## 4 Factors Influencing Reactivity
### Impact of pH on PMS Activation  
The behavior of PMS is affected by the pH of the solution. Alkali conditions can activate the PMS. In acidic conditions (pH below 5), hydrogen ions operate as quenchers for \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\)[168; 169]. Overall, PMS can be applied across a broad pH range. The \begin{table}
\begin{tabular}{c c c c c} \hline \hline \begin{tabular}{c} **Scavenging** \\ **Agents** \\ \end{tabular} & \begin{tabular}{c} **Specific ROS** \\ \end{tabular} & \
\begin{tabular}{c} **Kinetic Constants** \\ **(M\({}^{-1}\)s\({}^{-1}\))** \\ \end{tabular} &
\begin{tabular}{c} **Determination of** \\ **ROS** \\ \end{tabular} & **Ref.** \\ \hline \multirow{3}{*}{MeOH} & \(SO_{4}^{\bullet-}\) & \(3.2\times 10^{6}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [36] \\  & \(HO^{\bullet}\) & \(9.7\times 10^{8}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [155] \\ \hline EtOH & \(SO_{4}^{\bullet-}\) & \(1.6\)–\(7.7\times 10^{7}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [155] \\  & \(HO^{\bullet}\) & \(1.2\)–\(2.8\times 10^{8}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [158] \\ \hline TBA & \(SO_{4}^{\bullet-}\) & \(4\)–\(9.1\times 10^{5}\) & \(HO^{\bullet}\) & [158] \\  & \(HO^{\bullet}\) & \(3.8\)–\(7.6\times 10^{8}\) & \(HO^{\bullet}\) & [166] \\ \hline p-BQ & \(O_{2}^{\bullet-}\) & \(1.0\times 10^{9}\) & \(O_{2}^{\bullet-}\) & [166] \\ \hline FFA & \({}^{1}O_{2}\) & \(3.2\times 10^{7}\) & \({}^{1}O_{2}\) & [160] \\ \hline NaN\({}_{3}\) & \({}^{1}O_{2}\) & \(1.2\times 10^{8}\) & \({}^{1}O_{2}\) & [162] \\ \hline Phenol & \(SO_{4}^{\bullet-}\) & \(8.8\times 10^{9}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [165] \\  & \(HO^{\bullet}\) & \(6.6\times 10^{9}\) & \(SO_{4}^{\bullet-},HO^{\bullet}\) & [165] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The rate constants of scavenging agents often employed with active species. influence of pH across a broad spectrum (pH 3-11) on the activation of PMS through various Cu-based catalysts has been examined [93; 170]. As an example, [PERSON] et al. [170] investigated a pH range spanning from 3 to 11 and identified the most suitable pH as 6.5. The optimal pH values vary among different experiments ranging from pH 2 [171] to 5 [172] and to 11 [93]. There appears to be no consistent pattern regarding the effects of pH, possibly owing to the subsequent causes: Initially, a low pH leads to the dissolution and subsequent liberation of \(Cu^{2+}\) within the aqueous solution. In consequence, this could modify the surface structure of Cu-based materials, resulting in a reduction in reactivity. At low pH values, the abundance of protons could potentially challenge the Cu-based materials for PMS binding, thereby impeding the catalytic reactivity (Equations (10) and (11)). \[SO_{4}^{\bullet-}+H^{+}+e^{-}\to HSO_{4}^{\bullet-} \tag{10}\]  
\[HO^{\bullet}+H^{+}+e^{-}\to H_{2}O \tag{11}\]  
Furthermore, the reactivity can be influenced by the catalytic interactions with PMS. Cu-based catalysts exhibit varying pHpzc (pH at the point of zero charge) values, leading to distinct surface charges at different pH levels. The variation in surface charges will influence the interaction between the Cu-based catalyst and PMS through electrostatic repulsion, consequently impacting the overall reactivity [173]. Thirdly, the transformation of radicals under different pH environments can lead to varying pH influences. The depletion of sulfate radicals can occur through their reaction with \(HO^{-}\), bringing about the formation of hydroxyl-based radicals (Equation (1)) [174]. The redox potential of \(HO^{\bullet}\) varies with pH, potentially affecting its reactivity. As an example, the redox potential of \(HO^{\bullet}\) has been observed to be 2.7 V in acidic solution but decreases to 1.8 V at neutral pH [175; 176]. Moreover, the half-life of \(SO_{4}^{\bullet-}\) is longer (30-40 \(\upmu\)s) in water compared with that of the conventional \(HO^{\bullet}\) (\(10-3\)\(\upmu\)s) [177].