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Ecotoxicological information

Short-term toxicity to fish

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Description of key information

The endpoint is covered by a weight of evidence approach including one short-term fish study for zirconium dioxide (Bazzon, 2000) and two short-term fish studies for calcium hydroxide (Egeler et al., 2007a; Locke et al., 2009). Zirconium dioxide did not cause any adverse effects in Brachydanio rerio up to and including the limit test concentration of 100 mg/L (nominal loading rate). For calcium oxide, two studies performed with calcium hydroxide were added to the dossier because calcium oxide is transformed to calcium hydroxide when in contact with water. Both calcium oxide and calcium hydroxide will initially increase the pH of the aqueous medium in which they are dissolved. The observed adverse effects in the two studies added to the dossier could be ascribed to this pH increase, with initial pH being > 10 in test solutions close to the reported median effect concentrations. However, since Eidam (2014, 2015) demonstrated that only a limited amount of calcium is released (in pure water) from calcium zirconium oxide, no drastic pH increase is to be expected from adding the substance to aqueous media. Therefore, taking into account all abovementioned information, calcium zirconium oxide can be concluded to be not toxic or harmful to fish. 

Key value for chemical safety assessment

Additional information

1. Information on zirconium dioxide

The 96-h acute toxicity of zirconium dioxide to Brachydanio rerio was studied (Bazzon, 2000) under static conditions, according to OECD guideline 203. Fish were exposed to control and test chemical at a nominal concentration of 100 mg/L. Mortality/immobilization were monitored daily. No mortality was observed during the test, neither in the control group nor in the group exposed to the test item. The 96-h LC50 was thus > 100 mg/L.

2. Information on calcium oxide

For calcium oxide, data obtained from tests performed with calcium hydroxide were added to the dossier. The rationale behind this is that in the environment, CaO will result in Ca(OH)2 formation when in contact with water, according to the following (general) reaction:

CaO + H2O <--> Ca(OH)2

Two studies performed with Ca(OH)2 were added to the weight of evidence approach. The first study is a short-term toxicity study with the freshwater fish rainbow trout (Egeler et al., 2007a) which was executed according to OECD guideline 203. The biological findings (96-h LC50 = 50.6 mg Ca(OH)2/L) appeared to be closely related to the initial pH of the test solutions, which was > 10 at concentrations close to the LC50. The second study is a short-term toxicity study performed with the marine species Gasterosteus aculeatus (threespine stickleback) (Locke et al., 2009). Also in this study, a concentration-response relationship was established, yielding a 96-h LC50 of 457 mg Ca(OH)2/L. Here too, increased pH levels (> 10) were observed in test solutions where mortality occurred.

In the environment, lime substances rapidly dissociate or react with water. These reactions, together with the equivalent amount of hydroxyl ions set free when considering 100 mg of the lime compound (hypothetic example), are illustrated below:

Ca(OH)2 <--> Ca2+ + 2OH-

100 mg Ca(OH)2 or 1.35 mmol sets free 2.70 mmol OH-

CaO + H2O <--> Ca2+ + 2OH-

100 mg CaO or 1.78 mmol sets free 3.56 mmol OH-

From the dissociation in the aquatic environment, it is clear that the effect of calcium oxide or calcium hydroxide must be caused either by calcium or hydroxyl ions. Since calcium is abundantly present in the environment and since the 96-h LC50 reported by Egeler et al. (2007a) is within the same order of magnitude of its typical natural concentrations, it can be assumed that the adverse effects are mainly caused by the pH increase and not by calcium.

3. Conclusion on calcium zirconium oxide

Calcium zirconium oxide is zirconium dioxide with calcium partly replacing zirconium in the crystal lattice. Zirconium dioxide has been demonstrated not to cause any adverse effects in fish. Calcium oxide (as an individual substance) on the other hand is expected to be hydrolysed to calcium hydroxide, which will in its turn be subject to dissociation, releasing OH- ions and resulting in a pH increase. The observed effects in the short-term toxicity studies in fish were ascribed to this pH increase. Calcium as such is abundantly present in the environment and the lowest LC50 (reported by Egeler et al., 2007a) is within the same order of magnitude of its typical natural concentrations, therefore, there is no reason to assume that the observed toxicity is caused by calcium. Further, it was demonstrated by Eidam (2014, 2015) that only limited amounts of calcium are released (in pure water) from calcium zirconium oxide. Consequently, no dramatic pH increase is to be expected from the limited dissolution of calcium zirconium oxide in water and therefore the substance is considered to be not toxic or harmful to fish.