Solubility and hydrolysis of tetravalent actinides

16 Pages·2001·0.26 MB·English

Radiochim.Acta89,1(cid:50)16(2001)
(cid:211) byOldenbourgWissenschaftsverlag,München
Solubility and hydrolysis of tetravalent actinides
By V.Neck*and J. I. Kim
ForschungszentrumKarlsruhe,InstitutfürNukleareEntsorgung,Postfach3640,D-76021Karlsruhe,Germany
(ReceivedMay22,2000;acceptedinrevisedformJuly4,2000)
Thorium(IV) / Uranium(IV) / Neptunium(IV)/ formation dependon the preparationmethod, pretreatment,
Plutonium(IV) / Hydrolysis/ Solubility / Oxides / alteration during the experiment, and temperature [3(cid:50)7].
Hydroxides As the An4(cid:49) ions hydrolyze even at low pH, there are no
solubility data for amorphous An(IV) hydrous oxides or
hydroxides with An4(cid:49) as a predominant equilibrium aque-
Summary. The solubility and hydrolysis of Th(IV), U(IV),
Np(IV), and Pu(IV) are critically reviewed and a comprehen- ous species. The solubility products evaluated from exper-
sive set of thermodynamic constants at I (cid:53) 0 and 25(cid:176)C is imental solubilities depend directly on the hydrolysis con-
presented. The hydrolysis constantsare selected preferentially stantsapplied to calculate the An4(cid:49) concentration from the
from experimental studies at actinide trace concentrations, total An(IV) concentration determined experimentally.The
where the interference of colloid formation can be excluded. pH-dependent solubility of An(IV) oxides or hydroxides is
Unknown formation constants of mononuclear complexes often described by simplified chemical models, neglecting
An(OH) 4(cid:50)n are estimated by applying a semi-empirical elec-
n many of the hydrolysis species, e.g. polynuclear species,
trostatic model and an empirical correlation with the known
themononuclear speciesAn(OH) 2(cid:49), An(OH)(cid:49),andpartly
constants of other actinide ions. Based on the known and 2 3
even An(OH)3(cid:49) [8(cid:50)11]. As a consequence, the calculated
estimated hydrolysis constants, the solubility products of
An4(cid:49) concentrations and solubility products may be con-
An(OH) (am)or AnO ·xHO(am)are calculated from exper-
imental 4solubility data2 avail2able in the literature. The SIT is siderably overestimated. Despite these ambiguities such
usedfor ionicstrength corrections. modelsare useful for the estimationof solubilities, evenin
The solubilities of U(IV), Np(IV), and Pu(IV) hydroxides concentrated chloridebrines, where the hydrolysis equilib-
or hydrous oxides can be calculated by accounting only for ria are not known and possibly complicated by the forma-
mononuclearhydrolysisspecies.Theconsiderablyhighersolu- tion of ternaryhydroxide-chloride complexes at lowpH.
bilities of amorphous Th(IV) precipitates at pH (cid:44)5 include The objective of the present work is the evaluation of a
majorcontributionsof polynuclearspecies.The solubilitydata comprehensive set of thermodynamic data at I(cid:53)0 and
in acidic solutions depend strongly on the preparation and 25(cid:176) C for the hydrous oxides or hydroxides and aqueous
crystallinity of the solid phase. In neutral and alkaline solu-
hydrolysis species of tetravalent actinides. As the knowl-
tions, where An(OH) (aq) are the predominant aqueous spe-
4 edge of the hydrolysis constants is an indispensable pre-
cies, thesolubilitiesof AnO (cr)become equal to thoseof the
2 requisite for the evaluation of correct solubility products,
amorphous solids. This indicates that the crystalline dioxides
are coveredby amorphoushydroxide layers. the knownhydrolysis constants are critically selected from
the literature. Unknown formation constantsof mononuclear
hydrolysis species are estimated by applying well founded
1. Introduction models, correlations or analogies [12(cid:50)16]. Based on a
chemical model which includes all mononuclear species
Because of their high electric charge, tetravalent actinide
An(OH)4(cid:50)n up to n (cid:53) 4 and, if necessary, polynuclear
ions have a strong tendency toward hydrolysis in aqueous n
species proposed in the literature, the solubility products
solution and undergo polynucleation or further to colloid
are then calculated from the available solubility data. In
formation [1(cid:50)3]. As the solubilities of An(IV) hydroxides
order to minimize uncertainties arisingfrom ionic strength
or oxides arelow,the possibilityto investigate theaqueous
corrections, the present calculations are restricted to data
speciation by spectroscopic methods is rather poor. More-
I(cid:35)1mol/l.
over, U(IV), Np(IV), and Pu(IV) are easilyoxidized. Such
problems complicate the experimental investigation and
thermodynamic evaluation of the hydrolysis constants and 2. Hydrolysis of tetravalent actinides
solubility products. In particular the solubility products re-
ported for amorphous or microcrystalline An(OH)(am) or ThehydrolysisreactionsoftheAn4(cid:49)ions,i.e.theformation
4
AnO xH O(s) show considerable discrepancies. In many of hydroxide complexes An (OH) 4x(cid:50)y, are usually written
2 2 x y
cases the data do not refer to a well-defined unique solid as
phase,becausethedegreeofcrystallinity,particlesize,con- xAn4(cid:49)(cid:49)yH O↔An (OH) 4x(cid:50)y(cid:49)yH(cid:49)
2 x y
tent of crystal water and hence the molar Gibbs energy of
or
* Authorfor correspondence(E-mail:[email protected]). xAn4(cid:49)(cid:49)yOH(cid:50)↔An (OH) 4x(cid:50)y.
x y
2 V.Neck andJ.I. Kim
Table1. Ion interaction (SIT) coefficients at
i j (cid:229)ij 25(cid:176) C used in the present paper (from the
NEA-TDB[7,18],exceptotherwisestated).
H(cid:49) ClO(cid:50) 0.14(cid:54)0.02
4
H(cid:49) Cl(cid:50) 0.12(cid:54)0.01
OH(cid:50) Na(cid:49) 0.04(cid:54)0.01
Th(IV) U(IV) Np(IV) Pu(IV)
An4(cid:49) ClO(cid:50) 0.67(cid:54)0.1a 0.76(cid:54)0.06 0.82(cid:54)0.05 0.83(cid:54)0.1b
4
An(OH)3(cid:49) ClO(cid:50) 0.45(cid:54)0.1a 0.48(cid:54)0.08 0.5 (cid:54)0.1a 0.5 (cid:54)0.1a
4
An(OH)2(cid:49) ClO(cid:50) 0.3 (cid:54)0.1a 0.3 (cid:54)0.1a 0.3 (cid:54)0.1a 0.3 (cid:54)0.1a
2 4
An(OH)(cid:49) ClO(cid:50) 0.15(cid:54)0.1a 0.15(cid:54)0.1a 0.15(cid:54)0.1a 0.15(cid:54)0.1a
3 4
An4(cid:49) Cl(cid:50) 0.25(cid:54)0.03 0.36(cid:54)0.1a 0.4 (cid:54)0.1a 0.4 (cid:54)0.1a
An(OH)3(cid:49) Cl(cid:50) 0.2 (cid:54)0.1a 0.2 (cid:54)0.1a 0.2 (cid:54)0.1a 0.2 (cid:54)0.1a
An(OH)2(cid:49) Cl(cid:50) 0.1 (cid:54)0.1a 0.1 (cid:54)0.1a 0.1 (cid:54)0.1a 0.1 (cid:54)0.1a
An(OH)2(cid:49) Cl(cid:50) 0.05(cid:54)0.1a 0.05(cid:54)0.1a 0.05(cid:54)0.1a 0.05(cid:54)0.1a
3
An(OH)(cid:176) j 0 0 0 0
4
a: Estimatedaccordingtoanalogiesandsystematicsasoutlinedinthe text.
b:Capdevila and Vitorge [69] report two independently determined values: 0.85(cid:54)0.20 and
0.82(cid:54)0.07.TheformervaluefromVitorgeetal(1.03(cid:54)0.05)givenintheNEA-TDB[7,18]
isrevised.
