Adsorption of methylene blue onto poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) nanotubes: Kinetics, isotherm and thermodynamics analysis

•Polyphosphazene nanotube as an adsorbent could be facilely synthesized.•The adsorbent owns numerous electron-rich N and P atoms and hydroxyl groups.•The adsorbent was an efficient and specific adsorbent for the removal of MB.•The pseudo-second-order model could be better to describe the adsorption of MB.•The MB adsorption onto PZS nanotubes was endothermic and spontaneous. Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes, an excellent adsorbent, were successfully synthesized by an in situ template method and used for the removal of methylene blue (MB) from aqueous solution. The morphology and structures of as-synthesized PZS nanotubes were characterized by scanning electron microscopy, transmission electron microscope, Fourier transform infrared spectroscopy and N2 adsorption/desorption isotherms. The effects of temperature, concentration, pH and contact time on MB adsorption were studied. It was favorable for adsorption under the condition of basic and high temperature. The pseudo-first-order, pseudo-second-order and intraparticle diffusion models were used to fit adsorption data in the kinetic studies. And results showed that the adsorption kinetics were more accurately described by the pseudo-second-order model. The equilibrium isotherms were conducted using Freundlich and Langmuir models. It has been demonstrated that the better agreement was Langmuir isotherm with correlation coefficient of 0.9933, equilibrium absorption capacity of 69.16mg/g and the corresponding contact time of 15min. Thermodynamic analyses showed that MB adsorption onto the PZS nanotubes was endothermic and spontaneous and it was also a physisorption process.