Electrocatalytic Nitrate Reduction for Denitrifying Wastewater

Abstract

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.