Strontium oxide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The assessment of the ecotoxicological potential of strontium oxide is based on elemental strontium concentrations. Read-across of aquatic toxicity data available for soluble strontium substances is applied since the strontium ions determine the ecotoxicological potential of strontium oxide.

Read-across statement

Strontium oxide dissolves upon contact and during the reaction with water. The water solubility test of strontium oxide (OECD TG 105) indicates a high dissolution from strontium oxide (≤7.63 g/L at 20°C) and a corresponding increasing solution pH to a pH > 13. Due to the buffering capacity of most environmental systems, it may reasonably be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation. Strontium has very little tendency to form complexes with inorganic ligands, and its solubility is in consequence not driven by the presence of most inorganic anions (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 andstrontium sulfateat neutral to high pH. In solutions with a pH below 4.5, Sr2+ ions are predominant. Strontium sulfate forms at more neutral conditions (pH 5 to 7.5). Only under highly alkaline conditions, strontium carbonate controls dissolved strontium concentrations. 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 of strontium. Therefore, the assessment of the ecotoxicological potential of strontium oxide is based on elemental strontium concentrations. Read-across of aquatic toxicity data available for soluble strontium substances is applied since the strontium ions determine the ecotoxicological potential of strontium oxide.

Reliable acute aquatic toxicity data of strontium substances are available from GLP-conform guideline studies for algae, daphnia, fish and microorganisms. Referring to strontium ions, which determine the ecotoxicological potential of strontium oxide, a bounded value exceeding the acute OECD test limit of 100 mg/L could be identified for Daphnia magna (125 mg/L Sr corresponding to 147.8 mg/L SrO). Additional studies for fish, algae and microorganisms provide unbound E(L)C50 values > 47.1 mg/L Sr (corresponding to 55.7 mg/L SrO) represented by the highest concentrations tested in the respective test system. An overview of reliable short- and long-term data of the most sensitive species identified is provided in the table below:

Trophic level	Endpoint	Strontium oxide [mg/L]	Strontium [mg/L]
Short-term
Algae	72-h ErC50	> 55.70	> 47.1
Daphnia	48-h EC50	> 66.63 – 147.8	> 56.34 - 125
Fish	96-h LC50	> 66.63	> 56.34
Microorganisms	3-h EC50	> 193.71	> 163.8
Long-term
Algae	72-h EC10	28.7	24.3
Daphnia	21-d NOEC	24.83	21.0
Microorganisms	3-h EC10	> 193.71	> 163.8

According to the Guidance on information requirements and chemical safety assessment (Part B; chapter B.8 Scope of exposure assessment), an environmental exposure and risk assessment is mandatory for a substance if it is classified as hazardous to the aquatic environment according to Regulation (EC) No 1272/2008, meets persistence and bioaccumulation criteria of a PBT and vPvB substance or is a non-classified hazard to water, sediment and/or soil and a PNEC can be derived. However, thresholds for identifying as a “non-classified hazard to water, sediment and/or soil” are not provided by the guidance. The limit concentration of 100 mg/L for the acute toxicity of algae, daphnia and fish, i.e., the OECD test limit, and 10 mg/L for chronic toxicity may serve as threshold values. The only bounded acute EC50 value and the only bounded chronic EC10/NOEC value are well above these respective limits.

Based on The ECHA guidance Part B chapter 8.4.2.2 (Version 2.1, December 2011),“toxicity to aquatic organisms is used as an indicator of concern for sediment and soil organisms and a screening risk characterisation is undertaken using the equilibration partitioning method to derive PNECs for sediment and soil.”Strontium oxide is not a classified or non-classified acute and chronic hazard to aquatic organisms. Therefore, strontium oxide is also not an unclassified hazard to sediment and soil organisms.

Further, Strontium oxide does not meet persistence and bioaccumulation criteria of a PBT and vPvB.

Also, using the lack of short-term and long-term toxicity to aquatic organisms as an indicator of concern (or the lack thereof) for sediment and soil organisms, unclassified hazards to the sediment and soil compartment can be excluded.

Thus, since strontium oxide does not meet classification criteria as hazardous for the aquatic environment and is not considered to be a non-classified hazard, a PNEC derivation is not required.

Conclusion on classification

Strontium oxide completely dissolves upon contact and during the reaction with water.Therefore, the aquatic hazard potential is assessed based on toxicity data available for soluble strontium substances.

Reliable acute aquatic toxicity data of strontium are available from studies with algae, daphnia and fish; respective EC/LC50 values are unbounded and well above 10 mg/L or well above 100 mg/L. Therefore, strontium oxide does not meet classification criteria as short-term hazard to the aquatic environment under Regulation (EC) No 1272/2008 and subsequent adaptations.

Reliable chronic aquatic toxicity data of strontium are available from studies with algae, daphnia and fish; the respective NOEC/EC10 values are unbounded and/or well above 10 mg/L. Therefore, strontium does not meet classification criteria as long-term hazard to the aquatic environment under Regulation (EC) No 1272/2008 and subsequent adaptations.

Solely, long-term toxicity to fish has not been quantified. Criteria given in Table 4.1.0(b) iii) are applied in accordance with Figure 4.1.1 of Regulation (EC) No 1272/2008. The 96-h LC 50 for fish of strontium is > 56.34 mg/Lcorresponding to > 66.63 mg/L strontium oxide in the acute toxicity test with freshwater fish. A study available for marine fish with an LC50 of > 92.80 mg/L Sr (corresponding to a LC50 of > 109.75 mg/L SrOand thus above the classification criteria for substances for which adequate chronic toxicity data are not available) further points to a very low potential for acute toxicity to fish.Therefore, strontium oxide does not meet classification criteria as long-term hazard to the aquatic environment under Regulation (EC) No 1272/2008 and subsequent adaptations