Microbial incorporation of 13C-labeled acetate at the field scale: detection of microbes responsible for reduction of U(VI)

A field-scale acetate amendment experiment was performed in a contaminated aquifer at Old Rifle, CO to stimulate in situ microbial reduction of U(VI) in groundwater. To evaluate the microorganisms responsible for microbial uranium reduction during the experiment, 13C-labeled acetate was introduced into well bores via bio-traps containing porous activated carbon beads (Bio-Sep). Incorporation of the 13C from labeled acetate into cellular DNA and phospholipid fatty acid (PLFA) biomarkers was analyzed in parallel with geochemical parameters. An enrichment of active sigma-proteobacteria was demonstrated in downgradient monitoring wells: Geobacter dominated in wells closer to the acetate injection gallery, while various sulfate reducers were prominent in different downgradient wells. These results were consistent with the geochemical evidence of Fe(III), U(VI), and SO(4)2- reduction. PLFA profiling of bio-traps suspended in the monitoring wells also showed the incorporation of 13C into bacterial cellular lipids. Community composition of downgradient monitoring wells based on quinone and PLFA profiling was in general agreement with the 13C-DNA result. The direct application of 13C label to biosystems, coupled with DNA and PLFA analysis,     

Effects of progressive anoxia on the solubility of technetium in sediments

Technetium is a significant radioactive contaminant from nuclear fuel cycle operations. It is highly mobile in its oxic form (as Tc(VII)O4-) but is scavenged to sediments in its reduced forms (predominantly Tc(IV)). Here we examine the behavior of Tc at low concentrations and as microbial anoxia develops in sediment microcosms. A cascade of stable-element terminal-electron-accepting processes developed in microcosms due to indigenous microbial activity. TcO4- removal from solution occurred during active microbial Fe(III) reduction, which generated Fe(II) in the sediments and was complete before sulfate reduction began. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, PauII, a broad range of NO3-, Mn(IV), Fe(III), and SO4(2-) reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II), although no differences in the microbial population were discerned as anoxia developed. When sterilized sediments were incubated with pure cultures of NO3(-)-, Fe(III)-, and sulfate-reducing bacteria, TcO4- removal occurred during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal was due to reduction to hydrous Tc(IV)O2 in Fe(III)- and sulfate-reducing estuarine sediments.     

Biogeochemical processes in ethanol stimulated uranium-contaminated subsurface sediments

A laboratory incubation experiment was conducted with uranium-contaminated subsurface sediment to assess the geochemical and microbial community response to ethanol amendment. A classical sequence of terminal electron-accepting processes (TEAPs) was observed in ethanol-amended slurries, with NO3- reduction, Fe(III) reduction, SO4(2-) reduction, and CH4 production proceeding in sequence until all of the added 13C-ethanol (9 mM) was consumed. Approximately 60% of the U(VI) content of the sediment was reduced during the period of Fe(III) reduction. No additional U(VI) reduction took place during the sulfate-reducing and methanogenic phases of the experiment Only gradual reduction of NO3-, and no reduction of U(VI), took place in ethanol-free slurries. Stimulation of additional Fe(III) or SO4(2-) reduction in the ethanol-amended slurries failed to promote further U(VI) reduction. Reverse transcribed 16S rRNA clone libraries revealed major increases in the abundance of organisms related to Dechloromonas, Geobacter, and Herbaspirillum in the ethanol-amended slurries. Phospholipid fatty acids (PLFAs) indicative of Geobacter showed a distinct increase in the amended slurries, and analysis of PLFA 13C/12C ratios confirmed the incorporation of ethanol into these PLFAs. A increase in the abundance of 13C-labeled PLFAs indicative of Desulfobacter, Desulfotomaculum, and Desulfovibrio took place during the brief period of sulfate reduction that followed the Fe(III) reduction phase. Our results show that major redox processes in ethanol-amended sediments can be reliably interpreted in terms of standard conceptual models of TEAPs in sediments. However, the redox speciation of uranium is complex and cannot be explained based on simplified thermodynamic considerations.     

