Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 81-84, 2005

Accepted
Separation of Rare Earth Elements by Microorganisms
Takehiko Tsuruta*Department of Applied Chemistry, Tohwa University, 1-1-1 Chikushigaoka, Fukuoka 815-8510, Japan
Received: November 15, 2004; In Final Form: April 21, 2005
The selective accumulation of rare earth elements in Gram-positive bacteria and actinomycetes was examined.
The resting cells of 18 strains having high capacities to accumulate rare earth elements were screened for selectivity using a solution containing 5 elements: Y, La, Sm, Er, and Lu. Among the strains tested, Bacillus megaterium accumulated Sm, Streptomyces albus accumulated Lu, and Arthrobacter nicotianae accumulated both Sm
and Lu in higher quantities than the other metals. Similar results were also obtained from a solution containing Y
and 14 rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). The amount of Sm
accumulated in B. megaterium and that of Lu accumulated in S. albus increased as the initial metal concentration
increased. S. albus and B. megaterium cells show the highest capacity to accumulate Lu and Sm, respectively,from a solution containing 5 elements, Y, La, Sm, Er, and Lu when each metal concentration ranged from 20 to100 µM. These results suggest that the separation of these two rare earth elements using microorganisms should
be possible.

1. Introduction
The separation of rare earth elements from solutions containing
many kinds of rare earth elements is difficult because of theirvery similar chemical properties.1 Solvent extraction methods
performed for the separation of rare earth elements pose environmental problems because a large amount of organic solvent isinevitably used. As a consequence, many researchers have been
studying the removal of uranium using microorganisms suchas actinomycetes,2-5 bacteria,3, 6−10 fungi,3, 11−15 and yeasts.3, 7, 16
The adsorption of several actinides (thorium and uranium)
and lanthanides (lanthanum, europium, and ytterbium) by
Mycobacterium smegmatis has been investigated; however, theamount of adsorbed lanthanide on cells was low and there wasalmost no difference in the amounts of different lanthanides
extracted. 10 Recently, it was reported that Gram-negative
bacteria, such as Variovorax paradoxus, reduced the amount oflight rare earth elements such as Y, La, Ce, and Nd, more thanthat of the other rare earth elements from solutions containing
all of the lanthanides.17 It was also reported that Streptomyces
sp. decreased the amount of Yb.18
We have investigated the accumulation of rare earth elements
using various microorganisms from solutions containing only
one kind of rare earth element (not published). The amounts ofrare earth elements accumulated by Gram-positive bacteria weremuch higher than those by Gram-negative bacteria, fungi, andyeasts. Most species of the Gram-positive bacteria have higher
rare-earth-element accumulating abilities than actinomycetes.
However, actinomycetes and other Gram-positive bacteria
showed different accumulating features in the case of theuranium.3, 19 The amounts of uranium accumulated at pH 5.8
using Gram-positive bacteria such as Arthrobacter nicotianae
IAM12342, Bacillus subtilis IAM1024, and Micrococcus luteus
IAM1056, were higher than those using all the actinomycetes
strains tested (20 strains).3 On the other hand, the amounts ofuranium accumulated at pH 3.5 using half strains of actinomycetes tested were higher than those using all the Gram-positive bacteria except A. nicotianae.19 Therefore, actinomycetes
and other Gram-positive bacteria are expected to have different*Corresponding author. E-mail: ttsuruta@tohwa-u.ac.jp. FAX:
+81-92-552-2707.

features for accumulating rare earth elements from solutions
containing many kinds of rare earth elements. Accordingly,
in order to clarify the possibility of separating rare earth
elements, actinomycetes and other kinds of gram-positivebacteria that have a high accumulating ability for rare earth
elements were examined in this study. First, the screening ofmicroorganisms for the selective accumulation of Y, La, Sm,
Er, and Lu from a solution containing these 5 kinds of metals
was examined using 18 kinds of strains. After that, the accumulation pattern of Y and all lanthanides elements and theeffect of the concentration of rare earth elements on the accumulation of these elements were studied using strains thatshowed both high accumulating ability and selectivity.
2. Experimental
Materials. The strains used in this study were generously
donated by the IAM Culture Collection, Center for Cellularand Molecular Research, Institute of Molecular and Cellular
Biosciences, The University of Tokyo (IAM); and the Facultyof Engineering, Hiroshima University (HUT). The chemicals(guaranteed reagents) were obtained from Nacalai Tesque,Inc., Kyoto.
Culture conditions of microorganisms. The medium forgrowing bacteria contained 3 g/L meat extract, 5 g/L peptone,
and 5 g/L NaCl in deionized water. The medium for growingactinomycetes contained 4 g/L yeast extract, 10 g/L malt
extract, and 4 g/L glucose in deionized water, pH 7.1. 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 bycentrifugation (for bacteria) or by filtration through filter paper(for actionomycetes) and washed thoroughly with deionizedwater for use in the accumulation experiments.
Metal accumulation experiments
Screening of microorganisms for selective accumulation ofY, La, Sm, Er, and Lu. In order to determine which types ofmicroorganisms have greater separation ability, 18 strains of
15 species (9 actinomycetes, 9 other kinds of Gram-positivebacteria) were examined. The metal was supplied as
Ln(NO3)3. The accumulation experiments were conducted asfollows. Resting microorganisms (15 mg dry wt. basis) were

