- Email: [email protected]
Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2
Accepted Manuscript Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2 Lei Zhang, Ewa A. Dzik, Ginger E. Sigmon, Jennifer E.S. Szymanowski, Alexandra Navrotsky, Peter C. Burns PII:
S0022-3115(17)30882-6
DOI:
10.1016/j.jnucmat.2017.10.034
Reference:
NUMA 50566
To appear in:
Journal of Nuclear Materials
Received Date: 16 June 2017 Revised Date:
10 October 2017
Accepted Date: 11 October 2017
Please cite this article as: L. Zhang, E.A. Dzik, G.E. Sigmon, J.E.S. Szymanowski, A. Navrotsky, P.C. Burns, Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2, Journal of Nuclear Materials (2017), doi: 10.1016/j.jnucmat.2017.10.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2
Navrotsky b, and Peter C. Burns a,c
a
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame,
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Notre Dame, IN 46556, USA b
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis,
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Davis, CA 95616, USA c
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Lei Zhang a,*, Ewa A. Dzik a, Ginger E. Sigmon a, Jennifer E.S. Szymanowski a, Alexandra
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
Keywords: Thermochemistry, formation enthalpy, neptunium oxides.
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To whom correspondence should be addressed. Email: [email protected].
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*
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Abstract Neptunium (Np) compounds are important in the nuclear fuel cycle because of the buildup and long half-life (2.14 Ma) of Np-237 in nuclear waste, especially during long-term disposal in a
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geological repository. Neptunium in environmental conditions exists mainly in two oxidation states (+5 and +4) and can substitute for uranium and/or rare earths in solid phases. Yet thermochemical data for solid neptunium compounds are scarce, despite being critical for
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evaluating the environmental transport of this radioactive and toxic element. Although high temperature oxide melt solution calorimetry has proven very useful in obtaining thermodynamic
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data for the formation of uranium and thorium oxide materials, it has not yet been applied to transuranium compounds. Continuing a program at Notre Dame to study the thermodynamics of transuranium compounds, we report the first determination of the enthalpies of drop solution of well-characterized neptunium oxides (Np2O5 and NpO2) using oxide melt solution calorimetry in
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molten sodium molybdate solvent at 973 K. The enthalpy of the decomposition reaction, Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) at 298 K, is determined to be 7.70 ± 5.86 kJ/mol, and this direct measurement is consistent with existing thermodynamic data. The calorimetric methodology is
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straightforward and produces reliable data using milligram quantities of radioactive materials,
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and can be applied to many other transuranium compounds.
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1. Introduction Neptunium-237 is of concern for the long-term geological disposal of spent nuclear fuel because of its long half-life (2.14 Ma) and potential mobility, especially under oxidizing
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conditions because the neptunyl (NpO2+) ion is soluble in water. Neptunium forms various compounds with peroxide 1, sulfate 2-3, phosphate 4-6, arsenate 4, and other oxyanions, as well as simple oxides. Studies have shown that neptunium oxides can precipitate from neptunyl
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solutions under mildly oxidizing conditions 7-8. Neptunium may also partition into uranyl phases, such as studtite or metastudite (alteration phases of UO2 fuel), by substitution for U(VI) with 9-13
. Therefore it is crucial to understand the
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appropriate charge balance mechanisms
geochemical stability of Np-containing solid materials as well as species in aqueous solution. However, there are very limited reported thermodynamic data for solid neptunium compounds, owing to the scarcity of facilities and techniques that are equipped for handling this radionuclide.
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Neptunium exists in environmental conditions in primarily in two oxidation states (+5 and +4) and has two stable oxides, Np2O5 and NpO2. Np2O5 has a monoclinic structure (space group P2/c) 14 and is stable at low temperature (it decomposes to NpO2 and O2 between 700 and 973 K ), whereas NpO2 in the cubic fluorite structure (space group Fm3m) is stable to its melting
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15
point of 3070 ± 62 K 16. The enthalpy of formation of Np2O5 has been measured by acid solution
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calorimetry by Belyaev et al. (∆fH°298(Np2O5) = -2148 ± 12 kJ/mol) (∆fH°298(Np2O5) = -2162.7 ± 9.3 kJ/mol)
18
17
and by Merli and Fuger
. However, these reported values are in poor
agreement. The only reported enthalpy of formation of NpO2 was measured by oxygen bomb calorimetry by Huber and Holley ((∆fH°298(NpO2) = -1074.2 ± 2.5 kJ/mol) 19, using two grams of material. The heat capacities of Np2O5 (from 350 to 750 K) 20 and NpO2 (from 5 to 1770 K) 21-24
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have also been measured. However, the low temperature (0 to 298 K) heat capacities of Np2O5, needed to obtain standard entropies, have not been reported. High temperature oxide melt solution calorimetry has been successfully utilized to study the 25-28
. Here, we describe the
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thermochemistry of actinide oxides containing thorium and uranium
further development of this technique at the University of Notre Dame to study the formation enthalpies of neptunium compounds. Neptunium oxides (Np2O5 and NpO2) have been
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synthesized and characterized for calorimetric study. Their drop solution enthalpies (enthalpy of reaction in which a solid oxide pellet is dropped from room temperature into a molten oxide
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solvent at calorimeter temperature and dissolved) into molten sodium molybdate solvent at 973 K have been measured. The decomposition enthalpy of Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) at 298 K is determined to be 7.70 ± 5.86 kJ/mol. This value agrees well with those calculated using previously reported formation enthalpies of the two neptunium oxides. These experiments are a
2. Materials and Methods
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2.1 Materials preparation
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first step in the calorimetric study of heats of formation of more complex neptunium materials.
Caution: Nepunium-237 is radioactive and it decays by emission of alpha radiation. Its decay
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product is protactinium-233, which emits a strong gamma that is dramatically more penetrating than alpha. Therefore all experiments were done in purpose-designed laboratories at the University of Notre Dame with proper equipment. Conduction of experiments of this sort requires monitoring of the area and personnel, and a comprehensive program of radiation worker training and institutional oversight.
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Np2O5 was prepared by hydrothermal reactions of NpO2+ solutions with natural calcite crystals, as previously reported by our group for the single-crystal structure determination of this compound 14. The reactants were placed in Teflon cups with screw-top lids that were then placed
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inside Teflon-lined 125 mL stainless steel Parr reaction vessels, together with water to provide counter pressure in the outer vessel. The vessels were heated at 473 K for 7 days. Monitoring for unintended release of radionuclides is essential during disassembly of this apparatus, which
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should be conducted in a fume hood under negative pressure relative to the laboratory ambient. Dark brown Np2O5 crystals were recovered, and washed using 1N HCl to remove any remaining
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calcite, followed by washing with ultrapure H2O. The crystals were dried inside a glove box. NpO2 was prepared by the decomposition of Np2O5 during heating to 1173 K.
2.2 Thermogravimetric analysis
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Thermogravimetric analyses were performed using about 10 mg of dried Np2O5 crystals in a TA Instruments Q50 located in a radiological hood from 303 K to 1173 K at a heating rate of 5 K/min under N2. The sample was held at 423 K for 30 minutes to eliminate surface water on the
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Np2O5 crystals. The mass loss was in agreement with the end product being NpO2 (Figure S1).
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2.3 Powder X-ray diffraction
Np2O5 and NpO2 (remaining powder from TGA of Np2O5 crystals) products were ground using an agate mortar and pestle inside a glove box under negative pressure, and were deposited onto an airtight sample holder that provides containment of the material. PXRD patterns were collected from 5 to 90 ° (2 theta) with CuKα radiation using a Bruker D8 Advance diffractometer with a step width of 0.02 ° and 1 s counting per step (Figure S2 and S3). The patterns were
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processed using PDF-4+ to confirm the identities of both neptunium oxides and the absence of secondary phases.
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2.4 Calorimetry
Enthalpies of drop solution of the neptunium oxides into molten sodium molybdate (3Na2O·4MoO3) at 973 K were measured using a Setaram AlexSYS 1000 high temperature
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Tian-Calvet calorimeter. The calorimeter was calibrated using the heat content of 5 mg α-Al2O3 pellets 29-31. In order to minimize the risk of neptunium dispersal, silica glass crucibles were used
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as the molten solvent container (rather than platinum), so that the solidified sodium molybdate solvent and the crucibles are suitable for disposal or neptunium recovery as a single unit after the experiment. The sodium molybdate melt is not corrosive to the silica crucibles. Operating procedures for calorimetric experiments using neptunium materials were first developed for non-
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radioactive samples, and personnel completed multiple non-radioactive experiments of this sort before transitioning to hot work. The neptunium oxide sample pellets were prepared in a designated negative pressure glove box and were transported to the nearby calorimetry lab in
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triple containment. For each experiment, about 5 mg of neptunium oxides were pressed into a 1.5 mm diameter pellet, weighed accurately on a Mettler-Toledo XSE105 microbalance (± 0.01 mg),
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and dropped into 10 g of molten sodium molybdate solvent contained in silica glass crucibles in the AlexSYS calorimeter. The calorimetric assembly was flushed continuously with oxygen at 43 mL/min and oxygen was bubbled in the molten solvent at 5 mL/min through silica glass tubes to facilitate dissolution of neptunium oxides and provide an oxidizing atmosphere to oxidize all neptunium to the stable pentavalent state in the solvent. The final oxidation state of neptunium was confirmed to be pentavalent by UV-Vis spectra collected for the solvent subsequent to
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cooling to room temperature (Fig. S4). The thermochemical cycles listed in Tables S1 and S2 were used to calculate the enthalpies of reactions of interest.
