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Determination of Alpha-Emitting Nuclides of Thorium and Uranium in Soil and Sediment Samples
13 1
DETERMINATION OF ALPHA-EMITTING NUCLIDES OF THORIUM AND URANIUM IN SOIL AND SEDIMENT SAMPLES
P. SPEZZANO N. SILVESTRI ENEA, CRE Saluggia, Fuel Cycle Department, Radiotoxicological and Environmental Laboratories,
13040 Saluggia, Vercelli, Italy
ABSTRACT A procedure for the radiochemical determination of 228 Th, U, and 238 U in soil and sediment samples is described.
230 Th, 232 Th, 234
U,
235
The analytical method is based on the decomposition of the sample by Auoridepyrosulphate fusion technique to allow total dissolution and chemical exchange between natural isotopes and added yield tracer nuclides. Uranium and thorium are coprecipitated as hydrolyzable metals with natural Fe and A1 acting as carriers. Separation of thorium and uranium is carried out by reversed-phase chronlatography using a column filled with Microthene impregnated with tri-n-octylphosphine oxide (TOPO) as stationary phase followed by final purification by anion exchange chromatography. Each fraction is electrodeposited and final measurements are made by alpha spectrometry with surface barrier detector. Overall chemical yields generally are better than 60%. For individual isotopes, the lower limit of detection at the 95% confidence level is 10W3 Bq/g for a counting time of 1000’. One of the most important applications of the present procedure is its use in environmental monitoring, particularly in the vicinity of nuclear facilities and uranium mills, and for the study on the distributions and geochemical behaviors of natural actinides.
1.
INTRODUCTION
The determination of small concentrations of natural thorium and uranium in environmental samples has been object of several investigation for many years. Colorimetric or fluorometric procedures give good sensitivity, precision and accuracy, but
132 they do not supply any informations about isotopic distribution. Such inform* tions are of great interest in radio-ecological research and to understand geological process 111. Moreover, analysis of actinides nuclides in environmental materials are important for control measurements associated with discharge of these radionuclides from nuclear facilities and uranium mills. Many methods used for determination of thorium and uranium isotopes in environmental samples by alpha spectrometry include barium sulfate or cerium flu* ride precipitation for the initial separation of the actinides and final purification by solvent extraction 12-61 or they involve a sequence of purification steps by ion exchange chromatography 17-91. Methods involving sequential separation of both natural and artificial actinides allow their determination simultaneously on a single sample but they are generally useful when high levels of man-made alpha-emitting nuclides are encountered, as in locally contaminated areas. When only thorium and uranium isotopes have to be determined, it is possible to use a simply analytical procedure. In this paper a procedure for the sequential determination of alpha-emitting isotopes of thorium and uranium in soil and sediment samples is described. In the presented procedure, tracers are added to the sample at the earliest possible stage and potassium fluoride and pyrosulfate fusion [lo] was chosen as a reliable and effective method to bring tracer and total sample into solution simultaneously. Thorium and uranium are then coprecipitated in a mixed hydroxide precipitate using iron and aluminium naturally present as carrier. Separation of thorium and uranium is carried out by extraction chromatography using tri-n-octylphopsphine oxide (TOPO) followed by final purification with anion exchange chromatography. Each fraction is electroplated to obtain suitable thin sources for final measurements by alpha spectrometry.
2.
2.1.
EXPERIMENTAL Instrumentat ion
Alpha spectrometry measurements were performed using a 4-channel spectrometer comprising Tennelec TC 256 alpha spectrometers with a 450 mma surface barrier detectors (Ortec) connected through a Tennelec TC 306 Mixer Router to the quarters of a 2048-channel analyzer (Silena) adjusted to 10 keV/channel. The spectral region covered extended from 3 to 8 MeV. The resolution obtained with samples electroplated on stainless steel planchets with an active area of 1.6 cm in diameter was 40-60 keV (FWHM) at a distance source detector of about 1 cm. Counting efficiency of the detectors ranged between 19.1 and 24.1%. Occasionally, measurements of gamma activity were performed using a Ge detector connected to a 4096-channel pulse-height analyzer.
