Table I. Recovery of Added64Mn from Sea Water Equilibration time, hr
2 4
6 8
16 16
16 16 16
Recovery,
70
Efauent
Sample solution
Resin
93 94 95 96 99 99 98 99 99
6.8 6.0 5.2 4.8
0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1
1.0 1.0 1.0
1.0 1.0
Ortec single channel analyzer. Results shown in Table I indicate quantitative recovery in 16 hours with less than 1%of the manganese remaining in the water and less than 0.2% remaining on the resin. The recovery efficiency for five samples, each with 10 pCi of 54Mn and each equilibrated for 16 hours. had a coefficient of variation of
=t0.6%. The actual analysis of the samples by atomic absorption was not necessary since previous studies have proved its utility (9). Results in Table I indicate that optimum recovery may conveniently be achieved by equilibrating the samples overnight and eluting them the following morning. This batch-wise method extends the analytical use of Chelex100 to the smaller resin sizes and should be suitable for the concentration of other metals ( I O ) . ACKNOWLEDGMENT The author wishes to thank Robert Rahn and Elizabeth Waiters for their assistance in this work. My special thanks to Herbert Windom for making this work possible and for his review of the manuscript. Received for review June 11, 1973. Accepted November 26, 1973. This work was partially supported by EPA Grant (R-800372). (10) J. P. Riley and D Taylor, Ana/ Chim. Acta, 40, 479 (1968).
Separation of the Trivalent Actinides from the Lanthanides by Extraction Chromatography Terry D. Filer Health Services Laboratory. U.S. Atomic Energy Commission, Idaho Falls. ldaho 83401
One of the more difficult problems in actinide chemistry is the separation of the trivalent actinides from the trivalent lanthanides. The solution of this problem is necessary in the analysis of greater than gram quantities of soil because of the presence of milligram quantities of the rare earths. Since even microgram quantities of the rare earths have been shown to produce serious losses in the electrodeposition of the trivalent actinides ( I ) , a separation procedure with high decontamination factors is needed. Single-step liquid-liquid extraction procedures did not provide sufficiently large decontamination factors. Anion exchange involving ammonium thiocyanate ( 2 ) and cation exchange involving hydrochloric acid-ethanol ( 3 ) provide excellent decontamination factors but are cumbersome and time-consuming. Extraction chromatography provides an attractive means of separating metal ions of close chemical similarity because it combines the multiplate process of ion exchange without sacrificing to a great extent the simplicity, selectivity, and speed of liquid-liquid extraction. Bis(2-ethylhexyl)orthophosphoric acid (HDEHP) is an excellent reagent for intragroup separations of the trivalent lanthanide or actinide ions ( 4 , 5 ) because separation factors are higher than those observed in ion exchange methods. Also, the high reaction rate of HDEHP with these ions permits relatively high flow rates. Extraction (1) K. W. Puphal and D. R. Olsen. Ana/. Chem.. 4 4 , 284 (1972). (2) J. S. Coleman, L. B. Asprey. and R. C. Chisholm, J , Inorg. NucI. Chem.. 31, 1167 (1969). (3) K. Street, Jr., and G. T. Seaborg, J. Amer. Chem. SOC.,72, 2790 (1950). (4) D. F. Peppard. G. W. Mason, J. L. Maier, and W. J, Driscoll, J. Inorg. Nuci. Chem.. 4, 334 (1957) (5) D. F. Peppard, G. W. Mason, W. J. Driscoll, and R. Sironen, J. Inorg. Nuci. Chem.. 7, 276 (1958).
