Solubilization of lead and cadmium during the percolation of EDTA through a soil polluted by smelting activities

EDTA was percolated in laboratory columns through a soil polluted by heavy metals to investigate the efficiency of and processes involved in soil decontamination by chemical extraction. At high EDTA concentration (10−2 M), elution of Pb and Cd was very efficient for one pore volume, after which it decreased to almost zero due to depletion of available Pb and Cd and to competition with Ca and Fe slowly solubilized during the passage of the EDTA front. Clogging occurred after the end of the EDTA plateau. At lower EDTA concentrations (10−3 and 10−4 M), elution was less efficient, but extraction decreased little with the volume percolated; moreover no Ca above background values was dissolved. The optimum EDTA concentration for heavy metal extraction ranges between 10−2 and 10−3 M. The higher the concentration, the greater the extraction efficiency, but as the EDTA concentration is increased there is an optimum point at which clogging takes place and permeability decreases.