# 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
To improve photocatalytic activity towards PFOA degradation, [PERSON] and coworkers [77] modified Ga\({}_{2}\)O\({}_{3}\) hierarchical nanosheets by doping a series of transition metals (Mn, Co,Zn, Cu, Ti, and In). Indium doping in the Ga\({}_{2}\)O\({}_{3}\) hierarchical composite resulted best, completely degrading PFOA within 1 h with degradation kinetics ~7.8 times higher than those of pristine Ga\({}_{2}\)O\({}_{3}\) (Figure 3a). By investigating the reaction mechanism, this considerable improvement was attributed to not only enhanced light absorption and charge separation but also to the activated surface that provided conducive conditions for better adsorption of PFOA. In\({}_{2}\)O\({}_{3}\) is highly active in removing PFAS, and it follows a similar photo-oxidation pathway as TiO\({}_{2}\), but with significantly higher photocatalytic performance with a reaction rate approximately ~59 times higher than that of TiO\({}_{2}\)[10]. As In\({}_{2}\)O\({}_{3}\) provides the surface with better adsorption capacity for PFOA, photogenerated holes directly attack the carboxyl group of PFOA rather than forming the HO\({}^{\bullet}\) radical as in TiO\({}_{2}\). Since the surface plays the prime role in In\({}_{2}\)O\({}_{3}\), substantial work on In\({}_{2}\)O\({}_{3}\) was dedicated to refining the surface area by fabricating different nano morphologies like nanospheres, nanoplates, microspheres, and nanocubes [9; 78; 79]. Among them, nanoplates showed the highest surface area of 156.9 m\({}^{2}\)/g, exposing many active sites and leading to the highest photocatalytic performance in the degradation of PFOA [79]. In recent work, In\({}_{2}\)O\({}_{3}\) nanospheres derived from MOF offered a surface of a super-hydrophilic nature to facilitate the adsorption and tight attachment of PFOA [80]. This behavior results in complete decomposition and mineralization of PFOA in 3 h and 6 h, respectively. Oxygen vacancies also play a vital role in these nanostructured In\({}_{2}\)O\({}_{3}\) photocatalysts by acting as the adsorption sites for PFOA on the In\({}_{2}\)O\({}_{3}\) surface. Higher photocatalytic activity has been recorded for nanostructures possessing greater amounts of oxygen vacancies. In addition to the adsorption of reactants, photogenerated charge separation is also essential for enhancing the photocatalytic reaction rate. As in other semiconductors, a strategy of heterojunction was also adopted for In\({}_{2}\)O\({}_{3}\) to avoid the recombination process. Composites of In\({}_{2}\)O\({}_{3}\)/graphene [81], In\({}_{2}\)O\({}_{3}\)/Ce\({}_{2}\)O\({}_{3}\) (Figure 3b) [82], and In\({}_{2}\)O\({}_{3}\)/MnO\({}_{3}\) (Figure 3c) [83], synthesized by cost-effective techniques, formed a heterojunction at the interface due to the difference in the energy levels, which drives the photogenerated h\({}^{+}\) and e\({}^{-}\) to spatially separated from each other. The synergic role played by both materials also escalated the adsorption of PFOA and the  
Figure 2: PFOA degradation using (**a**) TiO\({}_{2}\) quantum dots supported on sulfonated graphene nanosheets (reprinted from [64], copyright (2020) with permission from Elsevier) and (**b**) single Pt atoms loaded on the reductive sites of the TiO\({}_{2}\) facet (reprinted from [67], copyright (2021) with permission from ACS Publications). harvesting of visible light in the composite. All these promotive effects caused the complete decomposition of PFOA in 3 h for In\({}_{2}\)O\({}_{3}\)/graphene and In\({}_{2}\)O\({}_{3}\)/MnO\({}_{\rm x}\), and in 1 h for In\({}_{2}\)O\({}_{3}\)/Ce\({}_{2}\)O\({}_{3}\) under UV light irradiation (254 nm), but with a higher intensity used in the case of In\({}_{2}\)O\({}_{3}\)/Ce\({}_{2}\)O\({}_{3}\) composites. Oxy/hydroxide forms of In (InOOH) degrade PFOA, with a rate constant \(\sim\)27.6 times higher than that of the commercial P25 photocatalyst [84]. Enhanced adsorption of PFOA on the surface of InOOH is the prime factor responsible for higher photocatalyst performance compared to P25 under UV light irradiation.