Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 121-126, 2007

Microbial Transformations of Plutonium
A. J. Francis,*, a C. J. Dodge,a and T. OhnukibaEnvironmental Sciences Department, Brookhaven National Laboratory, Upton, New York 11973, USA
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japanb

Received: December 21, 2006; In Final Form: December 21, 2006
Microorganisms have been detected in transuranic- and low-level radioactive wastes, Pu-contaminated soils, andin nuclear waste-repository sites. Microbial activity could affect the chemical nature of the Pu by altering thespeciation, solubility, and sorption properties and thus could affect the stability and mobility of Pu in theenvironment. We investigated the biotransformation of Pu(IV)-citrate by Pseudomonas fluorescens under aerobic
conditions. Characterization of 242Pu(IV)-citrate by electrospray ionization-mass spectrometry (ESI-MS) showed
the presence of 1:1 mononuclear, 1:2 mononuclear biligand, and 2:2 dinuclear complex. X-ray absorption nearedge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) analysis of Pu-citrateconfirmed Pu in the IV oxidation state and predominantly present as a 1:2 mononuclear biligand complex. Citricacid was rapidly metabolized by P. fluorescens resulting in the formation of Pu polymer. Under anaerobic
conditions, reductive dissolution of Pu(IV) to Pu(III) by Clostridium sp. was observed. The presence of Pu(III)was confirmed by XANES. Addition of Pu(VI) to a mixture consisting of bacterial cells and kaolinite clay
showed that the Pu was preferentially associated with bacterial cells as Pu(IV) and that Pu(VI) was reduced toPu(V) and Pu(IV) only in the presence of bacteria. Studies with Pu contaminated soils from Nevada Test Site
showed remobilization of Pu and other radionuclides due to enhanced microbial activity.

1. Introduction
The presence of Pu in transuranic (TRU)- and mixed-wastes
is a major concern because of the potential for migration fromthe waste repositories and contaminated environments.
Plutonium exists in several oxidation states (III, IV, V, VI) andas various chemical species with a very complex chemistry andenvironmental behavior. Soil pH, presence of organic ligands,
redox conditions, mineralogy, and microbial activity can affectthe chemical speciation of Pu. Chemical characterization of
Pu at contaminated sites shows that it varies according to thesite and the waste stream. At Rocky Flats, CO, the predominant form appears to be PuO2 (s).1 Pu generally is considered tobe relatively immobile; however, the transport of Pu, albeit atvery low concentrations, as colloids was observed at Rocky
Flats, Los Alamos National Laboratory (LANL), Nevada Test
Site (NTS), and Mayak Production Association, Urals, Russia.1–4
Plutonium was found associated with mineral colloids at the
NTS; bound to iron oxide colloids at Mayak; at Oak Ridge
National Laboratory (ORNL) with organics and at Rocky Flats
as Pu(IV) mostly associated with organic colloids.3–6 At Maxey
Flats Kentucky, a former low-level radioactive waste site Pu
was present as dissolved species, primarily as Pu(IV) complexed with organic ligands.7 Chelating agents, such as citricacid, are present in TRU waste at the Waste Isolation Pilot
Plant (WIPP).8 Citric acid forms a strong complex with Pu(IV)and has been used in the extraction of 239Pu from contaminatedsoil.9,10 In this paper, we briefly review the microbial transformations of Pu and in particular describe our initial studies on
the biotransformation of Pu-nitrate and Pu-citrate, the interactions of Pu with bacteria and clay, and the remobilization of Pu
from contaminated soil due to enhanced microbial activity.
2. Microbiology of Plutonium Contaminated Sites
Microorganisms have been detected in low-level radioactive*Corresponding author. E-mail: francis1@bnl.gov.
Fax: +1-631-344-7303.

