Anal. Cham. 1984, 56, 2851-2853
2851
Radiochemical Separation of Cobalt-60 in Seawater Using Continuous-Flow Coprecipitation-Flotation Masataka Hiraide,* Kenlchi Sakurai, and Atsushi Mizuike
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Faculty of Engineering, Nagoya University, Nagoya 464, Japan
Indium solution (100 mg/mL): Indium metal (100 g) was dissolved in 400 mL of 14 M nitric acid and diluted to 1 L with water. l-Nitroso-2-naphthol solution (5 mg/mL): The organic reagent was dissolved in ethanol. Surfactant solution (0.8 mg/mL): Sodium oleate (0.20 g) and sodium dodecyl sulfate (0.60 g) (Extra Pure Reagents, Nakarai Chemicals) were dissolved in 1 L of 70% ethanol. Scintillator solution: 2,5-Diphenyloxazole (PPO) (3.00 g) was dissolved in 1 L of xylene. Scintillation counting grade reagents (Nakarai Chemicals) were used. TOA solution (5% v/v): Tri-n-octylamine (Extra Pure Reagent, Nakarai Chemicals) was dissolved in xylene. Artificial seawater (3): Sodium chloride (3200 g), magnesium sulfate heptahydrate (1400 g), and sodium hydrogen carbonate (15 g) were dissolved in 100 L of water. The reagents used were of reagent grade unless otherwise stated. Recommended Procedure. Place 10 L of filtered seawater in a 20-L poly(ethylene) bottle. Add 50 mL of 12 M hydrochloric acid, 1 mL of cobalt carrier solution, 2 mL of indium solution, and 40 mL of l-nitroso-2-naphthol solution, and mix the solution till the yellowish-brown color of l-nitroso-2-naphthol becomes homogeneous. Add 10 mL of ethanol and 2 L of purified water to the flotation cell while bubbling nitrogen, and adjust the flow rate of nitrogen to 0.5-1 mL cm"2 min"1. Start the magnetic stirrer and peristaltic pumps to feed the sample at a flow rate of 0.5 L/min, aqueous 1 M ammonia at 35 mL/min, and the surfactant solution at 10 mL/min. The pH of the solution in the reaction bottle is between 8.5 and 9. After 2 min, when the reaction bottle becomes nearly full, close the stopcock to transfer the contents into the flotation cell. Switch on the suction pump to suck the floated precipitates and foam into the collection bottle. Coprecipitation-flotation continues for 20 min without attention of the analyst. After completion of sample feeding, switch off the peristaltic pumps and the magnetic stirrer. Suck the precipitates remaining on the solution surface by moving the bent glass tubing of the collection bottle. Take out the poly(ethylene) insert, and collect the adhering precipitates in the collection bottle by spraying with 10 mL of water. Add 10 mL of 6 M hydrochloric acid to dissolve the indium hydroxide precipitates and to rupture the foam. Transfer the solution and 15 mL of chloroform into a 300-mL separatory funnel, and shake the solution for 3 min to extract the cobalt chelate of l-nitroso-2-naphthol. Repeat the extraction twice more with 15 mL each of fresh chloroform. Evaporate the combined chloroform to near dryness, transfer the residue to a 15-mL crucible with a small amount of acetone, and evaporate the residue to dryness on a hot plate. Heat the crucible with a small gas burner flame and then as strongly as possible for 10 min. After the crucible is cooled, dissolve the residue in 5 mL of 12 M hydrochloric acid by heating on a hot plate, and evaporate the solution to dryness. Cool and dissolve the residue in 5 mL of 8 M hydrochloric acid. Transfer a 0.1-mL aliquot to a 5-mL volumetric flask, dilute the aliquot to the mark with water, and determine the cobalt by atomic absorption spectrometry (AAS) at 240.7 nm to obtain the chemical yield. Place the remaining solution and 5 mL of TOA solution in a 50-mL separatory funnel, and shake the mixture for 3 min to extract the cobalt. Collect the organic phase, and repeat the extraction once more with 5 mL of TOA solution. Transfer the combined organic phase into a 20-mL counting vial, and add 10 mL of scintillator solution. Count the activity of “Co for 400 min with a liquid scintillation counter, and correct the results for background, counting efficiency, and chemical yield.
