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Toxicity to soil macroorganisms except arthropods

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Description of key information

The LC0 for mortality of earthworms (Eisenia fetida) in nine chinese soils within 6 weeks was given as > 162 mg REE/kg dw.

Key value for chemical safety assessment

Additional information

There is one publication available which determined the effects of Lanthanum chloride on Caenorhabditis elegans (Zhang et al. 2010). Furthermore information is available for the toxicity to Eisenia fetida with a mixture of rare earth elements (Wen et al. 2006). The structure analogy of the rare earth elements with similar toxicity was described by diverse authors (e.g. Sneller et al. 2000).

The nematode bioassay with C. elegans was carried out in liquid medium by adding Lanthanum chloride directly into the medium (Zhang et al. 2010). Since C. elegans can form an arrested stage under starvation conditions, the medium also contained a bacterial suspension of Escherichia coli. The results showed that Lanthanum had significant effects on the growth and reproduction of worms above a concentration of 10 µmol La/L (corresponding to 0.138 mg La/L). A further test with only E.coli exposed to Lanthanum showed, that the La content in the bacteria significantly increased after Lanthanum treatment while the survival was not affected (up to 20 µmol La/L). In addition elemental mapping of C. elegans by µ-SRXRF showed that the uptake route via absorption of Lanthanum from the medium on the body surface was negligible, and the accumulation of Lanthanum is mostly due to the intake of E. coli containing high levels of Lanthanum.

For determination of toxic effects of sediment or soil samples on growth, fertility and reproduction of C. elegans normally sediment, soil, pore water or elutriates and aqueous extracts of sediments or soils are used (according to ISO 10872). In the present study the test substance was added directly to the medium without spiking soil or sediment followed by extraction before starting the test. Since Lanthanum highly adsorbs to the soil or sediment, it has to be taken into account, that the test concentration actually would be much higher in a sediment/water system to achieve the concentration of the water body in the present study. Furthermore it could be shown, that the effect of Lanthanum on C.elegans is not attributed to the intrinsic toxicity of the substance itself, but can be explained by a secondary effect via the food chain, since the Lanthanum uptake was due to the intake of Lanthanum enriched in E. coli. Therefore, the test with C.elegans cannot be taken for the assessment of the Lanthanum toxicity to soil macroorganisms except arthropods.

Further information is available for the toxicity to Eisenia fetida with a mixture of rare earth elements. No mortality of earthworms was observed in nine natural chinese soils within 6 weeks (Wen et al. 2006). The total content of Lanthanum, Yttrium, Cerium, Praseodymium and Neodymium in these soils was 18.61 - 162.28 mg/kg d.w., the concentration of Lanthanum was in a range of 4.6 - 71.9 mg La/kg dw. So generally, a LC0 > 71.9 mg La/kg could be stated. Although the total content of the rare earth elements can not be recalculated to Lanthanum, the more than two fold higher value of the total content of rare earth elements compared to the content of La showed, that the LC0 for Lanthanum is clearly underestimated.