Naphthenic acids, nickel salts

Endpoint summary

Description of key information

The environmental fate of naphthenic acids, nickel salts is most accurately assessed by separately assessing the fate of its moieties nickel cations and naphthenate anions. Since nickel 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.

 

Nickel

Abiotic degradation, including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including nickel ions. Abiotic degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

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

Transport and distribution: Nickel partitioning is quantified by the log Kp (soil/porewater) = 2.86; log Kp(sediment/freshwater) = 3.85 and the log Kp (suspended matter/freshwater) = 4.42, rendering it mostly immobile in the different environmental compartments.

Bioaccumulation: In general, Ni bioaccumulation is relatively low. Though nickel some bioaccumulation is observed in aquatic biota, bioaccumulation factors are typically low. Apparently, nickel does generally not magnify along food chains (McGeer et al. 2003).

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: Alog Koc of 4.7 was derived for naphthenate.

Additional information

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of naphthenic acids, nickel salts in water, a complete dissociation of naphthenic acids, nickel salts resulting in nickel 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 naphthenic acids, nickel salts 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 nickel to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as naphtenate anions are not expected to bind strongly with nickel, especially when compared to polydentate (chelating) ligands. Accordingly, protons will always out-compete nickel ions for complexation of monodentate ligands given equal activities of free nickel and hydrogen ions. The metal-ligand formation constants (log KML) of nickel with other carboxylic acids, i.e. acetic and benzoic acid, ranging from 0.41 to 1.81 (Bunting & Thong, 1970), further point to a low to moderate strength of the monodentate bond between carboxyl groups and nickel. 

 

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= 0.295 * 4.72 + 0.055

log KML= 1.45 (estimated nickel-naphthenate formation constant).

 

Thus, in the assessment of environmental toxicity and pathways ofnaphthenic acids, nickel salts, read-across to the assessment entities naphthenate and soluble nickel substances is applied since the individual ions ofnaphthenic acids, nickel saltsdetermine its environmental toxicity. Since nickel ions and naphthenate ions behave differently in the environment, regarding their 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, nickel 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. Bunting, J. W., & Thong, K. M. (1970). Stability constants for some 1: 1 metal–carboxylate complexes. Canadian Journal of Chemistry, 48(11), 1654-1656. Chemistry, 48(11), 1654-1656.