Molecular-scale characterization of uranium sorption by bone apatite materials for a permeable reactive barrier demonstration

Uranium binding to bone charcoal and bone meal apatite materials was investigated using U L-III-edge EXAFS spectroscopy and synchrotron source XRD measurements of laboratory batch preparations in the absence and presence of dissolved carbonate. Pelletized bone char apatite recovered from a permeable reactive barrier (PRB) at Fry Canyon, UT, was also studied. EXAFS analyses indicate that U(VI) sorption in the absence of dissolved carbonate occurred by surface complexation of U(VI) for sorbed concentrations less than or equal to 5500 mug U(VI)/g for all materials with the exception of crushed bone char pellets. Either a split or a disordered equatorial oxygen shell was observed, consistent with complexation of uranyl by the apatite surface. A second shell of atoms at a distance of 2.9 Angstrom was required to fit the spectra of samples prepared in the presence of dissolved carbonate (4.8 mM total) and is interpreted as formation of ternary carbonate complexes with sorbed MO. A U-P distance at 3.5-3.6 Angstrom was found for most samples under conditions where uranyl phosphate phases did not form, which is consistent with monodentate coordination of uranyl by phosphate groups in the apatite surface. At sorbed concentrations greater than or equal to 5500 mug U(VI)/g in the absence of dissolved carbonate, formation of the uranyl phosphate solid phase, chernikovite, was observed. The presence of dissolved carbonate (4.8 mM total) suppressed the formation of chernikovite, which was not detected even with sorbed U(VI) up to 12 300 mug U(VI)/g in batch samples of bone meal, bone charcoal, and reagent-grade hydroxyapatite. EXAFS spectra of bone char samples recovered from the Fry Canyon PRB were comparable to laboratory samples in the presence of dissolved carbonate where U(VI) sorption occurred by surface complexation. Our findings demonstrate that uranium uptake by bone apatite will probably occur by surface complexation instead of precipitation of uranyl phosphate phases under the groundwater conditions found at many U-contaminated sites.