A dynamic approach to the assessment of leaching behavior

This paper describes a set of experiments that were used to investigate the mechanisms that control the rate of leaching from municipal solid waste incinerator ashes and a numerical approach to the estimation of mass transfer coefficients. A small column reactor was used for the experiments, and chemical equilibrium speciation software was used to assist in analyzing the leachate composition. It was found that certain major ions and trace metals were limited by the solubility of various solid phases, while other major ions were limited in concentration by physical transport processes. The numerical model was able to satisfactorily predict the influence of the controlling solid phases, and was able to estimate the governing mass transfer coefficients. For example, the model accurately predicts that the release of lead from the ash is controlled early on (the first approximately 50 pore volumes) by the solubility of PbSO_4(s), and for the following approximately 3000 pore volumes was controlled by the solubility of Pb(OH)_2(s). Effective diffusion coefficients were significantly lower for trace metals and reactive major ions than for salts, indicating sorptive retardation within the ash matrix may be occurring.