Identification of indigenous phorate-degrading pathways during natural attenuation of contaminated soils

Soil contamination from organophosphorus pesticide production poses serious threats to human health. Understanding the degradation behavior of pesticides is crucial for developing effective remediation strategies. In this study, novel, highly effective extraction methods, including accelerated solvent extraction and online solid-phase extraction tailored for organophosphorus pesticides, were developed. Phorate and its degradation products were identified in a contaminated site of an organophosphorus pesticide factory in northern China using gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Key metabolites detected included phorate sulfone, phorate sulfoxide, phoratoxon, phoratoxon sulfone, and phoratoxon sulfoxide, along with other degradation products identified via GC-MS's unknown compound identification and deconvolution functions, and further analyzed with Compound Discoverer software. Various degradation pathways that led to the final formation of diethyl disulfide, formaldehyde, and hydrogen sulfide from phorate were uncovered. Among them, a novel degradation pathway for the intermediate O,O-diethyl thiophosphate was identified, where O,O-diethyl thiophosphate generated O,O,O-triethyl phosphorothioate in the presence of ethanol. Furthermore, four pathways leading to the formation of a newly identified compound, triethyl thiophosphate, were established: (1) phorate undergoing intramolecular nucleophilic hydrolysis via a six-membered ring transition state to form O,O,S-triethyldithiophosphate, which was subsequently oxidized; (2) O,O-diethyl thiophosphate, generated through phorate hydrolysis, reacting with ethanol; (3) diethyl chlorothiophosphate reacting with ethyl thiol; and (4) diethyl chlorothiophosphate being oxidized to diethyl chlorophosphate. Collectively, these findings provide essential evidence for understanding the fate of phorate during the natural attenuation of contaminated soils.