## 4 Factors Influencing Reactivity
### Anions  
In Cu/PMS systems, anions such as \(Cl^{-}\), \(HCO_{3}^{-}\), \(PO_{4}^{3-}\), and \(NO_{3}^{-}\) are able to interact with active oxidants, thereby impacting the efficiency of catalytic reactions. Chloride ions (\(Cl^{-}\)) are among the significant inorganic negatively charged ions present in water and in nearly all natural water sources [186]. On the other hand, \(SO_{4}^{\bullet-}\), \(HSO_{5}^{-}\), and \(HO^{\bullet}\) possess the capability to oxidize \(Cl^{-}\) into less reactive chlorine or hypochlorous. For this reason, the existence of \(Cl^{-}\) in the systems demonstrates a restraining influence [187,188]. Researchers have clarified that the impeding influence of chloride is linked to the depletion of \(SO_{4}^{\bullet-}\) and PMS in the process, which subsequently led to the generation of \(Cl^{\bullet}\) and \(Cl_{2}\), less active oxidizers [187,189]. This inhibitory effect can be succinctly illustrated in (Equations (17)-(21)).  
\[SO_{4}^{\bullet-}+Cl^{-}\to SO_{4}^{2-}+Cl^{\bullet} \tag{17}\]  
\[HSO_{5}^{-}+Cl^{-}\to SO_{4}^{2-}+HOCl \tag{18}\]  
\[HSO_{5}^{-}+2 Cl^{-}+H^{+}\to SO_{4}^{2-}+Cl_{2}+H_{2}O \tag{19}\]  
\[HO^{\bullet}+Cl^{-}\to HOCl^{\bullet-} \tag{20}\]  
\[HOCl^{\bullet-}+H^{+}\to Cl^{\bullet}+H_{2}O \tag{21}\]  
Moreover, bicarbonate ion (\(HCO_{3}^{-}\)) is indeed one of the frequently encountered inorganic anions found in wastewater and water. It is formed by the combination of carbon dioxide (\(CO_{2}\)) and water (\(H_{2}O\)). According to research findings [190], \(HCO_{3}^{-}\) and dihydrogen phosphate ions (\(H_{2}PO_{4}^{-}\)) have a dual influence on the removal of pollutants. They can quench \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) produced during the process and convert them into \(HCO_{3}^{\bullet}\) and \(H_{2}PO_{4}^{\bullet}\), respectively (Equations (22)-(24)).  
\[HCO_{3}^{-}+SO_{4}^{\bullet-}\to SO_{4}^{2-}+CO_{3}^{\bullet-}+H^{+} \tag{22}\]  
\[HO^{\bullet}+HCO_{3}^{-}\to H_{2}O+CO_{3}^{\bullet-} \tag{23}\]  
\[HO^{\bullet}+H_{2}PO_{4}^{-}\to HO^{-}+H_{2}PO_{4}^{\bullet} \tag{24}\]  
However, the oxidation potential of \(HCO_{3}^{\bullet}\) and \(H_{2}PO_{4}^{\bullet}\) is less than that of \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\), which hinders the overall reaction [191,192]. A large concentration of \(HCO_{3}^{-}\) can adjust the system's pH to a basic environment around pH 8.5 [193]; this condition is advantageous because it promotes the conversion of \(HSO_{5}^{-}\) to peroxymonosulfate (\(SO_{5}^{2-}\)), generating more opportunities for the generation of \({}^{1}O_{2}\)[92].  
\[SO_{4}^{\bullet-}+H_{2}PO_{4}^{-}\to SO_{4}^{2-}+H_{2}PO_{4}^{\bullet} \tag{25}\]  
Furthermore, the non-symmetric structure of PMS makes it susceptible to attack by nucleophiles like \(HCO_{3}^{-}\) and \(H_{2}PO_{4}^{-}\), leading to rapid decomposition. As a result, \(HCO_{3}^{-}\) and \(H_{2}PO_{4}^{-}\) can directly activate PMS [194]. Studies conducted by [PERSON] et al. [195] and [PERSON] et al. [196] reported that \(H_{2}PO_{4}^{-}\) and \(HCO_{3}^{-}\) have a beneficial impact on the elimination of pollutants. However, typically, \(H_{2}PO_{4}^{-}\) and \(HCO_{3}^{-}\) still act as inhibitors, hindering the overall degradation process.