wastes, TRU wastes, Pu-contaminated soils, and in wasterepository sites being considered for disposal of nuclear waste.
Leachates collected from the low-level radioactive-waste sites
contained 238,239,240Pu (gross activity of 1.7 × 105 pCi/L), aerobic- and anaerobic bacteria (Bacillus sp., Pseudomonas sp.,
Citrobacter sp., and Clostridium sp.) and organic compounds.11,12 The radioactive- and organic chemicals in theleachate were not toxic to the bacteria. Metabolically active
microbes were identified at the LANL’s TRU waste burial site
containing 239Pu-contaminated soil.13 Microorganisms present
in the radioactive wastes can affect the long-term stability ofradionuclides.14–17
3. Microbial Mobilization and Immobilization of
Plutonium
Under appropriate conditions, dissolution or immobilizationof Pu is brought about by direct enzymatic or indirect nonenzymatic action of microorganisms. These include changes
in the Eh, pH, oxidation state, production of extracellular
metabolites (e.g., citric acid, siderophores), biosorption, bioaccumulation, biotransformation of Pu complexed with organicand inorganic ligands, and Pu colloid formation or destabilization. Plutonium can exist simultaneously as Pu(IV), Pu(V),and Pu(VI) under oxidizing conditions. Reduced Pu species
has a high ionic charge, and tends to undergo hydrolysis, leading to the formation of polymers in systems with pH > 2.
Microorganisms directly or indirectly affect the oxidation
and reduction of Pu (Figure 1). A slight increase in microbialactivity (respiration) can alter the oxidation state of Pu(VI) toPu(IV) because of the very small differences in the reduction
potential between Pu(VI), Pu(V), and Pu(IV). The chemical

Pu(VI)

Abiotic /biotic+ 0.60VPu(V)abiotic+ 0.70V

abiotic /biotic

Pu(IV)- 0.39V

Pu(III)+0.65V

Figure 1. Plutonium oxidation states and redox potentials at pH 8.18

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

122 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
form of Pu (e.g., oxide, ionic, organic- or inorganic complexes,
co-precipitates), the availability of electron donors, electron
acceptors (Fe3+, Mn4+, NO3–, SO42–, organic compounds), nutrients (nitrogen, phosphorus), and environmental factors (pH,Eh, temperature, moisture) all can affect microbial activity andthe biotransformation of Pu. The extent of Pu dissolution could
rise considerably with an increase in microbial activity, particularly under anaerobic conditions due to the formation of stablecomplexes with metabolites and organic degradation products.
Conversely, microorganisms can bring about a decrease in soluble Pu or immobilization due to reduction from higher tolower oxidation state, bioaccumulation and bioprecipitation
reactions.
4. Dissolution of Plutonium by Microbial Extracellular
Metabolites
The dissolution of Pu by microorganisms is brought about
by their production of organic acids, such as citric acid, extracellular metabolites, and siderophores. Microorganisms grownin the presence of plutonium produced unidentified compounds
capable of dissolving and mobilizing Pu in soils. These compounds also may be involved in transporting plutonium into thecells.19–23
Siderophores are low-molecular-weight iron chelators produced by microbes in response to a limited availability of soluble iron. As Pu(IV) and Fe(III) have chemical and biochemicalsimilarities, and because of the ubiquity of siderophore-producing microbes, iron-sequestering agents produced by suchmicroorganisms could be important in the complexation of Pu
and other actinides, thus increasing their solubility and bioavailability. Desferal, a polyhydroxamate chelate produced by
microorganisms enhanced the dissolution of PuO2.13 The production of extracellular chelating agents resembling siderophores was observed with Pseudomonas aeruginosa grown in
the presence of uranium and thorium.24 Brainard et al.22 determined rate-constants for the solubilization of hydrous PuO2 (s)by the siderophores enterobactin and desferrioxamine B, andby selected carboxylate amino polycarboxylate, and catecholate
ligands. They showed that siderophores are extremely effective
in solubilizing actinide oxides; on a per-molecule basis, enterobactin was ~103 times more effective than the other chelators
tested in increasing the rate of solubilization of hydrous plutonium oxide.
Several bacteria and fungi grown in the presence of Pu produced extracellular Pu complexes that increased the concentration of Pu in soil-column eluates relative to controls. Elution
through soil effectively removed positively charged Pu complexes. The increased mobility of Pu in soil resulted from theformation of neutral and negatively charged Pu complexes. In
the presence of known microbial metabolites such as citric acidand synthetic ligands (DTPA, EDTA, EDDHA), Pu(VI) was
reduced to Pu(IV) before complexation, suggesting that the latter valence state would be the dominant one associated withorganic complexes in soils. Studies with selected organisms
indicated that Pu was both actively transported into, and sorbed
onto the cell, and these phenomena, as well as complexation of
Pu by extracellular metabolites, were a function of the form ofPu supplied (hydrolyzed and complexed forms), the organisms’
type and growth characteristics, and the ability of the organism
to alter extracellular pH. 20,21,23,25
5. Immobilization of Plutonium by Biosorption andBioaccumulation
Immobilization of Pu, on the other hand, may be caused
indirectly, by changing the Eh, and facilitating precipitation ofPu, reduction of Pu(VI) and Pu(V) to Pu(IV) oxidation state,biosorption, and bioprecipitation reactions. Naturally occur-

