anodes with high O2 evolution overpotential (i.e., anodes that are poor catalysts for the OER) favor the complete oxidation of organics to CO2, thus becoming ideal electrodes for wastewater treatment as in the case of PbO2 and BDD. Their behavior is a consequence of the weaker M−•OH interaction, favoring higher anode reactivity for organics oxidation. A classification of the most usual anodes for EO with their basic properties can be found in Table 2. 2.2. Figures of Merit

The effectiveness of EO is usually evaluated on the basis of various figures of merit. The percentage of abatement is usually estimated for either the pollutant concentration or the chemical oxygen demand (COD) (eq 5), where c0 and cf are the initial and final concentrations of the organic pollutant or COD values, respectively: X = 100

(C 0 − C f ) C0

(5)

On the other hand, the current efficiency can be expressed by means of the instantaneous current efficiency (ICE), the general current efficiency (GCE), or the electrochemical oxidation index (EOI), as described below. 35−38 The mineralization current efficiency (MCE), which is a ratio between the experimental and theoretical total organic carbon (TOC) removal, is sometimes preferred. ICE is calculated from eq 6, where CODt and CODt+Δt are the chemical oxygen demands (g dm−3) at electrolysis times t and t + Δt, respectively, I is the current (A), F the Faraday constant (96,487 C mol−1), V the electrolyte volume (dm3), and 8 is the oxygen equivalent mass (g eq−1):

a M(•OH) + R → M + mCO2 + nH 2O + x H+ + ye− (4)

where R is an organic compound with m carbon atoms without any heteroatom, and needs a = (2m + n) oxygen atoms to be totally mineralized to CO2. In both kinds of anodes, the initial •OH may also be simultaneously oxidized [M(•OH) → M + 1/2 O2 + H+ + e−] or may undergo dimerization [2 M(•OH) → 2 M + H2O2], resulting in decreased process efficiency.3 It is well-known that the low adsorption ability of •OH on anode surface favors its dimerization to H2O2.6,22,36 On the basis of these assertions, the electrochemical activity (which is related to the overpotential for O2 evolution) and

ICE = FV [(CODt − CODt +Δt )]/8I Δt

(6)

37

The GCE is calculated from eq 7, where (COD)0 and (COD)t are the chemical oxygen demands (g dm−3) at pertaining times, and the other variables have the same meaning as described above. This equation is similar to that D


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Table 2. Classification of Anode Materials Based On Their Oxidation Power and Potential for O2 Evolution in Acidic Mediaa

a Adapted with permission from ref 3. Copyright 2010 Springer. Adapted with permission from ref 19. Copyright 2011 Sociedade Brasileira de Quimica. Adapted with permission from ref 34. Copyright 2008 John Wiley & Sons Ltd. bStandard potential for OER is 1.23 V versus normal hydrogen electrode.

activity and selectivity; and (4) low cost and high durability (i.e., long service life). Even if we are still far from meeting all the requirements of the ideal anode, significant steps have been made toward the production of better electrode materials.3 A huge number of them have been tested,4,5,18 including polypyrrole, granular activated carbon, activated carbon fiber (ACF), glassy carbon, graphite, platinized Ti and massive Pt, pure and doped-PbO2, and mixed metal oxides (MMO) of Ti, Ru, Ir, Sn, Ta, and Sb. A large variety of studies has demonstrated the much greater efficiency of the so-called “nonactive anodes” like PbO239,40 and BDD.41−43 Table 3 confirms the great mineralization attained for several pollutants by using these latter anodes in comparison with other materials.18,44−57 Results for different organic pollutants using different electrocatalytic materials have been summarized and discussed in detail in several authoritative reviews.3−6,8,9,11,18,39,41,43,58 In practice, however, most anodes will exhibit mixed behavior because of the coexistence of organic oxidation approaches and OER. Overall, dimensionally stable anodes (DSA), SnO2, PbO2, graphite, and BDD show greater chemical resistance during wastewater treatment. Carbonaceous electrodes are very cheap and have a large surface area and are therefore widely used for the removal of organics in three-dimensional (3D) electrochemical reactors (e.g., packed bed, fluidized bed, carbon particles, porous electrodes, and so on).5 However, at highly anodic potentials, low durability is observed (surface corrosion). In the case of PbO2 anodes,5,39,40 the generation of highly toxic Pb2+ is a concern because of serious secondary pollution. However, Ti/PbO2 has proved to be somewhat stable, although its performance and stability depend on the preparation method. With a similar behavior, Ti/SnO2 was reported to present a limited service life. Pt is one of the most commonly used anodes in both preparative electrolysis and synthesis because of its good

proposed for ICE but represents an average value between those at initial time and t. GCE = FV [(COD0 − CODt )]/8It

(7)

In the case of the total current efficiency (TCE), the COD values are considered at the initial and final times: TCE = FV [(COD0 − CODf )]/8I Δt

(8)

Subsequently, from the ICE versus time curves, the EOI value can be calculated. The EOI for a given organic compound reveals the reactivity of the considered species. Its evaluation during a short time period, in particular at the beginning of the electrolysis, can be considered as specific to the substrate.3,5,38 The specific energy consumption (EC) per unit volume (kWh dm−3) is estimated according to eq 9, where Ecell is the average potential difference between anode and cathode during the electrolysis (V). EC = [EcellIt ]/[1000V ]

(9)

2.3. Influence of Main Operating Parameters on the Performance of Anodic Oxidation

Many of the drawbacks mentioned in Section 1 (see Table 1) limiting the efficient performance of electrochemical technologies at real scale are also encountered in EO.6−8 In this section, the most relevant aspects are pointed out. 2.3.1. Effect of Anode Material. The percentage of abatement and the efficiency of EO depend to a great extent on the nature of the anode material (in combination with other operating parameters), and thus, its proper selection is extremely important.19,22 For this reason, the choice of electrocatalytic material is a matter of extreme importance. According to Walsh17 and Urtiaga,9 the electrode must have the following properties: (1) high physical and chemical stability, being resistant to erosion, corrosion, and formation of passive layers; (2) enough electrical conductivity; (3) high catalytic E


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Table 3. Selected Examples of Comparative EO Treatment of Organic Pollutants With “Nonactive Anodes” (PbO2 and BDD)a removal efficiency (%)b and electrolysis time (h) compound

PbO2c

BDD

removal efficiency (%) obtained with other anodes in the same workb

ref

− Ti−Ru−Sn (10)* (24 h) Ti/IrO2−Ta2O5 (70)*** (26 h) Pt (90)*** (13 h) Au (100)*** (12 h) IrO2 (50)* (15 h) DSA (30−40)* (6 h) Ti/Pt (90)*** (8.5 h) Ti/IrO2−Ta2O5 (35)*** (12 h) − Ti/Ti0.5Ru0.45Sn0.05O2 (20)* (11 h) Pt (50)* (11 h) Ti/SnO2−Sb (40)** (25 h) − − Pt (28)* (7 h) Au (0)* (7 h) Ti/IrO2−Ta2O5 (26)* (6 h) ebonex (24)* (7 h) stainless steel (40)* (4 h) − Ti/SnO2 (40)*** (0.5 h)

44 45 46

4-chlorophenol 2-naphthol oxalic acid

80* (23 h) 100* (24 h) 100*** (10 h)

100* (6 h) 100* (12 h) 100*** (12 h)

chloranilic acid cyanide waste oxalic acid

80* (11 h) 75−80* (6 h) 100*** (8.5 h)

100* (6.4 h) 80−90* (3 h) 100*** (8.5 h)

sodium dodecylbenzenesulfonate methyl red p-nitrophenol methyl red mecoprop 1,2-dichloroethane

25** (10 h) 100* (11 h) 80** (25 h) 100* (8 h) 96* (13.5 h) 28* (7 h)

75** (10 h) 100* (6 h) 90** (9.3 h) 100* (4 h) 100* (3.6 h) 97* (7 h)

phenol methamidophos

100** (7 h) 50−80*** (0.4 h)

100** (5 h) 90*** (0.2 h)

37 47 48 49 50 51 52 54 55

56 57

a c

Other anode materials are included for particular cases. bEfficiencies for the removal of (*) COD, (**) TOC, and (***) pollutant concentration. PbO2 anode with different metal electrode support.

example, Panizza and Cerisola treated 225 mg dm−3 COD solutions of methyl red in 0.5 M Na2SO4 with different anodes using a flow cell (Figure 2a) at 500 mA (corresponding to anode potentials in the O2 evolution region).50 As shown in Figure 2b, the ability to remove solution COD decreased in the order Si/BDD > Ti/PbO2 > Pt > Ti/TiO0.50Ru0.45Sn0.05O2. The same trend can be seen for CE in Figure 2c. In the case of Ti/ PbO2, COD decreased to almost zero at 11 h, meaning the almost complete oxidation of the azo dye and all its byproducts. However, the efficiency decayed from 24% to 7% in 9 h, as expected when resistant oxidation intermediates such as aliphatic acids are accumulated and slowly destroyed. These results, as well as those reported elsewhere,44−61 demonstrate that organic pollutants are efficiently incinerated on Ti/PbO2 and Si/BDD (nonactive anodes) mainly by “quasifree” electrogenerated •OH. However, several studies only explain the decontamination of synthetic or real effluents, considering that the anode can be active or nonactive, which is not appropriate since the performance of some electrodes in EO is affected by other experimental conditions. The correct electrochemical approach requires the evaluation of the potential for the OER for different anodes used, and when only one electrode is employed, exhaustive discussion about the influence of strong oxidizing species should take place. Superfluous EO discussions should be avoided when MMO anodes are used because some authors have demonstrated that metal-based oxides cannot be merely classified into active and nonactive categories but may exhibit characteristics of both kinds of electrodes.24 According to Rodrigo and Brillas,62 the same behavior could be observed at BDD anodes because of the effect of the sp3/sp2 ratio on their electrocatalytic properties.63 According to Comninellis,3,34 active anodes accumulate lower •OH concentration (almost zero) at their surfaces,

chemical resistance to corrosion even in strongly aggressive media.4,5,18 Its behavior in the EO of organic pollutants has been widely reported in the literature,4,5,18,22 showing a significant electrocatalytic activity in some cases. DSA consist of a Ti substrate coated with a thin conducting layer of metal oxide or MMO (Ti, Ru, Ir, Sn, Ta and/or Sb).9,11 Since their first manufacture, many research studies have been performed to find new coatings for many electrochemical applications.11 The development of anodes coated with a layer of RuO2 and TiO2 brought about significant improvements in the chlor-alkali industry (DSA-Cl2), while anodes coated with IrO2 have been commercially used for promoting the OER (DSA-O2) in acidic media for several electrochemical processes, such as water electrolysis and metal electrowinning. IrOx and Ti/IrOx−Ta2O5 electrodes are relatively expensive and often they do not allow the total mineralization of organics. Ti/SnO2 anodes are more effective for wastewater treatment, but they have been shown to have a limited service life. Alternatively, BDD has been found to be the most efficient anode material for degradation of refractory pollutants during electrolysis in the region of water discharge. The BDD anode produces a large quantity of loosely adsorbed •OH.21 Consequently, it has high reactivity for organics oxidation, eventually leading to efficient water treatment.33 Many papers3−6,8,18,19,22,33,37,41,42 have demonstrated that BDD anodes allow complete mineralization of several pollutants such as ammonia, cyanide, phenols, aniline, hydrocarbons, dyes, surfactants, drugs, pesticides, etc. The anode material determines not only the percentage of the organic matter removal in the bulk solution but also the CE.8,19 Nevertheless, this is also correlated to the current density and the combination with other operating parameters. The effect of the anode material in EO has been clearly confirmed by several authors for different model organics.18 For F


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Figure 2. (a) Flow cell with parallel plate anodes, and comparison of (b) COD removal and (c) current efficiency as a function of electrolysis time for the electrochemical oxidation of 300 cm3 of 225 mg dm−3 COD of methyl red in the presence of 0.5 M Na2SO4 at 25 °C using different anodes of 16 cm2 area with a stainless steel cathode at 500 mA and 180 dm3 h−1. Reprinted with permission from ref 50. Copyright 2007 Elsevier.

whereas higher accumulation of •OH is found on nonactive anode surfaces. Nevertheless, this behavior depends on the ability of an active anode to convert M(•OH) to MO before the reaction of the former with the pollutant when it is in the vicinity of the surface or the diffusion of the radical toward the bulk.8,20 Thus, not all the •OH radicals have strong interaction with the anode surface to produce MO. Bejan and co-workers24 have revealed this behavior by testing different electrodes and determining the relative reactivity of the •OH radicals toward organic pollutants. These outcomes are in agreement with the suggested dual behavior (active and nonactive) of BDD in some cases.62−64 On the other hand, the type of interaction between the organic model pollutant and the anode surface must be clarified.20,21 As stated by other authors,3,9,18 the concentration and nature of the organic pollutant can influence the efficiency of the process (parameters discussed below). However, the two most frequent justifications for the decay of anode efficiency are fouling and corrosion.22 Electrode passivation, which usually involves the formation of an inhibiting layer as a result of a polymer film formation or byproduct adsorption, is usually attained in active anodes.22,46 This phenomenon can be controlled by using the galvanostatic mode;65 however, in some cases, final intermediates of the EO such as carboxylic acids can be strongly adsorbed, passivating the surface. In the case of nonactive anodes, it has been reported that BDD does not tend to undergo fouling because of the electrogeneration of active hydroxyl radicals that can oxidize polymeric materials deposited on the anode surface.66 However, this behavior is questionable because several authors have reported evidence of BDD passivation by the formation of polymeric film that reduces its efficiency.43,45,46 In contrast, BDD electrodes are susceptible to deactivation by anodic corrosion.67 This behavior is also observed at other electrocatalytic materials such as graphite, amorphous carbon, and metal oxides.

2.3.2. Effect of Current Density and Fluid Dynamics. Operation current density (i) or, less usually, applied Ecell, are very important in EO because they are the only parameters that can be directly controlled.9,68 In EO systems, interelectrode spacing is fixed and current is continuously supplied (i.e., galvanostatic mode). Current density (intensity per unit area of electrode) may be the term most frequently referred to because it controls the reaction rate, and consequently, it commonly defines the efficiency of the process. Nevertheless, the efficiency also depends on other conditions, as discussed below. Generally speaking, in the low i range (i.e., when the EO is not kinetically limited by the mass transport of organics to the anode surface), an increase in i leads to greater pollutant removal. Conversely, in the high i range (i.e., when the process is mass transport controlled), an increase in i is expected to enhance the O2 evolution, thus giving rise to a decrease in current efficiency (CE) and increase in energy costs (see subsection 2.3.3). For intermediate i values, (i.e., when the process is under a mixed kinetic regime), higher i values should cause an increase in the pollutant removal but also a decay in CE.34 This apparently simple scenario can actually be complicated by the additional formation of other oxidants like chlorine, ozone, hydrogen peroxide, persulfate, peroxophosphate, as current rises.3−5 For example, this behavior has been confirmed during the EO treatment of a real effluent with a flow electrolytic cell equipped with a BDD anode,69 similar to that of Figure 2a. As observed in Figure 3, COD removal rate was significantly increased, achieving different values of TCE, when the applied current density was increased from 20 to 60 mA cm−2. This increase in COD removal was observed because the formation of strong oxidizing species was also favored, thus minimizing the mass transport limitations and also the parasitic OER. Mediated oxidation explains the degradation of large molecules such as dyes.18 It is important to note that an increase in current density does not necessarily enhance the oxidation efficiency or G


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°C, and 75 dm3 h−1 operating in batch mode at 30 mA cm−2. They demonstrated that the oxidation process started with the breakage of the azo group up to 10 Ah dm−3, with a large accumulation of carboxylic acids taking place during the final stages of the treatment. The removal efficiency for all dyes was mainly dependent on their concentration and current density (approximately 65% of CE under optimal experimental conditions), which were larger for high organic loadings. This was attributed to the change of the mediated oxidation under the action of •OH and S2O82− in the concentrated solutions and only in the presence of S2O82− in the diluted ones. After 4 h of electrolysis at 30 mA cm−2, all solutions were completely decolorized and mineralized with a high EC of about 800 kWh m−3. A similar behavior has been reported by the same group for the anthraquinone dye alizarin red treated under comparable conditions.73 As suggested above, unsuitable hydrodynamic conditions can cause the appearance of mass transport limitations that affect the action of current density in EO.8,34 The stirring rate is one of the most important parameters when batch cells are used. Proper stirring minimizes the appearance of concentration gradients in the electrolytic cell, thus ensuring homogeneous conditions. Furthermore, it favors the movement of the generated ions, an increase in the pollutant removal efficiency, and the transport of the strong oxidants from the anode surface toward the solution.8,68 Figure 5 shows the effect of stirring rate

oxidation rate. See, for example, the trends obtained for the EO treatment of 4-chlorophenol in Figure 4. For a given anode

Figure 3. Influence of current density on the evolution of COD and total current efficiency (inset), as a function of time, during the electrochemical treatment of actual textile water using BDD anode. Conditions: 25 °C, 5 g dm−3 of Na2SO4, and 250 dm3 h−1. Reprinted with permission from ref 69. Copyright 2012 Elsevier.

Figure 4. Influence of current density on the trends of COD and instantaneous current efficiency (inset) during the electrolysis of 7.8 mM 4-chlorophenol (4-CP) in 1 M H2SO4 with a BDD anode at 25 °C. i: (□) 15, (×) 30, and (●) 60 mA cm−2. Reprinted with permission from ref 70. Copyright 2001 Electrochemical Society.

