2556
Anal. Chem. 1987, 59, 2556-2558
Rapid Determination of Thorium-230 in Mill Tailings by a Spectrometry Stephen Donivan,* Mark Hollenbach, a n d Mary Costello
Analytical Chemistry Laboratory, USDOE Grand Junction Projects Office, UNC Geotech, Grand Junction, Colorado 81502
A rapid procedure for the determination of rjDrh In mM taillngs by (I! spectrometry followlng lithlum metaborate fusion and purlflcatlon by solvent extraction has been developed. The source for (I! spectrometry Is prepared by copreclpltatlon with cerlum fluorlde. The procedure has been applled to samples collected at Department of Energy remedlal action sites, demonstrating an accuracy of 2.5 % and a preclslon of 5 % The detection ilmit Is 0.3 pCi/g. With thls procedure, a slngle analyst can analyze 15 samples/day.
.
An efficient, reliable procedure for the determination of T h activities in soils and mill tailings was needed to support site characterization projects such as the Formerly Utilized Sites Remedial Action Program (FUSRAP),the Surplus Facilities Management Program (SFMP), and the Uranium Mill Tailings Remedial Action Program (UMTRAP). These programs are administered by the US. Department of Energy (DOE) Office of Remedial Action and Waste Technology. Site characterizations are conducted to identify the extent of contamination and involve demarcating the geographic boundaries where contaminant activities exceed the cleanup criteria. This information is required to develop engineering plans for site cleanup. Radioactive contamination at most sites comprises radionuclides of the uranium decay series. As a member of this series, T h activities are a concern because 230This one of the most toxic radionuclides known ( I ) and because %is the parent nuclide of 226Ra. The DOE cleanup criterion for 22sRain soils is 5 pCi/g. If 22eRaactivities are used as the sole criterion for cleanup, a situation where 22sRa activities are less than the cleanup criterion of 5 pCi/g and T h activities are 1000 pCi/g would result in a 226Raactivity of 10 pCi/g after 25 years due to ingrowth, and the site would fail to meet the cleanup criteria. Characterization of a typical remedial action site requires accurate and precise analysis of hundreds of samples. And while accuracy and precision are important, low cost and speed are essential since the results are usually needed within weeks. These factors must be considered when selecting an analytical procedure. Numerous laboratory procedures for the determination of thorium isotopes by CY spectrometry have appeared in the literature in recent years. Some typical examples include Sill’s (I) potassium fluoride-pyrosulfate fusion followed by barium sulfate precipitation and solvent extraction, the procedure of Durham and Joshi (Z),which utilizes solvent extraction with high molecular weight amines and anion-exchange chromatography, and the procedure of deJong and Wiles (3), involving barium sulfate and lanthanum fluoride precipitations. These and other procedures offer good accuracy and precision, but many are difficult to perform and are too time-consuming for the analysis of large numbers of samples. In a previous investigation ( 4 ) , a procedure was developed for the simultaneous determination of uranium and thorium isotopes by cy spectrometry. The procedure utilizes a lithium metaborate fusion, which is much faster and simpler to perform than a potassium fluoride-pyrosulfate fusion and
has proven effective on a variety of materials. This procedure has been modified considerably for application to samples requiring only 230Thanalysis, resulting in a procedure that is very fast, reliable, and easy to perform. Recently, this procedure was applied’to soil samples collected a t an UMTRAP site in Falls City, TX. Over 300 samples were submitted and the results were requested within 30 days. The results presented here were obtained during the analysis of the Falls City samples and illustrate the utility of the procedure in a production-type situation. EXPERIMENTAL SECTION Instrumentation. CY spectra were obtained by using 300-mm2 partially depleted