Comprehensive thermodynamic model applicable to highly acidic to basic conditions for isosaccharinate reactions with Ca(II) and Np(IV)

Article abstract of DOI:10.1023/B:JOSL.0000002988.99769.cb

Isosaccharinate (ISA^-), a degradation product of cellulose codisposed in low-level nuclear wastes, is expected to be one of the dominant complexing ligands for radionuclides, especially tetravalent actinides. This paper presents a comprehensive

Full text of DOI:10.1023/B:JOSL.0000002988.99769.cb

C
Journal of Solution Chemistry, Vol. 32, No. 8, August 2003 ( 2003)
Comprehensive Thermodynamic Model Applicable
to Highly Acidic to Basic Conditions for
Isosaccharinate Reactions with Ca(II) and Np(IV)
Dhanpat Rai,1, Nancy J. Hess,1 Yuanxian Xia,1 Linfeng Rao,1
Herman M. Cho,1 Robert C. Moore,3 and Luc R. Van Loon4
Received April 24, 2003; revised August 6, 2003
Isosaccharinate (ISA ), a degradation product of cellulose codisposed in low-level nu-
clear wastes, is expected to be one of the dominant complexing ligands for radionuclides,
especially tetravalent actinides. This paper presents a comprehensive thermodynamic
model for isosaccharinate reactions with Ca(II) and Np(IV). The model is valid for a
wide range of pH values ( 2–14), ISA concentrations (ranging up to 0.1 m), and
ionic strengths (ranging up to 6.54 m), and is based on (1) NMR investigations of
HISA(aq) ( -D-isosaccharinic acid) and ISL(aq) [dehydration product of HISA(aq)],
and the solubility of Ca(ISA)2(c) as a function of pH and concentrations of Ca and ISA ;
(2) NpO2(am) solubility in a wide range of pH values ( 2–14) and total ISA concen-
trations of 0.0016 and 0.008 m and at fixed pH values of approximately 5 and 12 with
total ISA concentrations ranging from 0.0001 to 0.1 m; and (3) solvent extraction of
Np-ISA solutions, containing fixed NaClO4 concentrations ranging from 0.103 to 6.54
m and at fixed pCH+ values ranging from 1.5 to 1.9, with dibenzoylmethane. Pitzer’s
ion-interaction approach was used to interpret the data. The different aqueous species re-
quiredtoexplainthesedataincludedHISA(aq), ISL(aq), Ca(ISA)+, Np(OH)3(ISA)(aq),
Np(OH)3(ISA)2 , Np(OH)4(ISA) , and Np(OH)4(ISA)2 . The values of equilibrium
constants for reactions involving these species and det2ermined from these data pro-
vided close agreements between the observed and predicted concentrations in all of the
systems investigated in this study and those reported previously.
KEY WORDS: Isosaccharinate; isosaccharinic acid; thermodynamics; ion-interaction
parameters, solubility; NpO2(am); solvent extraction; equilibrium constants; -D-
isosaccharinic acid; -D-isosaccharino-1,4-lactone.
1Pacific Northwest National Laboratory, Richland, Washington 99352.
2Berkeley National Laboratory, Berkeley, California.
3Sandia National Laboratories, Albuquerque, New Maxico.
4Paul Scherrer Institute, Switzerland.
665
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0095-9782/03/0800-0665/0 2003 Plenum Publishing Corporation

666
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
1. INTRODUCTION
Low- and intermediate-level nuclear wastes contain significant quantities of
organic materials such as cotton, wood, and paper that have high cellulose content.
The degradation products of these organic materials have been reported(1–12) to
contain soluble, low-molecular weight acids that have a high affinity for tetravalent
actinides, making the tetravalent actinides, which otherwise form fairly insoluble
hydrous oxides, more soluble. In particular, -D-isosaccharinic acid [HISA(aq)]
(Fig. 1), a degradation product of cellulose, has been reported as the dominant com-
plexing agent for tetravalent actinides under highly alkaline conditions.(1,2,4,5,10)
However, no comprehensive thermodynamic model applicable to a wide range
in pH and isosaccharinate concentrations is currently available for any of the
tetravalent actinides. Such models and data are needed to reliably predict tetrava-
lent actinide behavior in low- and intermediate-level nuclear waste repositories. It
is known that isosaccharinate under acidic conditions transforms into HISA(aq)
and ISL(aq) [ -D-isosaccharino-1,4-lactone; a dehydration product of HISA(aq),
Fig. 1).(13) However, reliable values are not known for the acid dissociation con-
stant of HISA(aq) or the hydrolysis constant of ISL(aq). Therefore, fundamental
data on the following aspects are necessary to determine the complexation effects
of ISA in a wide range of conditions: (1) data for protonation/deprotonation con-
stant of HISA(aq) and equilibrium constant of hydrolysis reaction of ISL(aq), and
(2) complexation constants of ISA with tetravalent actinides applicable to a wide
range in pH and ISA concentrations.
In an earlier study, Rai et al.(14) determined the protonation/deprotonation
constants of HISA(aq) through the measurement of Ca(ISA)2(c) solubility as a
Fig. 1. Structures of organic ligands discussed in the paper: HISA(aq) ( -D-isosaccharinic
acid), ISL(aq) [ -D-isosaccharino-1,4-lactone, a product of dehydration of HISA(aq)], and
ISA ( -D-isosaccharinate).

