(297b) Highly Selective and Efficient Ammonia Removal and Recovery from Hydrolyzed Urine Using Zinc Oxalate

Recovering valuable products from wastewater can reduce energy inputs, costs, and environmental damage associated with pollution mitigation and chemical manufacturing but requires highly selective separation materials integrated into effective unit processes. The advantages of selective resource recovery materials are especially apparent within the nitrogen cycle. Roughly 1% of global energy is used by the Haber-Bosch process to convert atmospheric dinitrogen gas (N₂) to ammonia (NH₃), producing carbon dioxide (CO₂) as a byproduct. Anthropogenic sources of reactive nitrogen have unbalanced the nitrogen cycle because 80% of wastewater globally is discharged without treatment, and reactive nitrogen threatens aquatic ecosystems with eutrophication. Moreover, conventional biological wastewater treatment converts 100-fold less reactive nitrogen back to N₂ per year than is generated by Haber-Bosch, necessitating additional techniques that supplement biological treatment. Materials capable of selectively recovering reactive nitrogen from wastewaters without chemical transformation can provide an economic incentive for expanding global environmental remediation and can mitigate dependence on both the Haber-Bosch process and biological wastewater treatment.

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 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. High valence, small hydration radii, and low hydration energies dictate high ion exchange selectivity. However, ammonium is monovalent and has intermediate hydration properties compared to other common wastewater cations like sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), and calcium (Ca²⁺).

To overcome the selectivity constraints of ion exchange, we synthesized zinc oxalate from oxalic acid and zinc sulfate. Because divalent zinc (Zn²⁺) is an ammine-complexing transition metal cation and zinc oxalate is extremely water insoluble (1.38*10^−9 g/100 mL), precipitated zinc oxalate is an effective aqueous ligand exchanger. While ion exchange involves purely outer-sphere electrostatic attraction, ligand exchange involves a ligand supplying both electrons to a coordinate covalent bond. 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.

Considering the optimal pH for ammonia adsorption (9-10), hydrolyzed urine, which contains 80% of nutrients found in municipal wastewater but comprises only 1% of wastewater volume, was identified as a promising wastewater candidate. 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%. Furthermore, negligible amounts (i.e., below the detection limit of IC) of the competing cations present in hydrolyzed urine, sodium and potassium, were removed. Above an ammonia to zinc ratio of 2, ammonia began to outcompete oxalate for zinc ligand sites, and the zinc-ammonia complex began to elute. 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. 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.