Journal of Nuclear and Radiochemical Sciences, Vol. 7, No.1, pp. 1-6, 2006

ArticlesAccumulation of Thorium and Uranium by Microbes – the Effect of pH, Concentration ofMetals, and Time Course on the Accumulation of Both Elements Using Streptomyces levoris
Takehiko Tsuruta*
Department of Applied Chemistry, Tohwa University, 1-1-1, Chikushigaoka, Fukuoka 815-8510, Japan
Received: June 1, 2005; In Final Form: March 9, 2006
The accumulation of thorium and uranium by various microorganisms from a solution containing both metals at
pH 3.5 was examined. Among the tested species, a high accumulation ability for thorium was exhibited by strainsof gram-positive bacteria, such as Arthrobacter nicotianae, Bacillus megaterium, B. subtilis, Micrococcus luteus,
Rhodococcus erythropolis, and Streptomyces levoris. Though uranium was accumulated in small amounts bymost of microorganisms, A. nicotianae, S. flavoviridis, and S. levoris had relatively high uranium accumulation
abilities. In these high performance thorium- and uranium-accumulating microorganisms, S. levoris, which accumulated the largest amount of uranium from the solution containing only uranium at pH 3.5, accumulated about
300 µmol thorium and 133 µmol uranium per gram dry weight of microbial cells from a solution containing boththorium and uranium at pH 3.5. The amount and time course of the thorium accumulation were almost unaffectedby the co-existing uranium, while those of uranium were strongly affected by the co-existing thorium. The effectsof pH, the thorium and uranium concentrations, and time course on both metal accumulations were also evaluatedby numerical formulas.

1. Introduction
The removal of radionuclides, such as uranium (U) and thorium (Th) from aqueous solutions, especially from contaminated sources, seems to be a significantly useful subject for
exploiting unused or abandoned energy resources. In thisregard, efforts have been especially concentrated on studying
the accumulation of U using microorganisms, such as the actinomycetes,1−3 bacteria,4−8 fungi,9−13 and yeasts.7, 14
Thorium accumulation has also been investigated by someresearchers. Tzezos and Volesky12, 15 reported the biosorptionof U and Th by some microorganisms and the mechanism of
Th biosorption by Rhizopus arrhizus. White and Gadds13
reported the biosorption of Th by fungal biomass. Andres et
al4 reported the adsorption of Th and U by Mycobacterium
smegmatis.
Recently, various species and strains of actinomycetes, bacteria, fungi, and yeasts were screened for their abilities to accumulate U or Th from a solution containing only each metal at
pH 3.5.16, 17 Among the microorganisms, a high Th accumulating ability was exhibited by the gram-positive bacterial strains,
especially Arthrobacter nicotianae IAM12342, Bacilles subtilis IAM1026, and Micrococcus luteus IAM1056. The amountof Th accumulated by the gram-positive bacteria was greaterthan those by actinomycetes, gram-negative bacteria, fungi,
and yeasts. On the other hand, high ability to accumulate U
was found in some actinomycetes strains, such as S. albus
HUT6047 and S. levoris HUT6156. The amount of U accumulated by actinomycetes was higher than those by most of theother gram-positive bacteria, gram-negative bacteria, fungi,
and yeasts. The effect of Th (or U) addition on the U (or Th)
accumulation rate and the amount of accumulated U (or Th)
using S. levoris cells was also investigated.16, 17 Thorium
strongly affected the U accumulation, whereas U had only avery slight affect on the Th accumulation. Additionally, the UTh exchange reaction occurred in the solution containing an
excess amount of U.
In this study, various species and strains of actinomycetes,*Corresponding author. E-mail: ttsuruta@tohwa-u.ac.jp. FAX:
+81-92-552-2707.

