Formation of N-Nitrosodimethylamine during Chloramination of

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Formation of N-nitrosodimethylamine during chloramination of secondary and tertiary amines: Role of molecular oxygen and radical intermediates Stephanie Spahr, Olaf Arie Cirpka, Urs von Gunten, and Thomas B. Hofstetter Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04780 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 27, 2016

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Environmental Science & Technology

Formation of N-nitrosodimethylamine during chloramination of secondary and tertiary amines: Role of molecular oxygen and radical intermediates Stephanie Spahr,†,‡, k Olaf A. Cirpka, ¶ Urs von Gunten,†,‡,§ and Thomas B. Hofstetter∗,†,§ 1

Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 Dübendorf, Switzerland, School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland, Center for Applied Geoscience, University of Tübingen, D-72074 Tübingen, Germany, and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CH-8092 Zürich, Switzerland E-mail: [email protected]

Fax: +41 58 765 50 28, Phone: +41 58 765 50 76

∗ To

whom correspondence should be addressed

† Eawag ‡ EPF

Lausanne of Tübingen § ETH Zürich k present address: Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA ¶ University

1 ACS Paragon Plus Environment


Abstract

2

3

N-nitrosodimethylamine (NDMA) is a carcinogenic disinfection by-product from water

4

chloramination. Despite the identification of numerous NDMA precursors, essential parts

5

of the reaction mechanism such as the incorporation of molecular O2 are poorly understood.

6

In laboratory model systems for the chloramination of secondary and tertiary amines, we

7

investigated the kinetics of precursor disappearance and NDMA formation, quantified the

8

stoichiometries of monochloramine (NH2 Cl) and aqueous O2 consumption, derived 18 O-kinetic

9

isotope effects (18 O-KIE) for the reactions of aqueous O2 , and studied the impact of radical

10

scavengers on NDMA formation. While the molar NDMA yields from five N,N-dimethylamine-

11

containing precursors varied between 1.4% and 90%, we observed the stoichiometric removal

12

of one O2 per N,N-dimethylamine group of the precursor indicating that the oxygenation of N

13

atoms did not determine the molar NDMA yield. Small 18 O-KIEs between 1.0026 ± 0.0003

14

and 1.0092 ± 0.0009 found for all precursors as well as completely inhibited NDMA formation

15

in the presence of radical scavengers (ABTS and trolox) imply that O2 reacted with radical

16

species. Our study suggests that aminyl radicals from the oxidation of organic amines by NH2 Cl

17

and N-peroxyl radicals from the reaction of aminyl radicals with aqueous O2 are part of the

18

NDMA formation mechanism.


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19

Introduction

20

N-nitrosodimethylamine (NDMA) and other N-nitrosamines are potent carcinogens that can be

21

formed as disinfection by-products (DBPs) during chlorination, chloramination, and ozonation

22

of drinking water and wastewater. 1–5 Despite the identification of numerous NDMA precursor

23

compounds and suggestions for NDMA mitigation under different treatment conditions, 2,6–17 many

24

aspects of the reaction mechanisms leading to NDMA remain elusive. In fact, the chloramination of

25

amine-containing organic compounds has been identified as a major source of N-nitrosamines. 2,18,19

26

Secondary amines such as dimethylamine (DMA) are reported as frequently occurring NDMA

27

precursors with molar NDMA yields of up to 4%. 20–22 Similar yields (< 6%) observed during

28

chloramination of tertiary amines with N,N-dimethylamine functional groups were interpreted

29

as evidence for their transformation to secondary amines prior to NDMA formation. 8,19–21,23–25

30

However, NDMA yields are substantially higher (>60%) if the tertiary N,N-dimethylamine moi-

31

ety is bound via one methylene group to a (hetero)aromatic ring such as in the pharmaceutical

32

ranitidine. 8,20,26–28 These high yields suggest that secondary amines are probably not central inter-

33

mediates in reactions leading to NDMA. It remains unclear, however, whether differing NDMA

34

yields from secondary and tertiary amines indeed reflect differing reaction mechanisms.

