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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
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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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Introduction
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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
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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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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
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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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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).
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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 .
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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
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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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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
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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
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Environmental Science & Technology
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).
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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