Oxidation of ammonium by Feammox Acidimicrobiaceae sp. A6 in anaerobic microbial electrolysis cells

Abstract

Anaerobic ammonium oxidation under iron reducing conditions, also referred to as Feammox, can be carried out by the recently isolated Acidimicrobiaceae sp. A6 (A6). Ammonium is a common water pollutant which is typically removed by nitrification, a process that exerts a high oxygen demand in waste treatment systems. A6 oxidizes ammonium anaerobically using ferric iron [Fe(III)] as an electron acceptor and has also been shown to be an electrode (anode) colonizing bacterium. Results presented here demonstrate that A6, in a pure or enrichment culture, can thrive in microbial electrolysis cells (MECs) by oxidizing ammonium, while using the anode as an electron acceptor. Results also show that current production and ammonium removal increase with the concentration of 9,10-anthraquinone-2,6-disulfonic acid (AQDS), a soluble electron shuttling compound, which is especially noticeable for the pure A6 culture. Electron microscopy of the anode's surface reveals attached cells in the pure culture MEC; however, over the time of operation there is no formation of a biofilm and the majority of cells are in the bulk liquid, explaining the need for AQDS. Maximum coulombic efficiencies of 16.4% and a current density of 4.2 A m⁻³ were measured. This is a first step towards the development of a Feammox bacteria-based bioelectrochemical system for anaerobic ammonium oxidation while reducing electrodes instead of Fe(III).