Presence of local concentration higher than injected for non-ionic surfactant to solubilize NAPL in porous media: a result of interplay between partition and solubilization kinetics

This study delves into resolving the knowledge gaps concerning the dynamic correlation between the bulk partition of nonionic surfactants and the solubilization of residual non-aqueous phase liquids (NAPLs) for the technology of surfactant-enhanced aquifer remediation (SEAR) through a delicate investigation on the elution of dodecane by low-concentration Triton X-100 (TX-100) in saturated porous media. First an interesting phenomenon, the presence of local concentration of TX-100 higher than injected (i.e., C/C₀>1), was observed and then the mechanism was disclosed through a series of demonstration experiments to be the interplay between the kinetics of bulk partition of the surfactant and solubilization of residual dodecane. The rate-limited micelle-based solubilization of residual dodecane by TX-100, which induced the release of TX-100 from the residual dodecane phase as the source established by quick bulk partition of TX-100, resulted in the emergence of C/C₀>1 under flow-interruption condition. Interface partition of the ionic surfactant of SDBS and bulk partition of the miscible solvent of n-propanol could not cause such a phenomenon, demonstrating the importance of the occurrence of both bulk partition and micelle-based solubilization processes. The knowledge obtained in this study supplement to the current cognition of the dynamics of surfactant partition and NAPL solubilization, which can be helpful to optimize the application of surfactants in in-situ groundwater remediation techniques.