Differences in the efficiency and mechanisms of different iron-based materials driving synchronous nitrogen and phosphorus removal

Iron-dependent denitrification has been substantially investigated worldwide due to the advantages of low cost, high efficiency, and synchronized phosphorous removal. However, differences in nitrogen metabolism processes with different iron-based materials as electron donors have not been systematically studied. This study investigated the efficacy of nitrogen and phosphate removal using various iron-based materials as electron donors. Substantial nitrogen removal was demonstrated, with complete TN removal with Fe/C powder as electron donor, while relatively lower TN removal was achieved in iron scrap, Fe/C granular, and pyrite systems with removal efficiency of 86 ± 3.5%, 78 ± 5.7%, and 61 ± 3.1%, respectively. However, the high efficiency could only be sustained for a short time and further microbial metabolisms were significantly suppressed by microbial encapsulation caused by iron-bearing minerals. The introduction of fresh electron donors revitalized denitrification activity, with TN removal improved back to more than 80%. Similar trends were observed in phosphate removal, with increased efficiencies corresponding to ferrous ion release, reaching the highest level of 94 ± 0.02% with iron scrap as electron donor. Microbial community analysis revealed distinct compositions, with sulfur autotrophic denitrifying bacteria prevailing in pyrite systems, hydrogenotrophic denitrifying bacteria dominating with Fe/C powder, and Fe(II) oxidizing bacteria (FeOB) governing in Fe scrap and Fe/C granular systems for denitrification. Predicted genomic analysis elucidated mechanisms underlying nitrogen and phosphorus removal, emphasizing the importance of direct electron transfer via cytochrome C and ferrous ion transportation. Furthermore, differences in nitrogen metabolism among systems were highlighted, with Fe/C powder facilitating hydrogen oxidation proteins expression, while pyrite systems favored sulfur oxidation process, and iron scrap selected Fe(II) transportation and oxidation. This study provided valuable insights into revealing mechanism for efficient nitrogen and phosphorus removal with iron-based materials as electron donors.