Separation of Calcium and Strontium by Liquid ion Exchange Determination of Total Radiostrontium in
Milk
F. E. BUTLER Savannah River Plant, E , 1. du Pont de Nemours and Co., Aiken, S. C.
b A rapid liquid ion exchange method is described for separciting quantities of calcium from strontium in dilute acid solution. Separate extractions were made with di-2-ethylhexyl phosphoric acid (HDEHP) dissolved in toluene after stepwise pH adjustments of the aqueous solution. Calcium, strontium, and other cations in 250 ml. of milk were first concentrated in 60 ml. of dilute HCI solution by a batch ion exchange resin technique. After separation of calcium and radiostrontium by the liquid ion exchange method, and SrgOwere stripped from the and final organic phase with 3N " 0 3 precipitated from fuming nitric acid. The counting planchet contained only 2% of the calcium and 85 f 5% of the strontium in the original milk sample. Decontamination factors for Ygo, Cs13', K", La140,and wei'e greater than 1Os. Ba,I4O if present, 'can be removed by homogeneous plrecipitation of barium chromate. in analysis of milk and other biological samples for total radiostrontium is the removal of the large quantities of calcium before counting. While S r s can be determined by carrier-free separaticm and counting of the Yga daughter ( 2 ) , Srf19must be counted directly in combination with SrB3. The solid material on the planchet must be small to prevent absorption of the low energy SrgObeta particles during counting. The clzssical fuming nitric acid method of Willard and Goodspeed (8) is frequently used to separate W 9and SrS? from calcium. Although this inethod is rapid and 3ffers good de(iontamination factors 'or other radionuclides, it is not conveiiient for separation of strontium from the large quantities of calcium found i,i milk. Use of solid cation exchange resins results in good separation, providel that only trace quantities of calcium me present (6). Weiss and Shipman (7) reported the selective precipitation of strontium as the rhodizonate and this procedure was adapted a t the Savannah River Plant for use with several ty>,es of biological MAJOR PROBLEM
samples (1). However, recovery of strontium from milk by this procedure is low. Liquid ion exchange is a relatively new technique that shows promise in many chemical separations, and its technology has been summarized in two recent reviews (3, 4). Despite successful use of liquid ion exchange in experimental and pilot plant separations (9), a review of extraction procedures (6) showed that the method has not been widely exploited for analytical purposes. Coleman, Blake, and Brown (3) stressed the potential usefulness of liquid ion exchange in analytical chemistry, indicating that most of the elements might be separated by one or more of 55 anion and cation exchange reagents. The purpose of this study was to develop a rapid method for separating large amounts of calcium from strontium, to, accurately, determine total radiostrontium, and to obtain an approximation of strontium-90 content. EXPERIMENTAL
Tests with Aqueous Solutions. Pre-
liminary experiments were made using dilute aqueous solutions. Fourteen radionuclides a t levels of 106 d.p.m. or higher were separately prepared in 0.08N HCl solution and extracted with 5 and 20% (0.16 and 0.64M) di - 2 - ethylhexyl phosphoric acid (HDEHP) dissolved in toluene. Various amounts of NH40H (28 to 30% "3) were added to the aqueous solutions to attain pH's of 1 to 5 (equilibrium pH) after 1 minute of shaking. Aqueous and organic volumes were 60 ml. Aliquots of one or both phases were counted as the liquid in a gamma well counter or evaporated on stainless stecl planchets and counted in a beta counter. A standard pH meter was used. Of the two exchange media conccntrations tested, 20'70 H D E H P displayed better exchange characteristics. Only Zr-K'bgS, FeS'J, and 8 1 1 6 5 exchanged to the 5Y0 organic phase a t pH I . Results of initial tests with 20% H D E H P are shown in Table I. Greater than 957, Ca45was exchanged a t p H S compared to 33% Sr85. The rate of exchange of Sr85 is of interest. Tests showed that at p H 5, greater than 99% of the nuclide was
