Photochemical Transformations of Dichloroacetamide Safeners

Mechanism of safener mefenpyr-diethyl biodegradation by a newly isolated Chryseobacterium sp. B6 from wastewater sludge and application in co-contaminated soil

Safener mefenpyr-diethyl (MFD) was applied to cereal crops along with herbicides to improve herbicide selectivity for crops and weeds. However, the degradation mechanism of MFD in the environment remains unclear. One MFD-degrading bacterium, Chryseobacterium sp. B6, was isolated from activated sludge. According to Box-Behnken's optimal design, the degradation efficiency of MFD can reach 92% under conditions of pH 7.5, 30 °C, and a MFD concentration of 184 mg L-1. The degradation half-life experiment showed that a high concentration of MFD (300 mg L-1) inhibited the degradation ability of strain B6. Additionally, strain B6 was resistant to Ba2+, Cr3+, Li+, Zn2+, and Cu2+. The MFD degradation products of strain B6 were detected by GC/MS and its degradation pathway was proposed. MFD was first hydrolyzed by a hydrolase to an intermediate (RS)-1-(2,4-dichlorophenyl)-5-methyl-2-pyrazoline-5-carboxylic acid ethyl ester-3-carboxylic acid, and then further degraded by a decarboxylase to form the intermediate (RS)-1-(2,4-dichlorophenyl)-5-methyl-2-pyrazoline-5-carboxylic acid ethyl ester, finally, it is completely degraded by strain B6. Furthermore, strain B6 could effectively remove MFD from MFD-contaminated soil, and the half-life of MFD was also significantly reduced in MFD and Cu2+ co-contaminated soil after inoculating strain B6. To our knowledge, strain B6 was the first strain reported to degrade safener MFD, and this study provides a valuable candidate to remediate the co-contaminated soil with MFD and Cu2+.

Humic Substance Photosensitized Degradation of Phthalate Esters Characterized by 2H and 13C Isotope Fractionation

The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (³HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using ²H and ¹³C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for ²H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for ¹³C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of ²H and ¹³C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.

Microbial Biotransformation Products and Pathways of Dichloroacetamide Herbicide Safeners

Dichloroacetamide safeners are common ingredients in commercial herbicide formulations. We previously investigated the environmental fate of dichloroacetamides via photolysis and hydrolysis, but other potentially important, environmentally relevant fate processes remain uncharacterized and may yield products of concern. Here, we examined microbial biotransformation of two dichloroacetamide safeners, benoxacor and dichlormid, to identify products and elucidate pathways. Using aerobic microcosms inoculated with river sediment, we demonstrated that microbial biotransformations of benoxacor and dichlormid proceed primarily, if not exclusively, via cometabolism. Benoxacor was transformed by both hydrolysis and microbial biotransformation processes; in most cases, biotransformation rates were faster than hydrolysis rates. We identified multiple novel products of benoxacor and dichlormid not previously observed for microbial processes, with several products similar to those reported for structurally related chloroacetamide herbicides, thus indicating potential for conserved biotransformation mechanisms across both chemical classes. Observed products include monochlorinated species such as the banned herbicide CDAA (from dichlormid), glutathione conjugates, and sulfur-containing species. We propose a transformation pathway wherein benoxacor and dichlormid are first dechlorinated, likely via microbial hydrolysis, and subsequently conjugated with glutathione. This is the first study reporting biological dechlorination of dichloroacetamides to yield monochlorinated products in aerobic environments.

