Adsorption and desorption of binary mixtures of volatile organic contaminants on soil

Abstract

The adsorption equilibria of four volatile organic compounds, n-hexane, toluene, methanol and chlorobenzene on Highbank #4471 soil were measured by a dynamic response technique based on frontal analysis chromatography. The adsorption isotherms were BET type 11 isotherms, which indicates multilayer adsorption on soil. The heats of adsorption indicate physical adsorption. Competitive adsorption of organic mixtures on soil was assessed by measuring the adsorption equilibria of binary mixtures of volatile organic compounds by using the same technique. The binary mixtures systems were hexane-toluene(non-polar, non-polar), methanol-toluene(polar, non-polar), and methanol-chlorobenzene(polar, polar). The presence of two organics reduced each organic's uptake on soil. The adsorption isotherms of hexane-toluene system were BET type 11. With the presence of methanol, the isotherms exhibited a different behavior. The isotherms progressively became BET type III when the concentration of methanol was high. The desorption of pure compounds and binary mixtures from the soil were also investigated. The desorption profiles were linked to the shape of the adsorption isotherms and showed a two-step behavior. The first step corresponded to the desorption of the multilayers and the second step to the monolayer desorption. The second step was the rate-controlling step for the overall desorption process. The adsorption isotherms of single components on soil were modeled by using the BET isotherin. The extended BET model, the potential theory and the ideal adsorbed solution theory for mixture adsorption were also evaluated for prediction of the experimental results for binary adsorption. All models failed for prediction of the BET type III isotherms when methanol was present at high concentrations. For the n-hexane and toluene system, the IAST and extended BET models can predict the amount adsorbed. The potential theory works well at a low concentrations of n-hexane and under-predicted at high concentrations of n-hexane. For cases when methanol was present, only the IAST model can predict the adsorbed amount at conditions where concentrations of methanol are low.