# Light-Induced Advanced Oxidation Processes as PFAS Remediation Methods: A Review
## 3 Advanced Oxidation Processes (AOPs) for PFAS
### Heterogeneous Photocatalysis Materials
#### 3.1.2 Other Photocatalysts  
Bismuth oxyhalide (BiOX: where X = F, CI, Br, and I) photocatalysts are effective for the degradation of PFAS under light irradiation. Different bandgap and energy levels can be obtained for different X combinations, with BiOF, BiOCl, BiOBr, and BiOI showing bandgaps of 3.99, 3.22, 2.64, and 1.77 eV, respectively [86]. The optical and electronic properties can be further tuned using two different combinations of X in the compounds. Thus, these compounds provide the advantage of harvesting visible light for photocatalytic reactions. Most importantly, the formation energy for oxygen vacancies (OVs) of BiOX is very low, generating an abundant quantity of OVs on the surface. This feature further modulates the bandgap, providing sites for electron capture to delay the recombination of photogenerated charge carriers. The oxygen-deficient sites further participate in binding PFAS pollutants, with optimum interactions as compared to metal oxide. The layered structure of BiOX also offers a large surface-to-volume ratio with extensive exposure of active facets like (001). Because of these merits, BiOX showed an exceptional capability towards the photodegradation of PFOA, as reported first by [PERSON] et al. [87] for BiOCl. In their subsequent work [88], the same group systematically performed experiments to prove the linear relationship between the OVs and the photodegradation of PFOA. Doubling the OVs in BiOCl led to a four-fold increase in efficiency of defluorination in PFOA. The wide-bandgap BiOF photocatalyst can be modified by inducting OVs, which introduce the intermediate band in the forbidden gap to lower the energy required to excite the electrons by irradiation [89]. A study conducted by [PERSON] and coworkers [89] demonstrated that OVs could be tuned on exposed (001) facets of BiOF by adjusting the concentration of ethylene glycol during synthesis. The BiOF (001) produced by using 50% ethylene glycol could completely decompose PFOA within 6 h based on developed OVs. BiOI material, even  
Figure 3: (**a**) Effect of transition metal doping in Ga\({}_{2}\)O\({}_{3}\) hierarchical composites on PFOA degradation (reprinted from [77], copyright (2020) with permission from RSC Publishing). Photogenerated charge separation across the heterojunction of a (**b**) In\({}_{2}\)O\({}_{3}\)/Ce\({}_{2}\)O\({}_{3}\) (reprinted from [82], copyright (2016) with permission from RSC Publishing) and (**c**) In\({}_{2}\)O\({}_{3}\)/MnO\({}_{\rm x}\) composite (reprinted from [83], copyright (2019) with permission from John Wiley and Sons). with the smallest bandgap of \(\sim\)1.8 eV, was not conducive to the photocatalytic removal of PFAS, mainly owing to the lower valence band edge than that of the redox potential of (HO\({}^{-}\)/HO\({}^{\bullet}\)) [90]. However, BiOI\({}_{0.95}\)Br\({}_{0.05}\) displayed considerably high photocatalytic activity, requiring only 2 h to decompose 96% and 3 h to mineralize 65% of PFOA, thus showing the significance of the small amount of Br doping [90]. Here, rather than ROS, the holes and electrons played a more dominant role in the photocatalytic process. Doping of Br led to a reduction in bandgap for enhanced light harvesting and increased exposure of the (001) facet to elevate the separation of h\({}^{+}\)/e\({}^{-}\) pairs. This promoting characteristic of BiOI\({}_{0.95}\)Br\({}_{0.05}\) produced a degradation rate constant \(\sim\)4.3 times higher than that of BiOI. A similar boost in photocatalytic performance was observed when BiOI was mixed with BiOF and loaded with Bi nanoparticles to provide Bi/BiOI\({}_{1-x}\)F\({}_{x}\)[91]. Integrating both these bismuth oxyhalides further reduced the bandgap to increase the visible light absorption and elevated the surface area by two times due to the formation of hollow flower-like microspheres. On the other hand, Bi nanoparticles on the surface give rise to a surface plasma resonance effect to favor photogenerated charge separation by generating an electric field near the junction.