## 3 Results and Discussion  
### Characterization of Fenton Iron Peat Catalyst  
Considering that the elemental composition of iron sludge can influence its potential for resource recovery, this study first conducted an elemental analysis of the raw Fenton iron sludge using XRF spectroscopy. The results are presented in Table 1. The most abundant element in the iron sludge was carbon (49.7%), followed by Fe (35.4%), N (3.43%), Ca (3.327%), Si (2.980%), and S (1.445%), with other elements present in relatively lower concentrations. The carbon and silicon content mainly originated from natural organic matter and colloidal particulate suspensions in the raw water. The presence of Fe and S was primarily attributed to the introduction of oxidants during the Fenton treatment process, while N mainly originated from the coagulants added in the sludge storage tank. The calcium content was largely due to the addition of alkali at the end of the Fenton reaction. These results indicate that the iron sludge contains a high proportion of iron, highlighting its significant potential for resource recovery. Figure 2 presents the XRD patterns of Fenton iron-peat catalysts pyrolyzed for different pyrolysis times. All five samples contained SiO\({}_{2}\) (JCPDS No. 83-2465), CaS (JCPDS No. 77-2011), Fe\({}_{5}\)O\({}_{4}\) (JCPDS No. 75-0033), FeO (JCPDS No. 75-1550), iron nitride (JCPDS No. 75-2131), and Fe\({}^{0}\) (JCPDS No. 87-0721) [16]. As the pyrolysis time increased, the peak intensities of Fe\({}^{0}\) and iron nitride gradually intensified, indicating that prolonged pyrolysis facilitated complete thermal decomposition of the iron sludge, thereby promoting the reduction of iron species. The specific surface area and pore characteristics of the Fenton iron-peat catalyst samples prepared at different pyrolysis times are summarized in Table S1. The specific surface area exhibited an initial decrease followed by an increase as \begin{table}
\begin{tabular}{c c c c} \hline
**Elemental Composition** & **Proportion (wt\%)** & **Elemental Composition** & **Proportion (wt\%)** \\ \hline C & 49.70 & S & 1.45 \\ Fe & 35.40 & Al & 1.03 \\ N & 3.43 & P & 0.89 \\ Ca & 3.33 & Na & 0.61 \\ Si & 2.98 & & \\ \hline \end{tabular}
\end{table} 
Table 1: Main elements of the original Fenton iron mud. the pyrolysis time extended. At shorter pyrolysis times, volatile components generated from the thermal decomposition of iron sludge were not fully expelled and remained trapped between crystallites, leading to a lower specific surface area. With prolonged holding time, the pyrolysis byproducts were able to escape more completely, facilitating pore development and increasing the specific surface area. Additionally, the average pore diameter decreased with longer pyrolysis times, suggesting an increase in the number of smaller pores. Figure 3 presents the surface morphologies of the samples prepared at different pyrolysis times. The results indicate that, compared to Cat\(-\)1 (Figure 3a), the Cat\(-\)2 sample (Figure 3b) exhibited smaller dispersed particle sizes on its surface, likely due to the escape of volatile components during pyrolysis. As shown in Figure 3c-e, further extension of the holding time led to a more developed pore structure on the sample surface, accompanied by an increase in surface roughness. Notably, the Cat\(-\)2 sample exhibited distinct groove-like or raised structures, contributing to an increased specific surface area. This structural evolution is expected to provide more active sites, which can enhance the catalyst's performance in pollutant removal. Figure 3: Scanning electron microscopy images of (**a**) Cat\(-\)1, (**b**) Cat\(-\)2, (**c**) Cat\(-\)3, (**d**) Cat\(-\)4 and (**e**) Cat\(-\)5. Figure 2: XRD patterns of iron mud-based catalysts obtained by pyrolysis at different pyrolysis times. Figure 4 presents the deconvoluted XPS spectra of C 1s, N 1s, O 1s, and Fe 2p for Fenton iron-peat catalysts.