The use of biocathodes in bioelectrochemical systems (BES) for the removal of nitrate in wastewater has become a vital field of research. However, the elucidation of the underlying extracellular electron transfer (EET) fundamentals of denitrifying biocathodes is still lacking, but required for a deeper BES understanding and engineering. This study reports for the first time on the thermodynamics of microbial cathodes for nitrate and nitrite reductions using microbial microcosms isolated from a running denitrifying BES. Cyclic voltammetry showed that nitrate and nitrite reduction proceed at - 0.30 V, and - 0.70 V vs. Ag/AgCl, respectively, by surface associated EET sites. The biocathodes were predominantly covered by Thiobacillus sp. contributing with a nitrate reductase (narG) to the major function of the microcosms. In conclusion, the EET characteristics of denitrifying biocathodes are demonstrated for the first time

The authors acknowledge the scientific assistance of Prof. M.D. Balaguer. F.H. acknowledges support by the BMBF (Research Award "Next generation biotechnological processes-Biotechnology 2020+") and the Helmholtz Association (Young Investigators Group). This work was supported by the Helmholtz Association within the Research Programme Terrestrial Environment and by the Spanish Government (CTQ2011-23632). N.P. was supported by the Catalan Government (2012FI-B00941) and the University of Girona (MOB2014-ref22)