Re-Examining the Role of Dichloramine in High-Yield N

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Letter Cite This: Environ. Sci. Technol. Lett. 2018, 5, 154−159

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Re-Examining the Role of Dichloramine in High-Yield N‑Nitrosodimethylamine Formation from N,N‑Dimethyl-α-arylamines Meredith E. Huang, Shiyang Huang, and Daniel L. McCurry* Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: N-Nitrosodimethylamine (NDMA) is a potent carcinogen associated with chloramination of wastewater and wastewaterimpacted drinking waters. Substantial effort has been expended to identify the precursors and mechanisms leading to NDMA formation. One of the major discoveries has been that molecules in the N,Ndimethyl-α-arylamine class, including the common pharmaceutical ranitidine, form NDMA in high yield during chloramination. Simultaneously, it was hypothesized that these precursors react with monochloramine, the dominant species in most chloramine mixtures, to form NDMA. This monochloramine hypothesis contradicts past mechanistic work with simple secondary amines, as well as practical experience showing that minimization of dichloramine reduces the level of NDMA formation during wastewater reuse and drinking water treatment. In this work, we address the contradiction between practical experience and model precursor studies by showing that N,N-dimethyl-α-arylamines form NDMA chiefly via reactions with dichloramine, rather than monochloramine. We experimentally demonstrate substantially higher NDMA yields from dichloramination than from monochloramination of four N,N-dimethyl-α-arylamine compounds, including ranitidine, and computationally rationalize declining NDMA yields at large dichloramine doses, which may explain past results reporting low NDMA yields from dichloramination of ranitidine. These results provide support for NDMA control strategies currently under evaluation at wastewater reuse facilities.

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INTRODUCTION Since the discovery of high concentrations of the potent carcinogen N-nitrosodimethyalmine (NDMA) in recycled wastewater effluent,1 considerable effort has been expended to characterize the precursors and formation mechanisms of NDMA during water treatment.2−4 Early research identified chloramination as the key step in NDMA formation in water and wastewater1 and postulated monochloramine (NH2Cl) as the key chlorine species.5,6 Subsequent work indicated that dichloramine, a relatively minor component of total chlorine, was the critical compound leading to NDMA formation.7 This dichloramine hypothesis has been exploited in NDMA control schemes; minimizing dichloramine formation has been successful at reducing NDMA formation during drinking water and wastewater chloramination.8−11 In parallel to research on oxidative species involved in NDMA formation, research seeks to identify the organic precursors of NDMA. NDMA precursors are generally postulated to be organic amines,2−4 such as dimethylamine.5,6 Dimethylamine has been widely shown to form NDMA in ∼2% yield during chloramination [range of 0.5−3% (Table S1)]12−17 but was determined not to be an important precursor in wastewater effluent.12,13 Many other compounds have since been identified as NDMA precursors, including pharmaceuticals,18−22 quaternary amine polymers,17,23−27 and a fungicide degradation product.28 Particular attention has been focused on © 2018 American Chemical Society

a class of structurally similar compounds, beginning with the pharmaceutical ranitidine, that form NDMA in high (≥40%) molar yields upon chloramination (Table S1).16−20,29−31 This category of compounds has grown to include, e.g., N,Ndimethylbenzylamine, 5-(dimethylaminomethyl)furfuryl alcohol, and methadone.16,22,29 The common structural motif conveying high NDMA formation potential is the N,Ndimethyl-α-arylamine functional group.16 Additionally, selected alkylamines (e.g., N,N-dimethylisopropylamine) were found to form NDMA in relatively high yields, indicating that the key property is a leaving group capable of stabilizing a positive charge (e.g., benzyl and isopropyl) bonded to a dimethylamine group.16,32 The revolution in NDMA research caused by the discovery of high-yield NDMA precursors led to a revisitation of the mechanism of formation of NDMA. Several reports of highyield NDMA formation from ranitidine implicated monochloramine (NH2Cl) as the reactive electrophile,16,20,29 in contrast with previous research demonstrating NDMA formation from simple secondary amines was caused by dichloramine (NHCl2) and contemporaneous evidence supporting the role of Received: Revised: Accepted: Published: 154

December 20, 2017 January 19, 2018 January 22, 2018 January 22, 2018 DOI: 10.1021/acs.estlett.7b00572 Environ. Sci. Technol. Lett. 2018, 5, 154−159

