Mechanistic Aspects of the Formation of Adsorbable Organic Bromine

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Mechanistic Aspects of the Formation of Adsorbable Organic Bromine during Chlorination of Bromide-containing Synthetic Waters Markus Langsa, Anna Heitz, Cynthia A. Joll, Urs von Gunten, and Sebastien Allard Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00691 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Environmental Science & Technology

Chlorine Subs6tu6on

Br-DOM-subs

Cl-DOM

Cl-Br-DOM HOCl DOM

Electrophilic Subs6tu6ons

Br-DOM

HOBr Oxida6on DOM Br-

HOCl

DOMox

+

Br-

Br- Recycling

ACS Paragon Plus Environment

AOBr


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1

Mechanistic Aspects of the Formation of

2

Adsorbable Organic Bromine during Chlorination

3

of Bromide-containing Synthetic Waters

4

Markus Langsa,1,2 Anna Heitz,3 Cynthia A. Joll,1 Urs von Gunten,4,5,* and Sebastien Allard1** 1

5

Curtin Water Quality Research Centre, Department of Chemistry, Curtin University, GPO Box U1987, Perth WA 6845, Australia

6

7

2

Jurusan Kimia, Fakultas Matematika dan Ilmu Pengetahuan Alam, Universitas Papua, Manokwari Papua Barat 98314, Indonesia

8 3

9 4

10

Department of Civil Engineering, Curtin University, Perth WA 6845, Australia

Eawag, Swiss Federal Institute of Aquatic Science and Technology, ETH Zürich, Zürich, Switzerland

11 12

5

School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale Lausanne (EPFL), Switzerland

13 14 15

Corresponding authors

16

*Urs von Gunten, phone: +41 58 765 5270; email: [email protected]

17

**Sebastien Allard, phone: +61 8 9266 7949 ; email: [email protected]

18 19 20 21


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22

Abstract

23

During chlorination of bromide-containing waters, a significant formation of brominated

24

disinfection by-products is expected. This is of concern because Br-DBPs are generally more

25

toxic than their chlorinated analogues. In this study, synthetic water samples containing

26

dissolved organic matter (DOM) extracts and bromide were treated under various disinfection

27

scenarios to elucidate the mechanisms of Br-DBP formation. The total concentration of Br-

28

DBPs was measured as adsorbable organic bromine (AOBr). A portion of the bromine

29

(HOBr) was found to react with DOM via electrophilic substitution (≤ 40%), forming AOBr,

30

and the remaining HOBr reacted with DOM via electron transfer with a reduction of HOBr to

31

bromide (≥ 60%). During chlorination, the released bromide is re-oxidised (“recycled”) by

32

chlorine to HOBr, leading to further electrophilic substitution of unaltered DOM sites and

33

chlorinated DOM moieties. This leads to an almost complete bromine incorporation to DOM

34

(≥ 87%). The type of DOM (3.06 ≤ SUVA254 ≤ 4.85) is not affecting this process, as long as

35

the bromine-reactive DOM sites are in excess and a sufficient chlorine exposure is achieved.

36

When most reactive sites were consumed by chlorine, Cl-substituted functional groups (Cl-

37

DOM) are reacting with HOBr by direct bromination leading to Br-Cl-DOM and by bromine

38

substitution of chlorine leading to Br-DOM. The later finding was supported by

39

hexachlorobenzene as a model compound from which bromoform was formed during HOBr

40

treatment. To better understand the experimental findings, a conceptual kinetic model

41

allowing to assess the contribution of each AOBr pathway was developed. A simulation of

42

distribution system conditions with a disinfectant residual of 1 mgCl2 L-1 showed complete

43

conversion of Br- to AOBr, with about 10% of the AOBr formed through chlorine

44

substitution by bromine.

45 46


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47

INTRODUCTION

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Disinfection processes are applied to provide hygienically safe drinking water.1-3 Worldwide,

49

the main chemical disinfectant used for drinking water treatment is chlorine (HOCl + OCl-).3

50

Chlorine is typically applied at much higher concentrations than the ubiquitously occurring

51

bromide, for which the concentrations range from 10 to 2000 µg L-1 in source waters.4-10

52

During chlorination, bromide is oxidised to hypobromous acid/hypobromite (HOBr/OBr-)

53

with a second order rate constant for the reaction of HOCl with bromide of kHOCl,Br-= 1550 M-

54

1 -1 11

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publication. Once formed, bromine reacts with organic moieties of dissolved organic matter

56

(DOM), analogous to the reactions of chlorine with DOM.12, 13 Bromine can react either by

57

electron transfer (ET) (redox reaction) with a release of bromide or by electrophilic

58

substitution (ES) leading to the formation of brominated organic compounds, some of which

59

can be measured as individual brominated disinfection by-products (DBPs).14, 15 The presence

60

of both chlorine and bromine in the water induces competition for reactions with DOM

61

moieties and subsequent formation of chlorinated-DBPs (Cl-DBPs), brominated-DBPs (Br-

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DBPs) or mixed products.16, 17 In one study, it has been reported that bromine reacts 10 times

63

faster than chlorine with DOM extracts.13 Moreover, Heeb et al.18 compiled the species-

64

specific second order rate constants for the reactions of bromine with a series of phenolate

65

compounds, representative of DOM reactive moieties, and showed that they are on average

66

3000 times higher than the corresponding second order rate constants for the chlorine

67

reactions. Bromine can also affect the yield/magnitude and species distribution of

68

halogenated DBPs.16, 19-22 Furthermore, bromine has been reported to increase the formation

69

of N-nitrosodimethylamine (NDMA) during ozonation or chloramination of specific

70

precursors.23,

71

compounds are expected to be formed more readily and with a higher yield than chlorinated

72

compounds. Bromide can also enhance some processes during chlorination such as the

73

oxidation of Cr(III) to Cr(VI),25 abatement of phenol-containing micropollutants,26 oxidation

74

of HOI to iodate thereby mitigating the formation of iodinated-DBPs,16,

s .

