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Anal. Chem. 1992, 64, 3013-3017
Selective Extraction of Cesium from Acidic Nitrate Solutions with Didodecylnaphthalenesulfonic Acid Synergized with Bis(tert=butylbenzo)-21-crown-7 W. Jack McDowell' and Gerald N. Case Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Joseph A. McDonough and Richard A. Bartsch' Department of Chemistry and Biochemistry, Texas Tech University, Lubbock Texas 79409
The behavlor of other crown ether-synergized sulfonic acid extractkn systems suggested that the title system woukl be selective for cesium. Synthesis of the new lipophilic crown ether, bk[4(5)-t~lbenzo).2l-crown-7 (D(tBB)21C7), alkwedtertlngofthkhypotheelaUndernonbadlngconditknr, the dktrlbutlon codfklent for wdum between a toluene rolutbn 0.025 M in didodtwyhaphtha~wllonlc acid (HDDNS) and D(tBB)21C7 and an aqueous phase 0.1 M in nitrk ackl k 100 with separation factors of 1.2 from rubidium, 5.6 from potasdun, and 294 from sodium. Under loading and competitiveextractlonconditlonr, the dktrlbutkncoefficlentr were lower (5 for cesium),but the separationfactorsremained in the same order and of useful magnitude, 1.5 from rubkllum, 6.4 from potarrlum, and 192 from rodlum. Increasing ihe concmtraknrofD(t68)2lC7andHDDNSintheorgalcp~ gives hlgher distribution coefficients for cedum as did lower aqueous acid concentrattlons.
INTRODUCTION The separation of 137Csfrom other fission products in acidic nuclear fuel reprocessingstreams is an important technological problem. Methods for the solvent extraction separation of cesium from the other alkali metals, all in solution at pH >12 were developed some years ago.14 In addition, methods for selective precipitation and ion exchange606are known. In a recent review of solvent extraction methods for separation of cesium from other fiision products, Schulz and Bray7 noted that little technology exists for extraction of cesium from highly acidic solutions. More recently some methods have been developed for the recovery of cesium from acidicnuclear fuel reprocessing solution^.^^^ One of the goals of the study of the fundamental chemistry of solvent extraction systems has been to obtain sufficient
* To whom correspondence should be directed.
(1)Roee, W. J.; White,J. C. Anal. Chem. 1964,36,1998. (2) Egan, B. 2.;Zingaro, R. A.; Benjamin, B. M. Znorg. Chem. 1966,4, 1055. (3) Arnold, W. D.; Crouee, D. J.; Brown, K. B.Znd.Eng. Chem.Process Des. Dev. 1965, 4, 249. (4) Roddy, J. W.; Coleman, C. F. Znorg. Nucl. Chem. 1973,35, 4271.
( 5 )Lee, L.-M.;Kilpatrick, L. L. APrecipitation Process for Supernate Decontamination, Report DP-1634; Savannah River Laboratory: Aiken, SC, 1982. (6) Baumgarten, P. K.; Ebra, M. A.; Kilpatrick, L. L.; Lee,M. A. Zon Exchunge Processes for Decontaminating Alkaline Radioactive Waste; Ahtract of paper presented at the American Nuclear Society meeting, 23-26 Feb, 1981, Tucson, AZ. (7) Schulz, W. W.; Bray, L. A. Sep. Sci. Techno!. 1987,22, 191. (8) Blasius, E.; Nillea, K. H. Radiochem. Acta 1984,36,207. (9) Gerow,I. H. The UseofMacrocyclic Polyethers t o Remove Cesium137 from Acidic Nuclear Wastes by Solvent Extraction; Doctoral Dissertation; University of South Carolina; Columbia, SC, 1980. 0003-2700/92/0364-3013$03.00/0
understanding of the chemistry of these systems to allow the prediction of a molecular structure of a ligand that would be selective for a given ion or molecule. This work represents one of the rare instances in which this has been possible. Systematicinvestigationsof the solvent extraction of the alkali and alkaline earth metals from acid nitrate solutions wing synergistic mixtures of various crown ethers mixed with didodecylnaphthalenesulfonic acid (HDDNS) indicated the following: (1)tert-Butylbenzo-substitutedcrown ethers of the general structure, (OCH&H& with n = 5-7, tend to be selective for the alkali metals while cyclohexano- or tertbutylcyclohexanesubtitutedcrownstend to be more selective for the alkaline earths and other divalent ions."J (2) The 21-crown-7 ring was the appropriate size for cesium. Consequently, we predicted in 1986 that the then-unavailable bis(tert-butylbenzo)-21-crown-7(D(tBB)21C7) should exhibit synergisticselectivityfor cesium when combined in an organic solution with an organophilic acid." We can now report both a viable synthetic route to D(tBB)21C7 and results for the synergistic solvent extraction with HDDNS (Figure 1)into toluene solutions from aqueous nitric acid solutions.
