(180i) Determination of Pitzer Parameters for Pb(II) and Fe(II) Solubilities in Concentrated Brines in the Presence of Environmental Ligands

Assessing the performance of the U. S. Department of Energy's Waste Isolation Pilot Plant (WIPP), a deep geologic repository for defense-related transuranic waste in southeastern New Mexico, requires a detailed understanding of the response of concentrated brines to both minerals found in the surrounding rock formation and man-made materials that are emplaced in the repository. The geochemistry model developed for the WIPP is based on the activity coefficient model of Kenneth Pitzer. Parameter sets are available for many of the chemical species in the repository, including both common brine constituents such as Na⁺, Mg²⁺, HCO₃^?, SO₄^2? and Cl^?, as well as radionuclides such as Am(III) and Th(IV). In some scenarios iron and lead could be available in significant quantities in the repository through their use in waste packaging materials. There is limited information available in the literature for Pb(II) and Fe(II) in the complex brines observed at the WIPP. In addition, because of the basic pH and the strongly reducing conditions expected in the repository, both Pb(II) and Fe(II) are expected to form complexes with inorganic ligands such as hydroxide and chloride, and organic ligands such as EDTA and oxalate.

Consequently, we have undertaken an experimental and modeling effort to determine Pitzer parameters for the interactions of Pb(II) and Fe(II) complexes with other major species in the brine (Na⁺, Mg²⁺, Cl^?, HCO₃^?, and SO₄^2?). Experimental measurement of Pb(II) and Fe(II) solubilities in relatively simple solutions are combined with a free-energy minimization scheme to determine the new Pitzer parameters for each pair of species studied. Subsequent validation testing will assess the influence of the new parameters for Pb(II) and Fe(II) complexes on the WIPP geochemical model.

Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. This research is funded by WIPP programs administered by the Office of Environmental Management of the U.S Department of Energy.