Naphthenic acids, copper salts

Endpoint summary

Description of key information

The fate of copper naphthenate in the environment is most accurately evaluated by separately assessing the fate of its moieties copper cations and naphthenate anions. Since copper 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. Please refer to the data as submitted for each individual assessment entity:

 

Copper

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including copper ions. In general, (abiotic) degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for metals and metal compounds. Copper as an element is not considered to be (bio)degradable but is removed from the water column. Copper is therefore considered rapidly removed, conceptually equivalent to “rapid degradation” for organic substances.

Transport and distribution: Copper adsorption is quantified by the log Kp (soil/porewater) = 3.33; log Kp(sediment/freshwater) = 4.39 and the log Kp (suspended matter/freshwater) = 4.48, rendering it mostly immobile in the different environmental compartments.

Naphthenate

Abiotic degradation: Due to structural properties, hydrolysis is not expected to be an important fate path.

Biotic degradation: Available data on model and commercially available naphthenic acids indicate an inherent to ready biodegradation. Thus, naphthenate is considered biodegradable.

Bioaccumulation: A range of BCF values from 3.2 to 56.2 was estimated based on QSAR. The Japanese METI-NITI database reports a range of BCF between 1.6 and 27 L/kg wet-wt for sodium naphthenate. Thus, available data point to a low potential for bioaccumulation.

Transport and distribution: A log Koc of 4.7 was derived for naphthenate.

Additional information

Read-across

 

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of copper naphthenate in water, a complete dissociation of copper naphthenate resulting in copper cations and naphthenate anions may be assumed under environmental conditions. 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 copper naphthenate 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 copper (Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)). However, based on an analysis by Carbonaro et al. (2007) of monodentate binding of copper to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as naphtenate anions are not expected to bind strongly with copper, especially when compared to polydentate (chelating) ligands. The metal-ligand formation constants (log KML) of copper with other carboxylic acids, i.e. butyric acid and benzoic acid amount to log KML values of 2.14 and 1.51 -1.92, respectively (Bunting and Thong, 1970; CRC, 1972) and point to a 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= 0.430 * 4.72 + 0.213

log KML= 2.24 (estimated copper-naphthenate formation constant).

Thus, in the assessment of environmental fate and pathways of copper naphthenate, read-across to the assessment entities naphthenate and soluble copper substances is applied since the individual ions of copper naphthenate determine its environmental fate. Since copper 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 copper naphthenate.

 

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.

CRC Handbook of Food Additives, 2nd ed. 1972. Butyric acid-copper formation constant.

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