Mn oxides changed nitrogen removal process in constructed wetlands with a microbial electrolysis cell

Abstract

Intensified Mn redox cycling could enhance nutrient removal in constructed wetlands (CWs). In this study, Mn oxides (birnessite-coated sand) were used as the matrix in horizontal flow CWs (HFCWs) with a microbial electrolysis cell (MEC) (E-B-CW) or without an MEC (B-CW). The model CWs were developed to investigate the capacities and mechanisms of nitrogen removal with increased Mn redox cycling. The results showed that E-B-CW had the highest average removal efficiencies for NH<inf>4</inf>-N, NO<inf>3</inf>-N and TN, followed by B-CW and control HFCW (C-CW). The Mn(III) oxides (MnOOH or Mn<inf>2</inf>O<inf>3</inf>) and the Mn(IV) oxide (MnO<inf>2</inf>) were all detected in E-B-CW and B-CW, while the matrix in E-B-CW had much more Mn(IV) oxides than B-CW. Interestingly, clustering heat map showed that ammonification and nitrate reduction were related to Mn-oxidizing bacteria and the relative abundance of Mn-oxidizing bacteria in E-B-CW was highest due to the re-oxidation of Mn(II) by the MEC.