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EC number: 939-967-7
CAS number: -
By treating Chlorella cells with ZrOCl2, growth rate (optical density
measurement) was inhibited and started at the lowest concentration used
(20 mg/L). However, the reduction of growth was due to the lack of
phosphate precipitated by ZrOCl2. In fact, an experiment performed by
treating the cells with 100 mg/L and 200 mg/L of ZrOCl2 in
phosphate-supplemented medium, displayed no impact on growth rate of
Chlorella sp. Therefore, the growth inhibition which was observed, was
considered due to the unavailability of phosphate and not to zirconium
toxicity. The NOEC value is assessed at > 200 ppm of ZrOCl2.
Table 1: Biomass of
Loading rate (mg/L)
Biomass of algae*
(relative Fluorescence units)
SD: Standard deviation
*: The biomass was
determined by fluorescence measurement (duplicate measurements) and is
given as relative fluorescence units (x 10 exp 3). At the start of the
test, the initial cell density was 5000 algal cells/mL, corresponding to 1.01
x 10 exp 3
relative fluorescence units).
Table 2: Areas under
the Growth Curves (AUC)
Areas under the growth curves AUC (10 exp 3 *day)
And inhibition of AUC (IAUC)
*: mean value
significantly lower than in the control
Dunnett's tests, one-sided, alpha = 0.05)
Table 3: : Average
Growth Rates (µ)
Average growth rate µ (day-1) and inhibition of µ (Ir)
Table 4: Yield (Y)
Yield Y and inhibition of Y (Iy)
Section-by-section growth rates
Section-by-section growth rates (day-1) and inhibition of the growth rates (Ir)
6: Phosphate concentrations in the test media and in the control
Phosphate (mg PO4/L)
Sample 1+ 2
In a 72-hour toxicity
study, the cultures of green algal species Scenedesmus subspicatus were
exposed to the reaction mass of cerium dioxide and zirconium dioxide at
the loading rates of 0.32,
1.0, 3.2, 10,
32 and 100 mg/L under static conditions in accordance with the EU
Commission Directive 92/69/EEC, C.3 (1992), and the OECD Guideline 201
(2006). The NOEC, the LOEC and EC50 values based on the growth rate
were 32 mg/L, 100 mg/L and > 100 mg/L, respectively.
This toxicity study is
classified as acceptable and satisfies the guideline requirements for
subspicatus) growth inhibition test toxicity study.
The concentration of phosphate was
statistically significantly reduced compared to the control in the WAFs
with the loading rate of 32 mg/L and above (results of a Student-t test
with Bonferroni correction, p<0.008). The loss of phosphate can be
explained by the formation of insoluble complexes of phosphate with the
test item (which is a well-known behavior of rare earth elements in the
environment) during stirring of the dispersion. The depletion of
phosphate in the test medium during the test was clearly the reason for
the inhibition of algal growth determined at this test concentration.
Thus, growth inhibition was due to a secondary effect (i.e. the
complexation of the essential algal nutrient phosphate by the test item)
which is not considered environmentally relevant.
Analysis of phosphate:
At 0 hours, phosphate concentration decreased with increasing test
A similar concentration dependent pattern was observed at 24, 48 and 72
hours, with measured phosphate concentrations for all but the 3.2%
saturated solution being less than the LOQ (0.021 mg/L). In the control,
phosphate decreased from 1.19 mg/L at 0 h to 0.039 mg/L at 72 h. The
decrease in phosphate concentration during the test was due to the use
of phosphate for algal growth.
The reduced level of phosphate (compared to control) shown already
before the start of the test, which is statistically significant at the
highest saturated concentration, was possibly the cause for the reduced
algal growth rather than true toxicity of the test compound.
1. Information on zirconium dioxide (CAS# 1314-23-4)
For toxicity to aquatic algae and cyanobacteria, three studies were included and used in a weight of evidence approach to cover the endpoint for zirconium dioxide. All three studies were performed with read across substances. On the one hand, a study with a 'water soluble' zirconium compound (zirconium dichloride oxide) was included. On the other hand, two studies with insoluble zirconium compounds (zirconium basic carbonate and a reaction mass of zirconium dioxide and cerium dioxide) were included.
A 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 to occur in natural systems.
In the study by Peither (2009; according to OECD 201 and GLP), a reaction mass of ca. 60% CeO2 and 30% ZrO2 was tested at loading rates up to 100 mg/L in Scenedesmus subspicatus for 72 hours. The concentration of phosphate was statistically significantly reduced compared to the control in the WAFs with the loading rate of 32 mg/L and above. The loss of phosphate can be explained by the formation of insoluble complexes of phosphate with the test item (which is a well-known behavior of rare earth elements as well as zirconium in the environment) during stirring of the dispersion.
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 is not expected to occur in natural systems.
Finally, in the study by Kumar and Rai (1978), it is shown that algae exposed to zirconium dichloride oxide up to 100 ppm show growth inhibition, especially at 60, 80 and 100 ppm. This effect is caused by precipitation of phosphates which are essential to algae. When algae are supplemented with phosphate in the medium after filtration, growth was comparable to controls. The results suggest that zirconium dichloride oxide is not toxic directly to algae at concentrations up to 100 ppm. In conclusion, zirconium dichloride oxide is not expected to be toxic to algae in the natural aquatic environment. The relation between zirconium dichloride oxide and zirconium oxide is that in a buffered test medium zirconium dichloride oxide hydrolysis will be completed, resulting in formation of zirconium dioxide which precipitates from solution. Exposing aquatic organisms to 'water soluble' or insoluble zirconium compounds will hence not result in significantly different test results.
2. Information on erbium oxide (CAS# 12061-16-4)
It was not deemed necessary to include studies on erbium oxide, as the behaviour of rare earth compounds in algal growth inhibition tests is similar as for zirconium, i.e., as long as phosphate is in excess of the rare earth element, no rare earth element can exist in the dissolved fraction (i.e., no exposure), whereas when the rare earth element is in excess, all phosphate will be depleted from the test medium, so that the algal growth is limited. The observed growth inhibition is hence the result of phosphate deprivation and not a primary effect of the rare earth element. This type of effect is not expected to occur in natural systems and is therefore not considered relevant.
3. Conclusion on erbium zirconium oxide
Based on the information available on zirconium compounds as well as the known similar behaviour of rare earth elements such as erbium, the growth inhibition effects observed in algal studies are not considered relevant, as they are due to phosphate deprivation, which is not expected to occur to a significant extent in natural systems. No direct toxic effects caused by erbium zirconium oxide are to be expected.
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