(245a) Developing a Continuous Electrocatalytic Reactive Separation System for Purified Ammonia Recovery from Wastewater Nitrate

Wastewater nitrate (NO₃^–) emissions overwhelm aquatic ecosystems, causing algal blooms and billions of dollars in damage to U.S. surface waters (Sobota, et al., 2015). The electrochemical nitrate reduction reaction (NO₃RR) can simultaneously remove wastewater nitrate and produce ammonia, remediating aqueous pollution and circularizing ammonia manufacturing. However, translating NO₃RR to practice is challenged by the dilute and impure nature of real wastewaters (UNESCO, 2017), and the need to incorporate electrocatalysis with separations (nitrate removal and ammonia recovery). To bridge these critical gaps, we engineered an electrochemical reactive separations unit process called Electrocatalyst-in-a-Box (ECaB) that performs three subunit processes in tandem: selective wastewater nitrate extraction, nitrate conversion to ammonia, and purified ammonia recovery. In this work, we rationally optimized ECaB performance to develop a continuous electrocatalytic reactor system. We quantified contributions of solution-phase mass transport and kinetics to Donnan dialysis (DD) extraction and NO₃RR conversion subunit process rates. For DD, we found solution-phase transport limitations account for 42% of the observed extraction rate at low linear flow rates (32 cm s^–1), but those limitations are completely overcome at higher flow rates (170 cm s^–1). For NO₃RR, we found a local optimum flow rate that balances nitrate delivery at high flow rates with hydrogen evolution suppression at low flow rates, achieving an ammonia partial current density of ~200 mA cm^–2 at a modest potential (–0.53 V_RHE). We incorporated mass transport and kinetic rate constants of each subunit process into a full process model, and minimized capital and operating costs to obtain operating parameters for a continuous reactor to treat real fertilizer runoff. Finally, we validated this model with a benchtop continuous ECaB experiment. Altogether, this work constitutes a use-informed approach to electrochemical reactive separations engineering that quantitatively motivates fundamental studies for translation of electrochemical nitrate remediation to practice.