Reduction and Reoxidation of Nontronite: Questions of Reversibility

Abstract

Abstraet--Redox cycles are common in nature and likely have a profound effect on the behavior of soils and sediments. This study examined a key component ofredox cycles in smectites, namely, the reoxidation process, which has received little attention compared to the reduction process. Unaltered (oxidized) and reoxidized ferruginous smectites (nontronites) were compared using infrared and M~ssbauer spectroscopies, and thermal gravimetric analysis. The infrared and thermal gravimetric data revealed that the structural OH content of reduced-reoxidized clay is about 15 to 20% less than in the original (oxidized) sample, indicating that the structure remains partially dehydroxylated even after reoxidation. M6ssbaner spectra of reoxidized samples consisted of larger quadrupole splitting for Fe(III) doublets than in the unaltered samples, suggesting that the environment of Fe(III) is more distorted after the reductionreoxidation treatment.

Reduction and Reoxidation of Nontronite: The Data Reassessed

“…For example, similar shifts of the principal Si-O band were observed for Fe(II)/Fe(III) ratios ranging from 0.8 (Rozenson and Heller-Kallai 1976) to 9 (Komadel et al 1995) for nontronite from the same locality. The band at 870 cm -1 disappeared in a hydrazine-reduced sample with an Fe(II)/Fe(III) ratio of only 0.08, but persisted with reduced intensity in a dithionite-treated sample with a much higher Fe(II) content (Rozenson and Heller-Kallai 1976).…”

“…Changes observed in the infrared spectra on reduction of nontronites include the following: 1) The band at 3560 cm -1, which comprises all of the OH stretching vibrations, is reduced in intensity and shifted to lower frequencies; 2) The band at 1030 cm -j, assigned to the principal Si-O vibrations, shifts to lower frequencies; 3) The band at about 870 cm -1 assigned to Fe-OH-A1 and the band at about 815 cm 1 assigned to Fe-OH-Fe bending vibrations are shifted and become weak or disappear (Rozenson and Heller-Kallai 1976;Stucki andRoth 1976, 1977;Russell et al 1979;Komadel et al 1995). These changes are only partially reversible upon reoxidation.…”

“…These changes are only partially reversible upon reoxidation. Whereas Fe(II) is readily reoxidized to Fe(III) and the principal Si-O band is restored to its original position, the intensity of the OH bands, and particularly that assigned to Fe-OH-Fe deformation, is not fully restored (Rozenson and HellerKallai 1976;Komadel et al 1995).…”

“…For example, for nontronite SWa-1, with an initial Fe(III):OH ratio of 2.73:4 and n/rn = 0.32 (Lear and Stucki 1985), 3.12 of the 4 OH groups are expected to remain after complete reduction, according to the mechanism described by Equations [1] through [3]. Komadel et al (1995), using a similarly buffered system and the same clay as Lear and Stucki (1985), observed no bands due to OH stretching or bending vibrations after ~90% reduction. Structural changes caused by the reduction of Fe may be expected to lead to shifts in position of OH bands, but are unlikely to eliminate them completely.…”

“…Recently Komadel et al (1995) reconsidered the reversibility of the redox reactions. However, despite the numerous investigations, some questions remain unanswered.…”