Coordination chemistry of surface-associated ligands for solid–liquid adsorption of rare-earth elements

General guidelines for the design of ligands for the enrichment of rare-earth elements by solid–liquid adsorption are described using coordination chemistry. Relevant properties of ligands include selectivity of metal ions based on adjustment of donor atom polarizability, denticity, and the pKa range of the binding sites. The selectivity of solid-phase materials for the enrichment of rare-earth ions by the ligand design guidelines is outlined, with special consideration of additional variable factors including steric hindrance, saturated binding sites, variability in speciation caused by the identity of counterions and ionic strength, and size-exclusivity in ligands stemming from differences in bite angle, preorganization of ligands, or intraligand interactions. This review analyzes some principles of selectivity of rare-earth elements with ligands organized by donor type from examples collected from reports published between 2009 and 2021.