2003 RESEARCH PROJECTS Integrative Studies (These Interdisciplinary projects integrate research from more than one NABIR element) PROJECT: Elucidating Bioreductive Transformations within Physically Complex Media: Impact on the Fate and Transport of Uranium and Chromium PRINCIPAL INVESTIGATOR: Scott Fendorf Integrative Studies In situ stabilization of toxic metals and radionuclides is an attractive approach for remediating many contaminated DOE sites. By immobilizing toxic metals and radionuclides in place, the removal of contaminated water to the surface for treatment as well as the associated disposal costs are avoided. A number of elements that are of particular interest within the DOE complex, such as uranium, chromium, and technetium, are highly mobile in their oxidized state but have limited solubilities in reduced (electron-rich) forms. Thus, microbiological reductive stabilization of contaminant metals has been, and continues to be, actively explored in order to enhance in situ remediaton. It is likely that surface and subsurface microbial activity can alter the redox state of toxic metals and radionuclides such as chromium and uranium, either directly or indirectly, so they are rendered immobile. Furthermore, anaerobic bacterial metabolic products will help to buffer pulses of oxidation, typically from fluxes of nitrate or molecular oxygen, and thus may stabilize reduced contaminants from oxidative mobilization. Imparting an important criterion on the probability that contaminants will undergo reductive stabilization, however, is the physical nature and heterogeneity of the media. Variations in hydrologic conductivity coupled with size exclusion will lead to gradients between reactants (inclusive of organisms) that will determine bioreduction pathways and progression, thus dictating contaminant sequestration. At present, there is a paucity of data regarding the influence of physical heterogeneity on biogeochemical processes within structured media representative of both surface and subsurface environments throughout the DOE complex. In response to this need, we will conduct an integrated geochemical-biological-hydrological study to elucidate the impact of physical heterogeneity on chemical and biological processes. Our investigations will commence with constructed systems and progress to contaminant transport of uranium and chromium through intact soil cores. Throughout our experiments we will define both solution and solid phase products using a host of techniques. A particular emphasis is made on resolving the spatial distribution of specific (process controlling) bacteria and their association with biogenic (biologically formed) reaction products using a suite of unique microscopic and micro-spectroscopic means. In combination, the results of this study will help to define (1) the reduction pathway/mechanism of uranium and chromium within natural settings, (2) spatial variation in sequestration mechanisms within physically complex (and by direct association, complex biological and chemical) media, (3) the impact of the hydrologic setting, inclusive of vadose versus saturated zones, and (4) the stability of reduced contaminants. Such information is necessary for predicting the fate of toxic metals within natural environment; it will further help to define remediation strategies and conditions that will lead to the least mobile (least soluble) reaction product. PROJECT: Biostimulation of Iron Reduction and Uranium Immobilization: Microbial and Mineralogical Controls PRINCIPAL INVESTIGATOR: Joel Kostka Integrative Studies Uranium is the most common radionuclide contaminant found within the U.S. nuclear weapons complex managed by the DOE, and nitrate is often a cocontaminant with uranium because of the use of nitric acid in the processing of uranium waste. Previous studies have demonstrated that metal-reducing microorganisms can effectively promote the precipitation and removal of uranium from contaminated groundwater, but nitrate must be removed prior to the onset of metal reduction. Our project will focus on the microbial activity and geochemical conditions which are likely to make strong contributions to the fate of uranium during in situ bioremediation. Studies will be conducted at the Field Research Center (FRC), Oak Ridge, Tennessee, designated by the U.S. DOE NABIR program. Specifically, the project will 1) quantify the relationships between active members of the metal-reducing microbial communities, iron mineralogy, and nitrogen transformations in the field and in laboratory incubations under a variety of biostimulation conditions, 2) purify and physiologically characterize new model metal-reducing bacteria isolated from moderately acidophilic FRC subsurface sediments, and 3) elucidate the biotic and abiotic mechanisms by which FRC aluminosilicate clay minerals are reduced and dissolved under environmental conditions resembling those during biostimulation. Active microbial communities, iron mineralogy, and nitrate utilization will be monitored using state-of-the-art microbiological and geochemical methodologies. Another strength of this project is that it will be conducted in the field or under real sediment conditions, whereas the majority of previous studies of uranium immobilization have been conducted under controlled conditions in the laboratory. The study will provide molecular markers that will shed light on how various microbial groups respond to environmental perturbations during in situ bioremediation. The influence of denitrification intermediates, which have been shown to oxidize uranium, on biostimulation mechanisms will be specifically targeted in sediment incubations. Through quantification of the activity of specific microbial populations and an in-depth characterization of iron minerals likely to catalyze uranium sorption/ precipitation, we will provide important inputs for reaction-based biogeochemical models which will provide the basis for development of in situ uranium bioremediation strategies.   PROJECT: Biogeochemical Coupling of Fe and Tc Speciation in Subsurface Sediments: Implications to Long-Term Tc Immobilization PRINCIPAL INVESTIGATOR: John Zachara  Community Dynamics and Microbial Ecology Technetium-99 is an important DOE subsurface contaminant. It is long-lived (t1/2 = 2.13 x 105 y), and exists in groundwater as the mobile pertechnetate anion [Tc(VII)O4-]. Pertechnetate can be immobilized by reduction to insoluble Tc(IV)O2•nH2O. The half-cell potential for this reaction is “intermediate” in environmental redox space. Our past NABIR research demonstrated that Fe(II) resulting from the activity of dissimilatory iron-reducing bacteria (DIRB) can reduce and immobilize pertechnetate from high nitrate waters in Hanford and Oak Ridge sediment. The reduction kinetics depend on the biogenic Fe(II) concentration and its molecular and mineralogic environment in the sediment. Newly proposed research will investigate the oxidation/remobilization reaction with oxygen, and quantify mineralogic, biogeochemical, and microbiologic factors that control it during extended in-ground residence times. Biotic, biogenic, and abiotic Tc(IV)O2•H2O phase assemblages will be generated in model systems and Oak Ridge and Hanford sediments using knowledge from our first 3 years of research. The oxidation kinetics of Fe(II) and Tc(IV) in these will be studied in batch and column systems before and after: i.) geochemical aging, and ii.) biogeochemical transformation by Fe(II) oxidizing, NO3- reducing bacteria. Spectroscopic and microscopic techniques will be used to define Fe and Tc speciation, and their physical locations and forms, as a basis for interpretation of oxidation rate. A linked equilibrium/kinetic biogeochemical transport model will be applied to identify the reaction network and quantify the interdependent kinetic reactions involved in the oxidation process.     [Back to Award Recipients Page]