Transformation Mechanism Of Sulfonamide Antibiotics In The Common Water Disinfection Process

Sulfonamides are typical antibiotics that are widely used.A large amount of use has led to its retention in surface water,groundwater and even drinking water sources,with residual concentrations of ng·L^-1 or even?g·L^-1.The accumulation of sulfonamides in water presents a huge potential risk to the environment and human health.Besides,it is difficult for the sulfa antibiotics to be effectively removed in the conventional treatment process of the water supply plant,so its'transformation principle and mechanism in the disinfection process has drawn much attention.In this paper,two widely used new-type sulfonamides,sulfacetamide?SFA?and sulfathiazole?STZ?,were selected to explore their transformation principle and mechanism in three common disinfection processes:NaClO,ClO₂ and UV/NaClO.The transformation processes of two sulfonamide antibiotics in the three disinfection process system all conform to the second-order kinetics model.The rate constants between the two sulfonamides?SFA and STZ?and the three disinfection systems in the descending order are ClO₂ system,UV/NaClO system and NaClO system.Moreover,the transformation rate of STZ in any disinfection system is faster than that of SFA.The activation energy of each reaction is consistent with the reaction rate.The reactions of trace SFA and STZ in three disinfection processes under simulated actual conditions almost completed in about ten minutes while almost no target object was detected in the system.There were some environment factors such as pH,NH₄⁺,HCO₃^-,humic acid and different water quality backgrounds had effects on the conversion of the two sulfonamides in the three disinfection systems.In the NaClO and UV/NaClO disinfection systems,the reaction rate at the neutral pH condition was absolutely faster than that at the acid and alkali conditons.However,in the ClO₂ disinfection system,the reaction rate was accelerated with the increase of pH.The conversion of SFA and STZ in ClO₂ system could be inhibited by NH₄⁺,and the degree of inhibition was UV/NaClO system>ClO₂ system>NaClO system.In addition,the conversion of SFA and STZ in the ClO₂ and UV/NaClO systems could be promoted by HCO₃^-,but be inhibited by humic acid.In the three disinfection systems,there were sulfonamides generated with exfoliation of the characteristic functional groups of SFA and STZ,and were further oxidized into the intermediates such as phenol and aniline.Then,there were different pathways in the different disinfection system.Chlorination substitution occurred in the NaClO system,and the intermediate products such as sulfonamid,phenol and aniline were chlorinated,generating chlorinated by-products with strong toxicty.However,in the ClO₂ and UV/NaClO systems,the benzene structure of intermediate products,phenol and aniline,could be destroyed to generate small molecule organic acids such as oxalic acid,lactic acid and fumaric acid,which to be relatively more safe.