,
..
1
Journal of So!ution Chemistq, Vol. 28, No. 5, 1999
Application of Pitzer’s Equations for Modeling the
Aqueous Thermodynamics of Actinide Species in
Natural Waters: A Review~
n
Andrew R. Felmy* and Dhanpat Rai
Received October 30, 1998; revised February 2, 1999
A review of the applicability of Pitzer’s equations to the aqueous thermodynamics
of actinide species in natural waters is presented. This review includes a brief
historical perspective on the application of Pitzer’s equations to actinides, information on the difficulties and complexities of studying and modeling the different
actinide oxidation states, and a discussionof the use of chemical analogs for
different actinide oxidation states. included are tables of Pitzer ion–interaction
parameters and associated standard state equilibrium constants for each actinide
oxidation state. These data allow the modeling of the aqueous thermodynamics
of different actinide oxidation states to high ionic strength.
KEY WORDS: Actinides; tkrnmdynamic dat& Pitzer ion-interaction pammete~,
trivatent actinidcs tetmvalent actinids, pentavalent acdnidrw hexavalent acrinides.
1. INTRODUCTION
Actinide speeies in aqueous solution can exist in a variety of oxidation
states (III, IV, V, and VI) and can strongly interact with several different
Iigands of importance in natural waters such as OH-, CO:-, SO%-, and F-.
This large combination of oxidation states and interacting Iigands makes it
necessary for aqueous thermodynamic models to include large numbers of
chemical species and treat a wide range of electrolyte types from simple 1:1
electrolytes to highly charged 4:2 and even 6:1 electrolytes. In addition,
Pacific Northwest National Laboratory, Battelle Boulevard, PO Box 999, Rtchland, Witshington 99352.
twe d~mte ~i5pawr tohe memo~ of Kenneth S. Pitzer in rWO’#titiOtS
of his manY
invaluable contributions to solution chemistry.
533
{XW5-9782YYW5(KK1533S i 6.(IM
Q IVYY Plenum Fubiisbhg Cmpmation
DISCLAIMER
This repoti was prepared as an account of work sponsored
byanagency of the United States Government. Neither the
United States Government nor any agency thereof, nor any
of their employees, make any warranty, express or implied,
or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information,
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or
otherwise does not necessarily constitute or imply its
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opinions of authors expressed herein do not necessarily
state or reflect those of the United States Government or
any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
produced from the best available original
document.
c.
.
534
Felmy and Rai
since many actinide compounds are insoluble, the chemical species of these
elements are usually present in solution at trace concentrations relative to
the major electrolyte components (e.g., Na+, Cl-, CO~-, . ..). This means
there are always minimums of at least three chemical species present in
solution. Within the Pitzer activity coefficient formalism, the activity coefficient for a trace species in these systems’
..
can involve several model parameters including mixing terms for ions of like
sign (i.e., (3and W) as well as mathematically redundant cation-anion terms
(i.e., ~(o)in the B,x term in Eq. 1) between the tracer cation or anion and the
oppositely charged bulk cation or anion. Such facts make it difficult to define
unambiguous or nonredundant electrolyte ion-interaction parameters for trace
actinide species when such parameters often must be fit from common-ion
ternary data. In addition to these complications, the necessity to treat highly
charged electrolyte species presents a particular challenge both in terms of
the magnitude of the higher order electrostatic terms in unsymmetrical mixtures and in terms of model parameterizations involving the ~(z) term originally proposed by Pitzer in the late 1970s.(3’4)
For these reasons, application of Pitzer’s equations to the study of the
a stringent
and
aqueous thermodynamics of actinide species has presented
challenging
test of the theory. Such applications are important given the need
to understand and predict the aqueous thermodynamics of actinide species
in nuclear waste disposal areas.
The remainder of this paper will begin with a brief historical perspective
of the development and application of Pitzer’s equations to actirtide aqueous
species followed by a detailed summary of the Pbsr ion–interaction parameters
for actinide species of importance in natural waters. Thk summary will be
presented for different oxidation states beginning with the trivalent actinides and
ending with the hexavalent actinides. Patarneter vahtes will be included for the
major ligands of importance in natural waters including OH-, CO;–, S@4-, F-,
and W4-, as well as for selected organic Iigands (i e., EDTA). Tables of modeling
pararnetets applicable to each oxidation state will also be presented.
*All of the terms in Eq. (1) are defined elsewhere.(12] Briefly, t represents the tracer cation, B
and C are second and third viriat coefficients specific for each cation–anion interaction, @
is a second virial coefficient specific for cation-cation or anion-anion interactions and is
numerically equal to 13ijplus ‘(jij. Oijis a constant model parameter for each cation-cation or
anion-anion interaction. %)ijis the higher order electrostatic term for unsymmetrical mixtures.
F is a modified Debye-Huckel activity coefficient term, z is the species charge, Z equals
Zlzilmi, and WMXis a constant third virial coeftlcient representing cation-cation-anion
or
anion-anion-cation interactions.
.
.
