# Light-Induced Advanced Oxidation Processes as PFAS Remediation Methods: A Review
## 3 Advanced Oxidation Processes (AOPs) for PFAS
### Heterogeneous Photocatalysis Materials
#### 3.1.1 Metal Oxides  
TiO\({}_{2}\) was the first photocatalyst investigated for the removal of PFOA in water bodies [56] due to its well-known high activity towards the degradation of pollutants. However, TiO\({}_{2}\) showed a very low reaction rate constant (\(\sim\)0.0001 min\({}^{-1}\)), and high half-life (\(\sim\)6931 min) for PFOA degradation [57]. Similar problems were observed in the presence of titanate nanotubes (TNTs) synthesized by hydrothermal methods to improve the surface area, which were even less effective in removing PFAS under UV light [58]. This inefficiency of Ti-based photocatalysts was attributed to the strong repulsion of anionic PFAS molecules from the negatively charged surface of TNT. Commercial P25 TiO\({}_{2}\), a benchmark in the field, was able to complete the degradation reaction of PFOA within 4 h under illumination with 254 nm UV light [59]. However, under higher wavelength light (315-400 nm), the degradation of PFOA decreased drastically. The photocatalytic ability of TiO\({}_{2}\) to degrade organic pollutants mainly relies on the formation of HO\({}^{\bullet}\) and O\({}_{2}^{-\bullet}\) radicals by oxidation of water and reduction of O\({}_{2}\) through photogenerated holes and electrons, respectively. Nevertheless, both these reactive oxidation species (ROS) are less active for the decomposition of PFOA. Thus, in the case of TiO\({}_{2}\), photogenerated h\({}^{+}\) and e\({}^{-}\) directly initiate the decarboxylation of PFAS to mineralize them to CO\({}_{2}\) and fluoride ions in a stepwise manner. For this phenomenon to occur, the absorption of PFAS molecule on TiO\({}_{2}\) surface becomes a necessary condition. This can be achieved by spreading positive surface charges on the TiO\({}_{2}\) by carrying out the photocatalytic reaction of PFAS in acidic conditions. [PERSON] et al. [56] showed that in the presence of 0.15 M perchloric acid, the rate constant of PFOA degradation increased by \(\sim\)46 times with respect to that performed in the absence of acid. Following this work, most of the research for TiO\({}_{2}\)-based photocatalytic degradation of PFAS was performed at a pH below 4. Acids such as oxalic acid increase the photoreduction capability of TiO\({}_{2}\) to degrade PFAS. [PERSON] et al. [60] demonstrated that adding oxalic acid leads to the formation of carboxyl anion radicals (CO\({}_{2}^{-\bullet}\)) by photogenerated holes that have a higher reduction potential (\(-\)1.85 V) than that of photogenerated electrons. The same work also reported that oxalic acid is \(\sim\)11.6 times more effective in PFOA decomposition than perchloric acid under the equivalent pH of the reaction. TiO\({}_{2}\) nanoparticles in the presence of oxalic acid effectively degrade 86.7% of PFOA in 180 min under N\({}_{2}\) bubbling. TiO\({}_{2}\) supported on carbon allotropes such as MWCNT [61], graphene [62], and reduced-graphene oxide (rGO) [63] also increased the direct interaction between PFOA and TiO\({}_{2}\) photocatalysis owing to the high adsorption capacity of the carbon-based materialprovided by the high surface area and surface functional group. Carbon can also trap the photogenerated electrons of TiO\({}_{2}\) and reduce the electron-hole recombination process during the photocatalytic reaction. TiO\({}_{2}\)-MWCNT and TiO\({}_{2}\)-rGO were able to degrade \(\sim\)100% of PFOA under UV irradiation in 8 h and 12 h, respectively. Recently, [PERSON] et al. [64] fabricated TiO\({}_{2}\) quantum dots of 2-5 nm in size on sulfonated graphene nanosheets using sodium dodecyl sulfate as a surfactant (Figure 2a). Due to the hydrophobic nature of sulfonate graphene and highly dispersed TiO\({}_{2}\) quantum dots, PFOA pollutants could instantaneously be adsorbed on the surface, followed by fast decomposition by eliminating CF\({}_{2}\) species.