2009. "Newly recognized hosts for uranium in the Hanford Site vadose zone." Geochimica et Cosmochimica Acta 73(6):1563-1576. Abstract Uranium contaminated sediments from the U.S. Department of Energy’s Hanford Site have been investigated using electron microscopy. Six classes of solid hosts for uranium were identified. Preliminary sediment characterization was carried out using optical petrography, and electron microprobe analysis (EMPA) was used to locate materials that host uranium. All of the hosts are fine-grained and intergrown with other materials at spatial scales smaller than the analytical volume of the electron microprobe. A focused ion beam (FIB) was used to prepare electron-transparent specimens of each host for the transmission electron microscope (TEM). The hosts were identified as: 1) metatorbernite [Cu(UO2)2(PO4)2·8H2O]; 2) coatings comprised mainly of phyllosilicates on sediment clasts; 3) an amorphous zirconium (oxyhydr)oxide found in clast coatings; 4) amorphous and poorly crystalline materials that line voids within basalt lithic fragments; 5) amorphous palagonite surrounding fragments of basaltic glass; and 6) Fe- and Mnoxides. These findings demonstrate the effectiveness of combining EMPA, FIB, and TEM to identify solid-phase contaminant hosts. Furthermore, they highlight the complexity of U geochemistry in the Hanford vadose zone, and illustrate the importance of microscopic transport in controlling the fate of contaminant metals in the environment.
2009. "Uranium Speciation As a Function of Depth in Contaminated Hanford Sediments - A Micro-XRF, Micro-XRD, and Micro- And Bulk-XAFS Study ." Environmental Science & Technology 43(3):630-636. Abstract Processing ponds at the Hanford, Washington Area 300 site were used for storing basic sodium aluminate and acidic U(VI)-Cu(II)-containing waste from 1943 to 1975. One result of this use is a groundwater plume containing elevated levels of U and Cu beneath the dry ponds and adjacent to the Columbia River. We have used synchrotron-based micro-X-ray fluorescence (XRF) imaging, micro-X-ray absorption fine structure (XANES) spectroscopy, and micro-X-ray diffraction (XRD) techniques combined with bulk U LIII-edge X-ray absorption fine structure (XAFS) spectroscopy to determine the distribution and speciation of U and Cu through the vadose and groundwater zones beneath North Processing Pond #2 (NPP2). Sediment samples were collected from the vadose zone (8’ and 12’ depths), and a sample from the groundwater zone was collected just below the water table (12’-14’ depth). XRF imaging revealed two major U occurrences within the vadose and groundwater zones: (1) low to moderate concentrations of U(VI) associated with mineral surfaces (particularly chlorite), and (2) high concentration U(VI)-containing micron-sized particles associated with surface coatings on grains of muscovite and chlorite. These U(VI) hot spots are frequently spatially correlated with Cu(II) hot spots. In the groundwater zone, these particles were identified as the copper-uranyl-silicate cuprosklodowskite and the cupper-uranyl-phosphate metatorbernite. In contrast, the U-Cu-containing particles are X-ray amorphous in the vadose zone. Fits of U LIII-edge XAFS spectra by linear-combination fitting indicate that U speciation consists of ~ 75% uranyl sorbed to clays and ~25% metatorbernite-like X-ray amorphous U-Cu-phosphates (8’ depth); nearly 100% sorbed uranyl (12’ depth); and ~70% sorbed uranyl and ~30% cuprosklodowskite/metatorbernite (ground water zone). These findings suggest that the dissolution of U(VI)-Cu(II)-bearing solids as well as the desorption of U(VI), mainly from phyllosilicates, are important sources of U(VI) in the Area 300 groundwater plume.
2009. "The Roles of Outer Membrane Cytochromes of Shewanella and Geobacter in Extracellular Electron Transfer." Environmental Microbiology Reports 1(4):220-227. doi:10.1111/j.1758-2229.2009.00035.x Abstract As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] and manganese [Mn(IV)] (hydr)oxides, outer membrane cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Cell surface-exposed MtrC and OmcA can transfer electrons directly to the metal oxides. S. oneidensis MR-1 cells also secrete flavins that can facilitate ET to the oxides. The secreted flavins are thought to serve either as chelators that form soluble Fe(III)/Mn(IV)-flavin complexes or as electron shuttles that ferry the electrons from cell-associated ET proteins to the metal oxides. Cell-surface localization may also permit MtrC and OmcA to transfer electrons extracellularly to either flavin-chelated Fe(III)/Mn(IV) or oxidized flavins. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as the intermediates to relay electrons to type IV pili, which are then hypothesized to transfer electrons directly to the metal oxides. Thus, cell surface-localization positions these outer membrane cytochromes to transfer electrons to Fe(III)/Mn(IV) oxides external to the bacterial cells either directly, indirectly, or both, demonstrating a common strategy shared by Shewanella and Geobacter for extracellular reduction of the oxides.
2009. "Inhibition Effect of Secondary Phosphate Mineral Precipitation on Uranium Release from Contaminated Sediments." Environmental Science & Technology 43(21):8344-8349. doi:10.1021/es9021359 Abstract The inhibitory effect of phosphate mineral precipitation on uranium release was evaluated using a U(VI)-contaminated sediment collected from the US Department of Energy (DOE) Hanford site. The sediment contained U(VI) that was associated with diffusion-limited intragrain regions within its mm-size granitic lithic fragments. The sediment was first treated to promote phosphate mineral precipitation in batch suspensions spiked with 1 and 50 mM aqueous phosphate, and calcium in a stoichiometric ratio of mineral hydroxyapatite. The phosphate-treated sediment was then leached to solubilize contaminant U(VI) in a column system using a synthetic groundwater that contained chemical components representative of Hanford groundwater. Phosphate treatment significantly decreased the extent of U(VI) release from the sediment. Within the experimental duration of about 200 pore volumes, the effluent U(VI) concentrations were consistently lower by over one and two orders of magnitude after the sediment was treated with 1 and 50 mM of phosphate, respectively. Measurements of solid phase U(VI) using various spectroscopes and chemical extraction of the sediment collectively indicated that the inhibition of U(VI) release from the sediment was caused by: 1) U(VI) adsorption to the secondary phosphate precipitates and 2) the transformation of initially present U(VI) mineral phases to less soluble forms.
2009. "Mineralogical transformations controlling acid mine drainage chemistry." Chemical Geology 262(3-4):169-178. Abstract The role of Fe(III) minerals in controlling acid mine drainage (AMD) chemistry was studied using samples from two AMD sites [Gum Boot (GB) and Fridays-2 (FR)] located in northern Pennsylvania. Chemical extractions, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to identify and characterize Fe(III) phases. The mineralogical analysis revealed that schwertmannite and goethite were the principal Fe(III) phases in the sediments. Schwertmannite transformation occurred at the GB site where poorly-crystallized goethite rich in surface-bound sulfate was initially formed. In contrast, no schwertmannite transformation occurred at the FR site. The goethite in GB sediments had spherical morphology due to preservation of schwertmannite structure by adsorbed sulfate. Results of chemical extractions showed that poorly-crystallized goethite was subject to further crystallization accompanied by sulfate desorption. Changes in sulfate speciation preceded its desorption, with a conversion of bidentate- to monodentate-bound sulfate surface complexes. Laboratory sediment incubation experiments were conducted to evaluate the effect of mineral transformation on water chemistry. Incubation experiments were carried out with schwertmannite-containing sediments and AMD waters with different pH and chemical composition. The pH decreased to 1.9-2.2 in all suspensions and the concentrations of dissolved Fe and S increased significantly. Regardless of differences in the initial water composition, pH, Fe and S were similar in suspensions of the same sediment. XRD measurements revealed that schwertmannite transformed into goethite in GB and FR sediments during laboratory incubation. The incubation experiment demonstrated that schwertmannite transformation controlled AMD water chemistry during “closed system” laboratory contact.
2009. "Electron donor-dependent radionuclide reduction and nanoparticle formation by Anaeromyxobacter dehalogenans strain 2CP-C." Environmental Microbiology 11(2):534-543. Abstract Anaeromyxobacter dehalogenans strain 2CP-C can rapidly reduce U(VI) or Tc(VII) to U(IV)O2(s) or Tc(IV)O2(S) using either acetate or H2 as an electron donor source. Kinetic studies reveal that the H2-driven reduction of either U(VI) or Tc(VII) is faster than the acetate-driven reduction. The sub-cellular localization of reduced UO2 is extracellular while TcO2 nanoparticles are both periplasmic and extracellular. While electron donor-specific differences in UO2 nanoparticle aggregate size were observed, the association of UO2 nanoparticles with an exopolymeric substance (EPS) was observed and found to be independent of electron donor source. Electron donor-specific localization differences were not observed in cells incubated with Tc(VII). These finding have direct implications on immobilization strategies for highly soluble radionuclide contaminants in subsurface waters and the development of microbially assisted biostimulation efforts.
2009. "Microbial Reduction of Intragrain U(VI) in Contaminated Sediment." Environmental Science & Technology 43(13):4928-4933. doi:10.1021/es8029208 Abstract The accessibility of precipitated, intragrain U(VI) in a contaminated sediment to microbial reduction was investigated to ascertain geochemical and microscopic transport phenomena controlling U(VI) bioavailability. The sediment was collected from the US DOE Hanford site, and contained uranyl precipitates in a form of Na-boltwoodite within the mm-sized granitic lithic fragments in the sediment. Microbial reduction was investigated in a culture of a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1, in bicarbonate solutions at pH 6.8 buffered by PIPES. Measurements of uranium concentration, speciation, and valence in aqueous and solid phases indicated that microbial reduction of intragrain U(VI) proceeded by two mechanisms: 1) sequentially coupled dissolution of intragrain uranyl precipitates, diffusion of dissolved U(VI) out of intragrain regions, and microbial reduction of dissolved U(VI); and 2) U(VI) reduction in the intragrain regions by soluble, diffusible biogenic reductants. The bioreduction rate in the first pathway was over 3 orders of magnitude slower than that in comparable aqueous solutions containing aqueous U(VI) only. The slower bioreduction rate was attributed to: 1) the release of calcium from the desorption/dissolution of calcium-containing minerals in the sediment, which subsequently altered U(VI) aqueous speciation and slowed U(VI) bioreduction and 2) alternative electron transfer pathways that reduced U(VI) in the intragrain regions and changed its dissolution and solubility behavior. The results implied that the overall rate of microbial reduction of intragrain U(VI) will be influenced by the reactive mass transfer of U(VI) and biogenic reductants within intragrain regions, and geochemical reactions controlling major ion concentrations that affect U(VI) aqueous speciation and microbial activity.
2009. "Kinetics of Uranium(VI) Desorption from Contaminated Sediments: Effect of Geochemical Conditions and Model Evaluation ." Environmental Science & Technology 43(17):6560-6566. Abstract Stirred-flow cell experiments were performed to investigate the kinetics of uranyl [U(VI)] desorption from a contaminated sediment collected from the Hanford 300 Area at the US Department of Energy (DOE) Hanford Site, Washington. Three influent solutions of variable pH, Ca and carbonate concentrations that affected U(VI) aqueous and surface speciation were used under dynamic flow conditions to evaluate the effect of geochemical conditions on the rate of U(VI) desorption. The measured rate of U(VI) desorption varied with solution chemical composition that evolved as a result of thermodynamic and kinetic interactions between the influent solutions and sediment. The solution chemical composition that led to a lower equilibrium U(VI) sorption to the solid phase yielded a faster desorption rate. The experimental results were used to evaluate a multi-rate, surface complexation model (SCM) that has been proposed to describe U(VI) desorption kinetics in the Hanford sediment that contained complex sorbed U(VI) species in mass transfer limited domains. The model was modified and supplemented by including multi-rate, ion exchange reactions to describe the geochemical interactions between the solutions and sediment. With the same set of model parameters, the modified model reasonably well described the evolution of major ions and the rates of U(VI) desorption under variable geochemical and flow conditions, implying that the multi-rate SCM is an effective way to describe U(VI) desorption kinetics in subsurface sediments.
2009. "Reduction and long-term immobilization of technetium by Fe(II) associated with clay mineral nontronite ." Chemical Geology 264(1-4):127-138. doi:10.1016/j.chemgeo.2009.02.018 Abstract 99Tc is formed mostly during nuclear reactions and is released into the environment during weapons testing and inadvertent waste disposal. The long half-life, high environmental mobility (as Tc(VII)O4-) and its possible uptake into the food chain cause 99Tc to be a significant environmental contaminant. In this study, we evaluated the role of Fe(II) in biologically reduced clay mineral, nontronite (NAu-2), in reducing Tc(VII)O4- to poorly soluble Tc(IV) species as a function of pH and Fe(II) concentration. The rate of Tc(VII) reduction by Fe(II) in NAu-2 was higher at neutral pH (pH 7.0) than at acidic and basic pHs when Fe(II) concentration was low (< 1 mmol/g). The effect of pH, however, was insignificant at higher Fe(II) concentrations. The reduction of Tc(VII) by Fe(II) associated with NAu-2 was also studied in the presence of common subsurface oxidants including iron and manganese oxides, nitrate, and oxygen, to evaluate the effect of the oxidants on the enhancement and inhibition of Tc(VII) reduction, and reoxidation of Tc(IV). Addition of iron oxides (goethite and hematite) to the Tc(VII)-NAu-2 system, where Tc(VII) reduction was ongoing, enhanced reduction of Tc(VII), apparently as a result of re-distribution of reactive Fe(II) from NAu-2 to more reactive goethite/hematite surfaces. Addition of manganese oxides stopped further Tc(VII) reduction, and in case of K+-birnessite, it reoxidized previously reduced Tc(IV). Nitrate neither enhanced reduction of Tc(VII) nor promoted reoxidation of Tc(IV). Approximately 11% of Tc(IV) was oxidized by oxygen. The rate and extent of Tc(IV) reoxidation was found to strongly depend on the nature of the oxidants and concentration of Fe(II). When the same oxidants were added to aged Tc reduction products (mainly NAu-2 and TcO2•nH2O), the extent of Tc(IV) reoxidation decreased significantly relative to fresh Tc(IV) products. Increasing NAu-2 concentration also resulted in the decreased extent of Tc(IV) reoxidation. The results were attributed to the effect of NAu-2 aggregation that effectively retained Tc(IV) in the solid and decreased its vulnerability to reoxidation. Overall, our results implied that bioreduced clay minerals could play an important role in reducing Tc(VII) and in maintaining the long-term stability of reduced Tc(IV).
