Adsorption and desorption of crystal violet and basic red 9 by multi-walled carbon nanotubes

Batch adsorption and desorption of crystal violet (CV) and basic red 9 (BR9) on multi-walled carbon nanotubes (MWCNTs) were conducted. To investigate the possible mechanisms of adsorption/desorption hysteresis, oxidized MWCNTs (O-MWCNTs) with more oxygen-containing groups were obtained by oxidizing as-purchased MWCNTs (A-MWCNTs) using nitric acid. The adsorption kinetics could be described by the pseudo-second-order model, suggesting that chemical reactions are the rate-limiting steps. The adsorption isotherms were fitted well by the Langmuir model, which suggests that, in addition to π-π interactions, chemical reactions significantly affect the adsorption. The adsorption capacity decreased in the order of CV on A-MWCNTs, BR9 on A-MWCNTs, and BR9 on O-MWCNTs, possibly because the amidation between BR9 and the surface groups of MWCNTs results in steric hindrance, which limits the adsorption of BR9 to inner grooves between CNT bundles. Adsorption/desorption hysteresis was observed for BR9 but not for CV. It was found that the π-π interaction and molecular entrapment were not responsible for the adsorption/desorption hysteresis. The hysteresis might be caused by the irreversible amide bonds between BR9 and MWCNTs. The results indicate that the steric hindrance due to the three-dimensional structure of organic compounds plays an important role in both adsorption/desorption kinetics and equilibria.