bacteria, fungi, and yeasts were screened for their ability toaccumulate Th and U from a solution containing the sameamount of both elements at pH 3.5. The ability to accumulate
Th and U, the effect of pH on the accumulation, and the timecourse of the Th and U accumulation were also investigated by
S levoris cells, which accumulated the largest amount of U froman aqueous solution containing only U at pH 3.5.16 Furthermore,
an attempt was made to evaluate these effects by numericalformulas.
2. Experimental
2.1. Materials. The strains used in this research were generously donated by the IAM Culture Collection, the Center for
Cellular and Molecular Research, the Institute of Molecular
and Cellular Biosciences, the University of Tokyo (IAM), theFaculty of Engineering at Hiroshima University (HUT), andthe Faculty of Agriculture at Hokkaido University (AHU).
The chemicals (guaranteed reagents) used were obtained from
Nacalai Tesque, Inc., Kyoto, Japan.
2.2. Culture of microorganisms. The medium for growing
the bacteria (except actinomycetes) consisted of 3 g/L meat
extract, 5 g/L peptone, and 5 g/L NaCl in deionized water.
The medium for growing the actinomycetes, fungi, and yeasts
contained 4 g/L yeast extract, 10 g/L malt extract, and 4 g/L
glucose in deionized water at pH 7.1 (for actinomycetes) andpH 5.7 (for fungi and yeasts). The microorganisms were maintained on agar slants and grown in 300 mL of the medium in a
500 mL flask with continuous shaking (120 rpm) for 72 h at 30
˚C. Cells were collected by centrifugation (for bacteria except
actinomycetes and for yeasts) or by filtration through filter
paper (for actionomycetes and fungi), washed thoroughly with
deionized water, and then used in the following accumulation
experiments.
2.3. Uranium and Th accumulation experiments. The U
and Th ions were supplied as Th(NO3) 4 and UO2 (NO3) 2. The
pH of the solution was adjusted to the desired values with 0.1
M HNO3 or 0.1 M NaOH. Unless otherwise stated, the accumulation experiments were conducted as follows. The resting
microorganisms (15 mg dry weight (wt.) basis) were suspended
in 100 mL solution at pH 3.5 containing each 50 µM Th and U,© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/18/2006

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TsurutaJ. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

and the suspension was shaken for 1 h at room temperature.
The microorganisms were then collected by filtration througha membrane filter (pore size, 0.2 µm). The amount of U and Th
accumulated by the microorganisms was determined by measuring their contents in the filtrate using an inductively coupledplasma quantometer (ICPS8000, Shimadzu Corporation, Kyoto).
Uranium and Th were not adsorbed on the filter and the wall of
the vessel under this experimental condition.
2.3.1. Effect of Th and U concentrations on both metal accumulation. The effect of external Th and U concentrations on
both metal accumulation was examined as follows. The resting
cells of S. levoris (15 mg dry wt. basis) were suspended in 100
mL of solution at pH 3.5 containing the desired amounts of Th
and U (10–200 µM), and their suspensions were shaken for 1 h
at room temperature.
2.3.2. Time course of Th and U accumulations on both
metal accumulation. The resting cells of S. levoris (15 mg dry
wt. basis) were suspended in 100 mL of solution at pH 3.5 containing each 50 µM of Th and U, and their suspensions wereshaken for the desired time at room temperature. Separation of
the cells from the solution and the analysis of the metals content are the same as those mentioned above.
3. Results and discussion
3.1. Accumulation of Th and U by microorganisms froma solution containing both metals. The amounts of accumulated Th and U by 41 strains of 37 species (9 actinomycetes, 12bacteria, 11 fungi, and 9 yeasts) are summarized in Tables 1–4.
A preliminary experiment showed that the amount of accumulated Th and U using these microorganisms increased withincreasing pH of the solution; however, thorium hydroxide was
precipitated at pH 4.0.12 Therefore, the pH of the solution was
adjusted to 3.5 throughout the screening. Among the tested
microorganisms, a high Th accumulation ability was exhibitedby certain gram-positive bacterial strains, notably A. nicotianae IAM12342, Bacillus megaterium IAM1166, B. subtilis
IAM1026, Micrococcus luteus IAM1056, Rhodococcus erythlopolis IAM1399, and Streptomyces levoris HUT6156. On theother hand, though U was accumulated in small amounts by allthese microorganisms, A. nicotianae IAM12342, S. flavoviridis HUT6147, and S. levoris HUT6156 had relatively high U
accumulation abilities.
The relative degree of Th accumulation by the microorganisms was observed in the following order: gram-positive bacteria (except actinomycetes) > actinomycetes > gram-negative
bactera, fungi, and yeasts, indicating that gram-positive bacteria can accumulate Th more readily than the other types of
TABLE 1: Accumulation of Th and U from the Solution
Containing Th and U by Actinomycete
Species and strains
Streptomyces albogriseolus HUT6045
S. albus HUT6047
S. flavoviridis HUT6147
S. fradiae HUT6054
S. griseoflavus HUT6153
S. levoris HUT6156
S. olivaceus HUT6061
S. scabies HUT6027
S. viridochromogenes HUT6030

