Direct iron-electron driven autotrophic denitrification for low-carbon nitrate wastewater treatment

Wastewater treatment is an important source of greenhouse gas emissions, with traditional biological denitrification relying on organic carbon sources contributing to over 15 % of its carbon footprint. Herein we develop a direct iron-electron driven autotrophic denitrification strategy with zero-valent iron (ZVI), Shewanella oneidensis MR-1 and Paracoccus denitrificans for low-carbon nitrate wastewater treatment. Without adding any organic carbon sources, this strategy reduced total nitrogen from 28.2 to 2.4 mg/L within 6 days through direct electron transfer between iron and microbes, different from traditional H₂-mediated mechanisms. The direct iron-microbe electron transfer depends on the outer membrane-bound cytochromes, where hemes coordinate with ZVI's oxidized shell via carboxyl groups, facilitating electron transfer from iron to S. oneidensis MR-1, which then converts bicarbonate into formate to drive autotrophic metabolism for simultaneous denitrification and CO₂ capture. Life cycle analysis reveals the carbon emission of direct iron-electron driven autotrophic denitrification is approximately 2.51 kg CO₂/kg N, significantly lower than that of sodium acetate electron-driven denitrification counterpart (approximately 4.07 kg CO₂/kg N). This study introduces a low-carbon strategy for autotrophic nitrate removal from wastewater, and highlights the importance of desirable electron transfer pathway in sustainable wastewater treatment.