For simplicity, the term bromine will be used for the sum of HOBr and OBr- in this

24

Overall, during chlorination of Br-containing waters, brominated organic

27

and Mn(II)

28

75

oxidation avoiding its carryover to distribution systems.

76

Even though inactivation of microorganims is the main objective during chemical drinking

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water disinfection, exposure to DBPs, some of which are of potential health concern, also has

78

to be considered. Therefore, it is crucial to understand the extent and mechanisms of DBP

79

formation, with a special emphasis on brominated compounds since they are generally of


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greater potential health concern compared to their chlorinated analogues and they are

81

preferentially formed during chlorination of bromide-containing waters.29-32

82 83

The application of advanced analytical methods for measurement and identification of DBPs

84

allows continuous and indefinite discoveries of new halogenated DBPs.33 To date, hundreds

85

of halogenated DBPs have been reported.34 Nevertheless, a large portion of DBPs remains

86

unidentified and uncharacterised.35-37 Therefore, quantifying regulated and non-regulated

87

known DBPs may underestimate the total formation of halogenated compounds. To

88

overcome this problem, the measurement of halogen-specific adsorbable organic halogen

89

(AOX, which is similar to total organic halogen (TOX)) can be used to quantify the total

90

halogen incorporation into DOM.20, 38 Halogen-specific AOX measurements are designed to

91

quantify adsorbable organic chlorine (AOCl), bromine (AOBr) and iodine (AOI). Substantial

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efforts have been made to reveal the contribution of single Br-DBPs to the total AOBr

93

concentration. However, most of these studies have focussed only on a few specific DBPs,

94

such as the regulated brominated trihalomethanes and brominated haloacetic acids, and the

95

non-regulated brominated haloacetonitriles, the sum of which have been found to constitute

96

less than 50% of the total AOBr.17,

21, 39, 40

The unknown fraction of AOBr may contain

37, 41

Yang et al.42 reported a good correlation

97

compounds with potential health effects.

98

between the formation of AOBr and AOI and an increase in cytotoxicity and genotoxicity for

99

chlorinated and chloraminated waters. Therefore, AOBr measurements may be appropriate as

100

a proxy for the formation of brominated DBPs of potential health concern.

101 102

A recent study by Zhu and Zhang (2016)43 reported the application of a kinetic model to

103

predict the formation of halogen-specific AOX compounds (AOCl, AOBr, and AOI) during

104

chlorination and chloramination of raw waters. This model involved a total of 47 reactions

105

with 25 rate constants which were determined using methods based on “best fit” with AOX

106

measurements. The outcomes of their model calculations for AOCl, AOBr, and AOI

107

formation, as well as predictions of total oxidant residual concentrations, fitted well with their

108

experimental data, indicating the usefulness of kinetic modelling for simulation of

109

halogenated DBP formation during oxidative treatment processes.43 However, it is

110

questionable whether such a modelling effort will be transferrable to other systems, because

111

of the large number of variables.

112

This paper focuses on the formation of brominated organic compounds under simulated

113

drinking water chlorination conditions. The influence of water quality parameters (i.e.,


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bromide/bromine concentration, DOM type and concentration, chlorine dose) on AOBr

115

formation was investigated for synthetic waters. Furthermore, the impact of bromide

116

recycling during chlorination of Br-containing waters on AOBr formation was assessed. To

117

evaluate the competition between chlorine and bromine for reactive sites in DOM,

118

chlorination of DOM (i.e., formation of chlorine-substituted functional groups) followed by

119

bromine addition was investigated. Finally, an empirical kinetic model was proposed to

120

explore pathways for AOBr formation during drinking water chlorination of bromide-

121

containing waters.

122 123

MATERIALS AND METHODS

124

Chemicals

125

Inorganic and organic chemicals used in this study were all of analytical grade quality.

126

Ultrapurified water (resistivity of 18.2 mΩ) produced with a Purelab Ultra Analytic

127

purification system (Elga, UK) was used for all experiments. Three commercial DOM

128

extracts from the International Humic Substances Society (IHSS) were used in this study:

129

Suwannee River DOM (SR-DOM; catalogue number: 2R101N), Nordic Reservoir DOM

130

(NR-DOM; 1R108N), and Pony Lake Fulvic Acid (PL-DOM; 1R109F). The SUVA254 values

131

of the SR-, NR- and PL-DOM samples determined in this study were 4.45, 4.85, and 3.06 L

132

mgC-1 m-1, respectively.

133 134

Preparation of Oxidant Solutions

135

Aqueous stock solutions of sodium hypochlorite (approximately 1300 mM, 90 g L-1 as Cl2)

136

were standardized (in the concentration range 5 to 30 µM, pH 11) by direct UV measurement

137

at 292 nm using a CARY 60 UV-VIS Agilent Technologies spectrophotometer

138

(ε292nm (ClO-) = 350 M-1 cm-1 44). A bromine stock solution of 500 µM was produced by

139

mixing HOCl with a slight stoichiometric excess of bromide (5%).28 The solution was

140

vigorously stirred for at least 15 min to allow complete oxidation of bromide to bromine. The

141

concentration of the bromine stock solution was standardized as OBr- at pH 11 by

142

measurement of the UV absorbance at 329 nm (ε329 nm = 332 M-1 cm-1

143

residual, measured as the sum of bromine and chlorine, was analysed by the N,N-

144

diethylphenylene-1,4-diamine (DPD) colorimetric method.45

145

Analytical Methods for Halogen-Specific AOX, Bromide, Trihalomethanes and

146

Dissolved Organic Carbon


27

). The oxidant

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Halogen-specific AOX (AOCl and AOBr) was measured using a method developed by Hua

148

and Reckhow20 and modified by Kristiana, et al.39 (see Text S1). The quantification limit was

149

5 (±0.2) µg L-1 as Cl- for AOCl and 2 (±0.1) µg L-1 as Br- for AOBr. Bromide was measured