Metals accumulated(µmol/g dry wt. cells)
Th
U
241±3
281±3
290±1
202±4
183±1
297±7
216±1
215±4
241±2

23±2
93±1
145±4
54±8
69±4
133±0
89±3
95±3
75±2

Resting cells (15 mg dry wt. basis) were suspended in 100 mL ofsolution (pH 3.5) containing 50 µM of Th and U for 1 h at room
temperature.

microorganisms. As shown in Table 5, there were significant
differences. On the other hand, half of the actinomycetes species accumulated U much more than most of the gram-positivebacteria species (except A. nicotianae). There were not significant differences between gram-positive and gram-negative bacteria in the amount of accumulated U from the solution
containing both elements. Additionally, all of the microorgaTABLE 2: Accumulation of Th and U from the Solution
Containing Th and U by Bacteria
Species and strains

Metals accumulatedGram(µmol/g dry wt. cells)
type
Th
U

Arthrobacter nicotianae IAM12342
Bacillus licheniformis IAM11054
B. megaterium IAM1166
B. subtilis IAM1026
Escherichia coli IAM1268
Micrococcus luteus IAM1056
Rhodococcus erythropolis IAM1399
Pseudomonas aeruginosa IAM1054
P. fluorescens IAM12022
P. saccharophilia IAM1504
P. stutzeri IAM12097
Starkeya novela IAM 12100

339±1
294±1
314±2
295±4
135±0
346±1
316±3
135±4
153±3
143±1
183±1
89±2

142±5
22±2
28±3
12±5
15±3
50±1
63±2
40±2
26±3
13±4
72±4
37±1

+
+
+
+
–
+
+
–
+
+
+
+

See footnote of Table 1.

TABLE 3: Accumulation of Th and U from the Solution
Containing Th and U by Fungi
Species and strains

Metals accumulated(µmol/g dry wt. cells)
Th
U

Aspergillus niger IAM2086

81±6

0±0

Giberella fujikuroi AHU9078
Mucor hiemalis IAM6088
Neurospora sitophira AHU9213
N. sitophira IAM5502
Penicillium chrysogenum IAM7114
P. lilacinum AHU8357
P. notatum IAM7168
Rhizopus arrhizus AHU6573
R. oryzae IAM6006
Trichoderma viride AHU9503

95±5
91±5
182±5
110±3
88±2
57±3
128±2
115±3
125±0
76±3

15±3
74±2
40±1
25±2
2±1
25±2
5±4
36±0
31±2
14±4

See footnote of Table 1.

TABLE 4: Accumulation of Th and U from the Solution
Containing Th and U by Yeasts
Species and strains

Metals accumulated(µmol/g dry wt. cells)
Th
U

Candida utilis AHU3210
C. utilis IAM4220
Cryptococcus laurentii AHU3671
C. laurentii IAM12264
Debaryomyces hansenii AHU3759
Endomycopsis fibligera AHU4113
Hansenula anomala AHU3702
Saccharomyces serevisiae AHU3818
S. serevisiae IAM4512

53±1
50±1
104±4
137±2
124±1
77±4
65±1
87±3
24±2

See footnote of Table 1.

12±2
5±2
22±4
8±6
30±2
1±1
10±3
11±1
7±3

Accumulation of Thorium and Uranium by Microbes – the Effect of pH

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

3

TABLE 5: T-authorization of the Difference in the Average Amount of Each Metal Accumulated by Various Microorganisms from the Solution Containing Both Metals
Group ofmicroorganisms

Amounts of thorium versus
GPB
GNB
F
A

Group ofmicroorganisms

A
GNB
F
Y

****
****
****
****

GPB
GNB
F
Y

****
****
****

**
****

−

Amounts of uranium versus
A
GPB
GNB
F
−
****
****
****

−
−
****

−
****

*

See footnote of Table 1.
Symbols: **** There are significant differences at a dangerous rate of 1%.
*** Dangerous rates 2%, ** 5%, * 10%.
− There are no significant differences (at dangerous rates of 10%).
GPB: gram-positive bacteria, A: actinomycetes, GNB: gram-negative bacteria, F: fungi, Y: yeasts.

