Environ. Sci. Technol. 2008, 42, 6340–6346
N,N-Dimethylsulfamide as Precursor for N-Nitrosodimethylamine (NDMA) Formation upon Ozonation and its Fate During Drinking Water Treatment C A R S T E N K . S C H M I D T * ,† A N D HEINZ-JÜRGEN BRAUCH Chemical Analysis Department, DVGW-Water Technology Center (TZW), Karlsruher Str. 84, 76139 Karlsruhe, Germany
Received December 6, 2007. Revised manuscript received February 3, 2008. Accepted February 21, 2008.
Application and microbial degradation of the fungicide tolylfluanide gives rise to a new decomposition product named N,N-dimethylsulfamide (DMS). In Germany, DMS was found in groundwaters and surface waters with typical concentrations in the range of 100-1000 ng/L and 50-90 ng/L, respectively. Laboratory-scale and field investigations concerning its fate during drinking water treatment showed that DMS cannot be removed via riverbank filtration, activated carbon filtration, flocculation, and oxidation or disinfection procedures based on hydrogen peroxide, potassium permanganate, chlorine dioxide, or UV irradiation. Even nanofiltration does not provide a sufficient removal efficiency. During ozonation about 30–50% of DMS are converted to the carcinogenic N-nitrosodimethylamine (NDMA). The NDMA being formed is biodegradable and can at least partially be removed by subsequent biologically active drinking water treatment steps including sand or activated carbon filtration. Disinfection with hypochlorous acid converts DMS to so far unknown degradation products but not to NDMA or 1,1-dimethylhydrazine (UDMH).
Introduction Nitrosamines are compounds bearing the general moiety NsNO as characteristic structural element (N-nitroso compounds). Although nitrosamines are not produced or commercially used nowadays (other than research purposes), they frequently occur in the environment as they arise unintentionally from a large array of processes (1). Nitrosamines were found in tobacco smoke, beer, smoked meat products, balloons, condoms, hand washing pastes and mascaras, among others (1). Their importance as a group of harmful substances is above all based on the pronounced carcinogenic effect of most of its representatives. The nitrosamine with the simplest structure and having been most intensively studied is N-nitrosodimethylamine (NDMA). NDMA belongs to the group of genotoxic carcinogens and is also considered a likely human carcinogen (1). The doses necessary for tumor induction are low, and thus on principle, the presence of NDMA in drinking water has to be limited to the lowest possible values for precautionary reasons. In * Corresponding author phone: +49 221 178 4714; fax: +49 221 178 2237;
[email protected]. † Current address: Water Laboratory, RheinEnergie AG, 50606 Cologne, Germany. 6340
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the United States, the Office of Environmental Health Hazard Assessment (OEHHA) released a public health goal (PHG) of 3 ng/L NDMA in 2006 (2). In Germany, the recommendation by the federal environmental agency gives an admissible health based precautionary value of 10 ng/L for lifelong oral NDMA exposure via drinking water. The nitrosamines that are formed unintentionally in various areas, due to their good water solubility, can reach drinking water resources via industrial and municipal waste waters (1). Moreover, investigations in Canada and the U.S. revealed that NDMA can also occur as a disinfection byproduct during water treatment with chloramines (3, 4). According to more detailed investigations, it became evident that the NDMA quantities formed during chloramination cannot be attributed solely to dimethylamine (DMA) but that tertiary amines with a dimethylamine structural element are of importance, too (5). In this article, for the first time, the possibility of NDMA formation during ozone treatment of surface waters, riverbank filtrates, and groundwaters is reported. Elevated NDMA formation potentials upon ozonation had been noticed, in particular, in German groundwater from areas with intensive agriculture and a cultivation of certain crops (strawberries, apples, and cherries). The goal of this study was to clarify the precursor substances responsible for NDMA formation, to determine their concentrations in the aquatic environment as well as to investigate their behavior during common treatment steps applied for drinking water production.
