The aim of this study is to investigate the utilization of the zinc oxide nanoparticles (ZnO) as an adsorbent for the removal of Zn(II), Cd(II) and Hg(II) ions from aqueous solution. Adsorption properties of ZnO towards Zn (II), Cd(II) and Hg(II) ions were systematically investigated, including pH effect, adsorption equilibrium, adsorption kinetics and temperature. The adsorption isotherms were well described by Langmuir isotherm model with correlating constant (R-2) higher than 0.99. The maximum adsorption capacity was determined at 303 K and were found to be 357, 387 and 714 mg/g for Zn(II), Cd(II) and Hg(II) ions respectively. The adsorption kinetics data were well fitted by the pseudo-second-order rate model with high regression coefficients. The adsorption capacity was decreased with the increase of temperature, and thermodynamic calculations suggested that the adsorption of Zn(II). Cd(II) and Hg(II) ions onto ZnO nanoparticles is an exo-thermic process. (C) 2011 Elsevier B.V. All rights reserved.