2009. "Oxidative Dissolution Potential of Biogenic and Abiogenic TcO2 in Subsurface Sediments." Geochimica et Cosmochimica Acta 73(8):962-976. doi:10.1016/j.gca.2009.01.027 Abstract Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Exhibiting an intermediate redox potential, Tc is highly mobile in its anionic, oxidized state [Tc(VII)O4-]; and less mobile as a poorly soluble oxyhydroxide precipitate [Tc(IV)O2•nH2O] in its reduced state. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state (FRC, RG). Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe-phyllosilicates of different types. Biogenic Tc(IV)O2•nH2O was oxidized in anoxic, but unreduced RG and FRC sediments through redox interaction with Mn(III/IV) oxides. Bioreduction by Shewanella putrefaciens CN32 dissolved Mn(III/IV) oxides and generated biogenic Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. Biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO2-like phase in both sediments. Redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular speciation to RG. X-ray microprobe, electron microprobe, x-ray absorption spectroscopy, and micro x-ray diffraction were applied to the whole sediment and isolated Tc-contained particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50-100 µm-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O2 into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)-Fe(III) molecular species.
2008. "Appendix G: Geochemistry." In The Resource Conservation and Recovery Act Facility Investigation (RFI) Report for Hanford Single-Shell Tank Waste Management Areas, DOE/ORP-2008-01 Revision 0, vol. Tier 2, ed. FM Mann. U.S. Department of Energy, Office of River Protection, Richland, WA. Abstract This appendix discusses the geology of the Hanford Site and singe-shell tank (SST) waste management areas (WMAs). The purpose is to provide the most recent geochemical information available for the SST WMAs and the Integrated Disposal Facility. This appendix summarizes the information in the geochemistry data package for the SST WMAs.
2008. "Uranium(VI) Release from Contaminated Vadose Zone Sediments: Estimation of Potential Contributions from Dissolution and Desorption." Chapter 14 in Adsorption of Metals to Geomedia II, ed. MO Barnett, DB Kent, pp. 375-416. Elsevier, Boston, MA. Abstract A key difficulty in developing accurate, science-based conceptual models for remediation of contaminated field sites is the proper accounting of multiple coupled geochemical and hydrologic processes. An example of such a difficulty is the separation of desorption and dissolution processes in releasing contaminants from sediments to groundwaters; very few studies are found in the literature that attempt to quantify contaminant release by these two processes. In this study, the results from several extraction techniques, isotopic exchange experiments, and published spectroscopic studies were combined to estimate the contributions of desorption and dissolution to U(VI) release from contaminated sediments collected from the vadose zone beneath former waste disposal ponds in the Hanford 300-Area (Washington state). Vertical profiles of sediments were collected at four locations from secondary pond surfaces down to, and slightly below, the water table. In three of the four profiles, uranium concentration gradients were observed in the sediments, with the highest U concentrations at the top of the profile. One of the vertical profiles contained sediments with U concentrations up to 4.2x10-7 mol/g (100 ppm). U(VI) release to artificial groundwater solutions and extracts from these high-U concentrations sediments occurred primarily from dissolution of precipitated U(VI) minerals, including the mineral metatorbernite, [Cu(UO2PO4)2⋅8H2O]. At the bottom of this profile, beneath the water table, and in all three of the other profiles, U concentrations were <5.88x10-8 mol/g (14 ppm), and U(VI) release to artificial groundwater solutions occurred primarily due to desorption of U(VI). When reacted in batch experiments with artificial groundwater solutions with compositions representative of the range of chemical conditions in the 2 underlying aquifer, all samples released U(VI) at concentrations greater than regulatory limits within a few hours. A semi-mechanistic surface complexation model was developed to describe U(VI) adsorption on sediments collected from near the water table, as a function of pH, alkalinity, and Ca and U(VI) concentrations, using ranges in these variables relevant to groundwater conditions in the aquifer. Dilute (bi)carbonate solution extractions and uranium isotopic exchange methods were capable of estimating adsorbed U(VI) in samples where U(VI) release was predominantly due to U(VI) desorption; these techniques were not effective at estimating adsorbed U(VI) where U(VI) release was affected by dissolution of U(VI) minerals. The combination of extraction and isotopic exchange results, spectroscopic studies, and surface complexation modeling allow an adequate understanding for the development of a geochemical conceptual model for U(VI) release to the aquifer. The overall approach has generic value for evaluating the potential for release of metals and radionuclides from sediments that contain both precipitated and adsorbed contaminant speciation.
2008. "Advective Desorption of Uranium (VI) from Contaminated Hanford Vadose Zone Sediments under Saturated and Unsaturated Conditions." Vadose Zone Journal 7(4):1144-1159. doi:10.2136/vzj2007.0166 Abstract Sedimentary, hydrologic, and geochemical variations in the Hanford subsurface environment, as well as compositional differences in contaminating waste streams, have created vast differences in the migration and mobility of uranium within the subsurface environment. A series of hydraulically-saturated and -unsaturated column experiments were performed to i.) assess the effect of water content on the advective desorption and migration of uranium from contaminated sediments, and ii.) evaluate the uranium concentration that can develop in porewater and/or groundwater as a result of desorption/dissolution reactions. Flow rate and moisture content were varied to evaluate the influence of contact time, pore water velocity, and macropore desaturation on aqueous uranium concentrations. Sediments were collected from the T-TX-TY tank farm complex and the 300 Area Process Ponds located on the Hanford Site, southeastern Washington State. The sediments vary in depth, mineralogy, and in contamination events. Experiments were conducted under mildly alkaline/calcareous conditions representative of conditions commonly encountered at repository sites across the arid western United States and, in particular, the Hanford site. Results illustrate the release of uranium from these sediments is kinetically controlled and low water contents encountered within the Hanford vadose zone result in the formation of mobile-immobile water regimes, which isolate a fraction of the reactive sites within the sediments, effectively reducing the concentration of uranium released into migrating porewaters.
2008. "Kinetics of Reduction of Fe(III) Complexes by Outer Membrane Cytochromes MtrC and OmcA of Shewanella oneidensis MR-1." Applied and Environmental Microbiology 74(21):6746-6755. doi:10.1128/AEM.01454-08 Abstract Shewanella Oneidensis MR-1 possesses up to 42 c-type cytochromes with heme content varying between 1 to as many as 37. Among them, the outer-membrane cytochromes, particularly MtrC and OmcA, are suspected to function as terminal reductases and are responsible for its enzymatic catalysis capability. So far, the mechanisms of metal reduction by these outer-membrane cytochromes are unknown. In this work, we report the study of reduction kinetics of a series of Fe(III) complexes with citrate, NTA and EDTA by abiotically reduced MtrC and OmcA using a stopped-flow technique in combination with theoretical computation methods within the framework of the electron transfer theory of Marcus and speciation calculations based on the current thermodynamic database. Stopped-flow kinetic data showed that the reaction was very fast and appeared to proceed in two stages, a fast stage that completes in much less than a second and a slower stage afterwards. For a given complex, the reaction is faster by reduction with MtrC than OmcA, while for a given protein, the reaction completes in the decreasing order of Fe-EDTA > Fe-NTA > Fe-citrate. All the stopped-flow kinetic curves could be modeled by two parallel second-order bimolecular redox reactions with second-order rate constants ranging from 0.872 µM-1s-1 for the fast reaction between MtrC with Fe-EDTA complex to 0.012 µM-1s-1 for the slow reaction between OmcA and Fe-citrate complex. Speciation calculations indicated that at both metal:ligand ratios, 1:1.5 and 1:10, a single dominant ferric complex was responsible for the observed reaction for each ligand and, therefore, the observed dual-reaction pathways was attributed to the differences in the reduction behavior among various heme groups within each protein. The results of redox potential calculations with known thermodynamic data show only small differences on the scale of a few millivolts among the three complexes, suggested that the observed differences in reaction rate cannot be explained by the overall redox reaction free energy. In contrast, reorganization energies () calculated based on DFT-COSMO model are substantially different between the complexes, with a larger reorganization energy and therefore a larger activation energy associated with the citrate complex, and progressively smaller ones for the NTA and EDTA complexes. In combination with approximate electronic coupling terms, the theoretical results show good agreement with the observed trend and implicate the reorganization energy as the key factor in the kinetic reaction.
2008. "A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate, and oxyhydroxide minerals." Radiochimica Acta 96(9-11):591-598. doi:10.1524/ract.2008.1541 Abstract In this work we have applied liquid-helium temperature (LHeT) time-resolved laser-induced fluorescence spectroscopy (TRLIF) to characterize a series of natural and synthetic minerals of uranium carbonate, phosphate and oxyhydroxides including rutherfordine, zellerite, liebigite, phosphuranylite, meta-autunite, meta-torbernite, uranyl phosphate, sodium-uranyl-phosphate, bequerelite, clarkeite, curite, schoepite and compregnacite, and compared their spectral characteristics among these minerals as well as our previously published data on uranyl silicates. For the carbonate minerals, the fluorescence spectra depend on the stoichiometry of the mineral. For the phosphate minerals the fluorescence spectra closely resemble each other despite the differences in their composition and structure. For all uranium oxyhydroxides, the fluorescence spectra are largely red-shifted as compared with those of the uranium carbonates and phosphates and their vibronic bands are broadened and less resolved. The much enhanced spectra resolution at LHeT allows more accurate calculation of the O=U=O symmetrical stretch frequency, ν1, corresponding to the average spacing of the vibronic peaks of the fluorescence spectra and the spectral origin as reflected by the position of the first vibronic band. It was found that both the average ν1 and λ1 values correlate well with the average basicity of the inorganic anion.
2008. " Direct Involvement of Type II Secretion System in Extracellular Translocation of Shewanella Oneidensis Outer Membrane Cytochromes MtrC and OmcA." Journal of Bacteriology 190(15):5512-5516. doi:10.1128/JB.00514-08 Abstract Outer membrane decaheme c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 are extracellular lipoproteins important for dissimilatory reduction of solid metal (hydr)oxides during anaerobic respiration. To investigate the roles of type II secretion system (T2S) in translocation of MtrC and OmcA across outer membrane, we measured the effects of deleting two T2S genes, gspD and gspG, on the secretion of MtrC and OmcA when cells were grown under anaerobic conditions. Deletion of gspD or gspG resulted in slightly yellowish supernatants, different from the pink supernatant of wild type (wt). Comparative proteomic analyses revealed that, although MtrC, OmcA and NrfA, a periplasmic nitrite reductase, were present the supernatants of wt and ΔgspD mutant, their peptides counts were much lower in ΔgspD than in wt. Subsequent analyses with heme-staining and Western blot not only confirmed that deletion of gspD or gspG reduced the abundances of MtrC and OmcA in the supernatants, but also revealed that the deletions consequently increased their abundances inside the cells. Complementation of ΔgspG mutant with functional GspG could reverse the effects of deleting gspG on the colors of the supernatants and the abundances of MtrC and OmcA. In contrast, Western results showed that the abundance of NrfA was reduced in the supernatant and the cells of ΔgspD mutant, suggesting that reduced NrfA in the periplasm, where MtrC and OmcA were accumulated, contributed to its reduction in the supernatant. Thus, our results demonstrate at the first time that T2S facilitates translocation of MtrC and OmcA across outer membrane.
2008. "Reduction of Tc(VII) by Fe(II) Sorbed on Al (hydr)oxides." Environmental Science & Technology 42(15):5499-5506. doi:10.1021/es8003156 Abstract Technetium speciation, solubility and sorption behavior is strongly dependent on its valence state. Under oxic conditions, Tc exists as the soluble, weakly-sorbing pertechnetate [TcO4-] anion. The reduced form of technetium, Tc(IV), is stable in anoxic environments and is sparingly soluble as TcO2·xH2O(s). Here we investigate the heterogeneous reduction of Tc(VII) by Fe(II) sorbed on Al (hydr)oxides [diaspore (α-AlOOH) and corundum (α-Al2O3)]. Experiments were performed to study the kinetics of Tc(VII) reduction, examine changes in Fe surface speciation during Tc(VII) reduction (Mössbauer spectroscopy), and identify the nature of Tc(IV)-containing reaction products (X-ray absorption spectroscopy). We found that Tc(VII) was completely reduced by adsorbed Fe(II) within 11d (diaspore suspension) and 4d (corundum suspension). Mössbauer measurements revealed that the Fe(II) signal became less intense with Tc(VII) reduction, and was accompanied by increase in Fe(III) doublet and magnetically-ordered Fe(III) sextet signals, with latter parameters close to those for hematite. Formation of magnetically ordered Fe(III) did not depend on the oxidant nature, as both Tc(VII) or O2 lead to the formation of a virtually identical hematite-like phase. The Fe(II) doublet displayed no differences in Mössbauer parameters before and after Tc(VII) reduction, likely due to Fe(II) adsorption to similar sites and no Fe(II) sorption to or precipitation within solid phases formed. Tc-EXAFS spectroscopy revealed that the final heterogeneous redox product on corundum was similar to Tc(IV) oxyhydroxide, TcO2·xH2O. The formation of precursor polymeric TcnOy (4n-2y)+ chains prior to TcO2⋅xH2O precipitation might explain the formation of the separate TcO2-like phase on corundum without coprecipitated Fe.
