Contaminant Leaching From Cement-based Waste Forms under Acidic Conditions

Abstract

<p>A waste form can be prepared by mixing a hydraulic cement and, if needed, a bulking agent with an aqueous waste to cause it to solidify. A mechanistic leaching model was developed based on describing the chemistry of the waste form-Ieachant system and the flow regime of the leachant and assuming that transport takes place via diffusive exchanges through the waste form-Ieachant Interface. This model was successful in predicting Ieaching from simpIe waste matrices and in identifying important containment mechanisms effective in the more complex matrices.</p> <p>The cement-based waste forms studied had porosities ranging from 40 to 60%. Portland cement provided acid neutralization capacity to maintain the high pH environment where the waste form is stable; a typical waste form contains enough cement to neutralize between 2000 to 3000 times its volume of a pH 3 leachant.</p> <p>In a mild environment, leaching was controlled by the diffusion of the soluble fraction of a contaminant present in the connected pores of the matrices. Immobile species instantaneously solubilized to maintain chemical equilibrium between the soluble and insoluble fractions. In tests conducted over a period of almost 2 years, less than 1% of the initial amount of cadmium, chromium and lead contained in a specimen leached out.</p> <p>In an acidic environment, the leaching rates were limited by the availability of acid to dissolve the matrix. The leaching process was similar to a surface corrosion process. However, since waste matrices are not completely soluble in a mild acid, a leached layer develops, eventually protecting the core of the waste form from direct contact with aggressive groundwaters. Contaminants solubilized at the leaching front are subjected to concentration gradients that force them to diffuse both inward and outward. Only a fraction of the total concentration therefore leaches out.</p> <p>The mechanistic knowledge developed through the experimental programme and the modelling effort was used to develop eight long term leaching scenarios covering a wide range of waste form and groundwater conditions. In addition to allowing the prediction of leaching rates for periods of up to 100 years, these scenarios were useful in developing recommendations to prepare more efficient waste forms and design better landfills.</p>