Enhanced and prolonged adsorption of ammonia gas by zeolites derived from coal fly ash

In this study, zeolites were synthesized from coal fly ash (CFA) using NaOH (Na-X type) and KOH (chabazite-K type) and used for the adsorption of ammonia gas in a fixed-bed reactor. Magnetic separation was used to decrease the Fe impurity content from 6.84 to 2.37 wt.%, and to increase the Si and Al content in non-magnetic fly ash (NMFA). BET surface areas of the zeolites increased significantly to 425.28 m²/g with an increase in micropore volume (ca. 0.12 cm³/g). FTIR and NH₃-TPD analyses revealed that the increased surface area provided more acidic sites for enhanced ammonia gas adsorption. In particular, the zeolite (i.e., ZNF-X) synthesized from NMFA and NaOH showed the highest ammonia gas adsorption capacity (64.9 mg/g), owing to its high BET surface area and micropore volume. Although the initial adsorption capacity of commercial 13X zeolite (74.5 mg/g) was higher than that of ZNF-X, a much higher recycling efficiency for ZNF-X (92.0%) was achieved than for 13X (31.5%) during the five recycling tests because of the lower ammonia desorption temperature of ZNF-X. The results provide new insights into the synthesis of zeolites by upcycling of CFA and demonstrate the potential use of the generated materials for enhanced and prolonged removal of ammonia gas.