Adsorption of Benzene, Toluene, and Xylene by Two Tetramethylammonium-smectites Having Different Charge Densities

-A high-charge smectite from Arizona [cation-exchange capacity (CEC) = 120 meq/100 g] and a low-charge smectite from Wyoming (CEC = 90 meq/100 g) were used to prepare homoionic tetramethylammonium (TMA)-clay complexes. The adsorption of benzene, toluene, and o-xylene as vapors by the dry TMA-clays and as solutes from water by the wet TMA-clays was studied. The adsorption of the organic vapors by the dry TMA-smectite samples was strong and apparently consisted of interactions with both the aluminosilicate mineral surfaces and the TMA exchange ions in the interlayers. In the adsorption of organic vapors, the closer packing of TMA ions in the dry high-charge TMA-smectite, compared with the dry low-charge TMA-smectite, resulted in a somewhat higher degree of shape-selective adsorption of benzene, toluene, and xylene. In the presence of water, the adsorption capacities of both samples for the aromatic compounds were significantly reduced, although the uptake of benzene from water by the low-charge TMA-smectite was still substantial. This lower sorption capacity was accompanied by increased shape-selectivity for the aromatic compounds. The reduction in uptake and increased selectivity was much more pronounced for the water-saturated, high-charge TMA-smectite than for the low-charge TMA-smectite. Hydration of the TMA exchange ions and/or the mineral surfaces apparently reduced the accessibility of the aromatic molecules to interlamellar regions. The resulting water-induced sieving effect was greater for the high-charge TMA-smectite due to the higher density of exchanged TMAions. The low-charge Wyoming TMA-smectite was a highly effective adsorbent for removing benzene from water and may be useful for purifying benzene-contaminated water. Key Wards--Adsorption, Aromatic hydrocarbons, Benzene, Charge density, Smectite, Tetramethylammonium, Water. I N T R O D U C T I O N The type of exchangeable cations on clays strongly influence their sorptive characteristics for non-ionic organic compounds (NOCs). In nature, the net negative charges of clays are usually balanced by inorganic exchange ions, such as Na + and Ca z+, which are strongly hydrated in the presence of water. The hydration of these exchangeable metal ions and the presence of SiP groups in clays imparts a hydrophilic nature to the mineral surfaces. As a result, the adsorption of NOCs by clays is suppressed in the presence of water because relatively nonpolar organic chemicals cannot effectively compete with highly polar water for adsorption sites on the clay surface. In the absence of water, the clay acts as a conventional solid adsorbent; the high adsorptive capacity for organic compounds is attributed to its large surface area (Call, 1957; Jurinak, 1957; Chiou and Shoup, 1985). 3 Corresponding author. 4 Present address: Department of Environmental Engineering and Water Resources, Tamkang University, Taipei, Taiwan, Republic of China. Copyright 9 1990, The Clay Minerals Society The inability of clays to remove substantial amounts of NOCs from aqueous solution can be remedied by replacing natural metal cations with larger organic cations through ion-exchange reactions (McBride et al., 1977; Wolfe et al., 1985; Boyd et al., 1988a). Earlier studies indicated that exchanging quaternary ammonium cations for metal ions on clays greatly modified the sorptive characteristics of dry clays for organic vapors (e.g., White and Cowan, 1958; Barrer and Perry, 1961; McAtee and Harris, 1977). Depending on the size of the organic cation, the exchanged organic ions have been shown to form either a microscopic organic phase, as in hexadecyltr imethylammonium-smecti te (Boyd et aL, 1988b), or discrete organic-modified surface adsorption sites as in TMA-smecti te (Lee et al., 1989). Importantly, the substitution of organic cations reduces the hydration of the clay and concomitantly decreases the "free" aluminosilicate mineral surface area (i.e., that surface not covered by the organic exchange ions). As a result, the surface properties of the clay may change considerably from highly hydrophilic, if the clay contains mainly inorganic cations, to increasingly organophilic, as the inorganic cation is progressively replaced by the organic cation. Mortland et al. (1986) and Boyd et al. (1988c) used