Zinc fluoride

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.03 mg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.01 mg/L

Assessment factor:

1

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.16 mg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

78.9 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

78.9 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

56.3 mg/kg soil dw

Assessment factor:

1

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

14.7 mg/kg food

Assessment factor:

30

Additional information

Read-across justification

Zinc difluoride – general considerations:

Zinc difluoride is an inorganic solid at room temperature and consists of zinc cations and fluoride anions. Based on the solubility of zinc difluoride tetrahydrate in water (15.5 g/L at 25°C) according to handbook data (CRC handbook, 2008), a complete dissociation of zinc difluoride resulting in zinc and fluoride ions may be assumed under environmental conditions. 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.

 

The metal-ligand equilibrium constant for the formation of zinc difluoride tetrahydrate is reported as follows (Clever, et al., 1991 and references therein):

Zn²⁺+ 2F^-+ 4 H₂O (l) <=> ZnF₂*4H₂O (logK =3.34; K = 4.45*10^-4mol³kg^-3)

Zn²⁺+ 2F^-<=> ZnF₂(logK =1.5; K = 3.0*10^-2mol³kg^-3)

 

Thus, it may reasonably be assumed that based on the zinc difluoride tetrahydrate formation constant, the respective behavior of the dissociated zinc cations and fluoride anions in the environment determines the fate of zinc difluoride 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 determine its ecotoxicological potential.

 

Zinc(II):

Read-across to environmental fate and toxicity studies of different inorganic zinc substances, including zinc dichloride is appropriate and scientifically justified.This read-across approach was already applied in the EU Risk Assessment of inorganic zinc substances (2007).

 

Zinc is a natural element, which is essential for all living organisms. It occurs in the metallic state, or as zinc compound in one stable valency state (Zn²⁺). The environmental concentration of Zn in solutions is controlled rather by adsorption to clay minerals Fe, Mn Al hydroxides and organic matter than by solubility of Zn carbonates, hydroxides and phosphates. Zinc mobility in the environment is greatest under oxidising, acidic conditions and more restricted under reducing conditions. Below a solution pH of 7.5–8.0, zinc occurs predominantly in the Zn2+ form. At higher solution pH, zinc forms low solubility complexes with carbonate and hydroxyl ions. Under reducing conditions as found in sediments, particularly in conjunction with low pH, sphalerite (zinc sulfide) may form. Zinc may exhibit amphoteric behaviour at a pH > 11, but these conditions are rarely found in nature. It is rapidly sorbed to secondary oxides, clay minerals and organic matter in all but the most acid conditions (pH < 4.5) (Salminen et al. 2005 and references therein).

 

All environmental concentration data are expressed as “Zn” and environmental toxicity would be caused by zinc ions. Thus, ecotoxicity of soluble zinc substances are applicable to all zinc compounds that release zinc ions into the environment.

 

When zinc ions are released into the environment, they further interact with the environmental matrix and biota. The concentration of zinc ions available to organisms, i.e. the bioavailable fraction, depends on processes such as dissolution, adsorption, precipitation, complexation, inclusion into (soil) matrix, etc. These processes are defining the fate of zinc in the environment and, ultimately, its ecotoxic potential.

Fluoride:

Fluoride is a natural element. All environmental concentration data are expressed as “F”, and environmental toxicity (if any) would be caused by fluoride ions. Thus, ecotoxicity of soluble fluoride substances are applicable to all fluoride compounds that release fluoride ions into the environment. Further, read-across to environmental fate and toxicity studies of soluble fluoride salts (predominantly sodium fluoride) and acid is appropriate and scientifically justified. This read-across approach was already applied in the EU Risk Assessment of hydrogen fluoride (2001)as follows: “All reported tests with aquatic organisms were performed with NaF. Because HF occurs in the aquatic compartment mainly as fluoride ion, the NaF tests can be used for the evaluation of HF effects in aquatic organisms. All reported test results were corrected for the fluoride ion.”

 

The transport and transformation of fluoride in soil are influenced by pH and the formation of predominantly aluminium and calcium complexes. Adsorption to the soil solid phase is stronger at slightly acidic pH values (5.5–6.5). Fluoride is not readily leached from soils.

 

In solution, fluoride ions form strong complexes with other ions, particularly Ca2+, Al3+, Fe3+, PO43-and B(OH)4-. The concentration of fluoride ions in solution is often controlled by the solubility of fluorite, and the concentration inversely proportional to that of Ca2+. Fluoride also sorbs to mineral surfaces such as gibbsite, kaolinite, halloysite, and freshly precipitated amorphous Al(OH)3. Sorption to these solid phases may be favored at lower pH. Fluorine, however, is an essential micronutrient for mammals, serving to strengthen the apatite matrix of skeletal tissues and teeth (Salminen et al. 2005 and references therein).

 

The behaviour of the dissociated fluoride ions in the environment determines the fate of fluoride upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.

