Radiochemica1 Determination of Technetium-99 in Environmental Water Samples Norbert W. Golchert and Jacob Sedlet Argonne National Laboratory, Argonne, Ill., 60439
THISNOTE describes a radiochemical procedure for the determination of technetium-99, a pure beta emitter (0.29 MeV maximum energy) with a half-life of 2.12 X 105 years, at levels as low as 0.5 dpm per liter of water. Most separation procedures in the literature are designed to separate much larger quantities of technetium, usually from fission products. Typical of these are the procedures collected by Anders (1). Where macroquantities are available, as in the determination of technetium for the measurement of nuclear fuel burnup ( 2 , 3), chemical-instrumental methods are employed. These methods cannot be used at the low technetium concentrations found in the environment. Separation procedures that are designed for low concentrations are those for urine ( 4 ) and rain (4, but the former method does not provide adequate separation from some interfering activities and does not have adequate sensitivity, while in the latter method the technetium recovery is low and uncertain. Parallel studies that might be modified for analytical work are the search for long-lived technetium in minerals (6), the determination of technetium from the spontaneous fission of uranium-238 in pitchblende ores (7,8), and the search for technetium produced in molybdenum ores by cosmic-ray neutrons (9). There is no published procedure for the determination of technetium-99 that is applicable to the analysis of large numbers of environmental water samples. With the increasing availability and use of gram to kilogram quantities of this fission product, such a procedure is needed. The present work provides a method for the determination of small amounts of technetium-99 in aqueous solutions, especially in naturallyoccurring water. EXPERIMENTAL
Reagents. Tri-isooctylamine (TIOA), Bram Metallurgical Chemical Co., Philadelphia, Pa., reagent grade CP, was used without further purification to form 5 (VjV) solutions with xylene. The ruthenium carrier was prepared by dissolving 10 grams of C. P. ruthenium trichloride, A. D. MacKay, Inc., New York, N.Y., in 6N HC1 and diluting to 500 ml with 6N HCI. The ruthenium concentration was about 10 mgjml. Tracers. Technetium-99 was obtained from Oak Ridge National Laboratory as an aqueous solution of ammonium pertechnetate. The concentration was determined by four-pi ~
(1) E. Anders, “The Radiochemistry of Technetium,” AEC Report, NAS-NS-3021, NOV.1960. (2) R. J. Meyer, R. D. Oldham, and R. P. Larson, ANAL.CHEM., 36, 1975 (1964). (3) R. E. Foster, Jr., W. J. Maeck, and J. E. Rein, ibid., 39, 563 (1967). (4) W. Fairman and J. Sedlet, AEC Report, TID-7696, 10, Oct. 1963. ( 5 ) M. Attrep, Jr., M. S. Thesis, The University of Arkansas, Fayetteville, Arkansas, 1962. (6) G. E. Boyd, and Q. V. Larson, J . Phys. Chem., 60, 707 (1956). (7) B. T.Kenna and P. K . Kuroda, J. Inovg. Nucl. Chem., 23, 142 (1961). (8) B. T. Kenna and P. K. Kuroda, ibid., 26, 493 (1964). (9) G. Goldstein, Ph.D. Thesis, The University of Tennessee, Knoxville, Tennessee, 1965.
