Metal oxide-coated anodes in wastewater treatment

Abstract

Electrochemical oxidation is an effective wastewater treatment method. Metal oxide-coated substrates are commonly used as anodes in this process. This article compiles the developments in the fabrication, application, and performance of metal oxide anodes in wastewater treatment. It summarizes the preparative methods and mechanism of oxidation of organics on the metal oxide anodes. The discussion is focused on the application of SnO₂, PbO₂, IrO₂, and RuO₂ metal oxide anodes and their effectiveness in wastewater treatment process.

Introduction

Dyes, toxic organic molecules, inorganic salts, adsorbable organohalogens (AOX), aromatic pesticide residues, drugs, and surfactants are the major components of wastewater released from textile, leather, pulp and paper, printing, photograph, cosmetic, pharmaceutical, and food industries (Fornazari et al. 2012; Zaviska et al. 2009). The environmental concern has led to stringent rules and it is mandatory to treat the wastewater before disposing into natural water streams. Chemical, physicochemical, biological/enzymatic, advanced oxidation processes, and electrochemical methods are commonly employed in wastewater treatment (Brungs et al. 1996; Cheng-chun and Jia-fa 2007; Guinea et al. 2009; Hou et al. 2009; Martínez-Huitle and Brillas 2009; Rosales et al. 2009). In the past few years, “electrochemical method” has drawn much attention. It is economical and easy to construct and operate. More importantly, electrodes serve as immobilized active surfaces for the oxidation of organics (Fornazari et al. 2012). In addition, unlike chemical methods, electrochemical method does not necessitate the addition of chemical reagents. However, the addition of electrolyte is inevitable if the wastewater sample is not sufficiently conducting. In many cases, the electrolyte is deliberately added to enhance the degradation process. The added electrolyte strongly influences the efficiency of the process. In exploiting this method as a powerful tool in wastewater treatment, the key part of the electrolytic system—“electrode”—acquires utmost importance.

On oxidation, the organic contaminant undergoes complete oxidation to CO₂ and H₂O (combustion) or results in the formation of simpler fragments (conversion) (Comninellis 1994). On the other hand, reduction of organics cannot completely lower the chemical oxygen demand (COD) of the wastewater sample, but only leads to poor decontamination (Martínez-Huitle and Brillas 2009). Electrooxidation favors the effective removal of organics over electro-reduction.

Anode is the oxidation site in any electrolytic system. Long service life, large surface area, wide operating potential window between hydrogen and oxygen evolution reaction (OER) overpotentials, high catalytic activity, physically stable, resistant to corrosion, cheap, and easy to fabricate are the prerequisite properties intended to be fulfilled by the anodes employed in electrochemical wastewater treatment.

The anode materials like stainless steel, glassy carbon, Ti/RuO₂, Ti/Pt-Ir, activated carbon fibers, MnO₂, Pt carbon black, porous carbon and reticulated vitreous carbon, and electrodes made of lead, iron, and graphite usually undergo deformation during electrolysis (Martínez-Huitle and Ferro 2006; Rodgers et al. 1999). Beer introduced the titanium substrate-based RuO₂ and TiO₂-coated electrode, referred as dimensionally stable anodes (DSA®s) which possessed high catalytic activity and stability (Beer 1980; Chandler et al. 1997). Figure 1 is the flow chart showing the development of DSA® and DSA®-type metal oxide electrodes (Chandler et al. 1997; Chen et al. 2001, 2002; Klamklang et al. 2012; Nikolić et al. 2012; Panić and Nikolić 2007; Trasatti 2000). The DSAs are good conductors, exhibit high catalytic activity, dimensionally stable, and contribute towards energy savings (see Table 1). They exhibit extended operating life of 8–9 years, without deformation, and their durability has been established in many fields (Chandler et al. 1997; Chen et al. 2002).

Fig. 1

Flow chart of development of DSA® and DSA®-type metal oxide electrodes

Table 1 Performance data of different anode materials used in chlor-alkali industry

The metal oxide electrodes with stability comparable with DSA tempted several research teams to investigate their performance in the electrochemical wastewater treatment. The metal oxide electrodes prepared by Comninellis and Vercesi (RuO₂, IrO₂, TiO₂, ZrO₂, and Ta₂O₅) over Ta substrate exhibited high oxygen evolution overpotential. But, due to high cost of Ta, the other metal Ti was selected as the substrate, and ever since, Ti has been the favorite substrate for the fabrication of metal oxide-coated anodes. Titanium is a cheap “self-healing” substrate which anodically generates a layer of TiO₂ and protects itself against corrosion (Beer 1980). Moreover, the water quality is not affected by the dissolution of Ti in low concentrations (Niu et al. 2012). Ti/tin dioxide (SnO₂), Ti/lead dioxide (PbO₂), and platinum group metal oxide coated on valve metals (Ti, Zr, Ta, Nb, Hf, W) (DSA®s) are the exhaustively studied metal oxide electrodes.

Numerous combinations of these modified metal oxides have been fabricated and tested for their stability, catalytic activity, and ability to incinerate the organics. The ultimate goal is to fabricate an electrode which satisfies the prerequisite properties of the anode. Though few research teams have been successful in fabricating the electrodes with excellent catalytic activity and stability, the search for the best recipe still continues! This review outlines the developments in the preparation, properties, and application of metal oxide-coated anodes in the electrochemical wastewater treatment.

Fabrication of metal oxide anodes

The metal oxide anodes are cheap and easy to fabricate (Gaber et al. 2012; Klamklang et al. 2012; Kusmierek et al. 2011; Yang et al. 2009). Designing and fabricating an electrode of desired composition and characteristics is a challenging task. Thin film metal oxide coatings are prepared by solution phase and gas phase chemical methods. The solution phase method involves dip coating, spin coating, painting, spray pyrolysis, and sol–gel techniques, which employ a precursor solution (Perednis and Gauckler 2005). The gas phase methods are chemical vapor deposition (CVD) and atomic layer deposition (ALD) (Perednis and Gauckler 2005). Electrochemical method (electrochemical anodization and electrochemical deposition) is also widely utilized in the preparation of metal oxide-coated electrode.

A precursor solution is generally prepared by dissolving stoichiometric quantities of metal salts in a solvent or mixture of solvents. The resultant solution is applied on the surface of a substrate using suitable coating techniques [dip coating (Watts et al. 2008; Yang et al. 2009), spin coating (Fierro and Comninellis 2010), or painting (del Río et al. 2009)]. The spin coating technique gives uniform coat on flat substrates. For uneven substrates, dip coating or painting is preferred. The process of coating and drying is repeated until a desired thickness of the coat is obtained. Finally, the coated substrate is annealed to obtain superior morphological characteristics and wear resistance (Comninellis and Vercesi 1991). Thermal decomposition method was adopted in the fabrication of Ti/SnO₂-Sb₂O₃ (Yang et al. 2009), Ti/TiO₂-RuO₂ (Kusmierek et al. 2011; Fachinotti et al. 2007), and Ti/SnO₂-Pt-antimony (Sb) (del Río et al. 2009) electrodes.

