Theoretical Study On Catalytic Degradation Of Sulfonamides By TiO₂

Objective:Sulfonamides are widely used in animal husbandry in Haihe River Basin.Drug active compounds are not completely metabolized in human or livestock,but are secreted or excreted by the body.Antibiotic residues and drug-resistant bacteria are released into the environment.High water solubility leads to the increase of concentration in surface water,which increases the risk of multiple drug-resistant animal pathogens.Sulfonamides have high ecological risk due to their high biological toxicity.In the traditional degradation methods,the adsorption method only transfers the pollutants from the aqueous phase to the solid phase,and does not really degrade.The biodegradation cycle of biological method is long and can not achieve good degradation effect.Advanced oxidation process is easy to control and has high stability,among which photocatalytic degradation has a higher advantage.Some studies have shown that·OH,·O₂^-and intermediate free radicals are effective degradation methods,but the specific degradation products and reaction mechanism are still unclear.In this study,based on TiO₂photocatalytic method of advanced oxidation technology,sulfanilamide（SA）,sulfacetamide（SAC）and sulfamethoxazole（SMX）were used as representative drugs of sulfanilamide antibiotics to carry out theoretical research on photocatalytic degradation,which pointed out the direction of research on degradation of sulfanilamide antibiotics in real life.Methods:three kinds of sulfonamides were studied by molecular simulation and density functional theory（DFT）.They were basic sulfonamides,sulfonamides;straight chain sulfonamides and sulfacetamide;five membered ring sulfonamides and sulfamethoxazole.Firstly,the structure optimization of TiO₂（001）and（101）crystal faces and sulfanilamide molecules was carried out by using the Perdew-Burke-Ernzerhof（PBE）method of generalized gradient approximation（GGA）in VASP program,and the adsorption configuration was calculated.The adsorption energy was refined by HSE06 calculation module to determine the optimal adsorption configuration.Using Materials Studio 7.0 of Dmol³module in was used to analyze the density of states and charge density.The most stable adsorption configuration of hydroxyl co adsorbent was used as the starting material of the reaction.The CI-NEB method was used to search the transition state of the reaction.The correctness of the reaction path was judged the difficulty of the reaction was determined by the energy barrier.Finally,the photocatalytic degradation was obtained Mechanism of reaction path of sulfonamides.Results:The adsorption characteristics and degradation mechanism of sulfonamides on TiO₂crystal surface were obtained by DFT calculation.Adsorption part:through the calculation of adsorption energy and single point energy,the most stable adsorption configurations are AE0,AA1,CB0,CB1,XA0,XA1.From the adsorption configuration,it can be concluded that O,N atoms and Ti_（5）atoms have adsorption effect on sulfanilamide molecule,and H atom has adsorption effect on O_（2）and O_（3）atoms,but the adsorption effect on O_（2）atom is stronger.After adsorption,the bond length of benzene ring was lengthened and shortened.However,according to the bond length analysis,its stability did not change much.With the increase of the branched chain,the C-S bond is elongated and the stability is weakened.According to the change of bond length,it can be concluded that the influence of TiO₂（101）crystal surface is greater in the adsorption process.In the part of degradation mechanism,the degradation path of six sites,the breaking of longly branch chain,the hydroxylation of two sites of benzene ring,the dropping of-NH₂and the breaking process of S,N sites were considered.The ring opening reaction of sulfamethoxazole was also analyzed.Conclusion:During the adsorption process,the adsorption mainly depends on the interaction between H and O_（2）on the benzene ring.The density of States and charge density of the adsorption configuration were analyzed,and the electron transfer between Sulfanilamide and TiO₂surface was found.With the change of branch chain size,the C-S bond is significantly elongated and the stability of the bond is weakened,which is more conducive to the reaction.Different branched chains also affect the hydroxylation of benzene ring.In the degradation mechanism of S-N bond breaking,it was found that the breaking energy of S-N bond was very high,which indicated that it was difficult to break the reaction at this site,which was consistent with the conclusion reported in experiments that the site of S-N bond breaking needed direct photocatalytic reaction.The indirect photocatalytic reaction with·OH and TiO₂as catalyst also had no effect on the reaction energy barrier.The best degradation reaction path was branch chain position fracture.Compared with the energy barriers of the three sulfonamides,the relative degradation effect is SA>SAC>SMX.The degradation effect of SA on the two surfaces of TiO₂is similar,and the degradation effect of sac and SMX on the（001）surface of TiO₂is relatively better.Our conclusion can explain the degradation pathway and degradation products of sulfonamides under TiO₂photocatalysis.The results show that anatase TiO₂can effectively adsorb and degrade sulfonamides.