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. Based on the solubility of 2-ethylhexanoic acid, cerium salt in water, a complete dissociation resulting in cerium ions and 2-ethylhexanoate ions may be assumed under environmental conditions. Since cerium cations and 2-ethylhexanoate anions behave differently in the environment, including processes such as stability, degradation, transport and distribution, a separate assessment of the environmental fate of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity. 

 

Cerium

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including cerium ions. In general, (abiotic) degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for metals and metal compounds. Cerium as an element is not considered to be (bio)degradable.

Transport and distribution: Cerium partitioning is quantified by the log Kp (soil/water) = 3.54; log Kp(sediment/freshwater) = 5.15 and the log Kp (suspended matter/freshwater) = 5.12, rendering it mostly immobile in the different environmental compartments.

Regarding the aqueous chemistry, cerium can exist in the trivalent cerous state (Ce3+) and the tetravalent ceric state (Ce4+). However, modelling of thermodynamic stability of cerium species suggests that aqueous Ce3+ and cerous oxides/hydroxides are the only dominant species at environmentally relevant conditions. Studies also point to Ce(III) species being the main driver of toxicity (Dahle and Arai, 2015; Pulido-Reyes et al., 2015).

 

2-ethylhexanoic acid

Abiotic degradation may affect the environmental fate of 2-ethylhexanoic acid since it is prone to slow degradation by photochemical processes. Hydrolysis, however, is not expected to be an important fate path.

Biotic degradation: 2-ethylhexanoate is readily biodegradable. Based on the biodegradation in water, biodegradation in soil and sediment is also expected.

Bioaccumulation: 2-ethylhexanoate has a low potential for bioaccumulation (logPow = 2.96)

Transport and distribution: According to predictions of the Level III fugacity model of EPI Suite (v4.11) for the partitioning between air, soil, sediment and water in an evaluative environment assuming steady-state but not equilibrium conditions, 2-ethylhexanoate will preferentially partition into water and has a low potential for volatilisation. A significant adsorption to solid phases is not expected.

Additional information

Read-across approach

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.

Dahle & Arai (2015) Environmental geochemistry of cerium: applications and toxicology of cerium oxide nanoparticles. International journal of environmental research and public health 12/2, p. 1253-1278.

Pulido-Reyes et al. (2015) Untangling the biological effects of cerium oxide nanoparticles: the role of surface valence states. Scientific reports 5, p. 15613.