Spray pyrolysis is an aerosol process. It is relatively simple and cost effective (Lipp and Pletcher 1997; Perednis and Gauckler 2005; Vicent et al. 1998; Yusta et al. 1997). It involves the atomization of precursor and conversion of these droplets into solid particles by heating. The nature of the deposit obtained on the substrate is dependent on the transformations undergone by the droplets. The temperature gradient between the spraying nozzle and the substrate, the carrier gas and precursor solution flow velocity, and shape and nature of the substrate influence the transformations undergone by the droplets (Correa-Lozano et al. 1996a, 1997). Correa-Lozano et al. (1996a) prepared Ti/SnO₂-Sb₂O₅ electrode using spray pyrolysis technique and have thoroughly studied its characteristics (Correa-Lozano et al. 1996b, 1997).

The sol–gel method is also widely used in the fabrication of metal oxide anodes (Attia et al. 2002; Brinker et al. 1991; Choi et al. 2007; Panić et al. 2003a, b, 2010; Lu et al. 2010; Ozer et al. 1995). In this method, the microstructure of the coating depends on pH and viscosity (Ozer et al. 1995). Generally, the precursor salts are hydrolyzed by dispersing them in an appropriate media and vigorously stirred for certain ageing time. The formation of solid phase particles of the sol from hydrolyzed precursor is dependent on ageing time. Electrodes prepared at different ageing time differ in their electrochemical behavior (Panić et al. 2003a).

A uniform metal oxide coating of desired thickness can be achieved by electrochemical method by adjusting its operating conditions. In electrochemical anodization, a pretreated metal substrate is anodized in a suitable electrolytic bath. A layer of metal oxide coat is generated on the surface of the anodized metal. In electrochemical deposition, precursor salts are dissolved in the electrolytic bath, which are electrodeposited on the substrate. The electrochemical method is advantageous in achieving effective doping and a uniform metal oxide coat on the substrate. Electrochemical method was adopted in the preparation of the following electrodes: Ti/SnO₂-Sb₂O₃-Nb₂O₅/PbO₂ (Yang et al. 2009), Er-chitosan-F-PbO₂ (Wang et al. 2010b), C/PbO₂, Pb + Sn/PbO₂-SnO₂, Pb/PbO₂ (Gaber et al. 2012), Ti/PbO₂ (Panizza and Cerisola 2008), Ti/SnO₂-Sb₂O₅/PbO₂ (Liu and Liu 2008), Pb/PbO₂ (Martínez-Huitle et al. 2008), and Ti/SnO₂-Sb/PbO₂-Ce (Niu et al. 2012).

The gas phase techniques are also well known, but they require sophisticated instruments and are relatively costly. In CVD, generally the reactant (precursors) gases are pumped into the reaction chamber maintained at optimum temperature and pressure, where these reactive gases adsorb and react with the substrate to form a thin film. Metal organic chemical vapor deposition utilizes organometallic compounds as precursors (Amjoud et al. 1998; Duverneuil et al. 2002; Devilliers et al. 2003). The ALD technique is closely related to CVD, but the factor which distinguishes it from CVD is that the precursors are led on to the substrate alternately, one at a time (Riihelä et al. 1996). Using CVD, thin films with high uniformity can be reproduced. However, it involves complex processes, high temperature, and toxic gases. Few teams have used gas phase methods for the preparation of thin film metal oxide-coated electrodes (Watts et al. 2008; Yusta et al. 1997; Amjoud et al. 1998; Duverneuil et al. 2002). However, the earlier studies revealed that in majority of the cases, solution method was preferred for the preparation of metal oxide electrodes. The conditions used in the fabrication of different metal oxide-coated electrodes by solution method are presented in Table 2.

Table 2 Fabrication of metal oxide-coated anodes

Mechanism of electrochemical oxidation of organics on metal oxide anodes

Figure 2 displays various electrochemical methods used for wastewater treatment. The electroflocculation and electrocoagulation methods produce large amount of sludge, whereas such problem is not encountered in the case of electrooxidation (Mao et al. 2008). The organics in the wastewater are converted into simple molecules and/or completely incinerated to CO₂ and H₂O by electrochemical oxidation (Comninellis 1994). The morphology and composition of the metal oxide coating, nature of the contaminating organic molecule, and wastewater matrix influence the mechanism of oxidation of organics and hence the products (Martínez-Huitle and Ferro 2006).

Fig. 2

Electrochemical methods. Adapted from Martínez-Huitle and Brillas (2009)

Mechanism of oxidation

Oxidation of organics proceeds as follows:

• By direct electron exchange with the anode (direct oxidation)

• By the active species during electrolysis (indirect oxidation)

The direct exchange of electrons between the anode material and the organic molecule at the electrode solution interface is referred as “direct oxidation.” This reaction is viable at potentials well below the oxygen evolution overpotential. But the anode surface gradually undergoes passivation due to adsorption of organic molecules (Panizza and Cerisola 2009).

A theoretical model for the mechanism of oxidation of organics on metal oxide anodes was proposed by Comninellis (1994), and it has been accepted by the scientific community. During electrolysis, hydroxyl (^•OH) radicals are generated on the anode surface by water discharge at high anodic potentials (Panizza and Cerisola 2009; Watts et al. 2008). These ^•OH radicals adsorb physically (Eq. 1) or chemically (Eq. 2) on the metal oxide anode (MO _x ).

$$ {\mathrm{MO}}_x+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{MO}}_x\left(\cdot \mathrm{OH}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(1)

$$ {\mathrm{MO}}_x\left({}^{\cdotp}\mathrm{OH}\right)\to {{\mathrm{MO}}_x}_{+1}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(2)

As a result of strong interaction between hydroxyl radical (^•OH) with the anode metal and availability of a higher oxidation state for the anode metal leads to the formation of MO _x + 1 (Martínez-Huitle and Ferro 2006). Electrodes which can undergo such interaction are referred as “active” (carbon, graphite, IrO₂, RuO₂, platinum) (Panizza and Cerisola 2009). “Non-active” anodes (SnO₂, PbO₂, or boron-doped diamond (BDD)) only act as inert electron sink for the removal of electrons from organics, without providing any adsorptive site (Comninellis 1994; Ozer et al. 1995). Chemisorbed oxygen in MO _x + 1/MO _x couple acts as an active site for the selective oxidation of organics (Eq. 5), whereas physisorbed ^•OH radicals completely incinerate the organics into CO₂ and H₂O (Eqs. 3 and 4) (Comninellis 1994).

$$ {\mathrm{MO}}_x\left({}^{\cdotp}\mathrm{OH}\right)+\mathrm{RH}\to {\mathrm{MO}}_x+{\mathrm{H}}_2\mathrm{O}+{\mathrm{R}}^{\cdotp } $$

