A closer look at the sorption behavior of nonionic surfactants in marine sediment

Abstract

The main focus of this thesis is to provide an improved scientific basis for the risk assessment of alcohol ethoxylates (AE) in marine sediment. The studies in this thesis focus on one of the basic assumptions that underlie the equilibrium partitioning theory (EqP), i.e., that understanding the freely dissolved concentration is a prerequisite for sediment risk assessment. A method with a polyacrylate Solid Phase Microextraction (SPME) fiber is validated as a passive sampling tool for the analysis of the freely dissolved concentrations for individual AE and linear alkylbenzene sulfonates (LAS). By testing with varying AE structures and different sediments and clay minerals, we investigated how the dissolved concentration depends on the concentration of AE sorbed to marine sediment. Adsorption to clay minerals is the dominating sorption process for AE in marine sediments, while the influence of organic matter on the overall sorption is negligible. A dual-mode sorption model fits and explains the nonlinear sorption behavior of AE well, even for simple AE mixtures that compete for adsorption sites. The adsorption affinity of AE for clay minerals increases with longer alkyl chain length as well as with higher numbers of ethoxylate units. Therefore, after correcting for the specific surface area, the sorption coefficients for most marine sediments can be predicted based on molecular properties. The importance of the freely dissolved concentration in the sediment phase is demonstrated in toxicity tests with the mudshrimp Corophium volutator. In accordance with the EqP, we showed that the freely dissolved AE concentration at which toxic effects occur is similar for organisms that are either exposed to spiked seawater only or to a layer of spiked marine sediment. However, since C. volutator appeared to be more exposed to the overlying water than to the pore water in the sediment, its use in sediment bioassays should be reconsidered.