# Solar Concentration for Wastewaters Remediation: A Review of Materials and Technologies
## 3 Advanced Oxidation Processes and Photocatalysis  
AOPs are commonly defined by the chemistry and chemical engineering community as water treatments aimed at the removal of pollutants via oxidation by highly-reactive radicals, such as the hydroxyl (OH\({}^{\bullet}\)) or others (e.g., superoxide, peroxide, sulphate). As we are interested here in processes which can be activated by sunlight, we will limit this review to those involving photocatalysis. In wastewater remediation, the most common process employs hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) as the hydroxyl radicals source, activated by UV light. Although oxidation reactions by OH\({}^{\bullet}\) have been known for more than a century, with the use of Fenton's reagent in analytical chemistry, the application to wastewater remediation was only considered when evidence of OH\({}^{\bullet}\) generation in \"sufficient quantity to carry out water purification\" was given in the late 1980s [73]. Even though AOPs are of special interest for several applications, among which aromatics and pesticides degradation in water purification processes [74], oil derivatives, and volatile compounds [75], they have not yet been employed for large-scale commercial use. AOPs offer important advantages in water remediation, such as the possibility to effectively eliminate organic compounds in the aqueous phase, rather than collecting or transferring pollutants into another phase. Indeed, the contaminants can, in principle, be converted by complete oxidation into inorganic compounds, such as carbon dioxide; this process is called mineralization. Also, OH\({}^{\bullet}\) can, in principle, react with almost every organic aqueous pollutant without discriminating, potentially targeting a wide variety of compounds. A notable exception is that of Perfluorinated compounds, which are not attacked by OH\({}^{\bullet}\) radicals due to the stability of the C-F bond [76]. Some heavy metals can also be precipitated as M(OH)\({}_{\rm x}\)[77]. AOPs currently have a number of serious drawbacks, affecting cost and limiting their large-scale application. For example, a constant input of reagents is usually necessary to keep an AOP operational, because the quantity of hydroxyl radicals in solution needs to be high enough to react with all the target pollutants at useful rates. Scavenging processes can occur in the presence of species which react with OH radicals without leading to degradation, for example, bicarbonate ions (HCO\({}_{3}\)\({}^{-}\)) [78] and chlorides [79] should be removed by a pre-treatment or the AOPs will be compromised. In particular, the interference role of bicarbonate, chloride and dissolved silica anions has been recently studied in depth [80; 81; 82]. Finally, most existing processes rely on TiO\({}_{2}\) as the catalyst material. Although it offers important advantages in terms of stability, robustness and catalytic activity, it also has the serious limitation of requiring activation by UV light [83]. As such, it is mostly employed with artificial UV-sources, which are expensive, have short operational lifetimes, and require a high energy input.  
For these reasons, it is not economically reasonable to use only AOPs to treat large amounts of wastewater, but AOPs can be conveniently integrated with conventional treatments as a final or intermediate step [84]. In this context, the use of solar radiation to promote AOPs could contribute to greatly reducing costs by relying on a free and renewable source of photons, even if the cost problems mentioned above must be mitigated in order to have an efficient and competitive water purification technology. This possibility, along with increasing efforts towards the implementation of water reuse worldwide, are currently accelerating research towards the implementation of large-scale AOPs [4].