Adsorption of cadmium on bed sediments of river Hindon: Adsorption models and kinetics

The adsorption characteristics of cadmium on bed sediments of river Hindon in western Uttar Pradesh (India) have been studied. The effect of various operating variables, viz., initial concentration, solution pH, sediment dose, contact time, particle size and temperature has been studied. The optimum equilibration time was found to be 60 min, which was independent of initial concentration of cadmium ions. The adsorption curves were smooth and continuous leading to saturation, suggesting the possible monolayer coverage of cadmium ions on the surface of the adsorbent. The adsorption of cadmium increased with an increase in pH. Furthermore the adsorption of cadmium increased with increasing adsorbent concentration and decreased with adsorbent particle size. The important geochemical phases, iron and manganese oxide, support the adsorption of cadmium ions. The adsorption data were analysed using the Langmuir and Freundlich adsorption models to determine the mechanistic parameters related to the adsorption process. Thermodynamic parameters, viz., free energy change, enthalpy change and entropy change, were also determined. The negative values of free energy change (DeltaGdegrees) indicated the spontaneous nature of the adsorption and positive values of enthalpy change (DeltaGdegrees) suggested the endothermic nature of the adsorption process. The intraparticle diffusion of cadmium through pores in the adsorbent was found be the main rate-limiting step.