Ion-exchange behaviour of the transuranium elements in LiNO3 solutions

J. Inorg. Nucl. Chem., 1963, Vol. 25, pp. 447 to 452. Pergamon Press Ltd. Printed in Northern Ireland

ION-EXCHANGE BEHAVIOUR OF THE TRANSURANIUM ELEMENTS IN LiNO3 SOLUTIONS* S. ADAR,t R. K. SJOBLOM,R. F. BARNES and P. R. FIELDS Argonne National Laboratory, Argonne, Illinois and E. K. HULET and H. D. WILSON Lawrence Radiation Laboratory, Livermore, California (Received 2 July 1962; in revised form 6 September 1962)

Abstract--The distribution coefficients(KD'S)of the trivalent actinide elements Pu, Am, Cm, and Cf between LiNOs solutions (containing 0-005 M H +) and Dowex-1 anion resin were measured. A column separation of Am and Cm using this system was investigated at 22-5°C and 87°C and the separation factor was found to be substantially higher than that obtained with the complexing agent ammonium ~-hydroxyisobutyrate and cation exchange resin. THE separation of the transplutonium elements is becoming an increasingly important problem as the production of these elements is expanded. For example, long lived isotopes of americium and curium in almost gramme quantities were produced by irradiating a plutonium fuel assembly in the Materials Testing Reactor for two years. Processing of the assembly yielded approximately 600 mg of 2'aCm and 1 . 3 g o f m A m . A more recent programme has produced severalhundred grammes of 243Am and 244Cm but these materials have yet to be separated from the plutonium fuel and the accompanying fission products. Re-irradiation of some of these elements will eventually yield hundreds of milligrams of berkelium and californium. To meet the increasing demand for a better actinide element separation, a new ion exchange system was investigated and the results are reported in this article. Earlier work by MARCUSC1'2) has shown that the rare earth elements can be separated from each other using Dowex-1 anion resin as an adsorbent and LiNO 3 solutions as an eluant. A similar technique applied to the separation of the actinide elements is described in this paper. Typical actinide elements such as plutonium, americium, curium and californium were studied. Particular emphasis was placed on the transplutonium elements americium and curium which are being produced in large quantities and are difficult to separate from each other. Equilibrium experiments were performed at room temperature to measure the distribution coefficients of the actinide elements between Dowex-1 resin and LiNO 3 solutions of varying concentrations. The resin used in these experiments was BioRad (Dowex) A G l-X8, 200-400 mesh. The resin was graded rather crudely by batch settling in distilled water to remove the very fine resin particles. The resin was then washed in a column with 2 M NH~OH, then washed successively with water, 1 M H N O 3, 15 M H N O 3, and again with water. Finally the resin was dried in an * Based on work performed under the auspices of the U.S. Atomic Energy Commission. t Present address, Israel Atomic Energy Commission, Tel Aviv, Israel. c1~y. MARCUSand F. N E L S O N , J. Phys. Chem. 63, 77 (1959). t2~ y. MARCUSand I. ABRAHAMER,J. Inorff. Nucl. Chem. 22, 141 (1961). 447

