Chlorate Formation Mechanism in the Presence of Sulfate Radical

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Environmental Processes

Chlorate Formation Mechanism in the Presence of Sulfate Radical, Chloride, Bromide and NOM Shaodong Hou, Li Ling, Dionysios D. Dionysiou, Yuru Wang, Jiajia Huang, Kaiheng Guo, Xuchun Li, and Jingyun Fang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00576 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Environmental Science & Technology

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Chlorate Formation Mechanism in the Presence of Sulfate Radical, Chloride,

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Bromide and NOM

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Shaodong Houa, Li Lingb, Dionysios D. Dionysiouc, Yuru Wangd, Jiajia Huanga, Kaiheng Guoa,

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Xuchun Lie, Jingyun Fanga,*

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a. Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

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Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou

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510275, China.

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b. Department of Civil and Environmental Engineering, the Hong Kong University of Science and

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Technology, Clear Water Bay, Kowloon, Hong Kong

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c. Environmental Engineering and Science Program, Department of Chemical and Environmental

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Engineering (DCEE), University of Cincinnati, Cincinnati, OH 45221, USA

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d. Department of Environmental Science, School of Geography and Tourism, Shanxi Normal

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University, Xi’an 710119, China

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e. School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou,

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310018, P. R. China

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*Corresponding author. Phone: +86 18680581522; e-mail: [email protected].

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TOC Art

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Abstract

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Halides and natural organic matter (NOM) are inevitable in aquatic environment and influence the

26

degradation of contaminants in sulfate radical (SO4•‒)-based advanced oxidation processes. This

27

study investigated the formation of chlorate in the co-exposure of SO4•‒, chloride (Cl‒), bromide (Br‒)

28

and/or NOM in UV/persulfate (UV/PDS) and cobalt(II)/peroxymonosulfate (Co/PMS) systems. The

29

formation of chlorate increased with increasing Cl‒ concentration in the UV/PDS system, however,

30

in the Co/PMS system, it initially increased and then decreased. The chlorate formation involved the

31

formation of hypochlorous acid/hypochlorite (HOCl/OCl‒) as an intermediate in both systems. The

32

formation was primarily attributable to SO4•‒ in the UV/PDS system, while Co(III) played a

33

significant role in the oxidation of Cl‒ to HOCl/OCl‒ and SO4•‒ was important for the oxidation of

34

HOCl/OCl‒ to chlorate in the Co/PMS system. The pseudo-first-order rate constants (k′) of the

35

transformation from Cl‒ to HOCl/OCl‒ were 3.32 × 10-6 s-1 and 9.23 × 10-3 s-1 in UV/PDS and

36

Co/PMS, respectively. Meanwhile, k′ of HOCl/OCl‒ to chlorate in UV/PDS and Co/PMS were 2.43 ×

37

10-3 s-1 and 2.70 × 10-4 s-1, respectively. Br‒ completely inhibited the chlorate formation in UV/PDS,

38

but inhibited it by 45.2% in Co/PMS. The k′ of SO4•‒ reacting with Br‒ to form hypobromous

39

acid/hypobromite (HOBr/OBr‒) was calculated to be 378 times higher than that of Cl‒ to HOCl/OCl‒,

40

but the k′ of Co(III) reacting with Br‒ to form HOBr/OBr‒ was comparable to that of Cl‒ to

41

HOCl/OCl‒. NOM also significantly inhibited the chlorate formation, due to the consumption of

42

SO4•‒ and reactive chlorine species (RCS, such as Cl•, ClO• and HOCl/OCl‒). This study

43

demonstrated the formation of chlorate in SO4•‒-based AOPs, which should to be considered in their

44

application in water treatment.

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INTRODUCTION

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Recently, sulfate radical (SO4•‒)-based advanced oxidation processes (AOPs) have attracted

47

increasing attention in both water treatment and groundwater remediation.1 SO4•‒ has a standard

48

reduction potential of 2.5-3.1 V, which is similar or slightly higher than that of hydroxyl radical (HO•)

49

(2.4-2.7 V).1-3 SO4•‒ can be readily produced from the activation of peroxodisulfate (PDS) or

50

peroxymonosulfate (PMS) by UV, heat or transition metals.4-7 Because of its high reactivity and

51

selectivity, SO4•‒-based AOPs have demonstrated their advantages in the destruction of a variety of

52

micropollutants.1 In addition, SO4•‒ has a wider operational pH range and its precursors (i.e. PDS and

53

PMS) are more stable and easier to transport than those of HO• (i.e. peroxide and ozone).4-7 Due to

54

these advantages, SO4•‒-based AOPs have been extensively investigated as an alternative to

55

HO•-based AOPs in some cases for the control of micropollutants such as algal toxins,8, 9 flame

56

retardants,10 pesticides11 and pharmaceuticals5, 7, 12 in water treatment.

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However, toxic oxidation byproducts, such as bromate and chlorate, are formed in some SO4•‒

58

-based AOPs, due to the inevitable reactions of SO4•‒ with halides in water. Chlorate, a harmful

59

chemical with a health reference level of 210 μg L-1 suggested by U.S. EPA13 and a drinking water

60

standard of 200 μg L-1 regulated in Switzerland,14 can form during the oxidation of chloride (Cl‒) in

61

the UV/PDS system.15 The increase of persulfate dosage and acidic condition favor the formation of

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chlorate.15 However, the mechanism of chlorate formation from the oxidation of Cl‒ in SO4•‒-based

63

AOPs remains unclear.

