Chromatographic Enrichment of Lithium Isotopes Bull. Korean Chem. Soc. 2001, Vol. 22, No. 5 503
Chromatographic Enrichment of Lithium Isotopes by Hydrous Manganese(IV) Oxide
Dong WonKim
Department of Chemistry, ChungbukNational University, Cheongju 361-763, Korea Received January 10, 2001
Separation of lithiumisotopeswas investigated by chemical ion exchange with a hydrous manganese(IV) oxide ion exchanger using anelution chromatography. Thecapacity of manganese(IV) oxide ion exchangerwas 0.5 meq/g. Onemolar CFkCOONasolution was usedas aneluent. Theheavierisotope of lithiumwasenriched in thesolutionphase, while thelighter isotopewas enrichedin the ion exchanger phase.Theseparationfactor was calculatedaccordingtothemethodofGlueckauf from theelutioncurveand isotopicassays. Thesinglestage separationfactor oflithium isotopepair fractionation was 1.021.
Keywords: Lithium isotopeseparation,MnO2 exchanger, Capacity, Isotope effects, Elution chromatography.
Introduction
Thechemistry of thehydrousmetal oxides has been stud
ied in a large number of scientific fields, especially, ion exchangeseparation, scavengingby sorption and coprecipi tation, separation ofmineral, nuclearfuels, antiperspirants, radioactive contamination, and scavengingof trace metals.1 The high adsorptive capacity ofcolloidal hydrous manga- nese(IV) oxide for cations was demonstrated by Posselt et al..2,3 Theynoted that thepHof theisoelectricpoint of man
ganeseoxide is quitelow,ranging from pH2.8 to pH 4.5. At higher pH,colloidal manganese(IV) oxide hasa net negative charge. They demonstrated that the mechanism of adsorp tion by MnO2 is primarily electrostatic by comparing the adsorptionofcationic, anionic, and unchargedorganiccom pounds. Only the organic cation showed any significant adsorption, and it was adsorbedtoaboutthe sameextentas the metal cationsexamined.Gadde and Laitinen4studied the adsorption of several metals on hydrous manganese oxide and hydrous ferric oxides. Takeuchi et al.5reportedthat the sorbents are prepared by extracting lithium ions from lith
ium manganese oxides with the lithium/manganese mole ratiovaryingbetween 0.42 and 0.84. Andtheyinvestigated lithium isotope selectivity on these sorbents. They also found that allthesorbents are 6Li-specific and the values of the 7Li-to-6Li isotope separation factor are between 1.0040 and 1.0092at25 oC. Ooi et al.6reportedthation exchange chromatography oflithiumwascarriedout tostudy the lith
iumisotopeeffectin an aqueous ion exchange system, using titanium phosphate exchangers granulated with polyvinyl chloride or an inorganic binder. The ion exchanger granu lated with polyvinyl chloride showed significant isotope fractionations when a 0.05M (NH4)2CO3 solutionwas used as aneluent. They found that thelithium isotope separation factor wasroughly evaluated as 1.007. Theyalso found that the lighterisotope, 6Li+ waspreferentially fractionatedinto
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2281; Fax: +82-43-267-2279; E-mail: chem131@trut.chungbuk.ackr
the ionexchanger phase. Oi et al.7investigated the lithium isotope effect in aqueous ion exchange systems by cation exchange chromatography. They obtained the value ofthe separation factor of1.00089-1.00171at 25 oC.
Kim et al. 8 also investigated the separation of lithium isotopes with 1,7,13-trioxa-4,10,16-triazacyclooctadecane bonded Merrifield peptide resin by ion exchange elution chromatography, and obtainedthe impressibly larger value of thesinglestage separation factorof 1.068. Thetheoretical maximumvalueof theelementary separation factorfor frac tionation of lithium isotopes by molecular distillation is
1.080.9Recently, Kimet al.10 reported that theseparationof lithiumisotopes by aminobenzo-15-crown-5 bonded Merri
field peptide resin was investigated, andobtained thevalue ofthe separationfactor of lithium isotopes of 1.026. They also found that the heavier isotope, 7Li+ wasconcentratedin the resin phase, while the lighter isotope, 6Li+ was concen
trated in the solution phase.