Using chemical and isotopic data to quantify ionic trapping of injected carbon dioxide in oil field brines

Injection of carbon dioxide into depleted oil fields or deep saline aquifers represents one of the most promising means of long-term storage of this greenhouse gas. While the ultimate goal of CO2 injection in the subsurface is mineral storage of CO2 as carbonates, short-term (<50 year) storage of injected CO2 is most likely to be accomplished by ionic trapping of CO2 as bicarbonate ions (HCO3-) and hydrogeological trapping of molecular CO2. Here, we demonstrate a technique for quantifying ionic trapping of injected CO2 as HCO3- using geochemical data collected prior to and during 40 months of CO2 injection into a hydrocarbon reservoir at the International Energy Agency (IEA) Weyburn CO2 Monitoring and Storage Project, Saskatchewan, Canada. As a result of injection of CO2 with a low carbon isotope ratio (delta13C value), fluid and gas samples from four selected production wells showed an increase in HCO3- concentration and a decrease in delta13C values of HCO3- and CO2 over the observation period. Isotope and mass balance calculations indicate that, after 40 months of injection, approximately 80% of the HCO3- in the reservoir brines sampled from the four wells formed via dissolution and dissociation of injected CO2. This chemical and isotopic technique should be applicable to CO2 injection and storage in oil fields and in deep saline aquifers, provided there is sufficient carbon isotopic distinction between injected CO2 and baseline aquifer HCO3- and CO2.     

Iron-mediated microbial oxidation and abiotic reduction of organic contaminants under anoxic conditions

In anoxic environments, the oxidation of organic compounds, such as BTEX fuel components, by dissimilatory Fe(III) reduction can generate reactive mineral-bound Fe(II) species, which in turn are able to reduce other classes of organic and inorganic groundwater contaminants. In this study, we designed and evaluated an anaerobic batch reactor that mimicks iron-reducing conditions to investigate the factors that favor the coupling of microbial toluene oxidation and abiotic reduction of nitroaromatic contaminants. We investigated the influence of different Fe(III)-bearing minerals and combinations thereof on the coupling of these two processes. Results from laboratory model systems show that complete oxidation of toluene to CO2 by Geobacter metallireducens in the presence of Fe(III)-bearing minerals leads to the formation of mineral-bound Fe(II) species capable of the reduction of 4-nitroacetophenone. Whereas significant microbial toluene oxidation was only observed in the presence of amorphous Fe(III) phases, reduction of nitroaromatic compounds only proceeded with Fe(II) species bound to crystalline Fe(III) oxides. Our results suggest that in anoxic soils and sediments containing amorphous and crystalline iron phases simultaneously, coupling of microbial oxidation and abiotic reduction of organic compounds may allow for concurrent natural attenuation of different contaminant classes.     

Microbial reductive transformation of phyllosilicate Fe(III) and U(VI) in fluvial subsurface sediments

The microbial reduction of Fe(III) and U(VI) was investigated in shallow aquifer sediments collected from subsurface flood deposits near the Hanford Reach of the Columbia River in Washington State. Increases in 0.5 N HCl-extractable Fe(II) were observed in incubated sediments and (57)Fe M?ssbauer spectroscopy revealed that Fe(III) associated with phyllosilicates and pyroxene was reduced to Fe(II). Aqueous uranium(VI) concentrations decreased in subsurface sediments incubated in sulfate-containing synthetic groundwater with the rate and extent being greater in sediment amended with organic carbon. X-ray absorption spectroscopy of bioreduced sediments indicated that 67-77% of the U signal was U(VI), probably as an adsorbed species associated with a new or modified reactive mineral phase. Phylotypes within the Deltaproteobacteria were more common in Hanford sediments incubated with U(VI) than without, and in U(VI)-free incubations, members of the Clostridiales were dominant with sulfate-reducing phylotypes more common in the sulfate-amended sediments. These results demonstrate the potential for anaerobic reduction of phyllosilicate Fe(III) and sulfate in Hanford unconfined aquifer sediments and biotransformations involving reduction and adsorption leading to decreased aqueous U concentrations.     