35

To date, there is only limited understanding of how NDMA is formed during the reaction

36

of chloramine with tertiary amines. Based on the detection of cationic dimethylhydrazine inter-

37

mediates during chloramination of ranitidine, Le Roux et al. proposed that NH2 Cl is attacked

38

through nucleophilic substitution by the tertiary amine moiety of the precursor compound. 29

39

Moreover, the extent of NDMA formation depends on the molecular structure of the tertiary

40

amine. 27,30 Heterolytic bond dissociation energies for the elementary reaction to NDMA suggest

41

that leaving groups capable of forming stable carbocations (e.g., methylfuranes) are key for high

42

yields of NDMA. 27,30 These studies provide important evidence for the initial chloramination

43

reaction and possible factors influencing the formation of NDMA. However, information about the

44

consumption of NH2 Cl and O2 in central reactions, such as the incorporation of aqueous O2 into the

45

N–O bond of NDMA, is scarce and mainly circumstantial due to the challenges of characterizing 3 ACS Paragon Plus Environment


46

transient reactive (oxygen) intermediates.

47

Regardless of the yield of NDMA formation during chloramination, reactions of aqueous O2 play

48

an important role in the NDMA formation pathway. While chloramination experiments with 18 O-

49

labeled H2 O showed no incorporation of 18 O into NDMA, the amount of NDMA formed increased

50

with increasing concentrations of aqueous O2 . 22,26 Although thermodynamically feasible, reactions

51

of ground state triplet oxygen (3 O2 ) with even-electron species such as the known NDMA precursors

52

are spin forbidden elementary reactions and thus too slow to lead to the oxygenation of organic

53

amines or NH2 Cl. 31–34 As a consequence, molecular O2 can only react after activation to singlet

54

31,34–36 oxygen (1 O2 ), through reduction to superoxide (O•− 2 ), or in reactions with radical species.

55

Previous work suggests that the latter is the most likely option for NDMA formation during

56

chloramination because neither the addition of β-carotene as 1 O2 scavenger nor the addition of

57

superoxide dismutase as O•− 2 quencher had an effect on NDMA formation during chloramination

58

of DMA. 22 In contrast, formation of NDMA through radical intermediates has been proposed for

59

breakpoint chlorination of DMA 37 as well as for chloramination of quaternary amines. 38 But direct

60

experimental evidence for radical reactions with aqueous O2 is still lacking.

61

The goal of this study was to elucidate the reactions of aqueous O2 and NH2 Cl with regard to

62

contributions of radical intermediates to NDMA formation during chloramination. To this end, we

63

performed laboratory experiments with five NDMA precursor compounds, namely ranitidine, 5-(di-

64

methylaminomethyl)furfuryl alcohol (DFUR), N,N-dimethylbenzylamine (DMBA), 2,4,6-tris(di-

65

methylaminomethyl)phenol (TDMAP), and dimethylamine (DMA). Evidence for potential NDMA

66

formation mechanisms was obtained from (i) the kinetics of precursor disappearance and NDMA

67

formation, (ii) the quantification of aqueous O2 and NH2 Cl consumption, (iii) the analysis of

68

oxygen isotope fractionation of aqueous O2 , and (iv) the study of the impact of radical scavengers

69

on NDMA formation. We used the analysis of oxygen isotope ratios (18 O/16 O) of aqueous O2 for

70

the first time in the context of DBP formation. This methodology is well established for studies on

71

activation of oxygen in enzymes and by transition metal complexes 39–42 and reveals the mechanisms

72

of O2 activation from the magnitude of 18 O-kinetic isotope effects.


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73

Experimental Section

74

Chemicals

75

A list of all chemicals including suppliers and purities is provided in the Supporting Information (SI).

