Transport and Sorption of Volatile Organic Compounds and Water Vapor within Dry Soil Grains

Mechanisms governing the rates of adsorption and desorption of benzene, trichloroethylene, and water vapor by dry soil grains are investigated. For three different test soils, determinations are made of organic carbon content, specific surface area, grain density, grain porosity, and pore size distribution. Experimental measurements of the rates of adsorption and desorption show that periods of hours are required to achieve equilibrium. In addition, adsorption is observed to be much faster than desorption. A porous sphere model accounting for internal diffusion in spherical grains coupled with a nonlinear Freundlich isotherm is used to interpret the sorption rate data. The model follows both adsorption and desorption trends reasonably well using a single fitted parameter, the effective diffusivity. The model also provides good resolution of the substantial asymmetry in the rates of uptake and release. In a refinement, a parallel pore model is used to account more fully for effects of the experimentally measured pore size distribution.