Extraction kinetics of six heavy metals from contaminated clay soils

The rate of the extraction of four heavy metals (Cu, Nl, Pb and Zn) from artificially polluted and waste site clay soils is determined in a 0.1 N HCl solution. A two‐reaction model is developed that describes the rate of extraction at a constant pH and at a constant temperature by two processes: a fast, irreversible reaction that is first‐order in the metal concentration in the clay and a slow, reversible first‐order reaction. The influence of the temperature on the extraction rate is measured over a range of 5 to 80°C for Ni extraction from an artificially polluted clay soil. Calculated values for the energy of activation suggest that the extraction process is rate‐limited by a diffusion process in the porous soil particles, but chemical limitations seem probable in case of the older contaminations in the waste site soils.