Remediation and Recovery of
Uranium from Contaminated
Subsurface Environments with
K E L V I N B . G R E G O R Y * A N D
D E R E K R . L O V L E Y
Department of Microbiology, University of Massachusetts,
203 Morrill 4 North, Amherst, Massachusetts 01003
Previous studies have demonstrated that Geobacter
species can effectively remove uranium from contaminated
groundwater by reducing soluble U(VI) to the relatively
insoluble U(IV) with organic compounds serving as the
electrodes might serve as an alternative electron donor
for U(VI) reduction by a pure culture of Geobacter
sediments. Electrodes poised at -500 mV (vs a Ag/AgCl
of cells. However, when the poise at the electrode was
removed, all of the U(VI) returned to solution, demonstrating
that the electrode did not reduce U(VI). If G. sulfurreducens
was present on the electrode, U(VI) did not return to
solution until the electrode was exposed to dissolved
reduced U(VI) to U(IV) which was stably precipitated
U(VI) was removed and recovered from groundwater
using poised electrodes. Electrodes emplaced in flow-
removed U(VI) from the groundwater, and 87% of the
uranium that had been removed was recovered from the
electrode surface after the electrode was pulled from the
sediments. These results suggest that microorganisms
can use electrons derived from electrodes to reduce U(VI)
and that it may be possible to remove and recover
uranium from contaminated groundwater with poised
Uranium contamination of groundwater is a widespread
environmental problem (1). The volume and areal extent of
uranium contamination often precludes pump and treat
remediation strategies. An alternative approach is to reduce
the soluble, and thus mobile, U(VI) to relatively insoluble
U(IV), which precipitates (2-4). This prevents further migra-
demonstrated that adding electron donors, such as acetate,
U(VI) reduction (2-6). In this manner, soluble uranium
contamination can be concentrated as a solid phase in a
discrete zone within the aquifer.
Although in situ uranium bioremediation with dissimila-
reduced as long as a community with a high proportion of
U(VI)-reducing Geobacter species was maintained (4). How-
growth of the Geobacter species (7), was depleted from the
declined and sulfate-reducing microorganisms, which did
not reduce U(VI), became the primary acetate-consuming
organisms. Thus, additional environmental manipulations
to sustain the Geobacteraceae for long periods of time are
Another potential limitation of this approach is that
although U(VI) reduction prevents the further mobility of
the uranium, uranium remains in the environment in the
that the U(IV) precipitates generated during in situ uranium
bioremediation can be stable in the environment (8).
U(IV) precipitates after a site is remediated. This could be
accomplished with microbial (7) or chemical (9) extraction
These extraction methods may cost as much or more as the
U(VI) reduction step. Therefore, a method for uranium
remediation which would permit a simpler method of
removing the precipitated uranium would be beneficial.
Although organic acids and hydrogen are the common
electron donors for Geobacter species (10), these organisms
can also accept electrons from electrodes (11). For example,
with a properly poised electrode as the sole electron donor,
Geobacter metallireducens reduced nitrate to nitrite and
raised the possibility that Geobacter species might also be
able to reduce U(VI) with electrodes serving as the electron
donor and that this could be an alternative strategy for
promoting reduction of U(VI) in contaminated subsurface
electrodes can extract various ions from water (12-15) and
is collectively known as capacitive deionization. Capacitive
deionization technology has been applied to remove heavy
electric potential at a cathode may reduce target contami-
nants such as pertechnetate (22), cadmium (23), and
chromate (24). During electrokinetic remediation of con-
taminated soil and groundwater, current is applied to
electrodes which are inserted into the ground and contami-
nants are actively transported to the anode or cathode via
electro-osmosis, electromigration, diffusion, and/or elec-
trophoresis (25-28). However, the presence of natural
electrolytes and humic substances often confounds this
found little change in microbial or fungal diversity after
treatment (30); however, it is known that members of
Geobacteraceae may be enriched from sediment on cathodi-
cally poised electrodes for nitrate respiration (11).
Here we report that electrodes can serve as an electron
donor for U(VI) reduction by Geobacter sulfurreducens and
* Correspondingauthorphone: (413)577-4669;fax: (413)545-1578;
Environ. Sci. Technol. 2005, 39, 8943-8947
10.1021/es050457e CCC: $30.25
Published on Web 10/13/2005
2005 American Chemical SocietyVOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY98943
promote the removal of uranium from contaminated ground-
of uranium groundwater contamination, which may have
several advantages over previously described approaches.
