Significantly\nEnhanced Nitrate Removal by Nanoscale\nZerovalent Iron–Reduced Graphene Oxide Composites via Biological\nDenitrification: Performance and Mechanism

To\naddress the challenges of rapid oxidation and aggregation of\nnanoscale zerovalent iron (nZVI) in biological denitrification, we\ninvestigated the effectiveness of nZVI–reduced graphene oxide\n(rGO) composites in NO<sub>3</sub><sup>–</sup>–N removal\nby biological denitrification for the first time. The results revealed\nthat the addition of nZVI–rGO composites exhibited superior\nbiological denitrification performances, which were 7 times and 3\ntimes higher than that of bacteria alone and the addition of nZVI\nwith bacteria, respectively. Furthermore, by comparing the Fe<sup>2+</sup> ion, iron oxide content, Tafel polarization curve, and Tafel\nslope of nZVI and nZVI–rGO composites, it was found that nZVI–rGO\ncomposites had excellent antioxidant properties with few iron oxides\nin the biological denitrification process. More importantly, nZVI–rGO\ncomposites not only addressed the aggregation issue of nZVI alone\nbut also maintained high dispersibility in the biological denitrification.\nIn addition, linear scanning voltammetry and electrochemical impedance\nspectroscopy curves, as well as the cytochrome <i>c</i> activity,\nverified that nZVI–rGO composites had more efficient electron\ntransfer capabilities during the denitrification process. Lastly,\nthe addition of nZVI–rGO composites significantly enhanced\nthe abundance of <i>nap</i>A by 1.55 times to the bacteria\nalone. These findings provide deep insights into the mechanisms of\nnZVI–rGO composites in enhancing biological denitrification.