Strontium

Endpoint summary

Description of key information

Read-across statement:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2 is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13)

Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation. 

 

The solubility of strontium is not greatly 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). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pH. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO₄forms. 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. 2015 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. 2015). 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.

 

Thus, the chemical safety assessment is based on elemental strontium concentrations and read-across of environmental fate and toxicity data available for soluble strontium substances as well as monitoring data of elemental strontium concentrations in the environment. The reliable data selected for the assessment of environmental fate of strontium, including adsorption/desorption and bioconcentration/bioaccumulation, are based on elemental strontium concentrations of water, soil, sediment, and biota.

 

Abiotic degradation

The endpoints "Phototransformation of an element in water, soil or air" are not relevant for substances that are assessed using a read-across approach on an elemental basis, i.e., based on the exposure and effects of strontium, expressed as elemental strontium.

 

The term ‘Hydrolysis’ refers to the “Decomposition or degradation of a chemical by reaction with water”, and this as a function of pH (i. e., abiotic degradation). The chemical safety assessment of strontium is based on elemental strontium concentrations, i.e., the assessment of strontium is conducted regardless of the (pH-dependent) speciation in the environment. Hence, as the assessment is based on the elemental concentration, physico-chemical processes such as decomposition and degradation by reaction with water are not relevant. In general, (abiotic) degradation is an irrelevant process for inorganic substances that are assessed on an elemental basis.

 

Biotic degradation

For an inorganic substance such as strontium 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, regardless of the environmental compartment that is under consideration: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the environmental 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.

 

Bioacumulation

Bioconcentration and bioaccumulation of strontium appears to be negligible in aquatic and terrestrial organisms. Limited information on transfer of Sr through the aquatic food chain indicates that strontium does also not biomagnify.

 

Adsorption / desorption

Partition coefficients for different environmental compartments (soil, sediment, suspended particulate matter) have been derived for strontium, based on literature data and the FOREGS monitoring survey as follows: soil: 157.03 L/kg; sediment: 861.2 L/kg, suspended particulate matter: 1291.8 L/kg