Atmospheric oxidation of fenthion initiated by hydroxyl radical and ozone

The reaction of pesticides with atmospheric oxidants, such as reactive oxygen species and reactive nitrogen species is a crucial factor in assessing their environmental persistence, long-range transport, degradation and impact. This study investigates the gas phase oxidation of fenthion, a widely used organothiophosphate pesticide initiated by hydroxyl radical (˙OH) and ozone (O₃) using density functional theory (DFT) calculations at M06-2X/6-311++G(d,p) level of theory. For the ˙OH initiated reactions 20 reaction pathways were studied and the ˙OH addition to the phosphorus atom of fenthion is found to be the dominant reaction. The overall rate constant for the degradation of fenthion by ˙OH is found to be 2.29 × 10^-9 cm³ per molecule per s at 298 K, leading to a short atmospheric lifetime of ∼4 minutes. Subsequent oxidation of the key intermediates formed from the reaction of fenthion with ˙OH leads to the formation of toxic products, notably fenthion oxon (P5 and P6) which is an acetylcholinesterase inhibitor, and O,O-dimethyl phosphorothioic acid (P3) known for its estrogenic activity. The results obtained from time-dependent DFT calculations show that neither fenthion nor its products undergo photolysis under tropospheric sunlight. For the O₃-initiated degradation, 25 reaction pathways were explored, including addition of O₃ to the aromatic ring and the PS bond. Although O₃-initiated reactions are exothermic, their high energy barrier limits their atmospheric significance. Overall, this study highlights the high reactivity of fenthion with ˙OH and the significant toxicity of its degradation products, emphasizing the need for evaluating both the atmospheric fate and environmental toxicity of organophosphorus pesticides.