# 2009

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Marian Borkowski Michael Richmann



# COMPARISON OF RECENT THORIUM THERMODYNAMIC DATA WITH THOSE USED IN THE WIPP FMT_050405.CHEMDAT DATABASE

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### Comparison of Recent Thorium Thermodyn amic 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 comp are 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 repo rt) used for actinide(IV) solubility modeling in WIPP brine. In this review, three areas were assessed: inorganic complexation data, Pitzer interaction parameters and orga nic 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 repor ted in the recent lite rature [1, 2] since
the noted lower value for th e pentacarbonato complex has no significant consequence,

2. Stability constants for thorium sulfate aqueous species and solids in the FMT database are greater than those re ported in the literature [2],

3. The FMT database remains as the most compre hensive 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, ar e not comparable to the FMT database.

4. Published stability constants for Th(IV) co mplexation with selected organic ligands cannot be directly compared with those in FMT since they only relate to lower ionic strengths. The FMT database thermodynamic st ability 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 comp lexation in defining the solubility of Th(IV). In this context, the identified di screpancies for these da ta do not significantly affect the WIPP actinide(IV) solubility model.4 1. Introduction

In this review the thermodynamic constants calcu lated 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 Da tabase [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 aque ous and solid species is the dimensionless standard chemical potential . Therefore, you will have to convert the dimensionless standard chemical po tentials 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,

HCO 3 - ⇄ CO 3 2- + H +

.

Log K for the dissociation constant for HCO 3 - is then given by the following equation:
log K =

 [(  o /RT) CO 3 2  + (  o /RT) H +

 (  o /RT) HCO 3  ] / 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 reactio ns with inorganic ligands

The inorganic aqueous and solubi lity-limiting species for Th(IV) ar e listed in Table 2 (column “Th(IV) Reactions”). Thermodynamic stability constants for the re actions listed in the column “log K 0 FMT” were calculated using standard ch emical 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

H 2 O -95.6635
OH - -63.435
H + 0
Na + -105.651
K + -113.957
SO 4 2- -300.386
HSO 4 - -304.942
HCO 3 - -236.751
CO 3 2- -212.944
Th 4+ -284.227 Th(CO 3 ) 5 6- -1411.378 Th(OH) 3 (CO 3 ) - -775.627 Th(OH) 4 (aq) -626.5853 Th(SO 4 ) 2 (aq) -911.69 Th(SO 4 ) 3 2- -1214 Th(Ac) 2 2+ -604.8 ThAc 3+ -448.525 ThCit + -285.898 ThEDTA(aq) -285.419 Th(C 2 O 4 ) 2+ -297.428 ThO 2 (am) -451.408 Th(SO 4 ) 2 .9H 2 O(s) -1775.9 Th(SO 4 ) 2 .8H 2 O(s) -1680 Th(SO 4 ) 2 .Na 2 SO 4 .6H 2 O(s) -2011.29 Th(SO 4 ) 2 .K 2 SO 4 .4H 2 O(s) -1837.57 Th(SO 4 ) 2 .2K 2 SO 4 .2H 2 O(s) -2181.81 2[Th(SO 4 ) 2 .7/2K 2 SO 4 (s)] -5581.666 Table 2. Complexation reactions and correspond ing stability constants or solubility products for thorium species expected under WIPP conditions. Th(IV) Reactions Aqueous Species log K 0

FMT log K NIST[1] log K 0 Therm. of Th [2] ThO 2 (am) + 2H 2 O ⇄ Th(OH) 4 (aq)

ThO 2 (am) + 2H 2 O ⇄ Th 4+ + 4OH - (am) - amorphous -7.0 -55.6 (0.5)* -8.1 *

-46.7  0.9 fresh

-47.5  0.9 aged

Th 4+ + 4OH - ⇄ Th(OH) 4 (aq)

Th 4+ + 4H 2 O ⇄ Th(OH) 4 (aq) + 4H + 38.5

-17.5 40.2 (3.0)* 38.6 *

-17.4  0.7

Th 4+ + 3OH - + CO 3 2- ⇄ Th(OH) 3 CO 3 -

Th 4+ + CO 3 2- + 3H 2 O ⇄ Th(OH) 3 CO 3 - +3H + 38.3

-0.5 (0)* 38.3  0.7 (40.8  0.6 include colloids)

Th 4+ + 5CO 3 2- ⇄ Th(CO 3 ) 5 6- 27.1 32.3 (0)* 31.0  0.7

Th 4+ + 2SO 4 2- ⇄ Th(SO 4 ) 2 (aq) 11.6 9.69  0.27

Th 4+ + 3SO 4 2- ⇄ Th(SO 4 ) 3 2- 12.4 10.75  0.076
Solids log K s 0 log K s log K s 0

Th 4+ + 2SO 4 2- + 9H 2 O ⇄ Th(SO 4 ) 2  9H 2 O(s) 13.0 11.28  0.14

Th 4+ + 2SO 4 2- + 8H 2 O ⇄ Th(SO 4 ) 2  8H 2 O(s) 12.9 11.42  0.50

Th 4+ + 2Na + + 3SO 4 2- + 6H 2 O ⇄ Th(SO 4 ) 2  Na 2 SO 4  6H 2 O(s) 17.6 13.84  0.17

