Strontium sulphide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

2.9 mg/L

Assessment factor:

10

Marine water

Hazard assessment conclusion:

no data: aquatic toxicity unlikely

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

5.7 mg/L

Assessment factor:

10

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

3 715 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

no exposure of sediment expected

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

449 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

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 (S^2-), 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 (SO₄^2-). 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

PNEC sediment:

The PNEC_sedimentcan be derived from the PNEC_aquaticusing the equilibrium partitioning method (EPM).

A distribution/partition coefficient (K_D) between the water and sediment compartment for strontium has been determined (see chapter 1.3). This resulted in a typical K_{D, susp-water}of 1,291.8 L/kg (logK_D: 3.11). In a first step the units have to be converted from L/kg to m³/m³using the formula below.

K_{D, susp-water}(m³/m³) = 0.9 + [0.1 x (K_{D, susp-water}(L/kg) x 2,500) / 1,000]

This results in a K_{D, susp-matter}of 323.9 m³/m³. This value can be entered in the equation below to calculate the PNEC_sediment:

PNEC_sediment= (K_{D, susp-water}/ RHO_susp) x PNEC_aquaticx 1,000

with the PNEC_aquaticexpressed as mg/L, RHO_susprepresenting the bulk density of wet suspended matter (freshly deposited sediment) (1,150 kg/m³), and a K_{D, susp-water}of 323.9 m³/m³, a PNEC_sedimentthat is expressed as mg/kg wet weight can be derived. This value can be converted to a dry weight-based PNEC, using a conversion factor of 4.6 (CONVsusp = RHOsusp/Fsolid-susp * RHOsolid) kg wet weight/ kg dry weight.

This results in aPNEC_sedimentof 2,720 mg Sr/kg dry sediment corresponding to 3,715 mg SrS/kg dry sediment.

PNEC soil:

The PNEC_soilcan be derived from the PNEC_aquaticusing the equilibrium partitioning method (EPM).

A distribution/partition coefficient (KD) between the water and soil compartment was derived for strontium of 157.03 L/kg (Log KD: 2.2). In a first step the units have to be converted from L/kg to m³/m³using the formula below.

KD,_soil(m³/m³) = 0.2 +[0.6 x (KD,_soil(L/kg) x 2,500) / 1,000]

This results in a KD,_soilof 235.75 m³/m³. This value can be entered in the equation below to calculate the PNEC_soil

PNEC_soil= (KD,_soil/ RHO_soil) x PNEC_aquaticx 1,000

With the PNEC_aquaticexpressed as mg/L, RHO_soilrepresenting the bulk density of wet soil (1,700 kg/m³) and KD,soil is 157.03 m³/m³, a PNEC_soilexpressed as mg/kg wet weight is derived. This value can be converted to a dry weight-based PNEC, using a conversion factor of 1.13 kg wet weight/ kg dry weight.

This results in aPNEC_soilof 329 mg Sr/kg dry soil, re-calculation to strontium sulfide resulted in 449 mg SrS/L.

Conclusion on classification

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 gonionotus(read-across from H₂S) being the lowest effect level.Long-term toxicity data are available for 3 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 29 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.