Transformation among Aromatic Iodinated Disinfection Byproducts in

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

Transformation among Aromatic Iodinated Disinfection Byproducts in the Presence of Monochloramine: From Monoiodophenol to Triiodophenol and Diiodonitrophenol Tingting Gong, Yuxian Tao, Xiangru Zhang, Shaoyang Hu, Jinbao Yin, Qiming Xian, Jian Ma, and Bin Xu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03323 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13

Environmental Science & Technology

Transformation among Aromatic Iodinated Disinfection Byproducts in the Presence of Monochloramine: From Monoiodophenol to Triiodophenol and Diiodonitrophenol Tingting Gong,† Yuxian Tao,† Xiangru Zhang,‡ Shaoyang Hu,† Jinbao Yin,† Qiming Xian,*, † Jian Ma,† and Bin Xu§ †

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China ‡ Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Hong Kong SAR, China § State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Abstract: Aromatic iodinated disinfection byproducts (DBPs) are a newly identified category of highly toxic DBPs. Among the identified aromatic iodinated DBPs, 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol have shown relatively widespread occurrence and high toxicity. In this study, we found that 4-iodophenol underwent transformation to form 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the presence of monochloramine. The transformation pathways were investigated, the decomposition kinetics of 4-iodophenol and the formation of 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol were studied, the factors affecting the transformation were examined, the toxicity change during the transformation was evaluated, and the occurrence of the proposed transformation pathways during chloramination of source water was verified. The results revealed that 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, which could account for 71.0% of iodine in the transformed 4-iodophenol, were important iodinated transformation products of 4-iodophenol in the presence of monochloramine. The transformation pathways of 4-iodophenol in the presence of monochloramine were proposed and verified. The decomposition of 4-iodophenol in the presence of monochloramine followed a pseudo-second-order decay. Various factors including monochloramine dose, pH, temperature, nitrite concentration, and free chlorine contact time (before chloramination) affected the transformation. The cytotoxicity of the chloraminated 4-iodophenol samples increased continuously with contact time. The proposed transformation pathways also occurred during chloramination of source water.

1

ACS Paragon Plus Environment


Page 2 of 33

34

INTRODUCTION

35

Iodide is widely present in both source water and wastewater effluents. It has been reported that

36

the typical iodide concentrations in surface and ground water are below 10 µg/L.1–3 Specifically,

37

for some source water with salt water intrusion, the iodide concentrations may become much

38

higher.4 In source waters of 23 cities in the United States and Canada, the iodide concentrations

39

were in the range of 0.4–104.2 µg/L.4 Urban wastewater effluents typically consist of freshwater

40

with relatively low levels of iodide, but can be salinized by the use of seawater for toilet flushing,

41

which introduces high levels of inorganic ions, including iodide.5 The iodide concentrations in

42

saline wastewater effluents were determined to be 5.0–26.4 µg/L.5 During drinking water or

43

wastewater disinfection, iodide may be oxidized to hypoiodous acid, which may further react

44

with natural or effluent organic matter to form iodinated disinfection byproducts (DBPs).

45

Iodinated DBPs have been reported to be significantly more toxic than their brominated and

46

chlorinated analogues4,6–8 and thus have been drawing increasing concern.

47

Gas chromatography-electron capture detection and gas chromatography-mass spectrometry

48

have previously been used to detect and identify iodinated DBPs in drinking water.4,9–13 However,

49

because these techniques are not amenable to polar iodinated DBPs, only a few iodinated DBPs

50

have been detected and identified, mainly including iodinated trihalomethanes (THMs),

51

iodinated aliphatic acids, and iodinated haloacetamides.4,9,10,14‒17 More recently, a novel

52

precursor ion scan (PIS) method has been developed to enable the rapid selective detection of

53

polar iodinated DBPs using an electrospray ionization-triple quadrupole mass spectrometer

54

(ESI-tqMS).18 The principle of the PIS method is illustrated in the supporting information (SI).

55

Furthermore, by coupling it with ultra performance liquid chromatography (UPLC) for

56

preseparation, the ESI-tqMS can be used to identify unknown polar iodinated DBPs. By using 2


Page 3 of 33


57

this method, several groups of aromatic iodinated DBPs have been newly identified in

58

disinfected drinking water as well as chlorinated saline wastewater effluents, mainly including

59

iodinated phenols, iodinated nitrophenols, iodinated hydroxybenzoic acids, iodinated

60

hydroxybenzaldehydes,

61

benzoquinones.7,18–22 Specifically, Pan et al.19 reported that 2,4,6-triiodophenol (up to 33 ng/L)

62

and 2,6-diiodo-4-nitrophenol (up to 30 ng/L) were widely present in drinking water samples.

