The reactivity of peroxymonosulfate towards sulfamethoxazole

In this study, an in-depth assessment of the peroxymonosulfate (PMS) reactivity towards sulfamethoxazole (SMX) is reported for the first time based on (i) a comparative kinetics of SMX recorded under different sources of light (simulated solar irradiation and UV-C) and in the dark, all under different operational conditions (PMS and SMX dosages, pHs, temperatures, presence of chloride and hydrogen carbonate ions) and (ii) monitoring the role of Reactive Oxygen Species (ROS) involved in each investigated case with the use of proper scavengers together with the recording of Electron Paramagnetic Resonance spectra. SMX degradation rates obtained in dark, at acid/neutral pH and in presence of selective ROS scavengers demonstrated that SO4•–, 1O2 and HO• played a minor role; while the degradation at different pHs suggested the presence of two operational mechanisms: direct oxidation of the substrate by direct electron transfer between PMS and SMX and indirect oxidation through HO• and 1O2 at basic pH. Moreover, thermal assessment in dark showed that the process followed the Arrhenius equation with an activation energy 36.0 ± 1.6 kJ/mol. Under artificial solar irradiation, the kinetics constant of SMX increased due to the reached temperature and similar results were obtained in the dark at the same temperature. Therefore, the solar light could be discarded as PMS photochemical activation method for ROS generation. On the contrary, SMX degradation activated by the 254 nm PMS photolysis was based on the homolytic cleavage of the O-O bond with the formation of SO4•– and HO• radicals with a ratio 1.2:1. HO• radicals prevails over SO4•– at basic pH where the oxidation of OH– to HO• by SO4•– is strongly favored. The findings obtained in this research stands out the basis of PMS reaction in water and the mechanisms governing the microcontaminants degradation in function of the type of irradiation source and under the most commonly natural occurring inorganic water constituents, demonstrating also the potential application of this oxidant in combination with natural sunlight for water purification.