The dichloroacetamide safener benoxacor is enantioselectively metabolized by monkey liver microsomes and cytosol

The metabolism and toxicity of current-use herbicide safeners remain understudied. We investigated the enantioselective metabolism of the safener benoxacor in Rhesus monkey subcellular fractions. Benoxacor was incubated with liver microsomes and cytosol from female and male monkeys (≤30 min). Benoxacor levels and enantiomeric fractions were determined with gas chromatography. Benoxacor was metabolized by microsomal cytochrome P450 enzymes (CYPs), cytosolic glutathione-S-transferases (GSTs), and microsomal and cytosolic carboxylesterase (CESs). CES-mediated microsomal metabolism followed the order males > females, whereas the CYP-mediated clearance followed the order females > males. CYP-mediated metabolism initially resulted in an enrichment of the second eluting benoxacor enantiomer (E₂-benoxacor), whereas the first eluting benoxacor enantiomer (E₁-benoxacor) was enriched after 10 or 30 min in female or male microsomal incubations. Benoxacor metabolism by GSTs was enantiospecific, with a total depletion of E₁-benoxacor after approximately 20 min. Thus, the enantioselective metabolism of benoxacor by GSTs and CYPs may affect its toxicity.

Comparative Toxicity of Herbicide Active Ingredients, Safener Additives, and Commercial Formulations to the Nontarget Alga Raphidocelis Subcapitata

Chloroacetanilide herbicides are used worldwide to control weeds that affect crops such as corn, soybeans, and cotton. These herbicides are frequently paired with a "safener," which prevents herbicidal damage to the crop without diminishing weed control. Formulated herbicide products that include safeners and other ingredients are infrequently assessed for toxicity. Our goal was to understand the potential toxicity of safeners and herbicide + safener formulations relative to the toxicity of associated active ingredients. We quantified the concentration of safeners in commercially available formulations and tested effects on nontarget algae, Raphidocelis subcapitata, when exposed to individual herbicide active ingredients, safeners, and commercial formulations. The median effective concentrations (EC50s) causing 50% reduction in population growth for the herbicide active ingredients S-metolachlor and acetochlor were 0.046 and 0.003 ppm, respectively. The safeners benoxacor, AD-67, furilazole, and dichlormid were all substantially less toxic than the herbicides and were not toxic at environmentally relevant concentrations. The commercial formulations Dual II Magnum®, Me-Too-Lachlor II®, Harness®, and Surpass EC® all resulted in EC50 values that fell within the 95% confidence interval of the associated active ingredient herbicide. Interestingly, a significant increase in cell size was observed when algae were exposed to all the formulations, herbicides (acetochlor and S-metolachlor), and safener (dichlormid). The safener furilazole caused a significant decrease in cell size, whereas benoxacor and AD-67 had no observed effect on algae cell size. Significant algae cell size effects all occurred at or above the EC50 concentrations for each chemical, suggesting that other morphological effects may be occurring. Importantly, safeners in commercial formulations appeared not to impact toxicity to R. subcapitata compared with the active ingredient alone. Environ Toxicol Chem 2022;41:1466-1476. © 2022 SETAC.

Reactivity of chloroacetamides toward sulfide + black carbon: Insights from structural analogues and dynamic NMR spectroscopy

Chloroacetamides are commonly used in herbicide formulations, and their occurrence has been reported in soils and groundwater. However, how their chemical structures affect transformation kinetics and pathways in the presence of environmental reagents such as hydrogen sulfide species and black carbon has not been investigated. In this work, we assessed the impact of increasing Cl substituents on reaction kinetics and pathways of six chloroacetamides. The contribution of individual pathways (reductive dechlorination vs. nucleophilic substitution) to the overall decay of selected chloroacetamides was differentiated using various experimental setups; both the transformation rates and product distributions were characterized. Our results suggest that the number of Cl substituents affected reaction pathways and kinetics: trichloroacetamides predominantly underwent reductive dechlorination whereas mono- and dichloroacetamides transformed via nucleophilic substitution. Furthermore, we synthesized eight dichloroacetamide analogs (Cl₂CHC(=O)NRR') with differing R groups and characterized their transformation kinetics. Dynamic NMR spectroscopy was employed to quantify the rotational energy barriers of dichloroacetamides. Our results suggest that adsorption of dichloroacetamides on black carbon constrained R groups from approaching the dichloromethyl carbon and subsequently favored nucleophilic attack. This study provides new insights to better predict the fate of chloroacetamides in subsurface environments by linking their structural characteristics to transformation kinetics and pathways.