Figure 5. Influence of the stirring rate on the time course of color (dashed lines), COD (continuous lines) and TOC (inset) removal during the EO treatment of 60 mg dm−3 methylene blue in 0.5 M H2SO4 at 30 mA cm−2 and 25 °C. Reprinted with permission from ref 74. Copyright 2011 Elsevier.

material, the effect of current density depends on the characteristics of the effluent to be treated as well. Also, the negative influence of rising current density during the EO treatment of some organic pollutants has been attributed, as mentioned above, to the occurrence of the OER and/or mass transport limitations.34 From a critical point of view, the variation of current density may affect the efficiency of the process; nevertheless, the action of other strong oxidants produced at the anode surface must not be disregarded. The production of such oxidants is controlled not only by the applied current but also by other operating conditions (e.g., dissolved O2, pH, temperature, and inorganic ions) as well as by the anode properties.8 Cañizares and co-workers71 and Faouzi and co-workers72 explored the influence of i and initial dye concentration on the mineralization of azo dyes such as eriochrome black T, methyl orange, and congo red. They employed an undivided flow cell with a p-Si/BDD anode and a stainless steel cathode, both 78 cm2 in geometric area, to degrade high (1800 mg dm−3) and low (100 mg dm−3) COD contents of each dye in 500 dm3 of solution containing 5 g dm−3 Na2SO4 at their original pH, 25

(300−620 rpm) on the color, COD, and TOC removals during the treatment of 60 mg dm−3 methylene blue solutions at Tisupported Pt with the application of 30 mA cm−2.74 As can be seen, the hydrodynamic conditions strongly affect the color removal rate. Faster stirring led to efficient quicker decolorization, which suggests that the EO of the dye was controlled by mass transport. In contrast, the authors found that the hydrodynamic conditions did not significantly affect the COD and TOC removal rates because the concentration of hydroxyl radicals produced at the Ti-supported Pt anode is insufficient to oxidize the dissolved organic matter, although it is sufficient to favor the fragmentation of the dye chromophore group. The effect of production of byproducts during methylene blue treatment also promotes the passivation of the Pt surface, reducing its efficiency, and consequently, no improvement on the H


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degradation was achieved when hydrodynamic conditions were modified. The effect of the flow rate during the oxidation of the same dye with an electrochemical flow cell at 20 mA cm−2 with the BDD anode is shown in Figure 6.75 The COD and dye

Figure 7. Comparative COD removal vs time during the EO treatment of petrochemical wastewater containing 5 g of Na2SO4 with Pt and BDD anodes by application of 15 and 60 mA cm−2 at 400 rpm and 60 °C. Reprinted with permission from ref 77. Copyright 2012 Elsevier. Figure 6. Influence of the electrolyte flow rate on the COD evolution and color removal (inset) during the EO treatment of 80 mg dm−3 methylene blue in 0.5 M Na2SO4 at 20 mA cm−2 and 20 °C. Reprinted with permission from ref 75. Copyright 2007 Elsevier.

conditions, and it is then more suitable for lab-scale work. In contrast, continuous systems are appropriate for treating large volumes aiming industrial use. A strong impact of stirring conditions in batch tank reactors and liquid flow rate in continuous flow systems has been described for the EO treatment of organic pollutants,3−5,18,33 since these affect the transport of oxidants and organics and/or the residence time in the cell. In conclusion, the fluid dynamics strongly affects the EO efficiency, being completely correlated to the cell or reactor design, and these engineering issues will be discussed in section 5. 2.3.3. Effect of the Nature and Concentration of the Organic Pollutant. This effect is directly related to the nature of the anode material. In the case of nonactive anodes, the nature of organic pollutants seems to exert only a small influence on the degradation rate and the efficiency of the process; however, it is relatively dependent on the reactive oxidizing species generated at the anode surface. Conversely, at active anodes, different behaviors and efficiencies can be expected when organic pollutants and their concentrations are altered.4−6,8,18 It is important to note that these effects are further complicated by the interaction with other operation factors such as pH, passivation or corrosion of the surface, temperature, stirring rate, and possible reactions of the organic pollutant at the anode surface (e.g., adsorption/desorption).3,5,8 As a general rule, the higher the concentration of organic matter to be removed, the greater the CE of the process.3−5,79 This can be clearly observed in Figure 8, which depicts the trends of COD and ICE during the EO of 2-naphthol.80 In this particular case, for high organic loads, COD decreased linearly and ICE remained constant at about 100%; conversely, for low organic loads, COD decreased exponentially and ICE began to fall.3,80 This behavior caught the attention of Comninellis’s group, who further developed a comprehensive kinetic model that allowed them to predict the trend of COD and ICE for the treatment of organics by EO with BDD.3,34,80 The EO of organics was assumed to be a fast reaction, and its oxidation in the bulk by means of electrogenerated oxidants was obviated. The formulation of the model starts with the estimation of the limiting current density from COD values:3,34

removals are faster at higher flow rates (as found for trials in batch mode), meaning that the oxidation is mass transport controlled. The increase in the flow rate favors contact between organics and electrogenerated hydroxyl radicals near to the BDD surface, thereby minimizing their decomposition to O2. Compared with the results obtained by De Oliveira and coworkers,74 in this case, the BDD−hydroxyl radical interaction is so weak that the •OH radicals can even be considered as quasifree with respect to the behavior attained at the Ti-supported Pt anode. These quasi-free radicals are very reactive and can result in the mineralization of the organic compounds. On the other hand, the electrochemical flow cell used by Panizza and coworkers75 reduces the distance between the electrodes and increases the turbulence conditions, favoring a fast reaction of • OH radicals with the organic pollutant. Nevertheless, Savaş Koparal and co-workers76 investigated the decolorization of basic red 129 solutions. The rise of i from 0.25 to 1.00 mA cm−2 and pH up to 5.8 accelerated the color removal of 125 cm3 of 20−60 mg dm−3 dye solutions in 0.03 M Na2SO4, whereas the decolorization did not vary for flow rates between 1.49 and 2.87 dm3 h−1. This was owed to the effective action of hydroxyl radicals on the chromophore group. Rocha and co-workers77 investigated the EO of a real effluent generated upon petroleum exploration by a Petrobras plant in Brazil, using Ti/Pt and BDD anodes in an electrolytic batch cell and adding Na2SO4 to promote persulfate production. Depending on the stirring rate, BDD can promote complete COD removal because of the high amounts of effective hydroxyl radicals and persulfates generated from water and sulfate oxidation (Figure 7). However, when the previous electrochemical treatment developed by Rocha and co-workers77 was investigated by using a flow reactor with Ti/Pt and BDD anodes, different results were achieved in terms of COD decay.78 COD was gradually removed even without addition of sulfates thanks to the hydrodynamic conditions (turbulent or laminar) in the continuous flow cell, which allowed the efficient elimination of organic matter by •OH. From these results, it can be inferred that a batch reactor enables the study of a range of operating

ilim(t) = 4Fk mCODt

(10) −2

where ilim(t) is the limiting current density (A m ) at a given time t, 4 is the number of exchanged electrons, km is the mass I


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than expected (this behavior is notorious for the Cl-mediated oxidation and is discussed in section 3).89 Predicting the EO profiles of organic pollutants using mathematical models has allowed the proposal of a new approach to maintain the current efficiency close to 100% even under mass transport limitation conditions. Fierro and coworkers90 worked under galvanostatic conditions with the potential “buffered” by the competing OER. In accordance with this process, the working potential is fixed by the nature of the electrode material and it is buffered during organic oxidation. For the EO treatment of phenol, its conversion to hydroquinone, p-benzoquinone and pyrocatechol achieved higher aromatic selectivity, but with a current efficiency minor than 1%. With these results, the authors demonstrated two main features: (1) a total aromatic selectivity close to one suggests that, at the working anodic potential (1.7 V) buffered by the side OER on the Ti/IrO2 anode, the oxidation products of phenol do not undergo further oxidation even after a quasicomplete conversion of phenol and (2) the high selectivity toward p-benzoquinone formation (para to ortho ratio equal to 3:1) reveals a specific orientation of phenol on the IrO2 surface during EO. In fact, in the absence of interaction with the electrode, the main product is pyrocatechol (para to ortho ratio equal to 1:2). With regard to the effect of the limiting current, when low CE is attained, a higher electrical charge is needed for complete mineralization because a great amount of •OH is wasted in parasitic nonoxidizing reactions such as OER. This behavior is typically attained when the electrolysis is under mass transport control and the iappl > ilim.3,34,69 Therefore, it is important to estimate ilim from the COD value by using eq 10 for the efficient performance of EO of a real wastewater, as suggested by Panizza and Cerisola.91 For EO with BDD, Panizza and Comninnelis proposed a “modulated” electrolysis, which involved the adjustment of current density during the electrolysis to close to the limiting value, thus enhancing the CE and the oxidation rate.65 However, it is important to note that ilim is affected by the mass transport mode in the electrochemical reactor (stirring, flow, turbulence, and so on). Therefore, the determination of km of the reactor before its use is an essential parameter in terms of improving the viability of this advanced oxidation technology, although only a few researchers consider it as mandatory.69,78,91 It is worth noting that ilim only affects the electrochemical processes that occur at the electrode surface, such as EO or IEO, because the main oxidant (•OH and active chlorine, respectively) is formed from the electrode reaction (i.e., the process depends on the charge transfer and mass transport).3−5,8 In contrast, the reactivity derived from the action of strong oxidants once produced at the electrode surface is not dependent on ilim. For example, in EF, H2O2 is assumed to be produced, despite its limitations concerning ilim for H2O2 production, but then the reaction between organics and the main oxidant (hydroxyl radicals) is not dependent on ilim because it is a homogeneous process (Fenton’s reaction). The EO of various aromatic compounds in acidic solution has been performed by varying the organic concentration and current density. However, the nature of the organic pollutant can also affect the efficiency of treatment. In the case of aliphatic acids, Comninellis and co-workers92 have demonstrated that the electrochemical treatment is independent of the chemical nature of the organic compound. Nevertheless, a change of solution pH can potentially modify the chemical

Figure 8. Influence of the initial 2-naphtol concentration on the evolution of COD and ICE (inset) during the EO with BDD in 1 M H2SO4 at 238 A m−2 and 25 °C. Pollutant concentration: (◊) 2, (○) 5, and (×) 9 mM. The solid line represents model prediction. Reprinted with permission from ref 80. Copyright 2001 Elsevier.

transport coefficient in the electrochemical reactor (m s−1), and CODt is the chemical oxygen demand (mol m−3) at time t. Depending on the applied current, two different operation regimes can be identified:80 (1) iappl < ilim: the electrolysis is under current control; the current efficiency is 100%, and COD decreases linearly with time and (2) iappl > ilim: the electrolysis is under mass transport control; secondary reactions (such as the oxygen evolution) occur significantly, resulting in an ICE decrease, whereas COD removal presents an exponential decay because of mass transport limitations. The equations that described the time course of COD and ICE were obtained from COD mass balances of the electrochemical reactor and the reservoir. Considering the above information, at lower current densities, a kinetically controlled process is attained, while mass transport limitation and the parasitic O2 evolution are attained upon application of higher current densities.34 The main failure of this model is that it does not consider the existence of different phenomena on the anode surface that involve other chemical species. Furthermore, it cannot be applied to anodes and pollutants that do not satisfy ICE = 100% for a process under current control, as often happens for electrodes other than BDD; fortunately, these considerations have been taken into account by other authors, such as Polcaro and co-workers81 and Cañizares and co-workers,82−84 who proposed other interesting theoretical models. In addition, a recent model has been published where the effect of operating parameters on the process performance was investigated by Scialdone and co-workers.85−88 For simplicity, the complete oxidation of organic pollutants proceeding in competition with the OER with no significant accumulation of intermediates in the bulk has been considered. In this case, the ICE for a process under galvanostatic mode can be rapidly estimated for both mass transport and oxidation reaction control regimes. A good agreement between experimental data and theoretical predictions was observed by substantially altering the current density, the flow rate, and the initial concentration of the carboxylic acids tested. Recently, the model was upgraded in order to take into account the accumulation of intermediates and the presence of many organic pollutants from the beginning of electrolysis, thus predicting their concentration profiles during electrolysis.87 Note that numerous kinetic oxidation models have not considered the participation of other oxidants than •OH, which results in higher ICE values J


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electrolyte flow disruption could become critical problems in applications with commercial cells with narrow interelectrode distances.91 The electrolyte composition may also determine the absence or presence of membranes or other separators in the electrochemical cell.8,18 As an alternative to the direct treatment of wastewater, a two-stage remediation approach can be devised: synthetic solutions can be used first to electrogenerate oxidants that are further added to the wastewater to achieve the chemical oxidation of pollutants.8 The issues related to the oxidants’ stability before being placed into the wastewater have not been widely studied so far, but there are many works on the production of oxidants that could have an environmental remediation application. These works deal with the production of persulfates, perphosphates, and even percarbonates.3,8,34 Unfortunately, only a few works are related to the application of the resulting oxidant solution for the treatment of wastewater.18 The main advantage of this alternative is that operating conditions for the production of oxidants, such as temperature, solution composition, and so on, could be optimized and a much more efficient treatment could be designed. In this case, and in contrast to what was described for the direct feed of wastewater, the membrane allows the obtention of high efficiencies in the production of oxidants, thus becoming a key component of the divided cell. Conversely, a clear disadvantage of this sequential process is that the electrochemical activation of oxidants in the bulk (i.e., the transformation of oxidants into more reactive species, typically radicals formed upon synergistic interactions between electrogenerated oxidants) is not promoted because the electrochemical process and the treatment of the waste are carried out in different operations. Then, the electrochemical activation of oxidants, which can be achieved by irradiation with ultrasound or UV light or in the presence of H2O2, is a very important way to enhance the efficiency of the electrochemical wastewater treatment process.8 2.3.5. Effect of pH. The pH solution is an important operational parameter in EO because in most cases the pollutant removal efficiency is maximal at an optimum pH value, decreasing at lower or higher pH values. This behavior is attributed to the chemical structure of the pollutant, since the electroactive species in alkaline medium (i.e., the unprotonated form) can be more easily oxidized than that in acid medium (i.e., the protonated form, or vice versa),18 as well as to the decay in the concentration of •OH and other oxidants (ClO−/ HClO, O3, H2O2, and S2O82−) in the bulk solution.3−5 No specific behavior is observed when the pH value is changed, thus becoming completely case-sensitive, but it mostly affects the IEO process, such as when Cl− ion is present in the medium (see section 3).93 Several authors have described the effect of pH in EO. For example, Ammar and co-workers94 investigated the effect of current and pH during the EO of 100 cm3 of 220 mg dm−3 of indigo carmine solutions with 0.05 M Na2SO4 at 35 °C in a stirred undivided cell with a 3 cm2 Si/BDD anode and a 3 cm2 stainless steel cathode. At pH 3.0, a faster TOC decay can be observed in Figure 9a with increasing current from 100 and 300 mA because of the greater production of active BDD(•OH) that accelerates the oxidation of organics. In contrast, at 100 mA, the solution became colorless more quickly at pH 10.0 (120 min) compared with pH 3.0 (270 min), as can be seen in Figure 9b, because the electroactive species in alkaline medium (the unprotonated form) was more easily oxidized.

structure of organic pollutants (protonated or unprotonated forms), favoring a decrease or increase of the removal rates. However, it is important to note the ultimate dependence on the nature of the electrode because after the chemical modification of the pollutant structure adsorption/desorption processes are feasible. 2.3.4. Effect of Supporting Electrolyte and Conductivity. The electrolyte in electrochemical wastewater treatment could be wastewater, either raw or modified, or a synthetic aqueous solution especially prepared first to promote the production of oxidants and then to be added to the wastewater.68 The choice is important in order to select divided or undivided cells.18 Typically, most lab-scale studies reported in the literature use synthetic solutions of a given pollutant (or a mixture of pollutants) and large concentrations of salts, added as supporting electrolyte to increase the ionic conductivity, thus reaching concentrations much higher than those found in real effluents.3−5,8,18 To attain the desired conductivity value, sodium chloride or sodium sulfate is routinely added.8,68 It is worth highlighting that when chlorides are used, the Cl-mediated oxidation will be favored.3−5,8,9,93 On the other hand, persulfates can be produced at nonactive anodes when sulfates are used instead.3−5,8,33,69 Also, the concentration of organic pollutants in these synthetic solutions is not always realistic. In particular, for a current hot topic like the depletion of anthropogenic pollutants with special environmental or health interest because of their potential toxicity, such as persistent pollutants, the studied concentrations tend to be extremely high. The major drawback is that the study becomes unrealistic and conclusions should then be carefully drawn especially if they are intended to be extrapolated to fullscale applications. In any case, this option is not as bad for a preliminary understanding of the process in terms of the elucidation of reaction pathways (which is the main aim of labscale assessments) because it allows researchers to characterize the reactivity and the main intermediates without significant analytical constraints (in terms of limits of detection and quantification) and avoid important operational difficulties like the extreme Ecell values and high temperatures because of ohmic drops. The treatment of actual wastewater is much more complex. As stated, studies with synthetic wastewater are far from the real scenario in which the concentrations of pollutants and salts are fully determined by the industrial process in which they are produced. Modifications of the composition of raw wastewater should be carefully managed.8 “Intensification” of the concentration is only interesting from the scientific point of view in order to check if there are differences in the treatment results that could be associated with the change from a synthetic to a real water matrix.3−5,8,18,33 However, it has no place in full-scale applications. The addition of large concentrations of salts to an actual effluent is not good practice either and should be avoided because it is a source of secondary pollution. If the wastewater conductivity is too low, three main alternatives can be suggested: (1) the electrochemical route must be discarded, (2) special microfluidic reactors must be used (see section 5), or (3) the electrochemical process can be employed as a post-treatment after a preconcentration step like membrane filtration (see section 5). In addition, the occurrence of particulate matter is usual in actual wastewater, although its effect is rarely studied in the literature. Obviously, this type of matter could be eliminated by physical separation methods in order to prevent significant operational problems during the electrochemical treatment. In particular, electrode fouling or K


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Figure 10. Influence of operational variables on TOC decay for the EO degradation of 1.8 dm3 of 178 mg dm−3 mecoprop solutions in 0.05 M Na2SO4 of pH 3.0 with a flow reactor with a 20 cm2 BDD anode. Plot (a) at temperatures (●) 15, (■) 40, and (⧫) 60 °C, at 150 mA cm−2 and 130 dm3 h−1. Plot (b) at flow rate (●) 75, (■) 130, (⧫) 170, and (▲) 230 dm3 h−1, at 150 mA cm−2 and 40 °C. Reprinted with permission from ref 53. Copyright 2006 Elsevier.

Figure 9. (a) Influence of current on TOC removal vs electrolysis time for the EO treatment of 100 cm3 of 220 mg dm−3 indigo carmine solutions in 0.05 M Na2SO4 at pH 3.0 and 35 °C with a stirred undivided cell with a 3 cm2 Si/BDD anode and a 3 cm2 stainless steel cathode. (b) Indigo carmine concentration decay at pH 3.0 and 10.0 at 100 mA. Reprinted with permission from ref 94. Copyright 2006 Springer.

