Treatment of Aqueous Bromate by Superparamagnetic BiOCl-Mediated Advanced Reduction Process

Abstract

Bromate (${\mathrm{BrO}}_3^{-}$) contamination in drinking water is a growing concern. Advanced 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 (${\mathrm{CO}}_2^{•-}$)-based ARP, was proposed to eliminate aqueous ${\mathrm{BrO}}_3^{-}$. Formate and citrate were found to be ideal ${\mathrm{CO}}_2^{•-}$ precursor, and the latter was found to be safe for practical use. ${\mathrm{BrO}}_3^{-}$ (10 μg·L⁻¹, WHO guideline for drinking water) can be completely degraded within 3 min under oxygen-free conditions. In this process, ${\mathrm{BrO}}_3^{-}$ degradation was realized by the reduction of ${\mathrm{CO}}_2^{•-}$ (major role) and formyloxyl radical (minor role) in bulk solution. The formation mechanism of radicals and the transformation pathway of ${\mathrm{BrO}}_3^{-}$ 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 for 10 cycles) and appeared to be more resistant to common electron acceptors (O₂, ${\mathrm{NO}}_3^{-}$, and Fe³⁺) than hydrated electron based-ARPs. Phosphate based-superparamagnetic hydroxyapatite, used to support BiOCl in this work, was believed to be applicable for resolving the recycling problem of other metal-containing catalyst.

1. Introduction

As a toxic byproduct of disinfection, bromate (${\mathrm{BrO}}_3^{-})$ in drinking water can find its sources from stock NaOCl and HOBr solution used as disinfectants [1], bromide (${\mathrm{Br}}^{-}$) oxidation by ozone, chlorine, or oxidative radicals (e.g., hydroxyl radical (HO·), sulfate radical (${\mathrm{SO}}_4^{•-}$)) [2], and contaminant of hypochlorite disinfectant used in water plants [3]. Compared with the phase-transferring purification technologies (such as adsorption [4,5], ion exchange [6], and membrane filtration [7]), solving methods based on conventional non-radical-based reduction (medium-pressure UV irradiation [8], electrochemical reduction [9], zero-valent metal reduction [10], and Fe(II)/SI(IV) reduction [11]) or advanced reduction (involves strong reactive reducing radicals (RRRs) such as H/${\mathrm{e}}_{\mathrm{aq}}^{-}$ [12,13,14]) seem to be more thoroughly because of conversion of ${\mathrm{BrO}}_3^{-}$ to ${\mathrm{Br}}^{-}$ in the latter methods. Advanced reduction processes (ARPs) involve fast degradation kinetics and exhibit a relatively weak sensitivity to dissolved oxygen (DO) compared with conventional non-radical-based reduction processes. As such, ARPs shed light on the efficient and economical removal of BrO₃⁻ 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·, ${\mathrm{e}}_{\mathrm{aq}}^{-}$, and ${\mathrm{O}}_2^{•-}$) can be produced by irradiation (X/γ-rays [15], vacuum UV [16], electron beams [17]), ultrasound, peroxide thermolysis/photolysis/transition metal activation [18], photosensitization [19], ozone chain decomposition [20], and $\mathrm{Fe}{\left(\mathrm{CN}\right)}_6^{4-}$/${\mathrm{SO}}_3^{2-}$/${\mathrm{I}}^{-}$ 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 (${\mathrm{Br}}^{-}$, ${\mathrm{SO}}_3^{2-}$, ${\mathrm{S}}^{2-}$, and formate) to quench ROSs to generate a system which dominated by RRRs. Our recent work concluded that the low-pressure UV photolysis of ${\mathrm{SO}}_3^{2-}$ (UV-L/${\mathrm{SO}}_3^{2-}$) is highly suitable for practical ${\mathrm{BrO}}_3^{-}$ degradation [13]. The process that uses ${\mathrm{e}}_{\mathrm{aq}}^{-}$ (−2.9 V) as reductant gains ${\mathrm{SO}}_4^{2-}$ and ${\mathrm{Br}}^{-}$ as end products. However, the possible formation of sulfur-containing organic intermediates discount the advantage of UV-L/${\mathrm{SO}}_3^{2-}$ [22]. Besides H·/${\mathrm{e}}_{\mathrm{aq}}^{-}$, another strong reductant carboxyl anion radical (${\mathrm{CO}}_2^{•-}$, −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 ${\mathrm{CO}}_2^{•-}$-based process may perform well in ${\mathrm{BrO}}_3^{-}$ removal in terms of thermodynamics (${\mathrm{E}}_{{\mathrm{BrO}}_3^{-}/{\mathrm{Br}}^{-}}$ = −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 ${\mathrm{CO}}_2^{•-}$.