Francis237

ring soluble organics affected Pu uptake by the fresh-water
bacterium Aeromonas hydrophila and by the alga Scenedesmus
obliquus.26 Low Pu (and Am) concentrations observed in thewater column corresponded with maximum bacterial activity
and it was attributed to bioaccumulation by the biomass.27 The
biosorption of 239Pu from aqueous nitrate medium by the fungus biomass Rhizopus arrhizus was maximal at pH 6–7 andthat under optimal conditions a strain of Rhizopus species
removed 99% of the 239 Pu from the wastewater.28,29 Alphaenergy spectral analyses in combination with SEM observations showed that the absorption occurred mainly on the cell
walls of the fungus. Halomonas sp. isolated from the WIPP
site biosorbed ~9% (0.17 × 10 –9 M) of the total 241Pu in solution(1.8 × 10 –9 M); Acetobacterium sp. isolated from Grimsel
groundwater biosorbed ~7% (0.11 × 10 –9 M) of the total 241Pu(1.5 × 10 – 9 M) in solut ion. 3 0 On a d r y-weight basis,
Acetobacterium sp. sorbed 145 ng 241 Pu g –1 dry cells, andHalomonas sp. sorbed 351 ng 241Pu g–1 dry cells.
Sorption studies of Pu(VI) with bacterial resting cells haveshown that the interaction with bacteria caused changes in theoxidation state to Pu(V) due to endogenous respiration. Due to
its weak complexation ability, the cell bound Pu(V) was
released into solution; disproportionation of Pu(V) to Pu(IV)and Pu(VI); and complexation of Pu(IV) with the biomass wasobserved.25,31 EXAFS analysis of Pu associated with the cells
showed that Pu(VI) is primarily bound to the phosphate groups
on the cell surface.32
6. Reductive Dissolution of Pu(IV) to Pu(III) by Bacteria
under Anaerobic Conditions
The mobility and bioavailability of Pu is limited by its solubility. Plutonium is present in the environment mostly as theoxides and hydroxides of Pu(IV) that have low solubility. The
solubility products of Pu(IV) oxyhydroxides are estimated tobe 10 –56.8 and 10 –57.8. The solubility of plutonium (III) hydroxide is much greater, Ksp = 10 –22.6. Consequently, the reductionof Pu(IV) oxyhydroxides to Pu(III) is expected to increase thesolubility of plutonium in the environment. The electrochemical potential for this reduction reaction is similar to that foriron reduced by many microorganisms suggesting that thosemicrobes able to reduce iron and other metals also might
reduce Pu(IV) oxyhydroxides under similar conditions and,thus, reductively solubilize Pu(IV) to Pu(III). The persistenceof the reduced soluble Pu(III) is affected by its stability, as well
as by its interactions with organic and inorganic materials.
Rusin et al.33 showed that an iron-reducing bacterium Bacillus
sp. solubilized up to 90% hydrous PuO 2 (s) in about 7 days
under anaerobic conditions in the presence of the chelating
agent nitrilotriacetic acid (NTA). In these studies, Pu(III) waspresent as a Pu-NTA complex. However, without NTA, only
40% of Pu was solubilized suggesting that a strong complexing
agent is needed to keep the Pu in solution. Little dissolution of
PuO2 was observed in sterile culture media or in the presenceof non-iron-reducing bacteria, such as Escherichia coli.
Francis et al. (manuscript in preparation) investigated the
reductive dissolution of Pu(IV) by the anaerobic spore-forming
bacterium Clostridium sp. known to reduce U(VI) to U(IV);
Tc(VII) to Tc(IV); and Fe(III) to Fe(II). Addition of 1 × 10 –7
M 242Pu (IV)-nitrate had no effect upon the growth and metabolism of glucose by Clostridium sp. Pu added to the bacterial
growth medium (uninoculated control) resulted in its precipitation and was removed by 0.4 µm filtration. Speciation calculations showed that Pu most likely existed as Pu(OH) 4 at pH 6.2due to hydrolysis and polymerization. The growth of
Clostridium sp. lowered the Eh (–180 mV), and the pH from
6.2 to 2.8, concomitant with the production of acetic andbutyric acids and of carbon dioxide (225 µmol) and hydrogen.
After 14 h of growth, 70% of the Pu passed through a 0.4 µm

