PNNL

Abstract

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 25C. The model is based upon 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 high chloride solutions required the inclusion of four Pb chloride species (i.e., PbCI+,PbCI2(aq),PbCI3 and PbCI42) 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 PbCI2(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 upon comparisons with our UV-Vis spectroscopy measurements, as well as the observed solubility's. The model also proved satisfactory in predicting the observed solid phase assemblages, with the possible exception of solutions high in KCI.