Leach Models for Contaminants Immobilized by pH-Dependent Mechanisms
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A component-based leach model for describing leaching of hazardous contaminants that are immobilized by pH-dependent reactions is presented, and applications to environmental problems are described. The model describes simple chemical systems in which a solid phase provides acid-neutralizing capacity, and the contaminants are immobilized by precipitation or sorption reactions that are dependent on pH. Results of model simulations are related to predictions based on simple analytical leach models. Internal concentration gradients within the solid are predicted by the model, and their forms are used to explain how contaminants are immobilized and re- immobilization within the solid and how the concentration of acid in the leaching solution affects leaching of contaminants. Simulations show that the concentration of acid in the leaching solution affects leaching of contaminants in a complex manner that cannot be simply related to leaching of acid-neutralizing capacity. Even with these complex chemical interactions, the simulation results show that simple half-order models can be used to describe leaching if the observed diffusivity can be measured accurately. However, simulations show that conventional leaching tests such as the ANS 16.1 Procedure are not likely to measure accurately the observed diffusivity when applied to contaminants that are effectively immobilized.
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