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A review of microbial redox interactions with structural Fe in clay minerals
- L. Pentráková, K. Su, M. Pentrák, J. W. Stucki
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- Journal:
- Clay Minerals / Volume 48 / Issue 3 / June 2013
- Published online by Cambridge University Press:
- 09 July 2018, pp. 543-560
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Virtually all 2:1 clay minerals contain some Fe in their crystal structure, which may undergo redox reaction with surrounding redox-active species causing potentially significant changes in the chemical and physical properties of the clay mineral and its surrounding matrix. This phenomenon was originally of interest mostly as a laboratory experiment using strong inorganic reduction agents, but the discovery that the structural Fe could be reduced by microorganisms in natural soils and sediments opened the way for this to become a practical method for altering the chemical and physical properties of soils and sediments in situ. The purpose of this report was to review the body of literature that has been published since the inception of this field of inquiry and to complement, update, and complete three other reviews that have been published during the intervening years. Studies of microbial reduction of structural Fe in smectites have revealed the extent of reduction, effects on chemical and physical properties, reversibility (or lack thereof) of microbial reduction, stoichiometry, possible reaction mechanism, and types of organisms involved. Some organisms are also capable of oxidizing structural Fe, such as in biotite or reduced smectite, while one appears to be able to do both. Illitic layers resist reduction by microorganisms, but this can be partially overcome by the presence of an electron shuttle compound such as anthraquinone-2,6-disulfonate, which also enhances the extent of reduction in smectites. Microorganisms may be employed as an in situ reducing agent to drive redox cycles for structural Fe in constituent clay minerals of soils and sediments, which in turn can serve as an abiotic source for redox-mediated remediation of environmental contaminants.
Methods for handling redox-sensitive smectite dispersions
- J. W. Stucki, K. Su, L. Pentráková, M. Pentrák
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- Journal:
- Clay Minerals / Volume 49 / Issue 3 / June 2014
- Published online by Cambridge University Press:
- 27 February 2018, pp. 359-377
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Redox activation (reduction of structural Fe) of smectites greatly alters their chemical reactivity and physical properties, which may be exploited for various environmental, agricultural or industrial purposes. Their re-oxidation during preparation, characterization, and use is, however, a significant risk to their utility. In this study, methods and apparatus were developed and described which mitigated reoxidation. Ferruginous smectite (sample SWa-1, Na saturated) was used as the model smectite. It was reduced with sodium dithionite in a citrate-bicarbonate buffer solution at 70°C for 4 h, which achieved a maximum Fe(II)/total Fe ratio of 0.9113 ± 0.0048. The first step in rendering reduced samples useful is to remove from them the reducing agents and other solutes present during reduction. This was accomplished in the present study by reducing the sample in an inert-atmosphere reaction tube (IRT) (a 50 mL centrifuge tube equipped with a removable septum cap), then removing solutes from the suspension by centrifuge washing. The washing steps were performed with the aid of a controlled-atmosphere liquid exchanger (CALE) which provided connections between the sample suspension and deoxygenated solutions. The reduced state was measured by 1, 10-phenanthroline or by Mössbauer spectroscopy at 77 K to give Fe(II)/total Fe ratios. Some samples were freeze dried after washing. Results revealed that if reduced smectites are washed without protection from atmospheric O2, the extent of reoxidation is on the order of 40 to 60%. If the sample is subsequently dried, reoxidation increases to more than 76%. If the sample is protected using the IRT and the CALE, however, reoxidation is decreased to less than 2%. Freeze drying in a glove box allowed reoxidaton to increase to slightly more than 10%. These results indicate that more reoxidation occurred during the drying stage than during the washing stage. These observations lead to the conclusions that (1) protection of reduced samples from atmospheric O2 is essential if extensive reoxidation is to be prevented, and (2) the methods and apparatus described herein are effective for accomplishing that purpose in abiotically reduced smectites. They may also be effective if applied to microbially reduced smectites.
Tetrahedral charge and Fe content in dioctahedral smectites
- S. Kaufhold, J.W. Stucki, N. Finck, R. Steininger, A. Zimina, R. Dohrmann, K. Ufer, M. Pentrák, L. Pentráková
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- Journal:
- Clay Minerals / Volume 52 / Issue 1 / March 2017
- Published online by Cambridge University Press:
- 02 January 2018, pp. 51-65
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Natural aluminosilicates can contain Fe in tetrahedral or octahedral coordination. Amongst smectites, tetrahedral iron is known to occur in Fe-rich nontronites but few indications exist for the presence of tetrahedral Fe in smectites of the montmorillonite–beidellite series. A set of 38 different bentonites showed a correlation of tetrahedral charge and Fe content in their smectites. All materials with large tetrahedral charge were rich in Fe. This could be explained by a general tendency of Fe to enter the tetrahedral sheet. To investigate this correlation, nine materials were selected and investigated by Mössbauer, UV-Vis, Fe K pre-edge and EXAFS spectroscopy with respect to tetrahedral Fe (Fe[IV]). The latter two methods were at the detection limit but Mössbauer and UV-Vis spectroscopy provided consistent results indicating the significance of both methods in spite of some scatter caused by the overall small amount of tetrahedral Fe. The results indicate the absence of any relation between Fe content and tetrahedral Fe. Tetrahedral Fe can be present in Fe-poor smectites and absent in the case of Fe-rich materials. This means that Fe-rich montmorillonites have a larger tetrahedral charge which is not caused by Fe[IV] but by Al[IV]. A possible explanation for this indirect relation is based on: the coordination of Al3+ in the weathering/smectite-forming solutions determines the coordination in the precipitates; and the Al[IV/VI] ratio increases with increasing pH. The correlation could thus be explained if the pH of weathering solutions generally was higher in Fe-rich parent smectite rocks than in more acidic smectite parent rocks. The relation between tetrahedral charge and Fe content can probably be explained by different geochemical contexts throughout the formation of smectites which affect the coordination of dissolved Al.