Recently, Panizza and Cerisola95 considered the influence of several experimental parameters on EO efficiency by exploring the mineralization of 300 cm3 of water containing acid blue 22 and 0.5 M Na2SO4 in a flow cell equipped with a Si/BDD anode and a stainless steel cathode of 25 cm2. A significant rise in current efficiency was obtained when the temperature dropped from 60 to 25 °C because of the slower chemical decomposition of S2O82− to bisulfate and O2 as follows:

2.3.6. Effect of Temperature. In contrast to other parameters, the effect of temperature on the overall efficiency of the EO process has not been widely studied.3−8,18 Usually, a change in temperature has a slight influence on the EO under the action of hydroxyl radicals. However, some authors have suggested that an increase in temperature causes a greater mass transport toward the anode because of the decrease of medium viscosity. An example of this phenomenon was reported by Flox and co-workers,53 and it is illustrated in Figure 10a for the removal of mecocrop. A faster TOC abatement is obtained at higher temperature, decreasing the required specific charge for total decontamination from 35 A h dm−3 at 15 °C to 20 A h dm−3 at 60 °C. Mecoprop mineralization was accelerated because of the faster reaction of pollutants with BDD(•OH) as a result of the inhibition of its wasting reactions since higher amounts of organics could be transported toward the BDD surface.53,54 This mass transport limitation undergone by organics to arrive at the anode was confirmed by increasing the flow rate of the solution, as shown in Figure 10b for the same solution electrolyzed at 150 mA cm−2 and 40 °C. Some authors have reported that the positive effect of increasing temperature may be mainly attributed to an enhanced anode activity. However, this is an incorrect conclusion because the efficiency in the elimination of pollutants is because of an increase of the indirect reaction of organics with electrogenerated oxidizing agents from electrolyte oxidation (especially when sulfates and chlorides are present in the effluent). However, at high temperatures, thermal decomposition of some oxidants could occur.

S2 O82 − + H 2O → 2HSO4 − + 0.5O2 −2

(11) −1

At 20 mA cm , 25 °C, and 300 dm h , 100% of color and 97% of COD were removed from the 0.3 mM acid blue 22 solution after 12 h with EC of 70 kWh m−3. Conversely, Figure 11 reports the effect of temperature on the oxidation of 1 g dm−3 of the nitro dye acid yellow 1 in 1 M 3

Figure 11. Effect of temperature on the evolution of acid yellow 1 concentration (1 g dm−3) during electrolysis with a BDD anode at 30 mA cm−2 and 300 dm3 h−1. Conditions: (Δ) 20 and (□) 40 °C. Reprinted with permission from ref 96. Copyright 2009 Springer. L


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HClO4.96 Such a medium is considered as inert and therefore higher temperatures do not enhance the oxidation rate. The small difference observed is only because of an increase in the diffusion rate with rising temperature because of the decrease of the medium viscosity. Articles reporting EO treatments at different temperatures without production of oxidants other than •OH reveal an enhancement of the degradation process when nonactive anodes are used upon increase of temperature and flow rate, yielding higher CE and lower EC.3,18,33 Nevertheless, operation at ambient temperature is usually preferred as it provides electrochemical processes with lower energy requirements than their nonelectrochemical counterparts (i.e., incineration and supercritical oxidation). These findings confirm the need to optimize the configuration of the electrolytic system and parameters such as pH and temperature to obtain the highest oxidation ability. 2.3.7. Role of Oxygen. The oxidation of water to O2 is typically considered an undesired side-reaction during the EO of pollutants because it seriously affects the efficiency of the process, causing a significant increase in the operation costs. Nevertheless, Kapałka and co-workers have demonstrated that dissolved oxygen can have a positive effect.97 These findings constitute further evidence of the importance of the nature of the electrode material and represent a new approach to the electrochemical oxidation mechanism,98 probably, under lower or higher pressures.

The chloride oxidation process is briefly expressed by eq 12, which represents a simplified version of a multistep reaction whose characteristics mainly depend on the chemical composition and structure of the working electrode.106,107 Dissolved chlorine, depending on the proton and chloride concentrations, may undergo a disproportionation reaction in aqueous solution, forming Cl− and HClO (eq 13). Moreover, since the produced HClO is a weak acid (pKa ≈ 7.5), it is in equilibrium with ClO−. The effect of pH on the relative concentrations of Cl2, HClO, and ClO− has been reported elsewhere.108 At low pH, if the local concentration of dissolved chlorine exceeds its solubility, then supersaturation drives the formation of bubbles of gaseous chlorine.108 3.1.2. Oxidation of Organics by Electrogenerated Active Chlorine. As shown in eq 15, active chlorine can oxidize the organic pollutants in the homogeneous liquid phase. Furthermore, it has been suggested that an important role can be also played by surface electrochemical reactions.102,109 Some authors109−112 have suggested the participation of adsorbed chloro and oxychloro radicals in the oxidation mechanism (see, for example, eqs 16 and 17 for the oxychloro radicals and Figure 12). organics + ClO− → intermediates → CO2 + Cl− + H 2O (15)

MOx (•OH) + Cl− → MOx (HOCl) + e−

(16)

organics + MOx (HOCl) → intermediates

3. OXIDATION OF ORGANIC POLLUTANTS BY ELECTROGENERATED ACTIVE CHLORINE

→ MOx + CO2 + Cl− + H 2O + H+

(17)

3.1. Process

As discussed in section 2, the effectiveness of the electrochemical oxidation of organics in water depends on many factors, including the presence in solution of species able to act as mediators. The most frequently studied electrogenerated oxidant is active chlorine, a term that encompasses free chlorine and HClO/ClO−, for three main reasons: (1) the addition of chloride ions can cause an increase in the removal of organic pollutants because of the involvement of active chlorine in the oxidation process; (2) chloride ions are often present in liquid effluents and natural water, thus making inevitable the appearance of active chlorine in these media;4 and (3) in some cases, the presence of chloride ions in water entails the formation of chlorinated byproducts, such as high oxidation state oxychlorine ions, organochlorinated compounds, and chloramines, during the EO of organic compounds, as also found during water disinfection by chlorination. Such products may be even more toxic than the parent contaminants.4,5,99−101 3.1.1. Electrochemical Generation of Active Chlorine. The potential positive effect arising from the presence of chlorides on the electrochemical oxidation of organics in water has been explained on the basis of the conversion of chlorides to chlorine, hypochlorous acid, and/or hypochlorite, depending on pH (eqs 12 to 14), which can oxidize the organics near the anode and/or in the bulk (see eq 15 in the case of the alkaline medium).102−105 2Cl− → Cl 2(aq) + 2e−

(12)

Cl 2(aq) + H 2O → HClO + H+ + Cl−

(13)

HClO ⇆ H+ + ClO−

(14)

Figure 12. Scheme of main reaction pathways for the electrochemical oxidation of organics in water in the presence of chlorides. The reaction between HO• and Cl− is likely to occur in two consecutive steps: HO• + Cl− → ClOH•− and ClOH•− → HOCl + e−.

The possibility that some role could be played by the anodic shift of the oxygen evolution reaction (OER), caused by Cl− ions in solution, has also been taken into consideration.19,109,111 On the basis of all these considerations, the simultaneous occurrence of all these routes in the anodic oxidation of organics in the presence of chlorides cannot be discarded. As a result, the oxidation of organics by electrogenerated active chlorine could coexist with the direct oxidation at the electrode surface, as well as with the reaction with hydroxyl, chloro, and oxychloro radicals (Figure 12), thus yielding a complex system which does not easily allow prediction of the role of operating parameters on the performance of the process. M


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well.5,113,123−125 During the treatment of a mixture of disperse yellow, disperse red 74, and disperse blue 139 with a Ti/TiO2− RhOx anode in 0.1 M NaCl at pH 4.0, a high number of mono/ dichlorinated intermediates such as 2-chloro-2-methylbutane, cis-3-chloropropanate, 2-chloroethenylbenzene, cis-1,3-dichlorocyclopentane, and trans-1,2-dichlorocyclopentane was detected.125 Regina Costa and co-workers showed that the addition of NaCl to acid black 210 solutions prepared in phosphate buffer for their treatment using BDD gave rise to the formation of organochlorinated compounds (measured as adsorbable organic halogens), but these compounds were progressively degraded at long electrolysis time.123 Similarly, Panizza and co-workers5 have shown that the anodic oxidation of 2-naphtol on a Ti/Ru/Sn electrode with 7.5 g dm−3 NaCl at pH 12 yields some organochlorinated compounds that disappear during the electrolysis. Boudreau and co-workers have reported the quicker degradation of sulfamethoxazole with BDD in the presence of NaCl, although it results in the formation of several chlorinated byproducts. Interestingly, these byproducts were completely mineralized after protracted electrolysis, whereas they accumulated in the solution upon comparative chemical hypochlorination with sodium hypochlorite.113

The coexistence of these oxidation routes makes this process completely different from the classical chemical oxidation with hypochlorite or chlorine dioxide. Thus, as confirmed by many experimental works, higher abatement of organic pollutants is a c h ie v e d b y m e a n s o f t h e e le c t r o c h em i c a l a p proach.19,102−105,109,113 3.1.3. Formation of Undesired Products. The oxidation of organics in water containing chlorides takes place in competition with a large number of chemical and electrochemical reactions,102−105 such as the oxygen evolution at the anode (eqs 18a and 18b) and the chemical114 and electrochemical formation of chlorate (eqs 19a, 19b and 19c). In the case of undivided cells, the cathodic reduction of oxidants can also take place (eq 20). 4OH− → 2H 2O + O2 + 4e−

(18a)

2H 2O → 4H+ + O2 + 4e−

(18b)

ClO− + 2HClO → ClO3− + 2Cl− + 2H+

(19a)

6ClO− + 3H 2O → 2ClO3− + 4Cl− + 6H+ + 1.5O2 + 6e−

(19b)

Cl− + 3H 2O → ClO3− + 6H+ + 6e−

(19c)

ClO− + H 2O + 2e− → Cl− + 2OH−

(20)

3.2. Influence of Main Operating Parameters

In accordance with the literature, several parameters can affect the performance of the oxidation of organics by electrogenerated active chlorine, including the type of electrochemical reactor (divided or undivided cell, interelectrode distance, etc.), the nature of the anode, the temperature, the pH, the applied current or potential, the flow rate and the fluid dynamics, the concentration of chlorides, and the nature and concentration of organic pollutants. As mentioned above, the coexistence of various degradation routes leads to a complex system, thus complicating the prediction of the role of operating parameters. Therefore, the optimization of the process is not an easy task at all. In fact, there is an apparent discrepancy among the literature data regarding the effect of the various operating parameters. This can be illustrated by analyzing the effect of i on current efficiency: (1) Polcaro and coauthors126 reported a higher abatement of 2,6-dichlorophenol and p-hydroxybenzoic acid at Ti/RuO2 in phosphate buffer at pH 7 in the presence of 1 g dm−3 NaCl for the same charge amount, when i increased from 100 to 200 A m−2; (2) an insignificant effect of i has been reported for the incineration of oxalic acid at BDD127 and 2naphtol at Ti/Ru/Sn ternary oxide in the presence5 of 5 g dm−3 NaCl in the range from 25 to 100 mA cm−2; and (3) Comninellis and Nerini104 showed a detrimental effect of i during the oxidation of phenol at Ti/IrO2 at 50 °C in the presence of 85 mM NaCl (in the range of 0.05 to 0.3 A cm−2). Similarly, no effect of chloride concentration was reported for the electrochemical incineration of phenol at Ti/IrO2 (NaCl concentration in the range of 17 to 433 mM)104 and at Ti/ TiO2−RuO2−IrO2 (in the range of 0.5 to 4.5 g dm−3),128 while higher NaCl concentrations gave rise to higher removal efficiencies for the treatment of landfill leachate at various anodes (NaCl concentration from 2.5 to 10 g dm−3)103 or phenol at PbO2-based anodes (NaCl concentration from 0.1 to 0.5 mol dm−3).129 As regards temperature, Neodo and coworkers115 have shown that lower values favor the formation of active chlorine. However, the abatement of three dyes at PbO2 anodes from 1.0 to 2.0 g dm−3 NaCl was favored at different temperatures: acid blue 62, reactive red 141, and acid blue 62

Furthermore, other species such as chlorine dioxide, chlorite, and perchlorate can also be formed.99,100,115−120 Chlorine dioxide and chlorite (as chlorous acid) can be formed, in particular, through the direct oxidation of chloride (eqs 21 and 22).115 Cl− + 2H 2O → ClO2 + 4H+ + 5e−

E 0 = 1.599 V (21)

Cl− + 2H 2O → HClO2 + 3H+ + 4e−

E 0 = 1.659 V (22)

However, the bulk concentrations of chlorine dioxide and chlorite are usually significantly lower than the active chlorine contents.115,119 This is probably because of their involvement in both electrochemical and chemical reactions.115−117 ClO4− is likely to be formed via a general multistep oxidation pathway starting with chloride,99,118 as shown in eq 23a, where the ratedetermining step is the oxidation of ClO3− to ClO4−. Chlorate and perchlorate are suspected of exerting toxic effects and, consequently, WHO recommends very low concentrations in drinking water.65 Furthermore, their oxidizing ability is rather low compared with HClO/ClO−. Hence, their formation reduces the oxidation power of the electrogenerated mixture. Alternatively or additionally, the formation of perchlorate may proceed through reaction paths involving the hydroxyl radicals generated by the oxidation of water (eqs 23b and 23c),5,120−122 which dramatically depends on the nature of the anode and on adopted operating conditions (see below).5,119,120 Cl− → ClO− → ClO2− → ClO3− → ClO4 −

(23a)

ClO− + 2•OH → ClO3− + 2H+ + 2e−

(23b)

ClO3− + •OH → ClO4 − + H+ + e−

(23c)

As mentioned above, potentially toxic and quite stable organochlorinated byproducts can be formed as N


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disappeared more quickly at T ≤ 25 °C, T ≥ 25 °C, and T between 10 and 40 °C, respectively. From these results, it was concluded that the optimal temperature depends on the dye structure.130,131 Below, the effect of the most relevant and most well-studied operating parameters (e.g., the nature of the anode, the pH, the current density, the flow or mixing rate, and the presence of NaCl) are discussed in detail, with the main aim of clarifying, when possible, the reasons for the apparent discrepancies observed in the literature. 3.2.1. Effect of Anode Material. The nature of the anode material has a remarkable influence on the several reactions and competitive routes illustrated in Figure 12, including: (1) EO by electrogenerated hydroxyl radicals, which is favored with nonactive anodes, as explained in section 2. (2) The competition between the formation of active chlorine and parasitic reactions such as the OER. As stated above, the instantaneous current efficiency for the formation of active chlorine (ICEAC) strongly depends on the nature of the anode.89,103,115,119,132 Metal oxide anodes are usually preferred to graphite, Pt and BDD for a higher ICEAC. In particular, among metal oxides, active anodes (such as Ir- and Ru-based electrodes) often yield higher ICEAC than nonactive ones (such SnO2 and PbO2).8 Figure 13 compares the dependence of the

Figure 13. Dependence of the electrochemical free chlorine production efficiency on the chloride content of the electrolyzed solution under standard conditions using four different anode materials (iridium oxide, mixed iridium/ruthenium oxides, Pt, BDD). Adapted with permission from ref 133. Copyright Johnson Matthey Plc. 2008.

free chlorine production efficiency on the chloride concentration for two DSA made with active coatings of IrO2 or IrO2/ RuO2, Pt, and BDD.133 It can be seen that both DSA outperform the other two anodes. The generation of active chlorine can depend not only on the chemical composition of the anode but also on its morphology. To understand this aspect better, it is important to remember that chloride oxidation is enhanced at low pH, and porous anodes can exhibit in the porous structure substantially lower pH than in the bulk of the solution. Thus, the OER at the anode causes the acidification of the aqueous medium, leading to a gradient of pH between the anode surface and the bulk solution. For porous anodes, a very low pH is maintained in the porous structure of the anode nearly independent of the bulk pH16,89,134 (see subsection 3.2.2 and Figure 14c), allowing the oxidation of chlorides to active chlorine even in nonacidic solutions. In this context, it is important to remember that metal oxide anodes often present a porous-like structure, thus favoring chlorine evolution at nonacidic pH. Tan and coauthors have shown that a nano-PbO2 anode, prepared by a pulse

Figure 14. (a) Electrode potential-pH diagram for (a) O2 and (b) Cl2 evolution reactions in acidic solutions. (−) Thermodynamic equilibrium conditions; (--) kinetic conditions of anodic gas evolution at 1 mA cm−2. The pH dependence is based on the form of the overall reactions in both cases. (•) Experimental points. Adapted with permission from ref 134. Copyright 1987 Elsevier. (b) Speciation diagram for the chlorine−water system calculated for the electrolysis of 0.1 M NaCl at 25 °C in an undivided cell with a conversion of 0.2. Reprinted with permission from ref 18. Copyright 2009 Elsevier. (c) Qualitative variation of pH (illustrated by the dotted line) in an undivided cell caused by oxygen and hydrogen evolution reactions at anode and cathode, respectively. O


