Iron’s Hidden Role: How Rust Chemistry Could Transform Wastewater Nitrogen Removal

Nitrogen removal is one of the most energy-hungry jobs in modern wastewater treatment, and a new critical review argues that iron, one of the most abundant elements on Earth, could help do that job far more cheaply and cleanly. Writing in the journal ENGINEERING Environment, a team from the Water Research Centre at the University of New South Wales in Sydney, led by Rui Guo and corresponding author Min Zheng, systematically evaluates a family of processes that couple the cycling of iron between its ferrous, Fe(II), and ferric, Fe(III), oxidation states to the transformation of nitrogen compounds. The review, published as an open-access article, synthesizes what is known about these iron-mediated pathways, exposes the engineering obstacles that have kept them largely confined to laboratories, and lays out a research agenda for turning iron redox chemistry into a practical tool for sustainable wastewater treatment.

The motivation is straightforward. Conventional biological nitrogen removal relies on two sequential microbial steps: aerobic nitrification, in which ammonium is oxidized to nitrate, and anoxic denitrification, in which nitrate is reduced to nitrogen gas. The first step demands massive aeration, often accounting for a large share of a treatment plant’s electricity budget, while the second requires external organic carbon, typically methanol or other dosed chemicals, because municipal wastewater after primary treatment often lacks sufficient biodegradable carbon. Both steps also generate nitrous oxide, a greenhouse gas roughly 300 times more potent than carbon dioxide. As utilities worldwide face pressure to cut energy use, chemical consumption, and emissions simultaneously, autotrophic processes that sidestep aeration and organic carbon dosing have become a central goal of environmental biotechnology, and iron redox cycling offers several routes toward that goal.

The review identifies four principal iron-coupled nitrogen conversion pathways. The first and most celebrated is Feammox, the anaerobic oxidation of ammonium coupled to the reduction of Fe(III) to Fe(II). Discovered in soils and sediments, including paddy fields and wetland environments, Feammox means ammonium, long considered inert without oxygen, can be oxidized under anoxic conditions when ferric iron serves as the electron acceptor. The second pathway is nitrate-dependent ferrous oxidation, often abbreviated NDFO, in which microorganisms oxidize Fe(II) to Fe(III) while reducing nitrate to nitrogen gas, effectively achieving denitrification with iron rather than organic carbon as the electron donor. The third is iron-enhanced anaerobic ammonium oxidation, or anammox, in which appropriate iron dosing stimulates the anammox bacteria that combine ammonium and nitrite directly into nitrogen gas. The fourth is chemodenitrification, a purely abiotic reaction in which Fe(II) chemically reduces nitrite, producing nitric oxide, nitrous oxide, or nitrogen gas along with ferric oxide minerals.

What distinguishes this review from earlier surveys, the authors argue, is its insistence that these pathways cannot be treated in isolation. In a real bioreactor, Fe(II) and Fe(III) coexist, nitrite and nitrate are both present, and the microbial community contains organisms capable of several different metabolisms at once. The result is intense competition for shared substrates. Feammox products can feed anammox, but the two processes can also compete for ammonium. Chemodenitrification can consume the nitrite that anammox bacteria need, or it can generate the nitrous oxide that plants are trying to eliminate. Kinetic mismatch compounds the problem: abiotic Fe(II)-nitrite reactions can be fast, while microbial iron reduction and oxidation are comparatively slow, so the balance between biological and chemical routes shifts with operating conditions in ways that are difficult to predict. Understanding this network of competing and cooperating reactions, rather than optimizing any single pathway, is presented as the key to stable system performance.

Iron bioavailability emerges as a central control variable. Ferric iron is poorly soluble at neutral pH, and much of it in a reactor exists as solid oxyhydroxide minerals such as ferrihydrite, goethite, and hematite. Microorganisms can only use the fraction they can access, whether by directly reducing soluble Fe(III) species, by attaching to mineral surfaces, or by using electron shuttles such as humic substances or activated carbon that ferry electrons between cells and insoluble oxides. Studies cited in the review show that ferrihydrite, the most amorphous and reactive ferric mineral, can markedly improve Feammox performance in continuous reactors, while more crystalline oxides are far less effective. Chelating agents can keep Fe(II) in solution for nitrate-dependent ferrous oxidation, and sponge iron or zero-valent iron has been used as a slow-release source of ferrous ions that sustains iron-driven denitrification over long operating periods. The choice of iron form, dose, and delivery strategy therefore determines whether the iron cycle turns productively or grinds to a halt.

