Modification of coal fly ash for high capacity adsorption of methylene blue

Coal fly ash (FA) waste from local thermal power plant has been modified by acid activation and SiO 2 enrichment using hydrochloric and acetic acids. The modified FA with acid activation (FAA_HAc and FAA_HCl) and SiO 2 enrichment (SiO 2 _HCl and SiO 2 _HAc) were applied in the adsorption of methylene blue (MB) from aqueous solution. The physicochemical and morphological characteristics of the modified FA were characterized by XRF, SEM-EDX, XRD, BET, and TGA. The adsorption study was carried out in a batch system by optimizing pH, adsorbent dosage, contact time, initial dye concentration and temperature. The optimum adsorption is achieved at pH 8.0, adsorbent dosage of 1.5 g/L and contact time of 75 min. The experimental data fits the pseudo-second-order models and the activation energies (E a ) are 48.71, 48.95, 32.26, and 30.61 kJ/mol for FAA_HAc, FAA_HCl, SiO 2 _HAc, and SiO 2 _HCl, respectively. Langmuir, Freundlich, or Temkin isotherm models described well the experimental data. The maximum adsorption capacities (q m ) of FAA_HAc, FAA_HCl, SiO 2 _HAc, and SiO 2 _HCl with Lamgmuir model at 318 K are 146.18, 151.33, 95.81, and 100.78 mg/g, respectively. The q m values increase with temperature, indicating endothermic adsorption process. Themodynamic study gives the positive value of enthalpy (ΔH°) and entropy (ΔS°), and the negative value of free energy (ΔG°) changes, suggesting an endothermic and spontaneous process. The lower activation energy and higher adsorption capacity of modified FA with a simple, low-cost and eco-friendly technique provide a prospective application of the adsorbents for the removal of dye waste from aqueous solution.