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a Department of Botany and Plant Sciences, University of California, Riverside, CA 92521
b Institute of Environmental Science, Zhejiang University, Hangzhou 310029, China
* Corresponding author (jgan{at}ucr.edu)
Received for publication May 13, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: NDMA, N-nitrosodimethylamine
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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To assure environmental safety, it is essential to understand the potential for NDMA to leach to ground water when the treated wastewater is used for irrigation. Earlier studies show that NDMA volatilizes readily from soil (Oliver, 1979) and may also be taken up by various plants (Dean-Raymond and Alexander, 1976). These processes will probably determine the fraction of NDMA reaching the unsaturated soil zone when NDMA-containing wastewater effluent is used for irrigation. Once in soil, the potential for NDMA to move to ground water will closely depend on its adsorption capacity and degradation rate. Due to its relatively high water solubility, retention of NDMA by soils is generally weak (Gunnison et al., 2000). Therefore, degradation of NDMA may act as one of the most important processes for influencing the potential for NDMA to leach to ground water. Inconsistent findings were reported on NDMA degradation in soil from previous studies. For instance, Mallik and Tesfai (1981) observed that soil microorganisms in general were incapable of degrading NDMA, and loss of NDMA in most selected microorganism cultures was only 6 to approximately 9% after 20 h of incubation at 30°C. Tate and Alexander (1975)(1976) reported long lag time and slow degradation of NDMA in soil at 60 mg kg–1, and difficulties in isolating NDMA degraders from soil or sewage. However, in a more recent study by Gunnison et al. (2000), NDMA biodegradation was found to occur with native microorganisms cultured from soils under both anaerobic and aerobic conditions. However, essentially no information is available on NDMA fate and behavior in landscaped soils.
The persistence of organic pollutants in soil is known to depend on soil chemical properties and microbial activity, which in turn are affected by planting and management practices. Unlike homogeneous planting in agriculture field, landscapes are comprised of heterogeneous plantings such as turfgrass, ground covers, mulched areas, and bare surfaces. In a previous study, the planting cover was found to greatly affect the degradation rate of a number of pesticides (Gan et al., 2003). In this study, NDMA adsorption and degradation were investigated in different landscape soils and under different conditions. Efforts were made to understand the effect of soil chemical and microbial properties on NDMA persistence. The results from this study will be useful for evaluating NDMA leaching potential in landscape soils and for identifying conditions under which NDMA leaching risk is minimized when NDMA-containing wastewater effluent is used as irrigation water.
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MATERIALS AND METHODS |
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Landscape Soils and Characterization
Three different landscape soils were collected from a field site located at the Agricultural Experiment Station on the campus of University of California in Riverside, CA. The original soil was a Hanford sandy loam (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents) containing 0.3% organic matter. In 1995, plots (8 x 8 m) were constructed and then planted with tall fescue grass (Festuca arundinacea Schreb.), the low-growing ground cover spring cinquefoil (Potentilla tabernaemontani Asch.), or pear trees (Pyrus calleryana Decne.). At the time of sampling, the plots of turfgrass and ground cover had 100% cover by vegetation, while the plots of pear trees had a single tree at the center of the plot and the soil surface was mostly bare. Soil cores (10-cm length, 5-cm i.d.) were randomly taken from plots with the same planting cover using a hand auger. The fresh soil samples were pooled for the same vegetation type, sieved through a 2-mm sieve without complete air drying, and stored in plastic bags at room temperature before use. Basic soil chemical and physical properties were analyzed by the University of California-Division of Agricultural and Natural Resources (DANR) Analytical Laboratory in Davis, CA (Table 1).
