Calcium zirconium oxide

Toxicity to aquatic algae and cyanobacteria

Description of key information

The endpoint is covered by a weight of evidence approach including three studies for zirconium compounds (Vryenhoef and Mullee, 2010; Peither, 2009; Kumar and Rai, 1978) and one study for calcium hydroxide (Egeler et al., 2007c). Data on calcium hydroxide were included in the dossier because calcium oxide is transformed to calcium hydroxide when in contact with water. All EC50 values were > 100 mg/L, consequently, the individual compounds calcium oxide/calcium hydroxide and zirconium dioxide are not considered as harmful to algae. This leads to the conclusion that calcium zirconium oxide is not expected to be harmful to algae either. 

Additional information

1. Information for zirconium dioxide

For concluding on the toxicity of zirconium dioxide to aquatic algae and cyanobacteria, three studies were included in the weight of evidence approach to cover this endpoint. All three studies were performed with substances other than zirconium dioxide. On the one hand, a study with a 'water soluble' zirconium compound (zirconium dichloride oxide) was included. On the other hand, two studies with insoluble zirconium compounds (zirconium basic carbonate and a reaction mass of zirconium dioxide and cerium dioxide) were included.

The first study (Vryenhoef and Mullee, 2010) investigated the effect of zirconium basic carbonate on the growth of Desmodesmus subspicatus over a 72-h period. As zirconium could not be detected (< LOQ) in the test solution, the results were based on nominal concentrations. The ErC50 was > 100 mg/L and the NOErC was 32 mg/L (based on zirconium basic carbonate). Phosphate monitoring during the test indicated that reduced growth rate was concurrent with phosphate depletion due to phosphate complexing with zirconium and precipitation of the formed complexes. The observed effect is clearly a secondary effect which is only considered relevant in limited test systems such as those used for standard aquatic toxicity testing.

In the study by Peither (2009; according to OECD 201 and GLP), a reaction mass of ca. 60% CeO2 and 30% ZrO2 was tested at loading rates up to 100 mg/L in Scenedesmus subspicatus for 72 hours. The concentration of phosphate was statistically significantly reduced compared to the control in the WAFs with the loading rate of 32 mg/L and above. The loss of phosphate can be explained by the formation of insoluble complexes of phosphate with the test item (which is a well-known behavior of rare earth elements as well as zirconium in the environment) during stirring of the dispersion. The observed algal growth inhibition was concurrent with the depletion of phosphate in the test medium and therefore the observed effect is considered a secondary effect and not environmentally relevant.

Finally, in the study by Kumar and Rai (1978), it is shown that algae exposed to zirconium dichloride oxide up to 100 ppm show growth inhibition, especially at 60, 80 and 100 ppm. This effect is caused by precipitation of phosphates which are essential to algae. When algae are supplemented with phosphate in the medium after filtration, growth was comparable to controls. The results suggest that zirconium dichloride oxide is not toxic directly to algae at concentrations up to 100 ppm. In conclusion, zirconium dichloride oxide is not expected to be toxic to algae in the natural aquatic environment. The relation between zirconium dichloride oxide and zirconium oxide is that in a buffered test medium zirconium dichloride oxide hydrolysis will be completed, resulting in formation of zirconium dioxide which precipitates from solution. Exposing aquatic organisms to 'water soluble' or insoluble zirconium compounds will hence not result in significantly different test results.

Despite the observation of a secondary effect on growth rate due to phosphate deprivation, the EC50 levels obtained from these read across studies would not give rise to classification of zirconium dioxide as harmful to aquatic organisms (all EC50 values > 100 mg/L).

2. Information on calcium oxide

As for the other aquatic toxicity endpoints, the results for calcium oxide were based on data from tests using calcium hydroxide as test substance. Only one study was included in the weight of evidence approach. This study (Egeler et al., 2007c) investigated the effect of calcium hydroxide on the growth rate of Pseudokirchneriella subcapitata and yielded a 72-h EC50 of 184.57 mg Ca(OH)2/L and a 72-h NOEC of 48 mg Ca(OH)2/L based on nominal concentrations. Unlike in the fish and daphnid studies however, the initial pH of the medium at concentrations resulting in a considerable growth inhibition, was below 8 and therefore the biological findings cannot be attributed to a pH effect. However, it was observed that, with increasing test item concentrations, precipitates were formed over time to which algae adhered, leading to their flocculation. The flocculation of algae is thus considered to be the predominant biologically relevant effect in this system test. The recovery of the test item at the end of the test was below 80% of the nominal concentration. This can be explained since the test item is known to react with CO2 to calcium carbonate, which is poorly soluble in water leading to the formation of precipitates.

Despite the observation of a secondary effect on growth rate due to adherance of algal cells to precipitates (most likely calcium carbonate) throughout the study, the EC50 obtained from this study would not give rise to classification of the substance as harmful to aquatic organisms (EC50 > 100 mg/L).

3.Conclusion on calcium zirconium oxide

The adverse effects of zirconium compounds in algae were concluded to be caused by phosphate depletion, whereas the adverse effects of Ca(OH)2 appeared to be caused by precipitation (most likely calcium carbonate) and flocculation and sedimentation of the algae due to adherence to the precipitates. Both effects are secondary effects and the EC50 values obtained from the studies included in the weight of evidence approach would not give rise to classification of zirconium dioxide or calcium oxide/hydroxide as harmful to aquatic organisms (all EC50 values > 100 mg/L). Moreover, considering the limited water solubility of calcium zirconium oxide, only very small amounts of the substance may be expected to reach the surface water, as any sedimentation step in an on-site or off-site waste water treatment plant will remove almost all of the substance from the waste water. The limited amounts of calcium and zirconium that may be dissolved in the waste water (see Eidam, 2014, 2015) are not expected to cause any significant adverse effects in algae via a secondary mechanism in receiving surface water.