Zinc 3,5,5-trimethylhexanoate

Endpoint summary

Description of key information

The fate of zinc 3,5,5-trimethylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and trimethylhexanoate. In the assessment of environmental fate and behaviour of zinc 3,5,5 -trimethylhexanoate data available for the zinc cation and the trimethylhexanoate anion indicate that biotic degradation in respective compartments do not contribute significantly to its fate in the environment.

 

Zinc: Hydrolysis and biodegradation are not relevant for zinc. Zinc is an essential element that is actively regulated by organisms. Bioconcentration and bioaccumulation are not considered to be relevant for inorganic zinc substances. 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 6010 L/kg was derived. For soil, a solids-water partitioning coefficient of 158.5 L/kg was determined experimentally.

 

Trimethylhexanoate (and its structural analogue neodecanoic acid):

Abiotic degradation is not expected to significantly affect the environmental fate of trimethyl hexanoic acid and neodecanoic acid since the acids are lacking hydrolysable functional groups. Further, neodecanoic acid does not absorb light within a range of 290 to 750 nm.

 

Biotic degradation: Neodecanoic acid is not readily biodegradable (11% biodegradation in 28 d) based on results from a standard OECD ready biodegradation test. Studies are not available to assess the biodegradability of neodecanoic acid under simulated conditions or in soil, but given the limited biodegradation in water, biodegradation under simulated conditions, or in soil is not expected to occur to a great extent. However, based on the QSAR prediction with BIOWIN (v4.10), 3,5,5-trimethylhexanoic acid may be considered as readily biodegradable.

 

Transport and distribution: The estimated Koc of neodecanoic acid is 121 and may be sensitive to pH. Based on the outcome of the Molecular Connectivity Index model of KOCWIN (v 2.00), the logKoc of trimethyl hexanoate is 1.48 (Koc = 30.4 L/kg). The vapor pressure of trimethylhexanoic acid and neodecanoic acid is very low, i.e. 6.5 Pa and 0.65 Pa, respectively suggesting a limited volatilization from soil. Henry’s Law constant for trimethyl hexanoic acid and neodecanoic acid is calculated with 0.40 and 0.54 Pa-m3/mole at 25 °C, respectively, indicating that volatilization from water is not expected to occur at a rapid rate, but may occur. Trimethyl hexanoic acid and neodecanoic acid are weak organic acids with estimated dissociation constants (pKa) of 5.23 and 4.69, respectively. Consequently, trimethyl hexanoic acid and neodecanoic acid at neutral pH, typical of most natural surface waters, is expected to dissociate to the ionised form and therefore to remain largely in water.

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 andtrimethylhexanoate.

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 trimethylhexanoate anions are not expected to bind strongly with zinc. 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-trimethylhexanoate 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 and 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 fateand 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.