## 3 Results and Discussion
### Performance Testing
#### 3.2.2. COD Removal Performance in the \(\mathrm{Cat-2/H_{2}O_{2}}\) System  
Table 2 presents the COD removal efficiency of actual secondary effluent from a wastewater treatment plant under different reaction conditions in the \(\mathrm{Cat-2/H_{2}O_{2}}\) system. As shown in Table 2, in the Fenton-like system with an initial pH of 4.0, the COD removal efficiency reached 33.0%, whereas at an initial pH of 5.0, the efficiency dropped to only 13.6%. These results indicated that adjusting the initial pH to 4.0 enhanced the COD removal efficiency. Additionally, this study investigated the effect of oxidant dosage on COD removal in the \(\mathrm{Cat-2/H_{2}O_{2}}\) system. As shown in Table 2, when the oxidant concentration was 1.0 mM, the COD removal efficiency was 33.0%. However, increasing the oxidant concentration to 1.5 mM led to a decrease in COD removal efficiency to 16.5%. Therefore, an oxidant concentration of 1.0 mM was found to be optimal for maximizing COD removal. To further enhance COD removal efficiency, the effect of catalyst dosage was also examined. As shown in Table 2, increasing the catalyst concentration from 0.5 g/L to 2.0 g/L resulted in a slight decrease in COD removal efficiency from 33.0% to 29.1%. This suggests that an excessive amount of \(\mathrm{Cat-2}\) does not improve COD removal performance, likely due to catalyst particle aggregation. This aggregation reduces the available catalyst surface area in contact with H\({}_{2}\)O\({}_{2}\), thereby inhibiting the generation of reactive oxygen species and ultimately lowering the COD removal efficiency. In addition, the \(\mathrm{Cat-2/H_{2}O_{2}}\) system exhibited the optimal COD removal performance under acidic conditions at pH = 4, which limited the application of the process. Therefore, it is necessary to explore the process for effective COD removal under partially neutral conditions. 3.2.3. Effect of pH on COD Removal Performance in the \(\mathrm{Cat-2/H_{2}O_{2}}\)/Fe(VI) System  
Studies have shown that ferrates exhibit a synergistic oxidation effect with H\({}_{2}\)O\({}_{2}\)[13]. In this study, Fe(VI) was introduced into the \(\mathrm{Cat-2/H_{2}O_{2}}\) system to investigate the COD removal performance of the \(\mathrm{Cat-2/H_{2}O_{2}}\)/Fe(VI) system. Table 3 presents the effect of different pH conditions on COD removal. The results indicate that in the Fenton-like system \begin{table}
\begin{tabular}{c c c c c} \hline
**pH** & **Catalyst Concentration (g/L)** & **[H\({}_{2}\)O\({}_{2}\)] (mM)** & **Removal Rate (\%)** & **COD (mg/L) (\(\pm\)1)** \\ \hline
4 & 0.5 & 1.0 & 33.0 & 48 \\
5 & 0.5 & 1.0 & 13.6 & 60 \\
4 & 0.5 & 1.5 & 16.5 & 58 \\
4 & 2.0 & 1.0 & 29.1 & 50 \\ \hline \end{tabular}
\end{table} 
Table 2: COD removal rate under different conditions in the \(\mathrm{Cat-2/H_{2}O_{2}}\) system (pH = 4). Figure 6: (**a**) Adsorption removal of MB by iron mud-based catalyst under different pyrolysis times; (**b**) Degradation of MB via H\({}_{2}\)O\({}_{2}\) activation by iron mud-based catalyst under different pyrolysis times. with an initial pH of 4.0, the COD removal efficiency was 7.7%, while at an initial pH of 5.0, no significant COD removal was observed. These findings suggest that lowering the initial pH can facilitate COD removal. However, at pH 6.0, the COD concentration remained unchanged after treatment. In addition, the COD removal efficiency increased to 11.8% at pH 7.5 and 31.3% at pH 8.1. These results demonstrate that the Cat\(-\)2/H\({}_{2}\)O\({}_{2}\)/Fe(VI) system exhibits enhanced oxidation performance under weakly alkaline conditions, which may be attributed to the greater stability of Fe(VI) in an alkaline environment. 2.4 Effect of Total Concentration of Fe(VI) and H\({}_{2}\)O\({}_{2}\) on COD Removal Performance in the Cat\(-\)2/H\({}_{2}\)O\({}_{2}\)/Fe(VI) System  
Table 4 presents the effect of the total concentration of ferrate and H\({}_{2}\)O\({}_{2}\) on COD removal. The results indicate that when [Fe(VI)] + [H\({}_{2}\)O\({}_{2}\)] = 0.9 mM, the COD removal efficiency was 31.8%.