Iron (Fe) in clay minerals represents important redox buffers in soils and aquatic environments that modulate the mobility and toxicity of many anthropogenic and geogenic contaminants. While the reactivity of Fe in the octahedral sheet of the silicate structure has been widely studied and is associated with structural rearrangements as a consequence of changes in the Fe redox state, little is known about the contributions and the reactivity of Fe bound in tetrahedral sheets of iron-rich clay minerals. In this study, we assessed the role of Fe in octahedral vs. tetrahedral sheets of nontronites for a better understanding of redox processes in subsurface environments. To that end, we characterized the redox properties and mineralogy over repeated reduction and re-oxidation cycles of a series of synthetic nontronites containing up to 40 wt% of Fe and between 0 and 44% of total Fe bound in tetrahedral sheets.
Using a suite of techniques including FTIR, XRD, TEM, XMCD, and Mössbauer spectroscopy, we observed substantial changes in mineralogy over repeated redox cycles that point to a decrease in particle size, formation of some hydroxy-Fe interlayers, as well as a decrease of the overall content of Fe bound in tetrahedral sheets. Conversely, the assessment of Fe redox properties of the nontronite particles by electrochemical methods revealed that reduced and (re)oxidized specimen show different apparent reduction potentials, whereas the overall redox buffering capacity is maintained over several redox cycles. Our data suggest that changes in mineralogical properties of iron-rich nontronites, including the different coordination of Fe in octahedral vs. tetrahedral sheets, may not necessarily be directly coupled to a change in redox properties. By linking mineralogical characteristics with redox properties, we could better understand the role of tetrahedral Fe in iron-rich clay minerals in environments subjected to dynamic redox conditions.

Iron (Fe) in clay minerals represents important redox buffers in soils and aquatic environments that modulate the mobility and toxicity of many anthropogenic and geogenic contaminants. While the reactivity of Fe in the octahedral sheet of the silicate structure has been widely studied and is associated with structural rearrangements as a consequence of changes in the Fe redox state, little is known about the contributions and the reactivity of Fe bound in tetrahedral sheets of iron-rich clay minerals. In this study, we assessed the role of Fe in octahedral _vs._ tetrahedral sheets of nontronites for a better understanding of redox processes in subsurface environments. To that end, we characterized the redox properties and mineralogy over repeated reduction and re-oxidation cycles of a series of synthetic nontronites containing up to 40 wt% of Fe and between 0 and 44% of total Fe bound in tetrahedral sheets.
Using a suite of techniques including FTIR, XRD, TEM, XMCD, and Mössbauer spectroscopy, we observed substantial changes in mineralogy over repeated redox cycles that point to a decrease in particle size, formation of some hydroxy-Fe interlayers, as well as a decrease of the overall content of Fe bound in tetrahedral sheets. Conversely, the assessment of Fe redox properties of the nontronite particles by electrochemical methods revealed that reduced and (re)oxidized specimen show different apparent reduction potentials, whereas the overall redox buffering capacity is maintained over several redox cycles. Our data suggest that changes in mineralogical properties of iron-rich nontronites, including the different coordination of Fe in octahedral _vs._ tetrahedral sheets, may not necessarily be directly coupled to a change in redox properties. By linking mineralogical characteristics with redox properties, we could better understand the role of tetrahedral Fe in iron-rich clay minerals in environments subjected to dynamic redox conditions.

How do repeated redox cycles affect the tetrahedrally coordinated iron and influence the mineralogy and redox properties of iron-rich clay minerals?

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