Performance and mechanisms of heterotrophic nitrification aerobic denitrifying bacteria in utilizing photogenerated electrons from magnetite for efficient denitrification

Heterotrophic nitrification-aerobic denitrification bacteria (HN-AD) have been demonstrated to possess denitrification potential. The limited effectiveness of HN-AD in remediating nitrogen-polluted surface waters is attributed to its low C/N ratio. In this study, a magnetite photogenerated electrons coupled HN-AD bacteria system was constructed and its nitrogen removal mechanisms were revealed. The results demonstrated the electrons photo-generated by magnetite can effectively stimulate the growth of the HN-AD (Delftia sp., Y19).The coupled system of the ammonium and nitrate reached removal rates of 80.1 % and 71.3 %, which were 4 times higher than the strain Y19 alone (20.3 %, 15.2 %). Compared with dark conditions, the activity of enzymes (AMO, HAO, NAR and NIR) related to nitrogen removal in Y19 was increased by 4.81, 4.75, 6.45 and 4.78 times under sunlight irradiation, respectively. This suggests that the electrons photo-generated from magnetite can enhance the metabolic activities of the Y19 strain. Furthermore, the concentration of ferric ions dissolved from magnetite has been detected as equaling 0.13 mg/L, which plays a crucial role in the reduction of nitrate. The denitrification mechanisms of the coupled system can be incorporated: heterotrophic nitrification and aerobic denitrification by strain Y19, photogenerated electrons reduction of magnetite, the reduction of ferric ions, and the adsorption of magnetite. After 9 days of running the simulator, the magnetite-Y19 coupled system achieved removal rates of 100 % for nitrate and chemical oxygen demand, and 36 % for ammonium. This study offers novel insights into the utilization of photogenerated electrons by microorganisms for remediating the low C/N ratio wastewater.