Modeling Multiphase Contaminant Transport in Porous Media Using First-Order Mass Transfer Kinetics

Abstract

This article presents the development of a multiphase contaminant transport model that incorporates non-equilibrium, first-order kinetics to the solution of mass transfer processes. The model is developed to analyze the mass transfer processes and simulate the contaminant removal by air sparging (AS) and soil vapor extraction (SVE). The model considers distinguished single-phase and multiphase domains. This capability is especially important in the case of remediation techniques that involve an advective air flux such as AS and SVE. In such systems, two domains may be considered: the advective domain, i.e., the air, and the non-advective domain, which may be either the domain outside the advective air domain but in the vicinity of the air plume, or any space or pocket inside the advective air domain that is not in direct contact with the advective air domain. The model considers the contaminant mass transfer among all phases involved: aqueous, non-aqueous, gaseous, and solid. The theoretical basis of the model is built on the concept that soil particles are surrounded by water films that isolate the gaseous and non-aqueous phase system. Therefore, the contaminant mass transfer can take place across the aqueous-solid (sorption/de-sorption), aqueous-gaseous (stripping), aqueous-NAPL (dissolution), and gaseous-NAPL (volatilization) interfaces. The governing equations for the mass transfer are solved numerically using Galerkin's finite element formulation. The numerical solution was verified against an analytical solution. The model was successfully used to analyze air sparging experimental data of toluene removal from two 350 mL reactors using airflows of 0.0432 and 0.0216 cm/s. The simulation revealed that an accurate determination of the first-order mass transfer coefficients is still needed to simulate the mass transfer processes.