Overlooked Role of Sulfur-Centered Radicals During Bromate

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Overlooked Role of Sulfur-Centered Radicals During Bromate Reduction by Sulfite Junlian Qiao, Liying Feng, Hongyu Dong, Zhiwei Zhao, and Xiaohong Guan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01783 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 1, 2019

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Environmental Science & Technology

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Overlooked Role of Sulfur-Centered Radicals During Bromate

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Reduction by Sulfite

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Junlian Qiaoa,b,c, Liying Fenga, Hongyu Donga, Zhiwei Zhaod, Xiaohong Guana,b,c*

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aState

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Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R.

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China

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bShanghai

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China

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cInternational

Key Laboratory of Pollution Control and Resources Reuse, College of

Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R.

Joint Research Center for Sustainable Urban Water System, Tongji

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University, Shanghai 200092, P.R. China

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dKey

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Ministry of Education, Chongqing University, Chongqing 400045, China

Laboratory of the Three Gorges Reservoir Region's Eco-Environment, State

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*Author to whom correspondence should be addressed

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Xiaohong Guan, email: [email protected]; phone: +86-21-65983869; Fax: +86-

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21-65986313.

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1

ACS Paragon Plus Environment


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TOC Art

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2


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Abstract In this work, the kinetics and mechanisms of the reductive removal of BrO3ˉ

25

by sulfite in air atmosphere were determined. BrO3ˉ could be effectively reduced by

26

sulfite at pHini 3.0–6.0 and the reduction rate of BrO3ˉ increased with decreasing pH.

27

The co-existing organic contaminants with electron-rich moieties could be degraded

28

accompanying with BrO3ˉ reduction by sulfite. The reaction stoichiometries of

29

−Δ[sulfite]/Δ[bromate] were determined to be 3.33 and 15.63 in the absence and

30

presence of O2, respectively. Many lines of evidence verified that the main reactions in

31

BrO3ˉ/sulfite system in air atmosphere included the reduction of BrO3ˉ to HOBr and its

32

further reduction to Brˉ, as well as the oxidation of H2SO3 by BrO3ˉ to form SO3ˉ and

33

its further transformation to SO4ˉ. Moreover, SO4ˉ rather than HOBr was determined

34

to be the major active oxidant in BrO3ˉ/sulfite system. SO3ˉ played a key role in the

35

over-stoichiometric sulfite consumption because of its rapid reaction with dissolved

36

oxygen. However, the formed SO3ˉ was further oxidized by BrO3ˉ in N2 atmosphere.

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BrO3 ˉ reduction by sulfite is an alternative for controlling BrO3 ˉ in water treatment

38

because it was effective in real water at pHini ≤ 6.0.

39

3



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INTRODUCTION

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Bromide (Brˉ) is widely present in water sources at concentrations ranging from

42

~10 to >1000 μg L-1 in fresh waters and ~65 mg L-1 in seawater.1 Although bromide in

43

waters is commonly not a hazard to human health due to its low concentration, it can

44

be a major concern for water treatment because of its role in disinfection byproducts

45

(organobromine and bromate) formation during chemical disinfection and oxidation.2,3

46

Bromate (BrO3ˉ) is generally formed by the reaction of ozone and hydroxyl radicals

47

(HO) with naturally occurring Brˉ in drinking water during ozonation process.4

48

Moreover, it has been reported that BrO3ˉ could be also formed in other chemical

49

disinfection and oxidation treatments of

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Co(II)/peroxymonosulfate,6

51

oxidation.9,10 Wastewater ozonation is also a potential BrO3ˉ source for surface

52

waters.11 Due to its genotoxicity and carcinogenicity,12,13 the drinking water standard

53

for BrO3ˉ is 10 μg L-1.14-16

metal

Brˉ-containing waters, e.g., UV/persulfate,5

oxides

(CuO/NiO)/HOCl,7,8

and

Fe(VI)

54

Three categories of methods have been developed for minimizing BrO3ˉ

55

concentration in finished drinking water,17 including BrO3ˉ precursor (Brˉ) removal,

56

optimizing disinfection/oxidation process to minimize BrO3ˉ formation, and BrO3ˉ

57

removal prior to water distribution. Membrane separation as well as electrochemical

58

and adsorptive techniques have been adopted for Brˉ removal from drinking water

59

sources.18 Besides, several treatment options have been tested to minimize BrO3ˉ

60

formation during ozonation, namely ammonia addition, pH depression, HO radical

61

scavenging, and scavenging or reduction of hypobromous acid (HOBr) by organic 4


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compounds.19 However, either Brˉ removal or minimizing BrO3ˉ formation during

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ozonation may be not feasible due to complicated procedure, harsh conditions, and high

64

maintenance costs.20 Thus, effective post-treatment technologies should be developed

65

to remove BrO3ˉ from drinking water. Many techniques, including ion exchange

66

membrane,21 ultraviolet irradiation,22 photocatalysis,23 electroreduction,24 chemical

67

reduction,25 activated carbon,26 and biodegradation27,28 have been developed for BrO3ˉ

68

removal in past decades. Recently, the sulfite-based advanced reduction process (ARP)

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(e.g., UV/sulfite system) has attracted much attention for the reduction of chlorinated

70

organics (e.g., monochloroacetic acid29 and 1,2-dichloroethane30), perfluorooctanoic

71

acid,31 and oxyanions (e.g., BrO3ˉ)32 due to its high reduction efficiency. The removal

72

of BrO3ˉ in UV/sulfite system has been attributed to the formation of reactive species

73

including hydrated electrons (eaqˉ) and hydrogen atom radicals (H) but not sulfite

74

radicals (SO3ˉ).20,33 Considering the fast reaction between these reactive reductants and

75

dissolved oxygen (DO),34 UV/sulfite process should be conducted in oxygen-free

76

condition,32 which limited the practical application of this process. Additionally, the

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UV exposure would inevitably increase energy input.

