## 2 Activation Persulfate by Various Iron-Based Catalysts
### Activation Persulfate by Fe\({}^{0}\)  
In recent years, activation persulfate based on Fe\({}^{0}\) (zero-valent iron, ZVI) have been widely used in chemical production and environmental remediation [71; 72]. As mentioned above, the activation persulfate/Fe (II) mechanism can cause secondary pollution to water, so ZVI/PS [73; 74] is used instead to reduce a series of problems caused by the reduction of Fe\({}^{2+}\) content due to the change of pH and other factors in water [71]. ZVI/PS system has strong reducibility (Fe\({}^{0}\),E\({}^{0}\) = \(-\)0.44 V) [75]. Compared with CuFe\({}_{2}\)O\({}_{4}\), its reaction process is more complex, as shown in Figure 5. Fe\({}^{0}\) is first converted to Fe\({}^{2+}\) in the presence of acid and oxidant, then further oxidized to Fe\({}^{3+}\) by Fe\({}^{2+}\), and finally to Fe(IV) [76; 77]. The reaction mechanism follows [78]: According to the reaction equation, the reaction is easily affected by pH, and the reaction will gradually slow with the increase of pH. [PERSON] et al. [79] point out that the Fe\({}^{0}\)/PS system exhibits two-stage kinetics. The kinetic first stage is mostly attributed to a heterogeneous reaction occurring on the surface of the Fe\({}^{0}\) aggregate. As the reaction proceeds, decolorization shifts from the slow kinetic first stage to the fast kinetic second stage when sufficient Fe\({}^{2+}\) ions are maintained in the system [80]. \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]  
\[2\mathrm{Fe}^{0}+\mathrm{O}_{2}+2\mathrm{H}_{2}\mathrm{O}\to 2\mathrm{Fe}^{2+}+4 \mathrm{OH}^{-} \tag{3}\]  
\[\mathrm{Fe}^{0}+\mathrm{S}_{2}\mathrm{O}_{8}{}^{2-}\rightarrow\mathrm{Fe}^{2 +}+2\mathrm{SO}_{4}{}^{2-} \tag{4}\]  
\[\mathrm{Fe}^{0}+\mathrm{H}\mathrm{S}_{5}{}^{-}\rightarrow\mathrm{Fe}^{2+}+ \mathrm{SO}_{4}{}^{2-}+\mathrm{OH}^{-} \tag{5}\]  
Figure 4: TEM images of (**a**,**b**) rGO/CuFe\({}_{2}\)O\({}_{4}\) nanostructures under different magnifications [60]. Figure 3: Chemical structural formula of CuFe\({}_{2}\)O\({}_{4}\)_rGO [60]. \[\text{Fe}^{2+}+\text{S}_{2}\text{O}_{8}{}^{2-}\rightarrow\text{Fe}^{3+}+ \text{SO}_{4}{}^{2-}+\cdot\text{SO}_{4}{}^{-} \tag{6}\] \[\text{Fe}^{2+}+\text{HSO}_{5}{}^{-}\rightarrow\text{Fe}^{3+}+\text{ SO}_{4}{}^{2-}+\cdot\text{SO}_{4}{}^{-}\] (7) \[\text{Fe}^{0}+\text{S}_{2}\text{O}_{8}{}^{2-}\rightarrow\text{Fe}^{ 2+}+2\cdot\text{SO}_{4}{}^{-}+\text{SO}_{4}{}^{2-}\] (8) \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]0 (9) \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]1 (10) \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]2  
Figure 6 shows the proposed degradation pathway of 2,4-D [82]. By examining Figure 6, it can further confirm that macromolecular organic matter is decomposed into small molecular organic matter, which is gradually mineralized. Table 3 shows the degradation effects of various types of polluted water bodies activated by PS/PMS based on elemental iron. Usually, an appropriate amount of H\({}_{2}\)O\({}_{2}\)[83] will be added to the water when PS is activated by Fe\({}^{0}\), so as to reduce the cost of oxidant. Through the analysis of the table, it can be seen that the effect of ZVI when used alone [84] is worse than when it is combined with the carrier or when other conditions exist. \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]3 Schematic of the formation of \(\cdot\text{SO}_{4}{}^{-}\) and Fe(IV) in nZVI/persulfate systems containing methyl phenyl sulfoxide [81]. \[\mathrm{Fe}^{0}+2\mathrm{H}^{+}\rightarrow\mathrm{Fe}^{2+}+\mathrm{H}_{2} \tag{2}\]4 The proposed degradation pathway of 2,4-D [82].