Letter

Environmental Science & Technology Letters dichloramine in NDMA formation from ranitidine.21 This monochloramine hypothesis has been taken as a starting point for later investigations attempting to clarify the mechanism of NDMA formation from N,N-dimethyl-α-arylamine precursors, including ranitidine.30,31,33 However, the apparent primacy of monochloramine in NDMA formation reactions from highyield precursors contradicts practical experience suggesting that minimization of dichloramine minimizes NDMA formation.8−11,34 This contradiction can be explained in at least two ways. (1) Precursors in the N,N-dimethyl-α-arylamine class are not important in practice, and/or (2) despite previous studies implicating monochloramine, dichloramine is the primary oxidant leading to NDMA formation from N,Ndimethyl-α-arylamines. In this work, we revisit the conclusion that monochloramine is responsible NDMA formation from N,N-dimethyl-α-arylamine precursors and investigate the hypothesis that dichloramine leads to NDMA formation. We experimentally compare the formation of NDMA from four N,N-dimethyl-α-arylamine precursors (Figure 1) by monochloramine and dichloramine and explore an explanation for past results consistent with the hypothesis presented here.

the basis of the measured chloramine concentration divided by the starting concentration of the precursor, prepared gravimetrically. Reagent grades, suppliers, and chloramine preparation methods are provided in Text S1 of the Supporting Information. NDMA Analysis. NDMA samples were analyzed within 24 h via high-performance liquid chromatography with ultraviolet absorbance detection (HPLC−UV; 1260 Infinity II, Agilent). More details about HPLC−UV analysis of NDMA are provided in Text S2 of the Supporting Information. NDMA standard curves were prepared from 0.1 to 100 μM (Figure S1), and the calculated detection limit was 1 μM. Molar yields throughout are reported as means ± the 95% confidence interval according to a two-tailed Student’s t distribution. Modeling of Chemical Kinetics. Chemical kinetics of formation of NDMA from dimethylamine were modeled with Kintecus software35 using reaction constants obtained from the literature (Table S2).11

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RESULTS AND DISCUSSION Method Validation with Dimethylamine. The mechanism and kinetics of NDMA formation from DMA are known.7 Therefore, DMA was used as a test case for the experimental approach of applying a range of stoichiometric doses of preformed monochloramine or dichloramine to model compounds and measuring NDMA yields, with the expectation that dichloramine would produce dramatically more NDMA than monochloramine at comparable doses. DMA was treated with 1−20 molar equivalents (M.E., i.e., stoichiometric ratio of oxidant to precursor) of preformed monochloramine or dichloramine. NDMA formation from monochloramine was low at a modest dose (e.g., 0.93 ± 0.11% molar yield at 3 M.E.) (Figure 2A) and consistently increased with respect to dose, reaching a maximum of 1.7 ± 0.05% molar yield at the largest monochloramine dose (20 M.E.), consistent with the literature value of ∼2%.12−17 In contrast, DMA treated with dichloramine formed higher NDMA concentrations even at small doses (e.g., 5.1 ± 0.44% molar yield at 1 M.E.) (Figure 2A). The level of NDMA formation peaked at 7.9 ± 0.42% at 5 M.E. and declined at larger doses. Declining NDMA yields at larger dichloramine doses were initially attributed to breakpoint reactions; however, the time scale of breakpoint is relatively slow compared to that of the nucleophilic substitution reaction initiating NDMA formation, as discussed below. NDMA Formation from N,N-Dimethyl-α-arylamines. The molar yield of NDMA from ranitidine increased monotonically with respect to monochloramine dose, from 14 ± 0.4% at 4 M.E. (close to the previously reported

Figure 1. Chemical structures of the five molecules evaluated as NDMA precursors in this study.

