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Title: Microbial Uranium Immobilization Independent of Nitrate Reduction

Abstract

At many uranium processing and handling facilities, including sites in the U.S. Department of Energy (DOE) complex, high levels of nitrate are present as co-contamination with uranium in groundwater. The daunting prospect of complete nitrate removal prior to the reduction of uranium provides a strong incentive to explore bioremediation strategies that allow for uranium bioreduction and stabilization in the presence of nitrate. Typical in-situ strategies involving the stimulation of metal-reducing bacteria are hindered by low pH environments at this study site and require that the persistent nitrate must first and continuously be removed or transformed prior to uranium being a preferred electron acceptor. This project investigates the possibility of stimulating nitrate-indifferent, pH-tolerant microorganisms to achieve bioreduction of U(VI) despite nitrate persistence. Successful enrichments from U-contaminated sediments demonstrated nearly complete reduction of uranium with very little loss of nitrate from pH 4.9-5.6 using methanol or glycerol as a carbon source. Higher pH enrichments also demonstrated similar U reduction capacity with 5-30% nitrate loss within one week. Bacterial 16S rRNA genes were amplified from uranium-reducing enrichments (pH 5.7-6.7) and sequenced. Phylogenetic analyses classified the clone sequences into four distinct clusters. Data from sequencing and T-RFLP profiles indicated that the majority of themore » microorganisms stimulated by these enrichment conditions consisted of low G+C Gram-positive bacteria most closely related to Clostridium and Clostridium-like organisms. This research demonstrates that the stimulation of a natural microbial community to immobilize U through bioreduction is possible without the removal of nitrate.« less

Authors:
 [1];  [2];  [2];  [1];  [1]
  1. ORNL
  2. Florida State University
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
961553
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Microbiology; Journal Volume: 9; Journal Issue: 9
Country of Publication:
United States
Language:
English
Subject:
10 SYNTHETIC FUELS; BACTERIA; BINDING ENERGY; BIOREMEDIATION; CAPACITY; CARBON SOURCES; CLOSTRIDIUM; ELECTRONS; GENES; GLYCEROL; METHANOL; MICROORGANISMS; NITRATES; REMOVAL; SEDIMENTS; STABILIZATION; STIMULATION; URANIUM; VALENCE; Uranium; bioremediation; nitrate

