Enhancing Resource Recovery through Electro-Assisted Regeneration of an Ammonia-Selective Cation Exchange Resin

Ammonia-selective adsorbents can manage reactive nitrogen in the environment and promote a circular nutrient economy. Weak acid cation exchangers loaded with zinc exhibit high ammonia selectivity but face two implementation barriers: the stability of the zinc-carboxylate bond in complex wastewaters and energy- and logistics-intensive adsorbent regeneration with acidic solutions. In this study, we examined the stability of the zinc-carboxylate bond in varying solutions (pure ammonium solution, synthetic urine, and real urine) and during electro-assisted regeneration. For electrochemical regeneration, both electrolyte concentration and current density influenced the tradeoff between ammonia regeneration and zinc elution. Using 10 mM K2SO4 anolyte at 0.08 mA/cm2 current density, we achieved 4% zinc elution and 61% ammonia regeneration. In contrast, using 100 mM K2SO4 at 4.96 mA/cm2 improved regeneration efficiency to 97% but eluted 60% of zinc. We found that the electrolyte concentration was the key factor influencing the regeneration efficiency of NH3-selective adsorbents. Due to prevalent zinc elution, we designed an in-situ procedure for reforming the zinc-carboxylate bond and achieved similar adsorption densities between pre- and post-regenerated resin, thus enabling multiple cycle resin use. Ultimately, this study advances the understanding of ammonia-selective resins that can facilitate high-purity, selective, and durable nutrient recovery from waste streams.