Cobalt-60 and stable cobalt carrier In a stream (0.5 L/mln) of seawater sample were converted Into the 1-nltroso-2naphthol chelate, coprecipitated with Indium hydroxide, and floated with the aid of anionic surfactants and tiny nitrogen bubbles. After removing Indium and 1-nltroso-2-naphthol by liquid-liquid extraction and dry oxidation, the eoCo activity was measured by liquid scintillation counting. The chemical yield was higher than 90%, and the counting efficiency was ca. 80%. For a 10-L sample, the separation required ca. 150 min and the detection limit was 50 fCI/L of seawater. The sample volume can be Increased up to 100 L, which allows detection of as little as 5 fCI/L of seawater.
Cobalt-60 is one of the typical radionuclides in wastewaters from nuclear power plants and has a long half-life (5.27 years) and high concentration factors for aquatic organisms. Although 60Co has been determined by gas-flow proportional counters, liquid scintillation counters, and thallium-activated sodium iodide scintillation counters, liquid scintillation counting is most suitable from the standpoint of both high counting efficiency and easy sample preparation. The “Co concentration is extremely low; hence radiochemical separation from large volume samples is necessary prior to the counting. For example, “Co in 10 L of seawater has been coprecipitated with magnesium hydroxide, separated from magnesium and radionuclides other than “Co by anion exchange, and determined by liquid scintillation counting at levels as low as 50 fCi/L of seawater (1). However, for the separation of the precipitates from the mother liquor, the necessary aging of the precipitates and centrifugation were rather troublesome and time-consuming. The present paper describes a new radiochemical separation technique using continuous-flow coprecipitation-flotation (2), in which “Co is coprecipitated with stable cobalt carrier, l-nitroso-2-naphthol, and indium hydroxide and floated to the solution surface with the aid of anionic surfactants and tiny nitrogen bubbles. This technique is more rapid and easier in operation than conventional coprecipitation techniques.
EXPERIMENTAL SECTION Apparatus. The flow system for coprecipitation-flotation is shown in Figure 1. The dimensions of the flotation cell and the position of the solution inlet and outlet were optimized in order that both loss of precipitates through the drain and redispersion
of floated precipitates into the bulk solution did not occur. The sample inlet was extended obliquely upward along the walls of the flotation cell so that the flotation was not disturbed by the sample stream. A 15-cm-wide detachable poly(ethylene) insert was placed near the solution surface to prevent adhesion of precipitates on the walls of the flotation cell. Other apparatus used included an Aloka 671 liquid scintillation counter and a Nippon Jarrell-Ash AA-1 Mark II atomic absorption spectrometer with an SA-61 slit burner. Reagents. Standard “Co solutions (0.5 pCi/mL and 10 nCi/mL): A standardized solution (The Japan Radioisotope Assoc.) was diluted with 1 M hydrochloric acid. Cobalt carrier solution (1 mg/mL): Cobalt(II) chloride hexahydrate was dissolved in 0.1 M hydrochloric acid. 0003-2700/84/0356-2851$01.50/0
©
1984 American Chemical Society
2852
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Peristaltic pumps
Table I. Coprecipitation-Flotation of 0.5 mg of Cobalt in L of Artificial Seawater by the Combined Use of l-Nitroso-2-naphthol and Metal Hydroxide Precipitates 1- nitroso2- naphthol
collector metal
recovered,
added, mg
added, mg
%
100 100
Al, 100 In, 100°
92, 93 92, 93
“With
100 mg
5
cobalt
of indium alone, the cobalt yield
was
20%.
Table II. Coprecipitation-Flotation of Cobalt in Large Volumes of Artificial Seawater sample vol, L 10
20
Figure 1,
Apparatus for continuous-flow coprecipltation-flotatiori.
