This study investigates the potential toxicity of TiO2 and ZnO nano-particles (nano-TiO2 and nano-ZnO) against Daphnia magna (D. magna) neonates before and after absorbing phenol. Although limited scientific investigation is conducted on possible hazards of nano-particles (NPs), no research has been carried out either on the toxicity of nano-ZnO and TiO2 in combination with other materials or on their ability to release the hazardous substances adsorbed on their surface in cells. Daphnia magna neonates exposed to different concentrations of phenol-free and phenol-loaded NPs and resulting mortality rates were recorded after 12 to 96 h. The results of mortality rates were applied to SPSS ver. 16.0 in order to calculate median lethal concentrations (LC50) of NPs. The results of experiments showed that phenol-free TiO2 was “practically nontoxic” according to EPA overview (48 h LC50: 2705 mg·L-1). But after exposure to phenol, its 48 h LC50 reached 414 mg·L-1, which means a 6-time increment in toxicity. 48 h LC50 of phenol-loaded and phenol-free nano-ZnO was 2.14 and 2.18 mg·L-1, respectively. This study showed, in contrast to nano-TiO2, that no significant difference is found between the toxicity of nano-ZnO before and after exposure to phenol. Researchers considered the amount of toxin absorbability of NPs to be one of the most important factors influencing the change in NP toxicity.
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