2009

LANL‐CO ACRSP

ACP‐0911‐01‐03‐01

Marian Borkowski

Michael Richmann

COMPARISON OF RECENT THORIUM

THERMODYNAMIC DATA WITH

THOSE USED IN THE WIPP

FMT_050405.CHEMDAT DATABASE  

LA‐UR 09‐07298                                             
2 
Comparison of Recent Thorium Thermodynamic Data with Those Used in the 

WIPP FMT_050405.chemdat Database 

Marian Borkowski and Michael Richmann 

October 27, 2009 

The goal of this review is to critically compare the new thorium thermodynamic data published 

recently in the literature [1, 2, 3] with those present in the FMT_050405.chemdat database 

(referred to as the FMT database in this report) used for actinide(IV) solubility modeling in 

WIPP brine. In this review, three areas were assessed: inorganic complexation data, Pitzer 

interaction parameters and organic complexation constants. As a result of this assessment we 

found that: 

1. Thermodynamic data used in the FMT database for hydrolysis and carbonate 

complexation are consistent with the data reported in the recent literature [1, 2] since 

the noted lower value for the pentacarbonato complex has no significant consequence, 

2. Stability constants for thorium sulfate aqueous species and solids in the FMT 

database are greater than those reported in the literature [2], 

3. The FMT database remains as the most comprehensive listing of Pitzer parameters for 

thorium. In the “Chemical Thermodynamics of Thorium” [2], SIT parameters were 

given for modeling ionic strength effects and these do not extend to WIPP-relevant 

ionic strengths and, in this context, are not comparable to the FMT database. 

4. Published stability constants for Th(IV) complexation with selected organic ligands 

cannot be directly compared with those in FMT since they only relate to lower ionic 

strengths. The FMT database thermodynamic stability constants appear to be greater 

than those present in the literature [2, 3] and if so, introduce additional conservatism 

to the WIPP model. 

5. The complexation of thorium with organics and sulfate is much less important than 

the effects of hydrolysis and carbonate complexation in defining the solubility of 

Th(IV). In this context, the identified discrepancies for these data do not significantly 

affect the WIPP actinide(IV) solubility model. 
4 

1. Introduction 

In this review the thermodynamic constants calculated on the basis of the standard chemical 

potentials for the appropriate species present in the FMT database were compared with those 

from the Critical Stability Constants NIST Database [1] and those reported in Chemical 

Thermodynamics of Thorium [2]. 

In response to Don Reed’s request of April 30, 2006, L. Brush wrote “... the FMT database does 

not contain formation constants. Instead, the data entered for each aqueous and solid species is 

the dimensionless standard chemical potential. Therefore, you will have to convert the 

dimensionless standard chemical potentials for the aqueous species to their formation constants, 

if you need them. As an example, according to Yong-Liang, consider the equilibrium constant 

(log K) for the second dissociation reaction of carbonic acid, 

HCO3

- ⇄ CO32- + H+

. 

Log K for the dissociation constant for HCO3

-

 is then given by the following equation: 

 log K = [(o/RT)CO3

2

 + (o/RT)H+  (o/RT)HCO3



] / 2.3026” (1) 

The approach suggested by Brush was used for the thermodynamic stability calculations using 

the FMT database. The thorium species and associated standard chemical potentials imported 

from the FMT_050405.chemdat database are reported in Table 1. 

2. Major thorium complexation reactions with inorganic ligands 

The inorganic aqueous and solubility-limiting species for Th(IV) are listed in Table 2 (column 

“Th(IV) Reactions”). Thermodynamic stability constants for the reactions listed in the column 

“log K0

 FMT” were calculated using standard chemical potentials for appropriate species 

imported from the FMT database (see Table 1) and equation (1). 
5 

Table 1. Thorium species and dimensionless standard 

chemical potentials present in FMT_050405.chemdat 

database used to calculate the thermodynamic stability 

constants shown in Table 2. 