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2.5 UV-Vis spectroscopy
Sodium molybdate solvent containing dissolved neptunium oxide was dissolved in ultrapure H2O in air, with dissolution complete in a few minutes. UV-vis spectra were collected
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immediately after dissolution from 400 to 1300 nm at room temperature using a Jasco V-670
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spectrophotometer (Fig. S4).
3. Results
Initially, Np2O5 was prepared for study, and NpO2 was subsequently produced by heating Np2O5. Powder X-ray diffraction (PXRD) confirmed that the starting material was Np2O5, with
structure
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the pattern agreeing with that calculated using the previously reported single crystal X-ray . Thermogravimetric analysis (TGA) of Np2O5 crystals (during which it was held at
423 K for 30 minutes) indicated incorporation of a small amount of water in the oxide,
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corresponding to composition Np2O5·0.077H2O. Decomposition of Np2O5 initiates at about 573 K and is complete above 973 K under N2, with a heating rate of 5 K/min, consistent with
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previous reports 15. Bagnall and Laidler reported that Np2O5 persists to 693 K under vacuum 32. Decomposition of Np2O5 to NpO2 is reported to occur between 703 and 973 K, depending on O2 pressure
33
. Richter and Sari showed that the conversion is irreversible (consistent with
thermodynamic data, see discussions below) and that NpO2 remains stoichiometric to 1743 K 15. TGA revealed that the weight loss through heating to 1073 K was 4.0 %, indicating that the
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remaining material was NpO2 (calculated weight loss for Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) is 2.9 %). Powder X-ray diffraction confirmed the heated powder to be NpO2. The calorimetric data for each drop of Np2O5·0.077H2O and NpO2 samples are listed in Table
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1. Both neptunium oxides dissolved quickly in the sodium molybdate solvent at 973 K, and the calorimetric baseline returned to the starting position within 20 min. Np2O5·0.077H2O gave a small heat effect of less than 0.5 J per experiment, while NpO2 gave an even smaller heat effect
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of less than 0.05 J per experiment. The intrinsically small drop solution enthalpies of both neptunium oxides require a highly stable baseline during the experiments. As a result, only four
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of eight experiments for Np2O5·0.077H2O and four of five for NpO2 were appropriate for further calculations. The four drops of each sample gave ∆HDS(Np2O5·0.077H2O) = 39.63 ± 5.34 kJ/mol, and ∆HDS(NpO2) = 7.81 ± 1.22 kJ/mol. The calorimetric method proved to be straightforward and can be applied to other neptunium compounds.
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The oxidation states of neptunium in the molten sodium molybdate solvent after each experiment are crucial for the interpretation of the calorimetric data. Hence, the solvent with dissolved neptunium oxides after being cooled to room temperature was dissolved in water and
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its UV-Vis spectrum was collected immediately. The final oxidation states of both neptunium oxides in the sodium molybdate solvent were determined to be pentavalent (Fig. S4). Therefore it
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is possible to use the thermochemical cycles listed in Tables S1 and S2 to calculate the enthalpies of reactions involving both neptunium oxides.
4. Discussion
To obtain ∆HDS of anhydrous Np2O5, the effect of the small amount of water, which is presumably on the surface of the crystals, must be taken into account. Assuming physical
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adsorption, the water adsorption enthalpy is -44.0 kJ/mol, which is the heat of water vapor condensation at 298 K. Thus, ∆HDS(Np2O5) is calculated to be 34.22 ± 5.34 kJ/mol using the thermochemical cycles listed in Table S1.
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The drop solution enthalpies ∆HDS measured here include ∆HHC and ∆HS, where ∆HHC is the heat content of oxide from 298 to 973 K, and ∆HS is the heat of solution in the molten solvent at 973 K. ∆HHC can be calculated from the heat capacity (Cp) data 34 and is shown in Table 2. With
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the measured ∆HDS listed in Table 2, we calculated ∆HS of the neptunium oxides along with those of uranium oxides and thorium oxide. For comparison, the results are given per mole of
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actinide metal. ∆HS(NpO2.5) is comparable to ∆HS(γ-UO3) and ∆HS(ThO2), where ∆HS of these oxides result from the heat of solution without oxidation. However, ∆HHC(NpO2.5) was estimated by high temperature heat capacity measured by drop calorimetry from 350 to 750 K by Belyaev et al.
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, which gives Cp(T)/(J·K-1·mol-1) = 99.2 + 98.6 × 10-3 (T/K), since Np2O5 is unstable at
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high temperature. ∆HHC(NpO2.5) is likely underestimated to be consistent with the variation of ∆HHC of uranium oxides, so ∆HS(NpO2.5) is likely more negative, and closer to the values of ∆HS(γ-UO3) and ∆HS(ThO2).
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The enthalpy of decomposition of Np2O5 to NpO2 and O2 at 298 K is obtained by the thermochemical cycles listed in Table S2 and is 7.70 ± 5.86 kJ/mol, obtained directly from the
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measurements of drop solution enthalpies of these two neptunium oxides. The enthalpy of formation of NpO2 was previously measured by Huber and Holley19 by oxygen bomb calorimetry using 1.5 to 2 g of neptunium metal pellets in each experiment, giving ∆fH°298(NpO2) = -1074.2 ± 2.5 kJ/mol
19
. The enthalpy of formation of Np2O5 has been
measured by solution calorimetry by two slightly different methods. Merli and Fuger 18 reported ∆fH°298(Np2O5) = -2162.7 ± 9.3 kJ/mol using 2 – 12 mg of material, and Belyaev
9
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reported
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∆fH°298(Np2O5) = -2148 ± 12 kJ/mol using 37 – 73 mg of material. The decomposition enthalpy of Np2O5 is also calculated from these reported data and compared with that from this work (Fig. 1 and Table 3). Our new value (7.70 ± 5.86 kJ/mol) is in reasonable agreement with both
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numbers (14.3 ± 10.6 kJ/mol and -0.4 ± 13.0 kJ/mol), and yields a smaller error while using only 5 mg of sample via direct calorimetric measurements.
Table 3 also lists the entropy at room temperature as S°298(Np) = 50.45 ± 0.4 J·K-1·mol-1
been measured, and Merli and Fuger
21
18
,
. The low temperature heat capacity of Np2O5 has not
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S°298(NpO2) = 80.29 ± 0.42 J·K-1·mol-1
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have estimated S°298(Np2O5) = 186 ± 16 J·K-1·mol-1,
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which is consistent with the variation of the standard entropy of uranium oxides based on the difference in oxygen stoichiometry between the two oxides
34
. Using measured thermodynamic
data for NpO2, we calculate the ∆fG°298(NpO2) and ∆fG°298(Np2O5) from the elements, and estimate the equilibrium decomposition temperature of Np2O5 to NpO2 (Table 3). It is predicted
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that NpO2 would be the thermodynamically stable solid at room temperature, since all the calculations show equilibrium decomposition temperature below 298 K. This is consistent with a recent study which has shown that neptunium oxides can precipitate from neptunyl solutions 7-8
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under mildly oxidizing conditions
. However, as shown in many previous works
15, 32-33
and
confirmed in this study, Np2O5 can persist up to 973 K; Np(V) solid phases precipitate in
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solubility experiments instead of NpO2 conducted with natural water 36. Therefore, NpO2 may be the stable neptunium solid oxide thermodynamically for the long term. Nevertheless it is still important to understand the thermodynamics of metastable Np2O5 phase through obtaining reliable thermodynamic data. Smith et al.
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recently optimized formation enthalpies ∆fH°298(NpO2) = -1076.8 kJ/mol and
∆fH°298(Np2O5) = -2172.0 kJ/mol by thermodynamic modeling using the CALPHAD
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methodology. Their calculated enthalpy and entropy values for NpO2 are in good agreement with reported data (Table 3). However, their optimized ∆fH°298(Np2O5) = -2172.0 kJ/mol is more negative than the two reported values (-2162.7 ± 9.3 kJ/mol and -2148 ± 12 kJ/mol). They also
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reported optimized entropies S°298(NpO2) = 80.3 J·K-1·mol-1, and S°298(Np2O5) = 242.2 J·K·mol-1 (as the low-temperature heat capacity of Np2O5 has not been measured). Hence we can
calculate ∆fG298°(NpO2) = -1024.6 kJ/mol, which agrees with the experimental data; and
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∆fG°298(Np2O5) = -2061.4 kJ/mol, which is significantly smaller than the other data (Table 3).
