Remediation of uranium contaminated soils with bicarbonate extraction
and microbial U (VI) reduction
Elizabeth J.P. Phillips, Edward R. Landa and Derek R. Lovley
Key words: Bioremediation; Uranium; Mill tailings; Desulfovibrio
SUMMARY
A process for concentrating uranium from contaminated soils in which the uranium is first extracted with bicarbonate and then the extracted uranium is
precipitated with U(VI)-reducing microorganisms was evaluated for a variety of uranium-contaminated soils. Bicarbonate (100 mM) extracted 20-94% of the
uranium that was extracted with nitric acid. The U(VI)-reducing microorganism, Desulfovibrio desulfuricans reduced the U(VI) to U(IV) in the bicarbonate extracts.
In some instances unidentified dissolved extracted components, presumably organics, gave the extract a yellow color and inhibited U(VI) reduction and/or the
precipitation of U(IV). Removal of the dissolved yellow material with the addition of hydrogen peroxide alleviated this inhibition. These results demonstrate that
bicarbonate extraction of uranium from soil followed by microbial U(VI) reduction might be an effective mechanism for concentrating uranium from some
contaminated soils.
INTRODUCTION ganisms. The uranium could then be recovered in a concen-
trated form for disposal or possible reuse. The bicarbonate
extractant could be recycled to extract more uranium. The
results from bench-scale studies reported here suggest that
such a treatment strategy could be feasible.
MATERIALS AND METHODS
Soil types
In this paper, both true soils containing uranium-bearing
materials and other crushed earth materials, such as uranium
ore and uranium mill tailings were studied (Table 1). For ease
of description, all are referred to as soils. Mined, unprocessed
uranium ore, subgrade ore, and uranium mill tailings can be
found at uranium mining and milling sites. These materials
themselves, and admixtures of them with ambient soils, are
potential materials requiring remediation. The Denver Radium
Superfund Site soil contained waste from the radium pro-
cessing industry (active 1914-1926), for which processing and
disposal procedures are very poorly documented. Typical
waste from the radium industry included residues and precipi-
tates, soils contaminated by contact with waste solutions, and
crude or partially processed uranium ores [6]. Soil from the
Aberdeen Proving Ground Artillery Range was contaminated
with uranium as the result of test firing artillery shells con-
structed with 235U-depleted uranium [3].
Methods are needed for removing radionuclides and heavy
metals from contaminated soils. The ideal process for treating
contaminated sites would selectively remove the contaminants
of concern in a readily recoverable form, without excessive
soil destruction. Sodium bicarbonate is known to effectively
leach uranium from rocks and soils [2,13,14]. The uranium is
held in solution in the form of U(VI)-carbonate complexes.
Compared to other potential extractants such as strong acids
which may have a number of deleterious effects on soil struc-
ture and chemistry, bicarbonate is relatively environmentally
benign.
It has recently been demonstrated that U(VI)-carbonate
complexes can be effectively removed from solution through
the activity of U(VI)-reducing microorganisms [4,8]. The
microorganisms reduce U(VI) to U(IV) which then precipitates
as uraninite. Microbial U(VI) reduction may be more effective
in removing U(VI) from bicarbonate solutions than are other
removal techniques such as ion exchange resins and biosorp-
tion [8]. Furthermore, the uraninite precipitate from microbial
U(VI) reduction is relatively pure and compact and more eas-
ily handled than the uranium that is adsorbed onto resins or
biomass [8]. Microbial U(VI) reduction can be used to remove
uranium from a variety of uranium-contaminated waters [8].
From these previous studies it was readily apparent that a
potential mechanism to concentrate uranium from contami-
nated soils was to first extract the uranium with bicarbonate
and then precipitate the uranium with U(VI)-reducing microor-
Extraction of uranium
Uranium was extracted with sodium bicarbonate (100 mM,
pH 8.4) or nitric acid (1 N). Soils (10 g) were suspended in
the extractant (50 ml) in polypropylene centrifuge bottles on
a wrist action shaker. For the alkaline and mixed tailings, the
nitric acid concentration was adjusted after determining the
Correspondence to: D.R. Lovley, US Geological Survey, 430 National
Center, Reston, V A 22092, USA.
