Effect of wavelengths on photocatalytic oxidation mechanism of sulfadiazine and sulfamethoxazole in the presence of TiO2

In this paper, the effect of wavelengths on photocatalytic oxidation kinetics and mechanisms of sulfadiazine (SD) and sulfamethoxazole (SMX) in the presence of Degussa P25 TiO2 were discussed, and the influence of humic acids (HA) on the degradation process was further explored. The photocatalytic degradation process of sulfonamides antibiotics (SAs) can be characterized by the pseudo-first-order model, and the removal rates for SAs under ultraviolet (UV) light irradiation were higher than those under visible light irradiation, with the values in the order of 365 nm > 254 nm > 420 nm > 475 nm. The degradation ratios of SAs could reach over 99% within 90 min under UV irradiation, whereas the SAs removal rates were less than 70% after 120 min visible light irradiation. The superoxide radicals (·O2−) were the most important components during the SAs degradation both under UV and visible light irradiation. Due to the existence of special groups, SD mainly performed desulfurization reactions of the S-N bond cleavage and recombination, while SMX underwent ring-opening reactions because of the cleavage of the N-O bond in the isoxazole ring. HA had an inhibitory effect on the photocatalytic oxidation of SAs under UV light irradiation, but had a promotional effect under visible light irradiation. This study and the consequent results would provide a data basis and theoretical reference for the photocatalytic oxidation mechanisms of SAs in real water.