A XANES and EXAFS study of hydration and ion pairing in ambient aqueous MnBr2 solutions

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure of Mn(II) in aqueous MnBr2 solutions at ambient conditions from very dilute to the near saturation limit. The Mn K-edge EXAFS spectra for 0.05 and 0.2 m solutions showed that there was no Br(-I) in the first shell, and that the Mn(II) was fully hydrated with six water molecules in an octahedral arrangement. In contrast, for 6 m solution, the coordination number of water was reduced to about 5, and an average of about one bromine atom was present in the first shell as a contact ion pair. The 1s --> 4p transition at 6545.5 eV confirmed the observation of Mn-Br contact ion pairs at high concentrations and the 1s --> 3d transition at 6539.5 eV showed that the first shell coordination symmetry remained octahedral even in the presence of Mn-Br ion pairs.