Mechanisms of bisulfite/MnO₂-accelerated transformation of methyl parathion

Although bisulfite is able to activate manganese oxides for enhanced oxidation of organic contaminants with donor-electron functions, the removal mechanisms for some esters remain poorly understood. In this study, we investigated the bisulfite/MnO₂-accelerated transformation of methyl parathion (MP), a recalcitrant and toxic organophosphorus pesticide (OPP). The removal rate constants of MP depended on pH, oxygen conditions, and the ratio between [HSO₃^-] and [MnO₂]. MP transformation declined by 36% with the addition of pyrophosphate as a scavenging agent for Mn(III)aq. [Mn(OH)(SO₃H)]⁺, a reactive intermediate, may be involved in enhancing the transformation of MP. The overall reaction can be divided into three distinct processes. The first process comprises two steps: the dissolution of MnO₂ reduced by HSO₃^- and the formation of a Mn-sulfite complex by a relatively fast substitution-controlled process. The second process is much slower and forms a precursor organometallic complex between the MP and Mn(IV/III). The third process involves a series of redox/hydrolysis reactions via aqueous and surface reactions. The mechanisms of each process were interpreted using kinetic observation and product identification data. This study improved the fundamental understanding of the MnO₂/HSO₃^- reaction process, thereby increasing the feasibility for remediating OPP pollution of the soil-water environment.