Transformation of Herbicide Propachlor by an Agrochemical

Environ. Sci. Technol. 2004, 38, 6855-6860 these anthropogenic chemicals via drinking water or agri- Transformation of Herbicide cultural products, even at very low concentration, can have Propachlor by an Agrochemical adverse effects on public health and the ecological environment (5). To minimize pesticide residues in agricultural Thiourea systems and the environment, it is of utmost importance to improve current integrated pest management (IPM) strategies ,²,³ ³ and develop contaminant remediation approaches to manage WEI ZHENG,* SCOTT R. YATES, pesticide residue. SHARON K. PAPIERNIK,§ AND Various treatment strategies have addressed the need to MINGXIN GUO²,³ prevent and mitigate pesticide contamination of the natural Department of Environmental Sciences, University of environment. These technologies involve physical, chemical, California, Riverside, California 92521, USDA-ARS, Soil microbiological, and integrated practices, including biore- Physics and Pesticides Research Unit, George E. Brown Jr. mediation (6-9), chemical remediation (10-13), phytore- Salinity Laboratory, Riverside, California 92507, and - USDA-ARS, North Central Soil Conservation Research mediation (14 16), and photocatalytic degradation (17). Laboratory, Morris, Minnesota 56267 Unfortunately, few currently available practices are both environmentally benign and economically feasible. Chemical remediation, defined here as the use of nucleophilic chemicals or redox reagents to destroy contami- Propachlor and other chloroacetanilide herbicides are nants in soil and aquatic environments, offers a rapid and frequently detected contaminants of groundwater and surface economical approach to clean up contaminated sources. To water in agricultural regions. The purpose of this work use the technology safely, it is essential to find appropriate remediation reagents. In previous studies, we utilized a was to develop a new approach to remove propachlor chemical remediation strategy to accelerate the transforma- residues from the environment via chemical remediation by tion of halogenated fumigants on the surface of soil and the nitrification inhibitor thiourea. The transformation control their atmospheric emissions (18, 19). Using agro- processes of propachlor and thiourea mixed in aqueous chemicals (e.g., fertilizers and nitrification inhibitors) as solution, sand, and soil were elucidated. Analysis of remediation reagents provides an innovative approach to transformation products and reaction kinetics indicated remove unwanted agricultural contaminants (such as pes- that an SN2 nucleophilic substitution reaction occurred, in ticide residues) with minimal change to the agricultural which the chlorine of propachlor was replaced by system. Thiourea, a sulfur-based nitrification inhibitor - thiourea, detoxifying the herbicide. It appears that propachlor (20 22), has been discovered to accelerate the degradation undergoes a catalytic reaction in sand or soil amended of halogenated fumigants in aqueous solution and soil via an S 2 nucleophilic substitution reaction (18, 23). Further- with thiourea, which results in a significantly accelerated N more, a catalytic process was found to increase the rate of transformation rate as compared to the reaction in fumigant dissipation in soils as compared to aqueous systems. aqueous solution. The second-order reaction process was This led to the development of a reactive surface barrier examined at different temperatures to investigate the (RSB) as an alternative to the use of plastic tarps to reduce role of the activation energy. The enthalpy of activation halogenated fumigant emissions from the soil surface (23). (∆H) for the reaction of propachlor with thiourea was In addition to halogenated fumigants, our recent explo- demonstrated to be significantly lower in sand than in ration revealed that thiourea may react with many other aqueous solution, which provides evidence that a catalytic organic pesticides that have halide substitution on aliphatic transformation mechanism occurs in thiourea-amended carbons, such as chloroacetanilide herbicides. Use of the sand. The chemical reaction rate increased proportionally agrochemical thiourea as a remediation reagent has great to the amount of thiourea added to the sand. Column potential to decontaminate these compounds via nucleophilic substitution reactions, reducing their adverse effects experiments further suggested that the remediation strategy on the environment. Previous studies implied that propachlor could be used to remove propachlor residues from sand was a more reactive herbicide than other chloroacetanilide or soil to reduce leaching and prevent contamination of surface herbicides (e.g., alachlor and metolachlor) for nonenzymatic water and