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This big discrepancy in Gibbs free energy of Np2O5 is the result of the authors using a bigger S°298(Np2O5) to accommodate the higher decomposition temperature (888 K) of Np2O5. This cannot justify their optimization of thermochemical data; as discussed above, NpO2 is the thermodynamically stable oxide while Np2O5 is metastable.
In addition to calculating ∆fG° by ∆fG° = ∆fH° - T∆fS°, we can also calculate ∆fG°298 from
Np2O5 (cr) + 2H+ = 2NpO2+ + H2O
(1)
NpO2 (cr) + 4H+ = Np4+ + 2H2O
(2)
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oxides,
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solubility experiments by ∆G° = -2.303RTlogK° for the following reactions involving neptunium
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where the ∆fG°298(NpO2+) and ∆fG°298(Np4+) are calculated from the ∆fG°298(NpO22+) 37, and the standard potential for the NpO22+/NpO2+ and NpO2+/Np4+ couples by ∆G° = nFE°. Table 4 lists the ∆fG°298(NpO2) and ∆fG°298(Np2O5) calculated by Kaszuba et al. E°(NpO22+/NpO2+) and E°(NpO2+/Np4+) summarized by Kihara et al.
39
38
. Using the
, we also calculate the
∆fG°298(NpO2) and ∆fG°298(Np2O5) (Table 4). Comparing the data for the standard formation Gibbs energy of both neptunium oxides, the data measured by solubility experiments (Table 4) agree within experimental error with the ones obtained by direct thermochemical measurements
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(Table 3). On the other hand, using a recent work 40 reporting standard potentials for neptunium redox reactions, which are claimed to have higher accuracy, the calculated ∆fG° values are much less negative (Table 4), and not consistent with any other reported values. Our results show that
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the direct calorimetric measurements are consistent with the existing thermodynamic database.
5. Conclusion
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High temperature oxide melt solution calorimetry of neptunium oxides is straightforward and produces reliable data, while requiring only milligram quantities of material. Calorimetric studies
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of heats of formation of neptunium compounds are ongoing for carefully characterized synthetic materials, such as analogs to uranyl minerals and nanoscale clusters. These thermodynamic data will establish the stability of neptunium materials, and increase understanding of processes that occur during the dissolution of spent nuclear fuel and the transport of actinides in an aqueous
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Acknowledgements
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environment.
This work was supported as part of the Materials Science of Actinides, an Energy Frontier
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Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001089. Calorimetry and diffraction data were collected at the Materials Characterization Facility in the Center for Sustainable Energy at the University of Notre Dame.
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Reference:
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1. Burns, P. C.; Kubatko, K. A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L., Actinyl peroxide nanospheres. Angew Chem Int Edit 2005, 44 (14), 2135-2139. 2. Charushnikova, I. A.; Krot, N. N.; Starikova, Z. A., Crystal structure and spectral characteristics of double sodium neptunium(IV) sulfate. Radiochemistry+ 2000, 42 (1), 37-41. 3. Charushnikova, I. A.; Krot, N. N.; Starikova, Z. A., Synthesis, crystal structure, and characteristics of double cesium neptunium(IV) sulfate. Radiochemistry+ 2000, 42 (1), 42-47. 4. Weigel, F.; Hoffmann, G., Phosphates and Arsenates of Hexavalent Actinides .2. Neptunium. J Less-Common Met 1976, 44 (Jan), 125-132. 5. Forbes, T. Z.; Burns, P. C., The crystal structures of X(NpO2)(PO4)(H2O)(3) (X = K+, Na+, Rb+, NH4+) and their relationship to the autunite group. Can Mineral 2007, 45, 471-477. 6. Forbes, T. Z.; Burns, P. C., Ba(NpO2)(PO4)(H2O), its relationship to the uranophane group, and implications for Np incorporation in uranyl minerals. Am Mineral 2006, 91 (7), 10891093. 7. Finch, R. J., Precipitation of crystalline NpO2 during oxidative corrosion of neptuniumbearing uranium oxides. Scientific Basis for Nuclear Waste Management Xxv 2002, 713, 639646. 8. Roberts, K. E.; Wolery, T. J.; Atkins-Duffin, C. E.; Prussin, T. G.; Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Finch, R. J.; Prussin, S. G., Precipitation of crystalline neptunium dioxide from near-neutral aqueous solution. Radiochim Acta 2003, 91 (2), 87-92. 9. Burns, P. C.; Ewing, R. C.; Miller, M. L., Incorporation mechanisms of actinide elements into the structures of U6+ phases formed during the oxidation of spent nuclear fuel. J Nucl Mater 1997, 245 (1), 1-9. 10. Klingensmith, A. L.; Burns, P. C., Neptunium substitution in synthetic uranophane and soddyite. Am Mineral 2007, 92 (11-12), 1946-1951. 11. Douglas, M.; Clark, S. B.; Friese, J. I.; Arey, B. W.; Buck, E. C.; Hanson, B. D., Neptunium(V) partitioning to uranium(VI) oxide and peroxide solids. Environ Sci Technol 2005, 39 (11), 4117-4124. 12. Finch, R. J.; Kropf, A. J., Neptunium substitution into the structure of alpha-U3O8. Mater Res Soc Symp P 2003, 757, 455-462. 13. Shuller, L. C.; Ewing, R. C.; Becker, U., Quantum-mechanical evaluation of Npincorporation into studtite. Am Mineral 2010, 95 (8-9), 1151-1160. 14. Forbes, T. Z.; Burns, P. C.; Skanthakumar, S.; Soderholm, L., Synthesis, structure, and magnetism of Np2O5. J Am Chem Soc 2007, 129 (10), 2760-+. 15. Richter, K.; Sari, C., Phase-Relationships in the Neptunium-Oxygen System. J Nucl Mater 1987, 148 (3), 266-271. 16. Bohler, R.; Welland, M. J.; De Bruycker, F.; Boboridis, K.; Janssen, A.; Eloirdi, R.; Konings, R. J. M.; Manara, D., Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements. J Appl Phys 2012, 111 (11). 17. Belyaev, Y. I.; Smirnov, N.; Taranov, A., Determination of formation heats of neptunium pentozide and neptunyl hydroxide. Radiokhimiya 1979, 21 (5), 682-686. 18. Merli, L.; Fuger, J., Thermochemistry of a Few Neptunium and Neodymium Oxides and Hydroxides. Radiochim Acta 1994, 66-7, 109-113. 19. Huber, E. J.; Holley, C. E., Enthalpy of Formation of Neptunium Dioxide. J Chem Eng Data 1968, 13 (4), 545-&.
