Octanoic acid, zinc salt, basic

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

25.6 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

192 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

2.56 µg/L

Assessment factor:

100

Extrapolation method:

assessment factor

PNEC marine water (intermittent releases):

19.2 µg/L

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

416.7 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.271 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.027 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:

0.04 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Read across statement:

 

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the water solubility of octanoic acid, zinc salt, basic (321 mg/L at 20°C), a complete dissociation of octanoic acid, zinc salt, basic resulting in zinc cations and octanoate anions 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 octanoic acid, zinc salt, basic could not be identified. Data for zinc appear to be generally limited. However, zinc cations 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. (2007) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as octanoic acid 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 octanoic acid of 4.89 results in:

log KML= 0.301 * 4.89 + 0.015

log KML= 1.49 (estimated zinc-octanoate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated zinc-octanoate formation constant, the respective behaviour of the dissociated zinc cations and octanoate anions in the environment determine the fate of octanoic acid, zinc salt, basic 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 order to evaluate the environmental fate and toxicity of octanoic acid, zinc salt, basic, information on the assessment entities zinc cations and representative fatty acid 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 octanoic acid, zinc salt, 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.

Conclusion on classification

OCTANOIC ACID, ZINC SALT, BASIC:

Octanoic acid, zinc salt, basic consists of fatty acid anions and zinc cations and is moderately soluble in water. For the conclusion on classification, aquatic toxicity data of octanoic acid, zinc salt, basic, and data for octanoic acid and more soluble zinc compounds are taken into account.

Based on available aquatic toxicity data of Octanoic acid, zinc salt, basic, i.e. the lowest EL50 and EL10 of 0.16 mg/L and 0.02 mg/L, respectively, for 72-h inhibition of growth rate of the algae P. subcapitata in standard test medium, and considering its rapid degradability, the substance classifies Aquatic hazard, category Acute 1 (M-factor 1) and Chronic 2 according to Table 4.1.0 (a) and (b) (ii) of Regulation (EC) No 1272/2008.

In the assessment of the aquatic hazard potential of octanoic acid, zinc salt, basic, read-across to the assessment entities soluble zinc substances and octanoic acid is further applied since zinc cations and octanoate anions determine its fate and toxicity in the environment.

Acute (short-term) toxicity:

Acute toxicity data for the assessment entity “octanoate” are available for three trophic levels. The lowest L(E)C50 for fish, crustacean or algae is a 48-h EC50 of 15 mg /L observed for the algal species P. subcapitata. Thus, all EC50/LC50 values are well above the classification cut-off value for acute (short-term) aquatic hazard Category 1 of 1 mg/L according to Table 4.1.0 (a) of Regulation (EC) No 1272/2008.

Thus, the acute aquatic hazard assessment is based on the more toxic moiety, i.e. the zinc cation, and acute ecotoxicity reference values of zinc are recalculated for octanoic acid, zinc salt, basic based on a maximum zinc content of 24 %. Octanoic acid, zinc salt, basic would meet based on i) the acute aquatic ecotoxicity values of 136 µg Zn/L and 413 µg Zn/L for the zinc ion at pH 8 and pH 6 respectively; ii) the zinc content of octanoic acid, zinc salt, basic of 24 %, and iii) the resulting acute ecotoxicity reference value of 567 µg/L octanoic acid, zinc salt, basic at pH 8 as worst case, classification criteria of acute (short-term) aquatic hazard Category 1 with an acute M-factor according to Table 4.1.0 (a) of Regulation (EC) No 1272/2008.

Chronic (long-term) toxicity:

Chronic toxicity data for the assessment entity “octanoate” are available for the three trophic levels. The lowest chronic NOEC/EC10 for fish, crustacean or algae is the 21-d NOEC of 200 µg/L observed for the crustacean Daphnia magna resulting in a self-classification of octanoic acid as Aquatic Chronic 3 according to Table 4.1.0 (b) (ii) of Regulation (EC) No 1272/2008. The self-classification of octanoic acid is in line with the harmonised classification of octanoic acid (CAS 124-07-2) as Aquatic chronic 3 (Annex VI of Regulation (EC) 1272/2008). The 21-d NOEC of 200 µg/L octanoic acid used as ecotoxicity reference value would result in a chronic ecotoxicity reference value of 256.4 µg/L for octanoic acid, zinc salt, basic based on a maximum octanoic acid content of 78 % meeting also classification criteria as Aquatic Chronic 3 according to Table 4.1.0 (b) (ii).

The lowest chronic aquatic ecotoxicity reference value for the assessment entity “Zinc” was observed for the algal species P. subcapitata (19 µg Zn/L) at neutral pH, which corresponds to a chronic ecotoxicity reference value of 79.2 µg/L octanoic acid, zinc salt, basic based on a maximum zinc content of 24 %. The long-term (chronic) aquatic hazard assessment is thus based on the most toxic moiety, i.e. the zinc cation. Octanoic acid, zinc salt, basic would meet based on i) the lowest chronic aquatic ecotoxicity reference value observed for the algae Pseudokirchneriella subcapitata (19 µg Zn/L) at neutral pH; ii) the zinc content of octanoic acid, zinc salt, basic of 24 %, and iii) the resulting chronic ecotoxicity reference value of 79.2 µg/L octanoic acid, zinc salt, basic, classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008.

Octanoic acid, zinc salt, basic is readily biodegradable according to results of an OECD 301F test. Nevertheless, the chronic ecotoxicity reference value of 79.2 µg/L is compared with the criteria for long-term aquatic hazard classification, taking into account if zinc, the toxic moiety of concern is considered rapidly degradable. The concept of “Degradability” was developed for organic substances and is not applicable to inorganic zinc substances. As a surrogate approach for assessing “degradability”, the concept of “removal from the water column” was developed to assess if a metal ion would remain present in the water column upon addition (and thus be able to exert a chronic effect) or would be rapidly removed from the water column. In this concept, “rapid removal” (defined as >70% removal within 28 days) is considered as equivalent to “rapidly degradable”. The rapid removal of zinc from the water column is documented in the section „Environmental fate“. Consequently, zinc is considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. Based on the chronic ecotoxicity reference value of 79.2 µg/l, octanoic acid, zinc salt, basic would meet classification criteria of long-term aquatic hazard Category 2 in accordance with Table 4.1.0 (b) (ii) of Regulation (EC) No 1272/2008.  

Altogether, based on reliable, adequate and relevant studies, including the substance-specific algae toxicity data according to OECD 201 and read-across to aquatic toxicity data of octanoic acid and soluble zinc substances, octanoic acid, zinc salt, basic meets classification criteria of Regulation (EC) No 1272/2008 as Acute (short-term) aquatic hazard Category 1 with an acute M-Factor of 1 and Chronic (long-term) aquatic hazard Category 2.