Effect of Manganese Oxide Mineralogy and Surface Mo Coverage on Mo Isotope Fractionation During the Adsorption Process

The large molybdenum (Mo) isotope fractionation from seawater is caused by the adsorption of Mo on manganese oxides. However, the effects of the manganese oxide mineralogy (crystal structure) and surface Mo coverage on Mo isotope fractionation have not been investigated. In this study, the isotope fractionation of Mo by adsorption on synthetic todorokite, birnessite, and δMnO2 was investigated under a wide range of surface Mo coverages. The Mo isotope fractionation changed from Δ98/95Mo = 2.18 ± 0.05‰ to 2.61 ± 0.06‰ for todorokite; from 1.25 ± 0.05‰ to 2.10 ± 0.05‰ for birnessite; and from 2.19 ± 0.07‰ to 2.73 ± 0.08‰ for δMnO2. The Mo isotope fractionations of the three manganese oxides were negatively correlated with surface coverage normalized to the specific surface area. The independence of the obtained correlation of the manganese oxide species indicates that the Mo isotope fractionation depends on the surface coverage but not on the mineralogy of the manganese oxides. The experimentally observed Mo isotope fractionation (<2.7‰) in manganese oxides generally underestimates the isotope fractionation in natural ferromanganese oxides (~3‰). According to the dependency of the Mo isotope fractionation on the surface coverage, the underestimation relative to previous experimental studies can be attributed to the lower Mo surface coverage of natural ferromanganese oxides.