Isosaccharinate Reactions with Ca(II) and Np(IV)
667
function of pH. From this study they also determined the solubility product of
Ca(ISA)2(c) and Ca-ISA complexes. These values were then used(15) to interpret
NpO2(am) solubility data in the presence of Ca isosaccharinate solutions in a
relatively acidic and neutral region (pH < 6.5). Since these publications,(15,14)
our, yet unpublished, data based on NMR studies [Cho et al.(16)] indicate that the
deprotonation constant value for HISA(aq), estimated in Rai et al.,(14) is about 1.2
orders of magnitude too low, and ISL(aq) species, hitherto ignored, play a dominant
role in the acidic conditions. Although Cho et al.(16) were able to determine the
*
)
equilibrium constant for hydrolysis reaction of ISL(aq) [ISL(aq)+ H O ISA +
2
H+], the value they obtained contained a large uncertainty due to the fundamental
difficulty of measuring ISA at low pH. In addition, recent data have also become
available(17,18) indicating that CaISA+ is an important complexed species at pH
values >5 in the Ca-ISA system. This discussion indicates that reliable values are
not available for equilibrium constants applicable to the Ca-ISA system, including
those for reactions involving ISA in equilibrium with HISA(aq) or ISL(aq).
The main objectives of this study are to develop comprehensive models for the
solubility of NpO2(am) in the Np(IV)-H+–Na+–Ca2+–HISA(aq)–ISA –ISL(aq)–
OH –H2O and for the solubility of Ca(ISA)2(c) in the Ca2+–H+– HISA(aq)–
ISA –ISL(aq)–OH –H2O systems. To accomplish this goal, we (1) conducted
experiments on the solubility of NpO2(am) as a function of time and in a range of
pH values from 5 to 14 and at a fixed isosaccharinate concentration of 0.0016 M,
and in a range of isosaccharinate concentrations varying from 0.0001 to 0.1 M at a
fixed pH value of 12, (2) conducted experiments on the solubility of Ca(ISA)2(c) at
afixedpHvalueofabout8.3asafunctionofisosaccharinateconcentrationsranging
upto0.2 M andinterpretedthesedatawithdatapreviouslyreportedforCa(ISA)2(c)
solubility(14) to obtain/validate equilibrium constant values for reactions involving
HISA(aq) and ISL(aq) and for Ca-ISA complexes and for use in interpreting Np
data, and (3) used Pitzer’s ion-interaction model to develop thermodynamic models
based on the NpO2(am) and Ca(ISA)2(c) solubilities reported in this study and
those reported previously for this system.(15,14) Thus, the model we developed for
the Np(IV)-ISA system is applicable to a wide range in pH values (2–14), HISA(aq)
concentrations (0.0001–0.1 M), and NaClO4 concentrations (0.103–6.54 m) in the
acidic region.
2. MATERIALS AND METHODS
2.1. Preparation of NpO2(am)
The Np(IV) stock solution in HCl was prepared electrolytically. Over time,
the Np stock that initially contained a significant amount of Np(III) converted to
Np(IV). UV-Vis-NIR spectroscopic analyses of the stock solution prior to each
use confirmed the absence of detectable amounts of all oxidation states other than
Np(IV). The acidic Np(IV) stock solution was spiked into 20-mL of degassed

668
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
water containing 0.01 M Na2S2O4 in 50-mL glass centrifuge tubes. NpO2(am)
was precipitated by titrating with 5 M NaOH to pH values of approximately 10–
10.5. The precipitate was washed twice with 20- to 30-mL aliquots of degassed
water containing 0.01 M Na2S2O4. The centrifuge tubes were kept tightly sealed
with Nalgene caps to minimize exposure to the chamber atmosphere.
2.2. Preparation of NaISA Stock Solution
A stock solution of NaISA for use with the Ca(ISA)2(c) and NpO2(am)
solubility studies was prepared using the following procedure: (1) dissolving
Ca(ISA)2(c) in 2 M NaOH by shaking the suspension for a few days, where
dissolved Ca precipitates as Ca(OH)2 leaving NaISA in the aqueous phase, (2) cen-
trifuging and separating the supernatant containing NaISA and a portion of un-
reacted NaOH, (3) analyzing the concentration of ISA by carbon analysis, and
(4) making appropriate pH adjustments, using HCl, and dilutions of NaISA needed
for experiments.
2.3. Procedures
Experiments were conducted in an atmosphere-controlled chamber under an
Ar atmosphere for Np experiments or an N2 atmosphere for Ca(ISA)2(c) experi-
ments. Deionized distilled water, degassed by boiling and cooling in the Ar or N2
atmosphere, was used in all cases. Solubility was approached from the undersat-
uration direction.
2.3.1. Solubility of Ca(ISA)2(c) in NaISA
The Ca(ISA)2(c) used in this study was prepared by the same method de-
scribed by Rai et al.(14) A 0.2 g of Ca(ISA)2(c) was added to 50-mL centrifuge
tubes containing NaISA solutions ranging in concentrations from 0.01 to 0.2 M.
The pH values of the suspensions were adjusted to 8.3 and the suspensions shaken
continuously until analyzed. After 48 days of equilibration, the pH values were
measured using a combination Orion–Ross glass electrode calibrated against stan-
dard pH buffers. The suspensions were centrifuged and aliquots of the supernatant
were withdrawn for C analyses to quantify isosaccharinate concentrations and to
filter them through CentriPrep membrane filters with 30,000 molecular weight cut-
offs. The filtered solutions were analyzed for Ca using inductively coupled plasma
spectroscopy. Data are listed in Appendix Table A.I.
2.3.2. Solubility of NpO2(am) in NaISA
Because Np(IV) can be oxidized to Np(V) by O2 and such oxidation pro-
ceeds more easily in solutions of higher pH, special precautions need to be taken
to maintain the Np(IV) oxidation state. Several measures developed in previous

Isosaccharinate Reactions with Ca(II) and Np(IV)
669
studies(15,19) were taken to minimize oxidation during the preparation of stock
solutions and throughout the solubility experiments. These measures included
(1) preparing the concentrated Np(IV) stock solutions ( 250 g Np/L) in approx-
imately 3.5 M HCl [where Np(IV) is relatively stable] and storing the solutions
under argon, (2) conducting the experiments in the presence of a reducing agent
(0.01 M Na2S2O4), and (3) sealing the sample tubes during equilibration and seal-
ing the ion-specific electrodes into the sample containers during measurements to
avoid O2 diffusion.
Two sets of experiments were conducted to determine NpO2(am) solubility
in the presence of isosaccharinate. For each sample in the first experimental set,
2.5-mL aliquots of 0.1 M Na2S2O4were added to 50-mL centrifuge tubes, followed
by 5.2 mL of 0.4 M NaISA and 5 mg of washed NpO2(am) precipitate. Degassed
water or standard NaOH solution was added to bring the sample volume to 25
mL. The pH of the solution was adjusted using NaOH or HCl to several pH
values ranging from 5 to 12. Additional samples were prepared in standard NaOH
solutions ranging in concentration from 0.01 to 1.0 M. Samples in the second set
were prepared similarly, except the pH was fixed (about 12) and the concentration
of NaISA was varied from 0.0002 to 0.1 M. For both sets, the centrifuge tubes were
tightly capped and placed on a shaker. Periodically, pH values of the suspensions
weremeasuredusinganOrion–Rosscombinationglasselectrodecalibratedagainst
pH buffers; the electrode was sealed in the tubes to avoid contact of suspensions
with the chamber atmosphere.
The suspensions were centrifuged at 2000/g for at least 10 min after different
equilibration periods (4–6, 12–14, 20, or 28 days). The supernatant was then
filtered through Amicon Centriplus filters (Amicon Corp.) with a 30,000 nominal
molecular-weight cutoff and approximately 0.004- m pore size. An aliquot of the
filtrate was spiked into a liquid scintillation cocktail for total Np analysis by liquid
scintillation counting.
Oxidationstateanalyseswereperformedonallsamplesusingasolventextrac-
tion technique involving thenoyltrifluoroacetone.(20) Data are listed in Tables A.II
and A.III. The percentage Np(IV) listed in these tables is based on the portion of
total soluble Np extracted by thenoyltrifluoroacetone. Using this technique, the
unextracted amount [100-% Np(IV)] is assumed to represent % Np(V).
2.3.3. Solvent Extraction Experiments
Our previous publication(21) describes the preparation of stock solutions of
Np(IV) ( 60,000 dpm in 10 L) and NaClO4 (0.103, 0.53, 1.07, 2.22, 3.48,
4.91, and 6.54 m), the purification procedure for dibenzoylmethane (DBM), the
quantification of H+ concentrations containing NaClO4 solutions, and the general
procedure for determining distribution coefficients (D, ratio of Np in the organic
to the aqueous phases). Only a brief description is provided here. The negative

