Integration of manganese ores with activated carbon into constructed wetland for greenhouse gas emissions reduction.

Manganese oxide and activated carbon (AC) are widely employed in constructed wetlands (CWs) to remove nutrients and reduce greenhouse gas (GHG) emissions, however, the effect and mechanism of AC combined with manganese ores (MO) on GHG emissions remain unclear. In this study, the mechanisms of nutrient removal and GHG emissions reduction were investigated by three vertical subsurface-flow CWs: gravel (CW-B), manganese ores (MO) uniformly mixing with gravel (CW-M), or activated carbon (CW-MC). The average removal efficiencies of chemical oxygen demand, total nitrogen and total phosphorus in CW-MC were markedly improved compared to CW-B and CW-M, reaching 82.72%, 95.72% and 93.43%, respectively. Moreover, the global warming potential (CO2 equivalent) of CW-MC was reduced by 52.80% and 36.88% relative to CW-B and CW-M, respectively. Mixing of MO with AC reduced the loss of manganese and further enhanced the manganese cycling process by X-ray photoelectron spectroscope and concentration of Mn(Ⅱ) in CWs analysis. The introduction of MO and AC enhanced the PN/PS ratio of extracellular polymeric substances and facilitated extracellular electron transfer (EET). Furthermore, metagenomic analysis showed that the abundances of denitrifying, manganese oxidizing and electroactive bacteria genera were enhanced in the CW-MC, which promoted the transformation of nitrogen and manganese. Meanwhile, high abundances of denitrification and EET related genes were observed in CW-MC, improving denitrification efficiency and reducing N2O emission. This study elucidated the impacts and mechanisms of MO and AC on GHG emissions, providing a new insight to improve manganese-based CW performance.