Aqueous degradation of tioxazafen: Kinetics, products, pathways, and toxicity

Tioxazafen (TXF) is the first 1,2,4-oxadiazole nematicide. In the present study, the aqueous degradation of TXF was investigated in terms of hydrolysis and photolysis. Under the irradiation of simulated sunlight, TXF degraded very fast in ultrapure water and buffers with half-lives (t_1/2s) <8.3 min. A sole photoproduct (PP) PP228a was isolated, and identified by spectroscopic means (UV, IR, HRMS, and ¹H NMR) to be the thiophen-3-yl isomer converted from its thiophen-2-yl parent. Comparing with TXF, PP228a had quite extended t_1/2s ranging from 6.9 to 7.9 d. The photolysis kinetics of TXF and PP228a showed no pH-dependence, and varied for each individual compound as affected by nitrate, fulvic acid, and humic acid. Besides, both compounds were hydrolytically stable. 6 PPs of PP228a were identified, with two of them being its isomers. The mechanisms involved in the process included the biradical photosensitization, photoinduced electron transfer, and ring contraction-ring expansion reactions. The 48 h-EC₅₀ to Daphnia magna was 0.808 mg/L for PP228a comparing to >1.12 mg/L for TXF, while the results of Vibrio fischeri assays indicated that one or more PPs of PP228a might have higher toxicity.