Removal of methylene blue dye from water with low cost Nigella sativa seeds waste: kinetic, isotherm and statistical modelling

Nigella sativa seeds waste (NSW) was characterized by Fourier transformed infrared spectroscopy, X-ray diffraction, thermogravimetric analysis-TDA, and scanning electron microscopy analysis and used for the adsorptive uptake of methylene blue (MB) from water. The specific surface area of NSW determined by the methylene blue adsorption method was equal to 465.95 m(2)/g. The experimental equilibrium data were investigated using the isotherm equations of Langmuir, Freundlich, Temkin, Redlich-Peterson, Sips, and Radke-Prausnitz. The models of pseudo-first-order, pseudosecond-order, Elovich, intraparticle diffusion, and Avrami were applied for the kinetic data modeling. The maximum MB adsorption capacity of NSW obtained by the Langmuir equation was 149.4 mg/g. The experimental kinetic data fitted very well to the Avrami model. Thermodynamic parameters indicate the spontaneous and endothermic nature of MB adsorption onto NSW. The statistical physics model with single energy was used to determine the adsorption mechanism of MB on NSW. The model confirms the endothermic and physical nature of the process with an anchorage of MB onto NSW. The receptor site density (N-M), the adsorbed dye quantity at saturation (N-sat), the concentration at half-saturation (C-1/2) and the adsorption energy (Delta E) values are equal to 128.23, 141.1 mg/g, 25.4 mg/L and 18.0 kJ/mol, respectively.