A speciated, conventional Pitzer ion-interaction model for the aqueous Nd3+–H+–Na+–K+–Ca2+–Cl−–OH− system at 298 K and 0.1 MPa☆
Introduction
Trivalent americium and plutonium account for more than 99% of the total radioactivity slated for disposal at the U.S. Department of Energy’s (USDOE) Waste Isolation Pilot Plant (WIPP), and present the greatest risk of environmental radiologic release from the geologic repository (USDOE, 2019a). Due to the inherent difficulties in acquiring high-quality thermodynamic data for aqueous solutions containing trivalent actinides [An(III)], neodymium or other lanthanide [Ln(III)] compounds are used as analogs in thermodynamic measurements and geochemical models of repositories hosted in evaporite deposits (Felmy et al., 1989, Felmy et al., 1990, Fanghänel et al., 1999, Neck et al., 2009). Ln(III) hydroxides and mixtures with chloride salts are also of interest to the processing of rare-earth element (REE) ores and intermediate solids produced during refining, e.g., REE(OH)3.
In this study, a conventional, speciated, Pitzer ion-interaction model is developed to predict the behavior of An(III) compounds in geologic environments near 298 K and 0.1 MPa. The resulting thermodynamic model accurately reproduces the properties of NdCl3(aq), and mixtures containing evaporite salts, over a range of pH, including those typical of geologic nuclear waste repositories that use engineered chemical barriers to minimize transport of actinides. The model is compared to a published speciated Pitzer model that is currently used for geochemical modeling in support of recertification of evaporite-hosted nuclear waste repositories.
Section snippets
Model description and parameter development strategy
The Pitzer activity coefficient model (Pitzer, 1973) is a virial type, excess Gibbs free-energy function with the form:where ww = 1 kg H2O, R = 8.31446 J·(mol·K)−1, T is temperature, m is molality, and I is ionic strength. A detailed description of Equation (1) and further thermodynamic relationships such as individual solute activity and osmotic coefficients are given in Pitzer (1991, pgs 84–93) and Appendix (A3) in Harvie et al. (1984) (hereafter
Validation of the HMW model parameters for supporting electrolyte systems
Model parameters for the underlying systems relevant to this study: HCl + H2O, NaCl + H2O, KCl + H2O, CaCl2 + H2O and all related “ternary” interaction parameters, e.g., , , , , , are from Harvie et al. (1984).
To avoid over-fitting, the upper molality bounds of the HMW model for the individual binary subsystems were determined by comparison to osmotic coefficients from high-quality measurements and comprehensive standard models. This was necessary as Harvie and
Previous Pitzer models for aqueous NdCl3
Conventional Pitzer ion-interaction models for the completely dissociated NdCl3 + H2O system have been presented by Pitzer and Mayorga, 1973, Pitzer et al., 1978, Kodýtek and Dolejš, 1986, Kim and Frederick, 1988, and May et al. (2011). Because these models do not include liquid phase equilibria, they do not accurately reproduce the pH of NdCl3(aq) solutions and do not reproduce the data within its accuracy over the full molality range. The Kodýtek and Dolejš (1986) model is invalid because it
Review of the Könnecke model for mixtures (M = Na+, Ca2+)
Könnecke et al. (1997) assumed that the mixture parameters, , , and were necessary and arbitrarily assigned = 0.2. Values for and were then calculated relative to (Table 2). They likewise assumed = 0.1 but did not calculate values for and . Although binary parameters involving charged species would generally be considered to be more significant than interactions involving neutral species, Könnecke et al. (1997)
Revised Pitzer model for the speciated NdCl3 + H2O system
Our model was derived using the INSIGHT software code developed by Sterner et al., 1997, Sterner et al., 1998. INSIGHT contains the NONLIN and GMIN codes (Felmy, 1990, Felmy, 1995, Felmy et al., 1990) that are “qualified” for WIPP applications. In contrast to NONLIN, INSIGHT includes temperature and pressure functions that allow fitting temperature and pressure-dependent Gxs properties such as enthalpy, heat capacity, and density. The only difference between the HMW implementation of the Pitzer
Speciated Pitzer model for mixtures (M = H+, Na+, K+, Ca2+)
After producing an initial fit to the NdCl3 + H2O data the ternary parameter was fitted to the NdCl3 + HCl + H2O emf and solubility data from Roy et al., 2005b, Shevtsova et al., 1968. The term was not necessary. Following the recommendation of Mioduski et al. (2009), the 298.15 K Spedding et al. (1976) NdCl3·6H2O solubility of = 3.835 mol·kg−1 was favored over the measurements of Shevtsova et al. (1968). Our ultimately fitted NdCl3·6H2O solubility is = 3.837 mol·kg
Solubility model development for Nd(OH)3 solids
Because Nd(OH)3(s) solubility in water and dilute NaCl(aq) is < 0.01 mol·kg−1 (Fig. 18) and Nd3+ comprises >80 mole% of the neodymium solute species at pH ≤ 7 (Fig. 19), most of the Nd(OH)3(cr) solubility data of Rao et al., 1996, Silva, 1982 can be reproduced through the addition of only the term. This result is similar to that of Felmy et al. (1989) and our fitted lg K = 14.95 for the protonation reaction,Nd(OH)3(cr) + 3H+ = Nd3+ + 3H2Ois similar to the Rao et al. (1996)
Conclusion
The geologic disposal of nuclear wastes related to nuclear power and weapons production and retirement is a challenging problem for present and future generations. Whether a repository is in salt, crystalline igneous rock, or argillaceous sediment, a common concern to the disposal concepts being considered by the international community is the need to simulate reactions between groundwaters of varying salinity, nuclear waste materials, and the rocks surrounding the repository. This study
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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