Calcium zirconium oxide

Short-term toxicity to aquatic invertebrates

Description of key information

The endpoint is covered by a weight of evidence approach including one short-term study for zirconium dioxide (Bazin, 1994) and two short-term studies for calcium hydroxide (Egeler et al., 2007b; Locke et al., 2009). Zirconium dioxide did not cause any adverse effects in daphnids 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 aquatic invertebrates.

Additional information

1. Information on zirconium dioxide

In a study from Bazin (1994), the acute toxicity of zirconium dioxide to Daphnia magna was studied under static conditions according to EU method C2. Daphnids were exposed to control and test chemical at an initial loading rate of 100 mg/L for 48 hours. No significant immobilization was observed up to and including the loading rate of 100 mg/L. The 48-h EC50 was thus superior to this value.

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 test with Daphnia magna (Egeler et al., 2007b) which was carried out according to OECD guideline 202. The biological findings (48-h EC50 = 49.1 mg Ca(OH)2/L) were closely related to the initial pH of the test solutions, which ranged from 7.7 in the control treatments to 9.5, 9.7, 10.1, 10.7 and 11.1 at 14.8, 22.2, 33.3, 50 and 75 mg Ca(OH)2/L, respectively. Therefore the initial pH is considered to be the main reason for the effects of calcium hydroxide on Daphnia magna. The second study is a short-term toxicity study performed with the marine species Crangon septemspinosa (Locke et al., 2009), which was conducted following a standard methodology developed by the laboratory. A concentration-response relationship was established, yielding a 96-h LC50 of 158 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 48-h EC50 reported by Egeler et al. (2007b) 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 aquatic invertebrates. 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 aquatic invertebrates were ascribed to this pH increase. Calcium as such is abundantly present in the environment and the lowest EC50 (reported by Egeler et al., 2007b) 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 aquatic invertebrates.