Barium sulphide

Endpoint summary

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.

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).

Sulfide/sulfate – acute toxicity data:

Table. Overview of reliable acute toxicity data of H₂S and Na₂SO₄, read-across to barium sulfide and applied in hazard assessment

Read-across of H2S

Parameter

Endpoint

Concentration (mg H2S/L)

Corresponding concentration (mg BaS/L)

Reference

Freshwater
Fish Puntius gonionotus	mortality	96h-LC50	0.0027	0.013	Yussoff et al (1998)
Invertebrate Baetis vagans	mortality	96h-LC50	0.02	0.099	Oseid and Smith, 1974
Marine water
Invertebrate Penaeus indicus	mortality	96h-LC50	0.032	0.159	Gopakumar and Kuttyamma, 1996
Algae Skeletonema costatum	growth rate	NOEC	0.041	0.204	Breteler et al., 1991

Read-across of Na2SO4

(mg Na2SO4/L)

Freshwater
Fish Pimephales promelas	mortality	96h-LC50	7960	9493	Mount et al., 1997
Invertebrate Ceriodaphnia dubia	mortality	96h-LC50	3080	3673	Mount et al., 1997
Marine water
Algae Nitzschia linearis	growth rate	120h-EC50	1900	2266	Patrick et al., 1968

Regarding reduced environments, reliable acute data for sulfide are available for three trophic levels: algae, invertebrates, and fish. The lowest effect value is the 96h-LC₅₀of 0.0027 mg H₂S/L for larvae of Javanese carp (Puntius gonionotus), corresponding to 0.013 mg BaS/L.

With regard to oxic conditions, reliable acute data for sulfate are available for three trophic levels: algae, invertebrates, and fish. The lowest effect value is the 96h-LC₅₀1900 Na₂SO₄/L for the marine diatomNitzschia linearis,corresponding to 2266 mg BaS/L.

Barium - acute toxicity data:

The table below provides an overview of reliable toxicity data of barium substances. Reported values are based on barium concentrations. 

Table. Overview of reliable acute toxicity data of bariumapplied in hazard assessment

Parameter

Endpoint

Concentration (mg BaS/L)

Corresponding concentration (mg BaS/L)

Reference

Danio rerio	mortality	96h-LC50	> 97.5 > 3.50 (dissolved)	> 120.3 > 4.32 (dissolved)	Egeler and Kiefer, 2010
Invertebrate Daphnia magna	mortality/ immobility	48h-LC50	14.5	17.9	Biesinger and Christensen, 1972
AlgaePseudokirchneriella subcapitata	growth rate	72h-ErC50	> 30.1 > 1.15 (dissolved)	> 37.1 > 1.42 (dissolved)	Egeler and Kiefer, 2010

Reliable acute data were available for three trophic levels: algae, invertebrates, fish. The lowest effect value (based on

Reliable acute data were available for three trophic levels: algae, invertebrates, fish. The lowest effect value (based on dissolved barium in the test medium) is a 72h-ErC₅₀of > 1.15 mg Ba/L, corresponding to > 1.42 mg BaS/L, for growth reduction in algae.

It should be noted that the outcome of fish and algae tests, when expressed as dissolved barium concentrations resulted in effect levels that are > 3.50 mg Ba/L and > 1.15 mg Ba/L, respectively, whereas these levels are approximately a factor of ~30 higher when expressed as total barium, i.e. > 97.5 mg/L and > 30.1 mg/L of total Ba, respectively.

The low recovery of dissolved barium in the algae and fish study may be explained with the precipitation of barium sulfate. Thus, the Chemical Safety Assessment is based on the dissolved barium concentration.

Barium - chronic toxicity data:

Reliable studies on chronic toxicity of barium to the aquatic environment are available for three trophic levels: algae, invertebrates and fish. The toxicity tests were performed with barium dichloride dihydrate as test substance.

- In the study of growth inhibition of the algae speciesPseudokirchneriella subcapitataperformed by Egeler and Kiefer (2010), all significant effect levels (acute and chronic) were ≥ 30.1 mg total Ba/L and ≥ 1.15 mg dissolved Ba/L. Thus, the 72-h NOEC is ≥ 30.1 mg total Ba/L and 72-h NOEC is ≥ 1.15 mg dissolved Ba/L corresponding to a 72-h NOEC of ≥ 37.1 mg total BaS/L and 72-h NOEC of ≥ 1.42 mg dissolved BaS/L, respectively.