Experimental Section Reagents and Analytical Methods. Sources and purities of all chemicals used are given in the Supporting Information (SI). The analysis of NDMA was performed as described recently via a GC/MS method after solid-phase extraction (SPE) using NDMA-d6 as internal standard (6). This method allows detection of NDMA in water down to levels of 1 ng/L (limit of quantitation, LOQ). Concentrations of N,N-dimethylsulfamide (DMS) were determined by means of a HPLC/MS-MS method after SPE using 200 ng/L 1,1-dimethylurea-d6 as internal standard. SPE was performed following the procedure used for NDMA enrichment (6), with the exception that the sample was adjusted to pH 5 prior to SPE, that elution from the cartridges was achieved with a mixture of dichloromethane and methanol (4:1 v/v), and that the eluate was concentrated to complete dryness followed by redilution in 1 mL of a water/methanol mixture (8:2 v/v). The extracts were analyzed by injection of a 20 µL aliquot into a HPLC/MS-MS instrument (Agilent, 1100 Series, Applied Biosystems API-2000) equipped with a Waters Spherisorb ODS-2 analytical column (150 × 3.2 mm, 3 µm particle size). A multilinear binary gradient was formed from eluent A, water with 2 mM ammonium acetate, and eluent B, a water– methanol mixture (10:90 v/v) with 2 mM ammonium acetate (flow rate: 0.2 mL/min). Using electrospray ionization in the positive mode, quantification and confirmation was performed based on two multiple reaction monitoring (MRM) transitions of DMS (m/z 125), i.e., m/z 108 and m/z 44. 1,1Dimethylurea-d6 was detected via MRM transition m/z 95 f m/z 78. The method showed a linear relation between peak area and concentration (ratios) in the investigated concentration range of 0-900 ng/L. For calibration, samples running through the whole procedure including sample preparation were used. Detection sensitivity was determined according to the German standard protocol DIN 32645 from the linear regression analysis giving a LOQ of 10 ng/L DMS (7). Using spiked drinking water with a known DMS con10.1021/es7030467 CCC: $40.75
2008 American Chemical Society
Published on Web 04/18/2008
centration of 200 ng/L, coefficients of variation for withinday repeatability and between-day precision were in general less than 8%. Analytical methods for tolylfluanid, its metabolite DMST, daminozide, and 1,1-dimethylhydrazine (UDMH) are described in the SI. NDMA Formation Potential by Ozone (NDMA-FP-O3). To measure the NDMA formation potential in water samples upon ozonation, a standardized NDMA formation experiment was established. In this, the water sample (500 mL), i.e., a natural water sample or a sample of nonchlorinated drinking water spiked with 2 µg/L of a test substance, was amended with an aliquot of an ozone stock solution (∼160 mL) to reach an initial ozone concentration of 6 mg/L. Ozone was freshly produced from pure oxygen by using an ozone generator (Ozomat COM-AD-02, Anseros, Tübingen, Germany) and then passed through a bubble column filled with MilliQ water cooled at 5 °C. The concentration of the resulting ozone stock solution (∼25 mg/L) was determined directly before use via a standardized indigo method (8). The dissolved organic carbon (DOC) content of the drinking water was 0.6 mg C/L, the alkalinity 260 mg CaCO3/L, and the pH 7.4. The bottles were kept at room temperature (21 °C), and the reaction was stopped by adding sodium sulfite (0.04 M) after 30 min (no reduction or reverse reaction of NDMA was observed upon sulfite addition). The test solutions were analyzed for NDMA, and concentrations were corrected for the respective dilution caused by the addition of the ozone stock solution. NDMA was also measured in nonozonated background controls prepared in parallel, but never detected. In the case of drinking water samples spiked with a test substance, also nonspiked drinking water samples were ozonated and analyzed for NDMA. Typically, these nonspiked samples showed low NDMA levels (11–14 ng/L) due to some background contamination of the used drinking water with DMS (∼50 ng/L). For calculation of the NDMA-FP-O3 (given in ng/L NDMA), found NDMA levels in the spiked samples upon ozonation were corrected for these background NDMA levels. Molar conversion percentages were calculated based on the molar masses of the test substance and of NDMA. Biodegradation Testing Unit. Biodegradation of the known tolylfluanid metabolite