Environ. Sci. Technol. 2003, 37, 1311-1318
Abiotic Degradation of Trifluralin by Fe(II): Kinetics and Transformation Pathways THEODORE P. KLUPINSKI Environmental Science Graduate Program, The Ohio State University, Columbus, Ohio 43210 YU-PING CHIN* Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210
The herbicide trifluralin (2,6-dinitro-N,N-dipropyl-4(trifluoromethyl)benzenamine) is widely used in agriculture and may pose toxic risks to some aquatic organisms. While its degradation has been investigated in field studies, this research is the first to elucidate the specific abiotic transformations that trifluralin may undergo in reducing environments such as flooded soils and wetland sediments. Kinetic data and product identities were determined for the degradation of trifluralin in Fe(II)/goethite suspensions at near-neutral pH values. Under these conditions, trifluralin is consumed rapidly through a surface-mediated process that includes three distinct reactions: reduction of nitro groups, dealkylation of propylamines, and cyclization to form benzimidazoles. All detected products are among those that have been reported in natural soils and sediments. Therefore, these transformation pathways may play a significant role in affecting the fate of trifluralin in the environment.
Introduction Contamination of surface waters and sediments from nonpoint sources is a major environmental concern. Pesticides contribute significantly to this problem because of their widespread usage on agricultural land from which they can be transported by surface runoff. A U.S. study (1) showed that pesticides were found in ∼40% of those watersheds identified as presenting the greatest risks to aquatic life or human health because of contaminated sediments. Trifluralin (2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine, TR-1 as shown in Table 1) is one of the most common herbicides (annual usage about 25 million lb) used to control grasses and weeds for a variety of agricultural crops (2). It has been characterized as moderately to highly toxic toward aquatic animals, including fish and invertebrates (2). Trifluralin has been frequently detected in studies on agriculture-related contamination of surface waters (3), thus demonstrating the validity of the concerns outlined above. In this research, our goal is to determine how wetland sediments may influence the degradation of trifluralin, with obvious implications for the water quality of contaminated wetlands. The environmental behavior of trifluralin has been investigated in numerous field studies, the results of which * Corresponding author e-mail:
[email protected]; phone: (614)292-6953; fax: (614)292-7688. 10.1021/es025673r CCC: $25.00 Published on Web 02/25/2003
2003 American Chemical Society
have been reviewed (4). In soils and sediments, field degradation studies have been performed over periods of weeks to years (5-11). The results generally revealed partial trifluralin loss and the formation of several products. Golab et al. (7) reported 28 products, identified by mass spectrometry (12), from degradation in a loam soil over 3 yr. (The product labels applied in these papers, of the form TR-#, will be used here to simplify comparison.) In this and the other studies product recovery is incomplete, and transformation pathways are inferred rather than proven. Furthermore, the degradation is often tacitly assumed to be a microbiological process. Despite some evidence to support the possibility of abiotic reactions (9, 11), such pathways have not been thoroughly investigated to date. In the presence of mineral surfaces, Fe(II) promotes the reduction of nitroaromatic compounds (13). Wetland sediments are anoxic environments, often containing high levels of Fe(II) in porewaters (14). For example, at Old Woman Creek National Estuarine Research Reserve in northern Ohio, Fe(II) concentrations can reach 1 mM at sediment depths of 15-25 cm (15). Under such conditions, chemical (i.e., abiotic) reactions may play an important role in contaminant fate. In this paper, we investigate the abiotic degradation pathways of trifluralin in systems representative of wetland sediments.
Materials and Methods Chemicals. Reference Standards (See Table 1). Trifluralin (TR1) and the degradation products TR-2, TR-4, TR-6, and TR15 (g99% purity for each) were provided by Dow AgroSciences and used without further purification. TR-7 was synthesized by the sulfide reduction of TR-1 (16). A solution was prepared by dissolving 331 mg of TR-1 in 3.0 mL of ethanol. To this was added an aqueous solution (1.6 mL) made from 797 mg of Na2S‚9H2O and 281 mg of NaHCO3. After heating to 60 °C for 1 h, the product mixture was purified by chromatography on a column (length ) 15 cm, diameter ) 1 cm) of C18end-capped silica taken from a Waters Sep-Pak. Solvent mixtures of methanol/water were used for elution. The TR-7 fraction was rotary evaporated to remove the methanol, and the compound was precipitated from cold water as thin cream-colored crystals. TR-5 was made by the sulfide reduction of TR-2, and TR13 was made by heating TR-5 to reflux in a solvent mixture of propionic acid and HCl, as described by Leitis and Crosby (17). TR-8 was made by reducing TR-2 with excess Na2S in a 50/50 ethanol/water mixture for 1 h at 60 °C. TR-16 was made by reducing TR-15 with excess Na2S in a 50/50 ethanol/ water mixture for 1 h at 60 °C. TR-14 was made by reducing TR-13 with excess Na2S in a 25/75 methanol/water mixture for 4 days at 5 °C. TR-9 was made by reacting TR-6 with Fe(II) and goethite, using Fe(II) and TR-6 at ∼1000× normal reaction concentrations (described below). It is notable that the benzimidazoles TR-13, TR-14, TR15, and TR-16 are also observed as byproducts in the sulfide reductions of TR-1 and TR-2. Qualitatively, their formation in these reactions is less favored than in the Fe(II)/goethite reaction media discussed in this paper. Other Compounds. MOPS (4-morpholinepropanesulfonic acid; >99.5%), MES (4-morpholineethanesulfonic acid; >99.5% as monohydrate), FeCl2‚4H2O (99.995%), Na2S‚9H2O (98%), and NaHCO3 (g99.7%) were purchased from SigmaAldrich. Acid and base solutions were made from concentrated hydrochloric acid (Fisher Certified ACS Plus) and solid NaOH (Mallinckrodt AR, 99%), respectively. Solvents included Milli-Q water (Mill-Q UV Plus, Millipore), methanol (J. T. VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Trifluralin and Degradation Products
a The molecular ion (M+) and major fragment peaks are listed, with the base peak marked by an asterisk. b The base peak in all cases is [M + H]+. Fragment peaks were determined by MS/MS analysis. Spectra were not measured for the compounds marked with NM.
