The current state of centralized nitrogen (N) management has destabilized global environmental cycles via Haber-Bosch (HB) ammonia-N manufacturing which contributes 1.2% of global anthropogenic CO₂-eq emissions.¹ The majority of this N that is discharged to wastewaters goes untreated, leading to harmful algal blooms that threaten coastal and river ecosystems, which already costs the U.S. an estimated $210 billion per year in health and environmental damages.² Furthermore, the production of HB ammonia, and the subsequent discharge of wastewater nitrogen, is expected to substantially increase in the next three decades as the human population climbs to 9 billion people.³ Simultaneously removing nitrogen pollutants and recovering value-added products can preserve national water quality and supplement supply chains of nitrogen consumables with renewably sourced electricity. The electrochemical nitrate reduction reaction (NO₃RR) can be leveraged in reactive separation processes to convert wastewater nitrates to commodity products, such as ammonia. Engineering catalytic NO₃RR processes that operate at feasible rates and faradaic efficiencies is challenging because the majority of nitrate-rich wastewaters (e.g., fertilizer runoff) are dilute in nitrate concentration (< 5 mM).⁴ Molecular catalysts are uniquely suited to reduce nitrate at low concentrations in real wastewaters due to their strong substrate recognition (reactant selectivity) and product selectivity. In this study, we benchmarked the performance of the molecular catalyst Co-DIM (a Co-N₄ macrocycle complex and the only known molecular NO₃RR catalyst selective for ammonia⁵) in a reactive separations process for the treatment of real, nitrate-rich wastewaters.

We first demonstrated by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) that selective Co-DIM-mediated NO₃RR is feasible in nitrate-rich secondary effluent (municipal wastewater after biological nitrification). We then employed Co-DIM in electrochemical stripping (ECS): a membrane-separated cell that facilitates reactive separation of produced ammonia.^6,7 From real secondary effluent (28 mg NO3-N/L), we achieved greater than 60% nitrate removal with a faradaic efficiency of 25% and ammonia selectivity of 98%. However, the energy consumed for ECS per unit mass of N is 16 times the combined energy requirement for conventional wastewater N removal and HB ammonia synthesis. By introducing a mixed feed of ammonia- and nitrate-rich wastewater and performing electrodialysis (ED) to concentrate the reactant nitrate before ECS, the energy requirement for N removal and ammonia recovery was decreased by three times while the ED process became the dominant energy consumer in the overall process. Additionally, the increase in nitrate removal could not be explained by an increase in nitrate concentration alone. The ED process changes the concentrations and relative ratios of competing anions and buffering species, which can inhibit or promote the molecular electrocatalytic activity. We therefore explored a matrix of anion identities and concentrations by rotating-disk voltammetry and CPE to elucidate plausible inhibition and promotion mechanisms associated with catalyst activation and NO₃RR catalysis. This study therefore (1) benchmarks current and future efforts to reactively separate ammonia from real nitrate-rich wastewater with a molecular catalyst and (2) highlights molecular and process-level improvements to realize a circular nitrogen economy.

References

1 C. Smith, A. K. Hill and L. Torrente-Murciano, Energy Environ. Sci., 2020, 13, 331–344.

2 D. J. Sobota, J. E. Compton, M. L. McCrackin and S. Singh, Environ. Res. Lett., 2015, 10, 025006.

3 J. W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont and W. Winiwarter, Nature Geoscience, 2008, 1, 636–639.

4 Unesco, Ed., Wastewater: the untapped resource, UNESCO, Paris, 2017.

5 S. Xu, D. C. Ashley, H.-Y. Kwon, G. R. Ware, C.-H. Chen, Y. Losovyj, X. Gao, E. Jakubikova and J. M. Smith, Chem. Sci., 2018, 9, 4950–4958.

6 W. A. Tarpeh, J. M. Barazesh, T. Y. Cath and K. L. Nelson, Environ. Sci. Technol., 2018, 52, 1453–1460.

7 M. J. Liu, B. S. Neo and W. A. Tarpeh, Water Research, 2020, 169, 115226.