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20. Belyaev, Y. I.; Dobretsov, V.; Ustinov, V., Enthalpy and Heat Capacity of Np205 over the Temperature Range 350-750 K. Sov. Radiochem 1979, 21, 386. 21. Westrum, E. F.; Hatcher, J. B.; Osborne, D. W., The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide. J Chem Phys 1953, 21 (3), 419-423. 22. Arkhipov, V.; Gutina, E. A.; Dobretsov, V.; Ustinov, V., Enthalpy and heat capacity of neptunium dioxide at 350-1100 0 K. Radiokhimiya 1974, 16 (1), 123-126. 23. Nishi, T.; Itoh, A.; Takano, M.; Numata, M.; Akabori, M.; Arai, Y.; Minato, K., Thermal conductivity of neptunium dioxide. J Nucl Mater 2008, 376 (1), 78-82. 24. Benes, O.; Gotcu-Freis, P.; Schworer, F.; Konings, R. J. M.; Fanghanel, T., The high temperature heat capacity of NpO2. J Chem Thermodyn 2011, 43 (5), 651-655. 25. Aizenshtein, M.; Shvareva, T. Y.; Navrotsky, A., Thermochemistry of Lanthana- and Yttria-Doped Thoria. J Am Ceram Soc 2010, 93 (12), 4142-4147. 26. Mazeina, L.; Navrotsky, A.; Greenblatt, M., Calorimetric determination of energetics of solid solutions of UO2+x with CaO and Y2O3. J Nucl Mater 2008, 373 (1-3), 39-43. 27. Zhang, L.; Navrotsky, A., Thermochemistry of rare earth doped uranium oxides Ln(x)U(1-x)O(2-0.5x+y) (Ln = La, Y, Nd). J Nucl Mater 2015, 465, 682-691. 28. Zhang, L.; Shelyug, A.; Navrotsky, A., Thermochemistry of UO 2-ThO 2 and UO 2-ZrO 2 fluorite solid solutions. The Journal of Chemical Thermodynamics 2017. 29. Navrotsky, A., Progress and New Directions in High-Temperature Calorimetry. Phys Chem Miner 1977, 2 (1-2), 89-104. 30. Navrotsky, A., Progress and new directions in high temperature calorimetry revisited. Phys Chem Miner 1997, 24 (3), 222-241. 31. Navrotsky, A., Progress and New Directions in Calorimetry: A 2014 Perspective. J Am Ceram Soc 2014, 97 (11), 3349-3359. 32. Bagnall, K. W.; Laidler, J. B., Neptunium + Plutonium Trioxide Hydrates. J Chem Soc 1964, (Aug), 2693-&. 33. Fahey, J. A.; Turcotte, R. P.; Chikalla, T. D., Decomposition, Stoichiometry and Structure of Neptunium Oxides. J Inorg Nucl Chem 1976, 38 (3), 495-500. 34. Konings, R. J. M.; Benes, O.; Kovacs, A.; Manara, D.; Sedmidubsky, D.; Gorokhov, L.; Iorish, V. S.; Yungman, V.; Shenyavskaya, E.; Osina, E., The Thermodynamic Properties of the f- Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides. J Phys Chem Ref Data 2014, 43 (1). 35. Konings, R. J. M.; Benes, O., The Thermodynamic Properties of the f-Elements and Their Compounds. I. The Lanthanide and Actinide Metals. J Phys Chem Ref Data 2010, 39 (4). 36. Efurd, D. W.; Runde, W.; Banar, J. C.; Janecky, D. R.; Kaszuba, J. P.; Palmer, P. D.; Roensch, F. R.; Tait, C. D., Neptunium and plutonium solubilities in a Yucca Mountain groundwater. Environ Sci Technol 1998, 32 (24), 3893-3900. 37. Fuger, J.; Oetting, F., The Chemical Thermodynamics of Actinide Elements and Compounds. Part2. The Actinide Aqueous Ions. IAEA, Int. Atomic Energy Agency, Vienna, Austria 1976. 38. Kaszuba, J. P.; Runde, W. H., The aqueous geochemistry of neptunium: Dynamic control of soluble concentrations with applications to nuclear waste disposal. Environ Sci Technol 1999, 33 (24), 4427-4433. 39. Kihara, S.; Yoshida, Z.; Aoyagi, H.; Maeda, K.; Shirai, O.; Kitatsuji, Y.; Yoshida, Y., A critical evaluation of the redox properties of uranium, neptunium and plutonium ions in acidic aqueous solutions. Pure and Applied Chemistry 1999, 71 (9), 1771-1807.
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40. Chatterjee, S.; Bryan, S. A.; Casella, A. J.; Peterson, J. M.; Levitskaia, T. G., Mechanisms of neptunium redox reactions in nitric acid solutions. Inorganic Chemistry Frontiers 2017. 41. Helean, K. B.; Navrotsky, A.; Lumpkin, G. R.; Colella, M.; Lian, J.; Ewing, R. C.; Ebbinghaus, B.; Catalano, J. G., Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization. J Nucl Mater 2003, 320 (3), 231-244. 42. Robie, R. A.; Hemingway, B. S.; Geological Survey (U.S.), Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (10 p5 s pascals) pressure and at higher temperatures. U.S. G.P.O.;Denver, CO, 1995; p iv, 461 p.
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43. Lemire, R. J. An assessment of the thermodynamic behaviour of neptunium in water and model groundwaters from 25 to 150 degrees C; Atomic Energy of Canada Ltd.: 1984.
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Figure Captions Fig. 1. Enthalpy of decomposition for the reaction Np2O5 → 2NpO2 + 1/2O2 at 298 K measured by high temperature drop solution calorimetry (this work), compared with values measured by
Method B: calculated from ∆fH°(NpO2)
19
and ∆fH°(Np2O5)
; Method C: calculated from
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∆fH°(NpO2) 19 and ∆fH°(Np2O5) 17.
18
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oxygen bomb calorimetry and solution calorimetry. Method A: direct measurement in this work;
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Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) @ 298 K 30
10
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∆rHο (kJ/mol)
20
0
-20
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-10
C
B Method
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A
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Table 1. Drop solution enthalpies of Np2O5·nH2O and NpO2 into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K. Np2O5·nH2O
NpO2
∆HDS (kJ/mol)
Mass (mg)
∆HDS (kJ/mol)
5.41
33.91
5.09
6.29
5.54
46.40
6.19
6.47
37.32
5.57
5.96
40.90
6.17
Average*
39.63
Error†
5.34
Error (%)‡
13.48
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Mass (mg)
9.01 8.55
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7.38
7.81
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1.22
Mean of the experiments listed.
†
Two standard deviations of the mean.
‡
The uncertainty at a 95 % level of confidence.
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*
15.68
Table 2. Comparison of the drop solution enthalpy, and heat of solution among actinide oxides into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K.
NpO2.5
∆HS
17.11 ± 2.67 †‡
54.63
-37.52 ± 2.67
7.81 ± 1.22 ‡
53.04
-45.23 ± 1.22
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NpO2
∆HHC *
∆HDS
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Phase
γ-UO3
9.49 ± 1.53 41
64.22
-54.73 ± 1.53
UO2
-136.4 ± 2.3 41
52.80
-189.2 ± 2.3
ThO2
0.89 ± 0.48 41
49.50
-48.61 ± 0.48
*
Heat content of oxides from 298 K to 973 K calculated using Cp data from Konings 34.
†
Values for NpO2.5 are half those for Np2O5. All data are in kJ/mol.
‡
This work.
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Table 3. Thermodynamic data at 298 K for neptunium oxides from thermochemical measurements and estimation of decomposition temperature of Np2O5. Thermodynamic Data
Selected or Calculated Values
∆fH°298(Np2O5) (kJ/mol) ∆rxnH°(Np2O5 = 2NpO2 + 1/2O2) (kJ/mol)
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-1074.2 ± 2.5 19
∆fH°298(NpO2) (kJ/mol) -2162.7 ± 9.3 18
-2148 ± 12 17
14.3 ± 10.6
-0.4 ± 13.0
50.45 ± 0.4 35
S°298(NpO2) (J·K-1·mol-1)
80.29 ± 0.42 21
S°298(Np2O5) (J·K-1·mol-1)
186 ± 15 34
*
205.043 42
S°298(O2) (J·K-1·mol-1)
-1022.0 ± 2.5
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∆fG°298(NpO2) (kJ/mol) ∆fG°298(Np2O5) (kJ/mol) Decomposition temperature (K) *
7.70 ± 5.86
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S°298(Np) (J·K-1·mol-1)
-
This work.
-2035.3 ± 14.5
-2020.6 ± 16.4
-2028.3 ± 13.6
185 ± 142
0 to168
100 ± 78
Table 4. Thermodynamic data at 298 K for neptunium oxides calculated from solubility
Thermodynamic Data ∆fG°298(NpO22+) (kJ/mol)
E°(NpO2+/Np4+) (V)
∆fG°298(NpO2+) (kJ/mol) 4+
logK1° logK2°
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∆fG°298(Np ) (kJ/mol)
Selected or Calculated Values -795.8 ± 5.4 43
1.161 ± 0.014 38
1.193 ± 0.043 39
0.96 40
0.596 ± 0.078 38
0.660 ± 0.067 39
0.07 40
-907.9 ± 5.8 38
-910.9 ± 6.8
-888.4 ± 5.4
38
-500.2 ± 9.4
-420.8 ± 5.4
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E°(NpO22+/NpO2+) (V)
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experiments.
-491.1 ± 9.5
5.2 ± 0.8 36 -9.9 ± 1.7 38 -1021.8 ± 13.5 38
-1031.0 ± 13.5
-951.6 ± 11.1
∆fG°298(Np2O5) (kJ/mol)
-2023.3 ± 12.4
38
-2029.4 ± 14.4
-1984.4 ± 11.7
Reference
Kaszuba et. al. 38
Kihara et. al. 39
Chatterjee et. al. 40
∆fG°298(NpO2) (kJ/mol)
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Supporting Information
101
100
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Np2O5
Weight (%)
99
98
97
96
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NpO2 95 0
200
400
600
800
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Temperature (°C)
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Fig. S1. TGA (thermogravimetric analysis) of Np2O5.
Fig. S2. PXRD pattern of Np2O5.
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Fig. S3. PXRD pattern of NpO2.
Np(V)
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Absorbance
0.4
0.3
0.1 400
600
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0.2
800
1000
1200
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Wavelength (nm)
Fig. S4. UV-Vis of NpO2 dissolved in sodium molybdate.
