Rare earth element separations
Argonne researchers performing Grazing-incidence (GI) X-ray Absorption Spectroscopy (XAS) at the XSD bending magnet beamline station 12-BM-B at the APS are searching for an efficient process for rare earth element separations.
The team is exploring metal-ion speciation of electrolyte solutions, including the development of a predictive understanding of solvent extraction (SX) processes, in their quest for metal purification.
The coordination chemistry of metal ions in aqueous electrolyte solutions plays a critical role in their reactivity, stability, solubility and transport. Predicting these behaviors is complicated by difficulties in obtaining metrical information with the precision necessary to develop an understanding of the interatomic level processes at play and their relative importance.
Grazing-incidence (GI) X-ray Absorption Spectroscopy (XAS) under conditions of total external reflection is used to explore the coordination environment of the trivalent erbium ion, Er3+, at an electrolyte-vapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that the Er3+ ions have a simple hydration shell with an average Er-OH2 bond distance of 2.33(1) Å, consistent with previous descriptions of the aquated cation, [Er(OH2)8]3+.
No other correlations are observed in the electrolyte EXAFS (extended X-ray absorption fine structure) data acquired at room temperature. In contrast, the coordination of the Er3+ ions at the electrolyte-helium interface—as interrogated by use of electron-yield detection—reveal correlations beyond the Er-OH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6–7 O atoms at 2.36(1) Å and a second one of 3 Cl atoms at 2.89(2) Å, suggesting the formation of a neutral [(H2O)6-7ErCl3] entity at the surface of the electrolyte.
The presence of a third, distant peak in the Fourier transform data is attributed to Er-Er correlations (in possible combination with contributions from distant Er-O and Er-Cl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) Å, confirming the near-surface-enrichment of Er3+ as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082).
Mrinal K. Bera, Guangming Luo, Mark L Schlossman, Lynda Soderholm, Sungsik Lee and Mark R. Antonioeparations, “Erbium(III) Coordination at the Surface of an Aqueous Electrolyte,” The Journal of Physical Chemistry B, Just Accepted Manuscript. DOI: 10.1021/acs.jpcb.5b02958. Published Online May 14, 2015.