Monodisperse chitosan-bound Fe3O4 nanoparticles were developed as a novel magnetic nano-adsorbent for the removal of heavy metal ions. Chitosan was first carboxymethylated and then covalently bound on the surface of Fe3O4 nanoparticles via carbodiimide activation. Transmission electron microscopy micrographs showed that the chitosan-bound Fe3O4 nanoparticles were monodisperse and had a mean diameter of 13.5 nm. X-ray diffraction patterns indicated that the magnetic nanoparticles were pure Fe3O4 with a spinel structure, and the binding of chitosan did not result in a phase change. The binding of chitosan was also demonstrated by the measurement of zeta potential, and the weight percentage of chitosan bound to Fe3O4 nanoparticles was estimated to be about 4.92 wt%. The chitosan-bound Fe3O4 nanoparticles were shown to be quite efficient for the removal of Cu(II) ions at pH > 2. In particular, the adsorption rate was so fast that the equilibrium was achieved within 1 min due to the absence of internal diffusion resistance. The adsorption data obeyed the Langmuir equation with a maximum adsorption capacity of 21.5 mg g(-1) and a Langmuir adsorption equilibrium constant of 0.0165 Lmg(-1). The pH and temperature effects revealed that the adsorption capacity increased significantly with increasing pH at pH 2-5. and the adsorption process was exothermic in nature with an enthalpy change of -6.14 kJ mol(-1) at 300-330 K. (C) 2004 Elsevier Inc. All rights reserved.