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 123

Microbial Transformations of Plutonium

Chelating agents are present in TRU and mixed wastes due
to decontamination of nuclear reactors and equipment, cleanup
operations, and chemical separation of radionuclides.
Plutonium forms very strong complexes with a variety oforganic ligands. Naturally occurring organic complexing
agents, such as humic and fulvic acids, and microbially produced complexing agents, such as citrate, and siderophores, aswell as synthetic chelating agents can affect the mobility of Pu
in the environment. The presence of organic ligands in radioactive wastes is a major concern because of the potential forincreasing the transport of radionuclides from disposal sites.
Biotransformation of the radionuclide-organic complex should
precipitate the radionuclide from solution thereby retarding its
mobility in the environment. Chelating agents, such as citricacid, are present in TRU and mixed wastes. In addition toother organic ligands, an estimated 1,950 moles of citric acid ispresent in the WIPP inventory of TRU waste.8 Citric acid forms
a strong complex with Pu(IV) and has been used in exploringchelation therapy, and in extracting 239Pu from contaminatedsoil.9,10,34,35 We investigated the biotransformation of Pu-citrate(cit) under aerobic conditions by Pseudomonas fluorescens.36
7.1. Characterization of Pu-citrate. The complexation ofPu(IV) with excess citric acid at pH 4.5 and 6.5 was determined by solvent extraction and potentiometric titration andthe mononuclear biligand PuOcit2 was proposed.34 Analyses ofTh(IV)-citrate by potentiometric titration and conductimetricmeasurements showed the formation of [Th2cit3] 4+ (Reference37). Tetravalent plutonium forms a 1:1 Pu:citric acid (K = 1015)
complex and a 1:2 Pu:citric acid complex (K = 1030 ).38 In thepresence of excess citric acid, Pu(IV) oxidation state is stable.19
At pH 6.5, we identified a monomeric [Pucit] species, twoforms of biligand [Pucit2] species, and a dimeric [Pu2cit2] species of tetravalent plutonium with citric acid. Speciation calculations showed that the biligand complex is the predominant
form. In addition, analysis of the complex over 100 h demonstrated that citric acid inhibits polymer formation. We determined the complexation of Pu-citrate by ESI-MS and EXAFS.
Figure 3 shows the ESI-MS spectra for 1.6 × 10 –4 M Pu-citratecomplex. The spectrum shows the presence of three forms of
the Pu-citrate complex. The dominant peaks in the spectrum
can be attributed to the formation of a monomeric Pu-citratecomplex (m/z = 473.0) [Pucit(H 2O)Na] + (Figure 4A) and thebiligand complex is indicated by the presence of the threepea ks at m / z = 686.9 [P u (H 2 cit) 2 NO 3 ] + , m / z = 709.0[Pu(Hcit)(H2cit)NaNO3]+, and m/z = 731.0 [Pu(Hcit)2Na2NO3]+(Figure 4B). The differences in molecular weight for eachform are due to the presence of anion (nitrate) and cation(hydrogen, sodium) species which result in formation of singlycharged species. The presence of a complex containing two Pu
atoms is noted at m/z = 966.9 [Pu2 (Hcit)(cit)(NO3) 2] + (Figure4C) indicating the formation of a dinuclear species. XANES
and EXAFS determine the oxidation state of the Pu and thenature of Pu complexation with citric acid. XANES analysis
confirmed that the Pu was present as tetravalent form (data notshown). Figure 5 shows the k3-weighted raw EXAFS spectrum

Normalized absorbance

7. Biotransformation of Plutonium-organic Complexes

2.0
1.5

Pu-nitrate (18.068 keV)

1.0

Clostridium sp. (18.064 keV)

0.5
0.0

18055

18070 18085
Energy (eV)18100

Figure 2. XANES analyses of Pu before and after bacterial action(Francis, Dodge, Gillow; manuscript in preparation).

