Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals

Stucki, J. W.; Lear, Paul R.

doi:10.1021/bk-1990-0415.ch017

Cited by 16 publications

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“…This explanation would be consistent with earlier studies (Stucki et al, 1984b;Lear and Stucki, 1985;Stucki and Lear, 1989) that strongly indicated, based on layer charge measurements, that some of the Fe is reduced by a source of electrons (Z) within the clay structure. Aluminum-substituted tetrahedral sites may provide such a source of electrons due to the lower-valent tetrahedral cation.…”

Section: Resultssupporting

confidence: 92%

“…This is consistent with the hypothesis of Stucki and Lear (1989) who stated that Na2S204 should be a better reducing agent than any others that they studied because of its free-radical activity. Earlier studies Heller-Kallai, 1976a, 1976b;Stucki and Roth, 1977) that compared the reductive capability of various reducing agents also provided findings consistent with the hypothesis of Stucki and Lear (1989).…”

Section: Resultssupporting

confidence: 90%

“…Rinker et al (1959) reported ESR results from Na25204 solutions, showing that the free radical also existed in aqueous solution at g = 2.0051. In a preliminary experiment, Stucki and Lear (1989), upon observing the ESR signal in freshly prepared Na2S204 solutions, hypothesized that the free radical could have sufficient reductive capability to reduce structural Fe from the basal surfaces of smectite in spite of the coulombic barrier. The purpose of the present study was to test the hypothesis of Stucki and Lear (1989) by establishing more definitively the relationship between free-radical behavior in the reducing agent, its ability to reduce structural Fe in ferruginous smectite, and the Fe reduction mechanism.…”

Section: Introductionmentioning

confidence: 99%

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Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

Gan

1992

Clays and clay miner.

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Abstract--The oxidation state of structural iron greatly influences the physical-chemical properties of clay minerals, a phenomenon that may have significant implications for pollutant fate in the environment, for agricultural productivity, and for industrial uses of clays. Knowledge of redox mechanisms is fundamental to understanding the underlying basis for iron's effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this variation may not have been a simple function of the reduction potential of the reducing agent. The objective of this study was to identify the relationship between the Fe reduction mechanism and free radical activity in the reducing agent. Several reducing agents and their mixtures with the Na-saturated, 0.5 to 2 #m size fraction of ferruginous smectite (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. Only NazS204 exhibited paramagnetic free-radical behavior with a signal at about g = 2.011, which was attributed to the sulphoxylate (SO2-.) free radical. The free radical was labile in aqueous solution, and the ability of NazSzO4 solution to reduce structural Fe in the smectite decreased with age of the solution and paralleled the disappearance of the free radical signal in the ESR spectrum. The paramagnetic species was preserved and enhanced ifNa2SzO4 was added to the clay suspension, indicating that either the clay surface stabilized the SO2 9 radical or the additional unpaired electrons were produced in the clay structure.

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Section: Resultssupporting

confidence: 92%

Section: Resultssupporting

confidence: 90%

Section: Introductionmentioning

confidence: 99%

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Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

Gan

1992

Clays and clay miner.

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“…An alternative explanation for the reduction of bulk phase Fe(III), is that electron transfer between sub-surface Fe(III) and inner sphere-complexed sulfide may occur (possibly analagous to the abiotic and biotic reduction of structural Fe(III) in clay minerals; e.g. Stucki and Lear, 1990;Kostka et al, 1999). It is possible that both of the above processes may be facilitated by higher concentrations of dissolved sulfide, potentially giving rise (in conjunction with the effects of surface saturation) to the fractional order dependency of sulfide oxidation rate on sulfide concentration observed for all minerals.…”

Section: Controls On Reaction Kineticsmentioning

confidence: 99%

A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide

Poulton

Krom

Raiswell

2004

Geochimica et Cosmochimica Acta

541

443

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The reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C. Electron transfer between surface-complexed sulfide and solid phase Fe(III) results in the oxidation of dissolved sulfide to elemental sulfur, and the subsequent dissolution of the surface-reduced has an effect on mineral reactivity. However, these effects are unlikely to have a significant impact on the relative reactivities of the two mineral groups.4

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“…The chemical reduction of Fe(III) to Fe(II) in the crystal structure of smectite clay minerals greatly influences the chemical and physical properties of the clay (Stucki, 1988;Stucki and Lear, 1989), and several studies have suggested that many of these effects are related to changes in the face-to-face attractive forces between clay layers. Stucki et al (1984b) and Lear and Stucki (1989) showed that the interlayer repulsive force in smectite, i.e., swelling in water, decreases with increasing Fe(II) content, and that the specific surface area, as measured by ethylene glycol-monoethylether adsorption, is greatly diminished.…”

Section: Introductionmentioning

confidence: 99%

Effects of Iron Oxidation State on the Texture and Structural Order of Na-Nontronite Gels

Stucki

1991

Clays and clay miner.

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--Aqueous gels of unaltered (oxidized) and chemically reduced ferrnginous smectite (SWa-1 from the Source Clays Repository of The Clay Minerals Society) were characterized by transmission electron microscopy, electron diffraction, and energy-dispersive X-ray fluorescence to establish details regarding their texture, inter-layer and inter-particle arrangements, and chemical composition. Micrographs revealed that the reduction of structural Fe(III) to Fe(II) caused a consolidation of smectite particles from an extensive network of smaU crystals (1-6 layers thick) to distinct particles of limited size in the a-b direction and about 20-40 layers thick. The interlayer distances in the reduced sample appeared to be more uniform than in the oxidized sample, but both exhibited spacings of about 12.6 A. Chemical analysis showed no qualitative differences as a result of oxidation state. Electron diffraction patterns displayed marked differences. The pattern of the oxidized sample consisted of homogeneous rings, indicating that the stacking order in the a-b plane was turbostratic or disordered, whereas the reduced pattern exhibited much more order as evidenced by distinct spots amid low-intensity rings, suggesting that inter-layer attractive forces were stronger if Fe(II) was present in the clay crystal.

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Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals

Cited by 16 publications

References 0 publications

Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide

Effects of Iron Oxidation State on the Texture and Structural Order of Na-Nontronite Gels

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