Journal of Nuclear and Radiochemical Sciences, Vol. 16, pp. 11-14, 2016

ArticlesThe 3rd Japan-China Academic Symposium on Nuclear Fuel Cycle, ASNFC 2015

Recent Studies on Ca Isotope Separation by Crown-Ether Resin Chromatography
Shin Okumuraa, Saori Umeharab, Tadafumi Kishimotob, Masao Nomuraa,
Yasuhiko Fujiia,*, Tatsuya Suzukic, Masaki Ozawaa
aLaboratory for Advanced Nuclear Energy, Tokyo Institute of Technology
Research Center for Nuclear Physics, Osaka University
cGraduate School of Engineering, Nagaoka University of Technologyb

Received March 15, 2016; Accepted October 11, 2016
Benzo-18-crown-6 ether resin was synthesized with the carrier of porous fine silica beads. By using synthesizedcrown ether resin packed in glass columns, 48Ca enrichment was studied. A high speed chromatography experiment was conducted at the band migration speed of 48 m day-1. In spite of the high speed of migration, enrichment of 48Ca was observed at the front boundary of the displacement type chromatography. Comparing withpreviously reported experiments of 1 m 3 m and 200 m migrations, we confirmed that separation coefficient εslightly decreases with increase in the band velocity. In addition, the enriched zone was found to increase withincrease in the migration length. The observed relation shows that the width of the enriched zone is proportional
to square root of the migration length.

1. Introduction
Chromatography is an effective chemical separation technique and has been broadly applied for chemical analysis,
purification of materials, etc. In particular, chromatographyusing ion exchange resin has been applied in broad area ofchemistry as a powerful technique of separation. This technique is applicable even for isotope separation.
For many years, Research Laboratory for Nuclear Reactors(Presently, Laboratory for Advanced Nuclear Energy) has conducted the study on isotope effects and isotope separation of anumber of elements including uranium by using ion-exchange
chromatography. The ion-exchange uranium enrichment ischaracterized as high proliferation resistant technology. In thecourse of the study on the ion exchange uranium enrichment,organic resin embedded in porous silica beads was synthesized. This technique has been applied to synthesis of crownether resin and used for isotope separation of zinc and calcium.
Isotope separation developed for nuclear engineering has
expanded its application to various fields. Research into double β decay is a typical area where isotope separation is anticipated. The isotope 48Ca is a double β nuclide with the naturalabundance of 0.187 %. Development of 48Ca enrichment technology is anticipated. Usual isotope separation techniques ofgaseous diffusion and centrifuge are not applicable for 48Ca,
because there is no gaseous compound usable as the working
medium. Chemical exchange methods, utilizing functional
materials like crown-ether and cryptand, are more appropriate
than the physical isotope separation methods.
Calcium isotope separation by using crown-ether wasreported by Heumann and Jepson. Heumann studied calcium
isotope separation using strongly acidic cation exchange resin,dibenzo-18-crown-6 ether resin, and [2B.2.2] cryptand [1].
Jepson reported calcium isotope separation using synthesizedcrown-ether resin packed in a column [2]. Synthesis of crownether resin is possible but synthesis of large amount of fineresin is very difficult. In our laboratory, benzo-18-crown-6*1Corresponding author. E-mail: fujiiyaxyz@road.ocn.ne.jp,
fax; +81 445558271

ether resin has been synthesized by condensation process.
Production of fine particle resin is very difficult by condensation reactions. In spite of the difficulty, we have developed
production technology of fine particle crown-ether resin
embedded in porous silica beads. By using this resin, chromatographic experiments have been conducted at different
operational conditions by the researchers including the present
authors to study 48Ca isotope separation [3-8]. In the present
paper, attention has been placed on the high speed chromatography.
2. Experimental
Benzo-18-crown-6 ether resin embedded in porous silica
beads was synthesized in our laboratory and packed in 10
units of glass columns; the diameter was 8 mm and the length
was 100 cm each. The packed resin was pretreated with
500cm3 of deionized water and rinsed with 500 cm3 of 9M HClsolution. Then the first six columns, connected in series, werecharged with Ca solution; 0.1 M (mol dm -3) Ca in 9M HCl.
The flow rate was 2 cm3 min-1. The column temperature waskept constant at 35 °C by circulating thermostated water.
When the front edge of the Ca adsorption zone reached thebottom of the fifth column, the outlet of the sixth column wasconnected to the top of the seventh column by using a flexible
tube. Then the first column was disconnected from the line,
and the Ca feed solution was charged into the line from theinlet of the 2nd column. In the same way, when the adsorption
band reached the bottom of the sixth column, the feed solution
was charged into the third column. In this manner, chromatographic operation was continued maintaining the connectionof six columns in series for formation of the Ca adsorption
band. At the initial stage of the chromatography before thesampling of the fractions of effluent, as seen in Figure 1, theeffluent volume 304 cm3 was collected from the bottom of thesixth column, and then 25 cm3 of effluent was collected fromthe seventh, the eighth and the ninth columns. From the 10thcolumn 80 cm3 of effluent was collected. The total volume ofeff luent collected by the graduated cylinders was 459 cm 3.
Thereafter the effluent taken from the outlet of the 10th column© 2016 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/31/2016

