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Neptunium(IV) oxalate

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Neptunium(IV) oxalate
Neptunium(IV) oxalate.jpg
Identifiers
3D model (JSmol)
  • InChI=1S/2C2H2O4.Np/c2*3-1(4)2(5)6;/h2*(H,3,4)(H,5,6);/q;;+4/p-4
    Key: KBWUHCITSWHCDN-UHFFFAOYSA-J
  • C(=O)(C(=O)[O-])[O-].C(=O)(C(=O)[O-])[O-].[Np+4]
Properties
Np(C2O4)2
Molar mass 413 g·mol−1
AppearanceGreen crystals
slightly soluble
Hazards
GHS labelling:
Danger
Related compounds
Other cations
Thorium oxalate
Uranium(IV) oxalate
Plutonium(IV) oxalate
Related neptunium oxalates
 Neptunyl(V) oxalate
Neptunyl(VI) oxalate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Neptunium(IV) oxalate is an inorganic compound and a salt of neptunium and oxalic acid, with the formula Np(C2O4)2. It is known to form several hydrates with formulas Np(C2O4)2·xH2O (x = 1, 2, 6). The hexahydrate (x = 6) is prepared by adding oxalic acid to aqueous solutions containing neptunium(IV) and nitric acid, and forms green crystals. It is frequently burned as part of the production of neptunium(IV) oxide, which is used in nuclear waste, the preparation of other neptunium compounds, and the production of plutonium-238.

Contents

Synthesis

Neptunium(IV) oxalate is formed by precipitating of neptunium(IV)-nitric acid solutions using oxalic acid:

Np(NO3)4 (aq) + 2 H2C2O4 (aq) → Np(C2O4)2 ↓ + 4 HNO3 (aq)

The presence of higher neptunium oxidation states like neptunium(V) reduces the amount of oxalate filtered out, so it must be ensured that neptunium is in the +4 state in the initial nitric acid solution. Hydrazine is added to the initial solution to stabilize the +4 state and prevent oxidation. Ascorbic acid is then added to reduce any neptunium(V) or neptunium(VI) to neptunium(IV):

2 NpO2+2 + H2A → 2 NpO+2 + A + 2 H+
2 NpO+2 + 6 H+ + H2A → 2 Np4+ + A + 4 H2O

Here, H2A represents ascorbic acid, and A represents dehydroascorbic acid. Ascorbic acid is a slow reducing agent, and the process is sped up with increased nitric acid concentration and/or temperature. Oxalic acid is often added in a two-step precipitation process. After oxalic acid is added, the solid neptunium(IV) oxalate can easily be filtered out. [1] [2] [3] [4] [5] [6]

Physical properties

Neptunium(IV) oxalate forms several hydrates, with formula Np(C2O4)2·xH2O (x=0, 1, 2, 6). When neptunium(IV) oxalate is precipitated from aqueous solution, green crystals of the hexahydrate (Np(C2O4)2·6H2O) are formed. [2] Other hydrates, such as the monohydrate (Np(C2O4)2·H2O) and dihydrate (Np(C2O4)2·2H2O), are formed by heating this compound. An anhydrous (lacking water) form, Np(C2O4)2, is formed on further heating. [7]

Neptunium(IV) oxalate hexahydrate is highly insoluble in water. A study on its solubility in nitric acid-oxalic acid solutions found that its value ranges between 4.0 and 37.4 mg/L at 22 °C, but it increases with increasing temperature (19.0–373.5 mg/L at 45 °C, 32.4–420.1 mg/L at 60 °C). When the ratio of neptunium(IV) ions to oxalate ions is close to 1:2, solubility decreases as neptunium(IV) oxalate is precipitated out, but when it is less than or more than 1:2, solubility increases due to the formation of Np(C2O4)2+ and Np(C2O4)2−3 ions, respectively. Equilibrium equations: [8]

Np(C2O4)2 (s) ⇌ Np(C2O4)2 (aq)
Np(C2O4)2 + 2 H+ ⇌ Np(C2O4)2+ + H2C2O4
Np(C2O4)2 + H2C2O4 ⇌ Np(C2O4)2−3 + 2 H+

Reactions

Reaction with ozone

When reacted with ozone while suspended in water, it is oxidized to neptunyl(V) oxalate, and then to neptunyl(VI) hydroxide. [9]

