Interactions of anionic surfactants with lead, protons, and naphthalene in aqueous and soil systems: An experimental and modeling study

Anionic surfactants have been found to possess abilities for solubilizing hydrophobic organic compounds (HOCs) in the micelle core and associating with cations at micelle surface. These characteristics of anionic surfactants have been applied to wastewater treatment to remove organics and metal cations simultaneously in a process known as Micellar Enhanced Ultrafiltration (MEUF). In this study, these characteristics of anionic surfactants were examined for possible application to remediation of soils contaminated with HOCs and metal ions. Several anionic surfactants were examined in a soil/surfactant/naphthalene/lead system. Interactions investigated include (1) surfactant solubilization of naphthalene; (2) surfactant sorption on soil; (3) surfactant association with lead (Pb), and (4) surfactant precipitation. The effect of pH on the solubilization of naphthalene was insignificant. A partitioning model was applied for solubilization of HOC by surfactants and was able to reasonably describe solubilization of naphthalene. Sorption of anionic surfactants was found to be a function of pH. While anionic surfactant sorption was insignificant at pH above pH_zpc of the soil, sorption increased as the pH decreased below the pH_zpc of the soil. A decrease in surfactant sorption at low pH was observed and is presumably due to the protonation of surfactant monomer. The acid/base chemistry of anionic surfactants was found to be important to anionic surfactant sorption. A simplified surface complexation model was developed and was able to match the experimental sorption data adequately. With addition of anionic surfactants, enhancement of Pb removal from an artificially contaminated soil was found. Presumably, it was due to the association of the Pb ion with negatively charged micelle surfaces that provides the driving force for Pb desorption from soil. Pb has a higher affinity to a carboxylate based surfactant than to a sulfonated based surfactant. A modified Langmuir equation was able to properly describe the complexation of Pb with anionic surfactants. By combining models developed for individual interactions, Pb treatability data were reasonably simulated. The overall modeling framework for the soil/surfactant/naphthalene/Pb system demonstrated how effects of aqueous chemistry and surfactant concentration could be examined in the context of soil remediation.