(609c) A Molecular Level Understanding of Structural Rearrangement in Ligand-Lanthanide-Water Systems for Selective Recovery of Lanthanides

Lanthanide ions (Ln) are a subset of rare earth elements (REEs) that are essential components in electric vehicles, smart phones and wind turbines. Selective separation of these lanthanides using methods that utilize charge and size is challenging due to their identical ionic charges (+3) and similar radii of hydration, respectively. This research investigates an alternative method of selective separation among lanthanides based on the thermodynamics of adsorption of lanthanides to ligands in aqueous environments. In aqueous environments, lanthanide ions have organized layers of water around them. Adsorption of these lanthanide ions to ligands leads to rearrangement of the surrounding layers of water. Water molecules in the layers maybe ejected and replaced with molecules of the ligand. This rearrangement is expected to have a significant influence on the thermodynamics of the ligand-lanthanide adsorption process and provides an opportunity to utilize thermodynamic properties as a driver of separation among different lanthanide ions. Establishing this separation mechanism requires molecular level insight into the coordination environment of aqueous ligand-lanthanide systems. In this research, we focus on gaining insight into lanthanide ion adsorption to two ligand systems: metal chelating ligand ethylenediaminetetraacetic acid (EDTA) and lanthanide binding peptide EF handloop 1 of Lanmodulin (LanM EF 1). These were chosen because they represent current and emerging technologies for adsorption-based separations of REEs. Specifically, we investigate Lanthanum (La³⁺), Cerium (Ce³⁺), Praseodymium (Pr³⁺) and Neodymium (Nd³⁺) adsorption using molecular simulations. Molecular Dynamics (MD) is used to generate molecular configurations of the aqueous ligand-lanthanide systems and Density Functional Theory (DFT) is used to determine the structures with a quantum-level accuracy. We show that the coordination environments of the adsorbed lanthanides comprise of different arrangements of water molecules and donor nitrogen and oxygen atoms of the ligands. The structural determination is then used to garner insight into the thermodynamics of lanthanide adsorption, which is a key step toward designing ligands for selective REE separations.