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The aquatic toxicity of zirconium sulfate (a 'water soluble' zirconium compound) is evaluated using data for zirconium sulfate as well as data from read across substances such as zirconium basic carbonate (a sparingly soluble zirconium compound) and zirconium dioxide (an insoluble zirconium compound). Read across from insoluble or sparingly soluble zirconium substances is considered acceptable because stirring experiments performed with zirconium sulfate in aquatic test media (e.g., Harris, 2014a,b; Vryenhoef and Mullee, 2014) indicated that zirconium from the 'water soluble' zirconium sulfate does not stay in solution at environmentally relevant pH levels. Rapid hydrolysis occurs, resulting in precipitation of zirconium hydroxides or zirconium dioxide. Further, heavy complexing will occur with phosphates, which results in 100% precipitation whenever phosphate is present in excess. Complexation with carbonates may also result in progressive precipitation with increasing pH levels. As a result, at environmentally relevant conditions, zirconium sulfate cannot be considered as a water soluble compound. Overall, zirconium is not bioavailable in the aquatic environment and therefore no adverse effects are to be expected in aquatic organisms.

Further argumentation for the read across approach is given in the read across document attached to IUCLID Section 13.

Zirconium sulfate is concluded not to be toxic (acutely) to fish and aquatic invertebrates. The studies that were used for endpoint coverage (performed with zirconium sulfate and/or read across substances) were performed according to internationally accepted guidelines and did not report any adverse effects (mortality in fish, immobilization in daphnids) at the limit test concentration of 100 mg/L or upon exposure to a 100% v/v saturated solution. In algal growth experiments with read across substances, it was demonstrated that the observed growth inhibition was concurrent with phosphate depletion. Toxicity to algae is therefore due to phosphate deprivation as no primary toxic effects have been observed. Although no phosphate monitoring was performed during the algal growth inhibition study with zirconium sulfate (Vryenhoef and Mullee, 2014), here too, the observed effect on growth of algae exposed to the 100% v/v saturated solution can be assumed to be due to phosphate deprivation. This is supported by the fact that no dissolved zirconium could be detected at levels > LOQ (20 µg Zr/L) in any of the test solutions. Phosphate deprivation is a secondary effect which is not considered relevant at a normal environmental scale. Therefore, no effects on algae are expected in the environment either. Finally, zirconium sulfate was not found to be toxic to aquatic microorganisms in an activated sludge respiration inhibition test.