Extraction of Hexavalent and Trivalent F-Block Elements Using Metal–Organic Framework-303: An Experimental and Theoretical Investigation

Abstract In this study, the adsorption behavior of uranium, U(VI), and neodymium, Nd(III), onto metal–organic framework (MOF)-303, exhibiting a high surface area and tunable functional groups, was systematically investigated. Batch adsorption experiments were performed to evaluate the effects of solution pH, initial metal ion concentration, and contact time on the adsorption performance of MOF-303. The results revealed that adsorption efficiency was strongly influenced by pH, with maximum uptake for both U(VI) and Nd(III) observed at pH = 2. This behavior is attributed to the protonation of MOF-303 surface functional groups, which enhances electrostatic interactions with the metal ions under acidic conditions. The Langmuir model provided an excellent fit for both U(VI) and Nd(III), suggesting monolayer adsorption onto a homogeneous surface with a finite number of binding sites. The maximum adsorption capacities were determined to be 450.45 mg g–1 for U(VI) and 95.78 mg g–1 for Nd(III), indicating the strong affinity of MOF-303 for these ions. Kinetic studies demonstrated that the adsorption process followed a pseudo-second-order model, implying that chemisorption is the rate-limiting step involving electron sharing or exchange between MOF-303 and the metal ions. The thermodynamic studies confirmed the spontaneous nature of the adsorption process. Density functional theory calculations and scanning electron microscopy coupled with energy-dispersive X-ray were employed along with positron annihilation lifetime spectroscopy for elucidation of the binding sites, the distribution of U(VI) and Nd(III), and modifications in the pore network of MOF-303, respectively.