TiO 2 nanoparticles – Relationship between dispersion preparation method and ecotoxicity in the algal growth test

Purpose Little is known about the ecotoxicity of nanomaterials and there are no specific guidelines for sample preparation and testing. We set out to establish whether the method used to prepare TiO 2 dispersions had a significant impact on aquatic ecotoxicity. We also followed the formation of agglomerates during the incubation period. Methods We applied the algal growth inhibition test (OECD test guideline no. 201). Dispersions were prepared by stirring and/or ultrasonication for different durations, and by filtration according to an OECD procedure recommended for testing difficult substances. Results Samples stirred for 7 d were not toxic, but EC 20 values could be calculated for all the other treatments. Shorter treatments generated EC 20 values in the range 1–27 mg/L. Only the shortest treatment (1 min stirring, 1 min ultrasonication) produced an unusually high EC 20 value, indicating low toxicity. Development of agglomerate size and of toxicity depends on the nanoparticles. We found that ecotoxicity was predominantly caused by a fraction of nanoparticles and agglomerates obtained by passing dispersions through a 0.22-µm filter. Conclusions We propose a short treatment regime to generate the most relevant ecotoxicity data for TiO 2 , for example stirring for 1 min followed by 3 min ultrasonication. Until more data concerning the ecotoxicity of different fractions are available, we recommend the testing of unfiltered dispersions rather than filtrates. Relating ecotoxicity to the total hydrodynamic surface of the nanomaterials rather than concentration does not seem to improve the accuracy of ecotoxicity assessments using the algal growth inhibition test.