Effects of Solution Chemistry on the Partitioning of Phenanthrene to Sorbed Surfactants

Solution chemistry effects on surfactant micelle formation, surfactant sorption on kaolinite, and phenanthrene partitioning to surfactant micelles and sorbed surfactants were studied. For the anionic surfactant sodium dodecyl sulfate (SDS), critical micelle concentration (cmc) values decreased with increasing ionic strength but were unaffected by pH changes. For the nonionic surfactant Tween 80, the cmc was unaffected by pH and ionic strength changes. SDS sorption on kaolinite showed strong pH and ionic strength dependency: sorption increased as pH decreased and ionic strength increased, respectively. For Tween 80, only pH changes affected its sorption on kaolinite. Micellar phenanthrene partition coefficients (Kmic) increased with increasing ionic strength for SDS but were not affected by changes in pH. For Tween 80 Kmic values, no pH or ionic strength effect was observed. Phenanthrene distribution coefficients (KD) between the solid and aqueous phases varied as a function of solution chemistry and were primarily dependent on the concentration of surfactant that was sorbed. Organic carbon-normalized partition coefficients (Koc) of the sorbed surfactants showed a much greater affinity for phenanthrene than that predicted for natural organic matter. For sorbed SDS aggregates, Koc values fell into two general regions depending on whether the pH was above or below the point of zero charge (PZC) of kaolinite. Plausible structural differences for sorbed SDS aggregates are discussed to explain these regional Koc values.