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 degrees 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+ approximate to 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. (C) 2012 Elsevier B.V. All rights reserved.