133
-
Tracer, K F , NaZS04
Sample fusion
HCl dil.. +
NH40H
supernatant discard
+
precipitation 4 M HNOj-
dissolution
-
1 M HN03 discard
u fraction
Th fraction
d3
elect rodeoosi tion
Th d e t e r m i n a t i o n
m
elect rodepositi o n
U determination
Fig. 1. Flow diagram of thorium and uranium determination
134
2.2.
Reagents and Tracers
Tri-n-octylphosphine oxide (TOPO), Eastman Kodak Co., was used as extractant in 0.2 M cyclohexane solution and Microthene 651/50, microporous polyethylene, 60-100 mesh, Columbia Organic Chemicals, was used as the inert support for chromatographic separations. The stationary phase was prepared by mixing 3 g of Microthene with 2.5 ml of 0.2 M TOPO solution and conditioning with 40 ml of 1 M HN03, as described in literature Ill]. Anion-exchange resin, AG 1-X8, 50-100 mesh, chloride form, Bio-Rad Lab., was used for final purification of both thorium and uranium fractions. The calibrated 232U/22sTh spike solution was prepared by dilution from an aged standard solution (C.E.A., France). The activity of the final solution in 4 M HNO3, was 0.343 0.008 Bq/ml. The remaining tracers used, 226Ra, 210Po,237Np/233Pa,238Pu and 241Am were studied only as radiochemical interferences and were prepared from standard solution obtained from Amersham (U.K.) and C.E.A. All other reagents were of analytical grade.
*
3. 3.1.
PROCEDURE Sample Dissolution
Soil and sediment samples were dried at 12OoC and ground to pass through a 100 mesh standard sieve. About 1 g of sample was transferred t o a 30 ml platinum crucible and 0.5 ml of 232U/22sTh tracer solution was added. Sample was then evaporated t o dryness. The residue was mixed with 3 g of anhydrous potassium fluoride and heated to fusion. After cooling, the cake was treated with 3 ml of concentrated sulfuric acid and 3 g of anhydrous sodium sulfate were added for the pyrosulfate fusion according to Sill’s procedure 110). After cooling, the platinum crucible was transferred in a 200 ml beaker containing 100 ml of water, 10 ml of concentrated hydrochloric acid and 1 ml of 30% hydrogen peroxide and heated until dissolution of the content.
3.2.
Separation of Thorium and Uranium
The solution was transferred to a 250 ml centrifuge tube and the hydrated oxides (mainly iron and aluminium) were precipitated by adding ammonia t o a final pH of 7-8. The precipitate was -.entrifuged, washed with a solution of NHIOH at pH 8 and centrifuged again. The precipitate was then dissolved in 20 ml of 4 M HN03. The sample solution was passed at a flow rate of 1-2 ml/min through a chromatographic column (height: 10 cm - diameter: 1 cm) filled with the MicrotheneTOPO stationary phase. Thorium and uranium were sorbed onto the column. After the column had been washed with 30 ml of 1 M H N 0 3 in small increments t o rinse the column wall, thorium was eluted with 30 ml of 0.3 M H2S04 and
135 uranium was eluted with 30 ml of 1 M HF, maintaining always the same flow rate. The eluates were evaporated to dryness.
3.3.
Thorium Purification
A few ml of concentrated HN03 were added t o the residue and again evaporated t o dryness. This step was repeated and finally the residue was dissolved in a few ml of 8 M HN03. The solution was passed through a column (Dowex AGlx8, 50-100 mesh, 5 x 0.5 cm, preconditioned with 8 M H N 0 3 ) at a flow rate of 0.5 ml per minute. The column was washed with 10 ml of 8 M H N 0 3 and finally thorium was eluted with 20 ml of 8 M HC1. The eluate was collected and evaporated t o dryness.
3.4.
Uranium Purification
After evaporation to dryness, uranium fraction was treated with concentrated HCl and evaporated t o dryness. The residue was dissolved in a few ml of 8 M HC1 and the solution was passed through a second column (Dowex AGlx8, 50-100 mesh, chloride form, 5 x 0.5 cm) at a flow rate of 0.5 ml per minute. The column was washed with 10 ml of 8 M HC1 0.1 M HF. Uranium was eluted with 10 ml of 1 M HCl and the eluate was evaporated t o dryness.
+
3.5.
