New insights for simultaneous nutrient removal enhancement and greenhouse gas emissions reduction of constructed wetland by optimizing its redox environment through manganese oxide addition

Abstract

Manganese oxide (MnO_x) is receiving increased interest in the nutrient removal of constructed wetlands (CWs); however, its service effectiveness for simultaneous greenhouse gas (GHG) emissions reduction is still vague. In this study, three vertical flow CWs, i.e., volcanics (CCW), manganese sand uniformly mixing with volcanics (Mn-CW) and MnO_x doped volcanics (MnV-CW), were constructed to investigate the underlying mechanisms of MnO_x on nutrient removal enhancement and greenhouse gas (GHG) emissions reduction. The results showed that the MnO_x doped volcanics optimized the oxidation-reduction potential surrounding the substrate (-164.0 ∼ +141.1 mv), and resulted in the lowest GHG emissions (CO₂-equivalent) from MnV-CW, 16.8-36.5 % lower than that of Mn-CW and CCW. This was mainly ascribed to mitigation of N₂O produced during the NO₃^--N reduction process, according to results of ¹⁵N stable isotope labeling. Analysis of the microbial community structure revealed that due to the optimized redox conditions through chemical doping of MnO_x on volcanics, the abundance of microbe involved in denitrification and Mn-oxidizing process in the MnV-CW was significantly increased at genus level, which led to a higher Mn cycling efficiency between biogenic MnO_x and Mn²⁺, and enhanced denitrification efficiency and N₂O emission reduction. This study would help to understand and provide a preferable reference for future applications for manganese-based CW.