Zinc fluoride

Endpoint summary

Description of key information

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 its fate.

Abiotic degradation: Physico-chemical processes other than dissolution reactions are not considered relevant for zinc difluoride since the chemical safety assessment of inorganic substances is typically based on total dissolved elemental concentrations without considering the (pH-dependent) speciation in the environment. Thus, (abiotic) degradation is not a relevant process for zinc and fluoride and the chemical safety assessment is based on total elemental concentrations.

Biodegradation: For an inorganic substance such as zinc difluoride for which the chemical assessment is based on the elemental concentration (i.e., pooling all inorganic speciation forms together), biotic degradation is an irrelevant process: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the terrestrial compartment by degradation or transformation. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.

Transport and distribution: 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.

Data available for zinc suggests that zinc does not bioaccumulate. For metals, the transport and distribution over the different environmental compartments e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) is described and quantified by the metal partition coefficients between these different fractions (see IUCLID 5.4.1).

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.