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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.

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

Sulfide/sulfate – acute toxicity data:

Table. Overview of reliable acute toxicity data of H2S and Na2SO4, read-across to strontium sulfide and applied in hazard assessment

Read-across of H2S

 Parameter

 Endpoint

 Concentration

(mg H2S/L)

 Corresponding concentration

(mg SrS/L)

 Reference

 

Freshwater

 

 

 

 

 

Fish

Puntius gonionotus

mortality

96h-LC50

0.0027

0.0095

Yussoff et al (1998)

Invertebrate

Baetis vagans

mortality

96h-LC50

0.02

0.07

Oseid and Smith, 1974

Marine water

 

 

 

 

 

Invertebrate

Penaeus indicus

mortality

96h-LC50

0.032

0.11

Gopakumar and Kuttyamma, 1996

Algae

Skeletonema costatum

growth rate

NOEC

0.041

0.14

Breteler et al., 1991

 

Read-across of Na2SO4

 

 

 (mg Na2SO4/L)

 

 

 

Freshwater

 

 

 

 

 

Fish

Pimephales promelas

mortality

96h-LC50

7960

6707

Mount et al., 1997

Invertebrate

Ceriodaphnia dubia

mortality

96h-LC50

3080

2595

Mount et al., 1997

Marine water

 

 

 

 

 

Algae

Nitzschia linearis

growth rate

120h-EC50

1900

1601

Patrick et al., 1968

Regarding reduced environments, reliable acute data for sulfide are available for three trophic levels: algae, invertebrates, and fish. The lowest effect value is the 96h-LC50of 0.0027 mg H2S/L for larvae of Javanese carp (Puntius gonionotus), corresponding to 0.0095 mg SrS/L.

With regard to oxic conditions, reliable acute data for sulfate are available for three trophic levels: algae, invertebrates, and fish. The lowest effect value is the 96h-LC501900 Na2SO4/L for the marine diatomNitzschia linearis,corresponding to 1601 mg SrS/L.

Strontium - acute toxicity data:

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

(i) An unbounded value of > 43.3 mg Sr/L (> 55 mg SrS/L) was identified for algae.

(ii) The only bounded acute value of 125 mg Sr/L (> 171 mg SrS/L) is available for the invertebrate Daphnia magna.

(iii) The lowest acute effect value (based on measured Sr in the test medium) was an unbounded value of > 40.3 mg Sr/L (> 59.1 mg SrS/L) for the fish Cyprinus carpio.

It should be noted that the actual E(L) C50 values 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. Overview of reliable acute toxicity data of strontium applied in hazard assessment

Species

Parameter 

Endpoint 

Value (mg Sr/L) 

Value (mg SrS/L) 

Reference 

Acute fish data 

 

 

 

 

 

Cyprinus carpio

mortality 

96h-LC50

> 40.3

> 55

Tobor-Kaplon (2010) 

Acute invertebrate data 

 

 

 

 

 

Daphnia magna 

mortality/immobility 

48h-LC50 

125 

171

Biesinger and Christensen (1972) 

Algal data 

 

 

 

 

 

Pseudokirchneriella subcapitata 

growth rate 

72h-ErC50 

> 43.3

> 59.1

Tobor-Kaplon (2010)

 

Strontium - chronic 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.

(i) In the study of growth inhibition of the algae species Pseudokirchneriella subcapitataby Tablor - Kaplon, 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 ≥ 59.1 mg SrS/L.

(ii) The study on the chronic toxicity of strontium to invertebrates (Biesinger and Christensen, 1972) reported a calculated NOEC for Daphnia magna(i.e., EC 16/2) of 21 mg Sr/L (28.7 mg SrS/L).

(iii) A chronic fish study according to OECD 210 (Egeler and Morlock, 2013) was performed with Danio rerio. The NOEC (nominal) was set to ≥100 mg/L for strontium nitrate, corresponding to a re-calculated NOEC (nominal) for strontium sulfide of ≥ 56.5 mg/L.

The PNECaquatic calculation will be conducted using the assessment factors method since a large dataset from long-term tests for different taxonomic groups is not available, a Species Sensitivity Distribution (SSD) cannot be developed and statistical extrapolation methods can thus not be used to derive the PNECaquatic. An overview of available long-term data used for PNEC-derivation 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) 

Value (mg SrS/L) 

Reference 

Chronic fish data 

 

 

 

 

 

Danio rerio

mortality 

34d-NOEC

≥ 41.4

> 56.5

Egeler and Morlock (2013)

Chronic invertebrate data 

 

 

 

 

 

Daphnia magna 

mortality

21d-NOEC 

21 

28.7 

Biesinger and Christensen (1972) 

Chronic algae data 

 

 

 

 

 

Pseudokirchneriella subcapitata 

growth rate 

72h-NOECr 

≥ 43.3

> 59.1

Tobor-Kaplon (2010)

 

Sulfide/sulfate – chronic toxicity data:

Toxic effects of released sulfide from SrS are not relevant for the chronic hazard assessment of SrS as it is oxidized to sulfate, and thus the toxicity of sulfate should be assessed. In freshwater, however, sulfate appears to be of low acute toxicity to fish, daphnia and algae, with consistent LC/EC50 values far above 1000 mg/L whereas the lowest EC/LC50 value of dissolved strontium amounts to 40.3 mg Sr/L. Further, the solubility product constant of strontium sulfate of ~3×10–7indicates that once sulfide released from SrS is oxidized to sulfate, celestine (SrSO4) precipitates. Further, sulfate is essential to all living organisms, their intracellular and extracellular concentrations are actively regulated and thus, sulfates are of low toxicity to the environment (OECD SIDS for Na2SO4). Therefore, it may conservatively be assumed that the toxicological moiety of concern for the long-term toxicity of SrS (if any) is strontium and further that the contribution of sulfate to the overall toxicity of SrS may be neglected.

Conclusion on C&L of SrS as aquatic hazard:

Acute toxic effects of strontium and sulfide released from SrS are relevant for the acute hazard assessment of SrS. Reliable acute toxicity data of strontium and sulfide are available for three trophic levels: algae, invertebrates and fish, respectively with the 96h-LC50of 0.0095 mg SrS/L for the fishPuntius gonionotusread-across from H2S) being the lowest effect level. Long-term toxicity data for strontium are available for three trophic levels and range from ≥ 21 mg Sr/L to 43.3 mg Sr/L, corresponding to ≥ 28.8 mg/L and 59.1 mg/L strontium sulfide.

Therefore, acute and chronic reference values based on the lowest sulfide effect level for acute toxicity and the lowest dissolved strontium effect concentration for chronic toxicity were read-across to strontium sulfide resulting in acute and chronic reference values of 0.0095 mg SrS/L and 28.8 mg SrS/L, respectively.

The lowest acute value of 0.0095 mg SrS/L meets the classification criteria of Aquatic Hazard Acute Category 1 with an M-factor of 100 according to Regulation 1272/2008, Table 4.1.0 (a) and Table 4.1.3.

In accordance with Regulation (EC) No 1272/2008, Table 4.1.0 (b) (i), classification for chronic aquatic hazard is not required for strontium sulfide as all chronic EC10/NOEC values are above the classification criteria of 1 mg/L.