Theoretical Simulation Degradation of Bromoxynil by Ozonation in Liquid Phase: Mechanism Pathways, Kinetics, and Ecotoxicity

Abstract

Ozonation has been identified as an effective technique to degrade benzene pollutants in the wastewater environment. Bromoxynil, widely employed in agriculture, poses a serious environmental concern. This study presents a comprehensive theoretical investigation of the bromoxynil and ozone reactions using high-level quantum chemical calculations, transition state theory simulations, and computational toxicology in the liquid phase. The bromoxynil and O₃ reaction follows the Criegee mechanism by forming primary ozonides (POZs). Density functional theory calculations indicated that the ozone addition to C3C4, C4C5, and C5C6 positions of the benzene ring of bromoxynil is predominant, forming the primary ozonides IM3, IM4, and IM5, respectively. IM4 is the most important primary ozonide, which predominantly yields the CI7 and CI8 Criegee intermediates. The formation pathways of POZs IM3 and IM5 compete with IM4; then IM3 and IM5 decompose into CI5, CI6, CI9, and CI10. The subsequent reaction channels of CI8 and CI10 include their further reactions with O₃, O₂, and H₂O. Transition state theory simulations based on the potential energy surfaces calculated here for the bromoxynil + O₃ reaction indicate that the IM4 reaction yields 42.69% at 298 K, and the branching fractions of IM3 and IM5 are 31.01% and 18.05%, respectively. According to the results of toxicity assessment, the acute and chronic toxicity of most degradation intermediates and byproducts are lower than bromoxynil for aquatic organisms after ozonolysis. The studied reaction mechanisms directly link the kinetics and toxicity of bromoxynil degradation. Our results have provided significant data for the degradation of bromoxynil, which are discussed.