Radiochemical determination of cobalt-60 in environmental samples

Radiochemical determination of cobalt-60 in environmental samples

0039-9140/82/100871-03$03.00/0 Copyright © 1982 Pergamon Press Ltd Ta/alita, VoL 29, pp. 871 to 873. 1982 Printed in Great Britain. Ail rights reserv...

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0039-9140/82/100871-03$03.00/0 Copyright © 1982 Pergamon Press Ltd

Ta/alita, VoL 29, pp. 871 to 873. 1982 Printed in Great Britain. Ail rights reserved

RADIOCHEMICAL DETERMINATION OF COBALT-60 IN ENVIRONMENTAL SAMPLES* C. D.

JENNINGS

Western Oregon State College, Monmouth, OR 97361, U.S.A. and

T. M. BEASLEY Oregon State University, Marine Science Center, Newport, OR 97365, U.S.A. (Received 20 Decemher 1981. Accepted 13 April 1982)

Summary-A procedure for the radiochemical determination of 60Co in low-activity samples of sediment and biological material is described. Cobalt recovery is high and decontamination from tervalent lanthanides and naturally-occurring radionuclides is complete. Cobalt is precipitated with I-nitroso-2naphthol, decontaminated from iron by precipitation of the iron as ferric phosphate, extracted into methyl isobutyl ketone, and finally precipitated as cobalt mercury(II) thiocyanate for yield determination and beta-counting.

Decreasing concentrations of radiocobalt (principally 60Co) in the environ ment during the past twenty years have made its accu rate measurement in natural materials increasingly difficult. Reliance on NaI(TI) gamma-ray spectrometry, a technique that worked weil wh en activities of radiocobalt were much higher than those of naturally-occurring gamma-emitters in the environment, is no longer of sufficient accuracy now that the isotopes of cobalt are often masked by the spectrum of natural radioactivity. Nor does the refinement of Ge(Li) detectors solve this problem: while affording much better resolution than Nal(TI) detectors, the efficiencies of Ge(Li) for the high-en'ergy gamma-ray transitions of 60Co (1.17 and 1.33 MeV, respectively) are low and make its use for quantification of trace amounts of this radionuclide questionable. The solution lies in the radiochemical separation of cobalt from interfering stable and radioactive elements, thus allowing the 60Co to be determined by low-Ievel beta-counting techniques, Previously published radiochemical separation procedures for cobalt radioisotopes have emphasized purification from highly radioactive fission-product or neutron-activation product matrices. 1- 4 In these instances, neither high chemical yield nor separation from natural radioactivities is important. The sa me cannot be said, however, for the determination of low levels of radiocobalt in environmental samples, This paper describes a simple, effective method of extracting cobalt from sediment and biological matrices in a form suitable for the 60Co beta-emissions to be counted, The advantages of the separation scheme described are (1) cobalt is isolated from a wide range of elements existing in natural sampi es, by pre-

ClpItation with I-nitroso-2-naphthol; (2) the difficult isolation of cobalt from iron is accomplished by precipitation of ferric phosphate; (3) it is the first procedure to confirm decontamination from residual fallout and naturally-occurring radionuclides present in environmental materials today. It should be mentioned that nuclear power plants produce more 57CO and 58CO than 60Co. In the radiocobalt analysis of samples collected from such locales, the purified cobalt fraction should be counted on a gamma-ray spectrometer to determine the isotopic composition of the sam pIe.

*Supported by the U.S. Department of Energy under contract DE-AT06-76EV70030.

