Toxicity evolution and control for the UV/HO degradation of nitrogen-containing heterocyclic compounds: SDZ and PMM.

This study aimed to achieve toxicity control of sulfadiazine (SDZ) and pirimiphos-methyl (PMM) via the UV/HO process by optimizing the reaction parameters. The results show that both drugs had a good degradation effect under the following parameters: a HO molar ratio of 1:200, and neutral conditions. SDZ and PMM could be degraded by more than 99% within 3 min, respectively. In the Daphnia magna acute toxicity assay and Vibrio fischeri inhibition assay, both SDZ and PMM exhibited a phenomenon of increasing toxicity. Additionally, through the use of density functional theory (DFT) calculation and HPLC-QTOF-MS, 21 transformation products (TPs) were identified, and the principal degradation pathways were proposed. The toxicity of the TPs was determined by comparing the QSAR prediction results with toxicity test data. As a result, under the higher UV light intensity (2300 μW/cm) and neutral conditions, SDZ showed highest toxicity, whereas PMM showed lowest toxicity under the lowest UV light intensity (450 μW/cm) and neutral conditions. Four main toxic TPs were identified, and their yields could be reduced by adjusting the reaction parameters. Therefore, the selection of appropriate reaction parameters could reduce the production of toxic TPs and ensure the safety of water environment.