Biological mechanisms affecting the release of greenhouse gases from microbial fuel cell-constructed wetland by simultaneously altering structure and electron shuttles

The utilization of microbial fuel cells (MFCs) to regulate greenhouse gas (GHG) emissions in constructed wetlands (CW) has garnered significant attention. The impact of structural modifications and the introduction of iron-carbon materials on pollutants removal and GHGs release from MFC-CW has not been systematically studied. In this study, four types of MFC-CWs, including aerobic and anaerobic zones separated (SC), aerobic and anaerobic zones unseparated (IC), SC added with iron-carbon (SFC), and IC added with iron-carbon (IFC) were constructed. The objective was to investigate the effect of structural changes and electron shuttle additions on the release of GHGs. The results revealed that IFC with added iron-carbon simultaneously enhanced pollutant removal and GHG emission reduction, compared to IC. However, no significant enhancements were observed in SC and SFC. The cumulative release of nitrous oxide (N2O) and methane (CH4) in SC was reduced by 51.83% and 89.33%, respectively, compared to IC. Structural alterations in MFC-CW influenced the correlation between functional bacteria (Chlorobium, Azospira, and Denitratisoma) and electrochemically active bacteria (EAB) and increased values of nirS/nosZ and pmoA/mcrA, consequently enhancing denitrification efficiency and reducing GHG emissions. However, the structural changes did not yield similar effects in iron-carbon systems (SFC and IFC). Despite an increase in denitrogenation efficiency, the release of N2O did not decrease. This was attributed to the interaction between structural alterations and iron-carbon additions, which altered the role of iron in the N2O conversion process. Specifically, in the IFC configuration, iron was directly involved in the process, while in the SFC configuration, ferric-mediated electron transfer processes were observed. The crucial role of electron shuttles and the structure of MFC-CWs in controlling contaminant removal and GHG emissions was confirmed in this study.