78

Compared with the UV/sulfite system, the removal of BrO3ˉ by sulfite alone

79

obviously has significant advantages of competitive price and easy operation, but it has

80

received little attention so far. Gordon et al. reported that BrO3ˉ (BrO3ˉ/Brˉ, E0 = 1.4

81

V35) could be reduced by sulfite with half-life of 25–259 min at pH 4.0–7.0 and the

82

final products were Brˉ and SO42ˉ, respectively.25 In that study, the authors reported

83

that the equation for reduction of BrO3- by sulfite was as follows (Eq. 1): 5



84

BrO3― +3SO23 ― ↔3SO24 ― + Br ―

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Eq. 1

85

However, the molar ratio of sulfite to BrO3ˉ used by these authors at pH 4.0–7.0

86

ranged from 5.02 to 22.14, which was much larger than the stoichiometry shown in Eq.

87

1, while the authors did not offer corresponding explanations.25 In addition, the authors

88

did not determine the influence of background matrix on the performance of BrO3ˉ

89

reduction by sulfite, which is crucial for real practice. NaBrO3 combined with NaHSO3

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was also reported to be an excellent oxidizing agent under mild conditions, which could

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selectively oxidize alcohols, diols, and ethers and thus was used in organic synthesis,

92

due to the generation of reactive bromine species (HOBr).36-38 However, Schlaf and his

93

co-workers concluded that [H2O-Br]+ or Br+ rather than HOBr was the actual oxidizing

94

agent.39 Although the active oxidants formed during the reduction of BrO3ˉ by sulfite

95

are of debate, their generation may contribute to the over-stoichiometric consumption

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of sulfite during the process of BrO3ˉ reduction by sulfite.

97

It is well known that various transition metals can react with sulfite to generate

98

unstable SO3ˉ, which is prone to oxidation by oxygen, resulting in the formation of

99

peroxymonosulfate radical (SO5ˉ) and sulfate radical (SO4ˉ) through chain

100

propagation steps.40-43 SO4ˉ can further react with H2O or OHˉ to generate HO.44

101

However, the involvement of these radicals in the process of BrO3ˉ reduction by sulfite

102

keeps unknown and warrants further investigation.

103

Therefore, experiments were carried out in this study to (1) determine the kinetics

104

of BrO3ˉ reduction by sulfite and the reaction stoichiometry with the presence or

105

absence of oxygen, (2) identify the reactive species formed in BrO3ˉ/sulfite process by 6


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collecting electron spin resonance (ESR) spectra, performing quenching experiments,

107

monitoring the oxidation products of probe organic contaminant, and kinetic modeling,

108

as well as (3) evaluate the feasibility of BrO3ˉ removal by sulfite in real water.

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EXPERIMENTAL SECTION

110

Chemicals and Reagents

111 112

A complete list of reagents is provided in Text S1 of the Supporting Information. Experimental Procedures

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Batch experiments were conducted in 150 mL beakers at 23 ± 2 °C under magnetic

114

stirring. Air-exposed working solutions containing BrO3ˉ were prepared and adjusted

115

to the desired initial pH value (pHini) using H2SO4 or NaOH. Reactions were then

116

initiated by adding NaHSO3 from a stock solution that was pre-adjusted to the same

117

pHini as working solution. Unless otherwise specified, no measures were taken to adjust

118

pH during the reaction to observe the change of pH so as to delineate the release of

119

protons arising from the reduction of BrO3ˉ by sulfite. Periodically, 5.0 mL sample was

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collected and rapidly transferred to a small beaker containing 100 μL H2O2 stock

121

solution (100 mM) to quench the residual sulfite. Then the samples were filtered with

122

0.22 µm filters before subject to analysis with ion chromatograph (IC). Separate

123

samples, without quenching with H2O2, were collected to analyze the residual sulfite

124

concentration. To evaluate the inﬂuence of dissolved oxygen (DO) on reactions,

125

reactions were conducted in N2-sparged solutions (30 min N2 sparging, DO < 0.10 mg

126

L-1) and the reactions proceeded under continuous N2 bubbling.

127

The procedures of examining the degradation of various organic contaminants 7



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(phenol, acetaminophen (ACT), bisphenol A (BPA), carbamazepine (CBZ), atrazine

129

(ATZ), benzoic acid (BA), nitrobenzene (NB), and norfloxacin (NOR)) during BrO3ˉ

130

reduction by sulfite, quenching experiments, and formaldehyde formation when

131

methanol was oxidized in BrO3ˉ/sulfite process are present in Text S2.

132

To identify the transformation products, solutions containing 0.10 mM phenol

133

were prepared and treated by HOBr alone (0.436 mM) or BrO3ˉ/sulfite process (1.0 mM

134

NaBrO3 and 10.0 mM NaHSO3) at pHini 4.0. Before analyzing the transformation

135

products with gas chromatography-mass spectrometer (GC-MS), samples were

136

extracted and desalinated by solid phase extraction (SPE).

137

The influence of water matrix on reductive removal BrO3ˉ by sulfite was examined

138

in three waters, Milli-Q water, tap water collected in our lab, and a water sample

139

collected from Qingcaosha reservoir in Shanghai (pH = 7.6, DOC = 6.5 mg C L-1,

140

[HCO3ˉ] = 0.18 mM, [Brˉ] = 41.88 μg L-1, [Clˉ] = 28.98 mg L-1, [NO3ˉ] = 4.61 mg L-

141

1),

142

the water sample from Qingcaosha reservoir was stored at 4 °C before used in

143

experiment.

144

at three different pHini levels (5.0, 6.0, 7.0). After filtration with 0.45 μm membrane,

All experiments were performed at least in duplicate, and the average values with

145

standard deviations are reported unless otherwise noted.

