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

Abstract

--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.

Effects of Structural Iron Reduction on the Hydraulic Conductivity of Na-Smectite

“…The effect of oxidation state on bulk density is understandable in light of earlier studies which revealed that Fe reduction decreases the water content of the clay at a given applied pressure (Stucki et al, 1984c), decreases the specific surface area (Lear and Stucki, 1989), and increases the particle size (Stucki and Tessier, 1991). The reduction of structural Fe(III) to Fe(II) consolidates smectite particles from an extensive network of small crystals (1-6 layers thick) to distinct particles of limited size in the a-b direction and about 20-40 layers thick (Stucki and Tessier, 1991).…”

“…The reduction of structural Fe(III) to Fe(II) consolidates smectite particles from an extensive network of small crystals (1-6 layers thick) to distinct particles of limited size in the a-b direction and about 20-40 layers thick (Stucki and Tessier, 1991). The stacking of layers is also more ordered when the clay is reduced.…”

“…Permeability to organic compounds seems to be greater than to water (Anderson, 1981), probably because of the hydrophilic nature of clay surfaces; but the accumulation of an organic compound over time may alter the water-retention capacity of the clay. Also, the permeability to water changes with the water content of the compacted clay (Monserrate, 1982 Studies by Stucki and co-workers (Stucki et al, 1984b(Stucki et al, , 1984cStucki, 1987, 1989;Stucki and Tessier, 1991;Khaled and Stucki, 1991) revealed that the oxidation state of structural Fe in smectite produces large differences in many properties which are fundamental to the performance of clay liners, swelling pressure, cation exchange capacity, total layer charge, specific surface area, and microstructure or texture.…”

Influence of γ-radiation on the reactivity of montmorillonite towards H2O2

“…A typical native bentonite, such as the widely studied Wyoming Bentonite MX-80, consists of nearly 3 wt% Fe, the major part being structural Fe(III) replacing Al 3+ in the aluminum octahedral sheet [3]. At elevated Fe(II)/Fe Tot ratios however, significant changes in several physical properties such as the swelling pressure, cation exchange capacity (CEC), total layer charge, specific surface area as well as microstructural properties has been found in previous studies [4][5][6][7][8][9]. After the initial saturation of the bentonite barrier, anoxic conditions are expected.…”

Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

“…The redox state of the clay and knowledge of how to control or exploit it are of great importance to agriculture, industry, and the environment because of their profound effects on many physical-chemical properties of the clay Lear and Stucki, 1989;Komadel et al, 1990;Khaled and Stucki, 1991;Stucki and Tessier, 1991).…”