Speciation of naturally-accumulated uranium in an organic-rich soil of an alpine region (Switzerland)

Details

Serval ID
serval:BIB_BF6F0D0E2E5E
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Speciation of naturally-accumulated uranium in an organic-rich soil of an alpine region (Switzerland)
Journal
Geochimica et Cosmochimica Acta
Author(s)
Regenspurg S., Margot-Roquier C., Harfouche M., Froidevaux P., Steinmann P., Junier P., Bernier-Latmani R.
ISSN
0016-7037
Publication state
Published
Issued date
2010
Peer-reviewed
Oui
Volume
74
Number
7
Pages
2082-2098
Language
english
Abstract
Very high concentrations of uranium (up to 4000 ppm) were found in a natural soil in the Dischma valley, an alpine region in the Grisons canton in Switzerland. The goal of this study was to examine the redox state and the nature of uranium binding in the soil matrix in order to understand the accumulation mechanism. Pore water profiles collected from Dischma soil revealed the establishment of anoxic conditions with increasing soil depth. A combination of chemical extraction methods and spectroscopy was applied to characterize the redox state and binding environment of uranium in the soil. Bicarbonate extraction under anoxic conditions released most of the uranium indicating that uranium occurs predominantly in the hexavalent form. Surprisingly, the uranium redox state did not vary greatly as a function of depth. X-ray absorption near edge spectroscopy (XANES), confirmed that uranium was present as a mixture of U(VI) and U(IV) with U(VI) dominating. Sequential extractions of soil samples showed that the dissolution of solid organic matter resulted in the simultaneous release of the majority of the soil uranium content (>95%). Extended X-ray absorption fine structure (EXAFS) spectroscopy also revealed that soil-associated uranium in the soil matrix was mainly octahedrally coordinated, with an average of 1.7 axial (at about 1.76 Å) and 4.6 to 5.3 equatorial oxygen atoms (at about 2.36 Å) indicating the dominance of a uranyl-like (UO22+) structure presumably mixed with some U(IV). An additional EXAFS signal (at about 3.2 Å) identified in some spectra suggested that uranium was also bound (via an oxygen atom) to a light element such as carbon, phosphorus or silicon. Gamma spectrometric measurements of soil profiles failed to identify uranium long-life daughter products in the soil which is an indication that uranium originates elsewhere and was transported to its current location by water. Finally, it was found that the release of uranium from the soil was significantly promoted at very low pH values (pH 2) and increased with increasing pH values (between pH 5 and 9).
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Create date
11/02/2010 14:56
Last modification date
20/08/2019 16:33
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