64

During the interaction of SO4•‒ with halides, a series of reactive halogen species (RHS) such as

65

halogen atoms (X•), dihalogen anion radical (X2•‒), XO• and free halogen (HOX/OX‒) can be

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generated as eqs 1-6.15-19

67

X‒ + SO4•‒ → SO42‒ + X•

(1)

68

X• + X‒ ↔ X2•‒

(2)

69

X2•‒ + X2•‒ → X2 + 2X‒

(3)

70

X2•‒ + X• → X2 + X‒

(4)

71

X2 + H2O → H+ + HOX

(5)

72

X• + HOX → XO•

(6)

73

RHS can be further transformed to halates such as bromate and chlorate through reacting with

74

SO4•‒.20,

75

Bromide (Br‒) can be converted to hypobromous acid/hypobromite (HOBr/OBr‒) as the primary

76

intermediate and to bromate as final product in UV/PDS and Co/PMS systems.20, 21 However, the

77

conversion pathways of Br‒ to HOBr/OBr‒ are different in the two systems. The conversion of Br‒ to

78

HOBr/OBr‒ in the UV/PDS system was primarily driven by SO4•‒,20 while that in the Co/PMS

79

system was driven by Co(III) as the major pathway and by SO4•‒ as the minor pathway.21 The

80

transformation pathways of Cl‒ to chlorate in SO4•‒-based AOPs might share some similarity with

81

that of Br‒ to bromate, but the answers are currently unknown.

21

The formation of bromate in the SO4•‒-based AOPs has been well investigated.20-24

82

In addition, Br‒ and natural organic matter (NOM) are co-present with Cl‒ in natural water, which

83

can affect chlorate formation. The rate constants of Br‒ and NOM reacting with SO4•‒ are 3.5 × 109

84

M-1 s-1 and 2.0 × 103 L mg-1 s-1, respectively. Thus, they can compete with Cl‒ to react with SO4•‒.20,

85

21, 25

86

and HOCl are 1.2 × 1010 M-1 s-1 and (1.55 - 6.84) × 103 M-1 s-1, respectively,26, 27 while the rate

Br‒ and NOM can also react with RHS. For example, the rate constants of Br‒ reacting with Cl•

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constants of NOM reacting with Cl•, ClO• and hypochlorous acid/hypochlorite (HOCl/OCl‒) have

88

been reported to be 1.3 × 104 L mg-1 s-1, 4.5 × 104 L mg-1 s-1 and 0.7-5 M-1 s-1, respectively.19, 26, 28

89

Thus, the presence of Br‒ and NOM further make the chemistry of the system more complicated,

90

which influence on the chlorate formation deserves investigation.

91

This study investigated and compared the chlorate formation in UV/PDS and Co/PMS systems,

92

as they are two of the most efficient SO4•‒-based AOPs.4, 6, 10, 12 The objectives of this study were: (1)

93

to investigate the kinetics and mechanisms of chlorate formation from the oxidation of Cl‒ in

94

UV/PDS and Co/PMS processes; (2) to investigate the effects of Br‒ and NOM on chlorate

95

formation.

96 97 98 99

MATERIALS AND METHODS Materials. All chemical solutions were prepared using reagent-grade chemicals and deionized water

(18.2

MΩ

cm)

from

a

Milli-Q

system

(Millipore).

Sodium

persulfate,

100

Oxone®(2KHSO5·KHSO4·K2SO4, 95%), cobalt(II) sulfate heptahydrate (CoSO4·7H2O), N,

101

N-diethyl-p-phenylenediamine (DPD) and fluorobenzene (FB) were purchased from Sigma-Aldrich.

102

Sodium chloride (NaCl), sodium bromide (NaBr), sodium sulfite (Na2SO3) and ethylenediamine

103

tetraacetic acid disodium salt (EDTA) were purchased from Sinopharm Chemical Reagent Co., Ltd

104

(Shanghai, China). Methanol at HPLC grade was obtained from Thermo Fisher. Sodium chlorate

105

(NaClO3), potassium bromate (KBrO3), 2,6-dibromophenol (2,6-DBP) and 2,4,6-tribromophenol

106

(2,4,6-TBP) were purchased from Aladdin. Suwannee River NOM (Cat. No. 2R101N), obtained

107

from International Humic Substances Society, was dissolved in pure water by stirring overnight, and

-6-


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was subsequently filtered through a 0.45 μm pore size fiber membrane (Whatman).

109

Experimental procedures. For the UV/PDS system, a 700-mL cylindrical, borosilicate glass

110

vessel with a low-pressure mercury UV lamp (254 nm, GPH 135T5 L/4, Heraeus Noblelight) was

111

used following Wu et al.29 The unit volume photon flux (I0) entering the solution and the optical path

112

length (L) were determined to be 1.45 μEinstein L-1 s-1 using iodide/iodate chemical actinometry,30

113

and 3.05 cm by measuring the photolysis rate of H2O2,31 respectively. A water sample containing a

114

specific makeup of Cl‒/Br‒/NOM was added to the vessel reactor. PDS at 200 μM was added to the

115

reactor and immediately subjected to UV irradiation. For the Co/PMS system, a 250-mL

116

batch-reactor was used with mixing provided by a magnetic stirrer. CoSO4, Cl‒/Br‒/NOM and PMS

117

were dosed sequentially in the reactor without any pH adjustment. Samples were taken at

118

predetermined time intervals and divided into two portions. One was immediately subjected to

119

HOX/OX‒ determination. The other was quenched with sodium sulfite for ion chromatography (IC)

120

analysis to measure the concentrations of Cl‒, Br‒, chlorate and bromate. An additional experiment

121

was conducted in the same manner by adding 1 mM FB or 0.2% methanol to the system to study the

122

contributions of Co(III) or SO4•‒ to the formation of HOCl/OCl‒ and chlorate in the Co/PMS system.

123

The solution pH was not adjusted in UV/PDS and Co/PMS systems, which was 4.2 ± 0.2 and 3.8 ±

124

0.2, respectively. All experiments were conducted at ambient temperature and at least duplicated.

125

Analytical methods. The concentrations of HOCl/OCl‒ and HOBr/OBr‒ were determined by

126

DPD colorimetric method when either HOCl/OCl‒ or HOBr/OBr‒ was presented alone.32 In the case

127

of the co-presence of HOCl/OCl‒ and HOBr/OBr‒, their total concentrations were determined by

128

DPD colorimetric method, and the concentration of HOBr/OBr‒ was determined following the

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129

method developed by von Gunten’s group.33 In this method, 2,6-DBP was used to react with

130

HOBr/OBr‒ and to produce 2,4,6-TBP with the stoichiometric relationship of 1:1. Then the

131

concentration of HOBr/OBr‒ could be quantified by measuring the concentration of 2,4,6-TBP

132

(details in Text S1). The concentration of HOCl/OCl‒ was then calculated by subtracting the

133

concentration of HOBr/OBr‒ from the total halogen concentration. The solution pH was recorded

134

using a Thermo Scientific Orion 420-A pH meter.

