PNNL

Abstract

X-ray absorption fine structure (XAFS) spectroscopy was used to probe the first-shell coordination structure of Mn2+ in aqueous MnBr2 solutions at ambient conditions. Samples included three aqueous solutions with MnBr2 concentrations at 0.05 m, 0.2 m, and 6 m (molality, mol/kg water), and four solid reference compounds, MnBr2·4H2O, anhydrous MnBr2, Mn(CH3COO)2·4H2O, and MnO. The Mn Kedge EXAFS spectra were collected at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. The Mn K-edge EXAFS spectra for 0.05 m and 0.2 m solutions showed that there was no Br- in the first shell, and that the Mn2+ was fully hydrated with 6 water molecules. In contrast, for 6 m solution, the coordination number of water was reduced to about 5, and about 1 bromine atom was present in the first shell as a contact ion pair. The nearest-neighbor distance for the oxygen of the water (Mn2+-H2O) was about 2.167 Å, and for Mn2+-Br-, 2.626 Å. From the pre-edge spectra, the 1s?4p transition at 6545.5 eV confirmed the observation of Mn2+-Br- contact ion pairs at high concentrations, and the 1s?3d transition at 6539.5 eV was consistent with a predominantly octahedral structure for all MnBr2 concentrations.