Thermodynamics of neptunium(V) fluoride and sulfate at elevatedtemperatures Page: 4 of 5
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2.2
NpO F(aq)
2.0-
1.2
40
Np F(aq)
30
20 *
10
0
-10
10 30 50 70 9010NpO SO,
30 50 70 90t /C
Fig. 4. Effect of temperature on the stability constants of
NpO2F(aq) and NpO2SO4 [8] and the enthalpy of
complexation (in kJ-mol-1). I = 1 M NaClO4.Data in Figure 4 indicate that the complexation of
Np(V) with fluoride and sulfate is enhanced at higher
temperatures. The stability constants of the complexes,
NpO2F(aq) and NpO2SO4 increase by about 3 times as
the temperature is elevated from 25 to 70 C. The
enthalpies of complexation for NpO2F(aq) and NpO2SO4
are both endothermic, but the trends with the change in
temperature are opposite. As the temperature is increased,
the enthalpy of complexation for NpO2F(aq) decreases
while that for NpO2SO4- increases. Assuming linear
correlations between AH and t, the heat capacity of
complexation for NpO2F(aq) and NpO2SO4- was
calculated to be -(267 20) J-K-t-mol-t and (244 51)
J-K--mol1, respectively.
It is desirable to extrapolate the equilibrium constants
at I = 1.0 M to the values in an infinite dilute solution (I -
0). We have used the SIT (Specific Ion Interaction)
approach described in the literature [9] to calculate the
equilibrium constants at I = 0. For the reactions shown in
Table 1, the equilibrium constants at I = 0 (log K0) are
related to log K at other ionic strengths by the following
equation:log K - AZ2 x D = log K - ASm
(1)
where AZ2 = {y(Z2products) - y(Z2reactants)}, D is the Debye-
Huckel term used in the SIT method and D = Almv2/(1 +
1.5AIml/2), Im is the ionic strength in molality, and cis the
ion interaction parameter used in the SIT method [9]. The12 interaction parameters at 25 C (in kg mol-1), taken from
Ref.[9], include the following: (H+, C104) = 0.14 0.02,
1(Na+, F-) = 0.02 0.02, (Na+, S042-) = -0.12 0.02,
8 (Na+, HSO4-) = -0.01 0.02 and (NpO2+, C104) = 0.25
0.05. s(Na+, NpO2SO4) is not available, but we
6 assumed that it is equal to c(Na+, NpO2CO3-) = -0.18
0.15 kg mol- [9]. For the calculation of log K at
temperatures other than 25 C, the Debye-Huckel term in
0.2 Eq. (1) was calculated with the values of A at different
50 temperatures [9] and the ion interaction parameters at
25 C were used because the values at other temperatures
were not known. Using the interaction parameters at 25 C
30 may introduce some errors into the log K at other
temperatures. However, the errors are probably small,
since the values of (8s/T), are usually < 0.005 kg mol-1
10 K1 for temperatures below 200 C [9]. Besides, the values
of (8x/T), for the reactants and products may balance out
0 each other so that the As for many reactions remains
approximately constant up to 100 C [10]. The calculated
values of log K at different temperatures are listed in
Table 1.
ACKNOWLEDGMENTSThis work was supported by the Director, OST&I
Program, Office of Civilian Radioactive Waste
Management, U. S. Department of Energy, under
Contract No. DE-AC02-05CH11231 at Lawrence
Berkeley National Laboratory.
REFERENCES
[1] OCRWM; "Yucca Mountain Science and
Engineering Report Rev.]I", DOE/RW-0539-1,
Office of Civilian Radioactive Waste
Management: North Las Vegas, NV. 2002.
[2] R. Guillaumont, T. Fanghanel, J. Fuger, I.
Grenthe, V. Neck, D. A. Palmer, M. H. Rand;
"Update on the chemical thermodynamics of
uranium, neptunium, plutonium, americium and
technetium ", (Mompean, F. J., Illemassene, M.,
Domenech-Orti, C., Ben Said, K., eds.),
Amsterdam: Elsevier B.V., 2003.
[3] L. Rao, T. G. Srinivasan, A. Yu. Garnov, P.
Zanonato, P. Di Bernardo, A. Bismondo;
Hydrolysis of Neptunium(V) at Variable
Temperatures (10 - 85 0C), Geochim.
Cosmochim. Acta, 68, 4821-4830 (2004).
[4] G. Gran; Analyst, 77, 661 (1952).
[5] P. Zanonato, P. Di Bernardo, A. Bismondo, G.
Liu, X. Chen, L. Rao; Hydrolysis of
Uranium(VI) at Variable Temperatures (10 - 85
0C), J. Am. Chem, Soc. 126, 5515-5522 (2004).
[6] R. Arnek; Arkiv Kemi, 32, 81 (1970).0
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Rao, Linfeng; Tian, Guoxin; Xia, Yuanxian & Friese, Judah I. Thermodynamics of neptunium(V) fluoride and sulfate at elevatedtemperatures, article, October 31, 2006; (https://digital.library.unt.edu/ark:/67531/metadc878003/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.