(710f) Molecular Electrocatalysis for Electrochemical Ammonia Recovery from Wastewater Nitrate

Rapid population growth and widespread water pollution have made the need for sustainable water purification and nitrogen fertilizer production more pressing than ever. Traditional Haber-Bosch (HB) ammonia production is highly energy− and emission−intensive. The majority of nitrogen (N) produced from HB and discharged to wastewaters goes untreated, threatening aquatic ecosystems and human health by inducing harmful algal blooms that have increased exponentially since the 1960s. This study investigates selective electrocatalytic nitrate reduction (NO₃RR) by molecular catalysis and electrochemical separations as an avenue to simultaneously remediate nitrogen pollution and electrify ammonia manufacturing.

Engineering NO₃RR processes that operate at practically needed production rates is challenging because the majority of nitrate-rich wastewaters (e.g., fertilizer runoff) have dilute nitrate concentrations (< 5 mM). Molecular catalysts are well−suited to reduce nitrate at low concentrations in real wastewaters due to tunable metal−ligand interactions that provide a well-defined coordination environment for reactant recognition and product selectivity. We use the Co−N₄ macrocyclic electrocatalyst CoDIM, a molecular NO₃RR catalyst selective for ammonia, as a model catalyst for the treatment of real, nitrate-rich wastewaters.

We study the activity, selectivity, and energy efficiency of CoDIM−catalyzed NO₃RR in electrochemical stripping (ECS), a process that facilitates reactive separation of produced ammonia from real wastewaters. From real secondary effluent, we demonstrate greater than 70% nitrate removal with a faradaic efficiency of 25% and ammonia selectivity of 98% in the cathode chamber. To utilize the anode chamber, we introduce a parallel feed of ammonium-rich reverse osmosis retentate for further N recovery. We demonstrate a peak ammonia yield of 2.9 nmol s^-1 cm^-2, ECS energy efficiency of 0.13 kWh / gram N removal, and no loss in nitrate-to-ammonia selectivity. To further benchmark CoDIM−catalyzed NO₃RR activity, we performed foot-of-the-wave and plateau current analysis. Our results highlight molecular and process-level insights that facilitate ammonia production from real wastewaters.