silicon detectors housed in either Ortec Model 576 or Tennelec Model E 2 5 6 CY spectrometers. A Canberra Series 90 multichannel analyzer (MCA) was used for data collection. Sixteen spectrometers were used for this work, and each was allocated 1024 channels of memory in the MCA, resulting in 5 keV per channel. Detector resolution is 24 keV at full width at half-maximum. Resolution of actual samples is usually about 40 keV at a detector-to-sampledistance of 0.5 in. where the counting efficiency is 15% . Reagents. Cerium Chloride. A thorium-free 1mg/mL solution of cerium chloride is prepared by dissolving 0.27 g of CeC1,.7H20 in 10 mL of 6 N HC1, extracting with 10 mL of 2% tri-n-octylphosphine oxide solution (in cyclohexane),and then diluting the aqueous phase to 100 mL with water. Cerium Substrate Suspension. A cerium substrate suspension is prepared by adding 5 mL of the 1 mg/mL cerium chloride solution to 500 mL of 1M HCl and mixing. Forty milliliters of 48% HF is then added. Sample Preparation. Upon receipt, samples are dried at 100 “C for 24 h, ground to -28 mesh, and blended. An aliquot is taken for ‘%Ra analysis by high-resolution y spectroscopy (5). Another aliquot is ground to -100 mesh, blended, and submitted to the laboratory for analysis. Procedure. A 0.5-g sample is weighed into a 10-mL platinum dish and moistened with a small amount of water to minimize any frothing that might occur. Approximately 25 pCi of 228Th are then added. The actual activity added must be accurately known. Approximately 5 mL of 48% hydrofluoric acid is cautiously added. The sample is evaporated to near dryness, an additional 5 mL of hydrofluoric acid is added, and the sample is taken to dryness to eliminate silica, which might otherwise interfere with the subsequent solvent extraction. One gram of lithium metaborate is added and the sample is fused, using an automatic fusion burner, until a uniform melt is obtained. The dish is dropped while very hot into a 250-mL beaker containing about 50 mL of 2 N HN03. The sample is heated and swirled until completely dissolved and the volume reduced to about 40 mL. After the sample has cooled, the solution is transferred to a 50-mL disposable centrifuge tube. Ten milliliters of 2 % tri-noctylphosphine oxide solution (TOPO) in cyclohexane is added, the tube is capped, and it is then shaken on a mechanical shaker for 3 min. The sample is centrifuged for 1 min to separate layers. The TOPO layer is removed, using an automatic pipet and a disposable tip, to a second centrifuge tube containing 10 mL of 0.3 M sulfuric acid-2% sodium sulfate solution. The disposable pipet tip makes a very convenient disposable “separatoryfunnel”. The second centrifuge tube is capped and shaken for 5 min to back-extract the thorium and centrifuged as before. The TOPO
0003-2700/87/0359-2558$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
layer is removed and discarded with the automatic pipet. Fifty micoliters of the 1mg/mL cerium chloride solution and 250 pL of 20% sodium nitrite solution are added to the tube and mixed. One milliliter of 48% hydrofluoric acid is added to the tube and the solution is mixed and allowed to stand for 30 min. A 25-mm, 0.1-pm polycarbonate filter is mounted in a plastic filtration funnel, and two 5-mL portions of the cerium substrate suspension are filtered through the funnel. The suspension should be shaken thoroughly before use. The sample solution is filtered through the prepared membrane filter, washing first with water and then with an 80% ethanol-in-watersolution. The filter is mounted on a 1-in.-diameter,round, gummed label, dried, and counted overnight in the CY spectrometer. After counting is completed, the zszTh,zao?lh,and 228Th peaks are integrated. The counts obtained in the 228Thpeak must be corrected for any naturally occurring 228Thin the sample by subtracting the counts from the %counts. This assumes that =Th and q h are in equilibrium in the sample. The result represents the net 228Thcounts due to the 22sThactivity added as an internal standard and can then be used to calculate the 23oTh activity.