63

Yang and Zhang7 also reported the presence of 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol

64

in chlorinated saline wastewater effluents. Moreover, toxicological studies have demonstrated

65

that these aromatic iodinated DBPs showed relatively high developmental toxicity and algal

66

growth inhibition among all DBP categories.7,8 Especially, Yang and Zhang7 reported that

67

2,6-diiodo-4-nitrophenol and 2,4,6-triiodophenol showed the highest developmental toxicity

68

among all tested DBPs except for 2,5-dibromohydroquinone, while Liu and Zhang8 reported that

69

2,4,6-triiodophenol showed the highest algal growth inhibition among all tested DBPs. Therefore,

70

there is a critical need to investigate these newly identified aromatic iodinated DBPs, especially

71

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, which showed widespread occurrence and high

72

toxicity in disinfected water.

iodinated

hydroxybenzenesulfonic

acids,

and

iodinated

73

Previous studies have reported that some DBPs underwent transformation and

74

decomposition in the presence of disinfectants (e.g., free chlorine). Zhai and Zhang23 reported

75

that some aromatic brominated DBPs (e.g., 2,4,6-tribromophenol) underwent transformation in

76

the presence of free chlorine to form other aromatic and aliphatic halogenated DBPs. Na and

77

Olson24 demonstrated that cyanogen chloride decomposed in the presence of free chlorine

78

through hypochlorite-catalyzed hydrolysis. The transformation and decomposition of DBPs in

79

the presence of disinfectants may occur during drinking water or wastewater disinfection and in 3



Page 4 of 33

80

drinking water distribution systems with a secondary disinfectant, which may affect the overall

81

toxicity of disinfected water, and are thus of great significance for DBP studies. However, few

82

studies to date have reported the transformation of aromatic iodinated DBPs in the presence of

83

disinfectants. Drinking water utilities have increasingly switched from purely free chlorine

84

disinfection to either chloramine disinfection or a combination of the two to comply with the

85

regulations for THMs and haloacetic acids (HAAs).4,10,11 It has been reported that the use of

86

monochloramine as a disinfectant favored the formation of aromatic iodinated DBPs,18‒20 and

87

thus the transformation of aromatic iodinated DBPs in the presence of monochloramine is a

88

critical issue for investigation.

89

In this study, we found that 4-iodophenol underwent transformation to form

90

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the presence of monochloramine. To disclose

91

details of this transformation, the purposes of this study were to explore the transformation

92

pathways, to investigate the decomposition kinetics of 4-iodophenol and the formation of

93

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, to examine the factors affecting the

94

transformation, to evaluate the toxicity change during the transformation, and to verify the

95

occurrence of the proposed transformation pathways during chloramination of source water.

96 97 98

MATERIALS AND METHODS Chemicals

and

Reagents.

4-Iodophenol

(99%),

2,4,6-triiodophenol

(97%),

99

2-iodo-4-aminophenol (≥95%), 2-iodo-4-nitrophenol (≥98%), L-ascorbic acid (reagent grade),

100

formic acid (≥98%), iodine (≥99.0%), NaBr (98%), KI (99%), NH4Cl (99.0%), 15N-NH4Cl (≥

101

98.0%), N2H4·H2O (80%), KNO2 (≥96.0%), NaHCO3 (≥99%), NaOH (anhydrous, ≥98%),

102

and H2SO4 (95.0‒98.0%) were purchased from Sigma Aldrich. 4-Chlorophenol (>98%) and 4


Page 5 of 33


103

4-nitrosophenol (98.0%) were provided by TCI (Japan). 2-Iodo-4-chlorophenol (98%) and

104

2,6-diiodo-4-chlorophenol (95%) were obtained from Quality Control Chemicals Inc. (USA).

105

Suwannee River natural organic matter (SRNOM, 2R101N) was purchased from the

106

International Humic Substances Society. A sodium hypochlorite stock solution (13%) was

107

purchased from J&K Scientific, diluted to around 2000 mg/L as Cl2, and standardized using the

108

N,N-diethyl-p-phenylene

109

2,6-Diiodo-4-nitrophenol (≥97%) was purchased from CHEMPARTNER (Chengdu, China).

110

Cell counting kit-8 (CCK-8) was provided by DOjinDO Molecular Technologies, Inc. (Japan).

111

The human hepatoma cells HepG2, Dulbecco’s modified Eagles medium (DMEM) (containing

112

10% fetal bovine serum) and phosphate buffered saline (PBS) were provided by KeyGEN

113

Biotech (China). Ethanol (≥99.7%) and dimethyl sulfoxide (DMSO) (≥99.0%) were purchased

114

from Sinopharm Chemical Reagent Co., Ltd. (China). Methanol (HPLC grade) and acetonitrile

115

(HPLC grade) were purchased from LiChrosolv. Methyl tert-butyl ether (MtBE) (99.9%) was

116

purchased from Tedia. Ultrapure water (18.2 MΩ·cm) was supplied by a Simplicity UV ultrapure

117

water system (Merck Millipore).

diamine

ferrous

titrimetric

method25

every

month.