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organics. Notably, the selection of the anode should be performed having regard to the formation of undesired organochlorinated compounds.99,139 In contrast, for the treatment of organics particularly resistant to active chlorine, the utilization of nonactive anodes such as BDD is preferable because of the intermediacy of hydroxyl radicals, although a proper selection of operating conditions (e.g., low current densities, see subsection 3.2.3) is mandatory in order to prevent or at least minimize the formation of perchlorate, among others. For example, chloroacetic acid is quite refractory to active chlorine, and thus, BDD yielded much better performance than metal oxide anodes even in the presence of high concentrations of chlorides.140 3.2.2. Effect of pH. The pH value is expected to affect various relevant reactions and equilibria, in particular, it has influence on: (1) the competition between water and chloride oxidation, with lower pH favoring the chlorine evolution reaction.134 The evolution of chlorine would be impossible on a thermodynamic basis across the pH range. However, it can take place in a narrow acidic pH range because the OER, which is thermodynamically favored, is hampered at low pH and shows a much higher overpotential (Figure 14a).134 (2) The relative concentrations of active chlorine species (Cl2(aq), Cl3−, HClO, and ClO−). Figure 14b shows the speciation diagram calculated during the electrolysis of 0.1 M NaCl with a conversion of 0.2 for Cl−.108 Cl3− is formed in a very low concentration up to pH ca. 4.0 and then disappears, Cl2(aq) being the predominant species up to pH near 3.0, HClO in the pH range from 3 to 8, and ClO− at pH > 8.0. The mediated oxidation of organic pollutants with these species is expected to be favored in acidic media141 because of the higher E0 of Cl2(aq) (E0 = 1.36 V vs SHE) and HClO (E0 = 1.49 V vs SHE) compared with ClO− (E0 = 0.89 V vs SHE).18 (3) Side-reactions. A low pH value gives rise to a lower concentration of species such as hypochlorite and hypochlorous acid that can be further oxidized to chlorate at the anode.142 The pH value has also an important effect on (1) the homogeneous decomposition of oxidants, as is the case of reaction between hypochlorous acid and hypochlorite (eq 19a) that is expected to reach the maximum rate at quasi-neutral pH, (2) the loss of gaseous chlorine that is maximized for very low pH values (1.5 g dm−3) for the treatment of the reactive red 141 dye at Si/BDD led to higher and lower decolorization at pH lower than 4.0 and higher than 7.5, respectively, as a result of the less effective action of active chlorine in alkaline conditions because of the formation of chlorate and perchlorate.143 Huang and co-workers144 reported on the electrochemical treatment of a secondary effluent of an industrial wastewater treatment plant using a Ti/PbO2 anode. No significant change of COD removal was obtained by increasing the pH from 3.0 to 8.5, while a further increase to 11.0 caused a lower COD abatement. Conversely, some authors have found a higher COD destruction under alkaline conditions, which can be accounted for by various reasons evidencing the complexity of chemical mineralization reactions and the potential effect of pH on the tendency of some pollutants to be oxidized.10,18,144−146 For example, Mohan and co-workers145 found that a solution of 100 cm3 of 160 mg dm−3 of the azo dye acid blue 113 containing 1.74 g dm−3 NaCl, electrolyzed in a stirred undivided cell equipped with a Ti/ RuO2 anode at 10 mA cm−2, was more rapidly mineralized at pH 9.0 than at pH 7.0 or 4.0. A similar result was observed for the degradation of pentachlorophenol (PCP) on a Ti/SnO2− Sb anode. According to the authors, the anion form of PCP at higher pH can be much more easily adsorbed on the anode surface, resulting in enhanced PCP degradation at pH 11.0.10 Some authors did not observe a significant effect of initial pH on the degradation of organics by electrogenerated active chlorine. As an example, Rajkumar and co-workers128 observed that the removal of COD by electrolysis of a NaCl (2500 mg dm−3) aqueous solution of 300 mg dm−3 phenol was not affected by the initial pH in the 3−10 range. It must be noted, however, that the final pH was always between 8.0 and 8.5 regardless of the initial one because of the probable formation of a buffer during the electrolysis.128 3.2.3. Effect of Current Density and Flow Rate. In order to rationalize the effect of current density and flow rate on the electrochemical abatement of organics in the presence of chlorides, it is important to consider two boundary situations:89 (1) under the existence of operating conditions that favor the water oxidation over that of chlorides (e.g., low concentrations of NaCl, high pH, and catalytic anodes that favor the OER) or in the presence of organics that are very resistant to oxidation by active chlorine, the main degradation processes should be those discussed in section 2, such as a direct electron transfer and EO mediated by •OH or chloro and oxychloro radicals, which take place in a reaction layer with a very small thickness compared with that of the diffusion layer. Under these conditions, the current efficiency regarding the mineralization of the organics should be enhanced by (see section 2): (1) large Q


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Table 4. Effect of Flow Rate and Current Density on the Electrochemical Incineration of Oxalic Acida entry

anode

[NaCl] (g dm−3)

flow rate (dm3 min−1)

current density (mA cm−2)

conversion (%)

current efficiency (%)

1 2 3 4 5

IrO2−Ta2O5 IrO2−Ta2O5 IrO2−Ta2O5 BDD BDD

40 40 40 10 10

0.2 1.2 1.2 0.2 1.2

17 39 17 39 17

82 60 42 74 67

55 42 27 50 44

Galvanostatic mode. Aqueous solutions of 250 dm3 of 100 mM oxalic acid in Na2SO4 at pH 12 and 25 °C. Charge passed = 7000 C = 1.45Qth, where Qth is the required charge for a total conversion of the oxalic acid with a current efficiency of 100%.89

a

such oxidants.126 A more plausible explanation is that higher flow rates favor the homogenization of pH, thus increasing local acidic pH values such as those encountered in the pores as well as in the anode diffusion layer. As a result, oxidation by active chlorine is less effective89,144 However, more focused studies are necessary to clarify this point. (b2) It is important to consider that when i > ilim, the formation of active chlorine occurs under mass transport control of the chlorides toward the electrode surface. Under these conditions, a further increase of i or a decrease of the mixing rate would yield lower ICEAC. A decrease of ICEAC with i was observed by Neodo and coworkers115 at a Ti/RuO2·2SnO2 electrode in 0.07 M NaCl and by Choi and co-workers147 at an Ir-based anode. The high values of i and the relatively low concentrations of NaCl adopted in these studies suggest that the process kinetics were likely to be controlled by the mass transport of chloride toward the electrode surface or under a mixed kinetics regime. ́ Martinez-Huitle and co-workers127 reported that an increase of the current density from 300 to 600 A m−2 did not accelerate the oxidation of oxalic acid at BDD in the presence of 5 g dm−3 NaCl. Thus, under employed conditions, the process was likely to be controlled by the mass transport of chlorides toward the anode. Therefore, a very different effect of current density and flow rate (or mixing rate) on the abatement of organic pollutants can be expected depending on the adopted operating conditions, which helps in explaining the apparent disagreement in the literature (see section 3.2). Summarizing the previous considerations, when organics are mainly oxidized by active chlorine and the oxidation of chloride is not limited by mass transport, the mineralization of organics is likely to increase upon enhancing the current density and, for many systems, decreasing the flow or the mixing rate. Conversely, if the oxidation of organics occurs by “direct” processes or by active chlorine whose formation is mass transport controlled, the current efficiency for the mineralization of organics should decrease upon enhancement of the current densities and decrease of the flow or the mixing rate. Interestingly, these considerations on the effect of various operating parameters were confirmed by various studies,139,142 including one on the electrochemical incineration of oxalic acid at BDD and Ir-based anodes.89 In the absence of chlorides, a high current efficiency was obtained when the process was mainly under charge transfer control (i.e., when low current densities and high flow rates were imposed).89 On the other hand, in the presence of high concentrations of NaCl (40 mg dm−3), a higher efficiency was achieved upon increase of the current densities and decrease of the flow rate at both IrO2 (Table 4, entries 1 to 3) and BDD-based anodes (Table 4, entries 4 and 5). Notably, an increase of i and a decrease of flow rate lead to a higher generation of chlorates and (for BDD anodes) perchlorates.119−122 Vacca and co-workers148 reported that, for very high concentrations of NaCl (20 g dm−3), the

formation of chlorate at BDD is prevented by the utilization of low current densities (2.5 mA cm−2). Furthermore, Iniesta and co-workers129 observed that, at PbO2-based anodes, the electrochemical oxidation of phenol led the formation of chloroform, whose concentration at the end of the electrolysis could be minimized by working at low current densities and low chloride concentrations. 3.2.4. Practical Aspects Related to the Presence of Chlorides. A key point still to be addressed is whether Cl− is beneficial or detrimental in terms of the abatement of organic pollutants. To answer this question, it is important to recall that the presence of chlorides gives rise to a competition between the “direct” anodic oxidation and the Cl−-mediated oxidation, which mainly depends on the extent of several factors (see previous sections and Figure 12): (1) the competition between the formation of active chlorine and the water oxidation, (2) the kinetic constants of direct and mediated oxidation reactions, (3) the competition between direct oxidation and oxygen evolution, and (4) the competition between the mediated oxidation and lateral reactions involving the active chlorine (see Figure 12). Therefore, the following scenarios are possible: (1) let us first consider a scenario where, after the addition (or already in the presence) of chlorides, water oxidation is still favored over that of chloride because of the utilization of a particular anode and operation/or certain operating conditions (such as high pH and low concentrations of chlorides). In this case, the effect of chlorides on the abatement of organics is expected to be small since the direct oxidation process remains the main oxidation path. For example, at BDD, under alkaline conditions (pH 12), quite a slight decrease in the abatement of oxalic acid was observed upon addition of a small amount of NaCl.89 Similar results were observed for the oxidation of the dimethyl phthalate ester on a fluoride-doped Ti/β-PbO2 anode.149 Cabeza and coauthors reported that the COD abatement during the electrochemical oxidation of landfill leachates using a BDD anode was slightly affected by the concentration of NaCl in the range from 1420 to 8570 mg dm−3.150 Similarly, Comninellis and coauthors did not observe a significant effect of NaCl concentration (in the range from 0 to 85 mM) on the anodic oxidation of phenol at Ti/SnO2, a nonactive anode that yields a high concentration of •OH but minimizes the generation of active chlorine.104 (2) Let us now consider the opposite situation (i.e., the oxidation of chlorides is favored over that of water because of the use of high concentrations of chlorides and/or proper operating conditions). In this case, the direct anodic oxidation becomes less relevant. The performance of the oxidation process will depend on the ability of the anode to generate active chlorine, as well as on the competition between the Cl-mediated oxidation of organics and the chemical and electrochemical destruction of the oxidants. Hence, two boundary conditions are expected: (a) R


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if, under the employed operating conditions, the pollutant is easily oxidized by active chlorine and the electrode presents a high ICEAC and a poor direct anodic oxidation in the absence of Cl− (such as Ir- and Ru-based electrodes), the presence of Cl− enhances the abatement. This behavior was observed in a very large number of cases with nonactive electrodes that often yield a poor direct anodic mineralization and moderate or good generation of active chlorine.89,112,151−153 For example, Grgur and co-workers112 have recently shown that the oxidation of methomyl (a typical thiocarbamate pesticide) at the Ti/RuO2 electrode does not occur in the absence of chlorides, while it takes place with good results in the presence of NaCl. Similarly, the addition of small amounts of NaCl (0.05 M) to a water solution of methyl orange dye promoted decolorization and mineralization at Ru-based DSA.151 (b) If, under the employed operating conditions, (i) the electrode presents a low ICEAC and an effective direct anodic oxidation of the organic pollutant in the absence of Cl− (such as BDD anode) and/or (ii) the pollutant is quite resistant to active chlorine, the presence of Cl− should be detrimental, as experimentally observed in many cases and, in particular, with BDD anodes.89,123,150,154 For example, Costa and co-workers123 showed that the addition of NaCl to a phosphate buffer solution of acid black 210 dye at BDD gave a decrease in the abatement of TOC in a large range of pH. Scialdone and co-workers showed that the addition of NaCl resulted in a lower COD abatement for solutions of acid orange 7154 or chloroacetic acid140 using BDD. On the other hand, when the direct process is under mass transport control, the addition of chlorides can result in a higher current efficiency, since the process is no longer affected by the limitations imposed by the transport of the pollutant toward the anode surface. In this context, Pereira and coworkers155 showed that the oxidation of bisphenol A on BDD was not affected by the NaCl addition when the applied current density i was lower than the limiting one ilim as a result of the fact that, under these conditions, the direct oxidation prevailed over the mediated one. In contrast, when i > ilim, the addition of NaCl significantly increased the COD abatement rate. If large molecules are involved, the mineralization pathway may yield intermediates that offer a very different resistance with respect to the parent pollutants. Furthermore, the chemical nature of the intermediates can depend on the adopted oxidation processes. Hence, the intermediates generated by electrogenerated active chlorine may be different from those formed by direct anodic oxidation.113,154 Quite interestingly, in some cases the addition of NaCl favors the abatement of the chosen organic pollutant but has a negligible or even negative effect on mineralization (Figure 15).123,130,131,156,157 For example, Aquino and co-workers157 showed that the addition of 1.5 g dm−3 NaCl for the electrochemical degradation of a real textile effluent using a BDD anode at 5 mA cm−2 and 55 °C increased the decolorization rate but did not improve COD removal. Conversely, Sales Solano and co-workers achieved a notable improvement of COD removal with different NaCl concentrations to treat a real textile effluent and using a BDD anode with an electrochemical flow cell. However, the increase in the NaCl concentration promotes a significant production of chloroform. For this reason, Cl-mediated oxidation must be carefully applied by using lower current densities and lower NaCl concentrations.158

Figure 15. (a) COD removal and current efficiencies (CE) after 1 h and (b) TOC removal after 5 h for the electrochemical oxidation of 500 mg dm−3 of acid black 210 in 0.20 mol dm−3 phosphate buffer at 25 mA cm−2 with a BDD anode, in the presence and absence of 0.10 mol dm−3 sodium chloride at different pH values. Adapted with permission from ref 123. Copyright 2009 Elsevier.

3.3. Advantages, Disadvantages, and Key Aspects to be Addressed and Perspectives

Electrochemical chlorination has been proposed for several purposes such as water disinfection,135 treatment of industrial wastewater with highly refractory organics and high salinity,144 reverse osmosis concentrates,124,159 landfill leachates,103,150 wastewater contaminated by synthetic dyes18 or pharmaceutical residues,149 olive-mill wastewater, and tannery effluents.4 Thus, the treatment of water contaminated with biorefractory organic pollutants by electrochemical oxidation in the presence of chloride offers relevant advantages compared with chemical chlorination routes:18,93,160 (1) faster destruction of organic matter because of the involvement of several oxidation routes (see subsection 3.1.2 and subsequent ones), (2) the avoidance of transport and storage of dangerous chlorinated oxidants, and (3) the reduction of total costs because of the low operation costs that compensate the higher investment costs. As mentioned above, the method also presents some disadvantages:18 (1) the potential formation of undesired toxic organochlorinated derivatives such as THMs, (2) the electrogeneration of oxychlorine anions such as ClO3− and ClO4−, with potential adverse effects on human health, (3) the need for less widespread reactors such as electrochemical ones, (4) the poor abatement of quite resistant molecules that can on the other hand be effectively mineralized by direct oxidation with BDD anodes (see below), and (5) the difficult optimization of the process given by the complex effect of the operating parameters on the various oxidation routes involved in this process (see section 3.2). However, chlorinated byproducts are also formed in chemical chlorination, and oxidation by electrogenerated active chlorine is expected to yield smaller amounts of undesired byproducts upon utilization of proper operating conditions (see above). This method also presents some relevant advantages in comparison with direct electrochemical oxidation: (1) the S


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is an inefficient way to make use of all the potential of an electrochemical system because the reactivity at the counter electrode is neglected. Sections 2 and 3 have described the main direct and mediated oxidation routes that can exist at the anode. The core of this section deals with the coupling or combination of anodic and cathodic processes, aimed at taking advantage of the synergistic action of each, and thus enhancing the current efficiency of the electrochemical treatment. Hereafter, the most relevant cathodic processes are first presented, with a review of their coupling with the aforementioned anodic processes. Focus is put on purely electrolytic processes, but it is not extended to coupled systems that include microbial fuel cells or photoelectrocatalysis, which are based on the use of bioelectrodes and photoelectrodes, respectively.

utilization of cheap and stable electrodes such as DSA ones, (2) the high current efficiency that can be achieved even in the presence of very low concentrations of organic pollutants, since the oxidation process is not kinetically limited by the mass transport of the organics toward the anode surface, and (3) the utilization of lower working potentials and power densities compared with EO with BDD anodes, which are the most effective electrodes for that process. For applicative purposes, easy optimization of the process is a very sensitive area. As described in detail in subsection 3.2.1, active DSA anodes are preferred to BDD because of the more efficient generation of active chlorine and the much lower formation of chlorate and perchlorate. At these electrodes, higher abatements and current efficiencies are expected for most systems upon increase in the chloride concentration and i, decrease in the flow rate (or the mixing rate) and use of acidic or neutral pH, although there are some exceptions, as explained in previous paragraphs. On the other hand, when organic pollutants are resistant to oxidation by active chlorine, operating conditions have to be selected in order to enhance the oxidation by electrogenerated hydroxyl radicals, thus using “nonactive” anodes like BDD and pH that is not too low and, if possible, a low concentration of chlorides. Low current densities and high flow rates should also be used in this case to increase the mineralization current efficiency and minimize the formation of chlorate and perchlorate. Interestingly, the utilization of such potent anodes could allow the mineralization of most chlorinated organic compounds.161,162 However, further studies are necessary to evaluate if a proper selection of operating parameters can reduce the formation of perchlorate upon use of a BDD anode. Overall, in spite of the very large number of papers devoted to this topic in the last two decades, current available data are not sufficient either to select the best operating conditions, in particular the most suitable anode, or to evaluate fully the perspectives of this process. At the moment, a large number of incomplete studies have invaded the existing literature, which unfortunately frequently leads to apparent contradictory conclusions and makes the treatment of real wastewater on the industrial scale a case-sensitive field with limited valid references. More specific and rigorous studies are necessary to draw the whole picture. These studies should systematically compare the performance of various anodes (in particular, various kinds of metal oxides) under the same operating conditions in terms of (1) generation of active chlorine, chlorate and perchlorate in the absence and in the presence of organics; (2) electrochemical abatement of specific organic pollutants and the overall solution TOC; (3) generation and abatement of stable and volatile organochlorinated byproducts in order to allow the determination of mass balances; (4) service life of anodes, especially in highly corrosive media like concentrated chloride solutions; and (5) cost. In addition, the effect of other operating parameters (e.g., current density, flow rate, adopted reactor, and initial pH) should be more systematically investigated with the main aim of assessing their impact on all the above-mentioned aspects.