146

Analytical Methods and Kinetic Modeling

147

The details of the analytical methods used in this study are present in Text S3.

148

Based on the proposed mechanisms of BrO3ˉ reduction by sulfite, a kinetic model was

149

compiled to simulate the kinetic data to verify the proposed mechanisms and the details 8


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150

are shown in Text S4.45

151

RESULTS AND DISCUSSION

152

Kinetics of BrO3ˉ Reduction by Sulfite

153

Figure 1 shows the kinetics of BrO3ˉ reduction by sulfite and Brˉ generation at

154

pHini 3.0–7.0 in the air and N2 atmosphere. It should be noted that sulfite is used to refer

155

to the equilibrium mixture of sulfurous acid (H2SO3), bisulfite (HSO3ˉ), and sulfite

156

(SO32ˉ) while chemical formula is used when one specific species is referred to. As

157

shown in Figure S1, HSO3ˉ is the major sulfite species between pH 1.8–7.2, whereas

158

5.93%, 0.63%, and 0.06% of sulfite exists as H2SO3 at pH 3.0, 4.0, and 5.0 respectively.

159

Under oxic conditions, the rate of BrO3ˉ reduction by sulfite decreased with increasing

160

pHini, and the amount of reduced BrO3ˉ at equilibrium dropped progressively from

161

0.071 to 0.019 mM with increasing pHini from 3.0 to 7.0. Sulfite has been employed for

162

reductive dehalogenation but it was reported that the reductive dehalogenation rates

163

increased with increasing pH,46-48 which is very different from the influence of pH on

164

BrO3ˉ reduction by sulfite and indicates the different mechanisms of halogenated

165

organics and BrO3ˉ reduction by sulfite. The kinetics of BrO3ˉ reduction was also

166

determined at various sulfite concentrations at pHini 4.0. As the sulfite concentration

167

increased from 0.5 to 2.0 mM, the removal efficiency of BrO3ˉ increased from 18.64%

168

to 100% at equilibrium (Figure S2). The initial stage of BrO3ˉ reduction by sulfite at

169

pHini 4.0 could be well simulated by pseudo-first-order rate law (Figure S3a), and the

170

pseudo-first-order rate constant (kobs) was found to elevate from 0.014 to 0.41 min-1 as

171

the sulfite dosage increased from 0.5 to 2.0 mM. kobs increased exponentially with 9



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172

increasing sulfite dosage (Figure S3b), and the amount of reduced BrO3ˉ was always

173

far below the theoretical value calculated from Eq. 1 open to the air (Figure S2),

174

implying that DO influenced the reduction of BrO3ˉ by sulfite.

175

Therefore, the kinetics of BrO3ˉ reduction by sulfite was also determined under

176

anoxic conditions at various pHini levels in this study (Figure 1). Similar to the case

177

open to the air, the BrO3ˉ reduction rate by sulfite under anoxic conditions dropped with

178

increasing pHini. Interestingly, the rate of BrO3ˉ reduction by sulfite at pHini 3.0 under

179

anoxic conditions was greater than its counterpart open to the air while the reduction

180

rates of BrO3ˉ in the initial reaction stage at pHini 4.0–7.0 under anoxic conditions were

181

slower than those open to the air. Moreover, BrO3ˉ (0.10 mM) could be completely

182

transformed to Brˉ at pHini 3.0–5.0 under anoxic conditions while only partial removal

183

of BrO3ˉ (0.10 mM) could be achieved by 1.0 mM sulfite under oxic conditions at pHini

184

3.0–7.0. However, the amount of removed BrO3ˉ was as low as 0.016 and 0.008 mM at

185

pHini 6.0 and 7.0, respectively, under anoxic conditions within 120 min. The different

186

behaviors of BrO3ˉ reduction by sulfite under oxic and anoxic conditions should be

187

ascribed to the influence of oxygen and the change of pH during reaction.

188

BrO3― +3HSO3― ↔3SO24 ― + Br ― +3H +

Eq. 2

189

O2 +2HSO3― ↔2SO24 ― + 2H +

Eq. 3

190

Since the major sulfite species in a solution prepared from sodium bisulfite is

191

HSO3ˉ at pH 3.0–7.0 (Figure S1), the oxidation of HSO3ˉ by either BrO3ˉ or O2 releases

192

H+ (Eqs. 2–3),49 resulting in a drop of pH. Comparing Figure 1 and Figure S4, it could

193

be concluded that H+ was released at a greater rate when BrO3ˉ was reduced at a greater 10


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194

rate and the decrease in pH would in turn facilitate the abatement of BrO3ˉ by sulfite.

195

When pHini was 3.0, pH gradually dropped to 2.75 and 2.85 under oxic and anoxic

196

conditions, respectively. Since the pH variation in these two conditions was very similar,

197

the slower BrO3ˉ reduction under oxic conditions at pHini 3.0 should be mainly ascribed

198

to the influence of oxygen. However, at pHini 4.0–5.0, pH under oxic conditions

199

dropped much faster than that under anoxic conditions, which may contribute to the

200

more rapid BrO3ˉ reduction under oxic conditions. It should be noted that BrO3ˉ

201

reduction experienced a lag phase before a rapid disappearance kinetics was observed

202

at pHini 5.0 under anoxic conditions. The self-accelerating behavior of BrO3ˉ reduction

203

by sulfite under this condition should be ascribed to the accumulation of H+, released

204

from the reduction of BrO3 ˉ by HSO3 ˉ (Eq. 2), in solution. When pHini was further

205

increased to 6.0–7.0, pH experienced little change under anoxic conditions compared

206

to the case under oxic conditions. Consequently, little BrO3ˉ was reduced under anoxic

207

conditions at pHini 6.0–7.0 within 120 min.

