The Effect of Redox Cycling on the Fate of Tetrahedral Iron in Smectites
Abstract
Redox reactions involving Fe-bearing clay minerals play a significant role in the cycling of elements such as carbon, and for the mobility of inorganic and organic contaminants in subsurface environments. The layered alumo-silicate structure of clay minerals can accommodate up to 30 wt% Fe in both octahedral and tetrahedral coordination. Tetrahedral Fe is naturally present in Fe-rich clay minerals like nontronite, yet the behavior and fate of tetrahedral Fe during redox reactions remains poorly constrained. Here, we synthesized a series of nontronites with 0 to 27% of the total Fe present as tetrahedral Fe(III) that we subjected to chemical reduction, re-oxidation, and re-reduction with dithionite and hydrogen peroxide, respectively. We used FT-IR and Mössbauer spectroscopies, SEM-EDS, HRTEM and XRD to monitor changes in crystal chemistry and the fate of tetrahedral Fe during the redox manipulation. Reduction of all nontronites resulted in almost all structural Fe becoming reduced (77-100% Fe(II)/Fe total ), including a portion of the initially present tetrahedral Fe, and re-oxidation restored all Fe to its ferric form. The initial 2:1 smectite structure was conserved throughout all redox manipulations, with the typical structural changes during Fe reduction, mainly trioctahedral domain formation and structural OH-group loss, being largely reverted by re-oxidation. However, the initial reduction led to significant loss of tetrahedral Fe from the nontronite structure, and the tetrahedral Fe content in all nontronites converged to the same value of ~0.2 per half formula unit. The concomitant decrease in particle size and number of stacked layers in a significant portion of each nontronite led to the presence of two distinct populations of particles and suggests partial reductive dissolution as a plausible mechanism for tetrahedral Fe release. While some of the Fe was present in the aqueous phase, increased interlayer distances in reduced and re-oxidized nontronites implies that 48-63% of the released Fe became intercalated, presumably in the form of hydroxylated pillars. Overall, highly similar structures, crystal chemistry, and tetrahedral Fe content resulted for all nontronites irrespective of their initial tetrahedral Fe content and remained stable in subsequent redox manipulations, indicating the reversible reduction and re-oxidation of both octahedral and tetrahedral Fe in the initially altered clay mineral structure. Our results provide compelling evidence for why naturally occurring clay minerals contain only low amounts of tetrahedral Fe and that both octahedral and tetrahedral Fe in clay minerals may contribute to electron transfer reactions in natural environments.
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BibTeX
@misc{baron2026effect,
title = {{The Effect of Redox Cycling on the Fate of Tetrahedral Iron in Smectites}},
author = {Fabien Baron and Jagannath Biswakarma and Meret Aeppli and Erwan Paineau and Vineeth Pothanamkandathil and Eric Ferrage and Sabine Petit and Thomas B Hofstetter and Anke Neumann},
howpublished = {HAL},
year = {2026},
month = may,
url = {https://hal.science/hal-05760849v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Effect of Redox Cycling on the Fate of Tetrahedral Iron in Smectites
AU - Fabien Baron
AU - Jagannath Biswakarma
AU - Meret Aeppli
AU - Erwan Paineau
AU - Vineeth Pothanamkandathil
AU - Eric Ferrage
AU - Sabine Petit
AU - Thomas B Hofstetter
AU - Anke Neumann
PY - 2026
DA - 2026/05/18
PB - HAL
UR - https://hal.science/hal-05760849v1
AB - Redox reactions involving Fe-bearing clay minerals play a significant role in the cycling of elements such as carbon, and for the mobility of inorganic and organic contaminants in subsurface environments. The layered alumo-silicate structure of clay minerals can accommodate up to 30 wt% Fe in both octahedral and tetrahedral coordination. Tetrahedral Fe is naturally present in Fe-rich clay minerals like nontronite, yet the behavior and fate of tetrahedral Fe during redox reactions remains poorly constrained. Here, we synthesized a series of nontronites with 0 to 27% of the total Fe present as tetrahedral Fe(III) that we subjected to chemical reduction, re-oxidation, and re-reduction with dithionite and hydrogen peroxide, respectively. We used FT-IR and Mössbauer spectroscopies, SEM-EDS, HRTEM and XRD to monitor changes in crystal chemistry and the fate of tetrahedral Fe during the redox manipulation. Reduction of all nontronites resulted in almost all structural Fe becoming reduced (77-100% Fe(II)/Fe total ), including a portion of the initially present tetrahedral Fe, and re-oxidation restored all Fe to its ferric form. The initial 2:1 smectite structure was conserved throughout all redox manipulations, with the typical structural changes during Fe reduction, mainly trioctahedral domain formation and structural OH-group loss, being largely reverted by re-oxidation. However, the initial reduction led to significant loss of tetrahedral Fe from the nontronite structure, and the tetrahedral Fe content in all nontronites converged to the same value of ~0.2 per half formula unit. The concomitant decrease in particle size and number of stacked layers in a significant portion of each nontronite led to the presence of two distinct populations of particles and suggests partial reductive dissolution as a plausible mechanism for tetrahedral Fe release. While some of the Fe was present in the aqueous phase, increased interlayer distances in reduced and re-oxidized nontronites implies that 48-63% of the released Fe became intercalated, presumably in the form of hydroxylated pillars. Overall, highly similar structures, crystal chemistry, and tetrahedral Fe content resulted for all nontronites irrespective of their initial tetrahedral Fe content and remained stable in subsequent redox manipulations, indicating the reversible reduction and re-oxidation of both octahedral and tetrahedral Fe in the initially altered clay mineral structure. Our results provide compelling evidence for why naturally occurring clay minerals contain only low amounts of tetrahedral Fe and that both octahedral and tetrahedral Fe in clay minerals may contribute to electron transfer reactions in natural environments.
ER -