2-ethylhexanoic acid, cerium salt

Endpoint summary

Description of key information

Metal carboxylates such as 2-ethylhexanoic acid, cerium salt are substances consisting of a metal cation and a carboxylic acid anion. In the assessment of the aquatic toxicity of 2-ethylhexanoic acid, cerium salt, read-across to the assessment entities soluble cerium substances and 2-ethylhexanoic acid is applied since the ions of 2-ethylhexanoic acid, cerium salt determine its fate and toxicity in the environment.

  

Cerium

Reliable acute aquatic toxicity data of cerium ions are available for two trophic levels, algae and invertebrates:

- a 96-h LC50 of 0.13 mg Ce/L was identified for the fish rainbow trout

- a 48-h EC50 of 6.9 mg Ce/L was identified for the invertebrate species Daphnia magna

- a 72-h EC50 of 0.63 mg Ce/L was identified for the green algae Pseudokirchneriella subcapitata (effects caused by phosphorous deficiency because of cerium phosphate precipitation)

Reliable chronic aquatic toxicity data of cerium ions are available for algae:

- a 72-h NOEC of 0.46 mg Ce/L was identified for the green algae Pseudokirchneriella subcapitata (effects caused by phosphorous deficiency because of cerium phosphate precipitation)

 

In sum, cerium ions have a potential for acute and long-term aquatic toxicity.

 

2-ethylhexanoic acid

Acute data are available for three trophic levels: algae, invertebrates and fish

- an unbounded value of > 100 mg/L (96-h LC50) was identified for the freshwater fish species Oryzias latipes based on data of an analogous substance (sodium 2-ethylhexanoate). Further studies of 2-ethylhexanoate with the freshwater fish species Oncorhynchus mykiss, Lepomis gibbosus and Leuciscus idus resulted in 96-h LC50 values > 100 mg/L.

- a 48-h EC50 of 913 mg/L represents the lowest value identified for the invertebrate species Daphnia magna (with pH adjustment)

- a 72-h ErC50 of 500 mg/L was identified for the green algae Pseudokirchneriella subcapitata (with pH adjustment)

 

In sum, 2-ethylhexanoic acid has a low potential for acute aquatic toxicity.

 

Chronic data are available for algae, aquatic invertebrates and microorganisms:

- a 21-d NOEC values of 18 mg/L and of 25.0 mg/L were identified for Daphnia magna

- a 72-h NOEC of 130 mg/L was identified for the green algae Pseudokirchneriella subcapitata (with pH adjustment)

- a 17-h EC10 of 71.7 mg/L was identified for the inhibition of bacterial growth (Pseudomonas putida)

 

In sum, 2-ethylhexanoic acid has a very low potential for chronic aquatic toxicity to algae and invertebrates and appears to be of low toxicity to microorganisms.

 

2-ethylhexanoic acid, cerium salt

Using existing aquatic toxicity data of the dissociation products of 2-ethylhexanoic acid, cerium salt, the aquatic hazard assessment is based on the most toxic moiety, i.e. cerium ions. Existing aquatic toxicity data for cerium ions are therefore recalculated for 2-ethylhexanoic acid, cerium salt based on a maximum cerium content of 26 % (w/w) of 2-ethylhexanoic acid, cerium salt.

 

Reliable acute aquatic toxicity data are available for two trophic levels, algae and invertebrates:

- a 96-h LC50 of 0.5 mg/L was identified for the fish rainbow trout

- a 48-h EC50 of 26.5 mg/L was identified for the invertebrate species Daphnia magna

- a 72-h EC50 of 2.42 mg/L was identified for the green algae Pseudokirchneriella subcapitata (effects caused by phosphorous deficiency because of cerium phosphate precipitation)

Reliable chronic aquatic toxicity data are available for algae:

- a 72-h NOEC of 1.77 mg Ce/L was identified for the green algae Pseudokirchneriella subcapitata (effects caused by phosphorous deficiency because of cerium phosphate precipitation)

Due to the toxicity observed for fish, 2-ethylhexanoic acid, cerium salt appears to have a potential for acute and long-term aquatic toxicity.