670
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
logarithm of H+ ion concentrations (pCH ) in different solutions were determined
+
+
from the observed pH electrode readings (pHobs) using the equation [pCH
=
pHobs + A]. The values of A for different NaClO4 solutions (0.03 for 0.103 m,
0.14 for 0.53 m, 0.27 for 1.07 m, 0.48 for 2.22 m, 0.69 for 3.48 m, 0.90 for 4.91 m,
and 1.12 for 6.54 m) were determined using a Gran type of titration procedure.(22)
Isosaccharinate stock solution was prepared from Ca(ISA)2(c). All extractions
were performed in borosilicate glass scintillation vials, which were silanized using
Caceci and Choppin’s(23) procedure. A reducing agent, NH2OH.HCl, was used to
maintain Np in the tetravalent oxidation state.
+
Five-milliliter aliquots of NaClO4 adjusted to the appropriate pCH and con-
taining 0.01 M NH2OH HCl, 10 L of Np(IV) stock solution, and a known con-
centration of ISA (but varying between 0.0002 to 0.0028 m) were shaken with 5.0
mL of DBM in toluene at room temperature (25 10C) for 1 hr. The concentra-
+
tion of DBM was adjusted depending upon the NaClO4 molality and pCH of the
solution so that the distribution ratio (D) fell between 0.1 and 10. Kinetic studies
confirmed that the extraction equilibrium was attained in about 10 min. The vials
were centrifuged and 1.0-mL aliquots were taken from both phases to measure
the alpha activity. A Wallac 1415 liquid scintillation counter was used to count
the activity in HiSafe cocktail (Wallac). The remaining aqueous phase was used
for pHobs measurements. Extraction data along with normalized Np(IV) aqueous
concentrations used in thermodynamic analyses are reported in Table A.IV.
2.4. Thermodynamic Model
The ion-interaction model of Pitzer(24,25) was used to interpret the solu-
bility data. This aqueous thermodynamic model emphasizes a detailed descrip-
tion of specific ion interactions in solution. The effects of specific ion interac-
tions on the excess solution free energy are contained in the expressions for the
activity coefficients. The activity coefficients can be expressed in a virial-type
expansion as
X
X X
DH
i
m
mj mk
ln = ln
+
ij(I)
+
+
(1)
Cijk
i
j
j
j
k
where m is the molality, DH is a modified Debye–Hu¨ckel activity coefficient that
i
is a universal function of ionic strength, and ij(I) and Cijk are specific for each ion
interaction and are functions of ionic strength. The third virial coefficient, C, is
understood to be independent of ionic strength. A detailed description of the exact
form of Eq. (1) is contained in the nonlinear least-squares programs [NONLIN
and INSIGHT](26) for estimating the activity coefficients and calculating chemi-
cal equilibria involving multiple solid and aqueous species. We have used these
programs extensively in the past to interpret solubility data for many systems in-
cluding those involving isosaccharinic acid and Np(IV).(14,15,21,27) For calculation

Isosaccharinate Reactions with Ca(II) and Np(IV)
671
and plotting purposes of NpO2(am) and Ca(ISA)2(c) data, it was assumed that the
measured molarities equal molalities because all of the solutions are either of very
low ionic strength or the calculated molalities (e.g., for 1.0 M NaOH) are not signif-
icantly different than the molarity values. Since the solvent extraction experiments
were conducted in relatively dilute to concentrated NaClO4 solutions, appropriate
corrections were made to convert concentrations from molarity to molality units.
3. RESULTS AND DISCUSSION
3.1. Acid Dissociation and Dehydrolysis Constants of HISA(aq)
and Complexation of Ca2+ with ISA
Rai et al.(14) obtained extensive data on Ca(ISA)2(c) solubility as a function
of pH and Ca concentrations. Because preparation techniques for NaISA were
unavailable at that time, they were not able to conduct studies as a function of
ISA concentrations. ThesedataarenowpresentedinFig. 2andshowaprogressive
decrease in Ca concentrations at a fixed pH value of 8.3 with an increase in NaISA
concentrations, similar to data presented recently by Van Loon et al.(17) These
data on Ca(ISA)2(c) as a function of NaISA concentrations are critical in defining
the Ca-ISA complexes in pH values of >5. Rai et al.(14) interpreted Ca(ISA)2(c)
solubility as a function of pH using HISA(aq), ISA , and Ca(ISA)2(aq) species.
However, this model does not explain the behavior observed in Fig. 2, predicts
constant Ca concentration as opposed to observed decrease in Ca concentrations
with the increase in NaISA concentrations, and is not consistent with NMR data,(16)
which definitively shows that both HISA(aq) and ISL(aq) are the important species
under acidic conditions, and not HISA(aq) alone. The NMR data(16) provided a
reliable value for the proton dissociation constant of HISA(aq). However, the
NMR data were not sufficient to provide reliable values for equilibrium constants
of reactions involving ISL(aq). To develop a model that is consistent with all of
these data, we reinterpreted the entire data set using the Pitzer model. For these
interpretations, we relied on the HISA(aq) dissociation constant value [log K0
*
of –3.27 0.02 for HISA(aq)
ISA + H+] based on our NMR data(16) and
)
fitted the values for other species. The new model (Tables I and II) is consistent
with all of the data, includes HISA(aq), ISL(aq), ISA , and CaISA+ species along
with a revised value for the solubility product of Ca(ISA)2(c) and does not require
Ca(ISA)2(aq). A comparison of experimental data reported in Rai et al.(14) and
obtained in this study to those predicted by the new model show close agreement
(Figs. 2 and 3). Our values of 1.44 0.07 and 6.26 0.07, respectively, for the
logoftheequilibriumconstantfor[Ca2+ +ISA
CaISA ]andfor[Ca(ISA)2(c)
+
*
)
2+
*
)
Ca + 2ISA ] are very similar to the values (1.7 and 6.54, respectively)
recently reported for these reactions by Van Loon et al.(17) and the values (1.7
and 6.36 0.10, respectively) reported by Vercammen et al..(18) A low CaISA+

