GAS-PHASE ADSORPTION AND DESORPTION OF SINGLE-COMPONENT AND BINARY-MIXTURES OF VOLATILE ORGANIC CONTAMINANTS ON SOIL

Competitive adsorption of volatile organic mixtures on soil was studie d employing the frontal analysis chromatography technique. The binary mixtures were hexane/toluene (nonpolar, slightly polar), methanol/tolu ene (polar, slightly polar), and methanol/chlorobenzene (polar, polar) . In all the mixtures, the soil uptake of one component decreases in t he 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, isothe rms exhibited a different behavior, progressively becoming BET type II I as the partial pressure of methanol was increased. Desorption of pur e compounds and binary mixtures from the soil were also investigated. Desorption profiles were linked to the shape of the adsorption isother ms and showed a two-step behavior. The first step corresponded to deso rption along the desorption branch of the isotherm (desorption hystere sis of the isotherm), and the second step corresponded to desorption a long the adsorption isotherm after the hysteresis closure (the point a t which the adsorption and desorption branches of the isotherm converg e). The second step was the rate-controlling step for the overall deso rption process. The ideal adsorbed solution (IAS) theory was used to p redict mixture isotherms from pure-component isotherms. For the n-hexa ne/toluene system, the predictions of the IAS theory were quite accura te; however, for cases when polar species are present, the model under predicted the adsorbed amount. The binary desorption profiles were als o measured, and they indicate that desorption of the strong adsorbing component controls the overall desorption process.