Dissolution and remobilization of NAPL in surfactant-enhanced aquifer remediation from microscopic scale simulations.

In this paper, the dissolution and mobilization of non-aqueous phase liquid (NAPL) blobs in the Surfactant-Enhanced Aquifer Remediation (SEAR) process were upscaled using dynamic pore network modeling (PNM) of three-dimensional and unstructured networks. We considered corner flow and micro-flow mechanisms including snap-off and piston-like movement for two-phase flow. Moreover, NAPL entrapment and remobilization were evaluated using force analysis to develop the capillary desaturation curve (CDC) and predict the onset of remobilization. The corner diffusion mechanism was also applied in the modeling of interphase mass transfer to represent NAPL dissolution as the dominant mass transfer process. In addition, the effect of pore-scale heterogeneity on mass transfer rate coefficient and recovered residual NAPL was considered in the simulations. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as the surfactant for the SEAR process. The results indicate that although surfactants enhance NAPL recovery during two-phase flow, surfactant-enhanced remediation of residual NAPL through dissolution is highly dependent on surfactant type. When SDS ─as a surfactant with high critical micelle concentration (CMC) and low micelle partition coefficient (Km)─ was injected into a NAPL contaminated site, the mass transfer rate coefficient decreased (due to considerable changes in interface chemical potentials) which leads to a significant reduction in NAPL recovery after the end of two-phase flow. In contrast, Triton X-100 (with low CMC and high Km) improved NAPL recovery, by enhancing solubility at surfactant concentrations greater than CMC which overcompensates the interphase mass transfer reduction.

Pore network and Darcy scale modelling of DNAPL remediation using ethanol flushing: Study of physical properties in DNAPL remediation.

Co-solvent flushing into contaminated soils is one of the most effective techniques for Dense Non-Aqueous Phase Liquid (DNAPL) remediation. In addition to the increase of DNAPL solubility, co-solvents (e.g. ethanol) can alter the viscosity and density of aqueous phase and diffusion coefficient of solute. Any changes in these parameters can change the flow behaviour and alter the upscaled DNAPL mass transfer coefficient which is a key parameter controlling soil and groundwater remediation at Darcy-scale. While numerous studies have investigated DNAPL remediation using co-solvents at the Darcy scale, pore-scale modelling of co-solvent enhanced DNAPL remediation has not been well investigated. In this work, a three-dimensional pore-network model was developed to simulate the 1,2-dichlorobenzene (DCB) remediation experiments using ethanol-water flushing solution. The model simulates the effect of changes in solubility, viscosity, density, and diffusion coefficient during co-solvent flushing of the DNAPL. The results of pore network modelling for ethanol-water flushing for the DCB remediation were also validated using the experimental data. In addition to pore-scale modelling, a continuum scale modelling (Darcy-scale) was used for the DCB remediation using ethanol-water flushing. The results of both pore network and continuum scale modelling demonstrated that the ethanol content and flushing velocity influence the interphase mass transfer and DNAPL dissolution process. The results indicated while the mass transfer coefficient decreased in the presence of ethanol, the process of NAPL remediation was improved due to the substantial increase of solubility in the presence of co-solvent. The large scale modelling showed that NAPL bank can be formed in the front of ethanol-water mixture flushing.