Fatty acids, C14-18 and C16-18-unsatd., zinc salts

Endpoint summary

Description of key information

Limited data of Fatty acids, C14-18 and C16-18-unsatd., zinc salts are available. Thus, in the assessment, read-across to analogue substances and/or the assessment entities soluble zinc substances and fatty acids is applied since the ions of the substance Fatty acids, C14-18 and C16-18-unsatd., zinc salts determine its fate and toxicity in the environment.

In brief, metal partition coefficients for the distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water) are controlling the fate and distribution of zinc in environmental compartments whereas biotic degradation is expected to control the fate of fatty acids in the environment. More than 70 % of zinc is removed from the water column under reference conditions for EU regional waters (EUSES) (please refer to the respective endpoint summary of the assessment entity "Zinc" and reports by Mutch Associates, LLC, 2010 a, b), and C14 -18 fatty acids are rapidly degraded. Considering also the essentiality of zinc and the fact that fatty acids are a natural component of the environment and part of the regular diet of aquatic and soil organisms, the potential for bioconcentration and bioaccumulation of Fatty acids, C14-18 and C16-18-unsatd., zinc salts is considered to be low. For details on the environmental fate and pathways of the individual moieties, please refer to the respective endpoint summaries of the assessment entities.

Additional information

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of Fatty acids, C14-18 and C16-18-unsatd., zinc salts in water, a dissociation resulting in zinc cations and fatty acid anions may be assumed under environmental conditions. 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 Fatty acids, C14-18 and C16-18-unsatd., zinc salts 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. (2007) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as fatty acids, C14-18 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, andαO andβO are termed the slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of Fatty acids, C14-18 and C16-18-unsatd. of 4.96 (QSAR estimate of MarvinSketch V.5.3.8, mean of individual C14 - 18 and C16 - 18-unsatd. fatty acid components) results in:

log KML= 0.301 * 4.96 + 0.015

log KML= 1.51 (estimated zinc - C14 - 18 and C16 - 18-unsatd. fatty acids formation constant). 

Thus, it may reasonably be assumed that based on the estimated “zinc- fatty acids, C14-18 and C16-18-unsatd.” formation constant, the respective behaviour of the dissociated zinc cations and C14-18 fatty acids in the environment determine the fate of Fatty acids, C14-18 and C16-18-unsatd., zinc salts 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 toxicity of Fatty acids, C14-18 and C16-18-unsatd., zinc salts, read-across to the assessment entities soluble zinc substances and C14-18 fatty acids is applied since the ions of Fatty acids, C14-18 and C16-18-unsatd., zinc salts determine its environmental fate. Since zinc cations and C14-18 fatty 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. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for Fatty acids, C14-18 and C16-18-unsatd., zinc salts.

 

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