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Toxicity to microorganisms

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Key value for chemical safety assessment

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In accordance with Column 2 of REACH Annex VIII, the activated sludge respiration inhibition study (required in Section 9.1.4) does not need to be conducted as the substance is readily biodegradable and the applied test concentrations are in the range that can be expected in the influent to a sewage treatment plant.

Trizinc dicitrate will dissociate in solution to the metal ion and citrate anion (H7C6O7 -), so exposure of microorganisms will be to the citrate and the metal ions separately. The effect of trizinc dicitrate is not expected to significantly differ from the effects of metal ion and citrate separately. While the toxicity of citric acid is considered to be negligible (toxicity threshold value of >10000 mg/l), zinc toxicity (EC50) has been found to be 5.2 mg/l (cited from zinc metal RAR 2008). Therefore, the toxicity of trizinc dicitrate will be driven by the toxicity of zinc.

It is possible to extrapolate the data from zinc to trizinc dicitrate by converting the zinc results taking into consideration the molecular weight of both substances*. The EC50 value of 5.2 mg Zn/l is equivalent to an EC50 value of 15.08 mg/l trizinc dicitrate. This value was obtained by multiplying with a conversion value of 2.9*.

* Molecular weight of trizinc dicitrate/(molecular weight of zinc)*3 = 547/(65*3) = 2.9

The EC50 for zinc metal was obatained from set of data on toxicity tests with bacterial and protozoans.The available toxicity data were subjected to pair review and the most reliable data from the peer reviwed published study (Zinc metal RAR 2008) has been selected as key study. The bacterial studies gave an EC50 value ranging from 0.74 mg/l to 900 mg/l. In another study conducted using bacterium Pseudomonas putida, an EC10 values of 1.8 and 0.3 mg/l was obtained (Zinc RAR 2008). The toxicity test results (LC50) of zinc on protozoans ranges from 0.25 mg/l to 50 mg/l.