Electrodeposition
Thorium was electrodeposited starting from (NH4)aS01 at pH 2-2.5 according t o the Talvitie procedure [12],which was lightly modified. The residue was dissolved in 1 ml of concentrated sulfuric acid. After heating until dense white fumes were obtained, 6 ml of distilled water and 100 p1 of 1 M oxalic acid were added. The solution was neutralized with ammonia t o pH 2.5 (Thymol blue as indicator) and transferred into the electrodeposition cell. The beaker was washed with little portions of 1:99 H2SO4 (6 ml in total) and the wash solutions were added t o the cell. The pH was readjusted t o 2-2.5 (salmon-pink end point) with ammonia and some drops of 1:99 H2SO4. Electrodeposition was performed at a current density of 300 mA/cm2 for 4 hours. A minute before current stop, 1 ml of ammonia was added into the cell. Uranium was electroplated starting from 0.35 M ammonium acetate taken t o pH 2 with H N 0 3 [13].The residue was dissolved in 5 ml of 0.35 M CH3COONH4 (pH = 2) with heating and transferred t o the electroplating cell. The beaker was washed with little portions of electrolyte (5 ml in total) which were also transferred into the cell. The electrodeposition was carried out for 4 hours at 200 mA/cm2. A flow diagram for the separation and purification of thorium and uranium is shown in Fig. 1.
136
Tab. I. Results of thorium and uranium ores analysis Uranium ore, Bq/g
Thorium ore, Bq/g Isotopes 2 3 8 ~
Calculated
Measured
4.93
5.19f0.16
Measured 123.2
5.00f0.17 2 3 5 ~
2 3 4 ~
0.23
126.0fS.5 5.75
0.24f0.01 0.23f0.01
-
-
5.24f0.17 5.04f0.16
'"Th 230Th
41.1
-
41.3f 1.2
5.78f0.24 5.72f0.25 124.8zk3.5 125.1f3.5 <0.05
41.75 1.3
-
5.1690.23
-
123.753.6
-
124.1f3.5 -
4.95f0.22 Activity ratio
125.4f3.4
-
1.993f0.017
22aTh/232Th
1.998f0.016
Tab. 2. Decontamination Factors from some alpha emitters
Fina raction Tracer
Thorium
Uranium
2lOPo
2 x 102
4 x lo3
226Ra
> I x lo5
> 1 x lo5 > 2.5 x lo3
228Th
-
232u
>I x lo3
-
'"pa
> I x lo3
> 1 x lo3
>30
5 x 102
237Np
23SPU
2x
'*'Am
>i x lo5
102
7x
>
102
1x
lo5
137 Tab. 3. Radiochemical results for triplicate analysis of thorium and uranium isotopes in surface soils (Bq/kg) 230~h
232~h
23pu
2 3 5 ~
2 3 8 ~
41.652.4 32.1f2.2 42.1f2.5 2.2i0.47 41.1f2.5 45.1f2.5 35.4f2.2 40.7f1.7 1.8k0.32 42.4f1.8 42.4f2.6 33.7f2.4 42.3f2.4 2.4f0.46 42.9f2.4 meanfs 43.0f1.8 33.7f1.6 41.7f0.9 2.lf0.31 42.1f0.9
63.5f2.3 48.2f1.8 68.4f3.8 2.8f0.68 67.2f3.6 60.3f2.7 46.2f2.1 64.3f3.2 3.1f0.57 63.7f3.1 64.4f2.2 49.3f1.7 65.9f3.3 2.4f0.45 65.1f3.2 meanfs
4.
62.7f2.1 47.9f1.6 66.2f2.1 2.8f0.35
65.3f1.8
RESULTS AND DISCUSSION
The method described above is based on the possibility of thorium and uranium extraction from a nitric acid solution using tri-n-octylphosphine oxide. In this system, TOPO gives higher distribution coefficients for actinides of valency state IV and VI respect to other matrix elements, giving effective separation from all significant interferences. TOPO has been studied as extractant for several elements [14]. Testa and coworkers (15-191 used TOPO for the determination of actinides in biological and environmental samples using extraction chromatography. In this work, TOPO has been used in order to separate thorium and uranium from radiochemical and chemical interferences in soil and sediment samples. After extraction, thorium was stripped with 0.3 M H2S04 while uranium was stripped with 1 M HF, as described by Testa et al. At these point, of the other alpha-emitting nuclides of interest, any polonium strips with the thorium fraction while protactinium follows uranium. Neptunium and plutonium, if present, split between the two fractions. Polonium constitutes a serious interference in thorium determination because the alpha-peak of 210Po, 5.30 MeV, is very c!ose to the 5.34 and 5.42 MeV alphapeaks of 228Th, and thus the chemical yields of thorium may be overestimated. Therefore, the anion-exchange purification step was necessary in order to remove 210Po. Purification of thorium fractions was also useful to remove any residual iron that might interfere during thorium electrodeposition. The anion-exchange purification steps were also useful to remove neptunium and plutonium in both thorium and uranium fractions, but this could be necessary only if the procedure is applied to samples containing high activities of transuranium elements.
