2003 RESEARCH
PROJECTS
Biogeochemistry
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| PROJECT: |
Novel
imaging techniques, integrated with mineralogical, geochemical
and microbiological characterizations to determine the biogeochemical
controls on technetium mobility in FRC sediments |
| PRINCIPAL
INVESTIGATOR: |
Jon
Lloyd |
| Biogeochemistry |
Technetium-99
is a priority pollutant at numerous DOE sites, due to a combination
of its long half life (2.1 x 105 years), high mobility as Tc(VII)
in oxic waters, and bioavailability as a sulfate analog. The aim
of this project is to use a highly multidisciplinary approach to
identify the biogeochemical factors that control the mobility of
environmentally relevant concentrations of Tc(VII) in FRC sediments.
This hypothesis-driven research programme will use a combination
of geochemical, mineralogical, microbiological and spectroscopic
techniques to determine the solubility and phase associations of
Tc in batch sediment experiments ("progressive microcosms"),
where a sequence of terminal electron accepting processes are separated
by time. Additional column experiments utilizing FRC sediments containing
discrete biogeochemical zones will be challenged with low concentrations
of 99mTc, and the mobility of the radionuclide imaged using a g-camera.
By comparing the pattern of Tc immobilization, with high resolution
studies of the mineralogy, geochemistry and microbial ecology of
the columns, we will further characterize the biogeochemical controls
on Tc mobility in FRC sediments. Column experiments will also utilize
similar approaches to determine the stability of immobilized reduced
phases of Tc in the presence of oxidizing agents including nitrate.
The effectiveness of strategies proposed to stimulate Tc(VII) reduction
and precipitation in the subsurface will also be determined in both
batch and column experiments. Finally the experimental results will
be used to calibrate a modelling approach employing an established
coupled speciation and transport code. This would provide parameters
that could potentially be used to make predictions of the mobility
of Tc in FRC sediments and other subsurface environments.
| PROJECT: |
Investigation
of the Transformation of Uranium under Iron-Reducing Conditions:
Reduction of UVI by Biogenic FeII/FeIII Hydroxide
(Green
Rust) |
| PRINCIPAL
INVESTIGATOR: |
Ed
O'Loughlin |
| Biogeochemistry |
Green
rusts are mixed FeII/FeIII hydroxides that
are found in many suboxic environments where they are believed to play
a central role in the biogeochemical cycling of iron. The recent identification
of green rusts as products of the reduction of FeIII oxyhydroxides
by dissimilatory iron-reducing bacteria (DIRB), coupled with the ability
of synthetic green rust to reduce soluble UVI species to
insoluble UO2, suggests that biogenic green rusts may play
an important role in the mobility of U in FeIII-reducing
environments. The objective of our research is to examine the potential
for biogenic
green rusts
to affect the speciation of UVI under FeIII-reducing
conditions. To meet this objective, we will test the following hypotheses:
(1)
the
formation of green rusts from dissimilatory FeIII reduction
is controlled by FeIII speciation, solution composition,
and microbial physiology;
(2) the chemical composition and structural properties of biogenic
green rusts are variable and depend on the conditions under which they
were formed; (3) the rate of UVI reduction by biogenic green
rusts varies depending on their chemical composition and structure;
(4) the
rate of UVI reduction by a given biogenic green rust is
affected by the solution composition, which affects both the speciation
of UVI and UIV and the stability of
the green rust; and (5) the reduction of UVI to UIV can
be coupled to dissimilatory FeIII reduction
under conditions that promote the formation of biogenic green rust
and other
reactive FeII species. Our research will examine the effects of growth
conditions on the formation of biogenic green rusts resulting from
the reduction of FeIII oxyhydroxides by DIRB and the effects
of U concentration, carbonate concentration, pH, and the presence of
reducible co-contaminants
on both the kinetics of UVI reduction by biogenic green
rusts and the identity of the resulting U-bearing mineral phases. The
results of
this research will significantly increase our understanding of the
coupling of biotic and abiotic processes with respect to the speciation
of U in iron-reducing environments. In particular, the reduction of
UVI to UIV by biogenic green rust with the subsequent
formation of
U-bearing mineral phases may be effective for immobilizing U in suboxic
subsurface environments. This information has direct applications to
contaminant transport modeling and bioremediation engineering.
| PROJECT: |
Integrated
investigation on
the production and fate of organo-Cr(III) complexes from microbial
reduction
of chromate |
| PRINCIPAL
INVESTIGATOR: |
Luying
Xun |
| Biogeochemistry |
Chromate is soluble and extremely toxic, but trivalent Cr, Cr(III),
is much less toxic. In a survey of 18 Departmetn of Engergy facilities,
chromium contamination exists at 13 of them. Microbial reduction of
chromate to insoluble Cr(III) is a potential treatment for such sites.
