Separation of Americium in High Oxidation States from Curium

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Separation of Americium in High Oxidation States from Curium Utilizing Sodium Bismuthate Jason M. Richards* and Ralf Sudowe Radiochemistry Program, University of Nevada Las Vegas, Las Vegas, Nevada 89154, United States ABSTRACT: A simple separation of americium from curium would support closure of the nuclear fuel cycle, assist in nuclear forensic analysis, and allow for more accurate measurement of neutron capture properties of 241Am. Methods for the separation of americium from curium are however complicated and time-consuming due to the similar chemical properties of these elements. In this work a novel method for the separation of americium from curium in nitric acid media was developed using sodium bismuthate to perform both the oxidation and separation. Sodium bismuthate is shown to be a promising material for performing a simple and rapid separation. Curium is more strongly retained than americium on the undissolved sodium bismuthate at nitric acid concentrations below 1.0 M. A separation factor of ∼90 was obtained in 0.1 M nitric acid. This separation factor is achieved within the first minute of contact and is maintained for at least 2 h of contact. Separations using sodium bismuthate were performed using solid−liquid extraction as well as column chromatography.

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nitric acid and it is difficult to maintain americium oxidation unless the solid is constantly present. This is a problem because large-scale solvent extraction processes are not equipped to operate with solids present in the system. Americium−curium separation methods that utilize higher oxidation states of americium typically involve an oxidation procedure followed by a separation procedure. Many of the oxidation procedures are complicated, multistep syntheses with limited efficiency. The separation procedures often require careful control of a variety of factors including pH, temperature, and contact time. One of the most significant challenges to any separation involving higher oxidation states of americium is maintaining oxidation throughout the separation procedure.3,4,8−12 In this work, a solid−liquid extraction and a chromatographic separation of americium from curium utilizing NaBiO3 to simultaneously perform the oxidation and the separation is presented. This approach provides a simple and effective method for the separation of americium from curium.

he separation of americium from curium has been an important challenge in radiochemistry ever since these elements were discovered in 1944. The difficulty of this separation extends to the separation of americium from higher actinides and the lanthanides. An efficient and effective separation of americium from curium would simplify designs for closure of the nuclear fuel cycle. It would also allow for the removal of isobaric interferences in curium isotope analysis in nuclear forensics and assist in the measurement of the 242m Am/242gAm isomeric ratio resulting from neutron capture on 241Am.1,2 This separation is difficult because americium and curium exhibit similar chemical properties, especially in the nitric acid solutions used in nuclear applications. Both elements are predominantly trivalent in acidic solutions and have very similar ionic radii. These similarities have generated a renewed interest in utilizing higher oxidation states of americium to facilitate a successful separation from curium.3 It has been shown that americium can be oxidized to the 5+ and 6+ oxidation states in acidic solutions with sodium bismuthate (NaBiO3). Oxidation of americium with sodium bismuthate was first reported by Hara and Suzuki in 1977 and has been used more recently by Mincher and co-workers to perform separations.4,5 Americium in its hexavalent state (AmO22+) has successfully been extracted from highly acidic solutions using diamylamylphosphonate (DAAP), bis(2,6dimethyl-4-heptyl) phosphoric acid (HD(DIBM)P), and tributylphosphate (TBP) in solvent extraction systems.4,6−8 The success of these extraction systems depends entirely on the efficiency of the oxidation and the stability of the higher oxidation states. Trace reducing agents must be avoided and contact times must be kept short since contact with organic extractants or diluents can result in reduction of the americium. Solubilized NaBiO3 has been shown to act as a holding oxidant for hexavalent americium.4 NaBiO3 is only sparingly soluble in © 2016 American Chemical Society

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EXPERIMENTAL SECTION

Materials. The peroxide-free NaBiO3 was obtained from Idaho National Laboratory and was used as received. The 241 Am, 244Cm, and 242Pu tracer solutions were diluted from stock solutions purchased from Eckert & Zigler Isotope Products. The nitric acid used in all experiments was TraceSELECT nitric acid from Sigma-Aldrich. All other reagents were reagent grade and were used as received. Received: March 15, 2016 Accepted: April 15, 2016 Published: April 15, 2016 4605

