Abstract:
The interaction of U(VI) species with the calcite surface in pre-equilibrated calcite suspensions at pH 7.4 and 8.3 and P(CO2 ) = 10-3.5 bar was characterized in situ using extended X-ray absorption spectroscopy (EXAFS) and luminescence spectroscopies. Results indicate that uranyl triscarbonate-like adsorption complexes dominate at U(VI) solution concentrations <500 μM, whereas the formation of U(VI) hydroxide and carbonato precipitates is observed at higher concentrations, consistent with isotherm data and aqueous speciation calculations. The EXAFS data indicate weak splitting in the equatorial O shell of the U(VI) adsorption complexes, which may indicate that the adsorption complexes are bound in an inner-sphere fashion at the calcite surface, although no Ca backscattering could be positively identified. The luminescence data indicate the presence of at least two adsorption complexes that change proportion with U(VI) loading. One species, dominating at low-surface coverage, is the uranyl triscarbonate complex. A second species is observed at higher surface loadings with a luminescence spectrum that is intermediate between the triscarbonate species found at the lowest loadings and uranyl incorporated into bulk polycrystalline calcite. The combined EXAFS and luminescence data indicate that the U(VI) adsorption complexes forming at the calcite surface are triscarbonate-like complexes, with a change in interaction with calcite surface sites as the surface loading increases, and the formation of U(VI) hydroxide/carbonato precipitates at high concentrations. Consequently, multiple uranyl species are likely to exist at the calcite surface during interaction of U(VI)- containing waters in the near-surface environment. Furthermore, complex sorption/desorption behavior and kinetics may be associated with differing stabilities of sorbed U(VI) species in calcite-containing materials.