135

The concentrations of Cl‒, Br‒, chlorate and bromate were measured by an ion chromatography

136

(IC) system (ICS-900, Dionex) equipped with a conductivity detector. A high-capacity

137

hydroxide-selective analytical column (AS19, 4 × 250 mm, Dionex) with a guard column (AG19, 4

138

× 50 mm, Dionex) was used for separation. A gradient KOH eluent (10 mM initially for 10 min, then

139

ramping to 45 mM from 10 to 25 min) at a flow rate of 1 mL min-1 was provided using an EluGen

140

EGC-KOH II cartridge. The injection volume was 250 μL.

141

Data simulation. Two kinetic models were conducted to simulate the sequential conversion

142

rates of Cl‒ to HOCl/OCl‒ and then to chlorate by different reactive species in UV/PDS (eqs. 13-18)

143

and Co/PMS (eqs. 19-20) systems, respectively, by using Kintecus v5.50.34 The pseudo-first-order

144

rate constants (k′) were determined with the experimental results by the data fitting function of

145

Kintecus v5.50.

146 147

RESULTS AND DISCUSSION

148

Chlorate formation in the co-presence of chloride and SO4•‒. Figure 1a shows the formation

149

of chlorate from the oxidation of Cl‒ in Co/PMS and UV/PDS systems. The yield of chlorate

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150

increased with increasing reaction time in both systems. While at different Cl‒ dosages, the chlorate

151

yield after 30 min, in the Co/PMS system, increased from 5.66 μM to 7.63 μM with increasing Cl‒

152

concentrations from 20 μM to 50 μM at first, and then decreased to 0.559 μM at a Cl‒ concentration

153

of 1 mM. However, in the UV/PDS system, no chlorate was detected when the Cl‒ concentration was

154

lower than 100 μM, while the formation increased from 0.11 μM to 3.27 μM with increasing Cl‒

155

concentration from 100 μM to 1 mM. The conversion rate of Cl‒ to chlorate decreased from 28.3% to

156

0.056% with increasing the Cl‒ concentration from 20 μM to 1 mM in the Co/PMS system, while in

157

the UV/PDS system, it increased from 0.11% to 0.33% with increasing the Cl‒ concentration from

158

100 μM to 1 mM (Figure S1).

159

HOCl/OCl‒ was also generated in the co-presence of Cl‒ and SO4•‒ (Figure 1b). In the Co/PMS

160

system, the yield of HOCl/OCl‒ at 30 min increased from 15.1 μM to 217.5 μM with increasing the

161

Cl‒ concentration from 20 μM to 1 mM. In the UV/PDS system, the yield of HOCl/OCl‒ at 30 min

162

was less than 3.5 μM at a Cl‒ concentration of 1 mM, and was undetectable at 100 μM. The

163

concentration of HOCl/OCl‒ increased with increasing reaction time in both systems when the Cl‒

164

concentration was higher than 50 μM. Moreover, the HOCl/OCl‒ concentration increased firstly and

165

then decreased with increasing reaction time when the Cl‒ concentration was lower than 50 μM in

166

the Co/PMS system (shown in the inset box in Figure 1b), demonstrating that HOCl/OCl‒ might be

167

the intermediate in the conversion of Cl‒ to chlorate.

168

Mechanisms of the chlorate formation in UV/PDS and Co/PMS systems. Figure 2 shows the

169

evolution of Cl‒, HOCl/OCl‒, chlorate and total chlorine (TCl, the total molar masses chlorine in Cl‒,

170

HOCl/OCl‒ and chlorate) during the oxidation of Cl‒ in Co/PMS and UV/PDS systems. In both

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171

systems, the concentrations of Cl‒ decreased, whereas those of HOCl/OCl‒ and chlorate increased

172

with increasing reaction time. TCl remained constant throughout the reaction, demonstrating that

173

HOCl/OCl‒ and chlorate were the major products in both systems. Along with the discussion above

174

(Figure 1), HOCl/OCl‒ can be confirmed to be the key intermediate during the conversion of Cl‒ to

175

chlorate. Therefore, Cl‒ can be converted to HOCl/OCl‒ firstly and finally transformed to chlorate in

176

UV/PDS and Co/PMS systems. Note that the contribution of HO• to the formation of chlorate in this

177

study was insignificant (Text S2).

178

Chlorate formation mechanism in the UV/PDS system. To clarify the roles of SO4•− and UV

179

photolysis on the formation of chlorate in UV/PDS, some additional tests were conducted (Figure

180

S2). In the PDS/Cl‒ system, no HOCl/OCl‒ was formed, demonstrating that Cl‒ could not be oxidized

181

by PDS. The addition of methanol, as a radical scavenger, significantly inhibited the formation of

182

HOCl/OCl‒ in the UV/PDS/Cl‒ system, indicating that SO4•‒ was essential in the conversion of Cl‒ to

183

HOCl/OCl‒. To determine the role of the reactive species attributable to the conversion of

184

HOCl/OCl‒ to chlorate, HOCl was used as the reactant instead of Cl‒ (Figure S2b). The formation of

185

chlorate in the UV/PDS/HOCl system was significantly inhibited by the addition of methanol,

186

indicating that SO4•‒ primarily contributed to the conversion of HOCl/OCl‒ to chlorate. Nevertheless,

187

small amounts of chlorate were still formed in the UV/PDS/HOCl system with the presence of

188

methanol through the UV photolysis of HOCl/OCl‒ (Figure S2b), demonstrating that UV photolysis

189

of HOCl/OCl‒ played a minor role on the chlorate formation. The photolysis of HOCl/OCl‒ formed

190

Cl‒ as the major product and chlorate as the minor product (Figure S3).

