Hexavalent chromium removal from aqueous medium by activated carbon prepared from peanut shell: Adsorption kinetics, equilibrium and thermodynamic studies

Activated carbon was prepared from peanut shell by chemical activation with KOH. Unoxidized activated carbon was prepared in nitrogen atmosphere which was then heated in air at a desired temperature to get oxidized activated carbon. The prepared carbons were characterized for surface area and pore volume and utilized for the removal of Cr(VI) from aqueous solution. The effects of pH, contact time, initial concentration of adsorbate and temperature on adsorption of Cr(VI) were investigated. Adsorption kinetics of Cr(VI) was analyzed by pseudo first order, pseudo second order and intraparticle diffusion kinetic models. Results showed that Cr(VI) adsorption on both oxidized and unoxidized samples followed the first and second order kinetics models most appropriately. Isotherm data were treated according to Langmuir and Freundlich models. The results showed that both Langmuir and Freundlich models fitted the data reasonably but the Langmuir adsorption isotherm model fitted better in the temperature range studied. The adsorption capacity was found to increase with temperature, showed endothermic nature of Cr(VI) adsorption. The thermodynamic parameters, such as Gibb's free energy change (ΔG°), standard enthalpy change (ΔH°), standard entropy change (ΔS°) were evaluated. The value of ΔG° was found negative for the adsorption of Cr(VI) which confirmed the feasibility and spontaneity of the adsorption process.