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Zinc Isotope Fractionation on Benzo-15-crown-5

Resin by Liquid ChromatographyXingcheng DING

a, Masao NOMURA

b& Yasuhiko FUJJI

b

aInstitute of Nuclear-Agricultural Sciences, Zhejiang University , Hangzhou , 310029 ,

ChinabResearch Laboratory for Nuclear Reactors , Tokyo Institute of Technology , O-okayama,

Meguro-ku, Tokyo , 152-8550 , Japan

Published online: 05 Jan 2012.

To cite this article:Xingcheng DING , Masao NOMURA & Yasuhiko FUJJI (2007) Zinc Isotope Fractionation on Benzo-15-crown-5 Resin by Liquid Chromatography, Journal of Nuclear Science and Technology, 44:4, 623-627

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Zinc Isotope Fractionation on Benzo-15-crown-5 Resin

by Liquid Chromatography

Xingcheng DING1;, Masao NOMURA2 and Yasuhiko FUJII2

1Institute of Nuclear-Agricultural Sciences, Zhejiang University, Hangzhou 310029, China2Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology,

O-okayama, Meguro-ku, Tokyo 152-8550, Japan

(Received July 21, 2006 and accepted in revised form January 22, 2007)

Chromatographic fractionation of zinc isotopes was performed on the synthesized benzo-15-crown-5

resin as a column packing material at 323 K in the breakthrough manner for both a frontal and a rear

bands. Zinc adsorption capacity was affected by anion chloride concentration and solvent dielectric con-

stant. The heavier zinc isotopes were found enriched to the solution phase and the lighter zinc isotope wasconcentrated on the resin phase. The frontal maximum enrichment ratio for isotopic pair of68Zn/64Zn was

1.0081. The isotope separation coefficients for isotopic pair of 68Zn/64Zn for frontal and rear band, were

5:3104, 4:5104, respectively.

KEYWORDS: zinc isotope, benzo-15-crown-5 resin, liquid chromatography, isotope fractionation,

separation coefficient

I. Introduction

The use of enriched isotopes in all branches of scientific

research has continued to expand and develop during recent

years. There were many methods for isotope separation andenrichment. The electromagnetic isotope separation (EMIS)

process was applicable to all elements in the periodic table.1)

All isotopes of each multi-isotopic element could be en-

riched simultaneously. The plasma separation process

(PSP) was based on the ion cyclotron resonance of a charged

particle in a uniform magnetic field to separate isotopes.2)

Chemical exchange methods, in particular, were found to

be adequate for the separation of many elements. Among

chemical exchange methods, crown ether has been consid-

ered to have a potential ability for isotope separation. Since

Pedersen synthesized crown ethers,3) intensive studies have

been done on the complex formation reactions of crown

ethers with various cations.4,5) Kimet al.synthesized a resinhaving benzo-15-crown-5 for lithium isotopes separation6)

and Nishizawa et al. studied many elements isotope effects

in liquid-liquid extraction with crown ethers.711)

Zinc isotopes can be used as an important material in

many scientific fields. In nuclear engineering, zinc is a nec-

essary additive material for a cooling water treatment in light

water nuclear power plants. Injection of trace amount of

soluble zinc has recently been proved effective to suppress60Co contamination to the primary coolant of a boiling water

reactor and to reduce the gamma ray dose from the piping

system carrying the coolant.12,13) So far, most boiling water

reactor plants in American have initiated zinc injection to re-

duce radiation build-up on the surface of recirculation pip-

ing. However, natural occurring zinc contains 48.6%

64

Znand the thermal neutron cross section of 64Zn is 0.46 barn.64Zn-depleted zinc is desirable to reduce the radiation in

the reactor system and the fractionation of zinc isotopes is

necessitated.

Recently, a number of papers on the benzo-15-crown-5

resin for application to the zinc isotope separation by chro-

matography have been published.1418) In the present work,

chromatography was performed on the synthesized benzo

crown resin. This is the first report on zinc isotope chromato-

graphic fractionation for both frontal and rear bands, and the

emphasis was placed on the separation coefficient observed

at both frontal and rear bands during the separation process.

II. Experiments

1. Materials and Apparatus

The chromatographic resin as the column packing materi-

al was synthesized by the reaction of the phenol formalde-

hyde, hexamethylenetetramine, and benzo-15-crown-5 in tri-

chloroacetic acid. The obtained polymer dissolved into high

porous silica beads (KIB, Asahi Chemical Industry) to devel-

op silica beads embedded benzo crown resin with a diameter

of 40 to 60 mm. The capacity of synthesized benzo-15-

crown-5 resin was 0.3 kmol/g. Other chemicals were of

the analytical grade, purchased from Wako Pure ChemicalIndustries Ltd., and were used without further purification.