exchanged from aqueous solution to the organic phase in a 5-second shaking period. Strontium-85 was also rapidly and quantitatively stripped from 20% H D E H P with an equal volume of 3N
HNOa. Additional tests were made with samples containing Ca46 and SF5. Aaueous-to-organic volume ratios were v&ed from 1rl to 1:2 using 5, 20, and 40% H D E H P . Results for 5 and 20% HD%HP are summarized in Table If, Although there was not complete separation of the radionuclides in one extraction, comparisons were made of the amounts of Ca45 exchanged with 10 and 15y0 of the Srf15. The optimum conditions resulted when equivolumes of aqueous and 20% HDEHP solutions were used. Forty per cent H D E H P showed separations similar t o those of 20% solution and are not shown. The complete data for the optimum Ca45and Sr85 separation are plotted in Figure 1, showing exchange of 94% Ca46and 10% Sr*b a t equilibrium pH 2.8. These exchanges were reversible on addition of ",OH or HC1. Similar separation mas achieved from a sample containing 200 mg. of calcium carrier and Sr*9 tracer. Calcium was exchanged first at pH 2.8. Strontium89 was then exchanged to a fresh solution of 20% H D E H P a t p H 5 . The Sr89 was stripped from the organic solution nith 31V "03, the acid was evaporated, and the residue ws
Table I. Extraction of Radionuclides from Aqueous Solutions Per cent exchanged t o 20% HDEHP phase Measured equilibrium pH of aqueous solutions Nuclide pII 1 p H T pH 4 pH 5 95 100 100 SP 99 Cs-Ba137 complex fract o m but often fail\ to give pure individual cornponent-. For example, bcr d(a)anthraceiic and chrysene are not ne11 resolved nor are benzo(a)pyrene, bens?(e)pyrene, and benzo(k)-fluoranthene. Further separations can sometimes be achieved by partition chromatography on payer or by gas-liquid chromatography. The mAHAiwN
latter methods are usually limited to the use of rather small quantities. The use of solvent extraction for the purification of polycyclic aromatic compounds has been reported by several workers. Steidle (14) utilized nitromethane to extract this type of compound from pentane. More recently, nitromethane has been used for the extraction of polycyclic aromatic hydrocarbons from cyclohexane solutions of cigarette smoke condensates (9, 11). Other solvents evaluated by Haenni, Howard, and Joe (10) are acetonitrile, dimethylformamide, and methylsulfoxide, which were applied t o the removal of polycyclic aromatic compounds from heptane solutions of paraffin wax. These solvents are useful for the separation of unsubstituted polycyclic aromatic hydrocarbons as a class but do not selcctivcly separate individual comJ)ounds. Golumbic (8) qtudied several solvent pairs to obtain a system suitable for the separation of individual polycyclic aromatic compounds by countercurrent distribution. These solvent systems included cyclohexane: 80% ethanol, cyclohexane: 98% acetic acid, benzene:
80% acetic acid, iso-octane: Si% ethanol and n-heptane: aniline. H e concluded that the first of these n a s t h e most selective for iiomeric methylnaphthalenes. Relatively little separation was observed for unsubstituted polycyclic hydrocarbons. In our approach to this problem, we felt that t h e use of a complexing agent selective for these compounds would be desirable. The solubilization of polycyclic aromatic hydrocarbons in water by purines had been demonstrated by Brock, Druckrey, and Hamper1 (4). Certain hydrocarbons of this elas$, including benzo(a)pyrene, were solubilized t o a greater extent than were others. Weil-Malherbe carried out extensive studies with several purines (16). H e found that tetramethyluric acid was the most effective of the several purines which he tested for dubilization of I)cnzo(a)pyrene. Wanlcss ( I 6 ) utilized aqueouq caffeine solutions to prepare a polycyclic aromatic concentrate from high boiling petroleum products. Solubilities of several carcinogenic aromatic amines were shown by Seish ( I S ) to be greatly increased by the addition of caffeine or tetramethyluric acid. SirniVOL. 35, NO. 13, DECEMBER 1963
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