191

Chlorate formation mechanism in the Co/PMS system. To ascertain the roles of reactive

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192

species such as SO4•− and Co(III) on the formation of chlorate in Co/PMS, additional tests were

193

conducted (Figure 3).

194

(1) To ascertain whether Co(III) can oxidize Cl‒ to HOCl/OCl‒ in the Co/PMS system, FB was

195

used to scavenge SO4•‒ (kFB, SO4•‒ = 9.8 × 108 M-1 s-1) but it was less reactive with RHS such as Cl•

196

(kFB, Cl• = 8.0 × 105 M-1 s-1) and HOCl/OCl‒.35, 36 In the Co/PMS/Cl‒/FB system, the formation of

197

HOCl/OCl‒ increased in the first 10 min and then reached a plateau, which was equal to that in the

198

Co/PMS/Cl‒ system in the first 5 min and higher than that thereafter (Figure 3a). The similar

199

HOCl/OCl‒ formation, with or without the presence of FB in the first 5 min indicated that Co(III)

200

played a significant role in the oxidation of Cl‒ to HOCl/OCl‒. The plateau of the HOCl/OCl‒

201

concentration after 10 min demonstrated that SO4•‒ was essential for the oxidation of HOCl/OCl‒ to

202

chlorate in the Co/PMS system.

203

(2) To further ascertain whether Cl• is an important intermediate during the oxidation of Cl‒ to

204

HOCl/OCl‒ by Co(III), methanol was used to scavenge both SO4•‒ and Cl• in the Co/PMS system,

205

with rate constants of 3.2 × 106 M-1 s-1 and 5.7 × 109 M-1 s-1, respectively.36, 37 0.2% methanol (49.4

206

mM) could scavenge both SO4•‒ and Cl• formed in the system. The formation of HOCl/OCl‒ was

207

significantly inhibited in the presence of 0.2% methanol, which was similar to that formed in

208

PMS/Cl‒ (Figure 3a), indicating that Cl• was a key intermediate during the oxidation of Cl‒ to

209

HOCl/OCl‒ by Co(III).

210

(3) In addition, a small amount of HOCl/OCl‒ (1.86 μM) was detected in the PMS/Cl‒ system

211

(Figure 3a), indicating that PMS could directly react with Cl‒ to generate HOCl (eq 7), which was

212

consistent with the literature.38

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213

Cl‒ + HSO5‒ → SO42‒ + HOCl

214

(4) To determine the role of the reactive species attributable to the conversion of HOCl/OCl‒ to

215

chlorate, HOCl was also used as the reactant instead of Cl‒ (Figure 3b). Chlorate formation was

216

much higher from the oxidation of HOCl than that from Cl−. The formation significantly decreased

217

to only 0.38 μM in the presence of 0.2% methanol in the Co2+/PMS/HOCl system. Note that no

218

chlorate was detected in Co2+/HOCl and PMS/HOCl systems, indicating that neither Co(II) nor PMS

219

could oxidize HOCl/OCl‒ to chlorate. This result indicates that SO4•‒ played a primary role in the

220

oxidation of HOCl/OCl‒ to chlorate, and Co(III) produced from the oxidation of Co(II) had an

221

insignificant contribution to the formation of chlorate.

(7)

222

Proposed pathways of chlorate formation in SO4•‒-based AOPs. Based on the above results

223

and relevant literatures,17, 20, 21, 38, 39 the pathways of chlorate formation in SO4•‒-based AOPs are

224

proposed in Scheme 1. The pathways include the formation of HOCl/OCl‒ as an intermediate and

225

chlorate as the final product. For the first step, SO4•‒, Co(III) and PMS contribute to the formation of

226

HOCl/OCl‒ in different ways. SO4•‒ reacts with Cl‒ to form Cl•, which is subsequently converted to

227

HOCl/OCl‒ through eqs. 8-12.

228

Cl‒ + SO4•‒ → SO42‒ + Cl•

229

Cl• + Cl‒ ↔ Cl2•‒

230

Cl2•‒ + Cl2•‒ → Cl2 + 2Cl‒

231

Cl2•‒ + Cl• → Cl2 + Cl‒

232

Cl2 + H2O → H+ + HOCl + Cl‒

233

k = 3.2 (±0.2) × 108 M-1 s-1

(8)

k+ = 6.5 × 109 M-1 s-1, k- = 1.1 × 105 s-1

(9)

k = 9 (±1) × 106 M-1 s-1

(10)

k = 2.1 (±0.05) × 109 M-1 s-1

(11) (12)

Cl• is also produced by the oxidation of Co(III), and then transformed to HOCl/OCl‒. Meanwhile, a

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234

small amount of Cl‒ is oxidized to HOCl/OCl‒ by PMS. For the second step, the HOCl/OCl‒

235

produced above is further oxidized by SO4•‒ to generate chlorate as the final product. In addition, the

236

conversion of HOCl/OCl‒ to chlorate by Co(III) is insignificant. UV irradiation plays a minor role in

237

the conversion of HOCl/OCl‒ to chlorate. SO4•‒ pathways outweigh the other pathways among all the

238

steps of chlorate formation in the UV/PDS system. Furthermore, Co(III) plays a more important role

239

in the oxidation from Cl‒ to HOCl/OCl‒, but SO4•‒ is essential for the oxidation from HOCl/OCl‒ to

240

chlorate in the Co/PMS system.

241 242

Scheme 1. Proposed pathways of chlorate formation from the oxidation of Cl‒ in SO4•‒-based AOPs.

243

Conversion Rate Estimation by SO4•‒, UV and Co(III). According to Scheme 1, a model

244

consisting of 6 reactions (eqs 13-18) was constructed by assuming that the two steps conversion of

245

Cl‒ to chlorate (eqs 13-14) by SO4•‒ could be described by pseudo-first-order kinetics in the UV/PDS

246

system.

247 248 249 250 251

SO•− 4

Cl‒ �⎯� HOCl/OCl‒

k1 = ?

(13)

SO•− 4

k2 = ?

