Calculating the partitioning of the isotopes of Mo between oxidic and sulfidic species in aqueous solution

The fractionation of the isotopes of Mo between different geological environments has recently been determined to high accuracy using mass spectrometry (Barling et al., 2001). Fractionation is observed between Mo in seawater, where it exists primarily in the form of the Mo(VI) anion molybdate, Moo 4 2, and in oxic sediments, where the Mo is isotopically lighter than in sea water by similar to 1.8 parts per thousand (in terms of the Mo-97, Mo-95 isotope pair). EXAFS evidence exists for a five- or six-coordinate Mo environment in the Fe,Mn oxyhydroxides of ferromanganese nodules (Kuhn et al., 2003). In sediment regimes which are anoxic and sulfidic (sometimes referred to as euxinic), 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 (Erickson and Helz, 2000) and then to other sulfides. If the conversion is stoichiometrically complete, mass balance requires the same isotopic distribution in reactant and product. This is a result of the very high equilibrium constant for this reaction. Thus, to understand isotopic fractionation processes both the equilibrium constants for the 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 Mo-92 and Mo-100 between MoO4-2, MoO3(OH)(-), MoO2(OH)(2), MoO3, MoO3(OH2)(3), MOS4-2 and a number of other oxidic and sulfidic complexes of Mo. The 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 suggest highly distorted six-coordinate MoO3(OH2)(3) or three-coordinate MoO3 as better representations of the species present. We have also calculated visible-UV absorption spectra to support our interpretation of the speciation. MoO3 is calculated to be isotopically lighter than MoO4-2 by 1.6 parts per thousand, consistent with the experimentally observed difference between sea water and oxic sediments. 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. Copyright (c) 2005 Elsevier Ltd.