150

by ion chromatography using an AG9H/AS9H column and a 9 mM Na2CO3 eluent with a

151

flow rate of 1 mL min-1. The injection volume was 100 µL. The quantification limit was 2.8

152

(±0.9) µg L-1. The 4 regulated THMs, i.e., CHCl3, CHBrCl2, CHBr2Cl, and CHBr3, were

153

analysed by headspace solid-phase microextraction followed by gas chromatography-mass

154

spectrometry according to a published method.46 The quantification limits for CHCl3,

155

CHBrCl2, CHBr2Cl, and CHBr3 were 0.3 (±0.06) µg L-1 (2.6 nM), 0.2 (±0.07) µg L-1 (1.2

156

nM), 0.4 (±0.08) µg L-1 (1.9 nM), and 0.5 (±0.05) µg L-1 (2.1 nM), respectively. DOC

157

concentrations were determined by the UV/persulfate oxidation method using a Shimadzu

158

TOC Analyser. The quantification limit was 0.1 (±0.06) mgC L-1.

159 160

Experimental Procedures

161

Kinetic Study of AOBr Formation

162

Synthetic waters were prepared with purified water containing bromide (500 µg L-1 (6.26

163

µM)) and SR-DOM or PL-DOM (4 mgC L-1). These solutions were chlorinated (75 µM, ~5

164

mg Cl2 L-1) and samples were withdrawn at 5, 15, 30, and 60 min.

165

Bromination (HOBr) versus Chlorination (Br-+HOCl) Experiments

166

For bromination experiments (HOBr), synthetic waters containing various concentrations of

167

DOM (1.0–8.0 mgC L-1) were prepared in purified water and HOBr added to achieve initial

168

concentrations of 1.0, 3.13, and 6.26 µM (80–500 µg L-1). For chlorination experiments (Br-

169

+HOCl), similar synthetic waters containing various concentrations of SR-DOM (1.0–8.0

170

mgC L-1) and differing concentrations of Br- ranging from 1.0 to 6.26 µM were prepared in

171

purified water. A chlorine solution was added to achieve initial concentrations of 30, 50, 75,

172

and 140 µM Cl2 for 1, 2, 4 and 8 mgC L-1 of SR-DOM, respectively. Analyses of AOBr and

173

bromide for bromination and chlorination experiments were performed after 1 h reaction

174

time.

175

Pre-Chlorination Experiments

176

Synthetic waters containing SR-DOM (4 mgC L-1) were pre-chlorinated at differing initial

177

concentrations (15, 30, 45, 60, 75, and 90 µM) for 1 h before addition of bromine or bromide

178

(6.26 µM) followed by a holding time of 1 h. Differing pre-chlorination scenarios were also

179

investigated including pre-chlorination with fixed initial concentrations (30 and 75 µM) at

180

differing chlorine contact times of 5, 15, 30, and 60 min before bromine addition for 1 h.


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Another experiment was conducted in which SR-DOM (4 mgC L-1) was pre-chlorinated until

182

the oxidant was completely consumed (24 h). Thereafter, bromine was added to the reaction

183

mixture in the concentration range of 5–50 µM for 1 h.

184

Simulation of Realistic Drinking Water Treatment Conditions

185

A solution containing SR-DOM (4 mgC L-1) and bromide (6.26 µM) was chlorinated at

186

differing initial concentrations (15, 30, 45, 60, 75, and 90 µM) for 1 h and 24 h.

187 188

For all experiments, the pH of the solution was kept constant at pH 8.0 using phosphate

189

buffer (1 mM). Chlorine equivalent residual was measured for all experiments. All samples

190

from all experiments were quenched using aqueous Na2SO3 solution (10% molar excess

191

calculated based on the chlorine equivalent residual) and stored at 4oC prior to analysis. The

192

samples were analysed for halogen-specific AOX, Br-, and THMs within 24 h. All

193

experiments were conducted in duplicate.

194 195

RESULTS AND DISCUSSION

196

Kinetics of AOBr Formation

197

In a solution containing bromide (6.26 µM), chlorine (75 µM) and SR-DOM (4 mgC L-1),

198

HOBr was formed rapidly and reacted with the SR-DOM to form AOBr.12, 13 This is shown in

199

Figure 1 by the evolution of AOBr and inorganic bromine species (HOBr+Br- measured as

200

Br- after quenching). Most of the HOBr (83%) was incorporated into DOM as AOBr in the

201

first 5 min, reaching a maximum of 88% after 15 min. The decrease in inorganic bromine

202

species was inversely correlated with the formation of AOBr, with 23% of the initial Br-

203

remaining after 5 min and 13% after 1 h. The bromine mass balance (recovery) ranged from

204

97% to 103%, indicating that AOBr and Br-/HOBr were the main forms of bromine in this

205

system and that bromine loss by volatilisation was minimal. The evolution of inorganic

206

bromine species and AOBr for the corresponding experiment with PL-DOM is presented in

207

the supporting information (SI) Figure S1. A slightly lower Br-incorporation (71% after 5

208

min) was observed for PL-DOM compared to SR-DOM, in agreement with previously

209

published data.15 However, after 1 h, the AOBr concentration was similar to the SR-DOM

210

experiment with 85% Br-incorporation.


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Bromine Species (%)

100

80

60 AOBr Bromide Total bromine

40

20

0 0

5

15

30

60

211

Reaction time (min)

212

Figure 1. AOBr concentrations as a measure for Br-incorporation (%) and the inorganic

213

bromine species measured as bromide after quenching during chlorination of SR-DOM.

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Experimental conditions: SR-DOM (4 mgC L-1), bromide (6.26 µM, 500 µg L-1), phosphate

215

buffer (1 mM), pH = 8, chlorine (75 µM, ~5 mg L-1), Na2SO3 solution (10% excess based on

216

chlorine equivalent residual) for quenching. Lines are shown to guide the eye.