The chromatographic operation of zinc isotope fractiona-

This article was received and accepted as Original Paper.

Atomic Energy Society of Japan

Corresponding author, E-mail: dingxchru@hotmail.com

Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 44, No. 4, p. 623627 (2007)

623

ARTICLE

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tion was composed of five Pyrex columns (0.8 cm I.D,

100 cm length) packed with crown ether resin. High pressure

pump (NP-KX-100, Nihon Seimitsu) and pressure gauge

were connected in series with polytetrafluoroethylene tubes

(1 mm in diameter) between feed solution and columns. A

fraction collector (FRC-2100, Iwaki Asahi Techno Glass, Ja-

pan) was used to collect the effluent samples.

2. Chromatography Procedure and AnalysesFive columns, a pump, a pressure gauge, and a fraction

collector were all connected to perform zinc isotopic chro-

matography. The resin charged in columns was rinsed by

ethanol solution to remove the impurities between the resin

beads. Zinc chloride ethanol solution which was used for

eluting solution was introduced into the top of the first col-

umn continuously to form a zinc adsorption band and pass

down to all the columns to develop a zinc frontal band.

The flow rate of feeding solution was set to 6:00:5

ml/h. The effluent emerging from the bottom of the last

column was collected into small fractions (ca. 1.0 ml) by us-

ing the fraction collector. The total collection volume of the

five meter columns for the frontal band was 230 ml (seeFig. 1). When frontal sampling was finished, the feeding

solution for rear band chromatography was changed from

zinc chloride ethanol solution to ethanol solution immediate-

ly to elute adsorbed zinc on resin phase in order to investi-

gate zinc isotopic distribution on a rear band. Both frontal

and rear band fractions were applied to the analysis of zinc

concentration and isotopic compositions. Zinc concentration

was measured by flame analysis with an atomic absorption

spectrophotometer (ANA-182F) at the wavelength of 213.9

nm. The isotopic abundance ratios were determined by mul-

tiple collectors inductively coupled plasma mass spectrome-

ter (ICP-MS, X7 Series and supported by Thermo Electron

Corporation) when zinc concentration was diluted to 30

ppb by using 2.0% nitric acid solution. A set of zinc isotopic

ion beam was scanned 200 times (30 s for each scanning) for

each sample and each sample was measured by five runs.

The final zinc isotopic ratio was the average value of each

measurement and the measurement error was within 0.2%.

The blank solution (2.0% nitric acid solution) and original

feed solutions were measured before and after effluent sam-

ples in order to correct the possible interference from other

atomic or molecular species. All natural available zinc iso-

topes were measured except for 67Zn and 70Zn due to thetoo low natural abundance. Table 1 summarized the exper-

imental conditions for zinc isotopes chromatographic frac-

tionation.

III. Results and Discussions

1. Zinc Adsorption Results

The total zinc adsorption capacity on the benzo-15-crown-

5 resin (Qtot, kmol), was calculated by the following equa-

tion:

Qtot CoVFB Vd 1

where C0 is the concentration of zinc in the feed solution

(mol.dm3), VFB is the breakthrough volume (the break-

through volume was defined as the volume at which zinc

starts coming out of the column, ml) andVdis the dead vol-

ume (ml), which means the volume was not occupied by res-

ins in the column. In the present study, zinc adsorption ca-

pacity Qtot in one meter column was calculated to be equal

to 0.39 kmol.Table 2gave zinc adsorption capacity reported

under the different operation conditions. Temperature affects

adsorption capacity and usually high temperature has large

adsorption capacity. Experiments of temperature effect on

zinc adsorption capacity were performed in our laboratory

and the results proved that zinc adsorption capacity on resinphase remained nearly constant with the temperature varia-

tion from 298 to 323 K. The effect of temperature on zinc ad-

sorption capacity was very small in ethanol solvent. Further

210 220 420 440 460 480 500 520

0.00

0.02

0.04

0.06

0.08

0.10

ZnConcentratio

n

mol.dm

3

Effluent Volume ml

Fig. 1 Frontal and rear band chromatographic breakthrough curve

of zinc isotope fractionation

Note: In the frontal band (until 230 ml), zinc chloride ethanol

solution was used as effluent solution; In the rear band (from230 ml to the end), ethanol solution was used as effluent solution.

