Equilibrium and Kinetics of Cadmium and Zinc Adsorption on Activated Carbon from Palm Kernel Shell

A series of batch laboratory studies were conducted in order to investigate the suitability of palm kernel shell (PKS)-based activated carbon for the removal of cadmium (cadmium ions) and zinc (zinc ions) from their aqueous solutions. Single and binary adsorption isotherms and kinetics were investigated. All batch experiments were carried out at a constant temperature of 30+1°C using an incubator shaker that operated at 150 rpm. A concentration ratio of cadmium and zinc used in the adsorption of binary system is 1:1. Batch equilibrium studies showed that adsorption of cadmium and zinc were highly pH dependent. Uptake of cadmium and zinc increased with pH with maximum removal observed at pH 7.0, and decreased thereafter with further increase in the initial pH. In a single component system, the adsorption capacity of cadmium was higher than zinc, whilst in a binary component system, the adsorption capacity of zinc was higher than cadmium. The single component equilibrium data was analyzed using the Langmuir, Freundlich and Redlich–Peterson isotherms. Overall, the Redlich-Peterson isotherm showed the best fit for all adsorption under investigation in terms of correlation coefficient as well as error analysis. For all the systems studied, the analysis of isotherm shape factor showed that the adsorption was favorable. Equilibrium isotherms for the binary adsorption of cadmium (II) and zinc (II) ions onto PKS-base activated carbon have been analyzed by using the ideal adsorbed solution (IAS) theory, extended Langmuir (EL), modified extended Langmuir (MEL), modified extended Freundlich (MEF) and modified extended Redlich-Peterson (MERP) models. The binary isotherm data could be described reasonably well by the modified extended Langmuir model. The kinetics of single and binary systems describing the sorption of cadmium and zinc have been investigated. The rate models evaluated include the pseudo first order, the pseudo-second order and the intraparticle diffusion models. Kinetic data showed that the adsorption rates were a function of initial adsorbate concentration. For both single and binary systems, the pseudo-second order model correlate excellently the experimental data, suggesting that chemisorption processes could be the rate- limiting step.

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