Prediction of metal leaching rates from solidified/stabilized wastes using the shrinking unreacted core leaching procedure

The use of cement based stabilization techniques is a common and cost-effective method for stabilization of heavy metals-contaminated sludges and soils. Little is actually known, though, about the mechanisms involved with the release, or leaching, of contaminants from the stabilized waste. Most studies of leaching behavior of cement based systems have assumed that bulk diffusion from the monolith is the main driving force for contaminant release. Recent research has shown that leaching of contaminants is actually a result of the dissolution of the outer shell of the waste form, which results in a solubilization and release of contaminants from the leached shell. The leaching behavior has been successfully modelled as a shrinking unreacted core (SUC). This model incorporates the concept of acid exposure, rather than time, as the master variable in evaluating leaching behavior. The rate of contaminant leaching is controlled by the inward diffusion of acid species into the alkaline depleted leached shell. In this research, the behavior of 'real-world' solidified wastes was studied in order to verify that previously observed behavior in synthesized waste forms applied equally as well to real world wastes. Cored samples of stabilized wastes taken from two sites were used to evaluate a new test procedure designed to model shrinking core behavior. Metals leaching behavior was evaluated as a function of the exposure. (C) 1997 Elsevier Science B.V.