Diammonium hexanitratocerate

Endpoint summary

Additional information

General considerations

The Pourbaix diagram (see further explanation in the read across document attached to IUCLID Section 13) indicates that Ce+IV is not stable in solution. Only Ce+III can occur in aqueous solutions and has been demonstrated to be able to cause toxicity in aquatic organisms (e.g., see dossiers for trivalent cerium compounds such as cerium trichloride, cerium trinitrate). Therefore, the extent to which cerium from tetravalent cerium compounds such as cerium ammonium nitrate is bioavailable for uptake (and causing toxicity) will depend on the extent to which Ce+IV is reduced to Ce+III (dependent on pH and redox potential) and on Ce+III speciation (dependent on pH, medium composition, etc.). If the dissolved cerium concentrations reach toxic levels as determined in tests with trivalent cerium compounds such as cerium trichloride or cerium trinitrate, toxicity will also be observed in toxicity tests with tetravalent cerium compounds.

Short-term toxicity to aquatic organisms

The abovementioned explanation on potential cerium toxicity after addition of a tetravalent cerium compound to standard test media was confirmed in a series of ecotoxicity experiments with fish, daphnids, and algae. In an acute toxicity to fish study performed by Hefner (2014a) according to OECD guideline 203, the 96-h EC50 for rainbow trout was determined to be 0.14 mg dissolved Ce/L (corresponding to 0.53 mg (NH4)2Ce(NO3)6/L). Based on this result, cerium ammonium nitrate is considered to be very toxic to fish. A very similar result was obtained in an identical study with cerium trinitrate (a trivalent cerium compound), resulting in a 96-h EC50 value of 0.13 mg dissolved Ce/L for the same test species. This confirms that toxicity to aquatic organisms depends on the extent to which Ce+IV is reduced to Ce+III. In this case, sufficiently high dissolved cerium (Ce+III) concentrations were obtained in the test medium to cause mortality among the exposed fish.

The key study for acute toxicity to aquatic invertebrates performed by Hefner (2014b) according to OECD guideline 202 yielded a 48-h EC50 of > 100 mg (NH4)2Ce(NO3)6/L. No significant immobilisation was observed in the exposed daphnids. Dissolved cerium concentrations were only measurable in the treatment with the loading rate of 100 mg (NH4)2Ce(NO3)6/L, the mean measured concentration being 3.1 µg Ce/L throughout the test. In all other treatments dissolved cerium concentrations were < LOQ. Because no significant immobilisation was observed, the (unbound) EC50 was based on nominal concentrations. As a result, cerium ammonium nitrate is considered not to be toxic to aquatic invertebrates under the conditions of the test up to the solubility limit of cerium ammonium nitrate in the test water at a nominal loading rate of 100 mg/L.

In an algal growth inhibition study performed by Hefner (2014c) according to OECD guideline 201, no dissolved cerium concentrations > LOQ were observed in any of the treatments at the start and end of testing. Nevertheless, significant growth inhibition was observed in the treatment with the highest nominal loading rate (100 mg (NH4)2Ce(NO3)6/L). Based on nominal concentrations, the 72-h EC50 (growth rate-based) was determined to be 93 mg (NH4)2Ce(NO3)6/L for the unicellular green alga Pseudokirchneriella subcapitata. However, these values will not be taken forward to classification and PNEC derivation because the effects on growth were observed to be concurrent with phosphate depletion in the test medium due to heavy complexation with cerium, suggesting that the observed effect on growth inhibition is due to phosphate deprivation rather than direct toxicity of the rare earth.

Finally, in an activated sludge respiration inhibition test performed by Eisner (2014) according to OECD guideline 209, no adverse effects on microbial respiration were observed in any of the treatments, including the treatment with the highest (nominal) loading of 1000 mg (NH4)2Ce(NO3)6/L. The 3-h NOEC was therefore >= 1000 mg (NH4)2Ce(NO3)6/L and the test substance was not considered to be toxic to aquatic microorganisms.

Long-term toxicity

In accordance with Column 2 of REACH Annex IX, long-term testing on aquatic invertebrates and fish, as listed under standard information requirements 9.1.5 and 9.1.6 respectively, is not required if the chemical safety assessment concludes that the substance is of no immediate concern to the environment. Since no risks to the aquatic environment were obtained for all exposure scenarios in the chemical safety assessment, no refinement of the aquatic hazard assessment with long-term toxicity testing is deemed necessary.