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Adsorption Experiments
Batch experiments were conducted at room temperature (21 ± 1°C) to determine NDMA adsorption in the landscape soils. Briefly, 10 g of soil (dry weight equivalent) and 10 mL of 0.01 M CaCl2 aqueous solution were mixed in 40-mL Teflon centrifuge tubes at low speed for 24 h on a mechanical shaker. An aliquot of 25, 50, 100, 250, 500, or 1000 µL of 50 mg L–1 NDMA aqueous solution was added into the slurry, and the spiked samples were mixed at high speed on the shaker for another 24 h. The initial solution concentrations therefore ranged from 125 to 5000 µg L–1. Preliminary experiments showed that the concentration in the solution phase became constant in <2 h. Phase separation was achieved by centrifuging the sample tubes at 1500 x g for 20 min. To determine NDMA concentration in the aqueous phase, a 1.0-mL aliquot of the supernatant was transferred to a 20-mL glass vial containing 5.0 mL of dichloromethane and 5 g of anhydrous sodium sulfate. After addition of 0.1 mL of d6-NDMA (10 mg L–1, in dichloromethane), the vial was capped with an aluminum seal and a Teflon-lined butyl rubber septum and was vigorously mixed by hand for 2 min. The dichloromethane phase was then transferred into a GC vial and analyzed on a GC–mass spectrometer (MS) system. After the solution was decanted, the remaining soil was mixed with 5 g of anhydrous sodium sulfate, 5.0 mL of dichloromethane, and 0.1 mL of the d6-NDMA solution (10 mg L–1, in dichloromethane) at high speed for 4 h on the shaker. The sample tube was centrifuged at 1500 x g for 10 min, and an aliquot of the solvent phase was analyzed by GC–MS. Three replicates were used for each concentration level.
Incubation Experiments
Degradation of NDMA in landscape soils was determined through incubation experiments under laboratory controlled conditions. The initial water content of all soil samples was adjusted to 12% (w/w) by adding deionized water. Fifty grams of soil (dry weight equivalent) was weighed into 125-mL glass serum bottles (Wheaton, Millville, NJ), and spiked with 2.5 mL of NDMA aqueous solution. The spiked bottles were closed by capping with aluminum seals and Teflon-lined butyl rubber septa, and mixed thoroughly by shaking with hand. The sample bottles were incubated in the dark and triplicate samples were removed 0, 3, 7, 14, 21, 28, and 56 d after the treatment. Soil samples were spiked with 0.1 mL of d6-NDMA in dichloromethane (10 mg L–1) as a surrogate and extracted with 50 mL dichloromethane by shaking at high speed for 4 h. The solvent extract was filtered through a funnel containing 20 g of anhydrous sodium sulfate, and the dried extract was concentrated to a final volume of about 1 mL under a stream of dry nitrogen. An aliquot of the final extract was used for analysis by GC–MS. The recovery of NDMA was determined to be 36.6 ± 6.6, 39.0 ± 5.4, and 37.4 ± 7.2% for the tree soil, turfgrass soil, and ground cover soil, respectively. However, as d6-NDMA was used as a surrogate, the results were not corrected for recovery. The detection limit of NDMA using the above protocol was 0.20 µg kg–1.
A number of paired treatments were used to understand the mechanism of NDMA degradation in soil and the effect of treatment and environmental conditions. In the first paired treatments, one set of soil samples was autoclaved to remove biological activity before NDMA addition, while another set was not sterilized. Sterilization was achieved by autoclaving the soil samples twice at 122°C and 118 kPa (17.2 psi) for 40 min in a SV-3033 scientific pre-vacuum sterilizer (Steris, Mentor, OH), with a 48-h interval between the first and second autoclaving. In the second paired treatments, the landscape soils were treated at two different concentrations of NDMA to understand the effect of NDMA concentration on its persistence. The higher concentration was 250 µg kg–1, while the lower concentration was 25 µg kg–1. In the third paired treatments, the treated samples were incubated at two different temperatures to understand the effect of temperature on NDMA persistence. The treated samples were incubated at either room temperature (21 ± 1°C) or at 10°C in an incubator. In the fourth paired treatments, nonsterilized turfgrass soil was amended with anthropogenic nutrient sources to understand the potential effect of fertilization on NDMA persistence. One set of samples was amended with compost steer manure (0.5 g per container; Earthgro, Marysville, OH), and the other set was treated with calcium nitrate (5 mg per container), one day before NDMA treatment.
