## 2 Advanced Oxidation Processes
### Photolysis and Photocatalysis  
Photolysis and photocatalysis are two technologies that use radiation for radicals generation, and they differ since photocatalysis also has a catalyst present, to increase the radical's formation rate.  
Photolysis can be classified as direct or indirect photolysis, and it usually uses UV radiation from 200-400 nm. Direct photolysis allows the degradation of pollutants due to the photons generated by the irradiation source, and indirect photolysis (known also as photochemical degradation) causes the conversion of pollutants by the presence of radiation and different reagents that enhance the hydroxyl radicals' production [48]. Nowadays, UV radiation is already used for disinfection (inactivation of pathogenic organisms) purposes [49].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Oxidant Species** & **Oxidation Potential/[V]** \\ \hline Hydroxyl (OH) & 2.80 \\ Fluorine (F) & 3.03 \\ Ozone (O\({}_{3}\)) & 2.07 \\ Sulfate (SO\({}_{4}\)\({}^{2-}\)) & 2.01 \\ Hydrogen Peroxide (H\({}_{2}\)O\({}_{2}\)) & 1.77 \\ Permanganate (MnO\({}_{4}\)\({}^{2-}\)) & 1.77 \\ Chlorine dioxide (ClO\({}_{2}\)) & 1.57 \\ Chlorine (Cl\({}_{2}\)) & 1.36 \\ Dichromate (Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\)) & 1.23 \\ Oxygen (O\({}_{2}\)) & 1.23 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Oxidant potential of different oxidizing species.  
Photocatalysis requires radiation to activate a catalyst, generally a heterogeneous semiconductor, and only this activation allows the radical's generation and the pollutants' elimination [50]. To activate the catalyst, the radiation source must emit photons with equal or higher energy than the catalyst's bandgap, which are absorbed into the catalyst and cause the excitation and migration of electrons from the valence band (VB) to the conduction band (CB). This electron migration generates electron-hole pairs (e\({}^{-}\)/h\({}^{+}\)), which will interact with the pollutants present in the reactional medium or in the catalyst surface [44; 47; 51]. This interaction causes the generation of radicals, which react with the pollutants and promote their oxidation.  
In photocatalysis, several catalysts, like TiO\({}_{2}\), ZnO, ZnS, or CdS, among others, have already been used [44; 50]. Titanium dioxide (TiO\({}_{2}\)) is one of the most common catalysts, due to its low cost, high photo-activity, and chemical stability [52; 53]. This catalyst can present three different crystallite phases: Anatase (the most wanted phase, since it is more stable at low temperatures and more photo-active), Ruelle (more stable at high temperatures with lower band-gap energy), and Brookite phase (does not have catalytic properties and is rare) [54; 55; 56].  
The costs related to photolysis or photocatalysis can be a drawback for its application, since the artificial UV light is expensive [57], and some semiconductor materials, like the TiO\({}_{2}\) P25, requires this type of light, since this catalyst presents high band-gap energy, which can only be activated by UV [57]. Furthermore, the UV radiation represents only 3-5% of the solar spectrum which can make the use of solar light inefficient, despite the fact that it ensures new advantages as mentioned before [57; 58; 59]. The solar spectrum is mainly constituted by visible radiation (about 45%) [44], so the use of catalysts that can be activated by this type of radiation can reduce the operational costs of photocatalysis.  
One possibility to decrease the catalyst's band-gap energy is doping the material with metals or non-metals elements due to the introduction of oxygen vacancies or impurity energy levels [59], although this makes the catalyst production more expensive. Cationic metals (such as Fe, Ag, Cu, Pd, Pt, and Au) can create an intermediate band between CB and VB, which will decrease the band-gap energy [44; 60]. Non-metal doping (as C, N, F, S,... ) allows the use of solar radiation and can promote better electron trapping [44; 61].  
Figure 1: Scheme of examples of Advanced Oxidation Process (AOP) technologies.