Enhanced remediation of residual DNAPL by ethanol cosolvent flushing: Mass transfer behavior and empirical correlation

Ethanol cosolvent flushing is an effective in-situ technique for enhancing remediation of groundwater contaminated by dense non-aqueous phase liquids (DNAPLs). The remediation efficiency critically depends on the dissolution and mass-transfer performance of residual DNAPL. However, due to the intrinsic opacity of aquifer materials, the sand-column systems commonly used in flushing experiments provide limited insight into the dynamic behaviors of DNAPL and the underlying mechanisms governing its removal. In this study, high-resolution X-ray micro-computed tomography was employed to visualize and quantify the dissolution and mass transfer of residual DNAPL in porous media under various flow rates and ethanol contents. Results indicate that increasing flow rate enhances both dissolution rates and mass transfer coefficients. The effect of ethanol content is non-linear: dissolution rates increase gradually at low contents but rise sharply when ethanol exceeds ∼30%, consistent with solubility trends. Ethanol also alters residual DNAPL morphology and flow paths, enlarging interfacial area and counteracting the expected increase in mass transfer resistance. Moreover, local heterogeneity in mass transfer triggers dynamic interfacial processes such as receding, snap-off, and breakup, which influence dissolution behavior. Furthermore, a new empirical correlation was then developed, integrating saturation, flow rate, and solubility, which can accurately predict mass transfer coefficients during both water and cosolvent flushing. This work elucidates key pore-scale mechanisms controlling residual NAPL mass transfer and proposes a universal model for solubilization-enhanced remediation.