Radiochemical Determination of Uranium and the Transuranium Elements in Process Solutions and Environmental Samples Claude W . Sill and Rodger L. Williams Health Services Laboratory, U S .Atomic Energy Commission, Idaho Falls, Idaho
A previous procedure for the separation and determination of uranium and the transuranium elements using barium sulfate has been improved greatly and applied to practical problems. By adding a more dilute barium solution dropwise to the sample solution at its boiling point, precipitation of the elements being carried has been increased to a remarkable 99.995%, or a decontamination factor of 2 X 104 in a single precipitation. Ten pure solutions taken through the entire procedure including fusion, separation, and determination gave a relative standard deviation of 0.9%. The same changes also increased the permissible level of the worst interfering elements to about 1 mg, with most of the elements in the periodic table having been tested. The procedure has been applied to many kinds of process solutions and environmental samples with excellent sensitivity, precision, and reliability. The radiochemical purity of all fractions is demonstrated by high-resolution alpha spectrometry. In addition, a procedure for the precise determination of gross alpha activity is presented and its use in complying with the summation rule for mixtures required by federal regulations is demonstrated. PREVIOUS WORK performed in this laboratory (I-4) has shown that small quantities of all ter- and quadrivalent cations larger than about 1.O a.u. (Goldschmidt values) can be precipitated with barium sulfate in the presence of potassium to better than 99.7 %. Thus, except for astatine, radon, and francium, small quantities of all elements from lead to at least californium in addition to barium, lanthanum, and the light lanthanides are carried efficiently on barium sulfate. The elements uranium through americium can be prevented from precipitating by oxidizing them to their highest states in which the oxygenated cations of low charge are too large to fit into the barium sulfate lattice. Previous work has shown that those elements precipitated by barium sulfate are their own worst interferences, the error being highly dependent on the order of addition of barium. For example, 0.1,0.3, and 1.0 mg of thorium produced losses of americium-241 of 1.0, 1.7, and 33.9%, respectively, if the barium was added before the acid cake was dissolved in water. This is not a prohibitive level of interference but is an undesirable order of addition of barium because of the insolubility of anhydrous sulfates of iron, aluminum, chromium, etc., in concentrated sulfuric acid, and the necessity for removal of silica and adjustment of oxidation states before precipitation of barium sulfate. On the other hand, if the barium was added after dissolution of the cake, the losses produced by the same quantities of thorium increased drastically to 49.6, 62.7, and 85.7z, respectively. The data for the latter order of addition were obtained by adding 1 ml of a 0.95z solution of barium nitrate at the normal speed of delivery of the pipet to 25 ml of solution containing 0.5 ml of (1) C. W. Sill and C. P. Willis, ANAL.CHEM., 36,622 (1964). (2) C. W. Sill and C. P. Willis, ibid., 38, 97 (1966). (3) C. W. Sill, ibid., 38, 1458 (1966). (4) C. W. Sill, Health Physics, 17, 89 (1969).
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excess concentrated sulfuric acid and 3 grams of anhydrous potassium sulfate at about 70 "C while swirling the solution continuously. Subsequently, it was found that even 0.3 mg of thorium produced only 1 loss if a barium solution only half as concentrated were used, the other conditions remaining the same. Furthermore, if the more dilute barium solution was added dropwise to the solution at its boiling point, even 1 mg of thorium produced a loss of only 0.3 Because thorium is one of the worst interferences, both intrinsically and in frequency and quantity encountered, this tolerable level is adequate to restore complete reliability and confidence in the results obtained from a wide variety of sample types. The remarkable effect of such a small change in the concentration of the barium solution added and/or the temperature at which precipitation is made is clearly due to the fact that formation of barium sulfate is visibly delayed thereby for a few seconds, long enough for the barium to be distributed throughout the solution before precipitation occurs and permit the precipitation to be made under essentially homogeneous conditions. The increased efficiency is undoubtedly due to more efficient utilization of the entire volume of the crystal lattice in contrast to the few surface layers that are available to the tracer ion being separated when the barium sulfate is allowed to precipitate heterogeneously and almost completely at the point of entrance into the solution. Both recovery of the radionuclides being sought and the permissible quantity of interfering cations are increased markedly even though the interferences themselves are also carried more efficiently. The present manuscript gives the results of a study of interferences and of the problems encountered in the determination of uranium and the transuranium elements in a wide variety of samples of practical importance. Thorium, protactinium, polonium, and radium are the main other alpha-emitting elements carried efficiently under the same conditions. The previous publication (4) should be consulted for detailed information on the techniques and instrumentation employed for gross alpha counting, alpha spectrometry, standardization, etc.
