Biotransformation of Herbicides in the Presence of Various Electron Acceptors

Abstract

Among the important factors affecting the biotransformation of xenobiotic chemicals upon their release into the environment are the dominant electron acceptor condition present and the presence of other, more readily degraded carbon sources. Here, glass-bead biofilm columns were used to investigate the effects of the presence of three different inorganic electron acceptor conditions (oxygen respiration, nitrate reduction, and sulfate reduction) on the biotransformation of the acetanilide herbicides alachlor and propachlor, and to determine the effects of two exogenous carbon sources (acetate and glucose) on their biotransformation under each of these conditions.Biotransformation of alachlor and propachlor occurred in the presence of both carbon sources and under each of the three electron acceptor conditions. Both were transformed most rapidly under sulfate-reducing conditions. Analysis by gas chromatography/mass spectrometry (GC/ MS) did not reveal any significant metabolic products. Both herbicides react abiotically with bisulfide, produced within the sulfate-reducing cultures, though most of the transformation was attributed to the microorganisms. The primary, readily degraded carbon source (acetate or glucose) was needed to establish each culture, and its continuous presence was required to sustain herbicide biotransformation in the sulfate-reducing reactors. Loss of either acetate or glucose from the column influent did not significantly affect herbicide biotransformation in the aerobic or nitratereducing reactors, at least for short periods. Temporary loss of the external electron acceptors (O 2 , NO 3 -, or SO 4 2-

Assessment of Woodchip Bioreactor Characteristics and Their Influences on Joint Nitrate and Pesticide Removal

“…Similar to the results under aerobic conditions, past studies found that degradation under anaerobic conditions typically occurs over much longer time scales (several days to months) irrespective of the throughput rate of woodchip bioreactors. 62,63 Integrating longer sequencing (HRT/AET) time scales into woodchip bioreactor treatment cycles may improve microbial degradation of pesticides.…”

Nucleophilic Aliphatic Substitution Reactions of Propachlor, Alachlor, and Metolachlor with Bisulfide (HS-) and Polysulfides (Sn2-)

“…Even though chloroacetanilides are slow to hydrolyze at ambient pH (for example, the hydrolysis half-life for acetochlor is > 6 years at pH 7 and 25 °C (25)), abiotic reactions could still be important under certain conditions. Wilber and co-workers (26,27) reported rate constants for the reactions of alachlor and propachlor with HS -, but the temperature at which the experiments were conducted was not specified, nor were the products characterized. Stamper et al (28) found that alachlor, metolachlor, and propachlor underwent facile abiotic reaction with HS -, but their experimental approach precluded extraction of rate constants from the results.…”

Characterization of Two Novel Propachlor Degradation Pathways in Two Species of Soil Bacteria

“…Controlled persistence and biodegradation of herbicides in soil and water is highly desirable for reducing contamination and protecting our food and environment (3,6,7,20). Acetamide herbicides are used as preemergence herbicides for selective control of monocotyledon and dicotyledon weeds.…”