Study On Degradation Kinetics And Mechanism Of Antibiotic Sulfadimethoxine During Chlorine Disinfection Process

As a kind of common antibiotics,Sulfonamides antibiotics（SAs）are widely used in animal husbandry and aquaculture.After entering the environmental water body,SAs will continue to be accumulated,causing many side effects on animals and plants.In recent years,trace antibiotics in environmental water have been detected frequently,which has gradually attracted people’s attention.Chlorine disinfection is a common disinfection method in traditional water treatment technology,which plays an important role in ensuring the safety of water quality.Traditional water treatment process units（coagulation,sedimentation and sand filtration）can not effectively remove sulfonamides in water.However,residual chlorine also reacts with organics in water to produce disinfection by-products,which affects the safety of water quality.Therefore,the behavior characteristics and potential risks of sulfonamides in the process of chlorine disinfection need to be deeply studied and evaluated.In this study,the degradation kinetics and mechanism of typical sulfonamide antibiotic sulfadimethoxine（SDM）were studied by chlorine disinfection process.The main research contents and conclusions are as follows:（1）Firstly,by analyzing the bond length,bond angle and dihedral angle of SDM,the results show that H28-N8 and H29-N8 bonds on SDM molecules are vulnerable to free radical attack.There are sp² and sp³ hybrid modes on the atoms of benzene ring and two methyl groups.According to the frontier orbital theory,N8 on the amino group is the most active reactive site.Molecular surface electrostatic potential data show that electrophilic reaction is easy to occur near thioacyloxy O3;Near hydrogen H28 on the amino group,nucleophilic reaction is easy to occur.There may be interaction and hydrogen bonding between H22-N6,H28-N8,C13-N7,C14-N8 and other bonds.（2）In the study of the degradation efficiency and reaction kinetics of sulfadimethoxine degraded by chlorine disinfection,it was found that the reaction process accorded with the second-order reaction kinetics.The second-order reaction rate constant is positively correlated with the initial concentration of residual chlorine and negatively correlated with the concentration of ammonia nitrogen.The effect of chlorine oxidation degradation is the best under neutral conditions.Cl^-,NO₃^-,SO₄^2-have no effect on the chlorine oxidation process,HCO₃^-and CO₃^2-have inhibitory effect on the chlorine oxidation process,and CO₃^2-has significant inhibitory effect.RSM response surface was used to optimize the experiment.The optimum experimental conditions were molar ratio 6 and p H=7.（3）By studying the chlorine oxidation degradation mechanism and risk assessment of SDM,nine intermediate products of SDM were identified based on high-resolution mass spectrometry,and the possible degradation paths were put forward.Luminescent bacteria experiment and ecological structure effect relationship software ecosar show that the toxicity of pollutants has not been effectively removed,and intermediate products with higher acute toxicity may be generated.The binding sites of SDM and its products to luciferase were predicted by molecular docking technology to clarify the toxicity mechanism.The results showed that the ligands SDM,Pr344 and Pr394-a stably bound to the proteins of Vibrio Harvey luciferase,Vibrio fischeri luciferase and Bacillus cereus luciferase through various interactions,in which hydrogen bonding was common and played a key role in the interaction.The ligands SDM,Pr344 and Pr394-a have strong inhibitory effects on three common luciferases.Therefore,the potential risks of SDM in the process of chlorine disinfection need to be further studied.