Chromatographic-Solvent Extraction Isolation of Berkelium-249 from Highly Radioactive Solutions and Its Determination by Beta Counting L. G. Farrar, J. H. Cooper, and F. L. Moore Analytical Chemistry Dicision, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 A small high-pressure ion-exchange system has been used together with liquid-liquid extraction to achieve decontamination sufficient to permit the determination by beta counting of Z49Bk isolated from highly radioactive solutions. A high-pressure pump forces solutions through a column of Dowex 50 resin (20- to 40-tdiam particles) at pressures from 1800 to 2500 psi. The eluent is a dilute aqueous solution of a-hydroxyisobutyric acid of pH 4.2. The eluted 249Bkis purified by extraction into 2mthenoyltrifluoroacetone-xylene. I n this way the *49Bk is separated sharply from many elements including the actinides, fission products, and corrosion products. Separations can be effected at eluent flow rates as fast as 18 m l ern+ min-l, thereby minimizing loading time, elution time, and radiation damage to the resin. High-pressure operation makes possible the use of a long column (-2 ft) having a small free column volume (-1 ml).
A SIMPLE RAPID method to isolate 249Bkfrom highly radioactive transplutonium process solutions and subsequently to determine it by beta counting has eluded analytical radiochemists. A workable method developed earlier (I) is long and tedious. Because of the tremendous decontamination required to make beta counting feasible (a decontamination factor often as high as lolo), currently used methods ( 2 , 3) are based on counting the alpha radiation from 249Cf(tip = 360 yr), the daughter of 249Bk,after a suitable ingrowth period, usually several weeks. Because >99 % of the decay of 249Bk occurs by emission of low-energy beta particles (E,,, = 125 keV, 3.7 X lo6dpm/ng), beta counting should be considerably more sensitive than alpha counting for measuring 249Bk. Moreover, in beta counting, a daughter ingrowth period is not required. Because beta counting has the potential advantages of high sensitivity and speed, we studied it as a method to determine Z49Bk. Before the 249Bkis beta counted, it must be separated completely from cerium. Because the chemistry of Bk(1V) is very similar to that of Ce(IV), it is very difficult to effect the high degree of purification of 249Bkrequired for beta measurements ; highly selective chemical-separation techniques are mandatory. For example, in process control work it is often necessary to determine the 249Bk content (subnanogram amounts) of solutions that contain as much as 1O1O dpm of alpha, beta, and gamma radioactivity. To isolate berkelium from mixtures of transuranium elements and fission products by ion-exchange chromatography, early investigators ( 4 ) used the position of the eluted berkelium relative to positions of the other elements eluted. Such a procedure is very timeconsuming, because it requires several chromatographic
cycles. In 1956 Choppin, Harvey, and Thompson (5) developed an excellent procedure to isolate individual trivalent actinide ions. This popular procedure is based on the chromatographic elution at 87 “C of trivalent actinide ions from a cationic resin with an aqueous solution of cu-hydroxyisoRecently, process chemists (6) apbutyric acid of p H -4.2. plied high pressure to this cation resin-isobutyrate system to separate rapidly and successfully transplutonium elements from each other. To accomplish this, they adopted a novel technique developed by biochemists ( 7 )for the analysis of body fluids-the use of a very fine cationic resin (20- to 40-p-diam particles) under high pressure. The rapid flow rate of eluent that is achieved at high pressure minimizes radiation damage to the resin when the sample is radioactive. Two recently developed techniques, high-pressure column chromatography and