Synthesis of pure NaA zeolites from coal fly ashes for ammonium removal from aqueous solutions

The pure NaA zeolites used as adsorbents for ammonium removal were successfully synthesized from coal fly ash by means of alkaline fusion followed by hydrothermal treatment. The experiment samples were characterized in terms of X-ray diffraction, X-ray fluorescence, Scanning electron microscopy, Fourier transform infrared spectroscopy, and Automated specific surface area and pore size analyzer. The results showed that the NaOH solution’s concentration, reaction temperature, and reaction time had significant effects on the species of the zeolite phases. Well-ordered cubic NaA zeolite with Brunauer–Emmett–Teller specific surface areas of 41.6 m²/g was prepared in 3 M NaOH solution at 353 K for 3 h. The adsorption behavior of NH₄ ⁺ from aqueous solution onto NaA zeolites was investigated at different initial concentrations, pH values, and contact times. The adsorbent equilibrium could be reached within 60 min, and the maximum removal efficiency of NH₄ ⁺ was achieved at pH 7.0. The Langmuir and Freundlich adsorption models were also applied to describe the equilibrium isotherms. The obtained results show that the experimental data have a better (R ² = 0.99) fit to Freundlich model and the adsorption of NH₄ ⁺ ions using synthesized NaA zeolite is favorable.