Preparation of highly ordered cubic NaA zeolite from halloysite mineral for adsorption of ammonium ions

Well-ordered cubic NaA zeolite was first synthesized using natural halloysite mineral with nanotubular structure as source material by hydro-thermal method. SEM and HRTEM images indicate that the synthesized NaA zeolite is cubic-shaped crystal with planar surface, well-defined edges and symmetrical and uniform pore channels. The adsorption behavior of ammonium ions (NH4+) from aqueous solution onto NaA zeolite was investigated as a function of parameters such as equilibrium time, pH, initial NH4+ concentration, temperature and competitive cations. The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms. A maximum adsorption capacity of 44.3 mg g-1 of NH4+ was achieved. The regeneration and reusable ability of this adsorbent was evaluated, and the results indicated that the recovered adsorbent could be used again for NH4+ removal with nearly constant adsorption capacity. Thermodynamic parameters such as change in free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) were also determined, which indicated that the adsorption was a spontaneous and exothermic process at ambient conditions. Compared with other adsorbents, the as-synthesized NaA zeolite displays a faster adsorption rate and higher adsorption capacity, which implies potential application for removing NH4+ pollutants from wastewaters.