A sulfate and darapskite solubility model with Pitzer interaction coefficients for aqueous solutions containing NaNO2, NaNO3, and NaOH

• Molal-based Pitzer model coefficients developed.
• Sulfate-anion interaction parameters are temperature independent.
• Gibbs energies and enthalpies of darapskite determined.
• Sodium sulfate salt enthalpies consistent with structural model.

Alkaline nuclear waste, and many other industrial processes, have sodium sulfate salts immersed in aqueous electrolyte solutions containing nitrate, nitrite, and hydroxide ions. A model is required to predict the solubility of sulfate salts in these environments. In order to develop a model for sulfate solubility, this study developed molal-based Pitzer model coefficients for interactions between sulfate and nitrate, nitrate and hydroxide. Model parameters were determined by optimizing them until the difference between predicted and measured solubilities was minimized. The sulfate-anion binary Pitzer parameters were all found to be temperature independent. The reduced chemical potentials of the double salt darapskite (Na3NO3SO4*H2O) as a function of temperature was also required and determined here by fitting the model to solubility data. The reduced chemical potentials for darapskite and other sodium sulfate salts were converted to standard state Gibbs energies and enthalpies. These thermodynamic quantities were consistent with other estimates in the literature when available.