Iron-reducing bacteria can convert the oxidized form of iron in clay minerals, called ferric iron, into the reduced form of iron, called ferrous iron, which can then reduce hexavalent chromium to trivalent chromium—the reduced, insoluble and less toxic form of the heavy metal that poses a lower risk of groundwater contamination.
Bacteria has important role in long-term
remediation
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Background:Hexavalent chromium is a major environmental contaminant at several
Department of Energy (DOE) sites as well as
other sites around the
world.
The new study:
This study sheds light on the poorly understood process by which
iron- reducing bacteria reduce ferric iron in clay minerals, resulting in
ferrous iron that could then immobilize and detoxify chromium.
Results of the new study suggest that the iron- reducing activity of bacteria could
be artificially stimulated to produce ferrous iron in clay minerals,
which can then reduce hexavalent chromium to trivalent chromium. This
promising strategy could potentially enable long-term remediation of
heavy metal-contaminated sediments and groundwater aquifers worldwide.
Researchers at Environmental Molecular Sciences Laboratory (EMSL), a
DOE national scientific user facility at Pacific Northwest National
Laboratory, and Miami University added the iron-reducing bacterium Geobacter sulfurreducens to
tubes filled with ferric iron-containing clay minerals such as
smectites and chlorite. The researchers used ultra-sensitive microscopy
and spectroscopy instruments located in the Quiet Wing, a specialized
facility at EMSL, at national scientific user facility at Pacific
Northwest National Laboratory. Specifically, they used scanning electron
microscopy (SEM) with focused ion beam milling for thinning the samples
and performing elemental mapping. They also used transmission electron
microscopy (TEM) with electron energy loss spectroscopy for
high-resolution imaging and for determining the valence state of
chromium and iron before and after the reduction of hexavalent chromium
by ferrous iron present in clay minerals.
The bacteria reduced ferric iron in the clay minerals at low rates
when they were not stimulated with a compound known as
anthraquinone-2,6-disulfonate (AQDS), which enhances their iron-reducing
activity. Upon artificial stimulation with AQDS, the bacteria reduced
ferric iron present in smectites and smectite-rich clays from DOE’s
Hanford site, but not chlorite, at significantly higher rates. The
resulting ferrous iron in the clay minerals reduced hexavalent chromium
at higher rates with increasing temperatures, and at higher rates in
smectites compared with chlorite. The observed hexavalent chromium
reduction kinetics were well described by a second order rate equation
with respect to concentrations of hexavalent chromium and ferrous iron.
SEM and TEM imaging revealed that trivalent chromium was intimately
associated with the clay minerals, possibly in the form of
sub-nanometer-sized chromium hydroxide embedded in the clay matrix. This
structural feature is expected to minimize the secondary contamination
risk of trivalent chromium being exposed to environmental oxidants and
subsequently converting back to hexavalent chromium. Taken together, the
findings suggest that in-situ stimulation of iron-reducing
bacteria in iron-bearing clay minerals widely distributed in soils and
sediments at contaminated sites may represent a promising strategy to
stably immobilize chromium for long-term remediation efforts.
Source: (Adapted) from EMSL News
The cited study:
M.E. Bishop, P. Glasser, H. Dong, B. Arey, L. Kovarik, Reduction and immobilization of hexavalent chromium by microbially reduced Fe-bearing clay minerals, Geochim. Cosmochim. Acta, 133 (2014) 186-203. DOI: 10.1016/j.gca.2014.02.040
Related studies (newest first):
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Mathur Nadarajan Kathiravan, Ramalingam Karthick, Karuppan Muthukumar, Ex situ bioremediation of Cr(VI) contaminated soil by Bacillus sp.: Batch and continuous studies, Chem. Engineer. J., 169 (2011) 107–115. doi: 10.1016/j.cej.2011.02.060
J. Jeyasingh, V. Somasundaram, Ligy Philip, S. Murty Bhallamudi, Bioremediation of Cr(VI) contaminated soil/sludge: Experimental studies and development of a management model, Chem. Engineer. J., 160/2 (2010) 556–564. DOI: 10.1016/j.cej.2010.03.067
Ahmed Zahoor, Abdul Rehman, Isolation of Cr(VI) reducing bacteria from industrial effluents and their potential use in bioremediation of chromium containing wastewater, J. Environ. Sci., 21/6 (2009) 814–820. DOI: 10.1016/S1001-0742(08)62346-3
Guojun Cheng, Xiaohua Li, Bioreduction of chromium (VI) by Bacillus sp. isolated from soils of iron mineral area, Eur. J. Soil Biol., 45/5–6 (2009) 483–487. DOI: 10.1016/j.ejsobi.2009.06.009
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Chirayu Desai, Kunal Jain, Datta Madamwar, Hexavalent chromate reductase activity in cytosolic fractions of Pseudomonas sp. G1DM21 isolated from Cr(VI) contaminated industrial landfill, Process Biochem., 43/7 (2008) 713–721. DOI: 10.1016/j.procbio.2008.02.015
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Zainul Akmar Zakaria, Zainoha Zakaria, Salmijah Surif, Wan Azlina Ahmad,
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Related Information
CDC: Hexavalent chromium
Strategic Environmental Research and Development Program (SERDP): Hexavalent Chromium
Worst Polluted: Tannerie Operations - Chromium Pollution
Worst Polluted: Top Six Toxic Threats: Chromium
Related EVISA Resources
Link Database: Toxicity
of hexavalent chromium (chromate)
Link Database: Industrial Use of chromate
Link Database: Occupational exposure of hexavalent chromium
Link Database: Legislation for hexavalent chromium at the workplace
Link Database: Methods for chromium speciation analysis
Brief summary: The role of elemental speciation in legislation
Brief summary: Speciation and Toxicity
Brief summary: Standard methods for chromium speciation analysis
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last time modified: June 21, 2014
