## 4 Conclusions  
This study transformed Fenton iron sludge into an efficient catalyst and combined it with the H\({}_{2}\)O\({}_{2}\)/Fe(VI) system to degrade COD effectively. The results showed that Cat\(-2\) with a pyrolysis time of 2 h exhibited excellent catalytic activity. The degradation rate of MB in the Cat\(-2\)/H\({}_{2}\)O\({}_{2}\) system was 31.6%. Fe(VI) could enhance the oxidative performance of the Cat\(-2\)/H\({}_{2}\)O\({}_{2}\) system, with COD removal up to 46.2% under weakly alkaline conditions (pH = 8.1). Mechanistic studies indicated that high-valent iron and \({}^{1}\)O\({}_{2}\), \({}^{\bullet}\)OH, and O\({}_{2}\)\({}^{\bullet-}\) are the primary active species in the degradation process. Continuous flow experiments showed that the COD in the secondary effluent could meet the discharge standard after treatment in the Cat\(-2\)/H\({}_{2}\)O\({}_{2}\)/Fe(VI) system. This study provided a scientific basis for the resource utilization of Fenton iron sludge and the application of heterogeneous Fenton-like systems in water treatment. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15115945/s1](https://www.mdpi.com/article/10.3390/app15115945/s1)]([https://www.mdpi.com/article/10.3390/app15115945/s1](https://www.mdpi.com/article/10.3390/app15115945/s1)), Table S1: Specific surface area and pore parameters of iron sludge-based catalysts at different pyrolysis times; Table S2: Cost of COD removal in sequential batch experiment. L.W. : conceptualization, methodology, analysis, writing--review. X.H. : formal analysis, writing--original draft. L.L. : writing--review and editing, writing--original draft. Y.W. : writing--review and editing. B.Y. : writing--review and editing. G.C. : writing--review and editing. N.L. : conceptualization, writing--review and editing, supervision. L.H. : formal \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Catalyst** & **Wastewater COD** & **Flow Rate** & **pH** & **[H\({}_{2}\)O\({}_{2}\)]** & **[Fe(VI)]** & **Reflux** & **Outlet COD** \\
**(g)** & **(mg/L)** & **(mL/min)** & **(\(\pm\)0.20)** & **(mM)** & **(mM)** & **Ratio** & **(mg/L)** \\ \hline Lower layer 60, & 69 & 40.12 & 8.10 & 0.8 & 0.1 & 1:2 & 47 \\ Middle layer 50, & 70 & 40.12 & 8.10 & 1.2 & 0.15 & 1:2 & 45 \\ Upper layer 30 & 68 & 32.98 & 8.10 & 0.8 & 0.1 & - & 45 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: COD removal at different oxidant concentrations. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Catalyst** & **Wastewater COD** & **Flow Rate** & **pH** & **[H\({}_{2}\)O\({}_{2}\)]** & **[Fe(VI)]** & **Reflux** & **Outlet COD** \\
**(g)** & **(mg/L)** & **(mL/min)** & **(\(\pm\)0.20)** & **(mM)** & **(mM)** & **Ratio** & **(mg/L)** \\ \hline Lower layer 60, & 62 & 32.98 & 8.10 & 0.8 & 0.1 & 1:3 & 48 \\ Middle layer 50, & 62 & 26.01 & 8.10 & 0.8 & 0.1 & 1:3 & 43 \\ Upper layer 50, & 67 & 40.12 & 8.10 & 1.2 & 0.15 & - & 42 \\ Upper layer 30 & 68 & 32.98 & 8.10 & 1.2 & 0.15 & - & 44 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: COD removal at different flow rates. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Catalyst** & **Wastewater COD** & **Flow Rate** & **pH** & **[H\({}_{2}\)O\({}_{2}\)]** & **[Fe(VI)]** & **Reflux** & **Outlet COD** \\
**(g)** & **(mg/L)** & **(mL/min)** & **(\(\pm\)0.20)** & **(mM)** & **(mM)** & **Ratio** & **(mg/L)** \\ \hline Lower layer 60, & 62 & 32.98 & 8.10 & 0.8 & 0.1 & 1:3 & 48 \\ Middle layer 50, & 62 & 26.01 & 8.10 & 0.8 & 0.1 & 1:3 & 43 \\ Upper layer 50, & 67 & 40.12 & 8.10 & 1.2 & 0.15 & - & 42 \\ Upper layer 30 & 68 & 32.98 & 8.10 & 1.2 & 0.15 & - & 44 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: COD removal by different catalyst concentrations. analysis, writing--review and editing. All authors have read and agreed to the published version of the manuscript.