# Activation of Persulfate for Groundwater Remediation:
## 3 Applications and Development of Persulfate in ISCO  
In clean-up activities at numerous contaminated sites over the past 20 years, persulfate activated by various techniques has been employed with a satisfactory removal result. To improve the remediation efficiency, the multiple activation methods discussed above are usually combined (Table 5). The application mechanism of ISCO using persulfate is shown in Figure 1.  
Because only the activator and persulfate need to be injected into the underground through injection well, chelated iron activation and alkaline activation are currently the two most widely employed technologies for persulfate-based ISCO applications. The base is often sodium hydroxide (NaOH) or potassium hydroxide (KOH), and typically very high concentrations are needed to obtain good remediation results at the field application [69]. A considerable removal efficiency was obtained in several site remediation cases with alkaline activations. After three months of persulfate and hydroxide injection in the vadose zone and treatment region, TPH in existing wells were completely depleted, and TPH mass was reduced by 84% [108]. Over 90% of ethylbenzene and 87% of benzene were removed from PAHs and BTEX-contaminated areas in Brownfield, New York, following the application of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Target Contaminats**} & \multicolumn{4}{c}{**Reaction Conditions**} & \multirow{2}{*}{**Removal**} \\ \cline{2-2} \cline{4-6}  & & **Reaction** & & **Temperature** & **(\({}^{\circ}\)C)** & **(\%)** & **Ref.** \\ \hline PAHs & 223 mmol/L & 56 & & 6.5 & 20 & 35 & [103] \\ TCE & 1 g/L & 14 & & 7.23 & 25 & 92.4 & [105] \\  & 1 g/L & 14 & nano- Fe3O4 & 7.23 & 25 & 97.4 & \\ PCBs & 30\% by weight & 10 & & 4.9 & 23 & 77.9 & [106] \\ Dimethl & 3.14 g/L & 28 & nano- Fe3O4 & 7.7 & 20 & 97.92 & [102] \\ phthalate & & & & & & \\ PCBs & 20\% by weight & 15 & pH = 12 & 12 & - & 40.5 & [107] \\ BDE-209 and Cu & 0.1 mol/L & 10 & Coupled with NaNO\({}_{3}\) & 5.1 & - & 88.6 & [104] \\ PHE & 10\% & 25 & 0.01 mol/L NaNO\({}_{3}\) & 7.99 & 15 & 78.8\% & [54] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Bench-scale and laboratory studies with electrically activated persulfate. Figure 1: The application mechanism of ISCO using persulfate. alkaline activation [109]. To obtain a satisfactory remediation performance from the alkaline activation, the pH needs to be raised to more than 12. However, the pH in actual soil cannot reach the desired value due to the strong buffering capacity of the soil. For example, the pH was raised to 9.1 after 6.5 weeks injection and then decreased to 6.8 in 2 months [110]. It is a feasible way to maintain the alkaline conditions of the site by constructing a waterproof curtain to prevent the dilution of groundwater. The use of thermally activated persulfate to degrade organic pollutants has good application prospects for site remediation. Depending on the features of the remediation site, different heating techniques are applied, such as steam-enhanced heating, electrical resistance heating, and thermal conductive heating [44]. To reduce the high cost of heating, the temperature of approximately 40 degrees C was recommended in the field [111]. Electro-activated remediation has only recently been used in site remediation, and most of the cases were on a pilot scale. [PERSON] (2020) applied electrokinetic-activated persulfate for chlorinated solvents remediation in a clay site [56]. It was reported t that the subsurface temperatures along the transect were higher than the 35-40 degree C target range for persulfate activation. Substantial decreases (over 80%) in total CVOCs were observed at the monitoring well with the persulfate breakthrough. Another way to add heat is by adding CaO, which can react with H\({}_{2}\)O to release a large amount of heat, and increases the site temperature. [PERSON] et al. (2004) used CaO with persulfate to remediate BTEX and naphthalene in a site application and 93% of the contaminant was removed [112].