Gas Chromatography–Mass Spectrometry Analysis of N-Nitrosodimethylamine
To analyze NDMA in the final sample extract, a 1.0-µL aliquot was injected into an Agilent (Palo Alto, CA) 6890N GC equipped with a split/splitless inlet and an Agilent 7683 autosampler. An Agilent 5973 mass selective detector operating in the electron-impact ionization mode (EI) was used in the selective ion monitoring (SIM) mode with selected ions at 74 (NDMA) and 80 m/z (d6-NDMA) for detection. A response ratio between d6-NDMA and NDMA and peak areas were used to derive the concentration of NDMA. The column was a 30-m-long x 0.25-mm-i.d. x 0.25-µm-film-thickness DB-1701 capillary column (stationary phase 14% cyanopropylphenyl and 86% dimethylsiloxane; Agilent). The interface temperature was maintained at 280°C. Ion source and quadrupole temperature were kept at 230 and 150°C, respectively. Pulsed splitless injection was used with a pressure of 0.17 MPa for 0.3 min and a splitless time of 0.4 min. Helium was used as the carrier gas with a flow rate of 1.0 mL min–1 and the inlet temperature was maintained at 210°C. The oven temperature was programmed with an initial temperature of 45°C for 2 min, increased at 50°C min–1 to 100°C, held for 2 min, increased at 50°C min–1 to 280°C, and finally held for 1.5 min. The retention times of NDMA and d6-NDMA were 4.49 and 4.47 min, respectively. The ratio of response factor between NDMA and d6-NDMA was 1.21 under the used conditions, which was used for quantification of NDMA.
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RESULTS AND DISCUSSION |
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The rate constant kb was estimated from the difference between the paired nonsterile and sterile treatments, from which the relative contribution of microbial degradation to the overall NDMA degradation was calculated. In all landscape soils, degradation of NDMA was largely attributable to biological degradation at both NDMA treatment levels. In the bare-surface soil, kb contributed for 85 to 91% of k, while in the ground cover and turfgrass soils, the contribution was >95%. The effect of sterilization is consistent with Mallik and Tesfai (1981), who investigated the loss of NDMA in three different soil types (sandy loam, silt loam, and clay) and found the loss of NDMA was higher in nonautoclaved soils than in autoclaved soils. Oliver et al. (1979) used 14C-NDMA in an aerobic degradation study and found that nitrosamines were degraded to CO2 in nonsterile soils, but not in sterilized soils. Similar results were also found in sewage samples by Tate and Alexander (1975), who reported that the rate of NDMA loss from sewage decreased after autoclaving treatment. Therefore, from this and other studies, it may be concluded that NDMA degradation in soils is mostly mediated by soil microorganisms.
Persistence of NDMA also differed significantly in different landscape soils. The overall rate of degradation followed the order ground cover soil > turfgrass soil > bare-surface soil. When the initial NDMA concentration was 250 µg kg–1, t1/2 was estimated to be 4.1 d for the ground cover soil, 5.6 d for the turfgrass soil, and 22.5 d for the bare surface soil. A very similar trend was observed also for the lower NDMA treatment level (Table 4). Linear regression analysis was performed between k values for the different soils and the corresponding soil chemical or biological properties (Table 1). A close dependence was identified for soil microbial biomass (r2 = 0.93–0.96) or soil dehydrogenase activity (r2 = 0.92–0.96) for both NDMA treatment levels at 21°C. Therefore, the different persistence of NDMA in the landscape soils was underlined by the different microbial activity in these soils. Analysis of relationship of k values and soil organic matter also showed a significant linear relationship, with r2 of 0.89. Numerous studies have shown that soil organic matter plays a critical role in soil microbial ecology. Therefore, it is likely that the different planting practices changed soil organic matter quality and quantity in the soil, which in turn changed the makeup and activity of soil microorganisms. The rapid degradation of NDMA in ground cover and turfgrass soils was probably a result of more accumulation of organic matter, which supported an active growth of soil microorganisms contributing to NDMA transformation. In contrast, because the roots were sparse and organic matter accumulation was minimal in the tree soil, the microbial activity was limited, which led to the slow degradation of NDMA. Boyle and Shann (1998) found that plant species difference could influence rhizosphere microorganisms, and planting tends to significantly increase the rate of xenobiotic mineralization because of its effect on rhizosphere microbial ecology. In a previous study, Gan et al. (2003) observed that herbicides 2,4-D and dicamba were degraded much more rapidly in the ground cover and turfgrass soils than in the bare surface tree soil under aerobic conditions.