118

Preparation of Chloraminated 4-Iodophenol Samples. The preparation of solutions is detailed

119

in the SI. Three series of chloraminated 4-iodophenol samples were prepared. For Series 1, ten

120

aliquots (100 mL) of a 4-iodophenol solution (100 µg/L) were prepared. For aliquot 1, no

121

monochloramine was dosed. For aliquots 2–10, monochloramine was dosed at 5 mg/L as Cl2.

122

Then, the pH of each aliquot was adjusted to 8 with diluted NaOH and H2SO4 solutions.

123

Chloramination was conducted in headspace-free amber glass bottles at 20 ºC. The contact times

124

of aliquots 2–10 were 5 min, 0.25 h, 0.5 h, 1 h, 2 h, 6 h, 12 h, 24 h, and 48 h, respectively. After

125

the contact times, the monochloramine residual in each aliquot was quenched with the requisite 5



Page 6 of 33

126

stoichiometric amount of ascorbic acid.26 For Series 2 and 3, the monochloramine doses were 10

127

and 20 mg/L as Cl2, respectively, while the other conditions were kept the same as Series 1.

128

Duplicate samples were prepared for each series.

129

To examine the effect of various factors on the transformation of 4-iodophenol to

130

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the presence of monochloramine, five series

131

of chloraminated 4-iodophenol samples were prepared to study the effect of monochloramine

132

dose, pH, temperature, nitrite concentration, and free chlorine contact time (before

133

chloramination). The details of the samples are illustrated in the SI.

134

To evaluate the toxicity change during the chloramination of 4-iodophenol, a series of

135

chloraminated 4-iodophenol samples with different contact times (0, 0.25, 0.5, 1, 2, 6, 12, 24,

136

and 48 h) were prepared, which is detailed in the SI.

137

Preparation of Chloraminated 2,4,6-Triiodophenol and 2-Iodo-4-aminophenol Samples. Two

138

important

intermediate

products,

2,4,6-triiodophenol

and

2-iodo-4-aminophenol,

were

139

chloraminated to verify the proposed transformation pathways. A series of chloraminated

140

2,4,6-triiodophenol samples and a series of chloraminated 2-iodo-4-aminophenol samples with

141

different contact times were prepared. For the chloraminated 2,4,6-triiodophenol or

142

2-iodo-4-aminophenol samples, six aliquots (100 mL) of a 2,4,6-triiodophenol or

143

2-iodo-4-aminophenol solution (100 µg/L) were prepared. For aliquot 1, no monochloramine

144

was dosed. For aliquots 2–6, monochloramine was dosed at 5 mg/L as Cl2. Then, the pH of each

145

aliquot was adjusted to 8 with diluted NaOH and H2SO4 solutions. Chloramination was

146

conducted in headspace-free amber glass bottles at 20 ºC. The contact times of aliquots 2–6 were

147

1, 6, 12, 24, and 48 h, respectively. After the contact times, the monochloramine residual in each

148

aliquot was quenched with the requisite stoichiometric amount of ascorbic acid.26 6


Page 7 of 33


149

Preparation of Chloraminated Source Water Samples. A series of chloraminated simulated

150

source water samples were prepared to verify the proposed transformation pathways during

151

chloramination of source water. Seven aliquots (1 L) of a solution containing SRNOM (3 mg/L

152

as C), NaHCO3 (90 mg/L as CaCO3), NaBr (2.0 mg/L as Br) and KI (200 µg/L as I) were

153

prepared.19 For aliquot 1, no monochloramine was dosed. For aliquots 2–7, monochloramine

154

(labeled 15N-NH2Cl) was dosed at 5 mg/L as Cl2. Then, the pH of each aliquot was adjusted to 8

155

with diluted NaOH and H2SO4 solutions. Chloramination was conducted in headspace-free

156

amber glass bottles at 20 ºC. The contact times of aliquots 2–7 were 0.25, 0.5, 1, 12, 24, and 48 h,

157

respectively. After the contact times, the monochloramine (labeled 15N-NH2Cl) residual in each

158

aliquot was quenched with the requisite stoichiometric amount of ascorbic acid.26

159

Pretreatment of Water Samples. The samples for the high performance liquid

160

chromatography-ESI-tqMS (HPLC-MS/MS) analysis were pretreated according to a previous

161

method.18 Briefly, each sample was adjusted to pH 0.5 with 7:3 (v/v) concentrated sulfuric

162

acid/water and saturated through the addition of Na2SO4. The sample was then extracted with

163

MtBE. The volume of MtBE was one-tenth of that of the sample. After extraction, the organic

164

layer was transferred to a rotary evaporator and concentrated to 1 mL. The 1 mL solution in

165

MtBE was mixed with 10 mL of acetonitrile, and the mixture was evaporated to 0.5 mL. The 0.5

166

mL solution in acetonitrile was diluted with 0.5 mL of ultrapure water and filtered with a 0.45

