## 2 Cu-Based Catalyst-Activated PMS Processes
### Copper Oxide  
Copper oxide refers to a compound composed of copper and oxygen. It has several important properties and applications. Copper oxide, as a reliable, cost-effective, and low-toxicity heterogeneous catalyst, exhibits an excellent catalytic efficiency [66]. CuO is recognized as being among the most significant catalysts and a strong contender to trigger PMS [30; 67; 68; 69]. Evaluating the two most stable copper oxide catalysts (CuO and \(Cu_{2}\)O), the catalytic activity of CuO is higher than that of \(Cu_{2}\)O [70; 71].  
[PERSON] et al. [72] synthesized CuO for phenol degradation in aqueous solution via PMS activation. The findings indicated that the prepared CuO was proficient in catalyzing the decomposition of PMS into \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\), demonstrating its effectiveness for phenol elimination. In this research, it was assumed that \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) are produced directly from the reaction, in which \(Cu^{2+}\) is initially reduced to \(Cu^{+}\) by \(HSO_{5}^{\bullet-}\)[73]. The outstanding catalytic performance of copper oxide in activating PMS has been well recognized. However, directly using CuO as a catalyst for activating certain oxidants in an aqueous solution still requires more attention because of the leaching of \(Cu^{2+}\) ions; CuO can only work efficiently within a limited pH range [72], and the catalysts are difficult to separate from the reaction system [73]. [PERSON] et al. [73] compared the stability of copper oxide and copper-substituted ferrite and reported that only 1.5 \(\pm\) 0.1 ug\(\cdot\)L\({}^{-1}\) of Cu ions leached from CuFeO, whereas 46 \(\pm\) 3 ug\(\cdot\)L\({}^{-1}\) leached from CuO, using identical parameters after catalytic PMS activation. This demonstrates that CuO exhibits lower stability and efficiency in redox processes as a metal catalyst. Nevertheless, the issues with the dissolution and aggregation of the copper ions curtail the real-world use and reduce the activity of copper oxide. Addressing such shortcomings and further improving the powdered CuO catalyst can be accomplished by integrating the metal oxide onto a substrate with high stability, strong adsorption affinity, and high surface area toward the contaminant, such as zeolite [74; 75],carbon fibers [76], and polymers [77]. To reduce CuO aggregation and increase its specific surface area for catalytic reactions, [PERSON] et al. [31] evaluated the catalytic performance of CuO/reduced graphene oxide (rGO) and reported that it exhibited much higher catalytic efficacy compared to CuO during the removal of 2,4,6-trichlorophenol. [PERSON] et al. [40] found that activated carbon-supported CuO could be recycled for three cycles, and the release of copper ions was insignificant. [PERSON] et al. [78] synthesized a composite of CuO supported on biochar as a PMS activator for acid orange 7 (AO7), methylene blue, atrazine (ATZ), rhodamine B (RhB), and ciprofloxacin removal in highly saline organic wastewater, which achieved excellent catalytic efficacy. All these findings indicated that incorporating CuO onto the supporter is an effective method for enhancing its catalytic performance.  
In terms of advantages, CuO is a stable catalyst that can be easily integrated into composite materials to enhance its performance. However, one of the main disadvantages of CuO is its tendency to leach copper ions into the water, which can limit its long-term applicability and may require additional treatment steps to mitigate this issue. Additionally, CuO's effectiveness is highly dependent on precise pH control, as its catalytic activity can significantly diminish outside of the optimal pH range.