Significantly Enhanced Nitrate Removal by Nanoscale Zerovalent Iron–Reduced Graphene Oxide Composites via Biological Denitrification: Performance and Mechanism

To address the challenges of rapid oxidation and aggregation of nanoscale zerovalent iron (nZVI) in biological denitrification, we investigated the effectiveness of nZVI–reduced graphene oxide (rGO) composites in NO3––N removal by biological denitrification for the first time. The results revealed that the addition of nZVI–rGO composites exhibited superior biological denitrification performances, which were 7 times and 3 times higher than that of bacteria alone and the addition of nZVI with bacteria, respectively. Furthermore, by comparing the Fe2+ ion, iron oxide content, Tafel polarization curve, and Tafel slope of nZVI and nZVI–rGO composites, it was found that nZVI–rGO composites had excellent antioxidant properties with few iron oxides in the biological denitrification process. More importantly, nZVI–rGO composites not only addressed the aggregation issue of nZVI alone but also maintained high dispersibility in the biological denitrification. In addition, linear scanning voltammetry and electrochemical impedance spectroscopy curves, as well as the cytochrome c activity, verified that nZVI–rGO composites had more efficient electron transfer capabilities during the denitrification process. Lastly, the addition of nZVI–rGO composites significantly enhanced the abundance of napA by 1.55 times to the bacteria alone. These findings provide deep insights into the mechanisms of nZVI–rGO composites in enhancing biological denitrification.