(292a) Electrodialysis and Nitrate Reduction: An Electrochemical Reactive Separations Platform for Distributed Ammonia Recovery from Wastewaters

Ammonia is an essential compound to modern day fertilizer production and chemical manufacturing. Global ammonia demand exceeds 150 million tons a year (market value 70 billion USD), with over 96% fulfilled by the Haber Bosch (HB) process. However, HB production plants are energy-intensive (1-2% annual global energy consumption), carbon-intensive (1.2-1.4% annual anthropogenic CO₂ emissions), and concentrated in developed countries, leading to inequitable pricing and distribution.^1–5 Electrifying ammonia production with renewable energy can help offset the energy and carbon-intensive HB process in a distributed fashion, where modular process units powered by renewable electricity leverage feedstocks to deliver ammonia at the source.

Wastewaters are an underutilized feedstock for electrified ammonia production. Large amounts of reactive nitrogen, such as HB ammonia, accumulate in the biosphere because 80% of wastewater globally is discharged without treatment.⁶ Recovering nitrogen (N) from wastewater can simultaneously fulfill the roles of traditional removal technologies such as nitrification-denitrification and of fertilizer production processes such as HB. Two forms of reactive nitrogen dominate aqueous nitrogen emissions: ammonium (NH₄⁺) and nitrate (NO₃^-). Wastewater ammonium can be recovered through processes that selectively separate it from other wastewater constituents. Meanwhile, wastewater nitrate can be recovered as ammonia through a selective electrocatalytic reduction reaction followed by a selective separation process. Ammonia recovery from ammonium- and nitrate-rich wastewaters therefore creates opportunities to co-locate selective reactions with selective separations. In this talk, we describe Electrodialysis and Nitrate Reduction (EDNR), a novel electrochemical reactive separation process that efficiently couples water treatment and ammonia production from wastewaters.

Methods

The EDNR reactor consists of three chambers and operates in two stages (Figure 1a). Wastewater influent enters the middle chamber while ammonia is electrochemically recovered in the left and right chambers. In Stage 1 (electrodialysis), influent and migrate to the left and right chambers, respectively, due to an applied potential. During this stage, hydrogen evolution in the right chamber creates an alkaline environment (pH ~ 12). As a result, influent ammonium deprotonates to yield ammonia (pKa ~ 9.25): NH₄⁺ <--> NH₃ + H⁺ . In Stage 2 (nitrate reduction), ammonia is selectively produced from the electrochemical nitrate reduction (NO₃RR) in the left chamber: NO₃^- + 8e^- + 9H⁺ --> NH₃ + 3H₂O. Cycling between the two stages generates concentrated ammonia solutions in the right and left chambers. EDNR leverages electrochemical swings to facilitate engineering of the reaction environment in each chamber such that (1) process control is independent of the influent wastewater and (2) a broad portfolio of products can be recovered. For example, phosphorus (P) and/or potassium (K)-containing electrolytes can be used in the left and right chambers to allow production of different fertilizer blends by tuning the N-P-K content (the three macronutrients needed for plant growth).

Based on process and product needs, several materials were employed for specific purposes EDNR experiments. In the left chamber, Ti/IrO₂-Ta₂O₅ mesh was used for electrodialysis (Stage 1). Meanwhile, Ti foil was used as the NO₃RR electrode (Stage 2). Our group has previously characterized the near-surface structure of Ti foil under NO₃RR conditions and identified an optimal NH₃ Faradaic efficiency at −0.8 V_RHE.⁷ Ti/IrO₂-Ta₂O₅ mesh was used in the middle chamber as a counter electrode for NO₃RR. Lastly, Pt foil was used in the right chamber for electrodialysis. An anion exchange membrane separates the left and middle chambers, and a cation exchange membrane separates the middle and right chambers. Simulated wastewater influent (1.6 mM KNO₃ + 13.9 mM (NH₄)₂SO₄) and real wastewater (reverse osmosis brine) were used. Electrolytes in each chamber were recirculated in batch.

Aqueous speciation was quantified with cation chromatography, anion chromatography, and flow injection analysis (to measure total ammonia nitrogen: sum of aqueous ammonium and aqueous ammonia; indophenol method).

