EXAFS investigation on U(VI) immobilization in hardened cement paste. Influence of experimental conditions on speciation

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the coordination environment of U(VI) in cementitious materials. The EXAFS measurements were carried out on U(VI)-doped samples prepared under varying conditions, such as samples from sorption, hydration and diffusion experiments, and using different cementitious materials, such as crushed hydrated hardened cement paste (HCP) and calcium silicate hydrates (C-S-H). The samples had U(VI) loadings ranging from 1700 μg/g to 45000 μg/g. Applying principal component analysis (PCA) on 13 EXAFS spectra (each spectra corresponding to a minimum of five different scans) of the low loading samples, one single species is obtained indicating a similar U(VI) coordination environment for both HCP and C-S-H samples. This result confirms that C-S-H phases control the uptake of U(VI) in the complex cement matrix. The coordination environment structure of this species is similar to a U(VI) surface complex or to U(VI) in uranyl silicate minerals (two axial O atoms at 1.82 ± 0.02 A; four equatorial O atoms at 2.25 ± 0.01 A; one Si atom at 3.10 ± 0.03 A). At high U(VI) loading, PCA revealed a second U(VI) species, with a coordination environment similar to that of U(VI) in calcium uranate (two axial O atoms at 1.94 ± 0.04 A; five equatorial O atoms at 2.26 ± 0.01 A; four Ca atoms at 3.69 ± 0.05 A and five U atoms at 3.85 ± 0.04 A). This study suggest that, at low U(VI) loading, U(VI) is bound to C-S-H phases in HCP while at high U(VI) loading, the immobilization of U(VI) in cementitious materials is mainly controlled by the precipitation of a calcium uranate-type phase. (orig.)