Environ. Sci. Technol. 2007, 41, 6170-6176
Photochemical Attenuation of N-Nitrosodimethylamine (NDMA) and other Nitrosamines in Surface Water MEGAN H. PLUMLEE AND MARTIN REINHARD* Department of Civil and Environmental Engineering, Terman Engineering Center, Stanford University, Stanford, California 94305-4020
The aqueous photolysis of seven alkyl nitrosamines was studied by irradiation in a solar simulator. Nitrosamines included N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosodi-n-butylamine (NDBA), N-nitrosopiperidine (NPip), and N-nitrosopyrrolidine (NPyr). Direct photolysis at irradiations of 765 W/m2, representing Southern California midsummer, midday sun, resulted in half-lives of 16 min for NDMA and 12-15 min for the other nitrosamines. The quantum yield for NDMA was determined to be Φ ) 0.41 and Φ ) 0.43-0.61 for the other nitrosamines. Quantified products of NDMA photolysis included methylamine, dimethylamine, nitrite, nitrate, and formate, with nitrogen and carbon balances exceeding 98 and 79%, respectively. Indirect photolysis of nitrosamines in surface water was not observed; increasing dissolved organic carbon (DOC) slowed the NDMA photolysis rate because of light screening. Removal of NDMA measured in tertiary treated effluent flowing in a shallow, sunlit engineered channel agreed with photolysis rates predicted based on the measured quantum yield and system parameters. Because biodegradation is relatively slow, aquatic photolysis of NDMA is generally expected to be more significant even at relatively low levels of solar irradiation (t1/2 ) 8-38 h at 244-855 W/m2, 51° N latitude, 1 m depth).
Introduction Given the high demand for freshwater, rivers and reservoirs that receive wastewater effluent are also often relied upon as a freshwater resource. The effluent is likely to contain trace organic contaminants, some of which may be carcinogens, toxins, or endocrine disruptors. N-nitrosodimethylamine (NDMA) is a dialkyl-N-nitrosamine frequently detected in municipal wastewater effluent and reclaimed wastewater at concentrations of 20-100 ng/L (1, 2), as well as in drinking water at lower concentrations. It is formed as a disinfection byproduct of chlorination and chloramination (1) and has been classified as a probable human carcinogen by the U.S. Environmental Protection Agency (3). While the potential for ecotoxicological impact has not been thoroughly investigated at environmentally relevant concentrations, in vitro bacterial and mammalian cell studies have shown NDMA to * Corresponding author phone: (650) 723-0308; fax: (650) 7237058; e-mail:
[email protected]. 6170
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be a mutagen and in vivo studies have also shown genetic effects (4). In addition to NDMA, other nitrosamines have recently been detected in effluent-impacted rivers (5) and in a drinking water distribution system (6). Nitrosamines resist acidic and basic hydrolysis (7, 8) and field and microcosm studies of NDMA have established that it does not readily sorb to sediments and is slow to biodegrade in soils, sediments, and surface water (9-13). Nitrosamines are photosensitive, however, and currently UV irradiation is the most commonly applied treatment method for NDMA removal in wastewater and drinking water (1). The majority of NDMA photolysis studies employ UV irradiation (7, 1418), and quantum yields of Φ ) 0.28-31 have been reported for neutral pH (15-17). Information regarding kinetics, quantum yields, and products for NDMA photolysis under environmental conditions is lacking. In addition to a UV absorption band at λ ) 230 nm (corresponding to a π f π* transition), NDMA can absorb in the range of natural sunlight due to a weaker absorption band at λ ) 330 nm (n f π* transition) (14, 19). In general, the photochemistry of an n f π* transition state differs from a π f π* excitation and can lead to a distinct quantum yield and products (20). To predict the photochemical fate of NDMA, it is important to know whether the photolysis of NDMA in the natural environment is as effective without UV irradiation or compared to other attenuation mechanisms. If the photochemistry of NDMA in the natural environment is like that observed with UV light, the photodegradation will give a known suite of products including dimethylamine, methylamine, nitrite, nitrate, formaldehyde, and formate (14). Quantum yields for other dialkyl nitrosamines such as N-nitrosodiethylamine (NDEA), N-nitrosodin-butylamine (NDBA), and N-nitrosopiperidine (NPip) have been determined for the vapor phase or in an organic solvent (7), but they have not been determined in water for either UV or natural irradiation. To predict the rate of aquatic photolysis in the environment, the rate of light absorption by a compound of interest can be calculated from absorbance and irradiance spectra and converted into transformation rates by multiplying by the quantum yield (21). Predicted rates are sensitive to factors such as latitude, season, depth of water, and concentration