Adsorption of Metal Ions on Bed Sediments

Abstract The adsorption characteristics of cadmium on bed sediments of the River Ganga (India) have been studied to determine the tolerance of the river system for the heavy metal load. The effect of various controlling parameters, initial concentration, solution pH, sediment dose, contact time and particle size, have been evaluated. The optimum contact time needed to reach equilibrium was approximately 60 minutes and was independent of the initial concentration of cadmium ions. The adsorption curves are smooth and continuous leading to saturation, suggesting a possible monolayer coverage of cadmium ions on the surface of the adsorbent. The extent of adsorption increases with pH. Furthermore, the adsorption of cadmium increases as adsorbent doses increase and as adsorbent particle size decreases. The two geochemical phases, iron and manganese oxides, probably support the adsorption of cadmium ions. The adsorption data have been analyzed by using the Langmuir and Freundlich adsorption models to determine the mechanistic parameters of the adsorption process. Isotherms have been used to obtain thermodynamic parameters, free energy change (ΔGo), enthalpy change (ΔHo) and entropy change (ΔSo). The negative values of free energy change (ΔGo) indicate the spontaneous nature of the adsorption of cadmium on bed sediments, and the positive values of enthalpy change (ΔHo) suggest the endothermic nature of the adsorption process. The uptake of cadmium is controlled by bulk as well as intraparticle diffusion mechanisms.