Uranyl(VI) sorption in calcium silicate hydrate phases. A quantum chemical study of tobermorite models

Using density functional theory, we present the first quantum chemical study on uranyl, UO₂ ²⁺, absorption into the interlayer region of calcium silicate hydrate (CSH) phases, the main constituents of fresh and degraded cement. We modeled CSH phases by tobermorite 14 Å with various ratios of Ca/Si (C/S). Uranyl (VI) sorbed in the interlayer exhibits mainly coordination number 4. At higher C/S values, uranyl hydrolyzes and exhibits OH ligands in the first coordination shell. EXAFS data compare well with calculated structural features of sorption complexes. However, none of the sorption complexes determined displays all measured distances, suggesting that several sorption complexes co-exist. Our results represent the first comprehensive atomic scale model of actinyl sorption complexes in CSH and, together with pertinent EXAFS results, supports the hypothesis that interlayer absorption contributes to a considerable extent to the uptake of uranyl by CSH. Characteristic U–Si distances are assigned to the coordination mode of uranyl sorbed at SiO₄ tetrahedra of CSH. Measured U–Ca distances appear due to U–O–Ca bridges, the number of which defines the U–Ca distance. We characterized the energy of UO₂ ²⁺ sorption by calculated exchange energies of Ca²⁺ or 2H⁺ by UO₂ ²⁺. For higher C/S this analysis reveals correlations between the preferred sites for uranyl sorption and the compensating charge, estimated empirically, of the functional groups forming the site. The sites with stronger compensation, mainly defect-derived and three-fold coordinative sites, were calculated to be the most favorable ones.