beta counting. Appropriate dilutions of the initial solution were prepared in 1MNH40Hfor routine use. In some cases, technetium-95m, produced by cyclotron irradiation of niobium with helium-3 or helium-4 ions, was used and measurements were made by gamma-ray counting. Equipment. Beta counting was done by a Sharp Laboratories (now Beckman Instruments, Fullerton, Calif.) Low-Beta Counter operated in the Geiger region on a 99.05% helium0.95% isobutane gas mixture at 1050 V. The average background for the 2-in. diameter detector, which contained a gold-plated 0.9 mg/cm2 Mylar window, was 0.75 cpm. The electrodeposition cell used in this work was an enlarged model of the one described by Schwendiman and Healy (IO). Sample Pretreatment. Water samples are acidified immediately after collection at the rate of 5 ml concentrated HN03/1 to inhibit hydrolysis of cations. Procedure. Ten milliliters of 12N NaOH, 5 ml of 5 % NaOCl, and 2 ml of Ru(II1) carrier are added to one liter of the water sample. The solution is evaporated to about 50 mi, 10 ml of absolute alcohol is added, and the evaporation continued until 20-25 ml remain. The solution is centrifuged and the precipitate washed with H,O. The combined supernatant and wash solutions are acidified with concentrated HC1 and stirred for 15 min to decompose the NaOC1. Five milligrams of Fe(II1) is added, and the hydroxide is precipitated from the hot solution by the dropwise addition of concentrated NH40H. The solution is centrifuged and the supernate is retained. The precipitate is dissolved in a minimum of concentrated HCl, 5 ml of H 2 0 is added, and the hydroxide is precipitated as before. The precipitate is centrifuged and the supernate combined with the initial supernate. The ferric hydroxide precipitation is repeated on the combined supernatant-wash solution. The supernate is then made 1N in HC1,lO mg of Cu(I1) and 100 mg of thioacetamide are added, and the solution is heated on a hot water bath for 30 min. , ml The precipitate is centrifuged; then 10 ml of 1N H 2 S 0 4 0.5 of 50% H 2 0 2 ,and 2 ml of xylene are added, and the mixture is heated on a hot water bath for 15 min. The cooled solution is shaken in a separatory funnel with 15 ml of 5z TIOA in xylene for 1 min. The aqueous phase is discarded and the TIOA solution washed with 10 ml of 1N H2SO4containing 1 drop of 50% Hz02. The wash solution is discarded, 15 ml of 2N NaOH are added, and the phases mixed for 1 min. The phases are separated and the TIOA solution contacted once more with 15 ml of 2N NaOH. The two portions of NaOH solution are transferred to the electrodeposition cell with a small volume of water and the technetium is deposited onto stainless steel at 100 mA and 3.0 V for at least 5 hr. The steel disk is washed with water and alcohol, dried, and counted for beta activity. RESULTS AND DISCUSSION
Water samples were acidified with nitric acid to prevent hydrolysis of trace elements. Because the levels of activity were very low, it was necessary to use a large volume and concentrate the sample for analysis. Direct evaporation of these nitric acid solutions would result in loss of technetium (10) L. C . Schwendiman and J. W. Healy, Nucleonics, 16 (6), 78 (1958). VOL. 41, NO. 4, APRIL 1969
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Table I. Loss of Technetium in Various Steps of Procedure Step Per cent Tc lost Evaporation < I Ruthenium dioxide precipitation 15 Ferric hydroxide scavengings 1 Copper sulfide precipitation 3 Copper sulfide dissolution < 1 1 Extraction and back-extraction Electrodeposition < 1 Table 11. Decontamination Factors for Technetium Procedure Tracer Decontamination factor 54 Mn 2 x 104 T o
6sZn 90Sr-gOY 95Zr-95Nb 106R~.’06Rh IZ5Sb 1311 137cs
144Ce 233U 239Pu
x lo4 x 104 x 104 x 105 x 104 x lo4 x 103 x 105 x 104 > 104 > 104
8 2 1.8 1 1.3 6 5 I 3
as the volatile heptoxide. For this reason, evaporations were carried out from a sodium hydroxide solution, although ammonium hydroxide would work eqaully well. Other experiments showed that technetium was retained if it was first reduced to the nonvolatile lower oxidation states, but reduction was not used in the procedure because of other complications. The initial ruthenium separation was necessary to obtain adequate decontamination from ruthenium activities. Trace quantities of ruthenium existed in the samples in a number of different oxidation states (or chemical forms). The ruthenium was converted to a single species by employing an oxidationreduction cycle in order to obtain reproducible and adequate ruthenium decontamination. All the ruthenium was oxidized by the sodium hypochlorite to ruthenium(VI1) and then reduced to the insoluble ruthenium(1V) oxide by the alcohol. This step alone provided a ruthenium decontamination of several hundred and could be repeated, if large quantities of ruthenium activity were present. After the insoluble ruthenium dioxide was removed, the remaining sodium hypochlorite was decomposed to prevent it from interfering with the copper sulfide precipitation by oxidizing the sulfide. The subsequent iron scavenging steps provided additional decontamination. It was more effective to dissolve and reprecipitate the ferric hydroxide than to wash the precipitate. According to Anders (I), copper sulfide will carry technetium(VI1) quantitatively, as technetium heptasulfide, from 0.2 to 3N acid. Most authors precipitate the sulfide from nitric acid. However, copper sulfide will dissolve in hot 2N nitric acid, and at lower nitric acid concentrations if large quantities of ammonium nitrate are present. The sulfide ion is oxidized to elemental sulfur under these conditions. To avoid this problem, the copper sulfide was precipitated from hydrochloric acid solution. The method of dissolving the copper sulfide is one of the significant innovations used in this procedure. Copper sulfide is generally dissolved by repeated treatment of the precipitate with ammoniacal hydrogen peroxide (4-8). A more effective and cleaner dissolution is obtained by treating the precipitate with 50% hydrogen peroxide in 1N sulfuric acid and heating 670
