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

The simulated coastal constructed wetlands supplemented with Fe 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 NO-N removal of 72-94% and lower NO emission flux, when compared with mono-P.A biomass(CW-bio), mono-Fe 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 NO-N removal of 94% and enhanced removal efficiency of 41% in CW-M. Fe and P.A biomass coupling could reduce the stress of salinity on denitrification. Batch experiments have demonstrated that Fe 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. Fe 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 NO-N removal in CW-M for treating saline, low C/N wastewater in coastal wetlands.