Accelerated Reduction of Bromate in Frozen Solution - Environmental

Article pubs.acs.org/est

Accelerated Reduction of Bromate in Frozen Solution Dae Wi Min and Wonyong Choi* Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea S Supporting Information *

ABSTRACT: Bromate is a common disinfection byproduct formed during ozonation. Reducing bromate into bromide can remove this toxic pollutant, however, not many studies have been done for its environmental fate. In this work, we demonstrate a new transformation pathway that bromate can be efficiently reduced to bromide in frozen solution in the presence of organic reductants like humic substances (HS). The results showed that bromate in frozen solution could be removed by 30−40% in dark condition and 80−90% in irradiation condition (λ > 300 nm) in 24 h, while around 1% bromate was reduced in aqueous solution. The bromate reduction by HS induced a partial oxidation of HS, which was confirmed by X-ray photoelectron spectroscopic analysis of the HS sample recovered from the frozen solution. Photoluminescence analysis of HS revealed that the fluorescence quenching by bromate was observed only with very high concentration of bromate (0.1−0.2 M) in aqueous solution whereas the quenching effect in frozen solution was seen with much lower bromate concentration (5−100 μM). The highly enhanced removal of bromate in ice is ascribed to the freeze concentration effect that bromate and HS are concentrated by orders of magnitude to accelerate the bimolecular transformation in the ice grain boundary region. Freezing process in cold environments would provide a unique chemical mechanism for the removal of persistent bromate.

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INTRODUCTION Bromate (BrO3−), a carcinogenic oxyhalide anion, is identified as a major disinfection byproduct (DBP). During the ozonation and chlorination treatment for drinking water, bromide in water is transformed into bromate.1,2 It has been reported that 6− 12% of bromide is converted into bromate2,3 in water treatment plants. The average concentration of bromate after ozonation in water treatment plants ranges in 1.9−3.87 μg/L.4−6 Since the ozonation in water treatment plants is growing, the discharge of bromate as a DBP is expected to increase gradually. Furthermore, natural production of bromate from the reaction between bromide and ozone in the atmosphere has been also reported.7 A significant amount of bromate would be generated and introduced into the environment and the U.S. and European governments set a bromate concentration limit in drinking water as 10 μg/L. Bromate induces the production of reactive oxygen species (ROS) in living cells8−10 which causes toxic effects in aquatic organisms11−14 and rodent.15 Bromate is also persistent in aquatic environment and classified as persistent pollutants.16,17 Bromate has a very low vapor pressure and is hardly volatilized even under boiling.18 The concentration of bromate in artificial seawater was not changed in more than 2 years.19 Therefore, once bromate is produced and released into aquatic environment, it persists for a long time causing significant environmental impacts. As a removal mechanism of bromate, bromate can be reduced into bromide which is much less toxic. Although the reduction of bromate to bromide is thermodynamically © XXXX American Chemical Society

favorable, this halide oxyanion is kinetically stable in aqueous phase,18 which makes it difficult to be degraded. The natural fate of bromate has not been extensively studied. Some studies showed that bromate can be reduced by denitrifying bacteria20−22 or other type of bacteria.23 Microbes extracted from river samples also showed the ability to reduce bromate.24 The biological reduction of bromate, which has been largely investigated in laboratory conditions, needs to be further studied to assess its contribution in the natural environment. On the other hand, abiotic conversion of bromate reduction in the presence of humic substances has been reported,25,26 but the reported reaction was very slow to exhibit 20% conversion over 12 days. There might be other abiotic transformation mechanisms of bromate. In this study, we suggest and investigate a new abiotic mechanism of bromate reduction which takes place in frozen solution. The bromate conversion in aqueous solution containing humic substances (HS), which was negligible slow, was highly accelerated when the solution was frozen and subsequently thawed. Several examples of accelerated redox conversion reactions in frozen aqueous media have been reported in the literature.27−34 Here, we report a new example of enhanced transformation in frozen solution, which may play a significant role in the transformation of bromate in natural environment. Received: Revised: Accepted: Published: A

