A key role of inner-cation-π interaction in adsorption of Pb(II) on carbon nanotubes: Experimental and DFT studies

Herein the adsorption and desorption of Pb2+ on oxidized (O-CNTs) and graphitized multi-walled carbon nanotubes (G-CNTs) were studied, and detailed adsorption mechanisms were discussed by experimental characterization and density functional theory (DFT) calculation. The adsorption of Pb2+ on CNTs was co-guided by complexation, ion exchange, electrostatic and cation-π interactions. According to the abnormally low release ratio of Pb2+ on both O-CNTs and G-CNTs (< 9.03%), the O-containing groups on CNTs surface are not the only key factor affecting the adsorption behavior. The pore filling and complexation are the main mechanisms leading to irreversible adsorption, especially the important role of the inner-cation-π interaction in Pb2+ adsorption into the inner channel of CNTs at the high initial Pb2+ concentration, and DFT calculations further confirmed this result. The adsorption energy of the inner-cation-π interaction between Pb2+ and CNTs can be as high as − 77.851 kJ/mol, which is much higher than other interactions (≤ − 41.488 kJ/mol). Moreover, the stability of various adsorption mechanisms by HOMO-LUMO energy gap (Egap), electronic chemical potential (µ) and global hardness (η) were quantitatively measured and further revealed the inner-cation-π interaction is more stable. This study provides a deeper understanding of the removal of heavy metals by porous carbon-based nanomaterials.