S. L. Maxwell, III and S. T. Nichols
Westinghouse Savannah River Company
Aiken, SC 29808

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There is a need to measure actinides in environmental samples with lower and lower detection limits, requiring larger sample sizes. This analysis is adversely affected by sample-matrix interferences, which make analyzing soil samples above five-grams very difficult. A new Actinide-Recovery Method has been developed by the Savannah River Site Central Laboratory to preconcentrate actinides from large-soil samples. Diphonix Resin^â (Eichrom Industries), a 1994 R&D 100 winner, is used to preconcentrate the actinides from large soil samples, which are bound powerfully to the resin’s diphosphonic acid groups. A rapid microwave-digestion technique is used to remove the actinides from the Diphonix Resin^â, which effectively eliminates interfering matrix components from the soil matrix. The microwave-digestion technique is more effective and less tedious than catalyzed hydrogen peroxide digestions of the resin or digestion of diphosphonic stripping agents such as HEDPA. After resin digestion, the actinides are recovered in a small volume of nitric acid which can be loaded onto small extraction chromatography columns, such as TEVA Resin^â, U-TEVA Resin^â or TRU Resinâ (Eichrom Industries). Small, selective extraction columns do not generate large volumes of liquid waste and provide consistent tracer recoveries after soil matrix elimination.

Diphonix Resin^â, a resin with geminally-substituted diphosphonic acid groups chemically-bonded to a styrene-divinylbenzene matrix, was developed by Argonne National Laboratory and the University of Tennessee.^{(1, 2, 3)} Diphonix exhibits a high affinity for actinide ions in the tri-, tetra and hexavalent oxidation states. Diphonix Resin^â has previously been used in a method to preconcentrate actinides from soil.⁽⁴⁾ This method is somewhat tedious, however, requiring elution of the actinides from the Diphonix Resin using 1-hydroxyethane-1,1-dipohosphonic acid (HEDPA) extractant, which must be destroyed via a manual hot plate digestion prior to further analysis of the actinides.

Other preconcentration approaches have been attempted. Dipex Resin^â (Eichrom Industries), a resin coated with diphosphonic acid extractant, was applied to five gram samples. ⁽⁵⁾ This method uses isopropanol to remove the extractant from the resin, oxidation of the Dipex extractant using a sodium hydroxide fusion, followed by a calcium phosphate precipitation to scavenge the actinides. Thorium tracer losses were encountered, presumably due to thorium precipitation on the resin support during the isopropanol removal step. Yields for plutonium and americium for five-gram samples were in the 25 to 50% range.

In the SRS Actinide-Recovery Method method, tracer yields greater than 80% were achieved for Pu, U and Am for 10-gram EML QAP samples. A rapid microwave digestion technique is used to remove the actinides from the Diphonix Resin^â, which effectively eliminates interfering matrix components from the soil matrix. The microwave digestion technique is much more effective and less tedious than manual hot plate digestions of the resin or diphosphonic-stripping agents such as HEDPA.

The SRS Actinide-Recovery Method recovers actinides from large-soil samples in a small volume of nitric acid that can be loaded onto small extraction chromatography columns for actinide-specific separations.

The resins employed in this work are Diphonix Resin, TEVA Resin^â (AliquatÔ 336), UTEVA Resin^â (diamylamylphosphonate) and TRU-Resin^â (tri-n-butylphosphate (TBP) and N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) ) available from Eichrom Industries, Inc., Darien, Illinois. Nitric, hydrochloric and hydrofluoric acids were prepared from high -purity Optima^Ô reagents (Fisher Scientific, Inc.). All water was obtained from a Milli-Q2 water purification system. All other materials were ACS reagent grade and were used as received. Radiochemical isotope tracers Pu-242, U-232 and Am-243 from Amersham that had been diluted to the 4 dpm/mL level were employed to enable yield corrections. Soil quality assurance program (QAP) standards were obtained from the Department of Energy Environmental Measurements Laboratory (EML) in New York, N.Y.

Column preparation. The Diphonix Resin (100-200 mesh) columns prepared contained approximately 2.8-mL of resin. Chromatographic columns were prepared by slurrying the appropriate resin in water, then transferring aliquots of the slurry under vacuum to a column body (Environmental Express, Mount Pleasant, SC). The TEVA, U-TEVA and TRU Resin columns were prepared using 2-mL of each resin. Chromatographic columns were prepared by slurrying the appropriate resin in water, then transferring aliquots of the slurry under vacuum to a column body (Image Molding, Commerce City, CO). or mini-cartridge (Applied Separations, Inc., Allentown, PA) until the desired bed height was reached. Small particle size (50-100 micron) was employed, along with a vacuum extraction system (Applied Separations, Inc.). Flow rates of 2 -3 mL/min were typically used, much faster than the 0.5 mL/min gravity flow rates observed. Empty Image Molding columns were used as reservoirs in the dual-column method when mini-cartridges were employed.

Sample Preparation and Actinide Recovery and Separation. Soil leachates (10 to 15- grams) were prepared by microwaving the soil sample in 13-mLs of 15.7M nitric acid (HNO₃) and 4-mLs of 12M hydrochloric acid (HCl) at 180°C for 20 minutes. The sample was filtered using a 0.45-micron filter and evaporated to dryness. Soil leachates were redissolved in a mixture of 1.5M HCl and 1M hydrofluoric (HF).

