Zirconium dioxide

Toxicity to aquatic algae and cyanobacteria

Description of key information

In view of the lack of algal test data on zirconium dioxide, a weight of evidence approach is proposed using read across studies performed with zirconium dichloride oxide (Kumar and Rai, 1978), a reaction mass of ZrO2 and CeO2 (Peither, 2009), and zirconium basic carbonate (Vryenhoef and Mullee, 2010).
Based on these studies it can be concluded that any adverse effect observed on algal growth is due to phosphate depletion through precipitation of zirconium-phosphate complexes, and that in the environment, the secondary effect of phosphate deprivation is not relevant. 
No direct toxic effects caused by zirconium dioxide are to be expected.

Additional information

For toxicity to aquatic algae and cyanobacteria, three studies were included in this dossier and used in a weight of evidence approach to cover this endpoint. All three studies were performed with read across substances. 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.

A 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 not considered environmentally relevant.

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 was 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.