217 218

During drinking water treatment, HOCl reacts with DOM but also oxidises Br- to HOBr (k=

219

1550 M-1 s-1)11 (reaction (1) Scheme 1).13, 18 HOBr can then react with DOM moieties via ES

220

to form AOBr (reactions (3), (4) and (5) in Scheme 1).47-49 Alternatively, HOBr can oxidise

221

the electron-rich moieties of DOM resulting in a release of Br- (reaction (6), Scheme 1).48, 49

222

The released Br- can then be re-oxidised to HOBr (reaction (1), Scheme 1) by the excess

223

HOCl. This re-formed HOBr can again react with DOM moieties with the formation of

224

AOBr.

225


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Scheme 1. Main reactions involved in a DOM-containing system during chlorination in the

228

presence of bromide (Br-+HOCl) or during bromination (HOBr). The numbers in brackets

229

refer to reactions in the kinetic model in the SI, Table S1.

230 231

The extent to which HOBr reacts with DOM moieties to AOBr via ES or ET whereby Br- is

232

released, depends on the composition and the concentration of the organic functional groups

233

in the DOM.12, 15

234 235

Effect of Water Matrix Constituents and Impact of Bromide Recycling on AOBr

236

Formation: Chlorination of Bromide-Containing Water (Br-+HOCl) or Bromination

237

(HOBr)

238

General Observations

239

To better understand the differing mechanisms involved during chlorination of bromide-

240

containing waters, synthetic waters containing DOM (1.0–8.0 mgC L-1) and Br- (1.0–6.26

241

µM) were treated with HOCl (30–140 µM) and compared to similar bromination experiments

242

for which synthetic waters were treated directly with the same HOBr doses (1.0–6.26 µM).

243

The formation of AOBr (presented as % Br-incorporation) for chlorination (Br-+HOCl) and

244

bromination (HOBr) of various types of DOM, differing concentrations of SR-DOM and

245

differing concentrations of HOBr/Br- is shown in Figure 2. Generally, AOBr formation was


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much higher for chlorination (Br-+HOCl, grey bars) compared to bromination only (HOBr,

247

black bars). The maximum % Br-incorporation obtained in chlorination experiments was

248

98%, while in bromination experiments, it only accounted for up to 42% of the initial

249

bromine concentration. As illustrated in Figures S2a-c, the bromide concentration after

250

quenching the oxidant residual reached a maximum of 72% in bromination experiments

251

compared to only 18% after chlorination for 1 h. The bromine mass balance was ranging

252

from 97% to 103% indicating that most of the initial bromide was accounted for by AOBr

253

and inorganic bromine. This indicates that the dominant reaction pathway during the

254

bromination experiments (HOBr alone) is an electron transfer (ET) between HOBr and

255

DOM, with bromide and oxidised DOM being formed (reaction (6), Scheme 1). Therefore,

256

the formation of AOBr is only a minor reaction pathway in the bromination experiments. For

257

chlorination experiments (Br-+HOCl), high residual concentrations of the oxidants (15–32

258

µM), mostly chlorine, were available during the 1 h contact time (Figures S2d-f). This

259

enabled re-oxidation of bromide to HOBr (bromide recycling; reaction (1), Scheme 1), which

260

could further react with DOM forming more AOBr and resulting in a much lower sum of

261

HOBr and bromide at the end of the experiment. For experiments with bromine only, no

262

oxidant was left in solution after 1 h (Figures S2d-f), indicating all HOBr had been

263

consumed. These results emphasize the fact that Br- re-oxidation (bromide recycling) is a key

264

factor that impacts the extent of AOBr formation.

265 266

Influence of DOM Type

267

In a previous study it has been shown that DOM type (i.e., structure and functionality) can

268

play a role in the extent of Br-incorporation.15 Bromination (HOBr only, black bars) of

269

various types of DOM showed that SR-DOM and NR-DOM had a higher yield of AOBr

270

compared to PL-DOM (Figure 2a). For SR-DOM and NR-DOM, the % Br-incorporation was

271

38% and 42%, respectively, while it was only 27% for PL-DOM. This observation coincides

272

with the SUVA254 values, which indicate that SR-DOM and NR-DOM contain more reactive

273

aromatic sites for AOBr formation (SUVA254 of 4.45 and 4.85 L mgC-1 m-1, respectively)

274

than PL-DOM (SUVA254 of 3.06 L mgC-1 m-1). This result is consistent with various studies

275

showing that DOM sources with high SUVA254 are more reactive with oxidants than DOM

276

sources with low SUVA254.50, 51 Furthermore, the electron donating capacity (EDC) of DOM,

277

a measure of electron rich and potentially bromine-reactive moieties, also reflects the same

278

trend.52

For chlorination experiments (Br-+HOCl, grey bars, Figure 2a), no significant


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279

differences in AOBr formation between the different DOM samples could be observed, with

280

an average % Br-incorporation for SR-DOM, NR-DOM and PL-DOM of 93%, 90%, and

281

87%, respectively.

Br-Incorporation (%)

120

(b)

(a)

HOBr Br-+HOCl

(c)

100 80 60 40 20

2

4

8

-D

O

O M

1

1

3.13

6.26

PL

-D

O M NR

-D SR

M

0

Types of DOM

[SR-DOM] (mgC L-1)

[HOBr/Br-] (µ µM)

282 283

Figure 2. AOBr formation presented as % Br-incorporation resulting from bromination only

284

(HOBr, black bars) or chlorination of DOM in the presence of bromide (Br-+HOCl, grey

285

bars). (a) DOM types (SR-, NR-, PL-DOM) (4 mgC L-1); for bromination: [HOBr] = 6.26

286

µM; for chlorination: [Br-] = 6.26 µM, [HOCl] = 75 µM. (b) SR-DOM (1, 2, 4 and 8 mgC L-

287

1

288

75 and 140 µM for 1, 2, 4 and 8 mgC L-1 of SR-DOM, respectively. (c) SR-DOM (4 mgC L-

289

1

290

µM, [HOCl] = 75 µM. Phosphate buffer (1 mM), pH = 8 for all experiments. Na2SO3

291

solution (10% excess based on chlorine equivalent residual) was used to quench the reactions.

