Intervalence Electron Transfer and Magnetic Exchange in Reduced Nontronite

--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.

Section: Methodsmentioning

confidence: 99%

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

Stucki

1991

“…This method is based on the intervalence electron transfer (IT) transition, which occurs between Fe(II) and Fe(III) in adjacent octahedral sites (Lear and Stucki, 1987). Twenty-five mg of sample was suspended in 2.5 ml of citrate-bicarbonate (CB) buffer solution and diluted with 37.5 ml of H20 in an inert-atmosphere reaction vessel (Stucki et al, 1984a).…”

Section: Methodsmentioning

confidence: 99%

Reduction and Reoxidation of Nontronite: Extent of Reduction and Reaction Rates

Komadel

Lear

Stucki

1990

107

Abstract--The reduction and reoxidation of three nontronite samples, GAN (API H-33a, Garfield, Washington), SWa-1 (ferruginous Washington smectite), and NG-1 (Hohen Hagen, Federal Republic of Germany) were studied with visible absorption and M6ssbauer spectroscopy. The intensity of the intervalence electron transfer (IT) band at 730 nm in these nontronites was monitored during reduction and reoxidation at 277, 294, and 348 K. The results showed that the intensity of the band followed the number of Fe(II)-O-Fe(III) groups in the clay crystal, increasing to a maximum at about Fe(II):total Fe = 0.4; upon complete reduction, the band decreased to about the intensity of the unaltered, oxidized sample. With reoxidation of the sample with O2, the intensity of the band increased sharply, followed by a gradual decay back to the original, oxidized intensity. The ultimate level of Fe reduction achieved was at least 92%. Concomitantly, the color changed from yellow through green, blue-green, dark blue, light blue, and light gray as the Fe(II) content increased. The GAN nontronite was more difficult to reduce than the SWa-1 or NG-1 samples. The rate and level of reduction increased with the amount of reducing agent added.

“…The conductivity of electrons from the layer edge through the octahedral sheet would depend heavily on the presence of the transition metal. But Lear and Stucki (1987), based on measurements of magnetic exchange interactions and of intervalence electron transfer, concluded that structural Fe is reduced nearly randomly within the octahedral sheet. Reduction only from layer edges would likely create a reducing front Lear and Stucki (1987) clearly reject this possibility in ferruginous smectite.…”

Section: Resultsmentioning

confidence: 99%

Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

Gan

1992

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.