(4) Fairman, W. D., U. S. At. Energy Comm. Rept. ANL-6887 (1964). (5) Farabee, L. B., in U. S. At. Energy Comm. Rept. ORNL-3347, p. 149 (1962). (6) Kahn, B., J. Agr. Food Chem. 13, 21 (19135). ( 7 ) Lamanna, A., Yancik, V., Abbott, T. P., Holmes, J., Ifealth Phys. 1 1 , 586 (1965).
(8) Murthy, G. K., in U.S. Public Health Serv. Publ. 999-R-2, p. 7 (1963). (9) Murthy, G. K., Gilchrist, J. E., Campbell, J. E., J. Dairy Sci. 45, No.
9, 1066 (1962). (10) Parsi, E. J., Ioconelli, W. B., U. S. At. Energy Comm. Rept. NP-13620
(1964):
(11) Smith, H., Whitehead, E. L., Nature 199, 503 (1963).
(12) U. K. At. Energy Authority, Prod. Group Rept. 204(W) (1961).
RECEIVEDfor review March 21, 1966. Accepted May 31, 1966. A portion of this work presented at the 1965 American Industrial Hygiene Conference, May 3-7, 1965, Houston, Texas. Work performed under the auspices of the U. S. Atomic Energy Commission.
Determination of Strontium-90 in Seawater after Concentration by Manganese Dioxide WILLIAM H. SHIPMAN U. S. Naval Radiological Defense laboratory, Son Francisco, Calif.
b The adsorption of strontium b y hydrous manganese dioxide was used to concentrate strontium in seawater. The important parameters involved in the concentration process were studied. A complete radiochemical procedure for SrgO in seawater using the concentration method as a first step is presented. The chemical recovery is simplified by the use of Srg5which can b e gamma counted after Yeois milked and beta counted in a low background counter. The procedure yields a good chemical recovery, satisfactory decontamination from radium and other isotopes, and a low reagent blank. This procedure has been applied satisfactorily to a large number of analyses for Srw in seawater.
T
is a need to assay the oceans for strontium-90, among other radionuclides, to evaluate the extent of global fallout and to test the use of these radioisotopes for describing oceanic circulation and mixing. The low levels of strontium-90 in the ocean (particularly a t great depth) lead t o the requirement that large volumes of seawater be analyzed. A radiochemical procedure was devised for the determination of strontium-90 in large volumes of seawater. The strontium is isolated from seawater by coprecipitation with manganese dioxide. Then the strontium is purified through standard radiochemical procedures. The small ratio of strontium to sodium, calcium, and magnesium in seawater complicates the isolation of strontium from seawater. Also, the amount of strontium present is insufficient to precipitate with any of the common reagents. Other investigators (3, 5 ) have coprecipitated the strontium with calcium by the addition of oxalate or carbonate. These methods either present HERE
94 I 35
difficulties in manipulating insoluble precipitates or involve massive amounts of reagents and unwieldy volumes of precipitates, thus increasing the probability that contamination is introduced during manipulation. To obviate these shortcomings, the procedure reported here was developed. It requires smaller quantities of reagents and produces smaller volumes of precipitate. It has been observed that hydrous manganic oxide coprecipitates strontium ( I , 2). When this technique was applied to seawater, it was found that by selective chelation, the magnesium interference could be eliminated. When potassium permanganate is stoichiometrically combined with manganous chloride a t p H 12, the hydrous manganese dioxide that forms from the resultant oxidation-reduction reaction carries the strontium almost quantitatively. EXPERIMENTAL
Apparatus. An International centrifuge was equipped with a 3-liter manganese bronze basket and draining chamber, which allowed the centrifuge to be used as a chemical centrifuge. Reagents. H E D T A (Hydroxyethylethylenediaminetriacetic acid, 41.3% solution as the trisodium salt) was obtained from Chas. Pfizer and Co. Strontium-85 was obtained from Nuclear Science and Engineering Corp., Pittsburgh, Pa. All other reagents were Reagent Grade and were obtained from J. T. Baker Chemical Co., Phillipsburg, N. J. Procedure. The procedure for the determination of strontium in seawater consists of six basic steps: strontium and calcium are selectively chelated with H E D T A and magnesium is removed as the hydroxide at p H 12; strontium is freed from the chelate by the addition of copper sulfate and strontium is coprecipitated with hydrous manganese dioxide. The bulk of the calcium is eliminated by a
