Formation and Transformation of Pesticide Degradation Products Under Various Electron Acceptor Conditions

Principal Investigators
G.F. Parkin, University of Iowa

Abstract

Goals: Objectives are (1) year one--develop and refine analytical techniques required for identification of pesticide degradation products, develop and operate batch reactors under each of the four electron acceptor conditions, screen each reactor for major metabolic products, (2) year two--conduct kinetic experiments to quantify rates of formation and transformation of metabolic products and determine kinetic expressions to describe those reactions, obtain and analyze field samples from research site for metabolites, and (3) year three--complete kinetic experiments, and develop and test a mathematical model.

Rationale: Recent research has shown that while atrazine and alachlor are transformed in the environment under a variety of conditions, their rates of mineralization are likely much slower than their rates of initial transformation. Thus a number of degradation products are being formed and perhaps accumulating in the environment, and the nature of these products will likely be a function of the particular environment in which they are formed (e.g., the dominant electron acceptor condition). Therefore, it is desirable to gain information regarding the effect of these different environments on the formation and subsequent transformation of major degradation products.

Approach: Proposed research will employ both batch and column reactor techniques, some with soil-water suspensions. Soil obtained from an Iowa agricultural field known to have been treated in the past with atrazine and/or alachlor will be used. Liquid media used in all experiments will be a "synthetic" ground water designed to maintain one of the electron acceptor conditions of interest. All experimental reactors will be run at 16°C in an effort to keep experimental conditions as close as possible to those of a typical Iowa ground water. Reactors will be seeded with cultures which have been growing under desired electron acceptor conditions and have been shown to transform atrazine and alachlor. Acetate will be fed as a carbon and energy source. Pesticide, acetate, microbial biomass, and electron acceptors will be monitored over time during each experiment. Once a significant fraction of the fed pesticide has been transformed, products of atrazine and alachlor will be assayed in the effluents and/or soil samples of each reactor. Standards for some of the expected transformation products will be obtained. A number of analytical techniques will be employed for identification and quantification of these metabolic products, including use of selective GC detectors, GC-MS, or, if necessary, LC-MS. Unknown metabolites may be further analyzed using NMR spectroscopy.

Status: Initially, alachlor and atrazine disappeared in reactors maintained under all electron acceptor conditions, with the exception of aerobic. Resazurin, a color indicator of redox condition, was found to be involved in the transformation of alachlor and atrazine under denitrifying conditions. Second-order degradation coefficients for the biological transformation of alachlor and atrazine under denitrifying, methanogenic, and sulfate-reducing conditions were determined. Over the course of four experiments, the rate of alachlor transformation decreased considerably under methanogenic and sulfate-reducing conditions. Several metabolites of alachlor were positively identified in these systems. Under denitrifying conditions with organisms and resazurin present, aniline, m-xylene, acetyl alachlor, and diethyl aniline were positively identified as products of alachlor degradation. No metabolite accounted for greater than 35% of the initial mass of alachlor. m-Xylene was also detected in an abiotic reactor containing only resazurin, atrazine, and alachlor in ground water medium under denitrifying conditions. Because this compound is readily degradable, it is unlikely that m-xylene would persist in aerobic ground water as a result of alachlor contamination and subsequent transformation. Experiments indicated that resazurin may serve as an electron donor for organism growth, but it was unclear whether resazurin itself, or organisms capable of growth on resazurin, were responsible for the formation of metabolites. It is also possible that resazurin facilitated electron transfer, as vitamin B12 is known to do, under abiotic conditions. In the methanogenic and sulfate-reducing reactors, diethyl aniline, acetyl alachlor, and an unidentified metabolite (called SM4 for the purposes of reporting) were detected. No metabolite accounted for greater than 30% of the initial mass of alachlor. Acetyl alachlor was an expected product, likely formed as a result of reductive dechlorination. Some toxicity was noted during the course of the experiments, possibly the result of accumulation of unidentified metabolites, SM1, SM2, and SM4. Batch experiments were run on methanogenic and sulfate-reducing reactors using acetyl alachlor. Acetyl alachlor was never detected in the sulfate-reducing reactor after the initial dosing, due either to poor dosing technique or immediate abiotic transformation by sulfide/bisulfide ions present in the reactor. Acetyl alachlor was detected and monitored in the methanogenic reactor. Diethyl aniline and aniline were expected metabolites, though aniline and an unidentified compound were the only two transformation products identified by gas chromatography. No transformation products of atrazine were identified under any of the conditions investigated. Since atrazine disappearance was measured in the denitrifying, methanogenic and sulfate-reducing systems, and complete mineralization to carbon dioxide and water was very unlikely, metabolites should have been formed in these reactors. It is likely, however, that the extraction method used did not capture polar transformation products. This project has received its third and final year of funding, but some work is continuing.

Clients/Users: This research is of interest to those who are responsible for non-point source pollution control including regulators, farmers, and U.S. Department of Agriculture.

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