In this study, we investigated the adsorption capacities and the mechanisms of adsorption of cationic [copper(II), zinc(II), nickel(II), lead(II), cadmium(II)] and oxyanionic [dichromate(-II), permanganate(-I)] metal ions onto hexadecyltrimethylammonium (HDTMA)-modified and unmodified NaY zeolites. To understand the surface characteristics of the modified adsorbents, we analyzed the NaY zeolite and the HDTMA-modified zeolite (HMZ) in terms of their surface areas, zeta potentials, Fourier transform infrared (FTIR), elemental composites and cation exchange capacities. The results reveal that HMZ behaved as a dual-electronic adsorbent. Because cationic heavy metals cannot replace the HDTMA forming admicelles on the external surface, the obtained adsorbent could simultaneously adsorb both cationic and anionic metal ions. We propose that ion exchange is the primary mechanism through which cationic metal ions adsorb onto the NaY zeolite with adsorption capacities, dependent on the ion radius, following the order Pb > Cd > Cu > Zn > Ni. The mechanisms for the adsorption of the cationic and anionic metal ions onto HMZ include ion exchange and complexation reactions and/or surface precipitation, with the adsorption capacities of the cationic metal ions following the order Pb > Cu > Cd > Zn > Ni and those of anionic metal ions following the order MnO4- > Cr2O72-). (C) 2012 Elsevier B.V. All rights reserved.