of chromophoric dissolved organic matter (cDOM), and they can be modeled using these inputs (21, 22). In addition, indirect mechanisms of photolysis are often important in natural systems (21, 23, 24) because of the presence of photosensitizers and quenchers, but this has not yet been investigated for nitrosamines. The objectives of this study were to determine rates of direct and indirect photolysis and quantum yields for NDMA and six other nitrosamines under conditions of simulated natural sunlight. Structures and occurrence information for the selected nitrosamines, most of which are listed on the EPA’s Unregulated Contaminant Monitoring Rule 2 (UCMR 2) (25), are given in Table S1 in the Supporting Information (SI). Given its recent attention as a drinking and wastewater contaminant, NDMA was selected for additional study. Photodegradation products reported for UV irradiation were identified for simulated sunlight. From the measured quantum yield, expected aquatic photolysis rates for NDMA were calculated for a range of representative environmental conditions and for a 1 m deep, sunlit channel fed with tertiary treated reclaimed wastewater by the Orange County Water District (OCWD). 10.1021/es070818l CCC: $37.00
2007 American Chemical Society Published on Web 08/02/2007
Experimental Section Materials and Methods. Information regarding chemicals and materials and a more detailed description of the analytical methods can be found in the SI. Briefly, NDMA and the other nitrosamines were analyzed by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) using a C18 column with methanol and 2 mM ammonium acetate as the mobile phase, according to the method described by Plumlee et al. (2). 50 µL samples were directly injected without preconcentration, with instrument detection limits of 2 µg/L for NDMA and 2-25 µg/L for the other nitrosamines. The method was adapted to include dimethylamine and methylamine. 4-nitroanisole (PNA) was analyzed using HPLC based on the method described by Dulin and Mill (26). Nitrate, nitrite, and formate were measured using ion chromatography. Photolysis Apparatus and Procedure. Solutions of nitrosamines were irradiated using an Atlas Suntest CPS+ photosimulator (Chicago, IL) equipped with a 1.1 kW xenon arc lamp according to the method described by Lin and Reinhard (27). The lamp was fitted with special glass filters to block the transmission of wavelengths below 290 nm to simulate natural sunlight (passing wavelength, 290 nm < λ < ∼800 nm). Nitrosamines included NDMA, N-nitrosomethylethylamine (NMEA), NDEA, N-nitrosodi-n-propylamine (NDPA), NDBA, NPip, and N-nitrosopyrrolidine (NPyr). The factory specified photosimulator intensity was verified by comparing the measured photolysis rate of NDMA to the rate predicted given the quantum yield. Sample solutions were kept in capped quartz tubes (Quartz Scientific, Fairport, OH) and placed horizontally in a constant temperature (20.0 ( 1.7 °C) water bath 25 cm directly below the photosimulator lamp, and an additional tube covered in aluminum foil was placed in the photosimulator to serve as a control. Each sample tube (n ) 2-4) represented a replicate photolysis experiment. For direct and indirect photolysis experiments, initial concentrations of nitrosamines were spiked to 100 µg/L (1.3 µM for NDMA, and 0.6-1.1 µM for the other nitrosamines) with the exception of NPip, which was spiked to 1000 µg/L (8.8 µM) due to its higher detection limit. Direct photolysis tests in Milli-Q water (pH 6, unadjusted) were performed at 765 W/m2, which is equivalent to midday, midsummer sun in California (27), and used alongside actinometry to determine nitrosamine quantum yields. To assess the potential for indirect photolysis, nitrosamines were irradiated (n ) four test tubes per nitrosamine) in filtered (0.45 µm) surface water (absorbance spectrum given in Figure S1, SI) with photosimulator intensity reduced to 250 W/m2 to slow the rapid nitrosamine photolysis and compared to irradiation of nitrosamines in Milli-Q water (n ) four test tubes). To assess light screening by DOC, NDMA (200 µg/L) was irradiated in solutions of increasing concentrations of Aldrich humic acid (0-35 mg C/L), also at 250 W/m2. The test tubes were simultaneously irradiated, one at each concentration. The uncertainty in the photodecay rate at a given concentration was calculated as the standard error in the slope (rate) of the linear regression instead of from variation of the mean of multiple rates. This error estimate does not take into account the effect of test tube position in the photosimulator, which typically resulted in a greater relative standard deviation of 5-10% of the measured rate. To test the effect of oxygen, additional experiments were performed for some nitrosamines by bubbling Milli-Q water solutions with either oxygen or helium to purge oxygen. To monitor the products resulting from the photochemical decay of NDMA, 500 µg/L NDMA (6.7 µM) was spiked into Milli-Q water, and expected products were monitored over 1 h.
