## 2 Activation Persulfate by Various Iron-Based Catalysts  
### MeFe\({}_{2}\)O\({}_{4}\) (Me = Cu, Co, Zn, etc.) In terms of activation mechanism, transition metal compounds react with PS to produce a large amount of \(\cdot\)SO\({}_{4}\)\({}^{-}\); the reaction equation follows:  
\[\mathrm{M^{n+}}+\mathrm{S_{2}O_{8}}^{2-}\rightarrow\mathrm{M^{(n+1)+}}+\mathrm{ SO_{4}}^{-}+\mathrm{SO_{4}}^{2-} \tag{1}\]  
As can be seen from the above reaction, metal ions are in a free state dispersed in the solution during the reaction process. Although the wastewater can be degraded by the activation persulfate mechanism, it belongs to homogeneous catalysis; metal ions will be dissolved in the aqueous solution, which causes difficult separation from solution. Therefore, the production cost is greatly increased due to its difficult recycling nature, and it is easy to cause secondary pollution to the environment. Therefore, MeFe\({}_{2}\)O\({}_{4}\) with a low metal leaching rate has become a new research direction. Through PS/PMS [36] heterogeneous catalytic technology, these problems can be effectively solved [22; 37; 38]. At present, there are several common methods for preparing iron-based catalysts: hydrothermal, solvothermal, sol-gel preparation, and coprecipitation methods. In the hydrothermal method, the solute is dispersed into the solution, stirred, and heated in the reactor, and finally washed and dried to obtain the required product [39]. Similar to the hydrothermal method, the solvothermal method changes water into an organic solvent. By dissolving one or more precursors in a nonaqueous solvent, the reaction occurs in liquid phase or supercritical conditions [40]. The sol-gel method is to dissolve the metal alkoxides in organic solvents, form homogeneous solutions, add other components, react at a certain temperature to form gels, and finally make products by drying [41]. Coprecipitation is an important method to prepare composite oxide ultrafine powder containing a large variety of metal elements [42]. The electron transfer between transition metal oxides is much higher [43] than that between single transition metal oxides. Generally, AB\({}_{2}\)O\({}_{4}\)[44; 45] structure is referred to as spinel structure. CuFe\({}_{2}\)O\({}_{4}\) is a typical spinel ferrite with a magnetic structure, which has high chemical stability and low metal leaching rate. Taking CuFe\({}_{2}\)O\({}_{4}\) as an example, compared with single transition metal oxides, Fe and Cu elements can play a role in the reaction; respectively, they can also activate PS to produce \(\cdot\)OH and \(\cdot\)SO\({}_{4}\)\({}^{-}\). [PERSON] et al. [46] comprehensively compared the catalytic degradation effects of CoFe\({}_{2}\)O\({}_{4}\), CuFe\({}_{2}\)O\({}_{4}\), MnFe\({}_{2}\)O\({}_{4}\), and ZnFe\({}_{2}\)O\({}_{4}\). In detail, CuFe\({}_{2}\)O\({}_{4}\) presented the best and fastest catalytic performance in organics removal. Almost 87.6% azo dye acid orange 7 (AO7) was removed in PS solution coupled with CuFe\({}_{2}\)O\({}_{4}\)[46]. Additionally, it was known that CuFe\({}_{2}\)O\({}_{4}\) had the best catalytic effect. Moreover, through the quenching experiment, it was not \(\cdot\)OH but \(\cdot\)SO\({}_{4}\)\({}^{-}\) that played a major role in the reaction. Table 1 shows the degradation effects of some different MeFe\({}_{2}\)O\({}_{4}\)-activated PS/PMS on different kinds of wastewater. It can be seen from the table that the iron-based catalyst with spinel structure mainly acts on \(\cdot\)SO\({}_{4}\)\({}^{-}\) in the mechanism of activation persulfate; the effect of \(\cdot\)OH is slightly worse [47]. Of course, there are also some nonfree radical pathways, which degrade pollutants in water by generating singlet oxygen \({}^{1}\)O\({}_{2}\)[48; 49; 50].