(102c) Selective Nitrogen and Phosphorus Removal and Recovery Using Hybrid Ion Exchangers and Electro-Assisted Regeneration

The presence of nitrogen (N) and phosphorus (P) species, such as ammonium and phosphate, in wastewater is concerning due to their contribution to eutrophication, which has a wide range of negative environmental effects. In addition to their role as water pollutants, N and P are key components in fertilizer. Thus, removal and recovery of both N and P not only reduces harmful environmental impacts, but also enables more sustainable fertilizer production.

Therefore, we have fabricated and characterized NH₄⁺ selective hybrid ion exchange adsorbents loaded with transition metal cations, for precision wastewater separation processes. High selectivity and capacity were achieved through ligand exchange of ammonia with ammine-complexing transition metals. Compared to commercial resins, metal–ligand exchange adsorbents exhibited higher ammonia capacity (8 meq g^-1) and selectivity (N/K⁺ equilibrium selectivity of 10.1) in binary equimolar solutions. Considering optimal ammonia concentrations (200–300 meq L^-1) and pH (9–10) for metal–ligand exchange, hydrolyzed urine was identified as a promising candidate for selective TAN recovery. Ultimately, metal–ligand exchange ad- sorbents can advance nitrogen-selective separations from wastewater.

Hybrid ion exchange adsorbents also enable phosphate removal from low-levels (< 5 mg P/L) to ultra-low levels (e.g., <0.2 mg P/L) for eutrophication control. However, chemical-intensive regeneration of ion exchangers undermines process sustainability and accessibility in remote locations. We probed an electro-assisted regeneration approach to electrify phosphate removal and recovery by an ion exchange treatment train. The treatment train contains a hybrid anion exchanger (HAIX) with doped ferric oxide nanoparticles (FeOnp) and a weak acid cation exchanger (WAC) with carboxylic functional groups. Using synthetic secondary effluent, HAIX selectively removed phosphate with < 0.2 mg P/L in the effluent due to Lewis acid-base interactions between FeOnp and phosphate; WAC selectively removed calcium due to both electrostatic interaction and bidentate complexation of calcium with carboxylic functional groups. Both HAIX and WAC have pH-dependent capacity, which enable regeneration using pH 11 and pH 3 mild regenerant, respectively. Via electrochemical water splitting of effluent from the column treatment train, we demonstrated that pH 11 catholyte and pH 3 anolyte can be produced within 60 minutes. 50% HAIX capacity and 80% WAC were maintained for multiple cycles in batch using pH 11 catholyte and pH 3 anolyte regenerant, respectively. To identify the limiting factor during regeneration with mild regenerants, we probed the intraparticle diffusivity and intraparticle diffusion path length of HAIX and WAC, and determined that intraparticle diffusion path length was the key parameter controlling HAIX and WAC regeneration.

We envision the combination of hybrid ion exchangers and electro-assisted regeneration approach will facilitate decentralized unit process development for nitrogen and phosphate recovery from wastewaters.