2008. "Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface." Geochimica et Cosmochimica Acta 72(6):1521-1539. doi:10.1016/j.gca.2008.01.004 Abstract Technetium-99 is a byproduct of uranium fission. It exists in two stable valence states (IV/VII) and exhibits a half-cell potential of intermediate value (Eo = 0.748 V). The oxidized form of 99Tc [pertechnetate, TcO4-] is an oxyanion that sorbs poorly to amphoteric surfaces and forms few solid phases with geochemically relevant cations. It is consequently highly mobile in oxic water-rock systems. The reduced valence state [Tc(IV)] is relatively insoluble (<10-8 mol L-1), and, hence immobile as an oxyhydroxide precipitate [TcO2•nH2O(s)]. In spite of favorable thermodynamics, Tc(VII) reacts slowly with Fe2+(aq) under anoxic conditions. Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) sorbed to hematite and goethite, and by structural Fe(II) in a dithionite-citrate-bicarbonate (DCB) reduced natural phyllosilicate mixture containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) sorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe2+(aq) adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by structural Fe(II) in the DCB-reduced phyllollsilicates, were not sensitive to pH or the concentration of ion-exchangeable Fe(II), and were orders of magnitude slower than observed for the Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mössbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by sorbed and structural Fe(II) in anoxic environments may be a very important geochemical process that will proceed at very different rates and that will yield different surface species depending subsurface pH and mineralogy.
2008. "Hydrogenase- and Outer Membrane c-Type Cytochrome-Facilitated Reduction of Technetium(VII) by Shewanella oneidensis MR-1." Environmental Microbiology 10(1):125-136. doi:10.1111/j.1462-2920.2007.01438.x Abstract Pertechnetate, 99Tc(VII)O4-, is a highly mobile radionuclide contaminant at U.S. Department of Energy sites that can be enzymatically reduced by a range of anaerobic and facultatively anaerobic microorganisms, including Shewanella oneidensis MR-1, to poorly soluble Tc(IV)O2(s). In other microorganisms, Tc(VII)O4- reduction is generally considered to be catalyzed by hydrogenase. Here, we provide evidence that although the NiFe hydrogenase of MR-1 was involved in the H2-driven reduction of Tc(VII)O4- (presumably through a direct coupling of H2 oxidation and Tc(VII) reduction), the deletion of both hydrogenase genes did not completely eliminate the ability of MR-1 to reduce Tc(VII). With lactate as the electron donor, mutants lacking the outer membrane c-type cytochromes MtrC and OmcA or the proteins required for the maturation of c-type cytochromes were defective in reducing Tc(VII) to nanoparticulate TcO2·nH2O(s) relative to MR-1 or a NiFe hydrogenase mutant. In addition, reduced MtrC and OmcA were oxidized by Tc(VII)O4-, confirming the capacity for direct electron transfer from these OMCs to TcO4-. c-Type cytochrome-catalyzed Tc(VII) reduction could be a potentially important mechanism in environments where organic electron donor concentrations are sufficient to allow this reaction to dominate.
2008. "Scale-dependent desorption of uranium from contaminated subsurface sediments." Water Resources Research 44:W08413. doi:10.1029/2007WR006478 Abstract Column experiments were performed to investigate the scale-dependent desorption of uranyl [U(VI)] from a contaminated sediment collected from the Hanford 300 Area at the US Department of Energy (DOE) Hanford Site, Washington. The sediment was a coarse-textured alluvial flood deposit containing significant mass percentage of river cobble. U(VI) was, however, only associated with its minor, fine-grained (< 2mm) mass fraction. U(VI) desorption was investigated both from the field-textured sediment using a large column (80 cm length by 15 cm inner diameter), and from its < 2mm, U(VI)-associated mass fraction using a small column (10 cm length by 3.4 cm inner diameter). Dynamic advection conditions with intermittent flow and stop-flow events of variable durations were employed to investigate U(VI) desorption kinetics and its scale dependence. A multi-component kinetic model that integrated a distributed rate expression with surface complexation reactions successfully described U(VI) release from the fine-grained, U(VI)-associated materials. The field-textured sediment in the large column displayed dual domain, tracer-dependent mass transfer properties that affected the breakthrough curves of bromide, pentafluorobenzoic acid (PFBA), and tritium. The tritium breakthrough curve showed stronger non-equilibrium behavior than did PFBA and bromide, and required a larger immobile porosity to describe. The dual domain mass transfer properties were then used to scale the kinetic model of U(VI) desorption developed for the fine-grained materials to describe U(VI) release and reactive transport in the field-textured sediment. Numerical simulations indicated that the kinetic model that was integrated with the dual domain properties determined from tracer PFBA and Br best described the experimental results. The kinetic model without consideration of the dual domain properties over-predicted effluent U(VI) concentrations, while the model based on tritium mass transfer under-predicted the rate of U(VI) release. Overall, our results indicated that the kinetics of U(VI) release from the field-textured sediment were different from that of its fine-grained, U(VI)-associated mass fraction. However, the desorption kinetics measured on the U(VI)-containing mass fraction could be scaled to describe U(VI) reactive transport in the contaminated field-textured sediment after proper consideration of the physical transport properties of the sediment. The research also demonstrated a modeling approach to integrate geochemical processes into field scale reactive transport models.
2008. "Advective Removal of Intraparticle Uranium from Contaminated Vadose Zone Sediments, Hanford, USA." Environmental Science and Technology 42(5):1565-1571. doi:10.1021/es071113m Abstract A column study on U contaminated vadose zone sediments from the Hanford Site, WA, was performed in order to aid the development of a model for predicting U(VI) release rates under a dynamic flow regime and for variable geochemical conditions. The sediments of interest are adjacent to and below tank BX-102, part of the BX tank farm that contained high level liquid radioactive waste. Two sediments, with different U(VI) loadings and intraparticle large fracture vs. smaller fracture ratios, were reacted with three different solutions. The primary reservoir for U(VI) appears to be a micron-sized nanocrystalline Na-U-Si phase, possibly Na-boltwoodite, that nucleated and grew on plagioclase grains that line fractures within sand-sized granitic clasts. The solutions were all calcite saturated and in equilibrium with atmospheric CO2, where one solution was simply DI-water, the second was a synthetic ground water (SGW) with elevated Na, and the third was the same SGW but with both elevated Na and Si. The latter two solutions were employed, in part, to test the effect of saturation state on U(VI) release. For both sediments and all three electrolytes, there was an initial rapid release of U(VI) to the advecting solution followed by a plateau of low U(VI) concentration. U(VI) effluent concentration increased during subsequent stop flow (SF) events. The electrolytes with elevated Na and Si appreciably depressed U(VI) concentrations relative to DI water. The effluent data for both sediments and all three electrolytes was simulated reasonably well by a three domain model (the advecting fluid, fractures, and matrix) that coupled U(VI) dissolution rates, intraparticle U(VI) diffusion, and interparticle advective transport of U(VI); where key transport and dissolution processes had been parameterized in previous batch studies. For the calcite-saturated DI-water, U(VI) concentrations in the effluent remained far below saturation with respect to Na-boltwoodite and release of U(VI) to the advecting domain was limited, in large part, by intraparticle diffusion of U(VI). In contrast, for the electrolytes with elevated Na and Si, the release of U(VI) to the advecting domain was shown to be limited by the solubility of Na-boltwoodite. The fact that one model, with the same key parameters, is able to simulate such a diverse geochemical and hydrodynamic conditions indicates that the approach is sound and that the model is robust.
2008. "Electron Transfer at the Microbe-Mineral Interface: A Grand Challenge in Biogeochemistry." Geobiology 6(3):245-253. Abstract The interplay between microorganisms and minerals is a complex and dynamic process that has sculpted the geosphere for nearly the entire history of the Earth. The work of Dr. Terry Beveridge and colleagues provided some of the first insights into metal-microbe and mineral-microbe interactions and established a foundation for subsequent detailed investigations of interactions between microorganisms and minerals. Beveridge also envisioned that interdisciplinary approaches and teams would be required to explain how individual microbial cells interact with their immediate environment at nano- or sub-molecular scales and that through such approaches and using emerging technologies that the details of such interactions would be revealed at the molecular level. With this vision as incentive and inspiration, a multidisciplinary, collaborative team-based investigation was initiated to probe the process of electron transfer at the microbe-mineral interface. This grand challenge to this team was to address the hypothesis that multi-heme c-type cytochromes of dissimilatory metal reducing bacteria localized to the cell exterior function as the terminal reductases in electron transfer to Fe(III) and Mn(IV) oxides. This question has been the subject of extensive investigation for years yet the answer has remained elusive. The team involves an integrated group of experimental and computational capabilities at DOE’s Environmental Molecular Sciences Laboratory, a national scientific user facility, as the collaborative focal point. The approach involves a combination of in vitro and in vivo biologic and biogeochemical experiments and computational analyses that, when integrated, provide a conceptual model of the electron transfer process. The resulting conceptual model will be evaluated by integrating and comparing various experimental, i.e., in vitro and in vivo ET kinetics, and theoretical results. Collectively the grand challenge will provide a detailed view of how organisms engage with mineral surfaces to exchange energy and electron density as required for life function.
2008. "The role of multihaem cytochromes in the respiration of nitrite in Escherichia coli and Fe(III) in Shewanella oneidensis." Biochemical Society: Transactions 36(Pt. 5):1005-1010. doi:10.1042/BST0361005 Abstract The periplasmic nitrite reductase system from Escherichia coli and the extracellular Fe(III) reductase system from Shewanella oneidensis contain multihaem c-type cytochromes as electron carriers and terminal reductases. The position and orientation of the haem cofactors in multihaem cytochromes from different bacteria often show significant conservation despite different arrangements of the polypeptide chain. We propose that the decahaem cytochromes of the iron reductase system MtrA, MtrC and OmcA comprise pentahaem ‘modules’ similar to the electron donor protein, NrfB, from E. coli. To demonstrate this, we have isolated and characterized the N-terminal pentahaem module of MtrA by preparing a truncated form containing five covalently attached haems. UV–visible spectroscopy indicated that all five haems were low-spin, consistent with the presence of bis-His ligand co-ordination as found in full-length MtrA.
2007. "Anaerobic Microbial-Mineral Processes with Fe(III) Oxides: Experimental Considerations and Approaches." Chapter 4 in Methods for Investigating Microbial-Mineral Interactions: CMS Workshop Lectures, vol. 14, ed. Patricia A. Maurice and Leslie A. Warren, pp. 117-165. The Clay Minerals Society, West Lafayette, IN. Abstract The biogeochemical cycle of Fe is a one of the dominant redox cycles operative in surface waters and sediments, soils and vadose zones, and groundwater systems. In this cycle which is pronounced at oxic-anoxic boundaries, Fe compounds and microorganisms couple to mediate the oxidation of organic matter by molecular oxygen. The cycle includes: i.) the reductive dissolution of Fe(III) oxides by biogenic organic acids and organic matter oxidizing-metal reducing bacteria yielding Fe2+(aq) and ferrous containing minerals as products, and ii.) the oxidation of dissolved and solid-phase ferrous iron by molecular oxygen and microaerophilic Fe(II)-oxidizing bacteria with subsequent precipitation of poorly crystalline Fe(III) oxides (e.g., ferrihydrite). These Fe(III) oxides; that may recrystallize slowly with time to goethite, hematite, and lepidocrocite; represent a primary redox buffering agent (in terms of electron equivalents) in soils, sediments, and subsurface materials. Manganese (III/IV) oxides are also important in this regard. Because of the high surface area and surface chemical reactivity of Fe(III) oxides and Fe(II) containing mineral phases, the Fe biogeochemical cycle is closely linked to those of the trace metals, phosphorous, and various organic and inorganic anthropogenic contaminants.
2007. "Reduction of Pertechnetate [Tc(VII)] by Aqueous Fe(II) and the Nature of Solid Phase Redox Products." Geochimica et Cosmochimica Acta 71(9):2137-2157. doi:doi:10.1016/j.gca.2006.10.025 Abstract The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is strongly dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 10-9atm O2) in the absence of atmospheric or aqueous carbonate. The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Tc(IV)/Fe form was observed when 11 µmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. The heterogeneous reaction was more rapid, but not quantified. The kinetics of homogeneous reduction were slow at pH 7, but increased dramatically at pH 8, and correlated with the concentration of Fe(OH)+ and Fe(OH)2o(aq). Wet chemical and Fe-x-ray absorption near edge spectroscopy measurements (XANES) indicated that both Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox reaction products. 57Fe- Mössbauer, extended x-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements on the solid phase redox products that contained 13-15% Tc indicated that they were poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with a Tc2O10 octahedral dimer bound in bidentate edge-sharing mode to an octahedral FeO6 surface or vacancy site in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2•nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2•nH2O(s) as a result of its intraparticle/intragrain residence. It is suggested that precipitates of this nature may form in anoxic sediments or groundwaters, and that the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.