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MATERIALS AND METHODS NDMA Yield Experiments. NDMA yield experiments were conducted in triplicate 25 mL amber borosilicate glass vials, capped with PTFE-faced septa. In each vial, 25 mL of phosphate buffer (10 mM, pH 7.0) was augmented with a precursor molecule (Figure 1), typically at a concentration of 100 μM, either monochloramine or dichloramine (0−2 mM), and held in the dark for 24 h before the reaction was quenched with excess ascorbic acid (1.2 × [NH2Cl]0 or 2.4 × [NHCl2]0). The oxidant dose ([oxidant]0/[precursor]0) was calculated on

Figure 2. (A) NDMA molar yield as a function of chloramine dose from dimethylamine. [DMA]0 = 100 μM, [PO4 buffer] = 10 mM, pH 7.0, 22 ± 1 °C, reaction time of 24 h. All plotted points are the means of three experimental replicates with 95% confidence interval error bars calculated on the basis of a Student’s t distribution (smaller than symbols when not shown). (B) Modeled NDMA formation from dimethylamine, using reactions and rate constants from refs 7 and 11. [DMA]0 = 100 μM, [PO4 buffer] = 10 mM, pH 7.0, 22 °C, reaction time of 24 h. 155

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Figure 3. NDMA molar yield as a function of chloramine dose from four model amines: (A) ranitidine, (B) 5-[(dimethylamino)methyl]furfuryl alcohol, (C) dimethylbenzylamine, and (D) 2-[(dimethylamino)methyl]aniline. [Amine]0 = 100 μM, [PO4 buffer] = 10 mM, pH 7.0, 22 ± 1 °C, reaction time of 24 h. All plotted points are the means of three experimental replicates with 95% confidence interval error bars calculated on the basis of a Student’s t distribution (smaller than symbols when not shown).

stoichiometric consumption of 4.4 M.E.)31 to 71 ± 2.5% at 20 M.E. (Figure 3A). In contrast, the molar yield of NDMA from ranitidine and dichloramine was 43 ± 0.3% at 1 M.E., reaching a peak value of 107 ± 3.3% at 5 M.E. of dichloramine (Figure 3A). A molar yield slightly in excess of 100% is likely due to low-yield NDMA formation from the two secondary amines in ranitidine (Figure 1) supplementing the near-quantitative yield from the N,N-dimethyl-α-arylamine functional group. The NDMA yield declined at larger dichloramine doses, likely due to oxidation of key intermediates to unknown side products, as discussed below. Because ranitidine has multiple functional groups capable of reacting with chloramines (i.e., tertiary amine, secondary amines, and thioether), DFUR was studied as a simplified analogue (Figure 1). DFUR, like ranitidine, formed NDMA at increasing molar yields with respect to monochloramine dose, from 19 ± 0.5% at 1 M.E. to 55 ± 0.6% at 20 M.E. (Figure 3B). Dichloramine formed NDMA from DFUR in a 45 ± 0.4% molar yield at 1 M.E.; the yield peaked at 82 ± 1% at 3 M.E. and declined slightly at larger doses. The peak yield of 82% agrees well with past work (84.6%).31 To verify that enhanced formation of NDMA from N,N-dimethyl-α-arylamines is not confined to the range of oxidant doses evaluated throughout the rest of the study, DFUR was treated with 0.01−1000 M.E. of chloramines in two separate experiments, as described in Text S3 and Figure S2 of the Supporting Information. NDMA formation from another commonly studied N,Ndimethyl-α-arylamine, dimethylbenzylamine, followed a similar pattern. The NDMA yield increased monotonically with respect to monochloramine dose, plateauing at ∼57% at 20 M.E. (Figure 3C). NDMA formation with respect to dichloramine dose reached a peak value of 83 ± 7.4% at 4 M.E. and declined slightly at larger doses. A fourth N,N-dimethyl-α-arylamine, DMAMA, was also evaluated. NDMA formation from monochloramine was undetectable below 10 M.E. and then slightly increased to 1.7 ± 0.03% at 20 M.E. (Figure 3D). Dichloramine produced a higher level of NDMA formation at smaller doses, peaking at 11 ± 0.17% at 5 M.E. and declining at larger doses. A lower yield

