Dissimilatory metal-reducing bacteria (DMRB) can utilize Fe(III) associated with aqueous complexes or solid phases, such as oxide and oxyhydroxide minerals, as a terminal electron acceptor coupled to the oxidation of H 2 or organic substrates. These bacteria are also capable of reducing other metal ions including Mn(IV), Cr(VI), and U(VI), a process that has a pronounced effect on their solubility and overall geochemical behavior. In spite of considerable study on an individual basis, the biogeochemical behavior of multiple metals subject to microbial reduction is poorly understood. To probe these complex processes, the reduction of U(VI) by the subsurface bacterium, Shewanella putrefaciens CN32, was investigated in the presence of goethite under conditions where the aqueous composition was controlled to vary U speciation and solubility. Uranium(VI), as the carbonate complexes UO 2 (CO 3) 3(aq) 4Ϫ and UO 2 (CO 3) 2(aq) 2Ϫ , was reduced by the bacteria to U(IV) with or without goethite [␣-FeOOH (s) ] present. Uranium(VI) in 1,4-piperazinediethhanesulfonic acid (PIPES) buffer that was estimated to be present predominantly as the U(VI) mineral metaschoepite [UO 3 ⅐ 2H 2 O (s) ], was also reduced by the bacteria with or without goethite. In contrast, only ϳ30% of the U(VI) associated with a synthetic metaschoepite was reduced by the organism in the presence of goethite with 1 mM lactate as the electron donor. This may have been due to the formation of a layer of UO 2(s) or Fe(OH) 3(s) on the surface of the metaschoepite that physically obstructed further bioreduction. Increasing the lactate to a non-limiting concentration (10 mM) increased the reduction of U(VI) from metaschoepite to greater than 80% indicating that the hypothesized surface-veneering effect was electron donor dependent. Uranium(VI) was also reduced by bacterially reduced anthraquinone-2,6-disulfonate (AQDS) in the absence of cells, and by Fe(II) sorbed to goethite in abiotic control experiments. In the absence of goethite, uraninite was a major product of direct microbial reduction and reduction by AH 2 DS. These results indicate that DMRB, via a combination of direct enzymatic or indirect mechanisms, can reduce U(VI) to insoluble U(IV) in the presence of solid Fe oxides.

To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separating or immobilizing 99 Tc in waste or groundwaters, the effects of electron donor and the presence of the bicarbonate ion on the rate and extent of pertechnetate ion [Tc(VII)O 4 ؊ ] enzymatic reduction by the subsurface metalreducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction products were evaluated through a combination of high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and thermodynamic calculations. When H 2 served as the electron donor, dissolved Tc(VII) was rapidly reduced to amorphous Tc(IV) hydrous oxide, which was largely associated with the cell in unbuffered 0.85% NaCl and with extracellular particulates (0.2 to 0.001 m) in bicarbonate buffer. Cell-associated Tc was present principally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(IV) hydrous oxide the dominant solid-phase reduction product, but in bicarbonate systems much less Tc(IV) was associated directly with the cell and solid-phase Tc(IV) carbonate may have been present. In the presence of carbonate, soluble (<0.001 m) electronegative, Tc(IV) carbonate complexes were also formed that exceeded Tc(VII)O 4 ؊ in electrophoretic mobility. Thermodynamic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of E h and pH conditions typical of natural waters. Thus, carbonate complexes may represent an important pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubility Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.Technetium (element 43) is present in the environment principally as a result of fallout from nuclear weapons testing, uranium enrichment, nuclear fuel processing, and disposal after pharmaceutical use (34). During nuclear fuel reprocessing, Tc is solubilized from spent fuels and is present in all waste streams (10) principally as the pertechnetate anion [Tc(VII)O 4 Ϫ ]. Over the pH and E h ranges typical of most groundwaters, Tc may exist in oxidation states VII, VI, V or IV, but in the absence of strong complexing agents Tc(VI) and Tc(V) may be expected to disproportionate to Tc(VII) and Tc(IV) (31). The dominant oxidation state under oxic conditions is Tc(VII), which is weakly sorbed by most soils and subsurface sediments at near neutral pH values (15,35). Under anoxic conditions and in the absence of aqueous complexing agents other than OH Ϫ , Tc(IV) is largely immobile because it forms concentration-limiting solid phases and strong surface complexes with hydroxylated surface sites on Al and Fe oxides and clays (5, 9, 24, 31). Organisms capable of anaerobic energy metabolism, including the dissimilatory metal-reducing bacteria (DMRB) Shewanella putrefaciens, Shewanella alga, ...

CH2M HILL Hanford Group, Inc. (CH2M HILL) is designing and assessing the performance of an integrated disposal facility (IDF) to receive low-level waste (LLW), mixed low-level waste (MLLW), immobilized low-activity waste (ILAW), and failed or decommissioned melters. The CH2M HILL project to assess the performance of this disposal facility is the Hanford IDF Performance Assessment (PA) activity. The goal of the Hanford IDF PA activity is to provide a reasonable expectation that the disposal of the waste is protective of the general public, groundwater resources, air resources, surfacewater resources, and inadvertent intruders. Achieving this goal will require predicting contaminant migration from the facilities. This migration is expected to occur primarily through the movement of water through the facilities and the consequent transport of dissolved contaminants through the vadose zone to groundwater, where contaminants may be reintroduced to receptors via drinking water wells or mixing in the Columbia River. 2005 IDF PA for the key contaminants of concern: Cr(VI), nitrate, 129 I, 79 Se, 99 Tc, and U(VI). This discussion provides the rationale for why some K d values have changed with time.

SummaryThe contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO

Pertechnetate ion [Tc(VII)O(4) (-)] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O(2) (<1 mg L(-1)) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 micromol (dry wt. sediment) g(-1)]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.