Micro- and Macroscopic Investigations of Actinide Binding in Cementitious Materials

Micro-X-ray fluorescence and micro-X-ray absorption spectroscopy (μ-XRF/XAS) have been used to determine the elemental distribution and the coordination environment of U(VI) and Np(V,VI) in intact hardened cement paste (HCP) samples at the atomic level. Micro-XRF maps reveal a heterogeneous distribution of the actinides. Micro-XAS measurements on a few U(VI) hot spots show that two main U(VI) species contribute to the spectra: an U(VI) species with a chemical environment similar to that in U(VI) loaded HCP and calcium silicate hydrates (C-S-H) sorption samples and an U(VI) species with a chemical environment similar to that of U(VI) in calcium uranate. Micro-XANES indicates presence of both Np(IV,V) oxidation states in Np(V) loaded intact HCP, presumably caused by beam damage. Earlier studies have shown that U(VI) is taken up into the interlayer of C-S-H phases, and recently it was suggested that the same mechanism controls Np(IV,V) uptake by these phases. The present study is complementary to earlier ones carried out in our research group and demonstrates that information on the coordination environment of actinides is essential with the aim of developing a mechanistic understanding of actinide interaction with cementitious materials. The micro-scale studies show that actinide uptake by cementitious materials has to be interpreted in terms of solid solution formation rather than surface binding (ion exchange, surface complexes).