A mathematical model is proposed and compared with experimental data for the migration of heavy metal ions (Pb and Cd) during the flow of aqueous solutions through calcium-saturated montmorillonite packing. The main system parameters are pH, flow rate and concentration. It is assumed that the Liquid phase in the porous montmorillonite can be divided into two regions, mobile and immobile; that adsorption and ion exchange on the clay surface are instantaneous, and that adsorption isotherms are Linear throughout the modelling considered. The Ranges of experimental parameters are: pH 2.5-7.3, solution flow rate of 0.1-1 cm(3) min(-1) and heavy metal concentrations of 5-10 ppm. Breakthrough curves (BTCs) obtained experimentally are affected by changes in pH and flow rates. Between pH 2.5 and 4, the model equations describe the process quite well but from 4 to 7 a large deviation between the model and experimental data is observed due to precipitation of metals. (C) 1999 Society of Chemical Industry.