Study On The Properties And Mechanism Of Lanthanum-based Metal-organic Framework Materials To Remove Inorganic Fluorine From Water

Fluoride pollution has become one of the environmental problems closely concerned all over the world.A series of diseases relate to teeth and the skeletal system caused by human long-term exposure and intake of excessive fluoride have been fully recognized and concerned by the society.Therefore,it is urgent to find an appropriate method to reduce the content of fluoride in water to an acceptable range as soon as possible.Rare earth metal lanthanum-based materials have become the research hotspot of removing fluoride because of their unique fluoride affinities,excellent fluoride removal performance and stable chemical properties.In this paper,lanthanum-based metal organic framework and its derivative were designed and synthesized by using simple and feasible experimental methods and low-cost raw materials,which were used to remove fluoride from water in adsorption and capacitive deionization technologies respectively.The specific research contents and results are as follows:Adsorption has become the most widely studied and recognized technology in the field of fluoride removal at present because of its advantages of high efficiency,low cost and simple operation.In this study,lanthanum-based metal organic framework（La-MOF-NH₂）adsorbent was prepared by solvothermal method with La（NO₃）₃·6H₂O as the main raw material.At the same time,a new adsorbent La-MOF-NH₂@g-C₃N₄ was synthesized by adding g-C₃N₄ which rich in various nitrogen sources to explore the role of different nitrogen in the adsorption process.Both adsorbents showed complete spherical morphology,excellent selectivity and regeneration ability.In the adsorption process,fluoride ion can be removed by ion exchange with amino groups which were rich in adsorbent.Pyridine nitrogen and graphite nitrogen promote the adsorption of fluoride ion,while pyrrole nitrogen and nitrogen oxide contribute less to the adsorption.When the p H of the solution was 3 and the dosage of adsorbent was 2 g/L,the maximum adsorption capacity of La-MOF-NH₂ and La-MOF-NH₂@g-C₃N₄ in fluoride solution with initial concentration of 100 mg/L and 50 mg/L was 91.6 mg/g and 41.8 mg/g respectively.The adsorption process of fluoride by La-MOF-NH₂ and La-MOF-NH₂@g-C₃N₄conformed to Langmuir isothermal adsorption model and pseudo second-order kinetic model.Based on the physical characterization and adsorption model,the main adsorption mechanisms of the two adsorbents were ion exchange and complexation.Capacitive deionization technology has become a new and promising fluoride removal technology due to its advantages of low energy consumption and environmental friendliness,but its removal effect has not been ideal.Therefore,the development of electrode materials with excellent performance was the key to promote the development of capacitive deionization technology.In this study,a highly efficient porous structure material（La-N-C-800°C）was prepared by pyrolysis of La-MOF-NH₂material at high temperature as an electrode material,which was designed from the perspective of material structure and composition regulation.The metal organic framework derivatives generated after high-temperature calcination still maintain the complete spherical morphology of the precursor.La-N-C-800°C material showed large specific surface area（94.6 m²/g）and porous structure,which provided rich active sites for reaction with fluoride ion in the process of capacitive deionization technology.The electrode material showed the best fluoride removal performance when the voltage was1.2 V and the solution p H was 3.When the initial fluoride concentration was 10 mg/L,the optimal capacitive deionization capacity was as high as 8.0 mg/g.The fluoride removal process of La-N-C-800°C electrode material conformed to Langmuir isothermal adsorption model and pseudo first-order kinetic model.The electrode material showed high specific capacitance and conductivity in the electrochemical performance test.The removal of fluoride ion by the electrode material mainly depends on the synergistic effect of electric double layer and La-F complex.