Metal-chelating membranes have advantages as adsorbents in comparison with conventional beads because they are not compressible and they eliminate internal diffusion limitations. The aim of this study was to explore in detail the performance of poly (2-hydroxyethyl methacrylate-methacryloylamidohistidine) [poly(HEMA-MAH)] membranes for the removal of three toxic heavy-metal ions-Cd(II), Pb(II), and Hg(II)-from aquatic systems. The poly(HEMA-MAH) membranes were characterized with scanning electron microscopy and H-1-NMR spectroscopy. The adsorption capacity of the poly(HEMA-MAH) membranes for the selected heavy-metal ions from aqueous media containing different amounts of these ions (30-500 mg/L) and at different pH values (3.0-7.0) was investigated. The adsorption capacity of the membranes increased with time during the first 60 min and then leveled off toward the equilibrium adsorption. The maximum amounts of the heavy-metal ions adsorbed were 8.2, 31.5, and 23.2 mg/g for Cd(II), Pb(II), and Hg(II), respectively. The competitive adsorption of the metal ions was also studied. When the metal ions competed, the adsorbed amounts were 2.9 mg of Cd(II)/g, 14.8 mg of Pb(II)/g, and 9.4 mg of Hg(II)/g. The poly(HEMA-MAH) membranes Could be regenerated via washing with a solution of nitric acid (0.01M). The desorption ratio was as high as 97%. These membranes were suitable for repeated use for more than three adsorption/desorption cycles with negligible loss in the adsorption capacity. The stability constants for the metal-ion/2-methacryloylamidohistidine complexes were calculated to be 3.47 x 10(6), 7.75 x 10(7), and 2.01 x 10(7) L/mol for Cd(II), Pb(II), and Hg(II) ions, respectively, with the Ruzic method. (c) 2005 Wiley Periodicals, Inc.