Zinc 3,5,5-trimethylhexanoate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

114.4 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

33.9 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

555.6 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

654.4 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

313.9 mg/kg sediment dw

Assessment factor:

1

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:

197.8 mg/kg soil dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

0.02 g/kg food

Assessment factor:

90

Additional information

The fate and toxicity of zinc 3,5,5 -trimethylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and trimethylhexanoate.

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on its water solubility, zinc 3,5,5 -trimethylhexanoate is expected to dissociate completely under environmental conditions resulting in zinc and trimethylhexanoate ions.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 zinc 3,5,5 -trimethylhexanoate could not be identified. Data for zinc appear to be generally limited. However, zinc tends 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. (2007) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as trimethyl hexanoate 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 3,5,5-trimethylhexanoic acid of 5.23 results in:

log KML= 0.301 * 5.23 + 0.015

log KML= 1.59 (estimated zinc trimethylhexanoate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated zinc-trimethyl hexanoate formation constant, the respective behaviour of the dissociated zinc cations and trimethyl hexanoate anions in the environment determine the fate of zinc 3,5,5-trimethylhexanoate 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 environmental fate and pathways of zinc 3,5,5-trimethylhexanoate, read-across to the assessment entities soluble zinc substances and 3,5,5-trimetylhexanoic acid (and its structural analogue neodecanoic acid) is applied since the ions of zinc 3,5,5-trimethylhexanoate determine its environmental fate. Since zinc cations and trimethylhexanoate 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 3,5,5-trimethylhexanoate, information on the assessment entities zinc cations and trimethylhexanoate anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for zinc 3,5,5-trimethylhexanoate. 

 

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.

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.

Conclusion on classification

Aquatic toxicity studies with zinc 3,5,5-trimethyl hexanoate are not available. The fate and toxicity of zinc 3,5,5-trimethylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its moieties zinc and trimethylhexanoate. Zinc 3,5,5-trimethylhexanoate dissolves and dissociates into zinc and trimethylhexanoate ions upon contact with an aqueous medium. Therefore, the aquatic hazard potential is assessed based on the toxicity data available for the assessment entities zinc and trimethylhexanoate since the ions of zinc 3,5,5- trimethylhexanoate determine its environmental fate and toxicity.

 

Acute (short-term) toxicity: EC/LC50 values of 3 trophic levels (algae, invertebrates and fish) for neodecanoic acid (the structural analogue of trimethylhexanoate) are > 100 mg/L and well above the classification cut-off value for acute (short-term) aquatic hazard category 1 of 1 mg/L. This is supported by QSAR-based estimations for trimethylhexanoic acid. According to the QSAR-based outcome of the model ECOSAR v.2.0, trimethylhexanoic acid has also a low potential for acute toxicity since the 48-h EC/LC50 values were estimated with 65.0, 52.3 and 81.4 mg/L for freshwater algae, daphnids and fish, respectively. Thus, 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 3,5,5-trimethylhexanoate based on a maximum zinc content of 18%.

Zinc 3,5,5-trimethylhexanoate 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 maximum zinc content of zinc 3,5,5-trimethyl hexanoate of 18%, and iii) the resulting acute ecotoxicity reference value of 755.6 µg/L zinc 3,5,5-trimethylhexanoate 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: Regarding the aquatic toxicity of neodecanoic acid (structural analogue of trimethylhexanoate), reliable data are available for invertebrates and fish. The respective NOEC/EC10 values are > 1 mg/L and indicate a low potential for chronic toxicity. Regarding algae, an EC10 or NOEC is not available for neodecanoate. However, based on the fact that the EC50 for growth rate of algae is > 100 mg/L, we may assume that it is unlikely that the EC10/NOEC < 1 mg/L. The low potential for chronic toxicity is supported by QSAR-based estimations for trimethylhexanoic acid. According to the QSAR-based outcome of the model ECOSAR v.2.0, trimethylhexanoic acid has also a low potential for toxicity since chronic effect values ChV (ChV = 10^([log (LOEC x NOEC)]/2)) were estimated with 22.4, 7,2 and 9.2 mg/L for freshwater algae, daphnids and fish, respectively. Thus, EC10/NOEC values of trimethylhexanoate and its structural analogue neodecanoate are well above the classification cut-off value for (chronic) long-term aquatic hazard categories of 1 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 3,5,5-trimethylhexanoate based on a maximum zinc content of 18%.

Zinc 3,5,5-trimethylhexanoate 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 maximum zinc content of zinc 3,5,5-trimethylhexanoate of 18%, and iii) the resulting chronic ecotoxicity reference value of 105.6 µg/L zinc 3,5,5-trimethylhexanoate, classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008.

The chronic ecotoxicity reference value of 105.6 µg/L is compared with the criteria for long-term aquatic hazard classification, taking into account whether the zinc, the toxic moiety of concern, 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 105.6 µg/l, zinc 3,5,5-trimethylhexanoate 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.