Selective crystallization strategies for lanthanide-lanthanide and lanthanide-actinide separations

Rare-earth elements (REEs), including lanthanides, are vital for high-tech applications; however, their nearly identical trivalent ionic radii and coordination behaviour make individual separation challenging. This difficulty is further intensified by the co-occurrence of actinides (e.g., Th) in REE ores, which share similar chemical properties and complicate purification. In spent nuclear fuel (SNF) reprocessing, efficient separation of lanthanides from actinides is also essential for actinide recycling, radiotoxicity reduction, and advancing closed fuel cycle strategies. Selective crystallization has emerged as a promising alternative to conventional separation techniques such as solvent extraction and ion exchange. By fine-tuning parameters like ligand identity, metal-ligand ratio, pH, solvent system, temperature, and so on, crystallization enables preferential formation of metal-specific solid phases even among chemically similar ions, thereby achieving lanthanide-lanthanide (Ln-Ln) and lanthanide-actinide (Ln-An) separations. This feature article reviews and summarizes the latest advances in crystallization-based Ln-Ln separation, including systems based on inorganic oxoanions, metal-organic frameworks (MOFs), molecular complexes, and supramolecular assemblies. For Ln-An separation, special emphasis is placed on both REE purification and SNF applications involving oxidation-state-specific separation. Structural insights from single-crystal products offer a deeper understanding of coordination preferences and separation mechanisms.