Sorption of nonionic surfactant oligomers to sediment and PCE DNAPL: Effects on PCE distribution between water and sediment

Introduction of surfactant mixtures to the subsurface for the purpose of su rfactant-enhanced aquifer remediation requires consideration of the effects of surfactant sorption to sediment and nonaqueous phase liquids. These eff ects include alteration of the solubilizing properties of the surfactant mi xture a nd a Iteration of the sorption properties of the stationary phase. Sorption of octylphenol ethoxylate (EO) surfactant oligomers to a low organ ic carbon content (f(oc)) aquifer sediment and to dense nonaqueous phase li quid (DNAPL) consisting of tetrachloroethene (PCE) was examined in batch ex periments. At aqueous surfactant concentrations far below the critical mice lle concentration (CMC) of the mixture, sorption to sediment was characteri zed by an initial steep isotherm for both high and low EO content oligomers , with somewhat greater uptake of high EO content oligomers. This stage of sorption resulted in mild increases in the equilibrium constant, K-d,K-PCE for distribution of PCE between solution (including surfactant) and sedimen t (including sorbed surfactant). As the aqueous surfactant concentration in creased, surface aggregation of low EO content oligomers on the sediment co mmenced, and a dramatic increase in K-d,K-PCE was observed. At aqueous surf actant concentrations increasing above the CMC, the formation of solution m icelles caused the sorbed surfactant concentrations to plateau and then dec rease. This decrease in sorbed surfactant, along with competition by micell es for contaminant, likely contributed to the observed rapid decrease in K- d,K-PCE toward zero. Surfactant sorption to PCE DNAPL was greater relative to sediment by 1-2 orders of magnitude, with much greater uptake of the low EO content oligomers. Sorption of all but the lowest EO content oligomers to the PCE DNAPL was terminated by micellization.