## 2 Cu-Based Catalyst-Activated PMS Processes
### Zero-Valent Copper
Nevertheless, the preferable technique is one that can significantly enhance efficiency, requiring less energy to initiate the activation process and being cost-effective compared to other activation methods. The utilization of metallic transition elements for activating PMS demonstrates significantly enhanced activation rates and user-friendliness. Many studies have indicated the performance of the combination of PMS with ZVCu [28; 59; 60]. The effectiveness of the coupling is associated with the production of extremely high levels of ROS such as \(SO_{4}^{\bullet-}\) or \(HO^{\bullet}\)[61]. ROS are powerful oxidants and can participate in the breakdown of pollutants in various environmental applications. Some research indicates that both radicals can coexist in the system, with elimination effectiveness linked to the generation of both species [62] or a predominant one [63]. Thus, in Figure 1, the ZVCu/PMS system can exhibit synergistic effects. PMS can activate ZVCu by transferring an electron to the ZVCu surface, generating Cu(I) species. These Cu(I) species can act as intermediates in catalytic reactions, facilitating the transformation of organic compounds. The reaction pH may influence the concentration of a particular species. Under alkaline conditions, the \(SO_{4}^{\bullet-}\) might undergo a reaction with the hydroxyl anion, leading to the production of the \(HO^{\bullet}\) radicals (Equation (1)) [64]. \[SO_{4}^{\bullet-}+HO^{-}\ \leftrightarrow\ HO^{\bullet}+SO_{4}^{2-} \tag{1}\]  
[PERSON] et al. [27] assessed the use of ZVCu as a catalyst for activating PMS in the elimination of benzene carboxylic acid (20 umol L\({}^{-1}\)). In this investigation, the corrosion of the transition metal was hastened by PMS via Equation (2). The proposed degradation mechanism resembled the one used with hydrogen peroxymonosulfate (\(HSO_{5}^{-}\)). Figure 1: Behavior model of ZVCu/PMS in solution proposed for pollutant degradation. \[2 Cu^{0}+HSO_{5}^{-}\ \leftrightarrow\ 2 Cu^{+}+OH^{-}+SO_{4}^{2-} \tag{2}\]  
To eliminate 2,4-dichlorophenol (2,4-DCP), [PERSON] et al. [28] utilized the ZVCu/PMS system. Under acidic conditions, the examination of the mechanism demonstrated that PMS accelerates ZVCu corrosion, releasing Cu(I). The produced Cu(I) can induce the production of ROS through chain reactions with oxygen or activate PMS. \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) radicals are among the ROS that participate in the elimination of 2,4-DCP. The steps involved in the removal process of the phenol derivative comprise dechlorination, dehydrogenation, hydroxylation, ring opening, and mineralization. Another study by [PERSON] et al. [65] evaluated the effectiveness of ZVCu in the decomposition of \(H_{2}\)O\({}_{2}\) and \(HSO_{5}^{-}\) for the removal of textile effluents. PMS/ZVCu exhibited higher efficiency in removing chemical oxygen demand and total organic carbon (TOC) compared to the \(H_{2}\)O\({}_{2}\)/ZVCu combination. Nevertheless, the authors did not scrutinize the species participating in the elimination to suggest a reaction mechanism model. [PERSON] et al. [29] examined the triggering of PMS using four distinct types of ZVCu, namely, copper foam, copper sheets, graphene-copper sheets, and graphene-copper foam, for the evaluation of the degradation mechanism of naproxen (NPX). ZVCu coated with graphene resulted in a 30% decrease in \(Cu^{2+}\) release and a 10% increase in NPX elimination efficiency. Additionally, electron spin resonance (ESR) and radical-scavenging analysis studies verified that hydroxyl radicals were the prevailing species accountable for the breakdown rather than sulfate radicals. It should be highlighted that the main advantages of ZVCu are its high capacity to generate \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) radicals and its relatively low-cost and easy production process, making it an attractive option for environmental remediation.