Department Environmental Chemistry

Understanding of the unusual redox buffer behaviour of clay minerals

Fig.: Thomas Hofstetter
Iron-containing clay minerals transfer electrons over a wide range of reduction potentials


Electron transfer reactions involving Fe(II)/Fe(III) couples associated with iron-bearing clay minerals profoundly impact the environment by influencing organic carbon cycling, microbial activity, rock weathering and diagenesis, and corrosion. These redox reactions are also particularly important for groundwater remediation efforts because they produce Fe(II) that can abiotically reduce several classes of oxidized environmental contaminants to less toxic or less mobile forms, including radionuclides, toxic metals and metalloids, and various organic pollutants. 

Rates and extents of these redox reactions strongly depend on the speciation and coordination of Fe(II)/Fe(III), which has important consequences for interpreting the roles that iron plays in biogeochemical cycles as well as pollutant dynamics. In contrast to iron minerals such as iron (oxyhydr)oxides, changes of Fe oxidation state in clay minerals are not necessarily coupled to reductive dissolution. Instead, electron transfer to and from Fe primarily leads to structural re-arrangements of the mineral structure. 

In our research, we evaluate whether this feature is responsible for observations of the unusually large range of reduction potentials over which Fe in clay minerals can be involved in redox reactions. Our work combines the evaluation of Fe binding and redox state by spectroscopic and microscopic analyses with the electrochemical characterization of Fe redox reaction over the reduction potential range found in subsurface environments. In addition to studying reference clay minerals, our recent work extents to synthetic materials with well-defined iron coordination.

Publications

Decreases in iron oxide reducibility during microbial reductive dissolution and transformation of ferrihydrite

Ferrous iron formed during microbial ferric iron reduction induces phase transformations of poorly crystalline into more crystalline and thermodynamically more stable iron (oxyhydr)oxides. Yet, characterizing the resulting decreases in the reactivity of the remaining oxide ferric iron toward reduction (i.e., its reducibility) has been challenging. Here, we used the reduction of six-line ferrihydrite by Shewanella oneidensis MR-1 as a model system to demonstrate that mediated electrochemical reduction (MER) allows directly following decreases in oxide ferric iron reducibility during the transformation of ferrihydrite into goethite and magnetite which we characterized by X-ray diffraction analysis and transmission electron microscopy imaging. Ferrihydrite was fully reducible in MER at both pHMER of 5.0 and 7.5. Decreases in iron oxide reducibility associated with ferrihydrite transformation into magnetite were accessible at both pHMER because the formed magnetite was not reducible under either of these conditions. Conversely, decreases in iron oxide reducibility associated with goethite formation were apparent only at the highest tested pHMER of 7.5 and thus the thermodynamically least favorable conditions for iron oxide reductive dissolution. The unique capability to adjust the thermodynamic boundary conditions in MER to the specific reducibilities of individual iron (oxyhydr)oxides makes this electrochemical approach broadly applicable for studying changes in iron oxide reducibility in heterogeneous environmental samples such as soils and sediments.
Aeppli, M.; Vranic, S.; Kaegi, R.; Kretzschmar, R.; Brown, A. R.; Voegelin, A.; Hofstetter, T. B.; Sander, M. (2019) Decreases in iron oxide reducibility during microbial reductive dissolution and transformation of ferrihydrite, Environmental Science and Technology, 53(15), 8736-8746, doi:10.1021/acs.est.9b01299, Institutional Repository

Redox properties of structural Fe in clay minerals: 3. Relationships between smectite redox and structural properties

Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe2+/Fetotal fractions to fundamental redox properties, such as reduction potentials (EH). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe2+/Fetotal) in smectites over a wide range of applied EH-values (−0.6 V to +0.6 V). We examined Fe2+/Fetotal – EH relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe2+/Fetotal – EH relationships, were redox active over wide EH ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe2+/Fetotal – EH relationships correlated well with both bulk and molecular-scale properties, including Fetotal content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe2+/Fetotal – EH relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to EH-values.
Gorski, C. A.; Klüpfel, L. E.; Voegelin, A.; Sander, M.; Hofstetter, T. B. (2013) Redox properties of structural Fe in clay minerals: 3. Relationships between smectite redox and structural properties, Environmental Science and Technology, 47(23), 13477-13485, doi:10.1021/es403824x, Institutional Repository

Project partner

Andreas Vögelin, Eawag