Alkaline pre-treatment of waste chestnut shell from a food industry to enhance cadmium, copper, lead and zinc ions removal

Chestnut (Castanea sativa) shell was subjected to an alkaline pre-treatment in order to increase its adsorption capacity, and the influence of the operating conditions (NaOH concentration and time) on cadmium ions adsorption was studied. The shell modified with a 4% NaOH aqueous solution at 25 °C for 4 h was selected to favour cadmium adsorption and was also used as adsorbent of copper, lead and zinc ions from aqueous solutions. The highest adsorption capacity was obtained for Cd2+ ions, 9.9 mg/g, and the order of cation affinity was Cd2+ > Cu2+ ≈ Zn2+ > Pb2+. All adsorption processes followed the pseudo-second order kinetic model and the equilibrium data were satisfactorily fitted to the Sips or Freundlich models. The alkali pre-treated chestnut shell was characterised before and after adsorption using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) spectrometry. Chemical blocking of carboxyl and hydroxyl groups of pre-treated shell reduced Cd2+, Cu2+ and Zn2+ adsorption, which confirmed the participation of these groups in the process.

► Chestnut shell pre-treatment with 4% NaOH at 25 °C for 4 h favoured Cd2+ adsorption.
► Alkali pre-treated shell was used to adsorb Cd2+, Cu2+, Zn2+ and Pb2+ from aqueous solutions.
► Kinetic data were fitted to the pseudo-second order kinetic model.
► Equilibrium data were fitted to Freundlich and Sips models depending on the cation.
► Adsorption process was analyzed by FTIR, chemical blocking of groups, SEM and EDX.