# Treatment of Aqueous Bromate by Superparamagnetic BiOCl-Mediated Advanced Reduction Process
## 2 Results and Discussion
### Substitutes of Formate  
Formic acid/formate is currently the most efficient hole scavenger for the semiconductor photocatalytic process [37]. This property is ascribed to the simple one-carbon molecular structure of formic acid/formate. As such, the formic acid/formate transforms into CO\({}_{2}^{\bullet-}\) straightforwardly and involves minimal intermediate products (Equation (3)). However, formic acid/formate remaining after reaction will pose a healthy risk and demand post-treatment [23]. Green precursors that produce no second pollution are thus needed. Citrate and acetate are reasonable alternatives because they are nontoxic, readily available [41; 42] and hold a low potential to form chlorinated disinfection byproducts during chlorination as aliphatic carboxylic acids [43]. BiOCl-HAP\({}_{\text{SM}}\)/citrate demonstrated a performance in BrO\({}_{3}^{-}\) degradation equivalent to that of UV/BiOCl-HAP\({}_{\text{SM}}\)/formate under the same conditions (Figure 9). By contrast, while UV/BiOCI-HAP\({}_{\rm SM}\)/acetate presented a much poorer performance than that of UV/BiOCI-HAP\({}_{\rm SM}\)/formate. This result is analogous to the finding of [PERSON] et al. (2014), who reported citrate significantly promoting the photoreduction of Cr(VI) by TiO\({}_{2}\)/UVA-vis. The enhancement of photocatalytic reduction incited by organic hole scavengers mainly depends on the following three aspects ([PERSON] et al., 2016): (I) adsorption capacity of hole scavengers onto the photocatalyst surface; (II) reduction potential of the formed reducing radical (CO\({}_{2}^{\bullet-}\), HCOO\({}^{\bullet-}\), CH\({}_{2}\)CH\({}_{3}\)OH, and CH\({}_{2}\)OH); and (III) accumulation of intermediate products that consume the photogenerated active species (e.g., h\({}^{+}\), e\({}_{\rm CP}^{-}\), and HO\({}^{\cdot}\)) but do not produce reducing radicals. The influence of (I) and (II) were certified by the observation that the \(\rm{BrO}_{3}^{-}\) degradation rates in the presence of the three SOCAs (formate, citrate, and acetate) are correlated with their redox potentials or complex formation constants ([PERSON] et al., 2016). For (III), suspicion should fall on the difference in chemical structure of the three SOCAs. In the presence of oxygen, acetate can generate CO\({}_{2}^{\bullet-}\) through HO-induced dehydrogenation and oxygen addition (Equation (10)) and subsequent decarboxylation reaction (Equation (11)). Under oxygen-free or anoxic conditions, only a small fraction of acetate can transform into oxalate, and the major products are glyoxylic acid and glycolic acid for the reaction of acetate with HO. ([PERSON] et al., 2016). Oxalate may form as a result of further oxidation of glyoxylic acid. Oxalate has been reported to transform into formate through the photo-Kolbe mechanism during semiconductor photocatalytic photocatalytic process ([PERSON] et al., 2016). This observation implies that the intermediate products that consume photogenerated active species (e.g., h\({}^{+}\), e\({}_{\rm CP}^{-}\), HO\({}^{\cdot}\)) but do not produce reducing radicals are generated when acetate acts as CO\({}_{2}^{\bullet-}\) precursor. These points hence partially explain the relatively weak promotion of acetate. According to the work of [PERSON] et al. (2016), the direct attack of HO- on the hydroxyl group of citrate will yield CO\({}_{2}^{\bullet-}\) and another product (1,3-acetonedicarboxylic acid). The abstraction of the \(\alpha\)-hydrogen (\(-\)CH\({}_{2}-\)COO\({}^{-}\)) of citrate also yields glyoxylic acid, which can further oxidize into oxalate (CO\({}_{2}^{\bullet-}\) precursor). These occurrences may explain the approximating enhancement of citrate as formate. Thus, citrate can be used as a harmless substitute for formate:  
\[\rm{HO}\cdot+H_{3}CCOO^{-}(Acetate)\stackrel{{ O_{2}}}{{\to}} \rm{Oxalate}, \tag{10}\]  
\[\rm{HO}\cdot+Oxalate\rightarrowCO_{2}^{\bullet-}+CO_{2}\,. \tag{11}\]  
Figure 9: Bromate degradation efficiency by in presence of different SOCA.