4.1. Cathodic Processes

The cathodic process is considerably less explored than its anodic counterpart, so that its contribution to the whole decontamination process is often disregarded. However, the cathode can sometimes play an important role in water treatment. In some cases, the cathodic reaction is used to produce energy in the form of H2 while performing the solution decontamination via the anodic process,163 but alternatively, it can become an interesting option for the degradation of organics as well. Direct reduction of the pollutant at the cathode, as well as mediated decontamination based on the cathodic electrogeneration of H2O2, are the two most relevant cathodic processes. Moreover, there are some far less explored processes that involve the reduction of some species at the cathode with the consequent enhancement of the degradation. For example, the cathodic production of reactive oxygen species (ROS) during the oxygen reduction reaction (ORR) is a hot topic nowadays. The ORR mechanism is multistep and still remains unclear because of its complexity, since it strongly depends on the cathode material used. Adsorbed species like O2•− and HO2•,164 and even •OH,165 can be formed on the cathode, although at the moment most of these findings have only been reported within the field of fuel cells. Mediated electroreduction of halocompounds has also been suggested by some authors. This process consists in the continuous regeneration of a mediator species on the cathode, which is then able to reduce the pollutant by electron transfer in the bulk and becomes reoxidized.166 Aromatic hydrocarbons and their cyano- and keto-derivatives are preferred, but the options are restricted because the radical anion formed from reaction 24 is irreversibly protonated in aqueous media.167 Med + e− → Med•−

(24)

4.1.1. Direct Reduction. Direct cathodic reduction has been suggested as a promising approach for the detoxification of wastewater for a long time. The goal of this process is to achieve partial degradation of the polluted solution rather than quantitative TOC or COD removal. Particularly, much emphasis has been put on the electrochemical dehalogenation of water containing organochlorinated pollutants because the process ensures the removal of chlorine atoms under mild conditions. The control of parameters such as pollutant concentration, applied current or cathode potential, temperature, and flow rate is essential in direct processes, because of their influence on the appearance of limitations by electron transfer and mass transport, which eventually determine the current efficiency and energy consumption.168 Also, the

4. COUPLING OF ANODIC AND CATHODIC PROCESSES In most cases, the electrochemical treatment of water containing organic pollutants is carried out either anodically or cathodically by using a divided reactor and/or an electrode material that favors one of these single processes. Actually, this T


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Figure 16. (Left) System with an iron anode and a copper foam cathode for the redox control of the electrochemical dechlorination of trichloroethylene (TCE) in aqueous medium. ORP is the oxidation−reduction potential. (Right) Performance of different anode materials regarding the decay of TCE vs time. Adapted from ref 183. Copyright 2011 American Chemical Society.

selection of the cathode material is extremely critical, as it governs selectivity through minimization of the wasting reactions. In aqueous media, as well as in protic solvents and/or in the presence of hydrogen/proton donors, the reduction reaction of interest is in occurrence with the HER as the main parasitic reaction, thus affecting the current efficiency. In the past, large H2-overpotential cathodes such as Pb and Hg were preferred in order to minimize the HER. However, they are not environmentally friendly, and thus new trends have emerged in recent years. As observed in EO, the electrode material may also have a strong influence on the degradation rate, which depends on its electrocatalytic ability.169 Sometimes, the cathode preferentially promotes polymerization reactions, which cause a decrease of efficiency that can end in the electrode fouling because of irreversible passivation. For example, during the reduction of an aminoderivative contaminant such as antibiotic sulfanilic acid, the progressive formation of a black coating on the cathode was observed, which was related to polymeric compounds generated from reduction of sulfanilic acid and/or its intermediates.170 Nowadays, electroreduction finds a very interesting application in the decolorization of wastewater containing dyes. In particular, traditional cathodes have been used for the effective destruction of azo dyes since the NN bond can be readily transformed and finally broken by the electrons with formation of amino derivatives as shown in consecutive reactions 25 and 26, although poor decontamination of the solutions is usually attained because the reaction byproducts tend to be much more refractory to reduction and thus accumulate.18

Fan and co-workers studied the electroreduction of solutions of 80 mg dm−3 amaranth azo dye in a divided cell equipped with an ACF cathode, achieving 95% color removal but only 62% COD decay working galvanostatically at −1.0 mA cm−2 for 8 h. The overall decolorization could also be reached potentiostatically at Ecat > − 0.5 V versus SCE.171 In order to stimulate the cathodic reduction of the azo dyes, multicathode divided cells with a large cathode area have also been devised.172 It must also be noted that the structure of the dye can play an important role, potentially impeding the second step shown in reaction 26 because of stereochemical interactions with the cathode surface, as in the case of the electroreduction of acid yellow 9 on carbon fleece. Even under the application of a large Ecell, no cleavage of the azo bond took place.173 On the other hand, solutions containing other classes of dyes such as anthraquinones tend to undergo only partial decolorization. Thus, a maximum of ca. 60% color removal was obtained by electroreduction of reactive blue 4 at reticulated vitreous carbon (RVC) working at Ecat of −0.85 V versus Ag| AgCl, which proceeded via a one-step reversible transformation into a hydroquinone derivative.174 The favored reduction of nitroaromatic compounds has long been known, as well. Nitrobenzene undergoes a four-electron reduction to phenylhydroxylamine in aqueous solution at pH > 4 and a six-electron reduction to aniline at lower pH.175 With a carbon nanotube (CNT)-modified cathode, 95% nitrobenzene removal with 46% average current efficiency was achieved at −1.2 V versus SCE in pH 5 after 50 min, with no other product than aniline.176 Such reduction reactions occur in several electrochemical and chemical steps. For example, Laviron and co-workers found that 4-nitropyridine is reduced to a dihydroxylamine, which then dehydrates to give a nitroso compound that is finally reduced to hydroxylamine.177 As in the case of azo dyes, the structure of the pollutant can have some influence on the degradation rate. In a comparative study with nitrophenols (NPs), a slower electroreduction in the sequence 2,4-diNP > 2-NP > 4-NP > 3-NP ≫ phenol was observed.178

ArNNAr′ + 2e− + 2H+ → ArNHNHAr′ (25)

ArNHNHAr′ + 2e− + 2H+ → ArNH 2 + Ar′NH 2

(26) U


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hydrogen followed by the cleavage of σ bonds such as C−Cl through a multistep mechanism whose key step is reaction 28:186

Since −NO2 is a withdrawing group, its presence favors reduction and its position determines the reaction rate. An interesting feature of nitroaromatics is that reduction takes place at low negative potentials, at which parasitic HER is minimized.175 Traditional cathode materials such as graphite or Ti mesh can be used, usually giving rise to amino compounds and/or oligomers.169 However, a significant decontamination can be attained in some cases. For instance, the total conversion of hexahydro-1,3,5-trinitro-1,3,5-triazine, a military explosive found in groundwater, to small molecules was achieved at constant current with a cascade of reactors equipped with RVC cathodes. The intermediates did not accumulate, but the nitro compound was converted to a nitroso derivative and then to a hydroxylamino compound, followed by ring cleavage to yield methylenedinitramine and formaldehyde.179 Probably, the most well-known application of cathodic reduction is the dehalogenation of organohalogenated pollutants, which are not readily biodegradable. Their massive use in many human activities, especially in the case of chlorinated compounds, gives rise to serious environmental concerns related to their potential toxicity. The reduction of organic halides to achieve complete dehalogenation is often considered a faster and more efficient strategy than the more popular EO explained in section 2.180 Some studies have reported their electroreduction on nonmetals, as in the case of trichloroethylene (TCE) on graphite.181 However, since carbonaceous materials require very large overpotentials for the reduction of chlorinated compounds (sometimes close to those of water discharge), they are considered as almost inert regarding C-X cleavage reactions. Consequently, they are most frequently used to support metals, thus reducing their loading. Most typically, electrodehalogenation has been performed by using metals or metallized surfaces. Trichlorethene has been reduced on Ti|MMO at more negative Ecat than −0.8 V versus SHE without the formation of chlorinated byproducts, yielding a heterogeneous rate constant comparable to that obtained on Ni. The good performance of such DSA is advantageous in terms of stability and reduced cost.182 Cu has also proven to be a promising cathode for the efficient dehalogenation of organic halocompounds. Mao and co-workers have reported the effective electrochemical reductive dechlorination of TCE to yield ethane and ethene by using a Cu foam cathode, with relevant importance for groundwater treatment.183 Very interestingly, the anode material had a dramatic effect. As shown in Figure 16, the use of an iron anode very significantly enhanced TCE removal as compared with PbO2 and MMO anodes, which was explained by the released Fe2+ ions that led to a highly reducing environment (i.e., lower redox potential), unlike the prevalent O2 formation on inert anodes. Cu was also found to be more effective than other materials like Au for the electrochemical removal of HAAs, leading to the complete dehalogenation of brominated HAAs.184 In contrast, the reduction of polychlorinated HAAs gave rise to monochloroacetic acid (MCAA), whose reduction was much more difficult than that of the parent compounds. Luckily, noble metals such as Pd, Rh, and Ag can effectively reduce chlorinated HAAs.185 Two feasible mechanisms can be hypothesized during the electroreductive dechlorination: (i) direct reduction at the cathode surface via a two-electron one-proton reaction 27, or (ii) indirect dehalogenation by electrocatalytic hydrogenolysis (ECH, also called electrocatalytic hydrodechlorination), for which Pd (among other noble metals) is the preferred catalyst, which involves the cathodic reduction of water to yield atomic

RX + H+ + 2e− → RH + X−

(27)

(RCl)ads M + 2(H)ads M → (RH)ads M + HCl

(28)

As demonstrated by Rondinini and co-workers during the last 15 years for bromo- and chloroorganics, either massive or nanoparticulated silver cathodes have a peculiar halide affinity that results in an extraordinary ability for their electroreduction and, consequently, in low energy consumption processes thanks to the reduced Ecell, as well as enhanced current efficiency and substrate conversion.187−190 For example, a very large electrocatalytic effect was found in water for the reduction of aliphatic chlorides like 1,2-dichloroethane (DCA), which displayed a peak at −0.9 V versus SCE in aqueous medium instead of −2.5 V versus SCE in CH3CN.191 Quantitative abatements (>90%) were obtained after prolonged electrolyses of DCA and 1,1,2,2tetrachloroethane (TCA). Recent studies have shown that cathodes made of Pd-loaded materials or supported Pd exhibit remarkable dechlorination efficiencies,192 leading to totally saturated products because of their highly catalyzed HER and promotion of hydrodehalogenation. For example, the current efficiency for the reduction of atrazine to dechlorinated atrazine on Pd-modified RVC increased at higher atrazine and Pd concentration and lower current density.193 Some (poly)haloaromatics such as 4-chlorobiphenyl and 2,4-dichlorophenoxyacetic acid have also been quickly transformed by using Pd/Ni foam.194,195 The enhancement of both noble metal− support interaction and surface area tends to increase the efficiency of the process because of the ameliorated surface catalytic properties. A good way to reduce the metal loading is via support on carbonaceous materials. Ag-loaded CNTs can be prepared by electroless deposition,196 among other methods. Also, carbon-PTFE gas-diffusion electrodes (GDEs) loaded with Pd and fed with H2 gas allowed the transformation of 4chlorophenol into 100% phenol after 60 min, along with 70% COD removal after 120 min,197 whereas the complete dechlorination of 0.15 mM pentachlorophenol was reached after 180 min by using a Pd-CNT/graphite cathode.198 Considering the great influence exerted by mass transport conditions on process performance in direct processes, such as anodic oxidation and cathodic reduction, it is worth highlighting the almost total absence of information on the effect of hydrodynamics in the latter case, in contrast to much more focused reports on the former methods. A gratifying exception to this rule is the recent system by Zhao and co-workers to remove HAAs using Pd-loaded granular activated carbon (GAC) particles and Pd-carbon paper as cathodic surfaces in a divided reactor.199 The removal efficiency was enhanced when the hydraulic retention time was increased to 20 min, whereafter it remained almost constant. In such a threedimensional reactor, the importance of controlling the hydrodynamic conditions became evident, especially at higher current density (>0.6 mA cm−2) because the electroreduction rate mainly depended on the mass transport rate of pollutant. 4.1.2. Cathodic Electrogeneration of H2O2. The on-site cathodic generation of H2O2 for the oxidation of pollutants is receiving increasing attention, since it may entail a considerable reduction in the costs associated with H2O2 production, transportation, storage, and handling. V


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also improves the durability.205 The anodization of the carbonaceous material can also enhance the H2O2 production.206 Because of its high but limited oxidation power, electrogenerated H2O2 has been primarily employed for the oxidative partial transformation of organics in divided cells, such as that of maleic acid to malic acid, formaldehyde to formic acid, and phenol to organic acids and ketones.207 However, the H2O2 electrogeneration for water decontamination finds its main application in the use of undivided cells since coupling with other reactions/processes, especially the EF-based methods, very significantly enhances the oxidation ability of the system (see subsection 4.2.2).

The leading technology for industrial H2O2 production is the anthraquinone process, which involves the hydrogenation of 2alkyl-9,10-anthraquinone, for example, with a catalyst such as Ni or Pd. On the other hand, a well-established electrochemical process for synthesizing H2O2 with high selectivity and efficiency at industrial scale is the Dow process, which is based on the ORR on a trickle-bed reactor consisting of graphite chips coated with carbon black and a fluorocarbon binder.200 Oxygen can be reduced via the two-electron reaction 29 and (30) in acidic and alkaline medium, respectively.6 For this purpose, the essential component in such systems is the cathode. O2(g) + 2H+ + 2e− → H 2O2 −

−

O2(g) + H 2O + 2e → HO2 + OH

(29) −

4.2. Coupled Redox Processes

Achieving the complete or at least very significant removal of the organic matter content is not always feasible or easily reached by means of an anodic or cathodic process alone, which is mainly because of the accumulation of more refractory byproducts than the parent pollutant. Consequently, several coupling strategies using both oxidation and reduction reactions among those discussed in previous sections have been proposed in recent years.208 This alternative is sometimes called “paired electrolysis”, which hereafter embraces those systems where the redox processes occur simultaneously by using either divided or undivided cells, as well as sequential setups where reduction is followed by oxidation or vice versa. Sometimes the coupling is not really the goal of the treatment because the process of interest is the anodic or cathodic one, and thus, the reactions taking place at the counter electrode in undivided cells are often disregarded, although they can actually play a role. However, usually, the coupling is done on purpose by selecting conditions to take advantage of the synergy between anodic and cathodic reactions, thus increasing the global efficiency. 4.2.1. Coupling of Direct Cathodic Reduction and Anodic Oxidation. The direct reduction of an electroactive pollutant at the cathode surface can be coupled to either direct or mediated anodic oxidation (explained in sections 2 and 3, respectively). To do this, the key requirement is the incorporation of an anode that is able to generate adsorbed • OH from water oxidation and/or active chlorine from the oxidation of Cl−. The main objective of many studies in the literature is reaching the total mineralization of the pollutants under the action of the anode in an undivided cell. It is usual, apparently, for no reduction of the initial pollutant to occur at the cathode, so that the reaction pathways are explained on the basis of the anodic reactions. However, it must be taken into account that the cathodic reduction of byproducts cannot simply be disregarded because even when their contribution to the whole degradation is small, their toxicity can be significant.209 The detection of the reduction of byproducts tends to be more difficult compared with the oxidation ones since their relative concentration is much lower. They are, however, sometimes identified, thus increasing the complexity of the pathways, as in the case of the treatment of pretilachlor herbicide in an undivided cell with a Ti/SnO2 anode and a stainless steel cathode.210 The main reaction pathways included hydroxylation at the anode and dechlorination at the cathode. The electroreduction and electro-oxidation pathways for reactive black 5 azo dye using Ni electrodes were also assessed, revealing a quick conversion to aromatic amines and amino-

(30)

Two main drawbacks are usually associated with the electrochemical synthesis of H2O2 on smooth electrodes such as planar graphite, Ti foil, BDD, and stainless steel plates, namely, the low solubility of gaseous O2 in water (about 40 or 8 mg dm−3 in contact with pure O2 or air, respectively, at 1 atm and 25 °C) and the sluggish kinetics of the ORR with low current efficiencies, thus being impractical for technological scale-up. Several alternative cathodes are therefore used with the aim of increasing the ORR rate: hydrophobized GDEs manufactured from a mixture of technical carbon (electrocatalyst) and PTFE, bed particles, and electrodes with a large surface area (RVC, carbon felts, ACF, CNTs, gauzes, etc.). The highest H2O2 concentration is attained with use of a carbonPTFE GDE. Because of pressurization (>1 atm) with O2 in such cathodes, the ORR to yield H2O2 is preferred over the normal reduction of H2O to H2, thus shifting the cathode potential to more positive values with a concomitant decrease in the potential difference between the anode and cathode. In divided cells, the H2O2 concentration increases linearly as predicted by Faraday’s law. In contrast, in undivided electrolytic cells, H2O2 can be further oxidized to hydroperoxyl radical (HO2•) at the anode surface via reaction 31.201 H 2O2 → HO2• + H+ + e−

(31) •

In addition, some authors defend the formation of OH from either the cathodic one-electron reduction of H2O2 or in alkaline medium from reaction 32, as further explained in subsection 4.2.2:202,203 H 2O2 + HO2− → •OH + O2•− + H 2O

(32)

Lately, chemically modified cathodes have been used in order to favor the electrocatalysis of O2 reduction, trying to shift the reduction potential to more positive values and enhancing the rate of H2O2 formation. Considering the current industrial manufacture of H2O2 (mentioned above), quinones are among the preferred modifiers. For example, a Pt cathode modified with quinones accelerated the decolorization of solutions of acid orange II because of the involvement of •OH,204 whereas graphite/PTFE modified with 2-ethylanthraquinone also demonstrated greater H2O2 production. GDEs have also been successfully modified with Co and Cu phthalocyanines, as well as with metal oxide nanoparticles that are electrocatalytically active for the desired two-electron reaction in alkaline media. Modification of GDE with noble metals such as Ag is also interesting, since its presence changes the mechanism and increases the extent of the four-electron reduction of O2, but it W


