Zero-valent iron powder- and sponge iron-biochar composites drive enhanced nitrate conversion in autotrophic nitrate reduction by distinct electron delivery strategies

Iron-mediated autotrophic denitrification is promising for low-carbon wastewater treatment because it requires little external carbon and avoids secondary pollution. Current studies show that enhanced electron transfer between iron and carbon may improve its nitrogen removal efficiency, while the intermediate processes and mechanisms of electron-transfer mechanisms remain poorly elucidated. Therefore, in this study iron powder (IP) and sponge iron (SI) were combined with conductive biochar to construct iron-carbon autotrophic denitrification systems. And we compared the system's denitrification performance, iron corrosion characteristics, and microbial responses. Results revealed that both iron sources enhanced nitrate reduction. However, IP was associated with more pronounced accumulation of NH₄⁺ and NO₂^-, whereas SI produced fewer intermediate metabolites. Corrosion of the iron substrates produced conductive Fe₃O₄, which bonded with organic matter coating the biochar surface, creating an "iron oxide-biochar-biofilm" conductive network thereby enhancing the potential for extracellular electron transfer within the system. The two systems showed different microbial responses. The IP system developed a community dominated by Thiobacillus, and genes associated with the dissimilatory nitrate reduction to ammonium (DNRA) were significantly enriched, indicating a strong tendency toward DNRA in the system. In contrast, the SI system supports diverse and synergistic microbial networks; the simultaneous enrichment of denitrification genes and electron transport genes promotes the relatively complete conversion of nitrogen into gaseous nitrogen for removal. This study provides a mechanistic basis for the design and operational optimization of iron-based autotrophic denitrification materials for low C/N wastewater.