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chromatographic methods based on HDEHP for the separation of lanthanides in mineral acid systems have been reviewed (6). Kooi and coworkers used the HDEHP-hydrochloric acid system for the chromatographic separation of several transplutonium elements (7, 8). Moore and Jurriaanse (9) extended the use of HDEHP in extraction chromatography to nitric acid systems, utilizing Teflon (Du Pont) powder as an inert support. All of these procedures, however, are based on the extraction of lanthanides and trivalent actinides by HDEHP from mineral acid solutions. The distribution coefficients of the two groups of elements overlap, with americium behaving most like cerium or praseodymium. Therefore, the separation of lanthanides from trivalent actinides is not clean even with the use of a multiplate process such as extraction chromatography. Weaver and Kappelmann (10) showed that the substitution of carboxylic acids for mineral acids shifts the americium distribution coefficients downward slightly relative to the lanthanides. In addition, the presence of strongly complexing aminopolyacetic acids causes a much larger shift. Percival and Martin (11) showed that the light trivalent lanthanides and actinium could be separated from americium by extraction in HDEHP from an aqueous solution containing diethylenetriaminepentaace(6) E. Cerrai. Chromatogr. Rev.. 6.129 (1964). (7) J. Kooi, R. Boden, and J. Wijkstra. J . Inorg. Nucl. Chern.. 26, 2300 (1964). (8) J. Kooi and R . Boden, Radiochirn. Acta. 3. 226 (1964). (9) F. L. Moore and A. Jurriaanse, Anal. Chern.. 39. 733 (1967). (10) B. Weaver and F. A. Kappelmann. J Inorg. Nucl. Chem . 30. 263 (1967). (11) D. R. Percival and D. B. Martin, "Sequential Determination of Radium-226, Radium-228, Actinium-227, and Thorium Isotopes in Environmental and Process Waste Sampies," U.S. Atomic Energy Commission, Idaho Falls. Idaho. in preparation.
tic acid (DTPA) in a chloroacetic acid buffer at pH 3. In the present work, the use of HDEHP in extraction chromatography was extended to include the chloroacetic acid system containing the ammonium salt of DTPA, utilizing Teflon powder as an inert support. The new system provides excellent separation of the trivalent actinides from the trivalent lanthanides in a relatively pure actinide-lanthanide fraction at room temperature with relatively high flow rates.
Table I. Recovery and Decontamination of Trivalent Actinides a s a Function of Eluate Volume Eluate, ml
20
Found in acetinide fraction, Actinides
Z41Am z44Cm 25 2 cf
'4
Lanthanides
99 . 0 96 . O 83.3
140La la4Ce 14ipm 152EU-164EU
EXPERIMENTAL Apparatus. A 2$-inch hemispherical gas flow proportional counter using methane counting gas was used for alpha counting. A NaI well-type scintillation counter with a 2- by 2lh-inch well in a 3- by 3-inch crystal was used for gamma counting. A 65-cm3 germanium detector coupled to a multichannel analyzer was used for gamma spectrometry. A Beckman liquid scintillation spectrometer (Model LS ZOO), which features automatic standardization by channel ratio and ambient temperature operation, was used for beta counting. An ion exchange column, 1-cm inside diameter, with a small glass wool plug at the bottom was used for the column. Reagents. HDEHP, 0.45M. Dilute 150 ml of HDEHP (V-C Chemical Company, Richmond, Va.,) to 1 liter with n-heptane and transfer to a 2-liter separatory funnel. Wash twice with 200ml portions of 1:l 2M diammonium citrate and concentrated ammonium hydroxide, twice with 200-ml portions of 4M nitric acid, and twice with 500-ml portions of water. Use mixing times of one minute (11). Eluting Solution. DTPA, 0.025M: I M Monochloroacetic Acid. Dissolve 10 grams of DTPA in 100 ml of water containing 30 ml of concentrated ammonium hydroxide. Add 100 grams of chloroacetic acid and dilute to 900 ml. Adjust the pH to 3.0 with 5 to 10 ml of concentrated ammonium hydroxide. Dilute to 1liter. Procedures. Column Preparation. Add 45 ml of 0.45M HDEHP to 25 grams of Tee Six, polytetrafluoroethylene powder, 70 to 80 mesh (Analabs, Inc., Hamden, Conn.) in a 4-02 glass bottle. Cap the bottle and shake vigorously for 5 minutes. Filter the slurry through a double glass filter paper in a small Biichner funnel. Allow the powder to air dry at room temperature. Add 5.0 grams of the dry powder to the column (column of Tee Six is.10 cm high) and wash three times with 25-ml portions of water. Condition the column by passing three 10-ml portions of the eluting solution through it. The flow rate should be adjusted to 10 to 1 2 drops per minute. Separation of the Tricalent Actinides jrom the Lanthanides. The procedure given below for the separation of the trivalent actinides from the trivalent lanthanides is that used in the development of the procedure with pure solutions containing only the actinide and 2 mg of the traced lanthanide and is to be followed on actual samples such as soil only after total decomposition of the sample and isolation of the actinide-lanthanide fraction from the sample. Add 1 ml of a 10% sodium hydrogen sulfate solution and 2 to 3 drops of concentrated sulfuric acid to the sample solution in a 30-ml beaker. Evaporate the solution to dryness on an asbestos-covered hot plate until evolution of sulfuric acid fumes has ceased. Cool the residue, add 5 ml of the eluting solution, and heat to boiling to dissolve the residue. Cool and add the solution to the conditioned column. Wash the beaker with 3- and 2-ml portions of the eluting solution and add to the column. Load the column with the 10 ml of solution at a rate of 10 to 12 drops per minute. Continue the elution with an additional 30 ml of eluting solution at a rate of 10 to 12 drops per minute to recover the transplutonium elements. Wash the column with 50 ml of 6 M hydrochloric acid to remove the lanthanides. Wash the column with 50 ml of water prior to the next separation. The columns have been used for as many as ten runs with no loss of efficiency; however, cross contamination must be considered.