February 19, 2017 June 18, 2017 June 26, 2017 June 26, 2017 DOI: 10.1021/acs.est.7b00915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

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EXPERIMENTAL SECTION Materials. Chemicals and reagents used in this work include the followings: sodium bromate (NaBrO3, Aldrich), sodium bromide (NaBr, Aldrich), sulfuric acid (H2SO4, Sigma-Aldrich), humic acid (Aldrich), methanol (CH3OH, Samchun), ethanol (C2H5OH, Samchun), 1-propanol (C3H7OH, Sigma-Aldrich), 2-propanol (C3H7OH, Sigma-Aldrich), 1-butanol (C4H9OH, Sigma-Aldrich), t-butanol (C4H9OH, Alfa Aesar), formaldehyde (HCHO, Kanto), acetaldehyde (C2H4O, Sigma-Aldrich), propionaldehyde (C3H6O, Sigma-Aldrich), acetone ((CH3)2CO, J.T. Baker), diethyl ether ((C2H5)2O, Sigma-Aldrich), formic acid (HCOOH, Kanto), acetic acid (CH3COOH, Samchun), propionic acid (C2H5COOH, Sigma-Aldrich), butyric acid (C3H7COOH, Sigma-Aldrich), and isobutyric acid (C3H7COOH, Sigma-Aldrich). All chemicals were of analytical grade. Suwannee River (SR) humic acid and fulvic acid were purchased from the International Humic Substances Society. Deionized water used was ultrapure (18 MΩ·cm) and prepared by a Barnstead purification system. Experimental Procedure. Stock solutions of bromate and organic compounds were prepared and those of humic/fulvic acid were made freshly every week. A sample solution with a desired concentration of substrate was prepared in a 12 × 125 mm quartz tube. Then the pH of the solution was adjusted with a standard sulfuric acid solution and sealed with a septum. The quartz tubes were put in a merry-go-around reactor in an ethanol bath maintained at −5 °C, which was gradually cooled down to −20 °C. Such slow freezing prevented the quartz tube from breaking due to fast cooling. The solution in the tube was completely frozen around 30 min after it was put in the ethanol bath. This moment was considered as “time zero (t = 0)”. For the photoreduction experiments, a 100-W mercury lamp (Ace Glass Inc.) in a pyrex jacket (transmitting λ > 300 nm) was used as a light source. The light intensity was measured to be around 1.1 × 10−3 einstein L−1 min−1 (300 nm < λ < 450 nm) by ferrioxalate actinometry. After reaction, the frozen solution was thawed in a water bath at 40 °C. For the control reactions in aqueous solution, all the procedures were the same except that the ethanol bath temperature was maintained at 25 °C. Analyses. The concentrations of bromate (BrO3−) and bromide (Br−) were measured by an ion chromatograph (IC, Dionex ICS-2100) which was equipped with a column Dionex IonPac AS 18 (4 mm × 250 mm) and a conductivity detector. The eluent solution was 10 mM KOH. Hypobromite (BrO−) was colorimetrically measured by using ABTS (2,2-azino-bis(3ethylbenzothiazoline)-6-sulfonic acid-diammonium salt).35 Five milliliter of the sample solution, 0.5 mL of ABTS solution (1 g/L), and 0.5 mL of sulfuric acid solution (0.05 M) were mixed in a 15 mL conical tube and then 4 mL of deionized water was added. When hypobromite reacts with ABTS, ABTS•+ is formed which has the molar absorptivity of ε (405 nm) = 31 600 M−1cm−1. The absorbance at 405 nm of the sample was measured by a UV/visible spectrophotometer (Agilent 8453). Humic and fulvic acids were analyzed by X-ray photoelectron spectroscopy XPS; (Theta Probe AR-XPS system, Al Kα source; Thermo Fisher Scientific) for the comparison of the carbon oxidation states before and after the reaction with bromate in the frozen solution. The fluorescence quenching effect of bromate on humic acid was monitored by a spectrofluorometer (FluoroMAx-4, HORIBA). For the fluorescence measurement of aqueous solution, a quartz cuvette (10 × 10 mm) was used. For the frozen solution, the quartz cuvette could not be used since the frozen solution was so

turbid that the excitation and emission light could not penetrate through. Instead, two quartz plates fixed with a 1 mm gap each other were used to hold the sample solution frozen in a freezer at −24 °C.