292

Reaction time: 1 h for both chlorination and bromination.

): for bromination: [HOBr] = 6.26 µM; for chlorination: [Br-] = 6.26 µM, [HOCl] = 30, 50, ): for bromination: [HOBr] = 1.0, 3.13, 6.26 µM; for chlorination: [Br-] = 1.0, 3.13, 6.26

293 294

Influence of DOM Concentration

295

Chlorination/bromination experiments were conducted to study the effect of differing SR-

296

DOM concentrations on AOBr formation (Figure 2b). The % Br-incorporation for the

297

bromination experiments slightly and continuously increased from 28% to 39% with


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298

increasing DOM concentrations from 1 and 8 mgC L-1, respectively. In parallel, the bromide

299

fraction of the initial bromine concentration decreased from 72% to 63% for 1 mgC L-1 and 8

300

mgC L-1, respectively (Figure S2b). Therefore, even for 8 mgC L-1, oxidation of DOM was

301

still the dominant pathway during bromination and, since bromide recycling was not possible,

302

only about 40% of bromine was incorporated into DOM as AOBr. Overall, the ratios between

303

ET and ES reactions are quite similar for all DOM concentrations. Even though, increasing

304

the SR-DOM concentration increases the number of reactive sites, a constant ratio of DOM

305

moieties reacting with ET or ES is available for reaction with bromine. For the chlorination

306

experiments, the chlorine dose was increased with increasing NOM concentrations to ensure

307

a chlorine residual for bromide recycling. An increasing chlorine dose may impact the rate of

308

bromide oxidation/recycling by increasing the HOCl/Br- ratios. However, based on kinetic

309

experiments, after 1 h contact time, only a small influence of the chlorine dose on AOBr

310

formation is expected. Increasing the concentration of SR-DOM from 1 to 2 mgC L-1

311

increased AOBr formation by around 20 % from 71 % to 90 %. Further increases in DOC

312

concentration to 4 mgC L-1 and 8 mgC L-1 resulted in only slight increases in % Br-

313

incorporation to 93% and 95%, respectively. When the SR-DOM concentration was 1 mgC

314

L-1, bromine appears to have been in excess compared to the concentration of reactive sites,

315

resulting in only 71% Br-incorporation. However, increasing the SR-DOM concentration to 2

316

mgC L-1 appeared to allow most of the bromine to be incorporated into the DOM. Therefore,

317

increasing the DOM concentration further to 4 and 8 mgC L-1 did not significantly influence

318

AOBr formation since almost all the HOBr could be incorporated into DOM for a DOM

319

concentration of 2 mgC L-1.

320 321

Influence of Bromide Concentration

322

Experiments to test the effect of the initial inorganic bromine concentrations (HOBr or Br-)

323

on AOBr formation showed that increasing the HOBr concentrations in bromination

324

experiments increased the absolute concentration of AOBr (results not shown) but did not

325

affect the fraction of bromine which formed AOBr as the % Br-incorporation remained

326

unchanged (37% to 39%) (Figure 2c, black bars). Oxidation of DOM (ET) was still the

327

dominant reaction in bromination with more than 60% of the initial HOBr being reduced to

328

bromide (Figure S2c, black bars). This indicates that even for the highest HOBr dose and 4

329

mgC/L of SR-DOM, the reactive sites were not limiting for the AOBr formation. This is

330

illustrated by the same partitioning between ET and ES (reactions (3), (4), (5), Scheme 1).


12


Page 14 of 25

331

Analogous results were obtained for the chlorination experiments, where bromine was

332

incorporated with a higher but similar yield (93 to 98%) for all bromide concentrations.

333 334

Overall, in bromination experiments, around 35 - 40% of HOBr reacted through ES, while 60

335

- 65% reacted through electron transfer reactions (ET). In chlorination experiments, the %

336

Br-incorporation reached almost 100%. This is the result of a re-oxidation of bromide formed

337

from the ET reactions by HOCl to HOBr (bromide recycling, reaction (1), Scheme 1), until

338

an almost complete bromine incorporation into DOM is achieved.

339 340

To better understand whether competition between chlorine (HOCl) and bromine (HOBr) for

341

DOM reactive sites affects the extent of AOBr formation, pre-chlorination experiments

342

followed by HOBr/Br- addition were conducted and are described in the next section.

343 344

Competition between Chlorine and Bromine for Reactive DOM Moieties

345

Influence of Bromine

346

The effect of the pre-chlorination conditions on the formation of AOCl and AOBr from SR-

347

DOM was investigated by pre-chlorination at varying doses for 1 h, before addition of the

348

same HOBr dose followed by a reaction time of 1 h. Figure 3a shows that increasing the

349

chlorine dose during pre-chlorination resulted in an expected increase in the AOCl

350

concentration but also led to an increase in AOBr concentrations. A pre-chlorination dose of

351

zero reflects the reactivity of the unaltered SR-DOM with HOBr (bromination of 4 mgC L-1

352

SR-DOM, Figure 2a). With increasing pre-chlorination doses (Figure 3a), the % Br-

353

incorporation increased from 40% (2.2 µM AOBr) for a HOCl dose of 15 µM to nearly 100%

354

(5.7 µM AOBr) with 90 µM of HOCl. In accordance with the % Br-incorporation, the

355

inorganic bromine species (measured as bromide after quenching) decreased with increasing

356

pre-chlorination dose, from 54% (3.0 µM) to 7% (0.4 µM) for 15 and 90 µM of HOCl,

357

respectively (Figure 3a). For low chlorine doses (15 - 45 µM), even though there was some

358

chlorine residual before HOBr addition (0.3 – 15.6 µM HOCl (Figure S3)), AOCl

359

concentrations after 1 h contact time (HOCl only) and after 2 h contact time (1 h HOCl and

360

1 h HOCl + HOBr) were similar (Figure 3a, right axis). This shows that HOBr-DOM

361

reactions are favoured over HOCl-DOM reactions since both HOCl and HOBr were present

362

in solution in the second hour, but minimal additional AOCl was formed during that time.

