Radiochemical separation of lead by amalgam exchange

Talanta. 1967. Vol. 14. pp. 323 to 327.

Per@mon

Pras Ltd.

Printed in Northern Irelutd

RADIOCHEMICAL SEPARATION OF LEAD BY AMALGAM EXCHANGE IQBAL H. QURESHIand FAZAL I. NAGI Atomic Energy Centre, Ferozepur Road, Lahore, Pakistan (Received 9 Awust 1966. Accepfed 25 November 1966) Summary-_An amalgam exchange technique has been used for the rapid radiochemical separation of lead. A number of variables which affect the exchange were studied and a lead yield of 885 ‘Awas obtained. At room temperature the procedure requires about 13 min, whereas at 40” it can be completed in about 9 min. Radioactive tracers of 16 different elements were used to check the selectivity of this procedure, which has also been applied for the separation of “*Pb from thorium nitrate solution. PREVIOUS work on isotopic exchange between a dilute amalgam of a metal and its ions in aqueous solution has proved very useful for radiochemical separations of cadmium,r indium,2 strontium,3 bismuth,4 and thallium.5 In the amalgam exchange technique, a simple procedure gives excellent decontamination which is much better than that obtained previously. These studies show that the heterogeneous isotopic exchange reactions are rapid and selective and can be employed for rapid radiochemical separations. A preliminary communication6 indicated that the amalgam exchange technique can also be applied to the radiochemical separation of lead. The aim of the present investigation is to develop an optimum procedure for the separation of radio-lead by this technique. The separation is carried out in two steps. In the first step a dilute lead amalgam is made to contact an aqueous solution containing radioactive isotopes of lead, in order to selectively exchange radioactive lead with the inactive lead in the amalgam. The exchange reaction may be represented by

Pb(Hg) + $bs+ + fib0

+ Pbs+

where the asterisk denotes radioisotopes of lead. In the second step the amalgam containing radio-lead is removed and contacted with a solution of copper(I1) nitrate or lead nitrate to strip the element from the mercury into the aqueous phase. EXPERIMENTAL Apparatus The separations were made in one-ounce uare-bottomed flint glass bottles with polyethylene insert caps. The bottles were clamped in a met%ui lcal shaker (Wrist-Action shaker, Burrell Corp., Pittsburg, Pa., U.S.A.), to which an extension arm of 9 in. was connected to give additional radial action. For high temperature studies the bottles were agitated in a thermostatically controlled tank. Gross gamma ray measurements were made with a Baird Atomic Model 810 scintillation well counter, and gamma ray spectrum measurements with a 3 in. X 3 in. Nal(T1) crystal coupled with a Nuclear Data 512 channel analyser. A Tracerlab Geiger tube TGC-1 was used for beta activity measurements. Reagents Mercury: triply distilled, CENCO, U.S.A., further pursed by washing with dilute nitric acid, followed by rinsing with distilled water. Lead metal: C.P., City Chemical Co., New York, U.S.A. Thorium nitrate: Reagent grade, May & Baker Ltd., England. 323

324

I. H. QURESHIand F. I. NAGI

All radioisotopes used as tracers were obtained from the Radiochemical Centre, Amersham, England and from the Union Carbide Nuclear Company, Oak Ridge, Tenn., U.S.A. The lead-212 tracer was prepared from thorium in equilibrium with its decay products, by adsorbing the lead, bismuth, thallium and polonium on a column of De-Acidite FF (100-200 mesh) anion-exchange resin. After the column had been washed with 2M hydrochloric acid, the lead-212 was eluted with distilled water.’ Preparation of lead amalgam The lead metal was washed with 2M nitric acid until etched, washed with distilled water and dried. The appropriate weight of lead was added to 20 g of purified mercury stored under 0.1 M nitric acid, heated and stirred.

Exchangeprocedure Lead-212 tracer was added to 4 ml of 1M hydrochloric acid solution for yield determination and interference studies. Radioactive tracers (lV-108 cpm) of contaminating ions plus 1 pg of inactive lead were used for decontamination studies. Lead amalgam (200 ~1 H 2.6 g) containing 2 % lead by weight was added and the mixture was mechanically agitated for 3 min. A 200 ~1 aliquot was removed for counting and the aqueous phase was removed by suction. The amalgam was transferred to another bottle and washed twice with distilled water. The amalgam was then transferred to a bottle containing 2 ml of copper(H) nitrate solution in 1M nitric acid (25 mg of copper per ml) and agitated for 8 min. A 500 ~1 aliquot of the aqueous Iayer containing the separated lead isotopes was counted in a scintillation well-counter. The procedure requires about 13 min at room temperature and gives a yield of about 8@5 %. Separation of lead-212 from thorium nitrate Lead amalgam (2 %, 200 ~1) was added to 5 ml of 50 % thorium nitrate solution in 1Mhydrochloric acid and agitated for 2 min at 40”. The aqueous layer was removed by suction and the amalgam was transferred to another bottle and washed twice with distilled water. The amalgam was then mixed with 2 ml of lead nitrate solution in 1M nitric acid (100 mg of lead per ml) and agitated for 5 min at

