Biogeochemical cycling of iron is crucial to many environmental processes, such as ocean productivity, carbon storage, greenhouse gas emissions and the fate of nutrients, toxic metals and metalloids. Knowledge of the underlying processes involved in iron cycling has accelerated in recent years along with appreciation of the complex network of biotic and abiotic reactions dictating the speciation, mobility and reactivity of iron in the environment. Recent studies have provided insights into novel processes in the biogeochemical iron cycle such as microbial ammonium oxidation and methane oxidation coupled to Fe(iii) reduction. They have also revealed that processes in the biogeochemical iron cycle spatially overlap and may compete with each other, and that oxidation and reduction of iron occur cyclically or simultaneously in many environments. This Review discusses these advances with particular focus on their environmental consequences, including the formation of greenhouse gases and the fate of nutrients and contaminants. In this Review, Kappler and colleagues highlight recent advances in our understanding of the biogeochemical iron cycle, exploring the great complexity of the processes involved and novel mechanistic insights that have been gained.

An evolving view on biogeochemical cycling of iron

Biogeochemical transformation (inclusive of dissolution) of iron (hydr)oxides resulting from dissimilatory reduction has a pronounced impact on the fate and transport of nutrients and contaminants in subsurface environments. Despite the reactivity noted for pristine (unreacted) minerals, iron (hydr)oxides within native environments will likely have a different reactivity owing in part to changes in surface composition. Accordingly, here we explore the impact of surface modifications induced by phosphate adsorption on ferrihydrite reduction by Shewanella putrefaciens under static and advective flow conditions. Alterations in surface reactivity induced by phosphate changes the extent, decreasing Fe(Ill) reduction nearly linearly with increasing P surface coverage, and pathway of iron biomineralization. Magnetite is the most appreciable mineralization product while minor amounts of vivianite and green rust-like phases are formed in systems having high aqueous concentrations of phosphate, ferrous iron, and bicarbonate. Goethite and lepidocrocite, characteristic biomineralization products at low ferrous-iron concentrations, are inhibited in the presence of adsorbed phosphate. Thus, deviations in iron (hydr)oxide reactivity with changes in surface composition, such as those noted here for phosphate, need to be considered within natural environments.

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Phosphate Imposed Limitations onBiological Reduction and Alterationof Ferrihydrite

Redox-driven biogeochemical element cycles play a central role in converting organic matter in aquatic ecosystems. They also perform key functions such as removing nitrate, mitigating the formation of greenhouse gases and weakening the effects of contaminants. Recent research has revealed the presence of redox-active compounds in these ecosystems with hitherto unknown redox properties. These substances are metastable (that is, non-equilibrium solid phases), which can both donate and accept electrons. They are highly redox reactive and recyclable and may act as biogeobatteries by temporarily storing electrons. Their lifetime, however, is limited, and with time they become more crystalline and less reactive. In this Review, we argue that these redox-active metastable phases require activation by fluctuating redox conditions to maintain their high reactivity. In aquatic ecosystems, switching between oxidizing and reducing conditions can be achieved only through hydrological perturbations at hydrological interfaces (for example, water level fluctuations). We present a novel framework that links microscale biogeochemical processes to large-scale hydrological processes, and discuss implications and future research directions for biogeochemical element cycles in aquatic systems exposed to frequent hydrological disturbances. Highly redox-active compounds play an important role in biogeochemical element cycles in aquatic systems that are exposed to frequent hydrological disturbances.

A biogeochemical–hydrological framework for the role of redox-active compounds in aquatic systems

Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques

Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques.

Ferrous iron formed during microbial ferric iron reduction induces phase transformations of poorly crystalline into more crystalline and thermodynamically more stable iron (oxyhydr)oxides. Yet, characterizing the resulting decreases in the reactivity of the remaining oxide ferric iron toward reduction (i.e., its reducibility) has been challenging. Here, we used the reduction of six-line ferrihydrite by Shewanella oneidensis MR-1 as a model system to demonstrate that mediated electrochemical reduction (MER) allows directly following decreases in oxide ferric iron reducibility during the transformation of ferrihydrite into goethite and magnetite which we characterized by X-ray diffraction analysis and transmission electron microscopy imaging. Ferrihydrite was fully reducible in MER at both pHMER of 5.0 and 7.5. Decreases in iron oxide reducibility associated with ferrihydrite transformation into magnetite were accessible at both pHMER because the formed magnetite was not reducible under either of these conditions. Conversely, decreases in iron oxide reducibility associated with goethite formation were apparent only at the highest tested pHMER of 7.5 and thus the thermodynamically least favorable conditions for iron oxide reductive dissolution. The unique capability to adjust the thermodynamic boundary conditions in MER to the specific reducibilities of individual iron (oxyhydr)oxides makes this electrochemical approach broadly applicable for studying changes in iron oxide reducibility in heterogeneous environmental samples such as soils and sediments.

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Sanja Vranic

Papers

Decreases in Iron Oxide Reducibility during Microbial Reductive Dissolution and Transformation of Ferrihydrite