138 The principal losses of thorium and uranium occur during the electrodeposition step since the actinides electrodeposition yield decreases in the presence of other elements presents at the microgram level, as it is possible t o obtain after their separation from soil and sediment samples. In the present procedure, the best results for thorium were obtained starting from ammonium sulfate at pH 2, while for uranium high electrodeposition yields were obtained using 0.35 hf ammonium acetate at pH 2. Accuracy and precision of analytical method were checked by analyzing two samples of standard thorium and uranium ores in which both natural chains were thought to be present in secular equilibrium. Analysis were performed, after dissolution, on an appropriate aliquote containing about 100 pg of both thorium and uranium and 10 mg of iron were added as carrier t o assure a complete recovery of the actinides. Determination of thorium and uranium activities were performed by alpha spectrometry on electroplated disks that were counted for the time necess a r y t o give t o statistic counting uncertainty a value of about 1%. Assessment of 228Th/232Th ratio was performed on separate aliquote of unspiked samples. The analytical values obtained with the present procedure are shown in Tab. 1. The uncertainties given are the standard deviations resulting from propagation of all uncertainties incurred in the entire measurement process. Agreement of analytical results with calculated values are within statistics uncertainties. Decontamination factors for possible radiochemical interferences were also determined by adding 20 - 200 Bq of each radionuclide t o 1 g soil samples which were then carried throught the entire procedure. The results axe shown in Tab. 2. After all developmental work had been completed, the procedure was applied in its final form on soil and sediment samples spiked with known quantities of 232U/228Th tracer solution. Assessment of 228Th/232Th ratio was performed on separate aliquote of unspiked sample and it was necessary t o calculate chemical yield for thorium analysis. Obtained separation chemical yields were generally good, with values for thorium of 48-85%, average 72%, and for uranium 53-72%, average 62%. Tab.3 shows the experimental values obtained on three assessments for thorium and uranium isotopes carried out, according to above mentioned procedure, on two surface soil samples collected at Saluggia (VC), Italy. All uncertainties were propagated t o the final values. Agreement among triplicate analyses of actual samples is further evidence of good precision of the method. The larger uncertainties in 235U determination is due to the much lower activities and, therefore, poorer counting statistics. Sensitivity of analytical procedure was also calculated assuming a background count rate of the detector of 0.005 cpm under the peaks of the isotopes being measured. The lower limit of detection at the 95% confidence level is 10-3Bq/g for a count time of 1000 min. and for sample size of 1 g. The present procedure provides a rapid and reliable method for the determination of thorium and uranium isotopes in soil and sediment samples. The entire procedure requires about 8 hours to obtain the final fractions ready for electrodeposition. The results indicate that the accuracy and precision of the determinations
139
by the proposed method are satisfactory to control the discharge of natural actinides in environmental monitoring and t o study their distribution and geochemical behaviours.
REFERENCES 1. Ivanovich M., Harmon R. S., Uranium series disequilibrium. Applications to environmental problems, Clarendon Press, Oxford, 1982. 2. Percival D. R., Martin D. B., Anal. Chem., 1742, 46, 1974.
3. Sill C. W., Puphal K. W., Hindman F. D., Anal. Chem., 1725, 46, 1974. 4. Sill C. W., Health Physics, 393, 33, 1977. 5. Sill C. W., Anal. Chem., 618, 49, 1977. 6. Bernabee R. P., Health Physics, 688, 44, 1983.
7. Anderson R. F, Fleer A. D., Anal. Chem., 1142, 54, 1982. 8. Thomson J., Anal. Chim. Acta, 259, 142, 1982.
9. Jiang F. S., Lee S. C., Bakhtiar S. N., Kuroda P. K., J. Radioanal. Nucl. Chem., 65, 100, 1986. 10. Sill C. W., Anal. Chem., 1684, 33, 1961.
