Kinetic and equilibrium modeling for the adsorptive removal of methylene blue from aqueous solutions on of activated fly ash (AFSH)

Fly ash is a byproduct generated during the coal combustion in thermal power plants. Application of activated fly ash (AFSH) for removal of a toxic dye, methylene blue (MB) from its aqueous solution has been investigated. Characterization of the adsorbent was carried out with X-ray fluorescence (XRF), X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Surface area of the AFSH was also determined. Batch studies were performed to evaluate influence of various experimental parameters like initial pH, contact time, initial concentration and temperature on the removal of MB. Optimum conditions for the removal were found to be as initial pH 9.0, contact time = 100 min and adsorbent dose = 20 g/L. The surface area of activated fly ash was found to be 58.16 m2/g. The removal followed second order kinetics. The equilibrium data was fitted by Langmuir, Freundlich and Redlich–Peterson isotherm models. Error analysis revealed that Freundlich model fits the data best. Thermodynamic parameters such as change in Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were also calculated. A single-stage batch adsorber design for the MB adsorption onto AFSH was also presented based on the Freundlich isotherm model equation. The above findings suggest that activated fly ash can be effectively used for discoloration of dye contaminated wastewater.