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

Abstract

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.