Selective nitrogen removal in constructed wetlands enhanced by MnX-GAC (X=Fe or Zn): Performance, mechanism and removal pathway.

Constructed wetlands (CWs) are widely used to treat wastewater containing nitrogenous due to their low energy consumption, low maintenance costs, and significant ecological benefits. However, they face challenges such as low denitrification efficiency and greenhouse gas emissions during treatment. Enhancing the synergistic performance of the "substrate-microorganism" system within CWs is considered an effective way to address these problems. Granular activated carbon-supported Mn-based composite fillers (MnX-GAC, X = Fe or Zn) were prepared and placed in the upper and lower positions of the CW. Mn and Fe within these fillers act as electron acceptors or donors to enhance nitrogen removal and reduce nitrous oxide (NO) emissions. MnX-GAC boosted microbial interspecies electron transfer and improved the total nitrogen removal efficiency to 77.1 %. Moreover, the CW with MnX-GAC reduced NO emissions during the denitrification process and increased nitrogen (N) selectivity by 43.4 % compared with the control. The addition of MnX-GAC enhanced microbial interspecies electron transfer by stimulating microorganisms to secrete more protein and increasing their Cytochrome c content. Macrogenomic analyses revealed that MnX-GAC increased the abundances of denitrifying microorganisms (Anaerolinea and Thauera), conducive to nitrogen removal, extracellular polymeric substance secretion or aggregation, and nitrite nitrogen (NO-N) reduction to N. In the nitrogen transformation pathway, MnX-GAC increased the abundance of functional genes related to nitrification (amoABC, nxrAB and hao), denitrification (napABC, narGHI, nirKS, norBC and nosZ) and anaerobic ammonia oxidation (hzsABC and hdh), resulting in high nitrogen removal and selectivity towards N. In this study, MnX-GAC showed excellent performance in selective nitrogen removal in CWs, providing a new strategy to improve the performance of wastewater treatment in CWs and reduce the emission of greenhouse gases.