76

Monochloramine (NH2 Cl) stock solutions (30 mM) were prepared daily by mixing hypochlorite

77

and ammonium chloride at pH 9.5 (molar Cl:N ratio of 1:1.05) as described previously. 43,44

78

Chloramination experiments

79

Chloramination experiments were carried out with five NDMA precursors as listed in the Introduc-

80

tion section and shown in Table 1. Unless stated otherwise, reactors contained 10 mM phosphate

81

buffer at pH 8.0 in amber glass bottles of volumes between 100 and 1000 mL. Organic amines were

82

added from either methanolic, ethanolic, or aqueous stock solutions to reach initial concentrations

83

of 15 µM (see Figure S4). Reactions were initiated by addition of NH2 Cl in 15- to 18-fold excess

84

corresponding to initial concentrations of 225 - 270 µM. For concentration analysis and reaction

85

product identification, 1 mL of aqueous sample was withdrawn at selected time points and the

86

reaction was quenched by adding 5 µL of a Na2 S2 O3 solution (100 g/L). Note that the use of Na2 SO3

87

is not recommended owing to its reactivity with DFUR (Figure S5). NH2 Cl concentrations were

88

quantified by either membrane introduction mass spectrometry (MIMS) or colorimetric methods

89

(see chemical analyses). For continuous monitoring of the concentration of aqueous O2 , we filled

90

an aliquot of the reaction solution in 11 mL amber glass vials that were closed without headspace

91

with butyl rubber stoppers and aluminum crimp caps, and immersed a needle-type fiber-optic

92

oxygen microsensor into each vial. Two types of control experiments were set up identically to

93

assess the stability of the organic amine in the absence of NH2 Cl and to quantify the self-decay

94

rate constant of NH2 Cl in the absence of the organic amine. Molar NDMA yields were calculated

95

by dividing the measured concentration of NDMA by the initial concentration and the number of

96

N,N-dimethylamine groups of the precursor.

97

To assess the reactivity of NH2 Cl with the furfuryl alcohol moiety of DFUR, we quantified the 5 ACS Paragon Plus Environment


98

concentration of NH2 Cl during the reaction of 3 µM furfuryl alcohol with 45 µM NH2 Cl in 10 mM

99

phosphate buffer at pH 8.0. To study the impact of radical scavengers on NDMA formation, we

100

added either the hydroxyl radical scavenger tert-butanol (t-BuOH, 40 mM final concentration) or

101

peroxyl radical scavengers, namely 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid, ABTS,

102

2 mM) or rac-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (trolox, 0.5 mM), to the

103

reaction of DFUR (3 µM or 15 µM) with NH2 Cl (45 µM or 225 µM). Note that we refer to ABTS

104

and trolox as reduced species of the radical scavengers. Experiments with radical scavengers

105

differed regarding to the sequence of reactant addition and NH2 Cl quantification method. t-BuOH

106

was spiked to a phosphate buffered solution containing DFUR before the addition of NH2 Cl. In

107

contrast, ABTS was added immediately after spiking the DFUR-containing buffer with NH2 Cl.

108

Trolox was dissolved in phosphate buffer to which DFUR as well as NH2 Cl were added. In the

109

presence of t-BuOH and trolox, we quantified the NH2 Cl concentration with the ABTS method and

110

MIMS, respectively. When ABTS was added as a radical scavenger, we observed the oxidation of

111

ABTS to the colored ABTS•− and used its absorbance to quantify the amount of consumed NH2 Cl

112

indirectly (Figure S21a). Control experiments were set up to assess the reactivity of the organic

113

amine or NH2 Cl with the radical scavengers.

114

To quantify the formation of H2 O2 during the reaction of DFUR (50 µM) with NH2 Cl (750 µM),

115

we filled the reaction solution into 11 mL amber glass vials that were closed headspace-free

116

and monitored the decrease of O2 concentration as described above. When the consumed O2

117

was stoichiometrically equal to the initial nominal concentration of DFUR, we added 100 µL

118

catalase from bovine liver (0.3 g L−1 final concentration). The observed increase in aqueous O2

119

concentration upon addition of catalase corresponds to half of the H2 O2 concentration. To assess the

120

stability of H2 O2 during the NDMA formation reaction, we added 50 µM H2 O2 immediately after

121

the reaction of DFUR (50 µM) with NH2 Cl (750 µM) was initiated and determined the recovery of

122

H2 O2 by addition of catalase after 3.3 h. A more detailed description of experiments with catalase

123

is provided in the SI (section S13).