z
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EXPERIMENTAL
General Procedure. Add 3 grams of anhydrous potassium sulfate, 3 ml of concentrated sulfuric acid, and 5 or 6 drops each of concentrated nitric and 72% perchloric acids to the sample in a 250-ml Erlenmeyer flask and evaporate the solution to fumes. Add additional nitric and/or perchloric acids as necessary to ensure complete oxidation of organic matter. Heat the solution over a Fisher high-temperature blast burner with a 4-cm grid while swirling the flask continuously until a pyrosulfate fusion is obtained to ensure complete dissolution of any refractory compounds. After the sample has dissolved completely, heat the fusion at the maximum temperature obtainable from the blast burner until most of the excess acid has been volatilized to ensure subsequent complete dissolution of anhydrous iron and aluminum sulfates. (Most problems with pyrosulfate fusions
result either from too much sample for the amount of flux used or application of too little heat.) Cool the melt to room temperature and add 0.5 ml of concentrated sulfuric acid and 35 ml of water. Boil the solution for 5 minutes to dissolve anhydrous metallic sulfates and to hydrolyze any condensed phosphates that might be present. Centrifuge to remove silica, Adjust the volume to 30 ml and treat the solution with the appropriate oxidizing or reducing agents depending on the separations desired. If all elements precipitable by barium sulfate including uranium are desired, boil the solution for 1 minute with 4 drops of 209;: titanium trichloride. If a permanent violet color of tervalent titanium is not produced with 4 drops, reduce the solution in a small Jones reductor as described previously ( 4 ) . If all precipitable elements except uranium are desired, add 1 ml of 3 0 x hydrogen peroxide before the 5-minute boiling period to oxidize uranium and to ensure complete reduction of plutonium. If all elements except uranium and neptunium are desired, boil the solution for 2 minutes with 1 ml of 1 potassium dichromate. If all elements except uranium, neptunium, and plutonium are desired, dilute the solution to 40 ml and boil gently for 10 minutes with 1 ml of 0.5 potassium permanganate. If all elements except uranium through americium are desired, adjust the volume to 30 ml and boil the solution for 2 minutes with 5 drops of 1 silver nitrate and 0.5 gram of potassium peroxydisulfate. Detailed directions for making all these separations in sequence on a single sample have already been published (4). ' After adjustment of the oxidation states, add 1.00 ml of a 0.48x solution of anhydrous barium nitrate (or 0.459;: barium chloride dihydrate when using tervalent titanium) to the boiling Solution at about 1 drop every 2 seconds while swirling the flask continuously. Boil the solution for 1 minute and repeat the addition of another 1.00 ml of barium solution by the same technique. Boil the solution again for 1 minute and cool for 10 minutes in a bath of cold running water. The quantity of most elements, particularly curium and californium, remaining unprecipitated can be further reduced by a factor of about 3 by boiling. thebsolution for an additional 30 seconds about halfway through the addition of each 1-ml portion but is somewhat tedious for routine use. Transfer the cold solution to a 40-ml heavy-walled centrifuge tube and rinse the flask with at least three small portions of water sufficient to give a total volume of about 30 ml. Centrifuge at 2000 rpm for 5 minutes. Decant the supernate back into the 250-ml Erlenmeyer flask for further treatment to recover uranium or transuranium elements present in the oxidized state, if desired. Wash the barium sulfate with 5 ml of 0.5x sulfuric acid added in a forceful jet from a polyethylene squeeze bottle, and centrifuge. Suspend the barium sulfate in alcohol and filter on a 47-mm vinyl membrane filter (VM-6, 0.45 p, Gelman Instrument Co., Ann Arbor, Mich.) using the filtering chimney described below. Count in a scintillation counter as described previously ( 4 ) . Allow low-counting samples to stand for at least 3 hours, or preferably overnight, after mounting before counting to permit adequate decay of radon daughters built up on the nonconducting vinyl filter during filtration and drying (4). With 100-ml samples of water, sewage, process wastes, etc., the detection limit for a 1-hour count is about 2 x 10-9 pCi/ml or only one-fifteenth of the maximum concentration permitted for unknown alpha emitters in uncontrolled areas and only one twenty-five hundredth of the MPC for most of the transuranium elements (5). However, some process wastes contain high concentrations of aluminum, calcium, thorium, etc., and smaller samples are preferable for general use, particularly since higher sensitivities are generally unnecessary on such types of samples.
