## 3 Results and Discussion
### Catalytic Degradation of SMX  
In the PMS system, the degradation efficiency of SMX by different catalysts is shown in Figure 5. As shown in Figure 5a, the adsorption of SMX was negligible by Bi@Fe/CN-1 and Bi@Fe/CN-2 within 30 min. In comparison, the adsorption efficiency of SMX by Bi@Fe/CN-3 reached 6.6%. The increased adsorption capacity of Bi@Fe/CN-3 was attributed to the large SSA. Only 21.7% of SMX were removed by PMS alone within 60 min. It is worth noting that the addition of Bi@Fe/CN enhanced the degradation of SMX significantly in the PMS system. The degradation of SMX by Bi@Fe/CN/PMS followed the order of Bi@Fe/CN-3/PMS (99.7%) > Bi@Fe/CN-2/PMS (88.4%) > Bi@Fe/CN-1/PMS (62.5%). In addition, the pseudo-first-order kinetic model (R\({}^{2}\) > 0.97) was used to fit the reaction rate constant (\(k\)), which was found to be in the order of Bi@Fe/CN-3/PMS (0.0911 min\({}^{-1}\)) > Bi@Fe/CN-2/PMS (0.0340 min\({}^{-1}\)) > Bi@Fe/CN-1/PMS (0.0156 min\({}^{-1}\)) > PMS (0.0029 min\({}^{-1}\)). Thus, it can be derived that Bi@Fe/CN-3 exhibits the best catalytic activity. As shown in Table 2, a comparison was made on the normalized degradation rates of SMX in different PMS systems. Compared with g-C\({}_{3}\)N\({}_{4}\), the degradation rate of SMX was improved after Bi-Fe oxide co-doped on g-C\({}_{3}\)N\({}_{4}\), and the normalized degradation rate of SMX in Bi@Fe/CN-3/PMS system is significantly higher than that of other systems. Hence, Bi@Fe/CN-3 exhibited superior activation properties in the PMS system. The effects of Bi@Fe/CN-3 dosage, initial solution pH, PMS concentration, and co-existing anions on the degradation of SMX were studied in Figure 6. The rate constant of SMX removal increased from 0.0046 to 0.194 min\({}^{-1}\) with the dosage of Bi@Fe/CN-3 increased from 0.025 to 0.200 g/L (Figure 6a). This is due to the fact that a higher catalyst dosage provides more active sites for PMS activation. A similar pattern was also seen in the degradation of SMX with different PMS doses (Figure 6b). SMX was nearly entirely \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**PMS Systems** & **[SMX] (mg-L\({}^{-1}\))** & **[PMS] (mM)** & **[Catalyst] (g-L\({}^{-1}\))** & **Time (min)** & **Degradation Rate \({}^{*}\) (\%)** & **Reference** \\ \hline g-C\({}_{3}\)N\({}_{4}\)/PMS & 5.10 & 0.15 & 0.200 & 60 & 1.089 & [37] \\ FeO\({}_{3}\)/g-g-C\({}_{3}\)N\({}_{4}\)/PMS & 10.0 & 0.8 & 0.500 & 60 & 0.417 & [38] \\ FNCN/PMS & 5.00 & 1.6 & 0.400 & 60 & 0.467 & [39] \\ Fe\(-\)Co\(-\)O\(-\)g-C\({}_{3}\)N\({}_{4}\)/PMS & 10.1 & 0.8 & 0.200 & 30 & 2.063 & [21] \\ Fe\({}_{3}\)O\({}_{4}\)/g-FeOOH/PMS & 5.0 & 1.3 & 0.200 & 30 & 2.333 & [40] \\ Bi@Fe/CN-3/PMS & 15.0 & 0.6 & 0.100 & 45 & 2.432 & This work \\ \hline \hline \end{tabular}  
* Degradation rate per unit concentration of SMX (mg L\({}^{-1}\)), PMS (mM), and catalyst (g L\({}^{-1}\)) and per unit time (min).  
\end{table} 
Table 2: Comparison of SMX degradation rates in various PMS activation systems. Figure 5: (**a**) Degradation of SMX in different Bi@Fe/CN/PMS systems; (**b**) Kinetics curves of different catalytic systems fitted by a pseudo-first-order kinetic model. Reaction conditions: [cataly] = 0.10 g/L, [PMS] = 0.60 mM, [SMX]\({}_{0}\) = 15.0 mg/L, initial pH = 5.18.  
degraded with 0.6 mM and 0.8 mM PMS. It is worth noting that the rate constants of SMX degradation were in the order of 0.2 mM (0.0179 min\({}^{-1}\)) \(<\) 0.4 mM (0.0419 min\({}^{-1}\)) \(<\) 0.6 mM (0.0911 min\({}^{-1}\)) \(<\) 0.8 mM (0.0921 min\({}^{-1}\)). The rate constants were observed to increase with PMS concentration increase from 0.2 to 0.6 mM. However, there was no discernible improvement in the rate constant of SMX removal, with a further increase in PMS concentration from 0.6 to 0.8 mM. At low PMS concentrations, an increase in the PMS concentration makes it easier to produce active species, which speeds up the degradation of SMX. However, at high PMS concentrations, PMS depletes the free radicals produced (Equations (1)-(3)) [41].