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RESULTS AND DISCUSSION Bromate Reduction by Humic and Fulvic Acid. Bromate (BrO3−) reduction into bromide (Br−) was highly enhanced in ice phase compared with that in aqueous phase. Figure 1a compares the amount of the removed bromate and the produced bromide after 24 h reaction in aqueous and frozen solution in the presence of different types of HS

Figure 1. (a) The concentrations of removed bromate (BrO3−) and produced bromide (Br−) under dark condition after 24 h in aqueous (25 °C) and ice (−20 °C) media in the presence of humic substances. The time profiles of bromate and bromide under dark condition in ice (−20 °C) in the presence of (b) humic and (c) fulvic acid from SR. Experimental condition: pHi 3 (adjusted with H2SO4), [BrO3−]0 = 20 μM, [humic/fulvic acid]0 = 10 ppm (for (a)). B

DOI: 10.1021/acs.est.7b00915 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article


Figure 2. pH dependence of bromate reduction and the accompanying hypobromite production under dark condition with humic or fulvic acid in (a, c) ice (−20 °C) and (b, d) aqueous (25 °C) phase. The pH in ice samples refer to the pH value in aqueous solutions before freezing and the actual pH in the ice grain boundary region could be different. Experimental conditions: pHi 3 (adjusted with H2SO4), [BrO3−]0 = 20 μM, [humic/ fulvic acid]0 = 10 ppm, reaction time 24 h.

rate of bimolecular reaction between a bromate ion and a reductant molecule. The most dominant solute present in this frozen solution is sulfate (around 50 ppm added to reach pH 3), which should decide the behavior of brine layer/ice system.43 Since the eutectic temperature of sulfuric acid (−61.9 °C)44 is much lower than the experiment temperature (−20 °C), the samples are not completely frozen at −20 °C with leaving liquid brine layer in the ice grain boundary where the observed bromate reduction should take place. The brine liquid layer content should decrease with decreasing the temperature and the freezing enhancement effect is markedly reduced below the eutectic temperature where no liquid content exists.30 Role of Functional Groups in Humic Substance and Electron Donors. Many functional groups in humic and fulvic acids may serve as a reductant for bromate. To analyze the functional group change of HS as a result of the reaction with bromate in ice, X-ray photoelectron spectroscopy (XPS) analysis was carried out. XPS analysis showed that the oxygen content increased in the order of humic (Aldrich) 18.6% < humic (SR) 27.8% < fulvic (SR) 30.0%, which implies that the oxygen-containing functional groups increase in this order. Since these functional groups should be oxidized as a result of bromate reduction, the change of the main functional groups in humic acid before and after the reaction with bromate was analyzed through the deconvolution of XPS C 1s band, which is shown in Figure 3. The XPS band of C 1s in humic acids can be deconvoluted into hydroxyl group (C−OH), ether group (C−O−C), carbonyl group (CO), and carboxyl group (HO−CO) (Figure 3b,c).45−47 The XPS analysis clearly showed that the carbons in humic acid (Aldrich) were more oxidized after the reaction with bromate in ice. The carbon content with higher oxidation state (i.e., higher C 1s binding energy) was enhanced after reaction. The outstanding change is that the band intensity for C−OH groups was markedly reduced but that of HO−CO groups are enhanced after the