(14)

ℎ𝜈𝜈

k3 = 5.52 × 10-6 s-1

(15)

ℎ𝜈𝜈

k4 = 4.76 × 10-4 s-1

(16)

k5 = negligible

(17)

HOCl/OCl‒ �⎯� ClO3‒

HOCl/OCl‒ �� ClO3‒ HOCl/OCl‒ �� Cl‒ ℎ𝜈𝜈

ClO3‒ �� Cl‒

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252

ℎ𝜈𝜈

ClO3‒ �� HOCl/OCl‒

k6 = negligible

Page 14 of 33

(18)

253

Where k1 and k2 were fitted with the experimental data in Figure 2a, and k3-k4 were obtained from

254

the results of UV photolysis of HOCl/OCl‒ (Figure S3), with the help of Kintecus v5.50.34 No Cl‒

255

and HOCl/OCl‒ were formed from the UV photolysis of ClO3‒, indicating that k5 and k6 were

256

negligible in the UV/PDS system. As shown in Figure S3, the UV photolysis of HOCl/OCl‒ followed

257

the first-order kinetics, and the rate constants for the formation of ClO3‒ (k3) and Cl‒ (k4) were 5.52 ×

258

10-6 s-1 and 4.76 × 10-4 s-1, respectively (Table S1). The value of k1 and k2 were simulated to be 3.32

259

× 10-6 s-1 and 2.43 × 10-3 s-1, respectively (Table S1). The simulated results were shown in Figure 2a,

260

which were in great agreement with the experimental results, demonstrating that the model and

261

assumptions were reasonable. According to the calculated results, k2 was three orders of magnitude

262

larger than k3, which further proved that the chlorate formation from HOCl/OCl‒ in the UV/PDS

263

system was primarily driven by SO4•‒ and the UV photolysis of HOCl/OCl‒ played a minor role.

264 265 266

As for the Co/PMS system, the stepwise formation of chlorate could be described by eqs 19-20. Co/PMS

Cl‒ �⎯⎯⎯⎯� HOCl/OCl‒ Co/PMS

HOCl/OCl‒ �⎯⎯⎯⎯� ClO3‒

k7 = ?

(19)

k8 = ?

(20)

267

Where k7 and k8 were the pseudo-first-order rate constants of the conversion of Cl‒ to HOCl/OCl‒

268

and the oxidation of HOCl/OCl‒ to chlorate, respectively, in the Co/PMS system. k7 and k8 were

269

simulated to be 9.23 × 10-3 s-1 and 2.70 × 10-4 s-1, respectively (Table S1), by using the experimental

270

data in Figure 1 and the kinetic model.

271

In the UV/PDS system, the SO4•‒- driven rate constant of Cl‒ (1 mM) to HOCl/OCl‒ (3.32 × 10-6

272

s-1) was much lower than that of HOCl/OCl‒ to chlorate (2.43 × 10-3 s-1). Meanwhile, the UV

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Page 15 of 33


273

photolysis of HOCl/OCl‒ also contributed to the depletion of HOCl/OCl‒ (4.81 × 10-4 s-1) to form

274

Cl‒ as a major product (4.76 × 10-4 s-1) and chlorate as a minor product (5.52 × 10-6 s-1). Thus, the

275

conversion of chloride to HOCl/OCl‒ was a rate limiting step for chlorate formation in UV/PDS,

276

whose kinetics increased with increasing Cl‒ concentration, resulting in the increase of chlorate

277

formation with increasing Cl‒ concentration (Figure 1a). However, in the Co/PMS system, the rate

278

constant of Cl‒ (20 μM) to HOCl/OCl‒ (9.23 × 10-3 s-1) was higher than that of HOCl/OCl‒ to

279

chlorate (2.70 × 10-4 s-1), which led to the significant accumulation of HOCl/OCl‒. Cl‒ has double

280

effects on the formation of chlorate in the Co/PMS system. On one hand, the formation of

281

HOCl/OCl‒ increased with increasing Cl‒ concentration (Figure 1b), which provided more precursors

282

for chlorate formation. On the other hand, the higher concentration of Cl‒ and HOCl/OCl‒ consumed

283

more SO4•‒, resulting in the decrease of the steady-state concentration of SO4•‒ with increasing Cl‒

284

concentration. Thus, the chlorate formation presented a first increase and then decrease trend with

285

increasing Cl‒ concentration in the Co/PMS system (Figure 1a).

286

Effect of bromide on the chlorate formation in the co-presence of chloride, bromide and

287

SO4•‒. In real water, Br‒ and Cl‒ co-exist, and the concentration of the former is much lower than the

288

latter. Figure 4 shows the effect of Br‒ on the chlorate formation in Co/PMS and UV/PDS systems.

289

In the presence of 20 μM Br‒, the formation of chlorate decreased by 45.2% in 30 min in the

290

Co/PMS system at a Cl‒ concentration of 20 μM. Meanwhile, the chlorate formation from 1 mM Cl‒

291

decreased by 100% with the co-presence of 20 μM Br‒ in the UV/PDS system. These results

292

demonstrate that Br‒ significantly inhibits the chlorate formation in both systems, and the inhibition

293

is much higher on the UV/PDS system compared to the Co/PMS system. Br‒ can be oxidized by

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Page 16 of 33

294

SO4•‒ and Co(III) to form HOBr/OBr‒ and then to form bromate by SO4•‒.20-22 Thus, Br‒ competes

295

with Cl‒ for SO4•‒ or Co(III), resulting in a decrease in chlorate formation, and leads to the formation

296

of bromate in both systems.20, 21 However, the presence of 1 mM Cl‒ did not affect the bromate

297

formation in the UV/PDS system, but the bromate formation decreased by 30.1% in the presence of

298

20 μM Cl‒ in the Co/PMS system after 30 min reaction (Figure S4).

