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No reliable short term and chronic toxicity data were identified for soil organisms (macro-organisms, micro-organisms, terrestrial plants). Therefore the equilibrium partitioning method will be used for the derivation of a PNEC for the terrestrial compartment.

However, in soils, strontium sulfide dissolves in the pore water releasing strontium cations and sulfide anions (see physical and chemical properties).

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 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 is also consumed by bacteria found in soil and water that oxidize hydrogen sulfide to elemental sulfur. Photosynthetic bacteria can oxidize hydrogen sulfide to sulfur and sulfate in the presence of light and the absence of oxygen. A number of microorganisms have been found to degrade hydrogen sulfide to elemental sulfur or sulfate. Among these are heterotrophic bacteria and fungi. Soils may sorb considerable amounts of hydrogen sulfide from the air, retaining most of it in the form of elemental sulfur. Manganese oxides in soils appear to catalyze the oxidation of hydrogen sulfide to elemental sulfur. 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 or water-looged soils 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. Further, living organisms are typically adapted to temporary fluctuations of H2S concentrations in soils, where such conditions naturally occur. The formation of H2S under such conditions is a natural process, and reduced sulfate is predominantly of natural origin.

Under oxic conditions, sulfides released from SrS are oxidized to 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 terrestrial plants and is further assumed to be of low toxicity to other terrestrial organisms (OECD SIDS for Na2SO4).

Hence, the toxicity of strontium will be relevant for risk assessment purposes.