Freely dissolved concentration profile and Hyalella azteca toxicity of cationic surfactant C12-benzalkonium in spiked-sediment toxicity test

The freely dissolved concentrations (Cfree) have been considered a useful metric for exposure of aquatic organisms to organic contaminants. However, Cfreefor cationic surfactants has rarely been measured, and its use in sediment toxicity tests has not been evaluated. In this study, Cfreeof the cationic surfactant benzyldodecyldimethylammonium (C12-benzalkonium; C12-BAC) in water-only and spiked-sediment toxicity tests with the amphipod Hyalella azteca was analyzed using a passive sampling method. Polyacrylate-coated glass fibers were adopted as the passive sampler. Sorption isotherms of C12-BAC to the polyacrylate fibers were measured in chemical conditions comparable to those of the toxicity tests and used for Cfreecalculation in both tests. Detailed concentration analysis in the sediment toxicity test demonstrated a high concentration gradient of C12-BAC between sediment and overlying water; Cfreein pore water was 17-78 times higher than Cfreein overlying water and was 7.2-13 times higher than Cfreeat the sediment-water inter-face. The 50 % lethal concentration and bioconcentration factor of H. azteca obtained in the water-only test (23 mu g/L and 140 +/- 70 L/kg-wet, respectively) agreed with those calculated based on Cfreein pore water in the sediment test (49 mu g/L and 140 +/- 90 L/kg-wet, respectively), indicating that H. azteca is exposed mainly to the freely dissolved fraction in pore water. We concluded that Cfreein pore water is a useful exposure metric for H. azteca to cationic surfactants.