Functionalization Of Zirconium-based Metal Organic Frameworks And Their Adsorption Properties For Uranium

Under the current international background of energy shortage and environmental constraints,nuclear power,as one of the energy forms that takes into account the contradictory of"low carbon,economy and safety",is favored by all countries.Uranium（U）is the main fuel in the nuclear energy system,and a large amount of uranium-containing radioactive wastewater was produced in the process of uranium beneficiation and metallurgy,fuel production,power generation,spent fuel reprocessing.Uranium is extremely chemically and radioactively toxic.Free U（Ⅵ）ions can pose serious threats to human health（for example,kidney damage）and the environment through surface and groundwater pollution.So,the effective treatment of uranium-containing wastewater is the guarantee of national radiation safety and nuclear industry development.Adsorption is the preferred method for the treatment of uranium-containing wastewater,but the successful application of adsorption mainly depends on the adsorption performance of the adsorbents,including adsorption capacity,adsorption selectivity,and reusability.Therefore,how to improve the adsorption performance of adsorbents for uranium has become a key problem to be solved urgently in the adsorption treatment of uranium-containing wastewater.Metal-organic frameworks（MOFs）have the excellent characteristics of extremely high specific surface area,regular porous morphology and tunable pore structure,special metal centers,structural diversity and designability,so MOFs has attracted the attention of many scholars in the field of adsorption and separation.Among them,zirconium-based MOFs have good thermal stability,chemical stability,and mechanical stability,even can maintain the integrity of crystals in acidic and neutral aqueous solutions,which provide the possibility to be used for the removal of uranium in wastewater.In this paper,Zr-MOFs were functionalized to improve their uranium adsorption capacity,selectivity,separability and reusability;batch experiments and modern instrument characterizations were used to explore the adsorption properties of different Zr-MOFs for uranium;The adsorption isotherms,adsorption thermodynamics,and adsorption kinetics were studied,and the mechanisms of their adsorption for uranium were explored.The research topics and the corresponding research results are described as follows:（1）The Langmuir specific surface area of UiO-67 is much larger than that of UiO-66,and UiO-67 crystals have an opening of 8（?）,which is more favorable for the adsorption of UO₂²⁺（ion size 6.44（?））in its micropores.However,there are few studies on the adsorption properties of UiO-67.Based on these,UiO-67 was synthesized in this work,and its adsorption properties for UO₂²⁺were investigated.Firstly,UiO-67 was successfully synthesized and characterized by XRD,TGA and nitrogen isotherm adsorption test,compared with UiO-66.Secondly,the stability of UiO-67 in various solvents was discussed.The effects of adsorption time,pH and initial U concentration on the adsorption of U（Ⅵ）by UiO-67 were also investigated.The results show that UiO-67 has high thermal stability and can still maintain the stability of the crystal structure at 500℃.The BET specific surface area of UiO-67 is up to2,000 m²/g,which is twice that of UiO-66.UiO-67 has certain stability in DMF,but the crystal structures collapsed completely in water,methanol and ethanol solvents.In adsorption experiments,UiO-67 material can quickly and efficiently remove U,and the adsorption capacity was significantly higher than that of UiO-66.The adsorption isotherms and kinetic data of U（Ⅵ）on UiO-67 were comparatively conformed to Langmuir adsorption isotherm and pseudo-second-order kinetic models,and combined with infrared spectroscopic analysis,it is speculated that the adsorption process is the chemical interaction between carboxyl groups on UiO-67 and uranyl ion.Stability and adsorption experiments confirmed that the extension of organic chain did not change the thermal stability of UiO series,decreased the chemical stability,and improved the adsorption performance of uranium.（2）According to the Lewis acid-base theory,UO₂²⁺can be considered as a strong Lewis acid,which has a strong affinity with electron-rich groups.However,cyano group,with a lone electron pair,is rarely used in the functional modification of uranium adsorbents.In addition,compared with the direct functionalization on the organic chain,the electron-rich group,which is modified on the coordinatively unsaturated metal center of MOFs,can maintain the independence and freedom of the group and is more conducive to its interaction with UO₂²⁺.Based on these,the water-stable UiO-66 was functionalized by grafting the cyano group of adiponitrile on coordinatively unsaturated Zr(（Ⅳ）centers,yielding cyano-UiO-66（UiO-66-CN）through a facile coordination-based post-synthetic modification strategy（PSM）.The obtained sample was characterized by thermogravimetric analysis（TGA）,Fourier transform infrared spectrometry（FT-IR）,scanning electron microscopy（SEM）and powder X-ray diffraction spectrometry（PXRD）,which confirm the successful modification of cyano groups and the preservation of crystal structure.UiO-66-CN exhibits excellent adsorption performance for uranyl ion from water with maximum adsorption capacity of10.6 mg/g,which is five times as high as the pristine UiO-66 at the initial concentration of 5.0 mg/L.Moreover,the adsorbed uranyl ion can be easily desorbed with 0.1 mol/L HCl elution,and the prepared material also displays a desirable selectivity toward U（Ⅵ）in a solution containing a series of divalent competing ions.This work establishes a simple and energy efficient route via the PSM strategy to synthesize a novel type of cyano functionalized metal-organic framework for U（Ⅵ）ions extraction from solution.（3）MOFs particles usually exist in the form of powders,which require complex separation procedures（tedious high-speed centrifugation or filtration）and cause material loss during the separation process.MMOFs composed of MOFs and magnetic materials combine the advantages of rapid separation of magnetic materials and the excellent adsorption properties of MOFs,which have demonstrated their prospective for pollutant sequestration.Based on these,Fe₃O₄@SiO₂@UiO-66 core-shell magnetic microspheres were synthesized by in situ growth on Fe₃O₄@SiO₂ and used for the removal of U（Ⅵ）from aqueous environment.The characterization by transmission electron microscopy（TEM）,XRD,FT-IR,vibrating sample magnetometry（VSM）,TGA and nitrogen isotherm adsorption test proved that the Fe₃O₄@SiO₂@UiO-66 core–shell nanocomposite was successfully prepared.Batch experiments have been conducted to study the effects of initial pH,shaking time,initial uranium concentration and temperature on uranium sorption efficiency.The isotherm and kinetic data were accurately described by the Langmuir and pseudo-second-order models.The maximum adsorption capacity was calculated to be 616.5 mg/g for U（Ⅵ）,by fitting the equilibrium data to the Langmuir model.Various thermodynamic parameters were evaluated which indicated the endothermic and spontaneous nature of adsorption.Infrared analysis speculates that carboxyl groups is mainly involved in the chemisorption of U（Ⅵ）and Fe₃O₄@SiO₂@UiO-66 The excellent adsorption capacity and the sensitive response to the magnetic field made Fe₃O₄@SiO₂@UiO-66 core-shell magnetic microspheres a promising candidate for the removal of uranium（Ⅵ）from aqueous solutions.