4 research outputs found
Natural speciation of Mn, Ni, and Zn at the micrometer scale in a clayey paddy soil using X-ray fluorescence, absorption, and diffraction
EXAFS-Untersuchungen zur Rolle von Silicium bei der Sorption von umweltrelevanten Schwermetallen (Zn, As, Pb) in Speichermineralen (FeOOH,CSH)
Bei den Sorptions- und Desorptionsmechanismen an Mineraloberflächen ist oft nicht bekannt, durch welchen Mechanismus bestimmte Spurenelemente gebunden werden (Adsorption, Ionenaustausch, Ausfällung). Heterogene Reaktionen an der Fluid/Mineral-Grenzfläche sind im Allgemeinen sehr komplex. Ein Grund dafür ist, dass mehrere Mechanismen simultan ablaufen können. Da verschiedene Reaktionstypen räumlich getrennt ablaufen, d.h. im mikroskopischen Maßstab an unterschiedlichen Reaktionsplätzen, müssen die individuellen Reaktionsmechanismen auf atomarem Maßstab untersucht werden, um die Komplexität der Gesamtreaktion zu verstehen. Mit Hilfe der EXAFS Spektroskopie wurde die Bindungsform und -art der
eingelagerten Zink- und Blei-Atome in die synthetische Calciumsilikathydratphase (CSH) und des adsorptiv gebundenen Arsens an der Oberfläche der natürlichen Eisenhydroxidphase bestimmt. Hierbei spielt Silizium eine relevante Rolle für die Immobilisierung von Schwermetallen. In beiden Fällen, in denen das Silizium einerseits als Adsorbat an der Oberfläche der Eisenhydroxidphase und andererseits in die Struktur der Mischphase gebunden vorliegt, trägt es wesentlich zur Form und Art der Bindung der Schwermetalle in die neugebildete Phase bei.In order to make a statement about the immobilization of heavy metals it is necessary to know detailed information about sorptions- and desorption mechanisms. The mobilization and transport of trace elements are controlled by sorption at mineral surfaces. It is often not known what kind of mechanims will take place (adsorption, ionic exchange, precipitation) at the fluid/
minerals interface. During the immobilization processes a lot of mechanisms can take place simultaneously. The only possibility to get information about these complex immobilization processes is the examination at atomic scale.The spectroscopic results at the atomic scale obtained with help of the EXAFS spectroscopy shows the important role silicium plays in heavy metals immobilization processes. EXAFS Zn-, Ca-, Si K-edge and PbLIII-edge analyses reveal the site occupation of Zn and Pb in CSH, whereby the Zn(O,OH)4 and Pb(O,OH)4 tetrahedra are bound in layer rather than interlayer positions substituting for the silicate bridging tetrahedra and/or at terminal silicate chain sites. EXAFS Fe-, As- and Si K-edge analyses of natural hydrous ferric oxides reveal the bidentate binuclear surface complexations of the oxoanions arsenate and silicate and in addition the bidentate mononuclear surface complexation of the oxoanion arsenate
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Speciation and oxidation kinetics of arsenicin the thermal springs of Wiesbaden spa, Germany
Since 1886 arsenic has been known to be present as a trace component in the Wiesbaden thermal waters at concentrations of over 100 μg L-1. In this study for the first time molecular level speciation of arsenic was measured both in the water (by HG–AAS) and in Wellstone scale deposits (by XANES). Most of the arsenic in the anoxic NaCl-type waters is in the reduced arsenite form. Hydrous ferric oxide (HFO) precipitates in the scale deposits scavenge only the minor dissolved arsenate portion which is, however, accumulated up to 3% w/w. Isothermal precipitation experiments at in-situ temperatures showed a difference between the progress of both arsenic and iron oxidation and precipitation. This can be explained in terms of adsorption of the aqueous arsenite and heterogeneous oxidation on the HFO surface, but subsequently rapid release of the arsenate thereby formed back into the aqueous phase at enhanced temperature and increased pH. Such relatively rapid pseudo-homogeneous arsenite oxidation is too slow to efficiently retard the As(III) load already on the wellhead, but fast enough to prevent arsenic seepage into ground water aquifers
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Texture development and elastic stresses in magnesiowustite at high pressure
Cubic magnesiowustite has been deformed in a diamond anvil cell at room temperature. We present results for (Mg0.4Fe0.6)O, (Mg0.25Fe0.75)O, and (Mg0.1Fe0.9)O up to 37, 16, and 18 GPa, respectively. The diffraction images, obtained with the radial diffraction technique, are analyzed using both single peak intensities and a Rietveld method. For all samples, we observe a [100] fiber texture but the texture strength decreases with increasing iron content. This texture pattern is consistent with {110}-(1-10) slip. The images were also analyzed for stress, elastic strains, and elastic anisotropy. In general, the stress measured in magnesiowustite samples is lower than previously measured on MgO. The elastic anisotropy deduced from the X-ray measurements shows a broad agreement with models based on measurements with other techniques