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Hazard for aquatic organisms

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Read-across approach

In the assessment of the ecotoxicity ofzinc dibenzoate, a read-across approach from data for the metal cation and the organic anion is followed. This read-across strategy is based upon the assumption that upon release to the environment and dissolution in aqueous media, zinc dibenzoate will dissociate and only be present in its dissociated form, i.e. as zinc cation and benzoate anions.

Upon dissolution in water, it is indeed predicted that metal carboxylates dissociate completely into the metal cation and the organic anion at environmentally relevant conditions. Predictions of stability of zinc debenzoate in a standard ISO 6341 medium (2 mMCaCl2, 0.5 mM MgSO4, 0.77 mM NaHCO3and 0.077 mM KCl, pH 6 and 8) clearly show that monodentate ligands such as benzoic acid have no potential for complexing zinc ions in solution (Visual minteq. Version 3.0, update of 18 October 2012.

The fate and behaviour (e.g. partitioning) in the environment for Zn2+and benzoate anions are predicted to be significantly different from each other, resulting in a different distribution over the environmental compartments (water, air, sediment and soil). Because the relative exposure to both constituent ions is hence predicted to be different from the original composition ofzinc dibenzoate, data for the ecotoxicological properties ofzinc dibenzoateas such are considered less relevant for effects and risk assessment and a read-across approach to separate data for both the zinc cation and benzoate anion is preferred.

For most metal-containing compounds, it is the potentially bioavailable metal ion that is liberated (in greater or lesser amounts) upon contact with water that is the moiety of ecotoxicological concern. The solubility ofzinc dibenzoate(see IUCLID section 4.8 or chapter 1.3 of the CSR) is above the range of effects concentrations for dissolved zinc in the aquatic environment (lowest acute and chronic reference values: 136 and 19 µg Zn/L, respectively, PNECfreshwaterfor Zn = 20.6 µg Zn/L) and therefore ecotoxicity data for soluble zinc salts can be directly used in a read-across approach forzinc dibenzoate. As a conservative approach also the ecotoxicological properties of the carboxylic acid are considered.

According to the REACH Guidance on information requirements and chemical safety assessment, chapter B.8 Scope of exposure assessment, an environmental exposure and risk assessment is mandatory for a substance if it is classified as hazardous to the aquatic environment or if it has another classification and an aquatic PNEC can be derived. The threshold for PNEC derivation is not reported in the guidance, and was set at the limit test concentration for acute toxicity tests with fish, daphnids and algae, i.e. 100 mg/L. Therefore if a substance is not classified as dangerous for the aquatic environment, but meets the criteria for at least one of the other hazard classes or categories and has L(E)C50 values < 100 mg/L, it was still considered for the environmental exposure assessment.

For zinc dibenzoate, both the Zn2+ion and benzoic acid are considered for the environmental exposure and risk assessment because zinc is classified as hazardous to the aquatic environment (as Aquatic Acute 1, Aquatic Chronic 1) while benzoic acid is classified as Skin Irrit. 2 and Eye Dam. 1 and has some L(E)C50 values < 100 mg/L. In case both moieties require a risk assessment, the dose additivity approach is used to explain the ecotoxicological effects of the metal carboxylate based on the data for the individual moieties. As stated in a toxicity assessment of chemical mixtures opinion for the European Commission (Scientific Committee on Consumer Safety (SCCS), Scientific Committee on Health and Environmental Risks (SCHER), and Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). 2011. Preliminary opinion on Toxicity and Assessment of Chemical Mixtures;, the dose/concentration addition method should be preferred over the independent action approach if no mode of action information is available.

Ecotoxicological data forzinc dibenzoateare only available for a standard OECD 201 algae test withPseudokirchneriella subcapitata (Wenzel A, 2014). The algae test was selected because the zinc moiety is predicted to be the driver for toxic effects of zinc dibenzoate in the environment and algae are the most sensitive aquatic organisms to zinc.

Read-across to the corresponding toxicity data for effect of zinc and benzoic acid on algae growth rate and the dose additivity approach (based on the assumption of complete dissolution and a worst-case zinc content of 22% in zinc dibenzoate) results in a predicted ErC50 for zinc dibenzoate that is similar to the experimentally derived ErC50 for this substance (Eqn. 1; Table 1).