The hydrolysis constants K¢ (in a given medium) and K(cid:176) coefficients are estimated according to known values for
xy xy
(at infinite dilution), andthe corresponding formation con- analog ionsof equal charge,similarsizeand structure,sys-
stants(cid:226)¢ and (cid:226)(cid:176) are defined by tematics in the series of tetravalent actinides, and differ-
xy xy
K¢ (cid:53)[An (OH) (4x(cid:50)y)][H(cid:49)]y/[An4(cid:49)]x (1) ences between the interaction coefficients (cid:229)(Mx(cid:49)/Cl(cid:50)) and
xy x y (cid:229)(Mx(cid:49)/ClO (cid:50)) (cf. (cid:229) values given in the NEA reviews [7,
K(cid:176) (cid:53)K¢ (ª )(ª )y/(ª )x(a )y (2) 18]). 4 ij
xy xy Anx(OH)y H An w
and
2.1 Discussion of literature data
(cid:226)¢ (cid:53)[An (OH) 4x(cid:50)y]/[An4(cid:49)]x[OH(cid:50)]y (3)
xy x y Thorium(IV)
(cid:226)(cid:176) (cid:53)(cid:226)¢ (ª )/(ª )x(ª )y (4)
xy xy Anx(OH)y An OH The hydrolysis behaviour of Th(IV) has been investigated
respectively.[i]denotesthe concentrationof speciesi,ª its bynumerous potentiometrictitrationexperiments inthepH
activity coefficient and a the activity of water. The ihy- range 2.5(cid:50)4. At thorium concentrationsof10(cid:50)4(cid:50)10(cid:50)2mol/l,
w
drolysis constants log K are related to log (cid:226) by the ion mononuclear species are usually found to be of minor im-
xy xy
product of water (log K(cid:176) (cid:53)(cid:50) 14.00(cid:54)0.01 [7]). portance or even negligible compared to the polynuclear
In the present workwthe specific ion interaction theory species Th (OH) 4x(cid:50)y (cf. summaries in [1, 20]). On the
x y
(SIT) [7, 17, 18] is applied for the calculation of activity other hand, the dimeric, trimeric, tetrameric, hexameric,
coefficients. The simple SIT equation is preferred to the and decameric species proposed in the literature [1, 20(cid:50)
more elaborate Pitzer approach [19], because the hitherto 24] are the result of curve fitting procedures. They are not
available data for An(IV) hydrolysis species are not suf- ascertained by spectroscopic or other methods. In addition
ficientto evaluateallnecessary ioninteraction coefficients. it is shown by laser-induced breakdown detection (LIBD)
SomeuncertaintiesarisingfromtheestimationofSITcoef- [25, 26] that, under the conditions usually applied in
ficients have no significant effect on the calculation of ac- potentiometric titration studies ([Th] (cid:53) 10(cid:50)4(cid:50)10(cid:50)2mol/l,
tivity coefficients at I(cid:35)0.1mol/kg, whereas an estimated pH (cid:53) 2.5(cid:50)4),a considerable amount of Th(IV)colloids is
Pitzer coefficient (cid:226)(1) has a considerable effect even at low formed.Accordinglyitisnot surprising thatpotentiometric
ionic strength, in particular for ions of high charge. As the titrationstudiesperformedbydifferent authors,in different
influence of the SIT coefficients increases with the ionic media,at differentionic strengthandtemperature or at dif-
strength, the present calculations are restricted to data at ferenttotal thoriumconcentrations are interpreted with dif-
I(cid:35)1mol/kg.Accordingtothe SIT,the conditionalequilib- ferentsets of species. The published stability constantsfor
rium constantK¢ is related to K(cid:176) at I(cid:53)0 by thepolynuclear speciescannot becompared directly,andit
is difficult to decide, which set of species and hydrolysis
logK¢ (cid:53)logK(cid:176) (cid:49)˜z2D(cid:50)˜(cid:229)·I (5)
constantsisthemostrealistic.Thefrequentlyaccepteddata
with ˜z2 (cid:53) (cid:211) z2(products) (cid:50) (cid:211) z2(educts) and ˜(cid:229) (cid:53) (cid:211) of Baes et al. [1, 21], Brown et al. [20] and Grenthe and
i i
(cid:229) (products) (cid:50) (cid:211) (cid:229) (educts); z is the charge of ion i, m Lagerman [23] arelisted in Table2,togetherwith the most
ij ij i j
(mol/kg H O) the molal concentration of ion j, D is the recent results of Ekberg et al. [24], who performed
2
Debye-Hückel term at 25(cid:176)C:D (cid:53) 0.509 (cid:33)I/(1 (cid:49) Ba˚ (cid:33)I), potentiometric titrations at 15,25 and35(cid:176) C with relatively
with Ba˚ (cid:53) 1.5. I is the molal ionic strength, and (cid:229) is the lowthorium concentrations of 10(cid:50)5(cid:50)10(cid:50)4 mol/l and in ad-
ij
interaction parameter for a pair of oppositely charged ions. ditionsolventextraction experiments with thoriumconcen-
The ion interaction coefficients used in the present study trations in the range of 10(cid:50)7(cid:50)10(cid:50)5mol/l.
are summarized in Table1. As far as available, they are The firstmononuclear hydrolysisconstantof Baesetal.
taken from the NEA-TDB [7, 18]. Unknown interaction [21] (log K(cid:176) (cid:53)(cid:50) 3.0) and Grenthe et al. [23, 30]
11
Solubilityandhydrolysisoftetravalent actinides 3
Table2. Hydrolysis constants log K¢ pro-
posed for Th(IV) at 25(cid:176) C. The constxaynts at Authors Brownetal. Ekbergetal. Baesetal. Grenthe and
I (cid:53) 0 (log K(cid:176) values in brackets) are calcu- (Medium) [20] [24] [1,21] Lagerman[23]
latedwiththexySITcoefficientsinTable1. (0.1MKNO3) (1MNaClO4) (1MNaClO4) (3MNaClO4)
Mononuclearspecies
Th(OH)3(cid:49) (cid:50)2.98 (cid:50)3.3(cid:54)0.1 (cid:50)4.12b (cid:50)4.35(cid:54)0.09
((cid:50)2.3(cid:54)0.1) ((cid:50)2.1(cid:54)0.2) ((cid:50)3.0(cid:54)0.2) ((cid:50)3.1 (cid:54)0.3)
Th(OH)2(cid:49) (cid:50)8.6(cid:54)0.1 (cid:50)7.81b
2
((cid:50)6.6(cid:54)0.2) ((cid:50)5.8(cid:54)0.2)
Th(OH)(cid:49) (cid:50)13.8a (cid:50)12.3 (cid:54)0.2
3
((cid:50)11.4) ((cid:44)(cid:50)11.7)c ((cid:50)9.3 (cid:54)0.4)
Th(OH)(aq) (cid:50)19.4(cid:54)0.5 (cid:50)16.65(cid:54)0.04
4
((cid:50)17.0(cid:54)0.5) ((cid:50)15.9(cid:54)0.3)c ((cid:50)13.6 (cid:54)0.4)
Polynuclearspeciesd
Th(OH)6(cid:49) (cid:50)4.61b (cid:50)5.10(cid:54)0.17
Th2(OH)25(cid:49) (cid:50)7.87(cid:54)0.05
2 3
Th(OH)8(cid:49) (cid:50)19.1(cid:54)0.1 (cid:50)19.01b (cid:50)19.6 (cid:54)0.2
4 8
Th(OH) 4(cid:49) (cid:50)30.55 (cid:50)34.86(cid:54)0.05
4 12
Th(OH) 10(cid:49) (cid:50)33.67(cid:54)0.05
6 14
Th(OH) 9(cid:49) (cid:50)34.41 (cid:50)39.5(cid:54)0.2 (cid:50)36.76b
6 15
Th(OH) 8(cid:49) (cid:50)42.9 (cid:54)0.4
6 16
a:Interpolatedfromthevaluesat15and35(cid:176) C.