N,N-dimethyl-N’-(4-methylphenyl)-sulfamide (DMST) was studied using a fixedbed bioreactor established for simulation of aerobic microbial degradation processes (9, 10). This bioreactor consisted of a filter column (diameter: 5 cm; height: 23 cm) with an apertured glass bottom filled with porous sintered glass beeds (SIRAN-Carrier no. 023/02/300, Schott Engineering GmbH, Mainz, Germany) as carrier material, a 15 L reservoir bottle with air supply and sampling device, and a circulation system with a controllable diaphragm pump (CFG Prominent, Heidelberg, Germany). The porous carrier material provides optimal conditions for microbial colonization. The test was executed by filling the reservoir with 10 L of nonfiltered surface water (Rhine river at Karlsruhe) and spiking it with DMST at a concentration of 2 µg/L. The water was pumped under recirculation at 20 °C and in the dark over the carrier material in the filter column. The flow rate was adjusted to 10 mL/min. The system was permanently aerated with ambient air to ensure aerobic conditions. During the test period of 35 days, samples of 500 mL were taken after defined periods of time for analytical determination of DMST concentrations and the NDMA formation potential upon ozonation. From the time-dependence of the concentration levels, the microbial degradability of DMST was assessed. For the purpose of comparison, a second test filter using nonspiked surface water was run in parallel (sampling at day 0 and day 35). Monitoring. To assess the pollution of the aquatic environment in Germany a number of grab samples taken in the period from October 2006 to September 2007 were
analyzed for DMS. The monitoring survey comprised 557 groundwater samples from 452 different sampling points, 252 surface water samples from 60 rivers and lakes (159 sampling points) and 224 drinking water samples from 180 sampling points. Riverbank Filtration. To investigate the fate of DMS during the riverbank filtration (RBF) step of waterworks and to evaluate its microbial degradability, repeated sampling (n ) 16) was carried out at a well characterized RBF site at the lower Rhine river over a time period of about one year (08/2006–07/2007). Samples were taken from the river and an observation well being located between the bank line and the production well gallery and providing 100% bank-filtered water without any admixture of land-sided groundwater. The residence time of the infiltrated water in the gravely sandy aquifer to reach the observation well is between 7 and 20 days. The redox milieu of the transect is characterized by aerobic conditions. Flocculation Jar Test. The removability of DMS by flocculation was investigated in a noncontinuous laboratoryscale test, the so-called jar test (11). Polyaluminum chloride (pAlCl3) and iron chloride (FeCl3) were tested as flocculants and were added at doses of 5 and 10 mg/L Al or Fe to drinking water samples spiked with 20 µg/L (also see SI). Activated Carbon. DMS was investigated in a small-scale filter column test established for classification of the removability of organic micropollutants by activated carbon based on breakthrough curves (12). For investigation of the adsorbability of DMS, drinking water spiked with 25 µg/L DMS and stabilized with 50 mg/L sodium azide (for suppression of microbial degradation processes) was pumped over a bed with a customary carbon type often applied in waterworks (Filtrasorb 300, Chemviron Carbon, Belgium) (also see SI). Membrane Testing Unit. The behavior of DMS during nanofiltration (NF) was investigated using a commercially available tight NF membrane (NF-90, Dow-FilmTec, Minneapolis, MN, molecular weight cut off: 200 Da, isoelectric point at pH 4) and an established laboratory-scale nanofiltration testing unit (13, 14). A solution of drinking water diluted 1:1 with MilliQ water (pH 7.3) and spiked with 20 µg/L DMS was filtered over the membrane at a permeate flux of 70 L m-2 h-1. Both permeate and concentrate were recirculated back to the feed reservoir (see SI for further details). Oxidation and Disinfection Procedures. The behavior of DMS during oxidation and disinfection procedures frequently used in the process of drinking water treatment was determined in laboratory trials using drinking water spiked with 20 µg/L DMS (also see SI). In each case, the investigations were carried out at two pH values (pH 7 and pH 8) and at two different doses of the respective