Sele,ctiveCation Exchange Separation of Carrier-Free Cesium437 and Sodium-22 Johann Korkischl and K. A. Orlandini Chemistry Division, Argonne National Laboratory, Argonne, Ill. From organic solvent media Containing 2-thenoyltrifluoroacetone and pyridine,. alkali metal ions such as cesium and sodium are strongly retained on the strongly acidic cation exchange resin, Dowex 50, while practically all other elements are not adsorbed under these conditions. This fact has been employed for the rapid and selective separation of cesium-137 from uranium fission products and of sodium-22 from deuteron-irradiated magnesium targets. The recovery of these two carrier-free radionuclides was found to be higher than 99.9% and their radiochemical purity was in the vicinity of 100%. An entire ion exchange separation such as the separation of cesium-137 can be completed in less than 3-4 hours.
MOSTION exchange methods so far reported for the rapid and selective isolation of alkali metals, especially of cesium, are based on the retention of the heavy alkali metal ions on inorganic cation exchange materials such as heteropolyacidse.g., ammonium molybdophosphate (AMP), complex cyanides of the transition metals, and zirconium phosphate ( I ) . Among these, AMP has been used most frequently for the isolation of cesium-137 from a great variety of materials including uranium fission product solutions ( I ) . The advantages of these materials over organic ion exchange materials are that they are highly resistant to radiation damage, and they selectively adsorb the heavy alkali metals under both acidic and neutral conditions. In the case of AMP, the disadvantages are that this exchanger has a finite solubility in aqueous solutions and peptizes slowly in pure water. Furthermore, AMP alone is impervious to liquids so that in a column process, supports such as asbestos or silica gel must be added to provide porosity. Another disadvantage which is most prominent with AMP and the complex cyanides is that the recovery of the alkali metal-e.g., cesium-once adsorbed on the exchanger, necessitates the use of strongly salted solutions, such as ammonium nitrate, or the dissolution of the exchange material in an alkaline solution. Thus, in both cases the isolated heavy alkali metal is accompanied by comparatively large amounts of chemicals. Consequently, the preparation of carrier-free alkali metal tracer solutions, such as one of cesium-137 which solely contains this radionuclide, is only possible after carrying out additional separation steps. Synthetic organic cation exchangers have not been used frequently for the isolation of alkali metal ions from complex mixtures, such as, for example, of cesium-137 from fission product solutions ( I ) , because of selectivity and susceptibility to radiation damage of these ion exchange resins. With respect to the adsorption of cesium-137 from dilute acid or neutral solutions, the selectivity is poor. Many other elements, including uranium and most of the metal ions contained in uranium fission products, are coadsorbed with the On leave from the University of Vienna, Analytical Institute,
Austria. (1) J. Korkisch, “Modern Methods for the Separation of Rarer Metal Ions,” Chap. 5 , Pergamon Press, Ltd., Headington Hill
Hall, Oxford, England, in press, 1968.
cesium so that lengthy chromatographic ion exchange procedures and additional separation steps are required to purify the cesium. On the other hand, many methods have been reported which allow the successful separation of alkali metals from each other on cation exchange resins from which these elements can be readily eluted with mineral acids ( I ) . The method described in this paper combines both the selectivity advantages of AMP and the ready recovery of the adsorbed alkali metals from an ion exchange resin by elution with a mineral acid (hydrochloric acid). Furthermore, the effect of radiation damage is greatly minimized because the very strong adsorption of cesium-137 on Dowex 50 allows cesium to be rapidly separated from the other fission products. EXPERIMENTAL
