Fatty acids, C18-24, zinc salts

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

208.1 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

61.1 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1 010.1 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

1 189.9 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

570.7 mg/kg sediment dw

Assessment factor:

1

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:

359.6 mg/kg soil dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Metal carboxylates are salts consisting of metal cation and carboxylic acid anion. Based on the solubility of substance Fatty acids, C18-24, zinc salts in water, a complete dissociation upon dissolution (even if low based on conservative water solubility limit of 0.63 mg/L) resulting in zinc and fatty acids, C18-24 ions may be assumed under environmental conditions. The respective dissociation is in principle 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. However, under environmental conditions, a reunion of the dissociated ions is highly unlikely and it may reasonable be assumed that the respective behaviour of the dissociated zinc cations and fatty acids anions in the environment determine the fate of substance Fatty acids, C18-24, zinc salts 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 the (eco)toxicological potential.

In general terms, the stability constant of a metal complex can be calculated as follows: K = [ML] / [M][L], where K is the stability constant (expressed as a logarithm); M is the amount of metal ion such as Zn2+ ion, and L is the amount of a ligand such as fatty acids, C18-24 anions. The total concentration of metal CM can be computed with specialized computation programs. The basic equation CM = [M] + [ML] with [ML] = K [M] [L] becomes CM = [M] (1 + K [L]); hence [M] = CM / (1 + K [L]) shows that the concentration of the metal depends on the stability constant of the complex and free concentration of the ligand which is dependent upon corresponding pK and pH values. However, a metal-ligand complexation constant of Fatty acids, C18-24, zinc salts or a single salt of the UVCB e.g. zinc didocosanoate could not be identified. Data for zinc appear to be generally limited. However, zinc ions 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 docosanoate anions 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 Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of docosanoic acid of 4.95 results in:

log KML = 0.301 * 4.95 + 0.015

log KML = 1.51 (estimated metal-ligand formation constant for zinc-docosanoate, representing the main single salt of the UVCB Fatty acids, C18-24, zinc salts).

Thus, it may reasonably be assumed that based on the stability constant for zinc didocosanoate, the respective behaviour of the dissociated zinc cations and fatty acids, C18-24 anions under physiological conditions and in the environment determine the fate of Fatty acids, C18-24, zinc salts 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 (eco)toxicological potential.

Therefore, in the assessment of the (eco)toxicity of Fatty acids, C18-24, zinc salts, a read-across to data for fatty acids, C18-24 and soluble zinc substances is applied since only the ions of zinc and fatty acids, C18-24 are available in the environment and systemically and determine the (eco)toxicological potential.

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 concept Category approach for substance Fatty acids, C18-24, zinc salts".

Conclusion on classification

Substance Fatty acids, C18-24, zinc salts is poorly soluble in water (water solubility limit of 0.63 mg/L as conservative assumption), and thus its bioavailability in environmental compartments is expected to be low. The ecotoxicity data available for analogue substances indicate that the acute aquatic toxicity of zinc salts of shorter- and similar chained (C12-C18) fatty acids to algae, daphnia and fish is above the water solubility limit:

Algae: In a reliable study with the analogue substance zinc dilaurate (zinc salt of a C12-fatty acid), the EL10 and EL50 amount to 3.73 mg/L and 12.87 mg/L WAF for the acute toxicity to freshwater algae (Pseudokirchneriella subcapitata), respectively. The toxicity of a similar-chained (C16-18) fatty acids zinc salt to Pseudokirchneriella subcapitata is also low (EL10= 3.31 mg/L WAF, EL50 > 100 mg/L WAF fatty acids, C16-18, zinc salts). Thus, zinc salts of shorter- and similar-chained fatty acids appear to have a low potential for toxicity to freshwater algae. A similar low potential is assumed for substance Fatty acids, C18-24, zinc salts, a zinc salt of similar- and longer-chained fatty acids (C18-24) with a lower zinc content and solubility.

Invertebrates: The EC50 values for the acute toxicity of the analogue substances zinc dilaurate (zinc salt of a C12-fatty acid), zinc bis(12-hydroxyoctadecanoate) (zinc salt of a C18-fatty acid with an additional OH-group) and Fatty acids, C16-18, zinc salts to Daphnia magna are > 10 mg/L, > 1000 mg/L and > 100 mg/L, respectively, and thus above the water solubility limit of substance Fatty acids, C18-24, zinc salts of 0.63 mg/L. Thus, zinc salts of shorter- and similar-chained fatty acids appear to have a low potential for acute toxicity to freshwater invertebrates. A similar low potential is assumed for substance Fatty acids, C18-24, zinc salts, a zinc salt of similar- and longer-chained fatty acids (C18-24) with a lower zinc content and solubility.