12J. Nucl. Radiochem. Sci., Vol. 16, 2015

Fujii

480.00200

Ca /40Ca0.00205

Ca concentration / M

0.15

Figure 1. The experimental apparatus for 10 m migration chromatography.

0.001950.10
0.00190
0.05
0.00

600
650
700
3Effluent volume / cm

750

Figure 2. Calcium concentration profile and 48Ca enrichment at thefront boundary of 10 m migration breakthrough chromatography.

was collected in fraction by a fraction collector; the volume ofeach fraction was 4 cm3.
The concentrations of calcium in the collected fractions
were measured by flame photometry at the wave length of 622
n m u s i n g a s p e c t r o p h o t o m e t e r A NA-182 F ( To k yo
Photoelectric Co. Ltd). From the collected fractions of effluent, a portion of the fraction was sampled and added with HI
solution, then heated to dryness. The sampled CaCl2 was converted to CaI2 and subjected to isotope analysis. The calcium
isotopic abundance ratio of [48Ca]/[40Ca] in each collected fraction was determined by a thermal-ionization mass spectrometry using a mass spectrometer Finnigan MAT 261. Moredetailed description was made on the mass spectrometric isotope analysis in a previous work [4].
3. Results and discussion
3.1. Separation Coefficient. The aim of the present work
is to evaluate the effects of migration speed on the enrichmentof calcium isotope. The migration speed of the adsorption
band is considered to affect the separation process. High
speed migration can enhance the productivity of separation.
However it is believed that the HETP is increased when theband speed is increased. The separation coefficient is theoretically independent of the band speed. However we have toverify if this is true or not in the practical case of high speed
migration.
In the previous work, we have conducted the experimental
work at the band speed up to ca. 20 m d-1 using this type ofresin embedded in the porous beads of MST-8 produced byMizusawa Chemical Co. In the present work, the calcium
adsorptive chromatography was conducted at the feed flowrate of 2 ml/min. The observed band velocity at the feed flowrate was 48 m d-1. Chromatographic profile of calcium elutionin effluent is shown in Figure 2. The front boundary is very
sharp. This fact suggests that the adsorption of calcium ionson benzo-18-crown-6 ether resin is very fast. The determinedisotopic abundance ratios of [48Ca]/[40Ca] of the front part of
the adsorption band are plotted also in Figure 2. It is clearlyseen that the heavier isotope 48Ca is accumulated in the front
region of the Ca adsorption band. This enrichment tendencyof heavy isotope at the front boundary is the same as ones
observed in the previous work on calcium isotope separation
by crown ether resin [3-7]. In spite of the high speed of theband migration, the enrichment of 48Ca was successfully realized at the front band boundary.
The most important factor to evaluate the chromatographic

isotope separation process is the separation coefficient ε. The
separation coefficient of this system is calculated by the following equation using the experimentally observed data of thecalcium concentration and the isotopic abundance ratios in thefractions collected from the effluent,ε = S qi (ri/ro - 1) / Q(1)where ro and ri are isotopic abundance ratio [48Ca]/[40Ca] of calcium in the feed and in the effluent fraction i, respectively.
The measured value of ro in the present work is 0.001935. The
adsorption capacity of Q is calculated as,
Q = Co ( VB – VD) ,(2)where Co is the concentration of calcium in the feed solution,
VB is the effluent volume of the break through point and VD isthe total dead volume of the columns connected in series.
Based on eq 1 and using the experimentally observed data, theseparation coefficient ε of the present system is calculated as0.0026 and presented in Table 1. In the present work the errorof the observed ε consists of uncertainties in Ca concentration
analysis (about 5%) and the isotopic abundance ratio analysis(about 7%). The range of error indicates 1σ.
Previously observed separation coefficients are presented
also in TABLE 1. The value of ε is 0.0036 and 0.0032 for themigration 1m and 3m, respectively. Although the error ranges
are large, apparently ε is decreased in the cases of chromatographic experiments at high migration velocity. Probably it isdifficult to fully attain the isotopic exchange equilibrium
between two phases within the short contact time, when theflow rate is very fast.
3.2. HETP. Another important factor of the chromatographic separation process is the height equivalent to a theoretical plate (HETP). HETP is a kinetic factor and reduced
when the isotopic exchange rate is fast between the solution
phase and the resin phase. The smaller HETP means the better separation efficiency.
The HETP is defined as the slope of the enrichment profile
created in the adsorption band after the isotopically steadystate is attained. Such a state is attained only in the case oflong migration and narrow adsorption band. In the usual casesof migration, isotopic enrichment process is not steady state.
In the isotopically non-steady state, the slope varies depending