208

To verify the critical role of pH in the BrO3ˉ reduction process, experiments were

209

conducted at pH 3.0–5.0, where pH was maintained constant by dropwise addition of

210

NaOH. As shown in Figure S5, BrO3ˉ could be reduced by sulfite at pH 3.0–4.5 with

211

the reaction rate decreasing with increasing pH. Negligible BrO3ˉ was reduced by

212

sulfite at pH 5.0 within 30 min. It was reported that BrO3ˉ has a poor reactivity with

213

HSO3ˉ (𝑘BrO3― ,HSO3― = 0.027 ± 0.004 M-1s-1) but considerably greater reactivity with

214

H2SO3 (𝑘BrO3― ,H2SO3 = 85 ± 5 M-1s-1) at pH 4.6.50 As revealed by the species

215

distribution of sulfite at various pH values (Figure S1), the fraction of H2SO3 increases 11



216

gradually from 0.06% to 5.93% as pH drops from 5.0 to 3.0, which indicated that H2SO3

217

rather than HSO3ˉ was the major species responsible for BrO3ˉ reduction.

218

Reaction Stoichiometry

219

The amount of BrO3ˉ reduction and sulfite consumption was quantified at pHini

220

4.0 under anoxic and oxic conditions to better elucidate the process of BrO3ˉ reduction

221

by sulfite. Figure 2a shows that the amount of residual BrO3ˉ dropped and that of the

222

Brˉ generation increased linearly with increasing sulfite doses (0–0.30 mM) when 0.10

223

mM BrO3ˉ was treated with varying sulfite doses for 4 h under anoxic conditions. The

224

applied sulfite was completely consumed within 4 h for all cases. The molar ratio of

225

consumed sulfite to reduced BrO3ˉ was determined to be 3.33 (−Δ[sulfite]/Δ[bromate])

226

and that of consumed sulfite to formed Brˉ was 3.23, which were close to the results

227

reported in literature (Eq. 2).50,51 Therefore, the reaction between BrO3ˉ and sulfite in

228

the absence of oxygen was basically in accordance with Eq. 2.

229

In the presence of oxygen, however, it was found that nearly all sulfite (0.25–1.50

230

mM) was consumed during its reaction with 0.10 mM BrO3ˉ at pHini 4.0 for 30 min and

231

−Δ[sulfite]/Δ[bromate] was as large as 15.63 (Figure 2b). In addition, the mass balance

232

of bromine indicates that Brˉ was the predominant reduction product of BrO3ˉ during

233

the reaction process. As shown in Figure S6, the change of DO concentration was < 0.2

234

mg/L when there was only sulfite, indicating that the direct oxidation of sulfite by DO

235

was very slow and did not contribute much to the over‐stoichiometric sulfite

236

consumption by BrO3ˉ in the presence of oxygen. Therefore, the over-stoichiometric

237

sulfite consumption by BrO3ˉ under oxic conditions should be mainly ascribed to the 12


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involvement of DO and the generation of reactive intermediates during the reaction of

239

BrO3ˉ with sulfite (see the following sections).

240

Identification of Reactive Intermediate Species

241

Since HOBr was reported to be easily generated from the reaction of NaBrO3 with

242

NaHSO3 for preparing bromohydrin derivatives,38 and HOBr was a requisite

243

intermediate in BrO3ˉ reduction,52-54 the possible generation of HOBr was investigated

244

in BrO3ˉ/sulfite process. However, negligible HOBr was detected within 30 min during

245

the reduction of BrO3ˉ by sulfite at pHini 4.0, which might be ascribed to the fast reaction

246

of HOBr with excess HSO3ˉ (𝑘HOBr,HSO3― = 1.0 × 109 M-1s-1)55. To further identify the

247

role of HOBr as an oxidant in BrO3ˉ/sulfite system, the decomposition of eight organic

248

contaminants by HOBr alone and BrO3ˉ/sulfite systems were compared (Figure S7).

249

HOBr is highly reactive toward phenolics1 and the removal efficiency of phenolics

250

(ACT, phenol, BPA) by HOBr oxidation was 85.0%–100%. Although the two systems

251

shared coincident oxidation selectivity for ACT, phenol, BPA, CBZ, and NB, the

252

different oxidation selectivity of HOBr and BrO3ˉ/sulfite system towards ATZ and BA

253

indicated that there is other reactive oxidant but not HOBr responsible for the oxidation

254

of specific contaminants (ATZ and BA) in BrO3ˉ/sulfite system.

255

Furthermore, phenol was selected as the model compound to investigate the

256

degradation products with the aim of identifying the role of HOBr. If HOBr was formed

257

in BrO3ˉ/sulfite system and was the major oxidant contributing to phenol oxidation, it

258

would lead to the formation of brominated products by electrophilic substitution

259

reaction.56 Thus, the degradation products of phenol in HOBr alone and BrO3ˉ/sulfite 13



260

system were identified based on the full scan mode obtained from the GC-MS analysis.

261

When 0.10 mM phenol was oxidized by 0.436 mM HOBr, four brominated phenolic

262

compounds were detected (Figure S8a). However, no brominated transformation

263

product was observed in BrO3ˉ/sulfite system (Figure S8b). In addition, there were

264

several polycyclic compounds with fully or partially unsaturated rings formed in

265

BrO3ˉ/sulfite system. These results confirmed that HOBr was not the major active

266

oxidant contributing to phenol decomposition in BrO3ˉ/sulfite system.

267

Additionally, in BrO3ˉ/sulfite system, 99.5% ATZ and 36.5% BA were degraded

268

at pHini 4.0 in air atmosphere, but negligible degradation for ATZ and BA in N2

269

atmosphere was observed (Figure S9), indicating that the degradation of ATZ and BA

270

was dependent on DO. It is well known that oxygen plays an important role in sulfur-

271

centered radical propagation reactions,57 which thus affects the oxidation of organic

272

contaminants. Therefore, the reactive sulfur free radicals were expected to generate in

273

BrO3ˉ/sulfite system.

