2-ethylhexanoic acid, copper salt

Endpoint summary

Description of key information

The fate of 2-ethylhexanoic acid, copper salt in the environment is most accurately evaluated by separately assessing the fate of its moieties copper cations and 2-ethylhexanoate anions. In the assessment of environmental fate and behaviour of 2-ethylhexanoic acid, copper salt data available for the copper cation and the 2-ethylhexanoate anion indicate that abiotic degradation in respective compartments does not contribute significantly to its fate in the environment. Whereas copper is removed from the water column, 2-ethylhexanoate is readily biodegradable.

 

Copper

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including copper ions. Abiotic degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for metals and metal compounds. Copper as an element is not considered to be (bio)degradable but is removed from the water column. Copper is therefore considered rapidly removed, conceptually equivalent to “rapid degradation” for organic substances.

Transport and distribution: Copper adsorption is quantified by the log Kp (soil/porewater) = 3.33; log Kp(sediment/freshwater) = 4.39 and the log Kp (suspended matter/freshwater) = 4.48, rendering it mostly immobile in the different environmental compartments.

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:

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of 2-ethylhexanoic acid, copper salt in water, a complete dissociation of 2-ethylhexanoic acid, copper salt resulting in copper and 2-ethylhexanoate ions may be assumed under environmental conditions. 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, copper salt could not be identified. According to the Irving-Williams series, stability constants formed by divalent first-row transition metal ions generally increase to a maximum stability of copper (Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)). However, based on an analysis by Carbonaro & Di Toro (2007) of monodentate binding of copper to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as 2-ethylhexanoate are not expected to bind strongly with copper, especially when compared to polydentate (chelating) ligands. The metal-ligand formation constants (log KML) of copper with other carboxylic acids, i.e. butyric acid and benzoic acid amount to log KML values of 2.14 and 1-6 -1.92, respectively (Bunting and Thong, 1972) and point to a moderately stable complexation.

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= 0.430 * 4.72 + 0.213

log KML= 2.24 (estimated copper-ethylhexanoate formation constant).

Thus, in the assessment of environmental fate and pathways of 2-ethylhexanoic acid, copper salt, read-across to the assessment entities 2-ethylhexanoate and soluble copper substances is applied since the individual ions of 2-ethylhexanoic acid, copper salt determine its environmental fate. Since copper 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, copper salt.

 

CRC Handbook of Food Additives, 2nd ed. 1972. Butyric acid-copper formation constant.

Bunting, J. W., & Thong, K. M. (1970). Stability constants for some 1: 1 metal–carboxylate complexes.Canadian Journal of Chemistry,48(11), 1654-1656.

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