Incorporation of trivalent actinides and lanthanides in calcite. A time resolved laser fluorescence spectroscopy (TRLFS) study

摘要: Although incorporation of radionuclides into the bulk structure of mineralphases represents a very important retardation process, very little is known about the reaction mechanism and about the structure of the involved incorporated species. Carbonates are among the most important secondary alteration products formed during the degradation of cement in radioactive waste repositories. Previous studies have shown that divalent and trivalent metal ions like Mg2+[1],[2], Cd2+[3],[4] and Rare Earth Elements (REE)[5],[6] in presence of calcite are efficiently removed from the aqueous phase. An important point for incorporation of foreign ions into a crystal structure is the similarity of the ionic radii of the cation in the bulk structure and of the substituting element. The size of trivalent actinide and lanthanide ions are close to that of Ca2+. Another important question for any heterovalent substitution is the charge compensation mechanism. In an Eu3+/calcite co-precipitation study, Stipp et al.[7] proposed the substitution of Ca2+ by EuOH2+. Structural parameters of REE [8] and Nd3+[9] incorporated into the calcite lattice were obtained by using extended X-ray absorption fine structure spectroscopy (EXAFS). The studies show that the local oxygen coordination of the incorporated ions change from six fold to seven fold with increasing ionic radius of the lanthanide ions. The aim of this study was to identify the Cm/calcite incorporation mechanism (charge compensation) and to characterize the involved species. Structural information of Cm (III) incorporated in calcite was obtained by using Time Resolved Laser Fluorescence Spectroscopy (TRLFS). From …