400

Accumulated metal/(µmol/g dry wt. cells)

nisms accumulated greater amounts of Th than U under theseexperimental conditions.
Generally, the teichoic acid polymers in gram-positive bacteria definitely confer a strong negative charge on the surfaceof the cell wall because of their high content of ionized phosphate groups, although little if any teichoic acid is found in thegram-negative bacteria.18 It is tentatively considered that thechelate formation between the cell surface of gram-positivebacteria and Th ions becomes stronger than that of the gramnegative bacteria and Th ion. Consequently, it is reasonable
that the amount of Th accumulated by the gram-positive bacteria
is greater than that of the gram-negative bacteria. It is tentatively considered that the chelate formation between the cellsurface of gram-positive bacteria and Th ion becomes stronger
t ha n t hat of t he g ra m-negat ive bacter ia a nd T h ion.
Consequently, it is reasonable that the amount of Th accumulated by the gram-positive bacteria is greater than that of thegram-negative bacteria.
In the case of the amount of accumulated U from the solution containing both metals, the effect of the kind of species on
U accumulation was not clear, because the effect of Th was farlarger.
3.2. Effect of pH on accumulation of Th and U by S. levoris cells. Recently, the effect of pH on the U accumulation froma solution containing only U using S. levoris was reported.16
Though the amount of U accumulated gradually increased withincreasing pH of the solution under pH 2.00 and above pH
2.50, it rapidly increased with increasing pH of the solution
between pH 2.00–2.50. On the other hand, the amount of Th
accumulated from the solution containing only Th rapidly
increased with increasing pH of the solution under pH 2.00,
whereas it gradually increased with increasing pH of the solution between pH 2.00–3.25.18
The amounts of Th (squares) and U (circles) accumulated by
the S. levoris cells from the mixed solution containing the same
molar amount of each metal linearly increased with increasing
pH of the solution (Figure 1). The amount of accumulated Th(QTh) (µmol/g dry wt. cells) was well described by QTh =84.0 ×
pH + 31.9, and that of accumulated uranium (Q U) (µmol/g dry
wt. cells) was well described by Q U = 44.5 × pH – 3.88. The
amount of accumulated Th was far greater than that of U at all
pHs from 1.00 to 3.75. As a result, the amounts of Th and U
accumulated from the solution containing each metal16, 17 andthe amount of Th accumulated from the mixed solution containing both metals were very similar. However, the amount of
U from the mixed solution containing both metals was far
lower than the amounts of Th and U accumulated from thesolution containing each metal only16, 17 and the amount of Th
accumulated from the mixed solution containing both metals.
3.3. Dependence of Th and U concentrations on accumulation by S. levoris cells. As shown in Figure 2-a), the amountof Th accumulated from the mixed solution containing the

300

200

100

0
0

1

2

3

4

5

pH
Figure 1. Effect of pH on Th and U accumulation by S. levoris cells.
Resting cells (15 mg dry wt. basis) were suspended in a 100 mL ofsolution containing 50 µM Th (squares) and U (circles) for 1 h atroom temperature.

same molar amounts of both Th and U by the S. levoris cells(µmol/g dry wt. cells) increased with increasing Th concentration. When the Th and U concentration was 150 µM (residual
concentration of Th was 93.5 µM and that of U was 141 µM), ahigh Th accumulation of about 365 µmol of Th/g dry wt. cells
was observed.
Figures 2-a) and b) indicate that the accumulation of Th
from the mixed solution containing Th and U by the S. levoris
cells does not obey the Langmuir isotherm over the entire concentration region (dotted line). However, the relationshipbetween the amount of accumulated Th (µmol/g dry wt. cells)
and the residual Th concentration from 0 to 10.0 µM (as theinitial thorium concentration from 0 to 51.3 µM), and from10.0 to 93.5 µM (as the initial thorium ion concentration at 51.3
to 150 µM) obeys different Langmuir isotherms (solid line)
respectively, which depend only on the residual Th concentration. The curve in Figure 2-a) indicates the values calculated
from each Langmuir isotherm, Ce (Th)/Q Th = m ThCe (Th)+nTh,
where Q Th indicates that the amount of accumulated Th (µmol
Th/g dry wt. cells), Ce (Th) is the residual Th concentration in
the solution (µM) and mTh and nTh are the Langmuir constants.
Accordingly, the result obtained in this experiment has dual
patterns. When the initial Th concentration is varied from 0 to
51.3 µM (as 0 to 10.0 µM of residual Th concentration), theamount of accumulated Th is shown on the left side in Figure2-a). When the initial Th concentration increased from 51.3 to

