## 3 Results and Discussion
### Catalytic Mechanisms in \(\mathrm{Cat{-}2/H_{2}O_{2}/Fe(VI)}\) System  
Figure 8 shows the effect of different quenching agents on the MB degradation in the \(\mathrm{Cat{-}2/H_{2}O_{2}/Fe(VI)}\) system. As shown in Figure 8a and Table 5, when no quenching agent was added, about 98.1% of the MB was degraded within 120 min with the kinetic constants (k) of 0.0533 min\({}^{-1}\). As indicated in Figure 8a, when FFA was added, the MB degradation rate significantly decreased to 52%. In addition, the \(k\) was reduced from 0.0533 min\({}^{-1}\) t 0.0055 min\({}^{-1}\). In addition, the addition of TBA and CF led to a slight decrease in the degradation rate, which dropped to 94.4% (0.0240 min\({}^{-1}\)) and 92.5% (0.0216 min\({}^{-1}\)), respectively. As shown in Figure 8b, the consumption of PMSO and the generation of PMSO\({}_{2}\) are nearly equal, indicating the formation of high-valent iron in the system. The PMSO probing experiment confirmed the presence of high-valent iron in the \(\mathrm{Cat{-}2/H_{2}O_{2}/Fe(VI)}\) system [17]. Therefore, the quenching experiments confirmed that high-valent iron, \({}^{1}\)O\({}_{2}\), \({}^{\bullet}\)OH and \({\mathrm{O}_{2}}^{\bullet-}\) are the main reactive species in the \(\mathrm{Cat{-}2/H_{2}O_{2}/Fe(VI)}\) system. To further verify the active species generated during the activation of \(\mathrm{H_{2}O_{2}/Fe(VI)}\) by \(\mathrm{Cat{-}2}\), EPR spectroscopy was employed. DMPO was used to detect the presence of \({}^{\bullet}\)OH, Fe(VI), and \({\mathrm{O}_{2}}^{\bullet-}\), while TEMP was used to identify \({}^{1}\)O\({}_{2}\) in the reaction system. As shown in Figure 9a, the EPR spectrum reveals a signal peak for DMPO-High valent iron [21], confirming the generation of high-valent iron. Additionally, as shown in Figure 9b, a TEMP-\({}^{1}\)O\({}_{2}\) signal peak with a 1:1:1 intensity ratio was observed, indicating the presence of \({}^{1}\)O\({}_{2}\)[22]. In Figure 9c, the EPR spectrum displays a sextet signal for DMPO-\({\mathrm{O}_{2}}^{\bullet-}\), confirming the existence of \({\mathrm{O}_{2}}^{\bullet-}\)[23]. In Figure 9d, the EPR spectrum reveals four characteristic peaks with a 1:2:2:1 intensity ratio, which correspond to the DMPO-\({}^{\bullet}\)OH adduct, indicating the presence of \({}^{\bullet}\)OH [23]. These results are consistent with the quenching experiments. Therefore, high-valent iron, \({}^{\bullet}\)OH, \({\mathrm{O}_{2}}^{\bullet-}\), and \({}^{1}\)O\({}_{2}\) all participate in the degradation process. Figure 7: Effect of ferrate to \(\mathrm{H_{2}O_{2}}\) concentration ratio on COD removal efficiency. The reaction condition: pH = 8.1, [Fe(VI)] + [\(\mathrm{H_{2}O_{2}}\)] = 0.9 mM. Based on the above results, the proposed mechanism for COD degradation in the Cat\(-2/\)H\({}_{2}\)O\({}_{2}\)/Fe(VI) system is illustrated in Figure 10. The addition of Fe(VI) oxidant accelerates the Fe(III)/Fe(II) cycle [14], promoting the generation of reactive species (\({}^{\bullet}\)OH, O\({}_{2}\)\({}^{\bullet-}\), and \({}^{1}\)O\({}_{2}\)). The mechanism can be described as follows: Fe(II) on the surface of the catalyst, along with the C=O group, can activate H\({}_{2}\)O\({}_{2}\) to produce \({}^{\bullet}\)OH radicals. These \({}^{\bullet}\)OH radicals can directly oxidize and degrade organic pollutants. \({}^{\bullet}\)OH can react with H\({}_{2}\)O\({}_{2}\) to form O\({}_{2}\)\({}^{\bullet-}\)[18]. Additionally, Fe(III) reacts with H\({}_{2}\)O\({}_{2}\) to produce O\({}_{2}\)\({}^{\bullet-}\), which  
Figure 8: (**a**) Degradation rate under different capture agents and (**b**) PMSO consumption and PMSO\({}_{2}\) production in the system. \begin{table}
\begin{tabular}{c c} \hline \hline
**Quencher** & \(k\) **(min\({}^{-1}\))** \\ \hline TBA & 0.0240 \\ CF & 0.0216 \\ FFA & 0.0055 \\ black & 0.0533 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Kinetic constants of quenching experiments.