Interspecies electron transfer between Geobacter and denitrifying bacteria for nitrogen removal in bioelectrochemical system

Groundwater nitrate pollution is a severe environmental problem causing global concern. Among others, autotrophic denitrification on polarized electrodes is one of the energy-efficient and sustainable solutions. However, the poor nitrogen-removal rate severely hinders the application of this process. Herein, biofilms consisted of the symbiotic growth of Geobacter and denitrifying bacteria with interspecies electron transfer ability were formed on carbon paper electrodes through periodic polarity inversion. The biofilm with an inversion period of 12 h obtained the highest cathode current density (2.19 A/m2 on average), and the total nitrogen removal rate reached 4.98 g/m2Ca/d, which was an order of magnitude higher than that of the control cathode (current density 0.13 A/m2, total nitrogen removal rate 0.52 g/m2Ca/d). The excellent performance of the biofilms was attributed to the symbiotic interactions between Geobacter and denitrifying bacteria, in which Geobacter transferred electrons from the electrodes to the denitrifying bacteria for denitrification via cytochrome c-like species. Such an electron transfer process was further verified by suppressing current generation and denitrification through dosing either electron transfer inhibitors or denitrifying inhibitors. This work reveals a new extracellular electron transfer mechanism underlying the autotrophic denitrification on the cathode electrode and provides an insight into the development of efficient biocathodes for treating nitrate pollution in groundwater.