Microbial Biocathode Catalyzing Oxygen Reduction and Processing Nitrification

Abstract

Oxygen biocathodes with electroactive biofilms on graphite electrodes were developed consistently and reproducibly from wastewater inoculum under a high ammonium content selective pressure in a batch microbial fuel cell configuration using ferrocyanide at the anode. The microbial biocathodes process nitrification reactions and catalyse the reduction of molecular oxygen with an onset potential of 0.0 V vs. Ag /AgCl, and a current density at -0.28 V of j = -296 μA/cm2 at pH 7 and 20 °C under oxygen saturation (Cf. Figure).These biofilms also reduce hydrogen peroxide (not shown) at +0.25 V, the potential of an electroactive species in the biofilm (Cf. Figure). The nitrification reactions performed by the biofilm are found to be associated with the catalytic reduction of dioxygen. Nitrification reactions were studied by assaying the nitrogen species (ammonium, nitrite and nitrate) and the effect of adding hydroxylamine, an intermediate of the enzymatic oxidation of ammonium. Metagenomic studies via 16S rRNA gene sequencing of the biofilms onto the colonized electrodes were performed 4 week after inoculation. These studies show that the cathodic biofilms were significantly enriched with Cupriavidus, a proteobacteria genus, that is likely to be responsible for the catalytic properties of the microbial biocathode.The Figure shows the reduction of molecular oxygen by a colonized electrode at pH 7 and 20 °C in a 0.1 M fresh phosphate buffer as recorded with slow scan rate cyclic voltammetry. Scan rate: 1 mV/s. Green: abiotic graphite electrode under oxygen saturation. Black: biocathode under argon saturation. Red: biocathode under air. Blue: biocathode under oxygen saturation. Note also the chemically reversible system found at +0.25 V and assigned to the biofilm. Catalytic hydrogen peroxide reduction (not shown) occurs at the potential of this system. Figure 1