# Activation of Persulfate for Groundwater Remediation:
## 2 Activation Mechanism of Persulfate for ISCO Application
### Alkaline Activation  
Persulfate is slowly activated under acidic and weak alkaline conditions, while it is rapidly activated under strong alkaline conditions [68]. The reaction mechanism of base activation can be described as Equations (5)-(8). The sulfate radical (SO\({}_{4}\)\({}^{-}\)), hydroxyl radical (OH-) and superoxide radical (O\({}_{2}\)\({}^{-}\)) are generated during base activation. SO\({}_{4}\)\({}^{-}\) can directly react with OH- and H\({}_{2}\)O to produce OH-, which results in a large amount of OH- in the system at a pH higher than 12 [6]. O\({}_{2}\)\({}^{-}\) is quickly scavenged by SO\({}_{4}\)\({}^{-}\) and OH- [28], which is described in Equations (9) and (10), respectively. \[2\mathrm{S}_{2}\mathrm{O}_{8}\mathrm{{}^{2-}}+4\mathrm{OH}^{-}\to\mathrm{O}_{2 }\mathrm{{}^{-}}+\mathrm{SO}_{4}\mathrm{{}^{-}}+3\mathrm{SO}_{4}\mathrm{{}^{2-} }+2\mathrm{H}_{2}\mathrm{O} \tag{5}\]  
\[\mathrm{OH}^{-}+\mathrm{S}_{2}\mathrm{O}_{8}\mathrm{{}^{2-}}\to\mathrm{HO}_{2 }\mathrm{{}^{-}}+2\mathrm{SO}_{4}\mathrm{{}^{2-}}+\mathrm{H}_{2}\mathrm{O} \tag{6}\]  
\[\mathrm{SO}_{4}\mathrm{{}^{-}}+\mathrm{H}_{2}\mathrm{O}\to\mathrm{OH}\cdot +\mathrm{SO}_{4}\mathrm{{}^{2-}}+\mathrm{H}^{+} \tag{7}\]  
\[\mathrm{SO}_{4}\mathrm{{}^{-}}+\mathrm{OH}^{-}\to\mathrm{OH}\cdot +\mathrm{SO}_{4}\mathrm{{}^{2-}} \tag{8}\]  
\[\mathrm{O}_{2}\mathrm{{}^{-}}+\mathrm{SO}_{4}\mathrm{{}^{-}}\to\mathrm{SO}_{4} \mathrm{{}^{2-}}+\mathrm{O}_{2} \tag{9}\]  
\[\mathrm{O}_{2}\mathrm{{}^{-}}+\mathrm{OH}\cdot \to\mathrm{OH}^{-}+\mathrm{O}_{2} \tag{10}\]  
Table 2 summarizes various studies of the use of alkali-activated persulfate to degrade organic contaminants. [PERSON] et al. (2013) reported that the base-activated persulfate showed a good performance for remediation of the PAH-contaminated soil. When the initial pH was raised from 10 to 12, the removal efficiency of PAHs increased from 55% to 65% [21], [PERSON] (2020) reported that the alkali can not only promote the degradation of HCHs by activating persulfate, but also directly lead to the rapid dehydrochlorination of HCHs and HeptaCHs, which are converted into trichlorobenzenes (TCBs) and tetrachlorobenzes (TetraCBs), respectively [57]. After 21 days of treatment, the chlorinated organic compounds(COCs) conversion reached about 96% and 70% in two different soil fractions (2 mm \(<\) dp \(<\) 0.25 mm, dp \(<\) 0.25 mm) when using PS and NaOH concentrations of 400 mmol\({}^{-1}\)[57]. Great degradation efficiency were also observed when alkaline-activated persulfate was used for 4-bromo-2-chlorophenol (BCP) [13] and TPH [58]. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Target**} & \multicolumn{4}{c}{**Reaction Conditions**} & \multirow{2}{*}{**Removal**} \\ \cline{2-2} \cline{4-6}  & & **Reaction** & & **Temperature** & **Efficiency (\%)** & **Ref.** \\ \hline PAHs & 0.5 mmol L\({}^{-1}\) & 3 & 40 & 87 & [21] \\  & & & 50 & 96 & \\  & & & 60 & 99 & \\ TCE & 20 mmol L\({}^{-1}\) & 1 & 20 & 7.6 & [61] \\  & & & 30 & 31.6 & \\  & & & 40 & 100 & \\  & & & 50 & 100 & \\ TCE & 15 mmol L\({}^{-1}\) & 32 & 20 & 100 & [9] \\  & & 0.12 & 40 & 100 & \\  & & 0.35 & 50 & 100 & \\  & & 0.013 & 60 & 100 & \\ PCE & 15 mmol L\({}^{-1}\) & 0.063 & 50 & 92 & [63] \\ TPHs & 1000 mmol L\({}^{-1}\) & 2 & 55 & 72.64 & [64] \\ HCHs & 10 mmol L\({}^{-1}\) & 9 & 55 & 83 & [65] \\ Chlorinated Solvents & 100 mmol L\({}^{-1}\) & 3 & 50 & 79.5 & [66] \\ BPA & 8 mmol L\({}^{-1}\) & 0.013 & 30 & 98.5 & [62] \\ DDTs & 100 mmol L\({}^{-1}\) & 7 & 50 & 92–94 & [67] \\ \hline \end{tabular} Note: PAHs—Polycyclic Aromatic Hydrocarbons; TCE—Trichloroethylene; PCE—Perchloroethylene; TPHs—Total petroleum hydrocarbon; HCHs—Hexachloroxyclohexane; BPA—bisphenol A; DDTs—Dichloroethylene.  
\end{table} 
Table 1: Bench scale and laboratory studies with heat-activated persulfate. Several studies in the literature showed that an increase of the ratio of base to persulfate increased the degradation of the pollutants in the soil [57; 58; 59].