Mercury in the lowest levels of concentrations is dangerous for human health due to its bioaccumulation in body and toxicity. This investigation shows the effective removal of mercury (II) ions from contaminated surface waters by modified magnetic iron oxide nanoparticles (M-MIONPs) with 2-mercaptobenzothiazole as an efficient adsorbent. The proposed method is fast, simple, cheap, effective and safe for treatment of mercury polluted waters. Preparation of adsorbent is easy and removal time is short. Non-modified magnetic iron oxide nanoparticles (MIONPs) can adsorb up to 43.47% of 50 ng mL(-1) of Hg (II) ions from polluted water, but modified magnetic ironoxide nanoparticles (M-MIONPs) improved the efficiency up to 98.6% for the same concentration. The required time for complete removal of mercury ions was 4 min. Variation of pH and high electrolyte concentration (NaCl) of the solution do not have considerable effect on the mercury removal efficiency. Loading capacity of adsorbent for Hg ions is obtained to be 590 mu g g(-1). (C) 2011 Elsevier B.V. All rights reserved.