oxidant or disinfectant. The doses applied were 0.5 and 5 mg/L for ozone, 0.2 and 1.2 mg/L of free chlorine for hypochlorous acid (HOCl), 0.1 and 0.4 mg/L for chlorine dioxide (ClO2), 2 and 17 mg/L for hydrogen peroxide (H2O2), 5 and 10 mg/L for potassium permanganate (KMnO4), and 990 and 30 000 J/m2 for UV irradiation at 254 nm. Chosen contact times were practice-oriented with 2 h for ozone, KMnO4, and H2O2 and 96 h for HOCl and ClO2. The time-dependency of DMS transformation by HOCl was studied over a time-period of 30 h using unbuffered drinking water (spiked with 17 µg/L DMS) and a dose of 0.6 mg/L Cl2. In order to check whether UDMH is formed from DMS upon HOCl addition, further drinking water samples (spiked with 20 µg/L DMS) were dosed with 1.2 mg/L Cl2. These experiments were conducted at different pH values and contact times (pH 7 and 8 with contact times of 2, 6, or 24 h; unbuffered drinking water with a contact time of 24 h). Investigations at Selected Waterworks. In order to evaluate the fate of DMS in water utilities utilizing ozonation VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Structural formulas of N-nitrosodimethylamine (NDMA), daminozide, UDMH, tolylfluanid, DMST, N,N-dimethylsulfamide (DMS), and further important substances as well as their corresponding molar conversions into NDMA upon ozonation in percent (NDMA-FP-O3, 6 mg/L ozone, 30 min contact time, matrix drinking water). for water treatment, raw, partially treated, and finished waters of three waterworks were sampled and analyzed for both NDMA and DMS concentrations. These waterworks differ in particular in the type of the raw water used, i.e. surface water, riverbank filtrate, and groundwater, and the treatment steps established (also see SI).
Results and Discussion Identification of DMS as Precursor of NDMA upon Ozonation. For precursor identification, drinking water samples were spiked with various compounds being potentially relevant and were investigated with regard to their NDMAFP-O3. Besides dimethylamine, which was added to the water samples alone as well as in combination with various inorganic nitrogen compounds (nitrate, nitrite, ammonium, and hydrazine), about 30 different organic nitrogen compounds with possibly relevant structural elements were tested (see SI). For each of the compounds that appeared to be relevant in these tests, an analytical method was developed in order to compare the NDMA formation potential observed during ozonation with the corresponding precursor substance concentrations in the water body. The investigations showed that, in contrast to chloramination, the number of precursor substances that are responsible for NDMA formation upon ozonation is very limited. No NDMA formation was observed upon ozonation of dimethylamine alone or in combination with inorganic nitrogen compounds. Since elevated NDMA formation potentials in the range of 340– 1600 ng/L NDMA were noticed particularly in groundwaters from areas with intensive agriculture (hot spots), various plant protection products were tested that contain terminal or heterocyclic amine or N-N-moieties. Upon ozonation, only two active substances out of the larger number of examined plant protection products yielded NDMA: the plant growth regulator daminozide (Alar, Dazide 85) and the fungicide tolylfluanide (Euparen M WG, Baymat WG, Bayer Garten Universal-Pilzfrei, Melody Multi, Monceren 6342
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Plus, Folicur EM) (Figure 1). In particular ozonation of daminozide and its hydrolysis product 1,1-dimethylhydrazine (UDMH) resulted in high NDMA contents of the spiked waters, with molar conversion percentages of 55 and 80%, respectively. A comparison of the structures of daminozide, UDMH and NDMA explains the high formation potential from these precursor substances (Figure 1). Especially UDMH can be directly converted into NDMA by simple oxidation. However, neither daminozide (limit of detection, LOD: 100 ng/L) nor UDMH (LOD: 1.5 ng/L) could be detected analytically in the groundwaters from the identified hot spots. In addition, there were no indications that during the last years daminozide had been applied to the locations concerned. Furthermore, daminozide is not registered as a plant protection product in Germany any longer, so that it can very likely be excluded as cause of NDMA formation during ozonation of the waters from the hot