Elucidating the role of carbon shell in autotrophic denitrification driven by carbon-coated nanoscale zerovalent iron

[Display omitted] •NO3-N and TN are removed by 95% and 84% without NO2-N detected.•CS isolates Fe0 from NO3-N to alleviate biological and chemical competition.•Fe0@C shows high electron selectivity in wide range of pH, temp. and NO3-N.•H2 is the main electron donor and dominates the autotrophic denitrification.•The sustained electrons released from Fe0@C promote TN removal capacity by Fe0@C. Iron-carbon micro-electrolysis (IC-ME) can promote Fe0 corrosion to provide H2 or e− as electron donors for autotrophic denitrifier under organic-limited wastewater, but also stimulate the chemical reduction of NO3-N to undesired NH4-N. Therefore, it is still a challenge to optimize the eletron pathways provided by Fe0 to improve its eletron selectivity for biodenitrification. Herein, a Carbon-coated Iron (Fe0@C) Autotrophic Denitrification (CCIAD) system was first established to achieve higher removal efficiency of NO3-N (95%) and TN (84%) without NO2-N accumulation, and the role of carbon shell (CS) in Fe0@C was systematically evaluated. Hydrophobic CS could prevent iron oxidation from oxygen and isolate Fe0 from NO3-N to alleviate the competition of biological and chemical processes for reducing nitrate. In addition, the CS promoted e− generation, while inhibited the adsorption of NO3-N, thereby increasing the H2 production. Therefore, the assembly of carbon coated Fe0 regulated the electron pathway so that H2 became the main electron donor and dominated the autotrophic denitrification, thereby improving the electron efficiency of Fe0 (82.7%). The sustained electron release of Fe0@C improved its TN removal capacity (78 mg N/g Fe). This study shed light on the electronic pathways provided by Fe0@C and advanced our understanding of Fe0@C-based bio-denitrification for the purpose of next-generation Fe0 design.