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Environmental fate & pathways

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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 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 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.

Therefore, the assessment of the environmental fate and subsequently ecotoxicity of strontium oxide is based on elemental strontium concentrations. Read-across of data available for soluble strontium substances is applied since the strontium ions determine the environmental fate of strontium.

Abiotic degradation: Abiotic degradation including hydrolysis or phototransformation in water, soil or air, are not relevant for the environmental fate of inorganic substances including strontium and strontium oxide.

Biotic degradation: Biodegradation is not relevant for metals and metal compounds, including strontium and strontium oxide.

Bioaccumulation: Strontium has a low potential to bioconcentrate or bioaccumulate in aquatic and soil organisms.

Adsorption / desorption: Geochemical background data providing a total of 757 paired samples resulted in a Europe-wide median stream water/sediment Kd for strontium of 915 L/kg. Additional published data reported Kd values for the soil compartment ranging from 27.5 to 310 L/kg strontium (median). A weight of evidence approach resulted in an overall median soil Kd value of 65 L/kg strontium.

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