Regulation of photogenerated electron transfer efficiency: Heterojunction enhanced photocatalysis coupled with anaerobic ammonium oxidation for nitrogen removal

Anaerobic ammonium oxidation technologies are limited by nitrate accumulation in the treatment of wastewater with low carbon-to-nitrogen (C/N) ratios, making it difficult to achieve efficient total nitrogen removal. Light-driven biohybrid systems are promising emerging nitrate reduction technologies, yet the performance is limited by electron transfer efficiency. In this study, type II, Z-scheme, and S-scheme heterojunction photocatalysts, g-C₃N₄@CdS, were synthesized and coupled with anaerobic ammonium oxidation bacteria (AnAOB) to construct a light-driven biohybrid system capable of simultaneously removing nitrate and ammonium through enhanced photogenerated electron transfer. Among them, the S-scheme heterojunction photocatalyst with unique internal electric field and band bending configuration, exhibited the most effective charge separation and redox capability, achieving a total nitrogen removal efficiency of 94.8% under intermittent light without organic carbon. The electron transfer system activity increased by 335.2% and charge transfer resistance decreased by 7.4-fold, confirming the improved electron transfer efficiency in the S-scheme heterojunction system. The binding of light stimulation to the S-scheme heterojunction significantly increased the abundance of functional genes associated with the electron transport chain (e.g., CYTB, cydB, and ccoQ), thereby enhancing ATP synthesis and energy metabolism. The same phenomenon was observed in the abundance of denitrification-related genes (nar and nirS) and anaerobic ammonium oxidation genes (hzsA and hdh). This study elucidates the critical role of heterojunction structural regulation in facilitating electron transfer at the semiconductor-microbe interface and provides new insights and technical support for achieving efficient light-driven nitrogen removal from low C/N-ratio wastewater.