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124

Oxygen isotope fractionation experiments

125

The fractionation of stable O isotopes of aqueous O2 was studied in amber reactors containing

126

a magnetic stir bar and 11 mL of a 3 mM NH2 Cl solution in 10 mM phosphate buffer (pH 8.0).

127

The vessels were closed without headspace and an oxygen microsensor was introduced to measure

128

continuously the concentration of aqueous O2 . 15 to 200 µM of the organic amine (ranitidine,

129

DFUR, DMBA, or TDMAP) was added to the reactor while stirring to initiate the reaction. Once

130

the amount of consumed O2 equaled the initial nominal concentration of the added organic amine,

131

the reaction was quenched by creating a N2 headspace following the procedure of Pati et al. 45 To this

132

end, 3 mL of solution was replaced by N2 gas with a gas-tight glass syringe. Partitioning of O2 to the

133

N2 -headspace was promoted through horizontal shaking for 30 min at 200 rpm while keeping the

134

vessels upside down. Control samples containing 3 mM NH2 Cl without organic amine or 200 µM

135

organic amine without NH2 Cl were prepared similarly. Owing to much slower reaction kinetics

136

of DMA and NH2 Cl, we prepared twelve replicate samples containing 200 µM DMA and 3 mM

137

NH2 Cl as well as twelve control samples with 3 mM NH2 Cl. Two reactive batches and one control

138

batch were processed at selected time points over a time period of 4 days. In experiments with

139

DMBA, TDMAP, and DMA, we additionally determined the concentration of NDMA immediately

140

after sample quenching in the withdrawn reaction solution.

141

Chemical analyses

142

Concentrations of ranitidine, DFUR, and NDMA were quantified with reverse phase HPLC coupled

143

to a UV-vis detector using previously described methods (see section S3 for details). 28 Analytical

144

errors of the reported concentrations are typically < 1%. Transformation products formed during

145

chloramination of ranitidine and DFUR were analyzed by liquid chromatography-high-resolution

146

tandem mass spectrometry (LC-HR-MS/MS) using an adjusted analytical method described in

147

Gulde et al. 46 (see section S4 for details).



148

Quantification of aqueous O2

149

Concentrations of aqueous O2 were continuously measured with needle-type fiber-optic oxygen

150

microsensors connected to a 4-channel transmitter (PreSens Precision Sensing GmbH, Germany).

151

Four oxygen sensors were operated simultaneously after daily calibration with air-saturated and

152

oxygen-free water. O2 concentrations were corrected for variations in temperature. The analytical

153

uncertainties of O2 concentrations are smaller than ±0.5 µM.

154

NH2 Cl concentrations

155

Concentrations of NH2 Cl were determined by membrane introduction mass spectrometry (MIMS) 47

156

as well as spectrophotometrically using either ABTS or N,N-diethyl-p-phenylenediamine (DPD)

157

with iodide (I – ) as catalyst, 48,49 or the indophenol method. 50 NH2 Cl does not react with ABTS

158

but with I – to form hypoiodous acid, which oxidizes ABTS2− to the colored ABTS•− . A detailed

159

description of the methods for NH2 Cl quantification can be found in section S5. A Varian Cary

160

100 Bio UV-visible spectrophotometer was used for colorimetric assays. MIMS analysis was

161

performed using a MIMS 2000 (Microlab, Aarhus, Denmark) equipped with a multi-port valve

162

to enable an automatic measurement of multiple samples. The membrane inlet temperature was

163

set to 40◦ C and the sample flow rate was 4 mL min−1 . Steady-state signals of m/z 51 and m/z 53

164

were reached after 2 minutes and measured for 6 - 8 minutes. The software used for analysis of

165

MIMS signals is described elsewhere. 47 We used NH2 Cl calibration rows from either 10 - 50 µM