- The study on the chronic toxicity of barium to invertebrates (Biesinger and Christensen, 1972) reports a calculated NOEC forDaphnia magna(i.e., EC₁₆/2) of 2.9 mg Ba/L (nominal) corresponding to 3.6 mg BaS/L.

- A chronic fish study according to OECD 210 (Gilberg, 2014) was performed withDanio rerio. A NOEC of ≥ 40.3 mg/L total barium was derived, corresponding to a NOEC of ≥ 49.7 mg/L total BaS. Further, the NOEC of ≥ 1.26 mg/L dissolved barium corresponds to a NOEC of ≥ 1.55 mg/L dissolved Ba. The low recovery of dissolved barium in the study may be explained with the precipitation of barium sulfate. Thus, the Chemical Safety Assessment is based on dissolved barium.

Table.Overview of reliable chronic toxicity data of barium applied in hazard assessment

Parameter

Endpoint

Concentration (mg BaS/L)

Corresponding concentration (mg BaS/L)

Reference

Fish Danio rerio	mortality	33d-NOEC	≥ 40.3 ≥ 1.26 (dissolved)	≥ 49.7 ≥ 1.55 (dissolved)	Gilberg, 2014
Invertebrates Daphnia magna	mortality	21d-NOEC	2.9	3.58	Biesinger and Christensen, 1972
Algae Pseudokirchneriella subcapitata	growth rate	72h-NOECr	≥ 30.1 ≥ 1.15 (dissolved)	≥ 37.1 ≥ 1.42 (dissolved)	Egeler and Kiefer, 2010

One additional reliable endpoint has been identified for the marine organismCancer anthonyi, for which a nominal 7d-NOEC of 10 mg/L has been reported (endpoint: embryonal hatching).

Sulfide/sulfate – chronic toxicity data:

Toxic effects of released sulfide from BaS are not relevant for the chronic hazard assessment of BaS as it is oxidized to sulfate, and thus the toxicity of sulfate should be assessed. In freshwater, however, sulfate appears to be of low acute toxicity to fish, daphnia and algae, with consistent LC/EC50 values far above 1000 mg/L whereas the lowest EC/LC50 value of dissolved barium amounts to 14.5 mg Ba/L. Further, the solubility product constant of barium sulfate of 1.1×10^–10indicates that once sulfide released from BaS is oxidized to sulfate, barite (BaSO₄) precipitates. Further, 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 (OECD SIDS for Na₂SO₄). Therefore, it may conservatively be assumed that the toxicological moiety of concern for the long-term toxicity of BaS (if any) is barium and further that the contribution of sulfate to the overall toxicity of BaS may be neglected.

Conclusion on C&L of BaS as aquatic hazard:

Acute toxic effects of barium and sulfide released from BaS are relevant for the acute hazard assessment of BaS. Reliable acute toxicity data of barium and sulfide are available for three trophic levels: algae, invertebrates and fish, respectively with the 96h-LC₅₀of 0.013 mg BaS/L for the fishPuntius gonionotus(read-across from H₂S) being the lowest effect level. Long-term toxicity data for barium are available for three trophic levels and range from ≥ 1.15 mg Ba/L to 2.9 mg Ba/L, corresponding to ≥ 1.42 mg/L and 4.6 mg/L barium sulfide (all dissolved).

Therefore, acute and chronic reference values based on the lowest sulfide effect level for acute toxicity and the lowest dissolved barium effect concentration for chronic toxicity were read-across to barium sulfide resulting in acute and chronic reference values of 0.013 mg BaS/L and 1.42 mg BaS/L, respectively.

The lowest acute value of 0.013 mg BaS/L meets the classification criteria of Aquatic Hazard Acute Category 1 with an M-factor of 10 according to Regulation 1272/2008, Table 4.1.0 (a) and Table 4.1.3.

In accordance with Regulation (EC) No 1272/2008, Table 4.1.0 (b) (i), classification for chronic aquatic hazard is not required for barium sulfide as all chronic EC₁₀/NOEC values are above the classification criteria of 1 mg/L.