Efficient nitrate removal by photo-assisted microbial denitrification using FexOy/g-C3N4-biofilm hybrid in a coiled flow inverter.

Nitrate contamination in aquatic systems poses serious environmental and public health challenge, necessitating efficient and sustainable denitrification technologies. In this study, a photo-assisted microbial denitrification system was developed by integrating an S-scheme FexOy/g-CN (FCN) photocatalyst with an in situ grown microbial biofilm in a coiled flow inverter (CFI) reactor. The FCN heterojunction enhanced visible-light absorption and charge separation, enabling efficient generation of photoinduced electrons for microbial nitrate reduction. The biofilm matrix provided a protective microenvironment, facilitating extracellular electron transfer while suppressing reactive oxygen species through the hole-scavenging role of extracellular polymeric substances. The CFI reactor significantly improved mixing, mass transfer, and photon distribution, resulting in enhanced photocatalyst-biofilm interactions. Under optimized conditions, the FCN/biofilm system achieved ∼98% nitrate removal with high N2 selectivity and an approximately 2-fold increase in reaction rate compared to batch systems. Kinetic and mechanistic analyses revealed that photogenerated electrons promoted intracellular NADH regeneration, thereby accelerating enzymatic denitrification pathways. Response surface methodology was used to optimize key operating parameters and validate system performance. The system exhibited excellent stability over multiple cycles and achieved 76.9% nitrate removal in real wastewater. Overall, this study demonstrates an efficient remediation of nitrate from wastewater using a scalable, energy-efficient photo-microbial platform.