(humic and fulvic acid) as an electron donor. While a negligible amount of bromate was removed in aqueous phase reactions, 30−40% of bromate was eliminated after freezing and thawing the solution. The bromate removal in the absence of HS was negligibly low even in ice phase, which indicates that the presence of HS as a reductant is critical. Bromate removal time profiles in frozen solutions (Figure 1b,c) show relatively fast decrease of bromate in the initial period (within a few hours) and then gradual saturation. Figure 2 shows that the reduction of bromate was more favored in acidic condition in both aqueous and ice phases. The half reaction of bromate reduction (eqs 1−3)36 indeed indicates that this reaction should be favored at an acidic condition. The bromide production shown in Figure 1a accounted for only 75% of the bromate removal. This implies the formation of other product, which should be mainly hypobromite (BrO−) as eq 1 and Figure 2c,d show. BrO3− + 5H+ + 4e− → HBrO + 2H 2O E0 = 1.45 VNHE (1)

HBrO + H+ + 2e− → Br − + H 2O E0 = 1.34 VNHE

(2)

BrO3− + 6H+ + 6e− → Br− + 3H 2O E0 = 1.41 VNHE (3)

In all pH range, the bromate removal and the concurrent production of hypobromite in ice were consistently higher than that in aqueous phase. This enhancement would be ascribed to the freeze concentration effect in ice grain boundary phase. When the solution is frozen, various solutes (e.g., bromate and HS in this work) are expelled from the ice crystal phase and concentrated within the liquid brine layer in ice grain boundary region.27−29,37−40 Such solute localization behavior in ice grain boundary was observed by fluorescence microscopic imaging.41,42 This would increase the concentration of the solutes in the ice grain boundary and, subsequently, accelerate the C


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Table 1. Bromate Reduction in Aqueous and Ice Phase with Various Organic Compounds As an Electron Donora −Δ[ BrO3− ] (μM) electron donor

aq

ice

alcohol

methanol ethanol 1-propanol 1-butanol 2-propanol t-butanol

0.75 0.68 0.82 0.57 0.55 0.09

(±0.27) (±0.03) (±0.01) (±0.00) (±0.04) (±0.34)

20 (±0) 20 (±0) 20 (±0) 19.8 (±0.1) 20 (±0) 1.7 (±0.9)

carbonyl

formaldehyde acetaldehyde propionaldehyde acetone

0.42 0.15 0.38 0.17

(±0.06) (±0.65) (±0.46) (±0.35)

20 (±0) 20 (±0) 20 (±0) 3.28 (±1.07)

ether

diethyl ether

0.11 (±0.29)

0.31 (±0.39)

carboxylic acid

formic acid acetic acid propionic acid butyric acid isobutyric acid

0.53 0.13 0.25 0.20 0.11

13.13 (±0.53) 0.42 (±0.14) 0.18 (±0.05) 0.71 (±0.62) 0.87 (±0.31)

(±0.67) (±1.02) (±0.84) (±0.57) (±0.51)

Experimental condition: [BrO3−]0 = 20 μM, [Electron donor]0 = 1 mM, pHi 3, reaction time of 24 h in ice (−20 °C) and aqueous (25 °C) phase under dark condition a

To investigate the effects of the functional groups more systematically, the reduction of bromate in the presence of various organic electron donors (alcohols, aldehydes, acids etc.) in aqueous and ice phases is compared in Table 1. Among alcohols, primary and secondary alcohols highly enhanced the bromate reduction significantly whereas a tertiary alcohol (t-butanol) was hardly efficient. Among carbonyl compounds, aldehydes were efficient but acetone was not. This implies that only the organic electron donors which can be easily oxidized (by bromate) into carboxylic acids can be an efficient reductant in ice (eqs 4, 5). RCH 2OH + H 2O → RCOOH + 4H+ + 4e−

(4)

RCHO + H 2O → RCOOH + 2H+ + 2e−

(5)

t-Butanol, acetone, diethyl ether, and carboxylic acids cannot undergo the simple oxidation like reactions 4 and 5. As a result, they are not an efficient reductant which can be coupled with the half reaction of bromate reduction (eqs 1−3). Formic acid that effectively reduced bromate in ice is an exception because it can undergo a simple two-electron oxidation (eq 6) unlike other organic acids.

