Adsorption Studies of Aqueous Solutions of Methyl Green for Halloysite Nanotubes: Kinetics, Isotherms, and Thermodynamic Parameters

Halloysite nanotubes (HNTs) were used to successfully remove methyl green dye from water. The HNTs were also characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Si and Al magic angle spinning nuclear magnetic resonance with magic angle spinning (MAS-NMR) and nitrogen adsorption at 77 K. SEM and TEM micrographs showed that HNTs have lengths of 0.2 to 1.5 μm, an outer diameter of 100 nm and lumen of 20 nm wide. X-ray diffraction patterns showed that the HNTs were totally dehydrated. HNTs may be regarded as a mesoporous material with a pore size distribution in the range of 1.5-150 Å and specific surface area of 34.49 m⋅g. The adsorption kinetics and equilibrium data of the dye, initial dye concentration, temperature, pH and contact time effect on removal efficiency were also investigated. Pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elrich models were evaluated in order to determine the rate parameters. The adsorption rate followed pseudo-second-order kinetic model. Adsorption revealed that methyl green was adsorbed as the Langmuir isotherm model describes and the maximum adsorption capacity of the HNTs was achieved (185 mg⋅g), being an efficient adsorbent for methyl green