The Adsorption Characteristics And Degradation Mechanism Of Nitroimidazole:A DFT Study

Objective:Nitroimidazole antibiotics are mainly used for bacterial infection caused by anaerobic bacteria.In addition,they are also growth promoter due to certain concentrations of nitroimidazoles have the effect of weight gain and increase feed conversion.As it is used in large amounts,it causes accumulation in animals.Alongside,with high water-solubility and low biodegradation rate,these drugs easily accumulate in the water body,and eventually accumulate in the human body through the food chain and water body.While pharmacological experiments have shown that these remains have neurotoxicity,genetic toxicity,and renal toxicity.Therefore,it is of great significant to the degradation for nitroimidazole in water.The traditional methods?adsorption method,biodegradation?cannot obatin good degradation effects,due to the remains with high water solubility and low biodegradation rate.Many studies have shown that advanced oxidation process is an effective method,which is based on the reaction between the the generated·OH,O₂^-,intermediate free radicals and pollutants to achieve the degradation.However,the specific degradation products and reaction mechanisms are still unclear.Our research is based on the TiO₂ photocatalytic method of advanced oxidation process,using ornidazole,secnidazole,and ronidazole as the representative drugs of nitroimidazole to investigate the micro-catalytic mechanism.Methods:The adsorption characteristics and degradation mechanism of nitroimidazoles on TiO₂ surface are based on density functional theory?DFT?.Firstly,three kinds of nitroimidazoles were optimized by B3LYP functional of Gaussian 09,and then the DMol³ in Materials Studio 7.0 of Accelrys was used to further optimize.The optimization of the adsorption configuration of drug molecules on the anatase TiO₂?101?and?001?surfaces,the adsorption site,adsorption energy,density of state,and electron density are all based on the DMol³.We selected the hydroxyl co-adsorbate of the optimal adsorption configuration as the starting material of the degradation reaction,and the transition state is searched by the LST/QST method.The correctness of the reaction path is determined by frequency analysis,and through the energy barrier to determine the difficulty of the reaction,finally the mechanism of photocatalytic degradation reaction is obtained.Results:The adsorption characteristics and degradation mechanism of nitroimidazoles on the TiO₂ surface were obtained by DFT.For adsorption part:the most stable adsorption configuration and the best adsorption site can be obtained through adsorption.When N₃ atom is adsorbed on Ti_5cc atom,the adsorption configuration is the most stable.Adsorption caused the bond lengths of C₂-N₃ and C₂-N₁ on the imidazole ring to be significantly lengthened,and the bond stability of the C₂-N₃ and C₂-N₁ bond was weakened,which was valuable for the photocatalytic reaction.The adsorption of nitroimidazole on TiO₂?001?surface is more stable,while for the utilization of visible light?101?surface is higher.The part of degradation mechanism:TiO₂?101?and?001?surfaces photocatalytic degradation of ornidazole and secnidazole can be performed along two reaction pathways,and the optimal pathway is along the first pathway on the TiO₂?101?surface.For TiO₂?101?and?001?surfaces photocatalytic degradation of ronidazole,the reaction can only proceed on the TiO₂?101?surface along the first pathway.Conclusion:When there exist adsorption between the N₃and Ti_5cc atoms can obtain most stable structure.After adsorption,the C₂-N₃ and C₂-N₁ bond stability is weakened due to the bond lengths are significantly extended which is conducive to the degradation reaction.Simultaneously,the electronic characteristics of adsorption configurations are show that it is feasible to use visible light to drive the photocatalytic reaction on the TiO₂ surface.In the ring-opening reaction,C-N is found to be the ring-opening site,which is consistent with the existing experimental reports that the degradation of nitroimidazole is mainly the C-N cleavage on the imidazole ring.At the same time,it was found that TiO₂?101?surface photocatalyzed three kinds of nitroimidazole drugs,all could proceed along the first reaction pathway and which was the best reaction pathway.Comparing the energy barriers of the three kinds of nitroimidazoles photocatalytic reactions,it is found that compare with the N₁ branch,the C₂ branch has a greater effect on the reaction.The longer the C₂ branch chain,the greater the steric hindrance to the·OH radical,the more difficult for the H₁ proton transfer,and the more difficult the reaction to proceed.