## 2 Advanced Oxidation Processes
### Persulfate, Peroxymonosulfate, and Hydrogen Peroxide  
Oxidizing agents can be used in AOP, such as the persulfate anion (S\({}_{2}\)O\({}_{8}\)\({}^{2-}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), or peroxymonosulfate anion (HSO\({}_{5}\)\({}^{-}\)), which are the most used oxidizing agents in wastewater treatment systems [73]. The objective is the same as other technologies, that is, the generation of hydroxyl and/or other radicals for the elimination of contaminants [42]. The persulfate anion is stable at ambient temperatures, is selective, and is a strong oxidant, which can work in a wide pH range, but it is cost-effective [74]. Its high solubility and longer lifetime allow it to decompose several organic pollutants [42]. Persulfate (PS) can produce SO\({}_{4}\)\({}^{-}\) radicals when activated by heat, UV light, bases, transition metals (catalysts), or ultrasound [42; 73; 74], and its reaction is shown in Equation (21) [42]. \[\text{S}_{2}\text{O}_{8}\)\({}^{2-}+hv\ (heat\ \text{or}\ 2\text{e}^{-})>\cdot 2\ \text{SO}_{4}\)\({}^{-} \tag{21}\]  
Persulfate can react in an acidic medium, enhancing the production of sulfate radicals as presented in Equation (22) [42]. \[\text{S}_{2}\text{O}_{8}\)\({}^{2-}+\text{H}^{+}->\text{H}^{+}+\text{SO}_{4}\)\({}^{2-}+\cdot\text{SO}_{4}\)\({}^{-} \tag{22}\]  
Usually, the performance of persulfate alone is low, so to achieve good efficiency, the presence of catalyst, heat, or radiation is desired [42]. The presence of catalyst enhances the sulfate radical's production. The use of Fe\({}^{2+}\) is an option, since this metal can activate the PS for contaminants degradation, but this is not so efficient, probably due to the generation and accumulation of Fe\({}^{3+}\)[42]. Peroxymonosulfate (PMS) is also a strong oxidant. It can be used in a wide pH range, and in the presence of light, heat, or catalysts, it is activated and can generate hydroxyl and sulfate radicals as presented in Equations (23) and (24), and in Equation (25) when exposed in an acidic medium [42], which may represent an advantage compared to the persulfate reaction. \[\text{HSO}_{5}\)\({}^{-}+hv>\cdot\text{OH}+\cdot\text{SO}_{4} \tag{23}\]  
\[\text{HSO}_{5}\)\({}^{-}+heat>\cdot\text{OH}+\cdot 2\ \text{SO}_{4} \tag{24}\]  
\[\text{HSO}_{5}\)\({}^{-}+\text{H}^{+}+\text{e}^{-}->\text{H}_{2}\text{O}+\cdot\text{SO}_{4}\)\({}^{-} \tag{25}\]  
Additionally, the sulfate radicals can be decomposed into more hydroxyl radicals in basic pH conditions, as shown in Equations (26) and (27) [42]. Furthermore, the radical'sproduction is favorable at basic pH regarding PS and PMS reactions, since this pH allows the formation of hydroxyl radicals, which are more reactive than the sulfate radicals formed in acidic media [42]. \[\cdot\mathrm{SO_{4}}^{-}+\mathrm{H_{2}O}\mathrm{->}\cdot\mathrm{OH}+\cdot\mathrm{ HSO_{4}}^{-} \tag{26}\]  
\[\cdot\mathrm{SO_{4}}^{-}+\mathrm{HO}^{-}\mathrm{->}\cdot\mathrm{OH}+\mathrm{SO_{4 }}^{2} \tag{27}\]  
Hydrogen peroxide is a reagent with relatively high oxidative power that is used for radical's generation but may be inefficient when used alone since it cannot produce hydroxyl radicals by itself. It can also decompose into water and oxygen (as mentioned in Equations (28)-(30)), showing low performance in the removal of recalcitrant and refractory compounds [42]. \[\mathrm{H_{2}O_{2}}\mathrm{->HO_{2}}^{-}+\mathrm{H}^{+} \tag{28}\]  
\[\mathrm{H_{2}O_{2}}+\mathrm{HO_{2}}^{-}\mathrm{->H_{2}O}+\mathrm{O_{2}}+\mathrm{ HO}^{-} \tag{29}\]  
\[2\mathrm{H2O2}\mathrm{->2H2O}+\mathrm{O2} \tag{30}\]  
The radicals generated by hydrogen peroxide are less reactive than \(\cdot\mathrm{OH}\) radicals [45], and its oxidation potential is higher at low pH, so this reagent is favored when used in an acidic medium, and the combination of \(\mathrm{H_{2}O_{2}}\) with other reagents or radiation can enhance its efficiency [42]. However, the presence of hydrogen peroxide must be carefully dosed, since high and low amounts can negatively affect its performance, as explained in Section 2.3.