Additional soil samples (10-grams) were fused with sodium hydroxide and actinides were precipitated as hydroxides. The hydroxide precipitate was acidified and dissolved with 6M HCl, then evaporated to dryness to lower volume and better adjust the solution acidity. The total dissolution residue was then redissolved in approximately 8 to 10 -mLs of 1.5M HCl, with no HF present to avoid any fluoride solids on initial redissolution.

The leachate or total dissolution residue solutions were adjusted to 0.4M to 0.75M HCL containing 0.5M to 1M HF. The total fluoride should be sufficient to complex aluminum ions, which adversely affect actinide retention on Diphonix Resin. Soil samples with smaller amounts dissolved solids required less fluoride, while those with higher dissolved solids required more. If a small precipitate formed upon addition of HF, the sample was centrifuged, the supernate was set aside to load to Diphonix column. The fluoride precipitate was redissolved in a small volume of 0.5M HCL-0.25M boric acid, heating as necessary. This solution was added separately to the Diphonix column after the supernate solution was added.

Lowering the acid strength from the 2M HCl level used in previous Diphonix and Dipex work was found to significantly increase the actinide recoveries for Pu⁺³ and Am⁺³. Solid ascorbic acid is added to adjust the digest to 0.075M to 0.15M ascorbic acid. The ascorbic acid is added to reduce Fe⁺³ to Fe⁺², since Fe⁺³ is strongly retained on Diphonix Resin^â and will interfere with actinide recoveries. A sequential multistage column method using TEVA Resin^â and UTEVA Resin^â in tandem with TRU^â Resin was employed to isolate actinides after the soil-matrix-elimination.^(6,7) An additional separation on TEVA Resin^â to optimize Am removal from interfering rare earth elements was employed that utilizes higher ammonium thiocyanate levels than has been previously reported.⁽⁸⁾

A predigest step is used to digest the 2.8-mL of resin in 15.7M nitric acid at 190°C for 20 minutes. This step allows a larger volume of resin to be used, since the vessel pressure is released between the predigest step and the primary digestion step. After cooling and releasing the pressure to the microwave vessels, the resin is digested at 220°C for 35 minutes. After cooling the vessels, 3.5-mL of 30 wt% hydrogen peroxide is added to each vessel and the resin is digested at 210°C for 15 minutes. After cooling the digested resin solution is transferred to a glass beaker, 5 to 10-mL of 30 wt% hydrogen peroxide is added and each solution is evaporated to dryness. The residue is wet-ashed with 5 mL of 30 wt% hydrogen peroxide three more times. A mixture of 5-mL of 15.7M nitric and 5 -mL of 30 wt% hydrogen peroxide was added and each sample was ashed to dryness as needed until the color of the residue cleared or did not lighten further.

The evaporated resin digest was redissolved in the appropriate acid solution for subsequent extraction column separations. In this work the residues were redissolved in approximately 6-mL of 5M nitric acid. The solution was warmed slightly to ensure complete redissolution and 4-mL of 2M aluminum nitrate and 2-mL of 0.1M nitric acid was added to rinse the residue container in preparation for subsequent column separations. The final solution contains 12-mLs of 2.5M nitric acid-0.67M aluminum nitrate. A three-column separations method was used instead of a two-column method (such as UTEVA Resin plus TRU Resin) because the large amounts of thorium present in 10-grams of soil was found to adversely affect uranium retention on UTEVA Resin.

Microwave digestion of the resin was performed using a Questron Q-Wave 3000 closed-vessel system with temperature and pressure monitoring. Microwave vessels that can handle up to 625 psi pressure were employed. Plutonium, americium and uranium measurements were performed by alpha-particle pulse-height measurements using surface- barrier-silicon detectors.

Table 1 shows results on EML QAP soil samples that were leached using the microwave leaching procedure described above. Pu-242 and Am-243 tracer recoveries are typically greater than 80%. The Pu results show good agreement with the EML values for these radionuclides. Table 2 shows EML QAP results for plutonium, uranium and americium. Pu and Am tracer recoveries averaged greater than 80%. The Pu, U, and Am results agree well with the EML reference values. Uranium shows higher retention on Diphonix Resin than plutonium and americium. U-232 and total uranium spike recoveries have been shown to be approximately 95% for leached samples for sample sizes up to 50 grams. Since Rongalite (sodium formaldehyde sulfoxylate) was not available in this initial work to reduce uranium from U⁺⁶ to U⁺⁴ to ensure complete hydroxide precipitation, the lower U-232 recoveries are likely due to incomplete precipitation of hexavalent uranium. Future fusion work will include rongalite to improve uranium recoveries. New microwave vessels on the market that can handle 1000 psi will enable larger volumes of Diphonix Resin to be employed and larger soil samples to be analyzed. This microwave technique has also been successfully applied to fecal sample analysis at the Savannah River Site.⁽⁹⁾

Diphonix Resin can be used to preconcentrate actinides from large-soil samples. The microwave-digestion method effectively removes the actinides from the resin and the soil matrix is essentially eliminated. Results on DOE-EML Quality Assurance Program soil standards have validated method performance.

This work was performed under the auspices of the Department of Energy, DOE Contract No. DE-AC09-96SR18500. The authors wish to acknowledge Robert Henderson, Priscilla Patterson, Robin Young, Joyce Ray, Elouise Holmes, Patricia Edey, John Thomas, Brian Crandall, and David Filler for their assistance in testing this method in the SRS Environmental Laboratory.

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