Coordination-regulated transport in rare earth chelation–membrane separation: From Chelator-assisted processes to functionalized membrane interfaces

Rare earth elements (REEs) are essential strategic resources, but their highly similar physicochemical properties make selective separation extremely challenging. Conventional membrane processes generally rely on size exclusion and electrostatic interactions, resulting in insufficient selectivity for high-resolution REE separation. Chelation–membrane separation introduces coordination interactions into membrane transport, providing a promising strategy for active and selective REE separation. This review presents a comprehensive overview of recent advances in chelation–membrane separation for REEs from the perspective of coordination-regulated transport. First, the molecular design principles of chelators are discussed, with emphasis on donor atom chemistry, denticity, molecular geometry, hydrophilic–hydrophobic balance, and reversible coordination behavior. The structure–transport relationships of representative chelator families, including polycarboxylates, organophosphonic acids, hydroxamic acids, and macrocyclic ligands, are then systematically analyzed. Subsequently, the transport mechanisms of chelation-assisted membrane processes and chelating-functionalized membranes are summarized, highlighting how coordination interactions regulate ion hydration, interfacial recognition, confined transport, and carrier-mediated migration. Particular attention is given to coordination-assisted ultrafiltration and nanofiltration, facilitated transport membranes, two-dimensional layered membranes, MOF/COF membranes, ion-imprinted membranes, and polymer inclusion membranes. Finally, the current challenges associated with selectivity–permeability trade-offs, membrane stability, complex industrial feed streams, and scalable fabrication are critically discussed, together with future opportunities in dynamic coordination interfaces, nanoconfined transport, artificial intelligence-assisted molecular design, and sustainable membrane engineering. This review establishes a unified coordination–transport framework for REE membrane separation and provides guidance for the rational design of next-generation high-selectivity membrane systems.