Electrochemical insights into the two-step electron transfer mechanism of denitrifying biocathodes

Microbial electrochemical denitrification (MED) is emerging as a promising approach for nitrogen removal. However, its efficiency is limited by the low-rate extracellular electron transfer (EET) that occurs between microorganisms and electrodes. Due to the complex nature of electrochemical biofilms, the electron transfer mechanism in MED remains unclear. This study performs electrochemical characterization to investigate the EET mechanism of the MED biocathode. Reduction peaks observed at −0.442 ​± ​0.13 ​V and −0.218 ​± ​0.08 ​V vs. Ag/AgCl in cyclic voltammetry indicate the involvement of H 2 ase and cytochrome c in EET process, corroborated by the liner relationship between peak currents and scan rates. The decrease in charge transfer resistance of 19.2 ​% and 51.6 ​% at −0.2 ​V and −0.4 ​V, respectively, in electrochemical impedance spectroscopy fitting, indicates that cytochrome c and H 2 ase function in series to facilitate electron transfer. Elevated abundances of genes associated with H 2 ase and cytochrome c in the electroactive biofilm further support the combination of these two compounds involved in EET process. Findings in this study highlight the importance of H 2 ase and cytochrome c in MED, potentially offering alternative strategies for optimizing the MED system. • Comprehensive electrochemical characterization on denitrifying biocathode. • Direct electron transfer was identified in denitrifying biocathode. • Combination of H 2 ase and cytochrome c is the foundation of direct electron transfer. • An H 2 ase-cytochrome c pathway was proposed.