Microbiology, Molecular Environmental Sciences, Environmental Research & Technology Division

The overall objective of this research is to determine the mechanisms of microbial transformations of Pu, Np, and Am in selected transuranic (TRU) and mixed wastes. In this study, Brookhaven National Laboratory (BNL) will investigate in a systematic manner the biotransformation of known chemical forms of actinides followed by more complex materials found in TRU wastes so that the mechanisms of biotransformation of complex mixtures of TRU wastes can be properly understood. Specifically, BNL will investigate (i) the mechanisms of microbial dissolution and stabilization of transuranics (Pu, Np, and Am), (ii) the biodegradation of representative bulk organic constituents of the waste, such as, contaminated cellulose-based materials, organic extractants, and chelating agents, (iii) the biotransformation of selected TRU wastes forms (Pu, Np, and Am contaminated sludge, materials, and soils), and, (iv) microbial transformations that result in reduction in waste volume and the removal of selected actinides with the potential for reclassification of the waste. Initial studies will focus on the transformations of several chemical forms of Pu, commonly present in TRU waste. The mechanisms of microbial dissolution or precipitation of Pu in the presence of electron donors and acceptors under various microbial process conditions will be investigated. The chemical speciation of Pu before and after microbial action will be determined by extended X-ray absorption fine structure (EXAFS) spectroscopy at the National Synchrotron Light Source (NSLS). This is a collaborative research effort involving BNL, University at Las Vegas (UNLV), State University of New York at Stony Brook (SUNY SB) and the Center for Environmental and Molecular Sciences (CEMS) at SUNY SB and BNL. The basic information derived from this research can pave the way for (i) treatment of TRU and mixed wastes resulting in the stabilization of actinides with reduction in waste volume, (ii) remediation of contaminated sediments, soils, sludges, and wastes, and, (iii) reclassification from TRU to low-level or hazardous waste category with considerable savings in disposal cost.

BIOGEOCHEMICAL CYCLING AND ENVIRONMENTAL STABILITY OF Pu RELEVANT TO LONG-TERM STEWARDSHIP OF DOE SITES

A.J. Francis (BNL), B .Honeyman (Colorado School of Mines) and P.Santschi (Texas A&M, Galveston)

Plutonium contamination is widespread in surface soils and subsurface sediments throughout the DOE complex. Until the last decade or so, Pu was generally considered to be relatively immobile in the terrestrial environment, with the exception of transport via aeolian and erosional mechanisms. More recently, however, the transport of colloidal forms of Pu has been invoked as providing a mobilization pathway in low intensity stream environments and the subsurface. Central to understanding the environmental behavior of Pu in vadose- and saturated-zones, as well as waste streams, is the contribution of microbial communities to Pu speciation. This proposed research would address the principal mechanisms by which naturally occurring microbial communities regulate transformations in Pu chemical speciation; such changes may lead to either enhanced Pu immobilization or its release from immobile phases and subsequent transport. The overall objective of this proposed research is to understand the biogeochemical cycling of Pu in environments of interest to long-term DOE stewardship issues. Central to Pu cycling (transport initiation, immobilization) is the role of microorganisms. The hypothesis underlying this proposal is that microbial activity is the causative agent in initiating the mobilization of Pu in near surface environments: through the transformation of Pu associated with solid phases, production of extracellular polymeric substances (EPS) carrier phases, and the creation of microenvironments. Also, microbial processes are central to the immobilization of Pu species, through the metabolism of organically complexed Pu species and Pu associated with extracellular carrier phases and the creation of environments favorable for Pu transport retardation. This is a multi-disciplinary joint research project involving expertise in actinide microbiology (BNL), surface chemistry/radiochemistry (Colorado School of Mines) and environmental radiochemistry/biogeochemistry and radiocolloids (Texas A&M).

MOLECULAR MECHANISMS OF URANIUM REDUCTION BY CLOSTRIDIA AND ITS MANIPULATION

A.J. Francis (BNL) and A.C. Matin (Stanford University)