Relative abundance (%)

filter and 55% through a 0.03 µm filter suggesting a significant
portion of Pu was solubilized. Solvent extraction by thenoyltrifluoroacetone (TTA) confirmed a decrease in the polymeric
form of Pu and an increase in the soluble fraction, suggesting
the presence of Pu3+. XANES analysis of the culture at the Pu
L III edge (18.057 keV) confirmed that the oxidation state was
Pu3+ (Figure 2). The Eh of the medium was low and the CO2
concentration high, thus favoring the reduction of Pu from thetetravalent to the trivalent state. These results suggest that
under appropriate conditions Pu can be reduced to Pu(III) by
anaerobic bacterial activity.

Bmode (+)

731.0

100
80

709.0

60

473.0
686.9 827.1

966.9

215.1

40
20
0

300

600

900
m/z

1200 1500

Figure 3. Electrospray ionization mass spectrum (ESI-MS) in the(+) mode showing the molecular ions (m/z) produced by citric acidand the Pu-citrate complex at pH 6.5.36

Pu

A

Pu

B

Pu

Pu

C

Figure 4. Proposed structures at pH 6 for the monoligand 1:1 Pu:citr ic-acid complex (A); biliga nd 1:2 P u:cit r ic-acid complex[PuO(cit) 2] 4– (B); and dimeric 2:2 Pu:citric-acid complex (C). The
open circles represent oxygen and the filled circles represent carbon
atoms.

and the Fourier transform for the Pu-citrate complex. Basedupon this analysis the best configuration for the molecule consists of a mononuclear biligand complex (Figure 4B). The
presence of 1:1 Pu-citrate complex at pH 4 was reported by
Kantar and Honeyman.39 However, the dimeric complex has
not been previously reported.36
7.2 . Biodegradation of citric acid and Pu-citratecomplexes. Figure 6 depicts the rate and extent of citrate degradation in samples containing 10 –6 M and 10 –8 M Pu present
as Pu-citrate complexes at an ionic strength of 0.18 M. Citric6

A

Kaolinite

2
3

4

0

. subtilis

-2

3

-4
2

2
1
0

Francis

4 B

5
k X(k)

Fourier transform magnitude

124 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

4

6

8
10
-1
k(Å )

12

14

1:2 Pu:citric acid01

2 3 4 5 6
Radial distance (Å)

78Figure 5. EXAFS analysis for 1:2 Pu:citric-acid complex at pH 6.5.
Experimental data (solid line); fitted data (dashed line).36

Al
Si
P

Intensity

C i t r i c a ci d d e g r a d e d ( % )

100
80

S

Pu
B. subtilis

Citric acid-8

10 M Pu-citrate

60

Kaolinite-6

10 M Pu-citrate140

3Energy(KeV)

4

5

Figure 7. SEM-EDS analysis of the bacteria and clay mixture showing that Pu was preferentially associated with bacterial cells.42

20
0

2

0

20

40

60
Hours

80

100

the 0.03 µ m filter (colloid fraction). Yong and Macaskie 40
showed that the Th(IV)-citrate complex, an analog for Pucitrate, does not interact with Citrobacter sp. immobilized
cells.

Figure 6. Effect of adding 10 –6 and 10 –8 M 242Pu on citrate metabolism. Citric-acid concentration is 10 –4 M in all treatments.36

8. Interactions of Plutonium with Bacteria and Clayacid (10 –4 M) in the absence of Pu was metabolized completely
at a rate of 4.9 µM/h. With 10 –6 and 10 –8 M Pu present as thePu-citrate complex we observed a slight decline on the rate andextent of citrate degradation in comparison to the sample lacking Pu. In both samples, citrate was degraded >96%, and at
the rate of 4.0 µM/h and 3.8 µM/h, respectively. In the presence of 10 –6 M Pu, 20% was retained on the 0.4 µm filter, and
27% on the 0.03 µm filter; similar results were found with 10 –8
M Pu, at 43% and 57%, respectively (data not shown).
Comparison of the 10 –6 M Pu samples inoculated with bacteriawith that of the control samples (uninoculated sample) showedthat there is greater retention on the >0.4 µm filter in the former. These data suggest that the soluble 10 –6 and 10 –8 M Pucitrate complexes underwent significant biodegradation with
Pu released as a particle-reactive species that adsorbed to thecells (>0.4 µm), and possibly as a colloidal species (>0.03 µm).
Although the Pu(IV)-citrate complex is stable in the absenceof bacteria, adding bacteria to the 1:100 and 1:10000 Pu(IV)citrate complexes resulted in the retention of Pu species at 20–
43% by the 0.4 µm filter (biomass associated), and 27–57% by