Acid- and Base-Mediated Hydrolysis of Dichloroacetamide Herbicide Safeners

Safeners are used extensively in commercial herbicide formulations. Although safeners are regulated as inert ingredients, some of their transformation products have enhanced biological activity. Here, to fill gaps in our understanding of safener environmental fate, we determined rate constants and transformation products associated with the acid- and base-mediated hydrolysis of dichloroacetamide safeners AD-67, benoxacor, dichlormid, and furilazole. Second-order rate constants for acid- (HCl) and base-mediated (NaOH) dichloroacetamide hydrolysis (2.8 × 10^-3 to 0.46 and 0.3-500 M^-1 h^-1, respectively) were, in many cases (5 of 8), greater than those reported for their chloroacetamide herbicide co-formulants. In particular, the rate constant for base-mediated hydrolysis of benoxacor was 2 orders of magnitude greater than that of its active ingredient co-formulant, S-metolachlor. At circumneutral pH, only benoxacor underwent appreciable hydrolysis (5.3 × 10^-4 h^-1), and under high-pH conditions representative of lime-soda softening, benoxacor's half-life was 13 h─a timescale consistent with partial transformation during water treatment. Based on Orbitrap LC-MS/MS analysis of dichloroacetamide hydrolysis product mixtures, we propose structures for major products and three distinct mechanistic pathways that depend on the system pH and compound structure. These include base-mediated amide cleavage, acid-mediated amide cleavage, and acid-mediated oxazolidine ring opening. Collectively, this work will help to identify systems in which hydrolysis contributes to the transformation of dichloroacetamides, while also highlighting important differences in the reactivity of dichloroacetamides and their active chloroacetamide co-formulants.

Tandem field and laboratory approaches to quantify attenuation mechanisms of pharmaceutical and pharmaceutical transformation products in a wastewater effluent-dominated stream

Evolving complex mixtures of pharmaceuticals and transformation products in effluent-dominated streams pose potential impacts to aquatic species; thus, understanding the attenuation dynamics in the field and characterizing the prominent attenuation mechanisms of pharmaceuticals and their transformation products (TPs) is critical for hazard assessments. Herein, we determined the attenuation dynamics and the associated prominent mechanisms of pharmaceuticals and their corresponding TPs via a combined long-term field study and controlled laboratory experiments. For the field study, we quantified spatiotemporal exposure concentrations of five pharmaceuticals and six associated TPs in a small, temperate-region effluent-dominated stream during baseflow conditions where the wastewater plant was the main source of pharmaceuticals. We selected four sites (upstream, at, and two progressively downstream from effluent discharge) and collected water samples at 16 time points (64 samples in total, approximately twice monthly, depending on flows) for 1 year. Concurrently, we conducted photolysis, sorption, and biodegradation batch tests under controlled conditions to determine the major attenuation mechanisms. We observed 10-fold greater attenuation rates in the field compared to batch tests, demonstrating that connecting laboratory batch tests with field measurements to enhance predictive power is a critical need. Batch systems alone, often used for assessment, are useful for determining fate processes but poorly approximate in-stream attenuation kinetics. Sorption was the dominant attenuation process (t_1/2<7.7 d) for 5 of 11 compounds in the batch tests, while the other compounds (n = 6) persisted in the batch tests and along the 5.1 km stream reach. In-stream parent-to-product transformation was minimal. Differential attenuation contributed to the evolving pharmaceutical mixture and created changing exposure conditions with concomitant implications for aquatic and terrestrial biota. Tandem field and laboratory characterization can better inform modeling efforts for transport and risk assessments.

