Strontium fluoride

Endpoint summary

Description of key information

For the assessment of the environmental hazard potential of strontium difluoride, the assessment entity approach is applied and data for fluoride and soluble strontium substances are read-across since only the ions of strontium 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 strontium 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 strontium and fluoride and the chemical safety assessment is based on total elemental concentrations.

Biodegradation: For an inorganic substance such as strontium 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 strontium suggests that bioconcentration and bioaccumulation of strontium is negligible. 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

Strontium difluoride – general considerations:

Strontium difluoride is an inorganic solid at room temperature and consists of strontium cations and fluoride anions. Based on the solubility of strontium difluoride in water (210 mg/L at 25°C g/L) according to handbook data (CRC handbook, 2008), a complete dissociation of strontium difluoride resulting in the release of strontium 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 strontium difluoride is reported as follows (CRC handbook, 2008):

Sr²⁺+ 2F^-<=> SrF₂(logK =8.4; K = 4.33*10^-9mol³kg^-3)

 

Thus, it may reasonably be assumed that based on the strontium difluoride formation constant, the respective behavior of the dissociated strontium cations and fluoride anions in the environment determines the fate of strontium 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.

 

Strontium(II):

The solubility of strontium is not affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Strontium(2+) cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high solution pH. In solutions with a pH below 4.5, Sr2+ ions are predominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2005 and references therein, Krupka et al. 1999. EPA 402-R-99-004B).

 

Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011.Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B).

 

In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2005). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium 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.

 

Regarding the partitioning in sediment-water, the assessment is based on Kd values derived from monitoring data for elemental Sr concentrations in stream water and corresponding sediments. The partition coefficient for soil was derived in a weight of evidence approach using published Kd values.

 

Fluoride:

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).

 

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 fluoride; 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 favoured at lower pH (Salminen et al. 2005 and references therein).

 

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.

 

Therefore, the assessment of the environmental fate of strontium difluoride is based on elemental strontium and fluoride concentrations. Read-across of environmental fate data available for soluble strontium substances and soluble fluoride substances is applied since released strontium ions and fluoride ions behave differently in the environment but determine the fate of strontium difluoride.