Aqueous Thermodynamics of Actinide Species
2. HISTORICAL PERSPECTIVE OF THE DEVELOPMENT
PITZER ION-INTERACTION PARAMETERS FOR
ACTINIDES
535
OF
The first work on modeling the aqueous solution thermodynamics of actinide species using Pitzer’s equations was done by Pitzer and Mayorga.[s) This
original work included model parameters
for hexavalent
uranyl
(UO~+) with Cl-, C1O;, and NO;, and tetravalent Th4+ with Cl- and NO;. In
addition, model parameters were developed for several trivalent rare-earth
cations with Cl-. One year later, Pitzer and Mayorga@ published the parameters
for the 2:2 electrolyte UO~+–SOj-. These initial studies largely completed the
parameterization of the few binary chemical systems where the actinides or
actinide analogs (such as trivalent rare-earths for trivalent actinides) are sufficiently soluble to allow the determination of unambiguous cation–anion interaction parameters. The vast majority of subsequent work has focused on
chemical systems where the actinides (An) species are present in trace concentration relative to the major electrolyte solution components.
The first of these trace activity coefficient studies was conducted in
1989 on the solubiiity of PU(OH)3(S) in complex brines!’) In this application,
ion–interaction parameters for Nd 3+–C1- were successfully used as analogs
for the unknown and difficult to experimentally measure (because of oxidation
ion–interaction parameters. These studies established
problems) I?u3+-C1the usefulness of trivalent analogs in estimating the Pitzer ion–interaction
parameters for diflicult to measure actinide species. The use of these parameters accurately described the two to three orders of magnitude increase in
solubility of PU(OH)3(S) in concentrated brine over that observed for diiute
solution (Fig. 1). These studies established that the use of the Pitzer equations
offered a simpler approach, over that of assuming the formation of multiple
PU3+–CI- complexes, and accurate formalism for modeling the volubility of
actinide species in complex brines. The success of this work resulted in the
early 1990s application of Pitzer’s equations to a wide range of chemical
systems involving actinide species. Much of this work was performed at the
Pacific Northwest National Laboratory (PNNL) for trivalent and tetravalent
actinides$7-24) the Lawrence Berkeley National Laboratory (LBL), Sandia
National Laboratory (SNL), Los Alamos National Laboratory (LANL) for
pentavalent species, principally Np(V)@-2*), and at the Institut fur Radiochemie in Karlsruhe Germany for trivalent and pentavalent actinides@-33).
These studies, combined with previous work by Pitzer, resulted in the development of a significant database of modeling parameters for the chemical system
An(III)–Cl–SOi–P04–C03–Mo04–Hz0
at 25°C, for the system Th(IV)-Cl–
F–S04–C03–H20
at 25”C, and for pentavalent species, principally Np(V).
In addiiion, development of Pitzer ion–interaction parameters for organic
,
536
Feimy and F&d
-3
I
I
\
\
-4
‘d-
\
Pu(OH)Jam)
-s
“1+ Pa
indilute NaCl
\
-6
h btha
.
+*
-7
-8
i
-------
-9
------
C@ec$ontimit
-\ ---- ‘+---------
r
I
-10
t
-11 1
o
2
1
1
4
6
!
.8
I
!
J
10
12
14
Pc”+
Fig. 1. The effect of PBB I brine ([N-a+] = 5.2 M, [Ca2+] = 0.034 M, [Cl’]
= 4.6 M,
[SO%-] = 0.W4 M, ionic strength - 5.5 M) on the volubility of PU(OH)l (am). From Felmy
et al., Ref. 7.
cttelate-actinide species has been undertaken recently at Florida State University and Sandia National Laboratory.c~J Other relevant studies involving
trivalent rare-earths have been previously conducted at the University of
Miami with applications to seawater systems.
These actinide studies in
conjunction with binary and ternary ion–interaction parameters for major
electrolyte ions (Appendix Tables A.I and A.II) are providing a useful and
reliable database for application to complex natura[ systems containing actinides. Specific aspects of the development of this database are discussed below
for each actinide oxidation state.
3. ION-INTERACTION
ACTINIDE SPECIES
PARAMETERS
FOR DIFFERENT
3.1. Trivalent Actinides
Of all of the actinide oxidation states, trivalent actinides, and rare-earths
have been the most extensively modeled using the Pitzer formalism. In fact,
with relatively few modeling parameters (Tables I and II), this combined
speciation-ion-interaction
approach has successful y modeled the available
volubility and other thermodynamic data for an extensive chemical system
.;...’
,, .,
.
.
Aqueous Tbermodynamies
Table f.
537
Binary and Ternary Ion–Interaction Parameters for An (111)Species
Binary parameters
,
Species
AS$+-C}AnClz+-ClAnClj-ClAnOH2’–CIAn(OH)~-ClAn3+-SOjAn3+-H2PO:
Na+-An(CO&
.
of Actinide Species
Nit+-An(OH)Jaq)
C1--An(OH)Jaq)
Na+-C1--An3+
Ca~+_C1--An~+
Ca~+-Cl--An@+
Caz+–Cl--AnCl~
Cl--An(COJ~Na+-Cl--An(COJ]-
($0)
0.5856
0.593
.516
–.035
-.616
3.0398
0.00
–0.256
pm
p(z)
5.60
3.15
I .75
1.6
–.45
0.00
0.00
5.0
0.0
0.0
0.0
0.0
0.0
–2500
–92.9
0.0
d’
Ref.