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Apart from the simultaneous anodic/cathodic treatments in undivided cells, sequential redox processes using divided systems have also been reported. Indeed, back reactions plausibly occurring in undivided cells, which can therefore cause a decrease of the efficiency, may be at least partially minimized by the use of sequential setups, although with an important potential penalty caused by the presence of a separator between the anolyte and catholyte and more complicated handling. Reduction followed by oxidation was effective for the treatment of nitrotoluenes,175 as well as for pnitrophenol.214 In the latter study, 22% TOC removal was achieved in an undivided cell with a Ti/Pt anode and a stainless steel cathode, which was better than single oxidation and reduction, whereas 40% mineralization was obtained through 240 min of reduction followed by 60 min of oxidation. On the other hand, a higher removal of nitro compounds (over 97% and 93% for trinitrotoluene and 1,3,5-trinitro-1,3,5-triazine, respectively) was achieved with MMO electrodes at constant voltage under a sequential oxidation−reduction treatment.208 In a very interesting recent study, the treatment of the iodinated X-ray contrast media diatrizoate, which contains three iodine atoms, benefited from coupling between reduction at a Pd-doped graphite felt cathode and oxidation at a BDD anode in a three-compartment cell operated in continuous mode. Following the reduction process, the solution was fed into the anodic compartment. The complete reductive deiodination at Ecat = −1.7 V versus SHE yielded 3,5-diacetamidobenzoic acid that was further oxidized on BDD.215 Coupling between direct cathodic reduction and oxidation mediated by active chlorine has been more rarely reported, as in the case of acid yellow 9 azo dye treated with a carbon fleece cathode and Ti/Pt anode in NaCl medium.173 4.2.2. Coupling of H2O2 Electrogeneration and Anodic Oxidation: Electro-Fenton-Based Processes. If H2O2 is cathodically electrogenerated via reactions 29 or 30 in an undivided cell where the contaminated solution is treated, the process is called AO-H2O2 since the anode can contribute to the oxidation of organic pollutants and their byproducts by means of adsorbed •OH and HO2•, as well as by active chlorine when Cl− is present.161 Probably, the most interesting feature of this coupled process is that it can be carried out over all the pH range, thus making unnecessary the strict regulation and continuous control of the acidity/alkalinity. Furthermore, some authors have suggested the production of • OH from H2O2, which enhances the oxidation ability of AOH2O2. This radical can be formed via chemical reaction 32, as demonstrated by electron spin resonance (ESR) measurements, thus improving the oxidation of chlorophenols in the cathodic compartment of a divided cell.203 In fact, the mineralization of organics in that side was higher than that found in the anodic compartment that contained a Ti/IrO2/RuO2 anode. The oxidizing species •OH can also arise from the catalytic decomposition of H2O2 on the surface of a heterogeneous catalyst in three-phase systems,216 as well as from the oneelectron cathodic reduction of H2O2. For instance, the treatment of diethyl phthalate in an undivided cell equipped with a Pd-modified GDE as a cathode that favored the formation of H2O2 and •OH and a DSA yielded 81% and 40% COD and phthalate removal after 9 h, respectively.217 The previous Pd-GDE has been further used for a combined direct reduction and •OH-mediated oxidation of chlorophenols in the catholyte of a divided cell.218

naphthalenesulfonates by cathodic reduction as well as to polar alkylsulfonyl phenol derivatives under the action of •OH at the anode.211 In some studies, not only are the cathodic and anodic byproducts carefully analyzed but also is a large degradation induced by using a coupling approach with a powerful anode. For reactive orange 4, the single direct reduction on a stainless steel cathode only yielded 10% COD removal at 120 mA cm−2 and some aromatic byproduct remained in solution, whereas anodic oxidation on SnO 2 reached 75%, the coupled oxidation−reduction treatment being the most powerful one since it yielded 94% COD removal as well as the fastest decolorization.212 In the case of organic halides, the aim of coupling is the attainment of dehalogenation by cathodic reduction at the least negative potential, along with the simultaneous or sequential deep oxidation of the dehalogenated byproducts under the action of the anode. DCA and TCA have been comparatively treated by reduction, oxidation, and coupled processes in divided or undivided cells equipped with an Ag cathode and a BDD anode. 213 Higher decontamination and current efficiency was attained in the undivided cell for the same charge passage, particularly for TCA, since it allowed the synergistic action of cathodic dechlorination and •OH-mediated oxidation at BDD. Very notably, the single reduction of TCA entailed the accumulation of chlorinated byproducts, whereas the coupled process allowed their total destruction, which confirms the great potential of combined processes to ensure total detoxification. Recently, the use of undivided microfluidic reactors for performing coupled treatments has allowed significant progress because of the overlap of cathodic and anodic diffusion layers that lead to the intensification of mass transport phenomena. Regarding the treatment of MCAA, these reactors led to a very remarkable enhancement of its direct reduction on graphite compared with a batch reactor or a standard macro filter-press cell.140 As shown in Figure 17, the coupled process with a BDD anode and a graphite cathode yielded the fastest MCAA removal compared with the single anodic and cathodic processes, with 100% abatement at low current density (5 mA cm−2). The flow rate is a key parameter in such continuous reactors because it determines the residence time.

Figure 17. Treatment of solutions of 5 mM MCAA at pH ∼ 3 in a microfluidic reactor (interelectrode gap: 100 μm; electrode surface: 5 cm2) at 0.1 cm3 min−1 using (●) compact graphite cathode and Ti/ IrO2−Ta2O5 anode with 0.5 mM FeSO4, (□) stainless steel cathode and BDD anode, and (▲) compact graphite cathode and BDD anode with 0.5 mM FeSO4. Adapted with permission from ref 140. Copyright 2014 Elsevier. X


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subsection 4.1.2. Alternatively, some authors have employed less efficient cathodes like Ti/MMO224 and BDD.225 Notably, some of these cathodes are especially useful since they favor continuous Fe2+ regeneration from reaction 34, thus maintaining the catalytic cycle and making it possible to run the process with a very small initial amount of iron ions. A large variety of applications of the EF process to the degradation of organic pollutants can be found elsewhere.6,149

Several authors have employed three-dimensional electrode reactors equipped with GAC as a particle electrode, packed between the anode and cathode. Under these conditions, the applied current causes the polarization of each particle of GAC, which behaves as an anode on one side and a cathode on the other to produce H2O2. In these systems, the GAC particle can additionally serve as a catalyst to decompose H2O2 to •OH.219 The multiple reactions occurring in such a complex 3D reactor are schematized in Figure 18.

Fe 2 + + H 2O2 → Fe3 + + •OH + OH−

(33)

Fe3 + + e− → Fe 2 +

(34)

Significantly higher MCE values are achieved when the solution is irradiated with artificial UVA light or natural sunlight in PEF and SPEF processes, respectively, since the UV photons favor the continuous regeneration of Fe2+ with additional formation of •OH from photo-Fenton reaction [i.e., reductive Fe(III) photolysis], as well as the photodecarboxylation of Fe(III)-carboxylate complexes that are refractory to oxidation by •OH in the bulk.6 Alternatively, the presence of cocatalysts like Cu2+ ions can also enhance the treatment efficiency because of their positive synergistic effect. Quicker decontamination is promoted because of the formation of specific Cu(II)carboxylate complexes like Cu(II)-oxamate that can be destroyed by •OH in the bulk.226 A different, simple, and very effective alternative for enhancing the performance of the original EF process is coupling with anodic oxidation by using a large O2-overpotential anode. The first setup of this kind was a tank reactor composed of a GDE and a BDD as the anode and cathode, respectively, for the treatment of chlorophenoxy herbicides.227 Under EF conditions with 1 mM Fe2+, all solutions were completely mineralized at constant current because the refractory organic molecules and their metal complexes could be oxidized by the synergistic action of heterogeneous BDD(•OH) adsorbed at the anode surface and homogeneous • OH generated in the bulk via Fenton’s reaction, as recently confirmed for other organic pollutants.228,229 Therefore, the extraordinary electrocatalytic properties of BDD foster the total mineralization of the polluted solutions, since it is able to destroy the stable degradation byproducts usually accumulated during the latter stages of the water treatment such as oxalic acid, oxamic acid, and other refractory carboxylics and their complexes with iron and other metal ions.230 Another commonly used cathode for performing the EF-BDD coupling is carbon felt.6 Apart from BDD, less expensive materials like Ti/SnO 2 −Sb 2 O 5 −IrO 2 and PbO 2 have been used for enhancing the EF treatment of organic pollutants as well.231,232 In contrast to the vast literature on EF treatments in undivided cells, few studies report the use of a divided cell. This can serve to perform EF in the cathodic compartment, thus avoiding the H2O2 oxidation at the anode, whereas the anodic oxidation is carried out in the anodic compartment.233 The cost related to the greater cell voltage cannot, however, be disregarded. A radically different, brand-new technological way to exploit the ability of EF in divided reactors is that of reverse electrodialysis (RED). Stacks equipped with anionic and cationic membranes allowing the production of electricity from salinity gradients have been employed for the treatment of solutions independently recirculated through the cathodic and anodic compartments by EF and direct/mediated oxidation, respectively.234

Figure 18. Coupled reactions that occur in a three-dimensional electrochemical reactor. Adapted with permission from ref 219. Copyright 2013 Elsevier.

There are a few cases in which cathodic H2O2 electrogeneration can be coupled with the production of active chlorine at the anode. In such an approach, a divided cell that prevents the destruction of the former oxidant by electrogenerated HClO/ClO− is required. Under these conditions, the penalty caused by the increased Ecell upon use of membranes can be justified by the faster and larger decontamination. This concept was used for the degradation of formaldehyde with a SnO2−PdO−RuO2−TiO2/Ti anode and a graphite cathode,220 and dyeing wastewater with a Pt/Ti anode and a graphite cathode.221 Much more powerful technologies than AO-H2O2 appear when iron ions are either added or already present in the effluent to be treated. These are known as Fenton-based electrochemical processes, EF being the original one that opened the door to new methods that take advantage of coupling with light irradiation (PEF, and solar PEF or SPEF), coagulation based on dissolution of iron anodes (peroxicoagulation), ultrasound irradiation (sonoelectro-Fenton),222 dissolution of heterogeneous catalysts that supply Fe2+ (pyriteEF,)223 and bioremediation (bioelectro-Fenton).58 The fundamental requirement in all these systems is the presence of a cathode able to electrogenerate H2O2 from the two-electron O2 reduction reaction 29 at acid pH. The mixture of H2O2 and Fe2+ is called Fenton’s reagent, which yields •OH in the bulk solution from Fenton’s reaction 33. Such homogeneous •OH is the main oxidizing species in EF when low oxidation power anodes like Pt are usually employed. In contrast to AO-H2O2, the treatment by EF and related technologies is usually restricted to effluents within the pH range from two to four, pH ∼ 3.0 being the optimum value for performing the reaction 33.6 Some efforts have been made to extend the working pH range to circumneutral values, mainly by using supported or chelated iron catalysts. The cathodes most typically used in Fentonbased technologies are the carbonaceous ones mentioned in Y


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cathode and thus the complexes of final carboxylates are mainly in the form of Fe(II)-carboxylate, which can be readily degraded by •OH formed in the bulk from Fenton’s reaction. The use of BDD instead of Pt tends to convert Fe2+ to Fe3+, which decreases the yield of •OH formed from Fenton’s reaction, so that the anode contribution of BDD(•OH) is slight and rarely justifies the cost of the investment.235 In order to avoid the detrimental wasting reactions on BDD, the use of a divided cell or a sequential strategy in such cases would probably enhance the decontamination process. As noted in subsection 4.2.2, coupling of EF with BDD is beneficial when the cathodic reaction 34 is not so efficient, thus giving rise to an accumulation of the iron ions as Fe(III), which can therefore lead to the formation of very refractory Fe(III)carboxylates to oxidation by homogeneous •OH. Heterogeneous BDD(•OH) is required in those cases. The combination is also beneficial in the case of iron-chelating organic pollutants such as phosphonate herbicides.236 For such contaminants, EFlike treatments can be carried out by using other metal catalysts, like Mnn+ or Cun+, but their kinetics is much slower than that of iron-catalyzed EF and thus coupling with BDD is a good alternative. Finally, powerful anodes like BDD are also required for treating pollutants that are refractory to •OH in the bulk. For example, the total mineralization of DCA and TCA by EF was only possible upon the incorporation of BDD, which was explained by the formation of mono/dichloroacetic acids.237 As can be seen in Figure 19a, for the treatment of DCA, about 90% TOC removal was achieved after 420 min in EF with BDD, whereas 3−4 should preferably be treated by these coupled redox processes. For acidic real effluents (pH ranging between two and four), the best options are the Fenton-based processes because they allow minimization of the mass transport limitations that are inherent in the aforementioned direct processes at the cathode or anode surface, thereby dramatically accelerating the decontamination. Moreover, the electrochemical Fenton processes can be considered as more environmentally friendly than their nonelectrochemical counterparts because the H2O2 and Fe2+ required to carry out the Fenton’s reaction can be continuously regenerated in situ. The EF and related technologies based on the H2O2 electrogeneration allow the management of large volumes of wastewater, thereby yielding quick and large percentages of organic matter degradation. On the other hand, although the sludge formation is much less significant than that observed in chemical Fenton technologies, the need for a suitable post-treatment is a factor to be considered. Coupling in Fenton-based treatments may sometimes be unnecessary. This is the case when undesired back reactions can take place at the large O2-overpotential anode whose purpose should be increasing the degradation ability of the process. In particular, this finding has been demonstrated in EF systems with a carbon-felt cathode. The oxidation ability of EF systems with such large surface area cathodes is high enough to reach the total mineralization even with low O2-overpotential anodes like Pt because Fe2+ is very efficiently regenerated at the

Figure 19. (a) Increase of Δ(TOC)exp and (b) concentration of monochloroacetic acid (MCAA) + acetic acid accumulated during mineralization with electrolysis time for the EF treatment of solutions of 4 mM dichloroethane (DCA) with 0.035 M Na2SO4 and 0.5 mM Fe2+ at pH 3.0 and 10 °C using a (●) BDD/ADE or a (○) Pt/ADE cell at 300 mA. Adapted with permission from ref 237. Copyright 2011 Elsevier. Z


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Figure 20. Electrochemical plants and cells used in the EO process in batch operation mode. Bench-scaled flow plants with (a) photo/ electrochemical divided reactor; (b) undivided cell with parallel plate electrodes, (c) bipolar trickle tower reactor, (d) stirred two-electrode undivided tank reactor with a jacket for external thermostatization, (e) filter-press flow cell (FM01-LC reactor), and (f) experimental setup for the electrochemical treatment of Amaranth with an ACF electrode using a three-electrode divided cell. Adapted with permission from ref 18. Copyright 2009 Elsevier.

for such purposes because of the potential combination of electroreductive dehalogenation, hydrogenation/hydrogenolysis reactions, and H2O2 electrogeneration. (2) The reactor, being necessary to further explore the advantageous characteristics of microreactors that allow the use of cathodic and anodic reactions much more efficiently. Their great performance in terms of coupling direct reduction with •OH-mediated oxidation reactions has been observed. Another rarely surveyed approach is the use for the cathodic production of H2O2 with anodically formed O2 quickly transported to the cathode.240 Indeed, the reduced interelectrode gap can allow overcoming the limitations related to the limited solubility of O2. (3) The operating conditions, focusing on the influence of hydrodynamics since a better design and/or operation mode could decisively improve the results. As in the case of direct cathodic reduction, only a few examples are available in the literature at the moment. In EF, this can be accounted for by the leading role of •OH in the bulk, thus making much less relevant the effect of mass transport toward/from the electrodes. Generally speaking, the stirring rate in tank reactors or liquid flow rate in flow cells is simply fixed at sufficiently high values so as to ensure fast homogenization of treated solutions to yield the maximum degradation rates. However, some authors have studied in greater detail the effect of mass transport conditions in EF reactors. According to Sanromán’s group,241,242 the percentage of color removal from dye solutions working at constant cell voltage in continuous mode directly depends on the residence time. Decolorization over time was satisfactorily predicted after confirmation that the hydrodynamic behavior of the EF airlift reactor resembled that of a continuous stirred tank (CSTR). Modeling and prediction are therefore both feasible and desirable. (4) A detailed analysis of the reaction intermediates is mandatory in many cases, especially for complex chemical structures. When the anodically or cathodically formed byproducts are refractory to further reduction/ oxidation at the counter electrode, divided cells or sequential treatments would be more convenient, despite the disadvantages associated with the presence of separators. (5) Given the high performance of UV-irradiated Fenton-based processes, the use of renewable energy sources such as sunlight would allow further development of more eco-friendly technologies like SPEF.243 At present, the integration of large O2-overpotential anodes like BDD in SPEF allows the fastest degradation of

of the main reaction byproducts, namely MCAA and acetic, in EF-BDD, compared with their great accumulation in EF-Pt. A recent work on the treatment of atrazine by EF with a carbonfelt cathode is also paradigmatic:238 chemical, photochemical, and photocatalytic AOPs, as well as EF with a Pt anode, are unable to degrade the pesticide completely, whereas coupling with BDD is the only alternative that ensures that >97% mineralization is achieved because of the action of BDD(•OH) on the final heteroaromatic byproduct (i.e., cyanuric acid). Special attention should be paid to the application of coupled systems with H2O2 electrogeneration to the treatment of effluents with a high content of chlorinated ions. In the EF process, the formation of soluble (FeCl)n complexes can enhance the removal of organics because they allow the increase of the soluble iron dose. However, the presence of such ions is rather detrimental for various reasons: (i) the formation of (FeCl)n complexes can cause a decrease in the regeneration rate of Fe(II) and (ii) some reactions that cause the destruction of H2O2 take place, such as those involving chlorine radicals (Cl2•−, etc.) or active chlorine species:239 HClO + H 2O2 → Cl− + O2(g) + H 2O + H+

(35)

Although such destruction of H2O2 while it is electrogenerated at the cathode is not instantaneous and is expected to react near the cathode with Fe2+ to produce •OH in the bulk, there is practically no accumulation of H2O2. Thus, coupling with BDD is necessary in order to have enough •OH in the reactor to perform the degradation.161 In such cases, either a pretreatment to remove Cl− (e.g., use of ion-exchange resins), the use of separators to avoid contact between H2O2 and active chlorine, or the replacement of the Fenton-based technology by anodic oxidation mediated by active chlorine can be a feasible alternative. The optimization of operating parameters in coupled processes is cumbersome because a potentially beneficial modification regarding the reactivity at the anode might have a negative impact on the cathodic one. Nonetheless, key aspects to be considered in order to open up new perspectives on the coupling of anodic and cathodic reactions within the field of electrochemical water decontamination are (1) the electrode material, for example, addressing the development of catalyzed cathodes that facilitate the synergistic action of various processes. Ag- and Pd-modified GDEs are promising materials AA


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application of this technology in suitable conditions. As previously observed for many other electrochemical technologies during the twentieth century, such as fuel cells or electrodialysis, the closer to the market, the more important the cell design and the scale up become. The main points to be accounted for in electrolytic cell design are electrodes, electrolyte, flow patterns, heat transfer, and scale-up. Electrodes and electrolytes have been discussed in previous sections of this review and so, in the next sections, the role and influence of the other elements on results are discussed.

organic pollutants among all the treatments available. Furthermore, the construction of sun-powered systems is probably the ultimate goal in electrochemical water treatment, with some interesting devices already tested at pilot scale for electro-oxidation and electrocoagulation (see section 5).

