Electron transfer processes associated with structural Fe in clay minerals

Fe-bearing clay minerals are widely distributed in soils, sediments, and rocks, representing a significant Fe pool in the Earth's crust. The electron transfer (ET) from/to structural Fe in clay minerals is a crucial electron and energy flux in the natural environment, which drives numerous biogeochemical processes and contaminant transformation. Depending on the types and properties of both clay minerals and exogenous reactants as well as aqueous chemistry, the ET processes could involve interfacial ET through edge/basal planes and interior ET inside clay minerals. This paper reviews the important ET reactions between Fe-bearing clay minerals and various reactants, including Fe-cycling microbes, redox-active organic compounds, and heavy metals. Moreover, we discuss the physical-chemical mechanisms of interfacial and interior ET processes and develop models to illustrate the thermodynamic and kinetic constraints on the ET rate and extent. On this basis, we emphasize the environmental implications of ET associated with clay minerals, such as their roles in serving as biogeobatteries for biogeochemical processes and contaminant transformation, coevolution with microbes, and regulation of greenhouse gas formation. Finally, research needs are proposed to advance our molecular-scale understanding of ET processes and utilize them for environmental mitigation and human health.