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Ecotoxicological information

Ecotoxicological Summary

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Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
110.75 µg/L
Assessment factor:
1
Extrapolation method:
sensitivity distribution

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
32.8 µg/L
Assessment factor:
1
Extrapolation method:
sensitivity distribution

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
537.63 µg/L
Assessment factor:
1
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
7.83 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.78 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
1.3 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential for bioaccumulation

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 always 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 zinc-ethylhexanoate formation constant, the respective behaviour of the dissociated zinc cations and 2-ethylhexanoic acid 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 and toxicity 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 toxicity. 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 for each individual assessment entity.

 

In order to evaluate the environmental fate and toxicity 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.

Conclusion on classification

Aquatic toxicity studies of zinc bis(2-ethylhexanoate) are not available. In the assessment of environmental toxicity 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 fate and toxicity in the environment.

 

Acute (short-term) toxicity: EC/LC50 values of 3 trophic levels (algae, invertebrates and fish) range for 2-ethylhexanoate from 49.3 mg/L to >100 mg/L. Thus, all EC50/LC50 values are well above the classification cut-off value for acute (short-term) aquatic hazard Category 1 of 1 mg/L. The acute aquatic hazard assessment is based on the most toxic moiety, i.e. the zinc cation, and acute ecotoxicity reference values of zinc are recalculated for zinc bis(2-ethylhexanoate) based on a zinc content of 18.6 %.

Zinc bis(2-ethylhexanoate) meets based on i) the acute aquatic ecotoxicity values of 136 µg Zn/L and 413 µg Zn/L for the zinc ion at pH 8 and pH 6 respectively; ii) the zinc content of zinc bis(2-ethylhexanoate) of 18.6 %, and iii) the resulting acute ecotoxicity reference value of 713 µg/L zinc bis(2-ethylhexanoate) at pH 8 as worst case, classification criteria of acute (short-term) aquatic hazard Category 1 of Regulation (EC) No 1272/2008 with an acute M-Factor of 1.

 

Chronic (long-term) toxicity: For 2-ethylhexanoate, NOEC/EC10 values of 2 trophic levels (algae and invertebrates) range from 18 to 32 mg/L. According to the QSAR-based outcome of the model ECOSAR v.2.0, 2-ethylhexanoate has a very low potential for chronic toxicity to freshwater and saltwater fish with respective chronic values (ChV = 10^([log (LOEC x NOEC)]/2)) well above 15 mg/L. The long-term (chronic) aquatic hazard assessment is based on the most toxic moiety, i.e. the zinc cation, and chronic ecotoxicity reference values of zinc are recalculated for zinc bis(2-ethylhexanoate) based on a zinc content of 18.6 %.

Zinc bis(2-ethylhexanoate) meets based on i) the lowest chronic aquatic ecotoxicity reference value observed for the algae Pseudokirchneriella subcapitata (19 µg Zn/L) at neutral pH; ii) the zinc content of zinc bis(2-ethylhexanoate) of 18.6 %, and iii) the resulting chronic ecotoxicity reference value of 102.2 µg/L zinc bis(2-ethylhexanoate), classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008.

The chronic ecotoxicity reference value of 102.2 µg/L is compared with the criteria for long-term aquatic hazard classification, taking into account whether the zinc, the toxic moiety of concern due to the ready biodegradability of 2-ethylhexanoate, is considered rapidly degradable or not.

The concept of “Degradability” was developed for organic substances and is not applicable to inorganic zinc substances. As a surrogate approach for assessing “degradability”, the concept of “removal from the water column” was developed to assess whether or not a given metal ion would remain present in the water column upon addition (and thus be able to excert a chronic effect) or would be rapidly removed from the water column. In this concept, “rapid removal” (defined as >70% removal within 28 days) is considered as equivalent to “rapidly degradable”. The rapid removal of zinc from the water column is documented in the section „Environmental fate“. Consequently, zinc is considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. 

Based on the chronic ecotoxicity reference value of 102.2 µg/l, zinc bis(2-ethylhexanoate) meets classification criteria of long-term aquatic hazard Category 3 in accordance with Table 4.1.0 (b) (ii) of Regulation (EC) No 1272/2008.

Zinc bis(2-ethylhexanoate) meets classification criteria of Regulation (EC) No 1272/2008 as Acute (short-term) aquatic hazard Category 1 with an acute M-Factor of 1 and Long-term aquatic hazard Category 3.