Aqueous Adsorption of Rare Earth Elements by Organophosphonate-Ligand-Modified Metal–Organic Frameworks: The Role of Ligand Loading in Surface Complexation

Rare earth elements (REEs) are essential to clean energy and many other technologies, but REE mines are concentrated in a few geographical locations in the world and their broad accessibility is susceptible to trade bottlenecks. Waste streams from mining and other industrial processes containing trace REEs are attractive REE sources for enhancing the supply chain, but their separation from the waste stream remains a key challenge. Metal-organic frameworks (MOFs) offer well-defined, nanoscale pore structures highly conducive to chemical functionalization, allowing the design of organic ligand-modified MOFs with favorable 3D coordination geometries to achieve preferential binding to REEs. In this study, we modified MIL-101 with different loadings of the organophosphonate ligand, phosphonomethyliminodiacetic acid (PMIDA), and investigated their efficacy to recover three trivalent REE cations (Nd3+, Gd3+, and Dy3+) in the absence and presence of a common competing cation, Fe(III). PMIDA-modified MIL-101 maintained the crystalline structure and high specific surface area of MIL-101 and exhibited high-capacity adsorption of all three REEs, up to 195 mg g-1, with a small reduction in capacity in the presence of Fe(III). The REE adsorption exhibited a nonlinear relationship with the PMIDA loading: a higher PMIDA loading led to only a moderate increase in REE adsorption capacity but a notably higher number of available coordination sites per adsorbed REE and hence stronger REE-PMIDA complexes. Molecular dynamics simulations confirmed the formation of polyhedral PMIDA-REE complexes within MIL-101 nanopores, with a higher coordination number and binding energy at a higher PMIDA loading. These results suggest that the PMIDA-modified MIL-101 can provide a coordination environment similar to that found in the solvent extraction process, and careful selection of ligands and the MOF pore structure could enable highly precise separation of target REEs. The adsorbed REE could be eluted using 0.01 mol L-1 HCl, albeit with a nonnegligible loss in adsorption capacity over repeated use.