DOI: 10.1021/acs.analchem.6b01026 Anal. Chem. 2016, 88, 4605−4608

Letter

Analytical Chemistry

Chromatography. Pure NaBiO3 powder is very fine and, as such, is difficult to use as a chromatographic material without modification. Celite 535 was used to aid in filtration and to improve flow properties of the material. Columns were slurry packed using 0.1 M nitric acid into 2 mL polypropylene columns from Eichrom. First, 75 mg of Celite were packed to form a Celite plug onto which 500 mg of a mixture of Celite and NaBiO3 powder (5% NaBiO3 by weight) was packed. Columns were placed on a vacuum box (Eichrom Part No. AR12-BOX) and washed with 10 mL of 0.1 M nitric acid to ensure acidification of the mixture. A 1.0 mL sample containing americium and curium (∼100 Bq each) was loaded onto the column followed by 15 mL of 0.1 M nitric acid to elute the Am, which was collected in a single fraction in order to maintain a consistent flow rate. The Cm was then eluted with 15 mL of 2.0 M nitric acid and was also collected in a single fraction. The fractions were analyzed using alpha spectrometry. Prior to sample preparation by CeF3 microprecipitation, each sample was spiked with a set amount of 242Pu (2.17 Bq) as an internal standard to account for losses during precipitation and differences in counting efficiency then evaporated to near dryness and reconstituted in HCl. A volume of 0.5 mL of 30% H2O2 was added to ensure reduction of the Am.

Batch Sorption Studies. The sorption of americium and curium to NaBiO3 at different nitric acid concentrations was determined using a batch contact method. A known amount of NaBiO3 (∼50 mg) was weighed into 2.0 mL polypropylene microcentrifuge tubes. A 0.2 mL aliquot of the appropriate nitric acid concentration was added to the NaBiO3. The resulting mixture was then contacted with three separate 0.8 mL nitric acid solutions of the same concentration and separated by centrifugation in order to acidify the surface of the NaBiO3. The appropriate amounts of tracer (∼200 Bq/mL 241 Am or 244Cm in 0.1 M nitric acid), water, and concentrated nitric acid were added to give a final acid concentration equal to the acid concentration of the preconditioning step and give a total volume of 1.0 mL. The tubes were allowed to contact on a shaker table for 1 h. The solutions were then transferred to a syringe equipped with a 0.45 μm PTFE syringe filter. A 0.75 mL aliquot of the filtered solution was removed for analysis by liquid scintillation counting. The weight distribution ratio for each sample was calculated using the expression: Dw =

A s /m A aq /V

where

A s = Ao − A aq

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The reversibility of the sorption was shown by first contacting the solid NaBiO3 with a nitric acid solution containing Am or Cm and then removing the supernatant and replacing it with fresh nitric acid of the same concentration. If the Dw of the first and second contact were statistically equivalent, the sorption was considered reversible. For the contact time dependence studies, the batch contact procedure was repeated at a nitric acid concentration of 0.1 M and the contact time was varied from 1 min to 2 h. Am/Cm Separation. Solid−Liquid Extraction. A known amount of NaBiO3 (20.0 ± 0.5 mg) was added to two 2.0 mL microcentrifuge tubes (Solid-1 and Solid-2). A 0.2 mL aliquot of 0.1 M nitric acid was added to the NaBiO3. The resulting mixture was then contacted with three 1.3 mL solutions of 0.1 M nitric acid and separated by centrifugation in order to acidify the surface of the NaBiO3. The appropriate amounts of tracer (∼100 Bq each 241Am and 244Cm per mL in 0.1 M nitric acid) and 0.1 M nitric acid were added to Solid-1 to give a total volume of 1.5 mL. The sample was contacted on a shaker table for 10 min and separated by centrifugation. A 1.3 mL portion of the solution was removed from Solid-1 and added to Solid-2, and 1.3 mL of 0.1 M nitric acid were added to Solid-1. The samples were contacted for 10 more minutes and separated by centrifugation. A 1.3 mL portion was removed from Solid-2 for analysis, a 1.3 mL aliquot of solution was removed from Solid-1 and added to Solid-2, and 1.3 mL of 0.1 M nitric acid were added to Solid-1. This process was repeated until Solid-1 and Solid-2 had been contacted with the original tracer solution plus four solutions of 0.1 M nitric acid. Finally, the NaBiO3 was contacted with 4.0 M nitric acid for 10 min to remove any americium or curium, after which the solution was separated by filtration through a 0.45 μm PTFE syringe filter. All five 0.1 M nitric acid solutions were filtered through 0.45 μm syringe filters prior to analysis. The resulting solutions were analyzed using alpha spectrometry. Prior to sample preparation by CeF3 microprecipitation, each sample was spiked with a set amount of 242Pu (2.17 Bq) as an internal standard to account for losses during precipitation and differences in counting efficiency and 0.5 mL of 30% H2O2 were added to ensure total reduction of the Am.

