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 (FeO_x) and Mn (MnO_x) for respiration. These alternate pathways comprise anoxic oxidation of NH₄⁺ to NO₂⁻, NO₃⁻, and N₂ using FeO_x/MnO_x 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.

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中文翻译：

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金属氧化物（FeOx/MnOx）介导的脱氮工艺及其在废水处理中的应用综述

硝化和反硝化在固定含氮化合物转化为氮气的过程中起着至关重要的作用。在缺氧环境中，氮损失归因于自养厌氧氨氧化和异养反硝化作用，而将 NO ₃ ^-再循环为 NH ₄ ⁺的主要还原途径是异化硝酸盐还原为氨。然而，我们对全球氮循环的了解是有限的，直到发现了一种替代的缺氧过程，该过程涉及使用氧化还原金属作为催化剂。例如，缺氧海洋沉积物包含不同的微生物群落，这些微生物群落依赖于 Fe (FeO _x ) 和 Mn (MnO _x ) 的金属氧化物/羟基氧化物。) 进行呼吸。这些替代途径包括使用FeO _x /MnO _x作为电子受体将NH ₄ ⁺缺氧氧化为NO ₂ ^-、NO ₃ ^-和N ₂。有趣的是，产生的 NO ₂ ^-和 NO ₃ ^{- 的}连续还原与Fe ²⁺ /Mn ^{2+ 的}再氧化有关，导致有效的氮去除和 Fe ³⁺ /Mn ^{4+ 的}补充，这提供了氧化物表面（电子受体）用于 NH ₄ ^{+ 的}连续循环氧化。因此，这种新途径在海洋氮循环中起着至关重要的作用。由于铁/锰氧化还原循环与同步硝化和反硝化的耦合，生化途径被短路，对氧和有机碳的需求大大降低。在这里，我们简要回顾了氮循环、流行的脱氮技术，并概括了我们目前对各种生态系统中金属氧化物介导的氮循环、微生物相互作用的作用和机制以及影响该过程的因素的理解。此外，还讨论了实现该过程在实际废水处理中应用的实验的最新进展和未来的研究前景。

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