167

µm membrane before HPLC-MS/MS analysis. The samples for the toxicity test were pretreated

168

following another previous method.27 Each sample was adjusted to pH 0.5 with 7:3 (v/v)

169

concentrated sulfuric acid/water and saturated through the addition of Na2SO4. The sample was

170

then extracted with MtBE. The volume of MtBE was one-tenth of that of the sample. Then, the

171

MtBE layer was subjected to rotary evaporation to reduce the volume to 5 mL. The 5 mL MtBE 7



Page 8 of 33

172

solution was sparged with nitrogen gas until all the MtBE was evaporated. The remaining solid

173

was stored at 4 ºC. Before the toxicity test, the remaining solid of each sample was redissolved in

174

a small volume of DMEM (containing 0.5% of DMSO) to prepare a stock solution.

175

(HPLC-)MS/MS Analysis. An AB Sciex ESI-tqMS (AB SCIEX API4000) was used to

176

analyze the pretreated samples through direct infusion PIS m/z 126.9. An Agilent HPLC system

177

(Agilent Technologies G1316A-1260 TCC) was coupled to the ESI-tqMS for HPLC-MS/MS

178

analysis. The parameters of the instrument are given in the SI. To identify an iodine-containing

179

molecular ion detected by the direct infusion PIS m/z 126.9, the retention time of the molecular

180

ion was confirmed through HPLC-MS/MS multiple reaction monitoring (MRM). Then, product

181

ion scans of the molecular ion were conducted at the specific retention time to obtain the

182

fragment information to propose a structure. Finally, the proposed structure was confirmed using

183

the corresponding standard compound.

184

Synthesis of Transformation Products. As elaborated later in the results and discussion

185

section, four transformation products in the chloraminated 4-iodophenol samples were proposed

186

to be 2-iodo-4-nitrosophenol, 2,6-diiodo-4-nitrosophenol, 2,4-diiodophenol, and 2,6-diiodo-4-

187

aminophenol. Their standard compounds were not commercially available and were hence

188

synthesized in the lab. The synthesis of the four compounds is detailed in the SI.

189

Cytotoxicity Test with HepG2 Cells. The HepG2 cells were maintained in DMEM at 37 °C in

190

a humidified atmosphere with 5% CO2. The toxicity tests of the samples were conducted

191

following a previous method.28 To initiate the bioassay of a sample, the cells were rinsed with

192

PBS, trypsinized, and then transferred into 96-well plates at a density of ~1.0 × 104 cells/well.

193

After 24 h of growth, the cells were exposed to DMEM containing the required volume of the

194

stock solution of the sample. After a 24 h exposure time, 10 µL of CCK-8 solution was added 8


Page 9 of 33


195

into each well and incubated for 1 h at 37 °C with 5% CO2. Finally, the cells were measured at

196

450 nm with a microplate reader (TECAN, infinite M200). Cell viability was calculated from the

197

relative absorbance. Sextuplicate tests were conducted. The stock solutions of samples were

198

diluted with DMEM (containing 0.5% of DMSO) to achieve different exposure concentrations

199

(concentration factors). The EC50 value of the sample was obtained by plotting the curve of cell

200

viability versus the concentrations (concentration factors) of the sample with Graphpad Prism 5.

201 202

RESULTS AND DISCUSSION

203

Detection and Identification of Polar Iodinated Transformation Products. According to the

204

method developed by Ding and Zhang18, all electrospray-ionizable iodinated compounds can be

205

selectively detected by PIS m/z 126.9. Figure 1 displays the PIS m/z 126.9 spectrum of the

206

chloraminated 4-iodophenol sample with a contact time of 48 h. Besides 4-iodophenol (m/z 219),

207

several ions/ion clusters with relatively high intensities were observed, including m/z 248, 264,

208

345, 374, 379/381, 390 and 471, indicating that several polar iodinated products were generated

209

during chloramination of 4-iodophenol.

210

The major ions/ion clusters in the PIS m/z 126.9 spectrum were identified through

211

HPLC-MS/MS MRM and product ion scans. The corresponding chemical structures are shown

212

in Figure 1. Identification of the ion with m/z 390 (the first peak in the MRM chromatogram) is

213

exemplified here. Figure 2 shows the MRM (390→126.9) chromatogram of ion m/z 390 of the

214

sample (Figure 2b) and its product ion scan spectrum (the first peak in the MRM chromatogram)

215

(Figure 2d). The relatively long retention time (4.65 min) indicated that this compound was

216

likely aromatic.23 In the product ion scan spectrum of m/z 390, two losses of 127 and one loss of

217

28 were observed, indicating that this ion might contain two iodine atoms (two losses of 127) 9



Page 10 of 33

218

and one hydroxyl group attached to the benzene ring (one loss of 28). After subtraction of one

219

benzene ring, two iodine atoms and one hydroxyl group from m/z 390, the remaining part was 46,

