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 (∼9 × 10−8 M) and as a function of U(VI) concentration (∼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 pCO2 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, SSOUO2+, SSOUO2OH0SSOUO2OH0, SSOUO2(OH)2- and SSOUO2(OH)32- on the strong sites. From the isotherms the formation of SW1OUO2+ and SW1OUO2OH0SW1OUO2OH0 on the weak sites was inferred. Two additional surface complexes on the strong sites, SSOUO2CO3- and SSOUO2(CO3)23- and one surface complex on the weak sites, SW1OUO2CO3-, 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 ∼3.09 Å and one Si/Al at ∼3.29 Å or one Fe shell at ∼3.42 Å, 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.