Naphthenic acids, zinc salts, basic

Endpoint summary

Description of key information

Reliable studies on the environmental fate of naphthenic acids, zinc salts, basic are not available. Thus, read-across to the assessment entities zinc cations and naphthenate anions is applied since the ions of the substance naphthenic acids, zinc salts, basic determine its fate and toxicity in the environment. Since zinc cations and naphthenate 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. The environmental fate and behaviour of zinc cations and naphthenate anions are predicted to be different, resulting in a different relative distribution in environmental compartments water, air, sediment and soil.

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 naphthenate in the environment. More than 70 % of zinc is removed from the water column under reference conditions for EU regional waters (EUSES), and naphthenates are considered biodegradable. Considering the essentiality of zinc and the low potential for bioaccumulation of naphthenate, the potential for bioconcentration and bioaccumulation of naphthenic acids, zinc salts, basic is considered to be low. For further details, please refer to the respective endpoint summaries of the assessment entities.

Additional information

Read-across

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of naphthenic acids, zinc salts, basic in water, a complete dissociation of naphthenic acids, zinc salts, basic resulting in zinc cations and naphthenate anions may be assumed under environmental conditions ultimately. 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 naphthenic acids, zinc salts, basic could not be identified. According to the Irving-Williams series, stability constants formed by divalent first-row transition metal ions generally increase to a maximum stability of zinc (Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)). 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 naphthenate anions are not expected to bind strongly with zinc, especially when compared to polydentate (chelating) ligands. The metal-ligand formation constants (log KML) of zinc with other carboxylic acids, i.e. acetic, formic and benzoic acid, range from 0.6 to 1.59, (Bunting and Thong, 1970) and point to a lowly-moderately stable complexation. 

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 naphthenic acid of 4.72 results in:

log KML= 0.301 * 4.72 + 0.015

log KML= 1.44 (estimated zinc-naphthenate formation constant).

Thus, in the assessment of environmental fate and pathways of naphthenic acids, zinc salts, basic, read-across to the assessment entities naphthenate and soluble zinc substances is applied since the individual ions of naphthenic acids, zinc salts, basic determine its environmental fate. Since zinc ions and naphthenate ions behave differently in the environment, regarding their fate and toxicity, a separate assessment 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 naphthenic acids, zinc salts, 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.

Bunting, J. W., & Thong, K. M. (1970). Stability constants for some 1: 1 metal–carboxylate complexes. Canadian Journal of Chemistry, 48(11), 1654-1656.