Performance And Mechanism For Degradation Of Sulfadiazine In Water By UV/O₃ Process

Antibiotics have been widely used for the prevention and treatment of diseases in human and animal, as well as in farming and agriculture. However, due to the unreasonable use, various antibiotics have been detected in the environment, especially in the water environment. Because of the potential harm to human body and ecological environment, it is urgently necessary to develop efficient and economic technologies to remove the antibiotics in water.In this study, a typical antibiotic, sulfadiazine（SDZ）, was choosen as the target compound. The degradation performance was investigated and compared under ozonation alone（O₃）, ultraviolet alone（UV） process and the combined ultraviolet and ozone（UV/O₃） process. The effects of key factors on degradation rate of sulfadiazine were also examined. After 10 minutes of reaction, O₃ could effectively remove the sulfadiazine in water while the removal efficiency was negligible under UV process. Increasing the concentration of the gaseous ozone or increasing the UV intensity could accelerate the sulfadiazine degradation rate. The degradation rate would decrease when the initial concentration of sulfadiazine increased. The p H values of solution had a significant impact on the O₃ process. On the one hand, p H will change the mode of action between ozone and SDZ. On the other hand, the p H value will affect the dissociation form of the SDZ and further affect its reaction activity. For the UV process, different photolysis activities were mainly attributed to the change of SDZ dissociation form caused by sulution p H. UV/O₃ process, a typical advanced oxidation process, could not only efficiently remove SDZ from water, but also reflect the synergistic effect compared with the individual process. The action of key factors on SDZ degradation under UV/O₃ process was similar with O₃ process.Density functional theory（DFT） was used to optimise the molecular configuration of sulfadiazine and global atomic charge distribution was obtained. Then Fukui function value of every atom was calculated, so as to predict the chemical reaction sites of sulfadiazine molecule with hydroxyl radical. The samples under three kinds of treatments were detected by liquid chromatography-mass spectrometry/mass spectrometry（UPLC-MS/MS） technology. Intermediates in the reaction process were determined in combination with the calculation of quantum chemistry calculation and the possible degradation pathways of SDZ under three diferent treatments were determined. The degradation of sulfadiazine under O₃ process was mainly attributed to the direct action with O₃ molecule causing S-N bond breaking up and the open reaction of pyrimidine ring.The degradation of SDZ under UV process was caused through the direct photolysis. The photolysis product was detected and determined a possiblely new reaction site of SDZ molecule. In the UV/O₃ process, hydroxyl radical（·OH） played an important role. More intermediate products were detected and three possible reaction pathways between SDZ and hydroxyl radical were determined.