Surface functionalization of multi-walled carbon nanotubes (MWCNTs) by ethylenediamine, via chemical modification of carboxyl groups, using O-(7-azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate, was performed. The resulting materials were characterized by different techniques, such as FTIR, TGA and elemental analysis. Biocompatibility studies showed that the functionalized MWCNTs, at concentrations between 1 and 50 mu g mL(-1), were not cytotoxic for the fibroblast L929 cell line. In batch tests, the influences of solution pH, contact time, initial metal ion concentration and temperature on the sorption of Cd2+ ions onto raw-MWCNTs (raw-MWCNT), oxidized MWCNTs (o-MWCNT) and ethylenediamine-functionalized MWCNTs (e-MWCNT) were studied. The adsorption of Cd2+ ions by o-MWCNT and e-MWCNT was strongly pH dependent. The time dependent Cd2+ sorption onto raw-MWCNT, o-MWCNT and e-MWCNT can be described by a pseudo-second-order kinetic model. The Langmuir isotherm model agrees well with the equilibrium experimental data. The maximum capacity was obtained for e-MWCNT, 25.7 mgg(-1), at 45 degrees C. The thermodynamic parameters were also deduced for the adsorption of Cd2+ ions on raw-MWCNT, o-MWCNT and e-MWCNT and the results showed that the adsorption was spontaneous and endothermic. (C) 2009 Elsevier B. V. All rights reserved.