Determination of Quantum Yield. The quantum yield (Φ) of a photochemical reaction describes the moles of reactant transformed per moles of photons absorbed (21). An environmental quantum yield, Φc, representing the averaged value over the wavelength range at which the nitrosamine absorbs sunlight, was calculated using the following equation (26, 28):
(
Φc ) Φa
)
∑ I (λ) (λ) k ∑ I (λ) (λ)
kcp
0
a p
0
a c
(1)
where Φa is the known quantum yield of the actinometer; kcp and kap are the rates of photolysis for the chemical and actinometer, respectively; a(λ) and c(λ) are the extinction coefficients for the actinometer and chemical, respectively; and I0(λ) is the solar irradiance. Chemical actinometry, in which a calibrated photochemical reaction is used to quantify light intensity, was performed using the binary actinometer PNA/pyridine described by Dulin and Mill (26). Φa for PNA is reported to be
Φa ) 0.44[pyr] + 0.00028
(2)
and was thus determined from the known experimental molar concentration of pyridine. The actinometer half-life was adjusted to match that of NDMA, requiring 981 mg/L pyridine (12.4 mM) and 1.2 mg/L PNA (7.8 µM). Rates of photolysis were determined by monitoring the disappearance of the nitrosamine and PNA, which have similar absorption spectra, and calculated from the slope of the linear regression using
()
ln
C0 ) kpt Ct
(3)
where C0 and Ct are the chemical or actinometer concentrations at time zero and at time t, respectively, and kp is the rate of photolysis for the chemical or actinometer. Irradiation data (I0(λ)) for the lamp with UV filters, simulating natural sunlight, was obtained from the photosimulator manufacturer. The molar extinction coefficients, (λ), were determined from measured ultraviolet/visible (UV-vis) spectra of each nitrosamine and the actinometer at known concentrations. The total absorbance of the reaction solutions were maintained below 0.02 to minimize light absorption by the system (29), except the absorbance of the actinometer solution reached a maximum of 0.08. Calculation of Solar Irradiance and Expected Photolysis Rate. Global horizontal irradiance at the earth’s surface over the wavelength range 280-2279 nm was calculated using SMARTS (Simple Model for the Atmospheric Radiative Transfer of Sunshine, v. 2.9.5) (30) for midlatitude and tropical locations so that the expected NDMA aquatic photolysis rate could be calculated for a range of environmental conditions. Inputs to SMARTS included geographical position, season, date and time, and specific parameters such as ozone abundance, atmospheric gases, and aerosol model. For the purposes of this nonspecific irradiance prediction, representative or average inputs recommended by SMARTS developers were assumed, and water (non-Lambertian) was selected as the surface. SMARTS was also used to calculate the irradiance at Orange County Water District (OCWD) Water Factory 21 (WF21) for June 23-30, 2000, 33° N latitude. During this time, OCWD monitored NDMA in the influent and effluent of a sunlit, open channel receiving Orange County Sanitation District (OCSD) tertiary treated effluent (31). In addition to geographical position and time, inputs to the SMARTS model included air temperature and relative humidity. These parameters were determined for the site on the June 2000 VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Absorption spectra for selected nitrosamines in Milli-Q water. Irradiance of the photosimulator used for photolysis experiments is shown on the secondary y axis. dates from the California Irrigation Management Information System (CIMIS) online database (32) for an Irvine monitoring station located just 17 miles from OCWD WF-21. The database also reports total irradiance, which was used to adjust the SMARTS irradiance calculated