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 pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. 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. 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.

 

Therefore, the assessment of the ecotoxicity of strontium is based on elemental strontium concentrations. Read-across of ecotoxicity data available for soluble strontium substances is applied since the strontium ions determine the ecotoxicological potential of strontium.

 

Short-term toxicity data

Reliable acute data are available for three trophic levels: algae, invertebrates and fish.

·        An unbounded value of >43.3 mg Sr/L was identified for algae.

·        The only bounded acute value of 125 mg Sr/L is available for the invertebrateD. magna.

·        The lowest acute effect value (based on measured Sr in the test medium) was an unbounded value of >40.3 mg Sr/L for the fishC. carpio.

It should be noted that the actual E(L)C₅₀values for fish and algae may be well above the reported values as not even partial effects (i.e., mortality or growth rate inhibition) were noted at the highest test concentrations.The table below provides an overview of the most sensitive, reliable, short-term toxicity freshwater data available for strontium.

 

Table: Most sensitive reliable short-term toxicity endpoints for strontium in freshwater

Species	Parameter	Endpoint	Value (mg Sr/L)	Reference
Fish: Cyprinus carpio	mortality	96h-LC50	> 40.3	Tobor-Kaplon (2010)
Invertebrates: Daphnia magna	mortality/immobility	48h-LC50	125	Biesinger and Christensen (1972)
Algae: Pseudokirchneriella subcapitata	growth rate	72h-ErC50	> 43.3	Tobor-Kaplon (2010)

 

Long-term toxicity data

Reliable studies on chronic toxicity of strontium to the aquatic environment are available for three trophic levels: algae, invertebrates and fish. The toxicity tests were performed using strontium nitrate or strontium chloride hexahydrate as test substance.

 ·        In the study of growth inhibition of the algal speciesPseudokirchneriella subcapitataby strontium nitrate, all significant effect levels (acute and chronic) were equal or higher than 43.3 mg Sr/L (conservative value). Thus, the 72-h NOEC is ≥ 43.3 mg Sr/L.

·        The study on the chronic toxicity of strontium to invertebrates (Biesinger and Christensen, 1972) reported a calculated NOEC forDaphnia magna(i.e., EC₁₆/2) of 21 mg Sr/L.

·        A chronic fish study according to OECD 210 (Egeler and Morlock, 2013) was performed withDanio rerio. The NOEC (nominal) was set to ≥100 mg/L for strontium nitrate, corresponding to a re-calculated NOEC (nominal) for strontium of ≥ 41.4 mg/L.

 

An overview of available long-term data considered for the hazard conclusions is provided in the Table below.

 

Table: Most sensitive reliable long-term toxicity data for strontium in freshwater

Species	Parameter	Endpoint	Value (mg Sr/L)	Reference
Fish: Danio rerio	mortality	34d-NOEC	≥ 41.4	Egeler and Morlock (2013)
Invertebrates: Daphnia magna	mortality	21d-NOEC	21	Biesinger and Christensen (1972)
Algae: Pseudokirchneriella subcapitata	growth rate	72-h NOECr	≥ 43.3	Tobor-Kaplon (2010)