Freely dissolved concentration profile and Hyalella azteca toxicity of cationic surfactant C 12 -benzalkonium in spiked-sediment toxicity test

The freely dissolved concentrations (C ) have been considered a useful metric for exposure of aquatic organisms to organic contaminants. However, C for cationic surfactants has rarely been measured, and its use in sediment toxicity tests has not been evaluated. In this study, C of the cationic surfactant benzyldodecyldimethylammonium (C -benzalkonium; C -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 C -BAC to the polyacrylate fibers were measured in chemical conditions comparable to those of the toxicity tests and used for C calculation in both tests. Detailed concentration analysis in the sediment toxicity test demonstrated a high concentration gradient of C -BAC between sediment and overlying water; C in pore water was 17-78 times higher than C in overlying water and was 7.2-13 times higher than C at the sediment-water interface. The 50 % lethal concentration and bioconcentration factor of H. azteca obtained in the water-only test (23 μg/L and 140 ± 70 L/kg-wet, respectively) agreed with those calculated based on C in pore water in the sediment test (49 μg/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 C in pore water is a useful exposure metric for H. azteca to cationic surfactants.