Effects of different iron compounds on the acclimation of Feammox sludge: Nitrogen-iron transformation, sludge physicochemical properties, and microbial communities.

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) plays a critical role in the natural nitrogen cycle, but the differential regulatory mechanisms of various iron sources that influence Feammox sludge acclimation remain systematically unexplored in wastewater treatment. In this study, four iron compounds (FeCl3, Fe2O3, Fe3O4, and Fe(OH)3) were individually introduced into anaerobic sequencing batch reactors to initiate Feammox processes. After 126 days of enrichment, 48.18 %, 29.38 %, 27.56 %, and 32.18 % of NH4+-N removal efficiencies were achieved, respectively. Notably, the FeCl3 reactor exhibited stronger dynamic interactions between Feammox and nitrate-dependent ferrous oxidation (NDFO), achieving superior NH4+-N and total nitrogen removal efficiencies. Additionally, prolonged acclimation led to a reduction in floc size and an improvement in the settling performance of Feammox sludge, while a large quantity of iron sediments accumulated on the surface of the sludge. High Fe3+ concentrations promoted extracellular polymeric substances transformation toward fulvic acid- and humic acid-like components, facilitating Feammox through electron shuttle-mediated indirect electron transfer. Additionally, the FeCl3 reactor effectively mediated the transformation of iron crystal into FeO(OH) and achieved more efficient reuse of iron sediments. Microbial analysis revealed the enrichment of iron-reducing bacteria (Ferruginibacter) and autotrophic denitrifier (Blastocatellaceae), with intensified microbial interactions driving effective nitrogen-iron cycling process. Furthermore, the FeCl3-enriched community exhibited higher potential for nitrogen and iron related metabolic functions related compared to the other reactors, enabling more efficient enrichment of Feammox microorganisms and improved nitrogen conversion rates. These findings provide critical insights for optimizing iron sources in Feammox-based wastewater treatment.