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Multiphysics modelling reveals how the electric double layer governs nitrate transport and how a more negative catalyst potential-of-zero-charge promotes ammonia formation. 

Electrocatalyst-in-a-box, a novel reactive separation process, enables a molecular catalyst to convert wastewater nitrate into purified ammonia. 

Underutilized wastewaters containing dilute levels of reactive nitrogen (Nr) can help rebalance the nitrogen cycle. 

Ammonia is an essential compound to modern society, underpinning fertilizer production and chemical manufacturing. Global ammonia demand currently exceeds 150 million tons a year and is projected to increase over 2% annually. Over 96% of ammonia is currently generated through the Haber-Bosch (HB) process, in which steam-reformed hydrogen reacts with nitrogen under reaction conditions that consume 1–2% of global energy and contribute 1.2–1.4% of anthropogenic CO2 emissions every year. In an environmental context, ammonia is a form of reactive nitrogen. Large amounts of reactive nitrogen, such as HB ammonia, accumulate in the biosphere because 80% of wastewater globally is discharged without treatment. The resulting skew in the global nitrogen cycle leads to imbalanced ecosystems and threatens water quality. Conventional water treatment removes reactive nitrogen by converting it to N2 (biological nitrification–denitrification); at HB facilities, the N2 is then cycled back to produce ammonia. Directly valorizing reactive nitrogen in waste streams would shortcut the use of N2 as an intermediate in water remediation and ammonia production, allowing savings in energy, emissions, and costs. Indeed, treating nitrogen as a resource to recover rather than simply a pollutant to remove aligns with the US National Academy of Engineering’s call to manage the nitrogen cycle, a challenge central to chemical manufacturing and ecosystem protection.