EC50ZnNeo = 1 / {(weight % Zn / EC50 Zn) + (weight % Neo / EC50 Neo)} (Eqn. 1)

It is therefore concluded that the read-across approach to the zinc and benzoic acid is conservative. The zinc moiety is the main driver for toxic effects of zinc dibenzoate to aquatic organisms and the ecotoxicity data for benzoic acid do not add significantly to the predicted toxicity for zinc dibenzoate.

Table 1: Acute toxicity data for effects of zinc dibenzoate and its moieties to aquatic organisms (only most sensitive species per trophic level).

 Trophic level


 Zinc dibenzoate (CAS: 553-72-0)

Benzoic acid (CAS: 65-85-0)

 Zinc ion


 72h ErC50

0.2 mg/L (experimental,  Pseudokirchneriella subcapitata)

0.62 mg/L (based on read-across to zinc only);

0.61 mg/L (based on read-across to both zinc and benzoic acid)

33.1 mg/L (Pseudokirchneriella subcapitata

 0.136 mg Zn/L (Pseudokirchneriella subcapitata)


 96h LC50

 No experimental data

 44.6 mg/L (Lepomis macrochirus)

47.3 mg/L (Oncorhynchus mykiss)

 0.169 mg Zn/L (Oncorhynchus mykiss)

 Aquatic invertebrates

 48h EC50

 No experimental data

 > 100 mg/L (Daphnia magna)

 0.147 mg Zn/L (Ceriodaphnia dubia)

Conclusion on classification

The classification as hazardous to the aquatic environment of zinc dibenzoate is based on a weight of evidence approach, taking into account the data for zinc dibenzoate itself and the classification of its moieties (zinc and benzoic acid):

• Only data for its effect on algae growth rate are available for zinc dibenzoate (ErC50 and ErC10 of 0.199 and 0,065 mg/L, respectively; Wenzel A, 2014). Algae are considered as the most sensitive aquatic organisms for toxicity of zinc dibenzoate because i) algae are the most sensitive aquatic organisms for zinc and ii) benzoic acid is less toxic to aquatic organisms compared to zinc. Therefore it is concluded that data on toxicity of zinc dibenzoate to fish and aquatic invertebrates are not critical for classification and the values of 0.199 and 0.065 mg/L are taken forward as the acute Exotoxicity Reference Value (ERVacute) and chronic Exotoxicity Reference Value (ERVchronic) for this substance, respectively.

• The ERVacute is < 1 mg/L and therefore does result in an acute 1 classification for zinc dibenzoate.

• The ERVchronic for zinc dibenzoate is ≤ 0.1 mg/L and hence results in a chronic 1 classification (M factor 1) for a substance that is not rapidly degradable. The zinc moiety is identified as the main driver for toxic effects of zinc dibenzoate to aquatic organisms and the ecotoxicity of benzoate is predicted not to add significantly to the predicted toxicity for zinc dibenzoate. The concept of “degradability” was developed for organic substances and is not applicable to inorganic substances like zinc. 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 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”. Under IUCLID section 5.6, the rapid removal of zinc from the water column is documented. Consequently, zinc is considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. Following this line of reasoning, it can be concluded that the corresponding chronic classification for zinc dibenzoate is aquatic chronic 2.

• The substance zinc dibenzoate has no official Annex VI classification and will dissociate into zinc and benzoate ions after dissolution in water and hence can be regarded as a mixture of both constituent ions. Zinc has an official Aquatic Acute 1 and Aquatic Chronic 1 classification (M factor 1; Annex VI of CLP Regulation EC No 1272/2008)), while benzoic acid is not classified for environment. For the reasons mentioned above, the zinc constituent is however considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. Considering this, in combination with the chronic ecotoxicity reference value for zinc of 19 µg/L, the classification of the zinc constituent for chronic aquatic effect should be “Aquatic Chronic 2”, rather than the previously mentioned official Aquatic Chronic 1 classification. Taking into account the weight of zinc in zinc dibenzoate (22% max.), the summation method results in an Aquatic Chronic 3 classification for zinc dibenzoate.

It is concluded that, based on the results of an Algae study and the concept of rapid removal from the water column, an Aquatic Acute 1, Chronic 2 classification for hazards to the aquatic environment is appropriate for zinc dibenzoate.