Zinc oxide

Endpoint summary

Description of key information

The data newly generated on D. subspicatus and D. magna did not show evidence for additional toxicity of ZnO-nanoforms tested.

Additional information

Evaluation requests 4&5: ZnO-nanoform ecotoxicity testing on D . subspicatus and D. magna

As part of the testing programme requested by the Decision of July 9^th, 2019 on Zinc Oxide (ECHA 2019), chronic aquatic toxicity testing has been performed on the freshwater algae Desmodesmus subspicatus, and the invertebrate Daphnia magna (requests 4 and 5 of the Decision).

The ecotoxic effect of the selected nanoforms were compared to ZnCl₂  in the same conditions to assess any possible additional effect than that of the ion.

It was evident from the suite of ecotoxicity testing on D. subspicatus and D. magna, that there were no nanoparticles dispersed in solution, instead they rapidly formed agglomerates larger than 0.2 µm (i.e., no difference observed in any of the dispersed vs. dissolved fractions).

Two main observations in the ecotoxicity tests lead to conclude to no additional effect of the nanoforms:

- EC_x values of the nanoforms expressed as nominal or total zinc are not different from ZnCl₂, or higher than the soluble reference compound ZnCl₂. This is observed for both the alga and the daphnid.

- EC_x results expressed as dispersed and soluble fractions are very similar throughout the tests. This observation supports the absence of nanoparticle shortly after entering the test solution, as shown also by the transformation/dissolution results (Cooper et al 2021). 

Considering the variability observed in these tests, the differences with the reference compound ZnCl₂ within a factor 2 (corresponding to variation of 38% on the mean value) reflect inherent test variability, rather than effect. The reverse pattern of results observed in some cases (e.g. slightly lower EC₁₀ versus higher EC₅₀) is also interpreted as reflecting test variability, rather than resulting from a nanoparticle effect.

In the algae tests, the results on the hydrophobic coated nanoform 369 were unreliable. Precipitates were visible during the algae test and were examined in more detail. It appears that the organic triethoxy(octyl) silane coating of the 369 nanoform, dissolved and resulted in a severe disturbance of the distribution of zinc over different fractions in the test, including precipitated layers. This phenomenon made unreliable both the measurements of zinc in the test solution, and, as a consequence, the EC_x values calculated on measured zinc.

In the D. magna tests, the 1st filtration step over the 0.2 µm filter resulted in a compelling loss of zinc. This test artefact had a significant effect on the calculated EC_x values of the dispersed and dissolved fractions. Finally, the hydrophobic coated 369 nanoform hardly dissolved in the (static) D. magna test, resulting in no effect at 100 mg/L.

In conclusion, the data generated on D. subspicatus and D. magna did not show any evidence for additional toxicity of the ZnO-nanoforms tested.

 

More comprehensive summary data, discussion of the choice of testing materials, discussion of the results and conclusions is provided in the Industry document “Summary background document on testing of aquatic toxicity of nano-ZnO in the Framework of the Decision of July 9^th 2019 on Zinc Oxide (CAS nr 1314-13-2) under article (46)1 of REACH”. Full detail of the tests and results is presented in the extensive testing reports from the test Institute Fraunhofer (Schlich 2022a-d; Vogt 2022a-d).

_________________________________________________________________

 

ZnO nano

Nano ZnO data set

The present short-term acute aquatic toxicity database for nano-ZnO, covers the main taxonomic groups (fish, invertebrates and algae): In total, 13 species are included, 1 freshwater fish, 5 invertebrates (2 freshwater and 3 marine) and 7 algae species (2 freshwater and 5 marine). In addition, 1 acute study on 1 sediment freshwater species is available. For reasons of comparison, in the tables below,the data are presented as normalized values with the ecotoxicity value observed for the soluble zinc salts (Zn²⁺) as reference (EC_x = 100%). The ratio EC_x Zn²⁺/EC_x nano ZnO is provided. The detail of the studies is in the IUCLID files. The full set of EC₅₀ values are presented in the IUCLID sections 6.1.1. & 6.1.2. (fish), 6.1.3. & 6.1.4. (invertebrates) and 6.1.5. & 6.1.6. (algae and plants).

 

Nano ZnO chronic toxicity

The present review focused notably on these chronic data, which are considered to be of major relevancy for the risk assessment and the related PNECs. Like for the acute toxicity,the data are presented as normalized values with the ecotoxicity value observed for the soluble zinc salts (Zn²⁺) as reference (EC_x = 100%). The ratio EC_x Zn²⁺/EC_x nano ZnO is provided. The detail of the studies is in the IUCLID files.

The freshwater chronic dataset now covers all main taxonomic groups, with 2 invertebrates (mollusc and crustacean), 2 algae, 1 fish species and 1 aquatic plant species. In addition, data on 1 sediment crustacean is also included. The full set of EC₁₀/NOEC values are presented in the IUCLID sections 6.1.1. & 6.1.2. (fish), 6.1.3. & 6.1.4. (invertebrates) and 6.1.5. & 6.1.6. (algae and plants).

The marine chronic dataset covers all main taxonomic groups, with 1 invertebrate (clam), 2 algae, 1 fish species.

For freshwater, the comparison of the chronic ecotoxicity data obtained within the same studies for the same species/endpoints after exposing the organisms to either the soluble Zn2+ion or the ZnO-NP form show that there is generally no difference in toxicity between the two Zn-forms.

The general observation of similar toxicity by the Zn-ion and the ZnO-NP is most relevant for the discussion on the aquatic PNEC developed for zinc substances; it can be concluded that the available data show that the general Zn-ion based PNEC for freshwater is relevant for the nano-ZnO, too.

While the data for the freshwater algae do not suggest a higher toxicity of the NP form, the marine algae data set shows a diatom where the Zn2+/ZnO-NP toxicity ratio is higher than 2. A similar observation was made on the acute dataset. Consequently, this observation for the marine risk assessment merits further consideration.