250-ml plastic beaker, add 1 ml of cobalt yield monitor, 50 ml of 8M nitric acid, 50 ml of concentrated hydroftuoric 871

EXPERIMENTAL

Reagents

Use reagent grade chemicals unless otherwise specifie d, and use doubly distilled water in the preparation of ail solutions and in steps requiring water washes. Ammonium mercury(II) thiocyanate. Dissolve 316.8 g of mercury(II) thiocyanate, in 250 ml of 1M ammonium chio ride. Cobalt carrier and yield monitor (Co 10 mg/ml). Dissolve 4.04 g of cobalt chloride hexahydrate in water and dilute to volume in a lOO-mi standard ftask. The solution can be standardized by precipitating the cobalt in al-ml aliquot with sodium hydroxide, centrifuging the precipitate and then washing it with water, transferring it with ethanol to a 2.5-cm stainless-steel planchet and drying it under a heat lamp. The weight of cobalt hydroxide obtained is reproducible. I-Nitroso-2-naphthol solution. Dissolve 10 g of 1-nitroso-2-naphthol in 100 ml of glacial acetic acid. Trisodium phosphate solution. Dissolve 23.05 g of trisodium phosphate (l2-hydrate), in 1 litre of water. Procedures Sediment dissolution. Weigh 10 g of sediment into a

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acid and evaporate to dryness under a heat lamp. Add about 100 ml of 6M hydrochloric acid and evaporate to dryness again. Transfer the residue to a glass beaker with 6M hydrochloric acid; add 100 ml of 6M hydrochloric acid to the plastic beaker along with 1 g of boric acid, warm under a heat lamp until the boric acid is dissolved, and then transfer to the glass beaker. Evaporate on a hot-plate to 50 ml and th en jEter through S&S white-band filter paper. Put the filter back into the glass beaker, add 10 ml of concentrated nitric acid and 10 ml of concentrated perchloric acid, boil till fumes of perchloric acid appear and filter through a glass fibre filter, combining this filtrate with the previous one. Retain the filtrate for cobalt determination. Total dissolution is preferred to leaching, because Duursma et al. 5 hilve shown that the longer clay sediments are in contact with a solution containing radioactive cobalt, the greater is the fraction that is not extracted by weak leaching agents. Moreover, 6M hydrochloric acid has been found to remove only 75-85% of the 55Fe and 65% of the stable iron from Columbia River sediments," and cobalt would be expected to behave similarly. Dissolution of hiological material. Place a known weight of dried biological sample in a beaker, add 1 ml of cobalt yield monitor, slowly bring the temperature up to 450 in a muffie furnace, and ash the sample for 24 hr or until a light grey ash forms. Cool the sample, dissolve the residue in 100 ml of 6M hydrochloric acid, then reduce the volume to 50 ml on a hot-plate. Filter through S&S white-band filter paper, and continue as described above for sediments, from the same stage in the procedure. Cohalt determil1atiol1. Dilute the filtrate from the dissolution procedure until its acid concentration is lM. Add approximately 2 g of hydroxylamine hydrochloride. Bring almost to the boil, add 7 ml of the 10% 1-nitroso-2-naphthol solution and boil for 2 min. Filter off on S&S whiteband filter paper, wash several times with water and disca rd the filtrate. Place the filter and precipitate in a 250-ml beaker, add 10 ml eaqh of concentrated nitric and perchloric acids and heat to fumes of perchloric acid to destroy the precipitate. Transfer to a 50-ml centrifuge tube, dilute to 20 ml and precipitate cob alto us hydroxide with sodium hydroxide. Centrifuge, discard the supernatant solution and wash the precipitate with water. Dissolve the precipitate in a minimum of concentrated hydrochloric acid with heating in a water-bath (80°). Dilute to about 20 ml with water. For each 20 mg of iron present, add 5 ml of O.1M trisodium phosphate. Adjust to approximately pH 5-6 with ammonia solution, then to pH 3-3.5 by adding 1 ml of glacial acetic acid. Heat in a boiling water-bath to coagulate the precipitate of ferric phosphate. Filter off on S&S white-band paper, collecting the filtrate in a clean 50-ml centrifuge tube, and wash the precipitate three times with hot, dilute (2.5% v/v) acetic acid. Discard the precipitate. Precipitate cobaltous hydroxide from the combined filtrate and washings with sodium hydroxide, centrifuge and wash the precipitate with water. Dissolve the precipitate in a minimum of concentrated hydrochloric acid (2 or 3 drops) with heating in a hot water-bath, dilute to 10 ml with water and add 2 ml of concentrated ammonia solution. Filter into a 125-ml separatory funne\. Discard the filter. Add 2 ml of glacial acetic acid, 20 ml of 25% ammonium thiocyanate solution and 20 ml of methyl isobutyl ketone (MIBK) to the separa tory funnel and shake it for 5 min. Allow the phases to separate and discard the aqueous phase. Add 20 ml of 25% ammonium thiocyanate solution and shake for 1 min. After phase separation, discard the aqueous phase. Repeat this washing step. Strip the cobalt by shaking with two 5-ml portions of water for 30 sec each time. Combine these two aqueous extracts in a clean 50-ml glass centrifuge tube. Add 3 drops of concentrated hydrochloric acid. Heat nearly to boiling and add 2 ml of lM ammonium mercuric thiocyanate. Cool in an ice-bath and 0

