Manganese ore enhanced nitrate removal by improving bioavailability of organics released from corncob: Denitrification performance, organics transformation and nitrogen metabolic pathways

The key limiting factor in advanced nitrogen removal of carbon-poor municipal tailwaters is deficient electron donor. Corncob, a low-cost external carbon source, can enhance heterotrophic denitrification by releasing organics sustainably. But its practical application is usually restricted to low bioavailability of organics and the risk of secondary pollution. Manganese oxides (MnOx) can regulate the organics released from corncob by altering the structure and bioavailability of organics. Herein, coupling Mn Ore (MnO2 content ∼35 %) with corncob as substrate for microorganisms, quite high nitrate removal (98.4 ± 1.2 %) and less residual organics (COD < 10 mg/L) were observed. Organics characterization showed that original organics released from corncob were mainly humic substants (B/C = 0.4 ± 0.05, MW = 1.5 ∼ 350 kDa), and could be oxidized by MnOx into small molecular organics with high bioavailability (B/C = 0.72 ± 0.08, MW = 1.5 ∼ 35 kDa). Mn oxidation state analysis revealed that Mn(III/IV) in MnOx played an important role in organics transformation. The addition of corncob and redox transition of Mn enriched various functional denitrifying bacteria and dissimilatory lignocellulose -degrading bacteria, thereby upregulating functional genes encoding vital enzymes involved in nitrogen/carbon transformation. Overall, a Mn cycle-driven mixotrophic denitrification process was achieved, low molecular organics formed via MnOx oxidation accelerated heterotrophic denitrification rate, the Mn(II) produced from Mn(IV) reduction drive autotrophic denitrification. This finding could guide the low-cost technologies development for deep nitrogen removal from carbon-poor waters.