Cerium(3+) neodecanoate

Endpoint summary

Description of key information

For the substance as such, cerium neodecanoate, aquatic toxicity studies are not available. Therefore, read-across to the assessment entities cerium cations and neodecanoate anions is applied since, upon dissolution of cerium neodecanoate, cerium cations and neodecanoic anions determine fate and toxicity in the environment. Reliable data available for soluble cerium substances and neodecanoate indicate that the moiety of ecotoloxicological concern are cerium cations.

 

Acute (short-term) toxicity:

Reliable acute toxicity data are available for two trophic levels (aquatic invertebrates and algae). The EC/LC50 values for dissolved cerium range from 0.63 mg Ce/L to 6.9 mg Ce/L whereas toxicity of neodecanoate to algae and daphnia was not observed up to the to the test limit of >100 mg/L neodecanoate.  

Chronic (long-term) toxicity:

Reliable chronic (long-term) toxicity data are available for one trophic level (algae). The NOErC (72-h) of 0.46 mg/L dissolved cerium is based on growth inhibition of the green algae Pseudokirchneriella subcapitata. However, the reduced growth rate may be an artifact of the experimental test system and due to phosphate depletion rather than primary toxicity. Nevertheless, the effect concentration is applied in the hazard assessment.

In addition, QSAR estimations for chronic toxicity of neodecanoic acid (CAS 26896-20-8) to algae performed with ECOSAR v2.0 result in a chronic aquatic toxicity estimate (ChV) of 12.3 mg neodecanoic acid/L.

In sum, cerium neodecanoate may have some potential for acute and chronic aquatic toxicity based on the toxicity of its dissociation products.

Additional information

Conclusion on classification

For the substance as such, cerium neodecanoate, aquatic toxicity studies are notavailable. However, the environmental fate and toxicity of cerium neodecanoate is most accurately evaluated by separately assessing the respective fate and toxicity cerium cations and neodecanoate anions. Reliable data available for soluble cerium substances and neodecanoate indicate that the moiety of ecotoloxicological concern are cerium cations.

 

Acute (short-term) toxicity data: Toxicity of neodecanoate to algae and daphnia was not observed up to the test limit of >100 mg/L neodecanoate. The aquatic hazard assessment is thus based on the most toxic moiety, i.e. the cerium cations. The EC50 values available for 2 trophic levels (algae and invertebrates) amount to 0.63 and 6.9 mg/L dissolved cerium, corresponding to 2.9 and 32.1 mg/L cerium neodecanoate, respectively, based on a maximum cerium content of 21.5 % (w/w) of cerium neodecanoate. Thus, all EC50 values are well above the classification cut-off value for acute (short-term) aquatic hazard category 1 of 1 mg/L. In accordance with Regulation (EC) No 1272/2008, Table 4.1.0 (a), classification for acute (short-term) aquatic hazard is not required for cerium neodecanoate.  

Chronic (long-term) toxicity: A NOEC value of 0.46 mg/L dissolved cerium was determined for the growth inhibition of algae, corresponding to 2.1 mg/L cerium neodecanoate, based on a maximum cerium content of 21.5 % (w/w) of cerium neodecanoate. Thus, the available NOEC value for algae is well above the classification cut-off value for long-term aquatic hazard category 1 -3. However, since long-term data are not available for fish or invertebrates, the surrogate approach is applied. In accordance with Regulation (EC) No 1272/2008, Table 4.1.0 (b) (III), Figure 4.1.1 and adequate acute toxicity data of aquatic invertebrates with an EC50 of 32.1 mg/L, cerium neodecanoate meets the classification criteria of long-term aquatic hazard category 3.

Read-across

 

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. In a water solubility test according to OECD TG 105, a solubility of 157 µg/L cerium neodecanoate was determined. Thus, cerium neodecanoate is expected to dissociate in environmental media resulting in cerium and neodecanoate ions. The respective dissociation is reversible, and the ratio of the salt /dissociated ions is dependent on the metal-ligand dissociation constant of the salt, the composition of the solution and its pH. Based on an analysis by Carbonaro et al. (2007) of monodentate binding of cerium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as neodecanoate anions are not expected to bind strongly with cerium.

 

The analysis by Carbonaro & Di Toro (2007) suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms of carboxylic functional groups:

log KML= αO* log KHL+ βO; where

KML is the metal-ligand formation constant, KHL is the corresponding proton–ligand formation constant, and αO and βO are termed the slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of neodecanoic acid of 4.69 results in:

log KML= 0.356 * 4.69 + 0.739

log KML= 2.41 (estimated cerium-neodecanoate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated cerium-neodecanoate formation constant, the respective behaviour of the dissociated cerium cations and neodecanoic acid anions in the environment determine the fate of cerium neodecanoate upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently its ecotoxicological potential.

 

In the assessment of environmental toxicity of cerium neodecanoate, read-across to the assessment entities soluble cerium substances and neodecanoic acid is applied since the ions of cerium neodecanoate determine its ecotoxicologic potential. Since cerium cations and neodecanoate anions behave differently in the environment, including processes such as stability, degradation, transport and distribution, a separate assessment of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity.

 

In order to evaluate the environmental fate and toxicity of the substance cerium neodecanoate, information on the assessment entities cerium cations and neodecanoate anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for cerium neodecanoate.

 

Reference:

Carbonaro RF & Di Toro DM (2007) Linear free energy relationships for metal–ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms. Geochimica et Cosmochimica Acta 71: 3958–3968.