672
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Fig. 2. Solubility of Ca(ISA)2(c) as a function of NaISA concentration and at a fixed pH
of approximately 8.3 and 48-day equilibration period. Solid line depicts predicted total
concentration; other lines represent concentrations of different species, as identified, using
modeling parameters reported in Tables I and II.
formation constant value shows that the isosaccharinate complexes with Ca2+ are
rather weak and that this species does not contribute significantly to the total Ca
concentrations (Figs. 2 and 3). The log K0 value of 0.37 0.07 for [HISA(aq)
*
)
ISL(aq) + H2O] determined from these data is dependent on the value selected
for the proton dissociation constant of HISA(aq). However, these data show that
ISL(aq), as compared with HISA(aq), are the dominant species, which is consistent
with the NMR studies(16) and lends support to the accuracy of these values for use
in developing reliable models for Np(IV) systems involving ISL(aq), HISA(aq),
ISA , and CaISA+ species.
3.2. Solubility of NpO2(am) in the Presence of ISA
Aqueous Np concentrations in most of the samples were relatively low
(Tables A.II and A.III), so direct spectrophotometric analyses for the oxidation

Isosaccharinate Reactions with Ca(II) and Np(IV)
Table I. Pitzer Ion-Interaction Parameters Used in the Modela
673
(0)
(1)
(2)
Species
C
Reference
Binary Interactions
+ ----
H
Cl
0.1775
0.0765
0.0864
1.644
2.26
0.2945
0.2664
0.253
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.0008
0.00127
0.0044
0.0995
0.0995
0.00
(32)
(32)
(32)
(28)a
+ ----
Na
Na
Cl
+ ----
OH
Np4+ Cl
15.5
----
Np4+ ClO4
----
17.53
7.856
9.0
This studyb
(28)a
This studyb
NpOH3+ Cl
----
1.0
1.4
NpOH3+ ClO4
0.00
----
Ternary Inte+ractions
+ ----
H
H
Na+
0.036
0.004
0.05
0.006
0.045
(32)
(32)
(32)
+ ----
Na - Cl
----
----
Cl
Cl
OH
OH - Na
+
(32)
Np(OH)3ISA(aq) ClO4
This studyb,c
----
aAssuming corresponding U(IV) parameters reported in the quoted reference apply to the Np(IV)
system; our recent solvent extraction studies(21) with Np(IV) in low to concentrated chloride
solutions are consistent with these assumptions.
bThese parameters are primarily important in solvent extraction data; see text for details.
c
The use of this value for the reported parameter or for Np(OH)3ISA(aq) Na+ provides identical
----
fits. However, due to the limitations of the experimental data, it is, at present, not possible to
assign this value to one of these parameters only.
state and speciation were not possible. Therefore, oxidation state distribution of
aqueous Np was determined by solvent extraction, and the results in nearly all
cases (different pH, ISA concentrations, and times) indicated that 75–90% of
total aqueous Np was present in the tetravalent state.
TheaqueousNp(IV)concentrationsinequilibriumwithNpO2(am)aresimilar
at different equilibration periods in each set containing (1) 0.0016 M NaISA in a
range of pH values (Fig. 4), or (2) variable NaISA concentrations at a fixed pH
value of about 12 (Fig. 5), indicating that steady-state conditions are reached in
less than 5–6 days. Our previous studies with NpO2(am)(27) and other tetravalent
actinide hydrous oxides(28,29) show that equilibrium between tetravalent actinide
hydrous oxides and a wide variety of electrolyte solutions with a broad range in
H+ concentrations is reached within a few days. The attainment of steady-state
conditions in this study and the use of sampling times that exceeded the minimum
required to reach equilibrium(27) provided assurance that equilibrium is attained
in our experiments.
There is a dramatic decrease in aqueous Np(IV) concentrations with increas-
ing pH at pH <7 (Fig. 4). The NpO2(am) solubility in the acidic region is several
orders of magnitude higher than that predicted in the absence of isosaccharinate(27)
(Fig. 4). The measured Np concentrations are in close agreement with the data

674
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Table II. Important Chemical Reactions Included in the Model
Reaction
log Ko
Reference
2+
*
)
Ca(ISA) (c)
Ca + 2ISA
6.26 0.07
6.54
This study
(17)
2
7.62
(14)
6.36 0.10
1.44 0.07
1.70
(18)
This study
(17)
+
Ca2+ + ISA
CaISA
*
)
1.7
(18)
This studya
(14)
Ca2+ + 2ISA
Ca(ISA) (aq)
NN
*
)
2
5.4
*
HISA(aq)
)
*
HISA(aq)
)
ISL(aq) + H O
0.37 0.07
3.27 0.02
4.46
0.5
2.57 0.37
2.76
4.68 0.37
4.76 0.37
2.90 0.37
This study
(16)b
(14)
(33)c
This study
(15)
This study
This study
This study
2
ISA + H+
Np4+ + H O
NpOH + H+
3+
*
)
2
NpO2(am) + ISA + H+ + H O
Np(OH) ISA(aq)
*
)
2
3
NpO2(am) + 2ISA + H+ + H O
Np(OH) (ISA)
*
)
2
3
2
*
)
NpO2(am) + ISA + 2H O
Np(OH) ISA
Np(OH) (ISA)
4
2
4
2
2
*
)
NpO2(am) + 2ISA + 2H O
2
aNN = not needed.
bBased on NMR data(16) and consistent with the data presented in this study.
cThe equilibrium constant value is assumed to be identical to that for corresponding U reaction
and as reported in Rai et al.(34) This reaction does not contribute significantly to Np(IV)
concentrations at any of the experimental conditions in the NpO2(am) solubility study, but
becomes important in solvent extraction study.
obtained by Rai et al.(15) under identical conditions in the presence of isosacchari-
nate and where the experimental pH values overlap in both studies. The significant
result obtained in this study is that the measured Np solubility remains low and
>
constant under alkaline conditions exhibiting no pH dependence at pH values 7.
A continued decrease in Np concentration with increasing pH is not observed un-
der alkaline conditions, as predicted by the solubility model developed from low
pH data only.(15) The low Np solubility and lack of a pH dependence under alkaline
conditions may truly be due to Np(IV)-ISA complexes or to detection limit prob-
lems and/or the presence of colloids. However, as is demonstrated by the results of
the variable ISA experiments (Fig. 5), the actual detection limit is approximately
an order of magnitude lower than the measured concentrations; only ionic Np(IV)
is quantified in solvent extraction tests used in this study. Therefore, the lack of
pH dependence under highly alkaline conditions is not an experimental artifact,
but is due to the presence of different Np-ISA species under alkaline conditions.
The Np(IV) concentration in equilibrium with NpO2(am) increases approx-
imately one order of magnitude with an order of magnitude increase in NaISA