Apparatus. For the ion exchange separations quartz ion exchange columns of 0.5-cm diameter and 25-cm length have been employed. The columns were constructed so that the flow rate could be increased by applying external pressure. Reagents and Solutions. IONEXCHANGE RESIN: Air-dried Dowex AG 50W-X8, 100-200 mesh; hydrogen form, was utilized for the column separations and for the batch experi: The following reagent grade ments. ORGANICSOLVENTS solvents were used : pyridine, methanol, ethanol, acetone, tetrahydrofuran, benzene, nitrobenzene, isoamylacetate, acetylacetone, and quinoline. TTA-PYRIDINE SOLUTIONS: 2-Thenoyltrifluoroacetone (TTA) mol wt = 222 was dissolved in pyridine to give solutions of different molarities-e.g., 0.1M and 1M. The TTA-pyridine solutions should not be prepared earlier than 2-3 days prior to their use. If necessary the concentration of TTA in the pyridine may be raised, for example to 5M, when it is required to complex larger amounts of metal ions accompanying the alkali metals. When more than 100 ml of TTA-pyridine solutions are required, it is recommended to purify the freshly prepared TTA-pyridine solution by passing it through a larger column of Dowex 50-e.g., a 10-gram column of the resin is suitable to purify one liter of 1 M TTA-pyridine. This purification step removes traces of alkali metals and ammonia that may be present in the TTA and pyridine, respectively. TRACERS: Hydrochloric acid and nitric acid solutions of many radioactive tracers were used. These included: 137C~, **Na, 9% SOY, 95Zr gjNb, l69Yb, I44Ce, 4eSc, lo6Ru, 99Tc, 241Am,*39Pu, *39Np, 1*1Hf, lZ4Sb,k4Mn, IlOAg, *04Tl, 6oCo, Ig2Ir,lo9Cd, *O7Bi,and 44Ti. The radioactive measurements were performed with standard counting techniques. HYDROCHLORIC ACID SOLUTION: For the elution of cesium and sodium, 6 M hydrochloric acid was employed. This solution was prepared from reagent grade concentrated hydrochloric acid. Determination of Distribution Coefficients. The batch distribution coefficients (Kd-values) of the elements were determined by the same technique as has been described earlier (2). (2) K. A. Orlandini and J. Korkisch, USAEC Report ANL 7415,
(1968). VOL. 40, NO. 7, JUNE 1968
1127
Separation of Carrier-Free Cesium-137 from Uranium Fission Product Solutions. PRETREATMENT OF RESIN BED: 1 gram of the resin is soaked for about 30 min in a few milliliters of 0.1M TTA-pyridine eluent solution and is then transferred to the ion exchange column. The resulting resin bed is supported by a pad of quartz wool and is subsequently washed with about 10-15 ml of eluent solution. The column is now ready for the sorption step, PREPARATION OF FEEDSOLUTIONAND SORPTIONSTEP: To 50 p1 of the uranium fission product solution, 2-3 ml of concentrated hydrochloric acid is added and the solution is evaporated to about 0.5 ml. After cooling down to room temperature, 9-10 ml of eluent solution are added. This feed solution is passed through the pretreated resin bed at a rate corresponding to the back-pressure of the resin column. WASHING STEPS: After completion of the above sorption step, the column is washed with 30-40 ml of the eluent solution to remove the last traces of uranium and fission products, with a flow rate of about 1.5 ml/min. The residual TTA is removed by washing the resin bed with 5 ml of pyridine which in turn is removed from the column by the subsequent passage of 5 ml of distilled water. The flow rate during these two washing steps is the same as during the sorption process.
Table I. Distribution Coefficients in Pyridine at Varying TTA Concentrations Molarity of TTA in pyridine Element O.OM 0.1M 1.OM 22Na 168,840 8,646 25 I T S 7,211 2,836 1,105 2 0 4 ~ 1 5,500 237 23 1Z4Sb 9.2 2.3 1.6 m7Bi 20 31 -1
5.OM 4.6 1,o@J
... ... ...
Table 11. Distribution Coefficients of Sodium and Cesium in 0.1M TTA-90% Organic Sohent-lO% Pyridine Solutions Distribution coefficients Solvent cs Na Methanol 4,810 2,010 Ethanol 67,526 4,434 33,768 6,530 Acetone 84,420 8,420 Tetrahydrofuran Benzene 8,530 >600,000 >105 3,200 Nitrobenzene 12,000 Isoarnylacetate 6,000 20,220 112,560 Acetylacetone 2,460 67,536 Quinoline
Table 111. Distribution Coefficients of Sodium and Cesium in 0.1M TTA-P yridine Solutions Containing Varying Percentages of Water VOl
z
water present in 0.1M TTA-pyridine 1 5
10 20 30 40
50 60 70 80
Distribution coefficients Na cs 3,234 11,550 2,140 9,727 1,300 8,265 646 1,632 369 626 270 323 183 193 110 131 83 91 69 78
ELUTIONSTEP: Cesium is eluted with 10-20 ml of 6 M hydrochloric acid, with the same flow rate as during its sorption. Under these conditions the Kd-values of the alkali metals are less than one. REGENERATION OF THE RESIN: To prepare the resin for another separation after the elution step, the resin bed is first washed with 5 ml of distilled water to remove residual hydrochloric acid, and then about 20 ml of eluent solution are passed through at a rate of 1.5-2 ml/min. This high flow rate is essential to remove air bubbles that are formed during this operation. Subsequently, the column is washed with 10-15 ml of eluent solution at a flow rate corresponding to its back-pressure. The column is now ready for another run.