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 TiO₂ (P25) [26,27,28]. For liquid-phase photocatalytic BiOX depollution processes, photoinduced holes ${\mathrm{h}}^{+}$/HO· (${\mathrm{h}}^{+}+{\mathrm{OH}}^{-}\to$ HO·) and superoxide radicals (${\mathrm{O}}_2^{•-}$), have been considered to play important roles [29]. Thus, photoexcited BiOX can be used as an efficient source of HO·. In calculations for practical application, modest effort has been devoted to load BiOX onto magnetic materials (e.g., spinel [30], Fe₃O₄ [31], and Fe₂O₃ [32]). Recently, phosphate was used as photocatalyst support to improve a photocatalyst’s resistance against water erosion and hydraulic shearing [33]. Therefore, magnetic hydroxyapatite (HAP; Ca₁₀(PO₄)₆(OH)₂) may be a proper support in producing recoverable BiOX.

Besides HO·, SOCAs are other precursors of ${\mathrm{CO}}_2^{•-}$. In the photocatalytic system, two mechanisms may underlie the conversion of organic carboxylic acids into organic radicals, such as ${\mathrm{CO}}_2^{•-}$. First, SOCAs initially complex with the metal ion harboured by the photocatalyst. Subsequent intramolecular electron transfer from carboxylic acid to the photocatalyst conducted band and photogenerated hole (${\mathrm{h}}^{+}$)-mediated oxidation lead to the formation of organic radicals [34]. This mechanism considers that the reduction process occurs at the catalyst surface. In this mechanism, the energy of the highest occupied molecular orbital and ionization potential of SOCAs determine the formation potential of the reducing organic radicals. Second, ROSs (such as ${\mathrm{h}}^{+}$/HO·) generated on the interface of the photocatalyst and then reacted with the free SOCAs. As a result, reducing organic radicals, such as ${\mathrm{CO}}_2^{•-}$, are produced [35]. To date, only a few works have distinguished these two aspects in the mechanism expounding. In particular, the contributions of the absorbed ROSs on the photocatalyst surface and free ROSs in bulk solution to the formation of reducing organic radicals have not been discussed.

Given the above-mentioned literature review, an UV/BiOX/SOCA process was proposed to degrade ${\mathrm{BrO}}_3^{-}$ in drinking water. BiOCl, which was loaded onto superparamagnetic HAP (HAP_SM), was selected as a typical of BiOX for experimentation. This work intends to (1) probe the potential of combining photoexcited BiOX and SOCAs for ${\mathrm{CO}}_2^{•-}$ generation and ${\mathrm{BrO}}_3^{-}$ removal; (2) clarify the formation mechanism of reductive radical and the main site where ${\mathrm{BrO}}_3^{-}$ degradation occurs; and (3) evaluate the influence of several common electron acceptors (O₂, ${\mathrm{NO}}_3^{-}$, and Fe³⁺), which may influence the decontamination efficiency of UV/BiOX/SOCA.

2. Results and Discussion

2.1. Characterization of the Photocatalyst

Peaks appearing in diffraction patterns for BiOCl-HAP_SM (Figure S1) can be indexed to BiOCl (JCPDS No. 73-2060), HAP (JCPDS No. 09-0432), and Fe₃O₄ (JCPDS No. 85-1436). SEM and TEM images show that a 3D-microflower morphology is formed for BiOCl-HAP_SM (Figure 1a) and the HAP_SM particles are uniformly embedded into the BiOCl structure (Figure 1b). Experiments were also conducted to directly test the magnetization of the photocatalyst. BiOCl-HAP_SM powder (50 mg) was ultrasonically dispersed in 4 mL ultrapure water filled with a glass vial for 5 min before separation (Figure 2a). Once a magnet approached the glass vial, a clear and transparent solution was obtained within 30 s (Figure 2b). This occurrence indicates that the BiOCl-HAP_SM powder can be separated and recovered easily from the solution. Figure 3 presents the magnetization (M–H) curves recorded with magnetic fields from −20 kOe to 20 kOe at room temperature. A saturation magnetization (Ms) of 16.8 emu·g⁻¹ was measured for BiOCl-HAP_SM. BiOCl-HAP_SM attained an extremely narrow magnetic hysteresis loop corresponding to low values in remanence and coercivity. This result suggests the mixture’s superparamagnetic properties, which is provided by the Fe₃O₄ constituent (Figure S1). Figure 4 demonstrates the UV-visible (UV-Vis) diffuse reflectance (DR) absorption spectra of BiOCl-HAP_SM. BiOCl-HAP_SM exhibited a strong UV absorption. These results clearly show that micron-sized superparamagnetic BiOCl-HAP_SM with outstanding UV absorptive ability was successfully prepared through the two-step method.