220

for which a reasonable combination should be a nitro group. Therefore, the ion with m/z 390 for

221

the first peak was proposed to be 2,6-diiodo-4-nitrophenol or its isomers. To confirm the

222

structure, the standard compound of 2,6-diiodo-4-nitrophenol was purchased. Figure 2 also

223

shows the HPLC-MS/MS MRM (390→126.9) chromatogram of a 2,6-diiodo-4-nitrophenol

224

standard solution (Figure 2a) as well as the sample spiked with 2,6-diiodo-4-nitrophenol (Figure

225

2c), and the product ion scan spectrum of ion m/z 390 of the 2,6-diiodo-4-nitrophenol standard

226

solution (Figure 2e). The same retention time and product ion scan spectra confirmed that the

227

compound with m/z 390 in the sample (corresponding to the first peak in the MRM

228

chromatogram) was 2,6-diiodo-4-nitrophenol. Similarly, the ions/ion clusters with m/z 248, 264,

229

345, 374, 379/381, and 471 were identified as 2-iodo-4-nitrosophenol, 2-iodo-4-nitrophenol,

230

2,4-diiodophenol, 2,6-diiodo-4-nitrosophenol, 2,6-diiodo-4-chlorophenol, and 2,4,6-triiodo-

231

phenol, respectively. Details pertaining to the identification of these six compounds are provided

232

in the SI. Notably, there were two peaks in the MRM chromatogram of ion m/z 390 in the sample,

233

but the intensity of the first peak was much higher than that of the second peak (Figure 2b). The

234

latter was tentatively proposed to be 2,4-diiodo-6-nitrophenol (SI), which is an isomer of

235

2,6-diiodo-4-nitrophenol (the first peak).

236

It should be emphasized that the ions with highest intensities in the PIS m/z 126.9 spectrum

237

corresponded to 2,6-diiodo-4-nitrophenol (m/z 390, the first peak in the MRM chromatogram)

238

and

239

2,6-diiodo-4-nitrophenol and 2,4,6-triiodophenol showed relatively high toxicity among all

240

tested DBPs,7,8 and thus there was a critical need for us to fully investigate the transformation of

2,4,6-triiodophenol

(m/z

471).

Previous

studies

have

demonstrated

that

10


Page 11 of 33


241

4-iodophenol to 2,6-diiodo-4-nitrophenol and 2,4,6-triiodophenol in the presence of

242

monochloramine.

243

Proposing and Verification of Transformation Pathways. Based on the structures of the

244

transformation products (Figure 1), the transformation pathways of 4-iodophenol in the presence

245

of monochloramine were tentatively proposed and are shown in Figure 3. First, 4-iodophenol

246

reacted with monochloramine via Cl[+1] transfer to generate 4-chlorophenol (Ⅰ) and I[+1]

247

(Reactions (1) and (2)).23,29 Then, 4-iodophenol reacted with I[+1] in the solution to form

248

2,4-diiodophenol (Ⅳ) and 2,4,6-triiodophenol (Ⅴ) (Reactions (3) and (4)). Similarly to

249

4-iodophenol, Ⅳ and Ⅴ also reacted with monochloramine via Cl[+1] transfer to form

250

2-iodo-4-chlorophenol (Ⅱ) and 2,6-diiodo-4-chlorophenol (Ⅲ), respectively (Reactions (5)–(8)).

251

Also, Ⅰ might react with I[+1] in the solution to form Ⅱ and Ⅲ (Reactions (9) and (10)). The

252

formed Ⅳ and Ⅴ continued to react with monochloramine through nucleophilic substitution by

253

H2N[–1] to generate 2-iodo-4-aminophenol (Ⅵ) and 2,6-diiodo-4-aminophenol (Ⅶ),

254

respectively,30‒32 as well as I[–1] (Reactions (11)–(14)). The iodide ions (I[–1]) in the solution

255

might be oxidized by monochloramine to form hypoiodous acid (I[+1]), which participated in the

256

reactions. Finally, Ⅵ and Ⅶ were oxidized by NH2Cl to form 2-iodo-4-nitrosophenol (Ⅷ), and

257

2,6-diiodo-4-nitrosophenol (Ⅸ), respectively (Reactions (15) and (16)), which were further

258

oxidized by NH2Cl to 2-iodo-4-nitrophenol (Ⅹ), and 2,6-diiodo-4-nitrophenol (Ⅺ) (Reactions

259

(17) and (18)). Furthermore, similarly to 4-iodophenol, Ⅵ, Ⅷ and Ⅹ might react with I[+1] to

260

form Ⅶ, Ⅸ, and Ⅺ, respectively (Reactions (19)–(21)). It should be noted that the nucleophilic

261

substitution by H2N[–1] might occur in the ortho- or para-position of the hydroxyl group,

262

resulting in the formation of the two isomers of diiodonitrophenol (m/z 390). The results 11