extraction with thenoyltrifluoroacetone (TTA), are highly selective for separating berkelium. The method described herein exploits these techniques. It consists in preliminary decontamination of 249Bkby high-pressure chromatography on a column of the cation-exchange resin, Dowex 50, with the eluent a-hydroxyisobutyric acid (aHIBA); final purification of the decontaminated 249Bk by liquid-liquid extraction into TTA ; and measurement of the purified 249Bkby beta counting. EXPERIMENTAL Apparatus. Proportional counter, internal-sample, gasflow type (10% methane-90x argon); used at voltage settings of 1650 and 2350 V for alpha counting and for beta counting, respectively. Scintillation counter, NaI(T1) well-type; used for gamma counting. Pump, Beckman Accu-Flo (Model No. Ap-lM-2); available from Spinco Division, Beckman Instruments, Inc., Palo Alto, Calif. Stainless-steel frits, Type 316, 5-p-diam pores; available from Crawford Fitting Co., Solon, Ohio. Constant-temperature circulator, Haake type; available from Polyscience Corp., Evanston, Ill. High-pressure ion-exchange column. Figure 1 is a schematic diagram of the high-pressure ion-exchange column, All lines are l/s-in.-o.d. stainless steel of 28-mil wall thickness. The solution is pumped by a positive-displacement pump rated to 1000 psi but used satisfactorily at pressures as high as 2500 psi. To vent the high-pressure side of the pump when low flow rates develop due to bubble entrainment, a three-way valve vented to atmosphere is used in the line to the column. A pressure gauge monitors pump operation and is filled with water to decrease gas volume in the system. -~ ~
(1) F. L. Moore and W. T. Mullins, ANAL. CHEM., 37,687 (1965). (2) D. F. Peppard, S. W. Moline, and G. W. Mason, J. Inorg. Nucl. Cliem., 4, 344 (1957). (3) F . L. Moore, ANAL.CHEM.,38,1872 (1966). (4) G. H. Higgins, “The Radiochemistry of the Transuranium Elements,” NAS-NS-3031(1960).
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ANALYTICAL CHEMISTRY
~
( 5 ) G. R. Choppin, B. G. Harvey, and S. G. Thompson, J . Znorg.
Nucl. CRem., 2,66 (1956). (6) S. R. Buxton and D. 0. Campbell, U S . At. Energy Comm. Rept. ORNL-TM-1876 (1967). (7) C. D. Scott, J. E. Attrill, and N. G. Anderson, Proc. SOC. Exp. Biol. Med., 125, 181 (1967).
I M
PRESSURES TO 2500p.I
Figure 1. High-pressu Figure 2 is a photograph of the resin column installed in a glove box. The column (0.07 in. diameter, 25 in. long) is jacketed so that it can he heated with recirculating water. The column is loaded with 1.36 ml of resin to give a 23-in.4ong bed and a free column volume (fcv) of 0.72 ml. The resin is supported at the bottom with a stainless-steel frit (5-p-diameter pores) pressed into a '/& tubing coupling. A flow rate of 18 ml cm-2 min-' is normally attained, A water slurry of resin is pumped into the column under pressure from a reservoir attached to the top of the column. The reservoir is a 4-in.-long section of '/r-in.-diameter stainless-steel tubing fitted with a '/&. tubing connection to facilitate incorporation of the reservoir into the high-pressure line. The column is conditioned by being washed with -8 ml of 0.25M a-HIBA of pH 4.2 and then with 8 ml of distilled water to remove the excess a-HIBA. Reagents. Dowex 50 X 12 cation resin; available from Bio Rad Laboratories, Richmond, Calif. Before the resin was used, it was graded by water settling. Europium-152-europium-154 solution; available from the Isotope Development Center, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830. a-Hydroxyisobutyric acid (a-HIBA, mol wt = 104); available from Eastern Chemical Corp., Pequannock, N. J. A 0.25M stock solution in water, adjusted to pH 4.2 with ammonium hydroxide and containing 0.1 phenol to inhibit mold growth, is used as an eluent. lanthanum carrier. Dissolve 31 g of lanthanum hexahydrate, La(N03),.6H20, in 1 liter of water that contains a few milliliters of concd HCI, which is added to prevent hydrolysis. 