299

In the UV/PDS system, the HOX/OX‒ formation in the co-presence of Cl‒, Br‒ and SO4•‒ are

300

shown in Figure 5a. HOCl/OCl‒ was not detectable in the first 7 min, while its concentration

301

increased with increasing time after 7 min. The concentration of HOBr/OBr‒ firstly increased and

302

then decreased with increasing reaction time. The concentrations of HOCl/OCl‒ and HOBr/OBr‒

303

were 0.1 μM and 5.9 μM, respectively, at 10 min, and the former was much lower than the latter

304

throughout 30 min. As discussed above, the rate constant of Cl‒ to HOCl/OCl‒ was three orders of

305

magnitude lower than that of HOCl/OCl‒ to chlorate in UV/PDS, demonstrating that the limiting step

306

of chlorate formation in UV/PDS was the formation of HOCl/OCl‒. The inhibited formation of

307

HOCl/OCl‒ resulted in the undetectable formation of chlorate in the co-presence of Br‒ and Cl‒ in the

308

UV/PDS system.

309

The inhibition of HOCl/OCl‒ formation and preferential formation of HOBr/OBr‒ (Figure 5a) in

310

the presence of Br‒ is likely attributable to the following aspects. (1) The conversion rate of Br‒ to

311

HOBr/OBr‒ by SO4•‒ is higher than that of Cl‒ to HOCl/OCl‒. The second-order rate constant of

312

SO4•‒ with Br‒ to form Br• (3.5 × 109 M-1 s-1) is around 12 times higher than that with Cl‒ to form

313

Cl• (3.0 × 108 M-1 s-1).3 Furthermore, the transformation rate of Br‒ to HOBr/OBr‒ by SO4•‒ was

314

calculated to be 378 times higher than that of Cl‒ to HOCl/OCl‒, at the same steady-state

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Page 17 of 33


315

concentration of SO4•‒, by using the kinetic modeling developed by Yang et al. (2014)16 (Details see

316

Text S3 and Figure S5). (2) The reaction rate of Br‒ with Cl• is two orders of magnitude higher than

317

that of Cl‒ with Br• (eqs. 21-22).26 (3) The oxidation of Br‒ by HOCl/OCl‒ with the rate constant of

318

(1.55 - 6.84) ×103 M-1 s-1.27

319

Cl• + Br‒ ↔ BrCl•‒

k+ = 1.2 × 1010 M-1 s-1, k- = 1.9 × 103 s-1

(21)

320

Br• + Cl‒ ↔ BrCl•‒

k+ = 1.0 × 108 M-1 s-1, k- = 6.1 × 104 s-1

(22)

321

As for the Co/PMS system, HOBr/OBr‒ was the primary HOX/OX‒ in the first 1 min and

322

subsequently decreased, accompanied by an increase of HOCl/OCl‒ in the co-presence of Cl‒ and Br‒

323

(Figure 5b). About 96.7% of Br‒ was converted to HOBr/OBr‒ in the first 1 min, followed with a

324

gradual decay of HOBr/OBr‒. The yield of HOBr/OBr‒ in the co-presence of Cl‒ and Br‒ was higher

325

than that of Br‒ alone throughout the reaction period. Meanwhile, the HOCl/OCl‒ formation was

326

retarded compared to that of Cl‒ alone in the Co/PMS system, but they became equal after 7 min. By

327

comparing the cases of Br‒ alone and Cl‒ alone, the accumulation rate of HOCl/OCl‒ was lower than

328

that of HOBr/OBr‒, as was the decay rate after reaching their peak concentrations (Figure 5b).

329

Nevertheless, the conversion of Cl‒ to HOCl/OCl‒ was retarded but still significant in the presence of

330

Br‒, resulting in the formation of chlorate in Co/PMS (Figure 4).

331

The results in Co/PMS are likely to be attributable to the following aspects: (1) The rate

332

constants of the oxidation of Cl‒ to HOCl/OCl‒ (k7 = 9.23 × 10-3 s-1) and the oxidation of HOCl/OCl‒

333

to chlorate (k8 = 2.70 × 10-4 s-1) was lower compared to that of Br‒ to HOBr/OBr‒ (k = 1.48 × 10-2 s-1)

334

and HOBr/OBr‒ to bromate (k = 1.38 × 10-3 s-1) in the Co/PMS system.21 Nevertheless, the

335

conversion of Cl‒ to HOCl/OCl‒ was still significant by Co(III) in the presence of Br‒. (2) Br‒ can

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Page 18 of 33

336

fast react with HOCl/OCl‒ to form HOBr/OBr‒,27 resulting in the increase of the HOBr/OBr‒ and no

337

HOCl/OCl‒ in the first 1 min. As the reaction proceeded and Br‒ was consumed, HOCl/OCl‒ was

338

accumulated gradually after 1 min.

339

Effect of NOM on the chlorate formation in the co-presence of chloride and SO4•‒. Figure

340

6a shows the effect of NOM on the formation of chlorate in the Co/PMS system. The chlorate

341

formation from 100 μM Cl‒ in 30 min decreased by 83.7%, 95.6% and 99.5% at NOM

342

concentrations of 1 mg L-1, 2 mg L-1 and 5 mg L-1, respectively, in the Co/PMS system. Meanwhile,

343

the chlorate formation was totally inhibited in the presence of NOM in the UV/PDS system. The

344

inhibitory effect of NOM on the chlorate formation is due to: (1) the consumption of SO4•‒ by NOM

345

with a rate constant of 2.0 × 103 L mg-1 s-1;25 (2) the consumption of reactive chlorine species (RCS)

346

such as Cl•, ClO• and HOCl/OCl‒ by NOM with the rate constants of 1.3 × 104 L mg-1 s-1, 4.5 × 104

347

L mg-1 s-1 and 0.7-5 M-1 s-1, respectively.19, 26, 28 The consumption of SO4•‒ and RCS by NOM also

348

resulted in the decreased formation of HOCl/OCl‒. As shown in Figure 6b, the formation of

349

HOCl/OCl‒ from 100 μM Cl‒ in 30 min decreased from 95.8 μM to 30.5 μM with increasing NOM

350

concentration from 0 to 5 mg L-1 in the Co/PMS system. In the UV/PDS system, no HOCl/OCl‒ was

351

detected in the co-presence of 1 mM Cl‒ and 1 mg L-1 NOM.

