Nanostructured TiO₂ and ZnO prepared by using pressurized hot water and their eco-toxicological evaluation

The eco-toxicological effects of unconventionally prepared nanostructured TiO₂ and ZnO were evaluated in this study, since both oxides are keenly investigated semiconductor photocatalysts in the last three decades. Unconventional processing by pressurized hot water was applied in order to crystallize oxide materials as an alternative to standard calcination. Acute biological toxicity of the synthesized oxides was evaluated using germination of Sinapis alba seed (ISO 11269-1) and growth of Lemna minor fronds (ISO 20079) and was compared to commercially available TiO₂ Degussa P25. Toxicity results revealed that synthesized ZnO as well as TiO₂ is toxic contrary to commercial TiO₂ Degussa P25 which showled stimulation effect to L. minor and no toxicity to S. alba. ZnO was significantly more toxic than TiO₂. The effect of crystallite size was considered, and it was revealed that small crystallite size and large surface area are not the toxicity-determining factors. Factors such as the rate of nanosized crystallites aggregation and concentration, shape and surface properties of TiO₂ nanoparticles affect TiO₂ toxicity to both plant species. Seriously, the dissolution of Ti⁴⁺ ions from TiO₂ was also observed which may contribute to its toxicity. In case of ZnO, the dissolution of Zn²⁺ ions stays the main cause of its toxicity.