Citation Formats

Madden, Andrew, Smith, April, Balkwill, Dr. David, Fagan, Lisa Anne, and Phelps, Tommy Joe. Microbial Uranium Immobilization Independent of Nitrate Reduction. United States: N. p., 2007. Web. doi:10.1111/j.1462-2920.2007.01347.x.
Madden, Andrew, Smith, April, Balkwill, Dr. David, Fagan, Lisa Anne, & Phelps, Tommy Joe. Microbial Uranium Immobilization Independent of Nitrate Reduction. United States. doi:10.1111/j.1462-2920.2007.01347.x.
Madden, Andrew, Smith, April, Balkwill, Dr. David, Fagan, Lisa Anne, and Phelps, Tommy Joe. Mon . "Microbial Uranium Immobilization Independent of Nitrate Reduction". United States. doi:10.1111/j.1462-2920.2007.01347.x.
@article{osti_961553,
title = {Microbial Uranium Immobilization Independent of Nitrate Reduction},
author = {Madden, Andrew and Smith, April and Balkwill, Dr. David and Fagan, Lisa Anne and Phelps, Tommy Joe},
abstractNote = {At many uranium processing and handling facilities, including sites in the U.S. Department of Energy (DOE) complex, high levels of nitrate are present as co-contamination with uranium in groundwater. The daunting prospect of complete nitrate removal prior to the reduction of uranium provides a strong incentive to explore bioremediation strategies that allow for uranium bioreduction and stabilization in the presence of nitrate. Typical in-situ strategies involving the stimulation of metal-reducing bacteria are hindered by low pH environments at this study site and require that the persistent nitrate must first and continuously be removed or transformed prior to uranium being a preferred electron acceptor. This project investigates the possibility of stimulating nitrate-indifferent, pH-tolerant microorganisms to achieve bioreduction of U(VI) despite nitrate persistence. Successful enrichments from U-contaminated sediments demonstrated nearly complete reduction of uranium with very little loss of nitrate from pH 4.9-5.6 using methanol or glycerol as a carbon source. Higher pH enrichments also demonstrated similar U reduction capacity with 5-30% nitrate loss within one week. Bacterial 16S rRNA genes were amplified from uranium-reducing enrichments (pH 5.7-6.7) and sequenced. Phylogenetic analyses classified the clone sequences into four distinct clusters. Data from sequencing and T-RFLP profiles indicated that the majority of the microorganisms stimulated by these enrichment conditions consisted of low G+C Gram-positive bacteria most closely related to Clostridium and Clostridium-like organisms. This research demonstrates that the stimulation of a natural microbial community to immobilize U through bioreduction is possible without the removal of nitrate.},
doi = {10.1111/j.1462-2920.2007.01347.x},
journal = {Environmental Microbiology},
number = 9,
volume = 9,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • No abstract prepared.
  • The daunting prospect of complete nitrate removal at DOE sites, such as the ERSP Oak Ridge Field Research Center (FRC), provides strong incentive to explore bioremediation strategies that will allow for uranium bioreduction and long-term stabilization in the presence of nitrate. The cost and effort required for complete nitrate removal from the FRC and similar DOE-contaminated sites may prove to be unworkable. For example, field tests of uranium bioreduction at the FRC have shown that nitrate levels rebound quickly and completely after cessation of active biostimulation.
  • There is a growing need for a better understanding of the biogeochemical dynamics involved in microbial U(VI) reduction due to an increasing interest in using biostimulation via electron donor addition as a means to remediate uranium contaminated sites. U(VI) reduction has been observed to be maximized during iron reducing conditions and to decrease upon commencement of sulfate reducing conditions. There are many unknowns regarding the impact of iron/sulfate biogeochemistry on U(VI) reduction. This includes Fe(III) availability as well as the microbial community changes, including the activity of iron-reducers during the uranium biostimulation period even after the onset of sulfate reduction.more » Up-flow column experiments were conducted with Old Rifle site sediments containing Fe-oxides, Fe-clays, and sulfate rich groundwater. Half of the columns had sediment that was augmented with small amounts of small-particle 57Fe-goethite to track continuously minute goethite changes, and to study the effects of increased Fe(III) levels on the overall biostimulation dynamics. The addition of the 57Fe-goethite did not delay the onset of sulfate reduction, but slightly suppressed the overall rate of sulfate reduction and hence acetate utilization, it did not affect the bacterial numbers of Geobacter-like species throughout the experiment, but did lower the numbers of sulfate reducers in the sediments. 57Fe-M√∂ssbauer analyses (a 57Fe-specific technique) confirmed that there was bioavailable iron present after the onset of sulfate reduction and that iron was still being reduced during sulfate reduction. Addition of the 57Fe-goethite to the sediment had a noticeable effect on the overall composition of the microbial population. 16S rRNA analyses of biostimulated sediment using TRFLP (terminal restriction fragment length polymorphism) showed that Geobacter sp. (a known Fe-reducer) was still active and replicating during the period of significant sulfate reduction. DNA fingerprints of the sediment-attached microbial communities were dominated by 5 TRFs, that comprised 25-57% of the total profile.« less
  • The subsurface at the Oak Ridge Field Research Center represents an extreme and diverse geochemical environment that places different stresses on the endogenous microbial communities, including low pH, elevated nitrate concentrations, and the occurrence of heavy metals and radionuclides, including hexavalent uranium [U(VI)]. The in situ immobilization of U(VI) in the aquifer can be achieved through microbial reduction to relatively insoluble U(IV). However, a high redox potential due to the presence of nitrate and the toxicity of heavy metals will impede this process. Our aim is to test biostimulation of the endogenous microbial communities to improve nitrate reduction and subsequentmore » U(VI) reduction under conditions of elevated heavy metals.« less