Kudo et al.41 investigated the interactions between the anaerobic sulfate-reducing bacteria and Pu in the presence andabsence of bentonite, a backfill material for nuclear waste disposal. The bentonite was obtained from Japan, and the anaerobic sulfate-reducing bacteria were isolated from a pulp- andpaper-wastewater treatment plant. Distribution coefficients(Kd , in mL/g) were used as indices of radionuclide behavior.
Pu Kd values for living bacteria ranged from 1.8 × 103 to 1.1 ×
105 depending on the pH, while the Kd values for dead bacteria
were 1.2 × 103 to 5.9 × 103. Furthermore, higher Kd values of
4.0 × 10 4 –1.1 × 105 for living bacteria were obtained for pHs
between 6.83 and 8.25, while pH had no effect on the dead bacteria. In contrast, Pu Kd values for mixtures of living bacteriawith bentonite, and sterilized (dead) bacteria with bentonite
were 1.2 × 103 to 8.4 × 10 4 and 6.2 × 102 to 1.7 × 10 4, respectively. The presence of live anaerobic bacteria with bentonite
increased the Kd values of 239Pu by 50-fold, in comparison to
the mixture of dead bacteria with bentonite. The Pu Kd values
for bentonite alone, both nonsterilized and sterilized, wereapproximately 1.0 × 10 4. The results suggest that anaerobic
bacteria within the engineered barrier system (in this case,bentonite) will play a significant role in modulating the behav-J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 125

Microbial Transformations of Plutonium

Characterization of Pu at contaminated sites shows that its
environmental form varies according to the site and the waste
stream. For example, at Rocky Flats, CO, the predominant
form appears to be as PuO2 (s)1 at the Nevada Test Site (NTS)
Pu was associated with mineral oxides;3 while, at Oak Ridge,
TN it is associated with organic matter.5 Pu is generally considered to be relatively immobile; however, its transport to varying distances at low concentrations as colloids was observed at
Rocky Flats (RF), Los Alamos National Laboratory (LANL),
and NTS.1–4,6 Plutonium in surface waters at the RF site wasshown to be associated with organic macromolecules.6 Studies
with Pu contaminated soils show that Pu and other radionuclides are remobilized due to enhanced aerobic or anaerobic
microbial activity (Francis and Dodge, unpublished results).
Several bacteria and fungi grown in the presence of Pu produced extracellular Pu complexes that increased the concentration of Pu in soil-column eluates relative to controls. Elution
through soil effectively removed positively charged Pu complexes. The increased mobility of Pu in soil resulted from theformation of neutral and negatively charged Pu complexes. In
the presence of known microbial metabolites and synthetic
ligands (DTPA, EDTA, EDDHA), Pu(VI) was reduced toPu(IV) before complexation, suggesting that the latter valence
state would be the dominant one associated with organic complexes in soils.20 We examined the effect of microbial activity
on the mobility of Pu from a contaminated soil.
9.1. Characterization of Pu-contaminated soil. Plutoniumcontaminated soil (HP-11) was obtained from Area 11 soil of
the Double Track test shot area at the NTS and had a grossactivity of 50 nCi/g. The individual α components of the total
activity were determined and are given in Table 1. The β -emitter 2 41 Pu has also been detected but was not quantified.
Minerals present in the soil were determined using µ-X-ray diffraction on beamline X7A at the National Synchrotron Light
Source (NSLS). The predominant minerals consisted of various forms of iron oxides and aluminosilicates. The uranium
was present in the soil as hexavalent form associated as them inerals schoepite (UO3 ·2H2O) and liebigite(Ca2UO2 (CO3)3 ·11H2O).
9.2. Mobilization of Pu, Am, and U in NTS soil due tomicrobial activity. The effect of bacterial activity on themobilization of radionuclides in NTS soil is presented in Figure
8. Under anaerobic conditions glucose was metabolized withdecrease in pH to 4.8 and an increase in α activity in solution.
However, the appearance of the actinide in solution was concomitant with decrease in pH (Figure 8). In addition, the alkaline nature of the soil suggests that mobilization may be due toα activity in NTS soil

