In this study, 4A zeolite was prepared from opal waste rock by hydrothermal method and applied in ammonium ion adsorption. To optimize synthesis conditions, the effect of crystallization time (1-8 h), crystallization temperature (65-115 C), Na 2 O/SiO 2 (0.6-2.0), H 2 O/Na 2 O (20-70), and SiO 2 /Al 2 O 3 (1.0-3.5) was investigated. X-ray diffraction, scanning electron microscope imaging, cation exchange capacity, static water adsorption, Fourier transform infrared spectroscopy, and N 2 adsorption-desorption isotherm were used for assessing properties of 4A zeolite. Adsorption experiments were performed by 1.0 g l À1 4A zeolite with NH 4 þ solution (5-300 mg l À1) for 4 h at room temperature. The experiment results revealed with a crystallization time of 3 h, a crystallization temperature of 85 C, Na 2 O/SiO 2 ¼1.0, H 2 O/Na 2 O ¼ 40, and SiO 2 /Al 2 O 3 ¼2.0, the 4A zeolite synthesized had excellent performance with cation exchange capacity of 2.93 mmol (g dry zeolite) À1 and static water adsorption of 22.3%. The adsorption process was described by Freundlich model (R 2 >0.99) and the maximum adsorption capacity could reach to 53.11 mg g À1. The experimental results provided a novel approach for the utilization of opal waste rock, which is produced during the mining of opal-rich palygorskite, and for the synthesis of 4A zeolite and the removal of ammonium ion.

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