Biotic and Abiotic Defined Reactors. The glass, dual-
the reactors were autoclaved, then flushed with sterile,
0.25; KCl, 0.1; and NaHCO3, 2.0. The pH of the medium was
6.9. The chambers were placed on a multiposition stir plate,
and the working chamber was stirred with a magnetic stir
allowed to equilibrate at -500 mV (vs a Ag/AgCl reference
soil or uranium. The control chambers were treated in an
identical fashion but were not connected to a potentiostat.
Current measurements and electron recovery conversions
from an aqueous UO2Cl2stock solution which was equili-
brated with anaerobic gas (80:20 N2/CO2). Attempts to
anaerobic N2/CO2 to prevent oxygen intrusion. Aerobic
recovery of U(VI) was performed by bubbling air through
the working and counter chambers.
Geobacter sulfurreducens strain PCA (ATCC no. 51573)
was obtained from our culture collection and grown in the
previously described medium (32) at 30 °C. Acetate served
poorly crystalline Fe(III) oxide as the electron acceptor and
were transferred 3 times into medium with 40 mM fumarate
to the working chamber containing a poised electrode.
Fumarate respiration and current consumption served as
evidence that G. sulfurreducens was respiring on the surface
of the electrode (11). Respiration of fumarate and current
consumption was observed within 48 h of inoculating the
working chamber with G. sulfurreducens. Before addition of
uranium, the medium was exchanged to remove any
remaining planktonic cells and the fumarate.
Batch Soil Experiments. Contaminated soil and ground-
water was collected from a former uranium ore processing
facility in Rifle, CO (4). Chambers were constructed from 60
mm i.d. borosilicate glass (see the Supporting Information,
access ports at 4.5 and 12.5 cm from the bottom. A graphite
electrode (working electrode) grade G-10 7.62 × 1.27 × 2.54
cm3(Graphite Engineering; Greenville, MI) was placed at
contaminated soil to cover the electrode. The depth of soil
was approximately 10 cm. An identical electrode (counter
electrode) was suspended above the working electrode. The
from the Rifle site. A Ag/AgCl reference electrode (World
closed with a rubber stopper, and the water above the
sediment was bubbled with N2gas. Uranium was added to
provide 10 µM U(VI), and the chamber was shaken periodi-
cally to mix the soil and water. After a period of 96 h, power
was supplied to the electrodes and poise at the working
electrode was established, initially at -500 mV (vs Ag/AgCl)
and decreased stepwise to -700 mV. Samples for U(VI)
shaking and filtered through a syringe filter (0.2 µm pore
Column Experiments. Columns were constructed of
ports at 1, 6, 11, 15, and 22 cm) (see the Supporting
Information, Figure S2). With the exception of electrodes
contaminated soil from Rifle, CO. The working electrode, 6
cm from the inlet, was a porous, cylindrical (4.75 cm o.d.
1.27 cm thick), grade G-10 graphite (Graphite engineering;
electrode. Contaminated groundwater from Rifle, CO was
fed via syringe pumps (Harvard Apparatus; Holliston, MA).
with UO2Cl2to provide a concentration of 80 µM. The flow
rate to the column was 1.0 mL/min. Water samples were
collected from the 11 cm port with a syringe.
Analytical Methods. Uranium was measured by kinetic
was analyzed with ion chromatography using a Dionex DX-
100 as described elsewhere (33).
The microbial community was assessed as previously
(pH 8) to produce a slurry of graphite and cells. DNA was
extracted from the graphite with a modified version of the
miniprep of bacterial genomic DNA protocol (35). 16S rRNA
genes were amplified with the primer 8 forward (36) with
519 reverse (36) or 338 forward (37) and 907 reverse (38) and
Archaeal primers 344 forward (39) and 915 reverse (40). PCR
(34). Clone libraries were constructed from the 16S rRNA
genes using the TOPO TA cloning kit version R (Invitrogen;
Carlsbad, CA) according to the manufacturer’s instructions.
A total of 144 clones under each condition (with and
genes were amplified from each clone using M13 forward
and reverse primers (Invitrogen) using whole-colony PCR.
PCR products were purified using the QIAquick PCR puri-
fication kit (Quiagen; Valencia, CA). Inserts were sequenced
at the UMASS Environmental Biotechnology Center’s se-
database with the BLASTN (41) algorithm.