2.2. Bromate Degradation by UV/BiOCl-HAP_SM in the Presence of Formate

The merit of BiOCl-HAP_SM is its ability to integrate the magnetic performance of HAP_SM and the outstanding photocatalytic capability of BiOCl. Given the affirmation of the mixture’s magnetic characteristic (Figure 2 and Figure 3), the photocatalytic efficacy of BiOCl-HAP_SM was further tested. Formic acid/formate is a common precursor of reductive radicals in photoreductive processes [34,35]. As such, formic acid/formate was selected herein to preliminarily evaluate the degradation efficiency of ${\mathrm{BrO}}_3^{-}$ in the SOCA-mediated UV/BiOCl-HAP_SM process. As shown in Figure 5, 79.0% ${\mathrm{BrO}}_3^{-}$ was degraded by UV/BiOCl-HAP_SM within 15 min of reaction time. Such performance is considered excellent, given that a UV lamp power of 10 W was used and the efficiency was about 35% as the manufacturer claims. That is, BiOCl-HAP_SM possesses a good photocatalytic capability. Direct photolysis (UV alone, 16.7%; UV + Formate, 19.3%), adsorption (BiOCl-HAP_SM + formate in dark, 1%), or reduction by formate (0.7%) caused slow ${\mathrm{BrO}}_3^{-}$ degradation (Figure 5). Hence, reductive radicals (e.g., ${\mathrm{CO}}_2^{•-}$(−2.0 eV), H· (−2.3 eV), and ${\mathrm{O}}_2^{•-}$(−0.28 eV)) and the conducted electron (${\mathrm{e}}_{\mathrm{CB}}^{-}$) were speculated to play important roles [23,36]. Notably, high initial bromate concentration ([${\mathrm{BrO}}_3^{-}$]₀ = 6.6 μM (1.0 mg·L⁻¹)) was used considering the inexpedience caused by the ultrafast degradation for low concentration (100% degradation of 10 μg·L⁻¹ bromate within 3 min reaction time; Figure 6). Such concentration was also adopted to maintain the consistency of [${\mathrm{BrO}}_3^{-}$]₀ across all experiments, including the kinetics and degradation product investigation.

