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

Abstract

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.

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

“…Importantly, dichloroacetamide safeners can undergo photolysis (Kral et al, 2019;Su et al, 2019), hydrolysis (McFadden et al, 2022), abiotic reductive dechlorination (Ricko et al, 2020;Sivey & Roberts, 2012;Xu et al, 2020Xu et al, , 2022, and *Indicates significant difference compared with controls (shown as 0 for each figure). AD, AD-67.…”

“…Importantly, dichloroacetamide safeners can undergo photolysis (Kral et al, 2019; Su et al, 2019), hydrolysis (McFadden et al, 2022), abiotic reductive dechlorination (Ricko et al, 2020; Sivey & Roberts, 2012; Xu et al, 2020, 2022), and biotransformation (Abu‐Qare & Duncan, 2002) in the environment. These processes can result in the safener changing into structures like those of herbicides, which could be more toxic than the parent (Abu‐Qare & Duncan, 2002; Sivey & Roberts, 2012).…”

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

“…For example, the additive (co-formulation compound) benoxacor, which is used as a safener (i.e. a compound used in combination with herbicides to reduce negative effects on crops) of the herbicide metolachlor, accelerates the photodegradation of the active ingredient on soil surfaces, lessening its toxicity [ 341 ]. The extent to which biocides are photodegraded is also highly dependent on where the biocide residues occur.…”

Acid- and Base-Mediated Hydrolysis of Dichloroacetamide Herbicide Safeners

“… 2 − 6 Given their widespread use (estimated >8 × 10 6 kg/year globally) and occurrence in the environment, there is a growing body of research regarding the environmental fate and effects of dichloroacetamides. 2 , 4 − 10 Recent studies have demonstrated moderate acute toxicity of dichloroacetamides toward model freshwater fish species (LC 50 values of 1.4–4.6 mg/L), 11 , 12 and toxicity screening data available from the U.S. Environmental Protection Agency (EPA) indicate the potential for benoxacor to interact with multiple human nuclear receptors. 13 The EPA has also identified AD-67 and furilazole as “likely to be carcinogenic to humans”.…”

“…The dichloroacetamide safeners AD-67, benoxacor, dichlormid, and furilazole are commonly included in chloroacetamide herbicide formulations to selectively protect crops against herbicide toxicity. , Dichloroacetamides (Figure ) are modestly hydrophilic (log K ow 1.84–3.19) and mobile in aqueous environments, which has led to their detection in Midwestern drinking water sources at concentrations of up to 190 ng/L. − Given their widespread use (estimated >8 × 10 6 kg/year globally) and occurrence in the environment, there is a growing body of research regarding the environmental fate and effects of dichloroacetamides. ,− Recent studies have demonstrated moderate acute toxicity of dichloroacetamides toward model freshwater fish species (LC 50 values of 1.4–4.6 mg/L), , and toxicity screening data available from the U.S. Environmental Protection Agency (EPA) indicate the potential for benoxacor to interact with multiple human nuclear receptors . The EPA has also identified AD-67 and furilazole as “likely to be carcinogenic to humans”. , Furthermore, some dichloroacetamide safeners, including dichlormid and benoxacor, can transform under environmentally relevant conditions to yield products with increased biological activity that may pose even greater risks to the environment and human health.…”