Page 12 of 33

263

indicated that the para-position substitution was dominant, which is consistent with the

264

molecular orbital calculations (SI), and thus only the para-position substitution was included in

265

the transformation pathways. Among the products in the proposed transformation pathways, Ⅲ,

266

Ⅳ, Ⅴ, Ⅷ, Ⅸ, Ⅹ and Ⅺ were already detected and identified in the chloraminated 4-iodophenol

267

samples, and the presence of the other products (Ⅰ, Ⅱ, Ⅵ, and Ⅶ) were also examined in the

268

chloraminated 4-iodophenol samples. The results indicated that all of the other four products

269

were detected and identified in the chloraminated 4-iodophenol samples. The details of their

270

identification are illustrated in the SI. Thus all of the products in the proposed transformation

271

pathways were detected and identified in the chloraminated 4-iodophenol samples, indicating

272

that the proposed transformation pathways are reasonable. Furthermore, the molar concentrations

273

of the products in chloraminated 4-iodophenol samples with different contact times were

274

determined and are shown in SI Figure S12a. The concentrations of the products without

275

commercial standard compounds are indicated by the MRM peak areas (SI Figure S12b). It was

276

found that except for 2,6-diiodo-4-nitrophenol, the concentrations of all other products first

277

increased and then decreased with contact time. Moreover, 2,4,6-triiodophenol (Ⅴ),

278

2,6-diiodo-4-nitrophenol (Ⅺ) and 2,6-diiodo-4-chlorophenol (Ⅲ) were the three products with

279

the highest concentrations, which also supported the proposed transformation pathways.

280

To further verify the proposed transformation pathways, chloramination of two

281

intermediate products, 2,4,6-triiodophenol (Ⅴ) and 2-iodo-4-aminophenol (Ⅵ), was conducted.

282

These two intermediate products were chosen because they were involved in typical reactions in

283

the pathways and also commercially available. For 2,4,6-triiodophenol, its transformation to

284

2,6-diiodo-4-chlorophenol (Ⅲ), 2,6-diiodo-4-aminophenol (Ⅶ), 2,6-diiodo-4-nitrosophenol (Ⅸ)

285

and 2,6-diiodo-4-nitrophenol (Ⅺ) was verified. The identification of the four products in 12


Page 13 of 33


286

chloraminated 2,4,6-triiodophenol samples is demonstrated in SI Figures S13‒S16, and the molar

287

concentrations or peak areas of the four products in chloraminated 2,4,6-triiodophenol samples

288

with different contact times are displayed in SI Figure S17. For 2-iodo-4-aminophenol, its

289

transformation to 2-iodo-4-nitrosophenol (Ⅷ) and 2-iodo-4-nitrophenol (Ⅹ) was verified. The

290

identification of the two products in chloraminated 2-iodo-4-aminophenol samples is shown in SI

291

Figures S18 and S19, and the molar concentrations or peak areas of the two products in

292

chloraminated 2-iodo-4-aminophenol samples with different contact times are displayed in SI

293

Figure S20. The results indicated that the transformation of 2,4,6-triiodophenol and

294

2-iodo-4-aminophenol in the proposed transformation pathways both occurred in the presence of

295

monochloramine. This suggested that Reactions (7), (8), and (13)‒(18) in the proposed

296

transformation pathways are reasonable, and thus similar reactions, including Reactions (1), (2),

297

(5), (6), (11), and (12) should be reasonable. Since both 2,4-diiodophenol (Ⅳ) and

298

2,4,6-triiodophenol (Ⅴ) were positively identified in the chloraminated 4-iodophenol samples,

299

Reactions (3) and (4) should be reasonable, and thus similar reactions, including Reactions (9),

300

(10), and (19)‒(21) should also be reasonable. In this way, all of the reactions in the proposed

301

transformation pathways were demonstrated to be reasonable. To sum up, all of the products in

302

the proposed transformation pathways were positively identified in chloraminated 4-iodophenol

303

samples, and all of the reactions in the proposed transformation pathways were demonstrated to

304

be reasonable, thus the proposed transformation pathways were verified to be reasonable.

305

Some important points were extracted from the proposed transformation pathways. First,

306

4-iodophenol transformed to 2,4,6-triiodophenol in the presence of monochloramine (in the

307

absence of iodide). The iodine atom in 4-iodophenol may leave the benzene ring through Cl[+1]

308

transfer or nucleophilic substitution reactions and then the iodine in the solution continued to 13



Page 14 of 33

309

substitute to the benzene ring of 4-iodophenol to form 2,4-diiodophenol and 2,4,6-triiodophenol.

