Role of Chlorine Dioxide in N‑Nitrosodimethylamine Formation
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Article pubs.acs.org/est Role of Chlorine Dioxide in N‑Nitrosodimethylamine Formation from Oxidation of Model Amines † ‡ † § Wenhui Gan, Tom Bond, Xin Yang,*, and Paul Westerhoff † SYSU-HKUST Research Center for Innovative Environmental Technology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡ Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, United Kingdom § School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona 85287-3005, United States *S Supporting Information ABSTRACT: N-Nitrosodimethylamine (NDMA) is an emerging disinfection byproduct, and we show that use of chlorine dioxide (ClO2) has the potential to increase NDMA formation in waters containing precursors with hydrazine moieties. NDMA formation was measured after oxidation of 13 amines by monochloramine and ClO2 and pretreatment with ClO2 followed by postmonochloramination. Daminozide, a plant growth regulator, was found to yield 5.01 ± 0.96% NDMA upon reaction with ClO2, although no NDMA was recorded during chloramination. The reaction rate was estimated to be ∼0.0085 s−1, and on the basis of our identification by mass spectrometry of the intermediates, the reaction likely proceeds via the hydrolytic release of unsymmetrical dimethylhydrazine (UDMH), with the hydrazine structure a key intermediate in NDMA formation. The presence fi − of UDMH was con rmed by gas chromatography mass spectrometry analysis. For 10 of the 13 compounds, ClO2 preoxidation reduced NDMA yields compared with monochloramination alone, which is explained by our measured release of dimethylamine. This work shows potential preoxidation strategies to control NDMA formation may not impact all organic precursors uniformly, so differences might be source specific depending upon the occurrence of different precursors in source waters. For example, daminozide is a plant regulator, so drinking water that is heavily influenced by upstream agricultural runoff could be at risk. ■ INTRODUCTION such as methadone and ranitidine were over 50%.9,10 Various mechanisms, including nucleophilic substitution11 and chlorine N-Nitrosodimethylamine (NDMA) is acutely carcinogenic, 8 with a concentration of only 0.7 ng·L−1 associated with a transfer followed by DMA release, have been proposed to 10−6 lifetime cancer risk level.1 California has issued a explain NDMA formation from tertiary amines. It has also been notification level of 10 ng·L−1 for NDMA in drinking waters, highlighted that both monochloramine and dichloramine may play important roles in NDMA formation, for example, from and the United States Environmental Protection Agency 9 (USEPA) included it in Contaminant Candidate List 3, for ranitidine and N,N-dimethylisopropylamine, respectively. 2,3 Besides chloramines, oxidation of water samples by chlorine pollutants which may be considered for future regulation. 12,13 dioxide (ClO2) can also form NDMA. A small number of Concern about NDMA has been heightened by the increasing fi prevalence of polluted sources being used to produce drinking amines have been identi ed which can generate low yields of water. For example, waters affected by wastewater effluents NDMA during oxidation by ClO2, for example, ranitidine and frequently produce higher NDMA concentrations.4 dimethylaniline (NDMA yields of 0.055% and 0.016%, mol/ mol, respectively).14 DMA does not have a high NDMA yield To minimize risks associated with NDMA, it is imperative we 15 improve our knowledge of NDMA precursors and the pathways from reaction with ClO2. ClO2 is a selective oxidant due to its ability to abstract one electron.16 This means mechanisms by which they generate NDMA. Previous work suggested that ff dimethylamine (DMA) and monochloramine reacted to form which generate NDMA are likely to be di erent from application of chloramination. ClO2 is applied for control of unsymmetrical dimethylhydrazine (UDMH), which could then 17 be oxidized to NDMA.5 Subsequently, chlorinated unsym- color, taste, and odor, and it is used as either a primary or a metrical dimethylhydrazine (Cl-UDMH) was suggested as an secondary disinfectant, in the former case typically followed by important intermediate after nucleophilic attack of DMA on a secondary disinfectant, such as chlorine and chloramine, to dichloramine.6 However, molar yields of NDMA from chloramination of DMA are low (∼1−2%7,8), it has been Received: April 6, 2015 demonstrated that some tertiary amines are far more potent Revised: August 31, 2015 precursors (i.e., higher molar yields upon chloramination). Accepted: September 3, 2015 NDMA yields from chloramination of specific tertiary amines Published: September 3, 2015 © 2015 American Chemical Society 11429 DOI: 10.1021/acs.est.5b01729 Environ. Sci. Technol. 