Enhanced Degradation of Phenanthrene by Soil Bioelectrochemical Systems: Synergistic Interaction between Humic Acids and Microorganisms

Polycyclic aromatic hydrocarbons (PAHs) pose a persistent threat to soil ecosystems due to their recalcitrant nature and widespread distribution. This study investigates the role of humic acids (HAs) in enhancing phenanthrene (PHE) degradation and electron transfer efficiency in soil bioelectrochemical systems (SBSs). A series of HAs-amended SBSs was constructed and systematically evaluated for electrochemical performance, PHE removal efficiency, and HA component evolution. Fluorescence excitation-emission matrix spectroscopy coupled with parallel factor analysis, Fourier transform infrared spectroscopy, two-dimensional correlation spectroscopy, and structural equation modeling were employed to reveal the redox transformation pathways of HAs and their interactions with microbial communities. The results revealed that Components C1 and C3 of HAs played distinct yet complementary roles in facilitating extracellular electron transfer (EET). C1, with its low molecular weight and high mobility, served as a diffusible electron shuttle promoting long-distance electron transport. C3, enriched with quinonoid moieties and a stable aromatic backbone, supported sustained redox cycling and long-term system stability. Although Component C2 did not directly contribute to EET, it underwent microbial- and electrochemically driven transformation into C1 and C3, reinforcing the formation of a robust electron transfer network. These findings highlight the functional differentiation and synergistic interactions between HA subcomponents in promoting PHE degradation. From an environmental perspective, this study offers a novel and practical strategy for enhancing in situ bioremediation of PAH-contaminated soils. The utilization of natural HAs as electron mediators not only improves remediation efficiency but also aligns with sustainable and eco-friendly treatment principles, providing theoretical support for the development of advanced soil bioelectrochemical remediation technologies.