Role of Structural Hydrogen in the Reduction and Reoxidation of Iron in Nontronite

Abstract

Abstract--The effects of reduction and reoxidation of octahedral Fe 3+ on the exchange of structural hydrogen in nontronite were determined using tritium (all) as a label element. The uptake of H from the surrounding solution of nontronite suspensions increased as the reduction of structural Fe 3+ increased. Similarly, the loss of H from the structure increased as the reduction increased. The results are generally consistent with a reduction mechanism involving the loss of structural OH, leaving the affected Fe sites with less than six-fold coordination. The attenuation of increased negative charge on the clay layer, however, was less than predicted by such a mechanism.During the reoxidation of reduced nontronite in suspension, about one-third of the H remaining as part of the structure following reduction was lost, whereas twice that amount of H was incorporated into the structure from the surrounding solution. A reoxidation mechanism is proposed whereby H20 from the surrounding solution is incorporated into the mineral structure followed by the elimination of a hydrogen ion, returning the Fe to six-fold coordination. This mechanism implies the reversibility of Fe reduction in nontronite.

Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

“…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.…”

“…According to this hypothesis, structural Fe is reduced partially by internal and partially by external reducing agents, and the reduction is accompanied by dehydroxylation and reprotonation reactions. Lear and Stucki (1985) determined that two of the stoichiometry coefficients are linearly related, viz., r = 0.32 m, but the value of a has yet to be confirmed. The point of contact between the clay and the reducing agent is still unknown.…”

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

“…Stucki et al (1984a) and Lear and Stucki (1985) observed that the Table 2 and the results of Stucki et al (1984b) demonstrate that 13 decreases with increasing Fe 2 § content. Eq.…”

“…The reduction of clay suspensions in citrate-bicarbonate buffer solution with sodium dithionite (Na2S:O4) salt was accomplished as described by Lear and Stucki (1985) and the swellability in water, by the method of Stucki et aL (1984b). The specific surface area of clay gels was determined using the method described by Odom and Low (1978), which was patterned after the 2-ethoxyethanol (also known as ethylene glycol-monoethyl ether, EGME) method of Carter et aL (1965).…”

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

“…The maximum theoretical b length for a completely reduced sample is 9.323 ~, so if the actual level of reduction is > 50%, agreement between theory and experiment would be even better. Some error (-< +5%) also exists in determining the crystallographic spacings from the electron diffraction patterns, and the Fe in the reduced sample likely existed in some distorted, five-coordinate sites (Lear and Stucki, 1985), the distortions being reflected in the unit cell b-edge length. Due to the presence of rings rather than spots in the electron diffraction patterns of the oxidized sample, the crystallographic parameters were more difficult to deduce.…”