363

However, HOCl is partly consumed by oxidation of the released bromide (from HOBr


13

Page 15 of 25


364

oxidation of DOM) to form additional HOBr. Nevertheless, for the highest chlorine doses (60

365

- 90 µM), small increases in AOCl concentrations were observed after bromine addition

366

(Figure 3a, right axis), since significant concentrations of chlorine were still present after 1 h

367

reaction time (Figure S3). In these cases, both HOCl and HOBr were in competition for

368

DOM reactive sites, i.e., both AOBr and AOCl concentrations increased.

369 370

Influence of Bromide

371

An additional set of pre-chlorination experiments was carried out for which bromide was

372

added instead of bromine (Figure S4). For a HOCl pre-chlorination dose of 15 µM, where no

373

oxidant residual was measured after 1 h (Figure S4), no AOBr was detected, because bromide

374

could not be oxidised to HOBr. However, as soon as a chlorine residual was measured after

375

1 h reaction time (for HOCl doses ≥ 30 µM), a formation of HOBr upon addition of bromide

376

occurs. This can react with DOM leading to AOBr concentrations after the second hour

377

similar to the corresponding pre-chlorination of DOM experiments with subsequent HOBr

378

addition (Figures S4, 3a left axis).

379

Under these experimental conditions, increasing the pre-chlorination dose increased the

381

formation of AOBr. It was expected that a pre-chlorination step would consume part of the

382

DOM sites, which can react with HOX, thereby decreasing the number of reactive sites

383

available for ES with bromine, resulting in a reduction in AOBr formation. However, since

384

such an effect could not be observed, it was hypothesised that HOCl activated DOM moieties

385

for reaction with HOBr, which then favoured AOBr formation. (a)

(b) b

14 12

80 10 60

8 AOBr Bromide AOCl after 1 hour pre-chlorination AOCl after 1 hour HOBr addition

40

6 4

AOCl concentration (µ µM) AOBr concentration (µ µ M)

100

AOBr Experimental data AOCl after pre-chlorination AOCl after 1 hour HOBr addition

Br-DOM1 Br-DOM1-subs Cl-Br-DOM1

14 12

6 10 8

4

6 4

2

20 2 0

0 0

15

30

45

60

75

Pre-chlorination dose (µ µM)

90

AOCl concentration (µ µ M)

8

-

Br-Incorporation and reduced bromine (as Br ) (%)

380

2 0

0 5

15

30

60

Pre-chlorination time (min)

386 387

Figure 3. Effect of pre-chlorination dose on AOBr and AOCl formation. (a) SR-DOM pre-

388

chlorinated with differing doses before bromine addition. (b) SR-DOM pre-chlorinated for


14


Page 16 of 25

389

differing durations with a fixed initial HOCl dose before bromine addition: experimental data

390

and kinetic model calculations are shown. Experimental conditions: (a) pre-chlorination

391

(doses: 15, 30, 45, 60, 75, 90 µM) for 1 h, (b) pre-chlorination (dose: 75 µM) for 5, 15, 30

392

and 60 min; SR-DOM (4 mgC L-1), phosphate buffer (1 mM), pH = 8, HOBr (6.26 µM) for

393

1 h, Na2SO3 solution (10% excess based on chlorine equivalent residual) for quenching. The

394

stacked bars (left side of each pre-chlorination time) represent the modeled data, the striped

395

bars (right side of each pre-chlorination time) represent the experimental data.

396 397

To investigate the role of the pre-chlorination contact time on AOBr formation during post-

398

bromination, SR-DOM was pre-chlorinated with a fixed chlorine dose of 75 µM for various

399

contact times (Figure 3b). This leads to differing (pre-)chlorine exposures, before bromine

400

addition (6.26 µM for 1 h). The (pre-)chlorination exposure (pre-chlorination times: 5-60

401

min) did not affect the AOBr formation, which was relatively high (approx. 6.0 µM) (Figure

402

3b, left axis). A slight decrease of AOBr formation from 3.70 µM to 3.35 µM for pre-

403

chlorination times of 5 min and 60 min was observed for a similar experiment with a lower

404

HOCl dose of 30 µM (Figure S5). Conversely, the concentration of AOCl increased

405

continuously with increasing pre-chlorination exposure (time), from 6.7 µM to 10.5 µM for

406

pre-chlorination times of 5 min or 60 min, respectively. A further slight increase of AOCl

407

was observed 1 h after HOBr addition (from 9.9 to 11.9 µM for pre-chlorination times of 5

408

min or 60 min, respectively) (Figure 3b, right axis), because of the high chlorine equivalent

409

residual in all experiments (Figure S6). However, even though the concentration of AOCl

410

increased 1 h after HOBr addition for low pre-chlorination contact times (0 to 30 min) for a

411

chlorine dose of 30 µM, for the highest contact time of 60 min, the AOCl concentration

412

decreased (Figure S7). The AOCl concentrations were 9.1 µM after 60 min contact time with

413

HOCl only and 6.4 µM after HOBr addition (overall 2 h contact time with oxidants) (Figure

414

S7). In this case, the total oxidant concentration (HOCl + HOBr) was consumed within 120

415

min (60 min pre-chlorination + 60 min after HOBr addition) (Figure S8). This suggests that,

416

as expected, HOBr reacted with reactive DOM sites to form AOBr. However, because AOCl

417

decreased for the highest contact time (Figure S7), HOBr may have led to a substitution of

418

chlorine by bromine.