Table 1 The experimental conditions for zinc isotopes chromatographic fractionation

Feed solution Flow rate Band velocity Migration distance Temperature

(ml/min) (cm/h) (cm) (K)

Frontal band 0.08mol.dm3

ZnCl2 in ethanol 0.1 14.2 500 323

solution

Rear band Ethanol solution 1.0 10.5 500 323

624 X. DINGet al.

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experiments are needed to find out the mechanism of temper-

atures effect on zinc adsorption capacity. Same type of

crown ether was employed for all the experiments referred

in Table 2. Zinc chloride was used as the chemical form

of zinc for all experiments. The chemical species of zinc

chloride complex in the different solvents were so compli-

cated that only very limited references existed presently.

However, anion chloride ion concentration could effectivelyaffect the zinc adsorption capacity. In a low concentration of

chloride solution, the main chemical specie of zinc chloride

may have existed as aqua complex Zn2(OH2)n while in a

higher concentration of chloride solution, the main chemical

specie was dichloro complex Zn2(Cl)2. Due to the short-

ened radius of dichloro complex compared with aqua solu-

tion, it may lead to the large adsorption of zinc chloride

on resin phase. In Table 2, there was adsorption capacity

in 1.0 mol.dm3 HCl solution larger than that of the present

work although zinc chloride concentration was smaller than

that of the present work. Such kind of large zinc adsorption

could be considered as the contribution of the chloride ionconcentration. Zinc adsorption on resin phase was so compli-

cated that it could not be explained only by anion concentra-

tion. When compared with the results obtained by organic

solvents in Table 2, acetone solvent exhibited the largest

adsorption capacity among them. It may be affected by the

dielectric constant of different organic solvents.19) Zinc ad-

sorption capacity changes with an dielectric constant and

usually large dielectric constant organic solvent had small

zinc adsorption capacity. In methanol and ethanol solvents,

there were small amount of complex species between zinc

ion and these two solvents, and zinc mainly make complex

with crown ethers. Since ethanol solvent had a smaller di-

electric constant than methanol solvent, it has large adsorp-tion capacity. While, to acetone solvent, acetone had the

smallest dielectric constant among the solvents in Table 2,

zinc ions could make complex both with crown ether and

acetone solvent, and it led to the largest adsorption capacity.

When compared with the effect of anion chloride concentra-

tion and dielectric constant in case of Refs. 15) and 17)

in Table 2, Zinc adsorption capacity in 1.0 mol.dm3 HCl

had relatively smaller adsorption capacity than the result in

acetone solvent, although 1.0 mol.dm3 HCl had a much

smaller dielectric constant value, it means an anion chloride

concentration has much larger contribution to zinc adsorp-

tion capacity than that of a dielectric constant. Accordingto the experimental results,18) distribution coefficient of zinc

in acetone solution was 1.0, and this means zinc and crown

ether can easily make complex with the ratio of 1:1; While

the distribution coefficient in ethanol solution was only

0.3, it indicates that zinc is difficult to make complex with

crown ether in ethanol and has small adsorption capacity.

A large distribution coefficient value means large zinc ad-

sorption capacity and zinc can easily make complex with

crown ethers.

2. Zinc Isotopic Effect Observed on Frontal and Rear

Bands

The zinc concentration breakthrough profile was plotted in

Fig. 1. The breakthrough volume of five meter zinc chroma-

tography was equal to 212 ml, and, after this volume, a sharp

increase in the zinc concentration was observed, showing

the formation of a self-sharpening frontal boundary. This

means the ideal displacement chromatography was obtained

at a frontal band. The flat shape was kept before the decrease

of zinc concentration when the feed solution was changed to

ethanol solution. The shape of a rear band was far away from

sharpness and could not form ideal displacement chromatog-raphy at presently condition. It indicates the desorption be-

havior of zinc on resin phase is difficult only by ethanol sol-

vent. Further experiments are needed in order to find out the

best solvent for ideal displacement chromatography at a rear

band. However, the present results proved that adsorbed zinc

can be completely eluted out by ethanol solvent.

A zinc isotopic ratio of the present five-meter experiment

was depicted inFig. 2. The isotopic ratio data of a fraction is

expressed as the enrichment ratio, , and is defined as

Eq. (2):

i HZn=64Zni

H

Zn=64

Zno

; where H66,68: 2

where the superscript H is the value of heavy zinc mass

number 66 or 68, the subscript i and o denote the effluent

fraction number and original feed solution, respectively.

