Euradwaste '08 - EU Bookshop - Europa

By μ-XANES it was also shown that As exists in two oxidation states, As(0) and As(V). The analyses
of a number of tomographic cross-sections of elemental distributions recorded over different
sample areas show a strong positive correlation between U and As(V). By further development of
the method, using new planar compound refractive lens (CRL) array at the Fluoro-Topo-Beamline
at the synchroton facility ANKA of the Forschungszentrum Karlsruhe, a higher spatial resolution
(focus beam spot size of 2 x 5 μm 2 (V x H)) was achieved. The high resolution made it possible for
the first time to discern an As-rich boundary layer surrounding Fe(II)-nodules, see Figure 3, right
[7]. This suggests that an arsenopyrite mineral coating of framboidal pyrite nodules is present in the
sediment. Uranium occurs in direct vicinity of the As-rich layers. In conclusion of these results a
driving mechanisms for uranium-enrichment by secondary uranium(IV) minerals in the sediment
was suggested. Mobile, groundwater-dissolved U(VI) was reduced on the arsenopyrite layers to
less-soluble U(IV), which formed U(IV) mineral phases. As(0) was oxidised to As(V). Uranium,
therefore, is associated with As(V).
The results from microscopic methods are supported by cluster analysis of sequential extraction results.
They also indicate that U occurs in the tetravalent state, since major part of uranium is extracted
in the respective steps for U(IV) forms and the residual fraction [4]. By cluster analyses,
performed to identify possible correlations between elements, a strong correlation of U with As and
P was found (see Figure 4), supporting the mechanism postulated above and the existence of uranium
phosphate mineral ningyoite identified by SEM-EDX.
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NA14: 725 mg/kg U
Na As P U K Al Fe S
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NA15: 50 mg/kg U
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Figure 4: Cluster analyses for extended SE results of samples from the boreholes NA14 and NA15
In order to separate U(IV) and U(VI), a wet chemical method [9] was applied for the first time to
Ruprechtov samples. A major result is that uranium in all samples consists of both U(IV) and U(VI)
[4]. Results from all analyses are summarised in Table 1. The extraction did not dissolve all uranium.
The content of uranium in this insoluble phase is denoted as U(res). In all phases the
234 U/ 238 U activity ratio was determined, which is denoted as AR. The AR differs significantly in the
U(IV) and U(VI) phases, with ratios 1 in the U(VI) phase in
nearly all samples. The AR of the U(res) phase is, with exception of NA12, similar to that observed
in the U(IV) phase. Different (higher) AR in the NA12 residue may indicate involvement of different
U compounds in the sample material, i.e. U(IV) and insoluble U(res) represent different compounds.
Taking into account the higher stability of U(IV) phases we assume that insoluble uranium
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