419 420

Chlorine Substitution by Bromine in DOM

421

Bromination of Chlorinated Water


15

Page 17 of 25


422

To further test the hypothesis of chlorine substitution by bromine in DOM, a subsequent

423

experiment was conducted in which the oxidant residual from pre-chlorination was

424

completely consumed before bromine addition to avoid competition between HOCl and

425

HOBr for DOM reactive sites. SR-DOM samples (4 mgC L-1) were pre-chlorinated (45 µM)

426

until full consumption of the oxidant. Thereafter, HOBr was added to the samples to achieve

427

differing doses (5–50 µM).

428 429

In these experiments, AOBr increased from 1 to 7 µM, for bromine doses ranging from 5 to

430

50 µM (Figure 4), while AOCl gradually decreased from 12 µM (without addition of HOBr)

431

to 8 µM for a HOBr dose of 50 µM. Furthermore, an increase of total AOX (sum of AOCl

432

and AOBr) with increasing doses of bromine was observed (∆Total AOX in Figure 4). This

433

was assigned to DOM reactive sites which were already chlorinated with only a low

434

reactivity with chlorine for a further chlorination. Therefore, these sites react preferentially

435

with bromine to form Cl-Br-DOM moieties (reaction (4) in Scheme 1). For example, second

436

order rate constants for the reaction of 4-chlorophenolate, 2,4-dichlorophenolate and 2,4,6-

437

trichlorophenolate with HOBr are around three orders of magnitude higher than for HOCl.53,

438

54

439

reaction (5) in Scheme 1), while the AOBr concentration increased.

Overall, AOCl decreased (~ 33% reduction of the AOCl concentration, ∆AOCl in Figure 4,

16 AOCl AOBr Total AOX

AOX Concentration (µM)

14

∆ Total AOX = Cl-Br-DOM1

12

∆ AOCl = Br-DOM1-subs

10 8 6 4 2 0 0

440

5

10

20

50

HOBr dose (µ µM)


16


Page 18 of 25

441

Figure 4. Pre-chlorination (complete depletion of chlorine) followed by HOBr addition:

442

Effect of HOBr doses on AOX formation. Experimental conditions: SR-DOM (4 mgC L-1),

443

phosphate buffer (1 mM), pH = 8, pre-chlorination (dose: 45 µM) for 24 h then HOBr (5 – 50

444

µM) for 1 h, Na2SO3 solution (10% excess based on chlorine equivalent residual) for

445

quenching.

446

Based on these results, a new pathway for the formation of AOBr is proposed which involves

447

the substitution of chlorine in AOCl by bromine. An experiment with a model compound

448

(hexachlorobenzene) was conducted to further investigate this hypothesis.

449 450

Bromination of hexachlorobenzene

451

Hexachlorobenzene (HCB) was chosen for the bromination experiments because its aromatic

452

ring is fully substituted by chlorine. HCB solutions (50 µM) were treated with differing

453

doses of HOBr (250 to 1000 µM) for 120 h (for experimental details see Text S2).

454

Bromoform was the only THM detected after this reaction time (Figure S9). Although these

455

experiments were carried out with high concentrations of reactants (due to the low reactivity

456

of HCB) and the yield of CHBr3 was low, they provide evidence that chlorine can be

457

substituted by bromine in organic compounds.

458 459

Modelling of AOBr Formation

460

For a better mechanistic understanding of AOBr formation, a conceptual kinetic model is

461

proposed to simulate the experimental data (see Table S1, Text S3 for the model assumptions,

462

Text S4 for a step-by-step description of the modelling process and Text S5 for a sensitivity

463

analysis of the rate constants). The kinetic model calculations were performed with the

464

software Kintecus.55 According to our experimental results, there are 5 main reactions

465

involved in the formation of AOBr (Scheme 1):

466

(i) Reaction of HOBr with DOM by electrophilic substitution (ES). DOM1 was

467

assigned to the fraction of sites reacting with both HOBr and HOCl (reactions (2, 3), Scheme

468

1 and Table S1), leading to the formation of Br-DOM1 and Cl-DOM1. Cl-DOM1 represented

469

intermediate products (chlorine-substituted functional groups) that may react further with

470

HOBr.

471 472

(ii) Reaction of HOBr with DOM by electron transfer (ET, redox reaction). DOM2 was allocated to ET reactions with HOBr (reaction (6), Scheme 1 and Table S1).


17

Page 19 of 25


473

(iii) Reaction of HOBr with chlorinated DOM moieties by electrophilic substitution

474

(ES). Reaction of Cl-DOM1 with HOBr leading to Cl-Br-DOM1 (reaction (4), Scheme 1 and

475

Table S1). Cl-Br-DOM1 accounted for both AOCl and AOBr.

476

(iv) Chlorine substitution by bromine in DOM (reaction of AOCl to AOBr). Based on

477

the experimental evidence, this reaction was used generically to encompass the decrease of

478

AOCl along with an increase of AOBr. Reaction of Cl-DOM1 with HOBr leading to Br-

479

DOM1-subs (reaction (5), Scheme 1 and Table S1).

480

(v) HOBr formation by reaction of bromide, formed from electron transfer reactions

481

(ET), with chlorine (reaction (1), Scheme 1 and Table S1).

482

The total AOBr concentration in the model calculations is the sum of Br-DOM1, Br-DOM1-

483

subs and Cl-Br-DOM1.

484

Reactions related to the reactivity of HOCl and the formation of AOCl are described in the SI

485

(see Table S1, Text S3 for the model assumptions, Text S4 for a step-by-step description of

486

the modelling process and Text S5 for a sensitivity analysis of the rate constants).

487 488

Simulation of AOBr formation

489

In bromination experiments in the absence of chlorine, AOBr was solely formed from

490

reaction of HOBr with DOM1 (Br-DOM1) (Figure S10). For the chlorination experiments

491

(Figures S12 and Figure 5), the ES was still the major contributor to AOBr formation.

492

However, because chlorine was in excess, the competition between chlorine and bromine for

493

reactive sites reduced the portion of Br-DOM1, while the proportion of Cl-Br-DOM1 to the

494

total AOBr became significant. Chlorine substitution by bromine (Br-DOM1-subs) increased

495

with bromide/bromine (Figure S12) and chlorine concentrations (Figure 5), even though it

496

represents only a small portion of the total AOBr.

