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 IT 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.ZVl GRAUER, DAVID AVNIR et SHMUEL YARIV. Can. J. Chem. 62, 1889 (1 984).Faisant appel a la diffraction des rayons-X et a la spectroscopie visible et de fluorescence, on a CtudiC l'adsorption du colorant cationique rhodamine 6G par la montmorillonite (bentonite du Wyoming) et par la laponite (hectorite synthCtique). L'adsorption du colorant se fait par un mCcanisme impliquant un Cchange de cations. Les donnCes de rayons-X indiquent que, dans le cas de chacun des deux minkraux, le colorant cationique est adsorb6 dans les espaces interstitiels. On a trouvt que l'adsorption empeche la dimCrisation du colorant. A la suite de l'adsorption, la principale bande d'adsorption du colorant se dCplace vers le rouge. Une ttude de l'effet d'un changement de concentration du colorant ou de l'argile sur la position et I'intensitC de cette bande s'avkre un outil utile pour dkterminer la capacitC d'adsorption de l'argile. Les changements spectraux obsewCs sont lies a la polarit6 de la surface et a la floculation de l'argile. Pour des suspensions concentries de montmorillonite, on observe une adsorption supplCmentaire faible qui apparait comme un Cpaulement a 470-490 nm et que I'on peut attribuer une aggregation de molCcuies de colorant qui sont probablement emprisonnCes dans les cavitCs des flocons. i a rhodamine 6G, a cause de I'encombrement stkrique, n'a pas d'interactions IT avec le plan d'oxygenes de la couche de silicate. Par conskquent, l'adsorption du colorant par la dilatation des minkraux argileux ne provoque pas de mCtachromasie. On rapporte la...

Spectroscopic study of the adsorption of rhodamine 6G on clay minerals in aqueous suspensions

“…This metachromasy is attributed to the aggregation of the dye after it is adsorbed on the clay surface. Grauer et al (1984) have proposed that the adsorption of R6G on Laponite and montmorillonite prevents the dimerization of the dye. This difference might be due to the different conditions used in the separate studies.…”

“…The R6G dimer adsorbed at the internal surface, with an intermonomeric distance shorter than the externally adsorbed dimer, is a more constrained aggregate, owing to the limitation of the interlamellar space. Grauer et al (1984) assumed the possibility of dye aggregation in the inter-particle space of flocculated samples. In the present study, however, flocculated samples are not considered because of the sample conditions.…”

Model for Competitive Adsorption of Organic Cations on Clays

“…Several studies have focused on the adsorption of cationic organic dyes to negatively charged smectites (Chu and Johnson, 1979;De et al, 1974;Ghosal and Mukherjee, 1972;Grauer et al, 1984;Pham Thi Hang and Brindley, 1970;Narine and Guy, 1981;Venugopal and Nair, 1974), Margulies et al (1985 suggested a method of photostabilization of pesticides which is based on coadsorbing on a clay surface the photolabile pesticide and an adequate organic cation that acts as energy acceptor. In such a system the organic cation must be strongly attached to the surface of the clay in order to prevent contamination of soil and water.…”

Metachromasy in clay-dye systems: the adsorption of acridine orange by Na-saponite

“…In excess AO, the clay is gradually peptized. At this stage the dispersed clay platelets form small tactoids with monomeric AO in the interlayer space and at the same time adsorb dimerie and polymeric AO cationic species at the solidliquid interface.The adsorption of metachromic dyes by the trioctahedral smectite-like phase 'Laponite' and the effects of adsorption on the electronic absorption and emission spectra of the clay-dye suspension have been studied by several investigators (Avnir et al, 1986; Cenens & Schoonheydt, 1988;Cenens et al, 1990;Endo et al, 1986Endo et al, , 1988Grauer et al, 1984; Viaene et al, 1987; Tapia Estevez et al, 1994).It was shown in previous publications that the mechanism of the Laponite flocculation in the presence of aromatic organic cations can be studied from the visible spectra of clay suspensions to which metachromic dyes were added (Yariv et al, 1990). The following spectroscopic features were applied for the study of the colloid behaviour of Laponite during the adsorption of acridine orange [AO, I 3,6 -bis (dimethylamine) acridine hydrochloride] (Garfinkel-Shweky & Yariv, 1995).…”

“…The adsorption of metachromic dyes by the trioctahedral smectite-like phase 'Laponite' and the effects of adsorption on the electronic absorption and emission spectra of the clay-dye suspension have been studied by several investigators (Avnir et al, 1986; Cenens & Schoonheydt, 1988;Cenens et al, 1990;Endo et al, 1986Endo et al, , 1988Grauer et al, 1984; Viaene et al, 1987; Tapia Estevez et al, 1994).…”