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Decontamination of metal surfaces contaminated with low levels of radionuclides is a major concern at Department of Energy facilities. The development of an environmentally friendly and cost-effective decontamination process requires an understanding of their association with the corroding surfaces. We investigated the association of uranium with the amorphous and crystalline forms of iron oxides commonly formed on corroding steel surfaces. Uranium was incorporated with the oxide by addition during the formation of ferrihydrite, goethite, green rust II, lepidocrocite, maghemite, and magnetite. X-ray diffraction confirmed the mineralogical form of the oxide. EXAFS analysis at the U L III edge showed that uranium was present in hexavalent form as a uranyl oxyhydroxide species with goethite, maghemite, and magnetite and as a bidentate innersphere complex with ferrihydrite and lepidocrocite. Iron was present in the ferric form with ferrihydrite, goethite, lepidocrocite, and maghemite; whereas with magnetite and green rust II, both ferrous and ferric forms were present with characteristic ferrous:total iron ratios of 0.65 and 0.73, respectively. In the presence of the uranyl ion, green rust II was converted to magnetite with concomitant reduction of uranium to its tetravalent form. The rate and extent of uranium dissolution in dilute HCl depended on its association with the oxide: uranium present as oxyhydroxide species underwent rapid dissolution followed by a slow dissolution of iron; whereas uranium present as an inner-sphere complex with iron resulted in concomitant dissolution of the uranium and iron.

Speciation of uranium in cultures of Clostridium sp. by X-ray absorption near-edge spectroscopy (XANES) at the National Synchrotron Light Source and by X-ray photoelectron spectroscopy (XPS) showed that U(VI) was reduced toU(IV). In addition toU(IV), a lower oxidation state of uranium, most probably U(III), was detected by XANES in the bacterial cultures. Reduction of uranium occurred only in the presence of growing or resting cells. Organic acid metabolites, the extracellular components of the culture medium, and heat-killed cells failed to reduce uranium under anaerobic conditions. The addition of uranyl acetate or uranyl nitrate (>210 µ U) to the culture medium retarded the growth of the bacteria as evidenced by an increase in the lag period before resumption of growth, by decreases in turbidity, and in the total production of gas and organic acid metabolites. These results show that uranium in wastes can be stabilized by the action of anaerobic bacteria.

Relatively recently, inorganic colloids have been invoked to reconcile the apparent contradictions between expectations based on classical dissolved-phase Pu transport and field observations of "enhanced" Pu mobility (Kersting et al. Nature 1999, 397, 56-59). A new paradigm for Pu transport is mobilization and transport via biologically produced ligands. This study for the first time reports a new finding of Pu being transported, at sub-pM concentrations, by a cutin-like natural substance containing siderophore-like moieties and virtually all mobile Pu. Most likely, Pu is complexed by chelating groups derived from siderophores that are covalently bound to a backbone of cutin-derived soil degradation products, thus revealing the history of initial exposure to Pu. Features such as amphiphilicity and small size make this macromolecule an ideal collector for actinides and other metals and a vector for their dispersal. Cross-linking to the hydrophobic domains (e.g., by polysaccharides) gives this macromolecule high mobility and a means of enhancing Pu transport. This finding provides a new mechanism for Pu transport through environmental systems that would not have been predicted by Pu transport models.

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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Biodegradation of metal-citrate complexes by Pseudomonas fluorescens depends on the nature of the complex formed between the metal and citric acid. Bidentate Fe(III)-, Ni-, and Zn-citrate complexes were readily biodegraded, but the tridentate Cd-and Cu-citrate, and U-citrate complexes were not. The biodegradation of Ni-and Zn-citrate commenced after an initial lag period; the former showed only partial (70%) degradation, whereas the latter was completely degraded. Uptake studies with 14 C-labeled citric acid and metal-citrate complexes showed that cells grown in medium containing citric acid transported free citric acid at the rate of 28 nmol min ؊1 and Fe(III)-citrate at the rate of 12.6 nmol min ؊1 but not Cd-, Cu-, Ni-, U-, and Zn-citrate complexes. However, cells grown in medium containing Ni-or Zn-citrate transported both Ni-and Zn-citrate, suggesting the involvement of a common, inducible transport factor. Cell extracts degraded Fe(III)-, Ni-, U-, and Zn-citrate complexes in the following order: citric acid ‫؍‬ Fe(III)-citrate > Ni-citrate ‫؍‬ Zn-citrate > U-citrate. The cell extract did not degrade Cd-or Cu-citrate complexes. These results show that the biodegradation of the U-citrate complex was limited by the lack of transport inside the cell and that the tridentate Cd-and Cu-citrate complexes were neither transported inside the cell nor metabolized by the bacterium.Citric acid is a multidentate ligand and forms stable complexes with various metal ions (19). The rate and extent of biodegradation of several metal-citrate complexes by microorganisms vary. For example, Pseudomonas pseudoalcaligenes degraded Mg-citrate at a much lower rate than Ca-, Fe(III)-, and Al(III)-citrate (29). Studies with a Klebsiella sp. showed that citric acid and Mg-citrate were readily degraded, whereas Cd-, Cu-, and Zn-citrate were resistant to biodegradation (9). Both these studies also showed that metal toxicity was not responsible for the lack of or the lower rate of degradation of certain metal-citrate complexes but gave no other explanation. Biodegradation studies with Pseudomonas fluorescens showed that bidentate complexes of Fe(III)-, Ni-, and Zn-citrate were readily biodegraded, whereas complexes that involve the hydroxyl group of citric acid, the tridentate Cd-and Cu-citrate complexes, and U-citrate complex were not biodegraded (17). No relationship between biodegradability and stability of the complexes was observed. The tridentate Fe(II)-citrate complex, although recalcitrant, was readily biodegraded after oxidation and hydrolysis to the bidentate Fe(III)-citrate form, denoting a structure-function relationship in the metabolism of the complex (16).Microorganisms have selective transport systems for the uptake of metals of known biological function (11,23,28). Heavy metals are transported into cells by preexisting transport systems and are subsequently either chemically neutralized, sequestered, or removed from the cells by rapid efflux (6,35). Metals affect the transport of citric acid in bacteria. For example, the...

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