2.3. Mechanism Clarification

To identify the radicals formed, electron paramagnetic resonance (EPR) tests were performed. The EPR data directly supported that only ${\mathrm{CO}}_2^{•-}$ was produced and no H·, ${\mathrm{O}}_2^{•-}$ and HO· was formed (Figure 7). DO stripping at the start of experiments rationalizes inability to trap the ${\mathrm{O}}_2^{•-}$-DMPO adduct signal. The scavenging rate of H· by 10 mM DMPO ([DMPO] × $k_{\mathrm{H}·-\mathrm{DMPO}}$, ~10⁷ s⁻¹) was much faster than those by 10 μM ${\mathrm{BrO}}_3^{-}$ ([${\mathrm{BrO}}_3^{-}$] × $k_{\mathrm{H}·-{\mathrm{BrO}}_3^{-}}$, ~10² s⁻¹) and 4 mM formate ([formate] × $k_{\mathrm{H}•-\mathrm{Formate}}$, ~10⁵ s⁻¹). A proper identification was allowed, given the adequately different hyperfine coupling constants of DMPO-${\mathrm{CO}}_2^{•-}$ (α^N = α_β^H = 15.3 G) and DMPO-H· (α^N = 16.7, α_β^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, ${\mathrm{CO}}_2^{•-}$ is the only trapped species in the bulk solution. This observation is consistent with several reported results, which found that ${\mathrm{CO}}_2^{•-}$ was the only active species during the TiO₂ photocatalytic oxidation of formate [37]. Considering that ${\mathrm{CO}}_2^{•-}$ presents a much stronger reducing power than that of ${\mathrm{e}}_{\mathrm{CB}}^{-}$ (−0.25 V, [37]) and a weak ${\mathrm{BrO}}_3^{-}$ adsorption onto the surface of BiOCl-HAP_SM particles (making the particle surface reduction by ${\mathrm{e}}_{\mathrm{CB}}^{-}$ negligible), the ${\mathrm{CO}}_2^{•-}$ in the bulk solution was thus considered as the main contributor of ${\mathrm{BrO}}_3^{-}$ degradation in the UV/BiOCl-HAP_SM/formate. Given the fast reaction rate constant of ${\mathrm{BrO}}_3^{-}$ with ${\mathrm{CO}}_2^{•-}$ ($k_{{\mathrm{co}}_2^{-},{\mathrm{BrO}}_3^{-}}$ = 1.37 × 10⁹ M⁻¹·s⁻¹, Appendix A and Figure S1), the superior degradation of ${\mathrm{BrO}}_3^{-}$ by UV/BiOCl-HAP_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 ($\mathrm{HO}{·}_{\mathrm{s}}$) diffuses into bulk solution to form free HO· (${\mathrm{HO}}_{\mathrm{f}}$) and formate directly reacts with ${\mathrm{HO}}_{\mathrm{f}}$ to produce ${\mathrm{CO}}_2^{•-}$ (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_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 (${\mathrm{HCOO}}^{•-}$) is produced. ${\mathrm{HCOO}}^{•-}$ can reduce bromate because of its low reduction potential [34]. ${\mathrm{HCOO}}^{•-}$ also injects an electron to the conduction band of BiOCl-HAP_SM to terminate the reaction (Equation (7), [39]). Previous studies proposed that the decomposition of ${\mathrm{HCOO}}^{•-}$ produces H· and then H· transforms into ${\mathrm{CO}}_2^{•-}$ by reacting with formate (Equations (8) and (9), [37,40]). However, Perissinotti 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 ${\mathrm{CO}}_2^{•-}$ can be excluded. Regarding the second pathway, the adsorption (complexation) of formate with BiOCl-HAP_SM is a premise step for the final ${\mathrm{CO}}_2^{•-}$production. Therefore, the concentration evolution of formate in bulk solution was monitored in the BiOCl-HAP_SM/formate system with and without competitors (Figure 8). Formate readily adsorbed onto BiOCl-HAP_SM and the presence of 4 mM ${\mathrm{F}}^{-}$ well inhibited formate’s adsorption onto BiOCl-HAP_SM (<4% for 20 min) (Figure 8a). Correspondingly, ${\mathrm{BrO}}_3^{-}$ degradation readily decreased by about 11% when the adsorption of formate onto BiOCl-HAP_SM was almost inhibited (Figure 8b). Given these results, one can conclude that the free formate acts as the source of ${\mathrm{CO}}_2^{•-}$ through HO·-induced hydrogen abstraction and the adsorbed formate primarily serves as origin of reductive ${\mathrm{HCOO}}^{•-}$ (−2.1 V, [39]) which also contributes to the bromate degradation:

$$\mathrm{BiOCl}+hv\to {\mathrm{h}}^{+}{+\mathrm{e}}_{\mathrm{CB}}^{-},$$

(1)

$${\mathrm{h}}^{+}{+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+\mathrm{HO}{·}_{\mathrm{s}}\left(\mathrm{diffusing}\mathrm{to}\mathrm{bulk}\mathrm{solution}\mathrm{to}\mathrm{form}\mathrm{free}\mathrm{HO}\text{·} \left({\mathrm{HO}\text{·}}_{\mathrm{f}}\right)\right),$$

(2)

$$\mathrm{HO}{·}_{\mathrm{f}}+{\mathrm{HCOO}}^{-}\to {\mathrm{CO}}_2^{•-}+{\mathrm{H}}_2\mathrm{O}k=3.8\times {10}^9 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1},$$

(3)

$${\mathrm{BiOCl}-\mathrm{HAP}}_{\mathrm{SM}}+{\mathrm{HCOO}}^{-}\left(\mathrm{Formate}\right)\to \equiv \mathrm{Bi}-\mathrm{O}_{}^{-}\mathrm{OCH}\left(\mathrm{Complex}\right),$$

(4)

$$\equiv \mathrm{Bi}-\mathrm{O}_{}^{-}\mathrm{OCH}\stackrel{\mathrm{electron}\mathrm{injecting}\mathrm{into}\mathrm{the}\mathrm{conducted}\mathrm{band} }{\to }{\mathrm{HCOO}}^{-}+{\mathrm{BiOCl}-\mathrm{HAP}}_{\mathrm{SM}},$$

(5)

$${\mathrm{h}}^{+}+\equiv \mathrm{Bi}-\mathrm{O}_{}^{-}\mathrm{OCH}\to {\mathrm{HCOO}}^{•-},$$

(6)

$${\mathrm{HCOO}}^{•-}\stackrel{\mathrm{electron}\mathrm{injecting}\mathrm{into}\mathrm{the}\mathrm{conducted}\mathrm{band} }{\to }{\mathrm{CO}}_2+{\mathrm{H}}^{+},$$