of NDMA from DMAMA than from other N,N-dimethyl-αarylamines can potentially be attributed to two effects. The lower pKa (4.87) of the anilinium ion (protonated aniline) than for a typical tertiary alkylammonium ion (e.g., 9.8 for trimethylammonium36 and 8.25 for protonated ranitidine20) means that the fraction of available reactive nitrogen at pH 7.0 is orders of magnitude higher on the aniline functional group than on the dimethylamine group. Therefore, most initial reactions between DMAMA and chloramines would be expected to occur on the aniline nitrogen, leading to nonNDMA-forming side reactions. At small doses (≤2 M.E. of chlorine), the aniline group could conceivably scavenge most chlorine. Higher chloramine doses may not produce high NDMA yields because the product of the aniline chlorination side reactions may not be as capable of stabilizing a positive charge as the aromatic rings on compounds such as DFUR and DMBzA are. Competition between NDMA-Forming and Side Reactions. Reaction kinetics of dimethylamine treated with a range of monochloramine or dichloramine doses were modeled, under the same conditions under which the experimental data in Figure 2A were produced, to evaluate whether breakpoint chlorination leading to a decline in dichloramine concentration could explain declining NDMA yields at large dichloramine doses. Modeled 24 h yields of NDMA generally agreed with experimental results (Figure 2B). The molar yield of NDMA from dichloramine peaked at 8.0% at 10 M.E. of dichloramine and declined slightly to 7.6% at 20 M.E., while the experimental NDMA yield peaked at 8.0% at 5 M.E. and declined to 3.9% at 20 M.E. (Figure 2A). Modeled NDMA formation from monochloramine increased monotonically to 2.6% at 20 M.E. compared to 1.7% experimentally. In the case of the largest dichloramine dose (2 mM), with which breakpoint reactions are expected to proceed most rapidly, the total chlorine residual declined by half to 1 mM in 8.9 h. However, by that time, the modeled concentration of dimethylamine had declined by 80% to 20 μM (Figure S3), suggesting that breakpoint chlorination was unlikely to be responsible for the declining yields at the large dichloramine 156

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dichloramine molecule formed. The chlorine-transfer hypothesis is consistent with our data showing NDMA formation from monochloramine at larger doses; at sufficient monochloramine doses, chlorine-transfer reactions could presumably provide enough dichloramine to lead to high NDMA yields. Chlorine-transfer reactions have previously been observed to lead to rapid, substantial dichloramine formation, even at neutral pH and relatively modest Cl/N ratios. Researchers have previously observed that in chlorine and ammonia solutions at pH 7, Cl/N ratios of >1.0 led to the formation of ∼20 μM dichloramine from ∼100 μM monochloramine within a few minutes.37 Dichloramine concentrations subsequently decline due to breakpoint; however, in the presence of a reactive nucleophile, rapidly generated dichloramine could be scavenged, preventing loss to breakpoint. Our modeling shows that under the conditions evaluated in Figure 2B, the formation of dichloramine from monochloramine increases with respect to monochloramine dose and throughout the 24 h reaction period (Figure S7). The production of dichloramine from applied monochloramine was observed for DMA, in which monochloramine dosing produces consistent NDMA formation in both model and experiment, even though the model includes no reaction between monochloramine itself and DMA leading to NDMA. The primary role of dichloramine in NDMA formation reactions may explain the wide range of molar yields of NDMA from ranitidine (42−97%) reported in the literature, because chlorine-transfer reactions leading to dichloramine are acidcatalyzed and are second-order with respect to monochloramine,37 so dichloramine concentrations would vary widely with respect to pH and monochloramine dose. After initial nucleophilic substitution between the dimethyl group and dichloramine to form a chlorinated hydrazine (rather than hydrazine) intermediate, subsequent steps may proceed along the mechanism proposed by Spahr et al.;31 the details of this mechanism are worthy of further investigation. Implications. Previous reports in the literature had led to an apparent contradiction with respect to the importance of dichloramine in NDMA formation reactions: practical observations showed that dichloramine minimization consistently reduces NDMA formation in real drinking water and recycled wastewater,8−11 but mechanistic work suggested that minimizing dichloramine would not reduce the level of formation of NDMA from N,N-dimethyl-α-arylamines,16,20,29 which are wellestablished as potent NDMA precursors. Resolving this conflict requires (1) that N,N-dimethyl-α-arylamines not be important precursors in practice and/or (2) that dichloramine react with N,N-dimethyl-α-arylamines to form NDMA. The results presented herein contradict previous assertions that monochloramine is chiefly responsible for NDMA formation from N,N-dimethyl-α-arylamine precursors, such as ranitidine, and hence support the primary role of dichloramine, although we cannot yet rule out a role for monochloramine with available evidence. While the importance of N,N-dimethyl-α-arylamines as NDMA precursors in real water sources remains an open question, and at the high chlorine/precursor molar ratios common to water treatment, complete dichloramine avoidance is unlikely, the results presented herein provide a firmer scientific underpinning to engineering practices for minimizing NDMA during water reuse. These results support dichloramine minimization, such as via preformed monochloramine addition when practical, or application of chlorine before ammonia in very well-nitrified wastewater, as a cheap and viable strategy for controlling nitrosamine formation.

