Study On Performance And Mechanism Of Sulfamethoxazole Degradation By Microbubble Ozonation

Antibiotics with high toxicity and poor biodegradability cause serious ecological risks in water environment.In this study,the degradation performance of typical antibiotic sulfamethoxazole（SMX）in microbubble ozonation（MB/O₃）process was investigated and compared with common bubble ozonation（CB/O₃）process.The parameters of MB/O₃process were also optimized.Further,the effect of water qualities on SMX degradation performance in MB/O₃ process was analyzed.The degradation pathways of SMX in MB/O₃ and CB/O₃ processes were proposed by determining the reactive oxygen species（ROS）generation,intermediates detection and density functional theory（DFT）calculation.The differences in degradation pathways and reaction mechanisms between the two processes were compared and analyzed,and the enhancement mechanism of MB/O₃process was clarified.Finally,the toxicity of SMX and its degradation intermediates were evaluated.The main research results are as follows:1)The optimal parameters for SMX degradation in MB/O₃process were determined as follows:ozone dosing of 15 mg·min^-1,initial SMX concentration of 50 mg·L^-1 and ozone gas flow of 0.3 L·min^-1.Under the optimal conditions,the SMX degradation efficiency in MB/O₃ process was significantly higher than that of CB/O₃ process,and reaction kinetic constants of SMX removal were 0.1417 min^-1 and 0.0434 min^-1,respectively,while reaction kinetic constants of TOC removal were 0.0045 min^-1 and0.0011 min^-1,respectively.In addition,the ozone utilization efficiency in MB/O₃ process was higher than that in CB/O₃ process,where the ozone utilization efficiency were83.30%and 57.58%after 90 min,respectively.2)The degradation efficiency of SMX in MB/O₃ process under different initial pH conditions was in the order of:pH=3>pH=5>pH=9>pH=7>pH=11,indicating that the acidic condition was favorable to SMX degradation.The SMX degradation were inhibited by SO₄^2-,NO₃^-,Cl^-and HCO₃^-at different concentrations.Among them,the degradation efficiency of SMX decreased gradually when the concentrations of SO₄^2-and HCO₃^-increased from 0 m M to 20 m M,while the degradation efficiency of SMX decreased and then increased when the concentrations of NO₃^-and Cl^-increased from 0 m M to 20 m M,and it was the lowest at the concentration of 5 m M.In addition,low concentration of humic acid（HA）could promote SMX degradation as a free radical initiator,while high concentration of HA could inhibit SMX degradation as a free radical scavenger.3)The enhancement generation of ROS in MB/O₃ process were proven by electron paramagnetic resonance（EPR）spectroscopy detection,including hydroxyl free radicals（·OH）,superoxide free radicals(O₂^·-),and singlet oxygen（¹O₂）.The·OH and ¹O₂ were dominant ROS,and their comprehensive contribution rates to SMX removal in MB/O₃and CB/O₃ processes were 84.82%and 67.74%,respectively.According to DFT calculation,the sulfonamide group,17（N）,13（C）,15（C）,and 9（C）regions of SMX molecule were the more active electrophilic reaction sites.The degradation pathways determined by intermediates detection and DFT calculations revealed that the MB/O₃process was more complex because the enhanced oxidation of ROS led to the attack of both active and inactive sites.The toxicity prediction and assessment of SMX and its degradation intermediates showed that some intermediates were more toxic than SMX,which may be caused by the addition of hydroxyl groups by hydroxylation reactions on the benzene ring or isoxazole ring.