(7)

$${\mathrm{HCOO}}^{•-}\to {\mathrm{CO}}_2+\mathrm{H}\text{·},$$

(8)

$$\mathrm{H}\text{·}+{\mathrm{HCOO}}^{-}\to {\mathrm{H}}_2+{\mathrm{CO}}_2^{•-} k=2.3\times {10}^8 {\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}.$$

(9)

2.4. 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 ${\mathrm{CO}}_2^{•-}$ 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_SM/citrate demonstrated a performance in ${\mathrm{BrO}}_3^{-}$ degradation equivalent to that of UV/BiOCl-HAP_SM/formate under the same conditions (Figure 9). By contrast, while UV/BiOCl-HAP_SM/acetate presented a much poorer performance than that of UV/BiOCl-HAP_SM/formate. This result is analogous to the finding of Marinho et al. [44], who reported citrate significantly promoting the photoreduction of Cr(VI) by TiO₂/UVA-vis.

The enhancement of photocatalytic reduction incited by organic hole scavengers mainly depends on the following three aspects [45]: (I) adsorption capacity of hole scavengers onto the photocatalyst surface; (II) reduction potential of the formed reducing radical (${\mathrm{CO}}_2^{•-}$, ${\mathrm{HCOO}}^{•-}$, ${\mathrm{CH}}_2{\mathrm{CH}}_3\mathrm{OH}$, and ${\mathrm{CH}}_2\mathrm{OH}$); and (III) accumulation of intermediate products that consume the photogenerated active species (e.g., ${\mathrm{h}}^{+}$, ${\mathrm{e}}_{\mathrm{CB}}^{-}$, and HO·) but do not produce reducing radicals. The influence of (I) and (II) were certified by the observation that the ${\mathrm{BrO}}_3^{-}$ degradation rates in the presence of the three SOCAs (formate, citrate, and acetate) are correlated with their redox potentials or complex formation constants [46]. For (III), suspicion should fall on the difference in chemical structure of the three SOCAs. In the presence of oxygen, acetate can generate ${\mathrm{CO}}_2^{•-}$ 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· [47]. 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 process [48]. This observation implies that the intermediate products that consume photogenerated active species (e.g., ${\mathrm{h}}^{+}$, ${\mathrm{e}}_{\mathrm{CB}}^{-}$, HO·) but do not produce reducing radicals are generated when acetate acts as ${\mathrm{CO}}_2^{•-}$ precursor. These points hence partially explain the relatively weak promotion of acetate. According to the work of Meichtry et al. [49], the direct attack of HO· on the hydroxyl group of citrate will yield ${\mathrm{CO}}_2^{•-}$ and another product (1,3-acetonedicarboxylic acid). The abstraction of the α-hydrogen ($-{\mathrm{CH}}_2-{\mathrm{COO}}^{-}$) of citrate also yields glyoxylic acid, which can further oxidize into oxalate (${\mathrm{CO}}_2^{•-}$ precursor). These occurrences may explain the approximating enhancement of citrate as formate. Thus, citrate can be used as a harmless substitute for formate:

$$\mathrm{HO}·+{\mathrm{H}}_3{\mathrm{CCOO}}^{-}\left(\mathrm{Acetate}\right)\stackrel{{\mathrm{O}}_2}{\to }\mathrm{Oxalate},$$

(10)

$$\mathrm{HO}·+\mathrm{Oxalate}\to {\mathrm{CO}}_2^{•-}+{\mathrm{CO}}_2.$$

(11)

2.5. Reusability of BiOCl-HAP_SM

Catalyst reusability is often assessed when considering a catalyst’s possible practical application. Thus, the BiOCl-HAP_SM powders were recycled in successive tests of bromate degradation (Figure 10). Recovered BiOCl-HAP_SM particles showed sustainable high activity in bromate degradation. Almost no obvious change (< 5%) was observed after 10 cycles when the unavoidable loss of BiOCl-HAP_SM powders was considered. No significant release of bismuth ion, iron ion, and phosphate ion (${\mathrm{PO}}_4^{3-}$) was also noted in the reaction solution. Such good stability may be ascribed to the strong interaction between the surface Bi atom of BiOCl oxide and the ${\mathrm{PO}}_5^{3-}$ on the surface of HAP_SM surface.

