Redox reactions of iron in clay minerals play an important role during soil development by weathering and diagenesis as well as in carbon cycling and contaminant mitigation. These minerals typically host structural iron in octahedral sheets (substituting for Al) and occasionally also in tetrahedral coordination (substituting for Si). Clay minerals can buffer microbially-catalyzed or abiotic redox reactions over an unusually wide range of reduction potentials due to changes in mineral redox properties arising from re-arrangement of the mineral structure to accommodate additional charge. Whereas such behavior has been reported for iron in octahedral sheets, the contribution of iron in tetrahedral sheets to mineral redox properties and, thus, the behavior of these minerals in the environment, are still unclear.
In this project, we are studying the redox cycling of synthetic iron-rich nontronites containing octahedral iron (2/2 of octahedral cations) and variable amounts of tetrahedral iron (0 to 1/4 of tetrahedral cations). We characterized mineral structure and redox properties in a comprehensive manner. Using evidence from FTIR, XRD, TEM, and Mössbauer spectroscopy, we found that the 2:1 clay mineral structure was conserved during redox cycling while mineral crystallinity decreased. Further, repeated redox cycling resulted in the convergence of tetrahedral iron content to a low value (0.2/4 tetrahedral cations) independent of the initial tetrahedral iron content. Assessments of the redox properties of the nontronites using mediated electrochemistry showed that despite major structural and morphological changes, the redox buffering capacity of the nontronites was maintained over several redox cycles and were independent of the initial amount of tetrahedral iron. We observed different redox envelopes for reduced an oxidized clays. We are currently testing the hypothesis that this hysteresis is due to disequilibrium conditions in the mediated electrochemical measurements used to characterize the redox properties of clay minerals. To address this limitation, we are applying a mechanistic model that accounts for limitations on the kinetics of redox reactions by the electron transfer from the reactant to the clay minerals and charge redistribution in the mineral structure.
People: Vineeth Pothanamkandathil
Collaborators: Thomas Hofstetter, Anke Neumann, Fabien Baron