Iron-carbon micro-electrolysis facilitates autotrophic denitrification and Feammox in tidal flow constructed wetlands for enhanced nitrogen removal and reduced N2O emissions

This study focused on developing a tidal flow constructed wetland (CW) with an iron-carbon (Fe-C) substrate to enhance nitrogen removal efficiency. The results demonstrated significant improvements in the removal of NH4+-N and TN in the iron-carbon tidal flow constructed wetland (IC-TFCW) compared to traditional gravel CW. IC-TFCW achieved a 23.82 % and 31.01 % more effective removal, resulting in reductions of 82.45 % and 74.13 %, respectively. The large specific surface area and electron transfer effect of the Fe-C substrate facilitated pollutant retention and proliferation of denitrifying microorganisms. However, the Fe-C substrate inhibited plant growth and rhizosphere microorganisms, reducing plant nitrogen uptake by 2.69-fold. Galvanic cell reaction in the Fe-C substrate facilitated the release of iron and its oxides and promoted iron cycling under the alternating aerobic-anoxic conditions induced by tidal flow. This led to the enrichment of autotrophic denitrifying and Fe-reducing bacteria by 14.89 and 1.1 times, and increase in the relative abundance of denitrification genes nar (narG, narH, narI) and nap (napA, napB) by 60.94 %. The Fe2+ released during the galvanic cell reaction contributed electrons to denitrification, while the reduction of Fe3+ facilitated the Anammox process, resulting in the Feammox reaction. This created an autotrophic denitrification pathway (autotrophic denitrification and Feammox), synergizing with the heterotrophic denitrification pathway and the iron cycling process to significantly increase TN removal and reduce N2O emissions per unit of TN removed by up to 36.99 %. These findings elucidate the various denitrification pathways present in CW and provide insights into improving denitrification efficiency.