RESULTS AND DISCUSSION Coprecipitation-Flotation in the Flow System. Several collector precipitates were examined to find the most suitable for the preconcentration of 60Co. The criteria included (1) a high cobalt yield, (2) small quantities of precipitates (collector precipitates plus coprecipitated alkali and alkaline earth metals), and (3) ease of subsequent radiochemical separations and counting sample preparation. First, aluminum and iron(III) hydroxides were tested by using 5 L of acidified artificial seawater containing 0.5 mg of cobalt carrier and 100-250 mg of aluminum or iron(III). The pH was adjusted to 9-9.5 in continuous-flow coprecipitation-flotation, and the floated precipitates were dissolved in 10 mL of 6 M hydrochloric acid for the cobalt determination by AAS. When 100-200 mg of aluminum or iron(III) was used, ca. 90% of the precipitates were floated but cobalt yields were 51-80% due to unsatisfactory coprecipitation. With 250 mg of aluminum, both coprecipitation and flotation were complete in batch operation, but, in the flow system, part of the precipitates passed through the drain and cobalt yields decreased to 68-71%. Next, coprecipitation with l-nitroso-2-naphthol precipitates was
tried;
an
ethanol solution of l-nitroso-2-naphthol
was
added to aqueous samples to collect cobalt on the resulting precipitates (4). In this technique, the large quantity of precipitate (0.7 g/L) of the organic reagent itself makes its decomposition (required for eliminating color quenching in liquid scintillation counting) troublesome. Therefore, the quantity of the organic reagent was reduced to 0.02 g/L to avoid the large quantity of precipitate. Under these conditions, ca. 70% of the cobalt chelate was floated in batch
cobalt recovered, %
sample vol, L
92, 95 93, 94
50 100
cobalt recovered, % 96
97, 99
operation but the yields decreased to 25% in the flow system. Finally, we tried to float the cobalt chelate of l-nitroso-2naphthol after capturing it in metal hydroxide precipitates. A ca. 20-fold excess of l-nitroso-2-naphthol (0.02 g/L) was used for rapid, quantitative formation of the cobalt chelate (5). Aluminum and indium hydroxides were used as collector precipitates, because they did not react with the organic reagent. The pH in the reaction bottle was adjusted between 8.5 and 9.0, where alkali and alkaline-earth metals accompanying cobalt were much less than Viooo or Vioo of those quantities in seawater, respectively (6, 7). Two anionic surfactants were used for rendering the precipitate surfaces hydrophobic and floatable (with sodium oleate) and producing stable foam which supports the precipitates on the solution surface (with sodium dodecyl sulfate) (7,8). Ethanol used as the solvent of the surfactants was essential to produce numerous bubbles having diameters of 0.5 mm or less (9). To 5 L of acidified artificial seawater, 0.5 mg of cobalt, 100 mg of aluminum or indium, and 100 mg of l-nitroso-2naphthol were added, and continuous-flow coprecipitationflotation was carried out. The cobalt chelate was captured in the interstitial spaces of flocculent metal hydroxides in the reaction bottle, and the reddish precipitates were floated in the flotation cell and collected into the collection bottle continuously. The precipitates were then dissolved in 5 mL of 6 M hydrochloric acid for the cobalt determination by AAS. As shown in Table I, combined use of l-nitroso-2-naphthol and metal hydroxides was effective. Compared with aluminum hydroxide, indium hydroxide was floated more rapidly and its volume was smaller (Vs-1/*)· Excess l-nitroso-2-naphthol was not floated and passed through the drain. If necessary, the l-nitroso-2-naphthol in wastewater can easily be removed by adsorption on activated carbon. Applicability to 10-100-L samples was examined, the quantities of 12 M hydrochloric acid, cobalt carrier, indium, and l-nitroso-2-naphthol being increased in proportion to the sample volume. Satisfactory cobalt yields were obtained as shown in Table II. The volume of the final solution was less than V200 of the original sample volume. Further Radiochemical Separations. Removal of indium and radionuclides other than 60Co and decomposition of l-nitroso-2-naphthol were required before liquid scintillation counting. The cobalt chelate of l-nitroso-2-naphthol was extracted into chloroform from the hydrochloric acid solutions obtained by treating 10-100 L of seawater samples by the recommended procedure; the amount of chloroform was increased in proportion to the sample volume. By analyzing the aqueous phase by AAS, it was found that more than 99% of the cobalt was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table III. Effect of the Quantity of Cobalt Carrier and PPO on Counting Efficiency (10 nCi of 60Co) cobalt carrier, mg
PPO, mg
10 10 10 10 10
10 30 50 100 300
counting efficiency, % 73
78, 78, 78, 79 79 79 78
cobalt carrier, mg 10
7.5 5.0 2.5 1.0
PPO, mg
counting efficiency, %
1000 30 30 30 30
78 81 81 80 81
Table IV. Determination of
60Co
in
10
·
2853
L of Seawater
"Co added,
"Co found,0
"Co added,
"Co found,0
pCi
pCi
pCi
pCi
0
0.0 0.5 0.8
3.0 5.0
2.9 4.6
0.5 1.0
“The standard deviation due to statistical fluctuation of counting
was
±0.2 pCi.