310

Second,

311

monochloramine. Through this pathway, monochloramine was the nitrogen source of

312

2,6-diiodo-4-nitrophenol (an aromatic iodinated nitrogenous DBP). Previous studies have

313

demonstrated that monochloramine was a nitrogen source of some nitrogenous DBPs along with

314

organic nitrogen.33,34

4-iodophenol

transformed

to

2,6-diiodo-4-nitrophenol

in

the

presence

of

315

Decomposition Kinetics of 4-Iodophenol and Formation of 2,4,6-Triiodophenol and

316

2,6-Diiodo-4-nitrophenol. The molar concentrations of 4-iodophenol, 2,4,6-triiodophenol and

317

2,6-diiodo-4-nitrophenol in the chloraminated 4-iodophenol samples with different contact times

318

were determined and are shown in SI Table S1 and Figure 4a. The initial concentration of

319

4-iodophenol was 0.455 µM (100 µg/L). Its concentration was relatively stable initially (0–0.25

320

h) but decreased sharply to 0.217 µM after a contact time of 0.5 h, and continued to decrease to

321

0.013 µM until a contact time of 12 h. Thereafter, its concentration remained low and relatively

322

stable until 48 h. The monochloramine dose was 5 mg/L as Cl2 (70.4 µM), highly in excess

323

compared with that of 4-iodophenol, and thus the concentration of monochloramine was

324

assumed to be constant throughout the reaction. As shown in Figure 4b, the concentration of

325

4-iodophenol followed a pseudo-second-order decay with respect to its own concentration: = −

326

The apparent rate constant, kobs, was obtained from the integrated form: 1 1 = +

327

The regression was conducted using the experimental data from 0 to 12 h as shown in Figure 4b: 1 = 6 × 10 + 2 × 10 14


Page 15 of 33


328

The unit of c is M, and the unit of t is hour. Thus, kobs was 6×106 M‒1·h‒1. To further verify that

329

the decomposition of 4-iodophenol was a pseudo-second-order decay with different

330

monochloramine doses, the decomposition kinetics of 4-iodophenol with monochloramine doses

331

of 10 and 20 mg/L as Cl2 were also determined and the results are shown in SI Figure S21a and

332

S21b. For both monochloramine doses, the decomposition of 4-iodophenol followed a

333

pseudo-second-order decay with kobs of 1×107 M‒1·h‒1 and 2×107 M‒1·h‒1, respectively,

334

suggesting that the decomposition of 4-iodophenol in the presence of monochloramine (5, 10,

335

and 20 mg/L as Cl2) was pseudo-second-order, and the kobs was linearly correlated with the

336

monochloramine dose (SI Figure S21c).

337

The concentration of 2,4,6-triiodophenol was close to 0 initially (0–0.25 h) but increased

338

sharply to 55.1 nM after a contact time of 0.5 h, and continued to increase with contact time until

339

a maximum (86.5 nM) at 12 h, but began to decrease with contact time until 48 h (Figure 4a).

340

This indicated that 4-iodophenol continued to transform to 2,4,6-triiodophenol within 12 h, but

341

with longer contact times, the decomposition of 2,4,6-triiodophenol in the presence of

342

monochloramine might dominate (SI Figure S17a), leading to the decreasing concentration of

343

2,4,6-triiodophenol. The concentration of 2,4,6-triiodophenol was in the range of 0–86.5 nM.

344

The concentration of 2,6-diiodo-4-nitrophenol was close to 0 initially (0–0.25 h) but increased

345

sharply to 0.49 nM after a contact time of 0.5 h, and continued to increase with contact time until

346

a maximum (10.1 nM) at 24 h, and then remained stable until 48 h (Figure 4a). However, its

347

concentration was much lower (0–10.1 nM) than that of 2,4,6-triiodophenol, indicating that the

348

formation of 2,6-diiodo-4-nitrophenol was much slower than that of 2,4,6-triiodophenol.

349

During chloramination, 4-iodophenol transformed to a number of iodinated products,

350

including 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol. To examine the iodine proportion of 15



Page 16 of 33

351

the two products, the iodine transformation percentage from 4-iodophenol to 2,4,6-triiodophenol

352

and 2,6-diiodo-4-nitrophenol was calculated as follows: Iodine transformation percentage =

,$,%&'(()*+,-. × 3 + ,%)(()%$%-(&'*+,-. × 2 455 − $%()*+,-.

353

The unit of the concentrations is nM. The iodine transformation percentages from 4-iodophenol

354

to 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the chloraminated 4-iodophenol samples

355

with different contact times are shown in Figure 4c. The iodine transformation percentages first

356

increased to a maximum (71.0%) at 1 h and then generally decreased with contact time. Notably,

357

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol could account for 71.0% of the transformed

358

iodine from 4-iodophenol, indicating that they were important iodinated transformation products

359

of 4-iodophenol in the presence of monochloramine, while 2,4,6-triiodophenol was the major

360

one with much higher iodine proportions.

