The copper ion adsorption from aqueous solutions on oxidized single-wall (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) was investigated. Two different oxidation methods were used on the carbon nanotubes. One is ultrasonication of CNTs with concentrated HNO3, and the other is the oxidation of CNTs under UV-light with concentrated HNO3. The efficiencies of the oxidation methods were determined using Cu (II) adsorption. The adsorption parameters were examined, such as Cu (II) ion concentrations, pH, adsorption time, adsorbent quantities and temperature. The adsorption mechanism between the carbon nanotubes and Cu (II) ions was modeled using Langmuir and Freundlich isotherms, Pseudo-second order kinetic model, and Bangham and Elovich models. (C) 2014 Elsevier B.V. All rights reserved.