Effect Of Organic Acids On Photocatalytic Degradation Of Ronidazole In Aqueous Solution

Nitroimidazoles are widely applied to prevent and treat animals'infections caused by protozoans and bacteria.High concentration of nitroimidazole has been frequently detected in drinking water and foods.Considering its potentially mutagenic,carcinogenic and bacterial resistance,removing the nitroimidazoles from waste water is an important issue.TiO2 heterogeneous photocatalysis has been regarded as a promising technology for removal of contaminants due to mild reaction conditions and no secondary pollution.However,photo-generated electrons and holes are easily recombined in the TiO2 surface leading to the low degradation efficiency.Furthermore,powder TiO2 is difficult to recycle and unable to use visible light,and all these limit the practical application of TiO₂ photocatalysis technology.In the present study,ronidazole?RNZ?was selected as the representative of nitroimidazoles,and the main research contents are as follows:In order to suppress the recombination of electron and hole to further improve the efficiency of photocatalysis,organic acids was used as electron donor to enhance the degradation efficiency of ronidazole.The effect of different type of small molecular weight organic acid on TiO₂ photocatalytic degradation performance of RNZ was studiedunder illumination of UV light,and the reaction mechanism was also analyzed in detail.The results showed that concentration of oxalic acid,solution pH and the structure of organic acids had influences on photocatalytic perforamcne.The results indicated that the synergistic effect of organic acids on degradation efficiency followed the order of oxalic acid>citric acid?malic acid>tartaric acid>lactic acid,meanwhile,carboxyl and hydroxyl exist alone can not promote the degradation of RNZ.The main active substances in TiO₂+oxalic acid system are CO2 ·-and e-.There are two degradation pathways of ronidazole in TiO₂+oxalic acid system:One is the double bond on the nitrogen heterocyclic ring could be cut off to form the aliphatic compound;the second is azo compound formed by the conversion polymerization of the nitro group and amino group on the branched chain,and then the C-O bond could be cut off.In addition,in order to extend the photoreponse range of TiO₂ and the recovery of catalyst,Fe2O3@TiO2@HGMs was synthesized by sol-gel method.The samples were firstly characterized by energy dispersive spectroscopy?EDS?,X-ray diffractometer?XRD?,X-ray photoelectron spectroscopy?XPS?and UV-visible diffuse reflectance spectrography?UV-Vis DRS?.A synergetic system of Fe2O3@TiO2@HGMs combined with oxalate was developed to investigate the decomposition performance and mechanisms of ronidazole.Fe and Ti were successfully loaded on the surface of the hollow glass microspheres?HGMs?,and the form of Fe and Ti were Fe2O3 and TiO₂,respectively.Fe2O3@TiO2@HGMs composites not only improved the absorption capacity of UV region,but also broadened the absorption range of the light.The initial pH value of the solution had significant influence on the degradation performance of RNZ,and the degradation rate decreased with increasing pH.The existence of synergistic effect of organic acids on the degradation of ronidazole must meet the following two conditions:?1?the added organic acids can form complexes with Fe???;?2?Fe???-organic acids complexes must be responsive to the visible light.In the Fe2O3@TiO2@HGMs and oxalate system,OH is the main active species and CO2·-plays a supporting role.The results showed that based on self-generating H2O2,photocatalytic process and photo-Fenton reaction occurs simultaneously which can effectively improve the removal efficiency of RNZ.There are three degradation pathways of ronidazole in the composite system of Fe2O3@TiO2@HGMs and oxalate:pathway 1 is the branched chain of parent compound RNZ cut off to form a new heterocyclic product?3-acetyl-2-oxazolidinone?;pathway 2 is the C=N on the heterocyclic ring of RNZ cut off to form product;and pathway 3 is the branched chain on the heterocyclic ring cut off to generate polymeric product.Finally,intermediates were further oxidated to CO2 and H2O.