Mechanisms of quinone-functionalized FeOOH for enhanced nitrogen removal in micro-aerobic activated sludge process...

Micro-aerobic biological nitrogen treatment technology for low carbon-to-nitrogen (C/N) ratio wastewater often faces issues such as high effluent nitrate concentration and unstable treatment performance. This is because the inoculated sludge is typically dominated by heterotrophic denitrification and lacks the special ecological niche of multiple functional microorganisms coexisting under microaerobic conditions. In this study, quinone-functionalized FeOOH (Q-FeOOH) was synthesized via a green method and added to a micro-aerobic bioreactor to investigate its effects and the mechanisms. The results showed that when the influent C/N ratio was 1 and the NH₄⁺-N concentration was 50 mg/L, the effluent NH₄⁺-N and NO₃^--N concentrations were reduced by 5.94 mg/L and 12.85 mg/L, respectively, compared with those without Q-FeOOH addition. Various test results indicated that Q-FeOOH contained multiple valence states of C, O, and iron elements, as well as CO, CH, and CC functional groups. After long-term operation, the electrochemical properties of Q-FeOOH remained stable. Q-FeOOH increased the electron transfer capacity and cyclic voltammetry values of sludge by 55.27 % and 51 µF. The abundance of ammonia-oxidizing bacteria (AOB), anaerobic ammonium-oxidizing bacteria (AnAOB), and iron reducing bacteria (FeRB) increased by 7.67 %, 1.34 %, and 6.93 %, respectively. The quinone group acted as an electron shuttle to promote extracellular electron transfer, while FeOOH could serve as a terminal electron acceptor for FeRB. In addition, the iron released by Q-FeOOH and the created micro-aerobic microenvironment promoted the proliferation of AnAOB and AOB, thereby synergistically improving the denitrification performance of the micro aerobic sludge system.