b:CalculatedbyBaesetal.[21]fromdataofKrausandHolmberg[22].
c:Evaluatedby Baesand Mesmer [1] from thesolubility data ofNabivanets and Kudritskaya
[28].
d:Application of the SIT equation (log K(cid:176) (cid:53) log K¢ (cid:50) y log a (cid:50) ˜z2D (cid:49) ˜(cid:229) I) to the
xy xy w
polynuclearhydrolysisconstantslisted inthistableleadstothefollowingconstantsatI(cid:53)0
and˜(cid:229) valuesinNaClO solution:
4
Th(OH) 6(cid:49)(˜z2(cid:53)6): log K(cid:176) (cid:53)(cid:50)5.7, ˜(cid:229) (cid:53)0.24
2 2 2,2
Th(OH) 8(cid:49)(˜z2(cid:53)8): log K(cid:176) (cid:53)(cid:50)20.4, ˜(cid:229) (cid:53)0.13
Th4(OH)84(cid:49)(˜z2(cid:53)(cid:50)36): log K(cid:176)4,8 (cid:53)(cid:50)26.7, ˜(cid:229) (cid:53)(cid:50)0.59
4 12 4,12
Th(OH) 9(cid:49)(˜z2(cid:53)0): log K(cid:176) (cid:53)(cid:50)34.0, ˜(cid:229) (cid:53)3.7(cid:54)1.9.
6 15 6,15
(logK(cid:176) (cid:53)(cid:50)3.1) arein goodagreement,butseveralorders product. They lead to values, which are not considered as
11
of magnitude lower than the corresponding constants for reliable [7]. Theonly datareported for polynuclear species
the other An4(cid:49) ions. Brown et al. [20] and Ekberg et al. is basedon a potentiometric titrationstudy in 3M NaClO
4
[24] found more significant contributions of the species [33],whichis interpretedwith theformation ofU (OH) 9(cid:49)
6 15
Th(OH)3(cid:49) and the hydrolysis constants K reported by [1, 7] and log K¢ (cid:53)(cid:50) 16.9(cid:54)0.6.
11 6,15
theseauthors areabout oneorderof magnitudegreater(log
K¢ (cid:53)(cid:50)2.98in0.1M KNO [20]and(cid:50)3.3in1MNaClO
11 3 4 Neptunium(IV) and Plutonium(IV)
[24], corresponding to log K(cid:176) (cid:53)(cid:50) 2.3 and (cid:50)2.1, respec-
11
tively.) Nakashima andZimmer[27] determined a compar- The initial mononuclear hydrolysis of Np(IV) and Pu(IV)
able value of log K¢ (cid:53)(cid:50) 3.28 in 0.5M KNO has also been investigated by means of absorption spec-
(log K(cid:176) (cid:53)(cid:50) 2.2) by solv11ent extraction with TBPat pH (cid:53)3 troscopy [34(cid:50)40] (cf. Tables 3 and 4). However, the
11
1.5(cid:50)2.7andthoriumconcentrations of 10(cid:50)2mol/l.The hy- concentrations usedin these studies([Np(IV)](cid:53) 1·10(cid:50)3(cid:50)
drolysis constants reported for Th(OH) 2(cid:49), Th(OH)(cid:49), and 7· 10(cid:50)3M, [Pu(IV)] (cid:53) 1.3·10(cid:50)4(cid:50)2· 10(cid:50)3M) consider-
2 3
Th(OH)(aq) differ several orders of magnitude. The log ably exceed the solubility limit of the Np(IV) and Pu(IV)
K(cid:176) and4log K(cid:176) values reported by Baes and Mesmer [1] hydrous oxides at pH (cid:53) 1(cid:50)2 (cf. Figs.8 and 9 in section
13 14
are based on solubility data from Nabivanets and Kudrits- 3). The same holds for the studies on the Pu(III)/Pu(IV)
kaya [28], which are conflicting with solubility data pub- redox couples [41, 42]. As demonstrated in the filtration
lished later by other authors (cf.discussion in section 3.1). and LIBD study of Knopp et al. [45], the formation of
Pu(IV) colloids, which remain in solution without precipi-
tation, is the predominant reaction when the concentration
Uranium(IV)
exceedsthe solubilitylimit.Theinterferenceof colloidfor-
TheformationofU(OH)3(cid:49)hasbeeninvestigatedbynumer- mation may have led to misinterpretation and hence to er-
ousauthors in different media, preferentially byabsorption roneous data in all these studies. This source of error is
spectroscopy in the pH range of 0(cid:50)2.The thermodynamic excluded in the solvent extraction studies of Guillaumont
constants selected in the comprehensive reviews of the et al. [43, 44]. The reported stepwise hydrolysis constants
NEA and IAEA (log K(cid:176) (cid:53)(cid:50) 0.54(cid:54)0.06 [7] and (cid:50)0.34 are determined using Np-239 and Pu-238 trace concen-
11
(cid:54)0.20 [32], respectively) are in reasonable agreement.At- trations at I (cid:53) 1.0mol/l (HClO/LiClO ): log *K¢ (cid:53)
4 4 1
tempts to calculate log K , log K or log K from solu- (cid:50)0.45, log *K¢ (cid:53)(cid:50) 0.75, log *K¢ (cid:53)(cid:50) 3.3, log *K¢ (cid:53)
12 13 14 2 3 4
bility data include the uncertainties in the solid phase and (cid:50)6.3 for Pu(IV) [43] and log *K¢ (cid:53)(cid:50) 0.5, log *K¢ (cid:53)
1 2
inthe redundanceof thehydrolysisconstantsandsolubility (cid:50)1.0for Np(IV) [44].
4 V.Neck andJ.I. Kim
Table3. Formation constants of Np(IV) hy-
Medium/Species logK¢1y logK(cid:176)1y log(cid:226)(cid:176)1y Methoda Ref. drolysisspeciesat20(cid:50)25(cid:176) C.
1.0MHClO/LiClO,
4 4
room temp.,10(cid:50)3(cid:50)1MH(cid:49),
239Np(IV)traceconc. extr [44]
Np(OH)3(cid:49) (cid:50)0.5 0.55 14.55(cid:54)0.2
Np(OH)2(cid:49) (cid:50)1.5 0.35 28.35(cid:54)0.3
2
Otherdatafor Np(OH)3(cid:49)
2.0MHClO/NaClO , 25(cid:176)C (cid:50)2.30 (cid:50)1.25 12.75 spec [34]
4 4
0.01(cid:50)0.1MH(cid:49),
(1.4(cid:50)2.7)·10(cid:50)3MNp(IV)
1.0MHClO/NaClO , 25(cid:176) C (cid:50)1.90 (cid:50)0.84 13.16 spec [35]
4 4
0.01(cid:50)0.2MH(cid:49),
7.5·10(cid:50)3MNp(IV)
1.0MHClO/NaClO , 25(cid:176)C (cid:50)2.25 (cid:50)1.19 12.81 spec [36]
4 4
a: extr(cid:53)solventextraction,spec(cid:53)spectroscopy.
In our recent paper [45] it is shown that the solubility are estimated by applying two different methods. The first
of Pu(OH) (am)in the range pH (cid:53) 0(cid:50)12agrees well with method is based on the empirical intercorrelation between
4
thepH-dependencepredictedbythehydrolysisconstantsof hydrolysis constants of actinide ions at different oxidation
MetivierandGuillaumont[43].Thehydrolysisconstantsof states. For the second method our recent semi-empirical
Guillaumont et al. [43, 44], with the log K(cid:176) values more approach[16]is applied,in whichthe decrease ofstepwise
11
than an order of magnitude greater than those reported in complexation constants for a given metal-ligand system is
the other studies, are therefore considered to be the most relatedtothe increasingelectrostatic repulsionbetweenthe
reliable values among the published data for Np(IV) and ligands.