Competitive reduction of pertechnetate (99TcO4-) by dissimilatory metal reducing bacteria and biogenic Fe(II)

The fate of pertechnetate ((99)Tc(VII)O(4)(-)) during bioreduction was investigated in the presence of 2-line ferrihydrite (Fh) and various dissimilatory metal reducing bacteria (DMRB) (Geobacter, Anaeromyxobacter, Shewanella) in comparison with TcO(4)(-) bioreduction in the absence of Fh. In the presence of Fh, Tc was present primarily as a fine-grained Tc(IV)/Fe precipitate that was distinct from the Tc(IV)O(2)·nH(2)O solids produced by direct biological Tc(VII) reduction. Aqueous Tc concentrations (<0.2 μm) in the bioreduced Fh suspensions (1.7 to 3.2 × 10(-9) mol L(-1)) were over 1 order of magnitude lower than when TcO(4)(-) was biologically reduced in the absence of Fh (4.0 × 10(-8) to 1.0 × 10(-7) mol L(-1)). EXAFS analyses of the bioreduced Fh-Tc products were consistent with variable chain length Tc-O octahedra bonded to Fe-O octahedra associated with the surface of the residual or secondary Fe(III) oxide. In contrast, biogenic TcO(2)·nH(2)O had significantly more Tc-Tc second neighbors and a distinct long-range order consistent with small particle polymers of TcO(2). In Fe-rich subsurface sediments, the reduction of Tc(VII) by Fe(II) may predominate over direct microbial pathways, potentially leading to lower concentrations of aqueous (99)Tc(IV).     

The effects of reaction-product formation on the reductive dissolution of MnO2 by Fe(II)

We conducted batch-reactor experiments to measure the reductive dissolution of pyrolusite-coated (beta-MnO2) quartz by Fe(II) under conditions representative of an acid mine-drainage subsurface plume. The results reveal that reductive dissolution rates were initially rapid but declined considerably as Fe(III)(aq), a product of the reductive-dissolution reaction, was removed from solution by heterogeneous precipitation. The inhibition of reductive-dissolution was attributed to blocking of the beta-MnO2 surface sites by the Fe(III)(s) precipitate. Calculations of a simple model that accounts for the effects of Fe(III)(s) precipitate formation on reductive dissolution rates closely match temporal changes in Mn(II), Fe(II), and Fe(II) concentrations measured in 10 experiments, distinguished on the basis of the initial Fe(II)-to-Mn(IV) mole ratio and the initial Fe(III)(aq) concentration. The model-data comparisons reveal that the initial reaction rate on a clean beta-MnO2 surface exceeds the long-term reaction rate by 3 orders of magnitude, highlighting the importance of linking Fe(III) precipitation with the reductive dissolution of beta-MnO2 by Fe(II).     

Kinetic analysis of microbial reduction of Fe(III) in nontronite

Microbial reduction of structural Fe(III) in nontronite (NAu-2) was studied in batch cultures under non-growth condition using Shewanella putrefaciens strain CN32. The rate and extent of structural Fe(III) reduction was examined as a function of electron acceptor [Fe(III)] and bacterial concentration. Fe(ll) sorption onto NAu-2 and CN32 surfaces was independently measured and described by the Langmuir expression with the affinity constant (log K) of 3.21 and 3.30 for NAu-2 and bacteria, respectively. The Fe(II) sorption capacity of NAu-2 decreased with increasing NAu-2 concentration, suggesting a particle aggregation effect. An empirical equation for maximum sorption capacity was derived from the sorption isotherms as a function of NAu-2 concentration. The total reactive surface concentration of Fe(III) was proposed as a proxy for the "effective" or bioaccessible Fe(III) concentration. The initial rate of microbial reduction was first-order with respect to the effective Fe-(III) concentration. A kinetic biogeochemical model was assembled that incorporated the first-order rate expression with respect to the effective Fe(III) concentration, Fe(II) sorption to cell and NAu-2 surfaces, and the empirical equation for maximum sorption capacity. The model successfully described the experimental results with variable NAu-2 concentration. The initial rate of microbial reduction of Fe(III) in NAu-2 increased with increasing cell concentration from 10(2) up to approximately 10(8) cells/mL, and then leveled off with further increase. A saturation-type kinetics with respect to cell concentration was required to describe microbial reduction of Fe(III) in NAu-2 as a function of cell concentration. Overall, our results indicated that the kinetics of microbial reduction of Fe(III) in NAu-2 can be modeled at variable concentration of key variables (clay and cell concentration) by considering the surface saturation, Fe(II) production, and its sorption to NAu-2 and cell surfaces.     

