## 2 Advanced Oxidation Processes
### Electrochemical Oxidation  
Electrochemical oxidation (EO) is the most used electrochemical technology for wastewater treatment [82]. In EO, an electric charge is produced between two electrodes (solid conductors) and is discharged in an electrolyte solution used as a medium [46]. One of the electrodes is positively charged (anode), while the other is negatively charged (cathode), and the oxidation of pollutants occurs in the anode, while the reduction occurs in the cathode [46]. This technology has the advantages of requiring less space, needing lower temperatures in the operation, and having a low by-product generation rate [83]. The organic compounds oxidation can occur by direct anodic oxidation (classified as \"heterogeneous\" when it occurs the direct electron transfer to the anode or classified as \"homogeneous\" when the oxidants are produced on anode surface from components of the electrolyte solution) or by chemical reaction with electrogenerated species (-OH radicals or other oxidizing species). The first one leads to weak removal, while the second leads to high-performance removal [41; 82; 84]. The -OH radicals generation starts with the oxidation of water in the anode, and only occurs at 2 V or above. Below this value, only direct oxidation can happen [46]. The indirect oxidation can also occur by the interaction of the pollutants with strong oxidants (such as chlorine), which must be generated during electrolysis by Cl\({}^{-}\)-containing species present in the water matrix and/or in the electrolyte solution [41; 82; 85]. The occurrence of oxidative species is dependent on the anode oxygen-evolution-overpotential, in which the anodes can be classified as \"active\", presenting low oxygen-overpotential, or as \"non-active\", showing high oxygen-overpotential [41]. The \"non-active\" anodes produce hydroxyl radicals adsorbed on their surface as presented in Equation (32), allowing the direct oxidation of the pollutants with M(-OH), while in the \"active\" anodes, the M(-OH) (M is the anode material) is chemisorbed and transformed into superoxide or a higher oxide (MO) as presented in Equation (33) [41; 82]. \[\text{M}+\text{H}_{2}\text{O}\text{\textminus}>\text{M}(\text{\textminus}\text {OH})+\text{H}^{+}+\text{e}^{-} \tag{32}\]  
\[\text{M}(\text{\textminus}\text{OH})\text{\textminus}>\text{MO}+\text{H}^{+}+ \text{e}^{-} \tag{33}\]  
MO can decompose into oxygen as presented in Equation (34) and M(-OH) can have side reactions as presented in Equations (35) and (36) [82]. \[\text{2 MO}\text{\textminus}>\text{2 M}+\text{O}_{2} \tag{34}\]  
\[\text{2 M}(\text{\textminus}\text{OH})\text{\textminus}>\text{2 M}+\text{O}_{2}+2\text{H}^{+}+2\text{e}^{-} \tag{35}\]  
\[\text{2 M}(\text{\textminus}\text{OH})\text{\textminus}>\text{2 M}+\text{H}_{2}\text{O}_{2} \tag{36}\]  
A \"non-active\" anode can only act as an inert substrate and a sink for electron removal, and, consequently, it cannot participate in direct anodic oxidation or provide catalytic active sites for the adsorption of organic molecules from the reactional medium [82]. One important parameter is the anode material, since it can influence the selectivity and efficiency of this technology [82]. The most used and investigated electrodes are doped-SnO\({}_{2}\), PbO\({}_{2}\), RuO\({}_{2}\), IrO\({}_{2}\), boron-doped diamond (BDD), sub-stoichiometric and doped-TiO\({}_{2}\), Pt, and dimensionally stable anodes (DSA) [41; 49; 86]. The BDD electrode presents the advantage of high stability even in strongly acidic media, low adsorption properties, inert surface, and high O\({}_{2}\) evolution overvoltage, being the most preferable anode of electrochemical oxidation. The most responsible pathway for pollutants degradation using this electrode is by OH generation, and the BDD oxidation power is higher than for other anodes [49; 82; 87]. However, this material also presents disadvantages such as high cost and electrode instability during operation, and it is not easy to obtain BDD with large dimensions [88].