Pu(IV).Asexperimentaluncertaintiesarenotgiveninthese
papers [43, 44], they are estimated to be about (cid:54)0.2 log
units for eachhydrolysis step. Estimationmethod (A)
Complexation and hydrolysis constants of metal ions with
2.2. Estimation of unknown constants
comparableelectronic configuration are knowntocorrelate
In orderto obtaina completeset offormation constantsfor with the electrostatic interaction energy between the metal
mononuclear species An(OH)4(cid:50)n, the unknown constants andligand ions [1, 12(cid:50)15]:
n
Table4. FormationconstantsofPu(IV)hydrolysisspeciesat20(cid:50)25(cid:176) C.
Medium/Species log K¢ logK(cid:176) log (cid:226)(cid:176) Methoda Ref.
1y 1y 1y
1.0MHClO/LiClO,roomtemp., extr [43]
4 4
10(cid:50)8(cid:50)1MH(cid:49),10(cid:50)8M238Pu(IV)
Pu(OH)3(cid:49) (cid:50)0.45 0.60 14.6(cid:54)0.2
Pu(OH)2(cid:49) (cid:50)1.2 0.63 28.6(cid:54)0.3
2
Pu(OH)(cid:49) (cid:50)4.5 (cid:50)2.25 39.7(cid:54)0.4
Pu(OH)3(aq) (cid:50)10.8 (cid:50)8.54 47.5(cid:54)0.5
4
Otherdatafor Pu(OH)3(cid:49) log K¢ logK(cid:176) log (cid:226)(cid:176)
11 11 11
0.5MHCl/NaCl,25(cid:176) C (cid:50)1.65 (cid:50)0.63 13.37 spec [37]
0.01(cid:50)0.5MH(cid:49),7.2·10(cid:50)4MPu(IV)
0.5MHClO/NaClO , 25(cid:176)C (cid:50)1.60 (cid:50)0.64 13.36 spec [37]
4 4
0.01(cid:50)0.5MH(cid:49),7.2·10(cid:50)4MPu(IV)
2.0MHClO/NaClO ,25(cid:176) C (cid:50)1.73 (cid:50)0.70 13.30 spec [38]
4 4
0.01(cid:50)2.0MH(cid:49),10(cid:50)3MPu(IV)
0.19MHClO, 23(cid:176)C (cid:50)1.96 (cid:50)1.19 12.81 spec [39]
4
1.8·10(cid:50)3MPu(IV)
0.06MHClO,23(cid:176)C (cid:50)1.48 (cid:50)0.94 13.06 spec [39]
1.6·10(cid:50)3M4 Pu(IV)
0.5MHClO/NaClO ,25(cid:176)C (cid:50)1.57 (cid:50)0.611 3.39 spec [40]
4 4
0.03(cid:50)0.5MH(cid:49),1.3·10(cid:50)4MPu(IV)
1.0MHClO/NaClO ,25(cid:176)C (cid:50)1.51 (cid:50)0.45 13.55 redox [41]
4 4
0.1(cid:50)0.2MH(cid:49),10(cid:50)3MPu(IV)
2.0MHClO/LiClO,25(cid:176)C (cid:50)1.27 (cid:50)0.24 13.76 redox [42]
4 4
0.1(cid:50)2.0MH(cid:49),8 · 10(cid:50)3MPu(IV)
a: extr(cid:53)solventextraction,spec(cid:53)spectroscopy,redox(cid:53)Pu(III)/Pu(IV)redoxpotential.
Solubilityandhydrolysisoftetravalent actinides 5
Table5. Distancesd andeffectiveradiiofactinideions. the known hydrolysis constants for Am(III) and Cm(III)
M-OH2
[18, 46, 47], Np(IV) [44], Pu(IV) [43], NpO(cid:49) [48] and
Md (A˚)a Effectiveionicradiusr (A˚) 2
M-OH2 M UO2(cid:49) [7, 49]. Since the quotient z /d increases
2 M M-OH
inaqueous Crystal slightlyintheseries:Th(IV)(cid:44)U(IV)(cid:44)Np(IV)(cid:44)Pu(IV),
solutionb radiusc a corresponding increase is expected for the constants log
(cid:226)(cid:176) . This assumptionholds quite well for log (cid:226)(cid:176) of U(IV),
Pu3(cid:49) 2.50(cid:54)0.01 1.12(cid:54)0.02 1.12(CN(cid:53)8) ln 11
Np(IV), and Pu(IV). The correlation may be used to esti-
Am3(cid:49) 1.10(CN(cid:53)8)
Cm3(cid:49) 1.09(CN(cid:53)8) matethe unknown constants log (cid:226)l(cid:176)n for U(IV)and Np(IV).
However, as shown in Fig. 1 for the data of Ekberg et al.
Th4(cid:49) 2.46(cid:54)0.02 1.08(cid:54)0.02 1.09(CN(cid:53)9)
U4(cid:49) 2.42(cid:54)0.02 1.04(cid:54)0.02 1.05(CN(cid:53)9) [24], the hydrolysis constants of Th(IV) are significantly
Np4(cid:49) 2.40(cid:54)0.01 1.02(cid:54)0.02 1.03(CN(cid:53)9) lower than the expected values from this correlation. The
Pu4(cid:49) 2.39(cid:54)0.01 1.01(cid:54)0.02 1.01(CN(cid:53)9) large differences between the hydrolysis constants of
NpO(cid:49) 2.50(cid:54)0.02 1.12(cid:54)0.02 Th(IV) and those of other tetravalent actinides cannot be
2
UO2(cid:49) 2.42(cid:54)0.01 1.04(cid:54)0.02 explained bydifferences in the size of An4(cid:49) ions.
2
PuO2(cid:49) 2.40(cid:54)0.01 1.02(cid:54)0.02
2
Estimationmethod (B)
a: From the recentcompilation ofXRD andEXAFS datain aqueous
solution [16]. The second estimation method applies an electrostatic ap-
b:C0.a0l2cuA˚la[te7d5].according to rM (cid:53) dM-OH2 (cid:50) rH2O with rH2O (cid:53) 1.38(cid:54) proach(NeckandKim[16]),whichcorrelatesthe mononu-
c: Crystal radii at given coordination number (CN) from Refs. [12, clearcomplexationconstantslog(cid:226)(cid:176)lnforagivenactinideion
76]. with an inter-ligand electrostatic repulsion energy term:
log(cid:226)(cid:176) (cid:53)nlog(cid:226)(cid:176) (cid:50)repE /RTln10. (7)
ln 11 L
log(cid:226)(cid:176) (cid:126)elE (cid:126)(z /d ). (6) TheCoulombrepulsionenergytermrepE iscalculatedfrom
(M) M-L M M-L L
the charge and inter-distance of the ligands involved in a
z is the charge of the metal ion and d the distance be-
M M-L given complex and theirangular distribution:
tween the centers of metal and ligand ions. This empirical
(cid:111) z z
linear correlation is usually applied to the first com- repE (cid:53)N e2·(1/2) L L¢ . (8)
plexation constant, as used by Choppin [12, 13] to derive L A L dL-L¢ (cid:229)L-L¢
effective charges of z (cid:53) 2.3(cid:54)0.1 and 3.2(cid:54)0.1 for the
M N is Avogadro’s number, e the elementary charge, z and
preesnpteac-tiavnedly.hTexhaevdailsetnatncaecstidnide i,oin.es.AthneOsu2(cid:49)maonfdthAeneOff2e2c(cid:49)-, zLA¢ the formal chargenumber of ligand ions ((cid:50)1 for OLH(cid:50)).
tive radii of the actinide anAdn-OOHH(cid:50) ions (the latter is as- TLh¢ eisdcisatlacnuclaeteddL-Lf¢robmetwtheeendtihsteancceentedrs o(cid:53)f thre li(cid:49)garndsanLdatnhde
sTuambleed5t.oFbige.e1qsuhaolwtostthhaetaopfpaliHca2tOionmoofletchuilse)coarrreelgaitvioenn tion angle ¨L-M-L¢ between L,M andL¢ : M-L M L
d (cid:53){(d )2(cid:49)(d )2(cid:50)2d d cos¨ }1/2.
