In the present study, cashew nut shell(CNS) was investigated as a biosorbent for the removal of copper ions from aqueous solutions. Batch experiments were carried out to investigate the effect of solution pH, CNS concentration, contact time, initial copper(II) ion concentration and temperature on sorption efficiency. The copper adsorption was favored with maximum adsorption at pH 5.0. The percentage of copper ion removal onto the CNS was decreased with increasing temperature. Biosorption equilibrium time was observed in 30 min. The equilibrium adsorption data were fitted to Langmuir and Freundlich adsorption isotherm models and the model parameters were evaluated. The kinetics of copper(II) ion was discussed using four kinetic models, the pseudo-first-order, the pseudo-second-order, the Elovich kinetic model and the intra-particle diffusion models. It was shown that the adsorption of copper ions could be described by the pseudo-second order kinetic model. Thermodynamic quantities such as Gibbs free energy (Delta G degrees), the enthalpy (Delta H degrees) and the entropy change of sorption (Delta S degrees) have also been evaluated and it has been found that the sorption process was feasible, spontaneous and exothermic in nature. The results showed that CNS could be employed as a low-cost alternative adsorbent for the removal of copper ions from aqueous solutions. (C) 2010 Elsevier B.V. All rights reserved.