2007. "Geochemical Processes Controlling Migration of High Level Wastes in Hanford's Vadose Zone." Vadose Zone Journal 6(4):985-1003. doi:10.2136/vzj2006.0180 Abstract High level nuclear wastes (HLW) from Hanford’s plutonium reprocessing are stored in massive, buried, single-shell tanks in eighteen tank farms. The wastes were initially hot because of radioactive decay, and many exhibited extreme chemical character in terms of pH, salinity, and radionuclide concentration. At present, 67 of the 149 single shell tanks are suspected to have released over 1.9 million L of tank waste to the vadose zone, with most leak events occurring between 1950 and 1975. Boreholes have been placed through the largest vadose zone plumes to define the extent of contaminant migration, and to develop conceptual models of processes governing the transformation, retardation, and overall transport of tank waste residuals. Laboratory studies with sediments so collected have shown that ion exchange, precipitation and dissolution, and surface complexation reactions have occurred between the HLW and subsurface sediments moderating their chemical character, and retarding the migration of select contaminants. Processes suspected to facilitate the far-field migration of immobile radionuclides including stable aqueous complex formation and mobile colloids were found to be potentially operative, but unlikely to occur in the field, with the exception of cyanide-facilitated migration of 60Co. Fission product oxyanions are the most mobile of tank waste constituents because their adsorption is suppressed by large concentrations of waste anions; the vadose zone clay fraction is negative in surface charge; and, unlike Cr, their reduced forms are unstable in oxidizing environments. Reaction/process-based transport modeling is beginning to be used for predictions of future contaminant mobility and plume evolution.
2007. "Respiration of metal (hydr)oxides by Shewanella and Geobacter: a key role for multihaem c-type cytochromes." Molecular Microbiology 65(1):12-20. doi:10.1111/j.1365-2958.2007.05783.x Abstract Dissimilatory reduction of metal (e.g. Fe, Mn) (hydr)oxides represents a challenge for microorganisms, as their cell envelopes are impermeable to metal (hydr)oxides that are poorly soluble in water. To overcome this physical barrier, the Gram-negative bacteria Shewanella oneidensis MR-1 and Geobactersulfurreducens have developed electron transfer (ET) strategies that require multihaem c-type cytochromes (c-Cyts). In S. oneidensis MR-1, multihaem c-Cyts CymA and MtrA are believed to transfer electrons from the inner membrane quinone/quinol pool through the periplasm to the outer membrane. The type II secretion system of S. oneidensis MR-1 has been implicated in the reduction of metal (hydr)oxides, most likely by translocating decahaem c-Cyts MtrC and OmcA across outer membrane to the surface of bacterial cells where they form a protein complex. The extracellular MtrC and OmcA can directly reduce solid metal (hydr)oxides. Likewise, outer membrane multihaem c-Cyts OmcE and OmcS of G. sulfurreducens are suggested to transfer electrons from outer membrane to type IV pili that are hypothesized to relay the electrons to solid metal (hydr)oxides. Thus, multihaem c-Cyts play critical roles in S. oneidensis MR-1-and G. sulfurreducens-mediated dissimilatory reduction of solid metal (hydr)oxides by facilitating ET across the bacterial cell envelope.
2007. "Geochemical Controls on Contaminant Uranium in Vadose Hanford Formation Sediments at the 200 Area and 300 Area, Hanford Site, Washington." Vadose Zone Journal 6(4):1004-1017. doi:10.2136/vzj2006.0184 Abstract differences in uranium contributed by contaminated vadose sediments at two locations was investigated. At the BX tank farms, alkaline waste was accidentally released to a thick vadose zone. At the 300 Area, waste of variable acidity was released by unintended infiltration through the base of settling ponds. The waste form at the BX site was devoid of dissolved silica, and reacted with fluids trapped in microfractures to precipitate uranyl silicates. These secondary deposits were isolated physically from the vadose pore space and are not readily leached into pore fluids. At the 300 Area, the aluminum-rich waste precipitated on the surfaces of sediment clasts, forming a microporous reservoir of solid-phase uranium. Interaction of this coating with water in transit through the vadose zone provides a persistent source of dissolved uranium to groundwater.
2007. "Cation exchange reactions controlling desorption of Sr-90(2+) from coarse-grained contaminated sediments at the Hanford site, Washington." Geochimica et Cosmochimica Acta 71(2):305-325. Abstract Nuclear waste that bore 90Sr2+ was accidentally leaked into the vadose zone at the Hanford site, and was immobilized at relatively shallow depths in sediments containing little apparent clay or silt-sized components. Sr2+, 90Sr2+, Mg2+, and Ca2+ was desorbed and total inorganic carbon concentration was monitored during the equilibration of this sediment with varying concentrations of Na+, Ca2+. A cation exchange model previously developed for similar sediments was applied to these results as a predictor of final solution compositions. The model included binary exchange reactions for the four operant cations and an equilibrium dissolution/precipitation reaction for calcite. The model successfully predicted the desorption data. The contaminated sediment was also examined using digital autoradiography, a sensitive tool for imaging the distribution of radioactivity. The exchanger phase containing 90Sr was found to consist of smectite formed from weathering of mesostasis glass in basaltic lithic fragments. These clasts are a significant component of Hanford formation sands. The relatively small but significant cation exchange capacity of these sediments was thus a consequence of reaction with physically sequestered clays in sediment that contained essentially no fine-grained material. The nature of this exchange component explained the relatively slow (scale of days) evolution of desorption solutions. The experimental and model results indicated that there is little risk of migration of 90Sr2+ to the water table.
2007. "Kinetics of Reductive Dissolution of Hematite by Bioreduced Anthraquinone-2,6-disulfate." Environmental Science & Technology 41(22):7730-7735. doi:10.1021/es070768k Abstract The reductive dissolution of hematite (-Fe2O3) was investigated in a flow-through system using AH2DS, a reduced form of anthraquinone- 2,6 disulfonate (AQDS), which is often used as an electron shuttling compound in studies of dissimilatory microbial reduction of iron oxides. Influent flow-rate, pH, Fe(II) and phosphate concentrations were varied to investigate the redox reaction kinetics. The effluent AH2DS, AQDS, and Fe(II) concentrations changed significantly within the first half hour of AH2DS reaction with hematite and then gradually evolved toward steady-state. The steady-state rates decreased with increasing pH from 4.5 to 7.6 and decreased with decreasing flow-rate. The rates also decreased with increasing influent concentration of Fe(II) or phosphate that formed surface complexes at the experimental pH. Mineral surface properties, Fe(II) complexation reactions, and AQDS sorption on hematite surfaces were independently investigated for interpreting hematite reductive dissolution kinetics. AH2DS sorption to hematite was inferred from the parallel measurements of AQDS and AH2DS sorption to -Al2O3, a redox stable analog of -Fe2O3. Decreasing Fe(II) and increasing AH2DS sorption by controlling flow residence time, influent pH, Fe(II) and phosphate concentrations increased the rates of reductive dissolution. The rates were also affected by the redox reaction free energy when reductive dissolution approached equilibrium, as shown by the effect of increasing the influent concentration of Fe(II).
2007. "Influence of Calcium on Microbial Reduction of Solid Phase Uranium (VI)." Biotechnology and Bioengineering 97(6):1415-1422. doi:10.1002/bit.21357 Abstract The effect of calcium on microbial reduction of a solid phase U(VI), sodium boltwoodite (NaUO2SiO3OH ∙1.5H2O), was evaluated in a culture of a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1. Batch experiments were performed in a non-growth bicarbonate medium with lactate as electron donor at pH 7 buffered with PIPES. Calcium increased both the rate and extent of Na-boltwoodite dissolution by increasing its solubility through the formation of a ternary aqueous calcium-uranyl-carbonate species. The ternary species, however, decreased the rates of microbial reduction of aqueous U(VI). Laser-induced fluorescence spectroscopy (LIFS) and transmission electron microscopy (TEM) revealed that microbial reduction of solid phase U(VI) is a sequentially coupled process of Na-boltwoodite dissolution, U(VI) aqueous speciation, and microbial reduction of dissolved U(VI) to U(IV) that accumulated on bacterial surfaces/periplasm. The overall rates of microbial reduction of solid phase U(VI) can be described by the coupled rates of dissolution and microbial reduction that were both influenced by calcium. The results demonstrated that dissolved U(VI) concentration during microbial reduction was a complex function of solid phase U(VI) dissolution kinetics, aqueous U(VI) speciation, and microbial activity.
2007. "Biostimulation of Iron Reduction and Subsequent Oxidation of Sediment Containing Fe-silicates and Fe-oxides: Effect of Redox Cycling on Fe(III) Bioreduction." Water Research 41(13):2996-3004. doi:10.1016/j.watres.2007.03.019 Abstract Microbial reduction of iron has been shown to be important in the transformation and remediation of contaminated sediments. Re-oxidation of microbially reduced iron may occur in sediments that experience oxidation-reduction cycling and can thus impact the extent of contaminant remediation. The purpose of this research was to quantify iron oxidation in a flow-through column filled with biologically-reduced sediment and to compare the iron phases in the re-oxidized sediment to both the pristine and biologically-reduced sediment. The sediment contained both Fe(III)-oxides (primarily goethite) and silicate Fe (illite/vermiculite) and was biologically reduced in phosphate buffered (PB) medium during a 497 day column experiment with acetate supplied as the electron donor. Long-term iron reduction resulted in partial reduction of silicate Fe(III) without any goethite reduction, based on Mössbauer spectroscopy measurements. This reduced sediment was treated with an oxygenated PB solution in a flow-through column resulting in the oxidation of 38% of the biogenic Fe(II). Additional batch experiments showed that the Fe(III) in the oxidized sediment was more quickly reduced compared to the pristine sediment, indicating that oxidation of the sediment not only regenerated Fe(III) but also enhanced iron reduction compared to the pristine sediment. Oxidation-reduction cycling may be a viable method to extend iron-reducing conditions during in-situ bioremediation.
2007. "Characterization of Shewanella oneidensis MtrC: a cell-surface decaheme cytochrome involved in respiratory electron transport to extracellular electron acceptors." Journal of Biological Inorganic Chemistry 12(7):1083-1094. doi:10.1007/s00775-007-0278-y Abstract Abstract MtrC is a decaheme c-type cytochrome associated with the outer cell membrane of Fe(III)-respiring species of the Shewanella genus. It is proposed to play a role in anaerobic respiration by mediating electron transfer to extracellular mineral oxides that can serve as terminal electron acceptors. The present work presents the first spectropotentiometric and voltammetric characterization of MtrC, using protein purified from Shewanella oneidensis MR-1. Potentiometric titrations, monitored by UV–vis absorption and electron paramagnetic resonance (EPR) spectroscopy, reveal that the hemes within MtrC titrate over a broad potential range spanning between approximately +100 and approximately *500 mV (vs. the standard hydrogen electrode). Across this potential window the UV– vis absorption spectra are characteristic of low-spin c-type hemes and the EPR spectra reveal broad, complex features that suggest the presence of magnetically spin-coupled lowspin c-hemes. Non-catalytic protein film voltammetry of MtrC demonstrates reversible electrochemistry over a potential window similar to that disclosed spectroscopically. The voltammetry also allows definition of kinetic properties of MtrC in direct electron exchange with a solid electrode surface and during reduction of a model Fe(III) substrate. Taken together, the data provide quantitative information on the potential domain in which MtrC can operate.
2007. "Hanford Site Vadose-Zone Studies: An Overview." Vadose Zone Journal 6(4):899-905. doi:10.2136/vzj2006.0179 Abstract Large quantities of radioactive and chemical wastes, created from plutonium production for nuclear weapons, are located in a remote desert setting at the U. S. Department of Energy’s Hanford Site, north of Richland, Washington, USA. Much of the waste currently resides in the vadose zone. At Hanford, the vadose zone is characterized by glacial-fluvial sediments that are often highly stratified. The extremely heterogeneous sediments give rise to complex subsurface-flow paths that contribute to uncertainty of contaminant fate and transport. Research efforts have focused on answering questions of contaminant transport from the viewpoint of geologic, biologic, geochemical and hydrologic controls. This special section of the Vadose Zone Journal highlights key research topics that are systematically addressing vadose zone problems at the Hanford Site. Research to date indicates that some of the contaminant species (e.g., Cs-137, Co-60, Sr-90, uranium, etc.) are highly reactive with Hanford sediments, as predicted by geochemical considerations, rendering them effectively immobile, except under extremely saline or acidic conditions, while other species (e.g., Tc-99, I-129, H-3) are typically mobile and have moved deep into the vadose zone and subsequently into groundwater. In addition, large quantities of organics, including carbon tetrachloride, have moved in complex ways as both vapor and liquid, and have reached the water table at some locations where they represent a potential long-term threat to groundwater. Observed transport of mobile species is linked to liquid discharges and to elevated recharge rates that occur primarily at waste sites where land surfaces are void of vegetation and where winter rains have subsequently penetrated the subsurface wastes. A series of papers in this special issue document progress to date in understanding transport rates at Hanford, why anisotropy strongly affects the distribution of subsurface contaminants, why organic contaminants such as DNAPLs (e.g., carbon tetrachloride) are so difficult to find in the deep vadose zone, and what are the impacts of hypersaline fluids on waste form degradation and subsequent transport.
