Mechanistic insights into iron cycling-driven nitrogen removal from biogas slurry via coupled iron-based denitrification and Feammox.

In this study, ferrous-based denitrification was combined with Feammox (Fe(III) reduction coupled with anaerobic ammonium oxidation) to trigger NH4+ removal through intermittently adding NOx- (NO2- and NO3-) into biogas slurry. The results showed that NOx- oxidized Fe(II), then the generated Fe(III) was reduced to Fe(II) again, resulting in a continuous iron cycling and nitrogen removal. On day 35, the total nitrogen removal efficiencies in the NO2- (67.52%) and NO3--added (52.32%) groups were significantly higher than that of the control (without NOx-) (P < 0.05). Nitrifying and Anammox microorganisms were not detected in the NOx--added reactors, while Feammox functional microorganisms (iron-reducing bacteria) were enriched (1.08%-1.51%), and the electron transfer capacities were also increased by 7.69%-16.08%. Metagenomic analysis showed that the NO3- group had more nitrate reductase genes but fewer downstream denitrification genes than the control group, indicating that NO2- accumulated as a key intermediate. NO3- could not directly oxidize Fe(II), and no nitrate-dependent Fe(II)-oxidizing microorganisms were detected. Moreover, the Fe(II) oxidation products in the NO3--added reactors were identical to those generated by abiotic NO2- oxidation, suggesting that NO2- produced via partial denitrification was likely responsible for Fe(II) oxidation. Based on this, a possible metabolic pathway coupling nitrogen and iron transformations was proposed, in which partial NO3- reduction to NO2- may contribute to Fe(II) oxidation and subsequent Fe(III)-mediated NH4+ removal via Feammox. This study provided a method for dealing with biogas slurry, and also offers a new approach for simultaneously removing NOx- and NH4+.