DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
Actinide(IV) in carbonate media
Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
CEA DCC/DESD/SESD Section de GéoChimie
BP6, 92265 Fontenay-aux-Roses, France.pierre.vitorge(at)cea.fr.
·
Critical review for the Thermodynamic Data Base of the Nuclear Energy
Agency (OECD)
Pierre Vitorge
·
Electrochemical preparation and spectra of Np(IV) in carbonate media
DEA (pre-PhD work)
Laetitia Delmau
Which soluble species of Np(IV) are formed in carbonate (natural) aqueous
solutions ?
How to measure their stoichiometry and their stability ?
Comparison with other Actinide(IV).
1 - Published works on Np(IV) solubility and solution chemistry:
Comparison of the carbonate speciation (chemical conditions).
2 - Published Np(IV) solubility: Testing their reproducibility and their
interpretation
3-
Published spectra: interpretation and experimental verification
4-
Published Np(V)/Np(IV)
experimental verification
redox
potential:
interpretation
and
Patrick Tran The help us to draw the figures. This program receives some financial suport from ANDRA
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
0
-1
-3
(NH4)2CO3
NaHCO3
Na2CO3
Na, lg pCO2 = -3,5
[ 71M 0S5] (NH4)2CO3
-4
[ 77UEN/ SAI] (NH4)2CO3, calculated
Np(CO3)4,5-
Np(OH)4,0
-5
-6
[ 79FED/ PER] Na2CO3 1M , + CO2 ?
Np(CO3)5,6-
lg(aOH-)
-2
[ 81WES/SUL] NaHCO3 or Na2CO3, pH=8.2 or 11.3
[ 85RAI/ RYA] NaHCO3 0,01M + NaOH
[ 89M OR/PRA] NaOH, 0.3M Na+, lgpCO2 = -3,5?
[ 90PRA/ M OR] NaHCO3 0,01M + NaOH
[ 93LI/ KAT] Na2CO3 pH measured
t ext
-7
-5
-4
-3
-2
-1
0
lg(aCO32-)
Comparison of the carbonate speciation
used in the published works on Np(IV)
solubility and solution chemistry in carbonate media.
Only the works of [85RAI/RYAN], [89MOR/PRA] and [90PRA/MOR] were
performed in similar chemical conditions, and can then be easily compared.
If only Np(OH)4, Np(CO3)44- and Np(CO3)56- soluble complexes are formed,
a predominance diagram is suggested (by the places, where the name of
these sêcies are written). If the limiting complex is Np(CO3)56-, it is stabilised
at high ionic strength, this reason, can explain, that it is obtained in
concentrated carbonate or bicarbonate media.
DATA.XLS
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
-1
-2
-3
(NH4)2CO3
Na2CO3, cl osed s y st em
Na, l gP CO2 = -3. 5
NaHCO3
[ 71M OS5] , (NH4)2CO3 s ol ut i on
[ 85RA I / RY A ] , NaHCO3 0, 01M +NaOH
-4
[ 89M OR/ P RA ]
l g pCO2 = -3, 5 ?
[ 90P RA / M OR] NaHCO3 0, 01M +NaOH
lg[Np(IV)]
[ 93/ LI / K A T ] Na2CO3 pH meas ur ed
-5
-6
-7
-8
-9
-3
-2
-1
0
1
2
3
4
lgaCO3 - lgaOH
Np(IV) solubility in carbonate media.
The cited authors (experimental points in the figure) assume, that the solid phase is
NpO2s or Np(OH)4s. When the predominating soluble complex is Npk(CO3)i(OH)j4k-2i-j-,
the Np(IV) solubility, S, is
lgS =k lgKs° + lgbijk + i k(lgaCO32- - (4 / k - j)/ i lgaOH-)
All the experimental points would then have fallen on the same line, if the major
soluble complex had the stoichiometry i = 4 / k - j, typically Np(CO3)2(OH)22(proposed in [90PRA/MOR]) or Np(CO3)44- (proposed in this review). The experimental
data of Prapoto's thesis [89MOR/PA] [90PRA/MOR] are too scattered to make any
conclusion.
Lines are drawn, assuming the formation of Np(OH)4, Np(CO3)44- and Np(CO3)56SOLIV.XLS
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
-4
-5
lg[Np(IV)]
(NH4)2CO3
-6
Na2CO3, closed system
Na, lgPCO2= -3.5
NaHCO3
[85RAI/RYA], NaHCO3 0,01M + NaOH
-7
[89MOR/PRA] lg pCO2= -3,5?