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

Tsuruta

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

suspended in a 100 mL solution (pH 5.0) containing 66.5 µMof Y, La, Sm, Er, and Lu, and the suspension was shaken for 3
h at room temperature. The pH of the solution was adjusted to5.0 with 0.1 M NaOH because the precipitation of hydroxidefrom NaOH occurred at pH 6.0, while, in a solution containing
only Sm, the amount of accumulated Sm increased withincreasing pH (data not shown). The microorganisms werethen collected by filtration through a membrane filter (poresize 0.2 mm). The amount of metal accumulated by the cells
was determined by measuring the metal content in the filtrateusing an inductively coupled plasma quantometer (ICPS8000,Shimadzu Corporation, Kyoto).
Selective accumulation of Y and rare earth elements in
microorganisms. In order to determine the selectivity of Sm
and/or Lu accumulation, the selective accumulation of Sm
and/or Lu was tested in S. albus, B. megaterium, and A. nicotianae. The resting cells (15.0 mg dry wt. basis) weresuspended in 100 mL of the solution (pH 5.0) containing 66.5
each µM of Y and all the lanthanides elements for 3h at room
temperature.
Rare-earth-element accumulation capacity of microorganisms. In order to determine the Lu accumulation capacity of S.
albus and the Sm accumulation capacity of B. megaterium cells,
the cell-specific accumulation effect of each equilibrium metal
concentration in solutions containing only that metal was examined. Resting cells (15.0 mg dry wt. basis) were suspended in
100 mL of the solution (pH 5.0) containing the desired amounts
of Sm or Lu for 3h at room temperature.
Effect of rare-earth-element concentrations on selective
accumulation in microorganisms. In order to determine theeffect of the initial concentration of the rare earth elements on
their accumulation in S. albus and B. megaterium cells, theaccumulation from a solution containing equivalent molar
amounts of Y, La, Sm, Er, and Lu were examined. Restingcells (15.0 mg dry wt. basis) were suspended in 100 mL of thesolution (pH 5.0) containing the desired amounts of each metal
for 3h at room temperature.

TABLE 1: Accumulation of Rare Earth Elements Using
Various Microorganisms from the Solution Containing
Five Kinds of Rare Earth Elements
StrainYLaSmErLuTotal7214290139524Bacillus megaterium IAM1166

4650106

6573342

B. subtilis IAM1026

5140916094335

B. subtilis IAM1633

3823715389275

B. subtilis IAM11060

6865

104

78

107422B. licheniformis IAM11054

45

40

8159

83

308

Brevibacterium helovolum IAM1637 35

2978

53

83

279

Corynebacterium equiIAM1038

3929744473

259

Nocardia erythropolis IAM1399

48

3877

59

105

328

Streptomyces albus HUT6047

53

3792

65

153

401

S. aurantiacogriceus HUT6201

2728563535

182

S. flavoviridis HUT6147

3933675375267

S. griseoflavus HUT6153

3027583944198

S. levoris HUT6156

634295

77

151

427

S. olivaceus HUT6061

43507460

52278

S. phaeochromogenus HUT6013

6864

100

7387392

S. viridochromogenes HUT6030

483678

53

68

282

S. viridochromogenes HUT6031

4939835666294(A)

60

40200

3. Results and Discussion(B)

Metals accumulated (µmol/g dry wt.cells)

Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

6040200Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu(C)Metals accumulated (µmol/g dry wt.cells)

