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Abiotic degradation

Abiotic degradation is an irrelevant process for inorganic substances that are assessed on an elemental basis, including strontium sulfide.

In the aqueous and terrestrial environment, strontium sulfide dissolves in water releasing strontium cations and sulfide anions (see physical and chemical properties).

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: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. The Henry's law constant increases linearly with temperature, indicating an increasing tendency to partition to the gas phase. Further, at higher pHs, such as seawater, which has a pH of 8 or greater, evaporation is enhanced due to an ionic species gradient in the water close to the surface.

Hydrogen sulfide is one of the principal components in the natural sulfur cycle and may stay present in water for several hours, depending on the oxygen level of the environment under consideration. In oxic systems, oxidation to polysulfides, elemental sulfur, thiosulfate, sulfite and - eventually - sulfate will occur. Half-lives of 0.4 to 65 h have been reported for sulfide oxidation in the aquatic environment. Sharma and Yuan (2010), for example, demonstrated that sulfide is oxidised to sulfate and other oxidised S-forms in less than one hour. The oxidation of sulfide is mediated via biotic (sulfur-oxidizing microorganisms) and abiotic processes in water, soil and sediments, and reported half–lives which are less than an hour in most aerobic systems, do not distinguish between these two types of oxidation. Hydrogen sulfide oxidation by O2readily occurs in surface waters. In well drained and oxic soils, released sulfides are oxidized very quickly and H2S is not present.

Sulfides may also be formed under reducing conditions, e.g. in organic-rich sediments or water-logged soils via reduction of sulfate. Dissolved bisulfide and sulfide complex with trace metal ions, including Fe, Zn, Co, and Ni, and precipitate as sparingly soluble metal sulfides. In iron-rich environments, FeS and - eventually - FeS2will be the most abundant reduced sulfur compounds. Concentrations of H2S are mostly negligible though there are conditions under which relatively high levels may be present for extended periods.

SrS is not expected to be released to the air as such because of its low vapour pressure. However, in case H2S is formed, H2S may be released to the air as itevaporates 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 below).In the atmospheric compartment, sulfur compounds such as H2S are oxidized to SO2and sulfate. Residence time of hydrogen sulfide in the troposphere has been calculated to be 18 h. Sulfur dioxide and sulfates are eventually removed from the atmosphere through absorption by plants, deposition on and sorption by soils, or through precipitation. Further, clay or organic matter may sorb considerable amounts of hydrogen sulfide from the air, retaining it as elemental sulfur.

ATSDR (2006) Toxicological profile for hydrogen sulfide.

Biotic degradation

According to Column 2 of the REACH annexes on information requirements, no biodegradability tests should be conducted when the substance is inorganic. SrS is considered to be an inorganic substance; however, because of the importance of microorganism-mediated reactions in the natural sulfur cycle, this endpoint is not entirely irrelevant. When sulfide compounds such as SrS are released to the environment, the sulfur in the compounds will enter the natural sulfur cycle. In this cycle, sulfur transformations are mediated to an important extent by sulfur oxidizing and reducing microorganisms. In aerobic environments, sulfur oxidizing microorganisms will transform sulfides into - eventually - sulfates, whereas in anaerobic environments, sulfur reducing microorganisms will reduce oxidized sulfur compounds in the presence of reducing agents. Oxidation of reduced sulfur compounds has been detected in soils, freshwater and marine ecosystems, and biological waste water treatment plants. These findings demonstrate the wide distribution of sulfide transforming microorganisms. Half-lives for sulfide oxidation between 0.4 and 65 h have been reported, depending on the environment under consideration (Bagarinao, 1992). These half-lives represent half-lives based on combined abiotic and microorganism-mediated oxidation reactions.

The guidance that is given in Column 2 of Annex VII if REACH legislation and Chapter R.7b (Endpoint Specific Guidance) of the ECHA REACH Guidance Document, (November 2012), is relevant for strontium that is released from SrS, i.e. that the requirements for “Ready biodegradability” can be waived if the substance is inorganic.

Annex VIII states that "Further degradation testing shall be considered if the chemical safety assessment according to Annex I indicates the need to investigate further the degradation of the substance. The choice of the appropriate test(s) will depend on the results of the chemical safety assessment." Waiving of the need for data for this environmental endpoint may be considered if “The substance is highly insoluble in water), or if “The substance is readily biodegradable” (ECHA 2012, Chapter R.7b– Endpoint Specific Guidance).

However, for an inorganic substance like strontium for which the chemical assessment is based on the elemental concentration (i.e., pooling all inorganic speciation forms together), biotic degradation in the environment is an irrelevant process: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the environment by degradation or transformation processes. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.