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MESG is part of Biosciences Division at Argonne National Laboratory. One of the main foci during the creation and growth of the Molecular Environmental Science Group has been the development of an internationally recognized integrated multidisciplinary scientific team focused on the investigation of fundamental biogeochemical questions. Presently, expertise that is represented by members of the MES Group includes high-energy x-ray Physics, Environmental Chemistry, Environmental Microbiology, and radiolimnology. Additional expertise in Geomicrobiology, electron microscopy, and x-ray microscopy often is provided by collaborations with scientists outside of our group. Partial support of the Molecular Environmental Science Group is provided by the US DOE Environmental Remediation Science Programs to facilitate the use of the Advanced Photon Source by scientists funded to do work within that program. Information about how to apply for General User Beam time at the Advanced Photon Source can be found at the Advanced Photon Source web site or by contacting Ken Kemner or Bruce Ravel. It is currently difficult to predict the behavior of contaminants (organic, heavy metal, and radionuclide) in the subsurface. Understanding the processes controlling the fate and transport of contaminants in the environment is of fundamental importance in the development and evaluation of effective remediation and sequestration strategies. Bacteria and the extracellular material associated with them are thought to play key roles in determining a contaminant's chemical speciation and its mobility in the environment. Additionally, the microenvironment at and adjacent to actively metabolizing cells can be significantly different from the bulk environment. Our group uses a number of analytical techniques (i.e. ICP-AES, HPLC, kinetic phosphorescence analysis, x-ray diffraction, etc.) to better understand the role of minerals, microbes, and microbial exudates in determining contaminant mobility and fate in the environment. Additionally, we make use of a number of synchrotron-based x-ray techniques to further our understanding of the processes occurring at physical, geological, chemical, and biological interfaces that determine contaminant fate. Hard x-ray absorption spectroscopy techniques such as extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray absorption near edge spectroscopy (XANES) can provide information on the local chemical environment, coordination, and valence of individual elements in soils and sediments. Additionally, hard x-ray micro-imaging techniques (i.e. x-ray fluorescence microscopy) enable element-specific investigation of complex environmental samples at the needed micron and submicron length scales. An important advantage of these techniques results from the large penetration depth of hard x-rays in water. | |||
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![[Ed]](Ed_web_files/image002.jpg) |
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Ken Kemner Physicist |
Edward O'Loughlin |
Deirdre Sholto-Douglas
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Maxim Boyanov |
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Bruce Ravel Physicist |
Shelly Kelly |
Kent Orlandini |
Kelly Skinner-Nemec |
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![[Shelly]](skelly.jpg)
![[Kent]](kent.jpg)
![[Kent]](Kelly.jpg)
The
ability of several dissimilatory Fe(III)-reducing bacteria to reduce U(VI)
and produce less-soluble U(IV) species is now well established, but the
mechanisms by which the enzymatic U(VI) reduction occurs are unclear.
Reduction rates appear dependent on the presence or absence of certain
c-type cytochromes in the cell membrane of Shewanella oneidensis
MR-1. We examined both wild-type cultures and targeted gene deletion
cultures lacking in certain cytochromes to determine the uranium
distribution within the cells and in the intercellular space. TEM imaging
showed distinct regions of uranium accumulation: A) in the periplasmic
space, and B) along 100 nm thin and several micron long strings in the
intercellular space, presumably associated with the extracellular polymeric
substances (EPS) exuded by cells. Using synchrotron X-ray fluorescence
imaging with 150 nm resolution we were able to correlate the regions of high
uranium accumulation to high Fe and P content in the fibroid structures (B)
but not in extracellular regions void of the structures (C), implicating
heme-containing outer membrane proteins in the reduction of U(VI) in the
extracellular EPS-UO2 matrix. 
We
have used hard x-ray fluorescence measurements,
with 150 nm spatial resolution, to obtain elemental maps and quantitative
chemical analyses of single planktonic and adhered cells of Pseudomonas
fluorescens str. NCIMB 11764. Differences between the planktonic and
adhered cells with regard to morphology, elemental composition, and their
sensitivity to Cr(VI). The ability to obtain this information at spatial
scales of 150 nm on living cells (i.e. without the need for high vacuum)
opens new vistas with regard to studying microorganisms under conditions
relevant to their activities in natural systems. 
The
formation of intra-cellular precipitates in living orgranisms is unusual
and far less common than the abundant external mineral formation during
dissimilatory Fe(III) reduction (DIR). The functional role of internal
precipitates is largely unknown, although a respiratory role is
hypothesized. We studied the valence state of internal Fe precipitates
formed during DIR inside Shewanella putrefaciens CN32 cells. The
spatial distribution of elemental content within single bacterial cells
was determined using synchrotron-based X-ray fluorescence microscopy
with 150 nm resolution, followed by micro-XANES scans of different
regions on the cell and from the extracellular precipitates. Results
show that the Fe valence state of external microprecipitates is
consistent with magnetite, whereas Fe associated with the cells is more
reduced. Within the cell, Fe is most reduced in regions free from
internal precipitates and more oxidized where the precipitates are
formed. These results suggest a respiratory role of the internal
precipitates. This study also demonstrates the chemical complexity of
microbial environments and the need for spatially resolved spectroscopic
methods to accurately investigate the processes.