352

Figure S6 shows the formation of chlorate and HOX/OX‒ in the coexistence of Br‒, NOM and

353

Cl‒ in UV/PDS and Co/PMS systems. In the UV/PDS system, the formation of HOX/OX‒ and

354

chlorate was completely inhibited in the presence of 2 mg L-1 NOM and 20 μM Br‒, similar like that

355

in the presence of Br‒ (Figure 4). In the Co/PMS system, the formation of HOX/OX‒ was inhibited

356

by 50%, with the addition of 2 mg L-1 NOM, while that of chlorate was inhibited by 93.2%,

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Page 19 of 33


357

compared to the Co/PMS/Cl‒/Br‒ system. This was due to the further consumption of SO4•‒ and RCS

358

by NOM.

359

Engineering Implications. Cl‒ is inevitable in water environment at moderate concentrations

360

of several mg L-1, which concentrations were reported to be 4-11.8 mg L-1 in surface water and 20-30

361

mg L-1 in groundwater.40 The interaction of Cl‒ with SO4•‒ and the formation of chlorate should be

362

taken into consideration during the application of SO4•‒-based AOPs in water treatment.

363

The mechanisms of chlorate formation were different in UV/PDS and Co/PMS AOPs. The

364

former was primarily attributable to SO4•‒ while the latter also involved the contribution of Co(III).

365

The conversion of Cl‒ to HOCl/OCl‒ was the rate limiting step for the chlorate formation in UV/PDS,

366

but that was several orders of magnitude faster in Co/PMS responsible for Co(III). Thus, HOCl/OCl‒

367

was significantly accumulated in Co/PMS, but not in UV/PDS. The accumulation of HOCl/OCl‒

368

consumed SO4•‒ much faster than Cl‒. The above mechanisms resulted in the lower formation rate of

369

chlorate in UV/PDS compared to Co/PMS system at lower Cl‒ concentration (< 0.1 mM), while in

370

opposite at higher Cl‒ concentration (i.e., 1 mM). Meanwhile, the significant accumulation

371

HOCl/OCl‒ in Co/PMS could induce the formation of chlorinated disinfection byproducts (DBPs),

372

particularly at higher Cl‒ concentrations.

373

Water matrices such as NOM, bromide, nitrate and pH affect the formation of chlorate in SO4•‒

374

-based AOPs. Both bromide and NOM reduced the formation of chlorate, but induced the formation

375

of bromate and DBPs, respectively. Nitrate slightly affected the chlorate formation in Co/PMS, while

376

decreased the formation by 35.6% in UV/PDS under the experimental condition (Text S4 and Figure

377

S7). pH affects the chemistry of radicals in SO4•‒-based AOPs and the dissociation of HOCl/OCl‒,

-19-



Page 20 of 33

378

which may result in the difference of chlorate formation. The formation of chlorate and HOCl/OCl‒

379

at pH 7 were lower than that at pH 4 in both systems (details see Text S5 and Figure S8). Thus, the

380

formation of chlorate in SO4•‒-based AOPs under acidic condition needs more attention.

381 382

ASSOCIATED CONTENT

383

Supporting Information.

384

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

385

Determination of HOBr/OBr‒ concentrations in the co-presence of HOBr/OBr‒ and HOCl/OCl‒,

386

calculation of HOBr/OBr‒ and HOCl/OCl‒ formation rate by Kintecus at the same steady-state

387

concentration of SO4•‒, the calculated pseudo-first-order rate constants using kinetic models, the

388

contribution of HO• to the formation of chlorate, the conversion rate of Cl‒ to chlorate in Co/PMS

389

and UV/PDS systems, time-dependent HOCl/OCl‒ formation with the presence of methanol and the

390

chlorate formation during the oxidation of HOCl/OCl‒ in the UV/PDS system, formation of Cl‒ and

391

chlorate by direct UV photolysis of HOCl/OCl‒, the formation of bromate in the co-presence of Cl‒

392

and Br‒ in Co/PMS and UV/PDS systems, the formation of chlorate and HOX/OX‒ in the coexist of

393

Br‒, NOM and Cl‒ in Co/PMS and UV/PDS systems, the effect of nitrate and pH on the formation of

394

chlorate and HOCl/OCl‒ in Co/PMS and UV/PDS systems (PDF).

395 396

AUTHOR INFORMATION

397

Corresponding Author.

398

*Phone: + 86-18680581522. E-mail: [email protected].

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Page 21 of 33


399 400

ACKNOWLEDGMENTS

401

This work was financially supported by Natural Science Foundation of China (21677181,

402

51378515), the National Key Research Development Program of China (2016YFC0502803), the

403

Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program

404

(2015TQ01Z552), Guangzhou Science Technology and Innovation Commission (201707010249),

405

the Science and Technology Project of Zhejiang Province (2017C33036), and the Fundamental

406

Research Funds for the Central Universities (17lgzd21). D. D. Dionysiou also acknowledges support

407

from the University of Cincinnati through a UNESCO co-Chair Professor position on “Water Access

408

and Sustainability” and the Herman Schneider Professorship in the College of Engineering and

409

Applied Sciences.

410 411

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-26-


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Lists of Captions Figure 1. The formation of chlorate (a) and HOCl/OCl‒ (b) in Co/PMS (closed symbols) and UV/PDS (open symbols) systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [PDS]0 = 200 μM, I0/V = 1.45 μEinstein L-1 s-1. ................................................................................ 2 Figure 2. Evolution of chlorine species during the oxidation of Cl‒ in UV/PDS (a) and Co/PMS (b) systems. Solid lines show simulated results. Conditions: (a) [PDS]0 = 200 μM, [Cl‒]0 = 1 mM, I0/V = 1.45 μEinstein L-1 s-1; (b) [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 0.1 mM. ........................................................................................................................................ 3 Figure 3. (a) Time-dependent HOCl/OCl‒ formation in the Co/PMS system with the presence of FB or MeOH. (b) Comparison of chlorate formation during the oxidation of HOCl/OCl‒ in different systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 20 μM (a) and 100 μM (b), [FB]0 = 1 mM, MeOH = 0.2%, [EDTA]0 = 20 μM, [HOCl/OCl‒]0 = 100 μM. 4 Figure 4. Effect of Br‒ on the chlorate formation in the Co/PMS (closed symbols) and UV/PDS (open symbols) systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM; [PDS]0 = 200 μM, I0/V = 1.45 μEinstein L-1 s-1. .......................................................................................... 5 Figure 5. Effect of Br‒ on the HOX/OX‒ formation in the UV/PDS (a) and Co/PMS (b) systems. Conditions: (a) [PDS]0 = 200 μM, [Cl‒]0 = 1 mM, [Br‒]0 = 20 μM, I0/V = 1.45 μEinstein L-1 s-1; (b) [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = [Br‒]0 = 20 μM. ............................ 6 Figure 6. The effect of NOM on the formation of chlorate (a) and HOCl/OCl‒ (b) in the Co/PMS system. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 100 μM. ......... 7

