Magnetic Bentonite Assisted Construction Of Z-scheme BiOBr/BiOI With Pinecon-like And Its Efficient Removal Of Tetracycline

To optimize the photocatalytic performance of BiOBr,it is vital to solve its problems of low visible light harvesting ability and pollutant adsorption capacity,the easy recombination of photo generated electron hole（e^-/h⁺）,the serious agglomeration phenomenon,and the difficult of solid-liquid separation and recovery.Thus,we synthesized magnetic bentonite（MBT）supported Z-scheme heterojunction BiOBr/BiOI with pinecone-like structure(MBT₂₅/BiOBr/BiOI)by in situ growth BiOBr and BiOI on the surface of MBT through combining the methods of heterostructure construction,morphology control and carrier loading.Then,the prepared materials were characterized,and their adsorption photocatalytic properties were investigated by the degradation of tetracycline（TC）.Besides,the degradation pathway of TC was analyzed by using the ultra-high performance liquid phase mass spectrometry（UPLC-MS）.Additionally,the main synthesis mechanism and photocatalytic enhancement mechanism of MBT₂₅/BiOBr/BiOI were proposed.The main conclusions were as follows:（1）MBT and MBT₂₅/BiOBr/BiOI were synthesized by simple precipitation combined with microwave hydrothermal method.The characterization results showed that BiOBr and BiOI exhibited smooth lamellar and serious agglomeration.However,after the introduction of MBT,BiOBr and BiOI were cutted on its surface with good dispersion and then obtained Z-scheme heterojunction BiOBr/BiOI with pinecone-like structure.MBT₂₅/BiOBr/BiOI possessed increased specific surface area,enhanced optical absorption and e^-/h⁺separation efficiency.Besides,MBT₂₅/BiOBr/BiOI can realize rapid magnetic separation and recovery,which was conducive to recycling.（2）MBT₂₅/BiOBr/BiOI possessed excellent adsorption and photocatalytic degradation performance for TC.After 80 min visible light irradiation,the photocatalytic degradation of 20 mg/L TC was 89%.And the corresponding degradation rate constant(0.021 min^-1)of MBT₂₅/BiOBr/BiOI was 2,10 and 1.5times larger than BiOBr,BiOI and BiOBr/BiOI,respectively,and the degradation was still 80%after five cycles.When the concentration of TC was less than 5mg/L,the degradation of TC was as high as 96%.In the range of p H 3～10,MBT₂₅/BiOBr/BiOI could still maintain good photocatalytic performance.Besides,the XRD results showed the structure of MBT₂₅/BiOBr/BiOI was stable.（3）Synthesis mechanism of MBT₂₅/BiOBr/BiOI:bentonite has the characteristics of large specific surface area,pore structure,swelling and surface electronegativity,which could provide sufficient loading sites for Fe₃O₄,BiOBr and BiOI;the BiOBr and BiOI inoculating crystals were directionally in situ grew on the surface of MBT,then grew into BiOBr/BiOI plates through a dissolution-recrystallization process,which were finally self-assembled to form Z-scheme BiOBr/BiOI heterojunction with pinecone-like structure.The enhanced photocatalysis mechanism was as follows:MBT₂₅/BiOBr/BiOI possessed a lower band gap energy（1.5 e V）,and the synergy of Z-scheme BiOBr/BiOI exhibitied pinecone-like structure with functional MBT increase the specific surface area of the material and realize the double adsorption for TC,improving the light capture ability and the separation efficiency of electron hole pairs,and maintain the redox ability of BiOBr and BiOI.（4）The degradation pathway of TC was as follows:During the whole degradation process,TC molecules would be attacked by active radicals of·OH,h⁺,e^–and·O₂^–,result in hydroxylation,dealkylation,deamidation,dehydration,ring opening and further oxidation reactions,thus produce intermediate products.These intermediate products and TC continued to be oxidized to smaller molecules,and finally degraded and converted into CO₂ and H₂O.In conclusion,MBT₂₅/BiOBr/BiOI exhibited the significantly enhanced adsorption and photocatalytic degradation for TC,possessed rapid magnetic separation recovery.The synthesis mechanisms and the methods of optimizing photocatalytic performance in this study could provide valuable references for other photocatalyst to optimize its photocatalytic performance.