Crystallographic Facet-Directed Selective Adsorption of Uranyl: Thermodynamics and Kinetics

Capping agents influenced the terminal atoms of crystals, thereby enabling the exposure of crystal facets with distinct surface energies. This fine structural regulation affected the surface charge distribution across different crystal planes. However, studies on the differences in the adsorption behavior of guest molecules among different crystal facets have been rarely reported. In this work, we systematically studied the adsorption kinetics and thermodynamics of uranyl ions on different ZIF-8 crystal facets through facet regulation. Experimental results revealed that the cubic crystals (C-ZIF-8), which predominantly exposed {100} facets, reached adsorption equilibrium within 40 min with an initial uranium concentration of 10 mg L^-1 and exhibited a maximum adsorption capacity of 846.5 mg g^-1. First-principles calculations indicated that the (100) facets possessed an abundance of charge-unbalanced Zn atoms, which readily coordinated with the axial oxygen atoms of the uranyl ions. Additionally, hydrogen-bonding interactions around these sites stabilized uranyl ions at bridge sites on the (100) surfaces via electrostatic-directional interactions. This work provides empirical data and theoretical insights into the facet-dependent adsorption kinetics and thermodynamics of guest molecules, and offers valuable reference for the rational design of heterogeneous crystal adsorbents with controlled morphologies for radionuclide removal.