Enzymatic versus nonenzymatic conversions during the reduction of EDTA-chelated Fe(III) in BioDeNOx reactors

Reduction of EDTA-chelated Fe(III) is one of the core processes in the BioDeNOx process, a chemically enhanced technique for biological NOx removal from industrial flue gases. The capacity of Escherichia coli, three mixed cultures from full scale methanogenic granular sludge reactors, one denitrifying sludge, and a BioDeNOx sludge to reduce Fe(III)EDTA- (25 mM) was determined at 37 and 55 degrees C using batch experiments. Addition of catalytic amounts of sulfide greatly accelerated Fe(III)EDTA- reduction, indicating that biological Fe(III)EDTA- reduction is not a direct, enzymatic conversion but an indirect reduction with involvement of an electron-mediating compound, presumably polysulfides. It is suggested that not thermophilic dissimilatory iron-reducing bacteria but reducers of elemental sulfur or polysulfides are primarily involved in the reduction of EDTA-chelated Fe(III) in BioDeNOx reactors.     

Anaerobic degradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA)

The potential for anaerobic degradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) was investigated in laboratory incubations of sediments from a petroleum-contaminated aquifer and in aquatic sediments. The addition of humic substances (HS) stimulated the anaerobic degradation of MTBE in aquifer sediments in which Fe(III) was available as an electron acceptor. This is attributed to the fact that HS and other extracellular quinones can stimulate the activity of Fe(III)-reducing microorganisms by acting as an electron shuttle between Fe(III)-reducing microorganisms and insoluble Fe(III) oxides. MTBE was not degraded in aquifer sediments without Fe(III) and HS. [14C]-MTBE added to aquatic sediments adapted for anaerobic MTBE degradation was converted to 14CO2 in the presence or absence of HS or the HS analog, anthraquione-2,6-disulfonate. Unamended aquatic sediments produced 14CH4 as well as 14CO2 from [14C]-MTBE. The aquatic sediments also rapidly consumed TBA under anaerobic conditions and converted [14C]-TBA to 14CH4 and 14CO2. An adaptation period of ca. 250-300 days was required prior to the most rapid anaerobic MTBE degradation in both sediment types, whereas TBA was metabolized in the aquatic sediments without a lag. These results demonstrate that, under the appropriate conditions, MTBE and TBA can be degraded in the absence of oxygen. This suggests that it may be possible to design strategies for the anaerobic remediation of MTBE in petroleum-contaminated subsurface environments.     

Pore-Scale Characterization of Biogeochemical Controls on Iron and Uranium Speciation under Flow Conditions

Etched silicon microfluidic pore network models (micromodels) with controlled chemical and redox gradients, mineralogy, and microbiology under continuous flow conditions are used for the incremental development of complex microenvironments that simulate subsurface conditions. We demonstrate the colonization of micromodel pore spaces by an anaerobic Fe(III)-reducing bacterial species (Geobacter sulfurreducens) and the enzymatic reduction of a bioavailable Fe(III) phase within this environment. Using both X-ray microprobe and X-ray absorption spectroscopy, we investigate the combined effects of the precipitated Fe(III) phases and the microbial population on uranium biogeochemistry under flow conditions. Precipitated Fe(III) phases within the micromodel were most effectively reduced in the presence of an electron shuttle (AQDS), and Fe(II) ions adsorbed onto the precipitated mineral surface without inducing any structural change. In the absence of Fe(III), U(VI) was effectively reduced by the microbial population to insoluble U(IV), which was precipitated in discrete regions associated with biomass. In the presence of Fe(III) phases, however, both U(IV) and U(VI) could be detected associated with biomass, suggesting reoxidation of U(IV) by localized Fe(III) phases. These results demonstrate the importance of the spatial localization of biomass and redox active metals, and illustrate the key effects of pore-scale processes on contaminant fate and reactive transport.     

