Sorption of (trivalent) actinides and lanthanides

Introduction
The study of trivalent actinides is of particular importance for the safety assessment of nuclear waste disposal sites due to the predominance of this valence in deep geological formations. In particular, studying the solution-solid interface chemistry of these trivalent radioelements in the aqueous phase with a mineral is fundamental for better understanding their interactions at or within the surface of a host phase in a repository. As a relevant near field material (geotechnical barrier) for nuclear waste disposal sites, clay minerals are very important due to their retardation properties. Muscovite, a phyllosilicate material of aluminum and potassium, is very similar to clay minerals but less complex, so we are able to assign results from muscovite to clay minerals. Additionally, investigations concerning trace concentration of actinides appearing in the far field of a nuclear waste disposal are also of interest. Site-Selective Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) is a characterizational technique that can probe the behavior of low concentrated actinides on a molecular level. As a complementary technique resonant anomalous x-ray reflectivity (RAXR) will be used to get a deeper insight and a verification of the TRLFS results.

TRLFS, the main tool
The aim of this study focuses on understanding the surface interactions of muscovite with aqueous trivalent actinides and lanthanides using Eu(III) and Cm(III), and characterization of the solid and aqueous phase species using TRLFS. Europium (III) is used as a non-radioactive homologue for trivalent actinides due to its similar chemical behavior and its spectroscopic properties as a probe for TRLFS. Direct excitation of the ⁷F₀→⁵D₀ electron transition and consecutive integration of the respective emission generates information pertaining to the chemical coordination and environment of the Eu(III). First investigations in the muscovite-europium system show that there appears one poorly defined species (broad excitation peak) present at one site. Lifetime measurements of the luminescence are used in accordance with the Horrocks equation (europium) [1] and the number of coordinated waters can be determined. The lifetimes between 208 and 230 µs indicates 4 to 5 coordinated water ligands in the inner sphere. As a consequence of this the europium species is interpreted as inner-sphere sorption on the surface of muscovite.

[1] Horrocks, W.D. and Sudnick, D.R. (1979) J. Am. Chem. Soc. 101, 334-340.