Denitrification performance of solid carbon-releasing particle electrodes in a bioelectrochemical system: iron-manganese oxide-enhanced electron migration.

The limited availability of biodegradable organic carbon in low C/N wastewater results in unsatisfactory denitrification efficiency and low operational stability. To address this, a novel three-dimensional biofilm electrode reactor (3D-BER) was constructed using Fe3O4 and MnO2 modified corncob as a slow-release carbon-based particle electrode for coupled denitrification and NH4+-N removal. A key feature of this design was that the modified corncob functioned simultaneouslyacted not only as a slow-release carbon source, a biofilm carrier, and a redox-active particle electrode, enabling coupled carbon release, microbial colonization, and Fe/Mn-mediated electron transfer. At current density of 0.1 mA cm-2, the system achieved stable TN, NO3--N, and NH4+-N removal efficiencies of 77%-82%, 87%-92%, and 40%-54%, respectively, outperforming conventional solid-phase carbon source system, which achieved 61%-66% TN removal. The superior electrocatalytic activity was primarily attributed to the synergistic effects of the external current and Fe/Mn oxides, whereby reversible redox cycling of iron and manganese ions allowed them to act as efficient electron shuttles. In their oxidized forms, these oxides functioned as electron acceptors to promote ammonia oxidation; in their reduced forms, they served as electron donors for nitrate reduction. Microbial analysis showed that the Fe/Mn-modified electrified system selectively enriched nitrogen-transforming genera compared with the non-electrified Fe/Mn-modified system, with Thermomonas increasing from 3.2% to 11.2% and Thauera from 5.4% to 7.2%. narG and feoA were also upregulated by 8.1% and 25.5%, respectively. These findings indicate that electrochemical stimulation and Fe/Mn oxides jointly reshaped the microbial community and enhanced nitrogen conversion potential, ultimately enabling the efficient transformation and removal of multiple nitrogen species. Overall, Fe/Mn-modified electrified 3D-BER accelerated reactor start-up and improved denitrification stability and efficiency, demonstrating strong potential for advanced treatment of low C/N wastewater.