Uranyl incorporation into calcite and aragonite: XAFS and luminescence studies

X-ray absorption, luminescence, and Raman spectroscopic studies of U(VI)-co ntaining calcite and aragonite show that the UO22+ ion, the dominant and mo bile form of dissolved uranium in near-surface waters, has a disordered and apparently less stable coordination environment when incorporated into cal cite in comparison to aragonite, both common polymorphs of CaCO3. Our findi ngs suggest that calcite, a widely distributed authigenic mineral in soils and near-surface sediments and a principal weathering product of concrete-b ased containment structures, is not likely to be a suitable host for the lo ng-term sequestration of U(VI). The more stable coordination provided by ar agonite suggests that its long-term retention should be favored in this pha se, until it inverts to calcite. Consequently, future remobilization of U(V I) coprecipitated with calcium carbonate minerals should not be ruled out i n assessments of contaminated sites. Our observation of a similar equatoria l coordination of UO22+ in aragonite acid the dominant aqueous species [UO2 (CO3)(3)(4-)] but a different coordination in calcite indicates that a chan ge in UO22+ coordination is required for its incorporation into calcite. Th is may explain the observed preferential uptake of U(VI) by aragonite relat ive to calcite.