11. Extraction Chromatography, (T. Braun and G. Ghersini Eds), Elsevier, Amsterdam, 1975. 12. Talvitie N. A., Anal. Chem., 280, 44, 1972. 13. Roman D., Int. J. Appl. Radiat. Isot., 990, 35, 1984. 14. Ross W. J., White J. C., USAEC Report NAS-NS 3102, 1961. 15. Testa C., Anal. Chim. Acta, 447, 50, 1970. 16. Testa C., Santori G., Giorn. Fis. San. Prot. Rad., 1, 16, 1972. 17. Santori G., Testa C., J. Radioanal. Chem., 37, 14, 1973. 18. Testa C., Delle Site A., J. Radioanal. Chem., 121, 34, 1976. 19. Delle Site A., Marchionni V., Testa C., Triulzi C., Anal. Chim. Acta, 217, 117, 1980.
DETERMINATION OF ALPHA-EMITTING NUCLIDES OF THORIUM AND URANIUM IN SOIL AND SEDIMENT SAMPLES
P. SPEZZANO N. SILVESTRI ENEA, CRE Saluggia, Fuel Cycle Department, Radiotoxicological and Environmental Laboratories,
13040 Saluggia, Vercelli, Italy
ABSTRACT A procedure for the radiochemical determination of 228 Th, U, and 238 U in soil and sediment samples is described.
230 Th, 232 Th, 234
U,
235
The analytical method is based on the decomposition of the sample by Auoridepyrosulphate fusion technique to allow total dissolution and chemical exchange between natural isotopes and added yield tracer nuclides. Uranium and thorium are coprecipitated as hydrolyzable metals with natural Fe and A1 acting as carriers. Separation of thorium and uranium is carried out by reversed-phase chronlatography using a column filled with Microthene impregnated with tri-n-octylphosphine oxide (TOPO) as stationary phase followed by final purification by anion exchange chromatography. Each fraction is electrodeposited and final measurements are made by alpha spectrometry with surface barrier detector. Overall chemical yields generally are better than 60%. For individual isotopes, the lower limit of detection at the 95% confidence level is 10W3 Bq/g for a counting time of 1000’. One of the most important applications of the present procedure is its use in environmental monitoring, particularly in the vicinity of nuclear facilities and uranium mills, and for the study on the distributions and geochemical behaviors of natural actinides.
1.
INTRODUCTION
The determination of small concentrations of natural thorium and uranium in environmental samples has been object of several investigation for many years. Colorimetric or fluorometric procedures give good sensitivity, precision and accuracy, but
132 they do not supply any informations about isotopic distribution. Such inform* tions are of great interest in radio-ecological research and to understand geological process 111. Moreover, analysis of actinides nuclides in environmental materials are important for control measurements associated with discharge of these radionuclides from nuclear facilities and uranium mills. Many methods used for determination of thorium and uranium isotopes in environmental samples by alpha spectrometry include barium sulfate or cerium flu* ride precipitation for the initial separation of the actinides and final purification by solvent extraction 12-61 or they involve a sequence of purification steps by ion exchange chromatography 17-91. Methods involving sequential separation of both natural and artificial actinides allow their determination simultaneously on a single sample but they are generally useful when high levels of man-made alpha-emitting nuclides are encountered, as in locally contaminated areas. When only thorium and uranium isotopes have to be determined, it is possible to use a simply analytical procedure. In this paper a procedure for the sequential determination of alpha-emitting isotopes of thorium and uranium in soil and sediment samples is described. In the presented procedure, tracers are added to the sample at the earliest possible stage and potassium fluoride and pyrosulfate fusion [lo] was chosen as a reliable and effective method to bring tracer and total sample into solution simultaneously. Thorium and uranium are then coprecipitated in a mixed hydroxide precipitate using iron and aluminium naturally present as carrier. Separation of thorium and uranium is carried out by extraction chromatography using tri-n-octylphopsphine oxide (TOPO) followed by final purification with anion exchange chromatography. Each fraction is electroplated to obtain suitable thin sources for final measurements by alpha spectrometry.
2.
2.1.
