## 2 Advanced Oxidation Processes
### Ozone Based Technologies  
Ozonation uses a powerful oxidant, the ozone, to remove organic pollutants. This technology is pH-dependent, since it may involve two different pathways: direct reaction between the ozone and the pollutants (that occurs at pH! 7), or indirect reaction, by radical generation and interaction (at pH? 7) [41,49]. The use of ozonation at pH? 7 is desired, since the hydroxyl radical generation leads to a quicker reaction and generally to better removal of pollutants [41]. Ozone has a special preference for electron-rich molecules, like double and triple bonds, amines, or aromatic rings, and reacts the most with these molecules [49]. Ozonation can present different advantages, such as pollutant oxidation into H\({}_{2}\)O and CO\({}_{2}\), quick reaction, easy ozone production, and oxidation of different and persistent molecules. However, as disadvantages, ozone is a corrosive, explosive, and toxic compound, its generation must occur in situ, it is expensive, and it is not efficient for the removal of saturated compounds. Furthermore, the ozonation system needs an O\({}_{3}\) destroyer for safety reasons, and this technology may not lead to high mineralization (COD and BOD removal rates are usually low) [46,62]. The combination of single ozonation with H\({}_{2}\)O\({}_{2}\), UV radiation, or catalyst can enhance the radical generation, can increase the pollutants removal rate, and can also improve the mineralization efficiency [41,63]. Heterogeneous catalysts may initiate the hydroxyl radical generation and help in the initiation of ozone decomposition, enhancing the production of radicals [41]. Peroxone (H\({}_{2}\)O\({}_{2}\)/O\({}_{3}\)) improves the generation of hydroxyl radicals and can increase the pollutants oxidation efficiency, since the transformation of O\({}_{3}\) into -OH, and the mass transference of ozone to the liquid phase is better [42]. H\({}_{2}\)O\({}_{2}\) interacts with O\({}_{3}\), or vice versa, and produces -OH or -HO\({}_{2}\). The existence of acidic pH can negatively affect the -OH generation, so this technology is better when used at higher pH, since the H\({}_{2}\)O\({}_{2}\) usually reacts and is transformed into HO\({}_{2}\), which in turn increases O\({}_{3}\) transformation into -OH and OH\({}^{-}\)[42]. Equations (1)-(3) present the general reactions of peroxone technology [42]:  
\[\mathrm{H}_{2}\mathrm{O}_{2}+\mathrm{O}_{3}>\cdot\mathrm{OH}+\cdot\mathrm{HO} _{2}+\mathrm{O}_{2} \tag{1}\]  
\[\cdot\mathrm{OH}+\mathrm{O}_{3}>\cdot\mathrm{HO}_{2}+\mathrm{O}_{2} \tag{2}\]  
\[\cdot\mathrm{HO}_{2}+\mathrm{O}_{3}>\cdot\mathrm{OH}+2\,\mathrm{O}_{2} \tag{3}\]  
However, the use of H\({}_{2}\)O\({}_{2}\) in peroxone must be careful, since this compound can act as a scavenger of hydroxyl radical and decrease its generation, which could lead to a worse technology performance than single ozonation [41]. Regarding this process, some researchers have considered the optimum H\({}_{2}\)O\({}_{2}\)/O\({}_{3}\) ratio to be 0.5 mol/mol [49]. The use of UV with peroxone can increase the pollutants removal, due to higher generation of OH radicals, as presented in Equation (4) [42]. \[\mathrm{H}_{2}\mathrm{O}_{2}+2\,\mathrm{O}_{3}+h\upsilon->2\,\mathrm{HO}\cdot +3\,\mathrm{O}_{2} \tag{4}\]  
Catalytic ozonation combines the power of ozone with a catalyst. The presence of catalysts can initiate and enhance the decomposition of ozone molecules into oxidizing radicals, can allow a better absorption of ozone into the liquid phase [41], and, in general, can lead to lower ozone consumption, when compared to single ozonation [49]. Photocatalytic ozonation has a synergetic effect, since it combines the power of photocatalysis with ozone, allowing a higher production of radicals [64]. In this technology, the ozone molecule will react with the catalyst conduction band forming ozonide radicals (-O\({}_{3}\)\({}^{-}\)), which will posteriorly generate -OH radicals [65].