Th 4+ + 2K + + 3SO 4 2- + 4H 2 O ⇄ Th(SO 4 ) 2  K 2 SO 4  4H 2 O(s) 18.1 15.0  0.05

Th 4+ + 4K + + 4SO 4 2- + 2H 2 O ⇄ Th(SO 4 ) 2  2K 2 SO 4  2H 2 O(s) 21.2 18.67  0.22

Th 4+ + 7K + + 5.5SO 4 2- ⇄ Th(SO 4 ) 2  3.5K 2 SO 4 (s) 24.7 22.38  0.39

* In parentheses are listed ionic strengths for constants re ported 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 pK w = 14.0.7 Discussion of inorganic ligan d interactions with thorium

The ThO 2 (am) dissolution constant reported in the NI ST database as log K = -55.6 measured in
0.5 M NaClO 4 is quite different than the constants reported in Chemical Thermodynamics of
Thorium and used in the FMT model. This cons tant was rejected from our assessment as an outlier and was also not accepted in the recentl y 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 th at there are small differences in dissolution constants for fresh and aged ThO 2 (am) solids.

The constant for the formation of the pentacarbona to 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 da tabase. 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 im portant role. One of these complexes, Th(OH) 3 CO 3 - , is present in the FMT database. The stab ility constant for this complex is in good agreement with those reported in both the NI ST database and Chemical Thermodynamics of Thorium. It may be advisable, purely as a simp lification of the current model, to replace the chemical potential currently used in the FM T model with the value that includes colloid formation corresponding to log K 0 = 40.8. Such an approach could eliminate an enhancement factor for An(IV) colloids. The stability consta nts for other ternary hydroxy-carbonate complexes of thorium, e.g. Th(OH)(CO 3 ) 4 5- , Th(OH) 2 CO 3 (aq), Th(OH) 2 (CO 3 ) 2 2- and Th(OH) 4 CO 3 2- , 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. A dditionally, Pitzer parameters for these species were not given in the review [2], but we cannot exclude the possibili ty of data that could provide a basis for estimating these parameters.

Other species considered in the model are the th orium 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, the8 most important reactions fo r thorium speciation at pC H+ ~ 9.5 are hydrolysis and carbonate complexation. Thorium sulfate complexes and thor ium sulfate solids are less important to the WIPP chemistry. However, they may contribute to so me extent to the total picture. The presence of thorium-sulfate thermodynamic data makes th e FMT database more robust and extends the applicability of the FMT mode l to broader range of pH.

3. Pitzer ion-interaction parameters for thorium species

Ion-interaction Pitzer parameters are not av ailable in the NIST database. In Chemical Thermodynamics of Thorium[2], SIT (Specifi c Ion Interaction Theo ry) parameters are predominantly used for ionic strength corrections . The SIT parameters are not relevant to the FMT database, since in the FMT calculations, Pit zer ion-interaction parameters are used. In Chemical Thermodynamics of Thorium, the only P itzer parameters listed are for the interaction
of Th 4+ 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]. Thor ium 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 or ganic 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 calculate d 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 da ta for organic ligands is very difficult, because the calculated FMT constants are thermodyna mic, but other values reported in the NIST9 Table 3. Logarithm of stability constants of thorium with diff erent organics as a function of NaCl ionic strength publis hed 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 ThAc 3+ 4.41  0.02 3.85  0.02 3.92  0.03 4.26  0.03 4.29  0.03 4.51  0.03 ThAc 2 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 ThCitr 2 2- 18.72  0.04 18.28  0.03 17.41  0.06 17.33  0.04 - 19.12  0.04 ThOx 2+ 7.08  0.04 7.07  0.02 7.04  0.02 7.13  0.01 7.26  0.03 7.47  0.02 ThOx 2 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 denote s citrate and Ox denotes oxalate ions.

Table 4. Thermodynamic stability constants fo r thorium–organic complexes used in the FMT model and reported in the NIST database. Complex ThL n log  n 0 FMT log  n (  ) NIST [1] log  n 0 Therm. of Th [2] ThAc 3+ 7.4 The same as in SAND99-0943 NA ThAc 2 2+ 11.2 The same as in SAND99-0943 NA ThCitr + 15.2 11.61(0.1 M Na + ) NA ThCitr 2 2- - 21.14(0.1 M Na + ) NA ThOx 2+ 11.4 8.8(0.1 M Na + ) NA ThOx 2 (aq) - 16.8(1.0 M NH 4 + ) NA ThOx 2 .6H 2 O(s) - -21.38(1.0 M NH 4 + ) NA ThEDTA 23.6 22.3(0.5 M Na + ) NA - where Ac denotes acetate, Citr de notes 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 ar e greater than those reported in the NIST database and SAND99-094 3 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 r eactions for thorium speciation at pC H ~ 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 Consta nts of Metal Complexes, NIST standard Reference Database 46 Version 8.0 (2004).

2. Chemical Thermodynamics of Thorium, OE CD 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 an d Development of Modeling Parameters . SAND 99- 0943 (2001). Albuquerque: Sandi a National Laboratories.

4. Roy et al. “Activity coefficients in electrolyte mixtures: HCl + ThCl 4 + H 2 O for 5-55 o C.” J. Phys. Chem. , vol. 96, pp.11065-11072 (1992).