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Table S1. Reactions and thermodynamic cycles used to calculate enthalpies of drop solution of anhydrous Np2O5 into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K. ∆H (kJ/mol)
(1) Np2O5(cr, 298 K) → Np2O5(soln, 973 K)
∆H1 = ∆HDS(Np2O5) = ∆H2 - n∆H3 + ∆H4
(2) Np2O5·nH2O(cr, 298 K) → Np2O5(soln, 973 K) + nH2O(g, 973 K)
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Reaction
∆H2 = ∆HDS(Np2O5·nH2O) = 39.63 ± 5.34
(3) H2O(g, 298 K) → H2O(g, 973 K)
∆H3 = ∆Hheat content(H2O) = 25.0 42
(4) Np2O5(cr, 298 K) + nH2O(g, 298 K) → Np2O5·nH2O(cr, 298 K)
∆H4 = n*∆Hwater condensation(H2O) = n*(-
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44.0) 42
Table S2. Reactions and thermodynamic cycles used to calculate enthalpy of decomposition of Np2O5 → 2NpO2 + 1/2O2 at 298 K. Reaction
∆H (kJ/mol) ∆H1 = -2∆H2 + ∆H3 – 1/2∆H4
(2) NpO2(cr, 298 K) + 1/4O2(g, 973 K) → 1/2Np2O5(soln, 973 K)
∆H2 = ∆HDS(NpO2)
(3) Np2O5(cr, 298 K) → Np2O5(soln, 973 K)
∆H3 = ∆HDS(Np2O5)
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(4) O2(g, 298 K) → O2(g, 973 K)
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(1) Np2O5(cr, 298 K) → 2NpO2(cr, 298 K) + 1/2O2(g, 298 K)
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∆H4 = ∆Hheat content(O2) = 21.8 42
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• •
We synthesize and characterize Np2O5 and NpO2. Enthalpy of decomposition of Np2O5 to NpO2 agrees with existing thermodynamic data. The calorimetric methodology is straightforward and reliable.
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S0022-3115(17)30882-6
DOI:
10.1016/j.jnucmat.2017.10.034
Reference:
NUMA 50566
To appear in:
Journal of Nuclear Materials
Received Date: 16 June 2017 Revised Date:
10 October 2017
Accepted Date: 11 October 2017
Please cite this article as: L. Zhang, E.A. Dzik, G.E. Sigmon, J.E.S. Szymanowski, A. Navrotsky, P.C. Burns, Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2, Journal of Nuclear Materials (2017), doi: 10.1016/j.jnucmat.2017.10.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Experimental thermochemistry of neptunium oxides: Np2O5 and NpO2
Navrotsky b, and Peter C. Burns a,c
a
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame,
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Notre Dame, IN 46556, USA b
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis,
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Davis, CA 95616, USA c
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Lei Zhang a,*, Ewa A. Dzik a, Ginger E. Sigmon a, Jennifer E.S. Szymanowski a, Alexandra
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
Keywords: Thermochemistry, formation enthalpy, neptunium oxides.
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To whom correspondence should be addressed. Email: [email protected].
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*
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Abstract Neptunium (Np) compounds are important in the nuclear fuel cycle because of the buildup and long half-life (2.14 Ma) of Np-237 in nuclear waste, especially during long-term disposal in a
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geological repository. Neptunium in environmental conditions exists mainly in two oxidation states (+5 and +4) and can substitute for uranium and/or rare earths in solid phases. Yet thermochemical data for solid neptunium compounds are scarce, despite being critical for
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evaluating the environmental transport of this radioactive and toxic element. Although high temperature oxide melt solution calorimetry has proven very useful in obtaining thermodynamic
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data for the formation of uranium and thorium oxide materials, it has not yet been applied to transuranium compounds. Continuing a program at Notre Dame to study the thermodynamics of transuranium compounds, we report the first determination of the enthalpies of drop solution of well-characterized neptunium oxides (Np2O5 and NpO2) using oxide melt solution calorimetry in
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molten sodium molybdate solvent at 973 K. The enthalpy of the decomposition reaction, Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) at 298 K, is determined to be 7.70 ± 5.86 kJ/mol, and this direct measurement is consistent with existing thermodynamic data. The calorimetric methodology is
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straightforward and produces reliable data using milligram quantities of radioactive materials,
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and can be applied to many other transuranium compounds.
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1. Introduction Neptunium-237 is of concern for the long-term geological disposal of spent nuclear fuel because of its long half-life (2.14 Ma) and potential mobility, especially under oxidizing
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conditions because the neptunyl (NpO2+) ion is soluble in water. Neptunium forms various compounds with peroxide 1, sulfate 2-3, phosphate 4-6, arsenate 4, and other oxyanions, as well as simple oxides. Studies have shown that neptunium oxides can precipitate from neptunyl
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solutions under mildly oxidizing conditions 7-8. Neptunium may also partition into uranyl phases, such as studtite or metastudite (alteration phases of UO2 fuel), by substitution for U(VI) with 9-13
. Therefore it is crucial to understand the
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appropriate charge balance mechanisms
geochemical stability of Np-containing solid materials as well as species in aqueous solution. However, there are very limited reported thermodynamic data for solid neptunium compounds, owing to the scarcity of facilities and techniques that are equipped for handling this radionuclide.
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Neptunium exists in environmental conditions in primarily in two oxidation states (+5 and +4) and has two stable oxides, Np2O5 and NpO2. Np2O5 has a monoclinic structure (space group P2/c) 14 and is stable at low temperature (it decomposes to NpO2 and O2 between 700 and 973 K ), whereas NpO2 in the cubic fluorite structure (space group Fm3m) is stable to its melting
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point of 3070 ± 62 K 16. The enthalpy of formation of Np2O5 has been measured by acid solution
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calorimetry by Belyaev et al. (∆fH°298(Np2O5) = -2148 ± 12 kJ/mol) (∆fH°298(Np2O5) = -2162.7 ± 9.3 kJ/mol)
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17
and by Merli and Fuger
. However, these reported values are in poor
agreement. The only reported enthalpy of formation of NpO2 was measured by oxygen bomb calorimetry by Huber and Holley ((∆fH°298(NpO2) = -1074.2 ± 2.5 kJ/mol) 19, using two grams of material. The heat capacities of Np2O5 (from 350 to 750 K) 20 and NpO2 (from 5 to 1770 K) 21-24
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have also been measured. However, the low temperature (0 to 298 K) heat capacities of Np2O5, needed to obtain standard entropies, have not been reported. High temperature oxide melt solution calorimetry has been successfully utilized to study the 25-28
. Here, we describe the
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thermochemistry of actinide oxides containing thorium and uranium
further development of this technique at the University of Notre Dame to study the formation enthalpies of neptunium compounds. Neptunium oxides (Np2O5 and NpO2) have been
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synthesized and characterized for calorimetric study. Their drop solution enthalpies (enthalpy of reaction in which a solid oxide pellet is dropped from room temperature into a molten oxide
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solvent at calorimeter temperature and dissolved) into molten sodium molybdate solvent at 973 K have been measured. The decomposition enthalpy of Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) at 298 K is determined to be 7.70 ± 5.86 kJ/mol. This value agrees well with those calculated using previously reported formation enthalpies of the two neptunium oxides. These experiments are a
2. Materials and Methods
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2.1 Materials preparation
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first step in the calorimetric study of heats of formation of more complex neptunium materials.
Caution: Nepunium-237 is radioactive and it decays by emission of alpha radiation. Its decay
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product is protactinium-233, which emits a strong gamma that is dramatically more penetrating than alpha. Therefore all experiments were done in purpose-designed laboratories at the University of Notre Dame with proper equipment. Conduction of experiments of this sort requires monitoring of the area and personnel, and a comprehensive program of radiation worker training and institutional oversight.
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Np2O5 was prepared by hydrothermal reactions of NpO2+ solutions with natural calcite crystals, as previously reported by our group for the single-crystal structure determination of this compound 14. The reactants were placed in Teflon cups with screw-top lids that were then placed
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inside Teflon-lined 125 mL stainless steel Parr reaction vessels, together with water to provide counter pressure in the outer vessel. The vessels were heated at 473 K for 7 days. Monitoring for unintended release of radionuclides is essential during disassembly of this apparatus, which
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should be conducted in a fume hood under negative pressure relative to the laboratory ambient. Dark brown Np2O5 crystals were recovered, and washed using 1N HCl to remove any remaining
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calcite, followed by washing with ultrapure H2O. The crystals were dried inside a glove box. NpO2 was prepared by the decomposition of Np2O5 during heating to 1173 K.
2.2 Thermogravimetric analysis
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Thermogravimetric analyses were performed using about 10 mg of dried Np2O5 crystals in a TA Instruments Q50 located in a radiological hood from 303 K to 1173 K at a heating rate of 5 K/min under N2. The sample was held at 423 K for 30 minutes to eliminate surface water on the
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Np2O5 crystals. The mass loss was in agreement with the end product being NpO2 (Figure S1).
