BACKGROUND: Nitrogen-doped multi-walled carbon nanotubes (CNx) and cup-stacked-type carbon nanofibres (CST) were modified with nitric acid, at 85 degrees C, for 1-3 h to enhance their cation adsorption capacity, and were used for cadmium removal from aqueous solutions. These carbon nanostructures were characterized by SEM, EDX, FTIR, BET surface area, TGA and acid/base titrations. In addition, cadmium adsorption experiments were carried out at different pH values and constant temperature. RESULTS: EDX and FTIR revealed a 0.4 times increase in oxygen content after acid treatment, which was reflected in an increment of up to 1.93 mmol g(-1) of acid sites and in a shift of about two units of the zero point charge to lower pH values. The BET surface area, in general, decreased as the oxidation time increased. Finally, batch sorption experiments indicated that CNx are more effective than CST to sequester Cd2+ at pH 7; the sorption capacities of these materials increased as pH rose. CONCLUSIONS: The results obtained indicate that it is feasible to modify both physical and chemical properties of the carbon nanostructures CNx and CST by HNO3 in such a way that they can be used to adsorb cations present in aqueous solutions. (c) 2008 Society of Chemical Industry