(3)

$$ {\mathrm{MO}}_x\left({}^{\cdotp}\mathrm{OH}\right)+{\mathrm{R}}^{\cdotp}\to {\mathrm{MO}}_x+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O} $$

(4)

$$ {{\mathrm{MO}}_x}_{+1}+\mathrm{R}\to {\mathrm{MO}}_x+\mathrm{Intermediates} $$

(5)

The oxygen evolution reaction (Eqs. 6 and 7) always competes with the oxidation reaction of organics.

$$ {\mathrm{MO}}_x\left({}^{\cdotp}\mathrm{OH}\right)\to {\mathrm{MO}}_x+\mathrm{\frac{1}{2}}\kern0.5em {\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(6)

$$ {{\mathrm{MO}}_x}_{+1}\to {\mathrm{MO}}_x+\mathrm{\frac{1}{2}}\kern0.5em {\mathrm{O}}_2 $$

(7)

Figure 3 is the schematic diagram of electrochemical oxidation of organics on metal oxide anodes.

Fig. 3

Electrochemical oxidation of organics on metal oxide MO _x anode. 1 Water discharge. 2 Combustion of R (organic molecule). 3 and 6 O₂ evolution. 4 Higher oxide formation. 5 conversion of R to RO. Adapted from Comninellis (1994) and Simond et al. (1997a).

Oxidation by electrochemically generated oxidants

On electrolysis, species generated by the discharge of water (hydroxyl radical, hydrogen peroxide, ozone) react with themselves or with the electrolytes to produce active oxidants. These oxidants are utilized in the oxidation of organics and this type of oxidation is referred as “indirect oxidation” (Jüttner et al. 2000; Li et al. 2009; Martínez-Huitle and Ferro 2006; Panizza et al. 2000; Scialdone 2009; Scialdone et al. 2009b; Simond et al. 1997b; Zhao et al. 2003). Mediated electrooxidation involves metal ions with high oxidation potential. In this method, oxidants are electrochemically generated in a closed cycle and can completely incinerate the organics (Jüttner et al. 2000).

Different electrolytes could be found in any industrial effluent. Chloride salts are most common. It is observed that the presence of chloride (Cl⁻) ions in the wastewater increases the organic removal efficiency (Comninellis 1994; Scialdone et al. 2009b). Electrochemically generated active chlorine species efficiently oxidize the organics as per Eqs. 8–11 (Jüttner et al. 2000; Martínez-Huitle and Ferro 2006; Panizza et al. 2000; Scialdone et al. 2009b).

$$ \begin{array}{ll}2{\mathrm{Cl}}^{-}\to {\mathrm{Cl}}_{2\left(\mathrm{aq}\right)}+2{\mathrm{e}}^{-}\hfill & \left( E \mathit{^{\circ}}=1.40\;\mathrm{V}\;\mathrm{vs}.\kern0.62em \mathrm{SHE}\right)\hfill \end{array} $$

(8)

$$ \begin{array}{ll}{\mathrm{Cl}}_{2\left(\mathrm{aq}\right)}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{HOCl}+{\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}\hfill & \left( E \mathit{^{\circ}}=1.49\;\mathrm{V}\;\mathrm{vs}.\kern0.62em \mathrm{SHE}\right)\hfill \end{array} $$

(9)

$$ \begin{array}{ll}\mathrm{HOCl}\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{OCl}}^{-}\hfill & \left( E \mathit{^{\circ}}=0.89\;\mathrm{V}\;\mathrm{vs}.\mathrm{SHE}\right)\hfill \end{array} $$

(10)

$$ \mathrm{R}+{\mathrm{OCl}}^{-}\to \mathrm{Intermediates}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{Cl}}^{-} $$

(11)

The equilibrium in Eq. 10 is pH dependent. HOCl predominates in the pH range 3–8 and ClO⁻ predominates above pH 8 (Fornazari et al. 2012; Malpass et al. 2012). Hence, oxidation of organics by HOCl is favorable at near neutral pH (Fornazari et al. 2012; Malpass et al. 2012). The Cl₂/Cl⁻ and HOCl/Cl₂ couples also oxidize the organics completely (Fornazari et al. 2012; Song et al. 2010b). The electrochemical reactions at the anode surface involving chloride ions are given in Eqs. 12 and 13 (Scialdone et al. 2009b):

$$ {\mathrm{MO}}_x\left(\cdot \mathrm{OH}\right)+{\mathrm{Cl}}^{-}\to {\mathrm{MO}}_x\left(\mathrm{HOCl}\right)+{\mathrm{e}}^{-} $$

(12)

$$ {\mathrm{MO}}_x\left(\mathrm{HOCl}\right)+\mathrm{R}\to \mathrm{Intermediates}\to {\mathrm{MO}}_x+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{Cl}}^{-}+{\mathrm{H}}^{+} $$

(13)

MO _x (HOCl) mediates unselective oxidation and thereby provides substantial total organic carbon (TOC) removal (Fornazari et al. 2012). However, the presence of chloride ions is associated with the following disadvantages: the formation of AOX (Malpass et al. 2012; Sakalis et al. 2007) and the generation of ClO^–. AOXs are toxic and recalcitrant to degradation, and ClO⁻ reduces the service life of the metal oxide anode (Costa et al. 2010). Several other electrolytes like ClO₄ ⁻, NO₃ ⁻, PO₄ ²⁻, SO₄ ²⁻, CO₃ ²⁻, and Br⁻ were tested for electrochemical degradation process (Fornazari et al. 2012; Gaber et al. 2012). But the degradation efficiency in presence of Cl⁻ ions was found to be better than that achieved in the presence of these electrolytes. The peroxidisulfate is a powerful oxidizing agent and as effective as hydroxyl radical (Malpass et al. 2012). Excellent degradation of organics is possible in presence of sulfate ions, provided that the electrode and the wastewater environment matrix favors the formation of sulfate radical or peroxidisulfate ion (Martínez-Huitle et al. 2008; Martínez-Huitle et al. 2008; Brillas et al. 2009). However, the COD/TOC removal not only depends on the electrode and electrolytes, but also on the organic molecule itself (Marugán et al. 2007).

Factors influencing the anode performance

The performance of an electrode is rated in terms of its ability to incinerate the organics with minimum consumption of energy and time. The nature of the electrode, applied current density, duration of electrolysis, supporting electrolyte, pH, temperature, and cell geometry significantly contribute to the efficiency and energy consumption of the electrochemical oxidation process. The nature of the electrode is dependent on its fabrication technique, composition, and structure.

Fabrication technique

Both physical (surface morphology, metal oxide film thickness, composition, dimension, microstructure) (Wang et al. 2009) and chemical (availability of higher oxidation state, its affinity towards hydroxyl radicals, stability in different pH, active species generated in presence of different electrolytes, and wear-tear) properties of an anode influence its ability to oxidize the organics (electrochemical activity) or to generate active oxidants (electrocatalytic activity). The fabrication technique can modulate both the chemical and physical nature of the coating and hence its electrocatalytic and electrochemical activities.