Benoxacor caused developmental and cardiac toxicity in zebrafish larvae

Benoxacor (BN) is a highly effective antidote of dichloroacetamide herbicides generally used to protect crops from herbicidal damage. As a commonly used agrochemical, this herbicide antidote is continuously discharged in watercourses thus causing toxicity to aquatic organisms, and ultimately leading to contamination of the food chain. To date, its potential toxicity to the cardiac development of aquatic organisms has not been evaluated. In the present study, we have selected the zebrafish as a model to study the impact of BN on embryonic developmental and cardiac toxicity. The zebrafish embryos were exposed in 0.5, 1.0 and 2.0 mg/L BN from 5.5 to 72 h post-fertilization (hpf). The results indicated that the exposure to BN led to increased mortality and diminished heart and hatching rates in the embryos. BN exposure also brought pericardial edema (PE) and linear stretching of heart. Besides, exposure to BN induced an excessive accumulation of reactive oxygen species (ROS) in the zebrafish embryos and abnormal activities of the antioxidant enzymes, including catalase (CAT) and malondialdehyde (MDA). Moreover, exposure to BN caused serious cardiac toxicity of the embryos, accompanied by abnormality of heart development- and apoptosis-related genes. Surprisingly, astaxanthin (ASTA), as a common antioxidant, was found to be able to partially rescue the cardiac toxicity caused by BN, which indicated that ROS are probably the major reason for the resulting cardiotoxicity in zebrafish embryos. Our results suggest the need for a comprehensive safety evaluation of the regular consumption of benoxacor, which provides scientific basis for the development of health standards and assessment of potential risk in aquatic organisms or even human.

Pesticides: formulants, distribution pathways and effects on human health - a review

Pesticides are commonly used in agriculture to enhance crop production and control pests. Therefore, pesticide residues can persist in the environment and agricultural crops. Although modern formulations are relatively safe to non-target species, numerous theoretical and experimental data demonstrate that pesticide residues can produce long-term negative effects on the health of humans and animals and stability of ecosystems. Of particular interest are molecular mechanisms that mediate the start of a cascade of adverse effects. This is a review of the latest literature data on the effects and consequences of contamination of agricultural crops by pesticide residues. In addition, we address the issue of implicit risks associated with pesticide formulations. The effects of pesticides are considered in the context of the Adverse Outcome Pathway concept.

Dissipation, Fate, and Toxicity of Crop Protection Chemical Safeners in Aquatic Environments

Safeners are a group of chemicals applied with herbicides to protect crop plants from potential adverse effects of agricultural products used to kill weeds in monocotyledonous crops. Various routes of dissipation of safeners from their point of applications were evaluated. Despite the large numbers of safeners (over 18) commercially available and the relatively large quantities (~2 × 10⁶ kg/year) used, there is little information on their mobility and fate in the environment and occurrence in various environmental matrices. The only class of safeners for which a significant amount of information is available is dichloroacetamide safeners, which have been observed in some rivers in the USA at concentrations ranging from 42 to 190 ng/L. Given this gap in the literature, there is a clear need to determine the occurrence, fate, and bioavailability of other classes of safeners. Furthermore, since safeners are typically used in commercial formulations, it is useful to study them in relation to their corresponding herbicides. Common routes of dissipation for herbicides and applied safeners are surface run off (erosion), hydrolysis, photolysis, sorption, leaching, volatilization, and microbial degradation. Toxic potencies of safeners vary among organisms and safener compounds, ranging from as low as the LC₅₀ for fish (Oncorhynchus mykiss) for isoxadifen-ethyl, which was 0.34 mg/L, to as high as the LC₅₀ for Daphnia magna from dichlormid, which was 161 mg/L. Solubilities and octanol-water partition coefficients seem to be the principal driving force in understanding safener mobilities. This paper provides an up-to-date literature review regarding the occurrence, behaviour, and toxic potency of herbicide safeners and identifies important knowledge gaps in our understanding of these compounds and the potential risks posed to potentially impacted ecosystems.

