Euradwaste '08 - EU Bookshop - Europa
Euradwaste '08 - EU Bookshop - Europa Euradwaste '08 - EU Bookshop - Europa
Colloid Concentration / μg/l 10000 1000 100 10 1 0.1 0.01 TGT/1 TGT/3 MZD Grimsel Volvic (PE) TGT/2 Ruprechtov GTS Bad Liebenzeller (glass) Volvic (glass) Gerolsteiner (PE) 1 10 100 1000 Ionic Strength / mmol/l 346 BDS LEU ZUR Äspö NaCl solution Data from C. Degueldre 1996 Forsmark Figure 2: Comparison of colloid concentration in different types of natural groundwater, mineral water and synthetic NaCl-solution versus ionic strength. For details see [3] 3.2 Characterisation of immobile uranium phases The application of macroscopic and microscopic methods provided detailed insight into the U enrichment processes at the Ruprechtov site. Confocal μ-XRF and μ-XANES notably contributed to the identification of uranium immobilisation processes. In good agreement with results from other spectroscopic methods like ASEM and electron-microprobe μ-XANES identified U as U(IV) [6]. As demonstrated in Figure 3 (left), the shape and intensities show the average valence state of the sampled volume to be U(IV). All three curves do not show the multiple scattering feature 10-15 eV above the white line (WL) characteristic for U(VI) nor do they show a significant decrease in the WL intensity, which would be expected for U(VI) as be seen in the schoepite spectrum. norm. Absorption 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 μm 40 μm 90 μm schoepite 17.12 17.14 17.16 17.18 17.20 17.22 17.24 Energy [keV] Figure 3: Results from μ-XANES (left) and μ-XRD (right) of a sample from borehole NA4 [6,7].
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. Similarity Similarity 0 0 -100 -200 -300 NA14: 725 mg/kg U Na As P U K Al Fe S Na As P U K 1 2 3 4 5 6 7 8 9 Al Fe S 347 Similarity 0 0 -10 -20 -30 -40 As NA15: 50 mg/kg U As U P S K Na Fe Al U P S K 1 2 3 4 5 6 7 8 9 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 Similarity Na Fe Al
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Colloid Concentration / μg/l<br />
10000<br />
1000<br />
100<br />
10<br />
1<br />
0.1<br />
0.01<br />
TGT/1<br />
TGT/3<br />
MZD<br />
Grimsel<br />
Volvic<br />
(PE)<br />
TGT/2<br />
Ruprechtov<br />
GTS<br />
Bad Liebenzeller<br />
(glass)<br />
Volvic<br />
(glass)<br />
Gerolsteiner<br />
(PE)<br />
1 10 100 1000<br />
Ionic Strength / mmol/l<br />
346<br />
BDS<br />
L<strong>EU</strong><br />
ZUR<br />
Äspö<br />
NaCl solution<br />
Data from C. Degueldre 1996<br />
Forsmark<br />
Figure 2: Comparison of colloid concentration in different types of natural groundwater, mineral<br />
water and synthetic NaCl-solution versus ionic strength. For details see [3]<br />
3.2 Characterisation of immobile uranium phases<br />
The application of macroscopic and microscopic methods provided detailed insight into the U enrichment<br />
processes at the Ruprechtov site. Confocal μ-XRF and μ-XANES notably contributed to<br />
the identification of uranium immobilisation processes. In good agreement with results from other<br />
spectroscopic methods like ASEM and electron-microprobe μ-XANES identified U as U(IV) [6].<br />
As demonstrated in Figure 3 (left), the shape and intensities show the average valence state of the<br />
sampled volume to be U(IV). All three curves do not show the multiple scattering feature 10-15 eV<br />
above the white line (WL) characteristic for U(VI) nor do they show a significant decrease in the<br />
WL intensity, which would be expected for U(VI) as be seen in the schoepite spectrum.<br />
norm. Absorption<br />
3.5<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
1.0<br />
0.5<br />
0.0<br />
0 μm<br />
40 μm<br />
90 μm<br />
schoepite<br />
17.12 17.14 17.16 17.18 17.20 17.22 17.24<br />
Energy [keV]<br />
Figure 3: Results from μ-XANES (left) and μ-XRD (right) of a sample from borehole NA4 [6,7].
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