Proper substrate pretreatment is the key to produce a strong adhesion of metal oxide coating on the surface of the substrate (Beer 1980). The solvents used in the precursor solution, dopant and its content, coating technique, dip withdrawal rate, and annealing temperature are crucial factors which contribute in altering the characteristics of an anode. The schematic representation of the steps involved in the fabrication of metal oxide anodes by thermal decomposition is shown Fig. 4 (Comninellis and Guohua 2010).

Fig. 4

Schematic diagram of steps involved in the fabrication of metal oxide electrodes by thermal decomposition technique. Adapted from Comninellis and Guohua (2010)

The solvents used in the preparation of precursor solution play a role of great consequence in altering the morphology and hence the characteristics associated with the morphology (exposure of catalytic and adsorptive sites, porosity, and effective surface area) of the metal oxide coating. The difference in the thermal expansion coefficient of the base metal and the coating leads to the formation of cracks in the coating and results in typical cracked mud coating (Jiang-tao et al. 2007; Wang et al. 2009). Cracked mud coating gradually undergoes erosion. The adsorption capacity is favored by the irregular electrode surface. But it involves undesired strong adsorption of hydroxyl radicals as well. Weak interaction between the electrode surface and hydroxyl radicals increases electrochemical activity of the electrode towards the oxidation of organics, whereas strong interaction results in oxygen evolution (Costa et al. 2010; Kapałka et al. 2008). The efficiency of the process decreases with oxygen evolution.

Three different precursor solutions prepared in ethanol, n-butanol and a polymeric precursor were used in the fabrication of SnO₂ + Sb₂O₃ interlayer by thermal decomposition (Wang et al. 2009). The scanning electron microscopy (SEM) images in Fig. 5 clearly reveal the morphological differences in the oxide interlayer obtained from these precursor solutions. The oxide interlayer from the polymeric precursor assisted the structural alignment and produced more agglomerated coating, whereas alcoholic precursors produced typical cracked mud coating.

Fig. 5

SEM micrographics of SnO₂ + Sb₂O₃ coatings prepared with different precursor solvents: a polymeric precursor, b ethanol, and c n-butanol. Reprinted from Wang et al. (2009). Copyright 2008 Elsevier

The nature of the precursor solution can significantly affect the host metal and dopant content in the coating. It was found that viscous precursor solutions aid in the formation of agglomerated uniform coating and accumulation of the dopant on the surface of the coating, whereas alcoholic precursor solutions lead to cracked mud coating. Precursor solution prepared in isopropanol and sol–gel was used in the fabrication of Sb-SnO₂/Ti electrode by thermal decomposition. The Sn/Sb content in the coating was found to be higher than in the alcoholic precursor solution (Wang et al. 2010a; He and Mho 2004). On the contrary, the Sb content was found to be higher in the coating than in the solution when sol–gel method was adopted (Wang et al. 2006).

Antimony is the most commonly used dopant in the preparation of SnO₂ electrodes and it is the hydroxyl radical generation site (Jiang-tao et al. 2007). Hence, its accumulation on the surface of the coating is recommended. Variation in the Sb content considerably influences the morphology of the coating.

The electrocatalytic activity can be enhanced by minimizing the interlayer thickness and maximizing the outer active surface area by choosing the right combination of techniques in the preparation of metal oxide electrodes (Yeo et al. 2010). Different preparative methods were used in combination by Yeo et al. (2010) to obtain a compact layer of metal oxide coat. They prepared Ti/Pb(E)/PbO₂(T,E) (E-electrodeposition, T-thermal decomposition) electrode by electrodepositing a layer of Pb on Ti, followed by coating a layer of PbO by thermal decomposition. The PbO was then electrooxidized to PbO₂. This electrode acquired large surface area and roughness. The relative roughness factor of this electrode was found to be 8.98, where the arbitrary roughness factor of Ti/Pb(E)/PbO₂ (E) electrode was set as unity. Watts et al. (2008) found that the dopant content, coating technique, and the annealing temperature are the major factors which contribute significantly in controlling the characteristics of a metal oxide coat. The effect of number of layers of the coating and dip withdrawal rate on the performance of the electrode in eliminating the organics was found to be very feeble.

Interfacial layer or undercoat

The oxygen which evolved at the anode surface creeps through the metal oxide coating and passivates the substrate metal. This is eliminated by the inclusion of an interfacial layer connecting the active metal oxide coating and the substrate. The chemical stability, resistance against corrosion, and service life of the anode are enhanced by the insertion of an interlayer (Kong et al. 2012). An undercoat of stable metal oxide is applied on a substrate and then an outer active layer is overlaid. It is represented as substrate/interlayer/active surface layer as shown in Fig. 6. The interlayer supports the active metal oxide layer to adhere strongly onto the substrate and helps in guarding the stability and enhancing the electrocatalytic activity of the electrode. It is important for the interlayer to be highly conducting and itself does not undergo passivation. The constituent metals of the interlayer should satisfy the Hume-Rothery limit (15 %) between their ionic radii, so that a solid solution between the three layers is possible.

Fig. 6

Representation of multilayer metal oxide-coated substrate

The direct deposition of PbO₂ on Ti substrate by electrodeposition leads to the formation of a passive layer of TiO₂ (Yang et al. 2009). This can be avoided by depositing an interlayer on Ti substrate. The interlayer acts as a barrier against such passivation of the substrate by avoiding the diffusion of oxygen. For the interlayer in Ti/solid solution interlayer/PbO₂ electrode, the metals chosen by Kong et al. (2012 were such that the ratios of their corresponding ionic radii (Sn/Sb, Ru/Ti, Ir/Ta) were below the Hume-Rothery limit. Figure 7 is the SEM images of the interlayer surfaces SnO₂-Sb₂O₅, RuO₂-TiO₂, and IrO₂-Ta₂O₅ (Kong et al. 2012). The ionic radii ratio of the metals in the interlayer to Ti⁴⁺ obtained from the oxidation of Ti substrate also lays within the limit; hence, excellent binding was obtained between the interlayer and the substrate. Similarly, the interlayer and the electrodeposited PbO₂-active layer formed a continuous solid solution, except in the case of RuO₂-TiO₂ interlayer, which provided only partial solid solution with the PbO₂ due to large difference in the ionic radii of Ti⁴⁺ and Pb⁴⁺. The interlayer enhanced the orientation of the crystal structure of the electrodeposited PbO₂. Ti/IrO₂-Ta₂O₅/PbO₂ electrode coating showed high catalytic activity for oxygen evolution. The inclusion of IrO₂-Ta₂O₅ interlayer in Ti/PbO₂ electrode increased its service life from 11 to 672 h as determined by the accelerated lifetime test in 1 M H₂SO₄ under galvanostatic condition (4 A cm⁻²). RuO₂-TiO₂ and SnO₂-Sb₂O₅ interlayers showed intermediate enhancement in service life of 207 and 58 h, respectively. The interlayer effectively eliminated the passivation of Ti substrate and enhanced the performance of the anode towards oxidation of organics. Similar results were obtained from the electrodes with interlayers of different composition (see Tables 2, 3, and 4).

