High-performance hybrid NaA/X zeolite from natural volcanic rock: Optimized synthesis and synergistic mechanism for ammonium-nitrogen adsorption

A high-performance hybrid NaA/X zeolite was directly synthesized from natural volcanic rocks using a facile hydrothermal method. Response surface methodology with a central composite design was used to optimize the synthesis parameters (NaOH:ore ratio = 2.15, Si:Al ratio = 1.58, and crystallization temperature = 104 °C). Under optimal conditions, the synthesized material exhibited 93.46% relative crystallinity and a 42.65 mg g −1 ammonium‑nitrogen adsorption capacity. The hybrid NaA/X zeolite displayed distinct bimodal micropores (~0.4 nm from NaA zeolite and 0.8–1.4 nm from NaX zeolite). Integrating experimental characterizations and molecular simulations, we revealed a specific synergistic mechanism: The NaX zeolite dominated the ion exchange of NH 4 + ions, whereas the NaA zeolite played a selective role, preferentially capturing trace molecular NH 3 using its narrow micropores and high binding energy. This selective mechanism effectively mitigated the competitive occupancy of NH 3 within the NaX pores, liberating excess active sites for NH 4 + exchange and enhancing the adsorption efficiency. Regeneration tests demonstrated that the material retained a 91.2% regeneration efficiency after five consecutive cycles using a sodium chloride solution. This study provides a strategy for designing hybrid zeolites for ammonium‑nitrogen wastewater treatment.