2.6. Influence of Common Electron Acceptors

For reduction, electron acceptors, such as oxygen (O₂) and nitrate (${\mathrm{NO}}_3^{-}$), usually show significant suppressive effects [13,23]. Therefore, the effects of common electron acceptors (O₂, ${\mathrm{NO}}_3^{-}$, and Fe³⁺) at a concentration level of drinking water on bromate degradation by UV/BiOCl-HAP_SM/formate was evaluated (Figure 11). No significant influence was noted from 30–120 mg·L⁻¹ ${\mathrm{NO}}_3^{-}$ until the ${\mathrm{NO}}_3^{-}$ concentration increased to 240 mg·L⁻¹. Moreover, 1–4 μM Fe³⁺ (${\mathrm{FeEDTA}}^{-}$) exerted a slight effect on ${\mathrm{BrO}}_3^{-}$ degradation. As regards the DO (${\left({\mathrm{O}}_2\right)}_{\mathrm{aq}}$), 8.3 mg·L⁻¹ DO cause about 20% decrease in ${\mathrm{BrO}}_3^{-}$ degradation relative to that in the absence of DO. After comparing the scavenging rate of ${\mathrm{CO}}_2^{•-}$ by ${\mathrm{NO}}_3^{-}$ ((0.3–1.3) × 10³ s⁻¹), Fe³⁺ ((0.1–2.0) × 10² s⁻¹), DO (6.0 × 10⁵ s⁻¹), and ${\mathrm{BrO}}_3^{-}$ (9.24 × 10³ s⁻¹) (

Table 1. Scavenging of ${\mathrm{CO}}_2^{•-}$ by different electron acceptors.

Electron Acceptors e	Concentration	Reaction Rate Constant with $C{O}_2^{•-}$	Scavenging Rate
${\mathrm{BrO}}_3^{-}$	6.6 μM	1.4 × 109 M−1·s−1	9.24 × 103 s−1
${\mathrm{NO}}_3^{-}$	30–120 mg·L−1	6.5 × 105 M−1·s−1 [52]	(0.3–1.3) × 103 s−1
DO	8.1 mg·L−1	2.4 × 109 M−1·s−1 [52]	6.0 × 105 s−1
${\mathrm{Fe}}^{3+}$	1–4 μM	5.0 × 107 M−1·s−1 [52]	(0.1–2.0) × 102 s−1

), one can easily understand the inconspicuous influence of ${\mathrm{NO}}_3^{-}$ and Fe³⁺. However, ${\mathrm{CO}}_2^{•-}$ scavenging kinetics could not explain the DO-induced ~20% inhibition. The 8.3 mg·L⁻¹ DO should completely suppress ${\mathrm{BrO}}_3^{-}$ degradation theoretically because the scavenging rate of ${\mathrm{CO}}_2^{•-}$ by DO is 65 times that by ${\mathrm{BrO}}_3^{-}$. Thus, besides scavenging ${\mathrm{CO}}_2^{•-}$, DO may also play a positive role in the photoreduction of ${\mathrm{BrO}}_3^{-}$ by UV/BiOCl-HAP_SM/formate. The electroreduction of DO (${\left({\mathrm{O}}_2\right)}_{\mathrm{aq}}$) by ${\mathrm{e}}_{\mathrm{CB}}^{-}$ is accepted to generate the superoxide radical (${\mathrm{O}}_2^{•-}$), and ${\mathrm{O}}_2^{•-}$ may convert into ${\mathrm{H}}_2{\mathrm{O}}_2$ through disproportionation (Equations (12)–(15), [38]). ${\mathrm{H}}_2{\mathrm{O}}_2$ can easily transform into free HO· ($\mathrm{HO}{·}_{\mathrm{f}}$) through UV photolysis and catalysed decomposition by surface metal ion of photocatalyst [50]. As discussed above, ${\mathrm{HO}}_{\mathrm{f}}$, rather than $\mathrm{HO}{·}_{\mathrm{s}}$ near the surface of BiOCl-HAP_SM ($\mathrm{HO}{·}_{\mathrm{s}}$), contributed to the formation of ${\mathrm{CO}}_2^{•-}$. The ~20% inhibition of ${\mathrm{BrO}}_3^{-}$ degradation indicates that the DO’s positive effect outcompetes its negative effect. Obviously, UV/BiOCl-HAP_SM/formate was more resistant to common electron acceptors than the typical hydrated electron based-ARPs. For example, 95 μM ${\mathrm{NO}}_3^{-}$ (5.9 mg·L⁻¹) induced a 75% inhibition of MCAA degradation by typical ARP UV/${\mathrm{SO}}_3^{2-}$ [51]. Besides O₂, ${\mathrm{NO}}_3^{-}$, and Fe³⁺, residual chlorine (also an electron acceptor) is usually found in the drinking water to depress the microorganism growth. To decrease the influence of residual chlorine, pretreatment by UV photolysis is feasible and convenient:

$${\mathrm{e}}_{\mathrm{CB}}^{-}+{\left({\mathrm{O}}_2\right)}_{\mathrm{aq}}\to {\mathrm{O}}_2^{•-},$$