2-Thenoyltrifluoroacetone(TTA, mol wt = 222); available from Columbia Organic Chemicals Co., Columbia, S. C. All other chemicals were reagent grade. Procedure. Figure 3 is a block diagram of the over-all berkelium procedure. Condition the column by passing through it 8 ml of 0.25M a-HIBA of pH 4.2 and then washing it with 8 ml of water to remove excess a-HIBA. To prepare the sample for addition to the column, proceed as follows. Into a 1 2 4 centrifuge cone, pipet from the sample solution an aliquot that is estimated to contain at least 0.06 ng of 24sBk. Add a volume of lanthanum carrier solution that contains -5 mg of La. Add concentrated ammonium hydroxide to cause a precipitate of metal hydroxides to form. Centrifuge the mixture and discard the supernatant solution. Dissolve the precipitate in 5 to 6 ml of 0.05M HNOs that contains a total of lo6 to 10' dpm of 1s2Eu-Ls'Eu. Onto the high-pressure column that contains Dowex 50 ammonium-form cation resin, load the final solution through the pump intake at a low flow rate (4 ml cm-* min-I), which gives sharp band loading. Beginning at this point, maintain the temperature of the column at 80 + 4 "C. Wash the col-
Figure 2. Photograph of the high-pressure ion-exchange column installed in a glove box umn with 10 fcv's (-8 ml total) of water to remove excess acid and then with 10 fcv's of 0.5M ammonium formate of pH 4.2 to convert the unused portion of the resin to the ammonium form. Desorb the berkelium as follows. Elute the column with 0.25M a-HIBA; collect the berkelium fraction (-12 ml). (The '5sE~-'S4Eu will load in a band near the top of the resin column and will immediately precede berkelium down the column during elution.) Determine the migration of the 'azEu-'5'Eu hand down the column by gamma monitoring with a Geiger-Mueller probe. Begin to collect the berkelium fraction when the 1 s 2 E ~ - ' 5 4band E ~ reaches the bottom of the column; continue the collection for 10 fcv's. Prepare the berkelium fraction for extraction as follows. Destroy the a-HIBA by evaporating the berkelium fraction just to dryness with HNOa-HCIOn(l :I). Dissolve the residue in 4.5ml of 1 M HNOT0.2M Na2Cr&,; transfer the solution to an extraction vessel using a small additional portion of the 1 M HNOa4.2M Na,Cr,O, solvent as a wash solution. Heat the solution of the residue in a water bath at -90 "C for 15 min. Add 0.5 ml of 5 M H2S04, washing down the sides of the vessel with it as it is added. Extract the berkelium into TTA as follows. After the solution has cooled to room temperature, add 5 ml of 0.5M TTA-xylene and extract for 10 min on a Vortex mixer. Remove and discard the aqueous phase and perform four 3min scrubs of the organic phase with 5-ml portions of 1 M H N O d . 2 M Na,Cr20,. At this point, monitor the organic phase for alpha and gamma radioactivity to determine whether additional purification is necessary. If so, strip the organic phase with 10M HNOI and repeat the extraction. If not,
w.,m
Figure 3. Block diagram of the over-all procedure for *'9Bk VOL 40, NO. 11, SEPTEMBER 1968
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20
Table I.
I
I
I
I
I
Typical Process Solution
Concn, Element(s) 6OCo
96Zr-Q5Nb 10
SRp lO8Ru
IlOmAg 14 I c e - 14 4&
244Cm
249Bk 252Cf 25
4Es
Fe Rare earth
dPm/d 3 x 107 7 x 108 2 x 108 1 x 107 2 x 108 9 x 10'0 5 x 107 8 X lo7 2 xi07 4 (msln-4 46 (mdml)
Table II. Decontamination of Berkelium-249 in the Procedure
Radionuclide 60Fe 60Co
Q5Zr-g6Nb 10 3Ru-106R~
1 . 7 x 105~ > 5 . 3 x 107 3 . 8 x 104a
2.7 x
1030
4 . 6 X los
1lOmAg 137cs 141f-e l44G 16 2Eu-16
4Eu
239Pu
2*3Arn 244crn
262Cf
3.6 X loe x 10'0 >6.2 x 107 2 . 7 x 104 > 5 . 7 x 107 >1.7 X 10'O > 7 . 3 x 107 2.1
a Value obtained after re-extraction of 1 0MH N 0 3strip solution with an equal volume of 0.5M solution of TTA in xylene.