Naturally occurring element lithium consists oftwo stable isotopes, 6Liand 7Li, and each isotope plays an important role in nuclear science andindustry. The heavy isotope of lithium, 7Li couldbe used as apHcontrol agent(7LiOH) of the coolant in nuclear fissionreactors.And thelithiumcom pounds rich in 6Li will be required for the tritium breeder blanket in deuterium-tritium fusion power reactors in the future.7 Because ofthe large cross section for thermalneu
tron in the nuclear reaction, 6Li(n, a)T and7Li(n, on)T in fast neutron,avariety of isotopicratioof the element lithium havebeen proposed for releasing energyor production of tri
tium moreeffectively.11
In this work, lithium isotope separation was examined using ahydrousmanganese(IV) oxide ionexchangerbyion exchangeelution chromatography.
Experiment지 Section
Materi지s and Methods. Manganese(IV) oxide (-60 + 230 mesh, 99+ %) was purchased from Aldrich Chemical Company, Inc., USA. Lithium chloride and sodiumacetate
504 Bull. Korean Chem. Soc. 2001, Vol. 22, No. 5 Dong Won Kim
werepurchased from SigmaChemical Co., USA. AnAtomic AbsorptionSpectrophotometer(Hitachi Z-8000)was used to determine thelithium ion concentration in the solution. Mea
surementofthe lithiumisotoperatiowas carriedout using a ThermalIonizationMass Spectrometer(Finnigan MAT262) with a rheniumdouble filament. Amountof 1.0-2.0 Rg lith
ium was used to determine the isotope ratio in each feed solution.
Determinationof the Tot지 WeightCapacity.The capac
ity of the manganese(IV) oxide exchanger was determined by the method given in the literature.12 Manganese(IV) oxide was washed to neutrality with distilled water, and dried in air. Ofthis quantity, 1.000± 0.005 g was weighed into a dry 250mLErlenmeyer flask containingexactly 200 mL 0.1 NNaOH with 5% sodium chloride, and wasallowed tostand overnight. Amount of 1.0 g ion exchangersample of the samematerial was separatelyweighted into a weighing bottle,dried at110 oCovernight, and weighed againto deter
mine the percentage ofsolids. Of the supernatant liquid in the Erlenmayer flask, 50 mL aliquots are backtitrated with 0.1 N HCl againstphenolphthalein. The capacity was then calculated bytheformula:
Capacity(meq/g)=
(200 • NormalityNaOH) — 4(］如心& ・ NormalityAcid) (1) Sample Weight• (%Solid/100)
(
)Itrepresents the totalweightcapacity of theexchangerin thedryH-form.The resinmust becompletely in theH-form before weighing ofthe sample, since difference in equiva lent weights of differentionswould lead to errors.The stan dardsodium hydroxidesolutionwas treatedwith 5% sodium chloride to obtain a completely exchangeequilibriumbythe excess ofsodiumions. A reproducibilityof 士 1% canconse quently be obtained.
Separation of Lithium Isotopes. Manganese(IV) oxide ionexchangerwas slurried in sodiumacetate solution. The slurried MnO2 was packed in a water-jacketedglasscolumn (0.2 cm I.D. x 35 cm height). The temperature was main tainedat20 oC withusing anWaterCirculator (HAAKE A- 80). The volume of 0.1 mL of 0.01 M LiCl solution was loadedonthetop of the MnO2 bed.The 1.0M CH3COONa solution (Kd =25) wasused as an eluent. The lithium feed solutionwas then passed throughthe column under gravity flow. The flow rate wascontrolledby a fine stopcockto be 0.6 mL/hr. Theeffluentwascollected as a fraction of 0.1 mL each with anAutomaticFractionCollector(PharmaciaLKB FRAC-100).
ResultsandDiscussion
The ion exchange capacity ofthe MnO2 ion exchanger was foundto be 0.5meq/g.Thechromatogramwasobtained from column operation with 1.0 MCH3COONa solution at 20 oC as shown in Figure 1. From the elution curve, the number oftheoretical plates in the column, was calculated bythe equation:13
Figure 1. Elution curve for the separation of lithium isotopes.
N=8• (느 j2 (2)
where Vnax is thepeakelutionvolume, and P the band width at the concentration C = Cmax/e, andCmax the concentration of soluteat themaximumpeak height of theelutioncurve.