Fig. 7

SEM images of the interlayer surface. a Ti/SnO₂-Sb₂O₅. b Ti/RuO₂-TiO₂. c Ti/IrO₂–Ta₂O₅. Reprinted from Kong et al. (2012). Copyright 2012 Springer Science + Business Media B.V.

Table 3 Performance of different modified SnO₂ anodes and working conditions

Table 4 Performance of different modified PbO₂ anodes and working conditions

Metal oxides in wastewater treatment

Several mixed metal oxide electrodes have been used in the electrochemical wastewater treatment. Nevertheless, it is found from the literature that majority of these are the derivatives of four metal oxides, SnO₂, PbO₂, RuO₂, and IrO₂, and hence we limit our discussion to these metal oxide electrodes.

SnO₂

Tin oxide is an insulator in its stoichiometric form. The distortions created by the oxygen vacancies make it an n-type and conductive material. Doped SnO₂ anodes (non-active) show excellent conductivity, high oxygen and chlorine evolution overpotential, electrocatalytic activity for the oxidation of organic compounds, ability to adhere strongly to the base metal, and stability over a large pH range (Correa-Lozano et al. 1997; del Río et al. 2009; Jiang-tao et al. 2007; Lipp and Pletcher 1997; Vicent et al. 1998; Xu et al. 2012; Yusta et al. 1997). The deviation in the native defects, nature, and effect of doping into the crystal lattice influences the activity of SnO₂. The oxygen vacant sites are responsible for the conductivity of SnO₂ and also the availability of these vacant sites determines the ratio of MO _x + 1 and MO(^•OH) on the anode surface. If the crystal defect increases with the formation of more of oxygen vacant sites, the formation of MO _x + 1 is favored. The physically adsorbed hydroxyl radical reacts chemically to fill the vacant sites of the crystal lattice to form MO _x + 1. MO _x + 1 favors only the conversion of organics into oxidized intermediates, whereas MO(^•OH) completely incinerates the organics into CO₂ and H₂O (Feng et al. 2008). The electrocatalytic activity of Ti/SnO₂ electrode is higher than that of PbO₂ and RuO₂/IrO₂ electrodes (Radjenovic et al. 2011; Mao et al. 2008). Moreover, it is comparatively cheaper than the noble metal oxide electrodes and easy to fabricate (Mao et al. 2008).

The empty electron orbitals available on the f-block elements can be used as channels for electron transaction from the substrates, if doped into metal oxide coating (Fenga and Li 2003). Electrocatalytic activity of the electrodes for the oxidation of organics markedly improves on doping (Fenga and Li 2003). The dopant and its molar ratio with the host metal oxide influence the physicochemical properties of the electrode. Among the various dopants (B, Bi, Ce, F, Fe, Gd, Ir, La, Ni, Pt, Ru, and Sb) used to enhance the characteristics of the SnO₂, Sb has shown remarkable impact (Jiang-tao et al. 2007; Costa et al. 2010). Sn and Sb ionic radii satisfy the Hume-Rothery limit, and hence, a homogeneous solid solution between the two metals is possible. A list of modified SnO₂ electrodes and conditions used in their fabrication is given in Table 2.

The content of the dopant has a critical role to play in directing the microcrystalline structure, morphology, electrocatalytic activity, and capability of generation of hydroxyl radicals and hence the oxidation of organics. Ti/SnO₂-Sb electrode possesses high OER potential which varies with the composition of Sn/Sb ratio. For the electrodes prepared with varying concentrations of Sb, OER showed potential greater than 1.9 V (vs standard calomel electrode (SCE)) in all the cases and it increased with an increase in Sb content (Mao et al. 2008). The highest catalytic activity was found with 8 mol % [SnO₂-Sb(0.08)] and generated high concentration of oxidants.

The competition between the oxidation reaction and the OER resulted in low efficiency of Ru_0.7Ir_0.3O₂ anode to degrade β-blocker metoprolol and led to the persistence of halogenated derivatives, whereas on SnO₂-Sb electrode, the production of hydroxyl radical was sufficient enough to degrade the metoprolol into simpler intermediates (Radjenovic et al. 2011).

Jiang-tao et al. (2007) doped SnO₂ with 5, 10, and 15 % of Sb and used these electrodes in the electrooxidation of 4-chlorophenol. The oxygen evolution potential for Ti/Sb-SnO₂ with 5 % Sb electrode skipped from 1.5 V (without Sb) to 1.65 V. The oxidizable species in this potential window are oxidized directly on the anode surface. COD removal (48.9 %) of 4-chlorophenol with 15 % current efficiency was obtained on 5 % Sb-doped electrode, whereas for 0, 10, and 15 %, Sb dopant the COD removal was 36.4, 44.8, and 26.6 %, respectively. However, it has also been reported in both cases (Mao et al. 2008; Jiang-tao et al. 2007) that effective and homogeneous distribution of Sb diminished with an increase in Sb content. This was attributed to the difference in the thermal expansion coefficients. This effect is not so strong at lower Sb contents, but at higher content of Sb, the variation in the surface morphology is clearly noticeable. On using high content of dopant, the electrode follows conversion mechanism, due to selective oxidation by the chemisorbed oxygen, as discussed earlier (Kong et al. 2012). At low content of dopant (Sb centers are sites for hydroxyl radical generation), physisorbed hydroxyl radicals are involved in the oxidation leading to complete combustion of the organics. The inhomogeneous distribution of Sb reduces the catalytic activity, and the base metal gets exposed to the electrolytic bath solution leading to its passivation. The dopant not only influences the morphology of the oxide coating, but also controls the mechanism of oxidation of organics on the anode surface. Hence, it is important to choose the right dopant and monitor its content very carefully during the fabrication of an electrode.

Gd (2 mol %)-doped SnO₂-Sb possesses low oxygen vacancies in the crystal lattice of SnO₂ than undoped SnO₂-Sb (Feng et al. 2008). Lower oxygen vacancies enhance the production of hydroxyl radicals on the surface of the electrode and hence the oxidizing ability. Gd covered the electrode surface with an increase in its content and reduced the availability of Sb (Feng et al. 2008). It is worth noting that not only the dopant, but also its content substantially influences the performance of an electrode.

