uranium isotopes are determined by alpha-spectrometry following extraction with TOPO "onto a polymeric membrane. Thorium isotopes are co-precipitated with ...
Journal of Radioanalytical mzd Nuclear Chemistry, Articles, VoL 182, No. 1 (1994) 165-169
D E V E L O P M E N T O F A S E Q U E N T I A L M E T H O D FOR T H E D E T E R M I N A T I O N O F 238U, 234U, 232Th, 230Th, 228Th, 228Ra, 226Ra, A N D 210pb. IN E N V I R O N M E N T A L S A M P L E S J. M. GODOY, D. C. LAURIA, M~LUIZA D. P. GODOY, R. P. CUNHA h~stitu/o de Radioprotef~o e Dosimetria, Comiss~o Nacional de Energia Nuclear, C.P. 37750 - C E P 22793 - Rio de Janeiro (Brazil)
(Received January 24, 1994) A sequential analytical method for the determination of 2:~U, Z-~U,7~
z-~r'h, Z~Th, Z~Ra,
Z26Ra and 21~ in environmental samples was developed. Uranium and thorium isotopes are first chromatographically separated using tri-n-octyt phosphine oxide (TOPO) supported on silica gel. The uranium isotopes are determined by alpha-spectrometry following extraction with TOPO "onto a polymeric membrane. Thorium isotopes are co-precipitated with lanthanum fluoride before counting in an alpha spectrometer. Radium isotopes and 21~ are separated lay co-precipitation/precipitation
with mixed barium/lead sulphate. Radium-226is determined by gross alpha counting of the final BaSO4 precipitate and ~SRa by gross beta counting of the same source. Lead-210 is determined through beta counting of its daught~ product 2~~
The knowledge about the distribution of the long-lived natural radionuclides between different environmental compartments is necessary.
Characterization of sites contaminated by
uranium, radium and monazite sand require radiochemical analyses of uranium, thorium and their daughter products, especially if remedial actions are required. Sequential analytical method could be a helpful tool. Several methods are reported on in the literature; Percival and Martin, ~ Sill et. al., 2 Godoy, 3 and Lauria and Godoy. 4 The present method is an optimization of that from Lauria and Godoy, 4 which improves the decontamination factor for Th from Po. The technique also improves the 21~
determination.
Experimental The column (TOPO/Silicagel) is prepared as described previously. 4 Uranium-232 and 2~I'h tracers for the yield determinations of uranium and thorium were prepared as described previously. 4 Stable barium and lead carriers were used for radium and 2~~
The chemical yield for radium
was determined by atomic absorption spectrometry and for lead gravimewically as PbCrO 4. The measurements of the thorium and uranium isotopes was carried out by alpha spectrometry using ORTEC 576 surface barrier detectors coupled to one ORTEC ADCAM 918 and
Elsevier Science S. A.. Lausanne AkadOniai Kiadr, Budapest
J. M. GODOY et al.: DEVELOPMENTOF A SEQUENTIALMETHOD
a personal microcomputer. For the gamma-spectrometric measurements of 234Th a 30% intrinsic germanium detector (ORTEC Gamma-X high purity Ge) was connected to an ORTEC ADCAM 918 and a personal microcomputer. For the 22~Ra, 228Ra and 2~~ determinations a 10channel Low-Background Berthold's gas-flow proportional counter was used. All the reagents used were of analytical grade. Results and discussion
The procedure is described in schematic form in Figure 1. The conditions of the uranium and thorium separations on the TOPO column are the same as described previously. 4 The only modification introduced is the deposition of polonium on copper, in order to increase the decontamination factor of thorium from polonium. 4 After the column percolation, barium and lead sulphates are precipitated from the (feed + wash) solution by the addition of H2SO4. The mixed sulphates are then centrifuged, separated, and redissolved in nitrilotriacetic acid (NTA). Barium containing radium sulphate is re-precipitated by adding acetic acid until a pH of 4.5-5.0 is reached, leaving lead in the aqueous phase. The barium and radium sulphates are re-precipitated again but now from an ethylenediamine tetra-acetic acid (EDTA) medium, in order to obtain a cleaner final precipitate. After one month ,of growth, 2~Ra is determined by gross alpha counting as described by Godoy and Schuttelkopf. 5 Radium-228 is determined by beta counting. Alpha particles of 2Z6Ra are stopped by the filter paper. Corrections are made if there is any contribution from the daughter products of ~Z6Ra. Lead is precipitated from the NTA solution as PbS. After elimination of the sulfur by nitric acid addition, lead is precipitated as PbCrO 4 from the solution at pH 4.5-5.0. About two weeks later, 2t~
is determined through its daughter product 2~~ by beta counting)
The entire procedure, including the overnight settling of precipitates, takes about 2-3 days, starting from the sample preparation to the precipitation and counting. The decontamination factors obtained are 'shown in Table 1. It can be noted that by the deposition of 21~ on copper increases the decontamination of thorium from polonium by a factor of 3.4x104 as compared to only 34 as reported in the earlier technique. 4 Decontamination for other elements is greater than 2.5x102. With the typical chemical yields of 85% for thorium, radium, and lead, and 70% for uranium, the minimum detectable activities, using 1000 minutes counting time and 95% confidence level, are: 238U, 234U : 1 mBq; 232Th, 23~
and 22STh : l mBq; ~SRa : 5 mBq, but increases with
the 2~6Ra content; 22~
: 5 mBq.
