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.
Abstract--Using Garfield, Washington, nontronite as the model mineral system, methods and apparatus were developed to prepare reduced suspensions in citrate-bicarbonate-dithionite (CBD) solution. These techniques were effective in removing excess, undesired solutes from reduced suspensions while maintaining a high Fe 2 § content. They also enabled the preparation of dried, reduced films preferentially oriented with respect to the crystallographic c-axis. Supernatant solutions were collected and analyzed for Fe, A1, and Si, from which the extent of dissolution of the clay as a result of CBD treatment was assessed. Results indicated that very little Fe and Si were released to solution, but as much as about 8% of the total A1 was solubilized. The highest levels of A1 in solution Were observed in CB treatments without dithionite.
The frequencies of Si-O stretching, νSi-O, in layers of montmorillonite and of H-O-H bending, ν2, in the interlayer water were investigated by infrared spectroscopy. Oriented gels of Li-and Na-montmorillonite equilibrated at different water contents, mw/mc, were obtained by using a miniature pressure membrane apparatus. One part of each gel was used for the gravimetric determination of mw/mc. The other part was transferred to an FTIR spectrometer where the spectrum of the gel was measured by attenuated total reflectance. Thus, the dependence of both νSi-O and ν2 on mw/mc was determined in the same samples. The absorption bands for Si-O stretching were resolved by using a curve-fitting technique to determine the frequency, area, and height of the component peaks. Two prominent peaks, labeled peak III and peak IV, were selected for analysis. It was found that ν Si-O for each peak and the ratio of the areas (or heights) of the two peaks were all exponential functions of mc/mw. Moreover, a linear relation was found between ln(νSi-O) and ln(ν2). These results were interpreted to mean that the Si-O stretching vibrations in the montmorillonite layers and the H-O-H bending vibrations in the interlayer water are coupled and that both change as the layers come closer together.
Abstract-Three montmorillonites and a nontronite were reduced by sodium dithionite to obtain different amounts of Fe 2+ in their octahedral sites. The mass ratio of water to clay, mJmo, was determined as a function of Fe z+ at several values of the swelling pressure, II. The value mJmo decreased markedly with increasing Fe 2 § at each value of II for each clay. Moreover, curves of II vs. mJmc for the different clays were displaced downwards as Fe 2+ increased. A straight line was obtained when ln(II + 1) was plotted against 1/(mJmc); however, at some oxidation states of three of the samples this line exhibited a sharp break at a specific value of 1/(mJmc). The slope of the line decreased for each clay as Fe 2+ increased, and an increase in Fe 2+ was accompanied by an increase in the cation exchange capacity. These observations are thought to be due to a collapse or partial collapse of the superimposed clay layers resulting from the increase in cation-exchange capacity.
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