Adsorption-Desorption Studies of Cadmium in Three Different Soil Orders

摘要

Sorption isotherms have been widely used to assess the heavy metals retention characteristics of soilparticles. Adsorption behaviour of cadmium (Cd) in soils is an important process which exerts a majorinfluence on its uptake by plant roots. Desorption behaviour of the retained metals, however, usuallydiffer from that of adsorption, leading to a lack of coincidence in the experimentally obtained adsorptionand desorption isotherms. Three soils differing in physico-chemical properties (pH 5.7 to 8.2) andvaried taxonomy (Typic Ustochrepts, Typic Rhodustalfs and Entic Chromusterts), were subjected toCd treatment at various concentrations (0, 2, 4, 8, 15, 30, 45, 75 mg Cd/L). The Cd adsorbed by eachsoil was calculated as the difference between the amount of Cd present in the solution initially and thatremaining after equilibration. Immediately after adsorption, desorption took place using successivedilution method with five consecutive desorption steps. Both, Cd adsorption and desorption data weredescribed by Freundlich equation. The adsorption and desorption reactions, however, did not providethe same isotherms, indicating that hysteresis occurred in Cd adsorption-desorption process. Resultsindicated that the adsorption capacity of the soils for Cd increased with an increase in the pH oralkalinity of the soils. The rate of adsorption was, however, found to decrease with increased pH. Butno specific trend was observed in case of desorption. All the three soils used in this study followedFreundlich adsorption isotherms. The adsorption data, in general, indicated that Cd was in a fixed format higher pH levels. The Cd adsorption was in the order of Entic Chromusterts > Typic Ustochrepts >Typic Rhodustalfs.