Baker HPLC solvent), acetonitrile (J. T. Baker HPLC solvent), ethanol (Pharmco Products, 190 proof, USP grade), hexanes (Fisher Optima), ethyl acetate (Sigma-Aldrich HPLC grade), and propionic acid (Fisher Certified). Goethite (R-FeOOH) was synthesized by Meier (18) and stored in a concentrated aqueous suspension. The goethite has a surface area of 63 ( 2 m2/g, as determined by the BET method for N2 ad1312
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sorption, and its integrity was verified by X-ray diffraction analysis (Cu KR radiation) of a random powder mount. Analytical Techniques. Electron-impact mass spectrometry (EIMS) analysis was performed with a Hewlett-Packard 6890 gas chromatograph (GC) and 5973 Mass Selective Detector in positive-ion mode. Aqueous solutions or suspensions were first extracted with 97/3 Hexanes/Ethyl
Acetate, using solvent-to-sample volume ratios of 0.5-1000, depending on the original concentration. The organic solution was injected onto the GC (HP-5MS capillary column: 30 m × 250 µm × 0.25 µm; He carrier gas at 1.0 mL/min), and compounds were separated by increasing the temperature from 55 to 250 °C at a rate of 20 °C min-1. The total ion chromatogram showed peaks that could be analyzed individually for MS data. The results for trifluralin products matched the published spectra (12) available for all compounds except TR-6, TR-8, TR-9, and TR-16. Spectra for these products were easily interpreted based on fragmentation patterns and comparison with the other results. Electrospray mass spectrometry (ESIMS) analysis was performed on the more hydrophilic compounds, as this technique does not require extraction into organic solvents. (This fact is particularly salient for TR-9 and TR-16, which are very difficult to extractseven from concentrated aqueous solutions.) Aqueous samples were mixed with methanol or acetonitrile to give solutions that were infused into the electrospray source at a rate of 5-10 µL min-1. Some samples were spiked with acetic acid to increase signal intensity. Most measurements were made with a Micromass Q-TOF II mass spectrometer equipped with an orthogonal electrospray source (Z-spray) operated in positive-ion mode. Optimal ESI conditions were capillary voltage ) 3000 V, source temperature ) 110 °C, and cone voltage ) 60 V. Extended MS/MSn studies were performed with a Bruker Esquire ion trap mass spectrometer equipped with an orthogonal electrospray source operated in positive-ion mode. Optimal ESI conditions were capillary voltage ) 3500 V and source temperature ) 250 °C. For all samples, spectra were acquired in continuum mode until acceptable averaged data were obtained. As we could find no references for ESIMS analysis of the degradation products, compound identities were verified by the fragmentation patterns determined with MS/MSn studies. HPLC analysis was performed with a Shimadzu lowpressure gradient flow system: LC-10AT pump, FCV-10AL flow mixer, DGU-14A degasser, SCL-10A controller, SPD10A UV/Vis detector, SIL-10A autoinjector, and Class-VP 4.2 software. Samples were injected in volumes of 50-400 µL, and the components were separated on a reverse-phase column (Waters Nova-Pak C18: 3.9 × 150 mm, 60 Å pore size, 4 µm particle size) and detected by UV absorbance. Concentrations of TR-1 and the quantified degradation products were calculated from linear calibration equations determined by the analysis of standard solutions. Compounds were eluted either isocratically (method A: flow ) 1.0 mL min-1, 75% acetonitrile/25% water, λ ) 237 nm) or in gradient mode (method B: flow ) 1.0 mL min-1, linear increase from 55% methanol/45% water at 0 min to 82% methanol/18% water at 27 min, λ ) 225 nm). Reaction Studies. To maintain anoxic conditions, all reaction media were prepared in an argon-filled glovebox. Reactions were conducted at normal lab temperature (2123 °C), and pH was measured with an Orion Sure-Flow Ross semi-micro pH electrode. Concentrated Fe(II) stock solutions were made from FeCl2‚4H2O in 1 mM HCl, passed through a syringe filter (0.45-µm, Gelman, IC Acrodisc, Supor membrane), and used within 12 h. A pre-reaction mixture was prepared by combining aqueous stocks of a sulfonate pH buffer (MOPS or MES), goethite, and Fe(II). The suspension was then equilibrated for 20-380 min. (Reactivity was not affected by the length of equilibration time.) To start a reaction, TR-1 was added in microliter quantities from a concentrated methanol solution so that the methanol content in the reaction mixture was