Isotope/ nCi g–1

/ µg g–1

U<0.18<6.1 × 10 –2

U

<0.15

<3.6 × 101

238

U

<0.14

<4.2 × 102

241

Am

3.8 ± 0.7

1.1 × 10 –3

0.44 ± 0.02

2.6 × 10 –5

69.2 ± 6.5

1.1 × 10 –1

233,234
235,236

238

Pu

239,240

Pu

120

35
30

Glucose

Alpha

25

80

20

60

15

40

10

Beta

5
0

100

0

5

20

Activity (dpm/ml)

9. Mobilization of Plutonium from Contaminated Soils
Due to Microbial Activity

TABLE 1: Total α activity in NTS soil

Glucose consumed (mM)ior of Pu in geologic repositories.
Exposure of kaolinite clay to Pu(VI) showed rapid adsorption and was desorbed readily without change in the oxidation
state. In contrast, Bacillus subtilis cells exposed to Pu(VI)showed absorption of Pu(VI) with subsequent reduction to
Pu(V) and then to Pu(IV). Incubation of Pu(VI) with a mixture consisting of bacterial cells and kaolinite showed that Pu
was preferentially associated with bacterial cells as Pu(IV) andlittle Pu was found with kaolinite as evidenced by SEM and
EDS analyses (Figure 7).42 The bacterial cell surfaces consistof the negatively charged anionic functional groups ofphosphoryl, carboxyl, and hydroxyl moieties with affinity tobind Pu. The mechanisms of accumulation of Pu(IV) may
entail its precipitation as insoluble Pu hydroxides and/or complexation with the functional groups of the cellular surfaces orphosphate released form the bacteria. Panak et al.32 showedthat Pu(IV) was predominantly associated with the bacterial
phosphates.

0
10 15 20 25 30 35 40
Days

Figure 8. Mobilization of actinides due to microbial activity (Dodge
and Francis, manuscript in preparation).

dissolution of carbonate species present in the soil.
Addition of citric acid to the soil showed metabolism of citric acid and an increase both α and β activity in solution.
However with increase in time there was sudden decrease in α
and β activity (data not shown). This loss of activity in solution coincided with the complete utilization of citric acid by thebacteria. There was only a slight decrease in pH to 7.7 in thebacterially active sample. This observation suggests that citricacid forms a soluble complex with the actinide and that its
removal due to bacterial metabolism releases the actinide,resulting in its precipitation. These results also suggest that thetype of carbon source will have a different effect on microbial
mobilization of actinides.
10. Summary
A slight increase in microbial activity can alter the oxidation
state of Pu(VI) to Pu(IV) because of the very small differencesin the reduction potential between Pu(VI), Pu(V), and Pu(IV).
We observed (i) reductive dissolution of Pu(IV) to Pu(III) by
the anaerobic bacterium Clostridium sp.; (ii) biodegradation of
Pu-citrate complexes by Pseudomonas fluorescens resulted in
the formation of polymeric Pu; and (iii) incubation of Pu(VI)with Bacillus subtilis, kaolinite clay, or their mixture showedthat Pu was reduced to Pu(V) only in the presence of bacteria
and that the oxidation state of Pu associated with Bacillus subtilis and the mixture was Pu(IV); no changes in Pu oxidation
state was observed with clay. Studies with Pu-contaminatedsoils from NTS show that Pu and other radionuclides are remobilized due to enhanced aerobic or anaerobic microbial activity
and the type of carbon source affected the rate and extent of Pu
in the solution phase.

126 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
Acknowledgements. This research was supported by theEnvironmental Remediation Sciences Program (ERSP),
Environmental Remediation Sciences Division, Office of
Biological and Environmental Research, Office of Science,
U.S. Department of Energy, under contract No. DE-AC0298CH10886.
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