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[MIS18-P06] Isotope fractionation analysis based on adsorption structure for group 6 elements on Fe/Mn-(oxyhydr)oxides
Keywords:isotope fractionation, group 6 element, Fe/Mn-(oxyhydr)oxide, adsorption structure
Adsorption structures of Cr, Mo, and W oxoanions were analyzed by XAFS for ferrihydrite, goethite, hematite, and δ-MnO2 (Kashiwabara et al., 2011 and 2013). Isotope fractionations were estimated using the equation based on mass-dependent isotopic differences in vibrational energies (Bigeleisen and Mayer, 1947) with DFT-calculated vibrational frequencies. The several oxoanion structures of hydrated and adsorbed on mineral surface models were examined for DFT calculations.
The results of XAFS analysis suggested that the adsorption structures for chromate and tungstate have Td and Oh symmetries, respectively, and molybdate has both Td and Oh symmetrical adsorption structures with various ratios depending on the minerals. From the DFT calculations, the large isotope fractionation was confirmed by the structural change of oxoanions from Td to Oh symmetry. The evaluated isotope fractionations for each mineral by DFT calculations combined with structural information by XAFS analysis agree with experimental results well. This fact indicates that the isotope fractionations of group 6 elements through the adsorption on minerals are caused by structural change of oxoanions.
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