Numerical analysis of sorption and diffusion in soil micropores, macropores, and organic matter

Abstract

Sorption and diffusion of volatile organic compounds (VOCs) in soil are important processes in subsurface contamination, but are complicated by the heterogeneous nature of soil. The soil pore-size distribution is herein approximated as a biporous geometry of micropores and macropores. The microspheres embedded in macrospheres contain micropores. Voids between microspheres constitute the macropores. Soil organic matter is assumed present on the surface of microspheres. We develop numerical solutions for the transient diffusion processes with linear partitioning in soil organic matter and nonlinear adsorption in the biporous soil particles. The three coupled nonlinear partial differential equations describe the diffusion in micropores and organic matter in series with diffusion in macropores. The computation uses an implicit Crank-Nicolson method with a predictor-corrector scheme for fast and efficient calculations. The solutions are applied to toluene and water sorption-desorption experimental data obtained by gravimetric uptake measurements. The procedure allows the estimation of key parameters, including micropore diffusivities.