Influence of distribution characteristics of residual DNAPL on mass transfer in porous media under ethanol co-solvent flushing

The remediation of residual dense non-aqueous phase liquid (DNAPL) enhanced by co-solvents or surfactants has been investigated in many studies. Nevertheless, the influences of the residual DNAPL distribution characteristics in a porous medium, and the concentrations of the co-solvents or surfactants on the mass transfer rate of DNAPL remain relatively unexplored. For this reason, existing empirical models neglect the effects of the heterogeneity in the distribution of residual DNAPL, and the compositions of flushing floods on the DNAPL mass transfer rate. In this work, a two-dimensional micro-model is used to quantify the lumped mass transfer rate coefficients of PCE. Results show that increasing the injection rate and the ethanol concentration of flushing floods improves the mass transfer rate of PCE. Moreover, this micro-model allows for accurate quantification of variations in morphological characteristics of PCE blobs. The relative standard deviation sigma of residual PCE saturation, as an indicator of morphological characteristics of PCE blobs, varies with decreasing saturation. Standard saturation S0 is used to represent the heterogeneity of residual PCE distribution. The more heterogeneous the PCE distribution, the larger the S0, and the slower the mass transfer rate. According to the variation of relative standard deviation sigma with saturation, the PCE dissolution process can be divided into three stages, where the strong convection, weak convection, and diffusion within a limited distance are the primary controlling mechanisms for PCE mass transfer, respectively. Based on our study, a new empirical model, considering the distribution characteristics of residual PCE and the ethanol concentrations, is proposed to predict the PCE mass transfer process dominated by different mass transfer mechanisms in different dissolution stages.