448 S. ADAS,R. K. SJOBLOM,R. F. BARNES,P, R. FIELDS,E. K. HULETand H. D. Wn_soN oven at 110°C to constant weight. The equilibrium experiments were usually agitated, at room temperature, for a minimum of two hours. Tracer tests with Pu(IV) showed that equilibrium was reached within 2 hours. Reproducible distribution coefficients, Concentration of solute/g of resin KD = Concentration of solute/ml of solution were obtainable only when considerable care was taken with the assaying of the very viscous salt solutions and when the amount of salt on the final assay plate was insufficient to cause substantial absorption of the ~-particles (less than 0.1 mg). Attempts to precipitate the actinide elements as hydroxides from the LiNO3 solutions using ferric ion as the carrier, followed by removal of the Fe(III) ions by solvent extraction, usually led to higher KD'S. Other techniques, such as direct solvent extraction of the actinide elements from the LiNO3 solutions, also had complications and gave higher KD's. The tracers used for these investigations were carefully purified by ion exchange techniques and analysed radiochemically for other radioactive species. Stock solutions of Pu(IV) were prepared by standard techniques such as treatment with H~O2 or potentiometric reduction of Pu(VI). The absorption spectrum of the stock solution to be used was analysed with a Cary Model 14 Spectrophotometer before tracer solutions of Pu(IV) were prepared. Pu(III) was prepared by reducing Pu(IV) solutions with ferrous sulphamate. An excess of ferrous sulphamate as a holding reductant was necessary to prevent the slow oxidation of Pu(III) to Pu(IV). To insure that only Pu(III) was represented in the measurements, the initial and final LiNO3 solutions from the equilibration experiments were extracted with thenoyltrifluoroacetone which, under the proper conditions, extracts only Pu(IV). Since Pu(III) will also extract from LiNOz (0.005 M H +) solution, the hydrogen ion concentration was adjusted to about 0.5 M before extraction. The distribution coefficients of the trivalent actinide elements plutonium, americium, curium and californium are shown as a function of lithium nitrate (containing 0-005 M H +) concentration in Fig. 1. The adsorption of Pu(IV) by the resin was approximately ten times that of Pu(III) at 1 M LiNOz, the only concentration where the two valence states were compared. It was found that the distribution coefficients decrease rapidly as the [H+] concentration of the LiNO3 solutions increases above 0.01 M. A more detailed study of this effect with rare earth elements has been reported by MARCUS.tl'2~ From Fig. 1 it can readily be seen that the distribution coefficients increase with increasing LiNO3 concentration, and at constant molarity the distribution coefficients decrease with increasing atomic number, a reversal of the order observed in LiCI solutions, t3~ Comparing the KD's reported here with the results of MARCtrStl'~, it is apparent that the LiNOa-Dowex-1 system will not be as effective as LiC1-Dowex-1 ts~ in separating the actinide elements from the rare earths due to the overlap in KD'S of many of the elements in both series. However, it should be noted that the ratio ts~ E. K. HULET,R. G. GUTMACHERand M. S. CooPs,J. lnorg. Nucl. Chem. 17, 350 (1961).

Ion-exchange behaviour of the transuranium elements in LiNO3 solutions I000

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DISTRIBUTION BETWEEN

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FIG. 1.--Distribution coefficients of trivalent actinides between LiNO3 (0.005 M H +) solutions and D o w e x l-X8 resin.

{KD[Am(III)]}/{KD[Cm(III)]} ~ 2 found in this work would predict a better separation of these two elements than in the a m m o n i u m ~-hydroxy isobutyrate-Dowex 50 c4-6~ system used so widely in separating trivalent actinides. Column elutions were made with tracer solutions to investigate the separation of A m and C m under dynamic conditions. In order to elute these elements in a reasonable time, the range of 4-5 M LiNO 3 as the eluant was chosen. Hydraulically graded Dowex-I resin with an 0.8 to 1.5 cm/min settling rate was selected as the adsorbent. The actinide elements were strongly adsorbed on the resin bed from 8 M LiNO 3 (0.01 M [H+]), and then eluted with the more dilute 4.2 M LiNO3 (pH 2.1). The ~4~ G. R. CHOPPIN, B. G. HARVEY and S. G. THOMPSON, J. Inorg. Nucl. Chem. 2, 66 (1956). c5~ H. L. SMITH and D. C. HOFFMAN, ,1".Inorff. Nucl. Chem. 3, 243 (1956). ~e~ j. MtLSTED and A. B. BEADLE, Y. Inorff. NucL Chem. 3, 248 (1956).

