Abstract
Ammonium removal from water is important for preventing eutrophication, and low-silica faujasite (FAU)-type zeolites are promising ion exchangers because of their high cation-exchange capacities. However, conventional FAU-type zeolites are generally synthesized from high-purity reagents, while the combined use of agricultural and metallurgical wastes as Si and Al sources remains insufficiently explored. In this study, FAU-type zeolites were synthesized using rice husk ash and aluminum dross as Si and Al sources, respectively, and compared with reagent-derived FAU-type zeolites in terms of structure and NH4+ exchange behavior. FAU-type zeolites were obtained over a broad range of gel Si/Al ratios, whereas insufficient alkalinity produced an amorphous product and excessive alkalinity promoted secondary-phase formation. Under standard synthesis conditions, the waste-derived zeolites exhibited a surface area and micropore volume comparable to those of the reagent-derived zeolites. The waste- and reagent-derived zeolites showed similar equilibrium NH4+ uptake amounts of 47.3 and 48.3 mg g−1, respectively. Their exchange kinetics were best described by the pseudo-second-order model, while the isotherms fitted well to the Langmuir model, giving maximum exchange capacities of 2.97 and 2.85 meq g−1, respectively. Appropriate prewashing of the waste precursors enabled the waste-derived sample to retain its microporosity during repeated exchange–regeneration treatments. These results demonstrate that rice husk ash and aluminum dross can be converted into FAU-type zeolites with NH4+ exchange properties comparable to those of reagent-derived materials.