Figure 3. (a) Comparison of XPS C 1s spectra of humic acid (Aldrich) before and after the reaction. Deconvoluted XPS C 1s spectra (b) before and (c) after the reaction with bromate. After the reaction in frozen solution, the thawed sample was dried in a vacuum oven at room temperature. The numbers in parentheses in (b) and (c) indicate the relative portion (%) of each functional group, which was determined through the deconvolution of the XPS C 1s band. The main bands that show the largest change before and after the reaction in ice are indicated in blue color. Experimental condition: [humic acid] = 100 ppm, [BrO3−] = 1 mM → [humic acid]0 = 100 ppm, [BrO3−]0 = 1 mM pHi 3, reaction time of 24 h in ice (−20 °C) under dark condition.

HCOOH → CO2 + 2H+ + 2e−

(6)

All these results are consistent with the above-mentioned XPS analysis, which showed that the decrease of the C−OH band intensity was accompanied by the increase of the HO−CO band intensity after the reaction of humic acid with bromate in ice. Photoreduction of Bromate by Humic and Fulvic Acid. The effect of light irradiation on bromate reduction was also tested with or without HS (Figure 4a). Like bromate reduction in dark condition, the removal of bromate and the concurrent production of bromide were clearly observed only in ice phase. The photochemical removal of bromate was 2.1−2.7 times higher than that in the dark condition.

reaction with bromate, which implies that the hydroxyl groups were consumed as a reductant for bromate with the concurrent production of carboxylic acid groups. As available hydroxyl groups in HS are depleted, the bromate reduction seems to be decelerated as Figures 1b and 1c exhibit. D


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Table 2. Concentration (μM) of Bromate and Its Reduction Products after 1 Day Reaction in Icea reaction condition humic (Al)/dark humic (SR)/dark fulvic (SR)/dark humic (Al)/light humic (SR)/light fulvic (SR)/light

[ BrO3− ] 13.9 12.6 11.7 4.3 3.8 2.5

± ± ± ± ± ±

0.2 0.3 0.8 0.2 0.4 0.6

[ Br− ] 1.9 2.7 3.0 9.2 10.7 11.7

± ± ± ± ± ±

0.2 0.0 0.4 0.6 0.6 0.5

[ BrO− ] 3.7 3.6 4.2 5.3 4.6 3.9

± ± ± ± ± ±

0.2 0.4 0.5 0.3 0.4 0.1

[Br− + BrO− + BrO3−] 19.4 18.8 18.8 18.9 19.1 17.8

± ± ± ± ± ±

0.3 0.1 0.9 0.7 0.7 1.3

Experimental condition: [Humic substance]0 = 10 ppm, [BrO3−]0 = 20 μM, pHi 3, reaction time 24 h, in ice (−20 °C)

a

reductive conversion of hypobromite to bromide (eq 2) is fast enough to maintain its steady-state concentration low during the overall reductive process of bromate. Under the light irradiation, about 10% of bromate was removed in ice even in the absence of organic electron donors. As the absorption spectrum of bromate shows a slight overlap (around 300 nm) with the light transmitting profile of the pyrex filter (SI Figure S1), the photoexcitation of a small fraction of bromates would lead to their decomposition.48 In the presence of HS, on the other hand, the photosensitization of HS should provide an alternative reaction pathway for bromate reduction. Humic and fulvic acids can absorb light transmitted through the pyrex filter (SI Figure S1). The role of HS as a photosensitizer in inducing various indirect photochemical transformations in aquatic environments is well-known.49,50 Therefore, the excited HS may sensitize the reductive decomposition of bromate (eqs 7 and 8) as well. (7)

