Adsorption characteristics of rhodamine 6G on montmorillonite and laponite, elucidated from electronic absorption and emission spectra

Abstract

The adsorption of the cationic dye rhodamine 6G by montmorillonite (Wyoming bentonite) and laponite (synthetic hectorite) was studied by visible and fluorescent spectroscopy and by X-ray diffraction methods. Adsorption of the dye takes place by the mechanism of cation exchange. X-ray data indicate that the adsorbed cationic dye is located in the interlayer spaces of both minerals. It was found that adsorption prevents the dimerization of the dye, and gives rise to red shifts of the principal absorption band of the dye. A study of the effect of varying concentration of dye or clay on the location and intensity of this band proved to be useful in determining the adsorption capacity of the clay. The observed spectral changes are linked to surface polarity and to clay flocculation. At concentrated suspensions of montmorillonite, an additional weak absorption shoulder is obtained at 470–490 nm, which may be attributed to aggregated dye molecules, probably trapped within the cavities of book-house structured flocs. Due to steric hindrance rhodamine 6G does not give π interactions with the oxygen plane of the silicate layer. Therefore the adsorption of the dye by expanding clay minerals does not give rise to metachromasy. Fluorescence of the adsorbed dye is reported: it is red shifted and quenched relative to aqueous solutions. The use of fluorescence spectra to determine the saturation point is described, and aging and ultrasonic effects on spectral features are presented and discussed.