EXPERIMENTAL SECTION Chemicals. The crown ethers 4-tert-butylcycloheul5crown-5, dicyclohexanel&crownn-6, bis[4(5)-tert-butylben]18-crown-6,and dicyclohexano-21-crown-7were purchased from Parish Chemical Co. (Orem,UT) and used ae received. Cesium carbonate was obtained from Henley Chemicals, Inc. (Montvale, NJ). Except ae noted, other solvents and reagentswere obtained from chemical suppliers and used as received. The didodecylnaphthalenesulfonic acid was obtained from King Industries of Norwalk, CT, through the aid of Dr. Larry Gallaher. The material as received was approximately40%active materialin heptane. The dodecyl group were mixtureaof various branched alkyls averaging 12 carbons, and the position of the alkylgroup on the naphthalene rings also varied.'* The heptane was removed by vacuum evaporation, and the remainingmaterial wae dissolved in ethanol. This solution was loaded on a column of a strong-baseanion exchange resin in the free h e form, the column was washed with ethyl alcohol, and the sulfonicacid wae eluted with HCl in alcohol. After removing the alcoholby vacuum evaporation, the acid was dissolved in toluene and titrated to obtain the empiricalneutral equivalent. The value obtainedwae approximately 6% higher than the theoretical value of 464 for didodecylnaphthalenesulfonicacid. The titration value was used for preparation of stock solutions of HDDNS in toluene. (10) McDowell, W. J.; Moyer, B. A.; Case, G. N.; Case, F. I. Solvent Extr. Zon Ezch. 1986,4, 217. (11) ORNL Chemistry Diu. Ann. Prog. Rept. for Period Ending 31 March 1987,72 (Aug 1987). (12) PersonalcommucationfromDr.Larry Gallaher,KingIndustries, 1988.
@ 1902 Amsrlcan Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 64. NO. 23, DECEMBER 1, 1992
R.
/
R2
(R,, R2 = 12 carbons) HOONS Figure 1. Structure of
extractants.
I
base so Ivent
xAoAonx
D(1BB)Pl C7
(X = CH3S03-or Q-CH~CEH~SO~-)
Bie[4(6)-tert-butylbenzo]-2l-crown-7.Under nitrogen, a mixture of bis[2-(4- or 5-tert-butyl-2-hydroxyphenoxy)ethyll etherl8 (2.00 g, 4.98 "01) and cesium carbonate (3.40 g, 10.45 "01) in 160mL of acetonitrile was stirred at reflux. A solution of dimethanesulfonate of triethylene glycol (1.52 g, 4.98 "01) in 30 mL of acetonitrile was added dropwise over a l-h period. The reaction mixture was refluxed for 72 h, cooled, and filtered, and the filtrate was evaporated in vacuo. The residue was dissolved in 40 mL of dichloromethaneand washed successively with 3 N sodium hydroxide (2 X 30 mL), 6 N hydrochloric acid (40 mL), and water (40 mL) and then dried over magnesium sulfate. The dichloromethane was removed by vacuum evaporation, and the residue was chromatographed on alumina with dichloromethane/diethylether (1:l)as an eluent to give a clear oil. Trituration with 4 drops of hot methanol gave 1.26 g (49%) of a white solid with amp of 86-97 OC: IR (KBr) 1100 (CO) cm-l; lH NMR (CDCS) 8 1.33 (8, 18H), 3.78-4.45 (m, 20 H), 6.80-7.19 (m, 6 H). Anal. Calcd for C&O,: C, 69.74; H, 8.58. Found C, 69.59; H, 8.57 (Figure 2). Solvent Extraction. Mixed solutions of a crown ether and HDDNS in toluene were prepared by dilution from more concentratedstock solutionsof the individualcomponents. The concentrations of stock solutions were determined by titration of the sulfonic acid in ethyl alcohol with aqueous sodium hydroxide. Solutions of crown ethers in toluene were prepared by weight. For nonloading condition experiments, aqueous solutions consisted of dilute nitric acid solutions containing one of the (13) Pedersen, C. J. J. Am. Chem. SOC.1967,89,7017.
radiotracers lS7Cs,lSCs, **Na,'OK,or W b . In some cases lo-' M of the nonradioactive element was added to check for possible extraction by trace impurities in the extractants. No differences were seen between the distribution Coefficients obtained with and without the added nonradioactive element. Thus, the presence of extractant impurities having very high a f f i t y for alkali metals was ruled out. In addition, the constancy of distribution coefficientsat low loading indicates that there is no self-aggregationof the alkali metal complexes in the organicphase. For loading condition experiments the aqueous solutions were 0.01 M in each of the alkali metal nitrates, Li, Na, K, Rb, and Cs. For the determination of D Mva nitric acid concentrationfor each of the alkali metals above, the appropriate radiotracer was added to a quantity of the neutral mixture (2 X concentration) of alkali metal salts, and then portions of that solution were adjusted to the required acidity and made to volume. Equilibrations of these mixed HDDNS/crown ether solutions with aqueous solutions were performed in %-dramvials closed withpolyeealcaps. Egualquantities (2-3mLeach) oftheaqueous and organic phases were pipetted into a vial and equilibrated by inversion approximatelyonce every 2 s for 10-20 min on a rotating device at ambient temperatures. Previous experienceindicated that equilibrium was reached in 1 min or less. After phase separation,both phases were sampled (1mL), and cesium (or Rb or K or Na) was determined radiometrically using a 3-in. welltype NaI(ThI) crystal as the detector with the spectra being displayed on a multichannel analyzer. Usually, two aliquots of both phases of each equilibration were counted. Many of the points obtained for the distribution of cesium that were determinedusing lmCswerecheckedusinglWstoavoidthedifficultiee and possible errors arising from the necessity of counting the lnmBadecay product when using ls7Cs. Barium was known to distribute differently from cesium. However, no differenceswere found, indicating that sufficient time had elapsed between equilibrationand counting in the 'Ws-traced samplesfor ls"Ba to come to secular equilibrium. Lithium was determinedby flame photometry.
RESULTS AND DISCUSSION Although earlier work had suggested that a 21-crown-7ring should be appropriate for the extraction of cesium1OJ1 and more recent work by others had likewise indicated the same thing,49the available dibenzo-21-crown-7 had very limited solubility in hydrocarbon solvents, e.g.
