Adsorption-Desorption Studies of Cadmium in Three Different Soil Orders

Sorption isotherms have been widely used to assess the heavy metals retention characteristics of soil particles. Adsorption behaviour of cadmium (Cd) in soils is an important process which exerts a major influence on its uptake by plant ischolar_mains. Desorption behaviour of the retained metals, however, usually differ from that of adsorption, leading to a lack of coincidence in the experimentally obtained adsorption and desorption isotherms. Three soils differing in physico-chemical properties (pH 5.7 to 8.2) and varied taxonomy (Typic Ustochrepts, Typic Rhodustalfs and Entic Chromusterts), were subjected to Cd treatment at various concentrations (0, 2, 4, 8, 15, 30, 45, 75 mg Cd/L). The Cd adsorbed by each soil was calculated as the difference between the amount of Cd present in the solution initially and that remaining after equilibration. Immediately after adsorption, desorption took place using successive dilution method with five consecutive desorption steps. Both, Cd adsorption and desorption data were described by Freundlich equation. The adsorption and desorption reactions, however, did not provide the same isotherms, indicating that hysteresis occurred in Cd adsorption-desorption process. Results indicated that the adsorption capacity of the soils for Cd increased with an increase in the pH or alkalinity of the soils. The rate of adsorption was, however, found to decrease with increased pH. But no specific trend was observed in case of desorption. All the three soils used in this study followed Freundlich adsorption isotherms. The adsorption data, in general, indicated that Cd was in a fixed form at higher pH levels. The Cd adsorption was in the order of Entic Chromusterts > Typic Ustochrepts > Typic Rhodustalfs.