The definition can be expressed as an isotopic ratio in a local

fraction sample divided by the corresponding ratio in the

feed solution. Isotope fractionation of zinc isotopes occurred

within 12 mlrange from the breakthrough point at the frontal

band, while in a rear band, zinc isotope fractionation hap-

pened in a large volume range. To each local fraction, the

enrichment ratio was a monotonous decrease with the efflu-

ent volume. The results that the enrichment ratio value was

larger than 1.000 in the frontal band indicated that the heav-ier isotope of zinc was preferentially fractionated into the

eluted samples; On the contrary, it was proved that the light-

Table 2 Zinc adsorption capacity in various chromatographic operation

Solution Zn con. Dielectric Temp. Qtot

Referencemol.dm3 constanta) K mmol

1.0 mol.dm3 HCl 0.05 4.6 313 0.94 17)

Acetone 0.46 20.7 298 12.2 15)

Methanol 0.09 32.6 298 0.09 18)

Ethanol 0.08 24.3 323 0.39 This work

a)The value of dielectric constant was measured at 20C and detailed information was in Ref. 20).

Zinc Isotope Fractionation on Benzo-15-crown-5 Resin by Liquid Chromatography 625

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er zinc isotope remained on resin phase in the rear band re-

gion. In the frontal of zinc adsorption zone, the maximum

value of the enrichment ratio was observed as 1.0081 and1.0042 for the isotopic pairs of68Zn/64Zn and 66Zn/64Zn, re-

spectively. As to the rear band depleted area, the maximum

value of the depleted ratio was much smaller than that of the

frontal band. The value of each frontal maximum enrichment

ratio for isotopic pair of 68Zn/64Zn was nearly twice that of66Zn/64Zn. This could be attributed to the differences in

mass differences. The enrichment value of the frontal zinc

adsorption band was much larger than that of the rear band.

The separation coefficient " is used to evaluate chromato-

graphic performance and can be calculated based on the fol-

lowing equation derived by Spedding and Kakihana:20,21)

" S 1 X

ciVijRi

Ro

j

QtotRo1Ro3

where the subscriptsiando denote the effluent fraction num-

ber and the original feed solution, respectively; S is the sep-

aration factor; c and V are zinc concentration (mol.dm3)

and volume (ml) of each effluent fractions, respectively; R

is the atomic fraction of isotopes andQtotis the total amount

of adsorbed zinc on resin phase (kmol). The summation in

Eq. (3) has taken place in all fractions in which isotope frac-

tionation was observed.

Table 3tabulated the results of separation coefficient and

separation coefficient per unit mass difference of zinc for

both the frontal and rear bands. The separation coefficientvalue for 68Zn/64Zn was 5:26104 in the frontal band.

Separation coefficient of the frontal band has larger value

than that of the rear band for both two isotopic pairs. When

compared with the separation coefficient per unit mass dif-

ferent, the frontal band has much larger deviation than that

of the rear band. The reason may be due to the large effluent

volume of the rear band. The separation coefficient value ob-

tained in this work was only half than the value which per-

formed on DC-18-crown-6 resin,9) but larger than what was

obtained by microporous strongly acidic cation exchange

resin.22) The value of this work agreed with the value by

the same type crown ether which was obtained in 1.0

mol.dm3 hydrochloric acid solution by Oi and his

group.15,16)

IV. Conclusions

The chromatographic fractionation of zinc isotopes was

performed on the synthesized benzo-15-crown-5 resin in

the breakthrough manner both for the frontal and the rear

bands at 323 K. Zinc chemical species in solution and zinc

adsorption capacity on resin were so complicated and itmay relate to the anion chloride concentration and solvent

dielectric constant. Anion chloride concentration has a larger

effect on zinc adsorption capacity than that of a dielectric

constant. Zinc adsorption capacity in the present study was

smaller than that in acetone and 1.0 mol.dm3 hydrochloride

solution. It was firstly reported that heavier zinc isotopes

were found enriched to the solution phase and lighter zinc

isotope was concentrated on the resin phase. The isotope

separation coefficient for isotopic pair of 68Zn/64Zn for the

frontal and the rear bands, were around 5:3104, 4:5

104, respectively. Separation coefficient per unit mass dif-

ference of zinc in the frontal band was larger than that ofthe rear band.

Acknowledgements

The present work was performed as a part of Innovative

and Viable Nuclear Energy Technology Development Pro-

ject, The Institute of Applied Energy, Japan.

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