# A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances
## 3 Treatment Techniques for Short-Chain PFASs
### Advanced Oxidation/Reduction Techniques
#### 3.2.2 Photocatalytic Degradation  
Direct photolysis. Photocatalytic degradation is an advanced oxidation technique that can mineralize target compounds by UV light and photocatalysts. However, PFAss cannot absorb light over 220 nm directly, and the degradation rate of PFBA and PFPeA was only 16.3% and 24.3% by direct photolysis, respectively [15]. [PERSON] et al. found that PFPeA could strongly absorb the light from the vacuum UV region to 220 nm due to the degradation of PFAss under VUV irradiation (<190 nm) by the active substances produced by hemolysis and ionization of water, involving a hydrated electron (\(\mathrm{e_{aq}}^{-}\)), hydrogen radical (H\(\cdot\)), and hydroxyl radical (-OH) [46]. Therefore, how to use catalysts to promote the generation of active substances about free radicals for the photodegradation of PFAss was imperative. The photocatalytic efficiency of PFAss with different catalysts is shown in Table 4.  
The sulfate radical (\(\mathrm{SO_{4}}^{-}\)) produced from persulfate (\(\mathrm{S_{2}O_{8}}^{2-}\)) under UV irradiation was observed to be more effective for PFAss degradation over -OH. Therefore, \(\mathrm{S_{2}O_{8}}^{2-}\) was used as an oxidant to degrade PFAS frequently [47]. For instance, [PERSON] et al. found that the short-chain PFCAs in aqueous solution were oxidized to \(\mathrm{CO_{2}}\) and F\({}^{-}\) by \(\mathrm{SO_{4}}^{-}\) when the concentration of \(\mathrm{S_{2}O_{8}}^{2-}\) was 50 mmol/L [46]. Subsequently, this group also found that Fe\({}^{3+}\) was an effective catalyst for photodegradation, and the photodegradation efficiency of PFBA and PFPeA were improved to 49.9% and 64.5% due to strong light absorption from the complexation of PFAss with Fe\({}^{3+}\)[47]. Water-soluble polyacid photocatalysts could also promote C-F bond cleavage of PFPrA, thereafter generated to TFA, \(\mathrm{CO_{2}}\), and F\({}^{-}\), but the energy consumption of the process was exorbitant [48]. Zero-valent iron (ZVI) reduction. Reductants including zero-valent iron particles or iodine salt could also enhance the photodegradation of PFAss. ZVI was used as a reductive agent for the photodegradation of PFAss due to its high reduction potential (\(-\)0.447 V) and reactive surface area [15]. For instance, [PERSON] et al. observed that the degradation efficiency of short-chain PFSAs (C2-C6) could be up to 95% using ZVI, owing to the generation of iron oxide in the ZVI surface, which could interact with PFAss ions synergistically, thus promoting defluoridation efficiently [49]. Photocatalytic degradation. Photocatalysts were scarcely implemented in short-chain PFASs; the photodegradation efficiency could be deduced by examining the degradation intermediates and degradation efficiency of long-chain PFAss. For example, [PERSON] et al. adopted TiO\({}_{2}\) photocatalysts for 120 mmol/L of PFOA degradation under UV irradiation at 254 nm within 7 h, 97% of PFOA were converted to short-chain intermediates, including PFPrA, PFBA, PFPeA, PFHxA, and PFHpA, in which PFHpA reached the maximum at 5 h [50]. Currently, the widely used method for modifying TiO\({}_{2}\) was doping with precious metals (Pt, Pd, Au) or other metals (Pb, Cu, Fe). [PERSON] et al. explored TiO\({}_{2}\) doped with Pt to completely decompose 144.9 mmol/L PFOA within 7 h under UV irradiation at 365 nm; the defluoridation efficiency was 34.8%. It was 12.5-fold faster than unmodified TiO\({}_{2}\) due to the deposited Pt particles could store excessive electrons and promote the electrons transferred to PFAS availably [51]. In addition, the composite materials were used to improve the photocatalytic degradation of PFAss, such as TiO\({}_{2}\)-MWCNT and TiO\({}_{2}\)-rGO. It was found that the degradation efficiency of PFOA could reach 100% when using TiO\({}_{2}\)-MWCNT after 8 h under irradiation of UV at 265 nm [52].