Photocatalytic And Magnetic Enrichment Behavior Of Nanoferrites@TiO₂

At present,environmental problems are becoming more and more serious.As one of the main problems,water pollution has been widely concerned.The common pollution water quality is complex and contains many organic pollutants that are difficult to degrade.Carbofuran is a carbamate broad-spectrum insecticide with strong residue and toxicity.As the use of pesticides increases year by year,it is necessary to find a method with high efficiency and no secondary pollution for degradation.As a new oxidation method,photocatalytic oxidation can decompose organic pollutants into small molecular inorganic substances without secondary pollution under the irradiation of sunlight.Among them,TiO₂ has been extensively studied and applied due to its stable chemical properties,non-toxicity and abundant sources.However,the development and application of TiO₂ are greatly limited because of its small particle size and wide band gap.In order to solve the above problems,the size controllable and monodisperse MFe₂O₄（M=Ni,Co,Mg,Cu,Ca）nanoparticles were prepared by sol-gel method,and the effect of external magnetic field on the magnetic recovery efficiency was discussed.And the relationship between the doping of rare earth elements and the photocatalytic activity of TiO₂ was studied by doping TiO₂ with rare earth elements.Furthermore,a NiFe₂O₄@TiO₂ composite magnetic photocatalyst having a core-shell structure was prepared.The effects of magnetic nucleation on the photocatalytic activity and long-acting properties of composite materials were investigated.The degradation mechanism of carbofuran was inferred.The main conclusions were as follows:1.The monodisperse spinel nanoferrite MFe₂O₄ with controlled particle size,no impurity phase,narrow particle size distribution and controllable particle size（15nm,45nm,110nm）can be obtained by template-assisted sol-gel method.The magnetic field and magnetic field gradient distribution around the magnetic bar was simulated and analyzed by Maxwell（16.0）finite element analysis software.On this basis,the behavior of ferrite nanoparticles in fluids was analyzed,and the enrichment model of nano-ferrite particles in external magnetic field was established by adsorption theory:（i）Freundlich adsorption model at low magnetic permeability（<10μemu/Oe）which leads to the enrichment percentage below 50%,（ii）mixed（multilayer）adsorption model at intermediate permeability（10-50μemu/Oe）,and（iii）a monolayer adsorption at high permeability（>100μemu/Oe）leading to the enrichment percentage above 90%.2.Under the influence of rare earth doping,the grain size and band gap of titanium dioxide are reduced,and the absorption range is broadened.When the doping ratio is0.025%Fe-0.067%Gd-titanium dioxide（molar ratio）,the narrowest band gap（2.97eV）is found in the heat-treated TiO₂ at 500℃.Through the optimal doping ratio of TiO₂ to the degradation of carbofuran,when the concentration of catalyst is 0.5 g（corresponding to 400 ml of 100 ppm carbofuran）,the degradation rate of carbofuran can reach 94.7%and half-life is 1.87 h.Through the analysis of the intermediate products of carbofuran by MS-GS,the complete degradation route of carbofuran was deduced based on the test results.3.The NiFe₂O₄@Fe-Gd-TiO₂（500）composite with core-shell structure was prepared by sol-gel method,consisting of spinel NiFe₂O₄ and anatase TiO₂.It was found that the magnetic properties of the composite decreased compared with NiFe₂O₄,which was due to the shielding effect of the shell TiO₂ on the magnetic core.At the same time,the photocatalytic activity of the composite decreased,and the degradation rate of carbofuran decreased from 94.7%to 90.1%.This is because some cations in the core migrate into the shell Fe-Gd-titanium dioxide,which reduces the photogenerated electrons-holes and leads to the reduction of photocatalytic activity.The magnetic recovery of the composites was 86%,and the degradation rate of carbofuran decreased from 90.4%to 86.9%after the first photocatalysis,and then remained stable.Infrared analysis showed that the first degradation consumed the hydroxyl groups on the surface of Fe-Gd-TiO₂ catalyst,which greatly changed the photocatalytic activity of the material.