J. Nucl. Radiochem. Sci., Vol. 16, 2015

Recent Studies on Ca Isotope Separation by Crown-Ether13

TABLE 1: Experimental results of calcium isotope separation by chromatographic migration conducted using
the eluent of 9M HCl at 35˚CMigration
length, mMigrationVelocity, m d-1

Separation
coefficient, ε ×103

HETP, mm

Enriched zone
width, cmRemarks

16.5

3.6±1.4

3.7

17

Ref. [5]322.5

3.2±0.6

4

20

Ref. [10]1048

2.6±0.3

654

Present work

200

20

3.0±0.1

8.3

238

Ref. [5]The errors of separation coefficient show 1σ.
The error range of HETP is approximately ±1 mm in each run.

ln (ri – ro) = k (Xi - L) + ln(rL – ro)(3)

H = ε / k + 1 / Lk2(4)

where k is an experimentally determined gradient of enrichment curve, H is a height equivalent to a theoretical plate, L is
the total length of the resin bed of the columns. In this theory,the volume of eff luent fraction, or the volume of movingphase, is converted to the location of the stationary phase. The
volume difference between the eff luent volume of breakthrough point and the eff luent volume of francion i is converted to |Xi – L|. The conversion factor CF is given as,
CF = L /VB(5)Xi - L = - CF(Vi – VB)(6)

To apply this theory to the actual systems, it is noted that theisotopic ratio in the solution phase is in equilibrium with theisotopic ratio in the adsorption phase. If the isotopic abundance ratio of adsorbed Ca is natural ro, the isotopic abundance ratio in the solution phase becomes ro a. Therefore thepractical value of the original isotopic ratio in the solution
phase is ro(1+ε) rather than ro. For the convenience, ro(1+ε) isexpressed as ro* and above mentioned eq 3 can be written as,ln (ri – ro*) = - k (L /VB) (Vi – VB) + c(7)where c is ln(rL – ro). According to eq 7, the experimental
data, (r i – ro*), of the cases of migration 10 m and 200 m areplotted in Fig. 3 as a function of (Vi – VB) of each fraction i.
The plots of (r i – ro*) of migration 200m in Figure 3 gave
HETP of 8 mm.
By using eqs. 4 and 7, HETP of the present system was calculated as 6 mm for the migration of 10 m. Separation coefficient and HETP of the present work are listed in Table 1 alongwith those of the other experimental runs previously conducted. It is seen that HETP is increased with increase in themigration speed among the chromatographic migrations of1m, 3m and 10m. In the case of 200m migration, HETP wasenlarged, which is probably due to the large number of connections between the columns.
3.3. Width of enriched zone. So far, in our previous work,-7-8

In (ri-ro*)on the position in the band, from the maximum value at theband front to practically zero at the far inside the band. The
HETP of the chromatographic enrichment process in the nonsteady state is very difficult to obtain by theoretical calculation. However, using the very simple assumption of the single
phase migration, the theory of HETP for displacement-type
isotope enrichment has been developed [8, 9]. According to
the theory, HETP of the experimental separation system is calculated by the following equations,

200 m migration-9

10 m migration-10-110100
200Effluent volume from boundary / cm3Figure 3. Increment of 48Ca/40Ca isotopic ratio of calcium in theeffluent; two cases of the migration lengths of 10 m and 200 m. (ro*= 0.001940)

