Adsorption kinetics and thermodynamics of Malachite Green from aqueous solutions onto expanded Graphite nanosheets

Expanded graphite nanosheets (EG-nanosheets) were used for adsorption of Malachite Green (MG) from aqueous solution. The influences of dye concentrations, absorbent dosage, pH values and the temperatures on the adsorption were investigated as well. The dye adsorption experiments were carried out by utilizing batch procedure. EG-nanosheets were initially characterized by scanning electron microscopy (SEM), fourier transform infrared (FT-IR) and X-ray diffraction (XRD). Adsorption efficiency increased with increment in initial pH, dye con‌centration and temperature, but decreased with increment in adsorbent dose. The rate parameters of adsorption were evaluated by First-order, pseudo-second-order, and intraparticle diffusion models. These data indicated an endothermic spontaneous adsorption process and kinetically followed the pseudo-second order model with activation energy of +17.44 kJmol-1. Langmuir, Freundlich and Temkin equations were used for analyzing of experimental isotherm data and found that the Langmuir isotherm model showed good fit to the equilibrium adsorption data. The maximum adsorption capacity of 158.9 mg g-1 of Malachite Green was achieved (dye initial concentration of 100 mg L-1). Thermodynamic parameters such as changes in the free energy of adsorption (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated. The negative values of ΔG° indicate that the malachite green adsorption process is spontaneous in nature and the positive value of ΔH° shows the endothermic nature of the process. Adsorption onto EG has proved to be highly efficient technique for the handling of dye-contaminated waters.