166
: 1 mBq and 21~
J. M. GODOYet al.: DEVELOPMENTOF A SEQUENTIALMETHOD I Feed Solution I IM ItNO3
I U, Th,
Ra, Pb, Po
I TOPOISilicagel Column Columi~
Solution Ra, Pb
U, Th, Po
(Ba, Pb) SO4 Precipitation I ~ Reprecipitation of BaSO4 from NTA solution I
.
I
I I Solution I
recip Ra '
Eluant I I lution ' Column (Po) (Po) l
,
Pb
I
Reprecipitation I of BaSO4 from EDTA solution
I
PbS Precipitation
Ra
226Ra and 228 Ra determination by I gross alpha/beta I counting
Dissol. HNO 3 precipit. PBCrO4 sol. pH 5
IM NtI4 Eluant
Deposition on Cu Solution
Th Microprecipitation with LaF 3
I Precip. I Precip,
U,
Th
I Th alpha spectrometry
Solution V(Po)
Evaporation of NH 4 F Residue
U(Po)
I
Deposition
on TOPO film and ashis8
~
Pr~biP'
] U
21%b determ, by
1
alpha ]
spectrometry
21%i beta counting
Figure 1. The developed sequential method.
167
J. M. GODOY et al.: DEVELOPMENT OF A SEQUENTIAL METHOD
TABLE 1 D e c o n t a m i n a t i o n factors
Element
Contaminant
U U
Th
Ra
2.5 x 102
2.0 x 104
1.0 x 104
4.0 x 103
3.4 x 104
Th
8.0 x 102
Ra
3.4 x 104
1.8 x 104
Pb
2.8 x 103
1.0 X 104
Pb
Po
7.0 x 103
2.8 x 104
6.6 x 102
TABLE 2 O b t a i n e d values for the I A E A - S D A 1 reference s a m p l e s
Radionuclide
Reference value
(mBq/g~,y)
(mBq/gdry)
23SU
7.0 • 0.4
7.3 (6.3 - 3.5)"
2~U
7.4 • 0.7
7.3 (7 - 7.7)
232Th
12.0 • 1.5
12.2 (10.5 - 13.8)
23~ Z2STh
104.2 • 8.5 12.6 + 1.9
101.4 (2 - 106) 12.3 ( 1 0 . 2 - 12.5)
Z~SRa
N.D.
12.3 (10.9 - 15.6)
226Ra
63.0 5 : 5 . 0
74.9 (55 - 85)
21~
70.0 + 6.8
70 (58 - 88)
N.D. = Not detectable * = confidence level
168
F o u n d value
J. M. GODOY et al.: DEVELOPMENTOF A SEQUENTIALMETHOD
Five aliquots of 3 grams each of IAEA-SDA1 (marine sediment) reference samples were analyzed in order to check the accuracy of the method and the results are shown in Table 2. With the exception of 2~SRa (15%) all errors relative to the reference values were lower than 5%. Even for ~ R a
the results obtained are within the range of the confidence interval.
For all the
radionuclides determined, the associated standard deviation was less than 10%.
Concludons The present method is an improvement over the previous method due to a greater decontamination factor for thorium from polonium and due to the inclusion of the determination o f 2~~
The achieved detection limits allow the determination of U, Th, Ra and 21~
from the
same sample aliquot of practically any kind of environmental sample without using very large amounts of samples. References
1. D. R. PERCIVAL, D. B. MARTIN, Anal. Chem., 46 (1974) 1742. 2. C. W. SILL,K. W. PUPHAL, F. D. HINDMAN, Anal. Chem., 46 (1974) 1725. 3. J. M. GODOY, KFK Report 3505 KernforschungszentrumKarlsruhe GmbH, Posffach 3640, 75 Karlsruhe 1, FRG. January 1983. 4. D. C. LAURIA,J. M. GODOY, Sci. Total Environm.,70 (1988) 83. 5. J. M. GODOY, H. SCHUITBLKOPF, J. Radio,anal.Nucl. Chem., 111 (1987) 329.
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