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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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The Role of Carbon to Nitrogen Ratio on the Performance of Denitrifying Biocathodes for Decentralized Wastewater Treatment
Bioelectrochemical systems with denitrifying biocathodes have been of interest for the removal of nitrate in decentralized wastewater treatment applications. Only a few studies have directly focused on this application, but the removal rates have been very low. This study evaluated the operational parameters that affect the nitrate removal of two-chambered microbial fuel cells (MFCs) with a biocathode, particularly, the carbon to nitrogen ratio (C:N) and proton diffusivity across electrode chambers. The results show that proton diffusion across a proton exchange membrane is not a limiting step in nitrogen removal performance. At C:N ratios of 4 and 8, biocathodes with a continuously supplied carbon source at the anode were able to achieve complete nitrogen removal at a rate of 0.97 ± 0.21 and 1.15 ± 0.13 mg N L−1 d−1, respectively. However, as the C:N ratio increased from 4, 8, 16, and 32, the electrode potentials decreased accordingly. Ratio 4 C:N had a cathodic reduction potential of +66.1 ± 5.3 mV vs. SHE and dropped to −78.6 ± 9.8 mV vs. SHE at 32 C:N. The cathode electrode potential can be controlled by way of the carbon concentrations at the anode, which can have major indirect implications on the evolution of cathodic microbial communities that have preference to particular ranges of reduction potentials. The cathodic biofilms in this study were dominated by the phyla Proteobacteria, Acidobacteria, Bacteroidetes and Nitrospirae, which are known to have key denitrifying microorganisms. The genus Stenotrophomonas was found in abundance within the attached cathode biofilm and to a lesser extent in the suspended biomass. Vibrio, Acidobacteria_Gp4, Nitrosomonas, and Candidatus Competibacter were also cultivated in both the suspended and attached biomass. Nitrospira was only found in the attached biofilm. Regardless of operational scheme, nitrogen removal was improved at low C:N ratios, with 8 C:N having the best performance overall. This indicates that higher C:N ratios than were previously explored (>4 C:N) provide sufficient coulombs to facilitate denitrification at the cathode even while the anodic CEs remain low. Reactor design modifications should be considered to fully support robust denitrifying communities, enhancing the overall nitrogen removal for decentralized wastewater treatment applications.
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- Award ID(s):
- 1930451
- PAR ID:
- 10555112
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Water
- Volume:
- 14
- Issue:
- 19
- ISSN:
- 2073-4441
- Page Range / eLocation ID:
- 3076
- 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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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/w14193076
