Preparation Of Lanthanum Modified Zeolite And Passivation Effect On Phosphorus In The Sediments Of The River

Water eutrophication is a global problem and an important cause of the deterioration of aquatic ecosystem.Nitrogen,phosphorus and other nutrients enter surface water mainly through exogenous input and endogenous release.When exogenous phosphorus is no longer the main factor of water eutrophication,reducing the release of endogenous phosphorus has become the main direction of prevention and control of surface water eutrophication.Was prepared in this paper,design a kind of lanthanum modified zeolite materials,the preparation condition was optimized by response surface method,and examines the environmental factors of lanthanum modified zeolite influence on phosphate adsorption,further by FT-IR and XPS characterization methods,lanthanum modified zeolite was studied on the mechanism of phosphate adsorption,and finally examines the lanthanum modified zeolite passivation of river flood sediment phosphorus efficiency.The following research results were obtained:（1）A kind of lanthanum modified zeolite was prepared by the method of"co-precipitation+hydrothermal synthesis".The optimal preparation conditions were obtained by response surface methodology:lanthanum chloride concentration was 0.691 M;hydrothermal reaction temperature was 160℃and hydrothermal reaction time was 26 h.The lanthanum modified zeolite has good adsorption performance at pH less than 8 under both acidic and neutral conditions,and the maximum adsorption capacity is 28.91 mg/g at 25℃.（2）by BET test method,Fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）,scanning electron microscopy-spectrometer coupled（SEM EDS）and X-ray photoelectron spectroscopy（XPS）characterization,such as lanthanum modified zeolite before than the surface of modified natural zeolite 13 times,increase the activity of a large number of sites,be helpful for phosphate on the surface of zeolite adsorption;According to SEM images,the rod-like lanthanum hydroxide load on the surface of zeolite can be clearly observed.And according to EDS,the content of La element on the surface of lanthanum-modified zeolite is8.29%.The crystal lanthanum hydroxide was successfully loaded on the surface of zeolite by XRD.A new characteristic peak appears at 3615 cm-1,which may be due to the hydroxyl radical generated on the surface after lanthanum hydroxide loading.Moreover,the asymmetrical stretching vibration between 547 cm^-1 and 769 cm^-1 increases,indicating that lanthanum enters the skeleton of Si-O tetrahedron.After modification,the characteristic peak of La3d spectrum peak appeared,and the O1s on the surface of natural zeolite and lanthanum modified zeolite also changed,which may be caused by the loaded lanthanum hydroxide.（3）Cl^-,NO₃^-and SO₄^2-have little effect on the removal of phosphate,but CO₃^2-,as a basic ion,mainly affects the adsorption effect of lanthanum-modified zeolite by affecting the pH of the solution.Langmuir model can better describe the adsorption characteristics of lanthanum-modified zeolite,the maximum adsorption capacity is 28.91 mg/g,and different temperatures have little effect on the adsorption capacity of lanthanum-modified zeolite.At low concentrations（5 and 20 mg/L）,it is more consistent with the quasi-first-order kinetics,and at high concentrations（50 mg/L）,it is more consistent with the quasi-second-order kinetics.（4）FT-IR results show that the adsorption of phosphate on the surface of the lanthanum modified zeolite after hydroxyl radicals characteristic peak disappear,that in the process of adsorption of phosphate,lanthanum modified zeolite surface hydroxyl radicals participated in lanthanum modified zeolite adsorption of phosphate,the results will lead to the solution pH increase after adsorption,consistent with the change of the Zeta potential.XPS results showed that the relative concentration of lattice oxygen（O_L）on the surface of lanthanum-modified zeolite decreased from 57.2%to 53.0%before adsorption,indicating that a large amount of La-O complexed with phosphate in the process of phosphate adsorption,resulting in a decrease in the relative content of O_L.Therefore,the main force for the removal of phosphate by lanthanum modified zeolite is the complex reaction between the hydroxyl radicals on the surface of lanthanum hydroxide and phosphate,and phosphate exchanges the hydroxyl groups on the surface of lanthanum modified zeolite.（5）Lanthanum-modified zeolite as a passivating agent is mainly through the conversion of unstable phosphorus into stable state of Ca-P,and the pH change of overlain water has little effect on the passivated phosphorus,mainly because of the stability of La-P and the adsorption capacity of lanthanum-modified zeolite there is a large surplus;Although the reduction percentage of unstable phosphorus decreases with the increase of sediment depth,the active metal such as lanthanum in the material migrates to the deeper sediment under the biological disturbance,resulting in the decrease of unstable phosphorus in the deeper sediment with the increase of passivation time.The lanthanum-modified zeolite is a kind of cheap and environment-friendly material with broad application prospect.In conclusion,lanthanum-modified zeolite has a high potential in controlling phosphorus release from sediment.