RESULTS AND DISCUSSION Batch Sorption Studies. The sorption of metal ions to solid NaBiO3 has been reported previously. However, a literature review yielded no examples of NaBiO3 being used to perform separations. B. J. Mincher et al. reported poor closure for mass balance of cerium and europium in an experiment involving sodium bismuthate and showed that the “missing” activity was found to be associated with the undissolved bismuthate solid.8 This effect was found to be most prevalent at low acid concentrations. In this work, a similar sorption behavior was observed for americium and curium at low acid concentrations. While both elements show some sorption to the solid NaBiO3, curium sorption was greater than americium sorption at nitric acid concentrations below 1.0 M. The weight distribution ratios of americium and curium on solid NaBiO3 are shown in Figure 1. No sorption of either americium or curium is observed at nitric acid concentrations of 4 M or greater. The solid NaBiO3 has an ilmenite structure.11 NaBiO3 is likely exhibiting ion-exchange properties. The negatively charged oxygens on the surface of the solid NaBiO3 could interact more strongly with the trivalent curium than the oxidized americium. As the nitric acid concentration is increased, the adsorbed americium and curium could be displaced by the hydronium ions and stabilized in solution by complexation with nitrate. Contact-Time Dependence Studies. The sorption of americium and curium to solid NaBiO3 was measured over the course of 2 h (see Figure 2). The distribution ratio for both americium and curium decreased slowly over the time measured, but the separation factor remained relatively constant. The separation factor for americium and curium on sodium bismuthate averages 90 ± 10 in 0.1 M nitric acid over the first 2 h. Am/Cm Separations. The utility of sodium bismuthate for the separation of americium from curium was tested using a solid−liquid extraction technique as well as chromatography. Solid−Liquid Extraction. The separation was performed with two solid phases and five solution phases at room 4606

DOI: 10.1021/acs.analchem.6b01026 Anal. Chem. 2016, 88, 4605−4608

Letter

Analytical Chemistry

Figure 1. Nitric acid dependency of Dw for Am and Cm on NaBiO3. The data are the average of three replicates. The error shown is 1σ. Figure 3. Alpha spectra of the 0.1 and 2.0 M nitric acid fractions eluted from a Celite column containing 5% NaBiO3 by weight compared to the precolumn sample.

resin utilizing Am(V). This separation achieved only a 50% yield and ∼70% recovery following elution of curium.12 In this separation system 97 ± 2% of the americium and 98 ± 2% of the curium were recovered. During the course of the separation visible amounts of gas evolved in the column, possibly from the reaction of bismuthate with the solutions. The separation did not, however, appear to be adversely affected by this phenomenon. Only a small fraction of the solid NaBiO3 is dissolved over the course of the separation. The separation should be viable as long as sufficient NaBiO3 solid is present. It is likely that these columns could be used multiple times before needing to replace the column material.

Figure 2. Contact-time dependence of Dw for Am and Cm on NaBiO3 in 0.1 M nitric acid and the separation factor (SF) defined as SF = Dw(Cm)/Dw(Am). The data shown are the average of three replicates and the error shown is to 1σ.

