New insights into U(VI) sorption onto montmorillonite from batch sorption and spectroscopic studies at increased ionic strength.

The influence of ionic strength up to 3 mol kg-1 (background electrolytes NaCl or CaCl2) on U(VI) sorption onto montmorillonite was investigated as function of pHc in absence and presence of CO2. A multi-method approach combined batch sorption experiments with spectroscopic methods (time-resolved laser-induced fluorescence spectroscopy (TRLFS) and in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR)). In the absence of atmospheric carbonate, U(VI) sorption was nearly 99% above pHc 6 in both NaCl and CaCl2 and no significant effect of ionic strength was found. At lower pH, cation exchange was strongly reduced with increasing ionic strength. In the presence of carbonate, U(VI) sorption was reduced above pHc 7.5 in NaCl and pHc 6 in CaCl2 system due to formation of aqueous UO2(CO3)x(2-2x) and Ca2UO2(CO3)3 complexes, respectively, as verified by TRLFS. A significant ionic strength effect was observed due to the formation of Ca2UO2(CO3)3(aq), which strongly decreases U(VI) sorption with increasing ionic strength. The joint analysis of determined sorption data together with literature data (giving a total of 213 experimental data points) allowed to derive a consistent set of surface complexation reactions and constants based on the 2SPNE SC/CE approach, yielding log K°≡SSOUO2+ = 2.42 ± 0.04, log K°≡SSOUO2OH = -4.49 ± 0.7, and log K°≡SSOUO2(OH)32- = -20.5 ± 0.4. Ternary uranyl carbonate surface complexes were not required to describe the data. With this reduced set of surface complexes, an improved robust sorption model was obtained covering a broad variety of geochemical settings over wide ranges of ionic strengths and groundwater compositions, which subsequently was validated by an independent original dataset. This model improves the understanding of U(VI) retention by clay minerals and enables now predictive modeling of U(VI) sorption processes in complex clay rich natural environments.

Sorption of trivalent lanthanides and actinides onto montmorillonite: Macroscopic, thermodynamic and structural evidence for ternary hydroxo and carbonato surface complexes on multiple sorption sites.

The credibility of long-term safety assessments of radioactive waste repositories may be greatly enhanced by a molecular level understanding of the sorption processes onto individual minerals present in the near- and far-fields. In this study we couple macroscopic sorption experiments to surface complexation modelling and spectroscopic investigations, including extended X-ray absorption fine structure (EXAFS) and time-resolved laser fluorescence spectroscopies (TRLFS), to elucidate the uptake mechanism of trivalent lanthanides and actinides (Ln/An(III)) by montmorillonite in the absence and presence of dissolved carbonate. Based on the experimental sorption isotherms for the carbonate-free system, the previously developed 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model needed to be complemented with an additional surface complexation reaction onto weak sites. The fitting of sorption isotherms in the presence of carbonate required refinement of the previously published model by reducing the strong site capacity and by adding the formation of Ln/An(III)-carbonato complexes both on strong and weak sites. EXAFS spectra of selected Am samples and TRLFS spectra of selected Cm samples corroborate the model assumptions by showing the existence of different surface complexation sites and evidencing the formation of Ln/An(III) carbonate surface complexes. In the absence of carbonate and at low loadings, Ln/An(III) form strong inner-sphere complexes through binding to three Al(O,OH)6 octahedra, most likely by occupying vacant sites in the octahedral layers of montmorillonite, which are exposed on {010} and {110} edge faces. At higher loadings, Ln/An(III) binds to only one Al octahedron, forming a weaker, edge-sharing surface complex. In the presence of carbonate, we identified a ternary mono- or dicarbonato Ln/An(III) complex binding directly to one Al(O,OH)6 octahedron, revealing that type-A ternary complexes form with the one or two carbonate groups pointing away from the surface into the solution phase. Within the spectroscopically observable concentration range these complexes could only be identified on the weak sites, in line with the small strong site capacity suggested by the refined sorption model. When the solubility of carbonates was exceeded, formation of an Am carbonate hydroxide could be identified. The excellent agreement between the thermodynamic model parameters obtained by fitting the macroscopic data, and the spectroscopically identified mechanisms, demonstrates the mature state of the 2SPNE SC/CE model for predicting and quantifying the retention of Ln/An(III) elements by montmorillonite-rich clay rocks.

Spectroscopic identification of ternary Cm-carbonate surface complexes.

The influence of dissolved CO(2) on the sorption of trivalent curium (Cm) on alumina (gamma-Al(2)O(3)) and kaolinite was investigated by time resolved laser fluorescence spectroscopy (TRLFS) using the optical properties of Cm as a local luminescent probe. Measurements were performed at T < 20 K on Cm loaded gamma-Al(2)O(3) and kaolinite wet pastes prepared in the absence and presence of carbonate in order to pictorially illustrate any changes through a direct comparison of spectra from both systems. The red-shift of excitation and emission spectra, as well as the increase of fluorescence lifetimes observed in the samples with carbonate, clearly showed the influence of carbonate and was fully consistent with the formation of Cm(III) surface species involving carbonate complexes. In addition, the biexponential decay behavior of the fluorescence lifetime indicated that at least two different Cm(III)-carbonate species exist at the mineral-water interface. These results provide the first spectroscopic evidence for the formation of ternary Cm(III)-carbonate surface complexes.

Site-selective time-resolved laser fluorescence spectroscopy of Eu3+ in calcite.

Three samples of calcite homogeneously doped with Eu(3+) were synthesized in a mixed-flow reactor. By means of selective excitation of the 5D0-->7F0 transition at low temperatures (T<20 K), three different Eu(3+) species (species A, B, and C, respectively) could be discriminated. For each one, the emission spectrum and lifetime were obtained after selective excitation of the single species. On the basis of these data, species C could be identified as Eu(3+) incorporated into the calcite lattice on the (nearly) octahedral Ca(2+) site. Species B was also identified as Eu(3+) incorporated into the calcite lattice, but the ligand field shows a much weaker symmetry. Species A, however, is not incorporated into the crystal's bulk, having 1-2 H(2)O ligands left in its first coordination sphere and showing very little symmetry, and is considered as Eu(3+) adsorbed onto the calcite surface. The emission spectra of species C for Eu:calcite grown in the presence of Na(+) were found to differ from those of Eu:calcite synthesized in the presence of K(+). The latter revealed a strong distortion in site symmetry, which was not observed in the samples grown in Na(+) solutions. This finding provides spectroscopic evidence in favor of an incorporation mechanism based on the charge-balanced coupled substitution of Na(+)+Eu(3+)<-->2Ca(2+).