Mechanisms of Nd(III) uptake by 11 Å tobermorite and xonotlite

The uptake of Nd(III) by the crystalline C–S–H phases 11 Å tobermorite and xonotlite has been investigated by the combined use of wet chemistry techniques, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray diffraction (XRD) in combination with Rietveld refinement. The results from XRD and EXAFS allowed the different modes of Nd–Ca substitution in tobermorite and xonotlite to be distinguished from each other. Wet chemistry and EXAFS data showed that the formation of any Nd solid phase with fixed stoichiometry could be ruled out. XRD studies on the samples with high Nd loading (350 μmol Nd/g solid phase) further showed that Nd was bound in the structure of C–S–H phases. The EXAFS data suggested that Nd could form several species on xonotlite and tobermorite at low loadings (7–35 μmol Nd/g solid phase). Neodymium was predominantly bound on the external surface of both crystalline C–S–H phases after 1 day of reaction time and predominantly incorporated in the Ca layers of the crystalline C–S–H phases in the long run (?60 days reaction time). The latter process was faster at low Nd loadings and was apparently controlled by re-crystallization of the C–S–H phases. Neodymium incorporation was accompanied by the release of “zeolitic” water (water molecules in the interlayer of C–S–H) and bridging Si tetrahedra, reflected by the formation of more disordered structures in both C–S–H phases. The Nd retention model proposed in this study helps to improve understanding of the immobilization of trivalent lanthanides and actinides in cementitious materials. This knowledge is essential for long-term predictions of radionuclide retention in conjunction with a more detailed assessment of the safe disposal of actinides in the cementitious near field of a repository for radioactive waste.