Sorption behavior and acute toxicity of cationic surfactants in the aquatic environment

Abstract

Cationic surfactants are widely used as detergents, fabric softeners and disinfectants. Due to the charged nitrogen atoms, they have a high potential to sorb to negatively charged sediments, soils and sludge. That is also the reason why they are frequently detected in sediment and sludge. Sorption to relevant environmental phases is one of the key factors that controls the toxicity and bioavailability of organic chemicals. The equilibrium partitioning (EqP) theory assumes that toxicity and bioaccumulation in a sediment or soil system is driven by the freely dissolved concentration in the aqueous phase. In this thesis, ‘solid phase microextraction’ (SPME) is applied as a simple extraction technique. Ion-exchange SPME was successfully employed to determine freely dissolved concentrations of cationic surfactants. We apply this sampling tool to investigate the sorption behavior and bioavailability of cationic surfactants and to validate the EqP theory for several chemicals from the specific class of cationic surfactants. The SPME measurements show strong sorption of cationic surfactant to (dissolved or bulk) organic matter as well as clay minerals. The contribution of sorption by clay minerals to sorption in whole sediments can be as important as that by organic matter. The sorption isotherms to these soil components are slightly nonlinear and are influenced by solution pH, concentration of dissolved inorganic cations, and type of inorganic cations. The sorption data indicate that ion-exchange processes play an important role with respect to the sorption to both SPME fiber and soil components. The NICA-Donnan model explained several of these specific effects on cationic surfactant binding, for example by accounting for different electrostatic attractions in the presence of dissolved Ca2+ compared to comparable concentrations of Na+. The acute toxicity results of C12-benzalkonium show that the freely dissolved concentration is a better indicator of the actual exposure concentration than nominal and total concentration in the applied bioassays with aquatic organisms, cells and algae. The acute Daphnia and Lumbriculus tests both show no enhanced toxicity from possible ingestion of sorbed C12- benzalkonium in comparison with water-only exposure, which is in accordance with the EqP theory. Commonly used sorbent phases, like sediment, organic matter and proteins, can affect bioavailability and as a consequence also the effect concentrations for toxicity of cationic surfactants. The developed extraction method for measuring freely dissolved concentration offers an excellent tool to study the bioavailability of cationic surfactants.