Effects of nonlinear sorption on the diffusion of benzene and dichloromethane from two air-dry soils

In order to test for the effects of nonlinear sorption on the diffusive transport of volatile organic compounds (VOCs) through air-dry soils, diffusion and sorption experiments were conducted with two air-dry soils using benzene and dichloromethane (DCM). Sorption isotherms in the presence of relative humidity were measured in both soils using a static sorption chamber and were in excellent agreement with the BET model. Diffusive emission rates from isothermal and uniformly contaminated air-dry soils were measured in a one-dimensional soil column under transient conditions. Matching the linear equilibrium sorption diffusion model to the emission rate data resulted in apparent diffusivities that were found to increase with increasing initial VOC concentration in the soil. This observation was also confirmed theoretically and attributed to nonlinear BET sorption. At low contaminant loadings, in the linear sorption range, the BET sorption parameters were successfully used to predict the apparent diffusivities for diffusion experiments.