for each wavelength and thus allowed a correction for cloud cover. Other required inputs included total ozone column and turbidity, which were estimated from historical data available from the Royal Netherlands Meteorological Institute (KNMI) Global Ozone Monitoring Experiment (GOME) (33) and using recent Irvine weather reports of visibility, respectively. An “urban” model was selected as the aerosol model. Given irradiance data predicted by SMARTS, expected rates of NDMA photolysis were calculated assuming a particular water depth and light screening by the water body and using the quantum yield and molar absorptivity of the compound of interest (21) using equations S1 and S2 (SI). The photolysis rates at nonspecific midlatitude and tropical locations were determined for water body depths of 10 cm and 1 m, and intermediate light screening by the water body was assumed. This was calculated using a spectral slope of 0.02 and λ280 ) 0.3; a smaller spectral slope and greater reference absorbance correspond to increased light screening by the water body (34). An approximate channel depth was used (∼ 1.1 m) for OCWD, and the UV-vis absorbance of a 2006 water sample taken from a similar stage in the treatment train was collected to determine the light screening (spectral slope ) 0.03 and λ280 ) 0.1). Rates of NDMA photodegradation were also predicted for the solar simulator experiment performed with increasing concentrations of Aldrich humic acid. The UV-vis absorbance of each solution was measured to determine the light screening, and the depth used was the inner diameter of the horizontally placed sample tubes (1.5 cm). Irradiance data (millieinsteins cm-1 d-1) was provided by the manufacturer of the solar simulator.
Results and Discussion UV-Visible Absorption Spectra. In agreement with literature reports (14, 19), the UV-vis absorption spectra for NDMA and the other nitrosamines (Figure 1) show two absorption bands with maxima near 230 and 330 nm corresponding to the π f π* and n f π* transitions, respectively. The 6172
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nitrosamine absorption band near 230 nm is below the range of natural sunlight, but the less intense band at 330-340 nm overlaps with the irradiance spectrum of natural sunlight and of the photosimulator (Figure 1) and thus is responsible for the photodecay of nitrosamines in the environment. The nitrosamine absorption spectra are quite similar, which can be expected due to the structural similarity of the homologues. The molar absorptivity maxima measured for the seven nitrosamines are given in Table S2 of the SI. Direct Photolysis and Quantum Yield. The rates of direct photolysis and quantum yields for NDMA and the other nitrosamines are reported in Table 1. All dark controls showed stable nitrosamine concentrations, confirming no unexpected loss pathways. Photolysis is quite rapid for all nitrosamines, resulting in half-lives less than 20 min for the irradiation conditions of the photosimulator. Stefan and Bolton (14) found zero-order kinetics for NDMA at concentrations of 0.01-1 mM (0.74-74 mg/L) and first-order kinetics at concentrations less than 0.01 mM; all rate measurements reported here were performed at less than 0.01 mM, and first-order kinetics were indeed observed. Nitrosamine quantum yields are 10-100 times larger than that of some other photosensitive environmental contaminants, such as the pharmaceutical naproxen (Φ ) 0.04) (24) and the surfactant metabolite nonylphenol (Φ ) 0.003) (23). The quantum yield for NDMA measured in the present study for sunlight, Φ ) 0.41 ( 0.02, is similar to that measured for UV photolysis studies (Φ ∼ 0.3) (16-18). Also using UV light, Ho et al. (7) report pH-dependent broadband quantum yields between 0.11 (>pH 7) and 0.27 (