stir vigorously with a glass rod until precipitation of the blue cobalt mercuric thiocyanate is complete. Centrifuge, and discard the supernatant solution. Wash the precipitate with co Id water, then with cold ethanol, discarding the washings. Add 1 ml of ethanol, shake to obtain a suspension of the precipitate and transfer this with a Pasteur pipette to a weighd 2.5-cm stainless-steel planchet. Rinse the centrifuge tube with 250 pl of ethanol and add this immediately to the planchet to ensure an even coating with the precipitate. Dry under a heat lamp and weigh as Co[Hg(SCN)4J (11.98% cobalt). Count on a low-background beta-counter, making appropriate corrections for chemical yield, self-absorption and efficiency.

RESUL TS AND DISCUSSION

A radiochemical procedure for cobalt can be in error if it fails to separate the cobalt from major stable element interferences or if it is not sufficiently specifie and contaminating radionuclides are isolated along with the cobalt. Part of the procedure described here has been shown to decontaminate cobalt from reactor fission products and activation products in samples cooled for only 5-24 hr,1 but no consideration was given to separation of cobalt from environmental samples or its isolation from natural radionuclides. Present-day environmental materials, such as Columbia River sediments, are much more likely to be contaminated by transuranic radionuclides (Pu and Am), and long-lived fission and neutron-activation products such as 137CS, 55Fe and tervalent lanthanides, especially radionuclides of Eu. This is evident in the work of Robertson et al. 7 and from the radionuclides certified by the National Bureau of Standards (NBS) for certain of their standard reference materials discussed below. As shown in Table 1, the extraction by MIBK is completely selective for separation from tervalent 152Eu; the slightly higher count-rate for 152Eu in the aqueous phase after the MIBK extraction is due to the slight decrease in volume of the aqueous phase. The efficiency of decontamination from naturallyoccurring radionuclides was confirmed by extracting cobalt from deep-sea sediments which measurements of 239. 240 pU activity had previously shown to be below the depth of man-made radioactivity. Replicate analyses produced count-rates indistinguishable from the background, The completeness of decontamination of cobalt from naturally-occurring radionuclides is further supported by the fact that our results on the Columbia River sediment standard reference material supplied by the NBS (Table 2) are in excellent agreement with the certified value. This material also contains substantial amounts of 228Th, 230Th and 232Th in radioactive equilibrium with their daughter products, as weil as 137CS, 152Eu and 55Fe, A 10-g sample was used for the sediment analysis because the amount available is often limited, but the procedure should be applicable to samples of up to 50 g. The procedure is applicable to the analysis of both sediments and biological materials (Table 2), attesting

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Table 1. Decontamination of Co from l52Eu by MIBK extraction l52Eu activity of aqueous fraction, cpm

