Infrared study of oxidized and reduced nontronite and Ca-K competition in the interlayer.

摘要

Iron reduction and reoxidation in Garfield nontronite were studied by infrared spectroscopy (IR). The mineral was first reduced with sodium dithionite for a period of 0 to 4 hr to obtain various Fe reduction levels. The reduced samples were then reoxidized by bubbling O{dollar}sb2{dollar} through the suspensions for 8-12 hr to achieve complete reoxidation. IR spectra of both reduced and reoxidized samples were collected. Changes were observed in the spectral regions for the structural O-H stretching and deformation, and for the Si-O stretching vibrational modes. These observed spectral features indicated that the clay structure was modified significantly beyond merely a change in Fe oxidation state when Garfield was reduced by this potent reductant. One component band in the O-H stretching region from the 4-hr reduced sample exhibited a pleochroic effect, which indicated the existence of the trioctahedral domains. This suggested that the trioctahedral domains were formed when Garfield was reduced extensively. A large frequency downward shift (45 cm{dollar}sp{lcub}-1{rcub}){dollar} in the Si-O stretching region was also observed after 4-hr reduction. Such a large shift in frequency indicated that the change in Fe oxidation state in the octahedral sheet strongly affected the structural properties of the tetrahedral sheet, which might further affect physical and chemical properties on the mineral surface. The spectral differences across all three studied regions between unaltered (unreduced) and reoxidized samples after 4-hr reduction indicated that some structural changes associated with the redox process involving sodium dithionite are irreversible.; Four Fe-rich beidellite-nontronite minerals ranging from 17% to 22% of total Fe were also studied using infrared spectroscopy (IR). Sample treatments (reduction and reoxidation) were the same as that of Garfield nontronite. The IR spectra of the reduced and reoxidized samples offered a brand new approach to confirm or clarify the assignment of absorption bands proposed in previous studies. At the maximum reduction level, two new peaks in the O-H stretching (3622 cm{dollar}sp{lcub}-1{rcub}){dollar} and librational (655.5 cm{dollar}sp{lcub}-1{rcub}){dollar} regions were observed for the first time. These two peaks indicated that more changes in crystal structure occurred than reported in previous studies of reduction of Fe-containing clay minerals. This study thus marks a significant step forward in the understanding of the effects of redox processes on the structural properties of layer silicates.; Ca and K selectivity in the interlamaller space of the unaltered, reduced, and reoxidized ferruginous smectite (SWa-1) was studied. The results of this study can be applied to estimate fertilizer K availability in the production field. Cation exchange method was used in this study. In the unaltered clay mineral, Ca displayed a strong preference over K. This preference, however, was reversed when the clay was chemically reduced with sodium dithionite. When the reduced clay was reoxidized, Ca, again, displayed a preference over K, but the ratio of Ca to K was not restored to the original extent. The selectivity of cations in the interlamaller space of SWa-1 also depended on the ratio of Ca to K in the exchanger solution, and on the total concentration of the exchanger solution. Those results indicate that the conventional estimation of the available K based solely on the mineral composition of the soil-clay fraction is insufficient. The redox state of soil-clay and the competing cations (Ca and Mg most common) also play very important roles in controlling K availability.