274

To provide direct evidence for the generation of free radical species in the process

275

of BrO3ˉ reduction by sulfite, ESR spectra were collected with DMPO as a spin trap.

276

The signal with the hyperfine coupling constants (αN = 14.43 G, αβ ― H = 15.83 G)

277

was observed in ESR spectra with the addition of 100 mM DMPO before the initiation

278

of reaction at pHini 4.0 and 2.5 (Figures 3a and S10a), which was consistent with those

279

reported previously for DMPO-SO3ˉ adducts58,59. Furthermore, the signal of DMPO-

280

SO3ˉ adduct was not observed in the ESR spectra collected at pHini 7.2 (Figure S10b)

281

when HSO3ˉ and SO32ˉ are the major sulfite species (Figure S1). Thus, it could be 14


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282


concluded that SO3ˉ mainly arose from the reduction of BrO3ˉ by H2SO3.

283

Because SO3ˉ reacts rapidly with DO at a diffusion-controlled rate (2.5 × 109 M-

284

1s-1)

285

HO,41 the evolution of DO concentration can also provide an evidence for SO3ˉ

286

generation. It was observed that the concentration of DO dropped rapidly from 8.97 to

287

3.08 mg L-1 within 10 min (Figure S6), confirming the participation of oxygen in the

288

chain reactions involving SO3ˉ. Subsequently, the DO concentration rebounded

289

increasingly, which should be mainly ascribed to the depletion of sulfite and dissolution

290

of oxygen from the overlying air.

to form reactive radicals SO5ˉ, which will be further transformed to SO4ˉ and

291

The signals arising from other radicals (SO4ˉ and HO) were not observed in the

292

ESR spectra (Figure 3a), because excess DMPO (100 mM) trapped all of the formed

293

SO3ˉ and terminated the subsequent radical propagation reactions. In order to trap

294

secondary radicals formed from the SO3ˉ chain reactions, 100 mM DMPO was added

295

after initiating the reaction for 30 min. As depicted in Figure 3b, typical DMPO-SO4ˉ

296

and DMPO-HO signals are observed, indicating the formation of secondary radicals in

297

BrO3ˉ/sulfite system. However, SO4ˉ-adducts can react with H2O/OHˉ via nucleophilic

298

substitution reaction to yield the corresponding HO-adducts at a considerably fast

299

reaction rate (e.g. t1/2 of DMPO-SO4ˉ = 95 s in water).60,61 Thus, the signal of HO can

300

be mainly attributed to the transformation of DMPO-SO4ˉ to DMPO-HO via

301

nucleophilic substitution and the formation of HO in BrO3ˉ/sulfite system needs further

302

verification.

303

To identify the principal reactive radicals in BrO3ˉ/sulfite system, EtOH and TBA 15



Page 16 of 32

304

were selected as radical quenchers. EtOH readily reacts with SO4ˉ (𝑘SO4 ― ,

305

× 107–7.7 × 107 M−1s−1) and HO (𝑘HO,EtOH = 1.2 × 109–2.8 × 109 M−1s−1),34,44 while

306

TBA has a poor reactivity with SO4ˉ (𝑘SO4 ― ,

307

considerably good reactivity with HO (𝑘HO,TBA = 3.8 × 108–7.6 × 108 M−1s−1).34,44 The

308

inhibitory effect of EtOH and TBA on phenol degradation in BrO3ˉ/sulfite system was

309

thus investigated at pHini 4.0 (Figure 3c). Theoretically, the oxidation of 5.0 μM phenol

310

by HO and SO4ˉ should be almost completely inhibited by 2.5 mM TBA ((𝑘HO,phenol

311

× [phenol])/(𝑘HO,TBA × [TBA]) ≤ 3.47%) and by 55 mM EtOH ((𝑘SO4 ― ,phenol ×

312

[phenol])/(𝑘SO4 ― ,EtOH × [EtOH]) ≤ 5.00%), respectively. The ratios and the rate

313

constants used to calculate them were listed in Table S2. It was found that the dosing

314

of 2.5 mM TBA only resulted in a slight drop of the degradation rate of phenol in BrO3ˉ

315

/sulfite system from 0.31 min-1 to 0.28 min-1 while the application of 55 mM EtOH

316

almost completely inhibited the degradation of phenol (Figure 3c). These phenomena

317

indicated that SO4 ˉ rather than HO was the predominant oxidant in BrO3 ˉ /sulfite

318

system. NB was also selected as a chemical probe of HO, because of its high reaction

319

rate constant with HO (𝑘HO ,NB = 3.9 × 109 M−1s−1) but very low reaction rate constant

320

with sulfate radical (𝑘SO•4 ― ,NB ≤ 106 M−1s−1).34,62 NB loss in the two systems was

321

negligible over 30 min at pHini 4.0, which also indicated that the formation of HO was

322

negligible in BrO3ˉ/sulfite system (Figure S7).

TBA

EtOH

= 1.6

= 4.0 × 105–9.1 × 105 M−1s−1) but



323

Based on the results of quenching experiments and the degradation of chemical

324

probe (NB), a preliminary conclusion could be obtained that SO4ˉ was the dominant

325

reactive oxygen species in BrO3ˉ/sulfite system. To further prove the production of 16


Page 17 of 32


326

SO4ˉ, 5.0 mM methanol was added to scavenge SO4ˉ and the oxidation product,

327

formaldehyde, was detected in BrO3ˉ/sulfite system.63 As shown in Figure 3d, no

328

formaldehyde was detected in the absence of sulfite. However, the concentration of

329

formaldehyde increased linearly with reaction time, and approximately 6.7 μM

330

formaldehyde was produced at the end of reaction, which also provided evidence for

331

the generation of SO4ˉ in BrO3ˉ/sulfite system.

