Effective Gas-phase Diffusion Coefficient In Soils
Author(s)
C.-S. Fen
Abstract
Determination of the effective gas-phase diffusion coefficient is important to
quantify gas diffusion for a number of environmental pollution problems
involving migration and release of hazardous gases or organic vapors in soils.
Conventionally, the only considered transport mechanism for this coefficient is
molecular diffusion. This study additionally considers the effects of Knudsen
diffusion and nonequimolar flux on this coefficient and proposes a relationship
of this coefficient with both molecular and Knudsen diffusion coefficients, ratio
of molar masses and composition of component gases for gas diffusion in soils.
Through investigating calculation results of this relationship for hypothetical
diffusion systems having different soil permeabilities and diffusing gases at a
range of water saturation levels, it is found that the Knudsen and nonequimolar
effects can only be substantial for systems with dry soil permeabilities less than
10-13 m2 at any saturation. The Knudsen effect on this relationship is further
demonstrated through comparing results obtained from this relationship to the
measured effective diffusion coefficients for 72 soil cores, which all have high
silt and clay contents, given in the literature.
Keywords: gas diffusion in soil, molecular diffusion, Knudsen diffusion,
diffusibility.
1 Introduction
Volatile organic compounds (VOCs) are common groundwater contaminants
threatening our environment. Volatile contaminants can quickly spread in the
subsurface by vaporization at dissolved or free phase and by gas phase transport
through soil pores. Gas diffusion is an important transport mechanism
controlling migration and distribution of volatile contaminants. Other
environmental pollution problems such as emissions of methane, CO2 or organic
Keywords
gas diffusion in soil, molecular diffusion, Knudsen diffusion,
diffusibility.
Related Book
Other papers in this volume
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