Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes

Carbon nanotubes (CNTs) were oxidized with H2O2, KMnO4, and HNO3. Their physicochemical properties were investigated by BET N-2 adsorption, laser particle examination, Boehm's titration, zeta potential measurement and cadmium(II) adsorption. The experimental results suggest that cadmium(II) adsorption capacities for three kinds of oxidized CNTs increase due to the functional groups introduced by oxidation compared with the as-grown CNTs. The cadmium(II) adsorption capacity of the as-grown CNTs is only 1.1 mg g(-1), while it reaches 2.6, 5.1 and 11.0 mg g(-1) for the H2O2, HNO3 and KMnO4 oxidized CNTs, respectively, at the cadmium(II) equilibrium concentration of 4 mg 1(-1). Adsorption of cadmium(II) by CNTs was strongly pH-dependent and the increase of adsorption capacities for HNO3 and KMnO4 oxidized CNTs is more obvious than that of the as-grown and H2O2 oxidized CNTs at lower pH regions. The experiments of CNT dosage effect on the cadmium(II) adsorption show that the adsorption capacity for KMnO4 oxidized CNTs has a sharper increase at the CNT dosage from 0.03 to 0.08 g per 100 ml than the as-grown, H2O2 and HNO3 oxidized CNTs and its removal efficiency almost reaches 100% at CNT dosage of 0.08 g per 100 ml. Analysis revealed that the KMnO4 oxidized CNTs hosted manganese residuals, and these surely contributed to cadmium sorption to a yet-undefined extent. (C) 2002 Elsevier Science Ltd. All rights reserved.