Application Of Ozone And UV/PMS For Degradation Of Sulfadimethoxine: Reaction Kinetics,Product Identification,Reaction Mechanism,and Toxicity Assessment

Nowadays,there has been tremendous concern about the widespread occurrence of antibiotics in the aquatic environment.The presence of these antibiotics in environment is creating the threat of antimicrobial resistance genes(AMR).Although many detailed studies have been conducted recently to degrade the antibiotics in water,but the findings are yet conflicting.Therefore,in this particular study we have taken sulfadimethoxine(SDM)as a probe antibiotic which is a member of sulfonamide(SA).The presence of residual sulfonamide antibiotics such as SDM in surface waters has become an emerging concern due to their adverse effects on the environment and human health.In our investigation,we have degraded the SDM by ozone(O₃)and UV/PMS methods to investigate their degradation products,reaction mechanisms and toxicity evolution by Ecological Structure Activity Relationships software(ECOSAR).Firstly,in this study,the degradation of 20 mg L^-1 of sulfadimethoxine(SDM)in different water matrices was carried out by ozonation.The degradation of SDM was more pronounced at acidic p H than under ambient environmental conditions,which was also dependent on different water matrices.Both direct and indirect oxidation of SDM by ozone were observed,and it was also shown that both ozone molecules and hydroxyl radicals were involved in the SDM degradation process,whereas it was found that the saturated ring of SDM made it O₃-recalcitrant.Seven transformation products(TPs)were identified during SDM ozonation,allowing three degradation pathways to be proposed.Additionally,the main reaction sites,including N(7)and C(2)on the aniline ring,and the-S-N-bond,were confirmed both experimentally and theoretically.The toxicity evolution during the degradation process was investigated,and the results showed nontoxic intermediate products obtained during ozonation.Secondly,in this study,the photodegradation of SDM in phosphate-buffered solution was investigated for the first time in UV alone,UV-activated peroxymonosulfate(PMS)and UV/H₂O₂systems.For UV alone,we studied the reaction kinetics,identified the reaction products,and assessed the toxicity of degradation products.We have explored that hydroxylated degradation products were generated,and toxic products were not found.For UV/PMS system,we comprehensively studied various factors that influenced the removal of SDM,including the p H(5.0,7.0 and 9.0),oxidant doses,humic acid(HA)and bicarbonate(HCO₃^-).The conditions of[SDM]₀:[PMS]₀=1:20,p H=7.0,and T=25.0±2.0?caused 100%removal of SDM(5.0 mg L^-1)after 7.0 min of irradiation by a 300 W mercury vapor lamp.It was found that the increase of the PMS dose could promote([SDM]₀:[PMS]₀,1:1 to 1:50)the degradation efficiency,while the presence of HA(1.0 mg L^-1,2.0 mg L^-1,5.0 mg L^-1)and HCO₃^-(0.5 m M,1.0 m M,2.0 m M)inhibited the degradation of SDM to a degree.Radical quenching experiments showed that·OH,SO₄·^-and direct photolysis occurred during the UV/PMS process and played an important role in the removal of SDM.Sixteen degradation products of SDM were identified using an electrospray time-of-flight mass spectrometer,whose 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·^-and·OH radicals,respectively.ECOSAR analysis revealed that the degradation products of SDM were not considered as toxic to three typical aquatic species.For our better understanding of the facts,we have also investigated the degradation products generated in the UV/H₂O₂ system and have assessed the toxicity of these products by ECOSAR.The findings have revealed that nitrated products were also generated in UV/H₂O₂ system and no toxic products were found.Thus,UV alone,UV/PMS and UV H₂O₂ could be applied as efficient techniques for the elimination of SDM from water.