## 2 Advanced Oxidation Processes
### PS, PMS, and H\({}_{2}\)O\({}_{2}\) Oxidation  
PS and PMS anions are widely used oxidizing reagents. In fact, these agents are applied as other compounds in their molecular form (such as sodium persulfate (SPS) or potassium peroxymonosulfate), in which their dissociation into the anions and their interaction with light, heat, or catalysts generates sulfate radicals that will promote the pollutant's oxidation. However, the efficiency of persulfate anions alone is not as good, and the presence of other materials is required to enhance the generation of sulfate radicals, so, in general, and as presented in the referred studies, the use of these agents is accompanied by the presence of radiation and/or catalyst. Furthermore, the basic pH is favorable when these anions are used, since at this pH it is possible to form OH radicals that are more reactive than the \(\cdot\)SO\({}_{4}^{2-}\) radicals. Hydrogen peroxide is another widely used reagent for the generation of oxidizing radicals, and, at low or high amounts, its efficiency can be negatively affected due to low pollutants oxidation or scavenging behavior of hydroxyl radicals. This reagent is mainly used in the presence of other materials considered as an AOP technology, and its presence can enhance the technology's efficiency, particularly with the use of UV radiation, which allows the generation of hydroxyl radicals. [PERSON] et al. [35] used hydrogen peroxide for the degradation of a mixture of five parabens and p-HBA. The individual parabens using UVC/H\({}_{2}\)O\({}_{2}\) technology occurred in about 6 min, and the mixture of parabens was totally removed in 30 min. [PERSON] et al. [73] tested ethylparaben conversion using radiation and three different reagents (persulfate, peroxymonosulfate, and hydrogen peroxide). In 90 min, EP removal was about 97%, 98.1%, 81.3% and 36% for UV/H\({}_{2}\)O\({}_{2}\), UV/PS, UV/PMS, and UVC photolysis, respectively. Furthermore, [PERSON] et al. [74] removed about 98.9% and 34% of methylparaben using UV/PS and UVC at the same time. These results are very similar and prove the good performance of persulfate anion when coupled with UVC light. Compared to photolysis results, the existence of oxidizing agent seems to significantly increase the oxidation of the parabens compounds, which may be related to a higher generation of oxidizing radicals. Moreover, the use of UV/PMS was less effective than UV/PS in the study of [PERSON] et al. [73], which is not expected, since PMS can generate hydroxyl and sulfate radicals and PS can only produce sulfate radicals [42]. Total removal of PP using UV/PS oxidation was obtained by [PERSON] et al. [107], which used UVA radiation and simulated solar light, but only 12% was achieved using Vis radiation. However, these results suggest a good performance of these technologies regarding the parabens abatement for different radiation sources, which may be an advantage for this purpose. The presence of a catalyst may also enhance this degradation. [PERSON] et al. [108] removed BuP by peroxymonosulfate oxidation with Mn-Fe oxycarbide catalyst, achieving the highest removal of 99% on 180 min, while [PERSON] et al. [109] used graphene-based catalysts with PS to remove 1 mg/L of PP, achieving 95% degradation for the best catalyst in 20 min. Moreover, the use of SPS alone was tested, reaching 10% degradation in 120 min, which shows the poor performance of this agent when used alone. [PERSON] et al. [110] used 1 g/L of SPS and different iron materials as a catalyst for the removal of 0.4 mg/L of PP, achieving the best degradation, 90%, in 90 min. Comparing these studies shows that the presence of catalyst led to almost total removal in general, which shows a good degradation performance when a catalyst material is present, and the study by [PERSON] et al. [109] also presented a poor performance for SPS when used alone, which is the expected behavior.