An aqueous thermodynamic model for the Pb2+–Na+–K+–Ca2+–Mg2+–H+–Cl−–SO42−–H2O system to high concentration: application to WIPP brines

The development of an aqueous thermodynamic model for the Pb2+–Na+–K+–Ca2+–Mg2+–Cl−–SO42−–H2O system is presented, which is valid to high ionic strengths at 25°C. The model is based on the equations of Pitzer and has been parameterized from existing solubility, osmotic, electromotive force (emf), and spectroscopic data. To accurately represent the aqueous thermodynamics of Pb2+ in concentrated chloride containing solutions required the inclusion of four Pb chloride species (i.e., PbCl+, PbCl2(aq), PbCl3−, and PbCl42−) along with the necessary Pitzer ion interaction parameters for these species with the major electrolyte ions. The reliability of the final equilibrium model is tested against experimental solubility data on PbCl2(c) and PbSO4(c) in high ionic strength Waste Isolation Pilot Plant (WIPP) brines obtained as part of this study. On an overall basis the model accurately predicted the aqueous speciation, based on comparisons with our UV-Vis spectroscopy measurements, as well as the observed solubility. The model also proved satisfactory in predicting the observed solid phase assemblages, with the possible exception of those found in solutions high in KCl.