# Activation of Persulfate for Groundwater Remediation:
## 2 Activation Mechanism of Persulfate for ISCO Application
### Electrical Activation  
The electrochemical activation of persulfate is a novel green technology. By applying a positive current using an iron anode, Fe\({}^{2+}\) is continuously produced by anodic corrosion (Equation (13). The regeneration of Fe\({}^{2+}\) from Fe\({}^{3+}\) can be enhanced by the reduction of Fe\({}^{3+}\) on the cathode (Equation (14), which further improves the utilization of Fe\({}^{2+}\)[99; 100]. As the production of Fe\({}^{2+}\) is proportional to the current based on Faraday law, in situ manipulation (i.e., increasing or inhibiting) of persulfate reactivity is theoretically made feasible by adjusting the current [101]. In addition, persulfate was injected to a subsurface matrix such as clay. The transport distance may be extremely low due to a low hydraulic conductivity [102]. Electrical activation seems to be a promising technology in in situ persulfate-based applications when facing such circumstances, since persulfate can be delivered under an electrical current [53]. \[\mathrm{Fe}-2\mathrm{e}^{-}\rightarrow\mathrm{Fe}^{2+} \tag{13}\]  
\[\mathrm{Fe}^{3+}+\mathrm{e}^{-}\rightarrow\mathrm{Fe}^{2+} \tag{14}\]  
For this technology, which was only developed a few years ago, a handful of studies in the literature reported the applications in groundwater remediation (Table 4). [PERSON] et al. (2007) used electro-activated persulfate to degrade PAHs, which was further comparted with electrokinetic and persulfate oxidation systems [103]. The results showed that the removal efficiency of PAHs in the electro-activated persulfate system was 35%, which was higher than that in electrokinetic (24%) and persulfate oxidation (12%) systems [103]. [PERSON] et al. (2015) studied the in situ persulfate oxidation process of TCE by electrical activation, which significantly improved the transfer of injected persulfate from the anode reservoir to the cathode reservoir via the soil compartment resulting in a good remediation efficiency [102]. Electrical activated persulfate coupled with NaNO\({}_{3}\) to flush the cathode and attain a higher conductivity, achieved a good removal efficiency for BDE (88.6% was removed) [104] and PHE (78.8% was removed) [54]. When electro-activation is used with persulfate, the location of the injection wells should be considered properly as the transport rates of persulfate ions in the subsurface are different, and mainly dominated by electroosmosis and electromigration. [PERSON] et al. (2014) pointed out that the preferred location for persulfate injection may be the cathode reservoir after comparing the transport distance of persulfate and degradation of the contaminant (PCB) under different persulfate injection locations including anode, cathode and both electrodes [101]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Target**} & \multicolumn{5}{c}{**Reaction Conditions**} & \multirow{2}{*}{**Removal**} \\ \cline{2-2} \cline{4-7}  & & **Reaction** & & & **Temperature** & **Rate (\%)** & **Ref.** \\ \cline{2-7}  & & **Time (d)** & & & **(\({}^{\circ}\)C)** & & **(\({}^{\circ}\)C)** \\ \hline PAHs & 1.85 mol/kg soil & 1 & Fe\({}^{2+}\) & 7.2 & 25 \(\pm\) 1 & 65 & [94] \\ TCE & 0.009 mol/kg soil & 1 & Fe\({}^{2+}\) & 4.2 & 20 & 100 & [76] \\ TPHs & 0.021 mol/kg soil & 30 & Fe\({}^{2+}\) & 5.63 & 25 \(\pm\) 1 & 40 & [15] \\ HCHs & 0.3 mol/kg soil & 1 & Fe\({}^{2+}\) & 2 & 25 \(\pm\) 1 & 89 & [95] \\ PCBs & 1 mol/L & 3 & Fe\({}^{2+}\) & 6.61 & 25 & 81 & [78] \\ Atrazine & 50 mmol/L & 0.42 & Fe\({}^{2+}\) & 3.0 & 23 & 80 & [96] \\ PAHs & 200 mmol/L & 25 & Fe\({}^{3+}\)/n2 VI & 7.5 & 25 & 81.42 & [97] \\ PHE & 12 mmol/L & 2 & n2 VI & 2.8 & 25 & 76 & [92] \\ TBBPA & 25 mmol/L & 0.5 & n2 VI & 5.5 & 25 & 78.3 & [93] \\ PAHs & 1.2 & 7 & nano-Fe\({}_{3}\)O\({}_{4}\) & \(1.3\pm 0.2\) & \(24\pm 2\) & 63 & [98] \\ \hline \hline \end{tabular} Note: PAHs—Polycyclic Aromatic Hydrocarbons; TCE—Trichroethylene; TPHs—Total petroleum hydrocarbon; HCHs—Hexachlorocyclohexane; PCBs—Polychlorinated biphenyls; PHE—Phenanthrene; TBBPA—Tetrabromobisphenol A.  
\end{table}