Interaction of Se with Boom Clay: XAS solid phase speciation

INTERACTION OF SE WITH BOOM CLAY: XAS SOLID PHASE SPECIATION ERIC BREYNAERT1*, DIRK DOM1, JACQUELINE VANCLUYSEN1, ANDREAS SCHEINOST2, ANDRÉ MAES1 1Center for surface Chemistry and Catalysis (M2S, KULeuven), B-3001Leuven,, Belgium 2Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany *eric.breynaert@biw.kuleuven.be The Boom Clay formation mainly consists of mixed clay minerals (illite, interstratified illite-smectite), pyrite and immobile and dissolved natural organic matter. Within the geological repository concept, the Boom clay formation itself is considered the primary barrier against radionuclide migration. It provides good sorption capacities, very low permeability, and chemically reducing conditions due to the presence of pyrite (FeS2) Since the mobility and consequently also the dose contribution 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. 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]. Since pyrite (FeS2) is one of the main minerals governing the redox conditions of the Boom Clay solid phases, it is expected that selenite will be reduced to Se0 in the Boom Clay system, thereby effectively limiting the solution concentration to the solubility of Se0, provided that there is no interaction with the mobile Boom Clay organic matter. 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 of 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. 1. Bruggeman C., Maes A., Vancluysen J. and Vandemussele P. (2005). Selenite reduction in Boom clay: effect of FeS2, clay minerals and dissolved organic matter. Env. Poll., 137: 209-221. 2. Scheinost A.C. and Charlet L. (2008). Selenite reduction by mackinawite, magnetite and siderite: XAS characterization of nanosized redox products. ES&T, 42(6): 1984-1989. 3. Breynaert, E., Bruggeman C. and Maes A. (2008) XANES-EXAFS analysis of se solid-phase reaction products formed upon contacting Se(IV) with FeS2 and FeS. ES&T, 42(10): 3595-3601. Keywords: EXAFS/XANES, selenium, solid phase speciation.status: publishe