166

or 50 - 300 µM. Note that ranitidine, DFUR, NDMA, and trolox did not produce any interfering

167

signals in the mass spectrometer. In NDMA formation experiments, we detected an interference at

168

m/z 51 that was presumably created from an unknown reaction intermediate (see Figure S2). In this

169

case, quantification of NH2 Cl was performed at m/z 53. All reported NH2 Cl concentrations were

170

corrected by the self-decay of NH2 Cl observed in controls containing chloramine only. Note that

171

the four methods used for determining NH2 Cl concentrations led to differing results when NH2 Cl

172

was quantified during NDMA formation (see discussion in section S5.3 and Figure S3). The total

173

turnover of NH2 Cl determined at the end of these experiments was, however, identical (section S5). 8 ACS Paragon Plus Environment

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Stable oxygen isotope analysis

175

18 O/16 O

176

trometry (GC/IRMS) following recently developed procedures of Pati et al. 45 Briefly, sample vials

177

to which a 3 mL N2 headspace had been introduced were mounted on a CombiPAL autosampler

178

(CTC Analytics) equipped with a 2.5 mL gas-tight headspace syringe. The syringe was flushed

179

with N2 gas for 1 min before withdrawing 250 µL of the headspace from the sample. The gaseous

180

sample was injected into a split injector (5 mm ID quartz liner, 200◦ C) of a Trace GC (Thermo

181

Fisher Scientific) that was equipped with a Rt-Molsieve 5 PLOT column (Restek, 30 m x 32 mm

182

ID, 30 µm film thickness at 30◦ C) to separate O2 from N2 . Helium (99.999%) was used as carrier

183

gas at 80 kPa and the split flow was 40 mL min−1 . O2 pulses were introduced into a GC combustion

184

III interface (Thermo Fisher Scientific) equipped with a Nafion membrane for removal of water and

185

subsequently entered a Delta Plus XL IRMS (Thermo Fisher Scientific).

ratios of aqueous O2 were measured with gas chromatography isotope ratio mass spec-

186

O isotope ratios are expressed in the delta notation as δ18 O in permil (eq. 1) relative to Vi-

187

enna Standard Mean Ocean Water (VSMOW). Sample sequences included standards generated

188

from air-saturated phosphate buffer, which were measured in triplicates after every 6 samples to

189

ensure accuracy of δ18 O measurements in a standard bracketing procedure. Uncertainties of δ18 O

190

measurements were below ±0.5h. Blank samples containing oxygen-free water were analyzed in

191

triplicates at the end of each sequence for blank corrections of diffusive O2 input during sample

192

preparation and injection (see Pati et al. 45 for details).

δ18 O =

(18 O/16 O)sample −1 (18 O/16 O)VSMOW

(1)

193

We used Igor Pro software (WaveMetrics Inc., Lake Oswego OR, USA) to derive O isotope

194

enrichment factors, εO , with a nonlinear least-square regression according to equation 2, where

195

δ18 O and δ18 O0 are O isotope signatures determined in samples from oxygen isotope fractionation

196

experiments and in standards, respectively. c/c0 is the fraction of remaining aqueous O2 .



δ18 O + 1 = (c/c0 )εO 18 δ O0 + 1 197

198

Average

18 O-kinetic

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(2)

isotope effects (18 O-KIE) of both O atoms were determined with equa-

tion 3. 41,51 Uncertainties of εO and 18 O-KIEs are reported as 95% confidence intervals.

18

O-KIE =

1 1 + εO

(3)

199

Results and Discussion

200

Chloramination of amines: Kinetics, reaction products, and stoichiometry of

201

NH2 Cl and O2 consumption

202

Chloramination of ranitidine

203

The reaction of ranitidine (15 µM) with NH2 Cl (270 µM) at pH 8.0 produced NDMA with a

204

molar yield of 89.9 ± 0.1% in agreement with previous observations. 8,26–28 Figure 1a shows the

205

kinetics of the disappearance of ranitidine, dissolved O2 , and NH2 Cl as well as the formation of

206

NDMA. Within one hour, ranitidine was depleted following pseudo-first order kinetics (Figure S8).

