Effects of Reduction and Reoxidation of Structural Iron on the Surface Charge and Dissolution of Dioctahedral Smectites

Abstract

Abstract--The effect of Fe oxidation state on the surface charge (CEC) and solubility of smectites were studied using the <2-t~m, Na+-saturated fraction of an Upton, Wyoming; a Czechoslovakian; and a New Zealand montmorillonite; and a Garfield, Washington, nontronite. The reduction of structural Fe 3+ in the octahedral sheet of each clay produced a net increase in the negative surface charge of the clay. The observed cation-exchange capacities deviated from the linear relationship predicted by charge-deficit calculations, assuming changes only in the Fe2+/Fe a+ ratio, and reversibly followed Fe reduction according to a 2nd-degree polynomial function. The deviations suggest reversible changes in mineral structiite and composition during Fe reduction.These clays were susceptible to partial dissolution in citrate bicarbonate (CB) and citrate-bicarbonatedithionite (CBD) solutions. Small amounts of Fe and Si dissolved as a result of Fe reduction in CBD, but affected < 1% of the total clay mass except for the Czechoslovakian clay in which 2% of the clay dissolved. Although slightly more Fe dissolved than Si, no change in surface charge was noted. Almost no dissolution of these elements was detected in CB solution. In contrast, significant Al was detected in the CB solution, suggesting a heterogeneous dissolution mechanism. The CEC, however, was unchanged by the CB treatment. These results may be explained by the adsorption of hydrogen ions into the vacated AP + sites in the mineral structure. Dissolution seems to have been independent of the effects of Fe oxidation state on surface charge.

Effects of Oxidation State of Octahedral Iron on Clay Swelling

“…It depended only on the state of oxidation, and not on the path by which that state was reached. Treatment of this clay with citrate-bicarbonate and with dithionite dissolves some Fe, Si, and AI (Stucki et al, 1984b) and raises the possibility that changes in mJmc might be attributable to mineralogical alterations other than the reduction of structural Fe 3+. The aforementioned reversibility, however, precludes this possibility.…”

“…[~..o......,,..=~176176176176 ,.[~=" Fe 2+ (mmole/g) Figure 5. The relation between the cation-exchange capacity, o~, and the octahedral Fe 2 § content for clays from Upton, Wyoming (UPM), Czechoslovakia (CZM), New Zealand (NZM), and Garfield, Washington (GAN) (from Stucki et aL, 1984b).…”

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

“…The molecular compositions of these clays are respectively (Goodman et al, 1976;Stucki et al, 1984a) The clays were saturated with Na § dialyzed, and freezedried.…”

B3LYP Periodic Study of the Physicochemical Properties of the Nonpolar (010) Mg-Pure and Fe-Containing Olivine Surfaces

“…Understanding these systems is of significant importance because they serve as an important source for electrons in redox reactions in various geochemical environments. [32][33][34][35] Moreover, the change of the Fe oxidation state has a profound impact on the electronic structure of the surfaces of these minerals, such as in their adsorption properties and chemical activity.…”