Leaching of Cd and Pb from a Polluted Soil during the Percolation of EDTA:  Laboratory Column Experiments Modeled with a Non-Equilibrium Solubilization Step

Abstract

Chelating agents such as EDTA, when present in water percolating through polluted soils, are capable of solubilizing heavy metals, increasing their downward mobility to groundwater as they form water-soluble and negatively charged complexes. The mobilization and movement of heavy metals in an EDTA flux is investigated with both laboratory experiments and mathematical modeling. The solubilization of Cd and Pb is monitored during the percolation of EDTA, in both pulse and step modes, through columns filled with a contaminated soil. The experimental breakthrough curves (BTCs) are used to validate a numerical model that links solute transport of EDTA and EDTA−metal chelates to the metal solubilization process. An implicit difference scheme is used to solve advection−dispersion equations for free and complexed EDTA with, for both, inclusion of a second-order kinetic law to express the extraction reaction. The hydrodispersive parameters of the column are calculated using the bromide BTCs. The ability of the model to simulate various EDTA injection modes (pulse or step) at different chelate concentrations is tested by fitting the kinetic rate constant K of solubilization with the observed Cd and Pb BTCs. Pulse and step experiments give the same K values, 2.4 × 10-6 s-1 for Cd and 2.1 × 10-5 s-1 for Pb. The model accounts for the diminishing metal extraction efficiency as the metal in the solid is depleted or as the available EDTA concentration decreases. Methodological suggestions for necessary investigations prior to remediation operations are described.