Chromium(VI) adsorption by sawdust carbon : Kinetics and equilibrium

The ability of sawdust carbon to remove chromium from aqueous solution by adsorption was investigated according to equilibrium and kinetics. The extent of removal of Cr(VI) is dependent on concentration, pH and temperature of the solution. With an initial concentration of 100 mg/L and at 60°C and pH 2.5, the removal was found to be 49.8 mg/g. The intra-particle diffusion of Cr(VI) through pores in the adsorbent was sbown to be the main rate limiting step. The higher uptake at pH 2.5 was attributed to chemical reduction of Cr(VI) to Cr(III) coupled with physico-chemical adsorption of Cr(VI) species. The Langmuir and Freundlich adsorption isotherms were used to represent the experimental data. The Langmuir and Freundlich constants were calculated at different temperatures and the adsorption capacity for Cr(VI) increases with temperature. The method was applied on synthetic wastewaters. Treatment of the exhausted carbon with 0.1 M NaOH removed only 87.2% of the adsorbed chromium, suggesting that the binding to the carbon involved strong chemisorption forces.