Preparation And Characterization Of BiVO₄/MnO₂ Composite Magnetic Photocatalytic-oxidant

With the rapid development of chemical,papermaking,textile and metallurgical industries,environmental and water pollution problems have emerged one after another.In particular,the complex refractory organic compounds in dyestuff wastewater entered the environment without being treated,which would have a serious impact on human health and ecological environment security.At present,composite semiconductor photo-catalytic oxidation technology is an environmentally friendly and efficient method to treat organic wastewater.BiVO₄ is n-type semiconductor material with visible light photocatalytic activity.Its chemical properties are stable and its price is inexpensive,but it has defects such as fast photo-generated charge recombination and lower catalytic activity.MnO₂ as transition metal,is also n-type semiconductor material with good conductivity and small band gap.And so,BiVO₄ usually doped inδ-MnO₂ to prepare BiVO₄/δ-MnO₂ composite with good catalytic activity.However,the composite catalyst BiVO₄/δ-MnO₂ has difficulty in solid-liquid separation after catalytic reaction and is not easy to recycle.Therefore,under the premise of maintaining the catalytic activity of the composite catalyst BiVO₄/δ-MnO₂,the magnetic matrix Mn_xZn_1-xFe₂O₄and BiVO₄/δ-MnO₂ were used to prepare ternary magnetic composite catalyst Mn_xZn_1-xFe₂O₄/BiVO₄/δ-MnO₂ to realize the recycling of composite catalyst BiVO₄/δ-MnO₂.The composite catalytic-oxidant BiVO₄/δ-MnO₂ was prepared by hydrothermal method.To obtain the appropriate preparation process,the effect of different mass ratio（5¹5:100）in the composite,reaction time,pH for reactive system on the RhB degradation were investigated by using the single factor method.The structure and properties of BiVO₄/δ-MnO₂ were characterized by XRD,BET,SEM,XPS and PL.The results showed that the crystal phase structure of BiVO₄ is not destroyed after BiVO₄doped withδ-MnO₂,the particles and needle-likeδ-MnO₂ are attached to the surface of flaky BiVO₄,and the agglomeration phenomenon ofδ-MnO₂ was reduced.BiVO₄/δ-MnO₂ has larger specific surface area which was conducive to RhB molecular（1.59×1.18×0.56 nm）diffusion and adsorption.BiVO₄ doped withδ-MnO₂ that produced electron coupling and chemical bond,which could reduce the photoelectric charge and improved the light quantum utilization efficiency of composite material BiVO₄/δ-MnO₂.BiVO₄/δ-MnO₂ has the best photocatalytic effect on RhB when the mass ratio of is 10:100,and the degradation rate of RhB reached 97.8%after 3 h illumination.In weak acid solution,δ-MnO₂ had special oxidation.So,BiVO₄/δ-MnO₂with photocatalytic oxidation activity as the solution pH was 5,and the degradation rate of RhB reached 98.4%for 20 min.The molecular structure of RhB was oxidized and decomposed byδ-MnO₂,and the formed organic small molecules were further oxidized to the inorganic non-toxic and harmLess small molecular substances by the photocatalytic degradation of BiVO₄.Magnetic oxidant Mn_x Zn_1-xFe₂O₄/δ-MnO₂ was prepared by one-step hydrothermal method,and its structure and property were characterized through XRD,FT-IR,SEM,EDS,XPS,VSM.The results showed that the crystal phase structure of Mn_xZn_1-xFe₂O₄/δ-MnO₂ is mainly similar toδ-MnO₂,and the flaky Mn_xZn_1-xFe₂O₄ was adhered in theδ-MnO2 flower cluster-like spherical surface.So,Mn_xZn_1-xFe₂O₄/δ-MnO₂ had magnetic properties,and the saturation magnetization value（Ms）is 7.70 emu/g.Mn_x Zn_1-xFe₂O₄﹕δ-MnO₂ is 0.08﹕0.85 in the composite,which is not much different from the theoretical value.Electron coupling and chemical bond occurred after the combination of Mn_xZn_1-xFe₂O₄ andδ-MnO₂.When the optimal mass ratio between Mn_xZn_1-xFe₂O₄ andδ-MnO₂ was 10:100,the best degradation efficiency of Mn_xZn_1-xFe₂O₄/δ-MnO₂ was obtained and RhB degradation rate reached99.4%when the solution pH was 1 for 15 min.The magnetic composite photocatalytic oxidant Mn_xZn_1-xFe₂O₄/BiVO₄/δ-MnO₂was prepared by hydrothermal method.The catalytic degradation effect of RhB by Mn_xZn_1-xFe₂O₄ and BiVO₄/δ-MnO₂ with different mass ratios（5³0:100）was studied.The structural properties of magnetic composites were characterized by XRD,SEM,EDS,BET,XPS,VSM,UV-vis,etc.Results showed that the small particle Mn_xZn_1-xFe₂O₄ andδ-MnO₂ were adhered to the surface of the sheet-like morphological BiVO₄ matrix,and the spherical morphology ofδ-MnO₂ didn’t found.It improved agglomeration phenomenon ofδ-MnO_2.The specific surface area of Mn_xZn_1-xFe₂O₄/BiVO₄/δ-MnO₂ increases to 17.2 m²·g^-1 and the pore diameter is 12.2nm,which is favorable for diffusion and adsorption of the molecular size of RhB,which enhanced the catalytic degradation of RhB.After the combination of BiVO₄,Mn_xZn_1-xFe₂O₄ andδ-MnO₂ occurred electronic coupling and chemical bond,and formed heterojunction.The photo-generated electron-hole pairs recombination is effectively inhibited,and the catalytic activity of the magnetic composite catalyst Mn_xZn_1-xFe₂O₄/BiVO₄/δ-MnO₂ is obviously improved.The saturation magnetization（Ms）of Mn_xZn_1-xFe₂O₄/BiVO₄/δ-MnO₂ was 3.90 emu·g^-1.When the pH value of solution was 1,the degradation rate of RhB reached 99%at 20 min.