Toxicity Of Two Size ThO₂ Nanoparticles To Chlorella Pyrenoidosa

Thorium?Th?is a natural radioactive element existing in the environment and has potential to be a nuclear fuel.At present,Th⁴⁺is the main concern of the aquatic eco-toxicity of Th,but in our previous studies,we found that Th⁴⁺could hydrolyze into ThO₂ NPs with very small particle size after entering the natural waters,and Th enters water mainly in the form of ThO₂.The study of the ThO₂ NPs is helpful to better evaluate the aquatic ecological toxicity of Th.In this paper,two different diameter of ThO₂ NPs were synthesized and their physical and chemical properties were characterized.Chlorella pyrenoidosa?C.pyrenoidosa?was used as model organism to study the water ecological toxicity of two kinds of ThO₂ NPs.The work carried out mainly includes:?1?Two different diameter ThO₂ NPs were synthesized:?1?s-ThO₂,the average particle size was 52±5 nm,spherical;?2?b-ThO₂,the average particle size was141±6 nm,spherical;?2?The hydration size,Zeta potential and ion dissolution of s-ThO₂ and b-ThO₂ in OECD?Organization for Economic Cooperation and Development?medium were characterized;?3?The aquatic ecological toxicity of s-ThO₂ and b-ThO₂ to C.pyrenoidosa and the mechanism of its toxicity were explored.The main research results summarized as follows:?1?By measuring the hydration size and Zeta potential of s-ThO₂ and b-ThO₂ in OECD medium,it was found that s-ThO₂ agglomeration was more obvious.ICP-MS was used to measure the Th⁴⁺dissolution of s-ThO₂ and b-ThO₂ in the medium.It was found that after 96hours,the maximum dissolution amounts of s-ThO₂ and b-ThO₂ were 4.5 and 2 ug/L,respectively.?2?After exposure for 24,48,72 and 96 hours,the biomass of C.pyrenoidosa after exposure to 0.1,1,10,100 and 200??of s-ThO₂ and b-ThO₂ were analyzed.It was found that there was a significant dose-effect.The calculated 96 h-EC₃₀ values were 64.1??for s-ThO₂and 100.2??for b-ThO₂,respectively.The calculated 96 h-EC₅₀ values were 235.2??for s-ThO₂ and 773.6??for b-ThO₂,respectively.The s-ThO₂ was more toxic to C.pyrenoidosa than b-ThO₂ and the particle size of ThO₂ NPs is a factor affecting toxicity.After exposure for96 hours,the algae cells were observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The treatments of s-ThO₂ and b-ThO₂ at 200??,ThO₂ NPs were adsorbed on the surface of algae cells.The cell wall of algal cells in the treatment of 200??s-ThO₂ showed partial deformation.?3?After exposure for 96 hours,the cell membrane permeability of the treatment group and the control group were measured.It was found that the permeability of the treatments of s-ThO₂ and b-ThO₂ at 200??showed a significant increase compared to the control.The levels of ROS in C.pyrenoidosa cells were measured.It was found that after exposure for 48,72 and96 hours,ROS levels increased significantly compared to the control both in the treatments of s-ThO₂ and b-ThO₂.In summary,the particle size effect of ThO₂ NPs on C.pyrenoidosa is proposed,the s-ThO₂ was more toxic to C.pyrenoidosa than b-ThO₂.The toxicity effect of ThO₂ NPs to C.pyrenoidosa may due to the oxidative stress induced by the adsorption of ThO₂ NPs on the cell wall of algae,which leads to the increase of ROS level and the increase of cell membrane permeability.