Isosaccharinate Reactions with Ca(II) and Np(IV)
675
Fig. 3. Solubility of Ca(ISA)2(c) as a function of pH and 75-day equilibration period
[experimental data of Rai et al.(14)]. Solid lines depict predicted total concentrations; other
lines represent concentrations of different species, as identified, using modeling parameters
reported in Tables I and II.
concentration at a fixed pH of 12 (Fig. 5). These Np(IV) concentrations are also
similar to those observed in equilibrium with 0.0016 M ISA as a function of pH
(an average of 18 data points at pH values >8.5 from Fig. 4, plotted as a solid
circle in Fig. 5), thereby providing additional assurance about the consistency

676
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Fig. 4. Aqueous Np(IV) concentrations in equilibrium with NpO2(am) as a function of pH and
at a fixed total ISA concentration of 0.0016 m. Relatively low pH data from Rai et al.(15) The
pH values > 12 reported in this figure are calculated from INSIGHT based on input molarities
of NaOH and other ions present in the system. Solid line represents total concentration,
while other lines represent concentrations of different species, as identified, predicted using
modeling parameters reported in Tables I and II. The line “Np(OH)3+)” represents predicted
Np concentrations in the absence of ISA.
and the reliability of the data as a function of NaISA concentrations. Increases in
Np(IV) concentrations with increases in ISA concentration were also observed
for NpO2(am) suspensions equilibrated at pH 5 (Fig. 6). However, the measured
Np concentrations are nearly three orders of magnitude higher at pH 5 (Fig. 6)
than at pH 12 (Fig. 5). Although increases in NpO2(am) solubility are observed
in both acidic and alkaline conditions, the dominant species must certainly be dif-
ferent in these two regimes. Reasons for these differences are (1) the solubility of
NpO2(am) decreases with the increase in pH at values between 2 and 7 (Figs. 4),
which would require species, such as Np(OH)x(ISA)4y–(x+y), where x would have
a value of less than 4 and y a value 1, and (2) the solubility of NpO2(am) is
nearly constant at pH values between about 8 and 14 (Fig. 4), which would require
y
species such as Np(OH)4(ISA)y with y 1. Consistent with this observation,
Rai et al.(15) proposed Np(OH)3ISA0 and Np(OH)2(ISA)2 species to explain the
observed NpO2(am) solubility behavior in the acidic region. Thermodynamic in-
terpretations of the data currently available are discussed in the next section.

Isosaccharinate Reactions with Ca(II) and Np(IV)
677
Fig. 5. Aqueous Np(IV) concentrations in equilibrium with NpO2(am) as a function of
time and NaISA concentrations at a fixed pH of 12. The solid circle with horizontal
lines represents the mean and standard deviation of Np(IV) concentration (log mNp(IV)
=
7.292 0.09) of 16 samples with pH values >8.5, which are plotted in Fig. 4. Solid
line represents total concentrations, while other lines represent concentrations of different
species, as identified, predicted using modeling parameters reported in Tables I and II.
3.3. Thermodynamic Analyses of NpO2(am) Solubility Data
Because of the revised value of the dissociation constant of HISA(aq) and
the new values involving CaISA+ and ISL(aq) determined in this study (Table II),
and because of the need to develop a comprehensive model for the Np(IV)-ISA
system, the experimental data and those presented by Rai et al.(15) were interpreted
together to develop a reliable model that would be consistent with all of the avail-
able data. The aqueous Np(OH)3ISA(aq), Np(OH)3(ISA)2 , Np(OH)4(ISA) , and
Np(OH)4(ISA)22 make up the combination of complex species that provided close

678
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Fig. 6. Aqueous Np(IV) concentrations in equilibrium with NpO2(am) as a function of NaISA
concentrations(15) at a fixed pH value of 5. Solid line represents total concentrations, while
otherlinesrepresentconcentrationsofdifferentspecies, asidentified, predictedusingmodeling
parameters reported in Tables I and II.
agreement with the observed NpO2(am) solubilities in a range of pH values varying
from 2 to 14 and at fixed NaISA concentrations of 0.0016 or 0.008 M (Figs. 4 and 7)
and NaISA concentrations ranging from 0.0001 to 0.1 M and at fixed pH values
of 5 or 12 (Figs. 5 and 6). The relative distribution of these species as a function
of pH and NaISA concentrations are shown in Figs. 4 through 7. These data show
that at a given isosaccharinate concentration, primarily two species are required
to explain the observed NpO2(am ) solubility from pH values of 2 all the way to
approximately 14. The species Np(OH)3ISA(aq) or Np(OH)3(ISA)2 are required
for pH values of 7 and Np(OH)4(ISA) or Np(OH)4(ISA)22 are required for
<
0
>
pH values of 7. The species Np(OH)3ISA or Np(OH)4(ISA) are important at
2
<
total ISA concentrations of 0.01 M, while Np(OH)3(ISA)2 or Np(OH)4(ISA)2
are important at total ISA concentrations of 0.01 M. The NpO2(am) solubility
>

Isosaccharinate Reactions with Ca(II) and Np(IV)
679
Fig. 7. Aqueous Np(IV) concentrations(15) in equilibrium with NpO2(am) as a function
of time and pH at a fixed total ISA concentration of 0.008 m. Solid line represents total
concentrations, while other lines represent concentrations of different species, as identified,
predicted using modeling parameters reported in Tables I and II.
reactions involving these species are reported in Eqs. (2–5)
+
0
*
)
NpO (am) + ISA + H + H O
Np(OH) ISA
(2)
(3)
(4)
(5)
2
2
3
+
*
)
NpO (am) + 2ISA + H + H O
Np(OH) (ISA)
2
2
3
2
*
)
NpO (am) + ISA + 2H O
NpO (am) + 2ISA + 2H O
Np(OH) (ISA)
2
2
4
2
2
*
)
Np(OH) (ISA)
2
2
4
The equilibrium constants for these reactions calculated from these data were
2.57 0.37, 4.68 0.37, 4. 76 0.37, and –2.90 0.37, respectively. When
these equilibrium reactions for the dissolution of NpO2(am) (Eqs. 2–5) are com-
bined with the NpO2(am) solubility product (log Ksp
=
54.9),(27) they provide

