A leach model for solidified/stabilized waste forms based on empirical partitioning of contaminants

Abstract

An Empirical Partitioning Leach Model (EPLEM), has been developed to describe the leaching of metal contaminants from solidified wastes. The model considers interactions between two components such as hydrogen ion and a metal. The hydrogen ion component in the bath diffuses into the solid to neutralize the high pH of the pore water. As a result, the metal contaminant mobilizes and leaches out from the solid into the bath. Empirical equations are used to describe chemical equilibrium in the model. One equation describes the partitioning of a metal contaminant between mobile and immobile phases in the solidified waste as a function of pH. Two types of partitioning are modeled. One describes reactions that are like precipitation. The other describes reactions that are like sorption. The second equation describes the change of solid phase acid neutralizing capacity (SANC) as a function of pH. This reaction determines pH of the pore water. These equations are combined with an existing diffusive transport model (Batchelor 1997) to make EPLEM. This model assumes that components are transported due to Fickian diffusion with local chemical equilibrium in the porous solid. The dynamic diffusion equation was solved by a modified Crank-Nicholson algorithm. The model has been applied to simulate a laboratory leach test (Monolith Leach Test) that was conducted by Kosson (I 993). The results of the model agree well with the laboratory leach data for copper, chromium, lead and zinc. Little difference was observed between simulations that assumed precipitation-like partitioning or sorption-like partitioning. Sensitivity analysis of four parameters was conducted to evaluate their effects on simulation results, which were expressed as observed investigate contaminant leaching behavior under the infinite bath assumption. Results were compared to experimental data from the Monolith Leach Test, which was conducted in a finite bath with specified leachate renewal frequency. Simulation results indicate that experimental data for Monolith Leach Test did not agree with simulation results of infinite bath condition over long-term period.