IMPORTANCE OF KNUDSEN DIFFUSION ON BENZENE VAPOR TRANSPORT IN UNSATURATED SOIL

Experimental data for unsteady state benzene vapor transport in large (10.5 cm i.d. and 100 cm long) columns packed with dry and wet soil were used to evaluate the importance of Knudsen diffusion. The computed Knudsen diffusion coefficient, D(B)K, was compared with the molecular diffusion coefficient, D(a), for benzene in soil. Since in all cases D(B)K >> D(a), it was concluded that benzene diffusion in these soils was dominated by molecular and not Knudsen diffusion. However, Knudsen diffusion may be important in high clay soil or high moisture content soil with air-filled pores of extremely small size. The experimental data were adequately described by the diffusion equation, taking into account local equilibrium for vapor sorption and vapor partitioning into soil moisture.