207

The second-order rate constant for the reaction of ranitidine with NH2 Cl was 6.1 ± 0.3 M−1 s−1 in

208

agreement with a previous study. 28

209

The formation of NDMA only started after 1.6 h (dashed vertical line in Figure 1a) when

210

ranitidine had disappeared completely suggesting the presence of a critical intermediate. Indeed,

211

NDMA formation was concomitant with the decline of a transient reaction product (yellow stars,

212

Figure 1a) with m/z [M+H+ ] = 365.1049 corresponding to the molecular formula C13 H21 N4 O4 SCl.

213

We report this intermediate with arbitrary peak areas due to the lack of standard materials. The

214

mass of C13 H21 N4 O4 SCl exceeds that of ranitidine (m/z [M+H+ ] = 315.1485) by 50 Da, which cor-

215

responds to the presence of an additional hydroxyl group (+OH), an additional Cl atom (+Cl), and a

216

loss of 2 hydrogen atoms (−2 H). The intermediate, which we refer to as Ran-OH-Cl, was identified 10 ACS Paragon Plus Environment

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(a) Ranitidine

(b) DFUR

20

10

15

8 6

10

4

5

2

0

0 0

2

4

6

Time (h)

8

Ranitidine NDMA

10

DFUR Ran-OH-Cl

t = 0.3h

25

12

Concentration (µM)

160

Area "Ran-OH-Cl"

14

250

200

240

t = 1.6h

160

260

240

250

200

Concentration (µM)

260

240

Concentration (µM)

280

Concentration (µM)

280

240 14

14

12

12

10

10

8

8 6

6

4

4

2

2

0

0 0

NH2Cl NH2Cl control

1

2

O2

3

4

Time (h)

5

6

O2 (NH2Cl control)

Figure 1. NDMA formation during the reaction of NH2 Cl (270 µM) with (a) ranitidine (15 µM) over 11 h and (b) DFUR (15 µM) over 6.5 h in 10 mM phosphate buffer at pH 8.0. Consumption of NH2 Cl (measured with MIMS) and aqueous O2 is depicted as grey circles and light blue line, respectively. Note break in y-axes and scale change.

217

in a previous study for the chloramination of ranitidine under similar experimental conditions. 29

218

MS/MS fragmentation patterns of Ran-OH-Cl suggest that the 2-(dimethylamino)methylfuran moi-

219

ety remained intact during chloramination (Figure S15). Apparently, chlorination and hydroxylation

220

of ranitidine occurred more rapidly at the thioethyl-N-methyl-2-nitroethene-1,1-diamine moiety

221

than at the 2-(dimethylamino)methylfuran group. Note that we measured only minor concentra-

222

tions of 5-(dimethylaminomethyl)furfuryl alcohol (DFUR, m/z [M+H+ ] = 156.1021, Figure 1a), a

223

known NDMA precursor. 27 Small amounts of DFUR as well as the coincidence of NDMA forma-

224

tion with the disappearance of Ran-OH-Cl suggest that the latter was the primary NDMA precursor

225

during chloramination of ranitidine.

226

The concentration trends of NH2 Cl and O2 reveal that the transformation of ranitidine within

227

the first 1.6 h involved NH2 Cl but not dissolved O2 because its concentration remained constant



228

until ranitidine disappeared. In contrast, the disappearance of Ran-OH-Cl and the concomitant

229

formation of NDMA after 1.6 h were accompanied by a decline in NH2 Cl and O2 concentration

230

(Figure 1a). The total amount of consumed NH2 Cl during chloramination of ranitidine amounted to

231

124 ± 2 µM corresponding to 8.7 ± 0.2 times the initial ranitidine concentration (Table 1). NH2 Cl

232

consumption rates were distinctly different before and after 1.6 h and enabled us to attribute the

233

loss of NH2 Cl to the transformation of ranitidine vs. the formation of NDMA (Figure S9a). To

234

compare the two kinetic regimes, we report operational pseudo-first order rate constants for the

