Kristallchemischer Einbau trivalenter f-Elemente in trioktaedrische Smectite

Abstract

In many concepts for geological disposal of radioactive waste will be disposed in underground repositories. The long term isolation of radionuolides 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 Na 0 . 7 [Li 0 . 7 Mg 5 . 3 Si 8 O 2 0 (OH) 4 ] is one of these secondary phases identified within the alteration layer of corroded HLW glass. Numerous studies have dearly 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. Americium is one of the abundant actinides in HLW-glass, and may be mobilized during corrosion. Its chemical homologue, Curium, is present In lesser waste amount, but sensitive for speciation based on time-resolved laser spectroscopy. The lanthanide Europium also exhibits favorable spectroscopic characteristics and has been used as a non radioactive chemical homolog to identify the sorption mechanism. Therefore, using Eu(lll) and Cm(III) is a convenient way to study the chemical behavior of trivalent f-elements during formation of clay minerals. This study focuses on the multi-step synthesis of organo-hectorite as a model system for f-element coprecipitation with clay minerals. The new method was modified to synthesize an Eu- as well as a Cm-containing hectorite. The Eu-containing reaction products were identified unambiguously as hectorite by X-ray diffraction, FTIR-spectroscopy, and atomic force microscopy. After formation of hectorite in the presence of f-elements was assured, the sorption mechanisms of the associated Eu-species were investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure spectroscopy (EXAFS). An unhydrated Eu-species and a partly hydrated Eu-species could be identified by TRLFS. The unhydrated Eu-species can be interpreted as incorporated Eu(III) into the hectorite structure, or a remaining amorphous silica phase. The TRLFS-spectra of Eu-hectorite and the Eu-silica complexation are too similar to differentiate between these species, but dialysis experiments demonstrate the close association of the unhydrated Eu-species with the crystalline hectorite phase. Samples for TRLFS-measurements were taken during the dialysis process, and the same incorporated Eu-species was identified in the solid phases, as long as the Eu-hectorite was stable under acidic conditions.