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

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• 作者：Mrinal K. Bera ; Guangming Luo ; Mark L. Schlossman ; L. Soderholm ; Sungsik Lee ; Mark R. Antonio
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：July 16, 2015
• 年：2015
• 卷：119
• 期：28
• 页码：8734-8745
• 全文大小：464K
• ISSN：1520-5207

文摘

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鈥搗apor 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鈥揙H₂ bond distance of 2.33(1) 脜, 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鈥揾elium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the Er鈥揙H₂ hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6鈥? O atoms at 2.36(1) 脜 and a second one of 3 Cl atoms at 2.89(2) 脜, suggesting the formation of a neutral [(H₂O)_6鈥?ErCl₃] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to Er鈥揈r correlations (in possible combination with contributions from distant Er鈥揙 and Er鈥揅l interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) 脜, 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 electrolyte鈥搗apor 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.