# A Review of Treatment Techniques for Short-Chain Perfluoroalkyl Substances
## 3 Treatment Techniques for Short-Chain PFASs
### Advanced Oxidation/Reduction Techniques
#### 3.2.1 Electrochemical Oxidation  
Electrochemical oxidation is an emerging advanced oxidation technique due to the advantages of high removal efficiency, strong oxidative ability, and low energy consumption. This technique was found to degrade long-chain PFAs effectively. The most commonly used oxidation anodes included Ti/SnO\({}_{2}\), Ce/PbO\({}_{2}\), boron-doped diamond (BDD) electrodes, and their modified electrodes. Comparisons on the removal efficiency of short-chain PFAs by different electrode materials are shown in Table 3.  
[PERSON] et al. 's research group found that the degradation efficiency of Ti/SnO\({}_{2}\)-Sb for 100 mg/L PFOA could reach 98.8% when the current density was 40 mA/cm\({}^{2}\), pH at 5, and the electrolyte with 10 mmol/L NaClO\({}_{4}\)[43]. Subsequently, the research group explored \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Anode**} & \multirow{2}{*}{**PFASs**} & **PFASs** & \multirow{2}{*}{**Experiment**} & \multirow{2}{*}{**Removal**} & \multirow{2}{*}{**References**} \\  & & **Concentration** & & & \\  & & **(mg/L)** & & & \\ \hline \multirow{4}{*}{Ti/SnO\({}_{2}\)-Sb/PbO\({}_{2}\)-Ce} & PFBA & & & 31.8 & \\  & PFPeA & & 20 mA/cm\({}^{2}\), & 41.4 & \\ \cline{1-1}  & PFHxA & & 10 mmol/L NaClO\({}_{4}\) & 78.2 & \\ \cline{1-1}  & PFHpA & & & 97.9 & \\ \hline \multirow{2}{*}{BDD} & \multirow{2}{*}{PFHxA} & \multirow{2}{*}{870} & 100 mA/cm\({}^{2}\), & \multirow{2}{*}{98} & \multirow{2}{*}{[41]} \\  & & & OM, inorganic salt & \\ \hline \multicolumn{2}{c}{Si/BDD} & PFBS & \multicolumn{1}{c}{\textgreater{}150} & rotating disk electrode & 90 & [42] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Removal efficiency of short-chain PFAs by different electrode materials. the Ti/SnO\({}_{2}\)-Sb/PbO\({}_{2}\)-Ce electrode to degrade short-chain PFASs of PFHpA and PFBA, with removal rates of 97.9% and 31.8%, respectively. This might be related to the high resistance of short-chain PFASs and the co-existence of multiple PFASs [40]. In addition, [PERSON] et al. used a BDD electrode to remove high concentrations of 870 mg/L PFHxA in a solution containing OM and inorganic salt, finding that the removal rate was 98% within 2 h when the current density was at 100 mA/cm\({}^{2}\), and energy consumption was 45 Wh/L [41]. [PERSON] et al. explored a Si/BDD electrode for the elimination of high concentration PFBS (>150 mg/L) at low current density; the removal rate was 90% within 1 h [42]. Based upon the above studies, the BDD and its modified electrodes achieved remarkable results compared with other electrodes involving SnO\({}_{2}\) and PbO\({}_{2}\). However, the base materials of Si were too weak and performed unfortunate electrical conductivity. Therefore, high manufacturing costs and lack of suitable base materials limited the large-scale application of BDD electrodes. At present, it is important to discover a cheap and stable base material for the industrial application of BDD electrodes [37]. The degradation of PFASs on the electrode surface was related to electron transfer and free radical oxidation; the proposed pathways for electrochemical oxidation of PFOS in water are shown in Figure 3.  
Initially, the electrons are transferred from the terminal functional group of PFASs to the anode driven by the electric field and formed PFAS radicals (-C\({}_{\mathrm{n}}\)F\({}_{\mathrm{2n+1}}\)COO or -C\({}_{\mathrm{n}}\)F\({}_{\mathrm{2n+1}}\)SO\({}_{3}\)), the extremely unstable PFAS radicals undergo decarboxylation or desulfation to form -C\({}_{\mathrm{n}}\)F\({}_{\mathrm{2n+1}}\). The generated -C\({}_{\mathrm{n}}\)F\({}_{\mathrm{2n+1}}\) could react with H\({}_{2}\)O, OH\({}^{-}\), or -OH, and finally -CF\({}_{2}\) groups were cut down gradually to generate short-chain PFASs with the specific reaction equations shown in (3)-(7) [44,45].