we have not yet discussed details about the length of theenriched zone developed in the chromatographic band. To
design the enrichment plant, an important factor is the lengthof the adsorption band created in the packed separation column. Sufficiently long adsorption band is necessary to reachthe desired enrichment at the front boundary. On the otherhand, the length of the band should be short, as short as possible, from the view point of column utilization efficiency. To
limit the length of the adsorption band, we have to know thewidth of the enriched zone. From the practical view point, wedefine the enrichment zone as the area where the increment ofenrichment degree of calcium in fraction i, (r i – ro), is larger
than 10 % of the front maximum value (rL – ro). This means
the enrichment zone is holding 90% of the enriched isotope
accumulated in the front boundary region, and therefore 10%of the enrichment capacity is left in the latter part of theadsorption band during the migration. The enriched isotope in
the latter part can not be recovered and regarded as being lost.
The width of the enriched zone defined by the criteria mentioned above was calculated for each chromatographic experiment and listed in TABLE 1. The calculated enriched zone
width for each chromatographic run is plotted against thesquare root of migration distance in Figure 4. It is seen theenriched zone width is linearly proportional to the square rootof migration distance. The results indicate the following
empirical relation,
We = 0.17 L1/2

(8)

Width of enriched zone / cm

14J. Nucl. Radiochem. Sci., Vol. 16, 2015

Fujii

250

increases with increase in the band velocity among the migration distances of 1m, 3m and 10m. Concerning the width ofthe enriched zone, it was observed that the enriched zone
width is proportional to the square root of migration distance.

200Acknowledgements
The benzo-18-crown-6-ether resin was synthesized by Mr.
Toshitaka Kaneshiki. The authors of the present work would
like to express gratitude to him for his efforts to develop thecrown ether resin. This work was financially supported by theJapan Society for the Promotion of Science, Grant-in-Aid forYoung Scientists (A) (JSPS K AK ENHI Grant Number23684015) and Grant-in-Aid for Scientific Research (S) (JSPS
KAKENHI Grant Number 24224007).

150

10050References

005100.5(Migration distance / m)

15

Figure 4. An empirical relation between the migration distance andthe width of enriched zone.

where We is the enriched zone width. If we use this eq 8, it issuggested that after the 3000m migration the enrichment zone
will reach the length of ca. 9 m. The length of 9 m is quite
long but may be within the acceptable length from the view
point of the column size, if the actual plant can be composedof columns whose height is approximately 10 m. Improvementin the HETP could make the enrichment plant more compact.
4. Conclusions
Calcium isotope separation was studied by using synthesized benzo-18-crown-6-ether resin in porous fine silica
beads. High speed chromatography was conducted at the bandmigration speed of 48 m/day using 10 units of 1m glass columns connected in series. In spite of the high speed of migration, enrichment of 48Ca was observed at the front boundary of
the adsorption band. Observed separation coefficient was
2.6×10 -3, and the value of HETP is 6 mm. By comparing withpreviously reported data, it is seen that ε slightly decreases
with increase in the band velocity. It is also seen that HETP

(1) Stable Isotope, edited by H.L. Schmidt, H. Foerstel, and K.
Heinzinger ( Elsevier, Amsterdam, 1982).
(2) B.E. Jepson, W.F. Evans, MLM-3654, pp.4-26 (1990).
(3) Y. Fujii, M. Nomura, T. Kaneshiki, Y. Sakuma, T. Suzuki,
S. Umehara, et al. Mass dependence of calcium isotope
fractionations in crown-ether resin chromatography.
Isotopes in Environmental and Health Studies 46, 233-241(2010).
(4) S. Okumura, S. Umehara, Y. Fujii, M. Nomura, T.
Kaneshiki, M. Ozawa, T. Kishimoto, J. Chromatogr. A1415, 67 (2015).
(5) S. Umehara, T. Kishimoto, H. Kakubata, M. Nomura, T.
Kaneshiki, T. Suzuki, Y. Fujii, S. Nemoto, Prog. Theor.
Exp. Phys. 053C03, doi: 10.1093/ptep/ptv063 (2015).
(6) Y. Fujii, T. Kaneshiki, M. Nomura, T. Suzuki, S. Umehara,
T. Kishimoto, Proceedings of 12th Intern. Workshop on
Separation Phenomena in Liquids and Gases (SPLG, June2012, Paris) 44-49.
(7) S. Nemoto, K. Suga, Y. Fukuda, M. Nomura, T. Suzuki, T.
Oi, J, Nucl, Sci. Technol. 49, 425 (2012).
(8) Y. Fujii, M. Aida, M. Okamoto, T. Oi. Sep. Sci. Technol.
20, 377, (1985).
(9) M. Aida , Y. Fujii, M. Okamoto, Sep. Sci. Technol. 21, 643(1986).
(10) S. Umehara, S. Okumura, M. Nomura, T. Kaneshiki. Y.
Fujii. T. Kishimoto, 13th Workshop on Separation
Phenomena in Liquids and Gases, Talk 5 (SPLG, June2015, Bariloche, Argentina).