Adsorption of perchlorate onto raw and oxidized carbon nanotubes in aqueous solution

Perchlorate (ClO4-) is an emerging trace contaminant. The adsorption of ClO4- on raw and oxidized carbon nanotubes (CNTs) was investigated to elucidate the affinity mechanism of CNTs with anion pollutants. The adsorption of ClO4- into different CNTs increased in the order multi-walled CNTs < single-walled CNTs < double-walled CNTs (DWCNTs). Co-existing anions (SO42-, NO3-, Cl-) significantly weakened ClO4- adsorption, while the co-existence of Fe3+ and cetyltrimethylammonium cations increased ClO4- adsorption 2- to 3-fold. ClO4- adsorption was promoted by oxidized DWCNTs due to the introduction of more oxygen-containing functional groups, which served as additional adsorption sites. The pH values significantly affected the zeta potential of raw and oxidized DWCNTs and thus ClO4- adsorption. The pH-dependent curves of ClO4- adsorption on CNTs were distinct from those of conventional sorbents (e.g., activated carbon and resin). Maximum ClO4- adsorption occurred at pH = the isoelectric point (pH(IEP)) + 0.85 rather than at pH < pH(IEP), which cannot be explained by electrostatic interactions alone. Hydrogen bonding is proposed to be a dominant mechanism at neutral pH for the interaction of ClO4- with CNTs, and variations of ClO4- affinity with CNTs in different pH ranges are illustrated. (C) 2012 Elsevier Ltd. All rights reserved.