Two top soils (0-30 cm) were sampled from Burg El-Arab (calcareous soil) and Moshtohor (alluvial, non calcareous soil). These soils besides their separated clay fractions, the organic matter-free soils and two commercially clay minerals namely attapulgite and bentonite were subjected to equilibration with solutions having different Cd/Ca ratios varied from 0.1:99.9 to 8:92. Soluble Cd and Ca were determined in equilibrium solution as well as on the exchangeable materials. Selectivity of the studied exchange materials for both Cd and Ca were calculated. The obtained results could be summarized in the following:
Values of exchangeable Cd fractions differed depending on Cd/Ca ration in equilibrium solution and type of exchangeable materials. Values of selectivity coefficient revealed always higher preference for Ca relative to Cd although very slightly higher affinity for Cd was shown by attapulgite over bentonite
Cd adsorption by all the exchange materials obeyed Fruendlish adsorption isotherm indicating the adsorption is the main process by which Cd is retained to soils at the concentrations normally encountered in soils or even polluted soils. It may be concluded that, soils and then different components especially of the clay minerals such as bentonite and attapulgite may be used as tools for immobilizing additives of heavy metals e.g. Cd through exchange with exchangeable cations on soil colloids such as Ca2+.