Surface Heterogeneity of Trimethylphenylammonium-Smectite as Revealed by Adsorption of Aromatic Hydrocarbons from Water

Abstract

Abstract--Adsorption studies of aromatic hydrocarbons of various molecular sizes on organo-clays in aqueous solution were carried out for characterizing the surface heterogeneity of organo-clays. Benzene, toluene, pxylene, ethylbenzene and n-propylbenzene adsorption by a smectite with 5 different exchange degrees of trimethylphenylammonium (TMPA) cations for Ca a+ was measured. The Langmuir isotherm equation did not adequately describe the experimental data, especially for small molecules, whereas the Dubinin-Radushkevich (DR) equation combined with a gamma-type adsorption energy distribution function described all experimental data well, suggesting the surface and structural heterogeneity of TMPA-smectites. The calculated adsorption energy distributions indicated that the apparent heterogeneity depends on the molecular size of adsorbates. Small adsorbate molecules such as benzene explore a highly heterogeneous surface of TMPA-smectites while large molecules such as n-propylbenzene detect a relatively homogeneous surface. The surface fractal dimension was dependent on the extent of TMPA exchange for Ca z+. When TMPA content is less than 75% of the cation exchange capacity (CEC) of the smectite, the heterogeneity decreases as TMPA content increases; it increases with TMPA content thereafter. These results are related to the size distribution of micropores in TMPAsmectites, which are defined by the 2 semi-infinite aluminosilicate sheets and the interlayer cations. The micropore size distributions and, hence, heterogeneity are created in part by the inhomogeneity of the charge density of clay surfaces and the tendency for cation segregation in these systems.

Transmission electron microscopy of hexadecyltrimethylammonium-exchanged smectite

“…In studies of NOC sorption on the small-size cation trimethylphenylammonium (TMPA)-exchanged smectite, Sheng et al (1997) investigated whether adsorbate molecules such as benzene seek the highly heterogeneous surface of organoclays. They insisted that non-uniform distribution of cations may lead to the overall energetic heterogeneity of solid surfaces.…”

Sorption of Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) Compounds by Hectorite Clays Exchanged with Aromatic Organic Cations

“…Lee et al (1989) found that water sorption by TMA limited the sorption of aromatic molecules larger than benzene by TMA-smectites. Sheng et al (1997) found that the heterogeneous charge density of smectites noted by Stul and Mortier (1974) leads to micropores with a heterogeneous size distribution in TMPA-smectites. Similarly, Sheng and Boyd (1998) examined mixed Ca/TMPA-smectites and found that TMPA cations function as nonhydrated interlayer pillars, and organic-solute adsorption occurs predominantly on the siloxane surfaces and is not associated with the TMPA cations.…”

Relation of Water and Neutral Organic Compounds in the Interlayers of Mixed Ca/Trimethylphenylammonium-Smectites

“…To illustrate this, our results are compared to those for reduced-charge TMPA-smectites having no exchangeable Ca 2+ . Although adsorption isotherms on TMPA-smectites for neutral organic compounds are better described by the Dubinin-Radushkevich (DR) equation (Sheng et al 1996b), the data can be adequately fitted to the Langmuir equation, and this allows comparison to the data reported by Jaynes and Boyd (1991), as described below. The monolayer adsorption capacities Qm of benzene, ethylbenzene and n-butylbenzene on the mixed Ca/TMPAsmectites are summarized in Table 2.…”