Adsorption kinetics of toluene on soil agglomerates: Soil as a biporous sorbent

Single-component vapor diffusion at low pressure into dry soil agglomerates provides a well-characterized system for investigating adsorption and diffusion mechanisms. Equilibrium and kinetic data for adsorption of toluene vapor on Yolo loam and montmorillonite clay were obtained in this study with a gravimetric method. More extensive equilibrium data were obtained with a headspace gas chromatographic technique. Freundlich isotherms provided a good fit to the equilibrium data for relative partial pressures less than 0.6. Adsorption of toluene from the gas phase occurred in two stages: the first was due to fast diffusion and adsorption in macropores, and the second was controlled by slower diffusion and adsorption in intragrain micropores. As several days are required to establish equilibrium, the effect of slow diffusion and adsorption would be overlooked in shorter studies. Numerical simulations described the isothermal adsorption of toluene on spherical soil particles subject to a step change in bulk concentration. Biporous diffusion models are necessary to interpret the distinct diffusional characteristics of toluene in macropores and micropores with nonlinear adsorption. The simulations quantitatively describe details of the adsorption history.