Iron-manganese synergistic enhanced denitrification in slow-release carbon-based fillers: Mechanisms and performance

Agricultural waste offers economic and environmental benefits as a slow-release carbon source for denitrification; however, its inherent release of ammonia nitrogen limits its effectiveness. In this study, a novel solid-phase slow-release carbon filler was developed by doping Fe₃O₄ and MnO₂ oxides into a corn cob matrix. Experimental results showed that the CC-Fe&Mn group achieved nitrate nitrogen and ammonia nitrogen removal rates of 98.52 % and 85.09 %, respectively, with significantly improved efficiency and the shortest reactor startup time. Mechanistic analysis indicated that the Fe-Mn oxides acted as both electron acceptors and electron shuttles by (1) facilitating electron transfer through Fe³⁺/Fe²⁺ and Mn⁴⁺/Mn²⁺ redox cycles, (2) establishing a nitrification-denitrification synergistic pathway to enhance nitrogen removal, and (3) suppressing ammonia nitrogen release via metal ion coordination (reduction rate: 29.6 %). This filler effectively addressed the ammonia release issue associated with traditional agricultural carbon sources, providing a cost-effective and functionally improved solution for low C/N wastewater treatment.