Enhancing Feammox efficiency through riboflavin and humic acid: Nitrogen and iron transformation, energy metabolism, and microbial response

Optimizing electron shuttles and revealing their mediating mechanisms are crucial for enhancing the ammonium (NH₄⁺-N) oxidation coupled with Fe (III) reduction. In this study, anthraquinone-2,6-disulfonate (AQDs), riboflavin (RF), and humic acid (HA) were optimized in batch tests. The optimal dosages of 6, 2, and 8 mg/L for AQDs, RF, and HA resulted in average maximum NH₄⁺-N removal of 80.2 %, 88.5 %, and 99.2 %, with 91.4 %, 88.5 %, and 74.7 % of the removed NH₄⁺-N converted to nitrate, respectively. In addition, an enhanced extracellular electron transfer was also observed, including an enlarged current, diversified REDOX pathway, and reduced resistance. Outperformed AQDs in nitrogen removal and microbial activity, HA and RF were selected for the subsequent 100-day long-term investigation. During this stage, excess influent Fe tended to be stored as insoluble coatings on the sludge surface, while RF and HA facilitated its use to compensate for the reduced influent Fe³⁺. Meanwhile, they led to an increase in iron-reducing (Comamonas) and NH₄⁺-N oxidizing bacteria (Nitropsira and Planctomycetes), as well as improvements in electrochemical characteristics and microbial activity. Moreover, microbial N and Fe metabolic potential were efficiently enhanced. Consequently, NH₄⁺-N and TN removal rates were obviously increased to approximately 90 % and 40 %, respectively. The addition of electron shuttles led to long-term improvements in extracellular mass transfer and microbial metabolism, which contributed more than bridging the extracellular electron transfer. These results deepened the understanding of the effect of electron shuttles on Feammox.