Zinc m-toluate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

105.6 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

31.3 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

512.8 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

604.1 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

289.7 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:

182.6 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

The fate and toxicity of zinc m-toluate in the environment is most accurately evaluated by separately assessing the fate of its constituents zinc and m-toluate.

 

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion.Based on the solubility of zinc m-toluate in water (6.593 g/L at pH 7.47), a complete dissociation of zinc m-toluate resulting in zinc and m-toluate ions 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 zinc m-toluate could not be identified. Data for zinc appear to be generally limited. However, zinc tends 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. (2011) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as m-toluate 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 slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of m-toluic acid of 4.25 results in:

log KML= 0.301 * 4.25 + 0.015

log KML= 1.29 (estimated zinc m-toluate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated zinc-toluate formation constant, the respective behaviour of the dissociated zinc cations and m-toluate anions in the environment determine the fate of zinc m-toluate 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 the assessment of enviromental fate of zinc m-toluate, read-across to the assessment entities soluble zinc substances and m-toluic acid is applied since the ions of zinc m-toluate determine its environmental fate. Since zinc cations and m-toluate anions behave differently in the environment, including processes such as stability, degradation, transport and distribution, 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.

In order to evaluate the environmental fate and toxicity of the substance zinc m-toluate, information on the assessment entities zinc cations and m-toluate 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 zinc m-toluate.

 

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.

Conclusion on classification

Aquatic toxicity studies with zinc toluate are not available. The fate and toxicity of zinc m-toluate in the environment is most accurately evaluated by separately assessing the fate of its moieties zinc and m-toluate. Zinc m-toluate dissolves and dissociates into zinc and m-toluate ions upon contact with an aqueous medium. Therefore, the aquatic hazard potential is assessed based on the toxicity data available for the assessment entities zinc and m-toluate ions since the ions of zinc m-toluate determine its environmental fate and toxicity.

Acute (short-term) toxicity: EC/LC50 values of 3 trophic levels (algae, invertebrates and fish) range for m-toluate from 17.87 mg/L to 81.40 mg/L m-toluate. 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. The acute aquatic hazard assessment is based on the most toxic moiety, i.e. the zinc cation, and acute ecotoxicity reference values of zinc are recalculated for zinc m-toluate based on a maximum zinc content of 19.5 %.

Zinc m-toluate 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 m-toluate of 19.5 %, and iii) the resulting acute ecotoxicity reference value of 697.4 µg/L zinc m-toluate 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.

 

Chronic (long-term) toxicity: For m-toluate, NOEC/EC10 values of 2 trophic levels (algae and invertebrates) range from 9.63 mg/L to 9.93 mg/L m-toluate. According to the QSAR-based outcome of the model ECOSAR v.2.0, m-toluate has a very low potential for chronic toxicity to freshwater and saltwater fish with respective chronic values (ChV = 10^([log (LOEC x NOEC)]/2)) well above 10 mg/L.

The long-term (chronic) aquatic hazard assessment is based on the most toxic moiety, i.e. the zinc cation, and chronic ecotoxicity reference values of zinc are recalculated for zinc m-toluate based on a maximum zinc content of 19.5 %.

Zinc m-toluate meets 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 maximum zinc content of zinc m-toluate of 19.5 %, and iii) the resulting chronic ecotoxicity reference value of 97.4 µg/L zinc m-toluate, classification criteria of long-term aquatic hazard of Regulation (EC) No 1272/2008.

The chronic ecotoxicity reference value of 97.4 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 due to the ready biodegradability of m-toluate, 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 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 97.4 µg/l, zinc m-toluate 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.