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Environmental fate & pathways

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The fate of zinc m-toluate in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and m-toluate. Since zinc cations and m-toluate 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.

 

Hydrolysis is not relevant for zinc. Further, biotic degradation is not relevant for inorganic substances such as 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.

 

M-toluate is readily biodegradable and its acid has a log Kow of ~2.4. Abiotioc degradation and adsorption to soil, sediments or suspended matter are not expected to affect the fate of m-toluate in the environment. Based on the log Kow, toluate is not expected to bioaccumulate.

Additional information

Read across

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of zinc m-toluate in water (6.593 g/L at pH 7.47), a complete dissociation of zinc m-toluate resulting in zinc and m-toluate ions may be assumed under environmental conditions upon contact with water. 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 m-toluate could not be identified. Data for zinc appear to be generally limited. However, zinc 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 m-toluate 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 m-toluic acid of 4.25 results in:

log KML= 0.301 * 4.25 + 0.015

log KML= 1.29 (estimated zinc m-toluate formation constant).

 

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

 

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.