Quantum Chemical Investigation Of The Selenite Incorporation Into The Calcite (10(1)Over-Bar4) Surface

Selenium is a common pollutant in soils and aquifers. The radioisotope Se-79, an abundant fission product of U-235, is of particular concern in the context of nuclear waste disposal safety due to its long half life and its expected high mobility in the multibarrier system around potential nuclear waste disposal sites. Oxidized selenium species are relatively soluble and show only weak adsorption at common mineral surfaces. However, a possible sorption mechanism for selenium in the geosphere is the structural incorporation of selenium(IV) (selenite, SeO32-) into calcite (CaCO3). We carried out a detailed quantum chemical study of the incorporation of SeO32- into the calcite surface and the subsurface layers. As the main result, we present the structural changes upon incorporation of selenite (SeO3-2) into the dry and hydrated calcite (10 (1) over bar4) surface. These results are complemented by energetic considerations and, in turn, used to estimate the thermodynamic partition coefficient D for SeO3-2. incorporation into the surface and subsurface layers. The results corroborate the recently proposed entrapment model for selenium(IV) coprecipitation with calcite and show that equilibrium incorporation of selenite into calcite may occur in the surface layer but is practically impossible in subsurface layers or the bulk.