Species μ°i/RT 

 H2O -95.6635 

 OH- -63.435 

 H+ 0 

 Na+ -105.651 

 K+ -113.957 

 SO4

2- -300.386 

 HSO4

-

 -304.942 

 HCO3

-

 -236.751 

 CO3

2- -212.944 

 Th4+ -284.227 

 Th(CO3)5

6- -1411.378 

 Th(OH)3(CO3)

-

 -775.627 

 Th(OH)4(aq) -626.5853 

 Th(SO4)2(aq) -911.69 

 Th(SO4)3

2- -1214 

 Th(Ac)2

2+ -604.8 

 ThAc3+ -448.525 

 ThCit+ -285.898 

 ThEDTA(aq) -285.419 

 Th(C2O4)

2+ -297.428 

 ThO2(am) -451.408 

 Th(SO4)2.9H2O(s) -1775.9 

 Th(SO4)2.8H2O(s) -1680 

 Th(SO4)2.Na2SO4.6H2O(s) -2011.29 

 Th(SO4)2.K2SO4.4H2O(s) -1837.57 

 Th(SO4)2.2K2SO4.2H2O(s) -2181.81 

 2[Th(SO4)2.7/2K2SO4(s)] -5581.66 
6 

Table 2. Complexation reactions and corresponding stability constants or solubility 

products for thorium species expected under WIPP conditions. 

Th(IV) Reactions 

Aqueous Species 

log K0

FMT

log K 

NIST[1] 

log K0

Therm. of Th [2] 

ThO2(am) + 2H2O ⇄ Th(OH)4(aq) 

ThO2(am) + 2H2O ⇄ Th4+ + 4OH- 

(am) - amorphous 

-7.0 

-55.6 (0.5)* 

-8.1*

-46.7  0.9 fresh 

-47.5  0.9 aged 

Th4+ + 4OH- ⇄ Th(OH)4(aq) 

Th4+ + 4H2O ⇄ Th(OH)4(aq) + 4H+

38.5 

-17.5 

40.2 (3.0)* 38.6*

-17.4  0.7 

Th4+ + 3OH- + CO3

2- ⇄ Th(OH)3CO3- 

Th4+ + CO3

2- + 3H2O ⇄ Th(OH)3CO3-

 +3H+ 

38.3 

-0.5 (0)* 

38.3  0.7 

 (40.8  0.6 

include colloids) 

Th4+ + 5CO3

2- ⇄ Th(CO3)56- 27.1 32.3 (0)* 31.0  0.7 

Th4+ + 2SO4

2- ⇄ Th(SO4)2(aq) 11.6 9.69  0.27 

Th4+ + 3SO4

2- ⇄ Th(SO4)32- 12.4 10.75  0.076 

Solids log Ks

0

 log Ks log Ks

0

Th4+ + 2SO4

2- + 9H2O ⇄ Th(SO4)29H2O(s) 13.0 11.28  0.14 

Th4+ + 2SO4

2- + 8H2O ⇄ Th(SO4)28H2O(s) 12.9 11.42  0.50 

Th4+ + 2Na+ + 3SO4

2- + 6H2O ⇄ 

Th(SO4)2Na2SO46H2O(s) 

17.6 13.84  0.17 

Th4+ + 2K+ + 3SO4

2- + 4H2O ⇄ 

Th(SO4)2K2SO44H2O(s) 

18.1 15.0  0.05 

Th4+ + 4K+ + 4SO4

2- + 2H2O ⇄ 

Th(SO4)22K2SO42H2O(s) 

21.2 18.67  0.22 

Th4+ + 7K+ + 5.5SO4

2- ⇄

Th(SO4)23.5K2SO4(s) 

24.7 22.38  0.39 

* In parentheses are listed ionic strengths for constants reported in Critical Stability Constants NIST Database 46. 

