Study On The Performance And Mechanism Of Degradation Of Perfluorooctanoic Acid By Mixed Crystal TiO₂/BiOBr Composite Photocatalytic System

Perfluorooctanoic acid（PFOA）is a new type of persistent organic pollutant that is of wide concern due to its potential toxicity and environmental accumulation.The pluralistic heterojunction photocatalyst can effectively improve the photogenerated charge separation and has a strong redox ability,which is expected to improve the PFOA removal rate.Therefore,in this study,we synthesized anatase-rutile mixed-crystal TiO₂（AR）by a simple hydrothermal method and prepared TiO₂（AR）/BiOBr ternary heterojunction catalysts using in situ precipitation method,and evaluated the photocatalytic performance of TiO₂（AR）/BiOBr catalysts by the degradation rate of PFOA under visible light irradiation.The main research contents are as follows:（1）TiO₂（AR）/BiOBr composites were prepared by hydrothermal method and in-situ precipitation method.The crystal form,morphology and composition of the materials were investigated by XRD,Raman,FT-IR,SEM,TEM,BET and XPS.XRD and Raman confirmed the mixed crystal structure of TiO₂（AR）,and the mass percentages of anatase and rutile were calculated to be 68.8%and 31.2%,respectively.The results of SEM and TEM show that TiO₂（AR）and BiOBr are uniformly compounded to form a tightly connected interface,and the material is not physically mixed.FT-IR confirmed that the functional groups of the material did not change,and the characteristic peaks showed that the composite was successful.XPS results further confirmed that TiO₂（AR）and BiOBr were successfully compounded and formed stable Ti-O-Bi bonds.（2）The photocatalytic performance and interfacial electron transfer mechanism of TiO₂（AR）/BiOBr composites for PFOA degradation were studied.The photoelectric properties and electron transfer mechanism of the materials were investigated by UV-vis DRS,EPR,I-T,EIS,PL and TRPL.UV-vis DRS test confirmed that TiO₂（AR）has a wider light absorption range and narrower band gap energy width（Eg）than anatase TiO₂（A）and rutile TiO₂（R）.The results of photocatalytic experiments show that the degradation rate of PFOA by TiO₂（AR）/BiOBr catalyst is up to 81%,and the reaction rate constants are 1.86,2.44 and 3.79 times that of pure BiOBr,pure TiO₂（AR）and P25,respectively.The enhancement of photocatalytic activity can be attributed to the synergistic effect of mixed crystal structure and ternary heterojunction.Free radical capture experiments showed that the efficient degradation of PFOA was the result of the interaction of superoxide radicals（·O₂^-）and photogenerated holes（h⁺）.Repeated use experiments confirmed that the photocatalyst has high stability and reusability,and is expected to be applied to actual environmental remediation.（3）In this study,the degradation mechanism,degradation pathway and toxicity prediction of PFOA were investigated in TiO₂（AR）/BiOBr photocatalytic system.PFOA and its degradation products were detected by high performance liquid chromatography-mass spectrometry（LC-MS）,and the degradation mechanism of PFOA was analyzed.The possible degradation pathway of PFOA was analyzed based on DFT calculations and LC-MS results.It was confirmed that PFOA removed one CF2 unit at a time to form short-chain perfluorocarboxylic acids（PFCAs）.The toxicity prediction experiments of Chlorella and Escherichia coli confirmed that the ecological toxicity of the intermediate product short-chain perfluorocarboxylic acid decreased after PFOA degradation.This study proposes a possible mechanism for improving the catalytic performance of photocatalysts,which has high application value in the field of environmental remediation.