L-L¢ M-L M-L¢ M-L M-L¢ L-M-L¢
(9)
Fig.2. Application of the electrostatic ligand repulsion approach of
Neck and Kim [16] to actinide hydroxide complexes. Evaluation of
the shielding coefficientsC(n) from the known hydrolysis con-
OH-M-OH
Fig.1. Correlation of the formation constants log (cid:226)(cid:176) of actinide hy- stants ofAm(III) and Cm(III) (as selected in our recent review[47])
ln
droxidecomplexeswiththeelectrostaticinteractionenergyelE be- andPu(IV)(fromthesolventextractionstudyofMetivierandGuillau-
An-OH
tweenthe actinideandOH(cid:50)ions. mont[43]).
6 V.Neck andJ.I. Kim
Table6.Formationconstantsformononuclear
log (cid:226)(cid:176)11 log(cid:226)(cid:176)12 log (cid:226)(cid:176)13 log(cid:226)(cid:176)14 An(IV) hydrolysis species at 25(cid:176) C: exper-
imentaldataa,estimatedbandselectedvalues.
Thorium(IV)
Baesetal. [21] 11.0(cid:54)0.2 22.2(cid:54)0.2
Grenthe,Lagerman[23] 10.9(cid:54)0.3 32.7(cid:54)0.4 42.4(cid:54)0.4
Nakashima,Zimmer [27] 11.8(cid:54)0.2
Brownetal.[20] 11.7(cid:54)0.1
Ekberg etal.[24] 11.9(cid:54)0.2 21.4(cid:54)0.2 30.6 39.0(cid:54)0.5
Estimation(A) 13.4 26.5 36.7 43.9
Estimation(B) 11.9 22.9 31.4 37.0
Selected 11.8(cid:54)0.2 22.0(cid:54)0.6 31.0(cid:54)1 (39.0(cid:54)0.5)
38.5(cid:54)1.0c
Uranium(IV)
NEA[7] 13.46(cid:54)0.1
IAEA[32] 13.66(cid:54)0.2
Estimation(A) 13.9 27.5 38.2 45.7
Estimation(B) 13.6 (fixed) 26.3 36.4 43.6
Selected 13.6 (cid:54)0.2 26.9(cid:54)1 37.3(cid:54)1 (44.7(cid:54)1)
46.0(cid:54)1.4c
Neptunium(IV)
Duplessis,Guillaumont[44] 14.55(cid:54)0.2 28.35(cid:54)0.3
Estimation(A) 14.2 28.0 39.0 46.6
Estimation(B) 14.6 28.3 39.4 47.5
Selected 14.5 (cid:54)0.2 28.3(cid:54)0.3 39.2(cid:54)1 (47.1(cid:54)1)
47.7(cid:54)1.1c
Plutonium(IV)
Metivier,Guillaumont[43] 14.6 (cid:54)0.2 28.6(cid:54)0.3 39.7(cid:54)0.4 47.5(cid:54)0.5
Estimation(A) 14.5 28.5 39.7 47.5
Estimation(B) 14.6 28.3 39.4 47.5
Selected 14.6 (cid:54)0.2 28.6(cid:54)0.3 39.7(cid:54)0.4 (47.5(cid:54)0.5)
48.1(cid:54)0.9c
a:ConstantsatI(cid:53)0arecalculatedwiththe ioninteractionSIT coefficientsinTable1.
b:Estimation (A) isbased onthe correlation log (cid:226)(cid:176) (cid:126) (z /d ). Estimation (B) isbased on
An An-OH
the electrostaticapproachofNeckand Kim[16].
c:Calculated from solubility data for AnO ·xHO(am) in neutral to alkaline solutions (c.f.
2 2
sections 3.1(cid:50)3.4).
The angle ¨ is given by the assumption of complex Theestimations(A)and(B) forthe unknownconstantslog
L-M-L¢
symmetries with minimum ligand repulsion for the com- (cid:226)(cid:176) and log (cid:226)(cid:176) of Np(IV) are also inreasonable agreement
13 14
plexes ML (linear), ML (trigonal planar), ML (tetra- with each other. In the case of U(IV)the discrepancies be-
2 3 4
hedral), ML (trigonal bipyramidal) and ML (octahedral). tween the two estimation methods are larger and thus the
5 6
The electrostatic shielding between the ligands L and L¢, mean values are selected for the unknown constants log
due to the metal ion and hydration water molecules be- (cid:226)(cid:176) and log (cid:226)(cid:176) . The mononuclear Th(IV) hydrolysis con-
12 13
tweenthem,is describedsemi-empiricallybya virialequa- stantsproposed in the literature differlargely, andthe con-
tion: stants interpolated with estimation method (A) are con-
siderablyoverestimated. Thereforethe ligand repulsionap-
(cid:229) (cid:53)C(0) (cid:49)C(1) (¨ /180(cid:176)) (10)
L-L¢ L-M-L¢ L-M-L¢ L-M-L¢ proach (estimation method (B)) is applied for their critical
(cid:49)... (cid:49)C(L2-0M)-L¢ (¨L-M-L¢/180(cid:176) )20 examination(Fig.3).TheapplicationofEqs.(7)(cid:50)(10)with
TlighaensdhsiebludtinigndceopeefnfidceinenttosfCth(Ln-e)M-aL¢ctairneidsepeiocnif.icThfoersdhiifefledrienngt eliinthee)rinlodgic(cid:226)at(cid:176)1e1s(cid:53)th1at1.t9he(sohliigdhelirneloign(cid:226)F(cid:176)ig.v3al)uoers1in1.0th(edarashnegde
11
coefficients for OH(cid:50) ligands (C(L0-)M-L¢ (cid:53)(cid:50) 23, C(L1-)M-L¢ (cid:53) 94, 11.7 to 11.9 [20,24, 27] andthe lowerlog (cid:226)1(cid:176) 3 andlog (cid:226)(cid:176)14
andC(20) (cid:53)(cid:50) 15) are derived fromthe knownhydrolysis values of Ekberg et al. [24] should be preferred. The con-
L-M-L¢
constants of Am(III) and Cm(III) [47] and Pu(IV) [43] as stants proposed by Grenthe and Lagerman [23] suggest a
shown in Fig.2. linear increase of log (cid:226)(cid:176) with the number of hydroxide
ln
In Table6, the formation constants log (cid:226)(cid:176) of mononu- ligands. This is in contradiction to the observations made
ln
clear An(IV) hydroxidecomplexes, both experimental data forthehydroxidecomplexesofother actinideions andalso
and estimated values, are summarized for the purpose of for actinide complexes with other inorganic ligands [16].
comparison. If the selected constant is based on the esti- As the extrapolation to log (cid:226)(cid:176) implies large uncertainties,
14
mations, the uncertainty is assumed to be (cid:54)1 logarithmic the constants log (cid:226)(cid:176) are evaluated in sections 3.1(cid:50)3.4
14
unit. The hydrolysis constants of Pu(IV) are used as input from the solubilities of AnO ·xHO(am) in neutral and
2 2
data in both estimation methods and accordingly the esti- alkalinesolutions,wheretheneutralcomplexesAn(OH)(aq)
4
mations for Pu(IV) are close to the experimental values. are the predominant aqueousspecies.
Solubilityandhydrolysisoftetravalent actinides 7
Table7. Solubilityproducts for crystalline An(IV)dioxides at25(cid:176) C,
calculatedfromthermochemicaldata.
