Fertilizer runoff and inadequate wastewater removal cause waterborne emissions of reactive nitrogen (e.g., NH₄⁺, NO₃^–) that induce harmful algal blooms. Meanwhile, both wastewater treatment and ammonia synthesis are conducted separately in centralized facilities with severe limitations. We developed a novel electrochemical reactive separation process, Electrodialysis and Nitrate Reduction (EDNR), to efficiently couple wastewater treatment and ammonia production from reactive nitrogen (N) polluted wastewaters. The EDNR reactor consists of three chambers and operates in two stages, with the influent entering the middle chamber and products obtained from the left and right chambers. In stage 1, influent NO₃^– and NH₄⁺ are separated via electrodialysis (ED) and NH₃ is recovered in the right chamber (NH₄⁺ + OH^– --> NH₃). In stage 2, NH₃ is synthesized from the electrochemical nitrate reduction (NR, NO₃^– + 8e^– + 10H⁺ --> NH₄⁺) in the left chamber. The unique combination of separation and reaction facilitates engineering of the reaction environment independent of the influent wastewater conditions and expansion of the product portfolio. We demonstrated that EDNR can effectively recover and synthesize NH₃ from N-containing synthetic and real wastewaters and enables many engineering opportunities to improve efficiency. In addition to separating reactive N from the influent into the left and right chambers, the ED stage controls pH for NH₃ recovery and the next stage of NH₃ synthesis. By tuning the applied current magnitude and duration, favorable pH was achieved and maintained, and 95% of influent NH₄⁺ was recovered into the right chamber. The unique reactor design enables flexible engineering of the NR electrolyte environment to substantially influence the activity and selectivity of this electrocatalytic process. We found that 1) ClO₄^– exhibited the least competition with NO₃^–, 2) a high concentration of Na⁺ promoted NR activity, and 3) periodic refreshment of the interfacial electrolyte improved NH₃ selectivity. Therefore, the left chamber electrolyte was chosen to be 1 M NaClO₄ and potential pulses were applied, converting ~100% influent NO₃^– to NH₄⁺ in the left chamber. Based on the understanding of each component in this unit process, we further investigated strategies to optimize energy efficiency by tuning operational parameters. By leveraging renewable electricity, the modular EDNR process enables sustainable and distributed NH₃ manufacturing.

Kate Maher

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