# 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  
The mineralization of PFOA in water can be summarized as in Equation (1)  
\[\text{CF}_{3}(\text{CF}_{2})_{6}\text{COOH}+7/2\;\text{O}_{2}+7\;\text{H}_{2} \text{O}\to 8\;\text{CO}_{2}+15\;\text{HF} \tag{1}\]  
The reaction has been investigated in irradiated aqueous suspensions of TiO\({}_{2}\), one of the most extensively investigated photocatalysts, and the commonly accepted mechanism is the result of a photo-redox and \(\beta\)-scission pathway [104]  
Upon irradiation of suitable energy, electrons (\(\text{e}^{-}_{\text{CB}}\)) and holes (\(\text{h}^{+}_{\text{VB}}\)) are localized respectively within the conduction and valence band of TiO\({}_{2}\)[105], according to Equation (2). \[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]  
It is worth noting that PFOA exists mainly in its deprotonated form under natural pH conditions. Deprotonated PFOA undergoes hole-induced Kolbe decarboxylation [60, 106, 107], giving rise to the C7 radical intermediate (Equation (3)), which in turn reacts with molecular oxygen, producing a peroxy-radical [108] (Equation (4))  
\[\text{CF}_{3}(\text{CF}_{2})_{6}\;\text{COO}^{-}+\text{h}^{+}_{\text{VB}} \rightarrow\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\cdot+\text{CO}_{2} \tag{3}\]  
\[\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\cdot+\text{O}_{2}\rightarrow \text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{OO} \tag{4}\]  
The coupling of two peroxyradicals forms molecular oxygen and two oxyradicals (Equation (5)). \[2\;\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{OO}\cdot\to 2\; \text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{O}\cdot+\text{O}_{2} \tag{5}\]  
The oxyradical undergoes electron-coupled hydrogen abstraction from water and evolves to an unstable primary perfluorinated alcohol [109] (Equation (6)), which eventually leads to acylfluoride and hydrogen fluoride [110] (Equation (7))  
\[\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{O}\cdot+\text{H}_{2}\text {O}+\text{e}^{-}_{\text{CB}}\rightarrow\text{CF}_{3}(\text{CF}_{2})_{5}\text {CF}_{2}\text{OH}+\text{OH}^{-} \tag{6}\]  
\[\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{OH}\rightarrow\text{CF}_{3 }(\text{CF}_{2})_{5}\text{C}(\text{O})\text{F}+\text{HF} \tag{7}\]  
Finally, the acyl fluoride undergoes hydrolysis to the C6 carboxylic acid (Equation (8))  
\[\text{CF}_{3}(\text{CF}_{2})_{5}\text{C}(\text{O})\text{F}+\text{H}_{2}\text {O}\rightarrow\text{CF}_{3}(\text{CF}_{2})_{5}\text{COOH}+\text{HF} \tag{8}\]  
This in turn can undergo the same photo-redox mechanism several times until the chain length is reduced step by step [111]. Another pathway relies on the capability of the oxyradical C7 generated in Equation (5) to undergo monomolecular \(\beta\)-scission by eliminating fluorofosgene (COF\({}_{2}\)) and giving rise to a C6 perfluoroalkyl radical [112, 113, 114] (Equation (9)). \[\text{CF}_{3}(\text{CF}_{2})_{5}\text{CF}_{2}\text{O}\cdot\rightarrow\text{ COF}_{2}+\text{CF}_{3}(\text{CF}_{2})_{4}\text{CF}_{2} \tag{9}\]  
Fluorofosgene is easily hydrolyzed to HF and CO\({}_{2}\) (Equation (10))  
\[\text{COF}_{2}+\text{H}_{2}\text{O}\rightarrow\text{CO}_{2}+2\;\text{HF} \tag{10}\]  
The perfluoroalkyl radical can undergo the reaction steps as described above, starting from Equation (4). The mechanism described above is schematically represented in \[\text{TiO}_{2}+\text{hv}\rightarrow\text{TiO}_{2}\;(\text{e}^{-}_{\text{CB} };\text{h}^{+}_{\text{VB}}) \tag{2}\]0  
Notably, the \(\beta\)-scission mechanism (Equations (9) and (10)) induces the decomposition of PFOA without forming carboxylic intermediates, which instead are produced when the photoredox mechanism (Equations (3)-(8)) occurs. Therefore, the concentration of shorter-chain perfluorinated acids should increase proportionally to fluoride formation if the photoredox mechanism preferentially occurs. [PERSON] et al.