Naphthenic acids, nickel salts

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

6.15 µg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.615 µg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.142 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

30.73 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

3.07 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

6.138 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

1.11 mg/kg food

Assessment factor:

10

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.

Conclusion on classification

Aquatic toxicity studies of naphthenic acids, nickel salts are not available. Thus, read-across to the assessment entities soluble nickel substances and naphthenic acid is applied since the ions of naphthenic acids, nickel salts determine its fate and toxicity in the environment. Reliable data available for soluble nickel substances and naphthenate indicate that the moiety of ecotoxicological concern are nickel cations. Thus, the aquatic hazard assessment is based on the most toxic moiety, i.e. nickel cations, and acute and chronic ecotoxicity reference values of nickel are recalculated for naphthenic acids, nickel salts based on a maximum nickel content of 10.8 %.

 

Acute (short-term aquatic) hazard: Based on the lowest identified acute ecotoxicity reference value of 68 µg Ni/L for nickel ions at pH 8 and a maximum nickel content of naphthenic acids, nickel salts of 10.8 %, the acute ecotoxicity reference value recalculated for naphthenic acids, nickel salts amounts to 629.6 µg/L naphthenic acids, nickel salts. Therefore, naphthenic acids, nickel salts meets classification criteria of acute (short-term) aquatic hazard Category 1 of Regulation (EC) No 1272/2008 with an acute M-Factor of 1. 

 

Long-term (chronic) aquatic hazard: Based on the lowest identified chronic ecotoxicity reference value of 2.4 µg Ni/L for nickel ions and a maximum nickel content of naphthenic acids, nickel salts of 10.8 %, the chronic ERV recalculated for naphthenic acids, nickel salts amounts to 22.2 µg/L naphthenic acids, nickel salts.

The chronic ecotoxicity reference value of 22.2 µg/L is compared to criteria for long-term aquatic hazard classification according to Regulation (EC) 1272/2008, taking into account that nickel ions are removed from the water column. Based on available evidence (please refer to “conclusions on classification” of the respective assessment entity nickel), more than 70% of dissolved nickel is removed within 28 days under most “environmentally relevant” conditions. Nickel is therefore considered rapidly removeable (i.e. equivalent to “rapid degradation” for organic substances).

Based on the chronic ecotoxicity reference value of 22.2 µg/l, naphthenic acids, nickel salts meets classification criteria of long-term aquatic hazard Category 2 in accordance with Table 4.1.0 (b) (ii) of Regulation (EC) No 1272/2008.

Thus, naphthenic acids, nickel salts meets classification criteria of acute aquatic hazard Category 1 (M-factor 1) and long-term aquatic hazard Category 2 according to Regulation (EC) No 1272/2008 and subsequent adaptations.