40”. The solutioncontainingseparatedlead-212wastransferred to a new bottle and 10pg of thallium carrier were added. Then 200pl of 2 % thallium amalgamwere added and the mixture agitated for one min at 40”to remove thallium from the solution. Total time of separation is about 12 min, and a yield of about 66% is obtained. DISCUSSION AND RESULTS In the search for a suitable exchange medium, the amalgam was agitated in various systems. In general, the acid media were found to permit maximum exchange. With 2 % lead amalgam and a 3-min contact time exchange yields of 93.2 %, 90.1% and 86.3 % were obtained for 1M solutions of hydrochloric acid, hydrobromic acid and nitric acid. When the amalgam was agitated in 1M sulphuric acid, a white turbidity due to the formation of lead sulphate was observed. Further experiments with different concentrations of hydrochloric acid showed that maximum exchange is obtained in 0*7--15M hydrochloric acid media, and 1M hydrochloric was therefore selected as the exchange medium. In order to determine the optimum separation procedure several preliminary experiments were performed in which a number of factors which affect the exchange such as the concentration of lead in the amalgam and in the aqueous phase, the agitation time and the effect of temperature were studied and optimized. As the amount of lead in the amalgam was gradually increased from 0.1 to 2% the exchange yield rapidly increased from 58.5 % to 93.2 %. Further increase in the concentration of lead amalgam did not show any significant increase in the yield. Studies made with different volumes of a 2% lead amalgam gave the best yields with 200 ,ul portions of the amalgam. Because for high exchange it is necessary that the concentration of lead in the amalgam should be much greater than its concentration in the aqueous phase,

Separation of lead

325

experiments were carried out to determine the maximum usable aqueous concentration of lead. In these experiments the aqueous concentration of lead was varied, the lead concentration in the amalgam being kept at 2 %. It was found that above an aqueous concentration of 125 ,~g of lead per ml the exchange starts decreasing. Further studies showed that the exchange decreased if more than 5 ml of aqueous phase were used, and the optimum ratio of the volumes of aqueous phase and amalgam is 25. The exchange yield depends upon the duration of agitation, increases rapidly with increasing agitation time and begins to level off at 34 min. On prolonged agitation, the exchange gradually decreases, probably because of oxidation of lead from the amalgam by air trapped in the bottle above the solution. The results are shown in Table I. TABLE L-DEPENDENCE OF LEAD YIELD ON AGITATIONTIME AND TEMPERAW*

Agitation time, mill 1 2 3 4 5 6 7 1:

Initial exchange, %

Back-exchange in copper nitrate system, %

Back-exchange in lead nitrate system, %

24°C

40°C

24OC

40°C

24°C

40°C

54.8 85.5 93.2 96.4 97.8 96.9 96.4 96

92,4 96.8 97.9 97.6 96.8 -

6;7

946 81.4

-

96.8 98.3 98.4 -

42.6 50.8 65.3 70.6 71.0 72.0

38.8 SO.4 61.5 72 72 -

9;2 -

80.3 88.5 9;1 95.0 96.0

* In these experiments 200 ~1 of 2 % lead amalgam were used. The back-extracting solutions contained 25 mg of copper or 100 mg of lead per ml.

In the initial exchange most of the radioactivity is transferred to the amalgam in a short time. The next step is then to reclaim this activity from the amalgam into an aqueous solution by selectively stripping the element. Of the several solutions tried for back-exchange, only lead nitrate and copper nitrate solutions in 1M nitric acid were efficient. The back-exchange increases with increasing concentrations of lead and copper in the solutions and above a certain concentration no more increase is observed. Therefore, solutions containing 100 mg of lead or 25 mg of copper per ml of 144 nitric acid were chosen as back-extra&ants. With 8-min agitation backexchange yields of 72% and 95% respectively were obtained when lead nitrate and copper nitrate were used. The dependence of back-exchange yield on the duration of agitation is shown in Table I. With increasing agitation time the yield increases and starts leveling off at 7-8 min agitation. Therefore, in the optimum separation procedure a 3-min agitation for the initial exchange and an 8-min agitation for the back-exchange were used. Both the initial and the back-exchange reactions were studied at 40”. The results are given in Table I. The exchange reactions proceed somewhat faster at high temperature and a relatively shorter time is required to attain equilibrium. Thus at 40” the agitation times for the initial and the back-exchange can be reduced to 2 and 5 min respectively. In the interference studies it was found that the exchange yield is not affected by

326

I. H. QURES~ and F. I. NAGI

alkalies in concentrations up to 1M. Acids such as sulphuric, phosphoric and oxalic below O.lM concentrations do not interfere whereas sulphites and oxidizing agents do interfere in the exchange. It is therefore necessary that the oxidizing agents should be reduced to their lowest oxidation state before the amalgam exchange. Decontamination The selectivity of this method was checked by making decontamination studies with radioactive tracers of a number of elements representative of the periodic table. The results are summarized in Table II. The elements have been listed in the order T-m

