Plutonium(III) oxalate

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Plutonium(III) oxalate
Plutonium(III) oxalate hydrate
Names
IUPAC name
Plutonium(III) oxalate
Identifiers
3D model (JSmol)
  • InChI=1S/3C2H2O4.2Pu/c3*3-1(4)2(5)6;;/h3*(H,3,4)(H,5,6);;/q;;;2*+3/p-6
    Key: PHGVNOKIRFLVGH-UHFFFAOYSA-H
  • C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].[Pu+3].[Pu+3]
Properties
Pu2(C2O4)3·xH2O (0 ≤ x ≤ 10.5)
Appearance Turquoise-blue solid[1]: 836 
Melting point 225 °C (437 °F; 498 K) (anhydrous; decomposes)
Highly insoluble
Structure[2][a]
monoclinic
P21/c
a = 11.246 Å, b = 9.610 Å, c = 10.315 Å
α = 90.00°, β = 114.477°, γ = 90.00°
1014.6 Å3
4
Related compounds
Other anions
 Plutonium(III) carbonate
Other cations
 Americium(III) oxalate
Curium(III) oxalate
Related plutonium oxalates
 Plutonium(IV) oxalate
Plutonyl oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Plutonium(III) oxalate is a chemical compound consisting of plutonium(III) (Pu3+) and oxalate (C2O2−4) ions, with the anhydrous form having a chemical formula Pu2(C2O4)3. It forms hydrates, which have a formula of Pu2(C2O4)3·xH2O (0 ≤ x ≤ 10.5), with the form with 9 ≤ x ≤ 10.5 being most commonly produced under standard conditions. The structure of this hydrate has been characterized through X-ray diffraction, showing it forms honeycomb-like layers. Like all plutonium compounds, it is radioactive.

Plutonium(III) oxalate is produced by adding oxalic acid (H2C2O4) to solutions containing plutonium(III), but due to plutonium(III)'s instability towards oxidation, reducing agents have to be added for stabilization. At high temperatures, the compound decomposes, going through several different phases before arriving at plutonium(IV) oxide (PuO2). Because of this, it is used for the production of plutonium(IV) oxide, along with the related compound plutonium(IV) oxalate (Pu(C2O4)2). Other plutonium compounds can also be made with plutonium(III) oxalate, like plutonium(III) fluoride (PuF3) or plutonium(III) chloride (PuCl3).

History

Plutonium(III) oxalate has been used since the Manhattan Project to precipitate plutonium as a salt.[1]: 836  The instability of plutonium's +3 oxidation state has made it difficult to prepare for applications such as production of plutonium(IV) oxide, for which plutonium(IV) oxalate has been used instead. However, a 1965 report provided methods to prepare plutonium(III) oxalate using ascorbic acid to achieve the necessary oxidation state.[3]

Synthesis

The synthesis of plutonium(III) oxalate requires an acidic solution containing plutonium ions. To set plutonium in the +3 oxidation state, reducing agents have to be applied, as plutonium(III) is unstable and susceptible to oxidation. An oxidation inhibitor has to be added as well to prevent formation of oxidizing agents that could potentially re-oxidize the reducing agents or plutonium(III). After the plutonium(III) solution has been prepared, the slow addition of oxalic acid precipitates highly insoluble[4] plutonium(III) oxalate hydrates with very low losses, which can easily be filtered:[3][1]: 836 

2 Pu3+ (aq) + 3 H2C2O4 + x H2O → Pu2(C2O4)3·xH2O ↓ + 6 H+ (9 ≤ x ≤ 10.5)[2]

The initial solution is usually a nitric acid solution[1]: 836 [5] prepared via ion exchange, but hydrochloric acid (HCl) and sulfamic acid (H3NSO3) solutions can be used as well, which are prepared by dissolving plutonium metal in the respective acid. For reducing agents, ascorbic acid (C6H8O6) is often used as it has been found to quickly reduce plutonium:[3]

2 Pu4+ + H2A → 2 Pu3+ + A + 2 H+[b]

Other agents like hydroxylamine (NH2OH) can be added as well. For oxidation inhibitors, sulfamic acid or hydrazine (N2H4) can be used.[3]

Properties

Structure

While the structure of anhydrous plutonium(III) oxalate is unknown, two possible structures have been predicted with density functional theory calculations. One structure consists of plutonium-oxalate layers in a honeycomb-like arrangement, and the other consists of 2D sheets which are connected into a 3D network via bridging oxalate groups. Both of these structures have been calculated to be stable.[7]

On the other hand, the structure of the plutonium(III) oxalate hydrate is known, and has been characterized through X-ray diffraction. It has been determined to adopt the same structure as lanthanum(III) oxalate decahydrate, La2(C2O4)3·10H2O.[8] It is made up of repeating honeycomb-like layers of composition Pu2(C2O4)3(H2O)6. Within these layers, each plutonium(III) center is chelated (bonded with as to form a ring) by three oxalate ions and coordinated by three water molecules, having a coordination geometry of tricapped trigonal prismatic. A variable amount of water molecules lie between these layers, depending on exact synthesis conditions. The hydration number ranges between 9 and 10.5 water molecules per formula unit, although this form is sometimes referred to as the decahydrate.[9][2][10]

