Electrochemical oxidation processes have gained great attention over the last decade for the decontamination of wastewater containing persistent organic pollutants and for disinfection purposes (Brillas et al., 2009; Särkkä et al., 2015). ECO processes are emerging as most appealing environmentally friendly and highly efficient electrochemical treatments (Martínez-Huitle and Brillas, 2009). The major benefit of electrochemical treatments is its ability to regulate and generate in situ •OH radicals without adding chemicals or large amount of catalyst (Oturan and Aaron, 2014). Instead, ECO process can also benefits on the prevention and remediation of pollution issues since the electron is a clean reagent (Panizza and Cerisola, 2009). In ECO treatments, the organic pollutants can be removed by: (i) direct anodic oxidation (or direct electron
transfer between anode surface and pollutants), which usually yields relatively poor oxidation rates, and (ii) chemical reaction with electrogenerated reactive oxygen species at anode surface such as physisorbed or chemisorbed •OH radicals, which leads to total or partial oxidation of pollutants, respectively (Martínez-Huitle and Brillas, 2009; Panizza and Cerisola, 2009).
In direct anodic oxidation, pollutants are oxidized at anode surface after adsorption solely due to the electron, without the involvement of any other substances. In this process, oxygen is transferred from water to the organic pollutant via electrical energy, which is called electrochemical oxygen transfer reaction as described by equation (6) (Panizza and Cerisola, 2009):
Rads – ne- → Pads (6)
where, Rads and Pads are adsorbed and oxidized adsorbed organic pollutants at anode surface. Direct electro-oxidation can be possible at low potentials, but usually possesses low kinetics, depending on the electrocatalytic activity of anode materials. The use of low cell potentials avoiding oxygen evolution could frequently cause the loss of anodic activity. This is called poisoning effect and occurs due to the formation of polymer layer on the anode surface, which has limited their application in wastewater treatment practices (Martínez-Huitle and Brillas, 2009). The anode surface deactivation depends on, (i) adsorption properties of the anode surface ,and (ii) nature and concentration of organic pollutants and their transformation products (Panizza and Cerisola, 2009).
On the other hand, electrochemical oxidation of organic pollutants can be obtained without the poisoning effect by electrolyzing the aqueous solution at high anodic voltages with the involvement of oxygen evolution reactions (OERs), which generates in situ •OH radicals with no oxidation catalysts. A number of anodes favored partial and selective oxidation of organic pollutants (i.e., conversion method), whereas many others favored complete mineralization (i.e., combustion/incineration) to CO2 (Brillas and Martínez-Huitle, 2015; Panizza and Cerisola, 2009). Comninellis found that the nature of anode material strongly effects both the selectivity and efficiency of the ECO process, and proposed a comprehensive model of organics oxidation in acidic medium including the competition with O2 evolution reaction and heterogeneous •OH radicals (Figure 4) (Comninellis, 1994). The model assumes that in the initial step of oxygen transfer reaction, water molecules split to form adsorbed •OH radicals at anode oxide surface (MOx) as equation (7):
MOx + H2O → MOx (•OH) + H++ e- (7)
Figure 4: Electrochemical oxidation scheme of organic pollutants on the metal oxide anode surface (Modified from (Comninellis, 1994)).
Then, the different behaviors of anodes in ECO process was described by considering two limiting cases of electrodes, defined as ‘’active’’ and ‘’non-active’’ anodes in the schematic model.
