Toward a Chemistry-Guided Framework for Nd(III)/Dy(III) Separation: Integrating Hydrometallurgy, Ionic Liquids, and Deep Eutectic Solvents

Rare earth elements (REEs), particularly neodymium (Nd) and dysprosium (Dy), are indispensable for permanent magnets, electric vehicles, wind energy technologies, and other advanced energy systems. However, the efficient separation of Nd(III) and Dy(III) remains one of the most persistent challenges in hydrometallurgy because of their closely similar physicochemical properties, strong hydration, and comparable coordination behavior, which fundamentally limit the selectivity of conventional solvent extraction processes. This review critically evaluates recent advances in sustainable Nd(III)/Dy(III) separation by integrating conventional hydrometallurgical principles with emerging green solvent systems, including ionic liquids (ILs) and deep eutectic solvents (DESs), particularly hydrophobic deep eutectic solvents (HDESs). Particular emphasis is placed on the molecular mechanisms governing lanthanide separation, including hydration thermodynamics, interfacial dehydration, ion pairing, hydrogen-bond organization, solvent structuring, and coordination stabilization. Unlike conventional descriptive reviews that primarily summarize extraction efficiencies and solvent compositions, this review proposes a chemistry-guided conceptual framework that interprets Nd(III)/Dy(III) solvent extraction as a sequential and energetically coupled process involving hydration thermodynamics, interfacial dehydration, and coordination chemistry. By explicitly linking solvent structure and molecular-level interactions with macroscopic extraction performance, the proposed framework establishes a unified mechanistic basis for understanding and rationally designing selective Nd(III)/Dy(III) separation across conventional and emerging solvent systems. The review also identifies key knowledge gaps, particularly regarding interfacial energetics, transient coordination processes, solvent structuring effects, and predictive thermodynamic modeling. Finally, future research directions are discussed to support the rational design of environmentally sustainable, mechanism-driven rare-earth separation technologies and the efficient recovery of critical elements from secondary resources.