Abstract: The expansion of the application of lanthanides (rare earth elements) has aroused wide attention to their biological safety. To explore the effects of environmental factors on the biological toxicity of lanthanides, using zebra fish (Danio rerio) as the test organism, we examined the effects of water environmental factors (pH, HCO₃^-, Ca²⁺Mg²⁺, K⁺, Cl^-, etc.) on biological toxicity and bioaccumulation of lanthanides, and analyzed the toxicity levels and trends of four representative lanthanides (La, Ce, Gd and Ho) under different water environmental conditions. The results showed that in standard water, the 96 h-LC₅₀ of La to adult Danio rerio was 165.6 μmol·L^-1. Its toxicity was significantly related to pH, HCO₃^- and Ca²⁺Mg²⁺ in the water, with its slopes being 0.912, 1.135 and 0.222, respectively, and had little relationship with Cl^-, Na⁺, K⁺ and SO₄^2-. At the beginning of the test, the four lanthanides could be complexed with HCO₃^- to form precipitated particles. A decrease in pH or HCO₃^- could promote the conversion of lanthanides from granular state to soluble state, with enhanced biological toxicity and bioaccumulation. Under the conditions of pH 5.0 and HCO₃^- 0.2 mmol·L^-1 (excluding the influence of lanthanide precipitation), four lanthanide elements showed a consistent trend of toxicity (6.11-6.43 μmol·L^-1 of 96 h-LC₅₀ for Danio rerio). We speculate that the difference in the toxicity of the four lanthanide elements in standard water may be due to the difference in the effective state of a single element rather than the different toxicity of the elements. Therefore, before the differences in biological toxicity mechanisms are revealed, lanthanides can be considered as a group of elements with a toxicity additive model for biological toxicity assessment. The evaluation process should focus on the effects of pH and carbonates.

Key words: lanthanides, zebra fish, toxic effect, environmental factor, action mode.