4.1.1 Active anodes
At active anodes, with low O2-overpotentials (such as IrO2, RuO2, and Pt) (Martínez-Huitle and Brillas, 2009), a strong interaction occurs between the anode surface (MOx) and •OH radicals (i.e., chemisorbed active oxygen in the lattice of metal oxide). The chemisorbed •OH radicals may interact with anode, forming so-called higher oxide (equation (8)). The surface redox couple MOx+1/MOx can act as a mediator in the conversion of selective oxidation of organics (R with m carbon atom and without heteroatoms) into short-chain carboxylic acids (equation (9)). Moreover, O2 evolution as side reaction due to chemical decomposition of higher oxides is also involved in this reaction (equation (10))(Martínez-Huitle and Ferro, 2006; Panizza and Cerisola, 2009) :
MOx (•OH) → MOx+1 + H++ e- (8)
MOx+1 + R → MOx + RO (9)
MOx → MOx + (1/2) O2 (10) 4.1.2 Non-active anodes
On the other hand, at non-active anodes, with relatively high O2-overpotentials (such as BDD, PbO2, and SnO2 (Brillas and Martínez-Huitle, 2015; Nidheesh et al., 2018)), weak interaction exists between physisorbed •OH radicals and the electrode surface. These anodes do not contribute to the direct anodic oxidation of organic pollutants and do not promote any catalytic sites for pollutant adsorption (Nidheesh et al., 2018). Therefore, the formation of higher oxide is excluded and allows the direct oxidation of organics with adsorbed •OH radicals, which may result in complete combustion to CO2 as follows (equation (11)):
𝛼MOx (•OH) + R → 𝛼MOx + mCO2 +nH2O + zH++ ze- (11) where, organic compound R needs 𝛼 = (2m+n) oxygen atoms for complete mineralization
to CO2. The oxidative reaction (9) is much more selective than mineralization reaction (6). Like in active anode behavior, reaction (11) also undergoes competitive side reaction, i.e. O2 evolution reaction, resulting in decreased anodic oxidation performance, either direct oxidation to O2 (equation (12)) or indirect consumption via dimerization to hydrogen peroxide (equation (13)).
MOx (•OH) → MOx + (1/2) O2 + H++ e- (12) 2 MOx (•OH) → 2 MOx + H2O2 (13)
According to the mechanisms, anodes with low O2-overpotentials (active anodes) (good catalysts for O2 evolution) allow only partial oxidation of organics, whereas anodes with high O2-overpotentials (non-active) (poor catalysts for O2 evolution) favor complete oxidation of organics to CO2 (Panizza and Cerisola, 2009). Therefore, non-active anodes are ideal for wastewater purification. Nevertheless, since both organic oxidation and O2
evolution reactions compete parallel, many anodes exhibit diverse behavior in practices (Nidheesh et al., 2018).
It is well confirmed that the type of anode material is the most important factor for determining the extent of organic pollutants degradation in ECO processes (Comninellis and Vercesi, 1991). There are wide variety of anode materials including doped and undoped PbO2, mixed metal oxides of Ti, Ru, Ir, Sn and Sb, Ti/Pt, carbon based anodes , and boron-doped diamond (BDD) thin films anodes (Brillas and Martínez-Huitle, 2015).
The PbO2 and BDD anodes, with high O2-overpotential, are the most commonly used anodes for the electrochemical degradation of organic pollutants. The PbO2 anodes are inexpensive, easy to prepare, chemically stable, low electrical stability, and a large surface area (Martínez-Huitle and Brillas, 2009). Likewise, BDD anodes are made by depositing a thin diamond films on non-diamond materials, usually silicon, tungsten,
titanium, tantalum, or glassy carbon, by energy-assisted chemical vapor deposition. BDD anodes are very promising anode material for the electrochemical treatment of wastewater contaminated with organic pollutants. They exhibit a great chemical and electrochemical stability, a wide electrochemical working range ( HER at -1.25 V vs SHE and OER at + 2.3 V vs SHE), high inert surface with low adsorption, and a great mineralization efficiency compared to other anodes (Oturan and Aaron, 2014; Panizza and Cerisola, 2009). The behavior of PbO2 and BDD anodes during the electrooxidation of organic pollutant is well confirmed by Panizza and Cerisola (Panizza and Cerisola, 2009).
Nevertheless, despite of the high O2-overpotential, PbO2 anodes show low durability by surface corrosion and leaching of highly toxic Pb2+ ions into to treated water (Brillas and Martínez-Huitle, 2015). Similarly, the major drawbacks of BDD anodes includes high material cost and difficulties in finding an appropriate substrate for thin diamond layer deposition (Panizza and Cerisola, 2009), which tend to compromise their applicability in large-scale applications.
Alternatively, MMO electrodes include a broad range of materials able to adsorb oxygen on their structure and more suitable for anodic oxidation of organic pollutants.
Dimensionally stable anodes (DSA), are the commercial terms for MMO electrodes, due to their excellent ECO properties and structural integrity (Shestakova, 2016). DSAs usually constitutes a Ti substrates deposited by a thin semiconducting layer of metal oxide or mixed metal oxides, such as Ti/TiO2-RuO2, Ti/Ta2O5-IrO2, Ti/IrO2-RuO2, Ti/SnO2 -Sb2O5, and Ti/Ir0.45O2-Ta2O2 (Brillas and Martínez-Huitle, 2015; Shestakova, 2016).