Coprecipitation of uranium(VI) with calcite: XAFS, micro-XAS, and luminescence characterization

X-ray absorption and luminescence spectroscopies have been used to characterize the local structure and coordination of uranium(VI) species coprecipitated with calcite (CaCO₃) from room-temperature aqueous solutions. Different solution chemistries and pHs are found to result in differences in the equatorial coordination of the uranyl species (UO²⁺₂) incorporated in the calcite, with multiple coordination environments of uranyl evident in one sample. Differences in the equatorial coordination between the aqueous uranyl species and those found in the calcite indicate that coordination changes occur during incorporation of at least some species. This contrasts with previous findings showing no change in equatorial coordination during uranyl incorporation into aragonite, and may explain the greater incorporation in this latter phase. The absence of calcium backscatterers and well defined structure beyond the equatorial shell is consistent with disorder associated with disruption of the local calcite structure. This may indicate an inability of the uranyl unit to assume a stable structural environment in the host calcite, which could decrease the stability of uranyl- containing calcite. Calcite single crystals grown in uranyl-containing solutions exhibit polygonized spiral growth hillocks on (1014) surfaces composed of four vicinal surfaces, consistent with face symmetry. Micro-X-ray fluorescence reveals that uranium is differentially incorporated between nonequivalent vicinal surfaces, reflecting step-selective incorporation of uranyl species during growth. Micro-X-ray absorption near-edge structure spectra from the nonequivalent vicinal faces fail to reveal any differences in speciation between the vicinals or that might account for the presence of the multiple coordination environments identified by luminescence and X-ray absorption spectroscopies.