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| Titel |
Enrichment mechanisms of tellurium in ferromanganese crusts |
| VerfasserIn |
A. Sakaguchi, T. Sugiyama, A. Usui, Y. Takahashi |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067961
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| Zusammenfassung |
| Marine ferromanganese crusts (FMCs) consist of iron (Fe) hydroxides and manganese (Mn)
oxides with various minor and trace elements. Especially for tellurium (Te), which is
recognized as one of the rare metals, it has been reported that this element is concentrated
about 105 times in FMCs compared with earth’s crust, and the host phase might be Fe
(oxy)hydroxide (Hein et al., 2003). Actually, in our previous study, the high concentration of
Te in very surface layers of FMCs was found from the top to halfway down of a seamount in
the Pacific Ocean. However, the concentration of Te in surface layers through the
seamount showed good correlation with that of Mn instead of Fe. In this study, we
attempted to clarify the enrichment mechanism of Te in FMCs with some methods
including X-ray absorption fine structure (XAFS) technique for synthesised /natural
samples.
Seventeen FMC samples were collected from the Takuyo-Daigo seamount, from 950 m
(summit) to 3000 m in water depth, with hyper-dolphin (remotely operated vehicle) equipped
with live video camera and manipulators. The growth rates of all FMC samples were
estimated to be about 3 mm/Ma. Very surface layer (less than 1 mm) of all FMC was
analyzed with XRD and XAFS to confirm the mineral composition and speciation of Te.
Furthermore, to serve as an aid to clarify the adsorption mechanism of Te on FMCs,
distribution coefficients (Kd) and oxidation states were determined through the
adsorption experiments of Te(IV) and Te(VI) on ferrihydrite and δ-MnO2. In all the
experiments, pH and ionic strength were adjusted to pH 7.5 and 0.7 M, respectively.
The oxidation state of Te in water phase was determined with HPLC-ICP-MS. As
for the analysis of oxidation and adsorption states on the solid phase, XAFS was
employed.
The major mineral composition of Fe and Mn had no significant variation through the
water depth of Takuyo-Daigo seamount. The oxidation state of Te in all samples showed
hexavalent, and there was no significant difference of adsorption state independent of the DO,
salinity and temperature in water. It has been reported that Te exists as tetravalent and
hexavalent in sea water of the Pacific Ocean (Nozaki, 1996). Thus, it can be said
that the Te in sea water is oxidised and incorporated into FMCs. As a result of the
adsorption experiments in laboratory, the Kd of Te on ferrihydrite was larger than that of
δ-MnO2, and Te(IV) was adsorbed to a larger degree than Te(VI) on both minerals. The
adsorption experiments of Te(IV) on δ-MnO2 showed that the solid phase has only
hexavalent Te, although the water phase has both tetra and hexavalent species of
Te. Te(IV) on ferrihydrite was not oxidized to Te(VI). From these results, it can
be suggested that Te(IV) was oxidized by δ-MnO2 and would be adsorbed onto
ferrihydrite. Actually, the results of double-cell adsorption experiments support this
hypothesis. The detail of our results and discussion will be given in the presentation. |
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