PAPER www.rsc.org/greenchem | Green Chemistry Mixture effects and predictability of combination effects of imidazolium based ionic liquids as well as imidazolium based ionic liquids and cadmium on terrestrial plants (Triticum aestivum) and limnic green algae (Scenedesmus vacuolatus) Marianne Matzke,* a Stefan Stolte, b Andrea B¨ oschen b and Juliane Filser a Received 12th February 2008, Accepted 24th April 2008 First published as an Advance Article on the web 4th June 2008 DOI: 10.1039/b802350f Up to now the issues of mixture toxicity and combination effects were neglected within a prospective hazard assessment of ionic liquids (ILs). However when being released to the environment, mixtures of ILs or mixtures of ILs and other pollutants are likely to occur and therefore the impact of mixture toxicity should be taken into account. Thus this study investigates the effects of three differently composed mixtures containing ILs as well as ILs and the heavy metal cadmium on limnic green algae (Scenedesmus vacuolatus) and wheat (Triticum aestivum). For an evaluation of the results, two well established concepts of concentration addition (CA) and independent action (IA) were used. Both (CA and IA) underestimated the effects of the mixtures consisting exclusively of ILs leading to the assumption that interactions between the mixture components or between the mixture components and the environmental matrix occurred. The general applicability of CA and IA must be questioned. For the mixture of ILs and cadmium the deviations from the predictions were moderate (and less toxic than expected) so that CA can be recommended as a starting point for the analysis of combination effects of ILs and heavy metals. In general the presence of cadmium reduced the toxicity for both the aquatic as well as the terrestrial organisms. The green algae reacted approximately two orders of magnitude more sensitive to the mixture scenarios than wheat and proved to be a good reference test system for the evaluation of measured effects within a prospective hazard assessment of ionic liquids. Introduction Ionic liquids (ILs) are a promising substance class regarding their interesting physicochemical properties. Manifold appli- cations of certain ILs have been described in the fields of e.g. synthesis, 1 electrochemistry 2 and (bio)catalysis. 3,4 The main advantage of certain ILs is their improved operational safety in comparison to conventional solvents. This is based on the negligible vapour pressure of ILs, resulting in reduced air emissions and non-flammability. Nevertheless, there is a great uncertainty regarding their hazard potential and up to now studies investigating the ecotoxicological hazard potential emanating from ILs focused on single substance toxicities only. 5–10 However aquatic and terrestrial ecosystems are exposed to various multi-component mixtures of pollutants differing in composition and concentration. It is impossible to test all possible mixtures of chemicals and in general the problem of chemical mixtures is neglected in regulation to a large extent due to the high complexity of combination effects. To deal with this a UFT—Centre for Environmental Research and Technology, Department 10: Ecology, University of Bremen, Leobener Straße, D-28359, Bremen, Germany. E-mail: matzke@uni-bremen.de b UFT—Centre for Environmental Research and Technology, Department 3: Bioorganic Chemistry, University of Bremen, Leobener Straße, D-28359, Bremen, Germany problem there are large efforts to reliably predict combination effects of xenobiotics based on single substance toxicities. Two well established concepts, originating from pharmacological research have successfully been used in ecotoxicology for the investigation of mixture toxicities in single species tests 11–16 as well as in community studies: 17,18 (a) Concentration addition (CA) describes similarly act- ing substances, first introduced by Loewe and Muischnek in 1926/1927 19,20 and (b) Independent action (IA) describes dissimilarly acting compounds, first introduced by Bliss in 1939. 21 Especially in aquatic toxicology the concept of concentration addition is mostly used for the prediction of combination effects. 22 Because an IL itself represents a mixture (consisting of a cationic and an anionic moiety) first attempts were made in previous studies to apply the concept of CA for the prediction of the toxicity of one single ionic liquid. The applicability of concentration addition was confirmed for the prediction of ionic liquid toxicity for the mammalian cell line IPC-81. 23 The here presented study investigates the applicability of the concepts of CA and IA to mixtures of selected imidazolium based ILs and to mixtures of selected imidazolium based ILs in combination with the heavy metal cadmium. Cadmium was chosen because it is widespread in the aquatic and terrestrial environment due to natural occurrence and anthropogenic pollution and due to 784 | Green Chem., 2008, 10, 784–792 This journal is © The Royal Society of Chemistry 2008