Other dopants like La and Ru are also advantageous in modifying the characteristics of an electrode. On doping SnO₂-Sb with La and Ru, distortion effects led to the hindrance in the crystal growth and the crystallite size was reduced (Xu et al. 2012). The smaller crystallite size not only increases the surface area, but also increases the number of active sites on the surface of the anode. XPS measurements of this electrode revealed two different oxygen 1s peaks, one with lower binding energy (BE) component of 530.08–530.36 eV (assigned to lattice oxygen O_lat) incorporated into SnO₂ lattice and the other higher BE component of 531.86–531.89 eV (assigned to adsorbed oxygen O_ads) is of the weakly bonded oxygen species. The adsorbed oxygen is very reactive towards the oxidation of organics. The O_ads content on the surface of Ti/ SnO₂-Sb-La was found to be the highest (41.78 %), followed by Ti/SnO₂-Sb-Ru (39.42 %) and Ti/SnO₂-Sb (29.13 %) electrodes. Evidently, the addition of La and Ru as dopants substantially enhanced the performance of the Ti/SnO₂-Sb electrode in terms of COD removal and current efficiency (see Table 3).

Zhuo et al. (2011) reported that the service of Ti/SnO₂-Sb electrode doubled on doping with Bi. The degradation of perfluorooctanoic acid (PFOA) was successfully achieved (~90 %) on both Ti/SnO₂-Sb and Ti/SnO₂-Sb-Bi. Liu et al. (2013) have recently shown that the OER potential for the carbon nanotube (CNTs) coated with SnO₂ nanoparticles is higher than that of bulk SnO₂. A comparison was made between SnO₂ (TO-CNT), SnO₂-Sb (ATO-CNT), and SnO₂-Bi nanoparticle-coated CNT (BTO-CNT) for the electrochemical filtration of oxalate. SnO₂-Bi/CNT showed 1.5 to 3.5 times greater current efficiency and 4 to 5 times lesser energy consumption as compared to bare CNT for the electrooxidative filtration of ethanol, methanol, formaldehyde, and formate. Also, BTO-CNT showed higher OER potential and TOC removal as compared to ATO-CNT, suggesting that Bi can be an alternative for Sb. However, Lin et al. (2013a) reported that the Ti/SnO₂-Sb-Ce electrode yielded lower perfluorocarboxylic acid (PFCA) (perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA)) removal and observed secondary products of Sb due to the dissolution.

Modifications in the composition of SnO₂ electrode fabrication and investigation of their performance in the wastewater treatment are summarized in Table 3. The conditions used in the electrochemical wastewater treatment process, maximum COD/TOC removal achieved, and percent current efficiency have been summarized in this table. As mentioned earlier, the extent of degradation of an organic molecule is dependent on its chemical nature, structure, and stability. The data in Table 3 is helpful in selecting a particular electrode for the degradation of different organic pollutant and vice versa.

PbO₂

Generally, there are two kinds of PbO_2. Orthorhombic α-PbO₂ (brown color) and tetragonal β-PbO₂ (black color), where β-PbO₂ is a disordered close packed structure, possessing comparatively higher conductivity and higher overpotential for oxygen evolution are highly preferred (Abaci et al. 2005; Aquino et al. 2010; Gaber et al. 2012; Li et al. 2011; Yang et al. 2009; Zhoua and He 2008). β-PbO₂ is a non-active anode, highly conducting, chemically inert, cheaper than noble metals, and possesses high oxygen evolution overpotential and high catalytic activity for the production of oxidants. Direct oxidation of organics on the surface of PbO₂ anode is allowed (Fenga and Li 2003; Liu and Liu 2008; Yang et al. 2009). Thermal decomposition, electrodeposition (both in acidic and alkali media), and electrooxidation (anodization) are the techniques commonly employed in the fabrication of PbO₂ electrodes (Fenga and Li 2003; Liu and Liu 2008; Martínez-Huitle et al. 2008; Niu et al. 2012; Panizza and Cerisola 2008; Panizza and Cerisola 2009; Sala and Gutiérrez-Bouzán 2012; Soloman et al. 2009; Yang et al. 2009). Liu et al. (2009) have been able to fabricate both kinds of PbO₂ coat on Ti substrate. Pavlov (1992) carried out detailed work on the PbO₂ electrodes and proposed a complex mechanism of formation of hydroxyl radicals on the surface of lead dioxide anodes. A gel-crystal system was suggested which follows the equilibrium shown below (Liu et al. 2009; Pavlov 1992):

$$ \begin{array}{c}\hfill {\mathrm{PbO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{PbO}{\left(\mathrm{OH}\right)}_2\leftrightarrow {\mathrm{H}}_2{\mathrm{PbO}}_3\hfill \\ {}\hfill \begin{array}{ll}\mathrm{Crystal}\ \mathrm{layer}\hfill & \mathrm{Hydrated}\left(\mathrm{gel}\right)\mathrm{layer}\hfill \end{array}\hfill \end{array} $$

(14)

Precisely, according to Pavlov, in the gel zone, the hydrated PbO₂ forms linear polymer chains which allow the electron hopping from one Pb⁴⁺ ion to the other along the chain and from one polymer chain to the other, which accounts for its conductivity. The crystal zone is made up of PbO₂ and allows electrical conductivity. There exists certain number of active centers on the surface of the PbO₂ crystal layer. The hydroxide ions in the solution give out the electrons to these active centers which lead to the formation of hydroxyl radicals and create a weak interactive bond with the active centers. These hydroxyl radicals are utilized in the oxidation of organics (Liu et al. 2009).

PbO₂ as itself has been used in the electrochemical treatment of wastewater and also in its doped form. On adding suitable dopant, PbO₂ exhibits exceptional electrochemical properties. Ni, Bi, Co, Er, and Ce are the most commonly used dopants (Yang et al. 2009; Liu and Liu 2008; Liu et al. 2009; Niu et al. 2012; Wang et al. 2010b). Niobium-doped PbO₂ has showed excellent hydrophilicity and conductivity due to the improvement in its microcrystal structure (Yang et al. 2009). It has also been reported that doping roughened the thin film surface and increased the effective surface area for the generation of hydroxyl radicals (Yang et al. 2009). The effect of bismuth and cobalt as dopants in the fabrication of PbO₂ by electrodeposition was studied by Liu et al. (2009) and Liu and Liu (2008). This electrode was used in the degradation of nitrophenols. The addition of bismuth into the PbO₂ matrix remarkably decreased the crystallite size of the deposit and improved its electrocatalytic activity. Out of Ti/β-PbO₂, Ti/Bi-PbO₂, Ti/Co-PbO₂, Ti/Bi-Co-PbO₂ electrodes, the morphology of Ti/Bi-PbO₂ was found to be compact and porous with small crystallite size. This was attributed to the variations caused by the formation of Bi₂O₅ in the nucleation and crystal growth of the film (Liu and Liu 2008; Liu et al. 2009). In addition, this anode showed the highest oxygen evolution overpotential of 1.75 V (vs. SCE) and stability. Ti/Bi-PbO₂ and Ti/Bi-Co-PbO₂ favored the generation of hydrogen peroxide and hypochlorite ion. In contrast, no effect on the PbO₂ crystallite size was found by doping cobalt. Ti/Bi-PbO₂ showed the highest electrocatalytic activity for the generation of ^•OH. Obviously, the COD removal was maximum in the case of Ti/Bi-PbO₂. Though Ti/Bi-Co-PbO₂ exhibited smallest removal rates, its current efficiency was higher than Ti/β-PbO₂ and Ti/Co-PbO₂. Thus, Bi doping into the PbO₂ matrix dramatically improves its characteristics, but Co doping does not make any such differences.

