Determination of the Second Hydration Shell of Cr3+ and Zn2+ in Aqueous Solutions by Extended X-ray Absorption Fine Structure

Abstract

The geometric structures of CflNOsk and ZnjNChk aqueous solutions in a wide range of concentrations 2.7-0.005 m have been determined by means of the extended X-ray absorption fine structure (EXAFS) technique. X-ray absorption spectra at the K-edges of Zn and Cr have been measured in the transmission and fluorescence modes at the Synchrotron Radiation Source (U.K.). The analysis of all the experimental data (13 solutions) is compatible with a unique structural model, which basically agrees with the concentric shell model of Frank and Evans for ionic hydration. Therefore, it is shown that the EXFAS technique allows the determination of a second hydration shell in a wide range of concentrations, from almost saturated to highly dilute solutions. -O distances (Cr-= 2.00 Á, Zn-= 2.05 Á) and the coordination number (6 for both cations) for the first hydration shell are not affected by concentration. A second hydration shell is detected in both cases, although for chromium solutions, this contribution to the EXAFS spectra is more important than for zinc. The distance is around 4.0 Á, but in Cr3+ solutions a slight increase in the Cr-On distance is observed with dilution (4.02 Á for 0.01 m, and 3.95 Á for 2.6 m). The Zn-On distance shows no systematic trend, the average distance being 4.1 Á. The coordination number for this shell is 13.3 ± 1 for Cr3+ solutions and 11.6 ± 1.6 for Zn2+ solutions. The most concentrated Zn2+ solution (2.7 m) presents a singular behavior, since its coordination number decreases to 6.8 ± 1.5. The data analysis procedure is thoroughly described, and the possibilities of an alternative hypothesis for the second contribution to the EXAFS spectrum, such as multiple scattering effects, are carefully discussed.

Determination of dissociation energy for ligand exchange reaction from EXAFS

“…The peak intensity decreases as temperature increases due to the thermal motion. We can find the clear peaks at about 3.75 Å for Cr 3+ (a) and at 4 Å for Ni 2+ (b) corresponding to the second hydration shell and the multiple scattering effect, which was pointed out by Munoz-Paez and coworkers (Munoz-Paez et al, 1995, Sakane et al, 1998. The differences of the distance and intensity between them correspond to that of ionic radius and the amplitude of the multiple scattering.…”

Reduction and Sorption of Chromium by Fe(II)-Bearing Phyllosilicates: Chemical Treatments and X-Ray Absorption Spectroscopy (XAS) Studies

“…For example, Cr in tetrahedral coordination exhibits a single intense pre-edge peak and a small shoulder at -10 eV above the pre-edge (Bianconi, 1988;Bianconi et al, 1992;Bajt et al, 1993). These typical pre-edge features are found in the reference SrfrO 4 and Na2CrO4 compounds (Figure 8), where Cr was shown by structure determination to be four-coordinated (Arcon et al, 1998;Munoz-Paez et al, 1995). The octahedral site is recognized by the lack of a pre-edge peak (Cr203 and uvarovite spectra, Figure 8) and by two small resonances in the pre-edge region (Bajt et al, 1993;Arcon et aL, 1998).…”

Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

“…45,54,[56][57][58][59] However, we also compared the x-ray scattering calculated from the simulated structures directly to the observed EXAFS spectra. 36 This type of analysis has recently been used to compare AIMD simulations of UO 2 2+ + 64H 2 O ͑UO 2 2+ + 122H 2 O͒ to EXAFS measurements with remarkable success.…”