Relating Batch and Column Diffusion Coefficients for Leachable Contaminants in Particulate Waste Materials

For constructed facilities in which waste materials are used as partial substitutes for traditional aggregates, it is usually necessary to perform contaminant leachability tests to assess the long-term emission of contaminants from the facilities during service. Such tests can be performed under batch and column flow-through conditions. It is usually desirable to establish the relationship between leached contaminant concentrations obtained through both tests. Using Al and Cu diffusion coefficients as the target parameter, an analytical model is developed and presented herein with experimental data from acidic solution (pH of 4.5) leach testing of asphalt concrete that was amended with municipal solid waste incinerator bottom ash in weight percentages ranging from 0 to 20 to assess the equivalence of both sets of leaching conditions. The results for Al show that at higher column flowrates indexed by Peclet numbers in excess of 5.5, there is no defined relationship between Al diffusion coefficients obtained through both methods. Fluid flow at lower Peclet numbers approach batch conditions and afford an opportunity for the use of the analytical model presented in this paper provided comparisons are made at equivalent liquid/solid ratios. The values of the batch diffusion coefficients obtained for Al are of the order of 10^-10-10^-6 cm^2/s. For column leaching of Al, the range is 10^-8-10^-7 cm^2/s. No measurable quantity of Cu was obtained under both batch and column leaching conditions.