HS + hv → HS* HS* +

BrO3−

→ HSox + Br

−

(8)

Figure 4b and c show the time-dependent concentration change of bromate and bromide in ice under light irradiation in the presence of different concentration (0, 1, 10 ppm) of humic and fulvic acids. The rates of bromate removal and bromide production progressively increased with increasing the concentration of HS. This confirms the photosensitizing role of HS. Compared with the dark time profiles shown in Figure 1b and c, which exhibited near termination of reaction beyond 5 h, the photoconversion continuously proceeded with the irradiation time up to 24 h. The electron transfer from the photoexcited HS (HS*) to bromate can be investigated by monitoring the fluorescence spectra of HS in the absence and presence of bromate. An efficient electron transfer from HS* to bromate should quench the fluorescence emission. Figure 5 compares the fluorescence emission spectra of humic acid in the presence of varying concentrations of bromate. As bromate concentration increased from 0 to 0.2 M, the emission intensity was gradually reduced, which indicates that the excited HS is quenched by bromate. However, this quenching effect was clearly visible only with high bromate concentrations (0.1−0.2 M) and acidic condition (pH 1.5). When the bromate concentration was in the millimolar range, such fluorescence quenching effect was not observed (see Figure 5b). The quenching effect was not observed either at less acidic condition of pH 3.0 even with the molar concentration level of bromate (see Figure 5c). The results imply that the efficient photoinduced electron transfer from excited HS to bromate is enabled only in the presence of excessive concentrations of bromate and proton in aqueous

Figure 4. (a) The concentrations of removed bromate (BrO3−) and produced bromide (Br−) under irradiation condition (λ > 300 nm) after 24 h in aqueous (25 °C) and ice (−20 °C) media in the presence of humic substances. The time profiles of bromate and bromide under irradiation condition in ice (−20 °C) with (b) humic and (c) fulvic acid from SR. Experimental condition: pHi 3 (adjusted with H2SO4), [BrO3−]0 = 20 μM, [humic/fulvic acid]0 = 10 ppm.

Table 2 lists the concentrations of three bromine species (BrO3−, BrO−, Br−) and the sum of them after 1 day reaction in ice in the dark and light condition. The total balance of the bromine species was around 95% of the initial bromate concentration. When comparing the bromine species concentration between the dark and light conditions in Table 2, it is interesting to note that the hypobromite concentration is little different between the dark and light conditions whereas the concentrations of bromate and bromide are significantly different between the two conditions. This seems to be related with the fact that hypobromite is highly reactive with HS. The E


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Figure 5. Fluorescence spectra of 10 ppm humic acid (Aldrich) in (a−c) aqueous and (d) frozen solutions in the presence of different concentration of bromate at (a, b) pH 1.5 and (c, d) pH 3.0. In (d), the pure ice background scattering spectrum was subtracted from the fluorescence spectrum of each frozen sample.

Then, the bromate reduction by humic acid was carried out in a frozen solution (at 251.2 K) containing 0.5 wt % glycerol which was added to control the liquid content in the frozen solution (see SI Figure S3). According to a previous study,43 a frozen solution of 0.5 wt % glycerol at 251.2 K should contain the liquid fraction of 0.01 compared with the initial solution volume (i.e., solutes concentrated by 100 times). Therefore, it is assumed that the concentrations of humic, bromate and proton are enhanced by 100 times in the liquid grain boundary within ice at 251.2 K. The control experiment without humic acid induced a negligible bromate reduction, which indicates that the presence of glycerol does not affect the bromate reduction process in ice. According to the experimentally determined rate law (eq S1 in SI Figure S2), the bromate reduction rate in the glycerol−water ice is calculated to be 6.2 × 10−10 mol·L−1·s−1 (based on the assumption that the solute concentrations are enhanced by 100 times in the grain boundary liquid fraction), which is very close to the experimental value of 6.0 × 10−10 mol·L−1·s−1 (shown in SI Figure S3). This supports the proposal that the observed acceleration of bromate reduction in frozen solution is mainly due to the freeze concentration effect. Effect of Dissolved Oxygen. The effect of dissolved oxygen on bromate reduction was tested by saturating the aqueous solution with argon or dioxygen gas before freezing (Figure 6). The bromate removal showed almost no difference between Ar- and O2-saturated conditions in both dark and irradiation experiments. This implies that dissolved

