Environmental assessment of a cement-based solidified soil contaminated with lead

The use of equilibrium-based and mass transfer rate extraction tests has been proposed to provide an integrated assessment of leaching processes from stabilized/solidified wastes. The objectives of the research presented here are to validate this assessment approach and to evaluate the applicability of (i) using models assuming diffusive controlled release or coupled dissolution and diffusion for estimating rates of constituent release from cement solidified matrices, and (ii) using results from batch equilibrium leaching tests and physical characterization to obtain parameters for release rate models. The test matrix consisted of soil fines contaminated with lead during manufacturing operations that were subsequently treated by two methods of Portland cement solidification/stabilization. Fundamental leaching parameters (i.e., acid neutralization capacity of the waste, constituent solubility as a function of pH, pore water properties and, release rates fi:om monolithic leach tests) were measured. Results were interpreted according to behavioral models based on mechanistic understanding of leaching processes. Results showed that (i) a good assessment of the release of contaminants from cement-based S/S treated wastes can be obtained by integrated use of equilibrium-leaching tests and mass transfer leaching tests, (ii) equilibrium-leaching tests are sensitive to the extent of particle size reduction and contact time and need therefore to be adapted according to waste properties co ensure equilibrium, (iii) mass transfer leaching tests need to be defined according to the specified scenario, (iv) estimation of the activity coefficient within the waste pore water is necessary for accurate prediction of constituent release rates, (v) results need to be interpreted according to the considered scenario and the relevant behavioral model,(vi) the release of lead was governed by solubilization phenomenon at the solid-liquid interface (i.e., at the interface between the matrix and the leaching solution), and(vii) when release of lead is controlled by solubilization at the leachant-monolith interface, the release flux of lead can be directly calculated from the release flux of hydroxide and consequently from pH data measured in the leachate. (C) 1999 Elsevier Science Ltd. All rights reserved.