RESULTS AND DISCUSSION Procedure Selection. The project requirements of cost, speed, and reliability and the nature of the types of sample encountered dictated many of the elements of the procedure. Because resistate minerals such as zircon are sometimes encountered (especially in tailings samples) and can contain thorium, sample-digestionprocedures that result in complete sample dissolution are required. A simple acid digestion such as that described by Peters and Gladney (6) is not adequate in this situation. The lithium metaborate fusion procedure selected here performs well on a wide variety of sample types, including zircon (3, and is faster and simpler to perform than other types of fusion procedures. Sixteen samples can be completely dissolved and readied for extraction in under 2 h. The well-characterized solvent-extraction system using tri-n-octylphosphine oxide (8)was used because it is fast and efficient and results in a solution that is amenable to the rest of the procedure without requiring “take-to-dryness” type steps, which often result in loss of analyte (9). In recent years, coprecipitation of actinides and filtration with 0.1-pm membrane filters have virtually replaced electrodeposition procedures. The procedure of Sill and Williams (10) was used with essentially no changes, except that the recommended Type HT Tuffryn filters are no longer available. The membrane fiiters used for this work were obtained from Nuclepore and are more difficult to handle than the Tuffryn filters. The use of 1-in.-diameter gummed labels as mounting media facilitates handling of the filters and makes their use practical. Figure 1 presents a typical spectrum obtained by this procedure. The analysis of 16 samples typically takes 8 manhours, and results can be available overnight, if required. Chemical Recovery. The chemical recovery of the procedure was checked by analyzing several samples by inductively coupled plasma (ICP) emission spectroscopy. Thorium was determined at the 283.73-nm wavelength, using a Perkin-Elmer Model 6000 spectrometer. Samples containing 100 mg of thorium were processed through the procedure, and the resulting membrane filters were digested in HN03, HC104, and HzS04,and then diluted to a 10-mL final volume for measurement. The results of the ICP measurements plus additional recovery checks by using the net 228Thcount rates obtained for the actual Falls City samples indicate that the recoveries were extremely consistent throughout this project, averaging 95 % . Interferences. No chemical interferences were encountered during the development and use of this procedure. The T O P 0 extraction procedure results in a solution free of elementa that could interfere with the cerium fluoride precipitation. The only exception is natural thorium (232Th),which
2557
zzeTh
will result in degraded resolution if present in amounts greater than 200 pg. A radiometric interference will result from high concentrations of uranium, because the 234Ua peak at 4.72 MeV cannot be adequately resolved from the 230Tha peak at 4.69 MeV. The cerium fluoride precipitation is performed from an oxidizing solution to ensure that uranium is in the hexavalent state, which does not precipitate as a fluoride. The determination of small amounts of 23@l?hcan be successfully accomplished in the presence of up to 5000 pg/g of uranium with this method. Another radiometric interference can result in the inappropriate use of 22aThas an internal standard. zz8This a member of the thorium decay series and is present in significant amounts in some samples. Low values for 230Thwill result if the gross counts in the 228Thpeak are not corrected for contributions from 228Thpresent in the sample. An error of this type would better explain the lower-than-expected results obtained for 230Thon samples containing high concentrations of 232Thby Peters and Gladney (6) than their speculation concerning thorium ore formation. The use of 2zaThas an internal standard assumes that equilibrium exists in the sample between 232Thand zzsTh. This assumption may not be true for all samples and can be the largest source of error when using this procedure. The amount of 228Thadded as an internal standard must be large compared to the amount occurring in the sample to minimize this potential error. For most samples, the correction required to the 228Thcounts obtained is typically about 5 % . For those samples which contain larger amounts of thorium and for which the state of equilibrium is suspect, the sample can be rerun without the addition of 228Th to determine the 2z8Th/232Th ratio. Reanalysis of this type was not required for any of the samples analyzed during this study. 228Thwas selected as the internal standard because of the lack of any other suitable isotope. The use of 234Thas in Sill ( 1 ) may result in higher accuracy and precision but is impractical for a rapid, low-cost procedure. 2 q h requires a separate y count of the 177-keV peak for a recovery determination and, as a result, the a-counting efficiency must be accurately known and very reproducible. This is impractical when operating 16 spctrometers. Wrenn et al. (11) suggest the use of “’Th as a tracer, which works well when the approximate concen-
2558 Table
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
I. Results o f R e f e r e n c e M a t e r i a l Analysis sample
EPA climax sand tailings EPA diluted pitchblende ore EPA diluted climax sand tailings SY-2 SY-3 UTS-1 UTS-2 UTS-3 UTS-4 DH-1 (unknown control) BL-1 (unknown control)
23@Thconcentration, p C i / g result” expected value n
263 f 1 251 f 1 34 f 1 104 f 1 203 f 12 93 f 9 120 f 8 317 f 12 639 f 14 595 f 23 77 f 2
267 f 12 253 & 15 35f2 100 f 5 202 f 10 97 f 16 119 f 30 305 f 22 619 f 70 578 f 12 75f4
2 2 2 3
3 3 3 3 3 5 3
“The result is the average of two t o five analyses. T h e error i s an estimate of the experimental standard deviation.