1



Chlorate concentration (μM)

10

(a)

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[Cl-]0 = 20 μM, Co/PMS [Cl-]0 = 50 μM, Co/PMS

8

[Cl-]0 = 100 μM, Co/PMS [Cl-]0 = 1 mM, Co/PMS [Cl-]0 = 100 μM, UV/PDS

6

[Cl-]0 = 1 mM, UV/PDS 4

2

0 0

10

20

30

[Cl-]0 = 20 μM, Co/PMS -

[Cl ]0 = 50 μM, Co/PMS

400

[Cl-]0 = 100 μM, Co/PMS [Cl-]0 = 1 mM, Co/PMS 300

[Cl-]0 = 100 μM, UV/PDS [Cl-]0 = 1 mM, UV/PDS

40


30


20

-

HOCl/OCl- concentration (μM)

(b)

HOCl/OCl concentration (μM)

Time (min)

[Cl ]0 = 100 μM, UV/PDS

10 0 0

10

20

30

Time (min)

200

100

0 0

10

20

30

Time (min)

Figure 1. The formation of chlorate (a) and HOCl/OCl‒ (b) in Co/PMS (closed symbols) and UV/PDS (open symbols) systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [PDS]0 = 200 μM, I0/V = 1.45 μEinstein L-1 s-1.

2



5

Chloride concentration (μM)

Chlorate Free chlorine Chloride TCl

(a)

1400

Chlorate HOCl/OClChloride TCl

1200

4

1000 3 800 600

2

400 1 200 0

0 0

20

5

10

15

HOCl/OCl- and chlorate concentration (μM)

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20

Time (min) 120

(b)

80

Chlorate HOCl/OClChloride TCl

60

40

Chloride concentration (μM)

Concentration (μM)

100

1200 1000 800 600 400

20

200

0

0 0

(b)

1400

5

10

15

20

0

Time (min)

Figure 2. Evolution of chlorine species during the oxidation of Cl‒ in UV/PDS (a) and Co/PMS (b) systems. Solid lines show simulated results. Conditions: (a) [PDS]0 = 200 μM, [Cl‒]0 = 1 mM, I0/V = 1.45 μEinstein L-1 s-1; (b) [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 0.1 mM.

3


5


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(a) HOCl/OCl- concentration (μM)

20

15

Co2+/PMS/ClPMS/ClCo2+/PMS/Cl-/FB Co2+/PMS/Cl-/MeOH

10

5

0 0

5

10

15

20

25

30

35

25

30

35

Time (min)


14

Co2+/PMS/ClCo2+/PMS/HOCl PMS/HOCl Co2+/HOCl Co2+/PMS/HOCl/MeOH

(b)

12 10 8 6 4 2 0 0

5

10

15

20

Time (min)

Figure 3. (a) Time-dependent HOCl/OCl‒ formation in the Co/PMS system with the presence of FB or MeOH. (b) Comparison of chlorate formation during the oxidation of HOCl/OCl‒ in different systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 20 μM (a) and 100 μM (b), [FB]0 = 1 mM, MeOH = 0.2%, [EDTA]0 = 20 μM, [HOCl/OCl‒]0 = 100 μM.

4


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Co/PMS, Cl- = 20 μM; Br- = 0 Co/PMS, Cl- = 20 μM; Br- = 20 μM UV/PDS, Cl- = 1 mM; Br- = 0 UV/PDS, Cl- = 1 mM; Br- = 20 μM


6

4

2

0 0

5

10

15

20

25

30

35

Time(min)

Figure 4. Effect of Br‒ on the chlorate formation in the Co/PMS (closed symbols) and UV/PDS (open symbols) systems. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM; [PDS]0 = 200 μM, I0/V = 1.45 μEinstein L-1 s-1.

5



HOX/OX- concentration (μM)

12

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(a)

Br- & Cl- - HOBr Br- & Cl- - HOCl Br- alone - HOBr Cl- alone - HOCl

10

8

6

4

2

0 0

5

10

15

20

25

30

35

Time (min)

HOX/OX- concentration (μM)

25

(b)

Br- & Cl- - HOBr Br- & Cl- - HOCl Br- alone - HOBr Cl- alone - HOCl

20

15

10

5

0 0

5

10

15

20

25

30

35

Time (min)

Figure 5. Effect of Br‒ on the HOX/OX‒ formation in the UV/PDS (a) and Co/PMS (b) systems. Conditions: (a) [PDS]0 = 200 μM, [Cl‒]0 = 1 mM, [Br‒]0 = 20 μM, I0/V = 1.45 μEinstein L-1 s-1; (b) [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = [Br‒]0 = 20 μM.

6


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8

(a)

NOM = 0 NOM = 1 mg L-1 NOM = 2 mg L-1 NOM = 5 mg L-1

6

4

2

0 0

5

10

15

20

25

30

35

25

30

35

Time (min)

HOCl/OCl- concentration (μM)

120

(b)

NOM = 0 NOM = 1 mg L-1 NOM = 2 mg L-1 NOM = 5 mg L-1

100

80

60

40

20

0 0

5

10

15

20

Time (min)

Figure 6. The effect of NOM on the formation of chlorate (a) and HOCl/OCl‒ (b) in the Co/PMS system. Conditions: [Co2+]0 = 20 μM, [PMS]0 = 200 μM, [Cl‒]0 = 100 μM.

7


Chlorate Formation Mechanism in the Presence of Sulfate Radical

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