497

For the pre-chlorination experiments, the unexpected trend obtained for differing pre-

498

chlorination times for which similar AOBr concentrations were measured (Figure 3b) could

499

be explained by the model. DOM1 was used up by competing reactions with HOCl (reaction

500

(2), Scheme 1), while the Cl-DOM1 concentration increased with increasing (pre-

501

)chlorination exposure. Therefore, in terms of the total AOBr, the increasing contribution of

502

both Cl-Br-DOM1 and the chlorine substitution by bromine pathway (Br-DOM1-subs)

503

(reaction (4-5), Scheme 1) with increasing (pre-)chlorination exposure was compensated by

504

the decreasing contribution of ES to form Br-DOM1 (reaction (3), Scheme 1). Overall, a

505

similar total AOBr was measured.


18


Page 20 of 25

506

For the experiments with complete chlorine consumption after pre-chlorination (Figures 4

507

and S11), DOM1 was totally consumed by HOCl. Therefore, after HOBr addition, AOBr was

508

formed by direct ES with chlorinated DOM moities (Cl-Br-DOM1) and through chlorine

509

substitution by bromine (Br-DOM1-subs).

510 511 512

Implications for Drinking Water Distribution Systems

513

To simulate real drinking water treatment conditions, synthetic waters containing bromide

514

were chlorinated with doses ranging from 15 to 90 µM (1 to 6.4 mgCl2 L-1) and sampled at

515

two contact times (1 h and 24 h) to mimic residence times in distribution systems. Kinetic

516

model calculations were performed to better understand the experimental results.

517 518

Figure 5 shows that the model is able to predict AOBr formation for these conditions AOBr

519

formation was similar for the two sampling times (1 h and 24 h). The formation of AOBr

520

increased from 1.75 µM (30% Br-incorporation) to 5.61 µM (94% Br-incorporation) for

521

HOCl doses of 15 µM or 90 µM, respectively. AOCl formation showed a different behaviour.

522

For low chlorine doses (15 - 45 µM) with low residual chlorine concentrations after 1 h

523

(Figure S17), the AOCl concentration was similar for 1 h and 24 h. For the highest chlorine

524

dose with a substantial residual chlorine concentration after 1 h, an increase in AOCl was

525

observed up to 24 h. For example, for a chlorine dose of 90 µM, AOCl increased from 11.5

526

µM to 14.5 µM, while the oxidant residual decreased from 33 µM (2.3 mgCl2 L-1) to 13 µM

527

(0.9 mgCl2 L-1), for contact times of 1 h or 24 h, respectively.


19

Page 21 of 25


25

20

80 15 60 10 40

AOCl concentration (µ (µM)

100

Br-Incorporation (%)

Br-DOM1 Br-DOM1-subs Cl-Br-DOM1

AOBr after 1h AOBr after 24h AOCl after 1h AOCl after 24h

5

20

0

0 0

15

30

45

60

75

90

528

HOCl dose (µ µM)

529

Figure 5. Experimental and modelled AOX formation during chlorination of a water

530

containing SR-DOM and bromide for 1 h and 24 h. Experimental conditions: SR-DOM (4

531

mgC L-1), Br- (6.26 µM, 500 µg L-1), phosphate buffer (1 mM), pH = 8, HOCl doses: 15, 30,

532

45, 60, 75, 90 µM, Na2SO3 (10% molar excess calculated based on chlorine equivalent

533

residual) for quenching. The symbols represent experimental results: AOBr, circles, left axis,

534

AOCl triangles, right axis. Stacked bars represent the modeled contribution of various

535

reactive DOM sites to AOBr formation after 24 h.

536 537

Model calculations (bars in Figure 5) show that for low HOCl doses (15 – 30 µM), Br-DOM1

538

(reaction (3), Scheme 1) and Cl-Br-DOM1 (reaction (4), Scheme1) are the two main

539

processes for AOBr formation. For increasing HOCl doses, AOBr increased with an

540

increasing contribution of the chlorine substitution by bromine pathway (Br-DOM1-subs,

541

reaction (5), Scheme 1). At high HOCl doses (> 60 µM), the contribution of Br-DOM1 to the

542

total AOBr concentration slightly decreased with increasing chlorine doses and Br-DOM1-

543

subs contributed increasingly to AOBr formation. This shows that in distribution systems for

544

which a residual disinfectant of ~ 1 mgCl2 L-1 (chlorine dose 90 µM in Figure S17) or higher

545

is needed due to long residence times, Br-DOM1-subs may contribute >10% to the overall

546

AOBr formation. Furthermore, almost 100% of the bromide is expected to be converted to


20


Page 22 of 25

547

AOBr, while the AOCl will continually increase with increasing residence time. This is in

548

agreement with a recent study where more than 90% of the bromide was present as AOBr in

549

a distribution system.40 However, the current study was carried out with synthetic waters and

550

additional experiments need to be performed with real waters or in real distribution system

551

samples to validate these findings. Furthermore, the model was validated only with the SR-

552

DOM extract, different rate constants and DOM fractions might be needed for other DOM

553

types.

554 555

Supporting information

556

5 texts, 1 table and 17 figures with further information addressing model calculations and

557

additional data are available in the supporting information.

558 559

Acknowledgements

560

The authors would like to acknowledge the Australian Research Council (ARC

561

LP100100285), Water Corporation of Western Australia, Curtin University, the Swiss

562

Federal Institute for Aquatic Science and Technology (Eawag) and Water Research Australia

563

for support for this project. The authors also acknowledge the Australian Government

564

through The Department of Foreign Affairs and Trade for providing a PhD scholarship

565

under the Australian Award Scholarship (AAS) scheme for M. Langsa.

566 567 568

References

569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584

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Mechanistic Aspects of the Formation of Adsorbable Organic Bromine

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