Partitioning of hydrophobic organic compounds to sorbed surfactants. 1. Experimental studies

Partitioning of two hydrophobic organic compounds (HOCs), phenanthrene and naphthalene, to surfactant micelles, kaolinite, and sorbed surfactants was studied to provide further insight on (1) the effectiveness of using sorbed surfactants to remove HOCs from water and (2) the feasibility of surfactant-enhanced remediation (SER) for contaminated subsurface systems. Sorbed surfactant partition coefficients, K-SS, showed a strong dependence on the surfactant sorption isotherms: at low sorbed surfactant levels K-SS values were at their highest and then decreased with increasing surfactant sorption densities. K-SS values for SDS were always larger than the corresponding micellar partition coefficient (K-mic) values; for Tween 80, however, K-SS values were higher than K-mic Values only at the lower sorbed surfactant densities. HOC distributions between the immobile and mobile phases varied with surfactant dose be ca use of the competition between sorbed and micellar surfactants for HOC partitioning: distribution coefficients increased initially with increasing surfactant concentrations before decreasing at higher doses. Overall results of this study demonstrate that surfactant sorption to the solid phase can lead to increases in HOC retardation when equilibrium conditions are applicable (e.g., slow advection rates). This effect is desirable wh en the treatment objective is to immobilize HOCs by removing them from water; however, the effect is undesirable in SER applications. Before any surfactant-enhanced mobilization/immobilization process is selected, appropriate consideration of surfactant sorption and HOC partitioning to immobile versus mobile phases pertinent to a specific subsurface system must be contemplated.