Tungsten species in natural ferromanganese oxides related to its different behavior from molybdenum in oxic ocean

• Published in: Geochimica et cosmochimica acta 2013-04, Vol.106, p.364-378
• Main Authors: Kashiwabara, Teruhiko, Takahashi, Yoshio, Marcus, Matthew A., Uruga, Tomoya, Tanida, Hajime, Terada, Yasuko, Usui, Akira
• Format: Article
• Language: English
• Subjects: adsorption; chemical interactions; ferrihydrite; goethite; hematite; iron; isotopes; manganese; manganese oxides; molybdenum; seawater; tungsten; wavelengths
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.12.026

The tungsten (W) species in marine ferromanganese oxides were investigated by wavelength dispersive XAFS method. We found that the W species are in distorted Oh symmetry in natural ferromanganese oxides. The host phase of W is suggested to be Mn oxides by μ-XRF mapping. We also found that the W species forms inner-sphere complexes in hexavalent state and distorted Oh symmetry on synthetic ferrihydrite, goethite, hematite, and δ-MnO2. The molecular-scale information of W indicates that the negatively-charged WO42− ion mainly adsorbs on the negatively-charged Mn oxides phase in natural ferromanganese oxides due to the strong chemical interaction. In addition, preferential adsorption of lighter W isotopes is expected based on the molecular symmetry of the adsorbed species, implying the potential significance of the W isotope systems similar to Mo. Adsorption experiments of W on synthetic ferrihydrite and δ-MnO2 were also conducted. At higher equilibrium concentration, W exhibits behaviors similar to Mo on δ-MnO2 due to their formations of inner-sphere complexes. On the other hand, W shows a much larger adsorption on ferrihydrite than Mo. This is due to the formation of the inner- and outer-sphere complexes for W and Mo on ferrihydrite, respectively. Considering the lower equilibrium concentration such as in oxic seawater, however, the enrichment of W into natural ferromanganese oxides larger than Mo may be controlled by the different stabilities of their inner-sphere complexes on the Mn oxides. These two factors, (i) the stability of inner-sphere complexes on the Mn oxides and (ii) the mode of attachment on ferrihydrite (inner- or outer-sphere complex), are the causes of the different behaviors of W and Mo on the surface of the Fe/Mn (oxyhydr)oxides.