Reactivity of Fe(II)-bearing minerals toward reductive transformation of organic contaminants

Fe(II) present at surfaces of iron-containing minerals can play a significant role in the overall attenuation of reducible contaminants in the subsurface. As the chemical environment, i.e., the type and arrangement of ligands, strongly affects the redox potential of Fe(II), the presence of various mineral sorbents is expected to modulate the reactivity of surficial Fe(II)-species in aqueous systems. In a comparative study we evaluated the reactivity of ferrous iron in aqueous suspensions of siderite (FeCO3), nontronite (ferruginous smectite SWa-1), hematite (alpha-Fe2O3), lepidocrocite (gamma-FeOOH), goethite (alpha-FeOOH), magnetite (Fe3O4), sulfate green rust (Fe(II)4Fe(III)2(OH)12SO4 x 4H2O), pyrite (FeS2), and mackinawite (FeS) under similar conditions (pH 7.2, 25 m2 mineral/L, 1 mM Fe(II)aq, O2 (aq) < 0.1 g/L). Surface-area-normalized pseudo first-order rate constants are reported for the reduction of hexachloroethane and 4-chloronitrobenzene representing two classes of environmentally relevant transformation reactions of pollutants, i.e., dehalogenation and nitroaryl reduction. The reactivities of the different Fe(II) mineral systems varied greatly and systematically both within and between the two data sets obtained with the two probe compounds. As a general trend, surface-area-normalized reaction rates increased in the order Fe(II) + siderite < Fe(II) + iron oxides < Fe(II) + iron sulfides. 4-Chloronitrobenzene was transformed by mineral-bound Fe(II) much more rapidly than hexachloroethane, except for suspensions of hematite, pyrite, and nontronite. The results demonstrate that abiotic reactions with surface-bound Fe(II) may affect or even dominate the long-term behavior of reducible pollutants in the subsurface, particularly in the presence of Fe(III) bearing minerals. As such reactions can be dominated by specific interactions of the oxidant with the surface, care must be taken in extrapolating reactivity data of surface-bound Fe(II) between different compound classes.     

Formation of layered Fe(II)-Al(III)-hydroxides during reaction of Fe(II) with aluminum oxide

The reactivity of aqueous Fe(II) with aluminum oxide in anoxic solutions was investigated with batch kinetic experiments combined with Fe K edge X-ray absorption spectroscopy measurements to characterize Fe(II) sorption products. Formation of Fe(II)-Al(III)-layered double hydroxides with an octahedral sheet structure similar to nikischerite (NaFe(II)(6) Al(3)(SO(4))(2)(OH)(18) (H(2)O)(12)) was observed within a few hours during sorption at pH 7.5 and aqueous Fe(II) concentrations of 1-3 mM. These Fe(II) phases are composed of brucite-like Fe(II)(OH)(2) sheets with partial substitution of Al(III) for Fe(II), charge balanced by anions coordinated along the basal planes. Their fast rate of formation suggests that these previously unrecognized Fe(II) phases, which are structurally and compositionally similar to green rust, may be an important sink of Fe(II) in suboxic and anoxic geochemical environments, and impact the fate of structurally compatible trace metals, such as Co(II), Ni(II), and Zn(II), as well as redox-reactive species including Cr(VI) and U(VI). Further studies are required to assess the thermodynamics, formation kinetics, and stability of these Fe(II) minerals under field conditions.     

