2000 RESEARCH PROJECTS Program Element 4 Biogeochemical Dynamics PROJECT: The Role of Biogenic Solids in the Reductive Stabilization of Metal Contaminants: Influences on Microbial Versus Chemical Pathways and Reaction Products PRINCIPAL INVESTIGATOR: Scott E. Fendorf PROGRAM ELEMENT 4 Biogeochemical Dynamics To enhance in situ remediaton, microbiological reductive stabilization of contaminant metals is actively being explored. It is unclear whether reduction of metal contaminants is mediated principally by direct, enzymatic reduction or by geochemical reduction dependent on products of microbial metabolism. Recent evidence has brought to light the preponderance of biogenic solids formed by metal or sulfate reducing bacteria; however, the reactivity of these reduced materials toward contaminants in oxidized states is not well described. The specific pathway by which reduction takes place will be defined by the operating reaction kinetics. Here we seek to define the kinetics of chromate and uranyl reduction by a number of Fe(II) bearing biogenic solids that are common in reduced environments. We will then compare reduction processes in mixed phase (biologically active with reduced solids) systems under both static and dynamic hydrologic conditions. Throughout our experiments we will define both solution and solid phase products using a host of spectroscopic and microscopic techniques. Because the time period of immobilization will in large part be determined by the stability of the solid-phase, we will directly measure the oxidative dissolution of the reduced contaminants using dissolved oxygen and Mn(III)-pyrophosphate under hydrodynamic conditions. PROJECT: Hydrogen as an Indicator to Assess Biological Activity During Trace-Metal Bioremediation PRINCIPAL INVESTIGATOR: Peter Jaffé PROGRAM ELEMENT 4 Biogeochemical Dynamics The design and operation of a trace-metal or radionuclide bioremediation scheme requires that specific redox conditions be achieved at given zones of an aquifer for a predetermined duration. Tools are therefore needed to identify and quantify the terminal electron acceptor processes (TEAPs) that are being achieved during bioremediation in an aquifer, and that this be done at a high spatial resolution. The proposed research addresses this need, which is one of the aims of NABIR’s Assessment Element.   Hydrogen holds the promise of being a key parameter that that can be used to identify TEAPs. Theoretical analysis have shown that steady-state hydrogen levels in the subsurface are solely dependent upon the physiological parameters of the hydrogen-consuming microorganisms, and that hydrogen concentrations increase as each successive TEAP yields less energy for bacterial growth. The assumptions for this statement may not hold during a bioremediation scheme in which an organic substrate is injected into the subsurface and where organisms may consume hydrogen and carbon simultaneously. The objective of the proposed research is to gain a basic understanding of the hydrogen dynamics in an aquifer during a trace metal/radionuclide bioremediation scheme. To address this objective we have formulated the following working hypotheses: Hydrogen can be used an indicator that is more accurate than redox couples or Eh measurements for identifying bacterial activities in the subsurface during a bioremediation scheme. Bacteria via the combined utilization of a carbon source and hydrogen set hydrogen levels in the subsurface. These levels can be estimated for each TEAP from the biokinetic parameters of the carbon- and the hydrogen-degrading organisms. In TEAP transition zones, hydrogen levels will not correspond to the characteristic levels of either TEAP. To test these hypotheses, we will conduct two different types of laboratory experiments, and combine them with analytical tools to interpret and test the results in a quantitative manner. We will conduct batch experiments where we will track the simultaneous utilization of hydrogen and acetate in suspended growth experiments in order to determine if the bacterial growth on these two substrates can be explained by a conceptual model that we have formulated. Column experiments will be designed to manipulate a continuous-flow porous media system in which first acetate and then lactate are injected as an electron donor. We will then monitor the spatial distribution of electron donors (including hydrogen), electron acceptors, and microbial populations. The columns will be operated so that the redox conditions will range from oxidizing to reduced and back to oxidizing, in order to simulate a groundwater environment from the source of a plume to it’s outer edge. The results from the column experiments and from their numerical analysis will give us a thorough insight on