2007. "Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site." Environmental Science & Technology 41(13):4633-4639. Abstract Uranium(U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300-Area of the Hanford site (WA, USA) was investigated using multi-scale techniques. In 30-day batch experiments, only a small fraction of total U (~7.4%) was released to artificial groundwater solutions equilibrated with 1% pCO2. Synchrotron-based micro X-ray fluorescence spectroscopy (XRF) analyses showed that U was distributed among at least two types of species: 1) U discrete grains associated with Cu, and 2) areas with intermediate U concentrations on grains and grain coatings. Metatorbernite (Cu[UO2]2[PO4]2•8H2O) and uranophane (Ca[UO2]2[SiO3(OH)]2•5H2O) at some U discrete grains, and muscovite at U intermediate concentration areas were identified in synchrotron-based micro X-ray diffraction (XRD). SEM/EDS analyses revealed 8-10 µm size metatorbernite particles that were embedded in C-, Al-, and Si-rich coatings on quartz and albite grains. In - and bulk-X-ray Absorption Structure (XAS and XAS) spectroscopy analyses, the structure of metatorbernite with additional U-C and U-U coordination environments were consistently observed at U discrete grains with high U concentrations. The consistency of the - and bulk-XAS analyses suggests that metatorbernite may comprise a significant fraction of the total U in the sample. The entrapped, micron-sized metatorbernite particles in C, Al, and Si rich coatings, along with the more soluble precipitated uranyl carbonates and uranophane, likely control the long-term release of U to water associated with the vadose zone sediments.
2006. "High-Affinity Binding and Direct Electron Transfer to Solid Metals by the Shewanella oneidensis MR-1 Outer Membrane c-type Cytochrome OmcA." Journal of the American Chemical Society 128(43):13978-13979. doi:10.1021/ja063526d Abstract The identification of electron transfer proteins that couple soluble redox carriers to electrode surfaces has great potential in permitting the development of scalable bioreactors. In this respect, the 85 kDa outer membrane decaheme cytochrome OmcA (SO1779) from Shewanella oneidensis MR-1 can reduce soluble Fe(III) chelates, and has previously been suggested to function in concert with other membrane proteins as one of the terminal electron donors in the metal reductase protein complex of Shewanella oneidensis MR-1.1 Shewanella is a facultative anaerobe that can reduce a range of different metal oxides, including iron [Fe(III)], manganese [Mn(III/IV)], chromium [Cr(IV)] and uranium [U(VI)]2, and whose metabolic diversity has considerable promise for both the bioremediation of organic and metal contaminants as well as in the design of microbial fuel cells3. Biofuel cells offer a potential means to couple the breakdown of bio-wastes to generate power4. Miniaturization of these fuel cells is dependent on the elimination of the currently necessary membrane between cathode and anode compartments.5 It has been demonstrated that the immobilization of redox active proteins, such as glucose oxidase, on electrodes with redox active polymer coatings renders the membrane unnecessary.6 The identification of a purified metal-reducing enzyme able to densely bind and directly donate electrons to iron-oxide coated electrodes (commonly used to increase electron transfer efficiencies7) has great potential to contribute to fuel cell design. To identify the terminal electron donors in the S. oneidensis metal reductase system, and to explore whether isolated proteins can directly bind and mediate electron transfer reactions to reduce solid metals, we have purified OmcA and measured its ability to bind and transfer electrons to solid Fe2O3 in the mineral hematite.
2006. "Isolation of a High-Affinity Functional Protein Complex between OmcA and MtrC: Two Outer Membrane Decaheme c-type Cytochromes of Shewanella oneidensis MR-1 ." Journal of Bacteriology 188(13):4705-4714. doi:10.1128/JB.01966-05 Abstract SUMMARY Shewanella oneidensis MR-1 is a facultatively anaerobic bacterium that is capable of using insoluble oxidized metals, such as manganese [Mn(III, IV)] and iron [Fe(III)] oxides and oxyhydroxides, as terminal electron acceptors during anaerobic respiration. The ability of S. oneidensis MR-1 to reduce oxidized Mn and/or Fe has previously been linked to OmcA and MtrC: two decaheme c-type cytochromes that are localized to the outer membrane. To investigate how the electron transport proteins OmcA and MtrC are organized, we expressed and purified recombinant OmcA and MtrC from wild type S. oneidensis MR-1 as well as a mutant that lacked OmcA and MtrC (ΔomcA/mtrC). After purification to the nearly electrophoretic homogeneity from the ΔomcA/mtrC mutant, the recombinant OmcA and MtrC exhibited the characteristics of c-type cytochromes, and each of their polypeptides was confirmed to contain 10 hemes. When purified from wild type cells, endogenous MtrC or OmcA was always co-purified with recombinant OmcA or MtrC, respectively. Fluorescence polarization experiment showed that recombinant OmcA bound to the FlAsH-labeled MtrC with a dissociation constant of 7 ×10-7 M. The purified recombinant OmcA or MtrC alone displayed intrinsic ferric reductase activity with NADH used as an electron donor. Ferric reductase specific activity increased by 35 to 41% when nearly equimolar concentrations of OmcA and MtrC were assayed relative to the two proteins assayed independently. These results demonstrate that OmcA and MtrC directly interact with each other to form a stable complex with high ferric reductase activity.
2006. "Microscale Controls on the Fate of Contaminant Uranium in the Vadose Zone, Hanford Site, Washington." Geochimica et Cosmochimica Acta 70(8):1873-1887 . doi:10.1016/j.gca.2005.10.037 Abstract An alkaline brine containing uranyl (UO22+) leaked to the thick unsaturated zone at the Hanford Site. X-ray and electron microprobe imaging showed that the uranium was associated with a minority of clasts, specifically granitic clasts occupying less than four percent of the sediment volume. XANES analysis at micron resolution showed the uranium to be hexavalent. The uranium was precipitated in microfractures as radiating clusters of uranyl silicates, and sorbed uranium was not observed on other surfaces. Compositional determinations of the 1-3 µm precipitates were difficult, but indicated a sodium potassium uranyl silicate, likely sodium boltwoodite. Observations suggested that uranyl was removed from pore waters by diffusion and precipitation in microfractures, where dissolved silica within the granite-equilibrated solution would cause supersaturation with respect to sodium boltwoodite. This hypothesis was tested using a diffusion reaction model operating at microscale. Conditions favoring precipitation were simulated to be transient, and driven by the compositional contrast between pore and fracture space. Pore-space conditions, including alkaline pH, were eventually imposed on the microfracture environment. However, conditions favoring precipitation were prolonged within the microfracture by reaction at the silicate mineral surface to buffer pH in a solubility limiting acidic state, and to replenish dissolved silica. During this time, uranyl was additionally removed to the fracture space by diffusion from pore space. Uranyl is effectively immobilized within the microfracture environment within the presently unsaturated vadose zone.
2006. "c-Type Cytochrome-Dependent Formation of U(IV) Nanoparticles by Shewanella oneidensis ." PloS Biology 4(8):1324-1333. Abstract Modern approaches for bioremediation of radionuclide contaminated environments are based on the ability of microorganisms to effectively catalyze changes in the oxidation states of metals that in turn influence their solubility. Although microbial metal reduction has been identified as an effective means for immobilizing highly-soluble uranium(VI) complexes in situ, the biomolecular mechanisms of U(VI) reduction are not well understood. Here, we show that c-type cytochromes of a dissimilatory metal reducing bacterium, Shewanella oneidensis MR-1 are essential for the reduction of U(VI) and formation of extracelluar UO2 nanoparticles. In particular, the outer membrane (OM) decaheme cytochrome MtrC, previously implicated in Mn(IV) and Fe(III) reduction, directly transferred electrons to U(VI). Additionally, deletions of mtrC and/or omcA significantly affected the in vivo U(VI) reduction rate relative to wild type MR-1. Similar to the wild type, the mutants accumulated UO2 nanoparticles extracellularly to high densities in association with an exopolymeric substance (EPS). In wild type cells, this UO2-EPS matrix exhibited glycocalyx-like properties, contained multiple elements of the OM, polysaccharide, and heme containing proteins. Using a novel combination of methods including synchrotron-based X-ray fluorescence microscopy and high resolution immune-electron microscopy, we demonstrate a close association of the extracellular UO2 nanoparticles with MtrC and OmcA. This is the first study to directly localize the OM-associated cytochromes with EPS, which contains biogenic UO2 nanoparticles. In the environment, such association of UO2 nanoparticles with biopolymers may exert a strong influence on subsequent behavior including susceptibility to oxidation by O2 or transport in soils and sediments.
2006. "Microscopic Reactive Diffusion of Uranium in the Contaminated Sediments at Hanford, United States." Water Resources Research 42(W12420):1-15. doi:10.1029/2006WR005031 Abstract Microscopic and spectroscopic analysis of uranium-contaminated sediment cores beneath the BX waste tank farm at the US Department of Energy (DOE) Hanford site revealed that uranium (U) existed as uranyl precipitates primarily associated with the intragrain fractures of granitic clasts in the sediment (McKinley et al. 2005). The dissolution of the precipitates appeared to be controlled by intragrain ion diffusion coupled with the dissolution kinetics of the uranyl precipitates most likely as Na-boltwoodite. Here we presented a coupled microscopic reactive diffusion model by independently characterizing the intragrain diffusion and dissolution kinetics of Na-boltwoodite. Diffusion characterization with a nuclear magnetic resonance (NMR) pulse gradient spin-echo (PGSE) technique showed that the intragrain fractures of the granitic clasts in the Hanford sediment contain two domans with distinct diffusivities. The fast diffusion domain has an apparent tortuosity of about 1.5, while the slow region has a tortuosity of two orders of magnitude larger. A two-domain diffusion model was assembled and used to infer the geochemical conditions that led to intragrain uranyl precipitation when the sediment was contaminated by U-containing wastes at the site. Rapid precipitation of Na-boltwoodite was simulated when a U-containing, alkaline caustic, and high carbonate tank waste solution diffused into intragrain fractures originally containing Si-rich solutions. The model was also used to simulate uranyl dissolution and release from the contaminant sediment to aqueous solutions. With independently characterized parameters for Na-boltwoodite dissolution, the model simulations demonstrated that diffusion could significantly decrease the rates of intragrain uranyl mineral dissolution due to diffusion-induced local solubility limitation, and the intragrain uranyl precipitates could serve as a long-term uranyl source for the vadose porewater and underlying groundwater at this site.
2006. "Kinetics of Microbial Reduction of Solid Phase U(VI)." Environmental Science and Technology 40(20):6290-6296. Abstract Sodium boltwoodite (NaUO2SiO3OH ∙1.5H2O) was used to assess the kinetics of microbial reduction of solid phase U(VI) by a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1. The bioreduction kinetics was studied with Na-boltwoodite in suspension or within alginate beads. Concentrations of U(VI)tot and cell number were varied to evaluate the coupling of U(VI) dissolution, diffusion, and microbial activity. Batch experiments were performed in a non-growth medium with lactate as electron donor at pH 6.8 buffered with PIPES. Microscopic and spectroscopic analyses with transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and laser-induced fluorescence spectroscopy (LIFS) collectively indicated that solid phase U(VI) was first dissolved and diffused out of grain interiors before it was reduced on bacterial surfaces and/or within the periplasm. The kinetics of solid phase U(VI) bioreduction was well described by a coupled model of bicarbonate-promoted dissolution of Na-boltwoodite, intraparticle uranyl diffusion, and Monod type bioreduction kinetics with respect to dissolved U(VI) concentration. The results demonstrated the intimate coupling of biological, chemical, and physical processes in microbial reduction of solid phase U(VI).
2006. "Reductive Biotransformation of Fe in Shale-Limestone Saprolite Containing Fe(III) Oxides and Fe(II)/Fe(III) Phyllosilicates." Geochimica et Cosmochimica Acta 70(14):3662–3676. doi:10.1016/j.gca.2006.05.004 Abstract A <2.0-mm fraction of a mineralogically complex subsurface sediment containing goethite and Fe(II)/Fe(III) phyllosilicates was incubated with Shewanella putrefaciens (strain CN32) and lactate at circumneutral pH under anoxic conditions to investigate electron acceptor preference and the nature of the resulting biogenic Fe(II) fraction. Anthraquinone-2,6-disulfonate (AQDS), an electron shuttle, was included in select treatments to enhance bioreduction and subsequent biomineralization. The sediment was highly aggregated and contained two distinct clast populations: i) a highly weathered one with “sponge-like” internal porosity, large mineral crystallites, and Fe-containing micas, and ii) a dense, compact one with fine-textured Fe-containing illite and nano-sized goethite, as revealed by various forms of electron microscopic analyses. Approximately 10 to 15% of the Fe(III)TOT was bioreduced by CN32 over 60 d in media without AQDS, whereas 24% and 35% of the Fe(III)TOT was bioreduced by CN32 after 40 and 95 d in media with AQDS. Little or no Fe2+, Mn, Si, Al, and Mg were evident in aqueous filtrates after reductive incubation. Mössbauer measurements on the bioreduced sediments indicated that both goethite and phyllosilicate Fe(III) were partly reduced without bacterial preference. Goethite was more extensively reduced in the presence of AQDS whereas phyllosilicate Fe(III) reduction was not influenced by AQDS. Biogenic Fe(II) resulting from phyllosilicate Fe(III) reduction remained in a layer-silicate environment that displayed enhanced solubility in weak acid. The mineralogic nature of the goethite biotransformation product was not determined. Chemical and cryogenic Mössbauer measurements, however, indicated that the transformation product was not siderite, green rust, magnetite, Fe(OH)2, or Fe(II) adsorbed on phyllosilicate or bacterial surfaces. Several lines of evidence suggested that biogenic Fe(II) existed as surface associated phase on the residual goethite, and/or as a Fe(II)-Al coprecipitate. Sediment aggregation and mineral physical and/or chemical factors were demonstrated to play a major role on the nature and location of the biotransformation reaction and its products.
