Exploring utilization of modified ferrous sulfide-based materials in constructed wetlands to enhance nitrogen removal: Performance, microbial species interactions and community assembly mechanisms.

Constructed wetlands (CWs), as an ecological remediation technology, have been widely applied in purifying nitrogen-containing wastewater. However, their denitrification efficiency is often limited in low carbon-to-nitrogen ratio (C/N) wastewater due to insufficient electron donors. This study innovatively prepared two different ferrous sulfide-based materials, namely sodium alginate (SA) coated ferrous sulfide (FeS) (CW2: SA@FeS400), as well as FeS and zero valent iron (CW3: SA@Fe0-FeS), and used them as the substrates in CWs. Moreover, long-term denitrification performances of different CWs, as well as their impact on denitrification enzyme activity, electron transfer activity, and microbial community composition were further explored. Results showed that CW2 (73.93 %) and CW3 (73.61 %) had higher nitrate removal efficiencies compared to control group (CW1: 45.53 %). As hydraulic retention time and carbon-to‑nitrogen ratio (C/N) decreased, the nitrate removal efficiency in CW2 was relatively stable and high, while nitrate removal efficiencies in CW1 and CW3 significantly decreased. FeS-based materials enhanced the electron transfer activity and denitrifying enzyme activity of CWs, as well as enriching functional microorganisms. Additionally, FeS-based materials can significantly enhance the nitrogen removal by promoting the redox cycling of FeS and strengthening the synergistic symbiotic relationships among nitrifying bacterial communities. This study could provide a new insight for the optimization of inorganic electron acceptors in CWs to improve the nitrate removal from low C/N wastewater.