groundwater. conjugation by glutathione (24) and nucleophilic transformation with thiosulfate (10) and bisulfide (11). Thus, propachlor was selected as a representative of chloroacetanilide Introduction herbicides to investigate chemical remediation in the study. The objectives of this work were (i) to systematically Propachlor (2-chloro-N-isopropylacetanilide) is one of several investigate the reaction kinetics of propachlor and thiourea chloroacetanilide herbicides that are widely applied in the in an aquatic environment, sand, and soil; (ii) to characterize United States for pre-emergent control of annual grasses the transformation products and elucidate their reaction and broad-leafed weeds in corn and soybean crops. Owing mechanisms in different environmental media; and (iii) to to their popularity in agricultural applications, chloroacet- perform laboratory column experiments to illustrate the anilide herbicide residues have been frequently detected in potential for thiourea to remediate soils contaminated by groundwater, surface water, and coastal marine environ- propachlor ments (1-4). There is increasing evidence that exposure to Experimental Section * Corresponding author telephone: (909)369-4878; fax: (909)342- Chemicals and Soils. Propachlor (99% purity) and CH I (99% 4964; e-mail: [email protected]. 3 ² University of California, Riverside. purity) were purchased from Chem Service (West Chester, ³ USDA-ARS, George E. Brown Jr. Salinity Laboratory. PA). Thiourea (99% purity) was obtained from Aldrich § USDA-ARS, North Central Soil Conservation Research Laboratory. Chemical (Milwaukee, WI). All chemicals were used as 10.1021/es049384+ CCC: $27.50 2004 American Chemical Society VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6855 Published on Web 11/03/2004 received. Propachlor stock solutions were prepared in a approaches were used prior to hexane extraction as follows: mixture of methanol (HPLC grade) and water (1:1, v/v) and (i) aliquots of the reaction mixtures were adjusted to pH 10; used for spiking all samples. (ii) aliquots of the reaction mixtures were heated in a water Silica sand (90 mesh, pH 8.1) obtained from P. W. bath at 60 °C for 30 min; and (iii) the transformation prod- Gillibrand Co (Simi Valley, CA) was washed several times uct was methylated using CH3I: a 2-mL aliquot of propa- with deionized water and then oven-dried at 105 °C. The soil chlor:thiourea reaction mixture was treated with excess used in this study was an Arlington sandy loam that was CH3I (0.1 M) and then heated in a water bath at 60 °C for 30 collected from the University of California, Riverside Agri- min. After being cooled to room temperature, all treated cultural Experiment Station. The soil had a pH of 7.2 and an samples were extracted with hexane. Hexane extracts were organic carbon content of 0.92%. analyzed using an HP 5890 GC coupled to an HP 5971 Experimental Systems. In the first experiment, second- quadropole mass spectrometer equipped with a 30 m order reaction kinetics of propachlor and thiourea were HP-5MS capillary column (0.25 mm i.d. × 0.25 µm film measured in buffer solutions (pH 6.9) with an initial molar thickness; Wilmington, DE). Electron impact mass spectra ratio (propachlor:thiourea) of 1:4. Reactions were initiated were generated using an electron energy of 70 eV, and data by combining propachlor stock solution and 100 mL of were collected in full-scan mode in the m/z range of 50-400. thiourea solution (1.0 mM) in serum bottles, which were The chemical reaction between propachlor and thiourea then capped. Samples containing only propachlor were also is presumably one of bimolecular substitution, which follows prepared as untreated controls. All bottles were placed in second-order kinetics. A second-order kinetic model (10, 25) incubators maintained at 15, 24, 35, 48, and 55 °C(( 0.5 °C). was fitted to the transformation data for each matrix (aqueous At intervals, 1.0-mL aliquots of solution were withdrawn from solution, sand, and soil). To better understand the trans- triplicate bottles and extracted with 2.0 mL of hexane. formation of propachlor in sand amended with different levels Preliminary experiments revealed that the reaction was of thiourea, a pseudo-first-order kinetic model was also used quenched immediately when hexane was added as an to obtain the half-life, for comparison. All statistical analyses extraction solvent because propachlor was efficiently ex- were performed at a 0.05 significance level. tracted into the organic phase, while thiourea remained in Column Experiments. To demonstrate a potential ap- the aqueous phase. The concentration of propachlor in each plication of thiourea to remediate pesticide contamination, extract was analyzed using an Agilent 6890

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