Sulfate radical anion oxidation of diclofenac and sulfamethoxazole for water decontamination

This study aimed at investigating the reactivity of aniline-based pharmaceuticals with sulfate radical anion generated by the peroxymonosulfate (PMS)/Cobalt(II) system and by taking sulfamethoxazole and diclofenac as probe compounds. Transformation pathways were proposed for both pharmaceuticals relying on the identification of the main reaction intermediate products using liquid chromatography coupled to time of flight mass spectrometry. For diclofenac, the major transformation pathway led to the formation of a quinone imine derivative while for sulfamethoxazole a nitro derivative was obtained as the major intermediate. However, in both cases, oxidative reactions started with the formation of a N-centered radical probably through an one-electron transfer from the pharmaceutical to sulfate radical anion. This was followed by further multistep degradation involving decarboxylation, hydroxylation and bond cleavage reactions in route to mineralization. From a kinetic point of view, aniline-based pharmaceuticals quickly reacted with sulfate radical anion with second order kinetic rate constants in the 9–10 × 109 M−1 s−1 range. Sulfate radical-based technologies show promising results for the removal of these particular pharmaceuticals from wastewater treatment plant effluents mainly because of the higher selectivity of sulfate radical anion over hydroxyl radical, limiting radical scavenging by natural organic matter and allowing for higher abatement and mineralization rates.