An(IV) logK(cid:176) (AnO(cr))
sp 2
Th(IV) (cid:50)54.2(cid:54)1.3[50]
U(IV) (cid:50)60.6(cid:54)0.5[50], (cid:50)60.86(cid:54)0.36[7]
Np(IV) (cid:50)63.7(cid:54)1.8[50]
Pu(IV) (cid:50)64.1(cid:54)0.7[50], (cid:50)63.8 (cid:54)1.0[3]
Am(IV) (cid:50)65.4(cid:54)1.7[18]
An(OH)(aq) is the predominant aqueous species. The dis-
4
solutionequilibrium canhence be writtenas
AnO ·xH O(s)(cid:49)(2(cid:50)x)H O↔An(OH) (aq)
2 2 2 4
and the pH-independent solubility in this range
(log [An(IV)] (cid:60) log [An(OH)(aq)]) is given by the so-
tot 4
lubility constant
Fig.3. Application of the electrostatic ligand repulsion approach of
Neck and Kim[16]to thehydroxide complexesof Th(IV).The con- logKs¢(14)(cid:53)log[An(OH)4(aq)](cid:53)logKs¢p(cid:49)log(cid:226)¢14.
stants log (cid:226)(cid:176) are calculated according to Eq.(7) for log (cid:226)(cid:176) (cid:53) 11.9 (15)
ln 11
(solidline)andalternativelywithlog(cid:226)(cid:176) (cid:53)11.0(dashed line).
11
In neutral and alkaline solution of low ionic strength the
solubility is independent of the medium and ionic strength
3. Solubility of An(IV) hydroxides and oxides (log K¢s(14) (cid:60) log K(cid:176)s(14)), because the water activity and the
activity coefficients of An(OH) (aq) are approximately
4
The chemical form of freshly precipitated or aged An(IV) equal to 1.
solid phases is not clear. In the literature, they are called Solubility data determined with crystalline AnO (cr) as
2
either amorphous hydroxides An(OH) (am) or amorphous, an initial solid phase imply a certain ambiguity, because
4
partly microcrystalline hydrous oxides AnO ·xH O(am). radiolytic amorphization and hydration on the surface can-
2 2
Possibly they do not have a unique composition but con- not be ruled out [3]. In the following sections the exper-
sist of a hydrated oxyhydroxide AnO(2(cid:50)n)(OH)2n(am) with imental solubility data measured with crystalline An(IV)
0 (cid:44) n (cid:44) 2, where n decreases with aging or temperature. dioxides are compared with the solubility products for
The preparation of water-free crystalline dioxide AnO(cr) AnO(cr) calculated from known thermochemical data for
requires heatingabove 700(cid:176) C [3, 6, 29]. 2 AnO2(cr) andAn4(cid:49)(aq) according to
The conditional solubility products K¢ and K(cid:176) (at 2
sp sp (cid:50)RTlnK(cid:176) (cid:53)˜G(cid:176)
infinite dilution) of amorphous An(IV) precipitates, sp r m
(cid:53)˜G(cid:176) (An4(cid:49)(aq))(cid:49)4˜G(cid:176) (OH(cid:50)(aq))
An(OH) (am)orAnO ·xH O(am),andcrystallinedioxides f m f m
4 2 2 (cid:50)˜G(cid:176) (AnO(cr))(cid:50)2˜G(cid:176) (H O(l)). (16)
AnO (cr)refer to the dissolution equilibria f m 2 f m 2
2
An(OH)(s)↔An4(cid:49)(cid:49)4OH(cid:50) Table7 shows the solubility products for ThO2(cr),
4 UO(cr), NpO(cr) and PuO (cr) calculated by Rai et al.
2 2 2
and [50] with critically evaluated standard data
˜H(cid:176) (AnO (cr)) and S(cid:176) (AnO(cr)) [51], S(cid:176) (An(cr)) [52],
AnO ·xH O(s)(cid:49)(2(cid:50)x)H O↔An4(cid:49)(cid:49)4 OH(cid:50) f m 2 m 2 m
2 2 2 ˜G(cid:176) (An4(cid:49)(aq)) [53], and S(cid:176) (O(g)), ˜G(cid:176) (OH(cid:50)(aq)),
f m m 2 f m
with ˜G(cid:176) (H O(l)) [54]. Slightly different values for
f m 2
˜G(cid:176) (AnO (cr)) and log K(cid:176) are calculated by Kim and
K¢ (cid:53)[An4(cid:49)][OH(cid:50)]4 (11) f m 2 sp
sp Kanellakopulos [3]for PuO (cr)andfrom thedata selected
2
and in the NEA reviewon uranium [7] for UO(cr).
2
K(cid:176) (cid:53)K¢ (ª )(ª )4 (forAn(OH) (am)) (12)
sp xy An OH 4
3.1 Solubility of Th(IV)
K(cid:176) (cid:53)K¢ (ª )(ª )4(a )(x(cid:50)2) (forAnO · xHO(s)).
sp xy An OH w 2 2
(13) The solubility of amorphous Th(IV) precipitates, con-
sidered as either amorphous Th(OH)(am) [28, 29]) or
4
If there are no complexes with other inorganic ligands or ThO ·xHO(am) ([8(cid:50)10] has been investigated by Nabi-
2 2
colloidal species presentin solution, the total An(IV) equi- vanets and Kudritskaya [28] at 17(cid:176)C in 0. 1M NaClO, by
4
librium concentrationis given by Moon[29] at18(cid:176) Cin0.5MNaClO ,andbyRaietal.[8(cid:50)
4
[An] (cid:53)[An4(cid:49)](cid:49)(cid:211)x[An (OH) 4x(cid:50)y] 10] at room temperature in 0.1M NaClO4, 0.6M NaCl,
tot(cid:53)K¢ [OH(cid:50)](cid:50)4(cid:49)(cid:211)xx (K¢ y[OH(cid:50)](cid:50)4)x(cid:226)¢ [OH(cid:50)]y). 0.6M KCl,andconcentratedNaClandMgCl2solutions.In
sp sp xy (14) these studies, the amorphous precipitates are not treated at
highertemperature but only washedwith water.Moon[29]
In neutral and alkaline solutions, which are of interest alsodetermined solubility datain 0.1M NaClO withcrys-
4
for natural goundwater systems, salt or cement brines, talline ThO (cr) prepared at 700(cid:176) C. At low pH, the solu-
2
8 V.Neck andJ.I. Kim
in the other studies, the thorium concentration was deter-
minedafterultrafiltration ata filterpore sizeof 2nm[8, 9,
29] to exclude colloidal particles.
Thefollowingcomparisonshowstheproblemconnected
with the evaluation of thermodynamic constants from the
experimentalsolubility data. Asshown in Fig.4b thesolu-
bilities of Th(IV) hydroxide or hydrous oxide determined
by Moon [29] and Felmy, Rai and Mason [9] at I (cid:53) 0.5
or 0.6M are in reasonable agreement. Despite of this, the
proposedsolubilityproductsdiffer bymanyorders ofmag-
nitude(cf.Table8).Raietal.[9,10]interpretethedecrease
of the thorium concentration with a slope of (cid:50)4 by as-
suming the non-hydrolyzed Th4(cid:49)(aq) to be the prevailing
aqueousspeciesandneglectedallhydrolysisspeciesexcept
Th(OH)(aq). This interpretation is not consistent with the
4
results of potentiometric titration and solvent extraction
studies [20(cid:50)24, 27], which clearly demonstrate that tho-
riumisstronglyhydrolyzedundertheseconditionswithpo-
lynuclear species being predominant. Moon [29] evaluated
the mononuclear hydrolysis constants from the solubility
data obtained with ThO (cr) in 0.1M NaClO . Assuming
2 4
the crystalline dioxide to be the solubility limiting solid
phaseoverthe wholepH range, theevaluated mononuclear
hydrolysis constants are several orders of magnitude too
large, as compared to those from other experimental me-
thods [20(cid:50)24,27].Since the solubilities inthe neutraland
alkaline range are practically the same for Th(OH) (am)
4
in 0.5M NaClO and ThO(cr) in 0.1M NaClO , Moon
4 2 4
concludes that the solubility products must be similar
as well. Using only slightly different constants log K¢
sp
and log (cid:226)¢ in 0.5M NaClO the higher solubilities of
1y 4
Th(OH)(am)atpH(cid:44)5arethendescribedsimplybyfitting
4
Fig.4.SolubilityofTh(IV)at17(cid:50)25(cid:176) Casafunction ofthe H(cid:49)con- an additional formation constants for the dimeric species
centration(a)in0.1MNaClO (above)and (b)atI (cid:53)0.5(cid:50)0.6mol/l Th(OH) 4(cid:49).Thisinterpretationdisregardsthatthehydroly-
4 2 4
(below).TheexperimentaldataarefromRefs.[8,9,28(cid:50)30].Thesolid sisequilibriainsolutionareindependent ofthe solidphase.
curves are calculated with log Ks(cid:176)p (cid:53)(cid:50) 47.0 for amorphous Th(IV) In the present review an attempt is made to evaluate a
hydroxide or hydrous oxide, the selected hydrolysis constants for
set of thermodynamic constants, which is consistent with
mononuclear species and log K¢ and log K(cid:176) from Brown et al.
