Effects of concentration and freeze-thaw on the first hydration shell structure of Zn²⁺ ions

Abstract

To investigate the effects of salt concentration and freeze-thaw (FT) on the first hydration shell of Zn²⁺ ions in Zn(NO₃)₂ aqueous solutions, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to examine Zn K-edge EXAFS spectra of Zn(NO₃)₂ aqueous solutions with various concentrations before and after FT treatment. The influences of salt concentration and freeze-thaw on the structural parameters of the first hydration shell of Zn²⁺ ions, including hydration number, Zn-O distance and thermal disorder, were analyzed. The results show that Zn²⁺ ions have 3.2–6.8 nearest oxygen neighbors with the Zn-O distance being 0.202–0.207 nm. In highly concentrated solutions, Zn²⁺ ions are hydrated with four water molecules in a tetrahedral form. The dilution of Zn(NO₃)₂ aqueous solutions increases the number of water molecules in the first hydration shell of Zn²⁺ ions to six with their octahedral arrangement. Both the hydration number in the first hydration shell of Zn²⁺ ions and the degree of thermal disorder increase when the FT treatment is operated in Zn(NO₃)₂ aqueous solutions.

Keywords

EXAFS / Zn(NO₃)₂ aqueous solution / first hydration shell / hydration number / concentration / freeze-thaw

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