## 3 Results and Discussion
### Influence of the Co-Existing Ions  
Typically, various anions in natural water have an impact on the activity of catalysts. Therefore, the effects of different concentrations (10 mM and 100 mM) of H2 PO4\({}^{-}\), HCO3\({}^{-}\), Cl\({}^{-}\), and NO3\({}^{-}\) on the degradation of SMX in Bi@Fe/CN-3/PMS system were studied. As shown in Figure 7a, the SMX removal rate dropped from 99.9% to 36.3-44.9% in the presence of H2 PO4\({}^{-}\) (10-100 mM). The inhibitory effect of H2 PO4\({}^{-}\) on the removal of SMX may be attributed to the fact that H2 PO4\({}^{-}\) can react with active species such as OH to generate low-oxidizing H2 PO4\({}^{\bullet}\) and HPO4\({}^{\bullet-}\) (Equations (4) and (5)) [44]. \[\mathrm{H_{2}PO_{4}^{-}+\ ^{\bullet}OH\to H_{2}PO_{4}^{\bullet}+OH^{-}} \tag{4}\]  
\[\mathrm{HPO_{4}^{2-}+\ ^{\bullet}OH\to HPO_{4}^{\bullet-}+OH^{-}} \tag{5}\]  
As shown in Figure 7b, HCO3\({}^{-}\) has a significant inhibitory effect on the degradation of SMX in Bi@Fe/CN-3/PMS system. The rate of SMX degradation was reduced to 11.2% in the presence of 10 mM HCO3\({}^{-}\) due to HCO3\({}^{-}\) being able to interact with free radicals to produce low-oxidizing HCO3\({}^{-}\) (Equations (6) and (7)) [45]. However, when the HCO3\({}^{-}\) concentration was further increased to 100 mM, the degradation rate of SMX increased to 48.4% since HCO3\({}^{-}\) can also directly interact with PMS to provide SO4\({}^{\bullet-}\) and HCO4\({}^{-}\), which further promotes the removal of SMX (Equations (8) and (9)) [46]. \[\mathrm{HCO_{3}^{-}+\ ^{\bullet}OH\to HCO_{3}^{\bullet}+OH^{-}} \tag{6}\]  
\[\mathrm{HCO_{3}^{-}+SO_{4}^{\bullet-}\to HCO_{3}^{\bullet-}+SO_{4}^{2-}} \tag{7}\]  
Figure 7: Effect of different anions (**a**) H2 PO4\({}^{-}\), (**b**) HCO3\({}^{-}\), (**c**) Cl\({}^{-}\)and (**d**) NO3\({}^{-}\) on SMX removal in the Bi@Fe/CN-3/PMS system. Reaction conditions: [catalyst] = 0.10 g/L, [PMS] = 0.60 mM, [SMX]\({}_{0}\) = 15.0 mg/L, initial pH = 5.18. \[\mathrm{HCO}_{3}^{-}+\mathrm{HSO}_{5}^{-}\rightarrow\mathrm{SO}_{4}^{\bullet-}+2 \mathrm{OH}^{-}+\mathrm{CO}_{2} \tag{8}\]  
\[\mathrm{HCO}_{3}^{-}+\mathrm{HSO}_{5}^{-}\rightarrow\mathrm{SO}_{4}^{2-}+ \mathrm{HCO}_{4}^{-}+\mathrm{H}^{+} \tag{9}\]  
\(\mathrm{Cl}^{-}\) exhibits a dual effect on the degradation of SMX in Bi@Fe/CN-3/PMS system (Figure 7c). The degradation rate of SMX in the presence of \(\mathrm{Cl}^{-}\) (10 mM) was 83.7% within 60 min. However, the degradation rate of SMX reached 98.1% within 30 min with the addition of 100 mM \(\mathrm{Cl}^{-}\). In comparison, the Bi@Fe/CN-3/PMS system without \(\mathrm{Cl}^{-}\) only managed 63.7%. The negative effect of the low concentration of \(\mathrm{Cl}^{-}\) on the degradation of SMX is due to its reaction with active free radicals (\(\mathrm{SO}_{4}^{\bullet-}\) and \({}^{\bullet}\)OH), which generates \(\mathrm{Cl}^{\bullet}\) and \(\mathrm{Cl}_{2}^{\bullet-}\) with lower oxidizing abilities (Equations (10)-(13)) [47]. The \(\mathrm{Cl}_{2}\) and \(\mathrm{HClO}\) with high oxidizing ability will be generated due to a further increase in \(\mathrm{Cl}^{-}\) concentration. (Equations (14) and (15)) [48]. \[\mathrm{Cl}^{-}+\ ^{\bullet}\mathrm{OH}\rightarrow\mathrm{ClOH}^{\bullet-} \tag{10}\]  
\[\mathrm{ClOH}^{\bullet-}+\mathrm{H}^{+}\rightarrow\mathrm{Cl}^{\bullet}+ \mathrm{H}_{2}\mathrm{O} \tag{11}\]  
\[\mathrm{Cl}^{-}+\mathrm{SO}_{4}^{\bullet-}\rightarrow\mathrm{Cl}^{\bullet}+ \mathrm{SO}_{4}^{2-} \tag{12}\]  
\[\mathrm{HPO_{4}^{2-}+\ ^{\bullet}OH\to HPO_{4}^{\bullet-}+OH^{-}} \tag{5}\]0  
\[\mathrm{HPO_{4}^{2-}+\ ^{\bullet}OH\to HPO_{4}^{\bullet-}+OH^{-}} \tag{5}\]1  
\[\mathrm{HPO_{4}^{2-}+\ ^{\bullet}OH\to HPO_{4}^{\bullet-}+OH^{-}} \tag{5}\]2  
In addition, the effect of \(\mathrm{NO}_{3}^{-}\) on the degradation of SMX in the Bi@Fe/CN-3/PMS system was studied (Figure 7d).