STRUCTURAL-PROPERTIES OF REDUCED UPTON MONTMORILLONITE

Reduction of octahedral Fe in the crystalline structure of smectites i nfluences, possibly controls, surface-sensitive physical and chemical properties. The purpose of this study was to investigate if reduction of structural Fe by Na-dithionite or bacteria affects the chemical env ironment of constituent cations in montmorillonite, employing solid st ate multinuclear (Si-29 and Al-27) magic angle spinning nuclear magnet ic resonance (MAS NMR) spectroscopy. Reduction of structural Fe result ed in a positive (down field) chemical shift of the main Si Q(3) (Q(3) (0Al)) site Which was strongly correlated with Fe(II) content and infe rred that distortions in Si-O-T (T=Si, Al) bond angles and Si-O bond l engths occur with increasing layer charge. The line width (W) of the S i-29 Q(3) Signal also increased with increasing levels of reduction. N o change occurred in the position of the peak maximum for the octahedr al Al(Al-27(VI)) signal; however, an increased W was observed for this peak with increasing Fe(II) content. These results are attributed to decreases in Si-O-T bond angles and Si-O bond distances, corresponding to a better fit between the tetrahedral and octahedral sheets brought about by the presence of Fe(II) in the clay structure. The increased Al-27(VI) Signal width (W) may also be due to a lessening of the param agnetic influence of Fe(III) nuclei and enhancement of Al-27(VI) signa ls with different quadrupole coupling constants (QCC). Multinuclear MA S NMR analyses of dithionite-and microbially-reduced montmorillonite i ndicate that reduction of structural Fe caused reversible changes in t he smectite structure, at least as far as this method could discern.