361

Factors Affecting the Transformation of

4-Iodophenol to 2,4,6-Triiodophenol and

362

2,6-Diiodo-4-nitrophenol in the Presence of Monochloramine. The effects of different factors,

363

including monochloramine dose, pH, temperature, nitrite concentration, and free chlorine contact

364

time (before chloramination), on the transformation of 4-iodophenol to 2,4,6-triiodophenol and

365

2,6-diiodo-4-nitrophenol in the presence of monochloramine were examined, and the results are

366

shown in Figure 5. The effect of nitrite concentration was studied in that nitrite has been reported

367

to enhance the formation of aliphatic nitro-group containing DBPs,35,36 while the effect of free

368

chlorine contact time (before chloramination) was investigated to simulate the disinfection

369

scenario of short chlorination followed by chloramination.

370

The effect of monochloramine dose is displayed in Figure 5a and 5b. The concentration of

371

4-iodophenol decreased as the monochloramine dose increased from 0 to 100 mg/L as Cl2, while

372

the concentrations of 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol first increased to a 16


Page 17 of 33


373

maximum and then decreased. The results revealed that increasing the monochloramine dose

374

initially enhanced the decomposition of 4-iodophenol as well as the formation of

375

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, but when the monochloramine dose was over

376

10 mg/L as Cl2, 4-iodophenol might transform to other products, or 2,4,6-triiodophenol and

377

2,6-diiodo-4-nitrophenol might continue to degrade (SI Figure S17a), reducing their

378

concentrations. A previous study has also reported that some iodinated DBPs might undergo

379

decomposition with high levels of monochloramine.20

380

The effect of pH is demonstrated in Figure 5c and 5d. The concentration of 4-iodophenol

381

decreased as pH increased from 5 to 10, indicating that the decomposition of 4-iodophenol was

382

favored in alkaline conditions, possibly due to the inhibited decay of monochloramine,37 the

383

increased fraction in the more reactive phenolate form of 4-iodophenol, as well as the enhanced

384

hydrolysis of 4-iodophenol in alkaline conditions. The concentration of 2,4,6-triiodophenol first

385

increased to a maximum at pH values of 7, 8 and 9, and then slightly decreased at pH 10,

386

whereas the formation of 2,6-diiodo-4-nitrophenol reached a maximum at pH 8. The results

387

indicated

388

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol, possibly due to the following reasons: first, the

389

acid-catalyzed

390

monochloramine,37 and thus the formation of the products decreased in acidic conditions; second,

391

as the pH increased, the fraction in the more reactive phenolate form might increase, enhancing

392

the decomposition of 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol; third, higher pH (e.g. 10)

393

might enhance the hydrolysis of the two products, leading to their decreasing concentrations in

394

highly alkaline conditions.

395

that

neutral

and

slightly

monochloramine

alkaline

conditions

disproportionation

led

to

favored

a

lower

the

formation

concentration

of

of

The effect of temperature is shown in Figure 5e and 5f. The concentration of 4-iodophenol 17



Page 18 of 33

396

decreased as temperature increased, while the concentrations of 2,4,6-triiodophenol and

397

2,6-diiodo-4-nitrophenol first increased and then decreased (reaching a maximum at 20 °C),

398

indicating that a certain temperature (i.e., 20 °C) favored the formation of 2,4,6-triiodophenol

399

and 2,6-diiodo-4-nitrophenol, possibly due to their enhanced decomposition at higher

400

temperatures.

401

The effect of nitrite concentration is presented in Figure 5g and 5h. The concentrations of

402

4-iodophenol and 2,4,6-triiodophenol did not show significant changes with the increasing nitrite

403

concentration, but the concentration of 2,6-diiodo-4-nitrophenol significantly increased with the

404

increasing nitrite concentration, suggesting that nitrite enhanced the formation of

405

2,6-diiodo-4-nitrophenol, possibly through the reaction of HOI and nitrite to form INO2,37 which

406

was consistent with previous studies revealing that nitrite enhanced the formation of some nitro

407

group-containing DBPs (e.g., halonitromethanes).35,36

408

The effect of free chlorine contact time (before chloramination) is presented in Figure 5i

409

and 5j. The concentrations of 4-iodophenol, 2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol all

410

decreased with the increasing free chlorine contact time (before chloramination), indicating that

411

short free chlorine contact times before chloramination significantly enhanced the decomposition

412

of these DBPs, and longer contact times reduced their concentrations.

413

Toxicity Change during Transformation. As elaborated above, 4-iodophenol transformed to

414

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol in the presence of monochloramine. Such

415

transformation may change the toxicity of disinfected water, therefore cytotoxicity tests were

416

conducted for both standard compounds and sample mixtures. The comparative toxicity of the

417

three standard compounds is presented in Figure 6a, and the EC50 values of 4-iodophenol,

418

2,4,6-triiodophenol and 2,6-diiodo-4-nitrophenol were calculated as 490, 204, and 126 µM, 18


Page 19 of 33


419

respectively. Thus the cytotoxicity order of the three compounds was 4-iodophenol

Transformation among Aromatic Iodinated Disinfection Byproducts in

Recommend Documents