235

consumption of NH2 Cl. During the transformation of ranitidine, 60.5 µM NH2 Cl were consumed

236

with a rate constant k obs,21 of (7.2 ± 0.5) · 10−5 s−1 . This share of NH2 Cl consumption equaled 4.3

237

times the initial ranitidine concentration. The overstoichiometric NH2 Cl consumption suggests that

238

multiple sites of the thioethyl-N-methyl-2-nitroethene-1,1-diamine moiety (e.g., S and N atoms)

239

were chlorinated as suggested previously by Le Roux et al. 29 During the formation of NDMA, 63 µM

NH Cl

240

NH2 Cl were consumed corresponding to 4.4 times the initial ranitidine concentration (Table 1).

241

The rate constant of NH2 Cl consumption (k obs,22 = (1.1 ± 0.04)·10−5 s−1 ) was 7-fold smaller than

242

the one observed during transformation of ranitidine implying differing reactions of NH2 Cl with

243

ranitidine and reaction intermediates such as Ran-OH-Cl, respectively. The overstoichiometric

244

NH2 Cl consumption during NDMA formation indicates that NH2 Cl might be involved in several

245

reactions of the multi-step NDMA formation pathway. Experiments with differing ranitidine to

246

NH2 Cl ratios underscore the importance of NH2 Cl because maximum NDMA yields from ranitidine

247

were only reached in presence of ≥15-fold excess of NH2 Cl (Figure S7).

NH Cl

248

In contrast to overstoichiometric NH2 Cl consumption, we observed a stoichiometric disappear-

249

ance of dissolved O2 during the formation of NDMA. After 10 h, NDMA formation was complete

250

and the amount of consumed O2 (14.3 ± 0.3 µM) matched the initial concentration of ranitidine

251

(14.2 ± 0.1 µM) within analytical uncertainty. The stoichiometries of O2 and NH2 Cl consumption

252

are compiled in Tables 1 and S1. To assess whether the molar reaction stoichiometry determined

253

in the ranitidine experiment also applies for other amine-containing NDMA precursors, we chlo-

254

raminated two tertiary amines, which are known to produce high molar NDMA yields, namely


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Page 13 of 36


255

DFUR and N,N-dimethylbenzylamine (DMBA) as well as 2,4,6-tris-(dimethylaminomethyl)phenol

256

(TDMAP) and dimethylamine (DMA), which are known to produce significantly lower molar

257

NDMA yields.

258

Chloramination of other tertiary amines with high molar NDMA yield

259

Chloramination of DFUR and DMBA under experimental conditions that were identical to ex-

260

periments with ranitidine (pH 8.0, 270 µM NH2 Cl, Figure 1b and S10) led to high molar NDMA

261

yields of 84.6% and 82.5%, respectively, in agreement with previously reported values. 20,27 DFUR

262

and DMBA are structurally similar to ranitidine and are likely transformed to NDMA by the same

263

reaction mechanism. This interpretation is supported by almost identical reaction stoichiometries

264

(Table 1). The consumption of aqueous O2 was stoichiometric (15.2 ± 0.3 µM vs. 14.5 µM of initial

265

DFUR concentration), while the consumption of NH2 Cl (68.2 ± 2.9 µM) exceeded the initial DFUR

266

concentration by a factor of 4.7 (Table 1, data for DMBA see Table S2). To assess whether reactions

267

of NH2 Cl with the heterocyclic ring of DFUR contributed to the overall NH2 Cl consumption, we

268

conducted experiments with 3 µM furfuryl alcohol and 45 µM NH2 Cl under otherwise identical

269

experimental conditions (Figure S17). No significant decrease in the NH2 Cl concentration was

270

observed within 30 h indicating that the initial reaction of NH2 Cl and DFUR happens exclusively

271

at the N,N-dimethylamine group of DFUR.