–0.0166
–0.006
.01
.05
.05
0.00
0.00
0.044
31
31
31
31
31
13
0
b
Ternary parameters
Ref.
–0.2
–0.2
0.1
0.2
–0.014
–0.196
0.168
0.0273.
31
31
31
31
31
31
c
c
I
to the parameter for N&l+–HzPO~ (Ref. 10).
to &
~nakgous
bAssumrxI to be analogous to the parameter for Na+–Nd(COJj - (Ref. 14).
cAssumed to be analogous to the parameter for Na+-Cl ‘– Nd(COJ~- (Ref. 15).
u ASSu~
Table II.
Log R Values (25”C) for the Formation of An(lII) Aqueous Species
@
Reaction
An’+ +
Ans’ +
Ar$+ +
An~+ +
.
.
HZO = An(OH)z+ + H+
2H20 = An(OH)~ -i- 2H+
3HZ0 = An(OH)$ + 3H+
CI- = An@+
Ar$+ + 2CI- = AnCl~
An3+ + CO;- = AnCO.~
An3+ -t- 2CO~- = An(COJ);
An3+ + 3CO~- = An(COJ$An3+ + 2MoO~- = An(MoO&
2Am’+ + M@02~OH)j- = AnzMo@Jaq)
Ar$+ ‘+ F- = AnFz+
An~’ + 2F- = AnF~
An3+ + 3F = AnFT(aq)
+ 4H+
~
–7.56
– 15.8
<–28.6
.24
-.74
7.6
12.3
15.2
11.2
3.85
3.4
5.8
<11.2
Ref.
31
31
8
31
31
.8
8
8
a
a
38
38
b
“Assumed to be analogous to the values for corresponding Nd(HI)/molybdate species. (Ref. 9).
hEstimated from the results for volubility of NdF3(c). (Ref. 12).
.
.
Felmy and I&ii
538
(i.e., An(III)-Cl-S04-POA-COJ-Mo04–HZ0
at 25”C) to high ionic strength.
The vast majority of this model development has focused on the rare-earths,
Am(III), or Cm(III). Very little work has been done with Pu(HI) because of
experimental difficulties in maintaining Pu as Pu(III). However, the limited
studies that have been done [i.e., PU(OH)3(S) in brines], along with the known
hydrolysis constants and soh.tbility products for Pu(IH) species, support the
use of modeling parameters developed from Am(III)/Cm(III)/trivalent
rareearths for Pu(HI). As an example, the sohibility data for the hydroxides of
Am(III), Pu(III), and Nd(III) are all similar (Fig. 2), resulting in similar
volubility products and hydrolysis constant<lon–interaction
parameters.
As previously described, the development of the Pitzer ion-interaction
parameters for trivalent actinides has been conducted at PNNL using rareearth analogs and at the Institut fiur Radiochemie using Cm(III) isotopes. The
development of the model parameters by both groups is consistent in that
similar sets of major electrol yte parameters (see Appendix) were used in the
model calculations. Table I presents the Pitzer ion–interaction parameters for
An(III) species and Table II the standard state. equilibrium constants. In some
cases (Ci-, F– and MoO%- parameters), the seiected values were the ordy
values in the literature that had been extensively compared to a wide range
of experimental data. In other cases (OH-, CO~- and SO~-), more than one
set of model parameters satisfied this criteria.
.
,
.
.
-1
-3
-s
. .
-7
.4
Nd(OH)Jc)m
m Nrt(oH)3(cp’)
+ Am(OH),(c)W
●
-9
o Ads
v Pu(OH)3(am)m
-11
o
2
4
6
8
10
12
14
pc”+
Fig. 2. The S.oiubltityof trivalent actinide aud Ianthanide hydroxides in dilute solutiow (Ref. 39).
—
,
.
Aqueous Thermodynamics of Actinide Species
.,
539
The hydrolysis constants and ion–interaction parameters for hydrolysis
species were taken from the recent compilation of Fanghanel and KimJ3’J
These data were selected since this is the only set of data with ion–interaction
parameters between the An(III) hydrolysis species and the major electrolytes
(at least Na+ and Cl-) and the parameter values have been tested to some
degree against independent experimental data [i.e., the volubility of
Am(OH)J(s) in NaCl]. The hydrolysis constants are also in reasonable agreement with other values in the literature.@5J
The parameters for carbonate and sulfate were more difilcult to select
since two different sets of model parameters are in the literature. The first
set was developed at PNNL by the authors of this paper and the second set
has been developed at the Institut fur Radiochemie. The data set developed
at the -Institut fiir Radiochemie contains significantly more modeling parameters than the PNNL data set, principally as a result of the emphasis by the
Institut fur Radiochemie group in explaining their time-resolved fluorescence
spectroscopy (TRLFS) measurements of Cm(III). The PNNL model parameters have also been compared to a significantly wider range of experimental
data and it is, therefor~, of interest to test the model parameters of the Institut
fiir Radiochemie group on some of these data sets. As part of this effort, we
have initiated this process.