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CONCLUSIONS A novel method for the separation of americium from curium in nitric acid media was developed using NaBiO3 to perform both the oxidation and the separation. At low acid concentrations curium is more strongly retained on NaBiO3 than americium, with a separation factor of ∼90 in 0.1 M nitric acid. This separation factor is present within the first minute of contact and is maintained for at least 2 h of contact. A separation of americium from curium performed using a solid−liquid extraction technique gave purity and recovery of well over 90% for both elements. Separation by chromatography was faster than solid−liquid extraction and gave improved separation and recovery. The high separation factors, short contact times and overall simplicity of these separation systems make them promising for the separation of Am from Cm in a variety of applications including nuclear forensic analysis, nuclear fuel cycle closure, and stewardship science. Future research will concentrate on studying the behavior of earlier actinides and fission product lanthanides in these systems for fuel cycle applications.

temperature and was completed in less than 2 h. Americium was detected in all five solutions of 0.1 M nitric acid. Americium accounted for 100 ± 4% of the activity in the five solution phases. Curium accounted for 95 ± 3% of the activity recovered from the two solid phases, the remaining activity being from residual americium. In this separation system, 97 ± 3% of the americium and 95 ± 2% of the curium were recovered. Chromatography. The chromatographic separation was performed with a column made with 25 mg of NaBiO3 powder dispersed in 475 mg of Celite (27 mm bed height) (see Figure 3). The vacuum was kept at 5−7 in. Hg vac, and the flow rate was 1.5−2.0 mL/min. The separation procedure was completed in under half an hour and was performed at room temperature. Americium accounted for 100 ± 3% of the activity in the 0.1 M nitric acid fraction. Curium accounted for 98 ± 4% of the activity in the 2.0 M nitric acid fraction, the remaining activity being from americium. Mincher and co-workers recently reported a separation of americium from curium on TRU 4607

DOI: 10.1021/acs.analchem.6b01026 Anal. Chem. 2016, 88, 4605−4608

Letter

Analytical Chemistry

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (435)881-8381. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Catherine L. Riddle from Idaho National Laboratory for providing the pure sodium bismuthate. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0000979 through the Nuclear Science and Security Consortium. This material is based upon work supported under an Integrated University Program Graduate Fellowship.

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REFERENCES

(1) Morss, L. R.; Edelstein, N. M.; Fuger, J. The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Springer: Dordrecht, The Netherlands, 2006. (2) Jandel, M.; Bredeweg, T.; Bond, E.; Chadwick, M.; Clement, R.; Couture, a.; O’Donnell, J.; Haight, R.; Kawano, T.; Reifarth, R.; Rundberg, R.; Ullmann, J.; Vieira, D.; Wilhelmy, J.; Wouters, J.; Agvaanluvsan, U.; Parker, W.; Wu, C.; Becker, J. Phys. Rev. C 2008, 78 (3), 034609. (3) Runde, W. H.; Mincher, B. J. Chem. Rev. 2011, 111 (9), 5723− 5741. (4) Mincher, B. J.; Martin, L. R.; Schmitt, N. C. Inorg. Chem. 2008, 47 (15), 6984−6989. (5) Hara, M.; Suzuki, S. J. Radioanal. Chem. 1977, 36, 95−104. (6) Mason, G. W.; Bollmeier, A. F.; Peppard, D. F. J. Inorg. Nucl. Chem. 1970, 32 (3), 1011−1022. (7) Kamoshida, M.; Fukasawa, T. J. Nucl. Sci. Technol. 1996, 33 (5), 403−408. (8) Mincher, B. J.; Martin, L. R.; Schmitt, N. C. Solvent Extr. Ion Exch. 2012, 30 (5), 445−456. (9) Burns, J. D.; Shehee, T. C.; Clearfield, A.; Hobbs, D. T. Anal. Chem. 2012, 84 (16), 6930−6932. (10) Burns, J. D.; Borkowski, M.; Clearfield, a.; Reed, D. T. Radiochim. Acta 2012, 100 (12), 901−906. (11) Kumada, N.; Kinomura, N.; Sleight, a. W. Mater. Res. Bull. 2000, 35 (14−15), 2397−2402. (12) Mincher, B. J.; Schmitt, N. C.; Schuetz, B. K.; Shehee, T. C.; Hobbs, D. T. RSC Adv. 2015, 5 (34), 27205−27210.

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DOI: 10.1021/acs.analchem.6b01026 Anal. Chem. 2016, 88, 4605−4608