Toxicity of cerium and thorium on Daphnia magna

• Ce and Th were present as different chemical speciation in the exposure medium.
• Cerium isn't a toxicological analogue of thorium under specific conditions.
• The detrimental effect of Th in the form of particulate ThO2 should be highlighted.

Cerium (Ce) and thorium (Th) are always thought to be chemically similar and have comparable toxic properties on living organisms. In the present study, the acute and chronic toxicity of these two elements to freshwater crustacean Daphnia magna were investigated in the modified reconstituted water (6 mg/L KCl, 123 mg/L MgSO4·7H2O, and 294 mg/L CaCl2·2H2O in Milli-Q water, pH 7.8). It seemed that Ce and Th had comparable acute toxicity on Daphnia: 24/48 h EC50 for Th and Ce were 7.3/4.7 μM and 16.4/10.7 μM, respectively. However, Ce was present as soluble ions while all of Th was present as particulate ThO2 in the exposure medium. Considering their different chemical forms and bioavailability, the toxic mechanisms of Ce3+ and ThO2 on Daphnia would be totally different. To our knowledge, this is the first time to investigate the aquatic toxicity of thorium and cerium based on their actual chemical speciation in the exposure medium. The results also suggest that more attention should be paid on the detrimental effect of Th in the form of particulate ThO2.

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