In our prior research on chromate reduction, we have discovered that
soluble organo-Cr(III) complexes are likely formed in bioreduction
and then, perhaps, further transformed to insoluble Cr(III) precipitates.
The production of organo-Cr(III) complexes from chromate reduction
is extensively studied in eukaryotic systems because the stable DNA-Cr
adducts are mutagenic. However, the formation of soluble organo-Cr(III)
complexes from microbial reduction of chromate has only recently been
discovered, bringing up the necessity for investigating the significance
of the soluble complexes in Cr bioremediation. The proposed research
is aimed at: 1) characterizing the scope and extent of organo-Cr(III)
complex formation by chromate reducing microorganisms; 2) evaluating
cellular components that can potentially form organo-Cr(III) complexes;
3) addressing the stability and biodegradability of these organo-CR(III)
complexes; and 4) assessing the fate and transport of these compounds
in soils. The results will provide scientific guidance on whether organo-Cr(III)
should be considered during application of Cr bioremediation. The information
will also help establish a more complete biogeochemical cycle for Cr,
which does not currently consider organo-Cr(III) complexes.
| PROJECT: |
High-Resolution
Mineralogical Characterization and Biogeochemical Modeling of Uranium
Reduction Pathways at the NABIR Field-Research Center |
| PRINCIPAL
INVESTIGATOR: |
Chen
Zhu |
| Biogeochemistry |
The effectiveness
and feasibility of bioremediation at the field scale cannot be fully
assessed until the mechanisms of immobilization and U speciation
are resolved. However, characterization of the immobilized U and
its valence states is extremely difficult, because microbially mediated
mineral precipitates are generally nanometer (nm)-sized, poorly crystalline,
or amorphous. In this one-year, proof-of-concept phase of the proposed
research, we will (1) develop combined field emission gun - scanning
electron microscopy (FEG-SEM) and FEG transmission electron microscopy
(TEM) to detect and isolate uranium containing phases; (2) method
developments for TEM sample preparations and parallel electron energy
loss spectroscopy (EELS) determination of uranium valence; and (3)
to determine the speciation, fate, reactivity, valence states of
immobilized uranium, using the state-of-the-art 300-kV, FEG-TEM at
Johns Hopkins University.
We have
already successfully identified uranium association with goethite
and a carbonate- and phosphate-rich phase in contaminated
sediments
from Area 3 at the Oak Ridge Field Research Center (FRC). In the
proposed study, the valence states of immobilized U will be
characterized by
using EELS. The structures, compositions, and surface areas of precipitated
U phases, as well as mineral by-products, will be characterized using
the imaging, electron diffraction, and analytical capabilities of
the TEM. In addition to an imaging information limit of 0.12
nm, the Hopkins
facility is capable of Z-contrast imaging and energy-filtered TEM
(EFTEM) mapping of compositional variations across interfaces
at a resolution
better than 1 nm. The uranium energy loss spectrum is ideally suited
for energy-filtered imaging. Thus, EFTEM can reveal how uranium is
distributed within a phase, in relation to bacterial cells (intracellular
versus extracellular) and mineral byproducts (e.g., armoring).
We will
to study samples from the Oak Ridge FRC before and after laboratory
and field biostimulation experiments performed for the
NABIR program
(co-PI Lee Krumholz is funded to conduct field and laboratory tests).
This experimental design will ensure that observed mineral transformations
result from biostimulation. High-resolution mineralogical studies
are critically needed for the FRC site, because laboratory research
sponsored
by NABIR shows that the presence of Fe and Mn oxides can influence
U bioreduction and their influence may be a function of crystallinity.
This research
will provide a crucial component for a complete understanding of
the efficacy of uranium bioremediation. The results obtained
in this study will greatly help all investigators who use the
FRC in
their research. The fundamental observations to be made in this
study and
the methodology developed will have great utility for our understanding
of subsurface systems to address many national needs (e.g., nanotechnology).
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