dose, because substitution reactions proceed faster than loss of dichloramine to breakpoint. Furthermore, modeled concentrations of the key intermediate, N-chloro unsymmetrical dimethylhydrazine (UDMH-Cl), increased monotonically with respect to dichloramine (Figure S4), suggesting that declining NDMA yields cannot be attributed to low dichloramine availability. An alternative hypothesis to explain how elevated dichloramine concentrations lead to lower NDMA yields was evaluated computationally. We hypothesized that side reactions involving oxidation of the key UDMH-Cl intermediate by excess dichloramine shift the product speciation away from NDMA toward unknown products at large dichloramine doses. Modeled yields of these unknown products, “prod7” in the computational model (Table S2), increased as a function of dichloramine dose, surpassing NDMA yield at sufficiently large dichloramine doses (Figure S5). These results suggest that side reactions, while less important at small dichloramine doses, can consume sufficient UDMH-Cl to depress NDMA formation at large dichloramine doses. We postulate that similar side reactions can explain the declining NDMA yield at large dichloramine doses from N,N-dimethyl-α-arylamines, as well, and suggest that this mechanism be elucidated in future work. Chemical Interpretation. Previous researchers discounted the importance of dichloramine during their investigations of NDMA formation from N,N-dimethyl-α-arylamines. In one study,16 augmentation of a chloramine mixture with 100 mg/L ammonia was presumed to suppress dichloramine formation, and approximately equivalent yields of NDMA were formed from ranitidine upon treatment with either chloramine mixture. However, supplemental ammonia does not substantially alter chloramine speciation when ammonia is already in substantial excess11 because the back reaction converting ammonia and dichloramine to monochloramine is slow at neutral pH.37 Another study showed that the yield of NDMA from chloramine mixtures and ranitidine reached a peak value at pH 8, declining at higher or lower pH values,20 which was attributed to a maximum in the rate law product of monochloramine and protonated ranitidine (RAN-H+; pKa = 8.25) concentrations. We propose that the observed maximum yield at pH 8 can be explained by the increasing level of dichloramine formation and decreasing level of ranitidine protonation with respect to pH (Figure S6). Substitution reactions of the sort widely presumed to form key hydrazine intermediates from tertiary amines and chloramines29−31 are unlikely when the nucleophilic nitrogen is protonated,38 because there is no lone pair of electrons on the nitrogen when pH < pKa of the protonated amine. In the same study, yields of NDMA from ranitidine and preformed dichloramine were lower than from preformed monochloramine at pH 8;20 however, this could be attributable to side reactions between dichloramine and a UDMH-Cl-like intermediate, as explored above. A recent study aimed at determining the mechanism of NDMA formation from ranitidine and analogues reported the stoichiometry of monochloramine consumption to be 3.9−4.7 M.E. per dimethyl group.30 Superstoichiometric consumption of monochloramine was attributed to side reactions.30 Evidence presented herein points to dichloramine as the key chlorine species leading to NDMA formation and suggests that monochloramine consumption via chlorine transfer to form dichloramine could explain most of this overconsumption, as two molecules of monochloramine are consumed for every 157

