Photodegradation of sulfonamides by g-C 3 N 4 under visible light irradiation: Effectiveness, mechanism and pathways

For the first time, the photocatalytic degradation of sulfamethoxazole (SMX), sulfisoxazole (SSX), sulfadiazine (SDZ), sulfamerazine (SMZ) by g-C3N4 under visible light irradiation was investigated. Results revealed that compared to photolysis, the photocatalytic degradation efficiencies of the four sulfonamides were significantly enhanced with the addition of g-C3N4, and more than 90% of photodegradation removal was obtained. The effects of typical water quality parameters, including solution pH, bicarbonate ion and humic acid, on the photodegradation process were discussed. It was found that although the photodegradation of the four sulfonamides exhibited different trends under the variation of the water quality parameters, an excellent photocatalytic removal could always be achieved, illustrating the robustness and effectiveness of the g-C3N4 photodegradation process. The ESR measurements showed that both OH and O2− were produced in the photocatalytic process of g-C3N4 under visible light irradiation. And trapping experiments confirmed that O2− and holes played a significant role in the photodegradation of SMX, SDZ and SMZ; but holes and OH were the main oxidative species for SSX degradation. Finally, according to the oxidation products detected by ultra-high performance liquid chromatography-tandem mass spectrometric (UPLC/MS/MS), the degradation pathways of the four sulfonamides were proposed and compared. It was found that there were some common pathways shared by the different sulfonamides, such as cleavage of S-N bond and hydroxylation of the benzene ring (the main degradation pathways for SSX). More importantly, some specific photodegradation pathways were also identified: (1) the nitration of amino group on the benzene ring occurred for SMX, SDZ and SMZ (the main degradation pathway for SMX, SDZ and SMZ), but not for SSX; (2) the bond cleavage between benzene ring and S occurred only in degradation of SSX; and (3) the carboxylation of the methyl group occurred only in degradation of SMZ.