Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Fe(II) Redox Chemistry in the Environment.

Abstract Mineral–microbe interactions play important roles in environmental change, biogeochemical cycling of elements and formation of ore deposits. Minerals provide both beneficial (physical and chemical protection, nutrients, and energy) and detrimental (toxic substances and oxidative pressure) effects to microbes, resulting in mineral-specific microbial colonization. Microbes impact dissolution, transformation and precipitation of minerals through their activity, resulting in either genetically controlled or metabolism-induced biomineralization. Through these interactions, minerals and microbes co-evolve through Earth history. Mineral–microbe interactions typically occur at microscopic scale but the effect is often manifested at global scale. Despite advances achieved through decades of research, major questions remain. Four areas are identified for future research: integrating mineral and microbial ecology, establishing mineral biosignatures, linking laboratory mechanistic investigation to field observation, and manipulating mineral–microbe interactions for the benefit of humankind.

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A critical review of mineral–microbe interaction and co-evolution: mechanisms and applications

Cadmium adsorption to clay-microbe aggregates: Implications for marine heavy metals cycling

Soils and sediments are major reservoirs of organic matter (OM), whose dynamic turnover has a major impact on carbon cycling and global climate. OM in soils and sediments is predominantly associated with minerals, which decelerate OM decomposition and could help store carbon. However, iron (Fe) minerals could also degrade OM and release a fraction of OM to the atmosphere as CO2 and CH4, but the coupling of these processes is only partly understood. In this Review, we describe the mechanisms and importance of coupled iron–carbon (Fe–C) cycles. Oxygenation of structural Fe(II) in minerals generates reactive oxygen species, which either degrades or synthesizes OM. Reactive oxygen species can also either decrease or increase extracellular enzyme activity and microbial activity, thus indirectly transforming OM. In addition, Fe(III) reduction contributes to OM oxidation through anaerobic respiration. By contrast, OM affects the redox properties of Fe minerals by serving as electron donor, acceptor, shuttle, buffer or conductor and by co-precipitation and complexation with Fe minerals. These feedback mechanisms can result in complex interconnected Fe–C cycling processes; hence, future work must focus on attaining the net impact of combined Fe–C cycles. Coupled Fe–C cycles are important considerations for carbon sequestration, soil fertility and ecosystem functions. This Review explores the role of Fe minerals in stabilizing and degrading organic matter and the role of organic matter in enhancing reactivity of Fe minerals under different conditions. 

Coupled iron cycling and organic matter transformation across redox interfaces

The surface reactivity of clay minerals remains challenging to characterize because of a duality of adsorption surfaces and mechanisms that does not exist in the case of simple oxide surfaces: edge surfaces of clay minerals have a variable proton surface charge arising from hydroxyl functional groups, whereas basal surfaces have a permanent negative charge arising from isomorphic substitutions. Hence, the relationship between surface charge and surface potential on edge surfaces cannot be described using the Gouy–Chapman relation, because of a spillover of negative electrostatic potential from the basal surface onto the edge surface. While surface complexation models can be modified to account for these features, a predictive fit of experimental data was not possible until recently, because of uncertainty regarding the densities and intrinsic pKa values of edge functional groups. Here, we reexamine this problem in light of new knowledge on intrinsic pKa values obtained over the past decade using ab initio...

Modeling the acid-base properties of montmorillonite edge surfaces

Bio-reduction of ferrihydrite-montmorillonite-organic matter complexes: Effect of montmorillonite and fate of organic matter

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Bio-reduction of ferrihydrite-montmorillonite-organic matter complexes: Effect of montmorillonite and fate of organic matter