Results and Discussion
Ag/AgCl) removed U(VI) in sterile medium from solution
(Figure 1, inset). An amount of 80 µM U(VI) was removed
from solution in 24 h, at which time U(VI) was added to give
removed within 48 h. No U(VI) was lost from solution under
(data not shown). When G. sulfurreducens was present on
was ca. 6 times slower than that when cells were not present
(Figure 1, inset).
When poise was removed from the working electrode in
the cell-free system U(VI) was released back into solution
(Figure 1). Identical U(VI) recovery results were obtained
glovebox (data not shown), as opposed to bubbling with
anaerobic gas. In contrast, no U(VI) returned to solution for
more than 600 h when G. sulfurreducens was present on the
chamber resulted in rapid and complete recovery of U(VI)
(Figure 1). These results suggest that the uranium on the
89449ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005
electrodes was in a different form in the presence of G.
sulfurreducens than in its absence.
The recovery of U(VI) under anaerobic conditions in the
the uranium was in the U(VI) state. Although U(V) is a
potential reduction product, it rapidly disproportionates to
the (VI) and (IV) valence states (42). Furthermore, previous
studies have shown that U(VI) reduction at a carbon-fiber
In contrast, the requirement for oxygen in order recover
U(VI) from the systems that contained G. sulfurreducens
is that G. sulfurreducens is able to use the electrode as an
electron donor for U(VI) reduction in a manner similar to
electron consumption associated with U(VI) removal was
monitored (Figure 2). In the presence of cells the removal of
55 µM U(VI) consumed 107 µequiv of electrons, or 97% of
the results summarized above suggested that U(VI) was not
reduced in the absence of cells, the removal of 53 µM of
into the working chamber. Since U(VI) did not appear to be
reduced to U(V) or U(IV), the observed current flow may
have been associated with maintaining the poised reducing
double layer in the presence of oxidized ions, rather than a
transfer of electrons from the electrode to U(VI). Current
consumption in the absence of reduction of the target ions
is commonly observed during capacitive deionization (13,
18, 44). Nonfaradaic current is observed when the voltage at
the working electrode, electrode surface area, or solution
composition is changed. Adsorption at the graphite surface
and solution composition. The process of adsorption is
nonfaradaic and is known to be accompanied by current
Batch Reactors with Contaminated Sediment. To de-
contaminated groundwater associated with sediments, elec-
trodes were placed in subsurface sediments and associated
groundwater from a uranium-contaminated subsurface site
in Rifle, CO. The working electrodes were buried in con-
taminated sediment and water, and the counter electrodes
were suspended in the overlying contaminated water.
Initially, no poise was placed on the working electrode
and U(VI) concentrations remained stable for 96 h (Figure
3). After 96 h, the poise of the buried, working electrode was
set at -500 mV. U(VI) concentrations had decreased, from
10.4 to 8.8 µM at 146 h. At 170 h, the working electrode
potential was reduced to -600 mV, but at 314 h, U(VI)
concentrations had only deceased to 8.3 µM. The poise at
the working electrode was then decreased to -700 mV. At
946 h later, U(VI) concentrations had decreased to 1.7 µM.
remained steady for 1474 h (Figure 3).
Poise to the working electrode was removed after 970 h.
Within 72 h, approximately 30% of the U(VI) removed had
returned to solution. U(VI) concentrations then remained
into the system. At 96 h after the introduction of air, 83% of
the U(VI) that had been removed had appeared in solution,
and all of the U(VI) was recovered in solution within 260 h.
The finding that recovery of U(VI) required the presence of
oxygen indicates that most of the U(VI) removed from
solution was in the form of U(IV). Nitrate (NO3-) and sulfate
(SO42-) were initially present in the groundwater at 0.1 mM
and 9.5 mM, respectively. At the end of the experiment, the
SO42-concentration had decreased to 4 mM and NO3-was
FIGURE 1. Uranium removal and recovery in the presence of a
of G. sulfurreducens. The potentiostat was poised at -500 mV (vs
for the same data set which better illustrates the addition and
removal of U(VI). Uranium was added at t ) 0 h (80 µM) and t )
24 h (120 µM).
FIGURE 2. Uranium removal and corresponding electron balance
using a poised graphite electrode (-500 mV) with and without G.
sulfurreducens. Filled and open data markers correspond to the
uranium concentration and the cumulative electrons added to the
working chamber, respectively, under each condition.
