A review on metal oxide (FeOx/MnOx) mediated nitrogen removal processes and its application in wastewater treatment

Nitrification and denitrification play a vital role in the conversion of fixed nitrogenous compounds to nitrogen gas. In anoxic environments, nitrogen loss is attributed to autotropic anammox and heterotrophic denitrification, while, the main reduction pathway that recycles NO3- to NH4+ is dissimilatory nitrate reduction to ammonia. However, our knowledge of the global nitrogen cycle was limited until the discovery of an alternative anoxic process that involves the use of redox metals as catalysts. For example, the anoxic marine sediments comprise different microbial communities that depend on metal oxides/oxyhydroxides of Fe (FeOx) and Mn (MnOx) for respiration. These alternate pathways comprise anoxic oxidation of NH4+ to NO2-, NO3-, and N-2 using FeOx/MnOx as electron acceptors. Interestingly, successive reduction of the produced NO2- and NO3- is linked to re-oxidation of Fe2+/Mn2+ leading to efficient nitrogen removal and replenishing of Fe3+/Mn4+, which provide oxide surfaces (electron acceptors) for continuing cycles of NH4+ oxidation. Thus, this new pathway plays a vital role in the oceanic nitrogen cycle. The biochemical pathways are short-circuited and the requirement of oxygen and organic carbon reduces greatly due to the coupling of Fe/Mn redox cycles with simultaneous nitrification and denitrification. Here, we give a brief review on the nitrogen cycle, prevalent nitrogen removal techniques and recapitulate our current understanding of metal oxide-mediated nitrogen cycle in various ecosystems, the role and mechanism of microbial interactions, and the factors affecting the process. Further, the recent advances in experiments to achieve the application of this process in real wastewater treatment and future research prospects are discussed.