(12)

$$2{\mathrm{O}}_2^{•-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_2^{-}+{\mathrm{O}}_2,$$

(13)

$${\mathrm{O}}_2^{•-}+{\mathrm{H}}^{+}+{\mathrm{e}}_{\mathrm{CB}}^{-}\to {\mathrm{HO}}_2^{-},$$

(14)

$${\mathrm{HO}}_2^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2.$$

(15)

2.7. Transformation Intermediates of $Br{O}_3^{-}$

The product of ${\mathrm{BrO}}_3^{-}$ (Br(+5)) reduction assumes several valence states (−1, 0, +1, +3). Hence, the formation of bromite (Br(+3)), HBrO/${\mathrm{BrO}}^{-}$ (Br(+1)), bromine (Br(0)), and ${\mathrm{Br}}^{-}$ (Br(−1)) was monitored. The decrease in ${\mathrm{BrO}}_3^{-}$ concentration was accompanied by the increase in ${\mathrm{Br}}^{-}$ concentration (Figure 12a), and concentration change of ${\mathrm{BrO}}_3^{-}$ linearly correlated with that of ${\mathrm{Br}}^{-}$ (Figure 12b). Additionally, the mass balance of bromine suggests that ${\mathrm{Br}}^{-}$ was the only detected bromine species during the degradation process. This result may be due to the reason that ${\mathrm{CO}}_2^{•-}$ possibly showed higher reactivity toward Br(+3) and Br(+1) than Br(+3) did (E_{Br(0)/Br(−1)} = 1.07 V, E_{Br(+1)/Br(−1)} = 0.77 V, E_{Br(+5)/Br(−1)} = 0.58 V, E_{Br(+3)/Br(+1)} = 1.52 V, [53,54]). These results mean that all of the degraded ${\mathrm{BrO}}_3^{-}$ transformed into harmless ${\mathrm{Br}}^{-}$.

3. Materials and Methods

3.1. Materials and Reagents

Nitrobenzene (NB), phosphoric acid, KBrO₃, Na₂SO₃, NaNO₃, humic acid (HA), and 3,4-dihydro-1-oxylato-2,2-dimethyl-2H-pyrrolium (DMPO) were of ACS reagent grade purchased from J&K Scientific Ltd. (Shanghai, China). High-performance liquid chromatography (HPLC)-grade methanol was ordered from Merck (Darmstadt, Germany). Other reagents were obtained from Aladdin (Shanghai, China). In addition, GF/C glass fiber filters were obtained from Whatman (Maidstone, England). HA was purified as described previously [55]. The concentration of HA stock solution was calibrated every time before use. All other reagents were used without purification, and Milli-Q water (18.2 MΩ cm) was employed in preparing solutions. High-purity (99.99%) nitrogen and helium gases were purchased from Jingong (Hangzhou, China).

3.2. Synthesis and Characterization of BiOCl-HAP_SM

HAP_SM was synthesised as follows. First, 0.025 mol FeCl₂·4H₂O and 0.05 mol FeCl₃·6H₂O were dissolved in 50 mL ultrapure water, and 0.03 mol H₃PO₄ was diluted with 50 mL ultrapure water. Then, the above-mentioned solution was mixed with continuous stirring at 200 rpm for 10 min. This mixture was stripped with nitrogen for 10 min to solution A. Afterwards, 0.5 mol Ca(OH)₂ was dissolved in 100 mL 70 °C Milli-Q water to produce solution B. Solution A was dosed into solution B with a peristaltic pump at a speed of 1.2 mL·min⁻¹. The whole process was maintained in continuous stirring (400 rpm) and thermostatic (70 °C) under nitrogen protection. Ammonia solution was used to adjust the solution pH to 12–13. The final reaction slurry was transferred to a 1000 mL beaker, sealed with sealing film and settled for 72 h at 25 °C. The deposits were rinsed repeatedly with Milli-Q water, dehydrated at 70 °C under vacuum, and then ground. Pure HAP crystals were synthesised following the same steps of the synthesis of n-Fe-HA, except replacing the solution A by 100 mL H₃PO₄ solution (0.3 M).

The obtained HAP_SM (0.5 g) was added to 50 mL 0.01 M ethylene glycol solution of Bi(NO₃)₃. The mixture was ultrasonicated for 10min to ensure uniform dispersion. Then, 50 mL 0.01 M ethylene glycol solution of KBr was dosed into the above solution with a peristaltic pump at a flow rate of 0.6 mL·min⁻¹ with mechanically stirring at room temperature. The mixed solution was then autoclaved at 160 °C for 12 h. The formed precipitate was washed repeatedly with copious amounts of ultrapure water and anhydrous ethanol until the conductivity of waste liquid was maintained constant. The precipitate was then dried under vacuum for 12 h at 60 °C.