Subsurface contamination by radionuclides and toxic metals is a major problem across the DOE complex. Removal of contaminated media is financially prohibitive. Consequently, innovative, cost effective, in situ stabilization technology by exploiting the natural attenuation processes must be developed. Microbial stabilization of actinides (U, Pu, Np) and fission products (Tc) in the subsurface environments is currently being investigated at DOE sites. A wide variety of bacteria, including the strict anaerobic spore forming Clostridia, are involved in the reductive precipitation of uranium and Tc in the subsurface environments. Although the mechanisms of U reduction by dissimilatory metal reducing bacteria (DMRB) Geobacter, and Shewanella, and sulfate-reducing bacteria (SRB) Desulfovibrio, have been extensively investigated, little is known of the mechanisms of uranium reduction by fermentative bacteria such as Clostridia. It is postulated that the excess of electrons generated during fermentation of organic materials are used in uranium reduction process. This research addresses the need for detailed studies of the enzymatic mechanisms for reduction of radionuclides and/or metals by fermentative microorganisms. The overall objective of this research is to elucidate systematically the molecular mechanisms involved in the reduction of uranium by Clostridia. We propose to (i) determine the role of hydrogenases in uranium reduction, (ii) purify the enzymes involved in uranium reduction, (iii) determine the mechanisms of reduction, e.g., one or two electron transfer reactions, and (v) elucidate the genetic control of the enzymes and cellular factors involved in uranium reduction. Speciation and intermediate oxidation states of uranium will be determined electrochemically and X-ray absorption near edge structure (XANES) at the National Synchrotron Light Source (NSLS). Fundamental knowledge of molecular assessment of radionuclide and metal reduction will allow us to exploit the naturally occurring processes to attenuate radionuclide and metal contaminants in situ in the subsurface dominated by low and high pH, high nitrate, and / or organic matter where the dissimilatory metal reducing bacterial activity will be limited. This is a collaborative study between BNL and Stanford University involving expertise in bimolecular science, biochemistry, microbiology and electrochemistry.

REDUCTIVE PRECIPITATION AND STABILIZATION OF URANIUM COMPLEXED WITH ORGANIC AND INORGANIC LIGANDS BY ANAEROBIC BACTERIA

A.J. Francis

This research addresses the need to understand the principal mechanisms in which microbes alter radionuclide-organic complexes, and the resultant impacts on radionuclide solubility and stability under anaerobic conditions. This work will (i) elucidate the mechanisms of biotransformation and fate of uranium complexed with organic and inorganic ligands under anaerobic conditions, (ii) identify the factors, which regulate the bioreduction of complexed uranium (U) leading to decomplexation and precipitation of reduced uranium, (iii) determine the interactions (complexation and immobilization) of phosphate, polyphosphates (PolyP) and calcium with U (VI) and U (IV), and, (iv) enhance the reductive precipitation and stabilization of soluble uranium organic and inorganic complexes under anaerobic conditions in the subsurface. This is a multidisciplinary collaborative research involving Brookhaven National Laboratory (BNL), Colorado School of Mines (CSM), and the State University of New York - Stony Brook (SUNY SB). In this study, the biotransformation of soluble U-organic and inorganic complexes by iron-, fermentative-, and sulfate-reducing bacteria, and mixed cultures of bacteria isolated from Natural and Accelerated Bioremediation Research (NABIR)-Field Research Center (FRC) will be examined. In addition, the influence of soluble and particulate organic matter, inorganic phosphate, calcium, zero-valent iron, pH, and ionic strength on the rate and extent of biotransformation and the stabilization of reduced U will be investigated. Speciation and characterization of soluble U complexes and precipitates will be accomplished using advanced spectroscopic techniques available at BNL, SUNY SB, and Environmental and Molecular Sciences Laboratory (EMSL). Spectroscopic data will be used to constrain postulated chemical species in chemical speciation models. Basic information obtained from this study can be used for in situ stabilization of radionuclides by enhancing the biotransformation of organic/inorganic radionuclide complexes in subsurface environments by anaerobic microorganisms and formation of stable actinide complexes.

THE MICROBIAL STABILIZATION OF PLUTONIUM IN THE SUBSURFACE ENVIRONMENT

A. J. Francis

Plutonium (Pu) contamination is widespread in the surface soils and subsurface sediments throughout the Department of Energy (DOE) complex. Pu is generally considered to be relatively immobile; however, transport of Pu, albeit at very low concentrations, has been observed at many DOE sites. While several processes responsible for mobilizing and transporting Pu have been hypothesized, the stabilization by microorganisms of the various chemical forms of Pu present in the contaminated environment over the long-term have not been fully evaluated. Understanding the processes that enhance Pu immobility (i.e., stabilization) is crucial for the long-term stewardship of DOE sites with environmental Pu contamination. The overall objective of this research is to elucidate the fundamental mechanisms of stabilization of soluble (organic- and inorganic-Pu complexes) and colloidal forms of Pu by naturally-occurring microbial communities. Results of this basic research should lead to (i) a better understanding of environmental conditions likely to foster retardation of Pu mobility and transport, and, (ii) strategies for engineering the long-term immobilization of Pu in soils and sediments. At most Pu-contaminated sites, removal of contaminated media is financially prohibitive; the development of methods for the in situ immobilization of Pu is crucial for the long-term stewardship of the Pu contaminated sites.

Last Modified: January 31, 2008
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