The abiotic degradation of methyl parathion in anoxic sulfur-containing system mediated by natural organic matter

•NOM significant enhanced the abiotic degradation of methyl parathion in sulfide solution.•Methyl parathion reductive transformation mediated by NOM was specific to sulfide rather than other sulfur species.•Removal efficiency of methyl parathion related remarkably with NOM concentration and pH value.•Both the nitro group reduction and hydrolysis (SN@C) were acknowledged to be two transformation mechanisms.

Although the kinetics and transformation of methyl parathion have been investigated extensively, its abiotic degradation mechanism in anoxic sulfur-containing groundwater system is still not clear. In this work, the abiotic degradation of methyl parathion in anoxic sulfur-containing system mediated by natural organic matter (NOM) was investigated in batch experiments. It was found that the removal of methyl parathion (up to 80.7%) was greatly improved in sulfide containing NOM compared to those in sulfide alone (with 15.5%) and in NOM alone (almost negligible). Various sulfur species presented significant differences in behaviors methyl parathion degradation, but followed by the pseudo-first-order model well. No facilitated degradation of methyl parathion was observed in sulfite (SO32-) or thiosulfate (S2O32-) containing NOM such as anthraquinone. Although elemental sulfur (S0) and cysteine could further improve the degradation rate of methyl parahtion, their impacts was very limited. The removal efficiency of methyl parathion in anoxic sulfur-containing system were related remarkably with NOM concentration and solution pH. Based on the transformation products identified by gas chromatography-mass spectrometer (GC/MS) and liquid chromatography high resolution mass spectrometer (LC/HRMS), both the nitro group reduction and hydrolysis (SN@C) processes by sulfide (HS−) were further proved to be two predominant reaction mechanisms for the abiotic degradation of methyl parathion in anoxic sulfur-containing system. The results of this study help to understand the natural attenuation of methyl parathion under anoxic sulfide-containing groundwater system mediated by NOM.