In the case of carbonate, we have compared the Institut fiir Radiochemie
model parameters to the experimental data of Rao et af.(14’15)on the volubility
of NaNd(C03)2 “ 6H20 sodium carbonate, sodium bicarbonate, and in mixed
electrolytes with NaC1. On an overall basis, the parameter set proposed by
the Institut fur Radiochemie group works quite well on this extensive set of
independent data. However, a couple of facts prevented us from selecting
their parameter set. Fkst, although their model gives quite satisfactory agreement with the NaNd(COJ2 “ 6H20 volubility data in NaHC03 solutions, the
calculated speciation does not appear reasonable in that An(C03); and
AnCOJ are the dominant species in 0.1 m NaHC03, but uncompleted An3+
is the dominant species in 1.Om NaHC0,3. In addhion, the model parameters
proposed by the Institut fur Radioehemie group slightly overpredict (-0.5
log units) the observed NaNd(C03)z “ 6H20 solubilities in NaCl + Na2C03
solutions at high NaCl (i.e., 4m).
In the case of sulfate, we have compared the Institut fiir Radiochemie
model parameters to the experimental data of Rai et uf.(’3) on the volubility
of NdP04 in Na2S04. The model parameters from the Institut fur Radlochemie
group work quite well for the dilute (-0.001 m sulfate) solution, but underpredict the volubility data at higher sulfate (-O. 1 m) by approximately and order
of magnitude. Although we cannot be absolutely sure, this underprediction
appears to be the result of a relatively low value for the stability constant of
the An(S04); complex (see Rai er aL(’3) for a review). Although further tests
..
<
,
.
540
Felmy and Rai
of the different modeling approaches would be valuable, both data sets reproduce a wide range of experimental data quite satisfactorily and this should
give users of these models confidence in using either approach.
This database of modeling parameters makes application of Pitzer’s
approach to predicting the aqueous thermodynamics of trivalent actinides in
natural waters to high ionic strengths a reliable possibility. This is clearly
demonstrated by the data and calculations (Figs. 3–5), which show the applicability of the model to concentrated Na2COs and NaHC03 solutions (Fig.
3), the predictions in solutions containing high concentrations of polymerized
molybdate species (Fig. 4), and the applicability to concentrated Na2S04
solutions (Fig. 5).
3.2. Tetravalent
,,
.. ...
Actinides
The vast majority of Pitzer modeling parameters for tetravalent actinides
is available for Th(IV). Th(IV) is the easiest tetravalent actinide to study
experimentally because the tetravalent state is the onl y oxidation state of Th
that is stable in aqueous solution. For example, modeling parameters for the
Th–carbonate system were developed based on extensive experimental data
on the volubility of Th02(am) as a function of H+, CO:–, and HCOI concentrations (Fig. 6). The direct use of Th(IV) model parameters for U(IV), Np(IV),
and Pu(IV) species is expected to be problematic, because of systematic and
-20
..
.,
.
f
-3.0
1
EEsl
,,. .
.:’:,...
;“,
4.0
-E
m
-5.0
.>..
.*:
‘>.
>.,. .
,.
-.. /
,
,
-6.0
.
:!
“:
,,
-7.0
‘.
1.
-6.0 \
-12
1
I
!
t
I
-1.0
4.8
-0.6
4.4
-0.2
log
I
0.0
02
0.4
(Cti c /mol din-’)
Fig. 3. Observed and predtcted soiubility of NaNd(COJz(s) in concentrated NazC03 and
NaHC03 solutions (Ref. 14).
.
.
.
.
Aqueous ‘1’bermodynamics of Actinide Speeies
541
●
I
●
✍✍✍✍✍✍✍✍✍✍✍✍✍✍✍✍✍●✍✎
4
8.
---
.“
#-- .
“f::wJ
Cywlafti
-6
-9 t
1
--
Mledialkra
_____
--------
-
\
(noMOOJ
--------
*-----
------
-----
---
-
!
I
I
I
I
1
2
3
4
5
6
7
pc”+
Fig. 4. Volubility of NdPOi(c) as functions of PCH+ and molybdate concentrations at fixed
[phosphate],(Halof 10-’5 M (Ref. 9).
I
large differences in charge to ionic radii, and other factors, for the different
tetravalent actinides. Nevertheless, for the few cases where data do exist for
Th(IV) and other tetravalent actinide complexes other than hydroxide [i.e.,
U(IV)], the Pitzer ion–interaction parameters for Th(IV) species have proved
to be reliable indicators of the expected magnitude of the parameters for the
other tetravalent actinides [e. g., compare the Th(IV) data in Tables III and
IV with U(IV) data in Table V]. Values for the Pitzer ion–interaction parameters for Th(IV) species with Cl-, SO~-, and CO:- are available at 25°C as
well as the necessary equi Iibnum constants for these and other species (Tables
111and IV). This model development effort represents a significant test of .
the Pitzer ion–interaction model given the potential number of chemical
species present and the high charge type for many of these ion interactions.
