Enhanced deep nitrogen removal from low C/N wastewater via manganese-catalyzed iron-carbon micro-electrolysis coupled biological system: performance, mechanism, and microbial adaptability

Efficient nitrate removal from low carbon-to-nitrogen (C/N) wastewater remains challenging due to the scarcity of electron donors in conventional biological denitrification. To address this limitation, a synergistic system coupling manganese-catalyzed iron-carbon micro-electrolysis (IMC-ME) with biological denitrification was developed. Under carbon-deficient conditions (C/N = 2), the coupled system achieved 90.1% nitrate removal and 77.2% total nitrogen (TN) removal, substantially outperforming the standalone biological process (49.4% TN removal). Mechanistically, manganese dioxide functioned as a cathodic catalyst, accelerating zero-valent iron corrosion and continuously generating Fe²⁺ and atomic hydrogen [H] as inorganic electron donors. This electrochemical process induced a shift in the microbial community from Proteobacteria to Bacteroidota dominance. Notably, the system selectively enriched Zoogloea, enhancing biofilm stability, and Thiobacillus, indicating the activation of iron-dependent autotrophic denitrification pathways. This integration of chemical reduction and biological metabolism provides a promising strategy for deep nitrogen removal in organic-substrate-limited waters.