RESULTS AND DISCUSSION Teflon powder was selected as the inert support because of its exceptional stability to chemical reagents and its strongly hydrophobic tendencies. The advantages of Teflon powder over various other supports in extraction chromatography have been discussed by Moore and Jurriaanse (9). Preliminary experiments were performed to evaluate
1ioTrn li7Lu
40
z41Am
244Cm 252Cf
100.2 100.0 96.5
1 40La
ld4Ce
laiPm 15 2Eu-15
1ioTm
liiLu
4EU
0.10 0.20 1.3 1.4 0.03 0 06 0.10 0.20
8.9 2.2 0 .08
0.07
the pertinent variables in the Teflon-HDEHP system. Americium-241 and cerium-144 were used to check the effectiveness of the separation because immediate results could be obtained by gamma-counting each fraction. With 0.45M HDEHP as the stationary phase, DTPA concentrations were vaned from 0.050M to 0.0025M. The optimum concentration range of DTPA for maximum separation of americium from cerium was 0.013M to 0.025M. Lower concentrations of HDEHP also gave excellent separation but the columns might become overloaded when separating milligram quantities of lanthanides from the actinides. Several experiments were performed in which the height of the column was varied from 40 mm to 100 mm. Column lengths of 60 mm gave quantitative separation of trivalent cerium from americium, but were inadequate to separate other transplutonium elements with less favorable distribution coefficients from cerium. A column length of 100 mm was necessary to adequately separate all of the trivalent actinides from the lanthanides. The normal flow rate used with the standard 10- by 100-mm column was 10 to 12 drops per minute. Such a relatively fast flow rate is of practical importance because it allows the complete separation of actinides from lanthanides in about 80 minutes. Flow rates of 1 to 2 drops per minute showed negligible improvement in separations while flow rates as fast as 30 drops per minute have acceptable separations; for example, 1% of the cerium was found in the americium fraction. On the basis of these studies, the following conditions were selected for the separation of the trivalent lanthanides from the trivalent actinides: 10- by 100-mm column of 70- to 80-mesh Tee-Six powder containing 0.45M HDEHP-heptane as the stationary phase, 0.025M DTPA in 1M monochloroacetic acid adjusted to a pH of 3.0 with ammonium hydroxide as eluant, and a flow rate of 10 to 12 drops per minute. To determine the precision obtained using the above conditions, six samples containing americium-241 and six samples containing cerium-144 with 2 mg of cerium carrier present in each were carried through the separation starting with the evaporation of the solutions to dryness in the presence of sulfuric acid. In each case, fresh columns were prepared before the solutions were passed through the columns. The mean for the americium-241 recovery was 98% with a standard deviation of an individual about the mean of 1%. The mean for the cerium-144 recovery was 0.2% with a standard deviation of an individual about the mean of 0.2%. The above conditions were selected on the basis of studies with trivalent americium and cerium. The procedure A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
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A P R I L 1974
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Table 11. Recovery of Americium and Decontamination f r o m C e r i u m in Soil Recovery, Yo Actinide fraction
Lanthanide fraction
Sample
2:lAm
14Ce
24IAm
1
99.1 99.6 99.2
< O .05 0.2
0.9
