# 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
Apart from carbon, molecule-imprinted polymer (MIS)-modified TiO\({}_{2}\) nanotubes also boosted contact between PFOA and TiO\({}_{2}\) and reduced the recombination of h\({}^{+}\) and e\({}^{-}\) pairs by capturing photogenerated electrons [65]. Noble metal nanoparticles deposited on TiO\({}_{2}\) can also act as an electron sink to increase the lifetime of photogenerated h\({}^{+}\) and e\({}^{-}\) pairs. Among Pt, Pd, and Ag nanoparticles deposited by TiO\({}_{2}\), Pt-TiO\({}_{2}\) showed a considerable improvement (\(\sim\)12.5 times) in the photocatalytic performance of PFOA removal as compared to pure TiO\({}_{2}\)[66]. With the highest work function, Pt can attract more electrons from TiO\({}_{2}\) to form a strong Schottky barrier at the interface, which improves h\({}^{+}\)/e\({}^{-}\) separation during irradiation. [PERSON] et al. [67] successfully loaded Pt nanoparticles on the reductive sites of the TiO\({}_{2}\) facet to precisely capture photoinduced electrons (Figure 2b). This charge separation led to an increase in the number of holes on TiO\({}_{2}\) sites which participate in PFOA degradation. At the same time, electrons on the Pt atom produced hydrogen atoms and led to hydrogen spillover onto the TiO\({}_{2}\) surface. The Ti-H bond formed on the surface facilitates hydrodefluorination of PFOA through the cleavage of the C-F bond. In addition to reducing recombination, doping with transition metals like Cu, Fe, and Pb also increased visible light absorption by lowering the band gap of TiO\({}_{2}\), leading to superior photocatalytic degradation of PFAS [57; 68]. Combining different strategies can lead to significant advancements in photocatalytic performance by creating a synergetic effect. Fe-deposited titanate nanotubes (TNT) supported on activated carbon (AC) (Fe/TNTs@AC) demonstrated highly efficient photodegradation of PFOA (\(\sim\)90%) in 4 h under UV light irradiation with 62% of the defluorination reaction [69]. In Fe/TNTs@AC, the nanotube architecture and AC provided a high surface area, while \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) particles and AC considerably boosted PFOA adsorption on the surface and separation of the h\({}^{+}\)/e\({}^{-}\) pair by acting as the electron acceptor. Fe/TNTs@AC was recycled without significant loss in adsorption and degradation capacity of the photocatalyst. Similarly, Ga/TNTs@AC showed faster adsorption kinetics and degraded \(\sim\)75.0% while mineralizing 66.2% of PFOS after 4 h of UV irradiation [70]. This type of study opens an avenue towards developing more such TiO\({}_{2}\)-based photocatalysts by integrating different strategies, with each modification step playing a specific role. Along with TiO\({}_{2}\), ZnO was also studied for remediating wastewater from PFAS but with limited success [71]. Although heterojunction between ZnO/rGO did degrade 90.9% of PFOA [72], no other work on ZnO has been reported. With a more negative conduction band edge and a wider band gap (4.8 eV) than TiO\({}_{2}\), gallium oxide (Ga\({}_{2}\)O\({}_{3}\)) displayed a better capability for removing PFAS than TiO\({}_{2}\). In contrast to TiO\({}_{2}\), the scavenging experiment proved that the breaking down of PFOA proceeds via a reduction reaction in Ga\({}_{2}\)O\({}_{3}\) involving photogenerated electrons to produce a rate constant \(\sim\)19 times higher than that of TiO\({}_{2}\)[73]. Nanostructured Ga\({}_{2}\)O\({}_{3}\) in nanonehedle [74], sheaf-like [75], and nanorod [76] forms with improved surface area presented escalation in the demolition of PFOA from wastewater. Among them, the sheaf-like nanostructure assembled with nanoplates of an average width of 100 nm and thickness of 10 nm exhibited the highest reaction rate constant (4.85 h\({}^{-1}\)). Compared to commercial Ga\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\) powder, this value was \(\sim\)16 and \(\sim\)44 times higher, respectively. Nevertheless, the mechanism involved in all the nanostructures remained same where photoinduced electrons were used for the decarbonization of PFOA followed by HF elimination.