(124b) Synthesis and Characterization of Ligand-Exchange Adsorbents for Wastewater Refining

Recovering reactive nitrogen species from wastewater can help rebalance the nitrogen cycle. Ion exchange can efficiently recover ionized and ionizable reactive nitrogen species like ammonium (NH₄⁺)/ammonia (i.e., total ammonia nitrogen or TAN). Ion exchange with fixed-bed column operation exhibits several process advantages for selective total ammonia nitrogen recovery, including low electricity requirements, flexible scale, and suitability for decentralized applications. However, existing ion exchange resins with fixed anionic moieties are not intrinsically selective for the ammonium cation over other cations. To overcome the selectivity constraints of ion exchange, we synthesized zinc oxalate from oxalic acid and zinc sulfate. Selectivity is achieved because zinc binds too strongly to oxalate for competing cations to ion exchange with zinc, but TAN in the neutral ammonia form bypasses ion exchange competition and binds to the zinc itself with its electron lone pair. Additionally, if trace amounts of zinc-ammonia complexes are removed by ligand exchange competition between ammonia and oxalate, then oxalate will dissolve because sodium oxalate, potassium oxalate, and ammonium oxalate are water soluble. Consequently, nearly all that is left in the precipitate is zinc oxalate.

Batch ammonia adsorption tests were conducted in real hydrolyzed urine with zinc oxalate and a commercial tertiary amine resin (AmberLite IRA67, pKa 9) to buffer pH and preserve TAN speciation as ammonia over ammonium without introducing additional cations to the solution. Initial and final ion concentrations were analyzed with ion chromatography (IC). As long as the dose of zinc oxalate was high enough to establish an ammonia to zinc ratio of 2 and below in the system, over 95% of ammonia was removed, and zinc elution was below 0.5%. To show effective TAN recovery as well as removal, ammonia-loaded zinc oxalate was placed in 250 mM acetic acid. Over 99% of ammonia was recovered as ammonium acetate, and zinc elution due to oxalate protonation was below 0.2%. Therefore, zinc oxalate is highly selective for ammonia removal in a real wastewater stream, mild acids are effective at efficient TAN recovery, and zinc oxalate is robust against low pH levels. We have supplemented batch studies with flow-based synthesis and continuous-flow performance testing in columns more representative of practice. These studies also enable tuning of adsorption kinetics relative to flow rate, and evaluating the effect of flow fields on observed adsorption performance. Ultimately, ligand exchange adsorbents can aid in advancing a future vision of a circular nitrogen economy that extracts ammonia pollutants from wastewaters and offsets anthropogenic imbalances to the global nitrogen cycle.