680
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
the logarithm of the equilibrium constant of 43.47 for [Np4+ + 3OH + ISA
–
–
*
)
Np(OH)3ISA0], 45.58 for [Np4+ + 3OH + 2ISA
Np(OH) (ISA) ], 50.14
–
–
*
)
3
2
–
–
2
4+
–
for [Np4+ + 4OH + ISA
Np(OH) ISA ], and 52.00 for [Np + 4OH +
*
)
4
–
*
)
2ISA
Np(OH) (ISA) ]. No values of isosaccharinate complexes of Np(IV)
4
2
or of other tetravalent actinides are available other than those Rai et al.(15) deter-
mined for Np-ISA complexes, based on the limited data under acidic conditions
or for those Moreton(5) and Greenfield et al.(3,4) proposed for Pu(IV)-ISA com-
plexes based on a few samples at pH 12, and Vercammen et al.(12) proposed for
Th(IV) based on studies with ion-exchange resin at pH 13.3. Rai et al.(15) proposed
two species [Np(OH)3ISA0 and Np(OH)2(ISA)2 ] and the value for the equilibrium
constant they determined for Np(OH)3ISA0 is very similar to the value determined
in this study (Table II). This is not surprising since both values are based on the
same experimental data. However, reanalyses of Rai et al.(15) data using a revised
thermodynamic model (Tables I and II) showed that the species Np(OH)2(ISA)2
were not required to fit the data.
As we observed in this study for Np(IV), Moreton(5) and Greenfield et al.(3,4)
also observed an increase in Pu(IV) concentration with the increase in ISA
concentration at pH 12. Rather than interpreting the data under highly alkaline
conditions, as we did using mixed hydroxy-ISA complexes [Np(OH)4ISA and
Np(OH)4(ISA)22 ], they interpreted their data assuming complexes with isosaccha-
rinate-containing deprotonated hydroxyl groups. The reactions they proposed are
–
–
–
*
)
PuO2(am) + ISA
PuO2(am) + 2ISA
Pu(ISAH ) + 2H2O
(6)
(7)
4
2–
*
)
Pu(ISAH
)
+ 2H2O
2
2
where ISAH m represents the deprotonation of m hydroxyl groups of isosacchar-
inate.5 Like Reactions 4 and 5, Reactions 6 and 7 have no explicit pH dependence,
so Pu(IV) solubility is expected to remain constant as a function of pH. Unfor-
tunately, Pu(IV) solubility measurements were not made as a function of pH at
constant isosaccharinate concentration and the predicted behavior cannot be ver-
ified. However, because the behavior of Pu(IV) and Np(IV) are expected to be
similar and Np(IV) solubility does not exhibit a pH dependence under alkaline
conditions (Fig. 4), the deprotonation reactions (Eqs. 6 and 7) of isosaccharinate
hydroxyl groups can also explain our solubility data under alkaline conditions.
Spectroscopic techniques, such as nuclear magnetic resonance (NMR), are re-
quired to distinguish between the mixed hydroxy-ISA complexes proposed in
Eqs. 4 and 5 and the hydroxyl deprotonation reactions proposed in Eqs. 6 and 7.
Our13C-NMR studies with Na and Ca solutions of isosaccharinate(16) show that
5Species similar to those in Reaction 6 for Th [Th(ISAH 4) ] were also proposed by Vercammen
et al.(12) to interpret their adsorption data for an ion-exchange resin at pH 13.3.

Isosaccharinate Reactions with Ca(II) and Np(IV)
681
deprotonation of the carboxylic acid is complete by about pH 5 and that no further
deprotonation reactions of isosaccharinate are apparent in the pH range from 5
to 12. These preliminary data support the formation of mixed hydroxy-ISA com-
plexes and are consistent with our modeling results, which indicated that species
y
of the type Np(OH)4(ISA)y (with y 1) control the Np solubility from pH 8
to 14 (Figs. 4 and 5). The complexation of isosaccharinate with tetravalent ac-
tinides may shift the deprotonation of the hydroxyl group to lower pH values such
that studies specifically with complexed tetravalent actinides would be required to
determine whether the complexation involves deprotonation of hydroxyl groups
or hydrolyzed tetravalent actinides. However, regardless of how the complexes
are formed, the overall net reactions are well understood and provide reliable
data for predicting Np behavior in a wide range of pH values and isosaccharinate
concentrations.
3.4. Extending the Thermodynamic Model to High Acidity
and Ionic Strength
The thermodynamic model described above is based on the NpO2(am) sol-
ubility data obtained in relatively dilute electrolytes. For this purpose, we con-
+
ducted extensive solvent extraction studies in the pCH range of 1.5 to 1.9, total
isosaccharinate concentrations varying from 0.0002 to 0.0028 m, and NaClO4
concentrations fixed at different molalities ranging from 0.103 to 6.54 m
(Table A.IV). Based on the model described above, the dominant Np-ISA com-
plex species in the solvent extraction data is expected to be Np(OH)3ISA0, and
Np4+ and NpOH3+ are expected to become more important than in the solubility
data. Therefore, Pitzer ion-interaction parameters cannot be ignored for Np4+ and
NpOH3+ in these concentrated electrolytes. Pitzer parameters are not available for
these species with ClO4 , the major electrolyte anion in this system. However, it
has been suggested(21) that binary parameters(28) for U4+–Cl and UOH3+–Cl
can be used for the ClO4 interactions with corresponding Np species. Defining
these parameters from these solvent extraction studies alone in relatively constant
+
pCH values and as a function of a limited range in ISA concentrations is diffi-
cult. Therefore, several different modeling runs were conducted to quantify these
parameters. The parameters that provided the best fits to all of the data included
(0)
(1)
(0)
(1)
= 2.26 and
= 17.53 for Np4+–ClO4 , and
= 1.4 and
= 9.0 for
NpOH3+–ClO4 , whereas the only other reported experimental values(28) for the
(0)
(1)
ion-interaction parameters of tetravalent actinides are
= 1.644 and
= 15.5
for U4+–Cl , and
= 1.0 and
= 7.856 for UOH3+–Cl , indicating that the
(0)
(1)
perchlorate system requires larger values for these parameters than the chloride
system. We believe that the values of these parameters with perchlorate are reason-
able because (1) the reported
(0)
(1)
and
values(25) for trivalent rare earths with

