The stability of biogenic uraninite with respect to oxidation is seminal to the success of in situ bioreduction strategies for remediation of subsurface U(VI) contamination. The properties and hence stability of uraninite are dependent on its size, structure, and composition. In this study, the local-, intermediate-, and long-range molecular-scale structure of nanoscale uraninite produced by Shewanella oneidensis strain MR-1 was investigated using EXAFS, SR-based powder diffraction and TEM. The uraninite products were found to be structurally homologous with stoichiometric UO 2 under all conditions considered. Significantly, there was no evidence for lattice strain of the biogenic uraninite nanoparticles. The fresh nanoparticles were found to exhibit a well-ordered interior core of diameter ca. 1.3 nm and an outer region of thickness ca ∼0.6 nm in which the structure is locally distorted. The lack of nanoparticle strain and structural homology with stoichiometric UO 2 suggests that established thermodynamic parameters for the latter material are an appropriate starting point to model the behavior of nanobiogenic uraninite. The detailed structural analysis in this study provides an essential foundation for subsequent investigations of environmental samples.

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO 2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of $10 À8 M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface areanormalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO 2.0 to UO 2+x . In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO 2.25 (U 4 O 9 ) coated by an outer layer of U(VI). This result is in accordance with flowthrough dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions.