## 2 Cu-Based Catalyst-Activated PMS Processes
### Magnetic Copper Ferrite  
The heterogeneous spinel-type ferrites \(MFe_{2}O_{4}\) (M=Co, Zn, Ru, Cu, Ni, Mn, etc.) have garnered notable interest due to their remarkable properties, including relatively high activation performance, specific structure, outstanding recyclability, and stability, in addition to magnetic retrieval capability [41; 79; 80]. Among them, copper ferrite (\(CuFe_{2}O_{4}\)) has been extensively researched for its ferromagnetic characteristics and inherent surface hydroxyl sites. It combines the benefits of Cu and Fe, offering a synergistic effect that enhances ROS generation and pollutant degradation. Its magnetic properties allow for easy separation and recovery from the treated water, making it highly reusable and cost-effective. \(CuFe_{2}O_{4}\) also exhibits high stability and efficiency under a wide range of operational conditions. These materials have been widely studied owing to their effective function in catalyzing PMS for the degradation of persistent pollutants [81; 82]. Considering other \(MFe_{2}O_{4}\), such as \(CoFe_{2}O_{4}\), \(Nife_{2}O_{4}\), \(MnFe_{2}O_{4}\), \(ZnFe_{2}O_{4}\), etc., even though its catalytic efficiency is relatively low compared to that of \(CoFe_{2}O_{4}\), the low toxicity of \(CuFe_{2}O_{4}\) makes it a significant activator of PMS [83]. Moreover, a notable linear correlation has been shown between the number of surface hydroxyl sites and the degradation rate [84]. The diversity of preparative methods for obtaining copper ferrite, including the hydrothermal method [85], solid-state reaction method [86], coprecipitation technique [87], sol-gel method [88], sonochemical method [89], microwave (MW)-assisted synthesis method [90], and combustion synthesis method [38], leads to lower operational costs and increased catalytic activity without additional pollution [91]. Therefore, \(CuFe_{2}O_{4}\) is highly regarded as an excellent catalyst for the elimination of contaminants of organic origin, as indicated by several previous studies. The literature reports several examples of its ability of activate PMS. Figure 2 presents a schematic illustration of the mechanisms of PMS activation by \(CuFe_{2}O_{4}\). [PERSON] et al. [36] observed that bisphenol A (BPA) was degraded within 1 h using 0.3 gL\({}^{-1}\) PMS and 0.6 gL\({}^{-1}\)\(CuFe_{2}O_{4}\) at a neutral pH, while the performance of a CuO/PMS system was only 26.1% at the same time. This highlights that \(CuFe_{2}O_{4}\), as a polymetallic catalyst, exhibits faster and better catalytic performance than single-metal catalysts due to the synergistic effect. A beneficial cycle is created between \(Cu^{3+}/Cu^{2+}\)and \(Fe^{3+}/Fe^{2+}\), enhancing the generation of radicals and facilitating the smooth removal process. It is notable that surface-bound radicals of \(CuFe_{2}O_{4}\) were primarily responsible for the removal of BPA with few contributions of radicals in the solution. [PERSON] et al. [37] observed that the \(CuFe_{2}O_{4}\)/PMS system could eliminate 99% of tetrabromobisphenol A (TBBPA) within 30 min. Similarly, [PERSON] et al. [92] reported that 98% of ATZ was removed within 15 min using 1 mM PMS and 0.1 gL\({}^{-1}\)\(CuFe_{2}O_{4}\). Additionally, the excellent stability of the chemical composition and surface functional groups of \(CuFe_{2}O_{4}\), coupled with its reusability, are important advantages that make \(CuFe_{2}O_{4}\) a competent activator for PMS. It was observedthat even after seven recycling cycles, \(CuFe_{2}O_{4}\) retained good crystallization and high activity during the degradation of isopromide [73]. \(CuFe_{2}O_{4}\) was characterized by Fourier transform infrared spectroscopy before and after reaction. The post-reaction spectrum depicted that the four adsorption bands changed minimally, indicating that the chemical composition and structure of \(CuFe_{2}O_{4}\) were not destroyed and that \(CuFe_{2}O_{4}\) exhibited excellent stability throughout the reaction process [93].