The review also examines the operational levers that engineers can actually turn. Iron concentration and speciation sit at the top of the list, because moderate Fe(II) additions are known to accelerate anammox growth, likely by supporting the synthesis of iron-rich enzymes such as the heme-containing enzymes of anammox metabolism, whereas excessive doses can inhibit activity or trigger mineral precipitation that buries biomass. Dissolved oxygen must be carefully managed: too much oxygen chemically oxidizes Fe(II) and favors conventional nitrification, but micro-oxygen conditions have recently been shown to stimulate Feammox, suggesting a narrow optimal window. pH governs both the solubility of iron minerals and the rate of chemodenitrification, which is strongly pH-dependent, and temperature shapes the microbial community, with cold conditions known to suppress anammox consortia. Electron-transfer mediators, from natural humics to engineered conductive materials, offer a further handle for accelerating electron flow between microbes and iron minerals.

Mineral encrustation is one of the most stubborn translational barriers the review highlights. When Fe(II) is oxidized, whether biologically or chemically, the resulting ferric oxides precipitate as fresh mineral phases that can coat cell surfaces and biofilm matrices. Encrusted cells lose access to substrates and electron acceptors, and reactor performance decays. Nitrite, a common intermediate in all of these systems, plays a double role here, because it both drives abiotic Fe(II) oxidation and promotes the cell encrustation observed in nitrate-reducing iron-oxidizing cultures. Managing where and how fast ferric minerals form, and preventing them from smothering the microbial catalysts, is identified as a prerequisite for long-term reactor stability. On the positive side, the secondary minerals produced during chemodenitrification have been shown to sequester other contaminants such as arsenate, hinting at co-benefit opportunities in waters with mixed pollution.

Greenhouse gas risk is another theme the authors refuse to gloss over. Chemodenitrification is a recognized source of nitrous oxide, and its contribution depends strongly on pH and on the relative rates of the abiotic and biological branches of the nitrogen network. Earlier work suggests that abiotic nitrous oxide production, while strongly pH-dependent, may contribute little to total emissions in conventional biological nitrogen removal systems, but iron-amended reactors change the chemistry substantially, and the review stresses that pathway partitioning, meaning how much nitrogen flows through Feammox, anammox, NDFO, and chemodenitrification respectively, remains uncertain in most systems. Without reliable tools to quantify that partitioning, engineers cannot yet guarantee that an iron-based process will meet increasingly strict emissions limits. Isotopic tracing techniques, including nitrogen-15 labeling, are among the methods being deployed to resolve these questions, and recent studies have used them to distinguish anammox-dominated from Feammox-dominated regimes.

The review closes with a set of research priorities intended to move iron redox-coupled nitrogen conversion from promising to practical. The unresolved microbial mechanisms come first: the organisms responsible for Feammox, including the characterized Acidimicrobiaceae strain A6, are still poorly understood, and their kinetics, yields, and ecological interactions need systematic quantification. Slow reaction kinetics must be overcome, whether through better inocula, optimized mineral substrates, electron shuttles, or reactor designs that concentrate active biomass. Robust models that predict pathway partitioning under real wastewater conditions are needed so that operators can steer the network toward nitrogen gas rather than nitrous oxide. Strategies that lower iron demand, such as coupling Feammox with anammox so that recycled Fe(II) is reoxidized internally, are highlighted as routes to high-level nitrogen removal with minimal chemical input. If those gaps can be closed, the authors argue, iron redox cycling could underpin a new generation of low-energy, low-carbon nitrogen removal technologies, turning one of the planet’s most common elements into an ally in the effort to make wastewater treatment genuinely sustainable.

Subject of Research: Iron redox-coupled nitrogen conversion pathways for sustainable wastewater treatment

Article Title: A critical review of iron redox-coupled nitrogen conversion for sustainable wastewater treatment

Article References: Guo, R., Cen, X., Li, C., Zuo, Z., & Zheng, M. (2026). A critical review of iron redox-coupled nitrogen conversion for sustainable wastewater treatment. ENGINEERING Environment, 20(12), Article 188. https://doi.org/10.1007/s11783-026-2288-4

Image Credits: AI Generated

DOI: 10.1007/s11783-026-2288-4

Keywords: iron redox cycling, Feammox, anammox, nitrate-dependent ferrous oxidation, chemodenitrification, nitrogen removal, wastewater treatment, autotrophic denitrification, nitrous oxide emissions, electron shuttles, mineral encrustation, sustainable engineering