Nitrogen removal performance and thermodynamic mechanisms of Feammox mediated by ferric pyrophosphate at various pHs

Fe(III) reduction coupled with anaerobic ammonium oxidation (Feammox) is an emerging nitrogen removal process that is favored under moderately acidic conditions, yet its performance and mechanisms in neutral and alkaline conditions are poorly understood. Ferric pyrophosphate (FP) with a non-redox active ligand was selected as a typical chelated iron to study the performance and thermodynamics of Feammox at various pH levels. Ammonia removal efficiency increased from 18.1 % at pH 5.0 to 40.1 % at pH 7.0 but dropped to 2.6 % at pH 9.0. Thermodynamic studies revealed that FP-mediated Feammox was the dominant ammoxidation process at pH > 4.97. X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) demonstrated that FP-mediated Feammox led to the precipitation of vivianite (Fe3(PO4)2·8H2O) in the sludge, thus facilitating phosphorus removal from the water. XPS indicated the presence of Fe(III) and Fe(II) in secondary minerals. 16S rRNA sequencing identified the dominant bacterial genera in the enriched sludges; these included norank_f_A4b, Arenimonas, and Limnobacter. This study improves our understanding of Feammox over a wide pH range and highlights the importance of Fe(III)-ligand complexes in this process.