Decomposition

Neptunium(IV) oxalate decomposes on heating. Starting with the hexahydrate, Np(C2O4)2·6H2O, four water molecules per formula unit are lost between 80 and 90 °C. The fifth one is split off at 100–120 °C, and the last is split off at 190–200 °C to form anhydrous neptunium(IV) oxalate. Between 280 °C and 300 °C, this compound begins to decompose. According to one study, [7] the decomposition product is neptunium(IV) oxide (NpO2), neptunium oxycarbonates, or neptunium carbonates. However, later studies [10] [11] suggest that the products formed instead contain neptunium(V), suggesting that formation of NpO2 is impossible. They instead link the decomposition to neptunyl(V) oxalate and neptunium(V) oxide. All studies agree that the end product of neptunium(IV) oxalate decomposition is neptunium(IV) oxide, which is formed at satisfactory purity between 500 and 550 °C. [7] [10] [11] [2] [3] [5] [12] : 726 

Np(C2O4)2·6H2O → Np(C2O4)2·2H2O + 4 H2O (between 80 and 90 °C)
Np(C2O4)2·2H2O → Np(C2O4)2·H2O + H2O (between 100 and 120 °C)
Np(C2O4)2·H2O → Np(C2O4)2 + H2O (between 190 and 200 °C)
Np(C2O4)2 → NpO2 + 2 CO + 2 CO2 (between 280 and 550 °C)

Structure

In the hexahydrate (Np(C2O4)2·6H2O), initial studies suggested that each neptunium atom was bonded to four oxalate groups in a cubic fashion, but later studies [13] suggest that each neptunium atom is bonded to two water molecules as well. Three of the oxalate groups per neptunium atom lie perpendicular to the layers, while the other lies parallel, providing room to fit the two water molecules. Between these ([Np(C2O4)2(H2O)]n) layers lie the rest of the water molecules, four per formula unit. Each oxalate group donates two oxygen atoms to neptunium, and each water molecule donates one atom, so neptunium atoms are bonded to ten oxygen atoms total. [13]

Pu coordination in Pu(C2O4)2 6H2O.png
Coordination of neptunium in neptunium(IV) oxalate hexahydrate. Blue is neptunium, grey is carbon, red is oxygen in oxalate, and sea green is oxygen in water. Hydrogen atoms are omitted.
Pu(C2O4)2 6H2O layer new.png
Single layer of neptunium(IV) oxalate hexahydrate. Blue is neptunium, grey is carbon, and red is oxygen. Hydrogen atoms are omitted.

Applications

Neptunium separation and purification

Neptunium(IV) oxalate is an intermediate in neptunium separation and purification. Adding oxalic acid to a solution of other metals, such as alkali, alkaline earth, or transition metals, will precipitate neptunium as neptunium(IV) oxalate, allowing it to be separated from most other metals. It is also used in the separation of neptunium from other actinides, where a modification of the PUREX process with adjusted acid and tributyl phosphate concentrations is used. Neptunium(IV) oxalate is the form of neptunium which is isolated in this process, and it can then be burned to produce the oxide. [6] [12] : 703 

Synthesis of other neptunium compounds

Because it can be easily filtered out and decomposed, the calcination of neptunium(IV) oxalate is the main method of producing neptunium(IV) oxide. After the precipitation method described above, neptunium(IV) oxalate is heated in a stream of air or nitrogen, where the temperature is raised to 100–150 °C over an hour. After being held at that temperature for another hour, the temperature is further increased 500–550 °C for an hour, held there for two hours, and then cooled to room temperature under the gas stream. This method has been used by the Savannah River Plant for production of plutonium-238 for radioisotope thermoelectric generators. Neptunium(IV) oxide is also a major form of neptunium in radioactive waste, and can be used to make other neptunium compounds, like neptunium(III) fluoride (via reaction with hydrogen gas and hydrogen fluoride), or neptunium(IV) chloride (via reaction with carbon tetrachloride). [2] [3] [4] [5] [14] [12] : 730,736 

Carbon tetrachloride can also be reacted with neptunium(IV) oxalate to produce neptunium(IV) chloride directly. [12] : 736 

Other oxalates where neptunium is in the +4 oxidation state include an acid oxalate with formula (H3O)2Np2(C2O4)5·7H2O. This compound can be formed in many of the same conditions as Np(C2O4)2·6H2O can, and can be obtained in pure form at nitric acid concentrations of >3 M and neptunium concentrations of 0.01–0.02 M (see synthesis of neptunium(IV) oxalate above). Other double salts with formulas M2Np2(C2O4)5 (M=Na, K, NH4) are known. [15]

Neptunium(IV) oxalate complexes with formula Np(C2O4)6−5 are known, and can be crystallized with hexaaminecobalt(III) as the compound [Co(NH3)6]2[Np(C2O4)5]·4H2O. The neptunium atoms bond ionically to the oxygen atoms in the oxalate groups, and the oxalate groups are non-bridging. [16]

