# Light-Induced Advanced Oxidation Processes as PFAS Remediation Methods: A Review
## 3 Advanced Oxidation Processes (AOPs) for PFAS
### Heterogeneous Photocatalysis Materials
#### 3.1.3 Reaction Mechanisms
(2014) observed that the shorter-chain carboxylic acids slowly started to form after switching on the lamp, but during the first hours of reaction, a direct correlation between PFOA mineralization and fluoride formation was evident. This suggests that the reaction takes place initially through the predominant \(\beta\)-scission mechanism, which efficiently competes with the photo-redox one. The authors also observed a slight loss of fluoride ions of ca. 3% in the aqueous phase, possibly due to fluorination of the surface of TiO\({}_{2}\). On the other hand, for longer reaction times the mineralization of PFOA nearly stopped, and an enhanced formation of shorter chain carboxylic acids was observed, with a concentration trend following the molecular weight of the intermediates. This indicates the preferential occurrence of the photo-redox mechanism with longer reaction times. Another mechanism proposed in literature relies on the initial one-electron reduction of PFOA by photogenerated electrons, which results in the defluorination of the terminal perfluorinated carbon atom and the consequent formation of the corresponding perfluoroxyl radical (Equation (11)). The latter in turn abstracts a hydrogen atom from the water, forming an OH radical and a monohydrogenated intermediate (Equation (12)). Repeating the same pathway results in the progressive hydrogenation of the alkyl chain, eventually leading to mineralization (Stein  
Finally, it is worth noting that direct photolysis of PFOA could take place only under irradiation shorter than 220 nm, which corresponds to the bond dissociation energy of the C-F bond, i.e., ca. 544 kJ/mol [102; 115]. Only ca. 10% of degradation was observed through photolysis at 254 nm [116; 117]. It is worth noting that PFOA is almost inert towards the OH radicals produced in the presence of irradiated TiO2 (\(k_{\text{OH+pFOA}}\leq 10^{5}\) M\({}^{-1}\) s\({}^{-1}\)) [118], due to the absence of hydrogen atoms prone to OH radical abstraction in the PFOA structure. Moreover, the initial decarboxylation (Equation (3)) is not likely to be induced by OH radical attack. In fact, the perfluorination of the alkyl chain reduces the electronic density of the carboxylic moiety, making the OH radical-mediated electron transfer unfavorable. Therefore, TiO2, generally very active for the degradation of a wide range of organic substrates, is rather poorly efficient in the degradation of PFOA. Its activity is slightly enhanced only under highly acidic pH conditions (HClO4, pH < 2) [56; 119] where the pH-dependent potential of the photogenerated holes is higher. An analogous situation is observed for the TiO2-induced degradation of trichloroacetate, which does not possess any hydrogen atoms [120]. Therefore, the lower the efficiency of hole-induced water oxidation to OH radicals, the better the efficacy obtained in terms of PFOA degradation. Accordingly, other semiconductors with a poor ability to produce OH radicals are instead more active in PFOA degradation. For instance, [PERSON] et al. [10] found that the apparent rate constant for the degradation of PFOA was 8.4 times higher in the presence of In2O3 than in the presence of TiO2. This result was justified by considering the peculiar adsorption mode of PFOA onto the In2O3 surface and the higher hole availability for PFOA oxidation with respect to TiO2, where water oxidation preferentially occurs. PFOA is bound to In2O3 in a bidentate or bridging configuration, resulting in a vertical and ordered arrangement of chains (Figure 7a). On the TiO2 surface, PFOA instead binds in a monodentate way (Figure 7b) which results in a tilted configuration of the chains. Consequently, some of the inner CF2 groups of PFOA can interact with the TiO2 surface through hydrogen bonding. The ordered and tight configuration of the PFOA chains onto the In2O3 surface favors direct hole oxidation with respect to water oxidation to hydroxyl radicals, and consequently the PFOA degradation occurs more efficiently on the surface of irradiated In2O3 than of TiO2.