Fromtheelution curve and isotopicassaydata,the separa tion factor oflithiumisotopeswascalculated by the Glueck- auf theory.14 The data were plotted on probability paper wherethe abscissa was a probability scale and the ordinate was a linear scale. The localenrichmentpercentagewasthe ordinate and the fraction of the eluted mixture was the abscissa. This gave a linear plot. The slope obtained willbe
e
」 N ,and separation factor is 1+ & The separation factor, a, was determined from the slope of a least square line drawn through the points as shown in Figure 2. This is a typical plot oftheisotopicassaysinthisexperiment. In my experiment,the single stageseparation factor of lithium iso
topeswasobtained as thevalue of 1.021. Thisvalue is larger thanthoseofthe valuesobtained by Takeuchi et al.,5 Ooiet al.,6 and Oi et al..7 The relatively larger separation factor obtained in thisexperimentcan beexplained by the fact that the difference inthe adsorbabilities of theions between 6Li+
and7Li+ on the MnO2 exchanger is significant. The larger the difference in theadsorbabilitiesoftheionsbetween lith
iumisotopes,thelarger theseparation factor is obtained.
From the experimental data, it appears that the isotope exchange reaction can berepresented bythefollowingequa tion:
^Li
Solution +'Li
Manganese Oxide 石'너^Li
Manganese Oxide + ‘Li Solution(3) The subscripted symbols such as solution and manganese oxide refer to the solution and manganese(IV) oxide ion
Chromatographic Enrichment of Lithium Isotopes Bull. Korean Chem. Soc. 2001, Vol. 22, No. 5 505
Figure 2. Enrichment of lithium isotopes by cation exchange chromatography. The isotopic ratio of a fraction can be expressed by 7Li/6Li, where 6Li and 7Li are the relative abundances of the light and heavy lithium isotopes. The isotopic ratio of the standard used in the mass analysis is denoted by (6Li/7Li)o. The local enrichment factor, R, is defined as (7Li/6Li)(6Li/7Li)o. The Am/m is the proportions of lithium in the individual fractions.
exchanger phases in this chemical isotope exchange. Kon- doh et al.,15 Heumannetal.,16 Oiet al.,17Ooi et al.,6 Jepson et al.,18 and Fujineetal.1also reported thattheheavier iso
topes were preferentially enriched in the solution phase of chromatography using strongly acidic cation exchangers, anionexchanger, titanium phosphate exchangers, and poly mer-bound crown ether. These results well agree with my work. On the other hand, Oi et al.,7 Klinskii et al.,20 Aal
tonenetal.,21 Heumannetal.,22 and Lee23reported that the heavier isotopes were preferentially enriched in the resin phase ofcation and anion exchange chromatography. The separation factor depended upon themass of the isotope as well asthedifference in themassesofthe isotopepairs.The cation species in the exchangerphase is less hydrated than
Table 1. Separation of lithium isotopes of some investigations by ion exchange chromatography
Separation
Factor Method Exchanger Reference
1.0092 Extraction Lithium manganese oxide 5
1.007 Elution Titanium phosphate 6
1.00171 Displacement Asahi LS-6, fiber 7
1.026 Elution Crown resin 10
1.068 Elution Azacrown resin 8
1.014 Displacement Cryptand resin 19
1.0026 Breakthrough Dowex 50 x 24 24 1.0016 Breakthrough Cubic antimonic acid 25
1.014 Extraction LiMn© 26
1.021 Elution MnO2 This Work
thecationspeciesinthesolution phase.Thiscontributestoa difference in bonding and subsequent enrichment of the lighterisotope in the exchanger phase.23 These phenomena well agree with my work. Someinvestigations forthesepa ration oflithiumisotopes by ion exchange chromatography are listed in Table 1. As shown in Table 1, the crown, cryptand, LiMmOq, and manganese(IV) oxide exchangers are more significant to separate lithium isotopes than the otherexchangers.
Conclusions
Separationof lithiumisotopeswasinvestigatedby chemi
calionexchange with thehydrousmanganese(IV)oxideion exchanger using anelutionchromatographictechnique.The capacity of the MnO2 ion exchanger was 0.5 meq/g. One molar CH3COONa solution was used as an eluent. The heavier isotope of lithium was enriched in the solution phase, while the lighter isotope was enriched in the MnO2
ionexchangerphase. Thesingle stage separationfactor was calculated according to the methodofGlueckauf from the elution curve and isotopic assays. The single stage separa tionfactor of lithiumisotopeswas 1.021.
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