In particular, published values for the Th(COJ$- complex represent the only
accurate values for this highly charged (6: 1) electrolyte type present in the
literature. Also note that when highly charged species are present in solutions,
the magnitude of the mixing terms (i.e., 6 and W) for such species with the
bulk cations or anions can be very large and important in predicting the
aqueous thermodynamics. For example, Felm y et aL(’9) have estimated the
value of 0 for Th(C03)~-–C10~ at 5.5. Such large mixing terms for highly
charged species means that models that attempt to describe the thermodynamic
.
Felmy and Rai
.
—
-4
Predicted using $0)= 3.0396 and pm = -2500 forflrn=-3
-2
tog304 (moi ● L-l)
*
-1
S’x?mlas
Fig. 5. Normalized equilibrium aqueous Am concentrations from solvent extraction data of
McDowell and Coleman, (Ref. 42), obtained at constant acid activity and extractant-phase
composition. The solid line represents the predicted concentrations (Ref. 13).
relations for such species based solely on binary (cation-anion) interactions
(such as, the S.I.T, modelt29)) will fail for highly charged electrolyte species
when applied to common-ion ternary or other multicomponent solutions. This
fact was discussed in detail by Felmy et d.(19) in describing the differences
in Th02(am) volubility in solutions with and without added NaCIOd. Further
evidence for this is demonstrated in Fig. 7, which shows experimental data
for Th02(am) in mixed NaCl and Na2C03 solutions are drastically different
based solely on binary data [i.e., only
than model predictions
Th(COJ$--Na+
interactions determined from NazC03 solutions] Qr data in
NaCIOg [i-e., setting $ for Th(COJ$--CIO;
to 5.5]. Both approximations
are in error by orders of magnitude, although the actual calculated parameters
from fits to these experimental data are in the same range of expected
magnitude [i.e., 0 for Th(COJ~-–Ci - and W for Th(COJ$--Cl--Na+
at 1.8
and 0.3, respectively]. Clearly, highly charged electrolytes interact strongly
with their ionic environment and this ionic environment for NaCl solutions
differs from NaCIOA or other solutions. These facts necessitate the inclusion
of mixing terms in the aqueous thermodynamic models. Such situations
present difficult challenges for any electrolyte model, challenges that can be
. ..
. . —... -_.—_-_:
-------- _____
.
.
.
Aqueous Thermodynamic
of Actinide Species
543
.
‘O
0.4
0.8
1.2
N~C03
t
1.6
2.0
(M)
—
-2 :
CeIculated
(l-his study)
● Experimental
-3 ~
-4 +
●
..
+
;0
.. .
..; .
-7 =
.~
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
Log HCOa(M)
-2 -
—
-3 :
.
CeIculeted
(TldSstudy)
Experimental
..
#
-4 –
-5 ~
.
I I , I , I I I t I
-8-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
Log
NaOH
0
(M)
.
..
Felmy and Rai
w
Table 111. Relevant Pitzer Ion-Interaction
p(o)
Species
H+-Th(SOJ~Na+-Th(SO.J~K+-Th(SOq)~-
0.84
0.12
Th4+-HSOZ
0.90
1.092
1.44
Th4+-so;Na+–Th(COJj-
1.56
1.31
Th4+-cl-
Parameters for Th(lV) Species
Binary parameters
$(2)
p(l)
C+
Ref.
0.00
0.00
16
16
16
0.00
0.00
0.00
0.00
0.00
13.7
0.00
–160
0.00
–0.112
46
0.00
0.00
30.0
0.00
0.00
0.00
0.00
0.00
0.00
16
16
19
Ternary parameters
H+_Th4+
Na#-~g+
Na+–Thg+_CiMu2+-Th4+
&+-~4+_CI-
Th(so4)2-clTh(SO,)z-HSO:
Table IV.
.
16
16
20
20
20
20
16
16
Log P (25”C) for Reactions Involving Solution Species of Th(IV).
Reaction
Tif+ +
Th4+ +
Ti#+ +
Th4+ +
Th4+ +
Th4+ +
Th4+ +
,
Ref.
0.60
0.08
0.42
0.21
0.60
0.21
0.29
0.68
H+-Th4+_(3-
.
2s0?- = Th(so&(aq)
4H20 = Tag
+ 4H+
3s0;- = Th(so4):3F- = ThF.:
4F- = ThF4(aq)
SF- = ThFI
6F- = ThF~-
Log r
11.59
=—19.7
12.42
18.89
22.33
24.76
25.56
Ref.
16
47
16
18
18
18
18
..
4
handled by the Pitzer approach, but cannot be handled by simpler approaches
(e.g., S.I.T) that include only cation–anion interactions.
In addition to the data available for Th(IV) (Tables III and IV), significant
Fig. 6. Experimental data of Rai et al. (Ref. 43) and predictions line based on the model of
Felmy er al. (Ref 19). Aqueous Th concentrations in equilibrium with ThOz(am) in NaZC03
solutions containing O.IM NaOH (top), NaHC03 solutions (middle), and NaOH solutions
containing 1.0 M NazC03 (bottom).
.