spots. Tolylfluanid, however, was regularly applied in the area of the identified hot spots. It is a broad-spectrum fungicide, which is applied to control mildew, gray mold, scrab and many other fungal diseases in vegetable crops, fruit, grape cultures, hops, and ornamental plants (15). The active substance enjoys great popularity, since noxious organisms cannot develop resistance due to its mechanism of multisite effectiveness. In Germany, tolylfluanid sales in the year 2006 were in the range of 100–250 tons (16). The fungicidal effect of the molecule originates from the dichlorofluoromethylthiol-moiety. Tolylfluanid is degraded in soil and the aquatic environment via initial cleavage of the active group, to from N,N-dimethyl-N’-(4-methylphenyl)-sulfamide (DMST) (15). Also the metabolite DMST is not very stable in the environment, its half-life (Dt50-value) in soils ranges from 1.3-6.7 days (15). Traces of tolylfluanid and DMST with levels groundwater), presumably also accompanied by higher assimable organic carbon (AOC) contents, and longer running times of the sand or activated carbon filters seem to improve the removal of NDMA in these steps. The residual DMS not being converted in the ozonation step cannot be removed by sand or activated carbon filtration but is converted during disinfection with chlorine.
Acknowledgments We thank William Arnold, Christine Baus, Egon Denecke, Ralph Fliege, Sonja Fliege, Silke Glökler, Britta Hackhofer, Katja Hauer, Bernd Kirchner, Eva Klamroth, Norbert Konradt, Pia Lipp, Thomas Lustinetz, Patrick Marcus, Peter Ohs, Bernhard Post, Muriel Sona, and Urs von Gunten for their technical assistance and/or many helpful discussions. This work was funded by a joint project of the Association of Rhine Waterworks (ARW) and the German Chemical Industry Association (VCI).
Supporting Information Available Chemical sources and purities; analytical methods for daminozide, UDMH, DMST, and tolylfluanid; overview of the compounds investigated with the NDMA-FP-O3 test; VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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experimental details: activated carbon small-scale filter column test, flocculation Jar test, membrane testing unit, oxidation and disinfection procedures; details of the treatment processes at the investigated waterworks; behavior of DMS during nanofiltration; time-dependency of DMS transformation by HOCl. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) WHO. N-Nitrosodimethylyamine Concise International Chemical Assessment Document 38; World Health Organization: Geneva, 2002. (2) OEHHA. Public Health Goal for N-Nitrosodimethylamine in Drinking Water; Office of Environmental Health Hazard Assessment: CA, 2006. (3) Mitch, W. A.; Sharp, J. O.; Trussell, R. R.; Valentine, R. L.; AlvarezCohen, L.; Sedlak, D. L. N-Nitrosodimethylamine (NDMA) as a drinking water contaminant: A review. Environ. Eng. Sci. 2003, 20, 389–404. (4) Choi, J.; Valentine, R. L. Formation of N-nitrosodimethylamine (NDMA) from reaction of monochloramine: A new disinfection by-product. Water Res. 2002, 36, 817–824. (5) Gerecke, A. C.; Sedlak, D. L. Precursors of N-nitrosodimethylamine in natural waters. Environ. Sci. Technol. 2003, 37, 1331– 1336. (6) Lee, C.; Schmidt, C.; Yoon, J.; von Gunten, U. Oxidation of N-nitrosodimethylamine (NDMA) precursors with ozone and chlorine dioxide: kinetics and effect on NDMA formation potential. Environ. Sci. Technol. 2007, 41, 2056–2063. (7) DIN 32645. Decision Limit, Detection Limit, And Determination Limit: Estimation in Case of Repeatability; Terms, Methods, Evaluation. Issue 1994–05; German Institute for Standardization (DIN): Berlin, 1994. (8) DIN 38408–3. Gaseous Components (Group G): Determination of Ozone (G 3). Issue 1993–11; German Institute for Standardization (DIN): Berlin, 1993. (9) Karrenbrock, F.; Knepper, T. P.; Sacher, F.; Lindner, K. Development of a standardized testfilter for the determination of microbial degradability of single compounds. Vom Wasser 1999, 92, 361–371. (10) Knepper, T. P.; Sacher, F.; Lange, F. T.; Brauch, H.-J.; Karrenbrock, F.; Roerden, O.; Lindner, K. Detection of polar organic substances relevant for drinking water. Waste Manage. 1999, 19, 77–99. (11) DVGW. Technical Rules, Working Sheet W218: flocculation during Water Treatment; Part 2: Flocculation Procedures; German Technical and Scientific Association for Gas and Water (DVGW): Bonn, 1998.