Conclusions on classification and labelling

Acute (short-term) toxicity:

EC/LC50 values of 3 trophic levels (algae, invertebrates and fish) for 2 -ethylhexanoate are > 10 mg/L and thus well above the classification cut-off value for acute (short-term) aquatic hazard category 1 of 1 mg/L. The aquatic hazard assessment is thus based on the most toxic moiety, i.e. cerium ions. Existing aquatic toxicity data for cerium ions are therefore recalculated for 2-ethylhexanoic acid, cerium salt based on a maximum cerium content of 26 % (w/w) of 2-ethylhexanoic acid, cerium salt. Reliable acute aquatic toxicity data are available for three trophic levels (fish, algae and invertebrates) and the lowest effect concentration is the 96-h LC50 of 0.5 mg/L 2-ethylhexanoic acid, cerium salt for rainbow trout and thus well below 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), 2-ethylhexanoic acid, cerium salt must be classified as Category Acute 1 for short-term hazard to the aquatic environment according to Regulation (EC) No 1272/2008. The corresponding M factor is 1 (0.1<L(E)C50 ≤1 mg/L).

Chronic (long-term) toxicity:

Reliable chronic aquatic toxicity data of 2-ethylhexanoate are available from guideline studies for algae and daphnia. The lowest respective EC10/NOEC values are 130 mg/L and 18 mg/L, respectively, and well above the classification cut-off values for long-term aquatic hazard category 1 to 3. Since long-term data are not available for fish, the surrogate approach is applied. Based on classification criteria for substances for which adequate chronic toxicity are not available in Table 4.1.0 (b) (III), Figure 4.1.1 and adequate acute toxicity data of fish with LC50 values > 100 mg/L, 2-ethylhexanoate does not classify as long-term aquatic hazard category 1 to 3. Criteria for classification in category 4 are also not met.

The aquatic hazard assessment is thus based on the most toxic moiety, i.e. cerium ions. Existing aquatic toxicity data for cerium ions are therefore recalculated for 2-ethylhexanoic acid, cerium salt based on a maximum cerium content of 26 % (w/w) of 2-ethylhexanoic acid, cerium salt.

Reliable chronic aquatic toxicity data for 2-ethylhexanoic acid, cerium salt are only available for algae. According to the guidance on classification criteria for substances for which chronic toxicity data are available for one or two trophic levels, criteria in Table 4.1.0(b) iii) based on acute data are applied in accordance with Figure 4.1.1 of Regulation (EC) No 1272/2008. The 96 -h LC50 for rainbow trout of 0.5 mg/L 2 -ethylhexanoic acid, cerium salt meets classification criteria of Category Chronic 1 (<1 mg/L), with corresponding M factor 1 (0.1<L(E)C50 ≤1 mg/L).

Therefore, 2-ethylhexanoic acid, cerium salt classifies as acute aquatic hazard Category 1 (M factor 1) and chronic aquatic hazard Category 1 (M factor 1) under Regulation (EC) No 1272/2008 and its subsequent adaptations.

 

Additional information

Read-across

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of 2-ethylhexanoic acid, cerium salt in water, a complete dissociation of 2-ethylhexanoic acid, cerium salt resulting in cerium and 2-ethylhexanoate ions may be assumed under environmental conditions. Similar to other rare earth elements, cerium ions tend to form complexes with ionic character as a result of their low electronegativity.

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. A metal-ligand complexation constant of 2-ethylhexanoic acid, cerium salt could not be identified. However, based on an analysis by Carbonaro & Di Toro (2007) of monodentate binding of cerium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as 2-ethylhexanoate are not expected to bind strongly with cerium, especially when compared to polydentate (chelating) ligands. 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 Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of 2-ethylhexanoic acid of 4.72 results in:

log KML= 0.356 * 4.72 + 0.739

log KML= 2.42 (estimated cerium-ethylhexanoate formation constant).

Thus, in the assessment of environmental toxicity of 2-ethylhexanoic acid, cerium salt, read-across to the assessment entities 2-ethylhexanoate and soluble cerium substances is applied since the individual ions of 2-ethylhexanoic acid, cerium salt determine its environmental fate and toxicity. Since cerium ions and 2-ethylhexanoate ions behave differently in the environment, regarding their fate and toxicity, a separate assessment of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for 2-ethylhexanoic acid, cerium salt.

 

Carbonaro & Di Toro (2007):Linear free energy relationships for metal ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms. GCA 71, p3958-3968.