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(13) Mitch, W. A.; Sedlak, D. L. Characterization and fate of Nnitrosodimethylamine precursors in municipal wastewater treatment plants. Environ. Sci. Technol. 2004, 38, 1445−1454. (14) Lee, C.; Schmidt, C.; Yoon, J.; von Gunten, U. Oxidation of Nnitrosodimethylamine (NDMA) precursors with ozone and chlorine dioxide: Kinetics and effect on NDMA formation potential. Environ. Sci. Technol. 2007, 41, 2056−2063. (15) Le Roux, J.; Gallard, H.; Croué, J.-P. Formation of NDMA and halogenated DBPs by chloramination of tertiary amines: The influence of bromide ion. Environ. Sci. Technol. 2012, 46, 1581−1589. (16) Selbes, M.; Kim, D.; Ates, N.; Karanfil, T. The roles of tertiary amine structure, background organic matter and chloramine species on NDMA formation. Water Res. 2013, 47, 945−953. (17) Selbes, M.; Kim, D.; Karanfil, T. The effect of pre-oxidation on NDMA formation and the influence of pH. Water Res. 2014, 66, 169− 179. (18) Shen, R.; Andrews, S. Demonstration of 20 pharmaceuticals and personal care products (PPCPs) as nitrosamine precursors during chloramine disinfection. Water Res. 2011, 45, 944−952. (19) Shen, R.; Andrews, S. NDMA formation kinetics from three pharmaceuticals in four water matrices. Water Res. 2011, 45, 5687− 5694. (20) Le Roux, J.; Gallard, H.; Croué, J.-P. Chloramination of nitrogenous contaminants (pharmaceuticals and pesticides): NDMA and halogenated DBPs formation. Water Res. 2011, 45, 3164−3174. (21) Shen, R.; Andrews, S. Formation of NDMA from ranitidine and Sumatriptan: The role of pH. Water Res. 2013, 47, 802−810. (22) Hanigan, D.; Thurman, E. M.; Ferrer, I.; Zhao, Y.; Andrews, S.; Zhang, J.; Herckes, P.; Westerhoff, P. Methadone contributes to Nnitrosodimethylamine formation in surface waters and wastewaters during chloramination. Environ. Sci. Technol. Lett. 2015, 2, 151−157. (23) Kemper, J. M.; Walse, S. S.; Mitch, W. A. Quaternary amines as nitrosamine precursors: A role for consumer products? Environ. Sci. Technol. 2010, 44, 1224−1231. (24) Krasner, S. W.; Mitch, W. A.; Westerhoff, P.; Dotson, A. Formation and control of emerging C-and N-DBPs in drinking water. J. Am. Water Works Assn. 2012, 104, E582−E595. (25) Park, S. H.; Padhye, L. P.; Wang, P.; Cho, M.; Kim, J.-H.; Huang, C.-H. N-nitrosodimethylamine (NDMA) formation potential of amine-based water treatment polymers: Effects of in situ chloramination, breakpoint chlorination, and pre-oxidation. J. Hazard. Mater. 2015, 282, 133−140. (26) Hanigan, D.; Zhang, J.; Herckes, P.; Zhu, E.; Krasner, S. W.; Westerhoff, P. Contribution and removal of watershed and cationic polymer N-nitrosodimethylamine precursors. J. - Am. Water Works Assoc. 2015, 107, E152−E163. (27) Zeng, T.; Li, R. J.; Mitch, W. A. Structural modifications to quaternary ammonium polymer coagulants to inhibit N-nitrosamine formation. Environ. Sci. Technol. 2016, 50, 4778−4787. (28) Schmidt, C. K.; Brauch, H.-J. N,N-dimethylsulfamide as precursor for N-nitrosodimethylamine (NDMA) formation upon ozonation and its fate during drinking water treatment. Environ. Sci. Technol. 2008, 42, 6340−6346. (29) Le Roux, J.; Gallard, H.; Croué, J.-P.; Papot, S.; Deborde, M. NDMA formation by chloramination of ranitidine: Kinetics and mechanism. Environ. Sci. Technol. 2012, 46, 11095−11103. (30) Spahr, S.; Bolotin, J.; Schleucher, J.; Ehlers, I.; von Gunten, U.; Hofstetter, T. B. Compound-specific carbon, nitrogen, and hydrogen isotope analysis of N -nitrosodimethylamine in aqueous solutions. Anal. Chem. 2015, 87, 2916−2924. (31) Spahr, S.; Cirpka, O. A.; von Gunten, U.; Hofstetter, T. B. Formation of N-nitrosodimethylamine during chloramination of secondary and tertiary amines: Role of molecular oxygen and radical intermediates. Environ. Sci. Technol. 2017, 51, 280−290. (32) Bond, T.; Simperler, A.; Graham, N.; Ling, L.; Gan, W.; Yang, X.; Templeton, M. Defining the molecular properties of N-nitrosodimethylamine (NDMA) precursors using computational chemistry. Environ. Sci.: Water Res. Technol. 2017, 3, 502−512.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.estlett.7b00572.

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Details of analytical methods (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (213) 740-0762. ORCID

Daniel L. McCurry: 0000-0002-5599-2540 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Stephanie Spahr of Stanford University (Stanford, CA) and Dr. John Sivey of Towson University (Towson, MD) for helpful discussions. M.E.H. was supported by a University of Southern California Provost’s Undergraduate Research Fellowship.

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