Degradation of sulfamethoxazole by UV/sulfite in presence of oxygen: Efficiency, influence factors and mechanism

In this study, the degradation efficiency and mechanism of sulfamethoxazole (SMX) by UV/sulfite system in presence of oxygen were investigated systematically. The results indicated that SMX could be effectively removed by UV/sulfite following a pseudo first-order kinetic model. Compared to SMX photolysis under UV irradiation, its removal in UV/sulfite system was significantly enhanced, which was mainly ascribed to the formed reactive oxygen species. Sulfate radical (SO4−) was proved to be the major active radical for SMX degradation by scavenging experiments using methanol and tert-butanol as scavengers. UV photolysis and radical oxidation were two degradation pathways of SMX in UV/sulfite system, and their contribution to SMX removal at pH 6.9 was 38.8% and 61.2%, respectively. The degradation rate constant of SMX was decreased from 0.1415 to 0.0358 min−1 as the pH increased from 4.0 to 9.3, because pH is an important influence factor on species distribution of both SMX and S(Ⅳ), further influencing direct SMX photolysis and formation of active species. SMX degradation was increased with the increase of S(IV) concentration from 0 to 1000 μM, while it was decreased with the increase of initial SMX concentration. The presence of HCO3−, Ca2+ and fulvic acid inhibited the degradation of SMX in UV/sulfite system, while an insignificant effect on SMX removal was observed with the addition of Cl−, SO42− and Cu2+. NO3− could facilitate SMX degradation in UV/sulfite process as a precursor to form hydroxyl radical. Under the direct UV photolysis and radical oxidation, five degradation products of SMX were produced by hydroxylation, S–N bond cleavage of sulfonamide, and oxidation of amino group. Consequently, UV/sulfite could be an efficient approach for SMX removal from polluted water.