Redox Properties of Structural Fe in Clay Minerals: 5. The Role of Fe in the Tetrahedral Sheets of Synthetic Smectites

Abstract Iron-bearing smectite clay minerals are important redox-active phases in soils and sediments. Yet, the influence of iron substitution in the tetrahedral sheets (TetFe) of smectites on their redox properties remains poorly understood. Here, we investigated the repeated reduction and re-oxidation of synthetic Fe-rich smectites, known as synthetic nontronites (SyN), with constant content of Fe in the octahedral sheet (OctFe) but variable initial content of TetFe. The redox properties of redox-cycled smectites were characterized using mediated electrochemical analysis. The resulting Fe2+/FeTotal ratios determined over a wide range of reduction potentials were analyzed using a process-based model to extract thermodynamic and kinetic descriptors of the interfacial electron transfer and subsequent charge redistribution processes that occur during the redox reaction. The fitted standard reduction potentials and apparent diffusion coefficients for charge redistribution were insensitive to TetFe content, indicating that thermodynamics and bulk charge transport within the smectite crystal are predominantly governed by the Fe–O–Fe networks in the octahedral sheet. Conversely, samples containing TetFe exhibited fitted interfacial electron-transfer rate constants approximately an order of magnitude higher than the TetFe-free sample. Comparison of these thermodynamic and kinetic descriptors between synthetic nontronites and natural reference smectites showed that parameter values converge within a narrow range across iron-rich dioctahedral smectites. These model-based results highlight the distinct roles of OctFe and TetFe in controlling smectite redox reactivity in subsurface environments, with OctFe governing the thermodynamics and TetFe primarily influencing interfacial electron-transfer kinetics.