450

S. ADAR, R. K. SJOBLOM,R. F. BARNES,P. R. FIELDS,E. K. HULETand H. D. WILSON

columns were operated at 87°C, principally to increase the rate of exchange and narrow the elution peaks. Fig. 2 shows a typical column run. Smaller A m and Cm distribution coefficients were found from numerous column elutions at 87°C than those obtained in equilibrium studies at r o o m temperature. In addition, column elutions at room temperature (Fig. 3) indicated not only greater adsorption by the |05

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FIG. 2.---Column separation of trace amounts of Am and Cm at 87°C. 4.2 M LiNO3 + 0.01 M H + and Dowex l-X8 resin. Resin settling rate 8-15 mm/min. Column size 9 cm × 6 mm. Flow rate 0.38 ml/cm=see and tube volume 0"433 ml/tube. resin but also a larger separation factor, {KD[Am(III)]}/{KD[Cm(III)]}, (Table 1) than could be obtained at elevated temperatures. Further reduction in peak widths would be of greater advantage in utilizing the superior separation found in room temperature elutions. A mixture containing 20 mg of ~WCm and 40 mg of r o a m was selected to test the usefulness of the LiNO a with macro amounts of americium and curium. The mixture, containing a large amount of inert salts, resulted from the isolation by ion exchange o f 600 mg of curium and 1.2 of americium from an irradiated plutonium fuel assembly. A feed solution made by dissolving the material in 6 ml of saturated

Ion-exchange behaviour of the transuranium elements in LiNOa solutions I0,000

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FIG. 3.--Column separation of trace amounts of A m and Cm at 22"5°C. 4.2 M LiNO3 ÷ 0-01 M H + and Dowex l-X8 resin. Resin grade 8-15 mm/min. Column size 9 cm × 6 mm. Flow rate 0.16 ml/cm 2 sec and tube volume 0'461 ml/tube.

LiNOa (pH 2.1) was transferred onto a 1 cm diameter by 11 cm long resin bed maintained at 87°C. After adsorption the actinides were eluted with 4.2 M LiNOa (pH 2.9) with the results shown in Fig. 4. A good separation of americium from curium as well as from the inert impurities was attained. It was observed that the salts remained on the top of the resin bed and were later eluted with 1 M HNO3; however, no 0cactivity was found in this fraction. Americium and curium were recovered from the LiNOs solutions by precipitation as the hydroxides with NH4OH. The LiNOa-Dowex-1 system appears promising as a separation technique for the transuranium elements. For high concentrations of americium and curium the TABLE l Temp. (°C)

KD[Am (III)]

KD[Cm (III)]

KD[Am (III)] K.[Cm (III)]

22-5 87

10.2 6.1

5-3 3.9

1 '9 1.6

452

S. ADAR,R. K. SJOBI.,OM,R. F. BARNES,P. R. FIELDS,E. K. HULETand H. D. WILSON I012

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FIG. 4.--Column separation of milligram amounts of A m and Cm at 87°C. 4.2 M LiNO3 + 0.01 M H + and Dowex 1-X8 resin. Resin settling rate of 0.5-0.8 cm/min. Column size 1 cm × 11 cm. Flow rate 4 ml/5 min and tube volume 2 ml/tube.

LiNO3-Dowex-1 system gave better separations than either the a m m o n i u m ~hydroxy isobutyrate-cation column or the oxidation of americium by persulphate a~ or ozone, cs) In the latter cases, the oxidation of americium is difficult in the presence of H 2 0 ~ generated by the intense alpha activity, whereas the LiNO 3 separation is not adversely influenced. Generally, ion-exchange column operations, such as the one described, are well suited for remote control use in shielded facilities. tT~L. B. ASPREY, S. E. STEPHANOUand R. A. PENNEMAN,.jr. Amer. Chem. Soc. 73, 5715 (1951). ta) R. A. PENNEMAN,J. S. COLEMANand T. K. KEENAN,J. Inorg. Nucl. Chem. 17, 138 (1959).