4
400

0.3

a)
300

200

100

0
0

30

60

90

120

150

400

Accumulated uranium/(µmol/g dry wt. cells)

Accumulated thorium/(µmol/g dry wt. cells)
concentration (L/g drt wt. cells)

Accumulated thorium/(µmol/g dry wt. cells)

TsurutaJ. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

b)

0.2

0.1

0?
0

Residual thorium concentration/µM

c)
300

200

100

0
25

50

75

Residual thorium concentration/µM

100

0

30

60

90

120

150

Residual uranium concentration/µM

Figure 2. Effect of metal concentrations on Th (closed squares) and U (closed circles) accumulation by S. levoris cells. Resting cells (15 mg dry
wt. basis) were suspended in 100 mL of solution (pH 3.5) containing the desired same molar amounts of Th and U for 1 h at room temperature.
Symbols indicate the amount of accumulated Th (squares) and U (circles) calculated by the Langmuir equations using entire concentration data
(dotted line) or using data from two separate concentrations (solid line).

150 µM (as 10.0 to 93.5 µM of residual Th concentration), theamount of accumulated Th is shown on the right side of Figure2-a). A similar result was obtained19 from the absorption ofpotassium using barley roots. This fact seems to indicate thatTh accumulation using S. levoris cells is not a monolayer accumulation but a dual-layer accumulation. The constants m Th,
n Th, and the maximum amount of accumulated Th Q max (Th)that was used to estimate 1/mTh are summarized in Table 6. As
a result, the amount of accumulated Th was almost unaffectedby the accumulation of U by the S. levoris cells.
TABLE 6: Estimated Constants and Qmax(Th) from theLangmuir Isotherm
Initial Th
concentration
(µM)
0– 50

mTh

nTh

3.74 × 10−3

5.69 × 10−5

267

−3

−2

382

7.01 × 10−3372

50– 150

2.61 × 10

0– 1502.69 × 10−3

1.19 × 10

Qmax(Th)(µmol/g dry wt. cells)

See footnote of Figure 2.

It was reported that the accumulation of U from the solution
containing only U by the S. levoris cells obeys the Langmuir
isotherm, depending on the U concentration.16 The amount ofU (µmol/g dry wt. cells) accumulated from the mixed solution
containing the same molar amount of Th and U is shown in
Figure 2-c). As shown in this figure, the amount of U accumulated was far lower than that from the solution containing only
U 16 due to the presence of Th. The amount of U increased withincreasing initial U and Th concentrations from 0 to 50 µM (theresidual concentration of U was from 0 to 31.3 and that of Th
was 0 to 10 µM). On the other hand, the amount of U decreased
with increasing initial U and Th concentrations from 50 to 150µM (residual concentration of U was from 31.3 to 141 µM andthat of Th was 10 to 93.5 µM).
It was reported that the accumulated U was easily desorbedby the co-existing Th.16 Therefore, the amount of accumulated
U from the mixed solution containing the same molar amountsof U and Th was assumed to be the value, (the amount of accumulated U from the solution containing only U) – (the amountof accumulated Th from the mixed solution containing U and
Th). Accordingly, Q U = C e (U)/(m U ·C e (U) + n U ) – C e (Th)/(mTh·Ce (Th) + nTh), where Q U indicates the amount of U accumulated (µmol U/g dry wt. cells) from the mixed solution
containing U and Th, Ce (U) is the residual U concentration in
the solution containing both metals (µM), Ce (Th) is the residual Th concentration in the solution containing both metals(µM), and m U and n U are the Langmuir constants calculated
from the solution containing only U 16 ; m Th and n Th are theLangmuir constants mentioned in the previous section. The
calculated amount of accumulated U based on the entire concentration data (dotted line) was much higher from the experimental data especially for low concentration regions. On theother hand, though the value of U accumulated based on thisequation using two separated concentration regions (solid line)
was slightly lower than the experimental data, both values are
relatively similar. Therefore, it seems almost reasonable thatthe amount of U accumulated from the mixed solution containing the same molar amounts of U and Th can be calculated
from the difference in the amount of U accumulated from thesolution containing only U and that of Th accumulated fromthe mixed solution containing both elements. Based on the
result in the literature16 and the evaluated and experimental
data, it was tentatively assumed that the 1:1 U–Th ion exchange
reaction occurred from the solution containing both elements
using S. levoris cells.
3.4. Time course of Th and U accumulations from thesolution containing both metals. In order to solve the timecourse of the Th and U accumulations, the relationships of theamounts of accumulated Th and U and the retention time wereanalyzed.
The accumulation rate of Th using S. levoris is considered tobe the difference in the real adsorption rate and the desorption
rate.20 The time course of the Th accumulation was almost
unaffected by the co-existing U.16 Therefore, the real accumulation rate of Th is considered to be proportional to the difference in the maximal amounts of the total metal accumulated
and the amount of Th accumulated at a certain time. The
amount of accumulated U is ignored in this consideration. On
the other hand, the real desorption rate is considered to be proportional to the amount of Th accumulated at the same time.
Accordingly,
dQTh /dt = k1Th (QmaxTotal – QTh) – k2Th QTh.
At equilibrium, dQTh /dt =0 and QTh = Qe (Th),Q maxTotal = (k1Th + k 2Th) Qe (Th)/ k1Th.
dQTh /dt = (k 1Th + k2Th) Qe (Th)– k1Th QTh – k2Th QTh= (k 1Th + k2Th) {Qe (Th)– QTh}.
QTh = Qe (Th)[1– exp{–(k1Th + k2Th) t}].
As QTh /Qe (Th) = θ Th, 1– θ Th = exp{–(k1Th + k2Th)t},ln{1/(1– θ Th)} = (k1Th + k2Th)t.

