Competitive Adsorption of Para-Xylene and Water Vapors on Calcium, Sodium, and Lithium-Saturated Kaolinite
- K.D. Pennell *,
- R.D. Rhue and
- A.G. Hornsby
Vapor-phase adsorption of volatile organic chemicals (VOC) on soils and clay minerals has been shown to be greatly reduced in the presence of water. Exchangeable metal cations are known to interact strongly with water, and thus may influence the competitive adsorption of water and organic vapors. Flow-equilibration experiments were conducted to measure the concentration of water and p-xylene vapors adsorbed on oven-dried Ca-, Na-, and Li-saturated kaolinite from single and binary vapor systems. Li-saturated kaolinite adsorbed significantly less water than Ca- and Na-kaolinite. This was attributed to a 50% reduction in cation exchange capacity (CEC) of Li-kaolinite following heating at 110°C. The adsorption of p-xylene vapors on Ca-, Na-, and Li-kaolinite at 0% relative humidity (RH) was also related to the CEC and exchangeable cation species, which suggests that specific interactions occurred between exchangeable cations and p-xylene molecules. When the RH was increased to 10 and 20%, p-xylene adsorption decreased in small, but successive, increments. The competitive adsorption of both water and p-xylene on Ca-, Na- and Li-kaolinite was underestimated by two-component adsorption equations based on the Brunauer, Emmett, and Teller (BET) model. At low RH's, water adsorption on kaolinite was hypothesized to occur primarily at cation exchange sites, allowing for the continued adsorption of p-xylene on exposed mineral surfaces. Thus, the assumption of complete monolayer coverage prior to the onset of multilayer formation, upon which the BET model is based, may limit the ability of these equations to accurately predict the competitive adsorption of polar and nonpolar vapors.
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