Modified solid carbon sources with nitrate adsorption capability combined with nZVI improve the denitrification performance of constructed wetlands

•Modified agricultural wastes with nitrate adsorption capability acted as SCS.•Modified peanut shells (MPS) can released more carbon to remove NO3−-N in CWs.•SCSs combined with nZVI enhanced autotrophic and heterotrophic denitrification in CW.•More autotrophic denitrifying bacteria and NRFOB were enriched in MPN-CW. In this study, various modified agricultural wastes (modified canna leaves (MCL), modified rice straw (MRS) and modified peanut shells (MPS)) as solid carbon sources (SCSs) were used to remove nitrate in constructed wetlands (CWs). Then, modified SCSs combined with nZVI (SCSN) as co-electrons further enhanced both heterotrophic denitrification (HD) and autotrophic denitrification (AD) performance of CWs. The results showed that NO3−-N removal efficiencies in CWs with SCSNs (75.3–91.1%) and in CWs with SCSs (63.3–65.5%) were significantly higher than that in CK-CW (47.0%). The presence of SCSs reduced the accumulation of NO2−-N in CWs. Compared to the addition of SCSs, the addition of SCSNs decreased the effluent COD concentration in CWs, avoiding secondary pollution. In addition, the solid-phase denitrifiers Silanimonas and Thauera were enriched in MPS-CW. Thermomonas, an autotrophic denitrifying bacteria (ADB), and Azospira, a nitrate-reducing Fe (II) oxidation bacteria (NRFOB), exhibited high relative abundance in MPN-CW.