sulfate precipitation of the strontium. The rest of the calcium and any rare earth activity are removed by two fuming nitric precipitations of the strontium. Remaining extraneous activities are removed by a ferric hydroxide precipitation, and radium by a double barium chromate precipitation. Added yttrium carrier is precipitated as the hydroxide, then the solution is acidified and yttrium carrier is added. The solution is held for 15 days for the growth of the yttrium-90 prior to final milking and counting. The detailed procedure is as follows: To 60 liters of seawater in equilibrium with Sr85 tracer, 390 ml. of HEDTA are added, the pH is adjusted to 12, and the magnesium hydroxide is isolated by centrifugation and discarded. To the supernatant solution, 81 grams of CuSO4.5H20, 57 grams of KaOH, 48 grams of K?vln04, and 90 grams of AMnC12.4H20are added in that order. Solution should be complete before each addition. The solution is stirred for 3 hours and the precipitate allowed to settle overnight. MnOs is isolated by centrifugation and dissolved in a minimum volume of concn. HC1. SO2 gas is bubbled through the solution until solution is complete. The volume is adjusted to 2.5 liters and 4.5 grams of SrCle.6Hs0 are added. Fifty grams of K;a2S04 and 200 ml. of ethanol are added. and the solution is allowed to settle. The supernatant liquid is decanted and discarded. The precipitate is isolated by centrifugation, metathesized to the carbonate with saturated sodium carbonate, and again isolated by centrifugation. The precipitate is dissolved in the smallest volume of 2.1N HNOI possible. Three milliliters of fuming H N 0 3 are added for each milliliter of the 2.1N HNOa added. The solution is chilled in ice and the precipitate is isolated by centrifugation. This operation is repeated. The strontium is dissolved in 10 to 15 ml. of distilled water. Ten milligrams of Fe+3 are added and ferric hydroxide is precipitated by the addition of carbonate-free ammonium VOL. 38,
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The supernate saved from the first yttrium precipitation is acidified with concn. HC1 and adjusted to a convenient volume. An aliquot is gammaray counted and compared with a standard. The chemical recovery of the strontium is calculated from these data. RESULTS AND DISCUSSION
To establish the conditions of the procedure, four parameters affecting the recovery of strontium from seawater were studied and optimized. The ability of hydrous manganese dioxide to adsorb other radionuclides found in the ocean environment was tested under the conditions for optimum strontium recovery from seawater. The first and perhaps the most critical of the parameters studied was the effect of the pH of the seawater on the recovery of strontium. As can be seen in Figure 1, the recovery of strontium is very sensitive to small changes of pH. To protect the solution from pH changes due to dissolved carbon dioxide, the container should be kept sealed as much as possible once the p H has been adjusted to 12. To find the amount of manganese dioxide needed for the maximum recovery of strontium at p H 12, various amounts of manganese dioxide were
DH
Figure 1.
Effect of pH
hydroxide. The precipitate is coagulated by boiling, isolated by centrifugation, and discarded. The solution is acidified with 6N HNO, to methyl red color change. Twenty milligrams of Ba+2, 1 ml.-of 6M CHd2OOH. and 2 ml. of 6M NH4C2H;O2 are' added, and the solution is heated to boiling. One milliliter of 1.5M Na&rOc is added. The precipitate is isolated by centrifugation and discarded. To the solution, 20 mg. of Ba+* are added and the solution is heated to boiling. The precipitate is isolated and discarded. Strontium is precipitated by the addition of sodium carbonate and the precipitate is isolated by centrifugation. The precipitate is dissolved in the smallest volume of 1N H N 0 3 possible and the solution boiled to expel carbon dioxide. Twenty milligrams of Y+3 are added and yttrium hydroxide is precipitated by the addition of carbonate-free ammonium hydroxide. The precipitate is isolated and discarded. The date and time of this separation are needed to calculate yttrium growth. The solution is acidified with 1N "Os, 20 mg. of Y+3 carrier are added, and the solution is held for yttrium-90 growth. After not less than 15 days, the yttrium is precipitated by the addition of carbonate-free ammonium hydroxide. The precipitate is isolated and the supernatant solution saved for chemical yield determination by counting the
on recovery
washing with 5 ml. of water and 5 ml. of ethanol. The precipitate is dried 10 minutes at 100' C. and weighed BS Yz(C20J3-9H20.
96
48
36 32
Srs5.
