Acid green crystal-based in situ synthesis of polyaniline hollow nanotubes for the adsorption of anionic and cationic dyes

Abstract

In this article, acid green (AG) dye played a dual pivotal role. The first role is as a structure-guiding agent and a soft template for the acid-free synthesis of polyaniline hollow nanotubes (PANI-HNTs) while the second role is as a target dye to be removed as a model of anionic dyes, alongside methylene blue (MB) as a model of cationic dyes. After characterization using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), nitrogen adsorption–desorption isotherms (BET), and UV-visible absorption spectroscopy techniques, the prepared PANI-HNTs were exploited for the adsorption of basic MB dye and acidic AG dye from aqueous media. To optimize the adsorption process, the effects of different variables, such as adsorbent dose, contact time and pH, were studied. Due to the structural differences between the MB and AG dyes, the obtained data revealed that the best pH for the medium for optimal adsorption was 9.0 and 3.0, respectively. The rapid sorption dynamics were found to proceed in a second-order kinetic model and the equilibrium data for the adsorption of MB and AG dyes were fitted well to the Langmuir isotherm. The maximum monolayer capacity, q_max, for MB and AG was calculated from the Langmuir model and equalled 69.4 and 57.8 mg g⁻¹, respectively. A thermodynamic study revealed that the adsorption of MB by PANI-HNTs was a feasible, spontaneous, and exothermic process. Investigation of the substrate regeneration revealed that PANI-HNTs can be reused for dye adsorption several times. Therefore, the synthesized PANI-NTs are highly efficient for the dual removal of basic and acidic dyes. TEM images showed that PANI-HNTs were formed with an external and internal diameter of 50–60 nm and 5–10 nm, respectively.