4:30 PM - 4:45 PM
[BGM02-09] Surface Complexation Modeling of Molybdenum Adsorption on Oxides to predict the isotope fractionation
Keywords:molybdate, adsorption, surface complexation modeling
Solid speciation of molybdenum (Mo) is strongly affected by the redox conditions of the ocean. In anoxic conditions, Mo is rapidly removed from the ocean by forming molybdenum sulfides. In oxidizing conditions such as the present, Mo is very stable, with a residence time of about 0.8 Ma (Siebert et al., 2003). It is known that the concentration of trace elements such as Mo are controlled by adsorption process with minerals. The concentration of Mo in the ocean is controlled by adsorption of manganese oxides (Kashiwabara et al., 2011). Mo has seven stable isotopes and is characterized by significant differences in isotope fractionation depending on the mineral species. It is known that the isotope fractionation is small for iron oxides and large for manganese oxides (Barling & Anbar, 2004; Goldberg et al., 2009).
It is desired that molybdenum isotope fractionation can be used as an alternative indicator for manganese oxides because the presence of manganese oxides indicates the occurrence of oxygenic photosynthesis (Czaja et al., 2012). The large isotopic fractionation is caused by the difference in the fraction of adsorbed forms among mineral species (Kashiwabara et al., 2011). Adsorption form can be divided into two main types. One is the inner-sphere complex, which forms direct chemical bonds. The other is an outer-sphere complex, which does not form direct chemical bonds with the surface but forms electrostatic bonds. As the percentage of inner-sphere complexes increases, the isotope fractionation tends to increase. However, the effect of water quality conditions on solid speciation of adsorbed Mo on oxides has not been investigated. Therefore, even iron oxides, which are known to adsorb light isotopes, possibly adsorb heavy isotopes depending on water conditions. It has been needed to understand how isotope fractionation changes with water conditions, but little research has been done. Surface complexation modeling is a theoretical and quantitative method to deal with elemental adsorption. By using appropriate models, it is possible to estimate not only the adsorption behavior of minerals but also the chemical forms (surface speciation) of adsorbate. This method has the potential to treat isotope fractionation as a function of water conditions.
In this study, the speciation change of Mo adsorbed on the mineral surface with water quality was investigated spectroscopically. Surface complexation modeling was developed to obtain surface speciation of molybdenum in consistent with spectroscopic results. Adsorption experiments of Mo on δMnO2 and ferrihydrite were carried out to obtain the adsorption data. These adsorption data was analyzed by Extended Triple Layer model of surface complexation modeling (Fukushi et al., 2021) and the ratio of outer-sphere complexes to inner-sphere complexes was calculated. Isotope measurements were performed to investigate the effect of water quality changes on isotope fractionation. The results are in good agreement with those obtained by XAFS analysis by Kashiwabara et al., 2011. The magnitude of the isotope fractionation changed with changes in water quality. The trend of change was consistent with predictions from surface complex modeling. These results show the possibility of predicting the isotope fractionation by the surface complexation model.
It is desired that molybdenum isotope fractionation can be used as an alternative indicator for manganese oxides because the presence of manganese oxides indicates the occurrence of oxygenic photosynthesis (Czaja et al., 2012). The large isotopic fractionation is caused by the difference in the fraction of adsorbed forms among mineral species (Kashiwabara et al., 2011). Adsorption form can be divided into two main types. One is the inner-sphere complex, which forms direct chemical bonds. The other is an outer-sphere complex, which does not form direct chemical bonds with the surface but forms electrostatic bonds. As the percentage of inner-sphere complexes increases, the isotope fractionation tends to increase. However, the effect of water quality conditions on solid speciation of adsorbed Mo on oxides has not been investigated. Therefore, even iron oxides, which are known to adsorb light isotopes, possibly adsorb heavy isotopes depending on water conditions. It has been needed to understand how isotope fractionation changes with water conditions, but little research has been done. Surface complexation modeling is a theoretical and quantitative method to deal with elemental adsorption. By using appropriate models, it is possible to estimate not only the adsorption behavior of minerals but also the chemical forms (surface speciation) of adsorbate. This method has the potential to treat isotope fractionation as a function of water conditions.
In this study, the speciation change of Mo adsorbed on the mineral surface with water quality was investigated spectroscopically. Surface complexation modeling was developed to obtain surface speciation of molybdenum in consistent with spectroscopic results. Adsorption experiments of Mo on δMnO2 and ferrihydrite were carried out to obtain the adsorption data. These adsorption data was analyzed by Extended Triple Layer model of surface complexation modeling (Fukushi et al., 2021) and the ratio of outer-sphere complexes to inner-sphere complexes was calculated. Isotope measurements were performed to investigate the effect of water quality changes on isotope fractionation. The results are in good agreement with those obtained by XAFS analysis by Kashiwabara et al., 2011. The magnitude of the isotope fractionation changed with changes in water quality. The trend of change was consistent with predictions from surface complex modeling. These results show the possibility of predicting the isotope fractionation by the surface complexation model.