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2.3 Powder X-ray diffraction
Np2O5 and NpO2 (remaining powder from TGA of Np2O5 crystals) products were ground using an agate mortar and pestle inside a glove box under negative pressure, and were deposited onto an airtight sample holder that provides containment of the material. PXRD patterns were collected from 5 to 90 ° (2 theta) with CuKα radiation using a Bruker D8 Advance diffractometer with a step width of 0.02 ° and 1 s counting per step (Figure S2 and S3). The patterns were
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processed using PDF-4+ to confirm the identities of both neptunium oxides and the absence of secondary phases.
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2.4 Calorimetry
Enthalpies of drop solution of the neptunium oxides into molten sodium molybdate (3Na2O·4MoO3) at 973 K were measured using a Setaram AlexSYS 1000 high temperature
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Tian-Calvet calorimeter. The calorimeter was calibrated using the heat content of 5 mg α-Al2O3 pellets 29-31. In order to minimize the risk of neptunium dispersal, silica glass crucibles were used
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as the molten solvent container (rather than platinum), so that the solidified sodium molybdate solvent and the crucibles are suitable for disposal or neptunium recovery as a single unit after the experiment. The sodium molybdate melt is not corrosive to the silica crucibles. Operating procedures for calorimetric experiments using neptunium materials were first developed for non-
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radioactive samples, and personnel completed multiple non-radioactive experiments of this sort before transitioning to hot work. The neptunium oxide sample pellets were prepared in a designated negative pressure glove box and were transported to the nearby calorimetry lab in
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triple containment. For each experiment, about 5 mg of neptunium oxides were pressed into a 1.5 mm diameter pellet, weighed accurately on a Mettler-Toledo XSE105 microbalance (± 0.01 mg),
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and dropped into 10 g of molten sodium molybdate solvent contained in silica glass crucibles in the AlexSYS calorimeter. The calorimetric assembly was flushed continuously with oxygen at 43 mL/min and oxygen was bubbled in the molten solvent at 5 mL/min through silica glass tubes to facilitate dissolution of neptunium oxides and provide an oxidizing atmosphere to oxidize all neptunium to the stable pentavalent state in the solvent. The final oxidation state of neptunium was confirmed to be pentavalent by UV-Vis spectra collected for the solvent subsequent to
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cooling to room temperature (Fig. S4). The thermochemical cycles listed in Tables S1 and S2 were used to calculate the enthalpies of reactions of interest.
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2.5 UV-Vis spectroscopy
Sodium molybdate solvent containing dissolved neptunium oxide was dissolved in ultrapure H2O in air, with dissolution complete in a few minutes. UV-vis spectra were collected
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immediately after dissolution from 400 to 1300 nm at room temperature using a Jasco V-670
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spectrophotometer (Fig. S4).
3. Results
Initially, Np2O5 was prepared for study, and NpO2 was subsequently produced by heating Np2O5. Powder X-ray diffraction (PXRD) confirmed that the starting material was Np2O5, with
structure
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the pattern agreeing with that calculated using the previously reported single crystal X-ray . Thermogravimetric analysis (TGA) of Np2O5 crystals (during which it was held at
423 K for 30 minutes) indicated incorporation of a small amount of water in the oxide,
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corresponding to composition Np2O5·0.077H2O. Decomposition of Np2O5 initiates at about 573 K and is complete above 973 K under N2, with a heating rate of 5 K/min, consistent with
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previous reports 15. Bagnall and Laidler reported that Np2O5 persists to 693 K under vacuum 32. Decomposition of Np2O5 to NpO2 is reported to occur between 703 and 973 K, depending on O2 pressure
33
. Richter and Sari showed that the conversion is irreversible (consistent with
thermodynamic data, see discussions below) and that NpO2 remains stoichiometric to 1743 K 15. TGA revealed that the weight loss through heating to 1073 K was 4.0 %, indicating that the
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remaining material was NpO2 (calculated weight loss for Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) is 2.9 %). Powder X-ray diffraction confirmed the heated powder to be NpO2. The calorimetric data for each drop of Np2O5·0.077H2O and NpO2 samples are listed in Table
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1. Both neptunium oxides dissolved quickly in the sodium molybdate solvent at 973 K, and the calorimetric baseline returned to the starting position within 20 min. Np2O5·0.077H2O gave a small heat effect of less than 0.5 J per experiment, while NpO2 gave an even smaller heat effect
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of less than 0.05 J per experiment. The intrinsically small drop solution enthalpies of both neptunium oxides require a highly stable baseline during the experiments. As a result, only four
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of eight experiments for Np2O5·0.077H2O and four of five for NpO2 were appropriate for further calculations. The four drops of each sample gave ∆HDS(Np2O5·0.077H2O) = 39.63 ± 5.34 kJ/mol, and ∆HDS(NpO2) = 7.81 ± 1.22 kJ/mol. The calorimetric method proved to be straightforward and can be applied to other neptunium compounds.
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The oxidation states of neptunium in the molten sodium molybdate solvent after each experiment are crucial for the interpretation of the calorimetric data. Hence, the solvent with dissolved neptunium oxides after being cooled to room temperature was dissolved in water and
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its UV-Vis spectrum was collected immediately. The final oxidation states of both neptunium oxides in the sodium molybdate solvent were determined to be pentavalent (Fig. S4). Therefore it
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is possible to use the thermochemical cycles listed in Tables S1 and S2 to calculate the enthalpies of reactions involving both neptunium oxides.
4. Discussion
To obtain ∆HDS of anhydrous Np2O5, the effect of the small amount of water, which is presumably on the surface of the crystals, must be taken into account. Assuming physical
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adsorption, the water adsorption enthalpy is -44.0 kJ/mol, which is the heat of water vapor condensation at 298 K. Thus, ∆HDS(Np2O5) is calculated to be 34.22 ± 5.34 kJ/mol using the thermochemical cycles listed in Table S1.
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The drop solution enthalpies ∆HDS measured here include ∆HHC and ∆HS, where ∆HHC is the heat content of oxide from 298 to 973 K, and ∆HS is the heat of solution in the molten solvent at 973 K. ∆HHC can be calculated from the heat capacity (Cp) data 34 and is shown in Table 2. With
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the measured ∆HDS listed in Table 2, we calculated ∆HS of the neptunium oxides along with those of uranium oxides and thorium oxide. For comparison, the results are given per mole of
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actinide metal. ∆HS(NpO2.5) is comparable to ∆HS(γ-UO3) and ∆HS(ThO2), where ∆HS of these oxides result from the heat of solution without oxidation. However, ∆HHC(NpO2.5) was estimated by high temperature heat capacity measured by drop calorimetry from 350 to 750 K by Belyaev et al.
20
, which gives Cp(T)/(J·K-1·mol-1) = 99.2 + 98.6 × 10-3 (T/K), since Np2O5 is unstable at
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high temperature. ∆HHC(NpO2.5) is likely underestimated to be consistent with the variation of ∆HHC of uranium oxides, so ∆HS(NpO2.5) is likely more negative, and closer to the values of ∆HS(γ-UO3) and ∆HS(ThO2).
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The enthalpy of decomposition of Np2O5 to NpO2 and O2 at 298 K is obtained by the thermochemical cycles listed in Table S2 and is 7.70 ± 5.86 kJ/mol, obtained directly from the
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measurements of drop solution enthalpies of these two neptunium oxides. The enthalpy of formation of NpO2 was previously measured by Huber and Holley19 by oxygen bomb calorimetry using 1.5 to 2 g of neptunium metal pellets in each experiment, giving ∆fH°298(NpO2) = -1074.2 ± 2.5 kJ/mol
19
. The enthalpy of formation of Np2O5 has been
measured by solution calorimetry by two slightly different methods. Merli and Fuger 18 reported ∆fH°298(Np2O5) = -2162.7 ± 9.3 kJ/mol using 2 – 12 mg of material, and Belyaev
9
17
reported
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∆fH°298(Np2O5) = -2148 ± 12 kJ/mol using 37 – 73 mg of material. The decomposition enthalpy of Np2O5 is also calculated from these reported data and compared with that from this work (Fig. 1 and Table 3). Our new value (7.70 ± 5.86 kJ/mol) is in reasonable agreement with both
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numbers (14.3 ± 10.6 kJ/mol and -0.4 ± 13.0 kJ/mol), and yields a smaller error while using only 5 mg of sample via direct calorimetric measurements.
