Erbium(III) Coordination at the Surface of an Aqueous Electrolyte

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 electrolytevapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that theEr(3+) ions have a simple hydration shell with an average ErOH2 bond distance of 2.33(1) angstrom, 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 electrolytehelium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the ErOH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 67 O atoms at 2.36(1) angstrom and a second one of 3 Cl atoms at 2.89(2) angstrom, suggesting the formation of a neutral [(H2O)(67)ErCl3] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to ErEr correlations (in possible combination with contributions from distant ErO and ErCl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) angstrom, confirming the near-surface-enrichment of Er3+ as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082). Here, the strong associations between the Er-aqua-chloro entities at the electrolytevapor interface are shown to be consistent with the formation of domains of polynuclear cluster motifs, such as would arise through hydrolysis reactions of the aquated Er3+ cations. The local structural results and the calculated surface coverage are of relevance to understand the myriad reactions involved in the hydrometallurgical process of solvent extraction (SX) for metal purification, which involves the transfer of a selected metal ion, like Er, across an interface from an aqueous electrolyte to an organic phase.