Adsorption of nickel, copper, zinc and cadmium from aqueous solutions on carbon nanotubes oxidized with concentrated nitric acid was carried out in single, binary, ternary and quaternary systems. TEM and adsorption of nitrogen were used to determine texture and structural parameters, respectively. The surface chemistry was evaluated using the pH at the point of zero charge, FTIR spectroscopy and XPS analysis. The experimental results showed that all isotherms for Cu2+(aq) fit to Langmuir model in each system. On the other hand, the isotherms for Ni2+(aq), Cd2+(aq) and Zn2+(aq) in multi-component systems reveal the effect of competition for adsorption sites seen as a decrease in the amount adsorbed. The uptakes at the equilibrium concentration of 0-0.04 mmol L-1 in single system and 0-0.15 mmol L-1 in binary system are in the order Cu2+(aq) > Ni2+(aq) > Cd2+(aq) > Zn2+(aq) while for the ternary and quaternary, the order is Cu2+(aq) > Cd2+(aq) > Zn2+(aq) > Ni2+(aq). The results indicate that the mechanism of adsorption is governed by the surface features, ion exchange process and electrochemical potential. The latter plays a significant role in multi-component adsorption where redox reactions, not only on the adsorbent surface but also between the adsorbates, are likely to occur. (C) 2009 Elsevier B.V. All rights reserved.