Computational investigation of HO•-induced atmospheric transformation and ecological risk assessment of 2-methyl-4-chlorophenoxyacetic acid

Abstract

2-Methyl-4-chlorophenoxyacetic acid (MCPA) is a pervasive phenoxyacetic acid herbicide, frequently identified in environmental matrices. This study undertakes a systematic investigation of the atmospheric environmental transformation behavior and potential ecological risks of hydroxyl radical (HO•) induced MCPA for the first time, employing computational chemistry and computational toxicology methodologies. Our findings indicate that the initial degradation of MCPA can be triggered by both HO•-addition and hydrogen-abstraction reactions, with the latter being more likely to occur. The primary reaction pathways were identified as R2, R3, R8, and R13. The total rate constant for the gas-phase reaction of MCPA with HO• was calculated as 8.59 × 10−12 cm3 molecule −1 s−1 at 298 K. This total rate constant displayed a distinct positive temperature dependence within the temperature range of 250–375 K. Structures of 25 transformation products, which include 7 experimentally observed compounds, were determined from further reactions of crucial intermediates. The toxicity assessment of MCPA and its transformation products indicated that most of the hydroxyl oxidation products exhibit toxicity to aquatic organisms and possess mutagenic, developmental toxic, non-genotoxic carcinogenic, and skin irritating or corrosive properties. This research contributes to a comprehensive understanding of the transformation mechanisms, kinetics, and environmental repercussions of gas-phase MCPA, enhancing our ability to predict and mitigate its ecological impact.