## 6 Mechanisms for PMS Activation by Copper-Based Catalysts
### Radical Pathways  
Copper-based catalysts trigger PMS to generate \(SO_{4}^{\bullet-}\), \(HO^{\bullet}\), and \(O_{2}^{\bullet-}\)[29]. Considering their strong redox aptitude, \(SO_{4}^{\bullet-}\), \(HO^{\bullet}\), and \(O_{2}^{\bullet-}\) could interact alongside specific contaminants and degrade them into lesser-sized molecules. Across most systems, \(SO_{4}^{\bullet-}\) plays the key role as the active agent. To illustrate, [PERSON] et al. [79] utilized a sol-gel combustion approach to synthesize a \(CuFe_{2}O_{4}\) catalyst for triggering PMS. In the same research, TBA and EtOH were employed to inhibit \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\). The inhibitory effect of EtOH on TBBPA was significantly stronger compared to that on TBA. An EPR experiment showed DMPO-X's characteristic peak, pointing out that DMPO was oxidized by \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\). Consequently, the system was overshadowed by a \(SO_{4}^{\bullet-}\)-centered radical system. [PERSON] et al. [95] fabricated \(CuFeO_{2}\) rhombohedral crystals to initiate PMS activation for the disintegration of SDZ. EtOH and TBA were opted for as agents to counteract the presence of \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) radicals. The results demonstrated that EtOH exhibited a higher inhibition effect on SDZ removal than TBA did, thereby affirming the prevalence of a radical mechanism primarily governed by \(SO_{4}^{\bullet-}\). Furthermore, \(HO^{\bullet}\) played a crucial function in certain systems, possibly influenced by the reaction pH or the specific characteristics of the specified substance [208]. [PERSON] et al. [29] utilized ZVCu for the activation of PMS and the removal of NPX. DMPO-\(SO_{4}^{\bullet-}\) and DMPO-\(HO^{\bullet}\) signals were observed by ESR, and DMPO-\(HO^{\bullet}\) signals were more intense than DMPO-\(SO_{4}^{\bullet-}\) signals. To additionally illustrate the presence of active species, MeOH and TBA were utilized, and the suppression effect of TBA on NPX in the system was slightly less potent that of MeOH. Hence, the two radicals, \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\), in the ZVCu/PMS system participate in the removal reaction. Low-valent transition metal ions catalyze the activation of PMS to generate \(SO_{4}^{\bullet-}\). Afterward, these radicalsreact with \(H_{2}O\) or \(HO^{-}\) in the solution, resulting in the generation of \(HO^{\bullet}\), which plays a primary role in the system. Nevertheless, quenching and EPR experiments cannot be used to investigate the presence of radicals in all PMS systems. In certain PMS systems, other approaches have been employed by some researchers to illustrate the reactive mechanism. A sol-gel method was utilized to synthesize a copper ferrite catalyst under the assumption of the sole existence of \(HO^{\bullet}\) in the reaction system [92]. Hence, the elimination of nitrogen-zene (NB) and ATZ by a \(CuFe_{2}O_{4}\)/PMS system was compared, and the contribution of \(HO^{\bullet}\) on the catalyst surface was investigated according to the ratio of the quasi-first-order rate constant of reaction kinetics of each system during the degradation process. The results showed that the actual value of the ratio of quasi-first-order rate constants (\(\mathbf{k}_{\text{ATZ}}/k_{\text{NB}}=6.4\)) was much higher than the theoretical value (0.5\(\sim\)1), which proves that \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) are the main active species on the catalyst surface.  
A copper-based catalyst triggers the activation of PMS, leading to the formation of powerful radicals such as \(SO_{4}^{\bullet-}\), \(HO^{\bullet}\), and \(O_{2}^{\bullet-}\). These radicals exhibit robust oxidative capabilities, enabling them to effectively target pollutants. The pollutants undergo degradation, breaking down into smaller molecules such as \(CO_{2}\), inorganic oxides, and \(H_{2}O\) in a short time. Overall, the radical pathway proves to be a highly efficient approach for eliminating pollutants through AOPs.