Y(OH)3 is dissolved in 2 ml. of 1N HC1 and 4.3 ml. of 2.1N HNOa are added. The solution is heated to boiling and 15 ml. of a saturated solution of ammonium oxalate are added. Digestion for 20 minutes, cooling, and filtering the precipitate are followed by 1176
ANALYTICAL CHEMISTRY
t
28
I-
24l 0
Figure 2. recovery
'
10
'
'
I " I I I I I 20 30 40 50 60 70 80 90 100 110 Mg Mn02/100 ml SEA WATER
Effect
of the
:0
amount of MnOz used on strontium
Table 1. Effect of HEDTA Concentration on Strontium Recovery
l-
z W V
a a W
&W
9
swa
Stoichiometric ratio of amt. of HEDTA amt. of strontium and calcium in seawater vol.
Strontium recovery, yo
0.5 1.0 1.5
100 80.5 36.5
2
Table II. Effect of Volume of Seawater on Recovery of Strontium (through the initial concentration steps with MnOz) HOURS OF STIRRING
Figure 3. Effect of time of stirring on strontium recovery
introduced and the recovery of the strontium was measured (Figure 2). Based on the desire to introduce the minimum amount of reagent with maximum recovery of the strontium, a concentration of 110 mg. of MnOs per 100 ml. of seawater was chosen as the optimum amount. The curve relating the strontium recovery with concentration of manganese dioxide suggests that large increases in the concentration of the dioxide would give little additional increase in recovery. The importance of the time of contact between the manganese dioxide and the seawater was studied. Figure 3 clearly shows that with 110 mg./100 ml. of manganese the bulk of the adsorption of strontium occurs in the first hour and that no significant amounts are added by prolonged stirring. I n fact, a long stirring interval would introduce the problem of p H control against carbon dioxide in the atmosphere. Three hours of stirring was chosen as a compromise. The last of the four parameters studied was the effect of excess HEDTA r e sulting from an insufficient addition of copper sulfate. Table I shows that small errors in the addition of copper sulfate are not important] because an error as large as 1 0 0 ~ in o the amount of HEDTA would result in only a 20% loss in the strontium recovery. Once the procedure has been optimized, the applicability of the procedure to volumes of seawater larger than 100 ml. was tested. Table I1 shows that for samples of up to 2 liters, the manganese dioxide continued to be
very efficient in removing the strontium from the seawater. Table I11 shows the recovery of strontium from larger volumes of seawater using the entire procedure. These values show that the manganese dioxide continued to efficiently adsorb the strontium at larger volumes. The ability of manganese dioxide a t pH 12 to carry other radioactivities that might be expected in a marine environment was tested. Table IV shows that under the conditions of the procedure the manganese would not scavenge other activities quantitatively. However, Yamagata (6) shows that a t p H 8.2 manganese dioxide is quite efficient for these same radionuclides. This procedure has been applied to the successful analysis of a large number of seawater samples from the North Pacific (4). Under conditions a t this laboratory, a stable reagent blank of 0.31 d.p.m. of Sr" per sample has been observed. This chemical recovery has been about 75% (Table 111). The results shown in Table I11 are representative of chemical recoveries obtained on actual samples taken during a survey of the North Pacific. LITERATURE CITED
( 1 ) Egorov, Yu. V., Pushkarev, V. V., Tkochenko, E. V., J . Inorg. Chem. ( U S S R )6, 256, 1961. (2) Egorov, Yu. V., Pushkarev, V. V., Tkochenko, E. V., Radiokhimiya ( U S S R ) 3, 87 (1961). (3) . , Rocco. G. G., Broecker, W. S.,. J . Geophys: Res. 68, 4501 (1963). (4) Shipman, W. H., U. S. Radiological
Vol. of seawater (liters)
Strontium recovery (%)
0.1 1.0 2.0 2.0
90 89.5 92.1 89.8
Table 111. Recovery of Strontium from Large Volumes of Seawater (through the entire procedure)
Vol. of seawater (liters)
Strontium recovery (%)
57 56.6 56.9 57.1 57.0 55.2 55 56
77.2 73.1 74.7 73.8 77.6 72.0 76.3 71.9
Table IV. Isotopes Carried b y Hydrous Manganese Dioxide from Seawater at pH 12
Isotope corn
Cs'3' Ce144,Pula4
Carried, % 48.8 11.0 0.0
Defense Laboratory, San Francisco, Calif., unpublished data. ( 5 ) Sugihara, T. T., James, H. I., Troianello, E. J., Bowen, V. T., ANAL. CHEM:31, 44 (1959). . (6) Yamagata, N., Iwashima, K., Nature 200, 52, (1963).
RECEIVED for review April 18, 1966. Accepted May 31, 1966.
VOL. 3 8 , NO. 9, AUGUST 1966
1177