682
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
ClO4 are approximately 30 and 17%, respectively, higher than the values for the
corresponding parameters with Cl ; the values that we determined in this study
with ClO4 are consistent with this expectation; (2) Felmy et al.(30) reported orders
of magnitude lower ThO2(am) solubility in NaClO4 solutions as compared with
(0)
(1)
NaCl, indicating that higher values for
and
will be required for NaClO4
solutions as compared with NaCl, again consistent with our findings; and (3) in try-
ing to explain the observed ThO2(am) solubility as a function of Na2CO3 in NaCl
----
solutions, Felmy et al.(31) determined = 1.8 for Th(CO3)56
Cl as compared
to = 5.5 for Th(CO3)65 ClO4 , indicating higher values for Pitzer parameters
in perchlorate than in chloride systems. In addition to parameters for Np4+–ClO4
and NpOH3+–ClO4 , the solvent extraction data also required a value of 0.045 for
Np(OH)3ISA0–ClO4 or for Np(OH)3ISA0–Na+ (because the experiments were
done in concentrated NaClO4 solutions only, it is currently not possible to defini-
tively assign the value to one of these parameters) to closely fit the data, especially
in relatively concentrated solutions. This value for Np(OH)3ISA0–ClO4 is in the
range of reported values for interactions of other neutral species with monovalent
ions.(25)
The normalized aqueous Np(IV) concentrations from solvent extraction ex-
periments in NaClO4 concentrations ranging from 0.103 to 6.54 m show close
agreement with the concentrations predicted using the thermodynamic data
(Tables I and II) described above (Fig. 8), indicating that the model developed
in this study is consistent with the solvent extraction data. Xia et al.,(21) in inter-
preting their solvent extraction data for Np(IV)-sulfate in NaClO4 concentrations
ranging from 0.103 to 6.54 m, assumed that the (0) and (1) for Np4+–ClO4 and
for NpOH3+–ClO4 are identical to those for the corresponding U(IV) system. It
is of interest to know whether the new parameters for Np4+–ClO4 and NpOH3+
–
ClO4 impact the predictions made in Xia et al..(21) Therefore, modeling runs with
the Xia et al.(21) data were performed and results indicated that the inclusion of
these parameters did not significantly impact the predictions made in Xia et al.(21)
This, we believe, is because the concentrations of both the Np4+ and NpOH3+
are very low as compared with the total soluble Np in the Np(IV)-sulfate system
investigated by Xia et al.(21)
4. CONCLUSIONS
The results presented in this paper show that significant complexation of
Np(IV) by ISA can occur under acidic as well as basic conditions and that the
overall effect in solubilizing NpO2(am) is dependent on the pH and ISA con-
centration. The data reported in this study in conjunction with the data reported
by Rai et al.(14,15) provide a basis to develop a comprehensive model for pre-
dicting NpO2(am) solubility behavior in a wide range of ISA concentrations

Isosaccharinate Reactions with Ca(II) and Np(IV)
683
Fig. 8. Normalized aqueous Np(IV) concentrations
([Np(IV)]n) as a function of total ISA concentrations.
Solid line represents total concentrations, while other
lines represent concentrations of different species, as
identified, predicted using modeling parameters re-
ported in Tables I and II.

684
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Fig. 8. Continued.
(0.0001–0.1 M), very acidic to highly alkaline conditions (pH 2–14), and in
dilute to concentrated NaClO4 (0.1–6.5 m) solutions in the acidic region. The
Pitzer ion-interaction parameters and the equilibrium constants developed/used in
this study for the Np(IV)–H+–Na+–Ca2+–HISA(aq)–ISA –ISL(aq)–OH –H2O
and the Ca2+–H+– HISA(aq)–ISA –ISL(aq)–OH –H2O systems are summarized
in Tables I and II.

Isosaccharinate Reactions with Ca(II) and Np(IV)
685
ACKNOWLEDGMENTS
This research was conducted at the Pacific Northwest National Laboratory
[operated by Battelle for the U.S. Department of Energy (DOE) under contract DE-
AC06-76RL01830]. The research was carried out for the project (82715) on Devel-
opment of Biodegradable Isosaccharinate-Containing Foams for Decontamination
of Actinides funded by the DOE under the Environmental Management Sciences
Program. Partial support for this research was provided by the Mitsubishi Mate-
rials Corporation (MMC), Tokyo, Japan. We thank Mr. Todd Gervais and Dean
A. Moore for analytical help, Ryan Moore for drafting figures, and Ms. Debra M.
Hinton for text processing of this document (NpISA 2003 30 revised JSC.doc).
APPENDIX
Table A.I. Ca(ISA)2(c) Solubility in NaISA So-
lutions of pH Values 8.2, Equilibrated for 48 Days
log mNaISA
Initially added
At equilibrium
log mCa
1.940
1.636
1.464
1.347
1.170
0.959
0.724
1.63
1.49
1.46
1.35
1.17
0.96
0.70
2.239
2.444
2.568
2.702
2.829
2.991
3.336
Table A.II. NpO2(am) Solubility in 0.0016 M NaISA Containing 0.01 M Na2S2O4
and as a Function of pH and Equilibration Period
5-6 Days
13 Days
log mNp
pH
log mNp
% Np(IV)
pH
% Np(IV)
5.04
5.56
6.03
6.51
7.03
7.49
5.534
6.292
6.070
6.437
7.214
6.561
89
54
24
20
58
29
5.09
5.55
6.02
6.54
7.01
7.48
5.433
5.975
6.341
6.239
6.775
6.752
96
94
91
93
83
25

