Wastewater Nitrate Reduction to Ammonia By a Co-Centered Molecular Catalyst

Abstract

The U.S. spends $2.2 billion annually on remediation of harmful algal blooms (eutrophication), primarily caused by anthropogenic nitrogen pollution in wastewater discharge.1 Eutrophication perturbs coastal ecosystems by disrupting food chains, decreasing biodiversity, and causing nutrient deficiencies.2 Selective nitrate reduction (NO3RR) of wastewater is an attractive remediation strategy that prevents eutrophication and recovers a value-added commodity chemical in the form of ammonia. Recovering ammonia from nitrate in fertilizer runoff and municipal wastewater could offset up to 13% of ammonia produced worldwide through the Haber-Bosch process.3–6 With renewable-sourced energy, this strategy would also offset 320,000 metric tons of CO2-eq in greenhouse gas emissions.7 In this study, we present a Co-centered macrocycle, called Co-DIM, as a homogeneous molecular catalyst for the selective reduction of nitrate to ammonia. The advantages of Co-DIM include high water solubility, tunable product selectivity by ligand modification, and a wide range of pH operability.8 Despite comprehensive studies of promising molecular catalysts performed in the CO2 reduction (CO2RR) literature,9 NO3RR has not received a similar level of attention, and comparative frameworks do not yet exist to critically evaluate catalyst activity, selectivity, and stability. In this study we combine electroanalytical techniques in model systems to bench-scale, membrane-separated electrolysis in real wastewater. By doing so, we quantify the performance of a model NO3RR electrocatalyst, and demonstrate a generalizable approach for comparative benchmarking against heterogeneous and homogeneous reduction catalysts for other reactions (e.g., CO2RR).Determining the reaction mechanism of homogeneous molecular electrocatalysis requires careful understanding of the reaction environment. Using electroanalysis techniques, such as cyclic voltammetry (CV), and electrode surface identity modification (through atomic layer deposition and self-assembled monolayers), we were able to probe the electron transfer kinetics of Co-DIM activation and NO3RR catalysis. We found that Co-DIM is activated by an outer-sphere electron transfer mechanism, thereby deemphasizing electrode material identity for catalysis. We also leveraged key features of the obtained sigmoidal CVs to build the first ever TOF-eta relationship for a NO3RR molecular catalyst. Finally, we demonstrated greater than 50% removal of nitrate using Co-DIM in electrolysis of municipal wastewater. With this experiment, we present the novelty of using rational cell architecture and observable electrocatalysis in real wastewater for substantial nitrate removal and ammonia recovery. This work aims to encourage a framework for NO3RR evaluation to foster next-generation catalyst design and device implementation for large-scale nitrate removal from wastewater.(1) Dodds, W. K.; Bouska, W. W.; Eitzmann, J. L.; Pilger, T. J.; Pitts, K. L.; Riley, A. J.; Schloesser, J. T.; Thornbrugh, D. J. Eutrophication of U.S. Freshwaters: Analysis of Potential Economic Damages. Environ. Sci. Technol. 2009, 43 (1), 12–19. https://doi.org/10.1021/es801217q.(2) Kemp, W. M.; Boynton, W. R.; Adolf, J. E.; Boesch, D. F.; Boicourt, W. C.; Brush, G.; Cornwell, J. C.; Fisher, T. R.; Glibert, P. M.; Hagy, J. D.; Harding, L. W.; Houde, E. D.; Kimmel, D. G.; Miller, W. D.; Newell, R. I. E.; Roman, M. R.; Smith, E. M.; Stevenson, J. C. Eutrophication of Chesapeake Bay: Historical Trends and Ecological Interactions. Mar. Ecol. Prog. Ser. 2005, 303, 1–29. https://doi.org/10.3354/meps303001.(3) Wastewater: The Untapped Resource; Unesco, Ed.; The United Nations world water development report; UNESCO: Paris, 2017.(4) Food and Agriculture Organization of the United Nations. World Fertilizer Trends and Outlook to 2020. 2020, 38.(5) Kato, T.; Kuroda, H.; Nakasone, H. Runoff Characteristics of Nutrients from an Agricultural Watershed with Intensive Livestock Production. J. Hydrol. 2009, 368 (1), 79–87. https://doi.org/10.1016/j.jhydrol.2009.01.028.(6) Lang, M.; Li, P.; Yan, X. Runoff Concentration and Load of Nitrogen and Phosphorus from a Residential Area in an Intensive Agricultural Watershed. Sci. Total Environ. 2013, 458–460, 238–245. https://doi.org/10.1016/j.scitotenv.2013.04.044.(7) Smith, C.; Hill, A. K.; Torrente-Murciano, L. Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape. Energy Environ. Sci. 2020, 13 (2), 331–344. https://doi.org/10.1039/C9EE02873K.(8) Xiang, Y.; Zhou, D.-L.; Rusling, J. F. Electrochemical Conversion of Nitrate to Ammonia in Water Using Cobalt-DIM as Catalyst. J. Electroanal. Chem. 1997, 424, 1–3.(9) Costentin, C.; Robert, M.; Savéant, J.-M. Catalysis of the Electrochemical Reduction of Carbon Dioxide. Chem Soc Rev 2013, 42 (6), 2423–2436. https://doi.org/10.1039/C2CS35360A.