LEACHING OF CD AND PB FROM A POLLUTED SOIL DURING THE PERCOLATION OF EDTA - LABORATORY COLUMN EXPERIMENTS MODELED WITH A NONEQUILIBRIUM SOLUBILIZATION STEP

Chelating agents such as EDTA, when present in water percolating throu gh polluted soils; are capable of solubilizing heavy metals, increasin g their downward mobility to groundwater as they form water-soluble an d negatively charged complexes. The mobilization and movement of heavy metals in an:EDTA flux is investigated with both laboratory experimen ts and mathematical modeling. The solubilization of Cd and Pb is monit ored during the percolation of EDTA, in both pulse and step modes, thr ough columns filled with a contaminated soil. The experimental breakth rough curves (BTCs) are used to validate a numerical model that links solute transport of EDTA and EDTA-metal chelates to the metal solubili zation process. An implicit difference scheme is used to solve advecti on-dispersion equations for free and complexed EDTA with, for both, in clusion of a second-order kinetic law to express the extraction reacti on. The hydrodispersive parameters of the column are calculated using the bromide BTCs. The ability of the model to simulate various EDTA in jection modes (pulse or step) at different chelate concentrations is t ested by fitting the kinetic rate constant K of solubilization with th e observed Cd and Pb BTCs. Pulse and step experiments give the same K values, 2.4 x 10(-6) s(-1) for Cd and 2.1 x 10(-5) s(-1) for Pb. The m odel accounts for the diminishing metal extraction efficiency as the m etal in the solid is depleted or as the available EDTA concentration d ecreases. Methodological suggestions for necessary investigations prio r to remediation operations are described.