Soluble-electron mediator assisted bioelectrochemical reduction of nitrate to ammonium for nitrogen recovery

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Bioelectrochemical reduction of dilute nitrate (NO 3 − ; sub-mM) to ammonium (NH 4 + ) is a promising route toward circular nitrogen management from nitrate-contaminated groundwater and surface waters. However, conventional bioelectrochemical systems (BES) rely on extracellular electron transfer through electrode-attached biofilms, restricting catalysis to a few electroactive bacteria in direct contact with the electrode. Here, we developed a soluble-mediator-assisted BES in which a diffusible mediator shuttles electrons to suspended cells throughout the catholyte, enabling weakly electroactive bacteria to serve as nitrate-reduction catalysts. Three biocatalysts ( Escherichia coli , Shewanella oneidensis , and Candidatus Brocadia sinica), nine soluble mediators spanning viologens, quinones, and phenazines, cathodic potentials from −0.3 to −0.9 V, and pH 6.5 to 8.5 were screened, and NO 3 − reduction to NH 4 + was confirmed using a 15 NO 3 − tracer. With E. coli cells and methyl viologen, NO 3 − was reduced to NH 4 + at a maximum volumetric rate of 10.9 mmol-N L −1 day −1 and a low NO 3 − -affinity constant ( K s = 0.19 mM). This rate was 8-fold higher than previously reported biofilm-based systems, and NO 3 − reduction was repeated over three successive NO 3 − additions during 48 h of operation. Furthermore, the soluble-mediator-assisted BES showed high performance even in natural groundwater and surface water, with a Faradaic efficiency of 50–59% and an electricity consumption of 188–192 kWh kg-NO 3 − -N −1 , substantially lower than metal-catalyst systems at comparable NO 3 − concentration. These findings demonstrate that soluble-mediator-assisted BESs provide an energy-efficient platform for converting NO 3 − in natural water into a recoverable NH 4 + form, with full nitrogen recovery additionally requiring downstream NH 4 + concentration or separation.