Fe(III)-mediated anaerobic ammonium oxidation: A novel microbial nitrogen cycle pathway and potential applications

Nitrogen Removal by an Anaerobic Iron-Dependent Ammonium Oxidation (Feammox) Enrichment: Potential for Wastewater Treatment

“…As for nitrate (NO 3 − ), an increase can be observed in all the different conditions, as in the process described in equation 3, although the direct production to N 2 is the most favorable and, therefore, the most expected. However, the different production rates in the studies are not always consistent with the theory, as Feammox to NO 3 − was also thermodynamically feasible under these conditions [18,22]. Furthermore, ammonium reduction under Fe-rich anaerobic conditions is consistent with what has been previously reported for Feammox processes in natural systems.…”

“…As for the iron source, Huang et al [21] and Huang et al [46] studied the effect of the iron source on Feammox enrichment from isolates, and proposed that the best iron source in terms of NH 4 + removal and Fe(III) reduction was ferrihydrite, followed by goethite, and finally, FeCl 3 . This can be explained due to the higher thermodynamic feasibility of ferrihydrite compared to goethite as a source of Fe(III), since ferrihydrite has a higher negative Gibbs free energy than goethite [18]. However, our results showed that when using goethite, the highest NH 4 + removal was obtained, followed by FeCl 3 and ferrihydrite in both cases, anaerobic and aerobic.…”

“…Iron (Fe) is the fourth most abundant element on earth, occurring in different environments, typically in the redox states as Fe(II) (ferrous iron) and Fe(III) (ferric iron) [18]. This process, denominated Feammox (as anaerobic ammonium oxidation coupled to Fe(III) reduction) [19,20], generates the process of anaerobic oxidation of NH 4 + coupled to the reduction of iron (Fe(III)) in the absence of oxygen, nitrate (NO 3 − ), or NO 2 − , which are required by annamox [21], as previously mentioned, and theoretically could occur abiotically or be microbially mediated [22].…”

“…Figure 1 illustrates the different general processes for the cycling of nitrogen species, as with Feammox, where Fe(III) is used and reduced together with the oxidation of NH4+ for the production of the different nitrogen species. The Feammox process has been shown to produce either N2, NO2 − , or NO3 − as the end-product in several environments, as shown in Equations ( 1)-(3) [18]. Among these reactions, the N2 pathway is energetically more favorable than NO2 − or NO3 − pathways, and can occur over a wider range of conditions [22].…”

“…Among these reactions, the N2 pathway is energetically more favorable than NO2 − or NO3 − pathways, and can occur over a wider range of conditions [22]. 3Fe(OH)3 + 5H + + NH4 + → 3Fe 2+ + 9H2O + 0.5N2 ΔrGm = 245 kJ × mol −1 The Feammox process has been shown to produce either N 2 , NO 2 − , or NO 3 − as the end-product in several environments, as shown in Equations ( 1)-(3) [18]. Among these reactions, the N 2 pathway is energetically more favorable than NO 2 − or NO 3 − pathways, and can occur over a wider range of conditions [22].…”