Heterogeneous reduction of PuO₂ with Fe(II): importance of the Fe(III) reaction product

Heterogeneous reduction of actinides in higher, more soluble oxidation states to lower, more insoluble oxidation states by reductants such as Fe(II) has been the subject of intensive study for more than two decades. However, Fe(II)-induced reduction of sparingly soluble Pu(IV) to the more soluble lower oxidation state Pu(III) has been much less studied, even though such reactions can potentially increase the mobility of Pu in the subsurface. Thermodynamic calculations are presented that show how differences in the free energy of various possible solid-phase Fe(III) reaction products can greatly influence aqueous Pu(III) concentrations resulting from reduction of PuO?(am) by Fe(II). We present the first experimental evidence that reduction of PuO?(am) to Pu(III) by Fe(II) was enhanced when the Fe(III) mineral goethite was spiked into the reaction. The effect of goethite on reduction of Pu(IV) was demonstrated by measuring the time dependence of total aqueous Pu concentration, its oxidation state, and system pe/pH. We also re-evaluated established protocols for determining Pu(III) {[Pu(III) + Pu(IV)] - Pu(IV)} by using thenoyltrifluoroacetone (TTA) in toluene extractions; the study showed that it is important to eliminate dissolved oxygen from the TTA solutions for accurate determinations. More broadly, this study highlights the importance of the Fe(III) reaction product in actinide reduction rate and extent by Fe(II).     

In situ bioreduction of uranium (VI) to submicromolar levels and reoxidation by dissolved oxygen

Groundwater within Area 3 of the U.S. Department of Energy (DOE) Environmental Remediation Sciences Program (ERSP) Field Research Center at Oak Ridge, TN (ORFRC) contains up to 135 microM uranium as U(VI). Through a series of experiments at a pilot scale test facility, we explored the lower limits of groundwater U(VI) that can be achieved by in-situ biostimulation and the effects of dissolved oxygen on immobilized uranium. Weekly 2 day additions of ethanol over a 2-year period stimulated growth of denitrifying, Fe(III)-reducing, and sulfate-reducing bacteria, and immobilization of uranium as U(IV), with dissolved uranium concentrations decreasing to low levels. Following sulfite addition to remove dissolved oxygen, aqueous U(VI) concentrations fell below the U.S. Environmental Protection Agengy maximum contaminant limit (MCL) for drinking water (< 30/microg L(-1) or 0.126 microM). Under anaerobic conditions, these low concentrations were stable, even in the absence of added ethanol. However, when sulfite additions stopped, and dissolved oxygen (4.0-5.5 mg L(-1)) entered the injection well, spatially variable changes in aqueous U(VI) occurred over a 60 day period, with concentrations increasing rapidly from < 0.13 to 2.0 microM at a multilevel sampling (MLS) well located close to the injection well, but changing little at an MLS well located further away. Resumption of ethanol addition restored reduction of Fe(III), sulfate, and U(VI) within 36 h. After 2 years of ethanol addition, X-ray absorption near-edge structure spectroscopy (XANES) analyses indicated that U(IV) comprised 60-80% of the total uranium in sediment samples. Atthe completion of the project (day 1260), U concentrations in MLS wells were less than 0.1 microM. The microbial community at MLS wells with low U(VI) contained bacteria that are known to reduce uranium, including Desulfovibrio spp. and Geobacter spp., in both sediment and groundwater. The dominant Fe(III)-reducing species were Geothrix spp.     

Microbial sulfate reduction at low pH in sediments of an acidic lake in Argentina

We measured sulfate reduction in the acidic (pH < or = 3) sediment of an Argentinean lake influenced by volcanism. Sulfate reduction rates of 2.04 mmol m(-2) d(-1) were determined with a 35SO4(2-) core injection method and confirmed by batch incubations and from H2S measurements in the sediment. H2S production stopped when iron reduction was stimulated by addition of ferric iron. The results suggest that sulfate reduction at pH values around 3 is possible and can probably be used in biotechnological strategies if competing microbial processes are inhibited and electron donors are highly available.     

