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EC number: 264-885-7
CAS number: 64417-98-7
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.
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.
Zirconium as well as rare earth elements are known to heavily complex with phosphate. This complexation is not dependent of pH at environmentally relevant pH levels. The complexation is so strong that whenever phosphate in the algal test medium is in excess of zirconium or the rare earth, all zirconium or rare earth is lost from the aqueous solution (hence no exposure), whereas whenever zirconium or the rare earth is in excess, all phosphate is depleted from the test medium, and phosphate deprivation effects are observed in the algae. This effect is demonstrated for zirconium by Vryenhoef and Mullee (2010), Peither (2009) and Kumar and Rai (1978). These studies were therefore included in this dossier. For yttrium oxide, no data were included in this dossier, however, Visual Minteq (v3.0) modelling supports the assumed complexation behaviour of yttrium and hence the technical difficulty of phosphate depletion would be expected in algal growth inhibition tests with yttrium compounds too. Because of this technical difficulty, it is not considered possible to obtain meaningful results from algal growth inhibition experiments with yttrium or zirconium compounds and therefore it is not deemed useful to perform new experiments with yttrium zirconium oxide either.
1. Information on zirconium dioxide (CAS# 1314-23-4)
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. 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)
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
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 behaviour 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 not environmentally relevant.
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 yttrium oxide (CAS# 1314-36-9)
No data are included in this dossier on the toxicity of yttrium oxide to
aquatic plants. As is the case for zirconium (as demonstrated above),
rare earth elements such as yttrium are also known to typically heavily
complex with phosphates. Due to the fact that, whenever phosphate is in
excess in the test medium, all rare earth will disappear from the
solution (hence no exposure), and whenever the rare earth is in excess
of the phosphate, all phosphate will disappear from the solution, the
observed adverse effects on algal growth in such studies are typically
concurrent with phosphate depletion, yielding the conclusion that the
observed effects are secondary effects due to phosphate deprivation.
This is a technical problem which cannot be resolved (e.g., by phosphate
dosing during the test), and therefore, it is not considered useful to
perform tests with such substances or to include experimental
information in this dossier.
To illustrate the heavy complexation with phosphate, and to additionally
demonstrate the pH dependency of yttrium dissolution in aqueous media,
Visual Minteq (v3.0) calculations were performed. For this exercise, the
composition of the algal medium according to OECD guideline 201 was used
as well as a loading rate of 100 mg Y2O3/L, which corresponds to
0.000886 M of Y (assuming all Y ends up in solution, which is of
course an overestimation since Y2O3 has a very low water solubility).
A model run was performed for each pH level between pH 5 and 12. Ionic
strength was not set to a fixed value, but the model was allowed to
calculate it (default). Temperature was set to 22°C. Three possible
solid phases were added for Y: Y(OH)3, Y2(CO3)3 and YPO4. When
solubility products are exceeded in the aqueous solution, the model
allows precipitation of these phases. Note that the nominally added
total phosphate (PO4 3-, total) concentration is 1.18E-05 M, hence no
more LaPO4 than that can be formed. Under the abovementioned conditions,
the model calculations can be summarised as follows:
At a nominal loading rate of 100 mg Y2O3/L, all phosphate is calculated
to immediately disappear from the test medium through complexation with
Y. At environmentally relevant pH levels, phosphate complexation does
not appear to be dependent of pH. When pH increases (from pH 6 on),
carbonate complexation starts to become important. Upon further increase
of pH (from pH 8 on), yttrium hydroxide starts to increasingly
precipitate, reducing the contribution of carbonate complexation and
eventually also that of phosphate complexation to zero at very high
(environmentally irrelevant) pH levels. Note that the standard test pH
of the OECD test medium is 8.3. Although at that pH some Y is expected
to be dissolved, whenever phosphate is in excess of Y, no exposure to Y
will occur, whereas when Y is in excess of the phosphate, phosphate
deprivation effects will be ovserved in the algae.
3. Conclusion on yttrium zirconium oxide (CAS# 64417-98-7)
Based on the information available on zirconium compounds as well as the
known similar behaviour of rare earth elements such as yttrium, 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 yttrium zirconium oxide are to be expected. The
strong complexation with phosphate represents a technical difficulty
which cannot be resolved (e.g., by phosphate dosing during the test) and
hence testing is not considered useful here.
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