US Geological Survey, 430 National Center, Reston, VA 22092, USA
(Received 22 March 1994; accepted 28 July 1994)

Remediation of uranium contaminated soils
EJP Phillips et al~
TABLE I
Extraction of uranium from soils and precipitation of uranium from bicarbonate extracts
Sample Reference Description Sample preparation Extractable uranium (JJ.mol g-l) % of
Nitric acid' Bicarllonateb extracted
uranium
precipitatedC
Ore Air dried, crushed to
less than 2 mm
4.8Stockpiled ore at
abandoned uranium
mine, South Dakota
Acid-leached uranium
mill tailings, Texas
Mixed acid- and aIkaline-
leached uranium mill
tailings, Utah
Alkaline-leached uranium
mill tailings, Utah
4. 99
Acid
tailings
Mixed
tailings
[5] Dried at 110 °C. crushed
to less than 0.6 rom
Dried to 100 °C, crushed
to less than 2 rom
0.40 0.36 100
[7] 0.81 0.59 94
Alkaline
tailings
[7] Dried to 100 °C, crushed to
less than 2.0 mm. we!
sieved to less than 44 11m
Air dried, crushed to
less than 2 mm
.8 0.61 22 (75)d
Superfund
site
[6] Soil contaminated by
the radium industry,
Denver, Colorado
Artillery Range soil
contaminated with
uranium artillery shells,
Aberdeen Proving
Grounds, Maryland
2.5 0.49 19 (97)
Artillery
range
Air dried, crushed to
less than 2 rom
0.31 0.29 22 (96)
al N nitric acid, 4 h.
blOO mM bicarbonate from Fig. 1.
cUranium not passing through a O.2-JLm pore diameter filter after incubation of bicarbonate extract for 24 h with D. desulfuricans and H2.
dNumber in () indicates percent removal when extract was pretreated with peroxide to remove organics.
consumption of acid by carbonates in the sample using the
acid neutralization method [I]. The extractions were at room
temperature (ca. 20 °C). Preliminary studies indicated that
extraction of the soils for 4 h with nitric acid yielded estimates
for total uranium that were within 84-100% of the total uran-
ium in the soils as determined by delayed neutron analysis.
The kinetics of U(VI) extraction with bicarbonate extracts
were determined by subsampling (0.1 ml) over time and ana-
lyzing for soluble U(VI) as outlined below. Where noted, the
solids were collected with centrifugation and all of the bicar-
bonate extract was removed and replaced with fresh bicarbon-
ate solution. At the end of the extractions the extracts were
passed through a cellulose nitrate filter (0.2-JJ,m pore diameter,
Nalgene, Rochester, NY, USA) and stored at 4 °C. The
extracts were diluted (1:2, v/v) with water prior to the precipi-
tation studies.
of uranium-contaminated waters [8]. As previously described
[9], D. desu(fiiricans was grown in medium with lactate as the
electron donor and sulfate as the electron acceptor. Washed
cell suspensions were prepared under anoxic conditions in
bicarbonate buffer [9] and added to the bicarbonate soiJ
extracts in order to provide 2.5 mg of cell protein per bottle
H2 (10 ml) was added to each bottle as the electron donor
for U(VI) reduction and the extracts were incubated at 35 °C
Subsamples (0.1 ml) were taken over time with a syringe and
needle and, in an anoxic chamber, were first diluted 1000 times
and then filtered (0.2-j1.m pore diameter Gelman polysulfone
filter, Gelman Sciences. Ann Arbor, MI, USA). U(VI) and
total uranium in the filtrates were determined as outlined
below.
As discussed belo\\'. the bicarbonate extracts from some
soils were yellow and rates of U(VI) reduction and/or U(IV)
precipitation were inhibited in the yellow extracts. Analysis 01
the yellow extracts with a Shimadzu TOC Total Organic Car-
bon Analyzer (Shimadzu Scientific Instruments, Columbia
MD, USA) indicated that they contained relatively higb
(>20 mg L -I) concentrations of dissolved organic carbon
Therefore, when noted, these yellow extracts were diluted witi-,
a hydrogen peroxide solution (30%) rather than water. Thl
extracts treated with hydrogen peroxide were incubated for 1~
Microbial precipitation of uranium
Aliquots (10 ml) of the diluted soil extract or of bicarbonate
buffer amended with uranyl acetate were added to 20-ml
serum bottles, bubbled with N2-CO2 (80:20) for 5 min, and
the bottles were sealed with thick butyl rubber stoppers. Desul-
fovibrio desulfuricans was used as the U(VI)-reducing micro-
organism as it was in previous studies on the bioremediation