Competitive retention of lead and cadmium on an agricultural soil.

Lead and cadmium contamination of an agricultural soil has been studied using batch and column experiments. Thermodynamics of the retention phenomena may be represented by a Langmuir isotherm for an aqueous metal concentration up to 100 mg L(-1). First order kinetics with respect to the solid phase yield good predictability for both batch and column experiments. Kinetics and thermodynamics of lead retention predominate over those of cadmium. As a consequence, lead is preferentially retained and can even displace sorbed cadmium. In the event of an spill involving both metals, cadmium would move further in the soil and its aqueous concentration downstream could be even higher than that of the influent solution, increasing potential risks. A two-region model has been used to fit all the experimental results. Satisfactory predictions for column experiments are obtained with parameters which are consistent with those obtained for the batch experiments, for which sorption is described by a Langmuir isotherm including competitive retention.