Most riverine sediments have the high capability to adsorb and store the heavy metal ions. In the present study, the adsorption capacity of the bed sediments collected from Karaj River (Iran), have been experimentally studied for the cadmium ion adsorption. Multi-stage batch adsorption experiments were carried out for a constant sediment concentration of 20 g/L and different initial cadmium concentrations of 0.2, 0.5, 1, 10, 20, and 50 mg/L. The cadmium solutions with known concentration were added to the bed sediment with the mean diameter of 0.53 mm in five stages to characterize what is the capacity of the sediments to adsorb cadmium. The batch adsorption experiments were conducted as both kinetics and equilibrium. The results showed that by adding cadmium ions to the sediment at each stage, the adsorption capacity is less than the previous stage, so that, for cadmium concentration of 0.2 mg/L the adsorption percent and the amount of adsorbed cadmium was reduced from 88 to 70% and 9 to 6.8 mg/kg, respectively. These changes was decreased with increasing initial Cd concentrations. This process is useful for the seasonal rivers in which a certain concentration of heavy metal pollution may occasionally flow over the bed.