Fish: The EC50 values for the acute toxicity of the analogue substances zinc dilaurate (zinc salt of a C12-fatty acid) and Fatty acids, C16-18, zinc salts to Danio rerio are > 10 mg/L and > 10,000 mg/L, respectively, and thus above the water solubility limit of substance Fatty acids, C18-24, zinc salts of 0.63 mg/L. Thus, zinc salts of shorter- and similar-chained fatty acids appear to have a low potential for acute toxicity to freshwater fish. A similar low potential is assumed for substance Fatty acids, C18-24, zinc salts, a zinc salt of similar- and longer-chained fatty acids (C18-24) with a lower zinc content and solubility.

Based on studies of zinc salts of shorter- and similar-chained (C12-C18) fatty acids, i.e. analogue substances, in which the acute toxicity to algae, invertebrates and fish is above 1 mg/L, substance Fatty acids, C18-24, zinc salts does not meet acute (short-term) aquatic hazard criteria of Regulation (EC) No 1272/2008.

The acute toxicity data further indicate that zinc salts of shorter- and similar-chained (C12-C18) fatty acids have a significantly lower aquatic toxicity than soluble zinc salts. It is therefore assumed that a high fraction of zinc from zinc salts of medium- and longer chained (>C12) fatty acids is not bioavailable.

Based on the hazard assessment of "Zinc" within the framework of Regulation (EC) No 1907/2006, ecotoxicity reference values for the zinc ion are:

Acute

-for low pH: 0.413 mg Zn/L (based on single lowest value for Ceriodaphnia dubia)

-for the neutral/high pH: 0.136 mg Zn/L (based on single lowest value for Pseudokirchneriella subcapitata)

Chronic

-for low pH: 0.082 mg Zn/L (Daphnia magna)

-for the neutral/high pH: 8.0: 0.019 mg Zn/L (Pseudokirchneriella subcapitata)

Thus, algae appear to be the most sensitive trophic level with regard to toxicity of zinc.

Substance Fatty acids, C18-24, zinc salts 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 maximum zinc content of substance Fatty acids, C18-24, zinc salts of 9.9 %, and iii) the resulting acute ecotoxicity reference value of 1373.7 µg/L substance Fatty acids, C18-24, zinc salts at pH 8 as worst case, does also not meet classification criteria of acute (short-term) aquatic hazard Category 1 of Regulation (EC) No 1272/2008.

Substance Fatty acids, C18-24, zinc salts would however meet based on i) the lowest chronic aquatic ecotoxicity reference value observed for the algae Pseudokircherniella subcapitata (19 µg Zn/L) at neutral pH; ii) the maximum zinc content of substance Fatty acids, C18-24, zinc salts of 9.9 %, and iii) the resulting chronic ecotoxicity reference value of 191.9 µg/L Fatty acids, C18-24, zinc salts as worst case, classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008. However, based on toxicity scores of the trophic level most sensitive to zinc, i.e. EL10 values of 3.31 and 3.73 mg/L for the algae Pseudokircherniella subcapitata , the analogue substances fatty acids, C16-18, zinc salts and zinc dilaurate (with zinc contents of 10% and 13%, respectively), and by read-across substance Fatty acids, C18-24, zinc salts (with a maximum zinc content of 9.9 %) do not require classification for long-term aquatic hazard.

In addition, the moieties of substance Fatty acids, C18-24, zinc salts are readily biodegradable (based on read-across of ready biodegradability observed for zinc salts of shorter-chained (C8) and similar-chained (C16-18) fatty acids in OECD 301 studies) or are removed from the water column. 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. 

The moieties of substance Fatty acids, C18-24, zinc salts are also not expected to bioaccumulate: bioaccumulation is not relevant for essential elements such as zinc due to homeostatic control mechanisms and fatty acids degrade by the ß-oxidation pathway.

It is therefore concluded that based on (i) studies of zinc salts of shorter- and similar-chained (C12-C18) fatty acids, i.e. analogue substances, in which the acute toxicity to algae, invertebrates and fish is above 1 mg/L, (ii) moieties of substance Fatty acids, C18-24, zinc salts being readily biodegradable or removed from the water column, (iii) moieties of substance Fatty acids, C18-24, zinc salts being not bioaccumulative, and (iv) the EL10 of 3.31 and 3.73 mg/L of analogue substances for algae, substance Fatty acids, C18-24, zinc does not require classification as long-term aquatic hazard in accordance with Figure 4.1.1 and Table 4.1.0 (b)(ii) and (iii) of Regulation (EC) No 1272/2008. 

In sum, substance Fatty acids, C18-24, zinc salts does not meet classification criteria of acute (short-term) and long-term aquatic hazard of Regulation (EC) No 1272/2008.

This conclusion is consistent with the conclusions from the EU risk assessment for the analogue substance zinc distearate conducted within the framework of EU Existing Chemicals Regulation (EEC) No 793/93 (Final report R074_0805_env, May 2008): "Zinc distearate is not classified for the environment."