Table A.II. Continued
5-6 Days
13 Days
pH
log mNp
% Np(IV)
pH
log mNp
% Np(IV)
7.99
8.47
8.81
9.24
9.64
10.18
10.78
11.87
12.98a
13.82b
6.744
6.880
7.155
7.247
7.319
7.307
7.371
7.433
7.256
7.262
37
54
62
55
66
69
68
60
71
69
7.96
8.39
8.72
9.03
9.36
9.94
6.940
6.976
7.214
7.557
7.342
7.206
7.243
7.284
7.360
7.262
88
85
81
74
73
78
77
76
72
71
10.6
11.8
12.98a
13.82b
apH calculated by NONLIN from 0.125 m NaOH initially added to the solutions.
bpH calculated by NONLIN from 1.00 m NaOH initially added to the solutions.
Table A.III. NpO2(am) Solubility at pH Values of 12 in 0.01 M Na2S2O4 and as a Function
of NaISA Concentrations
4 days
% Np(IV)a
12 days
20 days
% Np(IV)
log mISA
log mNp
log mNp % Np(IV) log mNp
3.699
3.398
3.097
2.796
2.495
2.201
1.900
1.600
1.301
1.000
8.490
8.227
8.051
7.666
7.365
7.137
6.604
5.929
5.077
4.758
NL
73
77
89
87
94
92
91
98
95
8.208
8.247
7.946
7.651
7.408
6.991
6.442
5.908
4.977
4.641
60
79
78
89
88
91
93
91
96
97
8.092
7.845
7.706
7.305
7.028
6.058
8.155
5.516
4.571
4.294
46
68
73
83
92
95
97
98
98
96
aNL represents a sample for which a value is not available.
Table A.IV. Solvent Extraction Dataa for Np(IV) from ISA Solutions in Different
Concentrations of NaClO4
log m+H
log mb[ISA]t
log mb[Np(IV)]O
log mb[Np(IV)]aq
D
log mb[Np(IV)]n
0.103 m NaClO4
1.890
0.000
3.686
3.385
3.209
3.084
2.987
2.908
4.887
5.134
5.301
5.432
5.531
5.638
5.687
4.723
4.596
4.538
4.516
4.524
4.545
4.517
0.685
0.290
0.173
0.121
0.098
0.081
0.068
5.836
5.462
5.238
5.084
4.993
4.908
4.830
1.897
1.899
1.905
1.908
1.914
1.919

Isosaccharinate Reactions with Ca(II) and Np(IV)
687
Table A.IV. Continued
log mb[Np(IV)]O log m[bNp(IV)]aq
log m+H
1.930
1.932
1.940
1.941
log mb[ISA]t
D
log mb[Np(IV)]n
2.841
2.783
2.732
2.686
5.779
5.811
5.875
5.916
4.525
4.519
4.521
4.534
0.056
0.051
0.044
0.041
4.746
4.708
4.647
4.618
0.53 m NaClO4
1.840
1.839
1.849
1.849
1.856
1.863
1.867
1.875
1.877
1.880
0.000
3.674
3.197
3.072
2.975
2.896
2.829
2.771
2.719
2.674
4.772
4.946
5.208
5.264
5.358
5.417
5.512
5.532
5.588
5.647
4.726
4.638
4.563
4.549
4.523
4.499
4.513
4.508
4.505
4.478
0.901
0.492
0.227
0.192
0.146
0.121
0.100
0.095
0.083
0.068
5.955
5.692
5.355
5.284
5.165
5.082
5.001
4.976
4.918
4.831
1.07 m NaClO4
1.840
1.839
1.840
1.840
1.847
1.851
1.857
1.868
1.879
1.881
1.886
0.000
2.766
2.740
2.715
2.692
2.648
2.628
2.609
2.590
2.573
2.556
5.004
5.496
5.530
5.542
5.581
5.616
5.604
5.619
5.634
5.650
5.701
4.721
4.583
4.582
4.582
4.586
4.581
4.570
4.574
4.574
4.572
4.569
0.520
0.122
0.113
0.110
0.101
0.092
0.093
0.090
0.087
0.083
0.074
5.716
5.087
5.052
5.040
5.004
4.965
4.966
4.954
4.940
4.922
4.867
2.22 m NaClO4
1.7917
1.7847
1.7867
1.7927
1.7947
1.8037
1.8197
1.8237
1.8287
1.8327
0.000
3.654
3.353
3.177
3.052
2.875
2.809
2.751
2.699
2.654
4.761
4.805
4.856
4.914
4.930
5.000
5.050
5.087
5.129
5.155
4.710
4.643
4.618
4.619
4.564
4.547
4.548
4.561
4.549
4.526
0.890
0.689
0.578
0.508
0.431
0.352
0.315
0.298
0.263
0.235
5.949
5.838
5.762
5.706
5.634
5.546
5.498
5.474
5.420
5.371
3.48 m NaClO4
1.7275
1.7275
1.7275
1.7305
1.7365
1.7415
0.000
3.635
3.333
3.157
3.032
2.936
4.697
4.736
4.761
4.785
4.811
4.845
4.661
4.622
4.595
4.562
4.568
4.565
0.919
0.769
0.684
0.599
0.572
0.526
5.963
5.886
5.835
5.777
5.757
5.721

688
Rai, Hess, Xia, Rao, Cho, Moore, and Van Loon
Table A.IV. Continued
log m+H
1.7455
1.7615
1.7675
1.7755
log mb[ISA]t
log mb[Np(IV)]O
log mb[Np(IV)]aq
D
log mb[Np(IV)]n
2.856
2.731
2.680
2.635
4.872
4.914
4.938
4.978
4.547
4.525
4.498
4.522
0.474
0.408
0.364
0.350
5.676
5.611
5.561
5.544
4.91 m NaClO4
1.642
1.642
1.645
1.650
1.658
1.667
1.674
1.678
0.000
3.609
3.308
3.132
2.910
2.831
2.764
2.655
4.617
4.642
4.642
4.656
4.652
4.696
4.718
4.751
4.675
4.644
4.619
4.623
4.614
4.610
4.631
4.594
1.143
1.003
0.947
0.928
0.917
0.820
0.818
0.698
6.058
6.001
5.976
5.968
5.962
5.914
5.913
5.844
6.54 m NaClO4
1.554
1.553
1.554
1.571
1.577
1.585
1.597
1.605
0.000
3.582
3.281
2.883
2.804
2.737
2.627
2.582
4.605
4.611
4.613
4.626
4.639
4.676
4.666
4.687
4.675
4.669
4.655
4.641
4.639
4.647
4.607
4.588
1.173
1.143
1.102
1.035
0.999
0.935
0.874
0.798
6.069
6.058
6.042
6.015
6.000
5.971
5.941
5.902
aExtraction with DBM in toluene using 1:1 volume ratio of aqueous to organic phases.
bmxi represents molality of the subscripted species x and the subscript i refers to total for i = t,
organic phase for i = o, aqueous phase for i = aq, and normalized for i = n. The normalized
6
Np(IV) aqueous molality [m[Np(IV)]n] was calculated by assuming [Np(IV)]0 = 1.0 10
and then dividing this number by D, the distribution ratio (m[Np(IV)]O/m[Np(IV)]aq).
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