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Metal carboxylates such as zinc neodecanoate, basic are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of zinc neodecanoate, basic in water, a complete dissociation of zinc neodecanoate, basic resulting in zinc cations and neodecanoate anions may be assumed under environmental conditions. Thus, fate and toxicity of zinc neodecanoate, basic in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and neodecanoate and read-across to the assessment entities soluble zinc substances and neodecanoic acid is applied for the assessment. Please refer to the data as submitted for each individual assessment entity.

 

Zinc

Abiotic degradation: Hydrolysis is not relevant for inorganic substances such as zinc substances.

Biotic degradation: Biotic degradation is not relevant for inorganic substances such as zinc substances. For water, information is available on the removal of metals from the water column. The removal from the water column was modeled referring to the EUSES model parameters and different conditions of pH. Zinc is removed by > 70% under the reference conditions for EU regional waters (EUSES).

Bioaccumulation: Zinc is an essential element which is actively regulated by organisms, so bioconcentration/bioaccumulation is not considered relevant for all inorganic zinc substances.

Transport and distribution: 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.

 

Neodecanoic acid

Abiotic degradation: Abiotic degradation is not relevant for neodecanoic acid since it does not contain any components that can hydrolyse in water at an environmentally relevant pH.

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.

Bioaccumulation: According to a bioconcentration study neodecanoic acid does not appear to have a high potential to bioaccumulate (BCF < 225 L/kg wwt fish).

Transport and distribution: The estimated Koc of neodecanoic acid is 121 L/kg (logKoc: 2.08) and may be sensitive to pH. The vapor pressure is very low, i.e. 0.65 Pa suggesting a limited volatilization from soil. Henry’s Law constant for neodecanoic acid is calculated with 0.54 Pa-m3/mole at 25 °C indicating that volatilization from water is not expected to occur at a rapid rate, but may occur. Neodecanoic acid is a weak organic acid with an estimated dissociation constant (pKa) of 4.69. Consequently, 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

Read across

The fate of zinc neodecanoate, basic in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and neodecanoate.

 

Metal carboxylates are substances consisting of a metal cation and a carboxylate anion. In the water solubility test according to OECD test 105 of zinc neodecanoate, basic a solubility of 0.776 g/L at pH 6.81 was determined. Thus, zinc neodecanoate, basic is considered to be soluble and expected to dissociate completely under environmental conditions resulting in zinc and neodecanoate 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 neodecanoate, basic 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. (2011) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as neodecanoate 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 neodecanoic acid of 4.69 results in:

log KML= 0.301 * 4.69 + 0.015

log KML= 1.43 (estimated zinc neodecanoate, basic formation constant).

 

Thus, it may reasonably be assumed that based on the estimated zinc-neodecanoate formation constant, the respective behaviour of the dissociated zinc cations and neodecanoate anions in the environment determine the fate of zinc neodecanoate, basic 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 toxicity of zinc neodecanoate, basic, read-across to the assessment entities soluble zinc substances and neodecanoic acid is applied since the ions of zinc neodecanoate, basic determine its environmental fate. Since zinc cations and neodecanoate 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 neodecanoate, basic, information on the assessment entities zinc cations and neodecanoate 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 neodecanoate, basic.

 

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.

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