J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Lithium Isotope Separation using Cation Exchange Resin and Cross-linkage
Effect on Isotope Fractionation
Tatsuya SUZUKIa, Yu TACHIBANAa, Masao NOMURAb, and Yasuhiko FUJIIbaDepartment of Nuclear System Safety Engineering, Nagaoka University of Technology, Nagaoka, JAPANb Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo, Japan(Received: 29 September 2014 / Accepted: 19 January 2015)

We have studied on the lithium isotope separation by the chromatographic techniques using cation exchange resin.
We observed the isotope fractionation by cation exchange reaction. The lithium-6 was enriched in the resin side. The
isotope fractionation coefficient was confirmed to depend on the cross-linking degree of cation exchange resin, andto increase linearly with the cross-linking degree. From the result of the chemical shifts by NMR spectrometry, we
deduced that the cross-linkage effect on the isotope fractionation is caused by the dehydration from the lithium aquacomplex ion.
Keywords: lithium isotope, isotope separation, isotope exchange reaction, cation exchange resin, isotope effect, crosslinkage, NMR.
plate. Lithium isotope separation factors in many system
are summarized in Tables of Ref. [10] very well. From thestanding points of engineering chromatography, usage oforganic cation exchange resin is generally appropriate. In
the case of using the organic cation exchange resin, it wasreported that the separation factors by using the cationexchange resin depend on their cross-linking degree [4]. In
this report, it is indicated that this cross-linkage effect isrelated to the hydration of lithium ion. It was reported thatthe degree of cross-linkage is evaluated by the chemical
shift of 6Li NMR [11]. While, in the case of using inorganic
cation exchanger, the lithium isotope fractionation wasreported to be related to the chemical shift of the 7Li NMR[6]. In this report, it was discussed that the isotope
fractionation is related with the dehydration of the lithium
ion in the solid phase.
In the present study, the lithium isotope separation by
the cation exchange chromatography was investigated.
The cross-linkage effect on the isotope fractionation wasdiscussed by using the 7Li NMR spectrometry. Fromobtained results, we suggest the lithium isotope separationby using cation exchange resin with high cross-linking
degree.

1. Introduction

Tritium is used as fusion fuel, but natural abundance
among hydrogen isotopes is too low. Thus, the tritium must
be produced artificially. The 6Li(n,D)T reaction is a mostwell-known tritium breeding method. For the effective
breeding the tritium, the enrichment of lithium-6 isnecessary, since the natural ratio of lithium-6 is about 7.5%.
On the other hand, the enriched lithium-7 is required forthe pH control chemical of the primary coolant in the PWR[1]. The enriched lithium isotopes are supplied by thechemical exchange using amalgam, since the amalgam
lithium isotope separation method has the high isotope
separation factor [2]. However, recently the new lithium
isotope separation method of mercury free is required. Oneof candidates of the lithium isotope separation substituting
for the amalgam method is a cation exchange method. The
lithium isotope separation methods using a cationexchange reaction is not a novel method. The first lithium
isotope separation was demonstrated by using the cationexchange reaction by Tayler and Urey [3]. And then, manystudies on lithium isotope separation by using a cationexchange reaction has been performed. The separation
factors obtained in the cation exchange reactions havewide-range values. The higher separation factors weregenerally obtained in the case of using inorganic cationexchanger than in the case of using organic cationexchange resin [4-6]. Many researchers are exploring thecation exchanger with the higher separation factor. Resins
with macrocyclic ligands such as crown ethers becomestudied in search of higher separation factor [7-9].
However, above mentioned system has difficult tasks, suchas expensiveness of macrocyclic ligands, and low ion
exchange rate, i.e., high height equivalent to a theoretical

2. Experimental

2.1 Cation Exchange Resins
The cation exchange resins used in the present study
were Dowex series (Dow Chemical Co.,(Bio Rad)), Diaion
series (Mitsubishi Chemical Co.), and SQS(Asahi KaseiCo.). All resins we used were the styrene-divinylbenzene
type strongly acidic cation exchange resins with sulfo
groups. Both the gel type and the porous type resins wereincluded in the used resins.

author’s e-mail: tasuzuki@vos.nagaokaut.ac.jp

45©2015 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchT. Suzuki et al., Lithium Isotope Separation using Cation Exchange Resin and Cross-Linkage Effect on Isotope Fractionation

obtained the resin used for 7Li NMR measurement. Thisadjusted resin is put into a 10 mm OD glass NMR sample
tube. 1.7 M butyl-lithium in hexane is used as reference. The
small NMR sample tube of 5mm OD enclosing thereference was set into the above mentioned large diameter
sample tube. The 7Li NMR spectrometry was conductedusing a Varian FT-NMR XL 300.