Black carbon-enhanced transformation of dichloroacetamide safeners: Role of reduced sulfur species

Dichloroacetamide safeners are commonly included in herbicide formulations to protect crops from unintended herbicide toxicity, but knowledge of their environmental fate is scarce. Hydrogen sulfide, a naturally-occurring nucleophile and reductant, often coexists with black carbon (e.g., biochar, soot) in subsurface environments and could influence the fate of these safeners. In this study, we demonstrated that graphite powder, a model black carbon, significantly accelerated the transformation of three dichloroacetamide safeners (AD-67, benoxacor, and dichlormid) and two chloroacetamide herbicides (metolachlor and acetochlor) by hydrogen sulfide. Chloride was formed together with an array of sulfur-substituted products, suggesting a nucleophilic substitution pathway. Our results suggest that the electron-accepting capacity of black carbon can oxidize hydrogen sulfide species to elemental sulfur, which can further react with bisulfide to form polysulfide, likely accounting for the observed accelerated transformation of (di)chloroacetamides in systems containing black carbon and hydrogen sulfide. Moreover, our product analyses indicate that dimerization and/or trimerization of (di)chloroacetamides is possible in the presence of hydrogen sulfide and graphite, which is anticipated to decrease the mobility of these products in aquatic environments relative to the parent compounds. Herein, we also discuss how the structure and concentration of (di)chloroacetamides can influence their reactivity in the presence of black carbon and reduced sulfur species.

Photolysis of Trenbolone Acetate Metabolites in the Presence of Nucleophiles: Evidence for Metastable Photoaddition Products and Reversible Associations with Dissolved Organic Matter

Photolysis of trenbolone acetate (TBA) metabolites in the presence of various nitrogen-, sulfur-, or oxygen-containing nucleophiles (e.g., azide, ammonia, or thiosulfate, respectively) results in rapid (half-lives ∼20-60 min), photochemically induced nucleophile incorporation across the parent steroid's trienone moiety. The formation of such nucleophile adducts limits formation of photohydrates, suggesting competition between the nucleophile and water for photochemical addition into the activated steroid structure. Analogous to previously reported photohydration outcomes, LC/MS analyses suggest that such photonucleophilic addition reactions are reversible, with more rapid elimination rates than thermal dehydration of photohydrates, and regenerate parent steroid structures. Beyond photonucleophilic addition pathways, we also found that hydroxylamine and presumed nucleophilic moieties in model dissolved organic matter (DOM; Fluka humic acid) can react via thermal substitution with TBA metabolite photohydrates, although this reaction with model DOM was only observed for photohydrates of trendione. Most nucleophile addition products [i.e., formed via (photo)reaction with thiosulfate, hydroxylamine, and ammonia] are notably more polar relative to the parent metabolite and photohydration products. Thus, if present, both nucleophilic adducts and bound residues in organic matter will facilitate transport and help mask detection of TBA metabolites in surface waters and treatment systems.

Reductive transformations of dichloroacetamide safeners: effects of agrochemical co-formulants and iron oxide + manganese oxide binary-mineral systems

The toxic effects of herbicides are often incompletely selective and can harm crops. Safeners are "inert" ingredients commonly added to herbicide formulations to protect crops from herbicide-induced injury. Dichloroacetamide safeners have been previously shown to undergo reductive dechlorination in anaerobic abiotic systems containing an iron (hydr)oxide mineral (goethite or hematite) amended with Fe(ii). Manganese oxides (e.g., birnessite) are important redox-active species that frequently co-occur with iron (hydr)oxides, yet studies examining the effects of more than one mineral on transformations of environmental contaminants are rare. Herein, we investigate the reactivity of dichloroacetamide safeners benoxacor, furilazole, and dichlormid in binary-mineral, anaerobic systems containing Fe(ii)-amended hematite and birnessite. As the molar ratio of Fe(ii)-to-Mn(iv) oxide increased, the transformation rate of benoxacor and furilazole increased. The safener dichlormid did not transform appreciably over the sampling period (6 hours). The concentration of pH buffer ([MOPS] = 10-50 mM), ionic strength ([NaCl] = 10-200 mM), and order of solute addition (e.g., safener followed by Fe(ii) or vice versa) do not appreciably affect transformation rates of the examined dichloroacetamide safeners in Fe(ii) + hematite slurries. The presence of agrochemical co-formulants, including the herbicide S-metolachlor and three surfactants, in solutions containing Cr(H₂O)₆²⁺ (as a model homogeneous reductant) also did not substantially influence rates of safener transformation. This study is among the first to examine laboratory systems of intermediate complexity (e.g., systems containing mixtures of agrochemical co-formulants or mineral phases) when assessing the environmental fate of emerging contaminants such as dichloroacetamide safeners.

