The magnetic chitosan nanoparticles of 13.5 nm were prepared as a magnetic nano-adsorbent by the carboxymethylation of chitosan and the followed binding on the surface of Fe3O4 nanoparticles via carbodiimide activation. Their saturation magnetization, remanent magnetization, coercivity, and squareness were 62 emu/g, 1.8 emu/g, 6.0 Oe, and 0.029, respectively, reflecting their superparamagnetic property. The binding reaction of carboxymethyl chitosan on the surface of Fe3O4 nanoparticles was much faster than the self-crosslinking of carboxymethyl chitosan, and the appropriate reaction time was I It. At low carboxymethyl chitosan concentrations, the binding efficiency could be as high as 100%. The maximum amount of chitosan bound on the Fe3O4 nanoparticles was 4.92 wt%. In addition, magnetic chitosan nano-adsorbent was shown to be quite efficient for the fast removal of Co(II) ions at pH 3-7 and 20-45 degrees C. The maximum adsorption capacity for Co(II) ions occurred at pH 5.5, and the adsorption process was exothermic in nature with an enthalpy change. of -12.04 kJ/mol. The equilibrium was achieved within 1 min, and the adsorption data obeyed the Langmuir equation with a maximum adsorption capacity of 27.5 mg/g (557 mg/g based on the weight of chitosan) and a Langmuir adsorption equilibrium constant of 0.034 I/mg at 25 degrees C. Such fast adsorption rate and high adsorption capacity could be attributed to the absence of internal diffusion resistance and the high specific surface area. (c) 2005 Elsevier B.V. All rights reserved.