Chain assembly of Rhodococcus bacteria with O-doped g-C₃N₄ for photocatalysis mediated high-performance partial...

Nitrogen conversion via partial nitrification-anammox (PN/A), utilizing nitrite as a key intermediate, is an ideal low-carbon approach for wastewater nitrogen removal. However, the partial nitrification process, which is rate-limited and relies on less common bacteria with slow reaction kinetics, poses challenges for high-throughput PN/A implementation. Herein, we developed a microbial/photocatalysis coupling system using Rhodococcus bacterial and O-doped g-C₃N₄ (OCN) photocatalysts. This approach leverages photogenerated electrons and free radicals from photocatalysts to directly activate microorganisms, enhancing the redox gradient. This intensification selectively inhibits the enzymatic conversion of nitrite to nitrate and its reduction to nitrogen in Rhodococcus bacteria. Consequently, it promotes a highly selective partial nitrification process, generating ample nitrite to facilitate anammox reactions. Transmission electron microscopy and electrochemical characterization showed bacteria forming chain-like assemblies on OCN particles, with the composite exhibiting a favorable redox profile, low impedance, and high stability. Ammonia conversion to nitrite reached 96 % in 3 days, with an enriched NO₂^- concentration of 36.3 mg/L, 10 times higher than the raw bacterial control. Hence, this strategy of constructing bacterial-photocatalysis system achieved high selectivity and efficiency in partial nitrification. Transcriptome and qPCR analyses showed upregulation of genes linked to the short-cut denitrification metabolic pathway. Photocatalyst band structure and redox potential analysis suggest a new bio-photoelectrochemical partial nitrification pathway. Finally, the feasibility and applicability in future industrial and ecological water treatment were validated through demo H-type reactors and aquarium experiments. These findings offer innovative perspectives for controlled modulation of ammonia nitrogen conversion focus on nitrite intermediate, advancing an energy-efficient, low-carbon nitrogen cycle.