Unveiling the effect of Fe(III) and sulfate on ammonium oxidation under anaerobic condition: interactions and extracellular electron transfer.

Enhanced ammonium (10.6 - 14.7%) and total inorganic nitrogen (TIN, 4.3 - 18.1%) removal are achieved with individual sulfate (500 mg/L) and ferric (5 mg/L) addition. The promotion of ammonium and TIN were even higher than the total promotion with individual sulfate and ferric addition, when sulfate and ferric are both supplemented probably demonstrate the cooperation between ferric and sulfate during nitrogen removal. The Feammox/Sulfammox, Anammox and Denitrification contributes 9% ∼ 39%, 53% ∼ 87% and 4% ∼ 13% of nitrogen loss, respectively. However, decline of both ammonium (10.5%) and TIN (20.2%) removal are observed when ferric concentration is over 10 mg/L due to inhibitory effect of ferric accumulation. Nevertheless, the harmful effect of ferric is weakened after sulfate supplement, because of the formation of a protection layer containing ferric sulfate hydrate and graftonite on the surface of microorganisms, which limits mass transfer and intercellular accumulation of iron (21%). The 15N stable isotope tracing and N2O via hydroxylamine oxidation (isotopocule) confirms enhanced Sulfammox and Feammox after sulfate and ferric addition. The Feammox may be conducted by cytochrome c and humic acids mediated extracellular electron transfer between aerobic ammonia-oxidizing bacteria (AOB)/anammox and ferric-reducing bacteria, or by anammox bacteria alone through hydrazine synthase catalyzed ammonium oxidation and cytochrome c conducted ferric reduction. The Sulfammox process may be performed by cytochrome c, humic acids and fulvic acids mediated between AOB/anammox and sulfate-reducing bacteria. Then, the produced ferrous and sulfide through Feammox and Sulfammox could be re-oxidized by autotrophic denitrification, where the N2O produced by denitrification pathway are stimulated. This study provides an energy-efficient nitrogen removal technology for wastewater treatment and new insights for understanding the interactions of sulfur, iron and nitrogen cycles in natural ecosystem.