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The nitrogen cycle plays a key role biological, energy, environment, and industrial processes. Breaking natural nitrogen cycle is leading to accumulation of reactive nitrogen chemicals in water and atmosphere, therefore, better management of N-cycle has emerged as an urgent research need in energy and environmental science. Removing excessive nitrate (NO3−) from wastewater has increasingly become an important research topic in light of the growing concerns over the related environmental problems and health issues. In particular, catalytic/electrocatalytic approaches are attractive for NO3− removal, because NO3− from wastewater can be converted to N2 and released back to the atmosphere using renewable H2 or electricity, closing the loop of the global N cycle. However, achieving high product selectivity towards the desirable N2 has proven challenging in the direct NO3−-to-N2 reaction. In this presentation, we will report our finding on unique and ultra-high electrochemical NO3−-to-NO2−activity on an oxide-derived silver electrode (OD-Ag). Up to 98% selectivity and 95% faradaic efficiency of NO2− were observed and maintained under a wide potential window. Benefiting from overcoming the rate-determining barrier of NO3−-to-NO2−during nitrate reduction, further reduction of accumulated NO2− to NH4+ can be well regulated by the cathodic potential on OD-Ag to achieve a faradaic efficiency of 89%. These indicated the potential controllable pathway to the key nitrate reduction products (NO2−or NH4+) on OD-Ag. DFT computations provided insights into the unique NO2−selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO3−-to-NO2−activity on OD-Ag, we designed a novel electrocatalytic-catalytic combined process for denitrifying real-world NO3−-containing agricultural wastewater, leading to 95+% of NO3− conversion to N2 with minimal NOx gases. Importantly, NO2− derived from nitrate may serve as a crucial reactive platform for distributed production of various nitrogen products, such as NO, NH2OH, NH3, and urea.
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Electrocatalytically Denitrifying Wastewater Based on Unique Nitrate-to-Nitrite Selectivity on Ag
In our study, an exceptionally high selectivity of the electrocatalytic nitrate-to-nitrite transformation was discovered on Ag surfaces, among eighteen metals screened. It was demonstrated that this electrocatalytic step on oxide-derived Ag (OD-Ag), which possesses extended surface area (13 times) and enhanced specific activity (3 times) relative to Ag foil, can be coupled with a catalytic nitrite-to-dinitrogen step on a Pd catalyst using renewable hydrogen generated on-site by a water electrolyzer. We thereby proposed and demonstrated a combined electrocatalytic-catalytic process as an alternative strategy for innovative nitrate removal from agricultural wastewater with high selectivity of >95%. With future research and development, the combined process may hold the potential of tackling the ever-increasing nitrate pollution in water bodies to address its linked environmental and health issues. Strategically returning reactive nitrogen from wastewater back to the atmosphere in the inert form, this combined process is well-positioned to help close the global nitrogen cycle, one of the grand engineering challenges in the 21st century. In parallel with the applications in wastewater treatment, the Ag-based electrocatalytic nitrate-to-nitrate conversion with ultrahigh selectivity may be widely employed for designing cost-effective and energy-efficient syntheses of various nitrogen-based compounds in a distributed manufacturing fashion. The kinetics studies and computational insights could also be beneficial to advancing nitrogen-centric electrochemistry, materials science, and technologies.
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- Award ID(s):
- 2036944
- PAR ID:
- 10285371
- Date Published:
- Journal Name:
- ECS Meeting Abstracts
- Volume:
- MA2021-01
- Page Range / eLocation ID:
- 1296
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Removing excessive nitrate (NO3−) from wastewater has increasingly become an important research topic in light of the growing concerns over the related environmental problems and health issues. In particular, catalytic/electrocatalytic approaches are attractive for NO3− removal, because NO3− from wastewater can be converted to N2 and released back to the atmosphere using renewable H2 or electricity, closing the loop of the global N cycle. However, achieving high product selectivity towards the desirable N2 has proven challenging in the direct NO3−-to-N2 reaction. In this presentation, we will report our finding on unique and ultra-high electrochemical NO3−-to-NO2−activity on an oxide-derived silver electrode (OD-Ag). Up to 98% selectivity and 95% faradaic efficiency of NO2− were observed and maintained under a wide potential window. Benefiting from overcoming the rate-determining barrier of NO3−-to-NO2−during nitrate reduction, further reduction of accumulated NO2− to NH4+ can be well regulated by the cathodic potential on OD-Ag to achieve a faradaic efficiency of 89%. These indicated the potential controllable pathway to the key nitrate reduction products (NO2−or NH4+) on OD-Ag. DFT computations provided insights into the unique NO2−selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO3−-to-NO2−activity on OD-Ag, we designed a novel electrocatalytic-catalytic combined process for denitrifying real-world NO3−-containing agricultural wastewater, leading to 95+% of NO3− conversion to N2 with minimal NOX gases. In addition to the wastewater treatment process to N2 and electrochemical synthesis of NH3, NO2− derived from electrocatalytic NO3− conversion can serve as a reactive platform for distributed production of various nitrogen products. Our new research progress along this direction will be briefly presented.more » « less
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Ion exchange is widely used to treat nitrate-contaminated groundwater, but high salt usage for resin regeneration and management of waste brine residuals increase treatment costs and add environmental burdens. Development of palladium-based catalytic nitrate treatment systems for brine treatment and reuse has showed promising activity for nitrate reduction and selectivity towards the N2 over the alternative product ammonia, but this strategy overlooks the potential value of nitrogen resources. Here, we evaluated a hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery during treatment and reuse of nitrate-contaminated waste ion exchange brines. In the first step of the hybrid process, a Ru/C catalyst with high selectivity towards ammonia was found to be effective for nitrate hydrogenation under conditions representative of waste brines, including expected salt buildup that would occur with repeated brine reuse cycles. The apparent rate constants normalized to metal mass (0.30 ± 0.03 mM min−1 gRu−1 under baseline condition) were comparable to the state-of-the-art bimetallic Pd catalyst. In the second stage of the hybrid process, membrane distillation was applied to recover the ammonia product from the brine matrix, capturing nitrogen as ammonium sulfate, a commercial fertilizer product. Solution pH significantly influenced the rate of ammonia mass transfer through the gas-permeable membrane by controlling the fraction of free ammonia species (NH3) present in the solution. The rate of ammonia recovery was not affected by increasing salt levels in the brine, indicating the feasibility of membrane distillation for recovering ammonia over repeated reuse cycles. Finally, high rates of nitrate hydrogenation (apparent rate constant 1.80 ± 0.04 mM min−1 gRu−1) and ammonia recovery (overall mass transfer coefficient 0.20 m h−1) with the hybrid treatment process were demonstrated when treating a real waste ion exchange brine obtained from a drinking water utility. These findings introduce an innovative strategy for recycling waste ion exchange brine while simultaneously recovering potentially valuable nitrogen resources when treating contaminated groundwater.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/https://doi.org/10.1149/MA2021-01401296mtgabs