4,12 6,15 both,hydrolysisconstants frompotentiometric titrationand
[20]. The upper and lower limits (dotted lines) are calculated with
logK(cid:176) (cid:53)(cid:50)46.2 and(cid:50)48.2,respectively. solvent extraction studies and solubilities of amorphous
sp
Th(IV) precipitates. For this purpose, the hydrolysis con-
stants log K¢ and log K¢ derived by Brown et al. [20]
4,12 6,15
bility data for ThO(cr)are essentially lowerthan those for (c.f. Table 2) from potentiometric titrations at pH (cid:53) 3(cid:50)4
2
Th(OH)(am), but at pH (cid:46)5 equal thorium concentrations in0.1KNO areusedincombinationwiththeselectedcon-
4 3
are observed. Similar observations are reported byÖsthols stants for the mononuclear hydrolysis species. From the
et al. [30] for the solubility of microcrystalline ThO · solubility data of amorphous Th(IV) hydroxideor hydrous
2
xH O(s)at25(cid:176) Cin0.5MNaClO .Inthisstudytheprecipi- oxide in the same pH range and at the same ionic strength
2 4
tateisdried atroomtemperaturefor oneweekin avacuum (0.1MNaClO )thefollowingsolubilityproductsarecalcu-
4
desiccator.X-raypowder diffractionindicates a lowdegree lated:
of crystallinity and the water content is determined to be
x (cid:60) 2.5. The solubility data measured at 17(cid:50)25(cid:176) C and Exp. data of Ryan andRai [8]:
I (cid:53)0.1M(NaClO ) or I(cid:53)0.5(cid:50)0.6M (NaClO,NaCland log Ks¢p (cid:53)(cid:50) 45.2 (log K(cid:176)sp (cid:53)(cid:50) 47.3)
4 4
KCl) are shown in Fig.4a and b, respectively.
Exp. data of Nabivanets and Kudritskaya [28]:
ThepH-independentsolubilitiesmeasuredbyMoonand log K¢ (cid:53)(cid:50) 44.7 (log K(cid:176) (cid:53)(cid:50) 46.8) .
Rai et al. in neutral and alkaline solutions (log [Th] (cid:53) sp sp
(cid:50)8.2(cid:54)0.3 in 0.1 and 0.5M NaClO [29], (cid:50)8.8(cid:54)0.2 in In Table 2, the hydrolysis constants log K¢ and log
4 4,12
0.1M NaClO [8] and (cid:50)8.5(cid:54)0.6 in 0.6M NaCl and KCl K¢ ofBrown etal. [20] are combinedwith literaturedata
4 6,15
[9]), are in fair agreement. The significantly higher so- in 1 and 3M NaClO to evaluate the constants at I (cid:53) 0
4
lubilities reported by Nabivanets and Kudritskaya [28] and the SIT parameters ˜(cid:229). These parameterizations imply
(log [Th](cid:53)(cid:50) 6.3at pH (cid:53) 5.5(cid:50)7 in0.1M NaClO,17(cid:176) C) unknown uncertainties, because the literature data refer to
4
may be ascribed to insufficient phase separation by centri- different hydrolysis schemes. Nevertheless, they allow the
fugation. These authors do not mention filtration, whereas estimation of log K¢ and log K¢ in 0.5M NaClO and
4,12 6,15 4
Solubilityandhydrolysisoftetravalent actinides 9
Table8.Differentsetsofthermodynamiccon-
stants (I (cid:53) 0) for modelling the solubility of Raietal. Moon [29]a Östholsetal. presentstudy
amorphousThO ·xH O(s)at25(cid:176)C [8,9] [30]c
2 2
logK(cid:176) (cid:50)45.5 (cid:50)52.9/(cid:50)53.6 (cid:50)48.7 (cid:50)47.0(cid:54)0.8
sp
log(cid:226)(cid:176) (cid:50) 13.3/13.8 10.9 11.8(cid:54)0.2
11
log(cid:226)(cid:176) (cid:50) 23.9/24.5 (cid:50) 22.0(cid:54)0.6
12
log(cid:226)(cid:176) (cid:50) 36.3/37.2 32.9 31.0(cid:54)1.0
13
log(cid:226)(cid:176) 36.7 44.7/45.4 42.1 38.5(cid:54)1.0
14
log(cid:226)¢ (cid:50) /59.1b
2,4
log(cid:226)(cid:176) 141.3d
4,12
log(cid:226)(cid:176) 176.0d
6,15
a: The conditional constantsreported by Moon[29] for 0.1M/0.5M NaClO are converted to
4
I(cid:53)0withtheSITcoefficientsinTable1.
b:In0.5MNaClO.
4
c: TheauthorsappliedthehydrolysisconstantsfromGrentheandLagerman[23]in3MNaClO
4
andtheirionicstrengthcorrectionsarebased onSITcoefficientssimilartothoseinTable1.
d:Theconstants(extrapolatedtoI(cid:53)0withtheSIT,cf.Table2)arebasedonexperimentaldata
in0.1MKNO [20],1MNaClO [21],and3MNaClO [23].
3 4 4
hence the evaluation of the solubility products in 0.5M pH 13 (cf. Fig.4) sets an upper limit for the formation of
NaClO from the data at pH (cid:44)5: the anionic complex Th(OH)(cid:50) (log (cid:226)(cid:176) (cid:44) 39.5).
4 5 15
Exp. data of Moon [29]:
log Ks¢p (cid:53)(cid:50) 43.5 (log K(cid:176)sp (cid:53)(cid:50) 46.6) Solubilityof crystalline ThO2(cr)
Exp. data of Östhols et al. [30]: The solubility data determined by Moon [29] in 0.1M
log K¢ (cid:53)(cid:50) 45.1 (log K(cid:176) (cid:53)(cid:50) 48.2) .
sp sp NaClO with ThO(cr)areshown in Fig.5.AtlowpHthey
4 2
The solubility measured by Östhols et al. [30] for micro- are comparablewith the results of Baeset al. [21] at95(cid:176) C
crystalline ThO .xHO(s) represents a lower limit for an in 1M NaClO .Thelatter data are measured afteraddition
2 2 4
amorphous precipitate. Atthis concentrationlevel, polynu- ofcrystalline ThO2(cr)tooversaturated acidicTh(IV)solu-
clear species are negligible. An absolute upper limit is ob- tions. In a most recent paper Bundschuh et al. [31] report
tAaicnceodrdwinigthtologthKess(cid:176)ep (cid:53)ca(cid:50)lcu4la6t.i2on(sc,f.thdeottseodlulbiinleitsyinproFdigu.c4tbo)f. t(hHeCslo/NluabCilli)tyanodf c2o5ll(cid:176)oCid.aIlnThthOis2(csrt)udpyartcioculelsomatetIri(cid:53)c p0H.5Mti-
amorphous Th(IV) hydroxide or hydrous oxide is con- trationiscombinedwithlaser-inducedbreakdowndetection
sidered to be
log K(cid:176) (cid:53)(cid:50) 47.0(cid:54)0.8 .
sp
In contrastto the largediscrepancies betweenthe solubility
productsreported bythedifferentauthors[8, 9,29,30] (cf.
