Removal of methylene blue by NaX zeolites synthesized from coal gasification fly ash using an alkali fusion-hydrothermal method

NaX zeolites were prepared via an alkali fusion-hydrothermal method from coal gasification fly ash. The effects of the adsorbent dosages, initial dye concentrations, adsorption temperatures and pH values on the adsorption capacity of methylene blue (MB) onto the NaX zeolites were evaluated in batch mode. The maximum adsorption capacity of MB onto the NaX zeolites is 127.13 mg/g, which is higher than the recently reported values using similar adsorbents. The adsorption isotherm data fitted well to the Langmuir model, and the adsorption kinetics followed the pseudo-second-order model. Negative changes in Gibbs free energy (Delta G degrees) and enthalpy (Delta H degrees), and a positive change in entropy (Delta S degrees), suggested that the adsorption of MB onto the NaX zeolites is an exothermic and spontaneous process.