# 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
Moreover, OH radical-trapping experiments through electron paramagnetic resonance spectroscopy showed that the photogenerated holes are easily transformed into OH radicals on the TiO2 surface and only slowly react with adsorbed water or hydroxyl groups on the surface of In2O3. The addition of PFOA enhances the amount of OH radicals produced in the presence of TiO2 due to the electron-withdrawing ability of fluorine atoms reported elsewhere [121]. This does not help PFOA degradation, as OH radicals are not effective to this end [122], and holes are preferentially consumed to oxidize water. On the other hand, the already-small amount of OH radicals generated in the presence of In2O3 further decreases in the presence of PFOA. This indicates the favored direct hole oxidation of PFOA at the surface of In2O3. Of several attempts to enhance the PFOA degradation rate in the presence of TiO2 under UV irradiation, Pb\({}^{2+}\) modification showed the most promising results, providing a degradation rate 32.5 times higher with respect to the bare material [68]. Authors attributed  
\[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]1 Adsorption modes of PFOA onto In2O3 (**a**) and TiO2 (**b**). the enhanced activity to the capability of Pb for acting as an electron trap, thus increasing the availability of holes for the degradation of PFOA. Persulfate (S2O82-) has also been used as a promising alternative for the degradation of PFOA. The preliminary activation of persulfate is required to generate sulfate radicals (Equation (13)), which in turn may directly induce oxidation or react with water, producing OH radicals (Equation (14)) [123]. This reaction is slow at room temperature (k = 2 \(\times\) 10\({}^{-3}\) s\({}^{-1}\)) but quickly proceeds at higher temperatures [124]. \[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]2  
\[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]3  
The activation of persulfate can be performed through thermal energy. For instance, complete defluorination of PFOA (0.1 mM) was observed after 18 h at 70 \({}^{\circ}\)C [125]. Several other methods to activate persulfate have been proposed to degrade PFAS, including ultraviolet, microwave, Fe2+, and ultrasound approaches. However, the highest defluorination efficiency was achieved by heat activation [126]. Sulfate radicals induce PFOA degradation but with a much lower defluorination ratio, indicating an insufficient degree of mineralization. Several short-chain perfluoroalkyl carboxylates (C2-C6) have been identified [127; 128]. The proposed reaction mechanism proceeds with a sequential elimination of CF2 groups [129]. Decarboxylation of PFOA is induced by sulfate radical attack through the formation of a radical cation intermediate (Equations (15) and (16)). \[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]4  
\[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]5  
The resulting radical couples with an OH radical, producing the unstable alcohol intermediate which eventually evolves according to Equations (7)-(10).