In a preliminary experiment (data not shown), the most probable number (MPN) method was used in an attempt to enumerate NDMA-degrading bacteria by using NDMA as the sole substrate (Alexander, 1982). However, no significant dissipation of NDMA occurred in NDMA-amended culture tubes even four weeks after the treatment. This observation suggests that although NDMA was degraded biologically in the landscape soils, the degradation was not caused by degraders that used NDMA as their growth substrates. This observation was consistent with Tate and Alexander (1975), who were unsuccessful in isolating NDMA-degrading microorganisms from soils or municipal sewages. A few known bacterial strains were tested but found unable to metabolize NDMA (Tate and Alexander, 1976). Therefore, it may be concluded that microbial degradation of NDMA in the landscape soils was probably the result of co-metabolism, and that NDMA degradation may be dependent on the general microbial structure in soil.
Effects of Treatment and Environmental Conditions
The effect of NDMA treatment level, nutrient amendment, and temperature on NDMA persistence was evaluated through paired treatments. Degradation of NDMA at the lower NDMA level (25 µg kg–1) remained unchanged in the ground cover soil and turfgrass soil, and was slightly enhanced in the bare surface soil, when compared with the higher NDMA level (250 µg kg–1) (Table 4). Kaplan and Kaplan (1985) studied the NDMA mineralization kinetics in soil for 98 d at various concentrations, and found that NDMA mineralization rate increased by five times when the initial concentration was decreased by four orders of magnitude from 100 mg kg–1 to 10 µg kg–1. In Gunnison et al. (2000), effect of NDMA concentrations was studied using slurries of contaminated subsurface soils, and about a threefold increase in NDMA mineralization rate was observed for a three-orders-of-magnitude decrease in concentration from 50 mg L–1 to 50 µg L–1. From these studies, it is apparent that the effect of NDMA concentration may be significant only when the concentrations cover a very wide range.
Addition of composted manure had no significant effect on NDMA persistence in the turfgrass soil when compared with the no-amendment treatment (Table 3). In a previous study, Kaplan and Kaplan (1985) investigated the effect of supplemental organic matter on NDMA mineralization, and did not observe any significant effect. Therefore, although NDMA degradation appears to be proportional to the indigenous soil organic matter content, addition of an anthropogenic source of organic matter may not stimulate NDMA degradation over a short time period. Addition of a nitrate source also had no effect on NDMA degradation in the landscape soils (Table 3). Previous studies showed that high nitrate concentrations inhibited mineralization of atrazine in soil (Katz et al., 2000). The reduction in atrazine mineralization was attributed to a potential inhibition of microorganisms' ability to use atrazine as a nitrogen source (Clausen et al., 2002). Therefore, the lack of effect of organic and inorganic nutrients on NDMA degradation in this study may be attributed to the fact that soil microorganisms did not use NDMA as a nutrient source. This observation is consistent with the finding from the most probable number experiment, in which soil microorganisms were found to be unable to use NDMA as the sole substrate.
Degradation of NDMA at 10°C was generally slower than that at room temperature for the same soil (Table 5). For instance, when the initial NDMA treatment level was 250 µg kg–1, persistence of NDMA in the landscape soils decreased by about 40 to 60% at 10°C when compared with 21°C. Similar effects were also observed for the lower level treatments, as degradation rate of NDMA in soils decreased by about 40 to 50% at 10°C when compared with 21°C. It also appears that temperature had a greater effect on NDMA persistence in the turfgrass soil than in the other soils. Therefore, it may be expected that NDMA would have a longer persistence in the landscape soils during cooler seasons, and that use of treated wastewater for irrigation during the cooler seasons would pose an enhanced NDMA leaching risk.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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