how hydrogen concentrations correspond to bacterial activities in a groundwater system that is being augmented by an external carbon source. PROJECT: The Role of Biogeochemical Dynamics in the Alteration of U Solid Phases Under Oxic Conditions PRINCIPAL INVESTIGATOR: Heino Nitsche PROGRAM ELEMENT 4 Biogeochemical Dynamics Although in-situ and ex-situ microbial reduction has been demonstrated to reduce actinide groundwater concentrations in anaerobic systems, such biological alterations must be considered temporary treatments unless long-term anoxia can be guaranteed. Under oxidizing conditions, the more mobile, higher oxidation states of the actinides such as uranium (U), neptunium (Np), and plutonium (Pu) are the thermodynamically favored species. For example, in oxic U ore deposits in which uraninite (consisting of reduced, tetravalent U as UO2+x) is the parent material, alteration to U6+ minerals is well-documented with the U6+-phosphates frequently defining the boundaries of the ore body. While microorganisms are undoubtedly present in such systems, their role in these transformations and the ultimate precipitation of the insoluble phosphate phases is not well understood. Furthermore, the impact of wet-dry cycling on such alterations and the presence of other transuranium actinide elements has not, to our knowledge, been studied to any significant extent. The purpose of our study is to investigate the role of biogeochemical dynamics in the alteration of U solid phases under oxic conditions to form U6+-phosphates. We are investigating the role of important bacterial strains (pure cultures) and consortia of bacteria (isolated from a DOE field site) on the alteration of simple U6+ oxide hydrates to U6+-phosphates and phosphate solid solutions of U6+ and other actinides. To address such transformations under both saturated and vadose zone conditions, systems of interest include those that remain constantly hydrated and those that are exposed to wet-dry cycling. The microorganisms under study include pure cultures of Bacillus sphaericus (ATCC 14577), Desulfovibrio desulfuricans, and Geobacter metallireducens. These strains of microbes were selected to reflect a variety of subsurface conditions including aerobic systems, temporarily anaerobic and/or microaerophilic. Results with pure cultures will be compared with results obtained using consortia isolated from a field site. We will determine transformation pathways between the initial U6+ oxide hydrate and the secondary solids that form, as well as rates for these transformations. We will monitor for changes in metal oxidation state, and provide coordination information about the actinides. Our research will link important geochemical and microbiological aspects of this problem, and provide a fundamental basis for predicting the complex and dynamic interplay of natural attenuation and potential biological treatment strategies. PROJECT: Stabilization of Heavy Metal Contaminants in Subsurface Environments: Microbially Mediated Precipitation of Metal Sulfides from Complexes with Organic Chelates PRINCIPAL INVESTIGATOR: Murthy A. Vairavamurthy PROGRAM ELEMENT 4 Biogeochemical Dynamics Heavy-metal contaminants can be immobilized in situ in the subsurface by precipitating them as metal sulfides by chemical or microbiological methods. Bacterial reduction of sulfate or partially oxidized sulfur species (e.g., thiosulfate) generates H2S, but the appropriate conditions for this transformation are not well understood because the growth of many bacteria, including sulfate reducers, is affected by the toxicity of various heavy metal pollutants. Our hypothesis is that the formation of highly stable complexes of heavy metals with chelates modulates their toxicity, thereby allowing sulfate reducing bacteria to grow and produce H2S. Our main goal is to carefully investigate the fundamental mechanisms of biologically mediated metal-sulfide precipitation under sulfate-reducing conditions wherein the toxicity of two highly toxic metals, cadmium and lead, has been modulated by chelation. Our studies will focus on the growth kinetics of different types of sulfate-reducing bacteria with these heavy metal ions present as chelates of several typical natural and synthetic ligands. The rate of biological conversions will be correlated with chemical equilibrium data on the metal complex. Sulfur-k-edge XANES spectroscopy and EXAFS spectroscopy will characterize the sulfur and metal forms generated. This research will illuminate the fundamental biological processes for precipitating heavy metals as sulfides in situ in sulfate-reducing subsurface systems. PROJECT:   PRINCIPAL INVESTIGATOR: John M. Zachara PROGRAM ELEMENT 4 Biogeochemical Dynamics Abstract to follow. [Back to Award Recipients Page]