Fe3O4 magnetic nanoparticles with different average sizes were synthesized and structural characterizations showed that the three kinds of nanoparticles had different sizes, i.e., an average particle size of 8 rim, 12 nm and 35 nm was observed for the nanoparticles prepared with the co-precipitation method, the co-precipitation combining a surface decoration process, and the polyol process, respectively. The synthesized Fe3O4 nanoparticles with different mean particle sizes were used for treating the wastewater-contaminated with the metal ions, such as Ni(II), Cu(II), Cd(II) and Cr(VI). It is found that the adsorption capacity of Fe3O4 particles increased with decreasing the particle size or increasing the surface area. Various factors influencing the adsorption of metal ions, e.g., pH, temperature, amount of Fe3O4, and contacting time were investigated to optimize the operation condition for the use of Fe3O4 nanoparticles with an average size of 8 nm. The obtained results indicated that the mechanism was strongly influenced by the pH and temperature of wastewater. The maximum adsorption occurred at pH 4.0 under room temperature (20 degrees C) and the adsorption capacity of Fe3O4 nanoparticles was as high as 35.46 mg/g, which is almost 7 times higher than that of the coarse particles. (C) 2009 Elsevier B.V. All rights reserved.