.
h
545
Aqueous Thermodynamics of Actinide Species
Table V. Pitzer Ion–lntemction Parameters and Log ~ (25”C) for U(lV) Species
L@g ~
Reaction
p(o)
tJ4+-cl-
1.644
UOH3+-CIusoi+–clNa+–U(COJ$K+–U(COJ!-
1.0
1.64
1.5
1.5
Binary parameters
@)
p(l)
0.00
0.00
0.00
0.00
0.00
15.5
7.856
0.00
31.3
31.3
u(so’J>-cl-
48
21
21
22
22
–0.50
9.0
11.7
31.29
4 I.33
U4++HZO=
UOH3+ +H+
L.P+ +So%-=uso;+
U4+ -t- 2SO~- = U(S04)z(aq)
U4+ + 5CO:- = u(coJgLP+ +2CO;-+
20 H-= U(OH)2(COJ3-
Species
Ref.
C+
Ref.
0.0995
0.00
–0.2635
0.00
0.00
20
20
21
21
22
Ternary parameters
Ref.
0.29
21
model parametrization activities also have been conducted for U(IV) with
Cl-, SO1-, and CO$- (Table V). No model parameter has been published
explicitly for Np(IV) or Pu(IV) species, although such investigations are
currently being conducted by the authors at PNNL. Values of the Pitzer
ion–interaction parameters for U(IV) and Th(IV) speeies appear to correlate
well and it is likely that will hold true for the Np(IV) and Pu(IV) species as
well. It should be pointed out that often there is a redundancy betweim the
calculated standard state equilibrium constant and the Pitzer ion–interaction
parameters, especially when data at high electrolyte concentration are used
in the model parameterization. Therefore, in developing the Pitzer ioninteraction parameters for U(IV) species, the Th(lV) values were often used
as a guide or were fixed at the Th(IV) values. This co+uldhave influenced
the numerical closeness of the parameters. Nevertheless, the large. magnitude
of the values of these parameters, particularly for the highly charged electrolyte types, clearly demonstrates the necessity for accurately determining these
model parameters.
3.3. Pentavalent
Actinides
Aqueous species of Np(V) and Pu(V) are the principal pentavalent
actinides of concern. Of these, Np(V) is stable over a broader range of pH
.
- .——..
——
--.---—
.,
:.... ..
546
Felmy and Rai
Calculated 4.67m NrtCl(no mixhg)
-1-1
-2
!
.* ● .t
●
..09
..- & -------- =
*.
-m
-*-
●
●
4
0 ““k
●
?
0
●
_-
Calculated 2.33m NaCl (no mixing)
.
✏
9
E -4g
c
+
.*
/
g-5A
New Model (4.67m NaCt)
-6
-7-
Calculated 2.33m NaCl (C104-terms)
-------m . . . . . .- .--.=
.
1
Calculated 4.67m NaCl (C104-terms)
-a
o
0.5
1
1:5
NazC03 (M)
2
is
Fig. 7. ThOz(am) volubility data in mixed NajC03 and NaCl solutions. Experimental data
(open symbols represent 2.33 m NaCl, closed symbols represent 4.67 m NaCl).
redox conditions in aqueous solution than is Pu(V). Hence, the only
Pitzer ion-interaction parameters available for pentavalent actinides are for
Np(V) species. To the best of our knowledge, there are no known values
available for Pu(V) species, although recent unpublished work “at PNNL
indicates that the values for NpO~–Cl - serve as useful analogs for the
corresponding PuO~ parameters. Unfortunately, ion–interaction parameters
and solution-phase equilibrium constants for NpOJ species, which would be
used to model the aqueous thermodynamics of Np(V) to high ionic strength,
are available only for Cl-, CO;-, &d selected organic chelates (Table VI).
The development of the model paritmeters ‘summarized in Table VI is
the result of efforts primarily at SNL, LBL, LANL; and Institut fiir Radiochemie. These studies began with the work of Novak and Roberts}25J Neck
et af. ,(29) and Fanghanel et al.(m) all in 1995. These onginal studies showed
quite similar trends in terms of the parameters that were required to describe
the experimental data and in terms of the absolute magnitude of the modeling
parameters. Later, Runde et a/.c2b)presented a somewhat modified model of
the same chemical system (i.e. Pu(V) species in Na-C03–HC03–Cl–CI01–
H–OH–HZO solutions at 25°C) as well as comparisons of their model predic-
and
.
>’.
-..
:.
.,
.
-
-
.
Aqueous Thermodynamics of Actinide Speeies
Table VI.
..
547
Pitzer Ion-Interaction Parametem and Log ~ (25”C) for Np(V) Species
Reaction
NpO~
NpOJ
NpO;
NpOJ
NpO:
N@:
NpO;
N@;
+ H20 = Np020H + H+
-1-2H20 = NpOz(OH); + 2H+
+ CO;- = Np02C0-;
+ 2COj- = Np(COJ]+ 2CO:- = Np(COJ;+ EDTA4- = NpOzEDTA~+ HEDTA3- = NpOzHEDTA2i- HJEDTA2- = Np02H2EDTA-
Species
B(o)
NpO~-ClNpOzEDTA’--Na+
NpOzHEDTA2--Na+
NpOzH2EDTA–-Na+
NpOzCO;–Na+
NpOz(COJ~--Na+
NpOz(COJj--Na+
0.1415
0.6830
0.4733
–0.8258
0.10
0.48
1.8
NpOJOH)j-ClNp02CO~-ClNpOz(COJ~--ClNpOz(CO&--CLCO~--Np02(CO&Np02(OH)-Cl-
Log k-’
Ref.
