Theoretical Study On The Photocatalytic Degradation Of Nitroimidazole Antibiotics By TiO₂

Nitroimidazole organic compounds are a kind of drugs with the structure of 5-nitroimidazole ring.The main varieties include metronidazole（MNZ）,dimetridazole（DMZ）,ornidazole（ONZ）and Tinidazole（TNZ）etc.These drugs have antibacterial effects,as well as antitumor,antiviral and antigenic activity.However,nitroimidazoles has mutagenicity and carcinogenicity.It has been banned as a prohibited drug in many countries.It is not allowed to promote animal growth for all food animals.Most of the drugs taken by human and animal are not completely metabolizing in the organism,which makes the residue excreted into the water environment.Imidazole ring is very stable and not easy to be destroyed and decomposed.Thetoxicityisstrongandeasytopollutethe environment.Photocatalysis,as the most promising way to treat antibiotic contaminants,has attracted much attention due to its economic,efficient and green advantages in the degradation of antibiotics under sunlight and natural environment.It has been proved that TiO₂ can effectively photocatalytic degradation of nitroimidazoles.In this paper,the adsorption of MNZ and DMZ on TiO₂（101）and（001）surface have been studied by DFT.The results show that the two drugs can be effectively adsorbed on the TiO₂ surface,both in the vacuum and in the neutral,acidic or basicsolution.In vacuum,when the N atom on the imidazole ring is adsorbed on the Ti atom,the adsorption energy is maximum and the C=N bond grows longer,which is beneficial to the attack of the subsequent hydroxyl radical.When change the environment,TiO₂（101）surface can effectively utilize the visible light.While The effect of TiO₂（001）surface on visible light is greatly influenced by the environment.This paper also studied the reaction mechanism of photocatalytic degradation of metronidazole,dimetridazole,ornidazole and Tinidazole on TiO₂（101）and（001）surface,and water as a solvent for the reaction effect.The main purpose is to open the ring of imidazole ring and reduce the toxicity of pollutants.The reaction mechanism first is the hydroxyl radical attacking C atom.Second there are two reaction paths:one is N―C bond breaking,and then the H atom on hydroxyl radical transfer to N.The other one is the H atom on the hydroxyl radical first transfers to N,and then the N―C bond breaks.From the activation energy,the activation energy required for N―C bond breaking is low,while the activation energy needed for proton transfer is higher.Under the conditions of water solvation,the activation energy of some steps are reduced.The four kinds of drugs can all open ring on the surface of TiO₂ to achieve the purpose of degradation.The photocatalytic degradation mechanism of MNZ,DMZ,ONZ and TNZ on TiO₂（101）and（001）surface have been studied.The mechanism of TiO₂ in the treatment of photocatalytic degradation of drug residues is revealed.At the same time,the effect of solvent environment on the catalytic degradation process is also studied.Our research provides a new way of thinking for the treatment of drug residues in the environment.