Effects of Iron Oxidation State on the Specific Surface Area of Nontronite

Abstract

Abstract--The effect of Fe oxidation state on the specific surface area, Sin, ofnontronite was studied using the < 2-tzm, Na § fraction of the SWa-1 and Garfield nontronite reference clays. The reduction of structural Fe 3+ in the octahedral sheet of the nontronite decreased Sm as measured by the adsorption of 2-ethoxyethanol (ethylene glycol-monoethyl ether, EGME). The swellability in water of the nontronite also decreased during reduction. The amount of nonexchangeable Na +, on the other hand, increased with increasing Fe 2+ content and was highly correlated with EGME adsorption (r = -.985). The relationship between Sm and Fe 2+ was attributed to the collapse of partially or fully expanded layers to unexpanded layers.

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

“…Because cohesion forces between oxidized clay layers are weaker than between reduced layers (Stucki et al, 1984a;Lear and Stucki, 1989;Khaled and Stucki, 1991), adjacent oxidized layers glide more easily over one another, are more easily curved, and thus form thin but relatively long ribbons. Conversely, the greater attraction between reduced layers inhibits lateral crystal growth while promoting thicker stacks of layers, which are more ordered in the a-b dimension.…”

“…Lear and Stucki (1989) observed a significant decrease in EGME adsorption in reduced, compared with oxidized smectite, and thereby deduced a decrease in specific surface area assuming that EGME wets the same inter-laminar surfaces as water. According to Tessier and P6dro (1987), specific surface area is a good indicator of the amount of inter-laminar surface exposed to water in Na-smectire; nevertheless, part of the surface may be inaccessible to relatively neutral molecules such as EGME.…”

“…In particular, those layers that collapsed to 12.6 A may exclude EGME, and thus represent the equivalent of completely collapsed layers insofar as this measurement is concerned. But cation fixation, which has been shown for Na +, K +, Ca 2+, Cu 2+, and Zn 2+ (Chert et al, 1987;Lear and Stucki, 1989;Khaled and Stucki, 1991) to be a result of the reduction of structural Fe, should require more complete collapse of the layers. In the samples used in the present study, about 12% of the interlayer N a + is known to be fixed ; but no evidence was found in the electron micrographs of layers significantly more collapsed than 12.6 A.…”

“…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. Chen et al (1987), Lear and Stucki (1989), and Khaled and Stucki (1991) also observed a marked increase in the fixation of interlayer cations (Na +, K § Ca 2+, Cu 2+, and Zn 2+) by the chemical reduction of structural Fe. X-ray powder diffraction (XRD) analysis of reduced and unaltered (oxidized) Na-nontronite gels revealed fewer fully expanded layers in the reduced than in the oxidized clay (Wu et aL, 1989), indicating that reduction imparts a more consolidated texture to the gel.…”

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.…”