Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions

During solvent extraction of rare earth ions, an aqueous electrolyte solution is placed in contact with an immiscible organic solution of extractants to enable extractant-facilitated transport of ions into the organic solvent. Although ex-perimental methodologies such as x-ray and neutron scattering have been applied to characterize ion-extractant complexes, identifying the site of ion-extractant complexation has proven challenging. Here, we use tensiometry and surface-sensitive x-ray scattering to study the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2-ethylhexyl) phosphate (HDEHP), in the absence of a coexisting organic solvent. These studies restrict interactions of HDEHP with trivalent lanthanide ions to the aqueous phase and the liquid-vapor interface, allowing us to explore the consequences that one or the other is the site of ion-extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid-vapor interface, with an overwhelming preference for a light lanthanide, Nd, when present in a mixture with a heavy lanthanide, Er. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in the presence of an organic phase. These results reveal the antagonistic role played by ion-extractant complexation within the aqueous phase and clarify the potential advantages of water-insoluble extractants that interact with ions primarily at the interface during the process of solvent extraction.