Chemical and toxicological evolution of the antibiotic sulfamethoxazole under ozone treatment in water solution

This work studied the elimination paths of the sulfonamide antibiotic sulfamethoxazole by ozonation in fast kinetic regime. The ozonation runs were performed in conditions favouring either the direct attack of the ozone molecule or the indirect attack by ozone-generated radical species with initial concentration of 0.150 mM. When doses of ozone were transferred to the liquid phase 0.2 mM, in no case did sulfamethoxazole remain in solution. Two main transformation pathways were found involving the preferential attack of molecular ozone or radical pathway and leading to the formation of six intermediates, which were identified by LC-ESI-QTOF-MS. Both routes took place simultaneously in the different conditions tested, leading to a hydroxylation reaction of the benzene ring, oxidation of the amino group on the benzene ring, oxidation of the methyl group and the double bond in the isoxazole ring and S–N bond cleavage. The most abundant reaction intermediate was that resulting from S–N bond cleavage. The toxicity of partially ozonated samples for Daphnia magna and Pseudokirchneriella subcapitata revealed the formation of toxic by-products during the early stages of reaction and the persistence of considerable toxicity after the total depletion of sulfamethoxazole.