II.dEPARATION OF LEAD AND cONTAMINANrS+~

Separation, % Tracer

Weight? Pg C.F. &. C.F. 13 CF. C.F. C.F. 1.4 0.3 C.F. 2;. C.F. 1.3 C.F. 16.1 0.6

Electrode potential% V

TI -2.92 -2.9, -2.89, -248, -1.53, -0,16 -0.40 -0.34 -0.28 -0.28 to.21 +0.32 +0.32 t0.61, to.77 to.79 to.80

-2.52 -2.34 -247 -1.1

+0*25

Exchange step 0.018 0.01 0.003 OGOl 0.005 0.15 0.05 2.1 4.4 0.036 93.2 0.98 95 95 1.15 28.6 24.4 77.8

Back-extraction step

57.1 0.02 68 95 2.91 84.2 0.001 1.1 0.84 0.41 0.047

Total separated, % <0.014 <0.02
* Average of duplicate runs. Lead is average of 7 runs, standard deviation 1.9 %. t Weight of inactive element present before separation. C.F. = carrier-free. $ Standard electrode potential for lowest stable oxidation state. Data taken from Latimer? 7 Iodine in its lowest reduced state.

of their reduction potentials and the data have been subdivided to show the selectivity in the amalgam exchange step and in the back-extraction step. It is observed that the elements above lead (in Table II), with the exception of thallium and cadmium, do not contaminate the initial exchange step within the sensitivity of the experiments i.e., with the amount of tracer used (105-lo6 cpm), whereas the other elements do contaminate the initial exchange step, probably because their ions are reduced by lead amalgam. With the exception of bismuth these activities remain in the amalgam and are not extracted when copper(I1) nitrate solution is used for selective removal of lead from the amalgam. Bismuth, however, is extracted together with lead and causes considerable contamination. Several unsuccessful attempts were made to reduce this contamination by bismuth. It was finally reduced to less than 0401% when lead nitrate solution was used for back-extraction; this exchanges with the lead without oxidizing bismuth from the amalgam. Although the back-exchange yield is only 72 % with lead nitrate solution, it gives very good separation of lead from bismuth.

Separation

of lead

327

The simple amalgam exchange procedure was applied to the separation of lead-212 from thorium nitrate. The gamma-ray spectrum of the separated sample showed only about 3 % contamination by thallium-208. To reduce this contamination an additional step was added. The separated sample was agitated with a 2 % thallium amalgam for 1-2 min to exchange thallium selectively from the solution. By this treatment the contamination by thallium was reduced to less than 1%. The amalgam exchange technique for the radiochemical separation of lead is simple and rapid and appears to have useful potential for the separation of shortlived isotopes of lead. The yield can be further increased by shaking longer, and the time of separation can be reduced by performing the exchange at high temperatures. Although very small amounts of lead can be separated by this technique, the fkutl sample counted contains a large amount of inactive lead, and self-absorption corrections may be necessary if beta-counting is used. Acknowledgements-Some of the preliminary experiments were performed by S. Mahmud Ahmad. This work was supported in part by the U.S. National Bureau of Standards, under International Research Grant Programme, Contract No. NBS(G)-46. Zusammenfassung-Ein Amalgam-Austauschverfahren wurde zur schnellen radiochemischen Abtrennung von Blei benutzt. Eme Anzahl Variabler, die den Austausch beeinilussen, wurde studiert und eine Bleiausbeute von S8,S”A erzielt. Bei Zimmertemperatur dauert das Verfahren etwa 13 min, wlhrend es bei 40” in etwa 9 min beendigt werden kann. Radioaktive Tracer von 16 verschiedenen Elementen wurden benutzt, urn die Selektivitiit des Verfahrens zu priifen, das such zur Abtrennung von 814Pb aus Thoriumnitratlosung angewandt wurde. R&nn&On a utilid une technique d’&ange par amalgame pour la separation radiochimique rapide du plomb. On a 6tudi6 un certain nombre de variables qui atfectent l&change et obtenu un rendement en plomb de 88,5x. A temperature ordinaire, la technique necessite environ 13 mn, alors qu’a 40” elle peut i%trer&ili& en 9 mn environ. On a utilise les traceurs radioactifs de 16 elements differents pour v&ifier la selectivite de cette methode, qu’on a aussi appliquQ B la separation du *lrPb dune solution de nitrate de thorium. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

J. R. DeVoe, H. W. Nass and W. W. Meinke, Anal. Chem., 1961,33,1713. R. R. Ruth, J. R. DeVoe and W. W. Meinke. Tufunta, 1962.9,33. I. H. Qureshi and W. W. Meinke, ibid., 1963,.10,737.F. E. Orbe. I. H. Oureshi and W. W. Meinke. Anal. Chem.. 1963.35. 1436. I. H. Qureshi and W. W. Meinke, Radiochim.‘Acta, 1963, i, 99. . ’ J. R. DeVoe, C. K. Kim and W. W. Meinke, Talanta, 1960,3,298. T. T. Gorsuch, Analyst, 1960,85,225. W. Latimer, The Oxidation States of the EIements and Their Potentials in Aqueous Solutions, 2nd Ed., Prentice-Hall, New York, 1952.