Similar influence of doping was observed when Er was used as dopant. The Er doping into F-PbO₂ improved the electrocatalytic activity and produced compact coating with decreased crystallite size. Wang et al. (2010b) prepared erbium-doped chitosan-F-PbO₂ electrode and compared it with the electrode without Er doping and without chitosan for the degradation of 2,4-dichlorophenol (2,4-DCP). Maximum 2,4-DCP removal was achieved on the electrode obtained from 10 mM Er electroplating solution. The order of 2,4-DCP removal on different electrodes was found to be Ti/ SnO₂-Sb₂O₃/Er-chitosan-F-PbO₂ > Ti/SnO₂-Sb₂O₃/Er-F-PbO₂ > Ti/SnO₂-Sb₂O₃/F-PbO₂.

PbO₂ was electrodeposited on Ti/SnO₂-Sb(0.08) (8 mol % of Sb) interlayer using different concentrations of NaF in the electrodeposition bath solution (Mao et al. 2008). The NaF concentration not only affected the stability of PbO₂, but also the morphology of the PbO₂ deposit. Without the addition of NaF, the PbO₂ coat obtained consisted irregular crystallite size and shape with dents of varying sizes on the surface. As the concentration of NaF increased, a more compact coat of PbO₂ was obtained. Only 55 % color removal of the substrate AO7 was achieved on PbO₂-F (0.75) electrode, whereas PbO₂-F (0) showed 94 % removal for the same time period. This was attributed to more compact and uniform deposition which resulted with the inclusion of NaF. The higher color removal in case of PbO₂-F (0.75) electrode is due to the rough surface area which facilitated the adsorption of dye. With 0.1 g L⁻¹ of NaF (PbO₂-F (0.1)), the PbO₂ coat obtained was most stable to anodic dissolution in 1 M HOCl₄ (Mao et al. 2008). β-PbO₂ coated on Ti with SnO₂-Sb₂O₃-Nb₂O₅ interlayer showed higher electrochemical activity than Ti/RuO₂ and Ti/Sb-Sn-RuO₂ anodes for the degradation of phenol. The interlayer enhanced the surface area, electrocatalytic activity, conductivity, and hence the efficiency. Evidently, the removal efficiency increased from 78.6 % (Na₂SO₄) to 97.2 % on adding 21.3 g L⁻¹ of NaCl.

Niu et al. (2012) reported that the toxic PFCAs like perfluorobutanoic acid (PFBA), PFPA, PFHxA, PFHpA, and PFOA, which are recently driving attention as serious contaminants in various environmental matrices, can be successfully mineralized by electrochemical method using Ce-doped porous nanocrystalline PbO₂ film electrode (Ti/SnO₂-Sb/PbO₂-Ce). The same electrode provided excellent mineralization of PFNA, PFDA, and sulfamethoxazole as reported by Lin et al. (2013a, b) under mild conditions.

The dissolution of PbO₂ in strong basic conditions and on specific anodic polarization limits its usage in wastewater treatment (Carvalho et al. 2011; Niu et al. 2012). However, by adopting suitable fabrication technique, the wear resistance of PbO₂ can be conditioned (Carvalho et al. 2011) and possible release of Pb²⁺ in acidic conditions can be avoided by the application of suitable anodic current (Niu et al. 2012). Unlike SnO₂, passivation of surface due to the formation of hydroxides is avoided by the continuous production of hydroxyl radicals on the surface of PbO₂ and offers high charge transfer. Table 4 summarizes the comparison between the various modified PbO₂ anodes and conditions used in the electrochemical wastewater treatment process.

Ruthenium oxide and iridium oxide

RuO₂- and IrO₂-based anodes are active anodes, known for their electrocatalytic activity for oxygen and chlorine evolution. IrO₂ is one of the cheapest dimensionally stable anodes and shows low chlorine and oxygen evolution overpotential used in the generation of active chlorine species (Zaviska et al. 2009). Both IrO₂- and RuO₂-coated Ti anodes undergo dissolution in basic medium on anodization (Li et al. 2009). Ti/RuO₂ undergoes corrosion during electrolysis on prolonged usage (Papastefanakis et al. 2010). However, the service life of RuO₂ enhances on doping with Ir (Chandler et al. 1997; Chen et al. 2001, 2002; Duby 1993; Nikolić et al. 2012). The electrochemical properties of iridium (Ti_0.6Ir_0.4O₂/Ti) and ruthenium (Ti_0.6Ru_0.4O₂/Ti) oxide anodes, prepared by sol–gel method, were investigated by Panić et al. (2003a, b, 2007, 2010). They found that the iridium oxide shows enhanced catalytic activity for both oxygen and chlorine evolution and stability against corrosion as compared to ruthenium oxide (Panić et al. 2003a, b, 2007, 2010). These electrodes provide only small range of operating potential within hydrogen and oxygen evolution reactions. It is difficult to realize the direct oxidation of variety of organics on these anodes within this potential range. Hence, practical relevance of these anodes in the wastewater treatment on large scale is unacceptable. However, few research teams have successfully degraded the organic pollutants using these anodes.

The low chlorine evolution overpotential on these anodes is advantageous in the production of active chlorine species in wastewater containing chloride ions. The addition of NaCl as supporting electrolyte improves the removal efficiency, indicating that electrochemical treatment using Ti/IrO₂ and Ti/RuO₂ involves both direct and indirect oxidation (Chatzisymeon et al. 2009; Li et al. 2009). Enhancement in the COD removal efficiency was observed due to oxidation mediated by the electrochemically generated oxidants in the presence of chloride ions using Ti/RuO₂ anode in olive mill wastewater treatment (Papastefanakis et al. 2010). The degradation process involved the formation of intermediates followed by further oxidation to CO₂ and water.

Due to high electrocatalytic activity and stability, IrO₂-Ta₂O₅ anode finds wide industrial applications (Scialdone et al. 2009a). IrO₂-Ta₂O₅ anode is known for its selective oxidation. It oxidizes the organics into simpler carboxylic acids, particularly oxalic acid (Scialdone et al. 2009a). IrO₂-Ta₂O₅ is the suitable electrode for the degradation of OA (Scialdone et al. 2009a; Ferro et al. 2010).

