Barium sulphide

Sediment toxicity

Description of key information

Reliable freshwater sediment toxicity studies could not be identified for barium, sulfide or sulfate.
In the aqueous environment, barium sulfide dissolves in water releasing barium 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 BaS will be driven by the barium 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 BaS is assumed to be driven by the barium 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 barium 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.

Additional information

Read across approach:

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

Barium:

For the assessment of the environmental fate and behaviour of barium substances, a read-across approach is applied based on all information available for inorganic barium 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., Ba²⁺). The dissolution of barium substances in the environment and corresponding dissolved Ba levels are controlled by the solubility of barite (BaSO₄) and to a lesser extent by witherite (BaCO₃), two naturally occurring barium minerals (Ball and Nordstrom 1991; Menzie et al, 2008). Aqueous environments especially containing chloride but also nitrate and carbonate anions increase the solubility of barium sulfate. The solubility of barium compounds increases as solution pH decreases (US EPA, 1985a). However, the concentration of dissolved Ba cations in freshwater is rather low– unless solutions are strongly undersaturated with respect to barite and witherite. In solutions, undersatured in barite and wiltherite, barium occurs largely as free Ba²⁺. Barium cations are not readily oxidized or reduced and do not bind strongly to most inorganic ligands or organic matter. Thus, the Ba²⁺ion is stable under the pH-Eh range of natural systems, and in the dissolved state, the divalent barium cation is the predominant form in soil, sediments and water.

In sum, transport, fate, and toxicity of barium in the aquatic compartment are largely controlled by the solubility of barium minerals, specifically barium sulfate. The barium cation is the moiety of toxicological concern, and thus the hazard assessment is based on Ba²⁺.

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 (S^2-), 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 (SO₄^2-). 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 BaS. Hence, the short-term aquatic toxicity values of H₂S, re-calculated to BaS are applied in the acute aquatic hazard assessment (see Table below). However, under oxic conditions, sulfides released from BaS 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 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 barium. 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.

Canadian Council of Ministers of the Environment (2013) Canadian Soil Quality Guidelines for the protection of environmental and human health: Barium.

US EPA (1985a) Health advisory — barium. Washington, DC, US Environmental Protection Agency, Office of Drinking Water.

US EPA (1984)Health effects assessment for barium,Cincinnati, Ohio, US Environmental Protection Agency, Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office (Prepared for the Office of Emergency and Remedial Responsible, Washington, DC) (EPA 540/1-86-021).