Environmental Science Water Research & Technology PAPER Cite this: DOI: 10.1039/c9ew00366e Received 1st May 2019, Accepted 9th July 2019 DOI: 10.1039/c9ew00366e rsc.li/es-water Oxidation of ammonium by Feammox Acidimicrobiaceae sp. A6 in anaerobic microbial electrolysis cells† Melany Ruiz-Urigüen, a Daniel Steingart b and Peter R. Jaffé * a Anaerobic ammonium oxidation under iron reducing conditions, also referred to as Feammox, can be car- ried 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 [FeIJIII)] 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 anaer- obic ammonium oxidation while reducing electrodes instead of FeIJIII). 1. Introduction Ammonium (NH 4 + ) can accumulate in soil and water 1 and can be detrimental to the environment, particularly water sys- tems. Nitrification, the conversion of NH 4 + to nitrite (NO 2 - ) and nitrate (NO 3 - ), is the most extensively used method to oxidize NH 4 + in engineered systems. However, nitrification is energetically intensive as it requires oxygen inputs, which can account for a substantial amount of energy usage in wastewater treatment plants during the operation of aera- tors. 2,3 To lower the energy consumption for wastewater treat- ment, anaerobic oxidization of NH 4 + is a worthwhile en- deavor. Anammox oxidizes NH 4 + anaerobically by coupling it to NO 2 - reduction, but some aeration is still required to form the needed NO 2 - . Anaerobic NH 4 + oxidation under iron reduc- ing conditions is commonly referred to as Feammox. The oxi- dation of NH 4 + via Feammox to NO 2 - (ref. 4–8) occurs in the absence of molecular oxygen, which makes it an attractive candidate for the development of an energy efficient NH 4 + re- moval method. However, it requires iron oxides [FeIJIII)] as electron acceptors in a stoichiometric ratio of 6 : 1 (ref. 4 and 9) (eqn (1)–(3)). NH 4 + + 2H 2 O → NO 2 - + 8H + + 6e - (1) 3Fe 2 O 3 ·05H 2 O + 18H + + 6e - → 6Fe 2+ + 10.5H 2 O (2) NH 4 + + 3Fe 2 O 3 ·05H 2 O + 10H + → NO 2 - + 6Fe 2+ + 8.5H 2 O (3) Iron is abundant in the environment and thus Feammox can be enhanced in systems such as constructed wetlands to treat some wastewaters. 10 However, adding a stoichiometric amount of FeIJIII), shown in eqn (1), to wastewater treatment Environ. Sci.: Water Res. Technol. This journal is © The Royal Society of Chemistry 2019 a Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA. E-mail: jaffe@princeton.edu b Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c9ew00366e Water impact Coupling the Feammox process to MECs to facilitate the removal of ammonium using the anode as an electron acceptor, instead of having to add solid- phase ferric iron to a reactor, is a first step for the development of new Feammox-based anaerobic methods for ammonium oxidation, which, if successful, are likely to result in significant energy savings over traditional aerobic nitrification methods. Open Access Article. Published on 26 July 2019. Downloaded on 7/27/2019 5:40:50 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal