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⁺.