Soils and Sediments. Fuse 0.5 gram of finely-powdered siliceous material in 3 grams of anhydrous potassium fluoride in a platinum dish and transpose to a pyrosulfate fusion as described previously for rocks and soil ( I ) , replacing the barium chloride solution called for with 1 ml of nitric acid. Cool the pyrosulfate cake, add 50 ml of water and boil the solution until the cake becomes detached from the dish. Remove the dish and boil the solution for 5 minutes to dissolve all anhydrous iron and aluminum sulfates and to ensure complete hydrolysis of condensed phosphates. Cool the solution and examine it carefully for any sign of turbidity. Any turbidity present at this point indicates presence of alkaline earths or rare earths, either of which will cause severe loss of the radionuclides being sought. If present, the insoluble material must be separated and retreated or asufficiently small aliquot taken to reduce the quantity of interfering element below the permissible limit. Transfer the solution to a 100-ml volumetric flask and dilute to volume. Transfer an appropriate aliquot of the solution to a 250-ml Erlenmeyer flask, add enough more anhydrous potassium sulfate to give a total of 3 grams plus 0.4 ml of concentrated sulfuric acid per gram of extra potassium sulfate added. Dilute to 30 ml, heat the solution to boiling and continue with adjustment of the oxidation states and precipitation of barium sulfate as described in the general procedure. With average soils, the entire 0.5-gram sample has been used with excellent results. However, again considering the probability of encountering high concentrations of calcium, aluminum, thorium, etc., particularly in sediments from process wastes, use of smaller samples is prudent. Air Dusts. If the uranium or transuranium elements might be associated with siliceous material, or if significant quantities of alkaline earths or rare earths might be present, dry-ash the filter paper in a platinum dish and dissolve the insoluble material in a potassium fluoride fusion as described for soils. However, in most cases, it will be adequate to wet-ash the filter paper with nitric, perchloric, and sulfuric acids ( I , 6) in a 250-ml Erlenmeyer flask, adding 3 grams of anhydrous potassium sulfate and making a pyrosulfate fusion. A volume of 100 cubic meters of air is required to detect one tenth MPC for unknown alpha emitters in uncontrolled areas (5)in a 1-hour count. Other Samples. Many other types of samples such as soft tissues, blood, foodstuffs, vegetation, etc., can be handled by the present procedure after appropriate sample decomposition ( I ) and with due regard to the maximum quantity of interfering ions permitted. Separation of Polonium. Recovery of polonium by barium sulfate is only 9 2 x under the present conditions, the only major alpha emitter to be precipitated so inefficiently. Precipitation can be increased to 99.5% in the gross alpha determination by adding 1 mg of tellurium as tellurous acid just before the reduction of uranium with titanium trichloride. However, the polonium cannot then be separated from barium sulfate by reprecipitation from alkaline diethylenetriaminepentaacetic acid (DTPA). Polonium can be removed conveniently by precipitation with metallic tellurium before precipitation of the other alpha emitters with barium sulfate. Loss of uranium or transuranium activities in the tellurium precipitate is only 0.1 %. Volatilization of polonium during a potassium pyrosulfate fusion in a 250-ml Erlenmeyer flask is only about 1 % in several minutes' heating at the maximum temperature obtainable from a blast burner, a much hotter fusion of longer duration than will normally be required for dissolution of most nonsiliceous samples. When polonium is either to be determined or eliminated, dissolve the pyrosulfate cake by boiling with 2 ml of concentrated sulfuric acid and 25 ml of water. Add 1.00 ml of a solution of tellurous acid prepared by dissolving 0.625 gram of tellurium dioxide in 10 ml of concentrated hydrochloric
( 5 ) Code of Federal Regulations, Title 10, Part 20, as revised
January 1, 1968.