332

Effect of Co-existing Organic Contaminants

333

Considering the degradation of co-existing organic contaminants in BrO3ˉ/sulfite

334

system in air atmosphere, the influence of phenol (0–100 μM) and NB (100 μM) on the

335

BrO3ˉ transformation was examined (Figure 4). The influence of 5.0 μM phenol on

336

BrO3ˉ reduction was negligible and the reduction of BrO3ˉ after 10 min was minor.

337

Although the presence of 100 μM phenol had little influence on BrO3ˉ reduction within

338

5 minutes, the reduction of BrO3ˉ was remarkably enhanced at longer time (Figure 4a

339

and 4b). To further clarify the roles of co-existing organic contaminants, the

340

concentrations of sulfite during the reaction process were determined accordingly. As

341

shown in Figure S11, the consumption kinetics of sulfite was only slightly affected by

342

5.0 μM phenol and sulfite was nearly depleted within 10 min, leading to minor BrO3ˉ

343

removal after 10 min. However, the consumption of sulfite was greatly retarded with

344

excess phenol (100 μM), which may be ascribed to the competition of excess phenol

345

with sulfite for reactive species and thus less consumption of sulfite. The significant

346

amount of residual sulfite in the presence of 100 μM phenol contributed to the

347

continuous reduction of BrO3ˉ after 10 min. Due to the negligible reactivity of NB in 17



Page 18 of 32

348

BrO3ˉ/sulfite system (Figures 4c and S7), it will not scavenge species that destroy

349

HSO3ˉ, so its presence had little effect on BrO3ˉ degradation.

350

Proposed Reaction Mechanism

351

Table 1 and Table S3 show the major proposed reactions in BrO3ˉ/sulfite system

352

under oxic and anoxic conditions. Since ESR spectra (Figures 3a, S10a, and S12)

353

revealed the generation of SO3ˉ in BrO3ˉ/sulfite system in both air and N2 atmosphere,

354

it was proposed that the reduction of BrO3ˉ by sulfite was initiated by the reaction of

355

BrO3ˉ with H2SO3 via one-electron transfer, leading to the formation of SO3ˉ and BrO2

356

(R1 and R1a), independent on the presence of DO. Concomitantly, in the presence of

357

DO, SO3ˉ reacted rapidly with DO at a diffusion-controlled rate to form SO5ˉ (R5),

358

which further reacts with HSO3 ˉ and produces SO4 ˉ (R6). Furthermore, the other

359

reactions (R7-R12) involved in the transformation of sulfur were listed, which have

360

been proposed in the literature.41 SO4ˉ is the active oxidant responsible for phenol

361

degradation (R13), which agrees well with the results of quenching experiments, ESR

362

spectra, and formaldehyde generation. In the absence of oxygen, the reaction of BrO3ˉ

363

reduction by SO3ˉ need to be considered to explain the observed reaction stoichiometry.

364

However, the self-combination of sulfite radicals to form dithionate is expected to play

365

a negligible role due to the low steady-state concentration of radicals.

366

The

formed

BrO2

was

further

reduced

to

BrO2ˉ

by

HSO3ˉ

and

367

the disproportionation of BrO2 was also considered in the kinetic model (R14, R5e, R15,

368

and R6f). Then, the unstable BrO2ˉ was quickly transformed to HOBr by HSO3ˉ (R16

369

and R7g ). It was found that the formed HOBr could be further reduced by HSO3ˉ with 18


Page 19 of 32


370

rate constant of 1.0 × 109 M-1s-1 (R17 and R8h). Consequently, the accumulation of

371

HOBr and its contribution to phenol oxidation is negligible, which was consistent with

372

the negligible formation of brominated byproducts and non-observation of HOBr

373

during the reaction.

374

It should be specified that the decomposition of H2SO3 to SO2 and the oxidation

375

of HSO3ˉ to SO42ˉ by O2 could not be neglected (R2, R2b, and R3). As illustrated in

376

Figure S13, the total amount of measured sulfite and sulfate (SO32ˉ, HSO3ˉ and SO42ˉ)

377

decreased to 0.74 mM and 0.97 mM, respectively, when the solution containing 1.0

378

mM sulfite was stirred alone open to the air at pHini 3.0 and pHini 4.0 for 30 min.

379

Considering the conversion of substantially all H2SO3 and aqueous SO2 to a mixture of

380

HSO3ˉ and SO32ˉ during analysis, the substantial loss of total sulfur species should be

381

due to the escape of SO2 gas from the solution. Besides, some HSO3ˉ will be converted

382

to H2SO3 as pH is lowered (R4 and R3c).

383

Kinetic Simulation

384

A kinetic model was developed based on the reactions and the corresponding rate

385

constants summarized in Table 1 and was used to simulate the experimental data. The

386

details of the kinetic model were described in Text S4. Under oxic conditions, as shown

387

in Figure 4, the kinetics of BrO3ˉ reduction and Brˉ formation could be well simulated

388

with the kinetic model in the absence of co-existing organic contaminants. Furthermore,

389

the kinetic model could successfully simulate the BrO3ˉ reduction, the Brˉ formation,

390

as well as the degradation kinetics of organic contaminants in the presence of 5.0 μM

391

phenol or 100 μM NB. Although the kinetic model slightly underestimated the effect 19



392

of 100 μM phenol on the kinetics of BrO3ˉ reduction and Brˉ formation, it showed a

393

correct trend. However, the degradation kinetics of phenol at high concentration (100

394

μM) could not be simulated well (not shown) because abundant degradation products

395

of excess phenol were expected to generate in BrO3ˉ/sulfite system but were not

396

included in the kinetic model. Nevertheless, the kinetic model could well simulate

397

phenol degradation at low concentration (5.0 μM) because the trace amounts of formed

398

degradation products had little effect on the model (Figure 4c). Another kinetic model

399

was developed to simulate the experimental data under anoxic conditions based on the

400

reactions and the corresponding rate constants summarized in Table S3, and the kinetics

401

of BrO3ˉ disappearance and that of Brˉ generation were well simulated in the process

402

of BrO3ˉ reduction by sulfite at pHini 3.0 (Figure S14).

