Assessment of the biotoxicity of lanthanides (La, Ce, Gd, and Ho) on zebrafish (Danio rerio) in different water environments

The expanding applications of lanthanides (Ln) in various aspects have raised concerns about their biosafety. Slight changes in the chemical composition of environmental media can significantly affect the biological effectiveness of poorly water-soluble Ln; however, the knowledge of the effects of environmental factors on Ln toxicity remains limited. Here, the effects of pH, HCO3–, Ca2+, Mg2+, Na+, K+, Cl–, and SO42– on the bioefficacy and biotoxicity of Ln (La, Ce, Gd, and Ho) were comparatively studied using zebrafish (Danio rerio) as the test organism. In the standard water, the toxicity of Ln in zebrafish was significantly correlated with pH, HCO3–, and Ca2+-Mg2+ levels in the medium but not with the levels of Cl–, Na+, K+, and SO42–. At the beginning of the test, the four Ln were complexed with HCO3– in the medium to form precipitates. A decrease in pH or HCO3- concentration can promote the conversion of granular Ln to a soluble state, thus enhancing their bioavailability, biotoxicity, and bioaccumulation. At a pH of 5.0 and 0.2 mmol·L–1 HCO3–, where Ln precipitates were not found, the four Ln showed a consistent trend of 96 h-LC50 in zebrafish. These data indicate that the differences in the toxicities of the four Ln in the standard water may be due to differences in the effective states of the individual elements rather than the different toxicities of the elements. Overall, in biological toxicity assessments, Ln can be regarded as a group of elements with additive patterns of toxicity until the differences in their biological toxicity mechanisms are revealed, and the effects of pH and carbonate should be considered.