2004 RESEARCH PROJECTS Biogeochemistry PROJECT: Reduction and Reoxidation of Soils During and After Uranium Bioremediation;Implications for Long Term Uraninite Stability and Bioremediation Scheme Implementation PRINCIPAL INVESTIGATOR: Peter Jaffe Biomolecular Science and Engineering The proposed research addresses a key scientific question dealing with the implementation of trace metal/radionuclide bioremediation schemes that has been posed by the NABIR Biogeochemistry Element, namely the conditions and rates under which uranium will be remobilized after it has been precipitated biologically, and what alterations can be implemented to increase its long-term stability in groundwater after the injection of an electron donor has been discontinued.  Furthermore, the proposed research also addresses short-term iron reoxidation as a mechanism to enhance/extend uranium bioremediation, without its remobilization.   The proposed research is driven by the following hypotheses: Ferric (oxy)hydroxides, produced during the reoxidation of reduced soil, will “protect” a significant fraction of the reduced species, including U(IV), from being reoxidized when the groundwater redox potential increases after bioremediation is terminated. Short-term oxygen pulses can regenerate bioavailable Fe(III), extending the operation of long-term schemes of uranium bioremediation under iron reducing conditions. FeS will buffer the reoxidation of U(IV) by preferentially reacting with oxygen when the groundwater redox potential increases after bioremediation is terminated. To test these hypotheses, a series of column experiments will be conducted.  Columns will be loaded with soils from the FRC, the Old Rifle UMTRA site, and possibly Hanford.  They will be seeded with Geobacter metallireducens and a solution containing acetate, trace nutrients, and uranyl acetate will be fed to the columns to induce iron reducing conditions and the reduction of U(VI).  After most bioavailable iron has been reduced, sulfate reducing conditions will be achieved in selected columns by switching the electron donor to lactate, augmented with L-cysteine, and seeding the columns with Desulfovibrio desulfuricans.  Some reduced columns will be sacrificed to allow for U, Fe and S speciation and mineralogical analysis, and the rest of the columns will be reoxidized.  This will be done by feeding oxygenated water to some columns, and deoxygenated water containing nitrates to others.  Columns will be sacrificed at various stages of the reoxidation phase to allow for mineralogical analyses.  Column effluent will be monitored continuously to determine when and how much oxidative uranium dissolution occurs.  Some iron reducing columns will receive a periodic pulse of oxic water allowing for the partial reoxidation of Fe(II) to extend iron-reducing conditions.  The reoxidation rates and mineralogy of soils reduced in the laboratory will be compared to reoxidation rates of soils reduced in the field at FRC and the Old Rifle site.  Characterization of the mineral phases by chemical extraction, X-ray diffraction, and Mossbauer spectroscopy, and speciation of U, Fe and S using in situ X-ray absorption spectroscopy will be performed for all soils and experiments at different stages of the soil’s reduction and oxidation. Finally, results from the column experiments and mineral phase analysis will be used to mathematically simulate the observed reoxidation dynamics and the simulations will be compared to field reoxidation experiments to be conducted at the Old Rifle Site. PROJECT: Subsurface Bio-Immobilization of Plutonium:  Experiment and Model Validation Study PRINCIPAL INVESTIGATOR: Don Reed Biomolecular Science and Engineering A concurrent experimental and modeling study centered on the interactions of Shewanella alga BrY with plutonium species and phases is proposed.  The goal of this research is to investigate the long-term stability of bio-precipitated “immobilized” plutonium phases under changing redox conditions in biologically active systems.  The longevity of the subsurface immobilization of plutonium (e.g., by bio-reduction) is a key consideration in the effectiveness of remediation/containment approaches used, affects the design/choice of immobilization approaches, and defines issue regarding the closure of contaminated sites (e.g., natural attenuation).  Plutonium is the key contaminant of concern at several DOE sites that are being addressed by the overall NABIR program.   The benefit of this research to the NABIR biogeochemistry element is a research and modeling study that builds on past research with actinide interactions with S. alga will be conducted to address a key issue regarding the utility and application of bioremediation approaches to the containment of plutonium in the subsurface – This is a key consideration and component of a defensible containment and site closure strategy when plutonium is present as a contaminant.  The overall hypothesis for the proposed research is that stable recalcitrant plutonium phases will prevail in biologically active systems where bio-reduction occurs.  PROJECT: Mesoscale Biotransformation of Uranium: Identifying Sites and Strategies Where Reductive Immobilization is Practical PRINCIPAL INVESTIGATOR: Tetsu Tokunaga   Biomolecular Science and Engineering Bioreduction of U in contaminated sediments is an attractive strategy because of its low cost, and because of short-term studies supporting its feasibility.  However, any in-situ immobilization approach for U will require assurance of either permanent fixation, or of very low release rates into the biosphere.  Our long-term (2 years) laboratory experiments have shown that organic carbon (OC) based U(VI) bioreduction to UO2 can be transient even under sustained reducing (methanogenic) conditions. The biogeochemical processes underlying this finding urgently need to be understood.  This proposed research is designed to identify mechanisms responsible for anaerobic U oxidation, and identify conditions that will support long-term stability of bioreduced U.  We will investigate: (1) effects of OC concentration and supply rate (at different remediation stages) on remobilization of bioreduced U,  (2) influences of calcium concentrations and pH on U(IV)/U(VI) redox equilibrium,  (3) the roles of Fe- and Mn-oxides as potential U oxidants in sediments, and (4) the role of microorganisms in U reoxidation. [Back to Award Recipients Page]