–11.31
–23.54
5.03
6.47
5.37
8.54
4.98
3.42
30
30
30
30
30
34
34”
34
Binary parameters
p(z)
$(1)
0.281
0.5911
– 1.504
0.2575
0.34
4.4
22.7
0.00
0.00
O.CQ
0.00
0.00
0.00
0.00
c+
Ref.
0.00
0.00
O.CQ
0.256
0.00
0.00
0.00
30
34
34
34
30
30
30
Ternary parameters
Ref.
-0.24
–0.21
-0.26
–0.26
-1.9
–0.19
30
30
30
30
27
30
.,..
-...
..
..
. ..
,.
‘,
tions with those of Novak and RobertsJ”) Later Novak et a/.t27J and, very
recently, Al Mahamid et uL@) have accepted many of the earlier values
determined by the Institut fiir Radiochemie group and have applied these
values to similar chemical systems containing K+ with reasonable success.
These parameter values are listed in Table VI. Ion-interaction parameters for
Np(v) species with EDTA determined at Flordia State University(~) are also
included in Table VI.
3.4. Hexavalent Actinides
In the case of hexavalent actinide species, U(VI) and Pu(VI), limited
ion-interaction parameters are available- only for U(W) (Table VII). No
parameters are available exclusively for PuO$+, although, based on structural
... .
.
.
Felmy and Rai
548
Table VII.
Pitzer Ion-Interaction Parameters and Log ~ (25°C) for U(W)
Reaction
UO;+ + EDTA4- = U02EDTA2UO;+ + HEDTA3- = UOzHEDTAEDTA4- + H+ = HEDTA3-
Species
uo~+-clUo;+-soi+
Na+-UOzEDTA2Na+-UOzHEDTA-
p(m)
0.4274
0.322
–0.1516
0.382
Log K-
Ref.
13.16
8.34
10.57
34
34
34
Binary parameters
@l)
9(2)
I .644
1.827
1.74
0.257
0.00
0.00
0.00
0.00
C+
-0.0184
-0.0176
0.095
0.172
,
Ref.
44
44
34
34
considerations, the parameters for UO~+ are expected to be good analogs for
the PuO~+ values.
One of the principal reasons for the limited availability of ion–interaction
parameters for U(W) species relevant to natural waters is the presence of
several mononuclear and polynuclear hydrolysis species in aqueous solution
in the neutral or near-neutral pH region!37) The possible presence of these
hydrolysis species has made it difficult to develop Pitzer ion–interaction
models for hexavalent actinides outside of the stability region of the uranyl
ion, because it is difficult to attribute observed changes in thermodynamic
data (EMF, volubility, solvent extraction) to individual ion-interactions or
complexation reactions. This situation has resulted in recent attempts to
estimate the Pitzer parameters based on correlations with charge type, differences in charges for reactions, and other variables!~s) In addition, the use of
molecular modeling approaches are currently being applied at PNNL to better
estimate standard state equilibrium constants. These values are then used to
reinterpret experimental data to evaluate Pitzer ion–interaction parameters,
as has been demonstrated for alkaline earth cations.@)
Hence, in the c;se .of hexavalent actinides, the presence of multiple
species in solution at neutral to near-neutral pH conditions has made it
difflcr.dt to develop accurate ion–interaction models that are valid to high
ionic strength. This is unfortunate because this pH region is also the region
most relevant to natural waters. Clearly more research is still needed in
this area.
—
.
4. CONCLUSIONS
Application of the Pitzer ion–interaction approach to the study of actinide
aqueous species has proved to be a challenging test because of the large
.
—.—.
,
.
549
Aqueous Thermodynamics of Actinide Species
number of chemical species in solution and the wide range of electrolyte
types involved. Fortunately, a significant database of model parameters has
been developed, especially for the trivalent and tetravalent actinides, and can
be used to model the aqueous thermodynamics of actinide species to high
electrolyte concentration. However, much work needs to be done on the
pentavalent [especially Pu(V)] and hexavalent actinides in near-neutral pH
conditions as well as the trivalent and tetravalent actinides under extreme
conditions, such as high base concentration. In addition, reliable data are not
available for actinides at higher than ambient temperature. However, bwause
of the progress that has been made on developing accurate modeling parameters at 25°C, it should now be possible to extend these models to higher
temperatures if appropriate experimental data become available.
ACKNOWLEDGMENT
This manuscript is dedicated to the memory of Professor Kenneth S.
Pitzer whose personal help and encouragement contributed immeasurably to
many of the studies cited in this work.
