Euradwaste '08 - EU Bookshop - Europa

sediment as U(IV), being associated with As(V) as a precipitate on arsenopyrite layers, which
formed on pyrite nodules as ningyoite (U phosphate) and uraninite (U oxide) minerals [5- 8]. A
typical μ-XRF elemental distribution map is shown in Fig 2. In order to separate U(IV) and U(VI),
a wet chemical method was applied for the first time to Ruprechtov samples as well, confirming
that the major fraction of immobile uranium occurs in the tetravalent state [9].
In the second step these single task results were put together with results from characterisation of
organic matter and isotope analyses in the system. This gave insight into the role of microbial activity
and organic matter in the uranium immobilization process. It could be shown that sedimentary
organic matter (SOM) contributed and still contributes to maintain reducing conditions in the
clay/lignite layers. [10, 11, 12]. The key processes involved in uranium immobilisation can be
summarised as follows:
- Oxidation of sedimentary organic carbon (SOC) and reduction of oxidising agents (SO4 2- )
- SOC is partly oxidised to inorganic carbon, dissolved in groundwater (DIC) and partly released
as dissolved organic matter (DOC)
- Increase in 34 S in dissolved sulphate in the clay/lignite groundwater by microbial sulphate
reduction [14]
- Formation of framboidal FeS2 by reduction of SO4 2- (see Fig. 2)
- Production of PO4 by degradation of organic matter
- U(VI) reduction on FeAsS sites and oxidation of As(0) to As(V)
- Precipitation of U(IV) phosphate / oxide mineral phases
-
Figure 2: Fromboidal shape of pyrite, typical for microbially driven sulphate reduction
A major result is that uranium in all samples consists of both U(IV) and U(VI), however predominantly
of U(IV) [5, 6, 9]. Activity ratios (AR) below one for U(res) and U(IV) in nearly samples are
a strong indicator for their long-term stability. AR values significantly below unity are caused by
the preferential release of 234 U, which is facilitated by �-recoil process and subsequent 234 U oxidation.
In order to attain low AR values as low as 0.2 in the U(IV) phase, it must have been stable for
a sufficiently long time, i.e. no significant release of bulk uranium has occurred during the last million
years. This is expected under the strongly reducing conditions (-160 mV to –280 mV) in the
clay lignite waters and is in good agreement with the hypothesis that the major uranium input into
the clay/lignite horizon occurred during Tertiary, more than 10 My ago [13].
Finally, the overall picture puzzle shaped up its outlines: After re-evaluation of new hydrological,
geochemical and environmental isotope data from groundwater those were combined with previous
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