Degradation of methyl parathion in thermally activated peroxymonosulfate processes: Kinetics, reaction mechanism and toxicity evaluation

Methyl parathion, as widely utilized organophosphorus pesticide, was commonly detected in aquatic environments. Heat-activated peroxymonosulfate (PMS) represented an emerging advanced oxidation process. In this study, the degradation efficiency and reaction mechanism of methyl parathion by heat-activated PMS system were investigated by experimental and theoretical aspects. The findings indicated that the degradation efficacy of methyl parathion improved with the increase in temperature, PMS concentration and pH. The radical quenching experiments revealed that the primary reactive radical initiating methyl parathion degradation was ·OH. Moreover, we determined that the coexistence mechanism existed in the heat-activated PMS process involving both PMS direct oxidative and radical oxidative degradation of methyl parathion. Then, the possible reactions of methyl parathion with ·OH and SO 4 ˙ˉ in the water environment were calculated at SMD/M06–2X/6–311++G(3df,2p)//SMD/M06–2X/6–311+G(d,p) level to clarify the detailed reaction mechanism. It was found that two reactive radicals were more prone to react with methyl parathion via addition reaction at the P=S bond. The total reaction rate constants of methyl parathion with ·OH and SO 4 ˙ˉ slightly decreased as the temperature increased from 298 K to 333 K. Eventually, the toxicity evaluation by the QSAR-based toxicity prediction software demonstrated that the aquatic toxicity of most products was lower than methyl parathion, but still has positive mutagenicity and toxic developmental toxicity. This work offered novel insights into the reaction mechanism of methyl parathion with activated PMS and could also help to clarify the fate of methyl parathion in the water environment.