Investigation of fundamental interactions between rare earth elements and a series of carboxylate ligands to enhance separation performance: An experimental and computational approach

Rare earth elements (REEs) are a class of critical materials vital to an array of applications such as electronics, batteries, and defense weapons systems. These elements coexist in minerals and ores, and due to their similar chemical properties, the challenge remains to find an effective means of separation. Current large-scale separation processes rely on slight differences in size and acidity between REEs, however these separations are inefficient and pose detrimental risks to the environment due to the large volumes of volatile organics and acids required to achieve separations. REEs can be separated on the basis of their migration under an applied electric field, i.e., their electrophoretic mobility, which depends on differences in size and charge and may be modified through coordination with ligands. The degree to which coordination with ligands may influence the transport properties of these metal-ligand systems remains, for the most part, unknown. Here, we investigate the fundamental transport properties of a series of REE-carboxylate ligand complexes in aqueous solution via capillary electrophoresis and molecular dynamics (MD) simulations are carried out to validate metal-ligand coordination geometry and diffusion properties. Iminodiacetic acid (IDA) is identified as a ligand that greatly enhances separation between light and heavy lanthanides by selectively coordinating with light lanthanides in a tridentate mode and heavy lanthanides in a bidentate mode.