U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study

The mechanism of U(VI) sorption on montmorillonite (Na-SWy-1) in the absence and presence of carbonate was investigated through a combination of different approaches: macroscopic sorption experiments, surface complexation modelling using the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange sorption model and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. U(VI) sorption measurements were performed in the absence of carbonate at fixed ionic strength (0.1 M NaClO4) as a function of pH at U(VI) trace concentration (similar to 9 x 10(-8) M) and as a function of U(VI) concentration (similar to 10(-7) -10(-4) M) at a fixed pH (5, 6.8 and 8). In the presence of carbonate, experiments were carried out in equilibrium with atmospheric pCO(2) and in 1, 3 and 5 mM NaHCO3. The pH dependent sorption measurements at trace concentration in the absence of carbonate were modelled by considering the formation of the following surface species, equivalent to(SOUO2+)-O-S, equivalent to(SOUO2OH0)-O-S, equivalent to(SOUO2)-O-S(OH)(2)(-) and equivalent to(SOUO2)-O-S(OH)(3)(2-) on the strong sites. From the isotherms the formation of equivalent to(SOUO2+)-O-W1 and equivalent to(SOUO2OH0)-O-W1 on the weak sites was inferred. Two additional surface complexes on the strong sites, equivalent to(SOUO2CO3-)-O-S and equivalent to(SOUO2)-O-S(CO3)(2)(3-) and one surface complex on the weak sites, (SOUO2CO3-)-O-W1, were necessary to reproduce the sorption data obtained in the presence of carbonate. The EXAFS measurements did not allow to verify the formation of ternary uranyl-carbonate complexes on the montmorillonite surface. However, the obtained fit results, i.e. splitting of the equatorial oxygen shell, one Si/Al shell at similar to 3.09 angstrom and one Si/Al at similar to 3.29 angstrom or one Fe shell at similar to 3.42 angstrom, clearly indicate that under the given experimental conditions (pH, U(VI) loading) U(VI) forms inner-sphere surface complexes on montmorillonite edge sites via binding to aluminum octahedra and/or silicon tetrahedra. (C) 2012 Elsevier Ltd. All rights reserved.