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Environmental fate & pathways

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Description of key information

The environmental fate of zinc bis(2-ethylhexanoate) in the environment is most accurately evaluated by separately assessing the fate of its dissociation products, zinc cations and 2-ethylhexanoic acid anions. Since zinc cations and 2-ethylhexanoic acid 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 individually for each assessment entity.

 

Zinc: For metals and metal compounds such as zinc substances, abiotic and biotic degradation is not relevant. However, information is available on the removal of zinc ions from the water column. The removal from the water column was modeled referring to the EUSES model parameters and different conditions of pH, resulting in a removal of > 70 % of total zinc ions under the reference conditions for EU regional waters (EUSES). Consequently, zinc is considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. Bioaccumulation is also not relevant for zinc since it is regulated by homeostasic mechanisms of the organisms. Regarding the distribution in environmental compartments, the coefficient for partitioning of zinc between particulate matter and water (Kpsusp) of 109,648 L/Kg was derived for EU waters whereas the Kp for the distribution between sediment and water (Kpsed) was estimated with 73,000 L/kg. For saltwater, a partition coefficient water/suspended matter of 6,010 L/kg was derived. A solids-water partitioning coefficient of 158.5 L/kg was determined for soil experimentally.

 

2-ethylhexanoic acid: 2-ethylhexanoate is readily biodegradable. Based on the biodegradation in water, biodegradation in soil is also expected.

However, abiotic degradation is not relevant due to slow degradation by photochemical processes and the absence of hydrolysable functional groups. Furthermore, 2-ethylhexanoate does not have a potential for bioaccumulation based on the low logPow of 2.96. Reliable data according to OECD TG 106 indicate that adsorption to the solid soil phase is also not expected for 2-ethylhexanoic acid.

Additional information

Read across

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the water solubility of zinc bis(2-ethylhexanoate) (5.586 g/L at pH 6.2-6.5), a complete dissociation of zinc bis(2-ethylhexanoate) resulting in zinc and 2-ethylhexanoate ions may be assumed under environmental conditions upon contact with water. The respective dissociation is in principle 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 zinc bis(2-ethylhexanoate) could not be identified. Data for zinc appear to be generally limited. However, zinc cations tend to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of zinc is typically described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions.

 

Based on an analysis by Carbonaro et al. (2011) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as 2-ethylhexanoic acid anions are not expected to bind strongly with zinc. Accordingly, protons will out-compete zinc ions for complexation of monodentate ligands given equal activities of free zinc and hydrogen ions. The metal-ligand formation constants (log KML) of zinc with other carboxylic acids, i.e. acetic and benzoic acid, ranging from 0.56 to 1.59 (Bunting & Thong, 1969), further point to a low strength of the monodentate bond between carboxyl groups and zinc.

 

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 2-ethylhexanoic acid of 4.72 results in:

log KML= 0.301 * 4.72 + 0.015

log KML= 1.44 (estimated zinc-ethylhexanoate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated zinc-ethylhexanoate formation constant, the respective behaviour of the dissociated zinc cations and 2-ethylhexanoate anions in the environment determine the fate of zinc bis(2-ethylhexanoate) 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 enviromental fate of zinc bis(2-ethylhexanoate), read-across to the assessment entities soluble zinc substances and 2-ethylhexanoic acid is applied since the ions of zinc bis(2-ethylhexanoate) determine its environmental fate. Since zinc 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.

 

In order to evaluate the environmental fate of the substance zinc bis(2-ethylhexanoate), information on the assessment entities zinc cations and 2-ethylhexanoic acid anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely "Report Read-across concept Category approach for zinc bis(2-ethylhexanoate)".

 

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.

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