One-step surface-functionalization of polydopamine-modified multiwalled carbon nanotubes for the selective removal of organic dyes from aqueous solutions

Multiwalled carbon nanotubes (MWCNTs) modified by polydopamine (PDA) (MWCNTs/PDA) were synthesized via oxidation polymerization of dopamine. Scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analyses were used to characterize the obtained materials. To study the effect of the mass fraction of PDA on the adsorption capacity, a series of MWCNTs/PDA composites with different mass ratios between MWCNTs and PDA, were prepared, and their adsorption selectivity toward six different dyes was investigated. The results indicated that the MWCNTs/PDA composite with a 1: 1 mass ratio between MWCNTs and PDA exhibits a higher adsorption capacity toward phenazine and phenothiazine dyes, which both contain the C=C-C=N 1,4-conjugate quinone structure and acid fuchsin, with a N atom in a C=C-C=N 1,4-conjugate structure connecting H atoms. The adsorption process could be well described by the pseudo-second-order kinetic model and by the Langmuir isotherm, respectively. Furthermore, thermodynamic parameters revealed that the adsorption was an endothermic and spontaneous process.