Acid-free synthesis of polyaniline nanotubes for dual removal of organic dyes from aqueous solutions

Polyaniline nanotubes (PANI-NTs) were prepared by a green approach via the aniline oxidation using ammonium peroxydisulfate as an oxidant in the presence of methyl orange (MO) as a structure-guiding agent without any acid for the dual removal of anionic and cationic dyes from aqueous solutions. The structure, morphology and the physicochemical properties of the synthesized PANI-NTs were examined by different techniques such as Fourier transform infrared (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), UV-vis spectroscopy, nitrogen adsorption-desorption isotherms, and thermal gravimetric analysis (TGA). The TEM image reveals the nanotubular morphology of the synthesized PANI. The prepared PANI-NTs were effectively used for dual adsorption of anionic acid green (AG) and cationic methylene blue (MB) dyes from aqueous medium. The effect of various parameters was studied to optimize the conditions of the adsorption process. The obtained results indicated that the optimum pH for adsorbing MB and AG dyes are 9.0 and 3.0, respectively. Based on the kinetic studies, the sorption dynamics followed the second-order model. In addition, the equilibrium data of the anionic and cationic dyes' adsorption were fitted well to the Langmuir isotherm. Furthermore, the maximum monolayer capacity, (q(max)), for MB and AG was calculated from the Langmuir model to be 91.1 and 58 mg g(-1), respectively. The thermodynamic study showed that the adsorption process of MB onto PANI-NTs is spontaneous, feasible and exothermic process. The regeneration study indicated that PANI-NTs can be reused for multiple dye adsorption cycles. Therefore, the synthesized PANI-NTs can be considered as a highly efficient adsorbent for both of anionic and cationic dyes from aqueous medium.