solutions. However, the quenching effect could be clearly observed with such a low concentration of bromate (5−100 μM) in frozen solution. Figure 5d shows the fluorescence spectra of ice sample containing humic acid (Aldrich) and bromate. Although the spectra of the frozen samples show low signal-to-noise ratios because of the strong scattering background of the ice,39,51 the trend of fluorescence quenching by bromate is clear. If the bromate concentration within the ice grain boundary in the frozen solution of 20 μM bromate is assumed to be comparable to 0.2 M bromate in aqueous solution (based on the comparison of Figure 5a and d), the bromate concentration might be enhanced by 104 times in the ice grain boundary. This is consistent with the previous report which estimated that the concentration of methylene blue dye molecules is enhanced by 3−6 orders of magnitude in the ice grain boundary compared to the liquid phase.39 The present observation that the photochemical reduction of bromate in frozen solution containing HS is highly accelerated should be related with the freeze concentration effect that HS, bromate, and protons are highly concentrated within the ice grain boundary region where the bimolecular interaction between HS* and bromate should be much enhanced. The above proposal that the observed acceleration of bromate reduction in frozen solutions is ascribed mainly to the freeze concentration effect in the ice grain boundary region was additionally tested by carrying out some kinetic analysis. First, the rate law of bromate reduction by humic acid (Aldrich) in aqueous solution was determined (see SI Figure S2). F


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reduction by humic acid in aqueous solution (Figure S2), the initial rate determination of bromate reduction by humic acid in frozen solution (Figure S3) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-54-279-2283; fax: +82-54-279-8299; e-mail: [email protected]. ORCID

Wonyong Choi: 0000-0003-1801-9386 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by Korea Polar Research Institute (KOPRI) project “Polar Academic Program (PAP)”. We appreciate Kitae Kim and Alok Bokare for helpful comments.