5. SIGNIFICANCE OF CELL DESIGN AND SCALE UP In order to obtain a successful electrolytic treatment of wastewater in terms of efficiency and, hence, of economy, one of the most significant factors to be taken into account is the electrochemical cell design.244,245 Many factors can be considered for this design, such as mass transport, heat transfer, reactions kinetics, and current density or potential distribution on the surface of the electrodes. They are known to have a paramount significance on the results obtained, especially when the process is going to be applied at full scale. In this case, even small improvements in the efficiency could entail a huge savings of money and hence the difference between profit or loss in the scale-up of an electrochemical remediation process. In the literature, many different types of electrochemical cells can be found for the treatment of wastewater. Figure 20 shows several types applied to operations in batch mode. However, despite the great diversity of setups found in the literature, very little attention has been paid to this very important point in recent years. Most scientific and technical works have focused on the optimization of one of its components, namely the electrode, neglecting the influence on results of other important components, as pointed out in previous reviews.5,18 This can be explained because of the small scale of the studies, typically lab or bench scale, which restricts the main aim of the investigations: to determine the influence of the electrode material and operating conditions on the degradability of pollutants. It is worth taking into account that the effect of the electrode characteristics on results is usually very important and its properties are key to understanding the mechanisms of the electrochemical processes, as stated in sections 2 to 4.4,149 In fact, most studies about electrolysis of wastewater detail the intermediate and final products produced with a given electrode and try to determine if direct and mediated reaction kinetics are favorable for such types of electrode under optimized operating conditions. Here, it is important to bear in mind that the composition of the electrode is the most well-studied parameter but not its shape or conformation, even though they could have a great influence on results in many cases.219,246 This fact explains the large number of papers (usually at lab scale) focused on electrodes with interesting properties like BDD and DSA, in which only the composition of the electrode is considered. Regarding bench-scale studies, their main aim is to characterize mass transport and find a way to improve efficiency.83,84,247 In these studies, not only the composition of the electrodes but also the flow characteristics of the wastewater inside the electrochemical cell are assessed. To our knowledge, very few studies carry out a scale-up from the bench to the pilot scale and, obviously, there are almost none from the pilot to the full-scale plant.248 This is a major drawback for the application of electrolytic technology in the remediation of actual effluents, and it is expected to be one of the hottest topics in research on environmental electrochemistry or, more accurately, on environmental electrochemical engineering from now on.244 Challenges are very important, and the results of research should contribute to the

5.1. Flow Pattern and Construction Details

Flow pattern is key in increasing the efficiency of electrochemical processes.249 As presented in previous sections, one of the main concerns of some electrochemical remediation technologies is that most relevant reactions occur on the electrode surfaces, and thus mass transport of species from and toward the electrode surface is usually the bottleneck in the technology. In such cases, this limitation should be resolved by proper cell design and appropriate choice of flow pattern. At this point, it is worth highlighting that the concentration range in which electrochemical technologies should be applied is still controversial. There are clearly better technologies than electrolysis for highly concentrated wastes; the reference technologies are the recovery of the organic pollutant as a product by a nonelectrochemical separation unit (distillation, extraction, etc.) or heat recovery after incineration. On the other hand, there are also clearly better technologies for low concentrated wastes (or when the discharge limit is low) because they are especially designed to attack organic matter in the bulk and no mass transport limitations are found. Therefore, a very reasonable range of applicability is 1500 to 20000 mg dm−3,248 which could be extended in the lower limit down to very low values if electrochemical technology is properly devised (see previous comments on Cl-mediated oxidation and processes based on H2O2 electrogeneration and/ or UV and ultrasound irradiation). Regarding the flow pattern, mixed and plug flow are the two main models in chemical reactor engineering and they lead to the two main types of electrochemical cells used in the removal of organic pollutants from wastewater in the scientific literature: mixed-tank and flow cells. Mixed tank cells (see Figure 21) are used in most research carried out at lab scale because they are the simplest cells for assessing the electrochemical remediation of wastewater, and the mathematical interpretation of results is also very simple. Hence, noncomplex simulation tools need to be applied in order to understand and discuss the results. In lab-scale work

Figure 21. Typical lab-scale mixed-tank cell used in the electrolytic treatment of wastewater (Metrohm Autolab). AB


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found in the literature, mixing is typically obtained by magnetic stirring rods, mechanical stirrers, or recycling pumping. It is interesting to observe that in contrast to what could be expected by a chemical engineer, no turbulence promoters (deflectors, flats, etc.) are usually applied in studies carried out with tank cells in order to increase the efficiency of the process. Consequently, the main drawback is the very low km yielded by these cells and hence the lower current efficiency compared with electrolyses in flow-pass cells. With regard to potential scale-up, km largely depends on the mixing rate, but its low value makes the process inefficient when the concentration of pollutant becomes low. For this reason, initially, this flow pattern can only be recommended at full scale for the coarse removal of pollution in which mass transport is not limiting the kinetics of the process. Flow cells are the most common alternative found in the literature to mixed-tank cells. This type of cell is typically associated with bench-scale studies in which not only kinetic data but also some additional information about process performance are desired which allow discussion of the further applicability of the technology on a larger scale. The main advantage exhibited by this type of cell is its enhanced mass transport properties, which makes it suitable for treating much lower concentrations than mixed cells. A remarkable comparison between mixed-tank and flow-pass cells for the treatment of actual petrochemical wastewater under similar conditions has recently been published.78,250 In this comparison (see Table 5),

Figure 22. Typical flow cell used in the electrolytic treatment of wastewater.

for electrosynthesis (production of chlorine, chlorate, etc.), they can easily be adapted to environmental application and their use has been widely described in the literature.250,251 However, in the literature, very few scientific studies can be found which focus on the assessment of stacking for the removal of organic species and usually they are limited to very few cells. This is discussed later on in the scale-up section. In any case, most studies are carried out in discontinuous operation mode, and in these cases, single-pass cells are connected to a reservoir auxiliary tank through a recirculation pump in order to increase the number of times wastewater passes through the cell and hence the flow rate (Figure 23).

Table 5. Comparison of Batch and Flow Modesa mixed cell % COD removal anode mass transport coefficient (m s−1) = 5.9 × 10−6 15 30 15 30

mA mA mA mA

−2

cm cm−2 cm−2 cm−2

(25 °C) (25 °C) (40 °C) (40 °C) flow-pass cell

Ti/Pt

BDD

33.8 46.5 35.2 47.9

50.3 57.5 51.8 59.1

% COD removal anode mass transport coefficient (m s−1) = 2.0 × 10−5

Ti/Pt

BDD

cm−2 cm−2 cm−2 cm−2

64.5 90.7 94.1 94.7

76.2 94.5 94.7 98.7

20 40 40 40 a

mA mA mA mA

(25 (25 (40 (60

°C) °C) °C) °C)

Figure 23. Pilot plant for the electrolysis of wastewater.

This helps to solve problems associated with gas evolution, dragging bubbles from the cell that otherwise would accumulate in it, increasing the ohmic resistance of the electrolyte and hence the cell potential. Likewise, with a high recirculation flow rate, mass transport is improved, and thus greater removal of organic pollutants is achieved. The application of this cell-tank configuration means that the system could be modeled from the macroscopic point of view as a mixed-tank reactor, although in this case efficiencies are much greater than those obtained with a conventional mixed tank. Key points in the design of electrochemical cells for the treatment of wastewater are the interelectrode distance and the consequent construction details. As is well-known, the space between two electrodes (gap) affects the total resistance of the cell and hence the ohmic losses. The lower the electrode gap, the lower the cell potential, because the electrolyte resistance depends almost linearly on this gap. Consequently, it also affects the reactions occurring in the bulk. In addition, the

Adapted with permission from ref 78. Copyright 2014 Springer.

COD removal was found to be more efficient when an electrochemical flow-pass cell was employed because of the enhanced mass transport even though similar values were retained for the other parameters. With regard to flow cells, many commercial models are now available in the market, such as the Electrocell (ElectroCell A/ S, Denmark) (Figure 22), the FM01-LC (formerly ICI Chemicals and Polymers Ltd., now INEOS Chlor-chemicals, U.K.) and the Diacell (formerly ADAMANT TECH, now WaterDiam, Switzerland). They are especially designed to allow their use as single cells or the stacking of single cells, most likely in filter-press mode, although they can also be staked in parallel flow, inter alia. Although most commercial cells were developed AC


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Figure 24. Construction details of the Diacell.

Figure 25. Construction details of the Electrocell.

interelectrode distance also affects the role of gases formed during the electrolysis, it being necessary to remove them as quickly as possible to avoid operating problems. An optimum distance between the electrodes is required for good pollutant removal efficiency. Most research works report the use of electrochemical cells with an interelectrode gap between 0.5 and 3.0 cm. However, current studies by Scialdone and coworkers252 have reported the successful performance of microfluidic reactors with a gap in the order of some tens of microns. In any case, the great significance of this mechanical parameter of the cell is palpable and thus commercial cells

exhibit a very low interelectrode gap in order to increase the efficiency compared with purpose-made cells. However, recirculation flow rate becomes critical when excessively narrow gaps are used because it determines the stripping of gas bubbles. In addition to the interelectrode gap, the design of proper turbulence promoters is also taken into account in several commercial cells. This special mechanical design can help to increase mass transport and hence to improve efficiencies in the treatment of lowly loaded wastewater. Figures 24 and 25 show construction details of the Diacell and Electrocell commercial cells, respectively. In these two very AD


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Figure 26. Two examples of cell stacking (photo of Electrocell stack taken from manufacturer Web site,http://www.electrocell.com/electro-prodcell.html). Reproduced with permission. Copyright 2015 ElectroCell A/S.

Figure 27. External aspect of Diacell stacks with mono and bipolar electrodes.

study the continuous process,255 possibly because the discontinuous mode allows researchers to obtain a complete set of data with less experimental effort, the continuous mode should only be applied at full scale in very large applications, and technology is not yet mature enough for such applications. A particular case in electrochemical cell technology appears when several electrodes are to be used in the same cell. Here, the choice between monopolar and bipolar connection is very important. As is well-known, in the monopolar connection, all anodes and cathodes are directly connected to the power supply. In contrast, in the bipolar connection only the outermost electrodes are connected to a power source, and the current passes through the other electrodes, polarizing them: in the bipolar systems, the side of the electrode facing an anode is negatively polarized and vice versa on the other side facing a cathode. The advantages and drawbacks of both types of connections have been studied in many applications of electrochemistry but mainly in electrocoagulation. Unfortunately, such is not the case for the electrochemical processes reviewed in this paper. However, the few results seem very promising in terms of reducing energy consumption.256,257 Bipolar electrodes have been demonstrated to be most efficient for depletion of organic pollutants, but the monopolar configuration entails a lower operation cost because of the resulting lower cell potential. In addition, the bipolar connection allows engineers to look for more interesting mechanical designs of the cell. An interesting example is the reactor used by Yavuz and Shahbazi258 for the oxidation of the dye reactive black 5 and by Savaş Koparal and co-workers76 to decolorize basic red 129 solutions. It consists of a bipolar trickle tower with two concentric glass pipes with inner diameters of 4 and 2.5 cm and BDD Raschig rings (inner and outer diameters

different technologies, the special mechanical designs in direct contact with the electrode surface especially developed to help to distribute flows, promote turbulence, and facilitate the removal of gas bubbles can be observed. All these commercial cells are ready to be used with or without membranes and can easily be stacked, as shown in Figure 26. An extreme case study is the concept of microcells in which the distance between the cathode and the anode is extremely small, down to a micrometric distance. In microfluidics, process engineers are working in various areas ranging from the food industry through biotechnology to pharmaceutical products, from analytical and laboratory-scale equipment through energy conversion to industrial chemistry applications for the production of millions of tons of chemicals. However, in the context of electrochemical processes, the development of microcells for applicative purposes is just in its infancy. Although for the moment, it is only fundamental research, these novel reactors are used to perform the electrochemical treatment of water contaminated by organic pollutants in the absence of added supporting electrolytes under a continuous mode with high abatement. The results are very promising.252,253 Given the state of the art, transformation of the discontinuous process into a continuous one is expected to be carried out with the same auxiliary tank, simply through the addition of an inlet of wastewater and an outlet of treated wastewater.254 This configuration has the same advantages: it allows the use of flow-pass cells with large flow rates, allowing the stripping of the gas and a very high km. Furthermore, the continuous mode is likely to present various advantages including higher productivities. In any case, very few papers AE


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production of oxidants is the most frequently applied technology. For this reason, from the reactor design no special consideration should be accounted at this point. Activation of oxidants in the bulk is the key to obtaining an efficient technology for water treatment. Many works report the production and occurrence of large concentrations of these oxidants during the electrolysis of wastewater,271 although this is not a symptom of an efficient treatment but merely of the improper application of these oxidants; the best oxidant is the one that it is not observed in the bulk because it reacts immediately after its production. Activation of an oxidant involves the transformation of a not very reactive species (which could have a very high oxidation potential but a slow reaction rate) from the kinetic perspective into a more active oxidant species. A very well-known case is the transformation of oxygen into hydrogen peroxide (see section 4). From the thermodynamic perspective, oxygen is much more powerful than H2O2 but, at room temperature, the oxidation power of H2O2 overcomes that of oxygen. In general, the technology described in research papers is still at a very early stage and, normally, standard cells with minor modifications to the mixedcell tanks or (less likely) flow-pass cells are used. A more interesting case in terms of reactor design is the activation of oxidants by coupling UV irradiation to the electrolytic reactor.272,273 This is very interesting when peroxosalts are formed in the reaction media because it allows the formation of anions radicals (sulfate, carbonate, or phosphate radicals) and •OH from these salts, thus improving the efficiency in the removal of organics. It is also interesting when chlorides are contained in large concentration because UV promotes the formation of chloro radicals and •OH in the bulk. Many types of prototypes are proposed in the literature for combining electrochemical and UV irradiated processes, either with the mixed-tank or the flow-pass approach. The simplest prototype consists in the addition of a UV lamp at the top of a mixed tank. More developed systems apply quartz plates and grid electrodes to modify flow-pass cells, allowing light to access the electrode surface (Figure 28) or irradiating light not to the cell but to the auxiliary tank. This is no small

of 6 and 8 mm, respectively) placed in the inner glass pipe and performing a bipolar connection. Electrode connection is an interesting topic, and much research is expected. Figure 27 shows the external aspect of two commercial cells with the same number of electrodes but with different types of connections. In addition to electrode configuration, turbulence promoters, and interelectrode gap, performance of flow-pass cells largely depends on many construction details, and the use of computational fluid dynamics (CFD) tools is expected to have a great influence on the performance of electrolytic cells as in the case of fuel cells during the past decade. However, these tools have only been used rarely, and currently cell design259−261 for the removal of organics is in the very early stages. Finally, in addition to the flow pattern optimization that could be obtained with CFD, many recent innovations try to improve mass transport through the special design of the 3D electrodes (including packed electrodes).262−265 However, they have rarely been applied to the removal of organics,265,266 and no prototypes are worth describing because of the difficulties of extrapolating their physical aspects to full scale. 5.2. Combination of Electrochemical Systems with Other Technologies

The combination of the electrochemical process with other technologies in order to try to increase efficiencies obtained by the single electrolytic processes is a hot topic nowadays. It is especially important when it is aimed at the removal of organics at very low concentrations. In these conditions, electrolytic technology turns out to be inefficient because it becomes diffusion-controlled, even working with flow-pass cells at very high flow rates. In this case, a secondary process that is always associated with the electrolysis of organics is suggested: the production of oxidants on the electrode surfaces and, if required, their activation in the bulk. Nonexternally promoted production of oxidants is associated with (1) the type of electrode (as discussed in previous sections), and (2) the occurrence of anions such as chloride (section 3), sulfate, phosphate, carbonate, etc., in the supporting electrolyte. The latter case is typical of most wastewater according to what has been discussed above. Otherwise, the production of oxidants could be promoted by selecting the proper anode material by taking into account the particular composition of the wastewater. Thus, in wastewater with a high concentration of chlorides, the use of Ru-based DSA promotes the production of large amounts of hypochlorite, whereas a BDD anode should be avoided because it leads to the formation of chlorate and perchlorate (see section 3). When wastewater contains sulfates, BDD anodes are the best choice because they promote the production of persulfates and hence a much higher efficiency compared with DSA can be obtained.251,267,268 An alternative to this nonexternally promoted production of oxidants consists of dosing a particular salt in order to foster the formation of very powerful oxidants. In this case, the process should also consider a later separation stage in which the reduced form of this oxidant could be efficiently removed. Several works have proposed the use of pairs such as Ag(I)/ Ag(II)269,270 for the treatment of refractory wastewater with good efficiency. However, the use of this technology is restricted to very special cases because of the difficulties of the second stage, and normally, the nonexternally promoted

Figure 28. Prototype of flow cell with quartz plate and grid cathode to allow UV irradiation coupling. AF


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exciting topic because, in spite of the great advantages demonstrated, the excessive removal of ions in the first separation stage could become a significant drawback by increasing the cell voltage and minimizing the role of mediated oxidation caused by electrogenerated species.

difference but a very important one because if light is irradiated directly to the electrode, the electrode kinetics will be enhanced but volumetric processes will not be affected. If the process is under diffusion control, this means that no improvements can be made. Conversely, if light is irradiated to the auxiliary tank, transformation of oxidants (previously produced on the electrodes) into more powerful species (from the kinetic perspective) is obtained in the bulk, enhancing the efficiency of the process if it is diffusion-controlled, but in this case, photocatalytic properties of the electrode are not used. Hence, if this electrode contains semiconductors such as TiO2, light has to be directly irradiated to the electrode surface.274 In this context, direct sunlight irradiation is becoming a very significant topic because of the interesting cost decrease associated with it.275 In this case, the electrochemical cell should be coupled with instrumentation to optimize the sunlight irradiation. In this regard, recent advances are being achieved for the UV and solar photoelectro-Fenton processes.275,276 A collateral use of UV light is encountered in the use of photovoltaic generators. This coupling cannot be considered as a new type of electrochemical reactor, but experiences described in the literature clearly show that this combination should be accounted for in future research.277−283 A smaller number of manuscripts have been published on the enhancement obtained when ultrasounds are irradiated in a combined sono-electrolytic process.284,285 In this case, the frequency of the ultrasound is a parameter of major significance, because high frequencies yield radicals while lower frequencies only improve mass transport by increasing turbulence.286 The problem associated with this technology is the high energy consumption, which could be minimized by optimizing the placement of the generator. Depending on the point of irradiation, results may be different. In this case, mixedcell technologies and irradiation in auxiliary tanks are those most often reported. Another energy-intensive technology like ozonation has also recently attracted the attention of researchers for its efficiency in the so-called electro-peroxone process.287 Usually, the yield of O3 produced via electrical discharge is very low, as most of the supplied O2 is wasted in the reaction system. However, when the O2/O3 mixture is sparged into a reactor equipped with a GDE as cathode, H2O2 is satisfactorily electrogenerated, as discussed in section 4. Such H2O2 then reacts with O3 to produce •OH, and thus the oxidizing ability of the system is greatly enhanced. In the presence of a metal catalyst, the resulting EF-ozonation method could be even more potent. A very interesting strategy for improving the results of the electrolytic treatment of wastewater polluted with organics is the integration of membrane filtration with electrochemical technology. This combination has been successfully evaluated in two different case studies: the treatment of cellulose bleaching and pharmaceutical effluents.288,289 The preliminary conclusions that can be drawn are that the separation technology may be applied (1) as a separation pretreatment in the case of highly concentrated wastes, when it causes a significant decrease in organic loads, thus reducing the energy required in the electrolytic post-treatment or (2) as a concentration stage for diluted wastes, when it helps to produce a concentrate for which mass transfer inefficiencies are lower in the later electrolytic stage. In both cases, the robustness of electrochemical oxidation ensures the complete mineralization and detoxification of permeates and/or concentrates. The efficient regulation of this combination is a very

5.3. Heat Transfer Issues in Electrochemical Cells

With regard to temperature effect on the removal of organics, it is worth bearing in mind that this may become a key point, especially if oxidants are produced electrochemically and reactions in the bulk are to be promoted. Usually, high temperature promotes mediated electro-oxidation processes, and hence, it enhances the decontamination. However, this is not always true because many factors may affect the treatments. For example, peroxocompounds, such as peroxodisulfate and peroxodiphosphate, can be transformed into less powerful H2O2 if temperatures surpass 50 °C.290 Likewise, the solubility of gases decreases with temperature, and this may become a problem when chlorine is produced because high temperature favors stripping rather than production of hypochlorite. Electrolysis causes the transfer of huge amounts of energy as a consequence of the ohmic losses. Cooling is required to keep the temperature at the desired set point. Taking into account that the typical desired range of temperature is between room temperature and ca. 60 °C, water is typically used as a heatexchange fluid. Because of the small scale of the studies reported in the literature, not much attention has been paid to the design of heat exchangers for the removal of organics. Typically, an external heat exchanger (within the recirculation circuit between the tank and the cell) and a jacket are used for flow-pass cells (Figure 29) and mixed-tank reactors (see Figure 21), respectively.