RESULTS AND DISCUSSION Determinations of the batch distribution coefficients on Dowex 50 of cesium, sodium, and many other elements in pyridine solutions of varying TTA concentrations showed that from these media only the alkali metals are very strongly retained by the resin. The Kd values that were measured for lithium, potassium, rubidium, and ammonium were found to be of the same order of magnitude as those of sodium and cesium. Appreciable adsorption is also shown by thallium and some retention is observed for antimony and bismuth. Results are presented in Table I in which several of those elements are listed which are retained by the resin. All other elements including the rare earths, Sc, Y,Sr, Zr, Nb, U, Th, Ru, Tc, Am, Pu, Np, Hf, Mn, Co, Ni, Fe, Cu, Ir, Zn, Cd, Ti, Ca, and Mg have distribution coefficients of less than one and hence are not retained by Dowex 50. Silver ion is also not retained when present as the chloride (which is soluble in pyridine) but it is adsorbed if silver nitrate is used (Kd = 16 in 0.1M TTA-pyridine solution). From the data shown in Table I, it is seen that cesium is very strongly adsorbed at all investigated TTA-concentrations so that the molarity of TTA in the eluent solution (see working procedure) can be varied within very wide limits. The decrease in cesium adsorption with an increase of TTA concentration from 0.0 to 5 M is very probably due to the fact that cesium is extracted by TTA into various organic solvents from alkaline solutions (3). A similar explanation can be given with respect to the decreasing retention of thallium, antimony, and bismuth when increasing the TTA concentration from 0.0 to 1.OM. A peculiar behavior is, however, shown by sodium which is more strongly adsorbed than cesium from 0.0 to O.1MTTA-pyridine, while from 1Mand 5MTTApyridine, the reverse is true (see Table I). However, an attempt to separate sodium from cesium with 5 M TTA-pyridine as the eluent was unsuccessful because only a very small fraction of the sodium activity appeared in the eluate in the range where it should be eluted according to its batch distribution coefficient of 4.6 (see Table I). To investigate the effect of various organic solvents on the adsorption of sodium and cesium on Dowex 50, the distribution coefficients of these two elements were measured in 90 vol organic solvent-10 vol pyridine solutions which were 0.1M in TTA. The results of these investigations are presented in Table 11. In all cases the distribution coefficient of cesium is lower than that of sodium. As the coefficients of both elements are very high in all systems, the organic solvents listed in Table I1 are also suitable media to adsorb the alkali metals on the resin. When the same percentage of water is used in place of the organic solvents, TTA is precipitated. This is, however, not (3) P. Crowther and F. L. Moore, ANAL.CHEM., 35,2081 (1963).
1128
ANALYTICAL CHEMISTRY
Table IV.
Distribution Coefficients of Several Elements in 90 % Methanol-10 % Hydrochloric Acid Solutions of Varying Acidity Composition of solutions Overall Overall Overall acidity, 0.01N 0.1MTTA acidity, 1.2N acidity, 0.1N Overall in pyridine 0.1M TTA 0.1M TTA 0.1M TTA acidity, 1.2N 8,646 11 60 835 9,420 2,836 42,000 100 250 4,775 >lo4 29 40 >io4 (1 >io4 25 32 >io3 io3 >io3
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36 4.4
the case when the solutions contain 80% or less water. The addition of this solvent causes a reversal of the adsorption sequence--i.e., in the presence of water sodium is less strongly adsorbed than cesium. This effect as well as the variation of the distribution coefficients of these two elements in the presence of varying percentages of water is illustrated by the data presented in Table 111. It is also seen that the adsorption of the alkali metals decreases with an increase of the water content of the mixtures. In order to investigate the effect of TTA on the adsorption of cesium, sodium, and various other elements on Dowex 50 also from acid media, their distribution coefficients were methanol-10 vol hydrochloric acid measured in 90 vol solutions of varying acidity in the presence and absence of 0.1M TTA. The results of these studies are listed in Table IV. For the purpose of comparison, the distribution coefficients which have been measured in 0.1M TTA-pyridine solution are also included. From the data shown in Table IV it is seen that the adsorption of the elements shows the expected trend, namely, an increase with decreasing acid concentration. This increase can most clearly be seen in the case of the adsorption of the alkali metals and ruthenium. The presence of TTA in the acid solutions has practically no effect on the adsorption of the metal ions, except on the adsorption of hafnium which first decreases in the presence of TTA at 1.2N overall acidity whereafter it increases with a decrease of the overall acidity. Consequently, in acid solutions TTA-complexing of most metal ions is negligible in the presence of Dowex 50 because these ions show high adsorbability on the resin under these conditions. Complexing with TTA, however, becomes the predominant factor in regulating adsorption when the basic TTA-pyridine solution is used from which only cesium and sodium are retained by the resin (see last column of Table IV). This high selectivity of Dowex 50 for sodium, cesium, and the other alkali metals in the TTA-pyridine medium is based on the inability of the alkali metals to form stable pyridine-soluble TTA-chelates in the presence of Dowex 50 while the majority of the other metal ions form strong complexes with TTA and hence are not retained by the resin. These complexes are readily soluble in the pyridine. Hence this solvent is acting as an extractant for this group of elements while the alkali metals are taken up by the resin. This is another example of the principle of combined ion exchange-solvent extraction (CIESE) earlier described ( 4 ) .
z
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41 41
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