FIGURE 3. Removal and recovery of U(VI) in batch incubations of
uranium-contaminated soil and groundwater from Rifle, CO. After
96 h, the initial poise was set at -500 mV (vs Ag/AgCl). It was
adjusted to -600 mV at 170 h and -700 mV at 314 h. Power was
turned off at 970 h, and air was bubbled into the system at 1474 h.
At 1738 h, more U(VI) was added and power was reapplied at a
potential of -700 mV.
VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY98945
of soluble Fe(II) was 14 µM and increased to 34 µM at the
end of the experiment. It is likely that the electrodes had
Fe(III), in addition to stimulating U(VI) reduction.
To evaluate what microorganisms might be involved in
was returned to anaerobic conditions, U(VI) was added to
to the working electrode to maintain the poise at -700 mV
(Figure 3). U(VI) concentrations steadily decreased until the
on the electrode.
Comparison of the 16S rRNA gene sequences recovered
in sediments with sequences recovered from the surface of
that there was a slight enrichment of sequences which were
species (Figure 4). These sequences were not detected on
the control electrodes. There was also an increase in the
percentage of sequences most closely related to δ-proteo-
bacteria and bacteria in the Chlorobium/Flavobacterium/
Bacteroidetes (CFB) classification; however, genus-specific
enrichment from these families was not observed. Several
sequences decreased on the connected electrode.
Although poised electrodes serving as an electron donor
for nitrate reduction were enriched with Geobacter species
emplaced in the Rifle sediments were not. Nitrate is an
electron acceptor for several Geobacter species (10), and
nitrate concentrations in that previous study were at mil-
limolar concentrations. However in the Rifle sediments
nitrate, as well as U(VI), the other potential soluble electron
acceptor for Geobacter species in this environment, were in
micromolar concentrations. In contrast, as noted above
being reduced in the sediments with poised electrodes.
Therefore, it might be expected that sulfate-reducing
microorganisms, such as Desulfotomaculum species, would
be enriched on the electrode. Nitrosococcus are ammonia-
and methane-oxidizing chemolithotrophs. Potential meth-
ane- and ammonium-oxidizing microorganisms were en-
riched on cathodes harvesting electricity from marine
evaluate the potential for U(VI) removal under the more
aquifers, studies were next conducted with flow-through
columns packed with sediments from the Rifle site (Figure
5). U(VI) concentrations in groundwater from the site were
increased to 80 µM. Slow removal of U(VI) in batch
experiments at -500 mV indicated that greater reducing
potential should be applied to the electrode in subsequent
of uranium from the columns, the electrodes in the experi-
removal began immediately and was sustained for over 40
days. The control column, which contained nonpoised
graphite electrodes, did not remove uranium. At the end of
the experiment, a column was sacrificed and the working
electrode was quickly removed from the column, detached
from the potentiostat, and immediately submerged in
aerobic, 50 mM bicarbonate buffer for oxidation and extrac-
from the oxidation and extraction, which represented 87%
of the total U(VI) removed in the column over the duration
of the experiment.
In summary, the results suggest that U(VI) can be
effectively removed from groundwater with graphite elec-
trodes poised at ca. -600 mV (vs Ag/CgCl). Whereas U(VI)
may only be adsorbed to the electrode in the absence of the
microorganisms, it appears to be reduced to U(IV) when
microorganisms are present. The U(IV) remains as a stable
precipitate on the electrode in the absence of oxygen. This
offers the possibility that once uranium is precipitated from
contaminated groundwater onto electrodes, the electrodes
can then be removed from the groundwater, extracting the
precipitated uranium from the subsurface. This contrasts
with delivering electrons to the subsurface in the form of an
organic electron donor in which the U(VI) can effectively be
removed from the groundwater, but the precipitated U(IV)
remains in the subsurface. Field studies to evaluate the
possibility of remediation and recovery of uranium from
FIGURE 4. Comparison of 16S rRNA clones found on poised and
unpoised (control) electrodes during remediation of uranium
contamination of soil and groundwater. A total of 144 clones from
on the closest percent similarity in the database as determined by
the BLASTN algorithm (41).
columns packed with uranium-contaminated soil from Rifle, CO.
CO. The working electrodes in the control column were not given
port. The results represent the mean and range of the results from
89469ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005
Acknowledgments Download full-text
This research was supported by the Office of Science (BER),
U.S. Department of Energy Grant DE-FG02-04ER63718.
Supporting Information Available
and groundwater from Rifle, CO. This material is available
free of charge via the Internet at http://pubs.acs.org.
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