tration of 23@’hl is previously known. The 2?l?hpeaks are not well resolved from the 230Thpeaks, resulting in peak overlap when the 22@l’h/230Th ratio is not approximately one. This then requires the reanalysis of samples when an inappropriate activity of 22gThwas selected. Thus, the use of 22eThwas ‘considered inappropriate for this application. Analysis of Control Samples. Approximately 60 control samples were analyzed concurrently with the 300 Falls City samples. There were two types of control samples: known controls, which were reference materials known to the analyst, and unknown controls, which were either reference materials or duplicate samples with identities unknown to the analyst. Table I presents the results of reference-material analysis obtained for known and unknown controls. The expected values for the Environmental Protection Agency (EPA) materials are the EPA certified values. The values for the UTS materials are the certified values provided by CANMET (Canada Centre for Mineral and Energy Technology) (12). Values for SY-2 and SY-3 are taken from Gladney et al. (13) and assume isotopic equilibrium between 23@l?hand zzsRa. Values for DH-1 and BL-1 are based on CANMET values, assuming equilibrium (14). Twenty-seven samples were analyzed in duplicate as unknown control samples. The average agreement obtained was 5%. Accuracy and Precision. The accuracy, based on the analysis of reference materials expressed as the average dif-
ference between the result and the expected value, is 2.5%. The precision estimated from duplicate sample analysis and replicate analysis of reference materials is k5%. It must be stressed that these results were obtained in a routine analysis environmentand that the precision is largely based on analysis of duplicate samples unknown to the analyst. These results represent what can be expected from the procedure in routine operation. A rapid, reliable procedure for the determination of 23’?l”h in samples of uranium mill tailings and contaminated soils has been presented. The 300 Falls City samples plus 60 control samples were analyzed in 24 working days by one analyst. None of the results were rejected by the laboratory’s rather stringent quality-control program, which requires the analysis of reference materials and duplicates unknown to the analyst, demonstrating the attributes of the procedure in actual use.
LITERATURE CITED ( 1 ) Sill, Claude W. Anal. Cbem. 1977, 49, 618-621. (2) Durham, R. W.: Joshi, S. R., J . Radloanal. Chem. 1979, 52, 181-188. (3) deJong, I. G.; Wiles, D. R. J . Radloanal. Nucl. Chem. 1984, 82, 309-318. (4) Donivan, Stephen; Hollenbach, Mark; Korte, Nic U.S. Department of Energy open file report oIBX-121(82), 1962. (5) Dechant, Gary; Donlvan, Stephen US. Department of Energy open file report GJBX-4(64), 1984. (6) Peters, Richard J.; Gladney, Ernest S. Geostand. Newsl. 1983, 7 , 319-320. (7) Korte, N.; Hollenbach, M.; Donlvan, S. At. Spectrosc. 1982, 3 . 79-80. (8) White, J. C.; Ross, W. J.; Nuclear Science Series Report 3102; National Academy of Sciences: Washlngton, DC, 196 1. (9) Sill, Claude W. N6S Spec. Publ. 1978, No. 422 463-490. (10) SIII, Claude, W.; Williams, Rodger L. Anal. Chem. 1981, 53, 412-415. (11) Wrenn, McDonald E.; Singh, Narayani P.; Ibrahim, Shawki, A,; Cohen, Norman Anal. Chem. 1978, 50. 1712-1713. (12) Smlth, C. W.; Steger, H. F., Bowman, W. S. CANMET Rep 1984; NUTP-2E. (13) Gladney, E. S.; Eberhardt, W.; Peters, R. J. Geostand. Newsl. 1982, 6, 5-6. (14) Ingles, J. C.; Sutarno, R.; Bowman, W.S.; Faye, G. H. CANMfTRep. 1977, 77-64.
RECEIVED for review April 30, 1987. Accepted July 10,1987. This work was performed at the US.Department of Energy Grand Junction, CO, Projects Office, which is operated by UNC Geotech under Contract DE-AC07-86ID12584. This article was supported by the Assistant Secretary for Nuclear Energy, Office of Remedial Action and Waste Technology.
