Zirconium

Toxicity to aquatic algae and cyanobacteria

Description of key information

No direct toxic effect has been attributed to zirconium on growth rate of algae. The NOEC value in terms of zirconium element has been found to be > 102.5 mgZr/L after 14d exposure. During all the duration of the test, the algae were in  exponential growth phase. It was observed that zirconium induces a precipitation (known property of Zr) of the posphate in the medium leading to a direct impact on growth of the algae. A further test clearly proved the non toxic effect of zirconium by adding suitable amount of phosphate in the test medium. In the latter case, the growth of algae was restored. 

Key value for chemical safety assessment

EC50 for freshwater algae:

102.5 mg/L

Additional information

Only one valid study performed on a Zr compound has been identified for read across for toxicity to aquatic algae.

The testing material used was Zirconium dichloride oxide (ZrOCl2) but since ZrOCl2 hydrolyses rapidly and precipitates into Zirconium dioxide and its hydrated forms under environmental conditions, this latter substance was actually tested. In the present study, no toxic effect was attributed to zirconium and a NOEC > 102.5 mgZr/L was determined. However, the author observed a growth inhibition that he clearly attributed to phosphate depletion in the culture medium induced by the complexation of phosphate by zirconium. To prove this, the author perform an additional test identical to the previous one but adding suitable amount of phosphate so as to enable to restore the growth of the algae. In conclusion, zirconium was proved to be non acutely toxic to algae.

Another study performed on the algae Selenastrum capricornutum exposed to ZrCl4 was identified but disregarded. An EC50 of 2.6 mg Zr/L was observed. Concerning the toxic effect recorded, authors concluded that zirconium has induced phosphate precipitation by complexation phenomenon in the medium but did not go further in the experiment. Therefore, the publication has been unvalidated.

This complexation phenomenon has been clearly identified by Ferrand et al (2005 ; "Etude de la spéciation en solution, de la rétention dans les sols et du transfert sol-plante du zirconium."- ANDRA thesis) who indicates that phosphate compounds drive the solubility of zirconium in the environment and that Zirconium has a natural tendency to precipitate phosphates.

There is no available study to assess the toxicity to algae of ZrO2 alone. Indeed, another study using reaction mass of a lanthanide element and zirconium dioxide displayed no toxic effect. However, since the testing material used is not the zirconium dioxide pure, results have not been considered for the present dossier.