We determined the association of uranium with bacteria isolated from the Waste Isolation Pilot Plant (WIPP), Carlsbad, New Mexico, and compared this with known strains of halophilic and non-halophilic bacteria and archaea. Examination of the cultures by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) showed uranium accumulation extracellularly and/or intracellularly to a varying degree. In Pseudomonas fluorescens and Bacillus subtilis uranium was associated with the cell surface and in the latter it was present as irregularly shaped grains. In Halobacterium halobium, the only archeon studied here, uranium was present as dense deposits and with Haloanaerobium praevalens as spikey deposits. Halomonas sp. isolated from the WIPP site accumulated uranium both extracellularly on the cell surface and intracellularly as electron-dense discrete granules. Extended X-ray absorption fine structure (EXAFS) analysis of uranium with the halophilic and non-halophilic bacteria and archaea showed that the uranium present in whole cells was bonded to an average of 2.4 ± 0.7 phosphoryl groups at a distance of 3.65 ± 0.03 Å. Comparison of whole cells of Halomonas sp. with the cell wall fragments of lysed cells showed the presence of a uranium bidentate complex at 2.91 ± 0.03 Å with the carboxylate group on the cell wall, and uranyl hydroxide with U−U interaction at 3.71 ± 0.03 Å due to adsorption or precipitation reactions; no U − P interaction was observed. Addition of uranium to the cell lysate of Halomonas sp. resulted in the precipitation of uranium due to the inorganic phosphate produced by the cells. These results show that the phosphates released from bacteria bind a significant amount of uranium. However, the bacterially immobilized uranium was readily solubilized by bicarbonate with concurrent release of phosphate into solution.

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Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, Pcoz, and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, Pcoz, and PH at 25"C. Metal adsorption is tYPical@ favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na + with the dolomite surflce, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca2 + in solution will decrease Kds. At the same time, t~the extent that high Pcojs increase Ca2 + levels, &S will decrease with C02 levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Amcarbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to resorption). This indirect C02 effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to atTect to actinide Kds

We investigated the transformations of uranyl nitrate, uranyl citrate, uranyl ethylenediaminetetraacetate (U-EDTA), and uranyl carbonate by a denitrifying halophilic bacterium, Halomonas sp. (WIPP1A), isolated from the Waste Isolation Pilot Plant (WIPP) repository. The addition of uranyl nitrate, uranyl citrate, or uranyl EDTA to the brine or bacterial growth medium resulted in the precipitation of uranium. Extended X-ray absorption fine structure (EXAFS) analysis of the precipitates formed in the brine and in the growth medium were identified as uranyl hydroxide [UO 2 -(OH) 2 ] and uranyl hydroxophosphato species [K(UO 2 ) 5 (PO 4 ) 3 -(OH) 2 ‚nH 2 O], respectively. Dissolution of the uranium precipitate was concomitant with the growth of the bacteria under anaerobic conditions. The UV-vis spectra of the culture medium during growth showed that a uranyl dicarbonate complex [UO 2 (CO 3 ) 2 ] 2was formed due to CO 2 production from the metabolism of the carbon source succinate. The bacterium completely metabolized the citrate released from the uranyl citrate complex but not the EDTA released from the U-EDTA complex. Adding uranyl carbonate to the growth medium caused no changes in the uranium speciation due to bacterial growth. Uranyl carbonate was not biosorbed by the growing culture nor by washed resting cells suspended in 20% NaCl brine (3.4 M) because the complex was either neutral or anionic. Our results demonstrate that bacterial activity can enhance the dissolution of uranium phosphate by forming uranyl carbonate species.