a 10-L aliquot, 0-5.0 pCi of "Co was added, and, after standing for 180 min, the "Co was separated and determined by the recommended procedure. The results are shown in Table IV. The chemical yields were 92-95%, and the lower detection limit was 50 fCi/L of seawater, based on 3 times the standard deviation of the background counting. The whole separation procedure required ca. 150 min; i.e., 20 min for coprecipitation-flotation, 30 min for extraction with chloroform, 90 min for evaporation and decomposition, and 10 min for extraction with a TOA solution. This technique can be applied to seawater samples of up to 100 L without difficulty, decreasing the detection limit down to 5 fCi/L of seawater. Since "Co can exist in different chemical forms in seawater, the behavior of each species in the radiochemical separation should be studied in detail. Acidification of filtered seawater samples desorbs cobalt from colloidal hydrated metal oxides, and the use of an excess of l-nitroso-2-naphthol, a powerful chelating agent, may form the chelate with cobalt which is weakly complexed or weakly associated with organic and inorganic materials. Therefore, part of these species may be collected together with inorganic cobalt ions. The proposed technique can also be applied to freshwaters by using a sodium oleate solution (0.2 mg/mL in 70% ethanol) as surfactant solution; greater than 95% yields were obtained for the preconcentration of "Co in 10-50-L samples by con-
To
extracted, leaving indium completely in the aqueous phase. Radionuclides such as 90Sr and 137Cs are not coprecipitated with indium hydroxide. Manganese-54,66Mn, and 85Zn are coprecipitated, but they are not extracted with chloroform. Iron-59 (half-life 44.6 days), MCu (12.7 h), and 95Zr (64.0 days) behave similarly to 80Co through the whole separation and interfere with the determination of 80Co. The interference, however, can be eliminated by using the decay of these short-lived radionuclides. Cobalt yields for removal and destruction of organic matter were measured as follows: 1-10 mg of cobalt (corresponding to 10-100-L samples) complexed with l-nitroso-2-naphthol was extracted into chloroform and decomposed by the recommended procedure, and the resulting 8 M hydrochloric acid solution was analyzed for cobalt by AAS. The yields were better than 97%. The cobalt was extracted from 8 M hydrochloric acid solutions with a xylene solution of TOA to prepare counting samples. The acidity was most suitable for the extraction (1), and more than 99% of 1-10 mg of cobalt was extracted. Chemical Yield through the Whole Procedure. Cobalt, 1 mg, with or without 0.5 nCi of "Co, was added to 10 L of artificial seawater, separated by the recommended procedure, and détermined by liquid scintillation counting or AAS. Cobalt yields were 96% and 95%. In the recommended procedure, chemical yields were obtained without the TOA extraction, because xylene is not suitable for AAS and the extraction of cobalt is quantitative as described previously. /8-Activity Measurements. To find the optimum quantity of scintillator PPO, 10 mL of TOA solution containing 10 mg of cobalt carrier and 10 nCi of "Co (prepared by extraction from 8 M hydrochloric acid solutions) was mixed with 10 mL of xylene containing 10-1000 mg of PPO. The results are shown in Table III. When the quantity of PPO exceeded 50 mg, precipitation occurred and the counting efficiency no longer increased. Therefore, 30 mg of PPO in 10 mL of xylene was used. The counting efficiency was constant between 1 and 10 mg of cobalt carrier. Determination of "Co in Seawater. Seawater was sampled on the coast of Ise Bay, Aichi Prefecture, Japan, and filtered through 0.45-µ membrane filters (Millipore HA).
tinuous-flow coprecipitation-flotation. Registry No. "Co, 10198-40-0; indium hydroxide, 20661-21-6; l-nitroso-2-naphthol, 131-91-9; water, 7732-18-5.
LITERATURE CITED (1) Ikeda, N.; Abe, S.; Seki, R. Radioisotopes 1975, 24, 857-860. (2) Mizuike, A.; Hlralde, M.; Mlzuno, K. Anal. Chlm. Acta 1983, 148,
305-309.
(3) Koroleff, F. In "Methods of Seawater Analysis"; Grasshoff, K., Ed.; Verlag Chemle: Weinheim, New York, 1976; p 152. (4) Mizuike, A.; Hlralde, M.; Suzuki, T. Bunseki Kagaku 1977, 26, 72-74. (5) Suzuki, N.; Yoshlda, H. J. Chem. Soc. Jpn. 1959, 80, 1005-1008. (6) Hlralde, M.; Yoshlda, Y.; Mizuike, A. Anal. Chlm. Acta 1978, 81,
185-189.
(7) Hlralde, M.; Ito, T.; Baba, M.; Kawaguchi, H.; Mizuike, A. Anal. Chem.
1980, 52, 804-807.
(8) Sonawane, N. J.; Hlralde, M.; Mizuike, A. Anal. Chim. Acta 1983,
149, 359-362.
(9) Hlralde, M.; Mizuike, A. Bunseki Kagaku 1977, 26, 47-50.
Received for review April 25,1984. Accepted August 1,1984.
