U(VI) sorption on montmorillonite: A macroscopic and microscopic study


U(VI) sorption on montmorillonite: A macroscopic and microscopic study

Marques Fernandes, M.; Baeyens, B.; Dähn, R.; Scheinost, A. C.; Macé, N.; Bradbury, M. H.

The mechanism of U(VI) sorption on montmorillonite (Na-SWy-2) in the absence and the presence of carbonate was investigated through a combination of different approaches: macroscopic sorption experiments, modelling with the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange model and Extended X-ray absorption fine structure (EXAFS) spectroscopy. U(VI) sorption measurements were performed both in the absence and in the presence of carbonate at fixed ionic strength (0.1 M NaClO4) as a function of pH at U(VI) trace concentration (~ 9x10-8 M) and as a function of U(VI) concentration (5x10-8-10-4 M) at pH ~ 8. In the presence of carbonate, experiments were carried out in equilibrium with atmospheric pCO2 and in 1, 3 and 5 mM NaHCO3. The sorption edge data in the absence of carbonate were modelled by considering the formation of the following surface species, ≡SSOUO2+, ≡SSOUO2OH0, ≡SSOUO2(OH)2-, ≡SSOUO2(OH)32-, and ≡SSOUO2(OH)32- on the strong sites. From the isotherm the formation of ≡SW1OUO2+ and ≡SW1OUO2OH0 on the weak sites were derived. Two additional surface complexes on the strong sites, ≡SSOUO2CO3- and ≡SSOUO2(CO3)23- and one surface complex on the weak sites, ºSW1OUO2CO3-, were necessary to reproduce the sorption data 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 parameters, splitting of the equatorial oxygen shell, two Si/Al shells at ~3.09 Å and 3.85 Å and one Fe shell at ~ 3.43 Å clearly indicate that U(VI) forms a mononuclear edge-sharing surface complex, sharing one octahedral Fe, one octahedral Al and one tetrahedral Si/Al. The lack of angular dependence of the Polarized–EXAFS spectra confirms that U(VI) is not sorbed to the montmorillonite surface in the continuity of the octahedral sheet.

Keywords: EXAFS; montmorillonite; sorption U(VI)

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Publ.-Id: 15268