2015, 49, 11429−11437 Environmental Science & Technology Article Figure 1. Structures of the model compounds. provide a distribution system residual.18 As a secondary dose, contact time, and pH were also examined for selected disinfectant, formation of NDMA has been observed in surface precursors. Finally, the influence of a reactive precursor on 12 water after ClO2 treatment. As a primary disinfectant, ClO2 NDMA-FP in authentic wastewater samples was estimated. pretreatment followed by chloramination has exhibited inconsistent results for NDMA formation. Lee et al. reported ■ MATERIALS AND METHODS that ClO pretreatment of surface waters dramatically decreased 2 Regents. NDMA formation,15 whereas Shah et al. found that ClO did Thirteen model compounds which possess a 2 variety of chemical functionalities were investigated (Figure 1). not effectively reduce NDMA formation, and even increased Ranitidine was purchased from Alfa Aesar (United States), with NDMA in certain waters impacted by wastewater.13 This the others obtained from J&K (China). Stock solutions (1 indicates that preoxidation with ClO can either deactivate 2 mM) of each model compound were prepared in either specific precursors or convert them into more potent forms, ultrapure water or acetonitrile. UDMH was obtained from depending on their identity. Furthermore, Selbes and co- Sigma (United States). Mixed nitrosamine calibration standards workers reported that NDMA yields of model tertiary amines were purchased from Supelco (United States), and d6-NDMA after preoxidation with ClO2 became similar to that from DMA, was obtained from Cambridge Isotope Laboratories (United i.e., that yields decreased for precursors more reactive than 19 States). Acetonitrile of HPLC grade was purchased from Merck DMA and increased for the less potent precursors. This (Germany). Dichloromethane was obtained from J&K. Resprep suggests NDMA formation proceeded through the intermediate EPA method 521 solid-phase extraction (SPE) cartridges (6 release of DMA. Nevertheless, the underlying oxidation fl mL/2 g) from Restek (United States) were used to mechanism responsible for the con icting observations preconcentrate NDMA before analysis. fl regarding the in uence of ClO2 pretreatment on NDMA Monochloramine stock solutions were prepared daily by formation remains unclear. mixing equal volumes of diluted sodium hypochlorite (NaOCl) Considering industrial and agricultural chemicals represent (Sigma) and ammonium chloride solution at a molar ratio of an important pool of NDMA precursors in surface waters, the 0.8:1 at pH 8. As with previous studies on oxidation by ClO2, aim of the present study was to assess the effect of ClO on 2 the ClO2 solution was prepared daily from gaseous ClO2 by NDMA formation through testing the NDMA formation slowly adding dilute H2SO4 to a sodium chlorite (NaClO2) potential (NDMA-FP) from amine precursors, with and solution using a set of glass gas diffusion bottle reactors, without postoxidation by chloramines. Model compounds according to the standard method.22 Consecutively, the were chosen to cover a variety of structures, including resulting gas was collected and pumped through saturated pharmaceuticals, personal care products (PPCPs), agricultural NaOCl2 solution and then collected into ice-cold ultrapure products, and industrial contaminants with DMA functional water to produce a ClO2 solution. The concentrations of ClO2 groups. Model compounds with hydrazine moieties were also and monochloramine were determined by the N,N-diethyl-p- chosen as they had structures similar to that of NDMA. The phenylenediamine (DPD)−ferrous ammonium sulfate (FAS) occurrence of these compounds in surface water has been titration method.18,22 shown to be in the range of several nanograms per liter to tens Experimental Procedures. All experiments were under- of micrograms per liter.20,21 For comparison, NDMA-FP with taken in duplicate, and the pH levels were controlled using ff chloramination was also determined. The impact of the ClO2 phosphate bu er. When ClO2 was used as the sole oxidant, 11430 DOI: 10.1021/acs.est.5b01729 Environ. Sci. Technol. 2015, 49, 11429−11437 Environmental Science & Technology Article a Table 1. Molar Yields of NDMA from Model Precursors in This Study and the Literature NDMA yield (%, mol/mol) this study − precursor ClO2 NH2Cl ClO2 NH2Cl previous studies ref DMA ND 0.71 ± 0.08 0.87 ± 0.06 0.24b 5 1.2b 48 3.0b 15 TMA 0.11 ± 0.01 0.59 ± 0.01 0.21 ± 0.03 1.2b 15 1.9b 34 DMBzA 0.10 ± 0.02 83.47 ± 0.34 27.45 ± 0.47 83.8b 34 DMDC 0.48 ± 0.01 2.15 ± 0.03 0.60 ± 0.04 2b 15 DMPD 0.13 ± 0.01 0.34 ± 0.02 0.67 ± 0.02 0.04c 49 DMS ND 1.08 ± 0.10 0.40 ± 0.05 52c 49 DMAI ND 5.61 ± 0.13 5.02 ± 0.04 6b 15 DMAP ND 11.96 ± 0.21 0.71 ± 0.01 4.2b 15 BACd ND 0.27 ± 0.01 0.20 ± 0.01 0.26b 35 ranitidine ND 65.66 ± 1.72 21.50 ± 0.83 40.2b 7 62.9b 36 82.7b 31 tetracycline 0.31 ± 0.01 0.54 ± 0.04 0.28 ± 0.03 1.2b 9 polyDADMACd ND 0.10 ± 0.01 0.04 ± 0.01 0.19b 35 daminozide 5.01 ± 0.96 ND 5.04 ± 0.21 55c 27 3.61 ± 0.10d 3.50 ± 0.01d UDMH 3.42 (± 0.04)d 0.47 ± 0.04d 2.44 ± 0.05d 0.51b 50 a ± μ The values show the average standard deviation for each yield.