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Toxicity to aquatic algae and cyanobacteria

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Description of key information

(72 h) NOEC 0.015 mg/L (r-a) P. subcapitata

Key value for chemical safety assessment

Additional information

No data on toxicity of trizinc dicitrate (CAS 546-46-3) to algae are available. A 72 h algae study is available for the parent acid of trizinc dicitrate, citric acid 77-92 -9 with Scendesmus quadricauda, showing the low toxicity of the parent acid (8 d NOEC 425 mg/L, estimated, Bringmann and Kuhn 1982, rel. 2, see the CSA for citric acid for further details). Additionally, studies with citric acid and algae have been waived and not taken into consideration when assessing the hazard of citric acid since citric acid is a complexing agent and complexes essential nutrients, namely zinc, in solution (see CSA for citric acid for further details).

However, trizinc dicitrate in solution is expected to behave in the same manner as the parent acid and zinc counter-ion separately under the conditions of the test. The toxicity of citric acid 77-92-9 to algae is negligible, while the toxicity of zinc ion to algae is a 72 h NOEC value of 0.005 mg/L (De Schamphaere et al. 2003, cited in Zinc RAR 2008); therefore the toxicity of trizinc dicitrate 546-46-3 is going to be driven by the zinc counterion and the assessment of this substance should be based on the toxicity of zinc.

The toxicity of zinc to aquatic organisms has been reviewed in zinc metal RAR (2008). The authors of the Zinc metal RAR (2008) have reviewed numerous studies available for zinc and selected reliable studies that fulfil the OECD criteria for validity. The reliable studies in the zinc metal RAR (2008) determine a range of NOECs for algae spp. in the range 0.005 to 0.124 mg/L (as dissolved zinc).

Based on consideration of the available data for soluble zinc salts, a key NOEC value for invertebrates of 0.005 mg Zn/L (based on dissolved zinc) has been selected since this is the lowest reliable NOEC value available in the Zinc metal RAR (2008). The NOEC was obtained in a 72 h study of the effects of Zinc chloride (CAS 7646-85-7).

By taking into consideration the NOEC of zinc ions and neglecting the citric acid NOEC, the 72 h NOEC for Trizinc dicitrate with Pseudokirchneriella subcapitata is 0.015 mg/L.

Calculations:

The 72 h NOEC for Zinc is 0.005 mg/L (Zinc RAR 2008), which is equivalent to a 72 h NOEC value of 0.08 * 2.93 = 0.015 mg/L of Trizinc dicitrate

[using MW(trizinc dicitrate)/MW(zinc*3) = 574.37/(65.38*3) = 2.93]

 

Considerations:

It is known that abiotic factors can greatly influence the toxicity of Zn and therefore the toxicity of trizinc dicitrate to aquatic species. For the purpose of this assessment some specific factors can be singled out.

 

Hardness

This is the most studied and important parameter. With increasing hardness Zn becomes less bioavailable and thus less toxic because the zinc ions form insoluble complexes with Ca and Mg ions. In a chronic study by Paulauskis and Winner (1988) with D. magnait was demonstrated that by increasing the total water hardness as CaCO3 y 50 to 200 mg/L, the NOEC for reproduction increased 6 -fold, i.e. a decrease in toxicity. The NOEC for survival was less affected

 

pH

Similarly to hardness, with increasing pH the toxicity of Zn towards aquatic organisms decreases since Zn becomes less bioavailable by binding to H+ ions. In a study from De Schamphelaere et al. (2003) it was demonstrated that pH changed the toxicity to invertebrates by a factor of 3 to 4 with D. magna, by a factor of 2 to 3 with rainbow trout Oncorhynchus mykiss, and by a factor of >20 with the algae Pseudokirchneriella subcapitata. However a clear relationship between toxicity and pH could not be established with the exception of algae (Zinc metal RAR 2008).

Background concentration

The background concentration of Zn will influence the test organism's sucsceptibility to the metal. Higher background concentrations of zinc ensure that the animals are more tolerant to zinc, and therefore a decrease in toxicity is observed (e.g. Muyssen and Janssen, 2000 with D. magna).

 

Other abiotic factors

Other factors affecting the bioavailability and thus the ecotoxicity of Zn to aquatic organisms are the DOC concentration (by Zn binding with the organic carbon, Zinc metal RAR 2008) and alkalinity (by Zn binding with the carbonates, Zinc metal RAR 2008).

 

Conclusions:Hardness is the most understood and studied factor influencing the toxicity of zinc. pH and alkalinity are however also considered important factors. Since the factors are inter-related it is difficult to establish the extent to which each individual factor influences aquatic ecotoxicity of zinc, and therefore also of trizinc dicitrate (CAS 546-46-3). The Biotic Ligand Model (BLM) uses a more holistic approach to define the influence of abiotic factors towards bioavailability and zinc toxicity. With the BLM it is possible to reduce the bioavailability-uncertainty of zinc in a more site specific approach.It can be concluded that abiotic factors are important in determining the toxicity of zinc to the extent that some water quality bodies provide different PNECs according to water hardness. The OECD guidelines for ecotoxicity testing provide a wide range of acceptable pH (6-9), water hardness (24-250 mg/L as CaCO3) and background zinc concentrations (around 1 µg/L) for testing which cover most European waters and sediments. The zinc EC50 used in this endpoint summary has been taken from the Zinc RAR (2008) and is relevant to the OECD guideline testing parameter ranges; extremely low hardness water areas, such as those in Scandinavia, may not be covered though most European waters will be.The Zinc metal RAR (2008) have taken all of these factors into consideration when developing a PNEC.