Abstract: ZIF-8, a classical metal-organic frameworks (MOFs) material, was used as an adsorbent to study the effect of particle size on its uranium adsorption performance. ZIF-8 particles with different sizes were synthesized by three methods, and characterized by scanning electron microscope (SEM), X-ray powder diffraction (XRD), surface area and porosity analyzer. The adsorption of ZIF-8 particles with different sizes for U(VI) from uranyl nitrate solution was tested under the same conditions. The parameters of adsorption isotherm and kinetics were calculated. The results show that ZIF-8 with high crystallinity and high purity is successfully prepared. The morphology of the product is rhombic dodecahedron, and the sizes of the particles are about 50 nm, 150 nm and 2 μm with narrow distributions. The three samples have single uniform micropore structures and their BET surface areas are highly related to the particle size. All the ZIF-8 particles with different sizes can quickly adsorb 90% of U(VI) from the initial concentration of 200 mg/L of U(VI) solution in 70 min (at room temperature, pH=3).The particles with smaller size (about 50 nm) show better adsorption performance, and the saturated adsorption capacity reaches 520.26 mg/g. The adsorption kinetics conforms to the second order kinetic equation, and the adsorption isotherm conforms to the Langmuir model, which indicate that the adsorption for U(VI) of ZIF-8 belongs to chemical monolayer adsorption. After 4 cycles of adsorption and desorption, their removal rates for U(VI) are still above 70%.

Key words: Size effect, ZIF-8, Nanostructures, Uranium adsorption