New preprint on the redox properties of iron in clay minerals

In the preprint "Redox Properties of Structural Fe in Clay Minerals: 5. The Role of Fe in the Tetrahedral Sheet of Synthetic Smectites", we asked whether the location of iron within the crystal structure of smectites — specifically whether it sits in the tetrahedral or octahedral sheet — influences how these minerals accept and donate electrons. To answer this question, we used a series of synthetic nontronites with constant octahedral Fe content but systematically varying amounts of tetrahedral Fe, and characterized their redox behavior using mediated electrochemical reduction and oxidation. We then applied the kinetic-thermodynamic model developed by Vineeth in his previous work to decouple thermodynamic properties from electron transfer kinetics and charge redistribution within the mineral structure — an approach that allowed us to extract standard reduction potentials, heterogeneous rate constants, and apparent diffusion coefficients for each sample. We found that tetrahedral Fe has no significant effect on the thermodynamics of the redox reaction, but accelerates interfacial electron transfer kinetics by approximately one order of magnitude. Comparison with natural reference smectites showed that redox parameters converge within a narrow range across iron-rich dioctahedral smectites, underscoring the dominant role of the octahedral Fe–O–Fe network in controlling smectite redox reactivity.

This paper is the result of a collaboration with Fabien Baron at University of Poitiers and Anke Neumann at PSI, who synthesized and structurally characterized the smectites, and Thomas Hofstetter and Michael Sander at Eawag and ETH Zürich. We are especially grateful to Vineeth for leading this work with great dedication and rigor!