ANALYTICAL CHEMISTRY
the mixture at 90 “C for 15 minutes. The xylene is present to dissolve elemental sulfur, which is usually produced, and thus frees any copper sulfide trapped in small sulfur particles, thereby allowing complete reaction. Any remaining peroxide is beneficial in maintaining the technetium as pertechnetate. Tracer studies showed that no technetium was lost by volatilization of the heptoxide. Extraction of pertechnetate into the 5 % TIOA in xylene solution is rapid and effective. Equilibrium is established within seconds for both the extraction and back-extraction. Although TIOA extracts numerous species, most cations require much higher acid concentrations or the formation of complexes (11). Back-extraction was effected by a simple change in pH to a basic solution. Although the extraction into the TIOA is quantitative, about 90% of the technetium is back-extracted with an equal volume of NaOH solution. The two back-extractions removed 99 % of the activity from the TIOA. The cathode of the electrodeposition cell was a 1.5-in. stainless steel disk, 25 mils thick, with the technetium deposited in a circle of 11/*-in. diameter using a platinum anode. The deposition half-time is 27 minutes. Previous experience with the electrodeposition of technetium revealed several problems. Electrodeposition of technetium metal onto platinum required solutions greater than 2N in sulfuric acid (12) and deposition times of three days for quantitative depositioc Electrodeposition from sodium sulfate solutions results in the hydrated dioxide ( I 3 , 1 4 ) . Although not observed with macroamounts of technetium, small solubility losses, about 5 %, occurred while washing the electrolyte from the disk when sodium sulfate solutions were used to deposit trace quantities. Electrodeposition from sodium hydroxide solutions onto stainless steel eliminated the solubility problem, allowed quantiative deposition in about four hours, and provided a source with the highest possible efficiency for the counting system. The deposition of technetium from sodium hydroxide has been extensively studied by Matsuura and Yumoto (15). The procedure as presented is a carrier-free radiochemical procedure. Recoveries were determined by adding technetium99 tracer to one of a pair of samples and analyzing both in an identical manner. The inherent weakness in any carrier-free radiochemical separation procedure is that it is assumed that the factors that influence recovery are the same in all samples. Several alternatives to a carrier-free procedure were considered and rejected. If technetium-95m is used as a n isotopic tracer, conversion electrons give sufficient beta counts to mask the technetium49 activity at low levels. About 10s dpm of technetium-95m give about 50 beta cpm. Rhenium could be used as a non-isotopic carrier because of the chemical similarity between technetium and rhenium, and the technetium recovery assumed to be identical with that of rhenium. This approach was rejected because the inevitable fractionation between rhenium and technetium would make it impossible to obtain accurate chemical yields ( I ) . The carrier-free approach, in the case of low-level samples, is at least equal to the alternatives, and the reproducibility of the recovery is determined primarily (1 1) G. H. Morrison and H. Freiser, “Solvent Extraction in Analytical Chemistry,” John Wiley and Sons, Inc., New York, N.Y., 1957. (12) W. D. Box, Nucl. Appl., 1, 155 (1965). (13) G. E. Boyd, Q . V. Larson, and E. E. Motta, J . Amer. Chem. SOC.,82, 809 (1960). (14) R. E. Voltz and M. L. Holt, J . Electrochem. Soc., 114, 128 (1967). (15) N. Matsuura and H. Yumoto, Radioisotopes (Tokyo), 8, 28 (1959).
by the care taken by the analyst to follow instructions. The over-all chemical recovery, as determined by periodic checks with known amounts of technetium-99, is 79.6 % (c = 3.4 %). Stepwise losses are summarized in Table I. Excluding evaporation, electrodeposition, and counting time, the analysis takes about three hours. The counting efficiency for electrodeposited technetium-99 in the beta counter is 4 1 z . For a 1000-minute count, the limit of detection is 0.5 pCi (2.7 X lO-Ilg)/liter at the 99% confidence level, about the same as the sensitivity by activation analysis (6). The relative error of a single determination is 30% at 0.5 pCi/l, 22% at 1 pCi/l, 14% at 5 pCi/l, and 13% above 10 pCi/l. Decontamination factors were determined in the following manner. An appropriate quantity of the tracer to be checked was added to the initial sample and the entire separation procedure run. The resulting sample was then beta counted, in the same manner as a routine sample, and the counts due to the added tracer were converted to dpm by using efficiencies of the counter as determined by the procedure outlined by Bayhurst and Prestwood (16). The ratio of the dpm added to the dpm recovered is then the decontamination factor for the particular isotope added. The decontamination factors in this procedure for a number of typical isotopes are presented in Table 11. The isotopes chosen either represent families of elements or are frequently present in typical samples. The decontamination factors are adequate for the samples encountered. One problem that occurs with any procedure for the separation of technetium-99 at very low levels is how to establish that the observed radioactivity is from technetium-99. At
(16) B. P. Bayhurst and R. J. Prestwood, Nucleonics, 17 (3), 82 (1959).
