Retention of trivalent Actinides by structural Incorporation

The incorporation of radionuclides into a host mineral’s crystal structure is a particularly efficient means of retention, due to the fact that the incorporated radionuclide is removed from the water path. The process is relevant, both, naturally occurring under repository conditions[1], and as a technical means for the sequestration of actinide waste streams. Consequently, it is of utmost importance to understand the processes leading up to the incorporation, as well as the structural properties of the formed solid solution at the molecular level[2].
We will give an overview on the incorporation of trivalent actinides and lanthanides into Ca- and Ln-bearing mineral phases of technical and geochemical relevance. The minor actinides dominate the radiotoxicity in a nuclear waste storage site over hundreds of thousands of years. For Am and Cm the trivalent oxidation states is the only relevant oxidation in aqueous solutions, and even Pu may be present as Pu(III) under reducing repository conditions. The trivalent actinides have ionic radii closely matched to their lanthanides homologues as well as calcium, making mineral phases with these host cations ideal matrices for the incorporation of the trivalent minor actinides.
To identify and characterize actinides in solid solutions at the trace concentration level, we make use of time-resolved laser fluorescence spectroscopy (TRLFS). TRLFS allows for speciation of Cm(III) at concentrations below 10-9 mol/L, or 1 ppm in the solid state. Cm TRLFS can be complemented by experiments with Eu(III), which, while less sensitive, are more sensitive to changes in the local coordination geometry of the fluorescent probe.
An overview will be given on the characterization of solid solutions of secondary phases relevant under repository conditions. We will compare a phase formed close to equilibrium (calcite) with a metastable phase undergoing a phase transition (vaterite[3]), as well as a phase from high-temperature synthesis (powellite[4]). The effect of various reaction parameters on the reactions, and their implications for the stability of the formed solid solutions, and hence the retention of the radionuclides will be discussed.
[1] T. Stumpf and Th. Fanghänel, J. Colloid Interf. Sci. 249, 119 (2002).
[2] H. Geckeis, et al., Chem. Rev. 113, 1016 (2013).
[3] M. Schmidt, et al., J. Colloid Interf. Sci. 351, 50 (2010).
[4] M. Schmidt, et al., Dalton Trans. 42, 8387 (2013).