Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization techniques, and environmental applications of magnetite and green rust in the environment. The aim is to present a detailed overview of the key aspects related to these mineral phases which can be used as an important resource for researchers working in a diverse range of fields dealing with mixed-valent iron minerals.

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Microorganisms are a primary control on the redox-induced cycling of iron in the environment. Despite the ability of bacteria to grow using both Fe(II) and Fe(III) bound in solid-phase iron minerals, it is currently unknown whether changing environmental conditions enable the sharing of electrons in mixed-valent iron oxides between bacteria with different metabolisms. We show through magnetic and spectroscopic measurements that the phototrophic Fe(II)-oxidizing bacterium Rhodopseudomonas palustris TIE-1 oxidizes magnetite (Fe3O4) nanoparticles using light energy. This process is reversible in co-cultures by the anaerobic Fe(III)-reducing bacterium Geobacter sulfurreducens. These results demonstrate that Fe ions bound in the highly crystalline mineral magnetite are bioavailable as electron sinks and electron sources under varying environmental conditions, effectively rendering magnetite a naturally occurring battery.

Poorly crystalline Fe(III) (oxyhydr)oxides like ferrihydrite are abundant in soils and sediments and are often associated with organic matter (OM) in the form of mineralorganic aggregates. Under anoxic conditions, interactions between aqueous Fe(II) and ferrihydrite lead to the formation of crystalline secondary minerals, like lepidocrocite, goethite, or magnetite. However, the extent to which Fe(II)-catalyzed mineral transformations are influenced by ferrihydrite-associated OM is not well understood. We therefore reacted ferrihydrite-PGA coprecipitates (PGA = polygalacturonic acid, C:Fe molar ratios = 0−2.5) and natural Fe-rich organic flocs (C:Fe molar ratio = 2.2) with 0.5−5.0 mM isotopically labeled 57 Fe(II) at pH 7 for 5 weeks. Relying on the combination of stable Fe isotope tracers, a novel application of the PONKCS method to Rietveld fitting of X-ray diffraction (XRD) patterns, and 57 Fe Mossbauer spectroscopy, we sought to follow the temporal evolution in Fe mineralogy and elucidate the fate of adsorbed 57 Fe(II). At low C:Fe molar ratios (0−0.05), rapid oxidation of surface-adsorbed 57 Fe(II) resulted in 57 Feenriched crystalline minerals and nearly complete mineral transformation within days. With increasing OM content, the atom exchange between the added aqueous 57 Fe(II) and Fe in the organic-rich solids still occurred; however, XRD analysis showed that crystalline mineral precipitation was strongly inhibited. For high OM-content materials (C:Fe ≥ 1.2), Mossbauer spectroscopy revealed up to 39% lepidocrocite in the final Fe(II)-reacted samples. Because lepidocrocite was not detectable by XRD, we suggest that the Mossbauer-detected lepidocrocite consisted of nanosized clusters with lepidocrocite-like local structure, similar to the lepidocrocite found in natural flocs. Collectively, our results demonstrate that the C content of ferrihydrite−OM coprecipitates strongly impacts the degree and pathways of Fe mineral transformations and iron atom exchange during reactions with aqueous Fe(II).

It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments, and thus their stability and capacity to prevent carbon mobilization during permafrost thaw, is poorly understood. We have followed the dynamic interactions between iron and carbon using a space-for-time approach across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost (41.8 ± 10.8 mg carbon per g soil, 9.9 to 14.8% of total soil organic carbon). During permafrost thaw, water-logging and O2 limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, this rusty carbon sink is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw.