Impact of Pore Geometry and Water Saturation on Gas Effective Diffusion Coefficient in Soil

Wulong Hu,Yongshui Lin,Qiang Han,Yao Jiang,Daoyi Chen,Yifei Cui

doi:10.3390/app8112097

Abstract

Gas flow in soil plays a crucial role in terrestrial ecosystems, and numerical simulation of their movement needs to know their effective diffusion coefficients. How pore structure influences the effective diffusion coefficient has been studied intensively for dry porous media, but much remains unknown for unsaturated soils. Here, we employed the X-ray tomography technique at the pore scale to directly obtain the soil structures, the geometry of their pores and the water distribution under different water saturation levels were calculated using a morphological model. The results show that pore structures including porosity, interface area of gas–solid–water and pore diameter are closely related to water saturation. The increase of mean pore diameter with gas saturation can be fitted into a power law. We also investigated the impact of pore geometry and water saturation on the effective diffusion coefficients, which is independent of the molecular mass of gas after normalization. As the normalized effective Knudsen diffusion coefficient increases with average pore diameter following a power law, with the scaling factor related to pore geometry and the exponent is a constant, we explained and proved that the Knudsen diffusion coefficient increases with gas saturation, also following a power law.

Highlights

Understanding the movement of gas in soil is of great importance in many fields, such as estimating the environmental effect of gas spills [1,2,3], designing engineering projects to inject and store CO2 [4,5,6,7,8], treatments of volatile constituents of hazardous wastes in landfills and disposal sites [9,10,11]
Valid forformula other soils, calculated normalized mean pore diameter under saturations in that the was valid for other soils, we calculated the normalized meandifferent pore diameter under another soil sample with contrasting pore geometrical shape and pore size distribution, as shown in different saturations in another soil sample with contrasting pore geometrical shape and pore size average pore diameter and gas saturation can be fitted into a power‐law
Two 3D images and simulations were used to evaluate the effect of saturation on pore structure and on the effective diffusion coefficient of soil

Summary

Introduction

Understanding the movement of gas in soil is of great importance in many fields, such as estimating the environmental effect of gas spills [1,2,3], designing engineering projects to inject and store CO2 [4,5,6,7,8], treatments of volatile constituents of hazardous wastes in landfills and disposal sites [9,10,11]. Owing to the intrinsic complexity of soil structure, the practical study of gas transport in soil is generally based on macroscopic approaches, wherein the microscopic features are volumetrically averaged by omitting the pore geometry [11,12,13]. Because only the void space allows gas to diffuse, gas diffusive flow in soil mainly depends on pore geometry and water saturation [14,15,16]. The gas effective diffusion coefficient (Deff ) in soil is usually assumed to be proportional to their bulk diffusion coefficient (d) in free air.

Methods

Results

Conclusion