2006. "The Dissolution of Synthetic Na-Boltwoodite in Sodium Carbonate Solutions." Geochimica et Cosmochimica Acta 70(19):4836-4849. doi:10.1016/j.gca.2006.06.1553 Abstract Uranyl silicates such as uranophane and Na-boltwoodite appear to control the solubility of uranium in the contaminated sediments at the US Department of Energy Hanford site (Liu et al., 2004). Consequently, the solubility of synthetic Na-boltwoodite was determined over a wide range of bicarbonate concentrations, from circumneutral to alkaline pH, that are representative of porewater and groundwater compositions at the Hanford site. Results show that Na-boltwoodite dissolution was nearly congruent and its solubility increased with increasing bicarbonate concentration. Calculated solubility constants varied by nearly 2 log units from low bicarbonate (no added NaCO3) to 50 mmol/L bicarbonate. However, the solubility constants only vary by 0.5 log units from 0 added bicarbonate to 1.2 mmol/L bicarbonate, where logKsp = 5.39-5.92 and the average logKsp = 5.63. No systematic trend in logKsp was apparent over this range in bicarbonate concentrations. LogKsp values trended down with increasing bicarbonate concentration, where logKsp = 4.06 at 50 mmol/L bicarbonate. We conclude that the calculated solubility constants at high bicarbonate are compromised by an incomplete or inaccurate uranyl-carbonate speciation model.
2006. "The effect of calcium on aqueous uranium(VI) speciation and adsorption to ferrihydrite and quartz." Geochimica et Cosmochimica Acta 70(6):1379-1387. Abstract Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO2(CO3)32− and Ca2UO2(CO3)30(aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO3 solutions equilibrated with either ambient air (430 ppm CO2) or 2% CO2 in the presence of 0, 1.8, or 8.9 mM Ca2+. Under conditions where the Ca2UO2(CO3)30(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca2UO2(CO3)30(aq) accurately simulated the effect of Ca2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.
2006. "The Effect of Calcium on Aqueous Uranium (VI) Speciation and Adsorption to Ferrihydrite and Quartz." Geochimica et Cosmochimica Acta 70(6):1379-1387. Abstract Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO2(CO3)32- and Ca2UO2(CO3)30(aq), on inhibition of microbial reduction (Brooks et al., 2003) and uranium(VI) aqueous speciation in contaminated groundwater (Davis et al., 2004; Wang et al., 2004). However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO3 solutions equilibrated with either ambient air (430 ppm CO2) or 2% CO2 in the presence of 0, 1.8, or 8.9 mM Ca2+. Under conditions where the Ca2UO2(CO3)30(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca2UO2(CO3)30(aq) accurately simulated the effect of Ca2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.
2006. "Adsorption of Uranyl on Gibbsite: A Time-Resolved Laser-Induced Fluorescence Spectroscopy Study ." Environmental Science and Technology 40(4):1244-1249. Abstract Uranyl adsorbed on gibbsite at pH 4.0-8.0 and ionic strengths (ISs) 0.001-0.4 M (NaClO4) in the absence of carbonate was studied using time-resolved laser-induced fluorescence spectroscopy (TRLIFS) under cryogenic conditions. TRLIFS data showed the presence of several distinct emission components. Their contributions were determined using the evolving factor analysis approach. Four components denoted as species A, B, C, and D were discerned. Each of them was characterized by a characteristic response to pH and IS changes and also by a unique combination of the values of the fundamental transition energy E0,0, vibronic spacing E, and half-width of the vibronic lines W. Species A and B were major contributors to the overall emission. They were mainly affected by the pH and predominated below and above pH 5.0, respectively. In contrast with that, the contribution of species C was noticeable only at IS = 0.001 M, while it was suppressed or absent at high IS values. It was concluded that species A and B are likely to correspond to inner-sphere surface aluminol complexes AlO-(UO2)+ and AlO-(UO2)OH, while species C was hypothesized to correspond to electrostatically bound uranyl complexes (predominantly [UO2(OH)3]-).
2006. "Changes in Uranium Speciation through a Depth Sequence of Contaminated Hanford Sediments." Environmental Science and Technology 40(8):2517-2524 . Abstract The disposal of basic sodium-aluminate and acidic U(VI)-Cu(II) wastes into the now-dry North and South 300 A Process Ponds at the Hanford site resulted in U(VI) groundwater plume. To gain insight into the geochemical processes that occurred during waste disposal and that will affect the future fate and transport of this uranium plume, the solid-phase speciation of uranium in a depth sequence from the base of the North Process Pond through the vadose zone to the water table was investigated using electron microprobe measurements and x-ray absorption fine structure spectroscopy. Uranium in sediments from the base of the pond was predominantly corprecipitated with calcite. From ~2 m below the pond base to the water table uranium occurred dominantly in a sorbed form, likely on the surface aluminosilicate clay minerals. The presence of a U(VI)-phosphate phase was also observed in this region, but it only occurred as a major uranium species at one depth. The initial sequestration of U(VI) in these sediments likely occurred through coprecipitation with calcite as conditions did not favor adsorption. As the calcite-bearing pond sediments have been removed as part of a remediation effort, future uranium fate and transport will likely be controlled primarily by adsorption/desorption phenomena.
2005. "Enzymology of Electron Transport: Energy Generation with Geochemical Consequences." Chapter 3 in Molecular Geomicrobiology, Reviews in Mineralogy & Geochemistry, vol. 59, ed. J.F. Banfield, J. Cervini-Silva and K.H. Nealson, pp. 27-52. Mineralogical Society of America, Chantilly, VA. Abstract Dissimilatory metal-reducing bacteria (DMRB) are important components of the microbial community residing in redox-stratified freshwater and marine environments. DMRB occupy a central position in the biogeochemical cycles of metals, metalloids and radionuclides, and serve as catalysts for a variety of other environmentally important processes including biomineralization, biocorrosion, bioremediation and mediators of ground water quality. DMRB are presented, however, with a unique physiological challenge: they are required to respire anaerobically on terminal electron acceptors which are either highly insoluble (e.g., Fe(III)- and Mn(IV)-oxides) and reduced to soluble end-products or highly soluble (e.g., U(VI) and Tc(VII)) and reduced to insoluble end-products. To overcome physiological problems associated with metal and radionuclide solubility, DMRB are postulated to employ a variety of novel respiratory strategies not found in other gram-negative bacteria which respire on soluble electron acceptors such as O2, NO3- and SO42-. The novel respiratory strategies include 1) direct enzymatic reduction at the outer membrane, 2) electron shuttling pathways and 3) metal solubilization by exogenous or bacterially-produced organic ligands followed by reduction of soluble organic-metal compounds. The first section of this chapter highlights the latest findings on the enzymatic mechanisms of metal and radionuclide reduction by two of the most extensively studied DMRB (Geobacter and Shewanella), with particular emphasis on electron transport chain enzymology. The second section emphasizes the geochemical consequences of DMRB activity, including the direct and indirect effects on metal solubility, the reductive transformation of Fe- and Mn-containing minerals, and the biogeochemical cycling of metals at redox interfaces in chemically stratified environments.
2005. "Oxidative Remobilization of Biogenic Uranium (IV) Precipitates: Effects of Iron (II) and pH." Journal of Environmental Quality 34(5):1763-1771. Abstract The oxidative dissolution of biogenic U(IV) precipitates was investigated in bioreduced sediment suspensions in contact with atmospheric O2 with an emphasis on the influence of Fe(II) and pH on the rate and extent of U release from the solid to the aqueous phase. The sediment was collected from the US Department of Energy (DOE) Field Research Center (FRC) site at Oak Ridge, Tennessee. Biogenic U(IV) precipitates and bioreduced sediment were generated through anaerobic incubation with a dissimilatory metal reducing bacterium Shewanella putrefaciens strain CN32. The oxidative dissolution of freshly prepared and aged biogenic U(IV) was conducted in 0.1 mol/L NaNO3 electrolyte with variable pH and Fe(II) concentrations. Biogenic U(IV)O2(s) was oxidized with the highest rate and extent at pH 4 and 9. U release to the aqueous phase was the lowest at circumneutral pH. Increasing Fe(II) significantly decreased the release of U(VI) to the aqueous phase. From 70 to 100% of the U in the sediments was extractable at the experiment termination (40-80 days) with a bicarbonate solution (0.2 mol/L), indicating that biogenic U(IV) was oxidized regardless of Fe(II) concentration and pH. Sorption experiments and modeling calculations indicated that the inhibitive effect of Fe(II) on U(IV) oxidative remobilization was consistent with the Fe(III) oxide precipitation and U(VI) sorption to this secondary phase.
2005. "Fluorescence Spectroscopy of U(VI)-Silicates and U(VI)-Contaminated Hanford Sediment." Geochimica et Cosmochimica Acta 69(6):1391-1403. doi:10.1016/j.gca.2004.08.028 Abstract Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ~ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O=U=O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm-1 for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60 % of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although, an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.
2005. "Cryogenic Laser Induced U(VI) Fluorescence Studies of a U(VI) Substituted Natural Calcite: Implications to U(VI) Speciation in Contaminated Hanford Sediments." Environmental Science and Technology 39:2651-2659. Abstract Time-resolved laser-induced fluorescence spectroscopy (TRLFS) and imaging spectromicroscopy (TRLFISM) were used to examine the chemical speciation of uranyl in contaminated subsurface sediments from the Hanford Site, Washington. Spectroscopic measurements for contaminant U(VI) were compared to those from a natural, uranyl-bearing calcite (NUC) that had been found via X-ray absorption spectroscopy (XAS) to include uranyl in the same coordination environment as calcium (1). Spectral deconvolution of TRLFS measurements on the NUC revealed the unexpected presence of two distinct chemical environments consistent with published spectra of U(VI)-substituted synthetic calcite and aragonite. Apparently, some U(VI) substitution sites in calcite distorted to exhibit a local, more energetically favorable aragonite structure. TRLFS measurements of the Hanford sediments were similar to the NUC in terms of peak positions and intensity, despite a small CaCO3 content (<0.1 to 3.2 mass%). Spectral deconvolution of the sediment measurements also revealed the presence of U(VI) in calcite and aragonite structural environments. TRLFISM measurements at multiple locations in the different sediments displayed only minor variation indicating a uniform speciation pattern. Collectively, the measurements implied that waste U(VI), long-resident beneath the sampled disposal pond (32 y), had co-precipitated within newly formed carbonates. These results have major implications for the solubility and fate of the contaminated U(VI).
2005. "Kinetic Desorption and Sorption of U(VI) During Reactive Transport in a Contaminated Hanford Sediment." Environmental Science and Technology 39(9):3157-3165. doi:10.1021/es048462q Abstract Column experiments were conducted to investigate U(VI) desorption and sorption kinetics in a sand-textured, contaminated (22.7 µmol kg-1) capillary fringe sediment that had experienced long-term exposure to U(VI). The clay fraction mineralogy of the sediment was dominated by montmorillonite, muscovite, vermiculite, and chlorite. Saturated column experiments were performed under mildly alkaline/calcareous conditions representative of the Hanford site where uranyl–carbonate and calcium–uranyl–carbonate complexes dominate aqueous speciation. A U(VI) free solution was used to study U(VI) desorption in columns where different flow rates were applied. Uranium(VI) sorption was studied after the desorption of labile contaminant U(VI) using different U(VI) concentrations in the leaching solution. Strong kinetic behavior was observed for both U(VI) desorption and sorption. Although U(VI) is semi–mobile in mildly alkaline, calcareous subsurface environments, our results showed substantial U(VI) sorption, significant retardation during transport, and atypical breakthrough curves with extended tailing. A distributed rate model was applied to describe the effluent data and to allow comparisons between the desorption rate of contaminant U(VI) with the rate of short-term U(VI) sorption. Desorption was the slower process. Our results suggest that U(VI) release and transport in the vadose zone and aquifer system from which the sediment was obtained are kinetically controlled.
2005. "Influence of Sediment Bioreduction and Reoxidation on Uranium Sorption." Environmental Science and Technology 39(11):4125-4133. Abstract The influence of sediment bioreduction and reoxidation on U(VI) sorption was studied using Fe(III) oxide-containing saprolite from the U.S. Department of Energy (DOE) Oak Ridge site. Bioreduced sediments were generated by anoxic incubation with a metal reducing bacterium, Shewanella putrefaciens strain CN32, supplied with an electron donor. The reduced sediments were subsequently reoxidized by air contact. U(VI) sorption was studied in Na-NO3-HCO3 electrolytes that were both closed and open to atmosphere, and where pH, U(VI) and carbonate concentration was varied. Mössbauer spectroscopy and chemical analyses showed that 50% of the Fe(III)-oxides were reduced to Fe(II) that was sorbed to the sediment during incubation with CN32. However, this reduction and subsequent reoxidation of the sorbed Fe(II) had negligible influence on the rate and extent of U sorption, or the extractability of sorbed U by 0.2 mol/L NaHCO3. Various results indicated that U(VI) surface complexation was the primary process responsible for uranyl sorption by the bioreduced and reoxidized sediments. A two-site, non-electrostatic surface complexation model best described U(VI) adsorption under variable pH, carbonate and U(VI) conditions. A ferrihydrite-based diffuse double layer model provided a better estimation of U(VI) adsorption without parameter adjustment than did a goethite-based model, even though a majority of the Fe(III)-oxides in the sediments were goethite.
