Wrapping carbon nanotubes with poly (sodium 4-styrenesulfonate) for enhanced adsorption of methylene blue and its mechanism

Improving the adsorption performance of carbon nanotubes (CNTs) to develop superior adsorbents is of practical interest. Surface modification or making CNTs-based hybrids is a promising way. In this paper, CNTs were noncovalently wrapped with poly (sodium 4-styrenesulfonate) (PSS) to obtain CNT/PSS hybrid. The hybrid was well-dispersed in water, but aggregated immediately upon adsorption of methylene blue (MB). Adsorption isotherms and kinetics indicated that CNT/PSS exhibited more than two times higher adsorption capacity and much faster adsorption kinetics than pristine CNTs. XPS was used to study the interaction between CNT and PSS and the adsorption mechanism of MB on CNT/PSS. The results suggested that CNTs were wrapped with PSS by pi-pi electron-donor-acceptor interactions and -SO3- groups could accept the pi-electrons from the conjugated system of CNTs. Therefore, CNTs in the hybrid may not only adsorb MB by it-it interactions but also contribute to the electrostatic attraction between -SO3- groups of PSS and dimethylamino (-NMe2) groups of MB. The electrostatic attraction was rather strong in view of the quaternization of -NMe2 groups, and it adsorbed more MB than pi-pi interactions. We showed that electrostatic attraction has advantages over pi-pi interactions in adsorption because of space-saving tilted orientation of MB plane on CNTs surface and weakened electrostatic repulsion between adjacent MB molecules. The electrostatic attraction was responsible for both enhanced adsorption and the aggregation of CNT/PSS. (C) 2014 Elsevier B.V. All rights reserved.