Abstract: The reaction of hydroxy-Al interlayers in montmorillonite with monosilicic acid was studied by chemical analysis, X-ray diffractometry (XRD), and Fourier-transform infrared (FTIR) spectroscopy. Hydroxy-Al interlayers in montmorillonite was prepared by treating Ca-montmorillonite (< 2 µm) with hydroxy-Al solutions at an initial Al concentration of 0.5 mM and OH/Al molar ratios of 1.0, 2.0, and 2.5. The resultant partially intedayered montmorillonite was reacted with 0.5, 0.9, and 1.4 mM monosilicic acid solutions.
A substantial amount of monosilicic acid was adsorbed by the interlayer hydroxy-Al polymers through the reaction of Si-OH groups with mainly Al-OH groups, resulting in the formation of silicated Al-interlayers with Si/Al molar ratios up to 0.19. The adsorption of silicic acid by interlayer hydroxy-Al polymers increased the basal spacings of the partially interlayered montmorillonite. This effect was most pronounced for the hydroxy-Al interlayered montmorillonite formed in the systems at an OH/AI ratio of 2.0, in which the d(001) reflections (110°C) shifted from 13.6 Å before silication to 14.1 Å with a shoulder at 17.0 Å after silication. The differential IR spectrum of the silicated hydroxy-Al interlayers in montmorillonite exhibited absorption bands at 935 and 3730 cm-1, which can be assigned to Si-O and Si-OH, respectively, of the adsorbed Si(OH)4. An interlayer structure analogous to that of “proto-imogolite” was, thus, proposed for the silicated interlayer hydroxy-Al polymers in montmorillonite. This study revealed a new mechanism through which Si enters the interlayer spaces of smectite. The silication of hydroxy-Al-interlayered clays in natural environments and the related modifications in surface properties of the clays should receive increasing attention.