## 4 Electrochemical Aqueous Remediation of Fluorinated Compounds  
Non-thermal degradation techniques used to remediate PFAS in water include techniques such as separation (electrocoagulation and fractionation) and electrochemical treatment (oxidation and reduction). In terms of electrochemical treatment, a promising approach is electrochemical oxidation (EO) using a boron-doped diamond (BDD) as an anode to treat PFAS-contaminated water. Previous studies have investigated the potential of this technique [108; 109; 110], among investigated materials to use as an anode, the BDD has been shown to have the highest reactivity for PFOA and PFOS degradation [110; 111; 112]. In this technique, the BDD anode potential induces oxidization that forces the transfer of electrons from oxygen atoms of the carboxylate group of PFOA or sulfonate group of PFOS to the BDD, resulting in the decomposition of PFOA and PFOS. However, a challenge associated with EO treatment using a BDD anode is that chloride in water is readily converted to perchlorate (ClO\({}_{4}\)\({}^{-}\)), and anodes with higher EO activity tend to produce more perchlorate. Most recently, [PERSON] et al., (2019) used the BDD anode as a model to treat PFOA and PFOS in a chloride-bearing electrolyte and found that by adding hydrogen peroxide in a quantity greater than or equal to 50 mM H\({}_{2}\)O\({}_{2}\), the formation of perchlorate can be largely inhibited [113]. In their study, they found that a standard EO process without hydrogen peroxide can result in 40% of chloride converting into perchlorate. Using similar methods to previous studies that used a BDD anode, adding hydrogen peroxide to their strategy suppressed 88% of perchlorate formation during treatment. This happens because hydrogen peroxide quenches chlorine and hydroxyl radicals, thus inhibiting perchlorate formation. Additionally, they observed that the oxidation of PFOA and PFOS is primarily determined by direct electron transfer oxidation, regardless of the dominant radical species, meaning hydrogen peroxide would not largely inhibit PFAS degradation. This is considered a promising technique because only 4.0 L of 30% H\({}_{2}\)O\({}_{2}\) can assist in the treatment of 1 m\({}^{3}\) of groundwater at only an additional cost of USD 0.6 to 1.2/m\({}^{3}\) with the current market price. While the addition of hydrogen peroxide to the BDD approach is inexpensive, BDD electrodes have a high cost of approximately USD 7000/m\({}^{2}\) which limits their use for large scale application [114]. Another promising anodic material, Ti\({}_{4}\)O\({}_{7}\), has been explored for use in the removal of PFOA and PFOS from contaminated water [115; 116; 117]. [PERSON] et al., (2019) used a Magneli phase Ti\({}_{4}\)O\({}_{7}\) reactive electrochemical membrane (REM) in a single-pass, flow-through operation mode as a function of anode potential and flow rate to investigate the extent of PFOA and PFOS degradation [112]. The criteria used to assess performance were PFOA/PFOS conversion, extent of defluorination, and energy consumption compared to other technologies. Samples were analyzed using ion chromatography and LC/MS/MS based on of EPA Method 537.1 [54]. Compared to BDD anodes, they found that Ti\({}_{4}\)O\({}_{7}\) anodes show the best performance with the lowest energy consumption due to their larger surface area. In this study, 5.1 \(\pm\) 0.1 kWh/m\({}^{3}\) for PFOA and 6.7 \(\pm\) 0.2 kWh/m\({}^{3}\) for PFOS were needed to remove initial concentrations of 4.14 mg/L for PFOA and 5 mg/L for PFOS to below detection limits (86 ng/L for PFOA and 35 ng/L for PFOS). They also noted that the highest removal rates they achieved were 3415 \(\pm\) 203 umol/m\({}^{2}\)/h for PFOA and 2436 \(\pm\) 106 umol/m\({}^{2}\)/h for PFOS at a residence time of 3.8 s. These values for removal rates and energy consumption are among the lowest reported for electrochemical oxidation currently. Future work needed for REM anodes involves analyzing the success of operating multiple REMs in series in real waste streams.