A single [Pu2(C2O4)3(H2O)6]n layer in plutonium(III) oxalate hydrate. Blue is plutonium, grey is carbon, and red is oxygen. Hydrogen atoms are omitted.[2]
Zoom-in on the coordination sphere of a single plutonium atom within the layers. Blue is plutonium, grey is carbon, and red is oxygen. Hydrogen atoms are omitted.[2]

Decomposition

Data on the thermal decomposition of plutonium(III) oxalate is reported for the nonahydrate (Pu2(C2O4)3·9H2O). Upon heating in air, it loses water between 30 and 225 °C, becoming an octahydrate, dihydrate, and monohydrate before reaching anhydrous plutonium(III) oxalate (Pu2(C2O4)3). Starting at 225 °C, Pu2(C2O4)3 begins to decompose to plutonium(IV) oxide, PuO2, though complete conversion only occurs between 400 °C[9] and 450 °C.[11] The transition from Pu2(C2O4)3 to PuO2 goes through several phases, with compositions Pu2(C2O4)2CO3, Pu2C2O4(CO3)2, and PuOCO3.[11][9]

Diagram of the reactions that occur during the dehydration of plutonium(III) oxalate at high temperatures.[11][9]

The thermal decomposition follows a different pathway in argon. The process starts from the heptahydrate, Pu2(C2O4)3·7H2O, which loses water to transform into Pu2(C2O4)3·2H2O at 80–135 °C, Pu2(C2O4)3·H2O at 135–160 °C, and Pu2(C2O4)3 at 160–275 °C. This compound is converted to PuO2 between 275 and 700 °C.[9]

At room temperature in air, plutonium(III) oxalate also decomposes, albeit much more slowly. After approximately 20 weeks it decomposes into PuOCO3, and over a period of a few months to years converts at least partially to PuO2.[12][13] Comparisons of the decomposition of the related compound plutonium(IV) oxalate with different isotopes of plutonium suggest that oxidation by air is the driving force behind this decomposition. However, the splitting apart of the waters of crystallization by alpha particles occurs as a secondary decomposition mechanism with more radioactive isotopes of plutonium like plutonium-240.[13]

Uses

Plutonium(III) oxalate is frequently used as a starting material for the preparation of other plutonium compounds. For example, plutonium(IV) oxide is mainly produced by the high-temperature heating of either plutonium(III) oxalate or the related compound plutonium(IV) oxalate. During this process, it is slowly heated up to 700 °C to avoid rapid decomposition and evolution of gases, and afterwards it is heated to 1000 °C to remove any carbon left behind.[1]: 1031–1032  In addition, plutonium(III) oxalate has been used to synthesize plutonium(III) perrhenate, via reaction with rhenium(VII) oxide.[1]: 1065 

Halide production

Plutonium(III) oxalate can also be used to produce plutonium halides. The compound plutonium(III) fluoride has been prepared at the 150–300 gram scale by heating plutonium(III) oxalate in a stream of hydrogen between 150 and 600 °C, and then in a stream of hydrogen fluoride between 200 and 300 °C:[1]: 1077–1078 

Pu2(C2O4)3 + 6 HF → 2 PuF3 + 3 CO + 3 CO2 + 3 H2O

Plutonium(III) oxalate as a hydrate can also be used to prepare plutonium(III) chloride (PuCl3) by reaction with chlorinating agents. Reaction with hydrogen chloride is considered the best method for PuCl3 for medium-scale reactions (between 1 and 10 grams):[1]: 1093 

Pu2(C2O4)3·10H2O + 6 HCl → 2 PuCl3 + 3 CO2 + 3 CO + 13 H2O

Alternatively, the liquid hexachloropropene can be used as a chlorinating agent to avoid working with hazardous gases:[14]

Pu2(C2O4)3·10H2O + 3 C3Cl6 → 2 PuCl3 + 3 C3Cl4O + 3 CO2 + 3 CO + 10 H2O

To prepare plutonium(III) bromide (PuBr3), it can be reacted with hydrogen bromide (HBr) between 400 and 600 °C.[1]: 1095 

Notes

  1. This data is reported for the nonahydrate, Pu2(C2O4)3·9H2O.
  2. Here, H2A represents ascorbic acid, and A represents dehydroascorbic acid, (C6H6O6). The reduction from Pu(IV) to Pu(III) is assumed to have the same reaction as the ascorbic acid reduction of other metals, as given by [6].

