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Acute toxicity to aquatic invertebrates:


Up to now, the constituent dicerium trisulphide (and by analogy reaction mass of dicerium trisulphide and dilanthanum trisulphide) was considered as not harmful to aquatic invertebrates up to the solubility limit in the test water at the loading rate of 100 mg/L (Bazin 1997, 1994).


Based on the disseminated dossier of H2S, the following range of L/C50 values were obtained for aquatic invertebrates: 48h to 96h-L/EC50 = 0.022 to 1.07 mg/L. This means that ecotoxicity of H2S occurs at very low concentrations for aquatic invertebrates as well. However, in the available studies on dicerium trisulphide, no toxicity was observed. As it was hypothetised for fish, different methodologies of test solutions preparation could lead to different releases of H2S and thus to variation of toxicity; that could explain the fact that no toxicity has been observed under certain conditions.

Taking into account the new, unexpected data obtained on fish exposed to reaction mass of dicerium trisulphide and dilanthanum trisulphide, no conclusion will thus be drawn on acute toxicity to aquatic invertebrates.


Toxicity to aquatic plants:


A known phenomenon which occurs when performing algal growth inhibition tests with rare earth compounds is the complexation of rare earths with phosphate from the test medium. This complexation leads to phosphate depletion. As phosphate is an important nutrient to algae, some indirect adverse effects were observed on algae growth. Rare earth sulphides do not escape to this rule because such a phenomenon was demonstrated for dicerium trisulphide (Cerbelaud, 1997).


Indeed, this last study demonstrates that there is probably an interaction between phosphate and cerium, inducing the precipitation of phosphate during the test solution preparation (as cerium is in excess), and the precipitation of cerium when nutrient salts are added to the test medium (as phosphate is in excess). The possible interaction between the test substance and the test medium prevents for making any conclusion regarding the inhibitory effect of dicerium trisulphide on the algal growth. As the growth of algae is not possible without phosphate, the growth inhibition observed without complementation of the medium with nutrient salts may be explained by the very low level of phosphate in the test solutions. This hypothesis is confirmed by the fact that the addition of nutrient salts removes the inhibitory properties of dicerium trisulphide (Cerbelaud, 1997).


The new, unexpected data obtained on fish exposed to reaction mass of dicerium trisulphide and dilanthanum trisulphide reveals a potential implication of H2S in the observed toxicity. For algae, such an impact of H2S could not be excluded. However, it could not be possible to evaluate it because potentially masked by the indirect phenomenon above described; which would applied as well to the reaction mass.

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