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Toxicity to aquatic algae and cyanobacteria

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A weight of evidence approach was used to cover this endpoint. Three studies were included, one with zirconium sulfate, which is a 'water soluble' zirconium compound (Vryenhoef and Mullee, 2014), one with zirconium basic carbonate, which is a sparingly soluble zirconium compound (Vryenhoef and Mullee, 2010) and one with a reaction mass of CeO2 and ZrO2, which is an insoluble zirconium compound (Peither, 2009). The latter two studies report an adverse effect on algal growth which is concurrent with phosphate depletion from the test medium through complexation and precipitation with zirconium. Although zirconium sulfate is a 'water soluble' zirconium compound, the similarity of its hazard profile with the profiles of insoluble or sparingly soluble zirconium compounds is very high, because zirconium precipitates from the solution at environmentally relevant conditions, as was demonstrated by zirconium analysis in the test media (in all samples, dissolved Zr was < LOQ, i.e. < 20 µg Zr/L). Therefore, read across from insoluble or sparingly soluble zirconium compounds is considered acceptable. Further, alhtough phosphate monitoring was not performed in the study with zirconium sulfate (Vryenhoef and Mullee, 2014), a similar phosphate depletion effect can be expected to have occurred, because zirconium is known to heavily complex all phosphates (independent of pH) whenever it is in excess of the phosphates (and vice versa). Further, pH-dependent formation of zirconium dioxide, zirconium hydroxide, and zirconium carbonate precipitates can be expected in environmentally relevant tes media. Based on the weight of evidence approach, the observed effects on algae are considered secondary effects which are not environmentally relevant. In conclusion, the test substance is not considered to be toxic to algae.

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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.

A recent study was performed using zirconium sulfate (Vryenhoef and Mullee, 2014). Based on results from the range finding study (no definitive test performed), the 72-h EC50 and NOEC (growth rate based) were determined to be 94 % v/v saturated solution and 10% v/v saturated solution, respectively. Algae exposed to 100% v/v saturated solution showed 53% inhibition of growth rate after 72 h. However, no dissolved zirconium above the LOQ (20 µg Zr/L) could be detected in any of the treatments. As discussed below, the (limited) effect on growth observed at the highest (nominal) test concentration (100 % v/v saturated solution) can be assumed to be due to phosphate depletion from the test medium through complexation with zirconium. No phosphate monitoring was done during this test, but the fact that no dissolved zirconium was observed at detectable/quantifiable levels supports this assumption (no exposure = no primary toxic effects, hence observed effects due to secondary effect (phosphate deprivation)).

Overall, based on the results of the study of Vryenhoef and Mullee (2014), zirconium from zirconium sulfate can be assumed to be unavailable, and read across from insoluble and/or sparingly soluble zirconium compounds can be justified. Consequently, an algal growth inhibition study performed with zirconium basic carbonate (a sparingly soluble zirconium compound) and another study performed with a reaction mass of cerium dioxide and zirconium dioxide (30% ZrO2, an insoluble zirconium compound) were included in the dossier with the intention to cover the endpoint using a weight of evidence approach.

The study performed by 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 performed by Peither (2009), cultures of the 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.