The Development of Aqueous Thermodynamic Models: Application to Waste Tank Processing and Vadose Zone Issues.

Abstract

The presence of a wide range of radionuclides, metal ions, inorganic ligands, and organic chelating agents combined with the high base and electrolyte concentration in the Hanford waste tanks creates some unique and difficult problems in modeling the aqueous thermodynamics of these solutions. Solving these problems is important since this can lead to better strategies for tank processing and predictions of subsurface transport. In addition, a large number of scientists and engineers at Hanford and other sites rely on these models for making accurate predictions of tank chemistry. In developing accurate thermodynamic models for these solutions one of the most important factors is identifying the aqueous species present. Identifying these species in tank solutions presents some difficult challenges since, (1) current analytical methods for examining speciation in simple solutions, such as UV-VIS, Raman, etc., develop difficulties in interpreting or analyzing spectral features w hen multiple complexes are present and contribute to the observed spectra, (2) the solubility of the compounds of many elements, including the actinide elements, is very low under high base conditions. Determining the aqueous speciation in such solutions, which is still needed to predict changes in solubility, is currently done largely by curve- fitting solubility data to hypothesized speciation schemes. Clearly, new and improved approaches to this problem are needed. In order to circumvent these difficulties we have implemented a comprehensive approach which involve coupled experimental solubility studies, spectroscopic measurements of solution speciation, molecular modeling studies which yielded information on species structure and energetics, and thermodynamic modeling efforts using the Pitzer thermodynamic model which is valid to high ionic strengths. In developing this approach we have classified the speciation reactions in tank solutions into three areas: simple inorganic speciation reactions involving the addition of one or more ligands to a metal center, organic chelate binding to a metal center where the chelate completely/incompletely wraps the metal center, and systems where the binding ligand polymerizes forming multiple species which can interact with the metal ion.