Interaction and thermodynamics of methylene blue adsorption on oxidized multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) and their oxidized derivatives were used as adsorbents for the removal of methylene blue dye from aqueous solution. CNTs have consistent surface and distinct structure, thus they were selected as the novel adsorbents in this study. The site-energy distribution analysis could provide significant information for adsorption mechanisms. Therefore, this study concentrated on the site-sorption energy distribution analysis in combination with thermodynamic behavior of methylene blue adsorption on CNTs. The single point adsorption coefficient K_d was in order of MWCNTs-Ox>MWCNTs following the temperature 308 K>298 K. Surface area could not be the only single factor for methylene blue adsorption on CNTs, which suggested that morphology of CNTs, π-π electron donor-acceptor, electrostatic interaction and functional groups were also contributing factors. Thus, these all of interactions may drive the methylene blue adsorption. The thermodynamic parameters showed that the adsorption mechanism is spontaneous and exothermic. The site-energy distribution suggested that higher energy active sites would be engaged first, and consequently, the methylene blue molecules adsorb on the comparatively lower energy active sites of CNTs.