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Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 mg L(-1) dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits.

The effects of diffusive transport limitations on the dissolution of UO(2) were investigated using an artificial groundwater prepared to simulate the conditions at the Old Rifle aquifer site in Colorado, USA. Controlled batch, continuously-stirred tank (CSTR), and plug flow reactors were used to study UO(2) dissolution in the absence and presence of diffusive limitations exerted by permeable sample cells. The net rate of uranium release following oxidative UO(2) dissolution obtained from diffusion-limited batch experiments was ten times lower than that obtained for UO(2) dissolution with no permeable sample cells. The release rate of uranium to bulk solution from UO(2) contained in permeable sample cells under advective flow conditions was more than 100 times lower than that obtained from CSTR experiments without diffusive limitations. A 1-dimensional transport model was developed that could successfully simulate diffusion-limited release of U following oxidative UO(2) dissolution with the dominant rate-limiting process being the transport of U(VI) out of the cells. Scanning electron microscopy, X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS) characterization of the UO(2) solids recovered from batch experiments suggest that oxidative dissolution was more evident in the absence of diffusive limitations. Ca-EXAFS spectra indicate the presence of Ca in the reacted UO(2) solids with a coordination environment similar to that of a Ca-O-Si mineral. The findings from this study advance our overall understanding of the coupling of geochemical and transport processes that can lead to differences in dissolution rates measured in the field and in laboratory experiments., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and tested using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.

The dissolution of UO(2) in a continuously stirred tank reactor (CSTR) in the presence of Ca(2+) and Zn(2+) was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO(2) dissolution. The experiments were performed under anoxic and oxic conditions. Zn(2+) had a much greater inhibitory effect on UO(2) dissolution than did Ca(2+). This inhibition was most substantial under oxic conditions, where the experimental rate of UO(2) dissolution was 7 times lower in the presence of Ca(2+) and 1450 times lower in the presence of Zn(2+) than in water free of divalent cations. EXAFS and solution chemistry analyses of UO(2) solids recovered from a Ca experiment suggest that a Ca-U(VI) phase precipitated. The Zn carbonate hydrozincite [Zn(5)(CO(3))(2)(OH)(6)] or a structurally similar phase precipitated on the UO(2) solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO(2) surface, inhibiting the oxidative dissolution of UO(2). Interactions with divalent groundwater cations have implications for the longevity of UO(2) and the mobilization of U(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores.

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.