Partitioning of ethoxylated nonionic surfactants in water/NAPL systems: Effects of surfactant and NAPL properties

Surfactant-based remediation has received considerable attention as a potential technology for enhancing conventional pump-and-treat remediation processes for aquifers contaminated with organic nonaqueous-phase liquids (NAPLs). In surfactant remediation, NAPL contaminants can be removed by two processes: solubilization and mobilization. Although these processes have been extensively studied, limited information is available regarding the partitioning of nonionic surfactants between aqueous and organic phases. Because surfactants are amphiphilic molecules that have a finite solubility in aqueous phases and NAPL, partitioning between the two phases is expected. The study presented here examines the effect of surfactant and NAPL properties on the partitioning of nonionic surfactants between different aqueous and nonaqueous phases using ethoxylated non ionic surfactants with different chemical structures and mixture distributions and NAPLs covering a wide range of NAPL/water interfacial tensions. In general, surfactant partitioning was found to be strongly correlated with surfactant hydrophobicity, mixture polydispersity, and NAPL/water interfacial tension. Partitioning at the high surfactant concentrations relevant in surfactant remediation (>2% surfactant) was most strongly correlated with surfactant hydrophobicity and NAPL water interfacial tension whereas low concentration partitioning was more strongly correlated with surfactant mixture polydispersity. A quantitative correlation was developed to predict the high concentration partitioning behavior of surfactant/NAPL systems, given the surfactant critical micelle concentration and the NAPL/water interfacial tension. Implications for aquifer remediation are discussed.