Chromium(VI) bioremoval by Pseudomonas bacteria: role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination

Laboratory batch and column experiments were conducted to investigate the role of microbial exudates, e.g., exopolymeric substance (EPS) and alginic acid, on microbial Cr(VI) reduction by two different Pseudomonas strains (P. putida P18 and P. aeuroginosa P16) as a method for treating subsurface environment contaminated with Cr(VI). Our results indicate that microbial exudates significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite the fact Cr(III) has a very low solubility under the experimental conditions studied (e.g., pH 7). The formation of soluble organo-Cr(III) complexes led to the protection of the cells and chromate reductases from inactivation. In systems with no organic ligands, soluble organo-Cr(III) end products were formed between Cr(III) and the EPS directly released by bacteria due to cell lysis. Our results also provide evidence that cell lysis played an important role in microbial Cr(VI) reduction by Pseudomonas bacteria due to the release of constitutive reductases that intracellularly and/or extracellularly catalyzed the reduction of Cr(VI) to Cr(III). The overall results highlight the need for incorporation of the release and formation of organo-Cr(III) complexes into reactive transport models to more accurately design and monitor in situ microbial remediation techniques for the treatment of subsurface systems contaminated with Cr(VI).     

Sustained removal of uranium from contaminated groundwater following stimulation of dissimilatory metal reduction

Previous field studies on in situ bioremediation of uranium-contaminated groundwater in an aquifer in Rifle, Colorado identified two distinct phases following the addition of acetate to stimulate microbial respiration. In phase I, Geobacter species are the predominant organisms, Fe(III) is reduced, and microbial reduction of soluble U(VI) to insoluble U(IV) removes uranium from the groundwater. In phase II, Fe(III) is depleted, sulfate is reduced, and sulfate-reducing bacteria predominate. Long-term monitoring revealed an unexpected third phase during which U(VI) removal continues even after acetate additions are stopped. All three of these phases were successfully reproduced in flow-through sediment columns. When sediments from the third phase were heat sterilized, the capacity for U(VI) removal was lost. In the live sediments U(VI) removed from the groundwater was recovered as U(VI) in the sediments. This contrasts to the recovery of U(IV) in sediments resulting from the reduction of U(VI) to U(IV) during the Fe(III) reduction phase in acetate-amended sediments. Analysis of 16S rRNA gene sequences in the sediments in which U(VI) was being adsorbed indicated that members of the Firmicutes were the predominant organisms whereas no Firmicutes sequences were detected in background sediments which did not have the capacity to sorb U(VI), suggesting that the U(VI) adsorption might be due to the presence of these living organisms or at least their intact cell components. This unexpected enhanced adsorption of U(VI) onto sediments following the stimulation of microbial growth in the subsurface may potentially enhance the cost effectiveness of in situ uranium bioremediation.     

Effects of well spacing on geological storage site distribution costs and surface footprint

Geological storage studies thus far have not evaluated the scale and cost of the network of distribution pipelines that will be needed to move CO(2) from a central receiving point at a storage site to injection wells distributed about the site. Using possible injection rates for deep-saline sandstone aquifers, we estimate that the footprint of a sequestration site could range from <100 km(2) to >100,000 km(2), and that distribution costs could be <$0.10/tonne to >$10/tonne. Our findings are based on two models for determining well spacing: one which minimizes spacing in order to maximize use of the volumetric capacity of the reservoir, and a second that determines spacing to minimize subsurface pressure interference between injection wells. The interference model, which we believe more accurately reflects reservoir dynamics, produces wider well spacings and a counterintuitive relationship whereby total injection site footprint and thus distribution cost declines with decreasing permeability for a given reservoir thickness. This implies that volumetric capacity estimates should be reexamined to include well spacing constraints, since wells will need to be spaced further apart than void space calculations might suggest. We conclude that site-selection criteria should include thick, low-permeability reservoirs to minimize distribution costs and site footprint.