Migration '09 edition:12 location:Kennewick, Washington, USA date:September 20 - 25
In Belgium, the Boom Clay formation is studied as a reference host formation for the geological disposal of high-level and long-lived radioactive waste for more than 30 years. This formation mainly consists of mixed clay minerals (illite, interstratified illite-smectite), pyrite and immobile and dissolved natural organic matter. Since it provides good sorption capacities, very low permeability, and chemically reducing conditions due to the presence of pyrite (FeS2), the Boom clay formation itself is considered to be the main barrier preventing radionuclide migration from the geological repository. Within this concept for geological storage Se79 has been identified as one of the critical elements contributing to the final dose to man.
Since the mobility and consequently also the dose is highly related to the speciation of selenium, the knowledge on its behaviour in the far field of the Boom Clay formation is very important for the safety case of the future Belgian high level waste disposal program. Although the sorption and migration behaviour of Se in the Boom Clay system has been thoroughly studied, the speciation of Se in the Boom Clay system has never been identified spectroscopically. In all previous studies, the interpretation of the behaviour of Se in Boom Clay conditions has always been based on circumstantial evidence such as solubility measurements or comparison with the spectroscopically identified speciation of Se in model systems [1-3].
Selenite was equilibrated with Boom Clay for 1 month, before phase separation into 4 size fractions (clay-rich, pyrite-rich, organic matter-rich, supernatant) was achieved using centrifugation. For the first time the solid phase reaction products for the interaction of selenite with the Boom Clay solid phase have been spectroscopically identified. Based on the XANES/EXAFS analysis, selenite is reduced to Se0 on all solid phase size fractions of the Boom Clay, confirming the reduction of selenite to metallic selenium in the Boom Clay system. Despite the low equilibrium concentration in the supernatant solution (5x10-6M of Se) XANES measurements were possible and unexpectedly indicated the presence of a selenide phase (FeSe or an organic selenide), although Se0 was observed on the respective solid phase fractions.