EXPERIMENTAL Instrumentat ion
Alpha spectrometry measurements were performed using a 4-channel spectrometer comprising Tennelec TC 256 alpha spectrometers with a 450 mma surface barrier detectors (Ortec) connected through a Tennelec TC 306 Mixer Router to the quarters of a 2048-channel analyzer (Silena) adjusted to 10 keV/channel. The spectral region covered extended from 3 to 8 MeV. The resolution obtained with samples electroplated on stainless steel planchets with an active area of 1.6 cm in diameter was 40-60 keV (FWHM) at a distance source detector of about 1 cm. Counting efficiency of the detectors ranged between 19.1 and 24.1%. Occasionally, measurements of gamma activity were performed using a Ge detector connected to a 4096-channel pulse-height analyzer.
133
-
Tracer, K F , NaZS04
Sample fusion
HCl dil.. +
NH40H
supernatant discard
+
precipitation 4 M HNOj-
dissolution
-
1 M HN03 discard
u fraction
Th fraction
d3
elect rodeoosi tion
Th d e t e r m i n a t i o n
m
elect rodepositi o n
U determination
Fig. 1. Flow diagram of thorium and uranium determination
134
2.2.
Reagents and Tracers
Tri-n-octylphosphine oxide (TOPO), Eastman Kodak Co., was used as extractant in 0.2 M cyclohexane solution and Microthene 651/50, microporous polyethylene, 60-100 mesh, Columbia Organic Chemicals, was used as the inert support for chromatographic separations. The stationary phase was prepared by mixing 3 g of Microthene with 2.5 ml of 0.2 M TOPO solution and conditioning with 40 ml of 1 M HN03, as described in literature Ill]. Anion-exchange resin, AG 1-X8, 50-100 mesh, chloride form, Bio-Rad Lab., was used for final purification of both thorium and uranium fractions. The calibrated 232U/22sTh spike solution was prepared by dilution from an aged standard solution (C.E.A., France). The activity of the final solution in 4 M HNO3, was 0.343 0.008 Bq/ml. The remaining tracers used, 226Ra, 210Po,237Np/233Pa,238Pu and 241Am were studied only as radiochemical interferences and were prepared from standard solution obtained from Amersham (U.K.) and C.E.A. All other reagents were of analytical grade.
*
3. 3.1.
PROCEDURE Sample Dissolution
Soil and sediment samples were dried at 12OoC and ground to pass through a 100 mesh standard sieve. About 1 g of sample was transferred t o a 30 ml platinum crucible and 0.5 ml of 232U/22sTh tracer solution was added. Sample was then evaporated t o dryness. The residue was mixed with 3 g of anhydrous potassium fluoride and heated to fusion. After cooling, the cake was treated with 3 ml of concentrated sulfuric acid and 3 g of anhydrous sodium sulfate were added for the pyrosulfate fusion according to Sill’s procedure 110). After cooling, the platinum crucible was transferred in a 200 ml beaker containing 100 ml of water, 10 ml of concentrated hydrochloric acid and 1 ml of 30% hydrogen peroxide and heated until dissolution of the content.
3.2.
Separation of Thorium and Uranium
The solution was transferred to a 250 ml centrifuge tube and the hydrated oxides (mainly iron and aluminium) were precipitated by adding ammonia t o a final pH of 7-8. The precipitate was -.entrifuged, washed with a solution of NHIOH at pH 8 and centrifuged again. The precipitate was then dissolved in 20 ml of 4 M HN03. The sample solution was passed at a flow rate of 1-2 ml/min through a chromatographic column (height: 10 cm - diameter: 1 cm) filled with the MicrotheneTOPO stationary phase. Thorium and uranium were sorbed onto the column. After the column had been washed with 30 ml of 1 M H N 0 3 in small increments t o rinse the column wall, thorium was eluted with 30 ml of 0.3 M H2S04 and
135 uranium was eluted with 30 ml of 1 M HF, maintaining always the same flow rate. The eluates were evaporated to dryness.
3.3.
Thorium Purification
A few ml of concentrated HN03 were added t o the residue and again evaporated t o dryness. This step was repeated and finally the residue was dissolved in a few ml of 8 M HN03. The solution was passed through a column (Dowex AGlx8, 50-100 mesh, 5 x 0.5 cm, preconditioned with 8 M H N 0 3 ) at a flow rate of 0.5 ml per minute. The column was washed with 10 ml of 8 M H N 0 3 and finally thorium was eluted with 20 ml of 8 M HC1. The eluate was collected and evaporated t o dryness.