References

  1. Clark, David L.; Hecker, Siegfried S.; Jarvinen, Gordon D.; Neu, Mary P. (2011). "Plutonium". The Chemistry of the Actinide and Transactinide Elements (PDF). doi:10.1007/978-94-007-0211-0_7. ISBN 978-94-007-0211-0.
  2. Runde, Wolfgang; Brodnax, Lia F.; Goff, George; Bean, Amanda C.; Scott, Brian L. (2009). "Directed Synthesis of Crystalline Plutonium(III) and (IV) Oxalates: Accessing Redox-Controlled Separations in Acidic Solutions" (PDF). Inorganic Chemistry. 48 (13): 5967–5972. doi:10.1021/ic900344u. OSTI 956410. PMID 19485387.
  3. Porter, J.A.; Symonds, A.E. Jr. (1965). "Precipitation of Plutonium(III) Oxalate and Calcination to Plutonium Dioxide". doi:10.2172/4649172. OSTI 4649172.
  4. Burney, G.A.; Porter, J.A. (1967). "Solubilities of Pu(III), Am(III), and Cm(III) oxalates". Inorganic and Nuclear Chemistry Letters. 3 (3): 79–85. doi:10.1016/0020-1650(67)80128-4.
  5. Vishnu Anand, P.; Pankaj; Panda, Saroj K.; Kumar, Amit; Mishra, Satyabrata; Gnanasoundari, J.; Rajeev, R.; Desigan, N.; Venkatesan, K. A.; Patwardhan, A. W. (2025). "Development of a glove box-adaptable continuous precipitation and sedimentation system for plutonium reconversion: Experimental evaluation of precipitator performance". Journal of Radioanalytical and Nuclear Chemistry. 334 (8): 5371–5388. Bibcode:2025JRNC..334.5371V. doi:10.1007/s10967-025-10258-0.
  6. Kim, Eung-Ho; Chung, Dong-Yong; Kim, Won-Ho; Shin, Young-Joon; Lee, Eil-Hee; Yoo, Jae-Hyung; Choi, Cheong-Song (1997). "Neptunium Oxalate Precipitation from the Simulated Radioactive Liquid Waste". Journal of Nuclear Science and Technology. 34 (3): 283–287. Bibcode:1997JNST...34..283K. doi:10.1080/18811248.1997.9733662.
  7. Isbill, Sara B.; Miskowiec, Andrew J. (2025). "Computational insights into the structure of anhydrous Pu(III) oxalate". Journal of Nuclear Materials. 605 155583. Bibcode:2025JNuM..60555583I. doi:10.1016/j.jnucmat.2024.155583. OSTI 2491455.
  8. Chackraburtty, D. M. (1963). "X-ray evidence of plutonium(III) oxalate decahydrate". Acta Crystallographica. 16 (8): 834. Bibcode:1963AcCry..16..834C. doi:10.1107/S0365110X63002127.
  9. Orr, R.M.; Sims, H.E.; Taylor, R.J. (2015). "A review of plutonium oxalate decomposition reactions and effects of decomposition temperature on the surface area of the plutonium dioxide product". Journal of Nuclear Materials. 465: 756–773. Bibcode:2015JNuM..465..756O. doi:10.1016/j.jnucmat.2015.06.058.
  10. Uríková, Daniela Veronika; Kampitakis, Giannis; Císařová, Ivana; Alemayehu, Adam; Kloda, Matouš; Zákutná, Dominika; Lang, Kamil; Demel, Jan; Tyrpekl, Václav (2025). "Lanthanide Oxalates: From Single Crystals to 2D Functional Honeycomb Nanosheets". Inorganic Chemistry. 64 (8): 3686–3695. doi:10.1021/acs.inorgchem.4c04293. PMC 11881034. PMID 39964120.
  11. Christian, Jonathan H.; Foley, Bryan J.; Ciprian, Elodia; Darvin, Jason; Dick, Don D.; Hixon, Amy E.; Villa-Aleman, Eliel (2023). "Probing the thermal decomposition of plutonium (III) oxalate with IR and Raman spectroscopy, X-ray diffraction, and electron microscopy". Journal of Nuclear Materials. 584 154596. Bibcode:2023JNuM..58454596C. doi:10.1016/j.jnucmat.2023.154596. OSTI 1993013.
  12. Corbey, Jordan F.; Sweet, Lucas E.; Sinkov, Sergey I.; Reilly, Dallas D.; Parker, Cyrena M.; Lonergan, Jason M.; Johnson, Timothy J. (2021). "Quantitative Microstructural Characterization of Plutonium Oxalate Auto-Degradation and Evidence for PuO2 Nanocrystal Formation". European Journal of Inorganic Chemistry (32): 3277–3291. Bibcode:2021EJIC.2021.3277C. doi:10.1002/ejic.202100511. OSTI 1808901.
  13. Darvin, Jason R.; Kersey, Weslee A.; Villa-Aleman, Eliel; Dick, Don D.; Shehee, Thomas C.; Foley, Bryan J.; Dorris, Austin L.; Hartig, Kyle C. (2025). "Radiolytic degradation of 240Plutonium and 242Plutonium oxalates". Journal of Radioanalytical and Nuclear Chemistry. 334 (12): 8929–8942. doi:10.1007/s10967-025-10487-3.
  14. "Plan for synthesis of actinide chlorides and fluorides without using gaseous agents" (PDF). inis.iaea.org.