Table 3 also lists the entropy at room temperature as S°298(Np) = 50.45 ± 0.4 J·K-1·mol-1
been measured, and Merli and Fuger
21
18
,
. The low temperature heat capacity of Np2O5 has not
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S°298(NpO2) = 80.29 ± 0.42 J·K-1·mol-1
35
have estimated S°298(Np2O5) = 186 ± 16 J·K-1·mol-1,
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which is consistent with the variation of the standard entropy of uranium oxides based on the difference in oxygen stoichiometry between the two oxides
34
. Using measured thermodynamic
data for NpO2, we calculate the ∆fG°298(NpO2) and ∆fG°298(Np2O5) from the elements, and estimate the equilibrium decomposition temperature of Np2O5 to NpO2 (Table 3). It is predicted
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that NpO2 would be the thermodynamically stable solid at room temperature, since all the calculations show equilibrium decomposition temperature below 298 K. This is consistent with a recent study which has shown that neptunium oxides can precipitate from neptunyl solutions 7-8
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under mildly oxidizing conditions
. However, as shown in many previous works
15, 32-33
and
confirmed in this study, Np2O5 can persist up to 973 K; Np(V) solid phases precipitate in
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solubility experiments instead of NpO2 conducted with natural water 36. Therefore, NpO2 may be the stable neptunium solid oxide thermodynamically for the long term. Nevertheless it is still important to understand the thermodynamics of metastable Np2O5 phase through obtaining reliable thermodynamic data. Smith et al.
36
recently optimized formation enthalpies ∆fH°298(NpO2) = -1076.8 kJ/mol and
∆fH°298(Np2O5) = -2172.0 kJ/mol by thermodynamic modeling using the CALPHAD
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methodology. Their calculated enthalpy and entropy values for NpO2 are in good agreement with reported data (Table 3). However, their optimized ∆fH°298(Np2O5) = -2172.0 kJ/mol is more negative than the two reported values (-2162.7 ± 9.3 kJ/mol and -2148 ± 12 kJ/mol). They also
1
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reported optimized entropies S°298(NpO2) = 80.3 J·K-1·mol-1, and S°298(Np2O5) = 242.2 J·K·mol-1 (as the low-temperature heat capacity of Np2O5 has not been measured). Hence we can
calculate ∆fG298°(NpO2) = -1024.6 kJ/mol, which agrees with the experimental data; and
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∆fG°298(Np2O5) = -2061.4 kJ/mol, which is significantly smaller than the other data (Table 3).
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This big discrepancy in Gibbs free energy of Np2O5 is the result of the authors using a bigger S°298(Np2O5) to accommodate the higher decomposition temperature (888 K) of Np2O5. This cannot justify their optimization of thermochemical data; as discussed above, NpO2 is the thermodynamically stable oxide while Np2O5 is metastable.
In addition to calculating ∆fG° by ∆fG° = ∆fH° - T∆fS°, we can also calculate ∆fG°298 from
Np2O5 (cr) + 2H+ = 2NpO2+ + H2O
(1)
NpO2 (cr) + 4H+ = Np4+ + 2H2O
(2)
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oxides,
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solubility experiments by ∆G° = -2.303RTlogK° for the following reactions involving neptunium
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where the ∆fG°298(NpO2+) and ∆fG°298(Np4+) are calculated from the ∆fG°298(NpO22+) 37, and the standard potential for the NpO22+/NpO2+ and NpO2+/Np4+ couples by ∆G° = nFE°. Table 4 lists the ∆fG°298(NpO2) and ∆fG°298(Np2O5) calculated by Kaszuba et al. E°(NpO22+/NpO2+) and E°(NpO2+/Np4+) summarized by Kihara et al.
39
38
. Using the
, we also calculate the
∆fG°298(NpO2) and ∆fG°298(Np2O5) (Table 4). Comparing the data for the standard formation Gibbs energy of both neptunium oxides, the data measured by solubility experiments (Table 4) agree within experimental error with the ones obtained by direct thermochemical measurements
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(Table 3). On the other hand, using a recent work 40 reporting standard potentials for neptunium redox reactions, which are claimed to have higher accuracy, the calculated ∆fG° values are much less negative (Table 4), and not consistent with any other reported values. Our results show that
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the direct calorimetric measurements are consistent with the existing thermodynamic database.
5. Conclusion
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High temperature oxide melt solution calorimetry of neptunium oxides is straightforward and produces reliable data, while requiring only milligram quantities of material. Calorimetric studies
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of heats of formation of neptunium compounds are ongoing for carefully characterized synthetic materials, such as analogs to uranyl minerals and nanoscale clusters. These thermodynamic data will establish the stability of neptunium materials, and increase understanding of processes that occur during the dissolution of spent nuclear fuel and the transport of actinides in an aqueous
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Acknowledgements
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environment.
This work was supported as part of the Materials Science of Actinides, an Energy Frontier
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Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001089. Calorimetry and diffraction data were collected at the Materials Characterization Facility in the Center for Sustainable Energy at the University of Notre Dame.
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Reference:
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20. Belyaev, Y. I.; Dobretsov, V.; Ustinov, V., Enthalpy and Heat Capacity of Np205 over the Temperature Range 350-750 K. Sov. Radiochem 1979, 21, 386. 21. Westrum, E. F.; Hatcher, J. B.; Osborne, D. W., The Entropy and Low Temperature Heat Capacity of Neptunium Dioxide. J Chem Phys 1953, 21 (3), 419-423. 22. Arkhipov, V.; Gutina, E. A.; Dobretsov, V.; Ustinov, V., Enthalpy and heat capacity of neptunium dioxide at 350-1100 0 K. Radiokhimiya 1974, 16 (1), 123-126. 23. Nishi, T.; Itoh, A.; Takano, M.; Numata, M.; Akabori, M.; Arai, Y.; Minato, K., Thermal conductivity of neptunium dioxide. J Nucl Mater 2008, 376 (1), 78-82. 24. Benes, O.; Gotcu-Freis, P.; Schworer, F.; Konings, R. J. M.; Fanghanel, T., The high temperature heat capacity of NpO2. J Chem Thermodyn 2011, 43 (5), 651-655. 25. Aizenshtein, M.; Shvareva, T. Y.; Navrotsky, A., Thermochemistry of Lanthana- and Yttria-Doped Thoria. J Am Ceram Soc 2010, 93 (12), 4142-4147. 26. Mazeina, L.; Navrotsky, A.; Greenblatt, M., Calorimetric determination of energetics of solid solutions of UO2+x with CaO and Y2O3. J Nucl Mater 2008, 373 (1-3), 39-43. 27. Zhang, L.; Navrotsky, A., Thermochemistry of rare earth doped uranium oxides Ln(x)U(1-x)O(2-0.5x+y) (Ln = La, Y, Nd). J Nucl Mater 2015, 465, 682-691. 28. Zhang, L.; Shelyug, A.; Navrotsky, A., Thermochemistry of UO 2-ThO 2 and UO 2-ZrO 2 fluorite solid solutions. The Journal of Chemical Thermodynamics 2017. 29. Navrotsky, A., Progress and New Directions in High-Temperature Calorimetry. Phys Chem Miner 1977, 2 (1-2), 89-104. 30. Navrotsky, A., Progress and new directions in high temperature calorimetry revisited. Phys Chem Miner 1997, 24 (3), 222-241. 31. Navrotsky, A., Progress and New Directions in Calorimetry: A 2014 Perspective. J Am Ceram Soc 2014, 97 (11), 3349-3359. 32. Bagnall, K. W.; Laidler, J. B., Neptunium + Plutonium Trioxide Hydrates. J Chem Soc 1964, (Aug), 2693-&. 33. Fahey, J. A.; Turcotte, R. P.; Chikalla, T. D., Decomposition, Stoichiometry and Structure of Neptunium Oxides. J Inorg Nucl Chem 1976, 38 (3), 495-500. 34. Konings, R. J. M.; Benes, O.; Kovacs, A.; Manara, D.; Sedmidubsky, D.; Gorokhov, L.; Iorish, V. S.; Yungman, V.; Shenyavskaya, E.; Osina, E., The Thermodynamic Properties of the f- Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides. J Phys Chem Ref Data 2014, 43 (1). 35. Konings, R. J. M.; Benes, O., The Thermodynamic Properties of the f-Elements and Their Compounds. I. The Lanthanide and Actinide Metals. J Phys Chem Ref Data 2010, 39 (4). 36. Efurd, D. W.; Runde, W.; Banar, J. C.; Janecky, D. R.; Kaszuba, J. P.; Palmer, P. D.; Roensch, F. R.; Tait, C. D., Neptunium and plutonium solubilities in a Yucca Mountain groundwater. Environ Sci Technol 1998, 32 (24), 3893-3900. 37. Fuger, J.; Oetting, F., The Chemical Thermodynamics of Actinide Elements and Compounds. Part2. The Actinide Aqueous Ions. IAEA, Int. Atomic Energy Agency, Vienna, Austria 1976. 38. Kaszuba, J. P.; Runde, W. H., The aqueous geochemistry of neptunium: Dynamic control of soluble concentrations with applications to nuclear waste disposal. Environ Sci Technol 1999, 33 (24), 4427-4433. 39. Kihara, S.; Yoshida, Z.; Aoyagi, H.; Maeda, K.; Shirai, O.; Kitatsuji, Y.; Yoshida, Y., A critical evaluation of the redox properties of uranium, neptunium and plutonium ions in acidic aqueous solutions. Pure and Applied Chemistry 1999, 71 (9), 1771-1807.
