This study was initiated to support the discussions on grouping regarding aquatic ecotoxicological effects and for the identification of relevant properties as well as the development of a grouping concept addressing aquatic ecotoxicity. A comprehensive and homogenous data set based on fourteen nanomaterials was established. The selected NMs were modifications of five chemical species (Ag, ZnO, TiO₂, CeO₂, Cu). As the focus was on the applicability for regulatory purposes, for ecotoxicity the OECD test guidelines 201 (algae), 202 (daphnids) and 236 (fish embryo) were considered. The physico-chemical properties of the chosen NMs were determined in deionized water and the test media applied for the ecotoxicological tests. Reactivity, ion release, morphology and ecotoxicity of the chemical composition (information from the bulk material) were identified as the most relevant grouping properties regarding nanomaterial's ecotoxicity. A grouping scheme and procedure was proposed considering these properties. The scheme was validated with a set of additional nanomaterials (TiO₂, SiO₂, Fe₂O₃).

A rough, but reliable grouping of NMs with different chemical composition was possible. The separation of NMs with the same chemical composition, into different groups was only feasible, if the NMs show major differences in one of the relevant properties (e.g. different shape). Based on the available data set it is unknown whether either further physico-chemical properties have to be considered or whether the impact of the selected variations on ecotoxicity is too minor to result in significant ecotoxicological differences. In order to further advance the grouping concept for regulatory testing, future developments should include the specification of threshold values with regard to the properties solubility and reactivity as well as for the characterization of the morphology. Additionally, test methods addressing the sorption tendency of NMs to algae could contribute to an improvement of the ecotox-scheme with regard to the consideration of physical effects by shading resulting in limited growth.