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Sediment toxicity

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Reliable freshwater sediment toxicity studies could not be identified for strontium, sulfide or sulfate.
In the aqueous environment, strontium sulfide dissolves in water releasing strontium cations and sulfide anions. Sulfide rapidly oxidizes to sulfate under environmental conditions that are relevant for the aquatic environment. As sulphate is of low toxicity, it is assumed that the toxicity of SrS will be driven by the strontium ion.
A reliable sulfide toxicity study in the marine environment was conducted by Thompson et al. (1991) yielding a 60-d NOEC of 1.1 mg H2S/L (pore water concentration) for survival of the sea urchin Lytechnius pictus. For the hazard assessment in reducing environments, a worst-case approach could be followed (because adsorption / desorption processes are less relevant for H2S), in which this critical effect concentration is recalculated to a sediment-based H2S concentration assuming that all H2S was present in the pore water. However, due to the low interaction between sulfide/sulfate and sediment particles, the aquatic compartment (water column) is the more relevant and most critical compartment for the hazard assessment of these substances. Thus, sediment toxicity of SrS is assumed to be driven by the strontium ion.
According to Annex X of REACH (section 9.5.1. Long-term toxicity to sediment organisms), long-term testing shall be proposed by the registrant if the results of the chemical safety assessment indicates the need to investigate further the effects of the substance and/or relevant degradation products on sediment organisms. The choice of the appropriate test(s) depends on the results of the chemical safety assessment. Reliable data on toxicity of strontium sulfide in the sediment compartment could not be identified. Therefore, the PNECsediment is derived from the PNEC for the aquatic compartment applying the equilibrium partitioning method.

Key value for chemical safety assessment

Additional information

Read across approach:

In the aqueous 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.

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.

One reliable study with regard to sulfide toxicity in marine sediment was identified. Thompson et al (1991) investigated the effects of hydrogen sulfide on survival, growth, and gonad production of the sea urchinLytechinus pictus. The 49d-NOEC values for survival, growth, increase of wet weight, female gonad production and male gonad production were 1.1, 3.1, <1.1, 3.1 and <1.1 mg H2S/L, respectively, based on pore water concentrations.

Reliable studies with freshwater sediment organisms could not be identified. Sulfide precipitation and H2S formation is not expected in sediments without reducing conditions in the upper sediment layer(s). Under oxic conditions, the potential effects to sediment organisms resulting from exposure to released sulfide could be evaluated using toxicity data for sulfate. 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. As essential nutrient, sulfate is not very toxic to plants and is further assumed to be of low toxicity to other organisms (OECD SIDS for Na2SO4). Indeed, the toxicity of sulfate is low as indicated by the 96h-LC50of 660 mg Na2SO4/L forTrycorythus sp. (Goetsch and Palmer, 1997 summarised in OECD SIDS for Na2SO4). Sulfate does not bind to sediment, and therefore the aquatic compartment (water column) is the relevant and most critical compartment for the hazard assessment.

Acute or chronic sediment toxicity data could not be identified for strontium. The PNEC sediment is derived from the PNEC for the aquatic compartment applying the equilibrium partitioning method and the same approach is followed in the CSA of the sediment compartment.

References:

ATSDR (2006) Toxicological profile for hydrogen sulfide.

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