Sam pie

Before Co extraction

After Co extraction

l52Eu remaining in aqueous fraction, %

1 2

877 ± Il 889 ± Il

904 ± Il 897 ± Il

\O3±2% 101 ± 2%

Table 2. Cobalt-60 determinations in reference mate rial

Sam pie

Measured activity, dpm/g

Accepted activity, dpm/g

0.28 ± 0.03t 0.27 ± 0.02t

0.28 ± 0.01 0.28 ± 0.01

3.68 ± 0.05t

3.65

32.9 ± 0.7 33.1 ± 0.8

33.1 ± 0.1 33.1 ± 0.1

Columbia River Sediment*

Vegetable Meal

Standard~

Lake Sedimentj

*Standard Reference Material 4350B from National Bureau of Standards. tStandard deviation calculated from sam pie and background count-rates and are 1" confidence intervals. §EMRM-VM-l from Environmental Measurements Laboratory BERU lntercalibration StudyB (no error terms reported). tStandard Reference Material from National Bureau of Standards (reference number not yet assigned). Errors about the measured values are calculated from the propagated sam pie and background count rates, and are 1" confidence intervals. to its utility in the analysis of environmental materials. When the procedure was traced with 60Co, 92 ± 2% of the cobalt was found to be carried through the extraction with MIBK, and the recovery of Co[Hg(SCNl 4 J was 95 ± 5"!." giving an overall chemical recovery of 86 ± 5%. With care and familiarity, the entire procedure should easily pro vide cobalt recoveries of at least 75%. Sorne notes from our testing of this procedure may be helpful in circumventing problems that occur if this analysis is attempted in series with other radiochemical separations. First, cobalt is not collected effectively by either calcium oxalate or calcium sul-

phate from dilute acid solutions. Secondly, a substantial but variable proportion of the cobalt is co-precipitated when hydrous ferric oxide is precipitated with ammonia as suggested by Marsh and Maeck. 1 To separate cobalt from iron we used Young and Hall's9 procedure precipitating ferric phosphate, which we found co-precipitated only 3/r, of the cobalt. Finally, the precipitate of Co[Hg(SCNl 4 J will not form easily unless the volume is kept low and 3-4 mg of cobalt are present in the sam pie. If a plastic centrifuge tube is used for this step, a substantial amount of the precipitate adheres to the walls and cannot be recovered. In our experiments, 55 7% of the cobalt was recovered in plastic tubes and 95 ± 5% in glass tubes.

±

REFERENCES

1. S. F. Marsh and W. J. Maeck, Talanta, 1962, 9, 285. 2. R. B. Hahn and D. L. Smith, ibid., 1961,7, 291. 3. W. H. Burgus, Col/ected Radiochemical Procedures, 1. Kleinberg (ed.), pp. 66-72. Atomic Energy Comm. Rept. LA-I721 (Rev.), 18 Nov. 1955. 4. L. C. Bate and G. W. Leddicotte, The Radiochemistry of Cobalt, National Academy of Sciences NAS-NS 3041, Sept. 1961. 5. E. K. Duursma, R. Dawson and J. Ros-Vincent, Thalassia Jugoslavica, 1975, 11, 47. 6. C. D. Jennings and W. M. Jones, in Studies on the Concentration of55Fe in South Pacific Ocean Water and Marine Organisms and in the Columbia River, V.S. ERDA Rept. RLO-2231-TI-ll, 1977, pp. 57-66. 7. D. E. Robertson, W. B. Silker, J. C. Langford, M. R. Peterson and R. W. Perkins, in Radioactive Contamination of the Marine Environment (Symposium Proceedings, Seattle, 1973) pp. 141-158. IAEA, Vien na. 8. H. L. Volchok and M. Feiner, A Radioanalytical Laboratory Intercomparison Exercise, V.S. Dept. of Energy Rept. EML-366, 1979. 9. R. S. Young and A. J. Hall, Ind. Eng. Chem., Anal. Ed., 1946, 18, 262.