# Treatment of Aqueous Bromate by Superparamagnetic BiOCl-Mediated Advanced Reduction Process
###### Abstract  
Bromate (BrO\({}_{3}^{-}\)) contamination in drinking water is a growing concern. In this reduction processes (ARPs) are reportedly promising in relieving this concern. In this work, UV/superparamagnetic BiOCl (BiOCl loaded onto superparamagnetic hydroxyapatite) assisted with small molecule carboxylic acid (formate, citrate, and acetate), a carboxyl anion radical (CO\({}_{2}^{\bullet}\)-)-based ARP, was proposed to eliminate aqueous BrO\({}_{3}^{-}\). Formate and citrate were found to be ideal CO\({}_{2}^{\bullet}\) precursor, and the latter was found to be safe for practical use. BrO\({}_{3}^{-}\) (10 \(\mathrm{\SIUnitSymbolMicro}\)-\(\mathrm{\SIUnitSymbolMicro}\), WHO guideline for drinking water) can be completely degraded within 3 min under oxygen-free conditions. In this process, BrO\({}_{3}^{-}\) degradation was realized by the reduction of CO\({}_{2}^{\bullet}\) (major role) and formyloxyl radical (minor role) in bulk solution. The formation mechanism of radicals and the transformation pathway of BrO\({}_{3}^{-}\) were proposed based on data on electron paramagnetic resonance monitoring, competitive kinetics, and degradation product analysis. The process provided a sustainable decontamination performance (<5% deterioration foroxygen (DO) compared with conventional non-radical-based reduction processes. As such, ARPs shed light on the efficient and economical removal of BrO\({}_{3}\)\({}^{-}\) from drinking water. In ARPs, one challenge is the efficient generation of RRRs under mild conditions and without secondary pollution. Generally, reactive free radicals (HO., H., e\({}_{\text{aq}}^{-}\), and O\({}_{2}^{\bullet-}\)) can be produced by irradiation (X/\(\gamma\)-rays [15], vacuum UV [16], electron beams [17]), ultrasound, peroxide thermolysis/photolysis/transition metal activation [18], photosensitization [19], ozone chain decomposition [20], and Fe(CN)\({}_{6}^{4-}\)/SO\({}_{3}^{2-}\)/I\({}^{-}\) photolysis [21]. RRRs usually form along with the formation of reactive oxidative species (ROSs) in most of these reported technologies. A popular manipulation procedure is using scavengers (Br\({}^{-}\), SO\({}_{3}^{2-}\), S\({}^{-}\), and formate) to quench ROSs to generate a system which dominated by RRRs. Our recent work concluded that the low-pressure UV photolysis of SO\({}_{3}^{2-}\) (UV-L/SO\({}_{3}^{2-}\)) is highly suitable for practical BrO\({}_{3}^{-}\) degradation [13]. The process that uses e\({}_{\text{aq}}^{-}\) (\(-2.9\) V) as reductant gains SO\({}_{4}^{2-}\) and Br\({}^{-}\) as end products. However, the possible formation of sulfur-containing organic intermediates discount the advantage of UV-L/SO\({}_{3}^{2-}\)[22]. Besides H.-/e\({}_{\text{aq}}^{-}\), another strong reductant carboxyl anion radical (CO\({}_{2}^{\bullet-}\), \(-2.0\) V) exists. This radical can be produced by reactions between RRRs/ROSs and small-molecular-weight organic carboxylic acids (SOCAs; e.g., oxalate, formate, and citrate) [23]. The CO\({}_{2}^{\bullet-}\)-based process may perform well in BrO\({}_{3}^{-}\) removal in terms of thermodynamics (E\({}_{\text{BrO}_{3}^{-}/\text{Br}^{-}}=-0.61\) V, [13]). As regards the method of production and level of understanding, HO. shows the most variety and thoroughness among the ROSs. Thus, HO. is an ideal precursor for CO\({}_{2}^{\bullet-}\). Semiconductor heterogeneous photocatalysis is an adaptable, low-cost, environmentally friendly treatment technology for various pollutants currently being commercialized in many countries [24]. Recently, a novel type of semiconductor photocatalyst, bismuth oxyhalide (BiOX, X = F, Cl, Br, I), has attracted great concern because of its potential to address environmental and energy problems. The bandgap energy for BiOX was about 1.85-3.64 eV [25]. Thus, BiOX can be photoexcited by UV/visible light irradiation. The photocatalytic ability of BiOCl, BiOBr, and BiOI were reported to be superior to that of commercial TiO\({}_{2}\) (P25) [26; 27; 28].