Self-assembled MOF microspheres with hierarchical porous structure for efficient uranium adsorption

Herein, the self-assembled zirconium (Zr)-MOF microspheres formed by the octahedral nano-building blocks (UiO-66 and its derivatives) to adsorb uranium (U) was presented for the first time. The self-assembled MOF microspheres achieved the growth across orders of magnitude from nanometers (∼50 nm) to micrometers (∼2.3 μm), and exhibited ordered hierarchical porous structure (from ∼0.6 nm to ∼340 nm). The self-assembled MOF microspheres combined the advantages of nanostructures with stability and microstructures with hierarchical porous structure for effective mass diffusion. The improved mass diffusion properties of amidoxime (AO)-appended MOF microspheres (P-UiO-66-AO) endowed them with superior adsorption performance and excellent selective adsorption of U compared with AO-appended MOF (UiO-66-AO) octahedron. The adsorption equilibrium was obtained within 1400 min, and the maximum adsorption capacity of U was 283.2 mg/g. The adsorption capacity of U in simulated seawater was determined to be 11.4 mg/g and the ratio of distribution coefficients of U and vanadium (V) (Kd/U/Kd/V) increased 1.75 times than that of UiO-66-AO octahedron, overcoming the challenge of competitive adsorption of U and V with MOF. Overall, the strategy to fabricate self-assembled MOF microspheres with nano-building blocks is expected to expand the diversity and range of potential applications of MOFs.