GRs typically form under near neutral to alkaline conditions in suboxic
environments and have been identified as products of both abiotic and
microbially induced corrosion of iron and steel as well as microbially mediated
and abiotic reductive dissolution of ferric oxyhydroxides by Fe2+. Direct
evidence for the presence of GRs in hydromorphic soils has recently been
reported (Trolard et al. 1997; Abdelmoula et al. 1998; Génin et al. 1998) and
there is evidence to suggest that GRs may control Fe solubility in these soils
(Bourrié et al. 1999). In addition, GRs are metastable intermediates in the
transformation of Fe(II) to magnetite and Fe(III) oxyhydroxides (e.g.
lepidocrocite and goethite) under near neutral to alkaline conditions and are
believed to play a central role in the redox cycling of Fe in many aquatic and
terrestrial environments. 
Current
research on bioremediation of uranium-contaminated groundwater focuses on
supplying indigenous metal-reducing bacteria with the appropriate metabolic
requirements to induce microbiological reduction of soluble uranium(VI) to
poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in
the presence of environmentally relevant levels of calcium revealed limited
and slowed uranium(VI) reduction and the formation of a Ca-UO2-CO3
complex. However, the stoichiometry of the complex is poorly defined and may
be complicated by the presence of a Na-UO2-CO3
complex. Such a complex might exist even at high calcium concentrations, as
some UO2-CO3 complexes will still be present. The
number of calcium and/or sodium atoms coordinated to a uranyl carbonate
complex will determine the net charge of the complex. Such a change in
aqueous speciation of uranium(VI) in calcareous groundwater may affect the
fate and transport properties of uranium. In this paper, we present the
results from X-ray absorption fine structure (XAFS) measurements of a series
of solutions containing 50 mM uranium(VI) and 30 mM sodium bicarbonate, with
various calcium concentrations of 0 to 5 mM. Use of the data series reduces
the uncertainty in the number of calcium atoms bound to the UO2-CO3
complex to approximately 0.6 and enables spectroscopic identification of the
Na-UO2-CO3 complex. At nearly neutral pH values, the
numbers of sodium and calcium atoms bound to the uranyl triscarbonate
species are found to depend on the calcium concentration, as predicted by
speciation calculations. 
Minerals commonly found in soils, bacteria and the extracellular
material associated with them are thought to play a key role in
determining an element's speciation and thus its mobility in the
subsurface. In addition, the size of contaminant-associated particles has
also been shown to be a factor that controls subsurface mobility via
colloid transport. We have performed a number of x-ray absorption
spectroscopy, x-ray microscopy, and electron microscopy studies of
biogeochemical systems and have identified nanoparticle formation in a
number of them. Such experiments include studies of the oxidation state
and local environment of uranium exposed to (1) green rusts (GR) in an
anaerobic environment, and (2) sulfate-reducing bacteria (SRB)
(Desulfosporosinus sp.). An additional study includes trace metal
analysis of biomineralization products created by SRBs (Desulfovibrio
sp.) isolated from an abandoned Pb and Zn mine. The XANES spectra of
the GR and Desulfosporosinus systems indicate that U(VI) is reduced
when exposed to GR and the SRB. Additionally, EXAFS and electron
microscopy studies verify that theUO2 moieties formed are nano-sized. XRF
microprobe and electron microscopy studies of biomineralization products
formed by Desulfovibrio sp. indicate the formation of micron-size
particles consisting of randomly-packed nano-sized ZnS moieties that also
contain appreciable quantities of As and Se.
Modeling the transport and fate of heavy metals in the environment is a
central issue in environmental engineering and geochemistry. Interaction with
diverse complexing media (minerals, biomass, etc.) must be considered under
ambient conditions. Fluorescence XAFS was used to identify and quantify
in-situ the adsorption channels of Cd to the isolated cell walls of a
common groundwater bacterium, Bacillus subtilis. The results indicate
that Cd binds predominantly to protonated phosphoryl ligands below pH 4.4, while
at higher pH adsorption to carboxyl groups becomes increasingly important. At pH
7.8 we observe the activation of an additional binding site, which we
tentatively ascribe to deprotonated phosphoryl ligands. This work is a
collaboration with the groups of Prof. Bruce Bunker and Prof. Jeremy Fein from
the University of Notre Dame, IN. 
