Zinc hydroxide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

21.9 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

10.9 µg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

152 µg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

223.3 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

246.6 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

126.3 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

A basic assumption made in this hazard assessment and throughout this CSR, (in accordance to the same assumption made in the EU RA process) is that the ecotoxicity of zinc and zinc compounds is due to the Zn²⁺ ion. As a consequence, all aquatic, sediment and terrestrial toxicity data in this report are expressed as “zinc”, not as the test compound as such, because ionic zinc is considered to be the causative factor for toxicity. A further consequence of this is that all ecotoxicity data obtained on different zinc compounds, are mutually relevant for each other. For that reason, the available ecotoxicity databases related to zinc and the different zinc compounds are combined before calculating the PNECs. The only way zinc compounds can differ in this respect is in their capacity to release zinc ions into (environmental) solution. That effect is checked eventually in the transformation/dissolution tests and may result in different classifications.

An elaborated justification for this read-across approach is attached in IUCLID section 13.

Conclusion on classification

Zinc hydroxide does not have a harmonized classification under the European CLP Regulation ((EC) No 1272/2008).

The ecotoxicity of Zn compounds is attributed to that of the Zn²⁺ cation and thus depends on the solubility of the compound (i.e. its capacity to release the Zn²⁺ cation). The following justification is given for classifying Zn hydroxide:

Classification for acute aquatic effects

Considering a) the lowest acute aquatic ecotoxicity (EC50) values of 41.4 µg Zn/l and 154 µg Zn/l for the zinc ion at pH 8 and 6 respectively, and b) the molecular weight ratio of zinc hydroxide versus the Zn²⁺ ion, resulting in an acute ecotoxicity reference value of 41.4 x MW (Zn(OH)₂/ MW (Zn) = 62.9 µg substance /l at pH 8 (as worst case), zinc hydroxide is classified "Acute 1" (H400: Very toxic to aquatic life), with M-Factor 10.

Classification for chronic aquatic effects

Considering a) the lowest chronic aquatic ecotoxicity values of 11 µg Zn/l and 99 µg Zn/l for the zinc ion at pH 8 and 6 respectively, and b) the molecular weight ratio of zinc hydroxide versus the Zn²⁺ion, results in a chronic ecotoxicity reference value of 11 x MW (Zn(OH)₂/ MW (Zn) = 16.7 µg substance /l at pH 8 (as worst case) for zinc hydroxide.

For determination of the chronic aquatic effects classification according to the 2^ndATP CLP criteria, it has to be considered further if the substance is rapidly degradable or not. The concept of “Degradability” was developed for organic substances and is not applicable as such 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 excert a chronic effect) or would be rapidly removed from the water column. In this concept, “rapid removal from the water column” (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 and zinc compounds are considered as equivalent to being ‘rapidly degradable” in the context of classification for chronic aquatic effects. 

Considering the chronic ecotoxicity reference value for Zn(OH)₂of 16.7 µg/l, and considering zinc and its compounds as equivalent to being rapidly degradable, the classification of the substance for chronic aquatic effect is “Chronic 2” (H411: Toxic to aquatic life with long lasting effects).

In conclusion, zinc hydroxide is classified for hazards to the aquatic environment as H400 ("Acute 1") and H411 ("Chronic 2"). The M-factor for acute aquatic effects is 10.

General discussion

A basic assumption made in this hazard assessment and throughout this CSR, (in accordance to the same assumption made in the EU RA process) is that the ecotoxicity of zinc and zinc compounds is due to the Zn²⁺ ion. As a consequence, all aquatic, sediment and terrestrial toxicity data in this report are expressed as “zinc”, not as the test compound as such, because ionic zinc is considered to be the causative factor for toxicity. A further consequence of this is that all ecotoxicity data obtained on different zinc compounds, are mutually relevant for each other. For that reason, the available ecotoxicity databases related to zinc and the different zinc compounds are combined before calculating the PNECs. The only way zinc compounds can differ in this respect is in their capacity to release zinc ions into (environmental) solution. That effect is checked eventually in the transformation/dissolution tests and may result in different classifications.