2.2 Chromatography
Each cation exchange resin was filled in a glass column,
1 cm ID and 1 m of length. The conditioning of the column
filled with cation exchange resin was carried out by
following methods; first, the 400 cm3 of 1 M (mol/dm3)
NaOH solution was flowed in the column, next about 1 dm3
of 1 M HCl solution was flowed, last the deionized water
was flowed until the acidity of effluent water changed intoneutral. The cation resin in the column was conditioned toH+ type resin by above treatments. The 120 cm3 of 0.5 M
lithium acetate solution was fed into the conditioned column
packed with cation exchange resin at a constant flow rate of
8 cm3/min. The lithium was eluted by flowing the 0.5 M
potassium acetated solution at a constant flow rate of 5
cm3/min. The concentration of lithium in the eluted
solutions was measured by a flame analysis. The isotope
ratio of lithium in the eluted samples was determined by athermal ionization mass spectrometry.

3. Results and Discussion

Three examples of the chromatogram and the lithium
isotope fractionation are shown in Fig. 1. The
chromatograms and the isotope fractionation curves aresimilar form, and the adsorption amounts are different. Fromthe isotope fraction curves, we confirmed that the lithium-6
is enriched in the resin phase. The isotope exchange reaction
we used for the chromatography is as follow;
7 +Li + CH3COO6Li ' 6Li+ + CH3COO7Li.
(1)
The above isotope exchange equilibrium reaction is foundto be biased toward the right-hand side, since lithium ion
binds with the cation exchange resin. The degree of isotope
separation is evaluated by the separation coefficient, H. The
separation coefficient is defined as the subtraction 1 fromthe separation factor on single stage. In other words, theseparation coefficient means the off degree from 1 of theisotope exchange equilibrium constant. The separation
coefficient of the chromatography was calculated byfollowing equation using the chromatogram and the isotope
fractionation curve,

2.3 NMR Analysis
Each cation exchange resin was pretreated using asmall glass column packed with the cation exchange resinby successive flowing 1M HCl solution and deionized water.
0.5 M lithium acetate solution was fed into this column inorder to adjust the resin to the Li+ type. Then, the Li typeresin was rinsed with deionized water. After that, we0.4
Resin:50W–X8Fig.1

ߝൌResin:MP50120ொோబ ሺଵିோబ ሻ,(2)0.005

0.20.1σ ஼೔ ௏೔ ሺோబ ିோ೔ ሻ

where Ci, Vi, and Ri are the lithium concentration, theSeparation Coefficient, H

Li concentration / mol/L0.3

6137

Resin:50W–X12Isotope ratio, Li/ Li

0.5

11500
1000
3Effluent volume / cm

Example of the chromatograms and the lithium
isotope fractionation curves by the lithium cationexchange chromatography. 50W-X8 and 50WX12 are similar gel type cation exchange resins
except for the degree of cross-linkage; X8 andX12 mean 8% and 12% of cross linking degree,respectively. MP-50 is the macro-porous
equivalent to 50W resin.

0.004
0.003
0.002
0.001
0Fig.2460102030
Degree of Cross-Linkage / %Cross-linkage effect of cation exchange resin
on isotope fractionation.

T. Suzuki et al., Lithium Isotope Separation using Cation Exchange Resin and Cross-Linkage Effect on Isotope Fractionation

