PNNL

Abstract

X-ray absorption fine structure (XAFS) spectroscopy was used to measure the first-shell structure about Ca2+ in high-temperature aqueous solution. XAFS spectra were acquired at the Ca K-edge at temperatures up to 400°C and pressures up to 350 bar. For the system at 400°C, both Ca (4038.5 eV) and Cl (2822.4 eV) K-edge data were acquired and a global model was used to fit the two independent sets of XAFS data. Measurements were made at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. Above 250°C, a significant number of Ca2+-Cl- direct contact ion pairs form in agreement with existing thermodynamic data for this system. For a 1 m CaCl2 solution at 400°C, the mean coordination structure about Ca2+ contains 3.2 ± 0.6 water molecules at an average Ca-O distance of 2.356 ±0.026 Å and 1.8 ± 0.7 Cl- at a Ca-Cl distance of 2.677 ± 0.007 Å. An evaluation of the Ca and Cl pre-edge and near-edge (XANES) spectrum provided further confirmation of the change in the Ca2+ first-shell structure and symmetry. Overall these measurements provide a structural basis for understanding solvation of Ca2+ in hydrothermal systems. These results also provide important new insights into the structural aspects of Ca2+ ion pairing that are the basis of many biological processes under ambient conditions. Work was supported by the Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy (DOE). PNC/XOR facilities at the Advanced Photon Source, and research at these facilities, are supported by the US DOE Office of Science Grant No. DEFG03-97ER45628, the University of Washington, a major facilities access grant from NSERC, Simon Fraser University and the Advanced Photon Source. Use of the Advanced Photon Source is also supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. Pacific Northwest National Laboratory is operated for DOE by Battelle.