2005. "Ferrous Hydroxy Carbonate is a Stable Transformation Product of Biogenic Magnetite." American Mineralogist 90(2-3):510-515. Abstract A ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10-15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO₄³⁻ at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fe(III) was rapid; with 60% of the total Fe (Feтот) reduced in one day. Only an additional 10% of Feтот was reduced over the next three years. A fine-grained (10 nm), cation-excess (CE) magnetite with a Fe(II)/Feтот ratio of 0.5-0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micron-sized ferrous hydroxy carbonate [FHC; Fe2 (OH)2CO3⒮], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe(II)/Feтот ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of stochiometric magnetite (0.33). To best of our knowledge, this is the first report of biogenic FHC, and only the third observation of this material in nature. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe(II)-excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of fine-grained, biogenic magnetite in anoxic sediments.
2005. "Influence of Calcite and Dissolved Calcium on Uranium(VI) Sorption to a Hanford Subsurface Sediment." Environmental Science and Technology 39(20):7949-7955. doi:10.1021/es0505088 Abstract The influence of calcite and dissolved calcium on U(VI) adsorption was investigated using a calcite-containing sandy silt/clay sediment from the U. S. Department of Energy Hanford site. U(VI) adsorption to sediment, treated sediment, and sediment size fractions was studied in solutions that both had and had not been preequilibrated with calcite, at initial [U(VI)] ) 10-7-10-5 mol/L and final pH ) 6.0- 10.0. Kinetic and reversibility studies (pH 8.4) showed rapid sorption (30 min), with reasonable reversibility in the 3-day reaction time. Sorption from solutions equilibrated with calcite showed maximum U(VI) adsorption at pH 8.4 (0.1. In contrast, calcium-free systems showed the greatest adsorption at pH 6.0-7.2. At pH > 8.4, U(VI) adsorption was identical from calcium-free and calcium-containing solutions. For calcite-presaturated systems, both speciation calculations and laser-induced fluorescence spectroscopic analyses indicated that aqueous U(VI) was increasingly dominated by Ca2UO2(CO3)3 0(aq) at pH<8.4 and that formation of Ca2UO2(CO3)3 0(aq) is what suppresses U(VI) adsorption. Above pH 8.4, aqueous U(VI) speciation was dominated by UO2(CO3)3 4- in all solutions. Finally, results also showed that U(VI) adsorption was additive in regard to size fraction but not in regard to mineral mass: Carbonate minerals may have blocked U(VI) access to surfaces of higher sorption affinity.
2005. "Impact of Highly Basic Solutions on Sorption of Cs+ to Subsurface Sediments from the Hanford Site, USA." Geochimica et Cosmochimica Acta 69(20):4787-4800. Abstract The effect of caustic NaNO3 solutions on the sorption of 137Cs to the Hanford site micaceous subsurface sediment was investigated as a function of time, temperature (10oC or 50oC), and NaOH concentration. At 100C and 0.1 M NaOH, the slow evolution of [Al]aq was in stack contrast to the rapid increase and subsequent loss of [Al]aq observed at 50oC (regardless of base concentration). At 50oC, dissolution of phyllosilicate minerals increased with [OH], at 1 and 3 M NaOH solutions, almost complete dissolution of clay-sized phyllosilicates occurred. At 0.1 M NaOH, a zeolite (tetranatrolite) precipitated after about 7 days, while an unnamed mineral phase (Na2Al2Si3O10•2H2O) precipitated after 4 and 2 days of exposure to 1 M and 3 M NaOH solutions. At 100C there was no conclusive evidence of secondary mineral precipitation. The effect of base dissolution on Cs+ sorption by the Hanford sediment was investigated via i) Cs+ sorption over a large concentration range (10-9 – 10-2 mol/L) to sediment after exposure to 0.1 M NaOH for 56, 112, and 168 days, ii) Cs+ sorption to sediment in the presence of NaOH (0.1 M, 1 M, and 3 M NaOH) at Cs+ concentrations selected to probe high affinity, transition, and low affinity cation exchange sites, and iii) the application of a two-site numeric ion exchange model (Zachara et al. 2002a). No effect on Cs+ sorption to the Hanford sediment was observed during the 168 days sediment was exposed to 0.1 M NaOH, at 10oC; Cs+ sorption in the presence of base was well described by the ion exchange model when enthalpy effects were considered. In contrast, at 50oC, there was a trend toward slightly lower (log ~ 0.25) conditional equilibrium exchange constants over the entire range of surface coverage, and a slight loss of high affinity sites (15%) after 168 days of exposure to 0.1 M base solution. However, model simulations of Cs+ sorption to the sediment in the presence of 0.1 M base for 112 days were good at the lower Cs+ surface densities. At the higher surface densities, model simulations under predicted sorption by 57%. This under prediction was surmised to be the result of tetranatrolite precipitation, and subsequent slow Na → Cs exchange. At higher OH concentrations, Cs+ sorption in the presence of base for 112 days was unexpectedly equal to, or slightly greater than that expected for a pristine sediment. The presence of neoforms, coupled with the fairly unique mica distribution and quantity across all size-fractions in the Hanford sediment, appears to mitigate the impact of base dissolution on Cs+ sorption.
2004. "Chromium Speciation and Mobility in a High Level Nuclear Waste Vadose Zone Plume." Geochimica et Cosmochimica Acta 68(1):13-30. Abstract Chromium Speciation and Mobility in a High Level Nuclear Waste Vadose Zone Plume
2004. "Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer." Geochimica et Cosmochimica Acta 68(8):1791-1805. Abstract Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer
2004. "Cryogenic Laser Induced Fluorescence Characterization of U(VI) in Hanford Vadose Zone Pore Waters." Environmental Science and Technology 38:5591-5597. Abstract Ambient and liquid helium temperature laser-induced time-resolved uranyl fluorescence spectroscopy was applied to study the speciation of aqueous uranyl solutions containing carbonate and phosphate and two porewater samples obtained by ultra-centrifugation of U(VI)-contaminated sediments. The significantly enhanced fluorescence signal intensity and spectral resolution found at liquid helium temperature allowed, for the first time, direct fluorescence spectroscopic observation of the higher aqueous uranyl complexes with carbonate: UO2(CO3)22-, UO2(CO3)34- and (UO2)2(OH)3CO3-. The porewater samples were non-fluorescent at room temperature. However, at liquid helium temperature, both porewater samples displayed strong, well-resolved fluorescence spectra. Comparisons of the spectroscopic characteristics of the porewaters with those of the standard uranyl-carbonate complexes confirmed that U(VI) in the porewaters existed primarily as UO2(CO3)34-. A small amount of the dicalcium-urano-tricarbonate complex, Ca2UO2(CO3)3, was also observed that was consistent with thermodynamic calculation. The U(VI)-carbonate complex is apparently the mobile species responsible for the subsurface migration of U(VI), even though the majority of the in-ground U(VI) inventory at the site from which the samples were obtained exists as intragrain U(VI)-silicate precipitates.
2004. "Microscale Distribution of Cesium Sorbed to Biotite and Muscovite." Environmental Science and Technology 38(4):1017-1023. Abstract Individual 1 – 3 mm biotite and muscovite clasts from Hanford sediment were contacted with 0.001 – 0.08 M CsNO--3, then either left whole or sectioned perpendicular to their basal planes for examination by electron or X-ray microprobe. Cs+ was observed to preferentially sorb to mica edges, steps on mica surfaces, or to fractured regions. The observed localization conformed to hypothesized strong binding to frayed edge sites in preference to sites on basal planes. In section, Cs+ was found to penetrate the mica interior, forming discrete zones of concentration, particularly in muscovite. In biotite, Cs was more abundant, permeating the clasts at low abundance and forming also discrete zones of higher concentration. Concentrated Cs on both clast edges and within clast interiors corresponded to microscopic but relatively extensive zones where K was depleted. The localization of sorbed Cs in areas where K was depleted suggested that weathering reactions caused the formation of frayed edges sites within the micas. Cs+ accessed crystal interiors by diffusion along channels following crystal defects, cracks, or partings where pore fluids had previously migrated to form the interior alteration zones. At nm scale, areas with localized Cs were disordered, confirming that FES were developed in clast interiors. Cs+ that was adsorbed in this manner could be very resistant to desorption, as has been often observed.
2004. "Dissolution of Uranyl Microprecipitates from Subsurface Sediments at Hanford Site, USA." Geochimica et Cosmochimica Acta 68(22):4519-4537. Abstract This paper describes a study of thermodynamics and Kinetics of uranyl microprecipitates from subsurface sediments at Hanford Site, USA.
2004. "An Electrodynamics-Based Model for Ion Diffusion in Microbial Polysaccharides." Colloids and Surfaces. B, Biointerfaces 38:55-65. Abstract An electrodynamics-based model was formulated for simulation of ion diffusion in microbial polysaccharides with fixed charges and elecrostatic double layers. The model extends a common multicomponent ion diffusion model that is based on irreversible thermodynamics under a zero ionic charge flux condition, which is only applicable to the regions without fixed charges and electrostatic double layers. An efficient numberical procedure was presented to solve the differential equations in the model. The model well described key features of experimental observations of ion diffusion in negatively charged microbial polysaccharides including accelerated diffusive transport of cations, exclusion of anions, and increased rate of cation transport with increasing negative charge density. The simulated diffusive fluxes of cations and anions were consistent with a classic exchange diffusion concept in negatively charged polysaccharides at the interface of plant roots and soils; and the developed model allows to mathematically study such diffusion phenonmena. Numerical simulations also showed that ion diffusive transport within a becterial cell wall polysaccharide may induce an ionic current that compresses or expands the bacterial electrostatic double layer at the interface of the cell wall and bulk solution.
2004. "A Cation Exchange Model to Describe Cs+ Sorption at High Ionic Strength in Subsurface Sediments at Hanford Site, USA." Journal of Contaminant Hydrology 68(3-4):217-238. Abstract A Cation Exchange Model to Describe Cs Sorption at High Ionic Strength in Subsurface Sediments at Hanford Site, USA
2004. "Biotransformation of Two-Line Silica-Ferrihydrite by a Dissimilatory Fe(III)-Reducing Bacterium: Formation of Carbonate Green Rust in the Presence of Phosphate." Geochimica et Cosmochimica Acta 68(13):2799-2814. Abstract The reductive biotransformation of two Si-ferrihydrite (0.01 and 0.05 mole% Si) coprecipiates by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ~7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate (electron shuttle) that stimulates respiration was present in the media. Experiments were performed without and with PO43- (ranging from 1 to 20 mmol/L in media containing 50 mmol/L Fe). Our objectives were to define the combined effects of SiO44- and PO43- on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction (XRD) and Mossbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl- and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of PO43- whereas carbonate green rust GR(CO32-) ([FeII(6-x)FeIIIx(OH)12]x+(CO32-)0.5x.yH2O) and vivianite [Fe3(PO4)2.8H2O], were formed when PO43- was present. GR(CO32-) dominated as a mineral product in samples with < 4 mmol/L PO43-. The Fe(II)/Fe(III) ratio of GR(CO32-) varied with PO43- concentration; it was 2 in the 1 mmol/L PO43- and approached 1 in the 4- and 10-mmol/L PO43- samples. GR appeared to form by solid-state transformation of ferrihydrite. Medium PO43- concentration dictated the mechanism of transformation. In 1 mmol/L PO43- media, an intermediate Fe(II)/Fe(III) phase with structural Fe(II), which we tentatively assigned to a protomagnetite phase, slowly transformed to GR with time. In contrast, in medium with >4 mmol/L PO43-, a residual ferrihydrite with sorbed Fe2+ phase transformed to GR. Despite similar chemistries, PO43- was shown to have a profound effect on ferrihydrite biotransformations while that of SiO44- was minimal.
2004. "Reduction of TcO4- by sediment-associated biogenic Fe(II)." Geochimica et Cosmochimica Acta 68(15):3171-3187. Abstract Abstract-The potential for reduction of 9~CO- (aq) to poorly soluble 9~COZ .nHzO(s) by biogenic sediment-associated Fe(ll) was investigated with three Fe(IlI)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy's Hanford and Oak Ridge sites contained significant amounts of Mn(IlI,IV) oxides and net bioreduction of Fe(llI) to Fe(ll) was not observed until essentially all of the 1 hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcOz .nHzO(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO-: (aq) at rates that generally increased with increasing concentrations of 0.5 N HC1F~(Il~o of the sediments showed a common relationship between th~~8I~e~oncentra-= e ~/.!I80 20 'rY'\rI) --' t~?_~ (~~1 and the first-order reduction rate (in h-1, whereas the third demonstrated a markedly different trend. A combination of chemical extractions and s7Fe Mossbauer spectroscopy were used to characterize the Fe(llI) and Fe(ll) phases. There was little evidence of the formation of secondary Fe(ll) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(ll) were predominantly surface complexes of different forms. The reduction rates of Tc(VIl)O-; were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(ll) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(IlI), either naturally or via redox manipulation, may be effective at immobiJizing TcO-: (aq) associated with groundwater contaminant plumes.
