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Short-term toxicity to fish

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Description of key information

Reliable acute toxicity data for fish are available for sulfide, sulfate and strontium. Strontium and sulfide are released upon dissolution of SrS in the aqueous environment. Sulfide is rapidly oxidised under natural environmental conditions to sulfate. Thus, only acute but not long-term effects due to sulfide exposure are expected. For long term effects data for sulfate are taken into consideration. For C&L acute toxicity of sulfide was considered. Hence, the 96h-LC50 of 0.0095 mg SrS/L for the fish Puntius gonionotus is used as reference value for acute aquatic toxicity. Sulfate is of low toxicity.
Regarding toxic effects of dissolved Sr, one reliable acute toxicity data point (Klimisch 1, GLP) for a freshwater fish species -the carp Cyprinus carpio - has been identified. Based on measured Sr-level in the water column, a 96h-LC50 of > 55 mg SrS/L (> 40.3 mg Sr/L) is reported by Tobor-Kaplon (2010), using strontium nitrate as test substance.
One reliable acute toxicity data point (Klimisch 2) for a saltwater fish species -morone saxatilits- has been identified. Based on measured Sr-level, an unbounded 96h-LC50 of > 126.3 mg SrS/L (> 92.5 mg Sr/L) is reported by Dwyer, F. J., et al. (1992), using strontium chloride hexahydrate as test substance.
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Key value for chemical safety assessment

LC50 for freshwater fish:
9.5 µg/L

Additional information

Read-across statement:

No ecotoxicological data are available for strontium sulfide itself. However,in the aqueous and terrestrial environment, strontium sulfide dissolves in water releasing strontium cations and sulfide anions (see physical and chemical properties).

Sulfide:Sulfide anions react with water in a pH-dependant reverse dissociation to form bisulfide (HS-) or hydrogen sulfide (H2S), respectively (i.e., increasing H2S formation with decreasing pH). Thus, sulfide (S2-), bisulfide (HS-) and hydrogen sulfide (H2S) coexist in aqueous solution in a dynamic pH-dependant equilibrium. Sulfide prevails only under very basic conditions (only at pH > 12.9), bisulfide is most abundant at pH 7.0 – 12.9, whereas at any pH < 7.0, sulfide (aq) is predominant. Temperature and salinity are other parameters that affect to a lesser extent the equilibrium between the different sulfide species. Hydrogen sulfide evaporates easily from water, and the rate of evaporation depends on factors such as temperature, humidity, pKa, pH, and the concentration of certain metal ions (see section on environmental fate).

Hydrogen sulfide is one of the principal components in the natural sulfur cycle. Bacteria, fungi, and actinomycetes (a fungus-like bacteria) release hydrogen sulfide during the decomposition of sulfur containing proteins and by the direct reduction of sulfate (SO42-). Hydrogen sulfide oxidation by O2readily occurs in surface waters. Several species of aquatic and marine microorganisms oxidize hydrogen sulfide to elemental sulfur, and its half-life in these environments usually ranges from 1 h to several hours. Sharma and Yuan (2010), for example, demonstrated that sulfide is oxidised to sulfate and other oxidised S-forms in less than one hour. Photosynthetic bacteria can oxidize hydrogen sulfide to sulfur and sulfate in the presence of light and the absence of oxygen. Thus, the oxidation of sulfide is mediated via biotic (sulfur-oxidizing microorganisms) and abiotic processes, and reported half–lives which are less than an hour in most aerobic systems, do not distinguish between these two types of oxidation.

Sulfides may also be formed under reducing conditions, e.g. in organic-rich sediments via reduction of sulfate. Dissolved bisulfide and sulfide complex with trace metal ions, including Zn, Co, and Ni, and precipitate as sparingly soluble metal sulfides. Concentrations of H2S are mostly negligible though there are conditions under which relatively high levels may be present for extended periods. In addition it should be pointed out, that sediments where such conditions occur naturally, living organisms are typically adapted to temporary fluctuations of H2S concentrations. The formation of H2S under such conditions is a natural process, and reduced sulfate is predominantly of natural origin. The short half-life of H2S under normal aerobic environmental conditions, however, implies that the toxic effects of H2S are relevant for the acute but not for the long-term hazard and risk assessment of SrS. Hence, the short-term aquatic toxicity values of H2S, re-calculated to SrS are applied in the acute aquatic hazard assessment (see Table below). However, under oxic conditions, sulfides released from SrS are oxidized to sulfate, and in these cases the risks entailed by the released sulfur should be evaluated using toxicity data for sulfate.

References:

ATSDR (2006) Toxicological profile for hydrogen sulfide.

Strontium: For the assessment of the environmental fate and behaviour of strontium substances, a read-across approach is applied based on all information available for inorganic strontium compounds. This is based on the common assumption that after emission of metal compounds into the environment, the moiety of toxicological concern is the potentially bioavailable metal ion (i.e., Sr2+).This assumption is considered valid as the ecotoxicity is only affected by the strontium-ion and not by the counter (sulfide) ion.The speciation and chemistry of strontium is rather simple.

As reactive electropositive metal, strontium is easily oxidized to the stable and colourless Sr2+ion in most of its compounds, the chemical behaviour resembling that of calcium and/or barium (Wennig and Kirsch, 1988). In the environment, the element only occurs in one valence state (Sr2+), does not form strong organic or inorganic complexes and is commonly present in solution as Sr2+(Lollar, 2005). Consequently, the transport, fate, and toxicity of strontium in the environment are largely controlled by solubility of different Sr-salts (e. g., SrCO3, Sr(NO3)2, SrSO4, …).

These findings are sufficient justification for the implementation of a read-across strategy with ecotoxicity results obtained in tests that were conducted with different strontium compounds that generate free Sr2+-ions in solution, and this for all relevant environmental endpoints that were considered.

In sum, the environmental hazard assessment is based on strontium.

References:Wennig, R.; Kirsch, N. (1988): Chapter 57 Strontium, In: Seiler, U. G. et al.(eds), Handb.Tox. Inorg. Comp. NY, 631-638

 

Tabor-Kaplon (2010) conducted an acute toxicity test withCyprinus carpio(test substance: soluble strontium dinitrate). No effects (mortality) were observed at the geometric mean measured test concentration of 40.3 mg Sr2+/L (97.5 mg/L Sr(NO3)2) that corresponds to an LC50of > 55 mg SrS/L.

The lowest reliable acute effect concentration for fish with regard to sulfide was reported by Yussoff et al. (1998) for the Javanese carpPuntius gonionotus, i.e. the 96h-LC50of 0.0027 mg H2S/L corresponding to 0.0095 mg SrS/L. In oxic environments, however, sulfide released from SrS is oxidized to sulfate, and the hazard of the released sulfur should be evaluated by reading-across the toxicity of sulfate. The study by Mount et al (1997) was identified in the OECD SIDS for Na2SO4(i.e., the most relevant substance for assessing the hazard of sulfate) as key study with regard to acute toxicity of sulfate to fish. This study reports a 96h-LC50of 7960 mg Na2SO4/L for the fathead minnowPimephales promelascorresponding to a 96h-LC50of 5624 mg SrS/L.