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

Abstract

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, Er(3+), 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 Er(3+) 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 Er(3+) 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 Er(3+) 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 electrolyte-vapor 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 Er(3+) 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.

Crystallizing Elusive Chromium Polycations

“…Consequently, proposal 1) is not supported by observations. As for proposal 2), Antonio and co-workers (Bera et al, 2015) recently characterized both the surface and the bulk of a drop of an aqueous Er(NO3)3 solution by X-ray absorption spectroscopy. The spectra show clusters exist near the surface, while only monomers exist in the bulk.…”

Origins of Clustering of Metalate–Extractant Complexes in Liquid–Liquid Extraction

“…There is a macroscale interface between the aqueous and the organic phases that the solutes must cross during LLE, which contains important molecular scale organizational features. [12][13][14][15][16][17][18] In addition to this macroscale interface, it is hypothesized that there are nanoscale interfaces within the organic phase in the form of reverse micellar aggregates. [19][20] The pertinence of this colloidal model of the organic phase is still debated [21][22] and is possibly system dependent.…”

Two-Step Adsorption of PtCl62– Complexes at a Charged Langmuir Monolayer: Role of Hydration and Ion Correlations

“…Extractant-ion complexes at the oil/water interface have been investigated with in situ scattering and spectroscopy experiments. [9][10][11] Also, experiments designed to model certain aspects of ion speciation [26][27][28] and extractant-ion interactions at the air/water [29][30] and solid/water 22 interfaces have been reported.…”

“…[33][34] The air/water interface is a good model system to study the aqueous side of the oil/water interface, since air has a similar dielectric constant ( = 1) to oil ( ~ 2), and allows us to tune the relevant parameters more easily. [26][27][28][29][30] Our aqueous subphase contains 0.5 M LiCl to simulate the process conditions, and the [PtCl6 2-]/[Cl -] ranges from 10 -5 to 10 -1 . Anomalous x-ray reflectivity (a-XR) and x-ray fluorescence near total reflection (XFNTR) measurements directly determine the amount of PtCl6 2complexes adsorbed in the diffuse and Stern layers, respectively.…”