(6) C. W. Sill and C. P. Willis, ANAL.CHEM., 37, 1661 (1965). VOL. 41, NO. 12, OCTOBER 1969
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I 5/8" Dia.-Porous Stainless S t e e l Disc
Chamfer t o Fit
Figure 1. Filtering chimney for deposition of barium sulfate for alpha counting acid and diluting to 100 m l ( 5 mg/ml of Te). Add 5 drops of a freshly-prepared 25 % solution of stannous chloride dihydrate in 10% hydrochloric acid while swirling the flask continuously. Boil the solution vigorously for 3 minutes to flocculate the metallic tellurium and minimize scum formation. Allow the solution to stop boiling and add 1 additional drop of stannous chloride to be sure that precipitation of tellurium is complete. If additional darkening is produced, or if reducible elements such as iron, copper, etc., are present, more stannous chloride will have to be added and boiling repeated. Place the flask in a bath of cold running water and rinse the sides with a little water to prevent the tellurium from drying on the sides. Cool the solution for 5 minutes, centrifuge, wash, mount, and count the precipitated tellurium as described for barium sulfate. By sloshing the solution sideways and rinsing the walls of the tube midway through the centrifugation, losses in the scum of metallic tellurium that collects at the surface can be kept down to about 0.5% for the main supernate and 0.2 % for the wash. If decontamination of the polonium from other alpha emitters greater than about 1 x lo3 is required, heat the metallic tellurium in the centrifuge tube with 3 grams of potassium sulfate, 3 ml of concentrated sulfuric acid, and 1 drop of concentrated nitric acid until the solution begins to fume. Cool, dissolve the solution in 25 ml of water and repeat the precipitation with stannous chloride before counting. If decontamination of other alpha emitters from polonium greater than about 2 X lo2is required, evaporate the supernate to fumes to oxidize stannous tin and metallic polonium and tellurium. Add enough concentrated sulfuric acid to give a total of 2 ml to prevent precipitation of the tellurium carrier, presumably as stannic tellurite. Add water and tellurium carrier and repeat the precipitation with stannous chloride to give a total decontamination of over 2 X lo4. Discard the precipitate. Add 5 ml of 48% hydrobromic acid and sufficient, 30 % hydrogen peroxide to produce a permanent orange color of free bromine to the main supernate and evaporate to fumes. Cool the solution and repeat the treatment with hydrobromic acid and hydrogen peroxide to ensure complete volatilization of tin. Fuse and continue with the determination of uranium and the transuranium elements as described. Separation of Heavy Elements from Barium Sulfate. Curl the membrane filter slightly and insert it into a dry 40-ml conical centrifuge tube. Rinse the precipitate from the filter 1626
ANALYTICAL CHEMISTRY
by adding water dropwise until the entire paper has been covered systematically. About 5 ml of water is adequate to obtain 98% recovery if the highly crystalline barium sulfate produced by precipitation from boiling solution is present. If the barium sulfate has been reprecipitated, the tightly packed precipitate will not wash off and must be dissolved in a warm (
