Removal of methylene blue from aqueous solution using by untreated lignite as potential low-cost adsorbent: Kinetic, thermodynamic and equilibrium approach

The adsorption mechanism of the cationic methylene blue dye to the abundant and inexpensive lignite as an adsorbent from aqueous solutions was investigated as a function of ionic strength, pH, temperature, initial dye concentration, contact time and stirring speed. The adsorption capacity increased with the increasing temperature, initial dye concentration, pH and ionic strength, although it was not affected by the stirring speed. The value of zeta potential decreased with the increasing pH. Experimental adsorption data were modelled by different equilibrium isotherms such as Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R), BET, Halsey, Harkins-Jura, Smith and Henderson isotherms. The adsorption process fitted well to pseudo-second-order kinetics and the Langmuir model. Activation energy of the adsorption processes was found to be 49.81 kJ mol(-1) initial dye concentration of 80 mg L-1 using the Arrhenius equation, indicating the strong electrostatic interactions between the adsorbent and dye. The isosteric enthalpy and entropy changes were calculated as 58.19 kJ mol(-1) and -0.186 kJ mol(-1) K-1, respectively, indicating that the adsorption process was endothermic and the orderliness of the adsorption system slightly decreases with adsorption of methylene blue molecules to the lignite surface. FT-IR results revealed that several functional groups on the coal surface are responsible for methylene blue adsorption. The observed differences in the reflection of the SEM images of lignite and methylene blue adsorbed lignite are due to the presence of methylene blue, which was adsorbed. BET isotherm and nitrogen-adsorption-desorption isotherms analysis results implied that lignite is a heteroporous material exhibiting microporous properties. (C) 2014 Elsevier Ltd. All rights reserved.