## 3 Results and Discussion
### Catalytic Mechanism  
TBA, MeOH, and L-histidine were used as scavengers to identify the active species involved in the Bi@Fe/CN-3/PMS system. Figure 9d demonstrates that the degradation rate of SMX in the presence of MeOH, TBA, and L-histidine (100 mM) dropped from 99.8% to 39.2%, 45.3%, and 0.18% within 60 min, respectively. Meanwhile, the rate constants of SMX removal were arranged in the following order: control (0.0911 min\({}^{-1}\)) > TBA (0.0095 min\({}^{-1}\)) > MeOH (0.0074 min\({}^{-1}\)) > L-histidine (0.0001 min\({}^{-1}\)). The rate constants of the reaction between MeOH and \({}^{\bullet}\)OH or SO\({}_{4}\)\({}^{\bullet-}\) have similar values (\(k_{\text{MeOH/}^{\bullet}\text{OH}}\)= (1.2-2.8) \(\times\) 10\({}^{9}\)M\({}^{-1}\)S\({}^{-1}\) and \(k_{\text{MeOH/SO}_{4}^{\bullet-}}=\) (1.6-7.7) \(\times\) 10\({}^{7}\)M\({}^{-1}\)S\({}^{-1}\)) [50]. In contrast, \(k_{\text{TBA/}^{\bullet}\text{OH}}\) = (3.8-7.6) \(\times\) 10\({}^{8}\)M\({}^{-1}\)S\({}^{-1}\), which is much higher than \(k_{\text{TBA/SO}_{4}^{\bullet-}}=\) (4.0-9.1) \(\times\) 10\({}^{5}\)M\({}^{-1}\)S\({}^{-1}\) [51]. As a result, the contributions of \({}^{\bullet}\)OH and SO\({}_{4}\)\({}^{\bullet-}\) can be examined individually by the combination of TBA and MeOH quenching tests. It is worth noting that the degradation rate of SMX was reduced by 54% in the presence of TBA. Meanwhile, in the presence of MeOH, the degradation rate of SMX was reduced by 60%, only 6% less than in the presence of TBA. This indicates that both \({}^{\bullet}\)OH and SO\({}_{4}\)\({}^{\bullet-}\) contribute to the degradation of SMX. The inhibition of SMX degradation in the presence of TBA or MeOH can be attributed to the scavenging of \({}^{\bullet}\)OH and SO\({}_{4}\)\({}^{\bullet-}\) by these species. \({}^{1}\)O\({}_{2}\) can be quenched by L-histidine, which prevents \({}^{1}\)O\({}_{2}\) contributing to the SMX degradation. In the presence of L-histidine, SMX was essentially undegraded, showing that \({}^{1}\)O\({}_{2}\) plays a dominant role in the degradation of SMX. These results indicate that the SO\({}_{4}\)\({}^{\bullet-}\), \({}^{\bullet}\)OH, and especially \({}^{1}\)O\({}_{2}\) made contributions to the removal of SMX in Bi@Fe/CN-3/PMS system. Figure 9: EPR spectra of (**a**) SO\({}_{4}\)\({}^{\bullet-}\) and \({}^{\bullet}\)OH, (**b**) O\({}_{2}\)\({}^{\bullet-}\) and (**c**) \({}^{1}\)O\({}_{2}\) in the Bi@Fe/CN-3/PMS system; (**d**) Degradation of SMX with different scavengers. Reaction conditions: [catalyst] = 0.10 g/L, [PMS] = 0.60 mM, [SMX]\({}_{0}\) = 15.0 mg/L, initial pH = 5.18. In this study, the active species in Bi@Fe/CN-3 were further studied by EPR analysis. The active species, including SO4\({}^{\bullet-}\), \({}^{\bullet}\)OH, and O2\({}^{\bullet-}\), were captured by DMPO, while \({}^{1}\)O\({}_{2}\) was captured by TEMP. As shown in Figure 9a,b, the characteristic signals of DMPO-O2\({}^{\bullet-}\), DMPO-\({}^{\bullet}\)OH, and DMPO-SO4\({}^{\bullet-}\) were observed, providing further evidence for the presence of O2\({}^{\bullet-}\), \({}^{\bullet}\)OH, and SO4\({}^{\bullet-}\) in Bi@Fe/CN-3/PMS system. Clearly, an identifiable 1:1:1 triplet signal peak related to TEMPO-\({}^{1}\)O\({}_{2}\) was observed in the EPR spectrum (Figure 9c), providing evidence for the presence of \({}^{1}\)O\({}_{2}\) in Bi@Fe/CN-3/PMS system [52]. The intensity of the \({}^{1}\)O\({}_{2}\) signal peak increased sharply from 2 to 10 min, indicating the continuously generation of \({}^{1}\)O\({}_{2}\) in Bi@Fe/CN-3/PMS system. In summary, the O2\({}^{\bullet-}\), \({}^{\bullet}\)OH, SO4\({}^{\bullet-}\), and notably \({}^{1}\)O\({}_{2}\) exhibit a major role in the degradation of SMX. Further investigation of the activation mechanism of PMS was conducted by XPS analysis of fresh and used Bi@Fe/CN-3.