3.4.
Uranium Purification
After evaporation to dryness, uranium fraction was treated with concentrated HCl and evaporated t o dryness. The residue was dissolved in a few ml of 8 M HC1 and the solution was passed through a second column (Dowex AGlx8, 50-100 mesh, chloride form, 5 x 0.5 cm) at a flow rate of 0.5 ml per minute. The column was washed with 10 ml of 8 M HC1 0.1 M HF. Uranium was eluted with 10 ml of 1 M HCl and the eluate was evaporated t o dryness.
+
3.5.
Electrodeposition
Thorium was electrodeposited starting from (NH4)aS01 at pH 2-2.5 according t o the Talvitie procedure [12],which was lightly modified. The residue was dissolved in 1 ml of concentrated sulfuric acid. After heating until dense white fumes were obtained, 6 ml of distilled water and 100 p1 of 1 M oxalic acid were added. The solution was neutralized with ammonia t o pH 2.5 (Thymol blue as indicator) and transferred into the electrodeposition cell. The beaker was washed with little portions of 1:99 H2SO4 (6 ml in total) and the wash solutions were added t o the cell. The pH was readjusted t o 2-2.5 (salmon-pink end point) with ammonia and some drops of 1:99 H2SO4. Electrodeposition was performed at a current density of 300 mA/cm2 for 4 hours. A minute before current stop, 1 ml of ammonia was added into the cell. Uranium was electroplated starting from 0.35 M ammonium acetate taken t o pH 2 with H N 0 3 [13].The residue was dissolved in 5 ml of 0.35 M CH3COONH4 (pH = 2) with heating and transferred t o the electroplating cell. The beaker was washed with little portions of electrolyte (5 ml in total) which were also transferred into the cell. The electrodeposition was carried out for 4 hours at 200 mA/cm2. A flow diagram for the separation and purification of thorium and uranium is shown in Fig. 1.
136
Tab. I. Results of thorium and uranium ores analysis Uranium ore, Bq/g
Thorium ore, Bq/g Isotopes 2 3 8 ~
Calculated
Measured
4.93
5.19f0.16
Measured 123.2
5.00f0.17 2 3 5 ~
2 3 4 ~
0.23
126.0fS.5 5.75
0.24f0.01 0.23f0.01
-
-
5.24f0.17 5.04f0.16
'"Th 230Th
41.1
-
41.3f 1.2
5.78f0.24 5.72f0.25 124.8zk3.5 125.1f3.5 <0.05
41.75 1.3
-
5.1690.23
-
123.753.6
-
124.1f3.5 -
4.95f0.22 Activity ratio
125.4f3.4
-
1.993f0.017
22aTh/232Th
1.998f0.016
Tab. 2. Decontamination Factors from some alpha emitters
Fina raction Tracer
Thorium
Uranium
2lOPo
2 x 102
4 x lo3
226Ra
> I x lo5
> 1 x lo5 > 2.5 x lo3
228Th
-
232u
>I x lo3
-
'"pa
> I x lo3
> 1 x lo3
>30
5 x 102
237Np
23SPU
2x
'*'Am
>i x lo5
102
7x
>
102
1x
lo5
137 Tab. 3. Radiochemical results for triplicate analysis of thorium and uranium isotopes in surface soils (Bq/kg) 230~h
232~h
23pu
2 3 5 ~
2 3 8 ~
41.652.4 32.1f2.2 42.1f2.5 2.2i0.47 41.1f2.5 45.1f2.5 35.4f2.2 40.7f1.7 1.8k0.32 42.4f1.8 42.4f2.6 33.7f2.4 42.3f2.4 2.4f0.46 42.9f2.4 meanfs 43.0f1.8 33.7f1.6 41.7f0.9 2.lf0.31 42.1f0.9
63.5f2.3 48.2f1.8 68.4f3.8 2.8f0.68 67.2f3.6 60.3f2.7 46.2f2.1 64.3f3.2 3.1f0.57 63.7f3.1 64.4f2.2 49.3f1.7 65.9f3.3 2.4f0.45 65.1f3.2 meanfs
4.
