Manganese-based activated carbon composites promote nitrogen removal in low temperature constructed wetlands via...

Constructed wetlands (CWs) provide cost effective, nature based wastewater treatment but suffer performance losses at low temperatures. We tested granular activated carbon-supported manganese composites (MnX-GAC; X = Fe or Zn) positioned within CWs to enhance microbial extracellular electron transfer (EET). MnX-GAC increased electron acceptors/donors, strengthened EET activity, and improved nitrogen removal while mitigating temperature impacts. At 15 °C, the MnX-GAC system (CW6) raised ammonium (NH₄⁺-N) removal by 31.0% versus CW1. Although CW6 generated more CO₂ due to intensified carbon mineralization, it effectively suppressed the emissions of potent greenhouse gases (N₂O and CH₄) and achieved the lowest GWP per unit of nitrogen removed of 4.53 mg CO₂-eq/mg N. Metagenomics showed the enrichment of key functional taxa (e.g., Chloroflexota, Thermodesulfobacteriota, and Bacteroidota) and the upregulation of nitrogen metabolism genes and carbon metabolism genes, alongside increases in electron-transport chain and mediator genes, collectively facilitated electron production and utilization. These changes indicate that MnX-GAC enhances EET mediated pathways to sustain nitrogen removal under low temperature. Overall, MnX-GAC offers a practical strategy to overcome low-temperature limitations in CWs, delivering higher nitrogen removal and lower life-cycle climate impacts.