Adsorption Mechanism of Molybdenum(VI) on Manganese Oxides Causing a Large Isotope Fractionation

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 experimental results. Two elemental properties of Mo­(VI), namely d⁰ electron configuration and appropriate ionic radius, led to the formation of its distorted Oh adsorbed species based on the observation of highly distorted adsorbed species with DFT calculation results. This finding indicates that molybdate adsorption on Mn oxide occurs via addition reaction, which can explain the following natural phenomena: (i) Mn oxide is the host phase of molybdate in marine ferromanganese oxides, although most negatively charged ions are preferentially adsorbed on Fe (oxyhydr)­oxides. (ii) The large isotope fractionation of Mo is observed during its adsorption on natural ferromanganese oxides through the formation of distorted Oh adsorbed species on Mn oxide.