## 2 Advanced Oxidation Processes
### Fenton's Reagent  
The traditional [PERSON]'s reaction uses H2O2 and Fe2+ as a catalyst for -OH radical generation. During this process, Fe2+ is oxidized to Fe3+ and regenerated to Fe2+, through the reaction of H2O2 and Fe3+ [40]. The decomposition of H2O2 is catalyzed by Fe2+, forming the oxidative radicals that are going to attack the pollutants. However, the use of Fe2+ must be done with care, since it can react with the generated radicals, producing Fe3+, and work as a scavenger of the reaction itself [46]. Furthermore, in [PERSON]'s technology, the iron can be used in homogeneous or heterogeneous form [48]. Equations (5) and (6) present [PERSON]'s process mechanism [40; 41]:  
\[\mathrm{H_{2}O_{2}+Fe^{2+}->OH+HO^{-}+Fe^{3+}} \tag{5}\]  
\[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]  
The Fenton's oxidation rate is higher at pH = 3 since the hydroxyl radicals are more present at acidic pH, allowing a fast reaction [48]. This technology is effective but can present negative aspects related to the operational costs, due to the reagent's consumption and pH adjustments [46]. As disadvantages, it also presents difficult recovery and reuse of iron and limited operational pH range, which can restrict Fenton's applications and decrease its efficiency [68; 69]. Moreover, the use of H2O2 can present negative effects. In low amounts, it can decrease the efficiency of pollutants degradation due to insufficient radical's production, and in excessive quantities, it can form less reactive radicals, such as HO2+, and decompose itself into water and oxygen, reducing the technology performance, as presented in Equations (7)-(11) [62; 70]. \[\cdot\mathrm{OH+\cdot OH\cdot>H_{2}O_{2}} \tag{7}\]  
\[\mathrm{H_{2}O_{2}+OH\cdot>H_{2}O+\cdot HO_{2}} \tag{8}\]  
\[\cdot\mathrm{HO_{2}+\cdot HO_{2}}\textgreater{H_{2}O_{2}+O_{2}} \tag{9}\]  
\[\cdot\mathrm{OH+O_{2}>\cdot HO_{2}+\cdot O} \tag{10}\]  
\[2\;\mathrm{H_{2}O_{2}>2\;H_{2}O+O_{2}} \tag{11}\]  
The advantages of Fenton's reaction are the use of environmentally friendly reagents, and the fact that the technology is easy to operate and implement, is efficient, and does not need energy to initiate the reaction. However, this technology also allows a quicker consumption of Fe2+ than its regeneration, has a limited operation pH range, and the complexation of iron species and the production of secondary sludge generally occur [71]. The use of photo-Fenton (UV and Fenton) could lead to lower reagent costs since the generation of radicals is enhanced, the reaction time is lower, and it requires less reagent compared to single Fenton's [48]. Photo-Fenton reactions are presented in Equations (12) and (13) [41]:  
\[\mathrm{H_{2}O_{2}+\mathit{h}v\textgreater{}>2\;HO\cdot} \tag{12}\]  
\[\mathrm{H_{2}O+Fe^{3+}+\mathit{h}v\textgreater{}-OH+H^{+}+Fe^{2+}} \tag{13}\]  
This technology also leads to a lower generation of secondary iron sludge, and to a faster iron regeneration [71]. The use of heterogeneous catalysts in [PERSON]'s technology allows the occurrence of this reaction at higher pH [46]. Other methodologies can be used to overtake the traditional Fenton's drawbacks. The use of other metals (such as Cu, Mn, Cr, Ce, Co, Ru, or Al) instead of Iron (II) can be an option, and this substitution is widely known as Fenton-like processes [68; 69; 72]. Equations (14)-(20) present the [PERSON]'s mechanism using Cu2+ and Mn2+ [69; 72]:  
\[\mathrm{Cu^{2+}+H_{2}O_{2}\textgreater{}Cu^{+}+\cdot HO_{2}+H^{+}} \tag{14}\]  
\[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]0\[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]1 \[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]2 (17) \[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]3 (18) \[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]4 (19) \[\mathrm{H_{2}O_{2}+Fe^{3+}->HO_{2}+H^{+}+Fe^{2+}} \tag{6}\]5  
Catalyst regeneration also occurs when Cu\({}^{2+}\) and Mn\({}^{2+}\) are used, due to the interaction with H\({}_{2}\)O\({}_{2}\).