This research was conducted at Pacific Northwest National Laboratory,
operated by Battelle for the U.S. Department of Energy. This research was
suppoied by the following projects: the Japan Nuclear Cycle Development
Institute, the U.S. Department of Energy Environmental Management Sciences Program (project #26753), and Sandia National Laborato~’s Actinide
Volubility-Source “Term Project (AT-8746).
. .
APPENDIX
>.
These appendix tables include Pitzer ion–interaction parameters used
in the development and parameterization of the ion-interaction parameters
for actinide species. ,
—-
Table A.L
Speeies
Na+-ClNa+-SO~
Na+-HSO~
Na+-OHNa+-HCO~
Na+-CO~K+–ClK+–SO~
Binary Ion-interaction
@o)
0.0765
0.01958
0.0454
0.0864
0.0277
0.0399
0.04835
0.04995
Parameters for Major Electrolyte Ions
p)
0.2644
1.113
0.398
0.2.S3
0.0411
1.389
().2122
0.7793
p(z)
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0
0.00127
0.00497
0.00
0.0044
0.00
0.0044
–0.00084
0.00
Ref.
2
2
2
2
2
2
2
2
550
Felmy and Rai
Table A.I.
Species
K+-HSO;
K+-OHK+-HCO~
K+-COj@+-clCa2+–SO~
Caz+-HSO~
Ca2+-OHCa*+–HCO:
M~-Cl Mga+.S@
M&-HSO~
Mg2+-HCO~
M@ H+-CIH+-CIH+-S~H+-HSO~
Na+-H$O~
Na+–HPO~Na+–POj-
. ..... ..-
Continued.
p(o)
p(l)
–0.0003
0.1298
0.0296
0.1488
0.3159
0.20
0.2145
–0.1747
0.4
0.3S235
0.2210
0.4746
0.329
–0.10
0.1775
().()298
0.1735
0.320
–0.013
1.43
1.614
3.1973
2.53
–0.2303
2.977
1.6815
3.343
1.729
0.6072
1.658
0.2945
0.00
0.5556
0.0396
1.4655
3.8513
0.2065
–0.0533
–0.0583
0.17813
B(2)
0.00
0.00
0.00
0.00
0.00
–54.24
O.CK)
–5.72
O.oa
0.00
–37.23
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
@
0.00
0.0041
–0.008
–0.0015
–0.00034
0.00
0.00
0.00
0.00
0.00519
0.025
0.00
0.00
0.00
0.0008
0.0438
0.00
0.00795
0.02938
0.05154
Ref.
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
5
5
5
.
.,:,
,.
Table A. II.
.+
TernaryIon–Interaction Parameters for Major Electrolyte Ions”
Species
Ternary parameter
Na+-K+
f.Ja+-K+-cjNa+-K+-SO~Na+-K+–HCO~
Na+–K+–@Na+-Caz+
Na+-Ca~+-ClNa’-Ca2+-SO&
Na+_MgZ+
Na+-Mg2’–ClNa’-Mgz+-S@4N&+-H+
Na+-H+–Cl Na+-H+–HSO;
K+_Ca2+
K+–Ca~+_C[K+_Mg~+_ClK+-Mg2+-SO~K+-H+
–0.012
–0.0018
–0.010
–0.003
0.003
0.07
–0.007
–0.055
0.07
–0.012
–0.0$5
0.036
–0.004
–0.0129
0.032
–0.025
–0.022
–0.048
0.005
.
..
—.
..
—.——.——.—.
.
..
:’.,.-.:,.,,-.:,
-.—
.
,.,
551
Aqueous Tbermodymamics of Actinide Species
Table A.If.
Species
K+-H+-ClK+-H+–SO~K+- H+-HSOZ
Ca~+-M$+
;.
..
..-.
,.
. .
-.-:
.
.+
Caz+–Mgz+-Cl&~+-Mg~+_S@Ca2+_H+
CaZ+_H+-ClMg2+-MgOH--ClMg2+–H+
Mgz+–H+_ClMg2+_H+_Hso:
cl-–so~C1--SO~--Na+
cl--soj--ca~+
C1--SO~--Mgz+
CI--HSOI
Cl--OHC1-–OH--Na’
CI--HCO;
CI-–HCO;–Na+
C1-–CO~--Na+
Cl-–CO:--K+
SO~--HSO;-Na+
S@--HSO;-K+
SO~-–HSO;-Mg2+
s@--OHS~--OH--Na+
SO~-_OH--K+
SO~--HCOI
SO~-–HCO;-Na+
SC&-HCO-~_M#+
Soi--co;SO~-–CO~--Na+
S@--C&-K+
Continued.
Ternary parameter
–0.01 i
0.197
–0.0265
0.007
–0.012
0.024
0.092
–0.015
0.028
0.10
–0.01 1
–0.0178
().o~
0.0014
–0.018
–0.004
–0.006
–0.050
–0.006
0.03
–0.015
0.0085
0.004
–0.0094
–0.0677
–0.0425
–0.013
–0.009
–0.050
0.01
–0.005
–0.161
0.02
–0.005
–0.009
: -.:.
----
_..
—.
a All data from Ref. 2.
,
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,.
------
- . ...-.
.
.