Table8),thecorrespondinglog K(cid:176) valuesareinreasonable
sp
agreement if evaluated with the present set of hydrolysis
constants including the literature data for Th (OH) 4(cid:49) and
4 12
Th (OH) 9(cid:49). The polynuclear species Th (OH)6(cid:49) and
6 15 2 2
Th (OH) 8(cid:49)proposedintheliterature[21(cid:50)24]are foundto
4 8
be less suitable for modelling the experimental solubility
data discussed above. As the solubilities in the pH range
3.5to4.5decreasewithaslopeofabout(cid:50)4,theyaremuch
better fitted with a model that includes a hydroxide com-
plexof charge(cid:49)4.In thiscontext,itshould beemphasized
thatthehigh thoriumconcentrationsin thispH rangemight
also include a considerable amount of small colloids.
According to the solubilities in neutral to alkaline solu-
tions(log [Th(OH) (aq)](cid:53)(cid:50) 8.5(cid:54)0.6 [8,9, 29])the solu-
4
bility constant log K(cid:176) and the formation constant of
s(14)
Th(OH)(aq) are calculated as
4
logK(cid:176) (cid:53)(cid:50)8.5(cid:54)0.6 Fig.5.ExperimentalandcalculatedsolubilityofThO(cr)incompari-
s(14) 2
son with Th(IV) hydroxideor hydrous oxide.The experimental data
and for ThO(cr) are fromMoon [29](0.1M NaClO, 18(cid:176) C),Baeset al.
2 4
[21](1MNaClO,95(cid:176) C) andfromthe LIBD studyofBundschuh et
log(cid:226)1(cid:176) 4(cid:53)38.5(cid:54)1.0. al.[31](0.5MNa4Cl,25(cid:176) C).ThesolidlineiscalculatedforI(cid:53)0.5M
withtheselected hydrolysis constantsandthe experimental solubility
The latter value is consistent with log (cid:226)(cid:176) (cid:53) 39.0(cid:54)0.5
14 productforsmallThO2(cr)particles(about20nm).Thedashedlineis
from Ekberg et al. [24]. The constant solubility up to basedonthethermochemicalvalueoflogK(cid:176) (ThO(cr))(cid:53)(cid:50)54.2[50].
sp 2
10 V.Neck andJ.I. Kim
(LIBD) to determine the initial colloid formation as a
function of the H(cid:49) concentration in a series of 3·10(cid:50)2(cid:50)
9· 10(cid:50)5M thoriumsolutions.Consideringcolloidsas small
solid particles, their formation indicates that the total tho-
rium concentrationreaches or just exceedsthe solubility at
givenpH.Thesolubilityproductdeterminedbythismethod
isfoundtobelog K¢ (cid:53)(cid:50) 49.54(cid:54)0.22in 0.5MNaCl (log
sp
K(cid:176) (cid:53)(cid:50) 52.8(cid:54)0.3). Because of the small diameter (about
sp
20nm) of thethoriumcolloidsformed inthese pHtitration
experiments, the equations of Schindler [55] are used to
estimatetheeffectofparticlesizeonthe solubilityproduct.
Accordinglythesolubility productof a bulkThO (cr)crys-
2
tal is expected to be about 1.2 orders of magnitude lower
than that of the observed ThO colloids. This result is in
2
goodagreementwith logK(cid:176) (ThO (cr))(cid:53)(cid:50)54.2(cid:54)1.3[50]
sp 2
as calculated fromthermochemical data.
In Fig.5 the solubility of ThO (cr) is calculated as a
2
function of the H(cid:49) concentration with the hydrolysis con-
stants selected in section 2. The two curves obtained with
the experimental solubility product of log K(cid:176) (ThO
sp 2
(coll)) (cid:53)(cid:50) 52.8 [31] (solid line) and the thermochemical
value of log K(cid:176) (ThO (cr)) (cid:53)(cid:50) 54.2 [50] (dashed line)can
sp 2
be considered as upper and lower limits, respectively. At
pH (cid:44)2.5, the experimental solubility data for bulk or col-
loidalThO(cr)decrease with a slopeof (cid:50)4.Thisindicates
2
that the experimental data actually refer to the equilibrium
between ThO (cr) and Th4(cid:49)(aq). However, with increasing
2
pH the solubility data measured by Moon [29] with
ThO(cr) deviate more and more from the expected solu-
2
bilityofThO(cr)andatpH(cid:46)6theybecomeequaltothose
2
ofTh(OH) (am).Comparableresults areobtainedwhen the
4
LIBD experiment of Bundschuh et al. [31] is extended to
lower thorium concentrations. Obviously the hydrolysis of Fig.6. Solubility of UO2(s) as a function of the H(cid:49) concentration at
the Th4(cid:49)(aq) ion at pH (cid:46)2.5 leads to increased Th(IV) 20(cid:50)25(cid:176) C,(a) atI (cid:53)0.03(cid:50)0.2M(above), (b)in1MNaCl (below).
TheexperimentaldataarefromRefs.[10,11,56(cid:50)58].Thesolidlines
concentrations, whicharenotinequilibriumwith ThO2(cr). are calculated for 0.1 and 1M NaCl, respectively, with log Ks(cid:176)p (cid:53)
For the pH range, where An(OH) (aq) is the predominant (cid:50)54.5(cid:54)1.0andthehydrolysisconstantsselectedinthepresentpaper.
4
aqueousspecies,itmustbeconcludedthatthebulk crystal- The dotted lines show the range of uncertainty. The dashed line is
line solid is covered with an amorphous surface layer of calculatedwiththemodel proposedbyRaietal.[10, 56].
Th(OH)(am).
4
magnitudebelow those observedby Raiet al. [10, 56] and
3.2 Solubility of U(IV)
Grambow et al. [57] for fresh precipitates.
The solubilities reported in the literature for hydrous In addition to the uncertainty involved in the solid
UO ·xH O(s), amorphous or microcrystalline UO (s) are phase,Raietal. [10,56]pointoutthedifficultytomaintain
2 2 2
extremely scattered. Aspointed outin the NEAreview[7], properlyreducingconditions.Eventracesofdissolvedoxy-
they probably do not refer to a unique material, but rather gen cause at least partly oxidation of U(IV) to U(VI). It is
to a range of solids with different thermodynamic stabili- clearly demonstrated that the high solubilities reported by
ties. The experimental data in solutions of I (cid:53) 0.03(cid:50) Bruno et al. [59] and Gayer and Leider [60] (log [U] (cid:53)
0.2mol/l and in 1.0M NaCl are shown in Fig.6a and b, (cid:50)4.4(cid:54)0.4 at pH 5(cid:50)10 in 0.5M NaClO and (cid:50)5.3 to
4
respectively.Raiet al.[10,56] determined thesolubilityof (cid:50)4.2 in alkaline NaOH solutions, respectively) are caused
freshly precipitated UO ·xH O(am) in NaCl and MgCl by oxidized U(VI) species. Rai et al. [56,58] estimate the
2 2 2
solutions of various ionic strength. The solid was X-ray concentration of U(OH) (aq) in equilibrium with
4
amorphous before and after the solubility measurements. UO ·xHO(am) to be about 10(cid:50)8.0mol/l, and a number of
2 2
Fe powder and EuCl were added to prevent U(IV) from data with greater concentrations in the neutraland alkaline
2
oxidation. An earlier study was performed in alkaline range are ascribed to the presence of U(VI). As discussed
NaOH/Na S O solutions containing Zn powder [58]. In by Rai et al. [56, 58] there is no experimental verification
2 2 4
comparablestudiesofYajimaet al.[11](in0.1MNaClO ) for the formation of U(OH) (cid:50) at high pH as claimed in
4 5
and Grambow et al. [57] (in 1M NaCl), redox conditions earlier papers [1, 60].
were controlled electrochemically. Yajima et al. [11] ob- In order to evaluate the solubility product of
served an increase in crystallinity with the time of aging. UO ·xHO(am), the present calculations refer to the solu-
2 2
Their solubilties in the range pH (cid:44)5 are several orders of bility data of Rai et al. [10, 56] and Grambow et al. [57]