Results

During Stage 1 (electrodialysis), we demonstrate effective pH control in the left and right chambers (Figure 1b). The left chamber acidifies to a pH slightly above 1.5, which has been identified as optimal for NO₃RR on Ti.⁸ The right chamber basifies to pH ~ 12, facilitating deprotonation of to yield ammonia. After three cycles of EDNR, >95% of influent is recovered as ammonia in the right chamber (Figure 1c). Meanwhile, ~100% of influent is electrochemically reduced to ammonia in the left chamber (Figure 1d). We found three factors that enabled full nitrate conversion to ammonia: (1) the choice of weakly adsorbing perchlorate in the supporting salt; (2) the use of a high concentration of sodium in the supporting salt; and (3) pulsed electrolysis to periodically refresh the interfacial electrolyte. Parametric analyses of additional operating parameters further optimize the energy efficiency, nitrate conversion, and ammonia recovery rate of EDNR.

References

(1) Smith, C.; Hill, A. K.; Torrente-Murciano, L. Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape. Energy Environ. Sci. 2020, 13 (2), 331–344. https://doi.org/10.1039/C9EE02873K.

(2) MacFarlane, D. R.; Cherepanov, P. V.; Choi, J.; Suryanto, B. H. R.; Hodgetts, R. Y.; Bakker, J. M.; Ferrero Vallana, F. M.; Simonov, A. N. A Roadmap to the Ammonia Economy. Joule 2020, 4 (6), 1186–1205. https://doi.org/10.1016/j.joule.2020.04.004.

(3) Kyriakou, V.; Garagounis, I.; Vourros, A.; Vasileiou, E.; Stoukides, M. An Electrochemical Haber-Bosch Process. Joule 2020, 4 (1), 142–158. https://doi.org/10.1016/j.joule.2019.10.006.

(4) Fernandez, C. A.; Hatzell, M. C. Editors’ Choice—Economic Considerations for Low-Temperature Electrochemical Ammonia Production: Achieving Haber-Bosch Parity. J. Electrochem. Soc. 2020, 167 (14), 143504. https://doi.org/10.1149/1945-7111/abc35b.

(5) Comer, B. M.; Fuentes, P.; Dimkpa, C. O.; Liu, Y.-H.; Fernandez, C. A.; Arora, P.; Realff, M.; Singh, U.; Hatzell, M. C.; Medford, A. J. Prospects and Challenges for Solar Fertilizers. Joule 2019, 3 (7), 1578–1605. https://doi.org/10.1016/j.joule.2019.05.001.

(6) Clark, B.; Tarpeh, W. A. Selective Recovery of Ammonia Nitrogen from Wastewaters with Transition Metal-Loaded Polymeric Cation Exchange Adsorbents. Chemistry – A European Journal 2020, 26 (44), 10099–10112. https://doi.org/10.1002/chem.202002170.

(7) Liu, M. J.; Guo, J.; Hoffman, A. S.; Stenlid, J. H.; Tang, M. T.; Corson, E. R.; Stone, K. H.; Abild-Pedersen, F.; Bare, S. R.; Tarpeh, W. A. Catalytic Performance and Near-Surface X-Ray Characterization of Titanium Hydride Electrodes for the Electrochemical Nitrate Reduction Reaction. J. Am. Chem. Soc. 2022, 144 (13), 5739–5744. https://doi.org/10.1021/jacs.2c01274.

(8) McEnaney, J. M.; Blair, S. J.; Nielander, A. C.; Schwalbe, J. A.; Koshy, D. M.; Cargnello, M.; Jaramillo, T. F. Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Ammonia on a Titanium Electrode. ACS Sustainable Chem. Eng. 2020, 8 (7), 2672–2681. https://doi.org/10.1021/acssuschemeng.9b05983.

Figure 1 Caption. EDNR process and performance for 3-cycle experiment. Left and right chamber electrolytes: 1 M NaClO₄. Each cycle consisted of a 1-hr ED stage (constant current: 22.5 mA) and a 2-hr NR stage (pulsed potential, cathodic potential: –0.8 V_RHE for 10 s, and returned to OCV for 10 s). (a) EDNR process schematic. (b) pH vs. time in the left, middle, and right chamber electrode. (c) ED efficiency (mol of NH3 in right chamber / mol of ammonium initially present in middle chamber) vs. cycle number. (d) NR efficiency (mol of NH3 in left chamber / mol of nitrate initially present in middle chamber) vs. cycle number