## 6 Mechanisms for PMS Activation by Copper-Based Catalysts
### Non-Radical Pathways  
Nonetheless, in other PMS systems activated by copper-based catalysts, radicals have a limited role; in this case, the degradation of contaminants is primarily prompted by the non-radical route containing \({}^{1}O_{2}\)[209, 210], electron transfer [211, 212], and complexes. \({}^{1}O_{2}\) is a powerful ROS with high reactivity due to two electrons occupying the same energy level. It is effective in oxidizing organic pollutants. Generally, there exist three origins of production sources of \({}^{1}O_{2}\), involving the transformation of superoxide anion radicals (Equations (26)-(28)) [213]. \[O_{2}^{\bullet-}+HO^{\bullet}\to 1O_{2}+OH^{-} \tag{26}\]  
\[2O_{2}^{\bullet-}+2H^{+}\to H_{2}O_{2}+1O_{2} \tag{27}\]  
\[2O_{2}^{\bullet-}+2H_{2}O\to 2 OH^{-}+1O_{2}+H_{2}O_{2} \tag{28}\]  
The activation of PMS happens immediately (Equations (29) and (30)) [214, 215, 216]. Moreover, the transformation of functional groups occurs. \[HSO_{5}^{-}\to SO_{5}^{2-}+H^{+} \tag{29}\]  
\[SO_{5}^{2-}+HSO_{5}^{-}\to O_{2}+HSO_{4}^{-}+SO_{4}^{2-} \tag{30}\]  
\({}^{1}O_{2}\) exhibits high selectivity in terms of the types of compounds it reacts with; electron-rich organic pollutants, like phenols, tend to be its target [217]. Understanding the selectivity of \({}^{1}O_{2}\) is crucial when designing and implementing oxidation processes for environmental remediation. It enables the development of tailored approaches for the efficient removal of specific pollutants while minimizing the impact on non-target compounds. Different types of catalyst materials based on copper can induce the generation of \({}^{1}O_{2}\) in PMS systems. For instance, copper-based catalysts containing graphite N (nitrogen) groups [218], as well as catalysts featuring C=O (carbonyl) groups, have been investigated. The carbonyl group, proposed as the active site on the aromatic ring, enhances PMS decomposition by forming a metastable dioxirane compound (\"C-O-O-C\") as the crucial intermediate for \({}^{1}O_{2}\) formation. Additionally, magnetic catalysts with carbon-based carriers were studied [219]. All these catalysts facilitate the production of \({}^{1}O_{2}\) when they interact with PMS. The generation of \({}^{1}O_{2}\) provides an alternative non-radical pathway for pollutant removal in PMS systems. Additionally, the pathway of electron transfer also contributes significantly to the removal of pollutants. In general, catalysts act as efficient electron transfer agents, trans-ferring electrons from adsorbed hazardous substances to PMS, thereby enabling redox reactions [220]. Complex formation typically occurs on carbon-supported magnetic materials. That is, positively charged carbon creates a stable complex with significant redox capacity owing to the intense electrostatic attraction interacting with PMS (Equation (31)) [33; 221]. \[Catalyst+PMS\to Catalyst-PMS^{*} \tag{31}\]  
In contrast to the radical pathways, the activation mechanisms investigated in the subsequent systems are primarily initiated by the non-radical pathways. For instance, [PERSON] et al. [209] prepared a CuO-_CeO\({}_{2}\)_ composite catalyst to activate PMS via an easy hydrothermal-calculation approach. In the degradation process of RhB, the authors found that \({}^{1}O_{2}\) was the dominant ROS in the CuO-_CeO\({}_{2}\)_/PMS system. X-ray photoelectron microscopy analysis revealed that electron transfer occurred between CuO and _CeO\({}_{2}\)_, along with the formation of oxygen vacancies in _CeO\({}_{2}\)_, which was responsible for enhancing the generation of \({}^{1}O_{2}\). This process introduces a new non-radical oxidation pathway for activating PMS using the CuO-_CeO\({}_{2}\)_ catalyst. [PERSON] et al. [212] developed a series of single-atom Cu embedded on porous N-doped biochar (SACu@NBC) as an economical catalyst to activate PMS to degrade large amounts of BPA. The presence of common radical scavengers did not hinder BPA elimination, indicating that radicals were not present.