DERIVATION OF PITZER INTERACTION PARAMETERS FOR AN AQUEOUS SPECIES PAIR OF FECITRATE- AND NA+

The Waste Isolation Pilot Plant (WIPP) is a deep underground repository for the disposal of transuranic (TRU) radioactive waste developed by the U.S. Department of Energy (DOE). The WIPP is located within the bedded salts of the Permian Salado Formation, which consists of interbedded halite and anhydrite layers overlaying the Castile Formation. The waste includes, but is not limited to, the salts of citric acid and iron.

To calculate the solution chemistry for brines of WIPP-relevance, WIPP Performance Assessment (PA) employs the Pitzer formulation to determine the activity coefficients of aqueous species in brine. The current WIPP thermodynamic database, however, does not include iron species and their Pitzer parameters, in spite of the fact that there will be a large amount of iron in the WIPP. Iron would be emplaced as part of the waste, as well as the containers for the waste.

The objective of this analysis is to derive the Pitzer binary interaction parameters for the pair of Na⁺ and FeCitrate^-. Briefly, an aqueous model for dissolution of Fe(OH)₂(s) in Na₃Citrate solution was fitted to the experimentally measured solubility data. The aqueous model consists of several chemical reactions and related Pitzer interaction parameters. Specifically, Pitzer binary interaction parameters for the Na⁺ and FeCitrate^- pair (β⁽⁰⁾, β⁽¹⁾, and C^φ) were fitted to the experimental data. Anoxic gloveboxes were used to keep the oxygen level low (less than 6 ppm) throughout the experiments. Aging time was more than 900 days to ensure the equilibrium.

EQ3NR packaged in EQ3/6 v.8.0a calculates the aqueous speciation and saturation index using an aqueous model addressed in EQ3/6’s database. The saturation index indicates how far the system is from equilibrium with respect to the solid of interest. Thus, the smaller the sum of squared saturation indices that the aqueous model calculates for the given number of experiments, the more closely the model attributes equilibrium to each individual experiment with respect to the solid of interest.

The calculation of aqueous speciation and saturation indices was repeated by adjusting β⁽⁰⁾, β⁽¹⁾, and C^φ in the database until the values are found that make the sum of squared saturation indices the smallest for the given number of experiments. Preliminary values are 1.389, -48.28, and -0.3675, respectively.