Engineered Crown-Ether-Functionalized Hydrogels Enable Selective Ammonium Extraction

Ammonium (NH4+) recovery is essential for shifting from conventional pollutant removal to resource recovery strategies. However, the effective capture of NH4+ is challenged by competition from coexisting ions such as K+ and Na+ in wastewater. In this study, poly(acrylic acid) (PAA) hydrogels were functionalized with 15-crown-5 (15C5) to enhance the NH4+ selectivity. While the unmodified PAA hydrogel exhibited an inherent cation preference of K+ > Na+ > NH4+, incorporation of 15C5 reversed the order to K+ > NH4+ > Na+, which demonstrated a notable improvement given the high abundance of Na+ in wastewater. Mechanistic analysis indicated that 2:1 (NH4+)2·15C5 complex formation, together with solvation effects, likely contributed to the enhanced NH4+ binding. Notably, an optimal NH4+ selectivity coefficient of 1.8 over Na+ was achieved at intermediate cation concentrations (∼1.8 mmol/L), highlighting concentration-dependent optimization opportunities. The NH4+ selectivity of the functionalized hydrogels was also found to be dependent on initial ion concentrations and contact time, underscoring the importance of optimizing the balance between 1:1 and 2:1 complexation to maximize selective capture in practical applications. These findings demonstrate a promising approach for NH4+ recovery using PAA-g-15C5 hydrogels and provide mechanistic insights with practical implications for sustainable NH4+ recovery in complex aqueous systems containing competing ions such as Na+.