Abstract
A large mass-dependent isotope fractionation of molybdenum (Mo) during its adsorption on marine ferromanganese oxides, which are mixtures of iron (Fe) (oxyhydr)oxide and manganese (Mn) oxide, is most likely caused by a geometrical change from tetrahedral (Td) to distorted octahedral (Oh) during adsorption based on X-ray absorption fine structure (XAFS) spectroscopy. However, the understanding of such fractionation can be enhanced further. In this study, density functional theory (DFT) calculations were conducted to examine models of distorted Oh species on Mn oxide which are related to our proposed adsorption mechanism of molybdate. Results showed that even monomeric adsorbed species can reasonably explain highly distorted geometries observed by XAFS spectroscopy with large isotope fractionation. This finding indicates that the formation of polymolybdate, such as heptamolybdate, under circumneutral experimental condition (pH ∼ 8) at low Mo concentration in seawater is not required to explain the experimen...
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