volume, and the isotope abundance, R = r (1+r), in thesample fraction i, respectively. r is the isotope ratio. R0 is theoriginal isotope abundance. Q is the adsorption amount. The
relationship between the calculated separation coefficientand the cross-linking degree of the used resin is shown inFig. 2. We can see that the separation coefficient have alinear relationship to the degree of cross-linkage. This resultshows that the separation coefficient can be raised byincreasing the degree of cross-linkage of resin. It was also
confirmed that the separation coefficient is not dependent on
the gel type resin or the porous type resin.
The chemical shift of 7Li NMR spectrum in each resin
was evaluated. The relationship between the chemical shift
and the degree of cross-linkage is shown in Fig. 3. We canconfirm that the chemical shift is linearly related to thedegree of cross-linkage, and that the resin with the higher
cross-linkage degree makes the 7Li spectrum shift to thelower frequency, this shift is called as the high magnetic
field shift. The high magnetic field shift means that theelectron density around the lithium nucleus increases withthe increase in the degree of cross-linkage. Thisphenomenon can be explained by the hydration of lithiumion. The lithium ion is well-known to be hydrated and toexist as the aqua complex ion in the aqueous solution.
Although the lithium ion is fully hydrated in the aqueoussolution, the dehydration from the lithium aqua complex ion
occurres in the cation exchange resin because of thehydrophobicity of the resin. Since the sulfonate anion of
cation exchange resin can get closer toward the lithium ion
in the case of removing the hydration water surrounding thelithium ion, the closing sulfonate anion is deduced to supply
the negative electron around the lithium nucleus. The

number of hydrated water molecules around lithium ion in
the resin with the higher degree of cross-linkage is lower[12], because the resin with the higher degree of crosslinkage has generally higher hydrophobicity [13].
The dependence of the separation coefficient on thedegree of cross-linkage can be also explained by thehydration and the dehydration of lithium ion in the cationexchange resin. The lithium ion in the isotope exchange
reaction (1) is not a bare lithium ion but a hydrated lithiumion. Since some hydrated water molecules are removed if
the lithium ion binds the cation exchange resin, the isotope
exchange reaction is occurred in the hydration anddehydration reactions in the time of the adsorption anddesorption reactions. The reaction (1) is replaced with thefollowing reaction adding the hydration and dehydration
reactions.
R7Li(H2O)n+ + CH3COO6Li(H2O)m' R6Li(H2O)n+ + CH3COO7Li(H2O)m,where R is resin, and n < m. The heavier isotope is enrichedin the side of the chemical form with the stronger bindingforce [14]. Since the hydration bonding is strong, thelithium-7 is enriched in the lithium acetate with the morenumber of hydrated water molecules which exits in aqueous
solution, and the lithium-6 is enriched in the less hydrated
water molecules which exits in the resin phase. The resultsof isotope fractionation shown in Fig. 1 consists of the above
explanation. The separation coefficient becomes larger if thedifference of the chemical condition in the isotope exchange
reaction is larger. Thus, the separation coefficient becomeslarger in the case of using the resin with the higher degree of0.005

Separation Coefficient, H-3.1-3

7

Li Chemical Shift / ppm-3.2-2.9
0Fig.3(3)102030Degree of Cross-Linkage / %

0.004
0.003
0.002
0.001
0-2.8

Relationship between cross-linkage of cationexchange resin and chemical shift of 7Li
NMR.

Fig.4

47

7-3-3.2
Li Chemical Shift / ppm

Relationship between 7Li Chemical shift andand isotope fractionation.

T. Suzuki et al., Lithium Isotope Separation using Cation Exchange Resin and Cross-Linkage Effect on Isotope Fractionation

cross-linkage, since the difference between m and n in thereaction (2) becomes larger on account of the dehydrationfrom the lithium acqua complex ion in the resin. Ooi et al.
theoretically indicate that the dehydration reaction oflithium ion has a high separation coefficient [15]. It issummarized the cross-linkage effect on the isotope
fractionation in the cation exchange reaction is due to thedehydration reactions in the resin. Since the degree ofdehydration in the resin is estimated by the chemical shift of7
Li NMR spectrum, it may be better to make the connection
between the isotope fractionation and the chemical shift asshown in Fig. 4. The dehydration is dependent on thechemical properties of the resin such as the hydrophobicity.
Therefore, we expected that the high separation coefficient
can be obtained by increasing the hydrophobicity of theresin. The increasing the degree of cross-linkage of the resin
is one of the relatively-easy methods.

4. Conclusion

The lithium isotope separation was carried out by thechromatography using the cation exchange reaction. We
confirmed that the 6Li is enriched in the cation exchange
resin phase. The cross-linkage effect on the lithium isotope
fraction was confirmed. The high isotope fractionation canbe obtained with using the cation exchange resin with highcross-linking degree. This cross-linkage effect can beexplained by the dehydration from the lithium aqua complex
ion in the resin. We proposed the using the resin with thehigh hydrophobicity to obtain the high separation
coefficient.
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