Degradation of sulfadimethoxine in phosphate buffer solution by UV alone, UV/PMS and UV/H2O2: Kinetics, degradation products, and reaction pathways

The presence of residual sulfonamide antibiotics such as sulfadimethoxine (SDM) in surface waters has become an emerging concern due to their adverse effects on the environment and human health. In this study, UV alone, UV/PMS and UV/H2O2 degradation processes were studied by comparing the degradation efficiency and reaction mechanisms for SDM. Moreover, we systematically studied various factors that influenced the removal of SDM by UV/PMS, including the pH (5.0, 7.0, and 9.0), oxidant doses, humic acid (HA) and bicarbonate (HCO3-). UV alone can highly degrade SDM, and the combination of PMS or H2O2 with UV radiation further facilitated the oxidation performance. The removal efficiency of SDM in the three degradation systems was recorded with the order of UV/PMS > UV/H2O2 > UV alone. The conditions of [SDM](0): [PMS](0) = 1:20, pH = 7.0, and T = 25 +/- 2 degrees C cause a 100% removal of SDM (5 mg L-1) after 7.0 min of irradiation by a 300 W mercury vapor lamp. Degradation products of SDM were identified using an electrospray time-of-flight mass spectrometer. Their structures were further elucidated using MS/MS spectra. Three initial reaction sites of SDM were suggested, which were the direct photooxidation of the -NH2 group, hydroxylation of the -NH2 group and aniline ring via attack by SO4 center dot- and (OH)-O-center dot radicals. It was found that nitrated products were only produced in UV/PMS and UV/H2O2 systems. Ecological structure activity relationships (ECOSAR) analysis revealed that the degradation products of SDM during the three oxidation processes were not considered as toxic to three typical aquatic species. Thus, UV alone, UV/PMS and UV/H2O2 could be applied as an efficient technique for the elimination of SDM from water.