Pyrogenic Carbon Structures Determined Electron Transfer Mechanism Enhancing Iron Autotrophic Denitrification: Role of Oxygen-Containing Functional Groups and Graphitic Structures

Pyrogenic carbon can regulate iron cycle due to their electrochemical properties, but the underlying mechanisms by which their different structure-dominated electron transfer continuously mediates iron autotrophic denitrification (IAD) remain unclear. Therefore, this study explored the influence of structural evolution of pyrogenic carbon on electron transfer and the iron-nitrogen transformation processes. When oxygen-containing functional groups dominated electron transfer, their high abundance (1.14 mmol/g), particularly -OH and C═O, endowed pyrogenic carbon with high electron exchange capacity, facilitating electron transfer for Fe(III) reduction and denitrification processes. As a result, cumulative nitrogen removal increased by 1.67-fold, and the abundances of IAD-related microorganisms and their contributions to functional genes were significantly increased. Moreover, the electron donating capacity increased by 2.51-fold, elevating the Fe(III) reduction rate constant by 3.07-fold and ensuring the long-term utilization of iron. When graphitic structures dominated electron transfer, the electrical conductivity of pyrogenic carbon reached 1.61 S/cm, rapidly facilitating electron transfer in IAD, but the large amount of released iron ions was quickly oxidized and densely covered the carbon and iron surfaces, causing a 54.86% decrease in nitrogen removal. This study provides mechanistic insights into the regulatory effect of different pyrogenic carbons on IAD.