Zirconium acetate

Toxicity to aquatic algae and cyanobacteria

Description of key information

Following the read across strategy, it is considered appropriate to cover this endpoint using a weight of evidence approach including algal growth inhibition studies performed with zirconium basic carbonate, a sparingly soluble zirconium compound (Vryenhoef and Mullee, 2010), and a reaction mass of cerium dioxide and zirconium dioxide (30% ZrO2, an insoluble zirconium compound) (Peither, 2009). These studies, yielding EC50 values > 100 mg/L and NOEC values of 32 mg/L demonstrated that the observed growth inhibition is concurrent with phosphate depletion, yielding a secondary effect which is not environmentally relevant at a normal scale. The results of a stirring experiment performed by Vryenhoef (2014) to determine the solubility of zirconium acetate in algal test medium indicated that zirconium concentrations were below the LOQ (11 µg Zr/L), supporting the read across strategy using data from insoluble and sparingly soluble zirconium compounds.  

Additional information

For toxicity to aquatic algae and cyanobacteria, three studies were identified as containing relevant information for inclusion in this dossier. Two of these studies are used in a weight of evidence approach to cover this endpoint.

In the study performed by Vryenhoef (2014), the solubility of zirconium acetate was investigated in algal medium in a preliminary stirring experiment. This experiment, applying a nominal concentration of 50 mg/L test item (i.e., ca. 20 mg/L anhydrous zirconium acetate), indicated that no measurable zirconium could stay in solution (dissolved Zr < LOQ, LOQ = 11 µg Zr/L). Therefore, no final algal growth inhibition test was performed. Because the ecotoxicity experiment was not performed, no robust study summary could be included in IUCLID Section 6.1.5 (Toxicity to aquatic algae and cyanobacteria), but the information from the stirring experiment was included in a robust study summary under IUCLID Section 5.6 (Additional information on environmental fate and behaviour). Based on the results of this study, zirconium from zirconium acetate 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.

The two latter studies were used for read across in a weight of evidence approach to conclude that a 'water soluble' substance such as zirconium acetate is expected to yield similar results in an algal growth inhibition test, i.e. growth inhibition is only expected to be observed at test concentrations in which the added zirconium is in excess of the phosphates in the test medium and quickly removes all phosphates from the test medium, hereby compromising algal growth. No primary toxicity is expected to occur, and the phosphate depletion effect is not considered to be relevant at a normal scale.