# Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst
## 3 Results and Discussion
### Characterizations of Catalyst
Comparing the catalyst spectra before and after use, it was found that the O-H peak of the used catalyst almost disappeared, which may be the O-H group involved in the adsorption of organic matter. In addition, the peaks of O=C-O and C=O of FSC were significantly weakened after the reaction, suggesting that O=C-O and C=O may be involved in the activation of oxidants. FTIR spectra showed significantly weakened peaks at 3403 cm\({}^{-1}\), 1520 cm\({}^{-1}\), 1427 cm\({}^{-1}\), and 523 cm\({}^{-1}\) in used catalysts. The attenuated peaks correspond to consumption of carboxyl groups, carbonyl groups, and iron-oxygen bonds, confirming surface functional group depletion during catalytic reactions. The significant consumption of surface functional groups might also be one of the reasons for the decline in COD removal rate in the later stage of the pilot operation, as mentioned in Section 3.3. Figure 2d shows the crystal structure changes in FSC before and after use. In the unused FSC, significant diffraction peaks appear at 20 = 22.6\({}^{\circ}\), 33.1\({}^{\circ}\), and 36.4\({}^{\circ}\), corresponding to SiO\({}_{2}\), CaO, FeO, and Fe\({}_{3}\)O\({}_{4}\) phases, respectively, indicating that FSC is composed of polycrystalline phases and provides multiple active sites for catalytic reactions. Compared with the XRD pattern of the used FSC, the corresponding peak intensity of FeO was significantly weakened, which was attributed to the reduction in high-value iron species to Fe\({}^{0}\) during the oxidation-reduction cycle of Fe species in the catalytic reaction, which promoted the activation of oxidants and strengthened the formation of active species. In addition, the peaks and strengths of CaO, SiO\({}_{2}\), and other carrier phases remain relatively stable, highlighting their structural stability and providing physical support for the catalytic system. The fitting results of XPS characteristic peaks of C1s, N1s, O1s, and Fe2p of FSC were shown in Figure 3. Figure 3a shows that N1s can be divided into pyridine nitrogen (398.5 eV), pyrrole nitrogen (399.8 eV), and graphite nitrogen (400.8 eV). Table 2 shows that the relative atomic percentages of pyridinium, pyrrole, and graphite nitrogen in unused FSC are 1.05%, 2.07%, and 1.39%, respectively. The relative atomic percentage of pyridinium nitrogen, pyrrole nitrogen, and graphite nitrogen in FSC was reduced to 0.78%, 1.56%, and 1.17%, respectively, indicating that nitrogen species were involved in oxidant activation. The results show that pyridinium, pyrrole, and graphite nitrogen may promote \({}^{1}\)O\({}_{2}\) formation with the activation of H\({}_{2}\)O\({}_{2}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Catalyst**} & \multirow{2}{*}{**Sites**} & \multicolumn{2}{c}{**Content (at.\%)**} \\ \cline{3-4}  & & **Fresh FSC** & **Used FSC** \\ \hline \multirow{4}{*}{C1s} & C-C/C=C & 26.01 & 30.31 \\  & C-N/C-O & 6.74 & 9.12 \\  & C=O & 6.79 & 5.67 \\  & COOH & 6.05 & 3.62 \\  & Fe(III) & 1.79 & 2.74 \\  & Fe(II) & 1.03 & 1.37 \\  & Pyridine N & 1.05 & 0.78 \\ N1s & Pyrrole N & 2.07 & 1.56 \\  & Graphite N & 1.39 & 1.17 \\  & Fe-O & 9.37 & 8.87 \\  & O\({}_{\text{ads}}\) & 6.27 & 15.89 \\ O1s & C=O & 27.19 & 10.79 \\  & O-C=O & 27.41 & 12.18 \\  & C-O & 20.14 & 8.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The relative atomic percentage of different active sites in used and fresh FSCs. Figure 3b shows the XPS sub-peak results of catalyst O1s. The peak of 530.1 eV corresponds to the Fe-O bond, 530.66 eV is surface-adsorbed oxygen (O\({}_{\text{ads}}\)), 531.3 eV and 532 eV belong to C=O and O-C=O, respectively, and 533 eV corresponds to the C-O bond. The results showed that the relative atomic percentage of C=O and O-C=O decreased from 27.19% and 27.41% before the reaction to 10.79% and 12.18%, respectively, indicating a role of the carbonyl functional groups in H\({}_{2}\)O\({}_{2}\) activation [24].