mount a suitable aliquot of the organic phase on a stainless steel plate for beta counting. Prepare the column for additional samples by cleaning it with 20 to 30 ml of a 0.25M a-HIBA adjusted to pH 6.8 with ammonium hydroxide. RESULTS AND DISCUSSION
The high-pressure ion-exchange resin column effects excellent resolution of Z49Bk from cerium and other actinides but not from europium; Figure 4 is a typical elution spectrum. Good separation is achieved with 0.30M a-HIBA of pH 4.8, but the resolution is better with 0.25M a-HIBA of pH 4.2. Decontamination factors-i.e., the total amount of contaminative radionuclide present in the feed solution divided by the amount of that nuclide found in the 249Bk product-of 106 to lo6 for americium, curium, californium, and einsteinium and of > l o 1 0 for cerium have beenachieved. Because the 1 5 2 E ~ - 1 5 4peak E ~ immediately precedes the *49Bk peak in the a-HIBA elution, it is convenient to determine the location of the 249Bkfraction by gamma monitoring for 1 6 2 E ~ 154E~,This technique eliminates uncertainties due to erratic flow rates at high pressure, bubble formation, or small fluctuations in volume that result from changes in operating conditions. The process engineer has exactly the same problems with larger columns but can monitor for 252Cf by neutron detection as it moves down the column and thus determine the point at which to take the 249Bkfraction. Usually, analytical solutions contain too little 252Cf to make this technique possible. The final purification by TTA extraction ( 3 ) readily removes the europium, the remaining actinide elements, and other metal ions present. 1604
ANALYTICAL CHEMISTRY
Figure 4. Typical elution spectrum
Decontamination factor Solutions that contain zirconium, niobium, and iron in very high concentrations often require additional decontamination. An effective method for the preliminary removal of the bulk of these elements is to precipitate the berkelium on lanthanum fluoride. In extreme cases, still further decontamination can be achieved by stripping the berkelium from the organic phase into 10M HNOa and reextracting it into 0.5M TTA-xylene. In certain situations (no cerium present), the TTA method (3) is adequate; in other cases (no rare earth elements present), the high-pressure ion-exchange separation is sufficient. The method described in this paper offers, for the first time, a relatively simple rapid way to determine 249Bk in highly radioactive solutions produced during the chemical processing of transplutonium elements. It has been used successfully to analyze various types of samples for process control. The composition of a typical process solution is shown in Table I. Excellent separation of 249Bkis achieved from fission products, transuranium elements, and metal ions often associated with berkelium; typical data are listed in Table 11. The complete procedure requires about 6.5 hr. For eight test portions of a solution of 249Bkthat had been analyzed by three other methods, the average recovery was 96.6%; the relative standard deviation was 0.6 %. Results obtained by this method of direct beta counting of 249Bkagree within 4 % with those by alpha measurement of 249Cfingrowth, the latter result being the lower. Such agreement is considered to be very good in view of the uncertainties in the values for the half-lives of the radionuclides involved. For quantities of 149Bk so isolated, the beta spectra as viewed on an anthracene crystal reveal no extraneous beta emitters. The method has also been used to isolate and purify the radionuclides of berkelium and californium prior to their mass spectrometric determinations. ACKNOWLEDGMENT
The authors gratefully acknowledge valuable discussions with D. 0. Campbell regarding the operation of the highpressure ion-exchange column. The capable assistance of C . C . Foust and G. I. Gault in the experimental work is greatly appreciated. RECEIVED for review May 10, 1968. Accepted June 13, 1968. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