[90PRA/MOR] NaHCO3 0,01M + NaOH
-8
-9
-6
-5
-4
-3
-2
-1
lgaCO3
Np(IV) solubility in carbonate media.
The cited authors (experimental points in the figure) assume, that the solid
phase is NpO2s or Np(OH)4s. When the predominating soluble complex is
Npk(CO3)i(OH)j4k-2i-j-, the Np(IV) solubility, S, is
log10S = k log10Ks° + log10bijk + i k(log10aCO32- - (4 / k - j)/ i log10aOH-)
All the experimental points would then have fallen on the same line, if
the major soluble complex had the stoichiometry 4 = k j, typically
Np(OH)40, Np(OH)4CO32- (proposed in [71MOS5]) or Np(OH)4(CO3)24(proposed in [90PRA/MOR]). The experimental data of Prapoto's thesis
[89MOR/PA] [90PRA/MOR] are too scattered to make a conclusion.
The spectrum of the supernatant solution, where lg[(NH4)2CO3] = 0.18, is
also shown in [71MOS5]. It has also been observed later [77SAI/UEN],
[81WES/SUL] and [93LI/KAT] : it is the spectrum of the Np(IV) carbonate
limiting complex. It is consistent with the solubility calculation shown on the
figure. For this calculation we assume, that this limiting complex is
Np(CO3)56-. It would be dissociated into Np(CO3)44- and Np(OH)40
concentration is negligible (less than 10-8 M). The calculations also indicate,
that the solubility is not controlled by Np(OH)4(s) solid phase.
(NH4))6Np(CO3)5.12H2O and other solid phases, where certainly slowly
precipitating.
DATA.XLS
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
-1
-2
-3
lg[Np(IV)]
-4
-5
(NH4)2CO3
Na2CO3, closed syst em
Na, lgPCO2 = -3.5
NaHCO3
-6
[ 71M OS5] , (NH4)2CO3 solut ion
[ 85RAI/ RYA], NaHCO3 0,01M + NaOH
[ 89M OR/ PRA] lgpCO2= -3,5?
[ 90PRA/ M OR] NaHCO3 0,01M + NaOH
-7
[ 93/ LI/ KAT] Na2CO3 pH measured
-8
-9
-6
-5
-4
-3
-2
-1
0
lgaOH
SOLIV.XLS
Np(IV) solubility in CO32-/HCO3- media.
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
10,5
10
Np4+
NH4NpO2CO3s
NpO2+
pH
NpO2(OH)2s
cl osed syst em
9,5
l g pCO2 = -3.5
9
8,5
0
1
2
[(NH4)2CO3] (M)
pH measured by Moskvin
[71MOS5] during its solubility measurement of Np(IV, V and VI) in (NH4)2CO3
solutions and calculated in this review. We assume, that there was either no
exchange of carbonic gas with the air (solid line) or (dashed line) equilibrium
with the air only for carbonic gas (not for ammoniac gas).
7MOS.XLS
DCC/DESD/SESD Section de GéoChimie, Pierre Vitorge, Hélène Capdevila, Laetitia Delmau
25èmes Journées des Actinides, April 7-11, 1995, L'Aquila - Italy
Y+7
9
8
0
1
2
3
Na (mole/kg)
SIT regression on the redox potential of the Np(V)/Np(IV) couple
in Na2CO3 media.
The measured potential Em, is corrected for PCO2 assuming equilibration with
the air E = Em - 2 ´ 0.05916 log10PCO2. Y = E / 0.05916 - 11 D. The
correction of Em for the [NpV)] / [Np(IV] ratio cannot be checked in
[79FED/PER] (the square point) and has not yet been checked in [95DEL]
(the other points). The junction potential and the calibration of the reference
electrode cannot be checked in [79FED/PER]; they seem to be correct in
[95DEL]. SIT regression (mNa+ , Y) gives De = -0.168; Y° = 8.767, then
E'°(PCO2 = 10-3.5 atm) = 0.519 V/SHE, is a formal potential in standard
conditions except for PCO2. E'°(PCO2 = 1 atm) = E'°(PCO2 = 10-3.5 atm) - 7 ´
0.05916 = 0.105 V/SHE. Equilibration with the air was not checked in these
works. log10PCO2 could then be smaller than -3.5. This would lower E and
then E'° by several orders of magnitude. The formation constant of the
Np(CO3)56- estimated from this type of measurement would then be higher by
several orders of magnitudes. This is "unbelievable" [85RAY/RIAN].
DATA.XLS