Measurement of mass-transfer rates for surfactant-enhanced solubilization of nonaqueous phase liquids

Surfactant-enhanced solubilization of residual, nonaqueous-phase liquid (NAPL) contaminants is an emerging, subsurface remediation technology. The potential for nonequilibrium conditions is investigated for surfactant-enhanced solubilization of a NAPL trichloroethylene (TCE), in a model porous medium. The surfactant formulation consists of an anionic surfactant, sodium dihexyl sulfosuccinate, an alcohol, and an electrolyte in aqueous solution. Batch solubilization experiments are conducted to assess the significance of chemical rate limitations. Surfactant flood experiments are conducted in packed columns with residual TCE. Mass-transfer rate coefficients are determined as a function of aqueous-phase pore velocity, NAPL volumetric fraction, and surfactant concentration. A correlation for predicting mass-transfer rate coefficients as a function of system properties is developed, The mass-transfer rate coefficients and correlation are obtained by fitting a transport simulator to the column effluent concentration results. Significant differences are found between the correlation developed here and correlations developed for other NAPL-surfactant systems. The correlation predicts near-linear dependences of mass-transfer rates on the NAPL volumetric fraction and pore velocity. Using the Damkohler number, the degree of nonequilibrium behavior in surfactant-enhanced NAPL solubilization is analyzed for a range of conditions. Nonequilibrium conditions are found to be significant at relatively low NAPL volumetric fractions.