 

In sum, upon release to the environment and dissolution in aqueous media, zinc difluoride dissociates and is only present in its dissociated form, i.e. zinc cations and fluoride anions, and ecotoxicity (if any) is driven by zinc cations and fluoride anions. Therefore, relevant ecotoxicity data of soluble inorganic zinc substances and soluble inorganic fluoride salts (mainly sodium fluoride) and acid are read-across to assess the ecotoxicity of zinc difluoride (tetrahydrate) on a conservative basis. All read-across substances used for the assessment of zinc difluoride are more soluble (≥ 41 g/L) than zinc difluoride tetrahydrate (15.5 g/L). Since, the environmental toxicity of zinc difluoride is based on the concentration of zinc cations and fluoride anions in solution, read-across is considered to be conservative and unrestricted read-across is fully justified with regard to environmental fate and toxicity. In conclusion read-across to fluoride and soluble inorganic zinc substances based on solubility is fully justified.

Conclusion on classification

For the assessment of the environmental hazard potential of zinc difluoride, the assessment entity approach is applied and data for fluoride and soluble zinc substances are read-across since only the ions of zinc difluoride are available in an aqueous environment and determine the toxicity.

Based on aquatic toxicity data of its moieties, i.e. zinc and fluoride, zinc difluoride appears to be toxic to aquatic organisms. Short-term toxicity data of zinc are available for freshwater organisms covering three trophic levels. The respective reliable IC/LC/EC50 values are below 1 mg/L. Hence, classification and labelling for aquatic acute toxicity were based on the lowest value determined.

Based on the EU RAR "hydrogen fluoride" (2001), EC50-values of fluoride toxicity for algae range from 43 - 122 mg/L, for daphnids from 97 - 352 mg/L, and for fish from 51 - 340 mg/L.

The lowest reported EC50 values are applied in the hazard assessment. Thus, short-term toxicity data covering three trophic levels are available for zinc and fluoride and are summarized as follows:

Table: Short-term toxicity data of zinc and fluoride for freshwater organisms

Trophic level	lowest EC value for zinc	lowest EC value for fluoride
Algae	72-h IC50 Pseudokircherniella subcapitata, neutral/high pH 0.136 mg Zn/L (Van Ginneken, 1994) 0.215 mg ZnF2/L	96-h EbC50 43 mg F/L 234 mg ZnF2/L
Daphnia	EC50 (mobility) Ceriodapnia dubia; neutral/high pH 0.147 mg Zn/L (geomean value) 0.232 mg ZnF2/L	48-h EC50 97 mg/L 528 mg ZnF2/L
Fish	96-h LC50 Oncorhynchus mykiss, neutral/high pH 0.169 mg Zn/L (Buhl, 1990) 0.267 mg ZnF2/L	96-h LC50 51 mg/L 278 mg ZnF2/L

 

Aquatic toxicity data of zinc and fluoride when expressed as zinc difluoride are available for algae, daphnia and fish; respective IC/EC/LC50 values are below 1 mg/L. Therefore, zinc difluoride meets classification criteria as short-term hazard to the aquatic environment under Regulation (EC) No 1272/2008 and subsequent adaptations and should be labelled as

“acute aquatic hazard Cat. Acute 1”

Based on aquatic toxicity data of its moieties, i.e. zinc and fluoride, zinc difluoride appears to have a medium potential for long-term toxicity to aquatic organisms.

Since the dossier for zinc difluoride covers only regulation (EC) No 1907/2006 Annex VIII data, chronic data for invertebrates as well as chronic fish are not available. However, based on the EU RAR, 2010 the algae (i.e.,Pseudokircherniella subcapitata) is the most sensitive species for zinc and used for further hazard assessment.

Based on the EU RAR “hydrogen fluoride” (2001), a long-term NOEC-value is available for fish; i.e. 21-d LC5 of 4 mg/L. For daphnia, NOEC-values of fluoride toxicity range from 3.7 to 14.1 mg/L and for algae from 50 - 249 mg/L. The lowest reported NOEC values are applied in the hazard assessment.

Thus, long-term toxicity data covering three trophic levels are available for zinc and fluoride and are summarized as follows:

Table: Long-term toxicity data of zinc and fluoride for freshwater organisms

Trophic level	lowest EC value for zinc	lowest EC value for fluoride
Algae	NOErC 0.019 mg Zn/LPseudokircherniella subcapitata; geomean of 27 data) 0.030 mg ZnF2/L	7-d NOEbC 50 mg F/L 272 mg ZnF2/L
Daphnia	Annex IX data EU RAR, 2010 NOEC 0.088 mg Zn/L Daphnia magna, geomean of 27 data 0.139 mg ZnF2/L	21-d NOEC (reproduction) 3.7 mg F/L 20 mg ZnF2/L
Fish	Annex IX data EU RAR, 2010 NOEC 0.189 mg Zn/L Oncorhynchus mykiss, geomean of 15 data 0.299 mg ZnF2/L	21-d NOEC 4.0 mg F/L 22 mg ZnF2/L

 

Taking into account the lowest chronic ecotoxicity value observed on a wide variety of species of different taxonomic groups (19 µg Zn/L, see table above), chronic aquatic toxicity classification of the substance in accordance to regulation (EC) No 1272/2008 is

“Long-term aquatic hazard Cat. Chronic 2”

The chronic ecotoxicity reference value for the substance is 30 ZnF2/L. This value should be compared with the criteria for chronic classification, also taking into account whether the substance is considered rapidly degradable or not.

However, 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 excerpt 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 section 4.6., 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. 

Considering this, in combination with the abovementioned chronic ecotoxicity reference value for zinc difluoride of 30 µg/L, the classification of the substance for chronic aquatic effect should be “Long-term aquatic hazard Cat. Chronic 2” (H411).