Three different DSA®s, Ti/(RuO₂)_0.70(Ta₂O₅)_0.30, Ti/Ru_0.30Ti_0.70O₂, and Ti/Ru_0.30Sn_0.70O₂, were used in the degradation of RB-4 and RO-16 (da Silva et al. 2011). The Ti/Ru_0.30Ti_0.70O₂ showed the highest current efficiency among these electrodes. Indirect electrochemical oxidation by the active chlorine species generated in presence of NaCl enhanced the COD removal on this electrode. Ti/Ru_0.30Ti_0.70O₂ anode showed higher OER potential as compared to the other two. The same electrode was used in the degradation of Reactive Blue 81 and Reactive Red 2 by Kusmierek et al. (2011). Complete mineralization was achieved when the electrochemical degradation was coupled with the UV irradiation. These results indicate that TiO₂-RuO₂ mixed metal oxide solid solution is effective in the generation of active chlorine species. As these dyes can be successfully mineralized by indirect electrochemical oxidation, Ti/Ru_0.30Ti_0.70O₂ anode is suitable for the treatment of wastewater containing these dyes.

Chu et al. (2010) prepared Ti/RuO₂/IrO₂/TiO₂ with two compositions: one electrode (#1) with 0.20, 0.85, and 0.40 mg cm⁻² of Ru, Ir and Ti, respectively, whereas the other electrode (#2) composed of 1.10, 1.20, and 0.55 mg cm⁻² of Ru, Ir, and Ti, respectively. The higher oxide loading in case of #2 led to relatively rough surface of the coating. Direct oxidation of 2,4-DCP was observed on this electrode (Chu et al. 2010). Pt doping increases the service life of IrO₂ and RuO₂ anodes and the inability of Pt to produce hydroxyl ions is compensated by the ruthenium and/or iridium oxides (Li et al. 2009).

Table 5 compares different modified RuO₂ and IrO₂ anodes. The conditions used in the electrochemical wastewater treatment process, COD and TOC removal, and current efficiency displayed are useful in comparing the affinity of electrodes for different pollutant molecules.

Table 5 Performance of different modified RuO₂ and IrO₂ anodes and working conditions

Comparison between main metal oxide anodes

Table 6 reveals that the potential corresponding to the commencement of oxygen evolution reaction on the anode is related to OER overpotential and adsorption enthalpy of hydroxyl radicals (Kapałka et al. 2008). The ability of the anode to oxidize the organics increases with an increase in OER overpotential. Evidently, IrO₂ and RuO₂ are active anodes with low OER overpotential and are best suited for the production of chlorine and oxygen in chlor-alkali industries and electroorganic synthesis. They provide only partial decontamination with high energy consumption. The OER is highest for SnO₂ anode, and hence, it possesses high electrocatalytic activity for the production of hydroxyl radicals. Therefore, complete incineration of organics is viable. Similarly, OER for PbO₂ is also considerable and it is only slightly less than that of SnO₂. The only problem encountered in the implementation of these anodes for the wasterwater treatment on large scale is their dissolution on electrolysis in severe wastewater matrix. Studies on appropriate modifications in the fabrication technique to enhance their stability are of much importance.

Table 6 Oxidation power of the anode material in acid media

BDD anode recognized as the highly efficient anode for the oxidative degradation of organics in the past few years has acquired a lot of attention. Hydroxyl radical adsorption is very weak on the surface of BDD. The potential window between OER (2.2–2.6 V) and hydrogen evolution reaction is wide (>3 V) (Brillas et al. 2009; Koparal et al. 2007; Martínez-Huitle and Ferro 2006; Panizza and Cerisola 2008). However, BDD fabrication is very expensive as compared to metal oxide anodes. If appropriate modifications of metal oxide anodes to meet the characteristics of BDD and operating conditions are optimized to stretch the performance to the maximum, it only takes a mere part of the cost to achieve the same effectiveness as on BDD.

Conclusion

The electrochemical oxidation involving metal oxide electrodes for the abatement of organics in wastewater is an effective tool to keep up the aesthetic and environmental health. Nature of the electrode, pollutant, and operating conditions should coordinate suitably to successfully accomplish the goal of complete elimination of organics. The physical and chemical properties of the electrodes can be tuned by adopting appropriate fabrication technique to serve their purpose.

The structure, size, alignment, and morphology of the coated oxide thin film command the mechanism followed by the electrode. Electrodes with smooth surface interact very weakly with the hydroxyl radicals, and hence, these radicals are available for the oxidation of the organics, whereas on the electrodes with small crystallite size, large and rough surface areas provide sites for the hydroxyl radicals and organic molecules to adsorb strongly. The availability of hydroxyl radicals for the oxidation of organics diminishes on strong adsorption. Furthermore, with an increase in active centers on the surface of the oxide electrode and applied current density, the rate of formation of hydroxyl radicals also increases. Hence, metal oxide electrodes with smooth surface, small crystallite size, and high surface area are more effective. These prerequisite properties can be furnished with the help of suitable alterations made during the fabrication of the electrode. Selection of proper fabrication method, doping agent, interlayer insertion, and composition can bring out the best results.

Indirect oxidation of organics by the electrochemically generated oxidants is also a best method. Chloride is the most favorite electrolyte and ClO₄ ⁻, NO₃ ⁻, PO₄ ²⁻, SO₄ ²⁻, CO₃ ²⁻, and Br⁻ are also effective under appropriate optimized operating conditions. Inclusion of chloride ions substantially enhanced the efficiency in color, COD, and TOC removal in many cases due to the formation of HOCl, ClO⁻, and Cl₂. However, the risk of formation of AOX should be considered. Sulfate radicals, peroxydisulfate, peroxydicarbonate, and peroxydiphospates are also strong oxidants and can be effective under suitable conditions.

Among the three main metal oxide anodes discussed in this review, IrO₂ and RuO₂ show lowest OER potential. The oxygen evolution reaction on these electrodes competes with the oxidation reaction of organics. The complete removal of organics using these electrodes is possible only by indirect electrochemical oxidation. However, these electrodes are best suited for Cl₂ generation and electroorganic synthesis. Both SnO₂ and PbO₂ exhibit very good electrochemical activity for the oxidation of organics and electrocatalytic activity for the production of hydroxyl radicals. The service life of PbO₂ is greater than that of SnO₂, and the OER for SnO₂ is higher than that of PbO₂. Metal dissolution is a serious concern in electrochemical treatment process. SnO₂-Sb, PbO₂, IrO₂, and RuO₂ electrodes undergo dissolution in basic medium and high anodic potentials. However, modified SnO₂ and PbO₂ electrodes show high stability, electrochemical, and electrocatalytic properties than modified IrO₂ or RuO₂ electrodes. The performance of SnO₂ and PbO₂ is comparable with the BDD electrode which is recognized as a highly efficient anode for the oxidative degradation of organics. This makes modified SnO₂ and PbO₂ electrodes as suitable candidates for the electrochemical wastewater treatment on large scale.