403

The influences of oxygen on the pathways of BrO3ˉ reduction and sulfur

404

transformation in BrO3ˉ/sulfite system are summarized in Figure 5. The rate constant

405

for the reaction of DO with SO3ˉ (R5, Table 1) is about 4 orders of magnitude larger

406

than that for the reaction of BrO3 ˉ with SO3 ˉ (R4d, Table S3). In addition, the

407

maximum concentration of DO (0.28 mM) is about 2.8 times higher than that of BrO3ˉ

408

(0.10 mM). Therefore, the reduction of BrO3ˉ by SO3ˉ is almost completely inhibited

409

in the presence of DO (𝑘R5[DO] ≫ 𝑘R4d[BrO3 ˉ]). A series of sulfur-centered radical

410

chain propagation reactions caused by DO led to the over-stoichiometric sulfite

411

consumption. However, in the absence of oxygen, the SO3 ˉ generated by the initial

412

reaction of H2SO3 with BrO3ˉ is further oxidized by BrO3ˉ, resulting in the

413

stoichiometry (−Δ[sulfite]/Δ[bromate]) following Eq. 2. 20


Page 20 of 32

Page 21 of 32

414


ENVIRONMENTAL IMPLICATIONS

415

In this study, we proposed a reductive strategy to transform BrO3ˉ to Brˉ with

416

sulfite. Different from the eaqˉ-based reductive strategy, sulfite could effectively reduce

417

BrO3ˉ over a wide pH range of 3.0–6.0 in pure water open to the air. SO3ˉ was

418

generated during BrO3ˉ reduction by H2SO3, and HSO3ˉ was involved in the sulfur-

419

centered radicals chain propagation reactions. Several lines of evidence unraveled that

420

SO3ˉ was generated in BrO3ˉ/sulfite system and its rapid reaction with DO was the key

421

leading to the over-stoichiometric sulfite consumption. Organic contaminants with

422

electron-rich moieties can be effectively degraded in BrO3ˉ/sulfite system and SO4ˉ

423

rather than HOBr was determined to be the active oxidant. The effective removal of

424

BrO3ˉ (25.0 μg/L) in real water by sulfite at pHini ≤ 6.0 could be achieved (Figure S15),

425

indicating that this method was an alternative for controlling BrO3ˉ in water treatment.

426

As pHini is elevated to 7.0, the reduction of BrO3ˉ in tap water and source water by

427

sulfite is not efficient although that in Milli-Q water is effective, which should be

428

ascribed to large buffering capacity of tap water and source water, and thus causing

429

high pH during the reaction process. In sum, reductive removal of BrO3ˉ with sulfite is

430

low-energy and environmentally friendly. However, further research is needed to

431

broaden the applicable pH range of this approach and improve the performance of this

432

process in real waters, especially under neutral and weak alkaline conditions. The

433

influence of co-existing solutes on BrO3ˉ reduction by sulfite also warrants further study

434

so as to predict the performance of this technique in different real waters.

435

ASSOCIATED CONTENT 21



436

Supporting Information

437

The Supporting Information is available free of charge on the ACS Publications website.

438

Texts S1–S4, Figures S1–S15, and Tables S1–S3.

439

AUTHOR INFORMATION

440

Corresponding Author

441

*Email: [email protected];

442

Phone: +86-21-65983869;

443

Fax: +86-21-65986313.

444

Notes

445

The authors declare no competing financial interest.

446

ACKNOWLEDGMENTS

447

This work was supported by the National Natural Science Foundation of China

448

(Grant 21522704).

449

REFERENCES

450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465

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26


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Page 27 of 32


Concentration (mM)

(a) pHini 3.0

(b) pHini 4.0 0.10

0.10

0.08

0.08

0.08

0.06

0.06

0.06

0.04

0.04

0.04

0.02

0.02

0.02

0.00

0.00 0

5

10

15

20

25

0.00 0

30

10

20

Concentration (mM)

(d) pHini 6.0

30

40

50

60

0.10

0.10

0.08

0.08

0.06

0.06

0.04

0.04

0.02

0.02

0

20

40

60

80

100

120

Time (min)

(e) pHini 7.0

0.00

BrO3- (Air) Br -3(Air) BrO3- + Br - (Air) BrO3- (N2) Br -3(N2)

BrO3- + Br - (N2) Total Br

0.00 0

614 615 616 617 618

(c) pHini 5.0

0.10

20

40

60

80

Time (min)

100

120

0

20

40

60

80

100

120

Time (min)

Figure 1. Effect of dissolved oxygen (DO) on the kinetics of BrO3ˉ reduction by sulfite and that of Brˉ generation at different pHini levels. Reaction conditions: [NaBrO3]0 = 0.10 mM, [NaHSO3]0 = 1.0 mM.

27



(a) w/o O2

(b) w/ O2 0.10

0.08

0.04

2

R = 0.99 Slope = 0.31

BrO3BrTotal Br

R2 = 0.98 Slope = -0.30

0.02 0.00 0.00

619 620 621 622 623 624

0.05

0.10

0.15

0.20

0.25

Concentration (mM)

Concentration (mM)

0.10

0.06

Page 28 of 32

0.08 0.06 0.04

R2 = 0.95 Slope = 0.065

BrO3BrTotal Br

R2 = 0.97 Slope = -0.064

0.02 0.00 0.00

0.30

0.25

Sulfite concentration (mM)

0.50

0.75

1.00

1.25

1.50

Sulfite concentration (mM)

Figure 2. Stoichiometries for the reaction of BrO3ˉ with sulfite in the absence of oxygen (a) and in the presence of oxygen (b). Reaction conditions: (a) pHini = 4.0, [NaBrO3]0 = 0.10 mM, [NaHSO3]0 = 0-0.30 mM, reaction time 4 h; (b) pHini = 4.0, [NaBrO3]0 = 0.10 mM, [NaHSO3]0 = 0-1.50 mM, reaction time 30 min.