Figure 29. Detail of the heat exchanger (filter-press type) of a pilot plant for the treatment of wastewater.

Special cell designs are proposed for very specific purposes, such as the electrochemical filter-press microreactors used for the electrosynthesis of molecules with a high added value in which a heat exchanger is integrated into the microstructure electrode to remove (or supply) rapidly the heat required in exo- or endothermic reactions.291 Obviously, no further applications have been found for the removal of organics. 5.4. Scaling-Up and Investment Cost of Electrochemical Cells

As has been pointed out in this discussion, scale-up from the lab to the full scale is an issue of major importance in AG


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Figure 30. Stacking of cells in a 1 m2 pilot plant for the electrolysis of wastewater.

electrochemical technology. There are two very significant points to be considered: increasing the electrode area for a single cell and stacking of cells (Figure 30). With regard to the stacking of cells, no great difficulty is expected, but very few works report results on the treatment of organics at the pilot plant scale,283,288,292,293 and they are not focused on these potential problems because they are only preliminary approaches to the scale-up. The main concerns regarding stacking should be related to the management of heat and hydrogen. Hydrogen evolving at the cathode is a key point in this technology, and it is still an unresolved problem, since large amounts of this gas are expected to be produced during the electrolytic treatment of wastewater at a large scale. In single undivided cells, hydrogen is mixed with oxygen and this mixture should be carefully managed in order to avoid hazardous operating conditions. In small-scale systems like those commonly reported in the literature for lab-scale studies, total amounts of hydrogen produced are very small and a good dissipation system (e.g., extractors, purge valves, etc.) prevents dangerous situations from arising. However, the significance of the problem becomes extremely important when the scale increases294 because removal is not as easy. Initially, the recovery of hydrogen and its valorization in a fuel cell could be hypothesized,295,296 but the idea is not simple and only attempts in very small devices have been studied up to now. The use of divided cells (with membranes) may reduce this problem, but it entails higher operation costs because of the increased cell potential. Hence, much work needs to be done in the near future to overcome this limitation of the technology. On the other hand, heat transfer either in wires or into the electrolyte is a serious problem in large-scale plants because of the huge amount of transferred energy and the huge amount of heat that must be dissipated. In this case, heat exchangers can help to diminish the problem, which is not as important as in the case of hydrogen production and management, but special care has to be taken during the design phase in order to avoid overheating caused by ohmic drops. This is especially important in the dissipation of heat from the stacks because a bad design could seriously damage the frame of the cells. With regard to the size of the electrode in the cell, electrochemical engineers know well that current and potential distribution patterns are not uniform but depend on many factors including the flow patterns, the uniformity of the electrode surface, the existence of turbulence promoters, the current feeders, bubble stripping, etc. In contrast to other electrochemical technologies such as electroplating or fuel cell technology, in which the main aim is to obtain a perfect layer and/or to minimize efficiency losses because of a bad current

distribution pattern, very little attention has been paid until now to the removal of organics via electrolysis despite the significance. This is probably because of the very low number of current applications of this technology at full scale. However, this is very important because most wastewater electrolytic processes operate under diffusion control and, even for processes in which mediated electrolysis is promoted, the role of mass transport is not negligible. This means that efficiency is limited by the rate at which pollutants arrive to the electrode surface, and maximum efficiency is obtained when the electric charge transfer rate and the pollutant’s mass transport rate are balanced. Otherwise, side-processes (e.g., water oxidation) become important, and the organic removal becomes less efficient. At this point, the maximum size of the electrode to be used in an industrial application is of major importance because the larger the electrode, the more difficult it is to obtain an even current and potential distribution on its surface and hence to work close to the maximum efficiency conditions. This means that differences between the efficiency of two processes with current distribution patterns on the anode surface like those shown in Figure 31 could be very important in spite of having

Figure 31. Illustration of current density distribution in a circular electrode with a central current feeder.

the same average current density because the local efficiency in the regions of the electrode in which the current density is higher would certainly be much lower than that in other regions, and this would yield a lower average efficiency. Conversely, a large electrode surface helps to minimize the number of cells that should be included in the stack to obtain the area required for a given process, and this reduces the investment cost of the cell.12,297 This point is critical in the scaling-up of technologies such as EO with BDD because of the high electric resistance of this electrode material, which makes it difficult to obtain a uniform current distribution. Current AH


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electrode fouling and corrosion phenomena (periodic cleaning is advised)3−6,8,67 high operation costs because of the high energy consumption (but coupling with renewable energy sources is possible)56 low conductivity of the effluent68 optimization of reactor hydrodynamic conditions expensive, high O2 overpotential anodes (depending on the purpose)67,68 robustness

versatility to treat large volumes3,8,32

ease of automation4,5 addition of nontoxic reagents for increasing conductivity

attractive compact technology65

production of strong oxidants from salts8

microorganism inactivation92 evolution versatility from 2D to 3D electrodes102

no pH restrictions8,57,103

application as pretreatment or post-treatment of effluents in combination with other depuration technologies:56 (a) biological treatment, (b) Fenton oxidation, (c) ion exchange, (d) membrane filtration, (e) membrane bioreactors, and (f) electrochemical technologies

potential formation of halogenated byproducts (if halogen atoms are present)67,92

Table 6. Main Characteristics of EO Technology

6. CONCLUDING REMARKS A survey of the literature in the last 30 years has been conducted. It was aimed at understanding the issues, concepts, methodologies, and applications of the electrochemical oxidation of organics, oxidation mediated by electrogenerated active chlorine, cathodic processes, and coupled ones, with particular attention to the broad abilities, advantages, disadvantages, and perspectives of these technologies. These experimental works and all results reported by a number of authors rely upon the close collaboration of electrochemists, engineers, and chemists to ensure the effective application and exploitation of electrochemical processes in managing important water pollutants and using the advances in electrocatalysis. The principal features of EO, which is the most widespread EAOP among those discussed, are shown in Table 6, which emphasizes its chemical and technological advantages and disadvantages. The role of operating parameters within the framework of the electrochemical oxidation and the oxidation

abilities

Figure 32. Cost of electrochemical cells for wastewater treatment. Sources of information: (■) Electrocell, Skjem, Denmark, 2005; (△) Electrosynthesis Company, Lancaster, NY, 2005; lines: http:// electrochem.cwru.ed/ed/encycl/). Reprinted with permission from ref 12. Copyright 2009 Elsevier.

ambient temperatures and pressure requirements32

advantages

disadvantages/challenges to be further studied

feeders are essential for obtaining a good distribution, but no work has been done in this regard. In fact, some of the differences reported in the literature in the electrochemical removal of pollutants could be related to an uneven current distribution rather than to other factors. Major work should be carried out in the future to optimize the electrochemical cell so as to obtain the highest efficiency that the electrochemical process can offer. Wastewater inlets and outlets have great significance in turbulence terms and hence in mass transport. The use of CFD simulation tools is becoming a hot topic in electrochemical engineering in the attempt to obtain the best configuration for high efficiency.259,288−300 However, very little attention is paid to the design of novel reactors for the removal of organics, and the most important references are focused on the removal of simpler pollutants like chromium(VI).259,298−301 As with many other industrial applications, it is not easy to obtain cost data for electrochemical cells. This cost is very important because in many cases it could be between 30 and 60% of the total treatment cost, being as important as the energy cost.302 In the literature, a Williams-type model can be found for this investment cost, obtained from data of several companies and also from data in the literature.12,297 The results are reproduced in Figure 32. The main problem is the difficulty of extrapolation to higher surface requirements.

effective degradation of different organic pollutants in wastewater:3−6,8,16,19,31,37,41,57,85,92,100−103 (1) chemical industry, (a) fine chemical industry, (b) pulp and paper industry, (c) petrochemical industry, (d) pharmaceutical industry; (2) textile industry; (3) tannery industry; (4) food industry; (5) landfill leachate; (6) agro-industry, (a) olive oil and (b) dairy manure; and (7) urban and domestic wastewater production of reactive oxygen species (•OH radicals)3

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by electrogenerated active chlorine has been critically evaluated. It has been shown that the effect of operating parameters can drastically change as a result of the different routes involved in these processes. Studies on the cathodic processes used for the abatement of organic pollutants in water have also been reviewed to see how cathodic and anodic processes can be successfully coupled to enhance the efficiency of the supplied current. Studies on the reactors that can be used for all these processes have been reviewed, suggesting that further studies are necessary in order to identify the best solutions from the reactor perspective. In conclusion, the investigated electrochemical approaches appear mature enough to become a very promising tool for the treatment of wastewater contaminated by recalcitrant pollutants, but at the same time, some crucial aspects that require further study in order to develop the process on an applicative scale have been identified.

Films as well as contributions to national and international meetings. His research interests include electrochemical technologies for water treatment, electrocatalytic materials, electrocatalysis, and electroanalysis.

AUTHOR INFORMATION Corresponding Author Manuel Rodrigo obtained his Ph.D. degree in the University of Valencia in 1997, with a research focused on the modelling and automation of biological nutrient removal processes. In 1997, he joined the University the Castilla−La Mancha and began researching electrochemical engineering. In this first postdoctoral stage, his research was focused on the electrolyses of industrial wastewater. After a first postdoctoral training in the Lab of Prof. Comninellis (EPFL, Switzerland), he started working with diamond electrodes, one of the key topics in his research. In 2000, he was appointed as Associate Professor at the University of Castilla−La Mancha and began researching on electrocoagulation and high temperature PEM fuel cells. Then, oxidants production, microbial fuel cells, and soil electro-remediation have also focused his research attention. In 2009, he became a Full Professor of Chemical Engineering at the UCLM. He maintains strong consultant collaboration with many companies in energy and environmental engineering. He is author of more than 220 papers in referenced journal and books (h index 40), more than 70 technical reports for companies, five patents, and he has supervised 11 doctoral theses. He has been an invited professor at the universities Paris Est Marne la Vallée (France) and Politécnica de Valencia (Spain). At present, he is the vice-dean of Chemical Engineering in the Faculty of Chemical Sciences of the University of Castilla−La Mancha. He has served as Chairman of the Working Party of Electrochemical Engineering of the European Federation of Chemical Engineering since 2011.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

́ Carlos A. Martinez-Huitle graduated in Chemistry at Universidad de las Américas-Puebla (México) under the supervision of Prof. Dr. Marco Antonio Quiroz Alfaro. After a work experience in Ciba Specialty Chemicals (currently acquired by the German chemical company BASF), he moved to Ferrara (Italy) where he received his Ph.D. in Chemical Sciences at the University of Ferrara under the supervision of Prof. Achille De Battisti. During the same period, he worked as a visiting scientist in the group of Prof. Christos Comninellis at the EPFL Institute, Switzerland. From 2005 to 2008, he has served as a faculty member in the Department of Chemistry at the University of Milan. In 2008, he also moved to Brazil where he currently is an Associate Professor in the Institute of Chemistry at the Federal University of Rio Grande do Norte. He was awarded with the “Oronzio and Niccolò De Nora Foundation Prize” by the Italian Chemical Society (2005) and the “Oronzio and Niccolò De Nora Foundation Prize on Environmental Electrochemistry” by the International Society of Electrochemistry (2009). The Mexican scientist was also recognized by the German Government in 2009 with the “Green Talent Award” for his contributions in the field of water disinfection-treatment by new electrochemical technologies. During these years, he has studied and worked in Mexico, conducted research in Switzerland, and both taught and researched in Italy, Chile, and Brazil. He has published 120 publications in international journals, 6 book chapters, and coauthored the book entitled Synthetic Diamond

Ignasi Sirés obtained his Ph.D. degree in Chemistry in 2007 from the Universitat de Barcelona (UB, Spain). He also became a Materials’ Engineer after conducting studies at the UB and the Universitat Politècnica de Catalunya. He has undertaken postdoctoral stays and AJ


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Spanish Government (MINECO, Ministerio de Economiá y Competividad) through projects CTM2013-45612-R, FEDER 2007-2013 PP201010, CTQ2013-48897-C2-1-R, and INNOCAMPUS. University of Palermo−Italy (FFR Scialdone 2012/ 2013) is acknowledged for its financial support. O.S. is also indebted to Professors Giuseppe Silvestri, Giuseppe Filardo, Christian Amatore, and especially, Alessandro Galia for their help and continuous encoraugement.

professor-researcher positions at Università degli Studi di Genova, Université Paris-Est, University of Southampton in the U.K., and Universidad de Guanajuato. Dr. Sirés was awarded the Oronzio and Niccolò De Nora Foundation Prize in Environmental Electrochemistry in 2010 (International Society of Electrochemistry), the CIDETEC 2011 Prize for Young Researchers in Electrochemistry (Spanish Electrochemistry Group of the Spanish Royal Society of Chemistry), and the prestigious Carl Wagner Medal of Excellence in Electrochemical Engineering 2014 [Working Party on Electrochemical Engineering (WPEE) of the European Federation of Chemical Engineering (EFCE)]. In September 2009, he became an Assistant Professor at the Department of Physical Chemistry in the Faculty of Chemistry of the UB, carrying out his research with Prof. Enric Brillas at the Laboratory of Materials’ and Environmental Electrochemistry. Since September 2014, he has worked as a Tenured Assistant Professor in the same department. His research interests mainly focus on all aspects of environmental electrochemistry for wastewater treatment: the development of new electrode materials, the study of reaction intermediates, the process scale-up, the study of coupled cathodic/anodic processes, and the coupling with solar energy and microorganisms. His major efforts have been devoted to the electrochemical advanced oxidation processes based on Fenton’s reaction chemistry.

GLOSSARY ACF activated carbon fiber AO anodic oxidation AOP advanced oxidation process BDD boron-doped diamond c0 and cf initial and final concentrations of the organic pollutant or COD values, respectively CE current efficiency CFD computational fluid dynamics CNT carbon nanotube COD chemical oxygen demand CSTR continuous stirred tank reactor DCA 1,2-dichloroethane DSA dimensionally stable anode (registered trademark) DSA-Cl2 DSA used in the chlor-alkali industry for chlorine evolution DSA-O2 DSA used for promoting the OER Ecell potential difference between the anode and cathode EAOP electrochemical advanced oxidation process EC specific energy consumption EC electrocoagulation ECH electrocatalytic hydrodechlorination ED electrodialysis EF electro-Fenton EFl electroflotation EO electrochemical oxidation or electro-oxidation EOI electrochemical oxidation index ER electroreduction ESR electron spin resonance F Faraday constant (96487 C mol−1) GAC granular activated carbon GCE general current efficiency GDE carbon-PTFE gas-diffusion electrode HAAs haloacetic acids HER hydrogen evolution reaction km mass-transport coefficient I current i current density ilim(t) limiting current density at time t iappl applied current density ICE instantaneous current efficiency ICEAC instantaneous current efficiency for the formation of active chlorine IEO indirect electro-oxidation M anode surface MCAA monochloroacetic acid MCE mineralization current efficiency Med mediator species MO metal oxide electrode MMO mixed metal oxide electrode NPs nitrophenols OER oxygen evolution reaction ORP oxidation−reduction potential ORR oxygen reduction reaction

Onofrio Scialdone studied Chemical Engineering at the University of Palermo. He obtained his Ph.D. degree in Electrochemical Engineering in the Politecnico di Milano in 1999 and a Master in Economy at the Scuola Mattei of ENI (2000). He is Professor of Industrial Chemistry at Università degli Studi di Palermo, and he leads the research activities on electrochemistry of the “Laboratory of Chemical and Electrochemical Technologies” at the same university. His main research interests are in electrochemical reactions engineering, with particular emphasis on synthesis of fine chemicals, environmental protection, and electrical energy generation. He has actively contributed on both direct and indirect oxidation processes and on their modelling and, lately, on particular kinds of cells such as the microfluidic ones. He is a member of the editorial board of ChemElectroChem, and he has been involved in various research programs funded by the European Community, private companies, and the Italian Government. At present, he is the vice-dean of Chemical Engineering at the University of Palermo.

ACKNOWLEDGMENTS C.A.M.-H. thanks National Council for Scientific and Technological Development (CNPq-446846/2014-7 and CNPq-401519/2014-7, Brazil) for funding. He is also indebted to Dr. Sergio Ferro and Professors Achille De Battisti (University of Ferrara, Italy) and Enric Brillas (Universitat de Barcelona, Spain) for encouraging him to continue this research. M.A.R. and I.S. thank funding from the EU and the AK


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