来源：ACS Publications
Rare earth elements (REEs) are critical for energy, defense, and information technologies. Their chemical similarity makes the separation of individual REEs among the most demanding problems in modern separation science. Industrial separations rely heavily on liquid–liquid extraction (LLE), but the molecular mechanisms that control selectivity, phase behavior, and transport are often inferred indirectly and consequently not well understood. Also, recently emerging concepts, such as membrane-based, electrochemical, and bio-inspired separations, bring up open questions about ion speciation, interfacial structure, and nonequilibrium dynamics under operating conditions. Here, we review synchrotron X-ray techniques that provide element specificity and in situ nanoscale structural sensitivity with emphasis on how these measurements inform macroscopic separations. We highlight methods that interrogate the bulk for quantifying coordination chemistry, solution nanostructure, and fluctuation-driven thermodynamics alongside interface-sensitive techniques that directly resolve adsorption, complexation, and organization at liquid interfaces. Finally, we outline opportunities enabled by fourth-generation synchrotrons and X-ray free-electron lasers to inform new REE separation technologies.