2004. "Geomicrobiology of High Level Nuclear Waste-Contaminated Vadose Sediments at the Hanford Site, Washington State." Applied and Environmental Microbiology 70(7):4230-4241. Abstract Abstract – Sediments from a high-level nuclear waste plume were collected as part of investigations to evaluate the potential fate and migration of contaminants in the subsurface. The plume originated from a leak that occurred in 1962 from a waste tank consisting of high concentrations of alkali, nitrate, aluminate, Cr(VI), 137Cs, and 99Tc. Investigations were initiated to determine the distribution of viable microorganisms in the vadose sediment samples, probe the phylogeny of cultivated and uncultivated members, and evaluate the ability of the cultivated organisms to survive acute doses of ionizing radiation. The populations of viable aerobic heterotrophic bacteria were generally low, from below detection to ~104 7 CFU g-1 but viable microorganisms were recovered from 11 of 16 samples including several of the most radioactive ones (e.g., > 10 µCi/g 137Cs). The isolates from the contaminated sediments and clone libraries from sediment DNA extracts were dominated by members related to known Gram-positive bacteria. Gram-positive bacteria most closely related to Arthrobacter species were the most common isolates among all samples but other high G+C phyla were also represented including Rhodococcus and Nocardia. Two isolates from the second most radioactive sample (>20 µCi 137Cs g-1) were closely related to Deinococcus radiodurans and were able to survive acute doses of ionizing radiation approaching 20kGy. Many of the Gram-positive isolates were resistant to lower levels of gamma radiation. These results demonstrate that Gram-positive bacteria, predominantly high G+C phyla, are indigenous to Hanford vadose sediments and some are effective at surviving the extreme physical and chemical stress associated with radioactive waste.
2004. "Spectroscopic and Diffraction Study of Uranium Speciation in Contaminated Vadose Zone Sediments from the Hanford Site, Washington State." Environmental Science and Technology 38(10):2822-2828. Abstract Contamination of vadose zone sediments under tank BX-102 at the Hanford site, Washington, resulted from the accidental release of 7-8 metric tons of uranium dissolved in caustic aqueous sludge in 1951. We have applied synchrotron-based X-ray spectroscopic and diffraction techniques to characterize the speciation of uranium in samples of these contaminated sediments. U LIII-edge X-ray absorption fine structure (XAFS) spectroscopic studies demonstrate that uranium occurs predominantly as a uranium-(VI) silicate from the uranophane group of minerals. XAFS cannot distinguish between the members of this mineral group due to the near identical local coordination environments of uranium in these phases. However, these phases differ crystallographically, and can be distinguished using X-ray diffraction (XRD) methods. As the concentration of uranium was too low for conventional XRD to detect these phases, X-ray microdiffraction (íXRD) was used to collect diffraction patterns on _20 ím diameter areas of localized high uranium concentration found using microscanning X-ray fluorescence (íSXRF). Only sodium boltwoodite, Na(UO2)(SiO3OH)â1.5H2O, was observed; no other uranophane group minerals were present. Sodium boltwoodite formation has effectively sequestered uranium in these sediments under the current geochemical and hydrologic conditions. Attempts to remediate the uranium contamination will likely face significant difficulties because of the speciation and distribution of uranium in the sediments.
2003. "Nonlocal Bacterial Electron Transfer to Hematite Surfaces." Geochimica et Cosmochimica Acta 67(5):1081-1087. Abstract Non-Local Bacterial Electron Transfer to Hematite Surfaces
2003. "Quantifying the effects of small-scale heterogeneities on flow and transport in undisturbed cores from the Hanford formation." Vadose Zone Journal 2(4):664-676. Abstract Analysis of vadose-zone transport properties of cores of Hanford formation from the Environmental Restoration Disposal Facility.
2003. "Transport of Multiple Tracers in Variably Saturated Humid Region Structured Soils and Semi-arid Region Laminated Sediments." Journal of Hydrology 275(3-4):141-161. Abstract Transport of Multiple Tracers in Variably Saturated Humid Region Structured Soils and Semi-arid Region Laminated Sediments
2003. "Effect of Temperature on Cs+ Sorption and Desorption in Subsurface Sediments at Hanford Site, U.S.A." Environmental Science and Technology 37(12):2640-2645. Abstract Effect of Temperature on Cs+ Sorption and Desorption in Subsurface Sediments at Hanford Site, USA
2003. "Desorption Kinetics of Radiocesium from Subsurface Sediments at Hanford Site, USA." Geochimica et Cosmochimica Acta 67(16):2893-2912. Abstract Desorption Kinetics of Radiocesium from Subsurface Sediments at Hanford Site, USA
2003. "Transformation of 2-line Ferrihydrite to 6-line Ferrihydrite under Oxic and Anoxic Conditions." American Mineralogist 88(11-12):1903-1914 pt. 2. Abstract New Insights into the Transformation of 2-line Ferrihydrite to Crystalline Iron Oxides: The First Observation of 6-line Ferrihydrite during Solid-state Transformation
2003. "Influence of Electron Donor/Acceptor Concentrations on Hydrous Ferric Oxide (HFO) Bioreduction." Biodegradation 14(2):91-103. Abstract Dissimilatory metal-reducing bacteria (DMRB) facilitate the reduction of Fe and Mn oxides in anoxic soils and sediments and play an important role in the cycling of these metals and other elements such as carbon in aqueous environments.
2003. "Microbial Reduction of Structural Fe(III) in Illite and Goethite." Environmental Science and Technology Vol. 37(7):1268-1276 . Abstract Microbial Reduction of Structural Fe(III) in Illite and Goethite
2003. "Inhibition of bacterial U(VI) reduction by calcium." Environmental Science and Technology 37(9):1850-1858. Abstract The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO2,cr) make this process an attractive option for removing uranium from groundwater. Nevertheless, conditions that may promote or inhibit U(VI) reduction are not well-defined. Recent descriptions of Ca-UO2-CO3 complexes indicate that these species may dominate the aqueous speciation of U(VI) in many environments. We monitored the bacterial reduction of U(VI) in bicarbonate-buffered solution in the presence and absence of Ca. XAFS measurements confirmed the presence of a Ca-U(VI)-CO3 complex in the initial solutions containing calcium. Calcium, at millimolar concentrations (0.45-5 mM), caused a significant decrease in the rate and extent of bacterial U(VI) reduction. Both facultative (Shewanella putrefaciens strain CN32) and obligate (Desulfovibrio desulfuricans, Geobacter sulfurreducens) anaerobic bacteria were affected by the presence of calcium. Reduction of U(VI) ceased when the calculated system Eh reached -0.046 +/- 0.001 V, based on the Ca2UO2(CO3)(3) --> UO2,cr couple. The results are consistent with the hypothesis that U is a less energetically favorable electron acceptor when the Ca-UO2-CO3 complexes are present. The results do not support Ca inhibition caused by direct interactions with the cells or with the electron donor as the reduction of fumarate or Tc(VII)O-4(-) under identical conditions was unaffected by the presence of Ca.
2002. "Sorption of Cs+ to Micaceous Subsurface Sediments from the Hanford Site, USA." Geochimica et Cosmochimica Acta 66(2):193-211. Abstract Sorption of Cs+ to Micaceous Subsurface Sediments from the Hanford Site, USA
2002. "Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)." Geomicrobiology Journal 19:179-207. Abstract Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)
2002. "Reduction Kinetics of Fe(III), Co(III), U(VI), Cr(VI), and Tc(VII) in Cultures of Dissimilatory Metal-reducing Bacteria." Biotechnology and Bioenegineering 80(6):637-649. Abstract Kinetics of Dissimilatory Metal Reduction with Variable Metals and Metal Reducing Bacteria
2002. "Modeling the Inhibition of the Bacterial Reduction of U(VI) by beta-MnO2(S)(g)." Environmental Science and Technology 36(7):1452-1459. Abstract Modeling the Inhibition of the Bacterial Reduction of U(VI) by b-MnO2(s)
2002. "Influence of Mn oxides on the reduction of U(VI) by the metal-reducing bacterium Shewanella putrefaciens." Geochimica et Cosmochimica Acta 66(18):3247-3262. Abstract Dissimilatory metal-reducing bacteria (DMRB) enzymatically reduce Fe(III), Mn(III/IV), U(VI), and other polyvalent metals during anaerobic respiration. Previous investigations of the bacterial reduction of U(VI) in the presence of goethite (a-FeOOH) found that, in spite of potential competition as an electron acceptor, goethite had little impact on the bacterial reduction of U(VI) to insoluble U(IV). Mn(III/IV) oxides are also electron acceptors for DMRB but are stronger oxidants than Fe(III) oxides. Differences in the solubility of oxidized Mn and U challenges predictions of their biogeochemical behavior during redox cycling. The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by a subsurface bacterium Shewanella putrefaciens CN32 was investigated using synthetic Mn(III/IV) oxides [pyrolusite (?-MnO2), bixbyite (Mn2O3) and K+-birnessite (K4Mn14O27?8H2O)]. In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO2(s)) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence, or in the presence of gibbsite [Al(OH)3] added as a non-redox reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43-100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial-Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. In the absence of Mn(III/IV) oxides, UO2(s) accumulated as copius fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments?..
2001. "Solubilization of Fe(III) Oxide-Bound Trace Metals by a Dissimilatory Fe(III) Reducing Bacterium." Geochimica et Cosmochimica Acta 65(1):75-93.
2001. "Modeling and Measuring Biogeochemical Reactions: System Consistency, Data Needs, and Rate Formulations." Advances in Environmental Research 5:219-237. Abstract Modeling and Measuring Biogeochemical Reactions: System Consistency, Data Needs, and Rate Formulations
2001. "Distribution and Retention of 137Cs in Sediments at the Hanford Site, Washington." Environmental Science and Technology 35(17):3433-3441. Abstract ABSTRACT-137Cesium and other contaminants have leaked from high level waste (HLW) single-shell storage tanks (SSTs) at the Hanford Site in southeastern Washington...
2001. "Uncertainties of Monod Kinetic Parameters Nonlinearly Estimated from Batch Experiments." Environmental Science and Technology 35(1):133-141. Abstract Uncertainties of Monod Kinetic Parameters Nonlinearly Estimated from Batch Experiments
2001. "Microbial Reduction of Fe(III) and Sorption/Precipitation of Fe(II) on Shewanella putrefaciens Strain CN32." Environmental Science and Technology 35(7):1385-1393. Abstract Microbial reduction of Fe3+ and sorption/precipitation of Fe2+ on bacteria, Shewanella putrefaciens, strain CN32
2001. "Kinetic Analysis of the Bacterial Reduction of Goethite." Environmental Science and Technology 35:2482-2490. Abstract Kinetic Analysis of the Bacterial Reduction of Goethite
2001. "Dissimilatory Bacterial Reduction of Al-Substituted Goethite in Subsurface Sediments." Geochimica et Cosmochimica Acta 65(17):2913-2924. Abstract Microbiologic reduction of the 0.2-2.0 mm size fraction of an Atlantic coastal plain sediment (Eatontown) was investigated using a dissimilatory Fe(III)-reducing bacterium (Shewanella putrefaciens, strain CN32) to evaluate mineralogic controls on the rate and extent of Fe(III) reduction and resulting distribution of biogenic Fe(II). Mössbauer spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) was used to show that the sedimentary Fe(III) oxide was Al-substituted goethite (11-17% Al) that existed as 1-5 mm aggregates of indistinct morphology. Bioreduction experiments were performed in two buffers [HCO3-, 1,4-piperazinediethansulfonic acid (PIPES)] both without and with 2,6-anthraquinone disulfonate (AQDS), an electron shuttle. The production of biogenic Fe(II) and the distribution of Al (aqueous and sorbed) were followed over time, as was formation of Fe(II) biominerals and physical/chemical changes to the goethite.
2001. "Biotransformation of Ni-Substituted Hydrous Ferric Oxide by an Fe(III)-Reducing Bacterium." Environmental Science and Technology 35(4):703-712. Abstract Biotransformation of Ni-Substituted Hydrous Ferric Oxide by an Fe(III)-Reducing Bacterium
2000. "The Effect of Biogenic Fe(II) on the Stability and Sorption of Co(II)EDTA2- to Goethite and a Subsurface Sediment." Geochimica et Cosmochimica Acta 64(8):1345-1362.
2000. "Use of the Generalized Integral Transform Method for Solving Equations of Solute Transport in Porous Media." Advances in Water Resources 23(5):483-492. Abstract Use of the Generalized Integral Transform Method for Solving Equations of Solute Transport in Porous Media
2000. "Reduction of U(VI) in Goethite (a-FeOOH) Suspensions by a Dissimilatory Metal-Reducing Bacterium." Geochimica et Cosmochimica Acta 64:3085-3098.
2000. "Mineral Transformations Associated with the Microbial Reduction of Magnetite." Chemical Geology 169:299-318. Abstract N/A
1998. "Bacterial Reduction of Crystalline Fe3+ Oxides in Single Phase Suspensions and Subsurface Materials." American Mineralogist 83(11-12):1426-1443. Abstract There is no abstract currently available for this item
1998. "Influence of Iron Oxide Inclusion Shape on Co(II/III)EDTA Reactive Transport Through Spatially Heterogeneous Sediment." Water Resources Research 34(10):2501-2514. Abstract There is no abstract currently available for this item