272

While the molar NDMA yield and the reaction stoichiometries were almost identical for raniti-

273

dine, DFUR, and DMBA, the reaction kinetics differed significantly. Figures 1b and S10 show

274

that NDMA formation during chloramination of DFUR and DMBA was completed within 4 h and

275

6 h, respectively, and was thus faster than NDMA formation from ranitidine (within 10 h). During

276

chloramination of DFUR, we observed a short lag phase within the first 0.3 h of the reaction in

277

which only 0.3 µM of NDMA were detected. This amount is too small to lead to measurable

278

changes of O2 and NH2 Cl concentrations. After 0.3 h, the reaction accelerated and the formation

279

of NDMA coincided with the disappearance of DFUR, O2 , and NH2 Cl (Figure S8b). NH2 Cl

280

was consumed with a pseudo-first order rate constant of (2.2 ± 0.1) · 10−5 s−1 , which is twice



Page 14 of 36

NH Cl

281

the kobs,22 observed during NDMA formation from ranitidine. Even after NDMA formation

282

ceased (after 4 h), the consumption of NH2 Cl continued with a smaller k obs 2

283

10−6 s−1 (Figure S9b). This finding suggests that some additional reactions of NH2 Cl contributed

284

to the observed overstoichiometric NH2 Cl consumption. A tentatively identified reaction product

285

with molecular formula C5 H5 ClO2 (m/z [M+H+ ] = 133.0053) indicates that NH2 Cl reacts with

286

intermediates of the NDMA formation reaction leading to chlorinated furfuryl alcohol as final

287

reaction product (for possible molecular structures see Figure S16).

288

Chloramination of tertiary and secondary amines with low molar NDMA yield

289

Molar NDMA yields from chloramination of TDMAP and DMA were normalized to the num-

290

ber of N,N-dimethylamine groups of the precursor molecule and amounted to 15.8% and 1.4%,

291

respectively. Our data is in good agreement with previously reported NDMA yields of 18.4%

292

for TDMAP 20 and 1.2 - 2.3% for DMA. 20,27 The kinetics of NDMA formation and concomitant

293

NH2 Cl and O2 consumption are shown in Figure S11. Despite significantly smaller molar NDMA

294

yields from TDMAP and DMA, the stoichiometries of NH2 Cl and O2 consumption were similar

295

to the one for ranitidine, DFUR, and DMBA. The consumed amount of O2 corresponded with the

296

initial concentration of N,N-dimethylamine groups, while the amount of reacted NH2 Cl exceeded

297

the initial concentrations of N,N-dimethylamine groups by a factor of 3.9 and 2.2 for TDMAP and

298

DMA, respectively (Tables 1 and S2). The overstoichiometric consumption of NH2 Cl in experi-

299

ments with low-yield NDMA precursors indicates that unidentified reactions other than NDMA

300

formation likely contributed to the disappearance of NH2 Cl. The stoichiometric O2 consumption

301

in experiments with high- and low-yield NDMA precursors hints at the same mechanism of N-

302

atom oxygenation of secondary and tertiary amines. However, reactions with dissolved O2 do not

303

necessarily lead to the formation of NDMA and it is likely that other factors such as the molecular

304

structure of the precursor molecule determine the molar NDMA yield. Note that chloramination of

305

DMA was too slow to carry out reliable O2 concentration measurements over the entire experiment

NH Cl

306

of (7.7 ± 0.7) ·

period (> 3 days). The reported reaction stoichiometry of > 0.6 in Table 1 indicates that a higher


Page 15 of 36


307

number should be expected based on an observed stoichiometric O2 consumption in experiments

308

with higher initial concentrations of DMA (43 µM) and NH2 Cl (1000 µM, Figure S12).


ACS Paragon Plus Environment

16 NH

N

N

N

N

N

OH

O

O

N

OH

S

molecular structure

H N NO 2

H N

1.4 ± 0.1

15.8 ± 0.01

1.0 h : >0.6 i : 2.2

1.0 h : 1.0 : 3.9

1.0 h : 1.1 : 4.7

n.m. f : n.m. : 4.0 0.9 : 0 : 16 0.12 : n.m. f : 7.5

80.6 ± 0.1

Formation of N-Nitrosodimethylamine during Chloramination of

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