Unraveling the synergistic promotion mechanism of Fe0 coupling phragmites australis biomass for nitrogen removal in coastal wetland: From low to moderate salinities

The simulated coastal constructed wetlands supplemented with Fe0 and phragmites australis (P.A) biomass (CW-M) were constructed to improve nitrogen removal under different salinities (0–15‰). Results showed that the denitrification performance of CW-M were improved significantly, with the higher NO3−-N removal of 72–94% and lower N2O emission flux, when compared with mono-P.A biomass(CW-bio), mono-Fe0 system (CW-Fe) and control system. The nitrogen removal showed a trend of first increasing (0‰–7‰) and then decreasing (7‰–15‰) with the highest NO3−-N removal of 94% and enhanced removal efficiency of 41% in CW-M. Fe0 and P.A biomass coupling could reduce the stress of salinity on denitrification. Batch experiments have demonstrated that Fe0 and P.A biomass could mutually stimulate more total organic carbon and total iron (TFe) release as electron donors for denitrification. Meanwhile, appropriate salinity could also promote the release of TFe. The typical heterotrophic denitrifying genera Bacillus and iron autotrophic denitrifying genera Thermomonas have the highest proportion in CW-M, with 21.83% and 0.10%, respectively. Fe0 and P.A biomass adding simultaneously promoted the carbon and iron metabolism, further enhancing the nitrogen metabolism process. The joint enhancement of autotrophic and heterotrophic denitrification contributes to NO3−-N removal in CW-M for treating saline, low C/N wastewater in coastal wetlands. [Display omitted] •Higher NO3−-N removal (72–94%) and lower N2O flux was achieved in CW-M.•The interaction between P.A biomass and Fe0 was the highest at 7 ‰ salinity.•Fe0 and biomass could mutually promote the release of more TOC and total iron.•Low salinity had a promotion on TFe release.•Bacillus and Thermomonas accounted highest in CW-M, with 21.83% and 0.10%.