Zinc dioctanoate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

108.4 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

32.1 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

526.3 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

620 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

297.4 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:

187.4 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 zinc dioctanoate in water, a complete dissociation resulting in zinc and octanoate 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 octanoate anions in the environment determine the fate of zinc dioctanoate 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.

A metal-ligand complexation constant of zinc dioctanoate could not be identified. Data for zinc appear to be generally limited. However, zinc 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 octanoate 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 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 zinc dioctanoate formation constant the respective behaviour of the dissociated zinc cations andoctanoateanions in the environment determines the fate of zinc dioctanoate upon dissolution. In the assessment of environmental fate and toxicity of zinc dioctanoate, read-across to analogue substances and/or the assessment entities soluble zinc substances and octanoic acid is applied since the ions of zinc dioctanoate determine the environmental fate and toxicity of zinc dioctanoate. Since zinc ions and octanoate ions behave differently in the environment, a separate assessment of the environmental fate and toxicity 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 zinc dioctanoate.

Conclusion on classification

Aquatic toxicity studies with zinc dioctanoate are not available. In the assessment of zinc dioctanoate, read-across to analogue substances and/or the assessment entities soluble zinc substances and octanoic acid is conservatively applied since the ions of zinc dioctanoate determine its fate and toxicity in the environment. Reliable data indicate that the moiety of ecotoxicological concern are zinc cations. Toxicity data of octanoic acid point to low potential for acute (short-term) aquatic toxicity to algae and invertebrates. According to the QSAR-based model EPI Suite Kowwin v1.68 (experimental database match), octanoic acid has a low potential for bioaccumulation since the estimated log Kow amounts to 3.05. Since octanoate is readily biodegradable and does not have a log Kow ≥4, the potential for long-term aquatic toxicity is also expected to be low. Thus, the ecotoxic potential of the fatty acid chain, i.e. octanoic acid, is assumed to be negligible. Fatty acids are generally not considered to represent a risk to the environment, which is reflected in their exclusion from REACH registration requirements (c.f. REACH Annex V (Regulation (EC) No 987/2008)). Consequently, the aquatic hazard assessment is based on the most toxic moiety, i.e. the zinc cation, and acute and chronic ecotoxicity reference values of zinc are recalculated for zinc dioctanoate based on a maximum zinc content of 19 %.

Zinc dioctanoate meets 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 zinc dioctanoate of 19 %, and iii) the resulting acute ecotoxicity reference value of 715 µg/L zinc dioactanoate at pH 8 as worst case, classification criteria of acute (short-term) aquatic hazard Category 1 of Regulation (EC) No 1272/2008 with an acute M-Factor of 1.

Further, zinc dioctanoate meets based on i) the lowest chronic aquatic ecotoxicity reference value of zinc observed for the algae Pseudokircherniella subcapitata (19 µg Zn/L) at neutral pH; ii) the maximum zinc content of zinc dioctanoate of 19 %, and iii) the resulting chronic ecotoxicity reference value of 100 µg/L zinc dioctanoate as worst case, classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008.

The chronic ecotoxicity reference value of 100 µg/L is compared with the criteria for long-term aquatic hazard classification, taking into account whether the zinc, the only toxic moiety of concern, is considered rapidly degradable or not.

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 whether or not a given metal ion would remain present in the water column upon addition (and thus be able to excert 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 sectopn „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 100 µg/l, zinc dioctanoate 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.

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

In sum, applying a conservative approach, based on reliable, adequate and relevant studies, zinc dioctanoate classifies as Aquatic hazard, category Acute 1 and Chronic 2 according to Regulation (EC) No 1272/2008.