Competitive and Non-competitive Adsorption Characteristics of Nickel and Cadmium in Two Soils

ABSTRACT: Adsorption isotherms studies were conducted on two soils (an acidic and a basic soil) for single (non-competitive) and binary (competitive) systems of nickel (Ni) and cadmium (Cd). Soil samples were equilibrated with solutions containing eight concentrations ranged from 0.15 to 3 mM for 24 h, the suspensions were centrifuged and the solution was filtered and the filtrate was analyzed for Ni and Cd. The adsorption data for metals were described by parameters through fitting the adsorption data to the Langmuir and the Freundlich isotherms. The fit of experimental data for both metals to the Freundlich data was better than the Langmuir model.The quantity of monolayer adsorption capacity, qm, derived from the Langmuir model in basic soil was higher than acidic soil. In both soils the amount of qm for Cd under competitive condition with Ni was lower than non-competitive condition. Decrease in adsorption capacity of Cd under competitive condition was attributed to the occupation of some adsorption sites by Ni. Changes in the qm values of Ni under different treatments was negligible. Bonding energy coefficient, KL, for Ni was higher in basic soil in comparison with acidic soil. In both soils the amount of KL for Ni under competitive condition with Cd was higher than non-competitive condition.The quantity of KF value, the coefficient related to the adsorption capacity derived from the Freundlich isotherm, for Ni was higher in basic soil as compared to acidic soil. Similar trend was observed for Cd. In both acidic and basic soils, the quantity of KF value for Ni was higher than Cd, demonstrating higher sorption capacity of the studied soils for Ni in comparison with Cod.