The X-ray powder diffraction pattern of HAP_SM powders was tested using a Rigaku Dmax-2000 diffractometer (Rigaku Co., Tokyo, Japan. The morphology and microstructure of the synthesized BiOCl-HAP_SM particles were observed by scanning electron microscopy (SEM; FEI Quanta FEG-650) and transmission electron microscopy (TEM; JEOL JEM-1230). X-ray photoelectron spectroscopy (XPS) analysis was conducted using an ESCALAB 250Xi spectrometer. Magnetization measurements were performed using a Lakeshore 7304 vibrating sample magnetometer (Lake Shore Co. Ltd., Columbus, OH, USA) at 25 °C.

3.3. Experimental Procedures

A similar photoreactor described in a previous work was used to perform experiments [22]. This photoreactor is cylindrical (4 cm internal diameter) and has a total volume of 1.8 L (1.5 L samples). The UV source is a Heraeus low-pressure mercury UV lamp (no ozone generation, light emission at 253.7 nm), which was put in a quartz tube located at the center of the reactor and caused a incident UV intensity of 4.0 mW·cm⁻² in the completely mixed sample. Sodium bromate (1 mg·L⁻¹) was first dissolved with oxygen-free water preloaded in the reactor. Afterwards, precalculated amount HAP_SM-BiOCl (0.5 g·L⁻¹) and SOCAs (4 mM) were added. The resultant solution was mixed thoroughly. Oxygen-free water was prepared by helium gas purging (30 min at 1.0 L·min⁻¹). In most of the reactions, borate was used to adjust the solution pH. Collected samples were filtered through a 0.22 μm membrane before analysis. All experiments were operated in a thermostatic room (25 °C). The UV lamp was turned on for 20 min until stable prior to experiments.

3.4. Analysis Methods

${\mathrm{BrO}}_3^{-}$ and bromide (${\mathrm{Br}}^{-}$) were analyzed through ion chromatography (Dionex ICS-2000). NB (column: 4.6 mm × 250 mm × 5 µm SB–C18, column temperature: 25 °C, mobile phase: water and methanol (55:45, v/v), flow: 1.0 mL·min⁻¹, and VWD signal: 263 nm) was measured by a Agilent 1200 HPLC (Agilent, Palo Alto, CA, USA). Br₂ and HBrO/BrO⁻ were determined by the chemical ionization mass spectrometry method [56]. Bromite (${\mathrm{BrO}}_2^{-}$) determination followed the method reported [57].

Radicals (HO·, ${\mathrm{CO}}_2^{•-}$ and H·) were trapped with DMPO and corresponding DMPO adducts were examined by a Bruker EPR A300 electron paramagnetic resonance spectrometer (Bruker, Rheinstetten, Germany). The parameters of the EPR spectrometer were as follows: microwave power, 45 mW; central field, 3445 G; modulation frequency, 100 kHz; sweep width, 70 G; scans, 1–25; sweep time, 1–5 s; modulation amplitude, 1.25 Gpp; receiver gain, (8–40) × 10⁵; and time constant, 1–50 ms. Tests on magnetic properties were performed using a MPMS SQUID magnetometer (Quantum Design, San Diego, CA, USA).

4. Conclusions

The formate- or citrate- mediated UV/BiOCl-HAP_SM ARP was proved to be a feasible method for the eliminating BrO₃⁻ possibly existing in drinking water. ${\mathrm{CO}}_2^{•-}$ reacted quickly with ${\mathrm{BrO}}_3^{-}$ (1.4 × 10⁹ M⁻¹·s⁻¹), and ${\mathrm{BrO}}_3^{-}$ degradation mainly resulted from the ${\mathrm{CO}}_2^{•-}$ reduction in bulk solution. A mechanism for ${\mathrm{BrO}}_3^{-}$ degradation was proposed. Among the common electron acceptors (${\mathrm{NO}}_3^{-}$, DO, and Fe³⁺), only DO displayed a relatively obvious negative effect (20% inhibition of ${\mathrm{BrO}}_3^{-}$ degradation under the current condition). Phosphate (${\mathrm{PO}}_5^{3-}$) based-BiOCl-HAP_SM catalyst also demonstrated a superior reusability owing to the possible strong coordination between the surface Bi atom and ${\mathrm{PO}}_5^{3-}$. These findings are beneficial for developing green and efficient technologies for controlling ${\mathrm{BrO}}_3^{-}$ levels in drinking water. Further work on the preparation of photocatalysts that would generate high yields of free HO· should be pursued in the future research.