Association of Linear Alkylbenzenesulfonates with Dissolved Humic Substances and Its Effect on Bioavailability

The association of C10-, C12-, and C14-linear alkylbenzenesulfonates (LAS) with natural and spec- imen-grade dissolved humic substances (DHS) was measured with fluorescence quenching and with ultracentrifugation techniques. Good agreement was obtained with both of the analytical methods, suggesting that fluorescence quenching could be used to measure aqueous-phase partition coefficients. LAS−DHS partition coefficients increased with increasing length of the alkyl chain. Partition coefficients for the sorption of LAS to alkylammonium surfactant-coated, phyllosilicate clays also increased with increasing length of the alkyl chain in the LAS molecules. Taken together, these data indicate the significance of nonpolar forces in LAS−organic matter interactions. Toxicity studies examined the effects of DHS on the bioavailability to the fathead minnow, Pimephales promelas. Changes in the uptake and toxicity of LAS resulting from the addition of DHS were used to calculate aqueous-phase LAS−DHS partition coefficients. Good agreement was found between the partition coefficients calculated from the response of the test organism and those obtained with fluorescence and ultracentrifugation measurements. The toxicity studies suggest that the association of LAS with DHS can play a significant role in reducing the biologically available fraction of LAS in surface waters.