# Activation of Persulfate for Groundwater Remediation:
## 2 Activation Mechanism of Persulfate for ISCO Application
### Iron Activation  
The transition metals and metal-chelate complexes can activate the persulfate at ambient temperatures and neutral pH without the need for additional additives or energy to regulate pH or temperature [5]. Fe\({}^{2+}\), Ag\({}^{+}\), and Cu\({}^{2+}\) metal ions have all been reported to significantly activate persulfate [31; 72]. Fe\({}^{2+}\) is widely used for persulfate-based ISCO, which is in contrast to Ag\({}^{+}\) and Cu\({}^{2+}\) of high toxicity and high cost [73]. The mechanism of Fe\({}^{2+}\) activate persulfate involves a one-electron transfer in which Fe\({}^{2+}\) provides an electron to persulfate anion, as shown in Equation (11) [74; 75]. \[\text{Fe}^{2+}+\text{S}_{2}\text{O}_{8}{}^{2-}\rightarrow\text{Fe}^{3+}+ \text{SO}_{4}{}^{\bullet-} \tag{11}\]  
Iron-activated persulfate has been shown to be suitable for the degradation of most refractory pollutants including PAHs [74], TCE [76],TPHs [75],HCHs [77] and PCBs [78] etc. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Target** \\ **Contaminats** \\ \end{tabular} } & \multicolumn{4}{c}{**Reaction Conditions**} & \multicolumn{1}{c}{**Removal**} & \multirow{2}{*}{**Ref.**} \\ \cline{2-2} \cline{4-6}  & **Persulfate Conc.** & & **Reaction** & & & **Type of Base** \\ \hline PAHs & 0.5 mmol/L & 3 & 10 & NaOH & 55 & [21] \\  & & & 11 & & 62 & \\  & & & 12 & & 65 & \\ HCHs and & & & & & & HCHs -90 & [57] \\ HeptaCHs & 100–400 mmol/L & 21 & Above 12 & NaOH & HeptaCHs -70 & [58] \\ TPHs & 420 mmol/L & 56 & 12 & NaOH & 98 & [58] \\ BCP & 0.21 mol/L & 3 & \textgreater{}10 & NaOH & 97 & [13] \\ Phenol & 210/420 mmol/L & 7 & 12 & NaOH & 100 & [59] \\ BDE & 0.2 mol/L & 0.25 & 12 & NaOH & 30.9 & [70] \\  & & & With 55 \({}^{\circ}\)C & NaOH & 46.6 & \\  & & & With Fe\({}^{2+}\) & NaOH & 51.5 & \\  & & & With ultrasonic & NaOH & 49.2 & \\ TCA and TCE & 10–75 mmol/L & 2 & 12 with 50 \({}^{\circ}\)C & NaOH & 100 & [71] \\ \hline \hline \end{tabular} Note: PAHs—Polycyclic Aromatic Hydrocarbons; TPHs—Total petroleum hydrocarbon; BDE—brominated diphenyl ethers; HCHs—Hexachlorocyclohexane; BCP—Bromocresol purple; TCE—Trichloroethylene; TCA—Trichloroanisole.  
\end{table} 
Table 2: Bench scale and laboratory studies with alkaline-activated persulfate. (Table 3). It was shown that the removal efficiency of organic pollutants increased first and then decreased with the increase the concentration of Fe\({}^{2+}\)[31; 73]. This is because the low concentration of Fe\({}^{2+}\) would cause inefficient utilization of persulfate, while a high concentration of Fe\({}^{2+}\) would lead to consumption of sulfate radicals by Fe\({}^{2+}\) (Equation (12)) [40]. \[\text{Fe}^{2+}+\text{SO}_{\text{4}}\bullet^{-}\rightarrow\text{Fe}^{3+}+\text{ SO}_{\text{4}}{}^{2-} \tag{12}\]  
To minimize the scavenging of SO\({}_{\text{4}}{}^{-}\) by Fe\({}^{2+}\), the molar ratio of Fe\({}^{2+}\) to persulfate must be optimized. [PERSON] and [PERSON] (2016) and [PERSON] et al. (2019) reported considerable contaminants degradation when the mole ratio of Fe\({}^{2+}\) to persulfate was 1:1 and 1:2 [41; 79]. In addition, multiple additions of Fe\({}^{2+}\) is another option. Several studies reported that multiple additions of Fe\({}^{2+}\) can maintain the continuous generation of SO\({}_{\text{4}}{}^{-}\) and has a higher pollutant degradation efficiency than a one-time addition of Fe\({}^{2+}\)[76; 80; 81]. [PERSON] et al. (2004) proved that the TCE removal efficiency reached 92% with two sequential additions of Fe\({}^{2+}\), which was higher than that of one addition of the equivalent dose of Fe\({}^{2+}\) (47%). [PERSON] et al. (2013) obtained a similar trend when multiple additions of Fe\({}^{2+}\) were applied to degrade bisphenol A. Due to the rapid reaction between metal ions and persulfate, the homogenous activation method is unable to achieve in situ and sustainable activation in the field practice [82; 83].