Other constants are listed as the thermodynamic constants, i.e. extrapolated to zero ionic strength. Constants 

printed in italic font are calculated in this assessment using the reported constant and pKw = 14.0. 
7 

Discussion of inorganic ligand interactions with thorium 

The ThO2(am) dissolution constant reported in the NIST database as log K = -55.6 measured in 

0.5 M NaClO4 is quite different than the constants reported in Chemical Thermodynamics of 

Thorium and used in the FMT model. This constant was rejected from our assessment as an 

outlier and was also not accepted in the recently published NEA review[2]. Other constants for 

the formation of the hydroxy-complexes are in good agreement with those used in the FMT 

model. It is necessary to note that there are small differences in dissolution constants for fresh 

and aged ThO2(am) solids. 

The constant for the formation of the pentacarbonato complex used in the FMT model is 4 orders 

of magnitude smaller than reported in Chemical Thermodynamics of Thorium and 5 orders of 

magnitude smaller than listed in the NIST database. The pentacarbonato complex is not likely 

the most important carbonate complex under WIPP-relevant conditions because of the low 

carbonate concentration expected in the WIPP. Therefore, other ternary thorium complexes with 

carbonate and hydroxide may play a more important role. One of these complexes, 

Th(OH)3CO3

-

, is present in the FMT database. The stability constant for this complex is in good 

agreement with those reported in both the NIST database and Chemical Thermodynamics of 

Thorium. It may be advisable, purely as a simplification of the current model, to replace the 

chemical potential currently used in the FMT model with the value that includes colloid 

formation corresponding to log K0

 = 40.8. Such an approach could eliminate an enhancement 

factor for An(IV) colloids. The stability constants for other ternary hydroxy-carbonate complexes 

of thorium, e.g. Th(OH)(CO3)4

5-, Th(OH)2CO3(aq), Th(OH)2(CO3)22- and Th(OH)4CO32-

, are 

also reported in this data review [2] and the presence of these species could improve the accuracy 

but not the conservatism of the FMT model. Additionally, Pitzer parameters for these species 

were not given in the review [2], but we cannot exclude the possibility of data that could provide 

a basis for estimating these parameters. 

Other species considered in the model are the thorium sulfate complexes and solids. All of these 

stability constants and solubility products are about two orders of magnitude greater as used in 

the FMT database than reported in Chemical Thermodynamics of Thorium. In our opinion, the 
8 

most important reactions for thorium speciation at pCH+ ~ 9.5 are hydrolysis and carbonate 

complexation. Thorium sulfate complexes and thorium sulfate solids are less important to the 

WIPP chemistry. However, they may contribute to some extent to the total picture. The presence 

of thorium-sulfate thermodynamic data makes the FMT database more robust and extends the 

applicability of the FMT model to broader range of pH. 

3. Pitzer ion-interaction parameters for thorium species 

Ion-interaction Pitzer parameters are not available in the NIST database. In Chemical 

Thermodynamics of Thorium[2], SIT (Specific Ion Interaction Theory) parameters are 

predominantly used for ionic strength corrections. The SIT parameters are not relevant to the 

FMT database, since in the FMT calculations, Pitzer ion-interaction parameters are used. In 

Chemical Thermodynamics of Thorium, the only Pitzer parameters listed are for the interaction 

of Th4+ with Cl-

 and are taken from Roy et al. 1992 [4]. These same binary and ternary 

parameters are used in the FMT database. The FMT database is currently the most 

comprehensive collection of WIPP-relevant Pitzer interaction parameters. 

4. Complexation of thorium by organic ligands 

 The major source of thorium complexation with organic ligand data as a function of NaCl 

ionic strength is the SAND99-0943 report [3]. Thorium data published in this report is collected 

in Table 3. Data for thorium complexation with EDTA is not available in this report. In Chemical 

Thermodynamics of Thorium, complexation with organic ligands is not discussed. Therefore, the 

FMT database can only be compared to the NIST database and to the data in the SAND99-0943 

report. The FMT stability constants were calculated using the standard chemical potential for the 

appropriate species and are given in Table 4. 

