Structural incorporation of trivalent f-elements into trioctahedral smectite

In many concepts for geological disposal the radioactive waste will be disposed in underground repositories. The long term isolation of radionuclides from the biosphere will be ensured by the presence of several technical and natural barriers. A common technical barrier for radionuclides remaining from reprocessing of spent fuel is high-level nuclear waste glass (HLW-glass). Glass is stable for millions of years under dry conditions, but aqueous solutions may penetrate the near-field confinement and initiate alteration/dissolution of the HLW-glass. During alteration/dissolution processes the contained radionuclides may be mobilized, and various secondary phases form. Some of these new mineral phases represent a significant retention potential for mobilized radionuclides. The trioctahedral smectite hectorite Na0.7[Li0.7Mg5.3Si8O20(OH)4] is one of these secondary phases identified within the alteration layer of corroded HLW glass. Numerous studies have clearly shown that many radionuclides are associated with clay minerals and the migration of radionuclides is strongly reduced by complexation. Due to the structural complexity and chemical variability of smectites, sorption of radionuclides involves several sorption mechanisms: (1) adsorption via inner-sphere and outer-sphere complexation, (2) cation exchange in the interlayer and (3) formation of new secondary clay minerals opens the possibility to even incorporate radionuclides into the clay structure. In contrast to adsorbed species, incorporated species are associated with a more independent retention potential regarding changing geochemical conditions. Therefore, they are relevant to many issues in environmental and geochemical sciences. Until recently, it was not known whether the high radio toxic actinides become incorporated into the crystal structure of clay minerals like hectorite. Due to the potential importance in controlling actinide mobility, it is necessary to understand the mechanism of f-element incorporation into the clay structure in detail. (orig.)