Modeling Sorption of Anionic Surfactants onto Sediment Materials: An a priori Approach for Perfluoroalkyl Surfactants and Linear Alkylbenzene Sulfonates

摘要

A mechanistically derived model predicting the sorption of anionic surfactants to sediments was developed and evaluated for three classes of surfactants: perfluoroalkyl carboxylates, perfluoroalkyl sulfonates, and linear alkylbenzene sulfonates. The model includes both hydrophobic and electrostatic components and estimates the contribution of each to the sediment-water distribution coefficient (K_d) using Gibbs free energy terms. The hydrophobic free energy term was calculated from the aqueous solubilities of non-charged alkylbenzene or perfluoroalkane analogs and prior observations of increases in K_d values with increasing chain lengths. The electrostatic term was calculated from aqueous solution measurements using the non-ideal competitive adsorption Donnan (NICA-Donnan) model. The NICA-Donnan calculations were performed using parameters previously derived for generic humic acids. These two terms were coupled by multiplying by the fraction of organic carbon in the sediment, f_(oc), and a single fitting parameter, F_(access), the volumetric fraction of organic carbon accessible to the sorbing surfactant. The combined model accurately predicted the sediment-water distribution coefficients for all three classes of anionic surfactants. In its current formulation, the model was unable to capture the slight degree of isotherm nonlinearity observed for these surfactants.