Discussion of organic ligand interactions with thorium 

 The comparison of the thorium thermodynamic data for organic ligands is very difficult, 

because the calculated FMT constants are thermodynamic, but other values reported in the NIST 
9 

Table 3. Logarithm of stability constants of thorium with different organics as a function 

of NaCl ionic strength published in the SAND99-0943 report [3]. 

Complex 0.3M NaCl 1 M NaCl 2 M NaCl 3 M NaCl 4 M NaCl 5 M NaCl 

ThAc3+ 4.41  0.02 3.85  0.02 3.92  0.03 4.26  0.03 4.29  0.03 4.51  0.03 

ThAc2

2+ 7.47  0.03 6.56  0.03 6.82  0.03 7.19  0.02 7.30  0.03 7.66  0.03 

ThCitr+ 9.67  0.03 9.56  0.03 9.31  0.04 9.55  0.02 10.07  0.02 10.18  0.03 

ThCitr2

2- 18.72  0.04 18.28  0.03 17.41  0.06 17.33  0.04 - 19.12  0.04 

ThOx2+ 7.08  0.04 7.07  0.02 7.04  0.02 7.13  0.01 7.26  0.03 7.47  0.02 

ThOx2 13.63  0.04 13.42  0.04 13.49  0.05 13.83  0.06 13.95  0.04 14.63  0.03 

- where Ac denotes acetate, Citr denotes citrate and Ox denotes oxalate ions. 

Table 4. Thermodynamic stability constants for thorium–organic complexes used in the 

FMT model and reported in the NIST database. 

Complex ThLn log n

0

FMT 

log n() 

NIST [1] 

log n

0

Therm. of Th [2] 

ThAc3+ 7.4 The same as in SAND99-0943 NA 

ThAc2

2+ 11.2 The same as in SAND99-0943 NA 

ThCitr+ 15.2 11.61(0.1 M Na+) NA 

ThCitr2

2- - 21.14(0.1 M Na+

) NA 

ThOx2+ 11.4 8.8(0.1 M Na+) NA 

ThOx2(aq) - 16.8(1.0 M NH4

+

) NA 

ThOx2.6H2O(s) - -21.38(1.0 M NH4

+

) NA 

ThEDTA 23.6 22.3(0.5 M Na+) NA 

 - where Ac denotes acetate, Citr denotes citrate, and Ox denotes oxalate ions 

database and SAND99-0943 report are the apparent stability constant measured at specific ionic 

strengths. A very rough comparison suggests that the FMT constants are greater than those 

reported in the NIST database and SAND99-0943 report and, in this context, introduce 

additional conservatism to the model. The most important constants are for the complexation of 

thorium with EDTA and these are in reasonable agreement. 

 In our opinion, the most important reactions for thorium speciation at pCH ~ 9.5 are 

hydrolysis and carbonate complexation. Therefore, thorium complexes with organic ligands are 

less important to the WIPP chemistry model and do not make a significant contribution to the 

total solubility of thorium. 
10 

5. References 

1. NIST Critically Selected Stability Constants of Metal Complexes, NIST standard 

Reference Database 46 Version 8.0 (2004). 

2. Chemical Thermodynamics of Thorium, OECD Nuclear Energy Agency, Data Bank Issyles-Moulineaux, France (2007). 

3. Choppin, G.R., A.H. Bond, M. Borkowski, M.G. Bronikowski, J.G. Chen, S. Lis, J. 

Mizera, O. Pokrovski, N.A. Wall, Y.X. Xia, and R.C. Moore. WIPP Actinide Source Term 

Test Program: Solubility Studies and Development of Modeling Parameters. SAND 99-

0943 (2001). Albuquerque: Sandia National Laboratories. 

4. Roy et al. “Activity coefficients in electrolyte mixtures: HCl + ThCl4 + H2O for 5-55o

 C.” 

J. Phys. Chem., vol. 96, pp.11065-11072 (1992).