Accumulation of Thorium and Uranium by Microbes – the Effect of pH

3

3

2

2

1

ln{1/(1– θ ’)}

ln{1/(1– θTh)}

From the equation, one can see that ln{1/(1– θ Th)} is proportional to the adsorbing time.
As shown in Figure 3, the relationship of ln{1/(1– θ Th)} and t
showed a good linearity; however, the line did not fit the original point but was fitted to another line under and over 60 min.
The accumulation was very fast after contact between the metals and the microbes. Furthermore, most of the metals wereaccumulated within 1 h; therefore, the accumulation rate after
1h became slower. Accordingly, in the case of within 60 min,ln{1/(1– θ Th)} was estimated to be 1.88 × 10−2 t +0.444. On theother hand, in the case of the time from 60 to 180 min, that wasestimated to be 7.95 × 10−3 t +1.14.
As shown in Figure 4, the accumulated Th was in good
agreement with the calculated curve, and the amount of accumulated Th at equilibrium was estimated to be 353 µmol/g dry
wt. cells. Therefore, the accumulation rate of Th from the solution containing Th and U using the S. levoris cells was almost
described by the amount of accumulated Th only. Accordingly,
in the case of within 60 min, the amount of accumulated Th,
Q Th, was estimated to be 353{1– exp(–1.88 × 10 −2 t – 0.444)}.
On the other hand, in the case of the time from 60 to 180 min,that was estimated to be 353{1–exp(–7.95 × 10−3 t –1.14)}.
The time course of U accumulation using S. levoris wasstrongly affected by the co-existing Th. Therefore, the real
accumulation rate of uranium is considered to be proportional
to the difference in the maximal amounts of the total accumulated metal and the amounts of Th and U accumulated at a certain time. The real desorption rate of U is considered to be
proportional to the amount of Th and U accumulated at thesame time.

1

0

0
0

30

60

90

120

150

180

Time/minFigure 3. Parameter determination for the time course of Th and U
accumulation by S. levoris. Resting cells (15 mg dry wt. basis) weresuspended in 100 mL of solution (pH 3.5) containing 50 µM of Th
and U at room temperature. Square and triangle symbols indicate theln{1/(1– θ Th)} and ln{{1/(1– θ’)}, respectively. θ Th expresses the ratioof accumulated Th at a time and that at equilibrium, Q Th /QeTh , θ’expresses the ratio of the amount of accumulated Th and U at a time
and that at equilibrium, (Q Th+Q U) / (QeTh+QeTh).

Accordingly,
dQ U/dt = k1U (QmaxTotal – Q U – QTh) – k2U(Q U + QTh}.
At equilibrium, dQ U/dt = 0, Q U = Qe (U), and QTh = Qe (Th),Q maxTotal = (k1U + k2U){Qe (U) + Qe (Th)}/k1U.
∴dQ U/dt = (k1U + k2U){Qe (U) + Qe (Th)}– (k1U + k2U)(QU + QTh)
= (k1U + k2U){Qe (U) + Qe (Th) – Q U – QTh}.

