Denitrification driven by varying concentrations of Mn (II) under no external carbon source conditions: Denitrification performance, nitrogen transformation and metabolic pathways

Manganese-driven denitrification has the potential to effectively remove nitrogen while reducing the demand for additional carbon sources. Under no external carbon source conditions, the effects of Mn concentration and hydraulic retention time (HRT) on manganese-driven denitrification, and optimal reaction conditions remain unclear. Therefore, this study constructed six denitrification filters, using different HRTs and a series of Mn2+ concentration gradients. The results indicated that Mn-5 (5 mg/L Mn2+) performed optimally (HRT = 24h), with the average removal efficiencies of nitrate nitrogen (NO-N) and total nitrogen (TN) increased by 27% and 21%, respectively. Microorganisms associated with manganese-driven denitrification are enriched in the filters. Notably, in the Mn-5 filter, the abundance of key functional genes associated with manganese transport system, nitrogen conversion and removal, and carbon metabolism increased significantly, thereby enhancing nitrogen removal. In summary, this study provides a novel approach to denitrification, supporting future research.