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(1) von Gunten, U.; Hoigne, J. Bromate Formation during Ozonization of Bromide-Containing Waters: Interaction of Ozone and Hydroxyl Radical Reactions. Environ. Sci. Technol. 1994, 28 (7), 1234−1242. (2) Weinberg, H. S.; Delcomyn, C. A.; Unnam, V. Bromate in Chlorinated Drinking Waters: Occurrence and Implications for Future Regulation. Environ. Sci. Technol. 2003, 37 (14), 3104−3110. (3) Bonacquisti, T. P. A drinking water utility’s perspective on bromide, bromate, and ozonation. Toxicology 2006, 221 (2−3), 145−148. (4) Gillogly, T.; Najm, I.; Minear, R.; Marinas, B.; Urban, M.; Kim, J. H.; Echigo, S.; Amy, G.; Douville, C.; Daw, B.; Andrews, R.; Hofmann, R.; Croue, J.-P. Bromate Formation and Control During Ozonation of Low Bromide Waters; AWWA Research Foundation, American Water Works Association: Denver, CO, 2001. (5) McGuire, M. J., M, J. L., Obolensky, A. Information Collection Rule Data Analysis; Awwa Research Foundation, American Water Works Association: Denver, CO, 2002. (6) Legube, B. A Survey of Bromate Ion in European Drinking Water. Ozone: Sci. Eng. 1996, 18 (4), 325−348. (7) Hara, K.; Osada, K.; Matsunaga, K.; Iwasaka, Y.; Shibata, T.; Furuya, K. Atmospheric inorganic chlorine and bromine species in Arctic boundary layer of the winter/spring. J. Geophys. Res. 2002, 107 (D18), AAC 4−1−AAC 4−15. (8) Ahmad, M. K.; Amani, S.; Mahmood, R. Potassium bromate causes cell lysis and induces oxidative stress in human erythrocytes. Environ. Toxicol. 2014, 29 (2), 138−145. (9) Chipman, J. K.; Parsons, J. L.; Beddowes, E. J. The multiple influences of glutathione on bromate genotoxicity: Implications for the dose−response relationship. Toxicology 2006, 221 (2−3), 187−189. (10) Ahmad, M. K.; Khan, A. A.; Ali, S. N.; Mahmood, R. Chemoprotective Effect of Taurine on Potassium Bromate-Induced DNA Damage, DNA-Protein Cross-Linking and Oxidative Stress in Rat Intestine. PLoS One 2015, 10 (3), e0119137. (11) Teixidó, E.; Piqué, E.; Gonzalez-Linares, J.; Llobet, J. M.; Gómez-Catalán, J. Developmental effects and genotoxicity of 10 water disinfection by-products in zebrafish. J. Water Health 2015, 13 (1), 54−66. (12) Crecelius, E. A. Measurements of Oxidants in Ozonized Seawater and Some Biological Reactions. J. Fish. Res. Board Can. 1979, 36 (8), 1006−1008. (13) Fisher, D.; Yonkos, L.; Ziegler, G.; Friedel, E.; Burton, D. Acute and chronic toxicity of selected disinfection byproducts to Daphnia magna, Cyprinodon variegatus, and Isochrysis galbana. Water Res. 2014, 55, 233−244. (14) Stewart, M. E.; Blogoslawski, W. J.; Hsu, R. Y.; Helz, G. R. Byproducts of oxidative biocides: Toxicity to oyster larvae. Mar. Pollut. Bull. 1979, 10 (6), 166−169.

Figure 6. Comparison of the removed bromate (BrO3−) concentrations in different gas (O2, air, N2) saturation under (a) dark and (b) irradiation condition after 24 h in ice (−20 °C) media in the presence of humic substances. Experimental condition: pHi 3 (with H2SO4), [BrO3−]0 = 20 μM, [humic/fulvic acid]0 = 10 ppm.

oxygen molecules do not interfere with the reaction of bromate with HS. Although bromate and O2 are competing electron acceptors, bromate is a more powerful oxidant than O2 (E0 (BrO3−/Br−) = 1.41 VNHE vs E0 (O2/H2O) = 1.23 VNHE). This implies that the freezing enhanced reduction mechanism of bromate can be effective in both oxic and anoxic environment. Environmental Implications. Bromate is mainly produced from ozonation and chlorination processes for water treatment and often found in drinking water, surface water, and aquifer.18,52,53 Furthermore, bromate is also found even at aerosols in Antarctic atmosphere, which provides an evidence of natural production of bromate.7 Humic substance is also ubiquitous organic compound which is widely present in aquatic environments. Therefore, bromate can coexist with HS in frozen environments (e.g., ice, snow, aerosols in upper atmosphere, and frozen soils) and its chemical fate might be considerably affected by chemical reaction occurring in ice while the environment undergoes freeze−thaw cycles. According to the present study, bromate reduction by HS with or without light is much enhanced in ice phase than in aqueous phase. From 30% to 90% of bromate could be eliminated in 1 day in frozen solution containing HS, while less than 1% of bromate was reduced in aqueous solution. This freezing-induced enhancement in the bromate conversion is ascribed to the freeze concentration effect that bromate ions, reductants (HS), and protons are highly concentrated in the ice grain boundary region.27−29,37−40,51,54 This phenomenon can be proposed as a new transformation pathway of “persistent” bromate in environment.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b00915. UV/visible absorption spectra of bromate and HS (Figure S1), the rate law determination of bromate G


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Accelerated Reduction of Bromate in Frozen Solution - Environmental

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