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Environmental fate & pathways

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Abiotic degradation

Abiotic degradation is not relevant for inorganic substances such as barium 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, barium sulfide dissolves in water releasing barium 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 aqueous solution in a dynamic pH-dependant equilibrium. In oxic systems, oxidation to - eventually - sulfate occurs.

Barium in water occurs as the divalent cation in combination with other elements. Barium does not hydrolyze appreciably except in highly alkaline environments (i.e., at pH levels ≥10). Speciation is typically controlled by barium sulfate, BaSO4(barite), and to a lesser extent by barium carbonate, BaCO3(witherite).In solutions, undersatured in barite and wiltherite, barium occurs largely as free Ba2+. The Ba2+ion is stable under the pH-Eh range of natural systems.

Biodegradation

Biodegradation is not relevant for inorganic substances such as barium 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, barium sulfide dissolves in water releasing barium 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 barium in the environment is predominantly controlled by abiotic processes.

Environmental distribution

Release to the aquatic environment is the most relevant route of release of BaS. In the aqueous environment, barium sulfide dissolves in water releasing barium 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 ofBrown (1982)[EBRC1] provides a thorough description of the sulfur cycle.

The distribution of barium is governed by different processes: partition coefficients of barium were determined in different environmental compartments and typical log Kvalues for sediment, suspended matter and soil amount to 3.54, 3.72 and 1.78, respectively. Some barium salts, including barium sulfate (barite) and barium carbonate (witherite), have a low solubility and may therefore precipitate at higher barium concentrations. Typical baseline levels of barium in European surface water, sediment and top soils are 38.9 ug Ba/L, 135.4 mg Ba/kg dw and 61.9 mg Ba/kg dw, respectively.

Bioaccumulation:

Food chain bioconcentration and biomagnification of sulfide are unlikely(ATSDR 2006). Marine invertebrate dataindicate that sulfides do not have a potential for bioconcentration/bioaccumulation. Data available for bioconcentration of barium in fish suggest that bioconcentration and bioaccumulation of barium are also negligible

 [EBRC1]reference