Benoxacor is enantioselectively metabolized by rat liver subcellular fractions

This study investigated the enantioselective metabolism of benoxacor, an ingredient of herbicide formulations, in microsomes or cytosol prepared from female or male rat livers. Benoxacor was incubated for ≤30 min with microsomes or cytosol, and its enantioselective depletion was measured using gas chromatographic methods. Benoxacor was depleted in incubations with active microsomes in the presence and absence of NADPH, suggesting its metabolism by hepatic cytochrome P450 enzymes (CYPs) and microsomal carboxylesterases (CESs). Benoxacor was depleted in cytosolic incubations in the presence of glutathione, consistent with its metabolism by glutathione S-transferases (GSTs). The depletion of benoxacor was faster in incubations with cytosol from male than female rats, whereas no statistically significant sex differences were observed in microsomal incubations. The consumption of benoxacor was inhibited by the CYP inhibitor 1-aminobenzotriazole, the CES inhibitor benzil, and the GST inhibitor ethacrynic acid. Estimates of the intrinsic clearance of benoxacor suggest that CYPs are the primary metabolic enzyme responsible for benoxacor metabolism in rats. Microsomal incubations showed an enrichment of the first eluting benoxacor enantiomer (E₁-benoxacor). A greater enrichment occurred in incubations with microsomes from female (EF = 0.67 ± 0.01) than male rats (EF = 0.60 ± 0.01). Cytosolic incubations from female rats resulted in enrichment of E₁-benoxacor (EF = 0.54 ± 0.01), while cytosolic incubations from male rats displayed enrichment of the second eluting enantiomer (E₂-benoxacor; EF = 0.43 ± 0.01). Sex-dependent differences in the metabolism of benoxacor in rats could significantly impact ecological risks and mammalian toxicity. Moreover, changes in the enantiomeric enrichment of benoxacor may be a powerful tool for environmental fate and transport studies.

Sunlight Photolysis of Safener Benoxacor and Herbicide Metolachlor as Mixtures on Simulated Soil Surfaces

Benoxacor is a safener paired with the high-use herbicide S-metolachlor. Commercial formulations containing both compounds are sprayed onto soil pre-emergence to enhance yields of corn. In this study, we evaluated the sunlight photolysis of metolachlor and benoxacor, individually and as mixtures, in three different reaction environments: in water and on two soil-simulating surfaces (quartz and kaolinite). When irradiated individually, benoxacor degraded at least 19 times faster than metolachlor in each reaction environment, consistent with its higher molar absorptivity within the solar spectrum than metolachlor. When metolachlor and benoxacor were irradiated as mixtures, benoxacor promoted metolachlor degradation on quartz and, to a lesser extent, in water, but not on kaolinite. On quartz, at a benoxacor/metolachlor molar ratio of 0.1:1, metolachlor degraded 1.8 times faster than in the absence of benoxacor; as the benoxacor/metolachlor ratio increased, metolachlor degradation rate also increased. The photolysis rate of benoxacor depended on its initial surface concentration and was promoted by metolachlor. Benoxacor photoproducts were capable of absorbing sunlight and serving as photosensitizers for metolachlor degradation. These results illustrate how a safener can influence the photochemistry of its coformulated herbicide and suggest that such mixture effects should be considered when evaluating the environmental fate of agrochemicals.