http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Experimental study and mathematical modeling of sorption and desorption of volatile organic contaminants on soil
xiii, 170 leaves : illustrations ; 28 cm
The adsorption equilibria of various volatile organic contaminants on EPA standard soil were measured by a dynamic response technique based on frontal analysis chromatography. The adsorption isotherms were BET type II isotherms, which indicate multilayer adsorption in the pores. The magnitude of the calculated heats of adsorption confirmed that the adsorption was physical adsorption. It was also shown that adsorption of relatively non-polar organics took place uniformly on the available external surface area whereas water adsorption was much greater probably due to the polar nature of water molecules that allows them access to the internal clay layers and/or capillary condensation. As moisture is always present in the natural soil environment, the effect of humidity on organics' adsorption was assessed by measuring the sorption equilibria of toluene and chlorobenzene at different relative humidity levels. The sorption isotherms at low relative humidity were BET type II isotherms whereas at high relative humidities, they exhibited a type III behavior. In all cases, the presence of humidity reduced the organic uptake on soil. At high relative humidity and low organic relative vapor concentration, there was almost complete suppression of organic adsorption. At high organic relative vapor concentration, there was competitive adsorption of water and organic. The shape of the desorption profile was very much 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. This last step was the rate-limiting step for the overall decontamination process. It was shown that using a water-saturated gas stream improved greatly the organic removal rates by accelerating the monolayer desorption. A model was proposed to simulate breakthrough curves and desorption profiles, assuming local equilibrium between the pore gas phase and the solid phase and an axially-dispersed flow pattern. The sensitive parameters of the model were the axial dispersion coefficient and the adsorption equilibrium. The local equilibrium assumption gave good results which proved that there was no slow diffusion necessitating the consideration of a kinetic/retardation term.