∫ [1/{Qe(U) + Qe(Th) – QU – QTh}]dQU = ∫ (k1U + k2U)dt.
Because the amount of Th accumulated was unaffected by theamount of co-existing U,–ln{Qe (U) + Qe (Th) – Q U – QTh} = (k1U + k2U)t + Const.
When t = 0, as QU = QTh = 0 and Const = –ln{Qe(U) + Qe(Th)},
therefore,–ln{Qe(U) + Qe(Th) – QU – QTh}= (k1U +k2U)t –ln{Qe(U) + Qe(Th)}.
ln[{Qe(U) + Qe(Th)}/{Qe(U) + Qe(Th) – QU – QTh}]= (k1U + k2U)t.
ln(1/[1–(Q U + QTh)/{Qe (U) + Qe (Th)}]) = (k1U + k2U)t.
As (QU+QTh)/{Qe(U) + Qe(Th)}= θ’,
ln{1/(1– θ’)}= (k1U+k 2U)t.
QU: amount of U adsorbed (µmol/g dry wt. cells)
k 1U : real adsorption rate constant of U
k 2U : real desorption rate constant of U
Qe (U) : amount of U adsorbed (µmol/g dry wt. cells) at
equilibrium
From the equation, one can see that ln{1/(1– θ’)} is proportional
to the adsorbing time.
As shown in Figure 3, the relationship of ln{1/(1– θ’)} and t
also showed a good linearity; however, the line also did not fit
the original point but was fitted to another line under and over
60 min. The accumulation was very fast after contact between
the metals and the microbes. Furthermore, most of the metals
were also accumulated within 1 h; therefore, the accumulation
rate after 1h became slower. Accordingly, in the case of within
60 min, ln{{1/(1– θ’)} was estimated to be 2.21 × 10−2 t +0.517.
On the other hand, in the case of the time from 60 to 180 min,that was estimated to be 6.62 × 10−3 t +1.14.

Accumulated metal/(µmol/g dry wt. cells)

QTh: amount of Th adsorbed (µmol/g dry wt. cells): real adsorption rate constant of Th
k 1Th
QmaxTotal : the maximum total amounts of Th and U
adsorbed (µmol/g dry wt. cells)
k 2Th: real desorption rate constant of Th
QeTh: amount of Th adsorbed (µmol/g dry wt. cells)
at equilibrium

5

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

300

Th
200

100

U
0
0

30

60

90

120

150

180

210

Time/minFigure 4. Time course of Th and U accumulation from the solution
containing both metals by S. levoris cells. The experimental conditions
were the same as in Figure 3. Symbols are the same as in Figure 1.

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J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

As shown in Figure 4, the accumulated U was in good agreement with the calculated curve, and the amount of accumulated
U at equilibrium was estimated to be 104 µmol/g dry wt. cells.
Therefore, the accumulation rate of U from the solution containing Th and U using the S. levoris cells was described by theamount of total accumulated Th and U. Accordingly, in the caseof within 60 min, the amount of accumulated U, Q U, was estimated to be 104 – 457exp(–2.21 × 10−2 t – 0.517)+353exp(–1.88
× 10−2 t –0.444). On the other hand, in the case of the time from60 to 180 min, that was estimated to be 104 – 457exp(–6.62 ×
10−3 t –1.44)+353exp(–7.95 × 10−3 t –1.14).
4. Conclusions
Among the species tested, a high Th accumulation ability
was exhibited by strains of gram-positive bacteria, such as
Arthrobacter nicotianae, Bacillus megaterium, B. subtilis,
Micrococcus luteus, Rh odococcus er ythropolis, a nd
Streptomyces levoris. Though U was accumulated in small
amounts by all these microorganisms, A. nicotianae IAM12342,
S. flavoviridis HUT6147, and S. levoris HUT6156 had relatively high uranium accumulation abilities. In these high performance metal-accumulating microorganisms, S. levoris could
accumulate about 300 µmol Th and 133 µmol U per gram dry
wt. of microbial cells from a solution containing both Th andU at pH 3.5. The accumulated amounts of both elements linearly increased with the increasing pH of the solution. The
accumulated amount of Th was increased with increasing Th
concentration; however, it was almost unaffected by the coexisting U. On the other hand, the accumulated amount of U isstrongly affected by the co-exsisting Th, and it can be evaluated by the U–Th ion exchange mechanism. The time courseof accumulated Th was also unaffected by co-exsisting U andthat of U was strongly affected by co-existing Th.
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