Combined Experimental and Computational Study on the Transformation of a Novel 1,3,4-Oxadiazole Thioether Nematicide in Aqueous Solutions

It has been demonstrated that Exianliuyimi (EXLYM) exhibits good nematocidal activity. As a potential nematicide, EXLYM and its transformation products (TPs) may generate emerging pollutants with hazardous effects on the ecosystem. In this study, the fate of EXLYM in aqueous solutions was investigated using experimental and theoretical approaches. Laboratory-scale experiments showed that EXLYM is hydrolytically stable. Microbial processes are primarily responsible for the oxidation of sulfur in aqueous solutions. Under simulated sunlight, the t 1/2 values of EXLYM in acidic, neutral, and alkaline buffer solutions were 5.02, 3.83, and 5.55 h, respectively. Six TPs were identified using a non-target screening strategy realized by ultra-high-performance liquid chromatography coupled with Q-Exactive Orbitrap high-resolution mass spectrometry and 18O-labeling experiments. Four of these were unambiguously confirmed using authentic standards. Reactive oxygen species scavenging experiments, 18O-labeling experiments, and quantum-theoretical calculations suggested that EXLYM could degrade mainly through four pathways: sulfur oxidation, nucleophilic aromatic photosubstitution, C–S bond cleavage, and oxidative ring-opening. The proposed degradation kinetics, TPs, and transformation pathways in aqueous solutions provide valuable information on the fate of EXLYM in aquatic ecosystems and lay the foundation for further toxicological tests.