Acute toxicity of nanosized TiO₂ to Daphnia magna under UVA irradiation

The production and use of nanoparticles (NPs), especially those of titanium dioxide (TiO₂) have increased steadily within the last decade. Common knowledge of photoactive nanosized TiO₂ (nTiO₂) effects on freshwater organisms is largely limited to acute toxicity tests without taking into consideration solar ultraviolet-A (UVA) irradiation. The authors' approach in the present study, based on a combination of a standardized ecotoxicology method (Organization for Economic Cooperation and Development [OECD] test 202) and the expected solar UVA contribution, will allow a more realistic understanding of the toxicity of TiO₂ to Daphnia magna. The results show that the contribution of UVA to TiO₂ toxicity cannot be ignored. The half-maximal effective concentration values (median effective concentration [EC50], immobilization as the endpoint) using Organization for Standardization (ISO) test water as well as river water decreased from 29.7 to 33.6mg/L under dark conditions to 1.2 to 3.4mg/L TiO₂ after exposure to 0.56mW/cm² UVA radiation. The authors also discuss possible mechanisms of NPs toxicity and the accuracy of results in terms of problems observed, such as solubilization and sedimentation of NPs, in ecotoxicological testing of nTiO₂.