28


Page 29 of 32


150

(a)

100

40

Intensity

Intensity

(b)

60

50 0

20 0

-50

-20

-100

-40

-150

-60 3340

3360

3380

3400

3340

Magnetic field (G)

3400

(d)

7.5 0.28 min-1

Formaldehyde (μM)

-1

kobs(min )

3380

Magnetic field (G)

(c) 0.31 min-1 0.32

3360

0.24 0.16 0.08

BrO3BrO3- /sulfite system

6.0 4.5 3.0 1.5

-1

0.01 min

0.00

625 626 627 628 629 630 631 632 633

No scavenger

TBA

0.0 0

EtOH

5

10

15

20

25

30

Time (min)

Figure 3. ESR spectra of DMPO-SO3ˉ (a), ESR spectra of DMPO-HO and DMPOSO4ˉ (b), effect of TBA and EtOH on kobs of phenol degradation (c), and the formation of formaldehyde (d) in air atmosphere. Reaction conditions: pHini = 4.0, [NaBrO3]0 = 0.10 mM, [NaHSO3]0 = 1.0 mM, (a) and (b) [DMPO]0= 100 mM, (c) [phenol]0 = 5.0 μM, [TBA]0 = 2.50 mM, [EtOH]0 = 55.0 mM, (d) [MeOH]0 = 5.0 mM. (● indicates DMPO-SO3ˉ adduct; ▲ indicates DMPO-HO adduct; ◆ indicates DMPO-SO4ˉ adduct).

29


0.06

5 μM phenol 100 μM phenol 100 μM NB

0.04

-

0.02 0.00

0.06 0 μM phenol or NB 5 μM phenol 100 μM phenol 100 μM NB

0.04 0.02

5

10

15

20

Time (min)

25

30

35

(c)

5 4

0

5

10

15

20

25

30

35

Time (min)

100 99

5 μM phenol 100 μM NB

3

98

2

97

1

96

0

0.00 0

634 635 636 637 638 639

0.08

Page 30 of 32

NB concentration (μM)

0 μM phenol or NB

(b)

0.10

Phenol concentration (μM)

0.08

Br concentration (mM)

(a)

0.10

-

BrO3 concentration (mM)


95 0

1

2

3 15

20

25

30

35

Time (min)

Figure 4. Simulation of BrO3ˉ removal (a), Brˉ formation (b) and contaminants degradation (c) in BrO3ˉ/sulfite system under oxic conditions. Symbols and dash lines represent measured data and the model simulations, respectively. Reaction conditions: pHini = 3.0, [NaBrO3]0 = 0.10 mM, [NaHSO3]0 = 1.0 mM.

30


Page 31 of 32

640 641 642 643 644


Figure 5. Proposed pathways of BrO3ˉ reduction and sulfur transformation in BrO3ˉ/sulfite system. The pathway numbers (R1, R5, R6, R11, and R13-17) correspond to the reactions in Table 1 and the reaction equation and the rate constant of R4d is shown in Table S3.

31



646 647

Table 1. The model equations and the corresponding rate constants in air atmosphere in BrO3ˉ/sulfite system. No. R1 R2 R3

648 649 650

Page 32 of 32

k (M-1 s-1)a

Reaction ˉ

Reference

210 This work -3 8.0 × 10 This work 11.24 This work 8 2.0 × 10 , 50,64 R4 HSO3ˉ + H+ ⇌ H2SO3 3.6 × 106 41 R5 2.5 × 109 SO3ˉ + O2 ⟶ SO5ˉ 3  +  2 R6 8.6 × 10 This work SO5 ˉ + HSO3ˉ ⟶ H + SO4 ˉ + SO4 ˉ 4   R7 3.6 × 10 This work SO5 ˉ + HSO3ˉ ⟶ HSO5ˉ+ SO3 ˉ 41 3 2 + R8 9.1 × 10 HSO5ˉ + HSO3ˉ ⟶ 2SO4 ˉ + 2H 7 41    R9 9.0 × 10 SO5 ˉ + SO5 ˉ ⟶ 2SO4 ˉ + O2 41 R10 SO5ˉ + SO5ˉ ⟶ S2O82ˉ + O2 1.3 × 108 8 41  2  + R11 SO4 ˉ + HSO3ˉ ⟶ H + SO4 ˉ + SO3 ˉ 7.5 × 10 41 R12 SO4ˉ + SO4ˉ ⟶ S2O82ˉ 5.0 × 108 65 9  R13 SO4 ˉ + phenol ⟶ products 8.8 × 10 R14 2BrO2 + HSO3ˉ + H2O ⟶ 2HBrO2 + SO42ˉ + H+ 7.5 × 109 This work 66 + R15 2BrO2 + H2O ⟶ HBrO2 + BrO3ˉ + H 4.2 × 107 55 R16 HBrO2 + HSO3ˉ ⟶ HOBr + SO42ˉ + H+ 3.0 × 107 9 55 2 + R17 HOBr + HSO3ˉ ⟶ Brˉ + SO4 ˉ + 2H 1.0 × 10 aSome rate constants were obtained from literatures (R4, R5, R8-13, and R15-17). The rate constants of R1-3, R6-7, and R14 were estimated based on fitting the experimental data with the constructed kinetic model to under specific conditions. BrO3ˉ + H2SO3 ⟶ SO3 + BrO2 + H2O H2SO3 ⟶ SO2 + H2O 2HSO3ˉ + O2 ⟶ 2H+ + 2SO42ˉ

32


Overlooked Role of Sulfur-Centered Radicals During Bromate

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