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(12) Marcus, P. Development and Validation of a Rapid Laboratory Test for Assessment of the Adsorbability of Organic Single Substances onto Activated Carbon. Thesis, Technical University Dresden, 2005. (13) Lipp, P. Investigating Novel UF and MF-Membranes for Drinking Water Production from Surface Water, Final report of BMBF project 02WT0027; BMBF: Bonn, 2005. (14) Lipp, P. Development and Testing of a Membrane Testing Unit for Preselection of Membranes for Their Application in Drinking Water Treatment. Final Report; German Technical and Scientific Association for Gas and Water (DVGW): Bonn, 2001. (15) EFSA. Conclusion on the peer review of tolylfluanid EFSA Scientific Report 2005, 29, 1–76. (16) BVL. Report s Absatz an Pflanzenschutzmitteln in der Bundesrepublik Deutschland Ergebnisse der Meldungengemäß § 19 Pflanzenschutzgesetz für das Jahr 2006; Federal Office of Consumer Protection and Food Safety (BVL): Braunschweig, 2007. (17) Arnold, W. A.; Schmidt, C. K.; von Gunten, U. Kinetics and mechanisms of N-nitrosodimethylamine formation upon ozonation of N,N-dimethylsulfamide Environ. Sci. Technol. 2007, In review. (18) Haist-Gulde, B.; Baldauf, G. Removal of pesticides by powdered activated carbon s Practical aspects. Water Supply 1996, 14, 201–208. (19) Haist-Gulde, B.; Baldauf, G.; Brauch, H.-J. Removal of pesticides from raw waters. Water Supply 1993, 11, 187–196. (20) Seitz, W.; Jiang, J. Q.; Weber, W. H.; Lloyd, B. J.; Maier, M.; Maier, D. Removal of iodinated X-ray contrast media during drinking water treatment. Environ. Chem. 2006, 3, 35–39. (21) Baus, C.; Hung, H.-W.; Sacher, F.; Fleig, M.; Brauch, H.-J. MTBE in drinking water production—occurrence and efficiency of treatment technologies. Acta Hydrochim. Hydrobiol. 2005, 33, 118–132. (22) Kimura, K.; Amy, G.; Drewes, J. E.; Heberer, T.; Kim, T.-U.; Watanabe, Y. Rejection of organic micropollutants (disinfection by-products, endocrine disrupting compounds, and pharmaceutically active compounds) by NF/RO membranes. J. Membr. Sci. 2003, 227, 113–121. (23) UBA. List of treatment substances and disinfection processes as per § 11 of German Drinking Water Ordinance; Federal Environmental Agency (UBA):Berlin, 2007. (24) Ohme, R.; Preuschhof, H. Hydrazines and azo compounds from diamides of sulfuric acid. Liebigs Ann. Chem. 1968, 713, 74–86. (25) Sharp, J. O.; Wood, T. K.; Alvarez-Cohen, L. Aerobic biodegradation of N-nitrosodimethylamine (NDMA) by axenic bacterial strains. Biotechnol. Bioeng. 2005, 89, 608–618.
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