INVESTIGATION OF THE EFFECT OF MOISTURE ON THE SORPTION AND DESORPTION OF CHLOROBENZENE AND TOLUENE FROM SOIL

The sorption isotherms of chlorobenzene and toluene on a standard EPA soil at different relative humidities were measured using a dynamic technique based on frontal analysis chromatography. At 0% relative humidity, the isotherms were BET (Brunauer-Emmett-Teller) type II isotherms, indicating monolayer coverage of the surface followed by multilayer coverage. As the relative humidity increased, the shape of the isotherm became progressively a type III isotherm, indicating weaker adsorbent/adsorbate interactions. The presence of water greatly reduced the organic uptake by soil, especially at low organic concentrations. The effect of humidity was attributed to adsorptive displacement of the organic by water. A mechanistic approach was considered to evaluate the contribution of the possible sorption mechanisms to the total sorption. It was found that adsorption at the gas-liquid interface was important and that dissolution in liquid water and partitioning into organic matter from the adsorbed water phase were negligible. The BET theory for adsorption of binary gas mixtures on solid surfaces successfully represented the data in the relative humidity range 0-50% and relative organic pressure range 0-30%. The effect of humidity on the desorption profile was investigated, and it was found that the efficiency of extraction was increased when using a water-saturated gas stream for desorption instead of a dry gas stream.