Partitioning of Hydrophobic Organic Compounds to Sorbed Surfactants. 2. Model Development/Predictions for Surfactant-Enhanced Remediation Applications

A one-dimensional numerical model was developed to simulate the performance of surfactant-enhanced remediation (SER) applications for saturated subsurface systems containing adsorbed hydrophobic organic compounds (HOCs). The model incorporates temporally and spatially dependent HOC and surfactant mass balance equations to compute distributions in the aqueous, micellar, sorbed surfactant, and subsurface solid phases. In particular, the model accounts for losses of surfactant by sorption to the subsurface media and for the subsequent partitioning of HOCs to sorbed surfactant. Parameter values for the model were estimated from experimental rate and equilibrium data from the literature, and sensitivity analysis was conducted to evaluate the model performance and potential SER applications. Simulation results show that the relative affinity of HOCs and surfactants for the immobile subsurface solid phase (i.e., the respective retardation factors) is critical for determining whether contaminant desorption can be enhanced by surfactants. For example, under the conditions simulated here, removal of phenanthrene and naphthalene from a representative sandy (i.e., low organic carbon) aquifer is actually hindered by flushing with surfactant solutions, whereas for more hydrophobic contaminants (e.g., pyrene) surfactant addition can enhance HOC removal. Likewise, an increase in the organic carbon content of the subsurface solid phase increases the effectiveness of SER processes. The important rate and equilibrium model parameters evaluated in this study provide useful guidelines for the design and application of SER processes for contaminated subsurface systems and for interpreting SER-related studies.