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40. Chatterjee, S.; Bryan, S. A.; Casella, A. J.; Peterson, J. M.; Levitskaia, T. G., Mechanisms of neptunium redox reactions in nitric acid solutions. Inorganic Chemistry Frontiers 2017. 41. Helean, K. B.; Navrotsky, A.; Lumpkin, G. R.; Colella, M.; Lian, J.; Ewing, R. C.; Ebbinghaus, B.; Catalano, J. G., Enthalpies of formation of U-, Th-, Ce-brannerite: implications for plutonium immobilization. J Nucl Mater 2003, 320 (3), 231-244. 42. Robie, R. A.; Hemingway, B. S.; Geological Survey (U.S.), Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (10 p5 s pascals) pressure and at higher temperatures. U.S. G.P.O.;Denver, CO, 1995; p iv, 461 p.
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43. Lemire, R. J. An assessment of the thermodynamic behaviour of neptunium in water and model groundwaters from 25 to 150 degrees C; Atomic Energy of Canada Ltd.: 1984.
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Figure Captions Fig. 1. Enthalpy of decomposition for the reaction Np2O5 → 2NpO2 + 1/2O2 at 298 K measured by high temperature drop solution calorimetry (this work), compared with values measured by
Method B: calculated from ∆fH°(NpO2)
19
and ∆fH°(Np2O5)
; Method C: calculated from
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∆fH°(NpO2) 19 and ∆fH°(Np2O5) 17.
18
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oxygen bomb calorimetry and solution calorimetry. Method A: direct measurement in this work;
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Np2O5(cr) = 2NpO2(cr) + 1/2O2(g) @ 298 K 30
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∆rHο (kJ/mol)
20
0
-20
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-10
C
B Method
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A
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Table 1. Drop solution enthalpies of Np2O5·nH2O and NpO2 into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K. Np2O5·nH2O
NpO2
∆HDS (kJ/mol)
Mass (mg)
∆HDS (kJ/mol)
5.41
33.91
5.09
6.29
5.54
46.40
6.19
6.47
37.32
5.57
5.96
40.90
6.17
Average*
39.63
Error†
5.34
Error (%)‡
13.48
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Mass (mg)
9.01 8.55
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7.38
7.81
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1.22
Mean of the experiments listed.
†
Two standard deviations of the mean.
‡
The uncertainty at a 95 % level of confidence.
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*
15.68
Table 2. Comparison of the drop solution enthalpy, and heat of solution among actinide oxides into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K.
NpO2.5
∆HS
17.11 ± 2.67 †‡
54.63
-37.52 ± 2.67
7.81 ± 1.22 ‡
53.04
-45.23 ± 1.22
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NpO2
∆HHC *
∆HDS
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Phase
γ-UO3
9.49 ± 1.53 41
64.22
-54.73 ± 1.53
UO2
-136.4 ± 2.3 41
52.80
-189.2 ± 2.3
ThO2
0.89 ± 0.48 41
49.50
-48.61 ± 0.48
*
Heat content of oxides from 298 K to 973 K calculated using Cp data from Konings 34.
†
Values for NpO2.5 are half those for Np2O5. All data are in kJ/mol.
‡
This work.
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Table 3. Thermodynamic data at 298 K for neptunium oxides from thermochemical measurements and estimation of decomposition temperature of Np2O5. Thermodynamic Data
Selected or Calculated Values
∆fH°298(Np2O5) (kJ/mol) ∆rxnH°(Np2O5 = 2NpO2 + 1/2O2) (kJ/mol)
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-1074.2 ± 2.5 19
∆fH°298(NpO2) (kJ/mol) -2162.7 ± 9.3 18
-2148 ± 12 17
14.3 ± 10.6
-0.4 ± 13.0
50.45 ± 0.4 35
S°298(NpO2) (J·K-1·mol-1)
80.29 ± 0.42 21
S°298(Np2O5) (J·K-1·mol-1)
186 ± 15 34
*
205.043 42
S°298(O2) (J·K-1·mol-1)
-1022.0 ± 2.5
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∆fG°298(NpO2) (kJ/mol) ∆fG°298(Np2O5) (kJ/mol) Decomposition temperature (K) *
7.70 ± 5.86
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S°298(Np) (J·K-1·mol-1)
-
This work.
-2035.3 ± 14.5
-2020.6 ± 16.4
-2028.3 ± 13.6
185 ± 142
0 to168
100 ± 78
Table 4. Thermodynamic data at 298 K for neptunium oxides calculated from solubility
Thermodynamic Data ∆fG°298(NpO22+) (kJ/mol)
E°(NpO2+/Np4+) (V)
∆fG°298(NpO2+) (kJ/mol) 4+
logK1° logK2°
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∆fG°298(Np ) (kJ/mol)
Selected or Calculated Values -795.8 ± 5.4 43
1.161 ± 0.014 38
1.193 ± 0.043 39
0.96 40
0.596 ± 0.078 38
0.660 ± 0.067 39
0.07 40
-907.9 ± 5.8 38
-910.9 ± 6.8
-888.4 ± 5.4
38
-500.2 ± 9.4
-420.8 ± 5.4
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E°(NpO22+/NpO2+) (V)
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experiments.
-491.1 ± 9.5
5.2 ± 0.8 36 -9.9 ± 1.7 38 -1021.8 ± 13.5 38
-1031.0 ± 13.5
-951.6 ± 11.1
∆fG°298(Np2O5) (kJ/mol)
-2023.3 ± 12.4
38
-2029.4 ± 14.4
-1984.4 ± 11.7
Reference
Kaszuba et. al. 38
Kihara et. al. 39
Chatterjee et. al. 40
∆fG°298(NpO2) (kJ/mol)
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Supporting Information
101
100
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Np2O5
Weight (%)
99
98
97
96
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NpO2 95 0
200
400
600
800
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Temperature (°C)
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Fig. S1. TGA (thermogravimetric analysis) of Np2O5.
Fig. S2. PXRD pattern of Np2O5.
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Fig. S3. PXRD pattern of NpO2.
Np(V)
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Absorbance
0.4
0.3
0.1 400
600
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0.2
800
1000
1200
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Wavelength (nm)
Fig. S4. UV-Vis of NpO2 dissolved in sodium molybdate.
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Table S1. Reactions and thermodynamic cycles used to calculate enthalpies of drop solution of anhydrous Np2O5 into molten sodium molybdate (3Na2O•4MoO3) solvent at 973 K. ∆H (kJ/mol)
(1) Np2O5(cr, 298 K) → Np2O5(soln, 973 K)
∆H1 = ∆HDS(Np2O5) = ∆H2 - n∆H3 + ∆H4
(2) Np2O5·nH2O(cr, 298 K) → Np2O5(soln, 973 K) + nH2O(g, 973 K)
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Reaction
∆H2 = ∆HDS(Np2O5·nH2O) = 39.63 ± 5.34
(3) H2O(g, 298 K) → H2O(g, 973 K)
∆H3 = ∆Hheat content(H2O) = 25.0 42
(4) Np2O5(cr, 298 K) + nH2O(g, 298 K) → Np2O5·nH2O(cr, 298 K)
∆H4 = n*∆Hwater condensation(H2O) = n*(-
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44.0) 42
Table S2. Reactions and thermodynamic cycles used to calculate enthalpy of decomposition of Np2O5 → 2NpO2 + 1/2O2 at 298 K. Reaction
∆H (kJ/mol) ∆H1 = -2∆H2 + ∆H3 – 1/2∆H4
(2) NpO2(cr, 298 K) + 1/4O2(g, 973 K) → 1/2Np2O5(soln, 973 K)
∆H2 = ∆HDS(NpO2)
(3) Np2O5(cr, 298 K) → Np2O5(soln, 973 K)
∆H3 = ∆HDS(Np2O5)
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(4) O2(g, 298 K) → O2(g, 973 K)
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(1) Np2O5(cr, 298 K) → 2NpO2(cr, 298 K) + 1/2O2(g, 298 K)
21
∆H4 = ∆Hheat content(O2) = 21.8 42
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• •
We synthesize and characterize Np2O5 and NpO2. Enthalpy of decomposition of Np2O5 to NpO2 agrees with existing thermodynamic data. The calorimetric methodology is straightforward and reliable.
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