these levels there is no nuclear property that can be used to identify technetium-99. The half-life is too long for practical measurement, there are no gamma-rays, and beta spectrometry is at best impracticable. Most evidence is indirect. What is known is that the observed activity followed the chemistry of technetium and that the half-life of the beta emitter, from a series of counts, is long. One method that has been used in this laboratory with excellent success is to characterize technetium-99 by measurement of the absorption half-thickness of its beta particle, 7.2 mg/cmz of aluminum in our counters. Counts are taken over the first few half-thicknesses with appropriate absorbers and compared to a technetium-99 source counted in the same manner. This technique has been used successfully with as little as 2 cpm of technetium-99. Surface water samples, collected on a weekly basis, have been analyzed for technetium-99 by this procedure since the fall of 1965. Of some 150 samples analyzed, approximately 80% of the results have been below the limit of detection. The positive values ranged from 0.5 pCi/liter to 49.4 pCi/liter with an average of 4.65 pCi/liter. An attempt was made to adapt the extraction step of Foster, Maeck, and Rein (3) to our problem. Their extraction of pertechnetate with tetrapropylammonium hydroxide from basic solution provides an excellent separation of technetium from essentially all other fission products. Unfortunately, the high salt concentrations encountered in evaporated water samples reduced the technetium extraction to 13 in spite of a scale-up in volume by a factor of five.
z
RECEIVED for review May 3, 1968. Accepted January 23, 1969. A preliminary report of this study was presented to the Division of Nuclear Chemistry and Technology, 153rd National Meeting, American Chemical Society, Miami Beach, Florida, April 1967. The work was done under the auspices of the U.S. Atomic Energy Commission.
Solvent Sublation of Iron( Ill) Chloride by Tri-n-Octylamine Jacob Elhanan and Barry L. Karger Department of Chemistry, Northeastern Unicersity, Boston, Mass. 02115
RECENTLY solvent sublation, a subdivision of the general method of adsorption bubble separation processes ( I ) , has been successfully demonstrated by Karger and coworkers ( 2 , 3 ) as a new tool in low concentration analytical separations. The technique was originally introduced by Sebba ( 4 ) as an auxiliary method to ion flotation. In solvent sublation gas bubbles generated in an aqueous phase are used to extract material selectively into a water immiscible organic phase. The aqueous phase will normally contain ionic nonsurface active solute in the presence of an oppositely charged surfactant. Several mechanisms of mass transfer from the aqueous to the nonaqueous phase are possible. The surfactant may adsorb at the gas-liquid interface as the bubbles travel through (1) B. L. Karger, R. B. Grieves, R. Lemlich, A. J. Rubin, and F. Sebba, Sep. Sci., 2, 401 (1967). (2) B. L. Karger, A. B. Caragay, and S. B. Lee, ibid., 2, 39 (1967). (3) A. B. Caragay and B. L. Karger, ANAL.CHEM.,38, 652 (1966). (4) F. Sebba, “Ion Flotation,” Elsevier, Amsterdam, 1962, Chap. X.
the aqueous phase. Being oppositely charged to the solute in the water, the surfactant may then form an ion pair with the solute at the gas-liquid interface, with the subsequent extraction of the ion-pair complex into the organic phase. Alternatively, a solute-surfactant ion pair may form in solution and be subsequently adsorbed at the gas-liquid interface. Nonsurface active soluble species may also be carried into the organic phase by bulk liquid surrounding the bubble being dragged into the nonaqueous layer. Solubilization of such nonsurface active species in the organic layer will depend on the interaction of the species with the organic solvent, free surfactant molecules, or solute-surfactant ion pairs. The controlled aspects of solvent sublation provide an opportunity to follow closely the various stages associated with this extraction technique. In the present work, the solvent sublation of ferric ion from hydrochloric acid-dioxane aqueous solutions into anisole by tri-n-octylamine was studied. The choice of this system was largely determined by the established data available on the liquid-liquid solvent extraction of group VI11 metals from VOL. 41, NO. 4, APRIL 1969
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