Acute Toxic Effects Of Cerium On Daphnia Magna And The Influence Of Toxicity Modifying Factors

As the most widely used rare earth elements(REEs),cerium(Ce)enters the environment in large quantities during the production and use of related products,which may cause potential toxic effects on aquatic ecosystems.However,the toxic effects of Ce on aquatic organisms and the relationship with environmental factors are still unclear,which seriously restricts the environmental management and pollution control of REEs.In this study,Ce was selected as the research object,and the acute exposure experiment of Ce on the Daphnia magna was carried out to study the acute toxicity of Ce on Daphnia magna and the effect on the antioxidant system,and to clarify the mechanism of the toxic effect of Ce on Daphnia magna.For the acute toxicity of Ce,the quantitative relationship between the acute toxicity of Ce in water and environmental factors was constructed,which provided a reference for revealing the mechanism of Ce ecotoxicity effect and establishing a biotic ligand model(BLM)for the toxicity of Ce in the water environment to the Daphnia magna.The conclusions of this study are as follows:(1)The inhibition rate of Daphnia magna increased gradually with the increase of Ce³⁺concentration.The EC50 based on the concetrations of Ce³⁺at 24 and 48 h were 23.4 and 3.73?M,respectively.The EC50 based on the concetrations of dissolved Ce at 24 h and 48 h were 60.6 and 10.9?M,respectively.(2)At 24 h exposure,SOD activity increased first and then decreased gradually with the concentration increase of Ce.At 48 h of exposure,SOD activity increased with the concentration increase of Ce,but the activity was only 0.493?0.815 times of that at 24 h.H₂O₂ and CAT decreased with the increase of Ce concentration,but compared with 24 h,H₂O₂ content and CAT activity decreased after 48 h exposure at different concentrations.The rate of CAT decomposing H₂O₂ may be higher than the rate of generating H₂O₂from decomposing ROS by SOD.SOD was inhibited and the production of H₂O₂ was reduced.There was no significant difference for the GSH content between treatement and control groups,thus the Ce exposure did not affect GSH content.The content of MDA increased with the increase of Ce,indicating that Ce might cause persistent oxidative damage to Daphia magna.(3)The effects of Ca,Mg,Na,K,p H,Fulvic acid(FA),citric acid(CA)and malic acid(MA)on the acute toxicity of Ce were investigated.It showed that Ca and Na inhibited the toxicity of Ce at low concentrations,and Ca²⁺and Na⁺participated in the competition between Ce³⁺and biological ligands.However,Mg enhanced Ce toxicity,while K had no significant effect on Ce toxicity.The toxicity of Ce was strongly inhibited by p H.FA,CA and MA can inhibit the acute toxicity of Ce to Daphia magna,and the concentration of Ce³⁺may be reduced by complexation.The Log K value for Ce³⁺toxicity to Daphia magna was 5.92 while the protective influence of Ca and Na were 3.73 and 2.78,respectively.The Log K value for Ce³⁺toxicity to FA was 2.37.The research results will elucidate the acute toxicity effect and mechanism of Ce on Daphia magna,and also provide references for the establishment of water quality criteria and standards of Ce in freshwater and the pollution control of REEs.