Abstract
ABSTRACT
Competitive adsorption of volatile organic mixtures on soil was studied employing the frontal analysis chromatography technique. The binary mixtures were hexane/toluene (nonpolar, slightly polar), methanol/toluene (polar, slightly polar), and methanol/chlorobenzene (polar, polar). In all the mixtures, the soil uptake of one component decreases in the presence of the coadsorbate. The adsorption isotherms of the hexane/toluene binary system were BET type II for both the single-component and the mixture. In the presence of the polar specie, methanol, isotherms exhibited a different behavior, progressively becoming BET type III as the partial pressure of methanol was increased. Desorption of pure compounds and binary mixtures from the soil were also investigated. Desorption profiles were linked to the shape of the adsorption isotherms and showed a two-step behavior. The first step corresponded to desorption along the desorption branch of the isotherm (desorption hysteresis of the isotherm), and the second step corresponded to desorption along the adsorption isotherm after the hysteresis closure (the point at which the adsorption and desorption branches of the isotherm converge). The second step was the rate-controlling step for the overall desorption process. The ideal adsorbed solution (IAS) theory was used to predict mixture isotherms from pure-component isotherms. For the n-hexane/toluene system, the predictions of the IAS theory were quite accurate; however, for cases when polar species are present, the model underpredicted the adsorbed amount. The binary desorption profiles were also measured, and they indicate that desorption of the strong adsorbing component controls the overall desorption process.
Key words:
adsorption; desorption; soil remediation; volatile organic compounds
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