Oxidative transformation of sulfamethoxazole during direct and radical Ozonation: Pathways, kinetics, and water quality effects

Sulfamethoxazole (SMX) is a widely used antibiotic that frequently persists in aquatic environments, and whose behavior during advanced oxidation processes remains complex. In this work, we examined its degradation by ozonation under two clearly differentiated pH conditions chosen to clearly separate a regime dominated by molecular ozone (pH 3.0) from one driven almost entirely by hydroxyl radicals (pH 13.0). Ozone behaved very differently in each system: at acid pH it remained relatively stable and accumulated in the liquid phase, whereas at high pH it decomposed almost immediately, leaving the process governed by radical formation and by a mass-transfer rate that hardly diminished over time. These contrasting oxidation environments strongly influenced how SMX transformed. Under acid conditions, SMX degradation followed apparent pseudo-first-order kinetics with a rate constant of 0.3541 min^-1, leading to rapid removal and well-defined accumulation-decay profiles of transformation products. In contrast, under alkaline conditions the degradation rate constant decreased to 0.2231 min^-1, and several intermediates persisted for longer periods due to the radical-dominated regime. Acid ozonation also resulted in substantially higher mineralization, with a marked TOC removal (36.9%), whereas negligible TOC reduction was observed under alkaline conditions (2.1%). These mechanistic differences were also reflected in water quality parameters, with the acid system showing more consistent improvements in color, aromaticity and TOC. Overall, the comparison illustrates how the oxidation regime governs both the nature of the intermediates and the pace at which they evolve throughout the process.