UV/Chlorine Treatment of Sulfamethoxazole: Removal, Transformation Products, Ecotoxicity, and Environmental Fate

UV/NaOCl and UV/H₂O₂/NaOCl treatments of sulfamethoxazole (SMX) were compared across several reagent ratios using pseudo-first-order kinetics, LC-DAD/LC-MS screening of transformation products (TPs), multi-trophic bioassays, and quantitative structure–activity relationship (QSAR)-based toxicity and fate screening. Increasing the NaOCl proportion from an SMX:NaOCl molar ratio of 1:1 to 1:10 increased the pseudo-first-order rate constant from 0.215 to 0.818 min⁻¹, but faster SMX removal did not correspond to a more favorable post-treatment bioassay profile. The 1:1 and 1:2 UV/NaOCl systems achieved substantial SMX removal within 10 min and gave the lowest responses among the treated systems in the Microtox^®, Daphtox F^®, and Lemna sp. assays. Higher NaOCl loadings and the mixed-oxidant systems produced stronger post-treatment bioassay responses and larger signals for ECOSAR-prioritized chlorinated aromatics and coupling products. Fate screening further distinguished more persistent, strongly sorbing products from TPs with lower sorption or greater predicted transport potential. The tested reagent ratios therefore revealed a trade-off between removal kinetics and the biological and environmental profile of the resulting mixtures.