# Treatment of Aqueous Bromate by Superparamagnetic BiOCl-Mediated Advanced Reduction Process
## 2 Results and Discussion
### Mechanism Clarification  
To identify the radicals formed, electron paramagnetic resonance (EPR) tests were performed. The EPR data directly supported that only CO\({}_{2}^{\bullet-}\) was produced and no H-, O\({}_{2}^{\bullet-}\) and HO- was formed (Figure 7). DO stripping at the start of experiments rationalizes inability to trap the O\({}_{2}^{\bullet-}\)-DMPO adduct signal. The scavenging rate of H- by 10 mM DMPO ([DMPO] \(\times\)\(k_{\rm H-DMPO}\), \(\sim\)10\({}^{7}\) s\({}^{-1}\)) was much faster than those by 10 uM BrO\({}_{3}^{-}\) ([BrO\({}_{3}^{-}\)] \(\times\)\(k_{\rm H-BrO_{3}^{-}}\) \(\sim\)10\({}^{2}\) s\({}^{-1}\)) and 4 mM formate ([formate] \(\times\)\(k_{\rm H\bullet-Formate}\), \(\sim\)10\({}^{5}\) s\({}^{-1}\)). A proper identification was allowed, given the adequately different hyperfine coupling constants of DMPO-CO\({}_{2}^{\bullet-}\) (\(\alpha^{\rm N}\) = \(\alpha_{\beta}\)\({}^{\rm H}\) = 15.3 G) and DMPO-H- (\(\alpha^{\rm N}\) = 16.7, \(\alpha_{\beta}\)\({}^{\rm H}\) = 22.4). Thus, a signal from DMPO-H- should be observed if H- is located in bulk solution. However, ESR tests did not detect the H- (Figure 7). Given the above argument, CO\({}_{2}^{\bullet-}\) is the only trapped species in the bulk solution. This observation is consistent with several reported results, which found that CO\({}_{2}^{\bullet-}\) was the only active species during the TiO\({}_{2}\) photocatalytic oxidation of formate [37]. Considering that CO\({}_{2}^{\bullet-}\) presents a much stronger reducing power than that of e\({}_{\rm CB}^{-}\) (\(-\)0.25 V, [37]) and a weak BrO\({}_{3}^{-}\) adsorption onto the surface of BiOCl-HAP\({}_{\rm SM}\) particles (making the particle surface reduction by e\({}_{\rm CB}^{-}\) negligible), the CO\({}_{2}^{\bullet-}\) in the bulk solution was thus considered as the main contributor of BrO\({}_{3}^{-}\) degradation in the UV/BiOCl-HAP\({}_{\rm SM}\)/formate. Given the fast reaction rate constant of BrO\({}_{3}^{-}\) with CO\({}_{2}^{\bullet-}\) (\(k_{\rm CO_{2}^{-}}\), BrO\({}_{3}^{-}\) = 1.37 \(\times\) 10\({}^{9}\) M\({}^{-1}\)-s\({}^{-1}\), Appendix A and Figure S1), the superior degradation of BrO\({}_{3}^{-}\) by UV/BiOCl-HAP\({}_{\rm SM}\)/formate can be reasonably explained. Obviously, two pathways for formate are involved in the degradation process of bromate. In the first pathway, HO- near the catalyst's surface (HO-s) diffuses into bulk solution to form free HO- (HOI) and formate directly reacts with HOI to produce CO\({}_{2}^{\bullet-}\) (Equations (1)-(3), [38]). This occurrence was proven by the ESR tests (Figure 7). In the second pathway, formate first adsorbs (complexes) onto the BiOCl-HAP\({}_{\rm SM}\) particle surface (Equation (4)). This action is followed by ligand-to-metal charge-transfer (Equation (5), [34]) or direct hole transfer reaction (Equation (6), [34]). Ultimately, formyloxyl radical (HCOO\({}^{\bullet-}\)) is produced. HCOO\({}^{\bullet-}\) can reduce bromate because of its low reduction potential [34]. HCOO\({}^{\bullet-}\) also injects an electron to the conduction band of BiOCl-HAP\({}_{\rm SM}\) to terminate the reaction (Equation (7), [39]). Previous studies proposed that the decomposition of HCOO\({}^{\bullet-}\) produces H- and then H- transforms into CO\({}_{2}^{\bullet-}\) by reacting with formate (Equations (8) and (9), [37, 40]). However, [PERSON] et al. [39] negated the occurrence of Equations (8) and (9) on the basis of nontrapping of the H- signal. Similarly, no EPR signal of H- was observed in our work (Figure 7). Thus, the H-derived formation of CO\({}_{2}^{\bullet-}\) can be excluded. Regarding the second pathway, the adsorption (complexation) of formate with BiOCl-HAP\({}_{\rm SM}\) is a premise step for the final CO\({}_{2}^{\bullet-}\)production. Therefore, the  
Figure 7: EPR spectra obtained from (**a**) UV/BiOCl-HAP\({}_{\rm SM}\)/Formate and (**b**) UV/BiOCl-HAP\({}_{\rm SM}\) systems in the presence of DMPO.