He jishu (Apr 2021)
Preparation of ZIF-8 loaded citric acid-coated nano-zerovalent-iron and its adsorption properties for U(VI)
Abstract
BackgroundThe separation, removal and recovery of U(VI) from water is of great significance to the sustainable development of nuclear energy.PurposeThis study aims to prepare a composite material ZIF-8 loaded by citric acid-coated nano-zero-valent iron (nZVI@CA/ZIF-8), and investigate its adsorption behavior and mechanism for U(VI).MethodsnZVI@CA/ZIF-8 was chemically synthesized and its adsorption properties for U(VI) were analysised by various characterization methods. The effect of time, initial pH, uranium concentration, temperature and ionic strength on Batch U(VI) adsorption were investigated from the aspects of adsorption kinetics, isotherms and thermodynamics. X-ray diffraction spectra (XRD), scanning electron microscopy - energy dispersive spectroscopy (SEM/EDS), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectra (FTIR) were used to characterize and analyse the adsorption behavior and mechanisms.ResultsThe results show that the equilibrium adsorption capacity of uranium is about 115.0 mg∙g-1 when the initial uranium concentration is 50.00 mg∙L-1 at pH 4.0 and 25 ℃. The U(VI) adsorption amount of nZVI@CA/ZIF-8 increases with the increase of initial pH, initial U(VI) concentration and temperature, and maintains good U(VI) adsorption performances even at high Na+ ionic concentration (0.5 mol·L-1). The equilibrium adsorption capacity of uranium is about (110.0±5.0) mg·L-1 when the initial uranium concentration is 50.00 mg·L-1 at pH 4.0 and 25 ℃. The adsorption is a spontaneous endothermic monolayer chemical process, which can be deduced by the pseudo-second-order kinetic and Freundlich models. Brunauer Emmett teller (BET) measurement suggests that nZVI@CA/ZIF-8 displays a porous structure and a large specific surface area (1 271 m²∙g-1). Results of XRD and SEM-EDS demonstrate that the crystal structure and microscopic morphology of ZIF-8 and nZVI are present in the composite. Test results of XPS and FTIR verify that U(VI) can be reduced to U(IV) by nZVI particles exists the nZVI particles could reduce on the surface of composite in the form of UO2. Moreover, nZVI@CA/ZIF-8 can also adsorb U(VI) from the solution by forming Zn-O-U coordination bonds.ConclusionsThis study can provide technical and theoretical references for the synthesis of metal-organic framework composites and the study of its U(VI) removal from radioactive wastewater.
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