Iron and tin isotope equilibrium fractionation factors from Mössbauer and synchrotron radiation data

Isotopic fractionation is observed between Mo in seawater, where it exists primarily in the form of the Mo(VI) anion molybdate, MoO4 -2 , and in oxic sediments, where the Mo is isotopically lighter than in sea water and evidence exists for a five- or six-coordinate Mo environment in Fe,Mn oxyhydroxides. In anoxic sediments, where the Mo(VI) is expected to exist as a sulfide, no fractionation is observed compared to seawater. This is presumably because of the stoichiometric conversion of the Mo from MoO4 -2 to MoS4 -2 and then to other sulfides. This results from the very high equilibrium constant for the sulfidation reaction. Thus, to understand isotopic fractionation both the equilibrium constants for isotopic fractionation reactions and the equilbrium constants for transformation of one chemical compound to another must be considered. We here present quantum mechanical calculations of the isotopic fractionation equilibrium constants for the isotopes 92 Mo and 100 Mo between MoO 4 -2 , MoO 3(OH) - , MoO 3(OH2)3, MoS4 -2 and a number of other oxidic and sulfidic complexes of Mo (and scale the results to give Mo 97, 95 fractionations). These fractionation equilibrium constants are calculated directly from the computed vibrational, rotational and translational contributions to the free energy in the gas-phase using quantum methods. Calculated vibrational frequencies and ratios of frequencies for different isotopomers are first obtained using a number of different quantum methods and compared with available experimental data to establish the most reliable methodology. We have also calculated free energy changes in aqueous solution for a range of reactions of MoO4 -2 and MoO2(OH)2 with H2O and H2S. We present evidence for the instability of the monomeric octahedral species Mo(OH)6 commonly assumed to exist in acid solution and propose highly distorted six-coordinate MoO3(OH2)3 or three-coordinate MoO3 as better representations of the species present. We explain the isotopic lightness of oxic sediments as arising from an intermediate step in which a three coordinate MoO3 species is formed in aqueous solution, and subsequently attaches to the surface of a Fe,Mn oxyhydroxide mineral.