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.含铁黏土矿物（Fe-bearing clay minerals）广泛分布于土壤、沉积物与岩石中，是地壳中铁的重要储库。黏土矿物中结构铁（structural Fe）的电子转移（electron transfer, ET）过程是自然环境中关键的电子与能量流，驱动着众多生物地球化学过程与污染物转化。根据黏土矿物与外源反应物的种类、性质以及水相化学条件的差异，电子转移过程可分为通过矿物边缘/基面进行的界面电子转移，以及黏土矿物内部的体相电子转移。本文综述了含铁黏土矿物与各类反应物间的重要电子转移反应，涵盖铁循环微生物（Fe-cycling microbes）、氧化还原活性有机化合物（redox-active organic compounds）以及重金属。此外，本文探讨了界面与体相电子转移过程的物理化学机制，并构建模型阐明了电子转移速率与程度所受的热力学与动力学约束。在此基础上，本文着重阐述了黏土矿物相关电子转移过程的环境意义，包括其作为生物地球化学电池参与生物地球化学过程与污染物转化、与微生物协同演化，以及调控温室气体生成的作用。最后，本文提出了未来研究方向，以期深化对电子转移过程的分子尺度认知，并将其应用于环境缓解与人类健康保障领域。