References

  1. ↑ Luerkens, D. W. (1983). "Two-stage precipitation of neptunium (IV) oxalate". Du Pont de Nemours (E.I.) and Co. Retrieved 6 August 2021.
  2. 1 2 3 4 Porter, J. A. (1964). "Production of Neptunium Dioxide". Industrial & Engineering Chemistry Process Design and Development. 3 (4): 289–292. doi:10.1021/i260012a001.
  3. 1 2 3 Porter, J. A. (July 1, 1961). Precipitation of neptunium oxalate and calcination to neptunium oxide (PDF) (Report). Du Pont de Nemours (E.I.) and Co., Aiken, S.C. (USA). Savannah River Lab. OSTI   4166658.
  4. 1 2 Peruski, Kathryn M.; Parker, Connor J.; Cary, Samantha K. (2023). "Analysis of neptunium oxides produced through modified direct denitration". Journal of Nuclear Materials. 587 154704. Bibcode:2023JNuM..58754704P. doi:10.1016/j.jnucmat.2023.154704. OSTI   2000387.
  5. 1 2 3 Peruski, Kathryn M.; Powell, Brian A. (2020). "Effect of calcination temperature on neptunium dioxide microstructure and dissolution". Environmental Science: Nano. 7 (12): 3869–3876. doi:10.1039/D0EN00689K. OSTI   1737655.
  6. 1 2 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. 1 2 3 Kozlova, R. D.; Karelin, A. I.; Lobas, O. P.; Matyukha, V. A. (1984). "Thermal decomposition of neptunium (4) oxalate". Radiokhimiya (in Russian). 26 (3): 311–316. ISSN   0033-8311 . Retrieved 6 August 2021.
  8. ↑ Luerkens, D. W. (1 July 1983). Neptunium (IV) oxalate solubility. [22, 45, 60/sup 0/C] (Report). doi:10.2172/5904308. OSTI   5904308 . Retrieved 6 August 2021.
  9. ↑ Fedoseev, A. M.; Gogolev, A. V.; Shilov, V. P.; Charushnikova, I. A.; Makarenkov, V. I.; Perminov, V. P. (2017). "Reaction of ozone with Np(IV) and Pu(IV) oxalates in water" . Radiochemistry. 59 (6): 570–578. Bibcode:2017Radch..59..570F. doi:10.1134/S1066362217060042.
  10. 1 2 Karelin, A. I.; Krot, N. N.; Kozlova, R. D.; Lobas, O. P.; Matukha, V. A. (1990). "Thermal decomposition of Np(IV) and Pu(III, IV) oxalates" . Journal of Radioanalytical and Nuclear Chemistry Articles. 143 (1): 241–252. Bibcode:1990JRNC..143..241K. doi:10.1007/BF02117565.
  11. 1 2 Bessonov, A. A.; Afonas'Eva, T. V.; Krot, N. N. (January 1989). "On formation of neptunium(5) and (6) during thermal decomposition of neptunium(4) compounds". Radiokhimiya; (USSR). 31 (5).
  12. 1 2 3 4 Yoshida, Zenko; Johnson, Stephen; Kimura, Takaumi; Krsul, John (2011). "Neptunium". The Chemistry of the Actinide and Transactinide Elements. doi:10.1007/978-94-007-0211-0_6. ISBN   978-1-4020-3555-5.
  13. 1 2 Sockwell, A. Kirstin; Sweet, Teagan F. M.; Barth, Brodie; Isbill, Sara B.; Diblasi, Nicole A.; Szymanowski, Jennifer E. S.; Sigmon, Ginger E.; Oliver, Allen G.; Miskowiec, Andrew J.; Burns, Peter C.; Hixon, Amy E. (2023). "Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries*". Chemistry – A European Journal. 29 (47) e202301164. Bibcode:2023ChEuJ..29E1164S. doi:10.1002/chem.202301164. PMID   37227412.
  14. ↑ Colle, J.-Y. (2011). "(Solid + gas) equilibrium studies for neptunium dioxide". Journal of Chemical Thermodynamics. 43 (3): 492–498. Bibcode:2011JChTh..43..492G. doi:10.1016/j.jct.2010.10.027.
  15. ↑ Bykhovskii, D. N.; Kuz'Mina, M. A.; Maksimov, V. F.; Novikov, G. S.; Smirnov, A. N.; Solntseva, L. V. (September 1988). "Synthesis conditions and some properties of neptunium (IV) oxalates in which C2O4Np = 2.5". Soviet Radiochemistry (Engl. Transl.). 30 (1). OSTI   5774959.
  16. ↑ Tamain, Christelle; Autillo, Matthieu; Guillaumont, Dominique; Guérin, Laetitia; Wilson, Richard E.; Berthon, Claude (2022). "Structural and Bonding Analysis in Monomeric Actinide(IV) Oxalate from Th(IV) to Pu(IV): Comparison with the An(IV) Nitrate Series". Inorganic Chemistry. 61 (31): 12337–12348. doi:10.1021/acs.inorgchem.2c01674. OSTI   1968736. PMID   35881850.
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