Zirconium oxide sulphate

Toxicity to aquatic algae and cyanobacteria

Description of key information

Following the read across strategy, it is considered appropriate to cover this endpoint by a weight of evidence approach including two read across studies performed with zirconium basic carbonate and a reaction mass of cerium dioxide and zirconium dioxide. Both read across substances are insoluble in water (as is the case for zirconium basic sulfate) and both tests (Vryenhoef and Mullee, 2010; Peither, 2009) yielded a 72h ErC50 and 72-h NOErC value of > 100 mg/L and 32 mg/L (nominal), respectively, in Desmodesmus subspicatus. Phosphate monitoring during the tests indicated that the observed effects on growth observed in the test solution with the highest nominal concentration of test substance were due to early phosphate depletion instead of inherent toxicity of the test items. Strong complexation with phosphates is a well known behaviour of both rare earths and zirconium and is not considered environmentally relevant.

Additional information

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

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 not considered environmentally relevant.

In the second study (Peither, 2009) cultures of green algal species Scenedesmus subspicatus were exposed to a reaction mass of cerium dioxide and zirconium dioxide (containing approximately 60% CeO2 and 30% ZrO2). The NOEC and EC50 values based on growth rate were 32 mg/L and > 100 mg/L respectively (based on nominal concentrations of reaction mass). Almost no test substance (monitored based on dissolved cerium measurements) was present in the test solutions. The concentration of phosphate was statistically significantly reduced compared to the control in the test solutions. Here also the loss of phosphate can be explained by the formation of insoluble complexes of phosphate with cerium and zirconium (which is a well-known behavior of rare earth elements as well as zirconium in the environment). 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.

As zirconium basic sulfate is also a zirconium compound with extremely low water solubility, the above studies were used in a read across approach to indicate that zirconium basic sulfate is not expected to cause environmentally relevant adverse effects in algae either.