Screening of microorganisms for selective accumulationof Y, La, Sm, Er, and Lu using various microorganisms.
The amounts of total accumulated metals in some Gram-positive bacteria and actinomycetes, such as A. nicotianae, B.
subtilis IAM11060, S. albus, and S. levers, were higher thanthose in others (Table 1). Among these, the strains S. albus
and S. levers had higher accumulating abilities for Lu than for
Y, La, Sm, and Er. These strains can accumulate Lu at about
150 µmol/g dry wt. cells. On the other hand, in A. nicotianae,
the amounts of accumulated Sm and Lu were higher than theamounts of Y, La, and Er. This strain can accumulate Sm andLu about 150 µmol/g dry wt. cells. The amount of Sm accumulated in B. megaterium was higher than the amounts of Y,
La, Er, and Lu, though the total amount of the rare earth
elements accumulated in this cell was lower than that in A.
nicotianae, B. subtilis, S. albus, and S. levoris.
The results of these experiments are quite different from thoseof Reference 17. Gram-negative bacteria, such as Variovorax
paradoxus and Comamonas acidovorans, show a higher capacity
to accumulate light rare earth elements, such as Y, La, and Nd
than to accumulate heavy rare earth elements.17
Selective accumulation of Y and rare earth elements in
microorganisms. As shown in Figure 1-(A), in S. albus, Lu
was accumulated at higher amounts than other elements whilein B. megaterium, Sm had the highest accumulation (Figure 1(B)). Furthermore, as shown in Figure 1-(C), the amounts of
Sm and Lu accumulated in A. nicotianae were higher than thatof Y and other rare earth elements. Based on the results ofthese experiments, Sm and/or Lu can be selectively accumu-Metals accumulated (µmol/g dry wt.cells)

Arthrobacter nicotianae IAM12342 82

Metals accumulated (µmol/g dry wt.cells)

82

60

40200
Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 1. Accumulation of rare earth elements using (A) S. albus (B)B. megaterium and (C) A. nicotianae from the solution containing
yttrium and all the lanthanoid elements.

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Separation of Rare Earth Elements by Microorganisms

83

●

● ●

●

200

●

100(A)

002040

Metals accumulated (µmol/g dry wt.cells)(A)

Sm accumulated (µmol/g dry wt.cells)

150
30060

□100
●

●

◆■
○

◆
■

◆
■

○

○

●

●
◆

50
○
■

◆
■
○

002550

75

100

125

Initial metal concentration (µM)

●

●

●

200 ●

100 ●

0102030405060(B)70

Metals accumulated (µmol/g dry wt.cells)

Lu accumulated (µmol/g dry wt.cells)●□

1503000Figure 2. Effect of the external (A) lutetium or (B) samarium concentration on each accumulation from the solution containing each metal
only using (A) S. albus and (B) B. megaterium. The lines were estimated by the Langmuir isotherms mentioned in Table 2.

lated in microorganisms, such as S. albus, A. nicotianae, andB. megaterium.
Rare earth elements accumulation capacity of microorganisms. The amount of Lu accumulated in S. albus and Sm
accumulated in B. megaterium cells increased with an increaseof each metal concentration (Figure 2). The amount of eachmetal accumulated obeys the Langmuir isotherm, Q M =Ce(M)/(mM Ce(M) + nM), where QM indicates the amount ofeach accumulated metal (µmol each metal/g dry cells), Ce(M)is the concentration of each residual metal concentration (µM),
and mM and nM are Langmuir constants. On the basis of thisfinding, it seems reasonable to postulate that the accumulationsof Lu and Sm by these microorganisms are mostly dependenton physico-chemical binding to the cell components. TheLangmuir constants and the maximum amounts of each metal
are shown in Table 2.
Effect of initial rare-earth-element concentration on selective accumulation in microorganisms. As shown in Figure 3,
TABLE 2: The Estimated Langmuir Constants and Qmax fromthe Langmuir Isotherm
Initial metal
concentration(µM)

Residual metal
concentration(µM)

Lutetium0 − 84.6

Samarium0 − 91.1

Qmaxµmol/gdry cells

0 − 39.9

3.35 × 10−3

4.86 × 10−3299

0 − 52.3

3.86 × 10−3

1.92 × 10−3259

13n68m■ Y● Sm○ La◆ Er□ Lu100●●●□●●50□◆■○□◆■○□□

■
◆○

◆○■

◆○■00

Residual Lu concentration (µM)

Metal□□

Residual Sm concentration (µM)(B)

□255075

100125Initial metal concentration (µM)
Figure 3. Effect of the concentrations of external rare earth elementson the accumulation of yttrium, lanthanum, samarium, erbium, andlutetium using (A) S. albus and (B) B. megaterium.

the amount of Lu accumulated in S. albus cells increased from91 to 127 µmol/g dry wt cells with an increase of initial Luconcentration from 20 to 94 µM. The amount of Sm accumulated in B. megaterium cells also increased from 61 to 96µmol/g dry wt cells with an increase of initial Sm concentration from 20 to 100 µM. The amount of Lu accumulated in S.
albus cells and that of Sm in B. megaterium cells were higher
than the accumulated amounts of the other 4 kinds of metal
combined in each cell within whole metals concentration. As aresult, the selective accumulation ability of Lu in S. albus andthat of Sm in B. megaterium cells were kept within the wholeconcentration.
Accordingly, from the results of these experiments, it isshown that the cells of these microorganisms can be used toseparate rare earth elements.
Acknowledgements. This work was supported by theMetal Mining Agency of Japan (MMAJ). We thank grateful toMMAJ.
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