Environmental occurrence, fate, and transformation of herbicide safeners

Monica E. McFadden

doi:10.17077/etd.006338

Herbicide safeners are a growing class of agrochemicals commonly included in commercial herbicide formulations to selectively protect crops from herbicidal toxicity. Due to their extensive use and hydrophilic nature, several species of safeners have been detected in surface waters throughout the Midwestern U.S. Yet, despite the widespread use and occurrence of safeners, their environmental fate and effects remain poorly understood. Recent studies have demonstrated that some safeners can transform under environmentally relevant conditions into products with increased biological activity that may pose human and environmental health risks. Despite these concerns, safener applications are not closely tracked, and there are limited available environmental data regarding safener occurrence, fate, and effects. Thus, there is a need to evaluate the presence, fate, and transformation of herbicide safeners and their transformation products in the environment. The overall goal of this work was to evaluate the presence of safeners near application sites, and to quantify the timescales, products, and pathways of safener transformation in natural and engineered systems. This work was accomplished through four major objectives, focusing on analytical methods development, identification of transformation products and pathways, and quantification of sorption parameters.The first objective was to establish and employ a new method for quantifying a recently-registered safener, cyprosulfamide, and two of its major degradates in water samples near cornfields where cyprosulfamide was applied. We developed a novel method using solid-phase extraction and liquid chromatography with tandem mass spectroscopy (LC-MS/MS) for quantifying cyprosulfamide and the products cyprosulfamide desmethyl and N-cyclopropyl-4-sulfamoylbenzamide, which have been recommended by the EPA as “residues of concern in drinking water”. To evaluate the potential for off-field transport and transformation of cyprosulfamide, the method was used to analyze groundwater and surface water samples collected near cornfields in the Midwestern U.S. where cyprosulfamide had been applied. All three compounds were detected in surface water samples and demonstrated seasonal detection trends. N-cyclopropyl-4-sulfamoylbenzamide was most frequently detected (56%), followed by cyprosulfamide (25%) and cyprosulfamide desmethyl (19%). Maximum concentrations ranged from 22.0 to 5185.9 ng/L. None of the target analytes were detected in groundwater. The second objective assessed the pH dependence of hydrolysis rates and products for four safeners of the popular dichloroacetamide class in natural and engineered systems and identified hydrolysis product structures and pathways. We determined second-order rate constants for acid- (HCl) and base-mediated (NaOH) dichloroacetamide hydrolysis that were, in many cases, greater than those reported for their structurally-related chloroacetamide herbicide co-formulants. Most notably, the rate constant for the base-mediated hydrolysis of benoxacor was two orders of magnitude greater than that of its active ingredient co-formulant, S-metolachlor; indeed, at pH 10, the half-life of benoxacor was approximately 0.5 days while that of metolachlor was estimated to be 110 days. Only benoxacor transformed by hydrolysis under circumneutral pH conditions, with a half-life of approximately 55 days. Under high pH conditions representative of lime-soda softening, the half-life of benoxacor was 13 hours–a timescale relevant to partial transformation during water treatment. Based on Orbitrap LC-MS/MS, we identified hydrolysis product structures and proposed three distinct, generalizable mechanistic pathways that depend on system pH and compound structure. Identified pathways include base-mediated amide cleavage, acid-mediated amide cleavage, and acid-mediated oxazolidine ring opening. Collectively, this work helps to identify environments where hydrolysis is most relevant to the fate of dichloroacetamide safeners and highlights important differences in the reactivity of safeners and their herbicide co-formulants. The third objective quantified microbial biotransformation rates, identified products, and elucidated pathways for dichloroacetamide safeners in aqueous systems. Using aerobic microcosms inoculated with river sediment, we demonstrated that microbial biotransformation of benoxacor and dichlormid proceeds primarily, if not exclusively, via co-metabolic processes. When a labile carbon source is present, we found that benoxacor is transformed by both hydrolysis and microbial biotransformation processes, and in most cases biotransformation rates were faster (half-life in systems with labile carbon was approximately 15 days) than those we measured for hydrolysis (half-life was 41 days). Further, we observed multiple microbial biotransformation products for benoxacor and dichlormid that are analogous to those reported for structurally-related chloroacetamide herbicides, thus indicating the potential for a conserved biotransformation mechanism across both chemical classes. Observed products include monochlorinated species such as the regulated herbicide CDAA, glutathione conjugates, and sulfonated species. We propose a transformation mechanism mediated by glutathione-S transferase in which the safener is first dechlorinated and is subsequently conjugated with glutathione. The glutathione conjugate would likely be further cleaved by other enzymes, such as carboxytranspeptidase, cysteine beta-lyase, and additional oxidase enzymes that are reported to play a role in the microbial biotransformation of structurally-related chloroacetamide herbicides. The fourth and final objective quantified soil partitioning coefficients to predict the fate and mobility of dichloroacetamide safeners and their transformation products in model soils of varying organic carbon content. Linear sorption isotherms and kinetics studies revealed that safener sorption scaled with soil organic carbon content, indicating that uptake is driven by hydrophobic partitioning and non-specific binding interactions. Calculated Kd values for dichloroacetamides and the transformation products of benoxacor and dichlormid demonstrate greater mobility for transformation products compared to safeners. The increased mobility of transformation products has important implications for safener monitoring efforts as well as exposure risk, as the products are more likely to move from the application site into water systems compared to the parent compounds. Collectively, this work highlights dichloroacetamide safeners as mobile, reactive, and biologically-active compounds that can yield products via hydrolysis and microbial biotransformation that may pose increased risk to humans and environmental health. By establishing a better understanding of safener presence, identifying relevant transformation processes and respective rates, and by quantifying the relative sorption parameters of safeners and their transformation products, this work improves our understanding of the environmental fate, risk, and potential effects of herbicide safeners and their transformation products.

Environmental occurrence, fate, and transformation of herbicide safeners

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