Strontium sulphide

Endpoint summary

Additional information

Abiotic degradation

Abiotic degradation is not relevant for inorganic substances such as strontium sulfide that are assessed based on environmental elemental concentrations (i.e., pooling all speciation forms together in the effects and exposure assessment). In the aqueous and terrestrial environment, strontium sulfide dissolves in 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 (S^2-), bisulfide (HS^-) and hydrogen sulfide (H₂S) coexist in aqueous solution in a dynamic pH-dependant equilibrium. In oxic systems, oxidation to - eventually - sulfate occurs.

Strontium exists almost exclusively in the environment as Sr⁺²cation and will form hydrated cation, SrOH⁺, strontium sulfate, SrSO₄(celestite), and strontium carbonate, SrCO₃(strontianite). Celestite and strontianite are the two common strontium minerals in nature and there are not very soluble in water: 125 mg/L (at 25 °C) for strontium sulfate and ~ 10 mg/L (at 25 °C) for strontium carbonate (IUCLID, 2014 and IUCLID, 2013).

Biodegradation

Biodegradation is not relevant for inorganic substances such as strontium sulfide that are assessed based on environmental elemental concentrations (i.e., pooling all speciation forms together in the effects and exposure assessment).In the aqueous and terrestrial environment, strontium sulfide dissolves in water releasing strontium cations and sulfide anions (see physical and chemical properties). Any sulfide released to the environment enters the natural sulfur cycle in which oxidation and reduction reactions are mediated through abiotic as well as biotic processes. Sulfur oxidizing and reducing microorganisms are omnipresent and determine the predominant state of the present sulfur depending on the prevalent conditions. The fate of strontium in the environment is predominantly controlled by abiotic processes.

Environmental distribution

Release to the aquatic environment is the most relevant route of release of SrS. In the aqueous environment, strontium sulfide dissolves in water releasing strontium cations and sulfide anions.

Depending on redox conditions sulfides remain in the system or oxidize to - eventually - sulfate. Upon release, sulfide added to the environment enters the natural sulfur cycle and anthropogenically released sulfur becomes indistinguishable from present sulfur. Consequently, the environmental distribution of sulfides is driven by the same reactions driving the natural sulfur cycle. The review of Brown (1982) provides a thorough description of the sulfur cycle.

The distribution of strontium is governed by different processes: partition coefficients of strontium were determined in different environmental compartments and typical log K_D values for sediment, suspended matter and soil amount to 2.94, 3.11 and 2.2, respectively. Some strontium salts, including strontium sulfate (celestine) and strontium carbonate (strontianite), have a low solubility and may therefore precipitate at higher strontium concentrations. Typical baseline levels of strontium in European surface water, sediment and top soils are 480 ug Sr/L, 621.8 mg Sr/kg dw and 76 mg Sr/kg dw, respectively (for detailed information please refer to IUCLID section 5.5 “environmental monitoring data”).

Bioaccumulation:

Food chain bioconcentration and biomagnification of sulfide are unlikely(ATSDR 2006). Marine invertebrate data indicate that sulfides do not have a potential for bioconcentration/bioaccumulation.

Data that were retrieved, suggest that strontium bioconcentration and bioaccumulation is negligible: internal concentrations of soft tissues remain situated between 0.5 and 5.7μg/g, regardless of the external concentration (9 – 8000μg/L). Whole body concentrations were considered less relevant due to the potential of strontium to replace Ca in the bones. Reported tissue BAFs vary more than 2 orders of magnitude, but remain below 100. Moreover, an inverse relationship between exposure concentration and BAF has been observed, i. e., decreasing BAFs with increasing Sr-levels in the water column (Moiseenko and Kudryavtseva, 2001).

The data indicate that strontium can is homeostatically controlled by aquatic organisms. The homeostatic control in soft tissues of strontium is observed to continue to function up to the milligramme range of exposure (8 mg/L in seawater; Ueda et al, 1973).

Limited information on transfer of strontium through the food chain indicates that strontium does not biomagnify in aquatic food chains.