ADSORPTION-KINETICS OF TOLUENE ON SOIL AGGLOMERATES - SOIL AS A BIPOROUS SORBENT

Single-component vapor diffusion at low pressure into dry soil agglome rates provides a well-characterized system for investigating adsorptio n and diffusion mechanisms. Equilibrium and kinetic data for adsorptio n of toluene vapor on Yolo loam and montmorillonite clay were obtained in this study with a gravimetric method. More extensive equilibrium d ata were obtained with a headspace gas chromatographic technique. Freu ndlich isotherms provided a good fit to the equilibrium data for relat ive partial pressures less than 0.6. Adsorption of toluene from the ga s phase occurred in two stages: the first was due to fast diffusion an d adsorption in macropores, and the second was controlled by slower di ffusion and adsorption in intragrain micropores. As several days are r equired to establish equilibrium, the effect of slow diffusion and ads orption would be overlooked in shorter studies. Numerical simulations described the isothermal adsorption of toluene on spherical soil parti cles subject to a step change in bulk concentration. Biporous diffusio n models are necessary to interpret the distinct diffusional character istics of toluene in macropores and micropores with nonlinear adsorpti on. The simulations quantitatively describe details of the adsorption history.