62.7f2.1 47.9f1.6 66.2f2.1 2.8f0.35
65.3f1.8
RESULTS AND DISCUSSION
The method described above is based on the possibility of thorium and uranium extraction from a nitric acid solution using tri-n-octylphosphine oxide. In this system, TOPO gives higher distribution coefficients for actinides of valency state IV and VI respect to other matrix elements, giving effective separation from all significant interferences. TOPO has been studied as extractant for several elements [14]. Testa and coworkers (15-191 used TOPO for the determination of actinides in biological and environmental samples using extraction chromatography. In this work, TOPO has been used in order to separate thorium and uranium from radiochemical and chemical interferences in soil and sediment samples. After extraction, thorium was stripped with 0.3 M H2S04 while uranium was stripped with 1 M HF, as described by Testa et al. At these point, of the other alpha-emitting nuclides of interest, any polonium strips with the thorium fraction while protactinium follows uranium. Neptunium and plutonium, if present, split between the two fractions. Polonium constitutes a serious interference in thorium determination because the alpha-peak of 210Po, 5.30 MeV, is very c!ose to the 5.34 and 5.42 MeV alphapeaks of 228Th, and thus the chemical yields of thorium may be overestimated. Therefore, the anion-exchange purification step was necessary in order to remove 210Po. Purification of thorium fractions was also useful to remove any residual iron that might interfere during thorium electrodeposition. The anion-exchange purification steps were also useful to remove neptunium and plutonium in both thorium and uranium fractions, but this could be necessary only if the procedure is applied to samples containing high activities of transuranium elements.
138 The principal losses of thorium and uranium occur during the electrodeposition step since the actinides electrodeposition yield decreases in the presence of other elements presents at the microgram level, as it is possible t o obtain after their separation from soil and sediment samples. In the present procedure, the best results for thorium were obtained starting from ammonium sulfate at pH 2, while for uranium high electrodeposition yields were obtained using 0.35 hf ammonium acetate at pH 2. Accuracy and precision of analytical method were checked by analyzing two samples of standard thorium and uranium ores in which both natural chains were thought to be present in secular equilibrium. Analysis were performed, after dissolution, on an appropriate aliquote containing about 100 pg of both thorium and uranium and 10 mg of iron were added as carrier t o assure a complete recovery of the actinides. Determination of thorium and uranium activities were performed by alpha spectrometry on electroplated disks that were counted for the time necess a r y t o give t o statistic counting uncertainty a value of about 1%. Assessment of 228Th/232Th ratio was performed on separate aliquote of unspiked samples. The analytical values obtained with the present procedure are shown in Tab. 1. The uncertainties given are the standard deviations resulting from propagation of all uncertainties incurred in the entire measurement process. Agreement of analytical results with calculated values are within statistics uncertainties. Decontamination factors for possible radiochemical interferences were also determined by adding 20 - 200 Bq of each radionuclide t o 1 g soil samples which were then carried throught the entire procedure. The results axe shown in Tab. 2. After all developmental work had been completed, the procedure was applied in its final form on soil and sediment samples spiked with known quantities of 232U/228Th tracer solution. Assessment of 228Th/232Th ratio was performed on separate aliquote of unspiked sample and it was necessary t o calculate chemical yield for thorium analysis. Obtained separation chemical yields were generally good, with values for thorium of 48-85%, average 72%, and for uranium 53-72%, average 62%. Tab.3 shows the experimental values obtained on three assessments for thorium and uranium isotopes carried out, according to above mentioned procedure, on two surface soil samples collected at Saluggia (VC), Italy. All uncertainties were propagated t o the final values. Agreement among triplicate analyses of actual samples is further evidence of good precision of the method. The larger uncertainties in 235U determination is due to the much lower activities and, therefore, poorer counting statistics. Sensitivity of analytical procedure was also calculated assuming a background count rate of the detector of 0.005 cpm under the peaks of the isotopes being measured. The lower limit of detection at the 95% confidence level is 10-3Bq/g for a count time of 1000 min. and for sample size of 1 g. The present procedure provides a rapid and reliable method for the determination of thorium and uranium isotopes in soil and sediment samples. The entire procedure requires about 8 hours to obtain the final fractions ready for electrodeposition. The results indicate that the accuracy and precision of the determinations
139
by the proposed method are satisfactory to control the discharge of natural actinides in environmental monitoring and t o study their distribution and geochemical behaviours.
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