Barium sulphide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

142 µg/L

Marine water

Hazard assessment conclusion:

no data: aquatic toxicity unlikely

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

77 mg/L

Assessment factor:

10

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

742 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

257 mg/kg soil dw

Assessment factor:

2

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Read across approach:

In the aqueous and terrestrial 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 soils, barium is strongly adsorbed to clay minerals and organic and fine structured soils and is not expected to be very mobile because of precipitation (sulfate and carbonate) and its inability to form soluble complexes with humic and fulvic materials. Further, barium easily precipitates as sulfate and carbonate and reacts readily with metal oxides and hydroxides, being subsequently adsorbed onto soil particles. Under acid conditions, however, some of the water-insoluble barium compounds may become soluble and move into ground water (Canadian Council of Ministers of the Environment, 2013; US EPA, 1984).

In sum, transport, fate, and toxicity of barium in the environment 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).

PNEC marine water:

A relevant PNEC for the marine environment cannot be determined for the following reasons:

(i)Barium levels in seawater range from 2 to 63μg/L with a mean concentration of about 13 μg/L (Bowen 1979).

(ii) Applying ECHA-guidance, the derived marine PNEC of 11.5μg/L for barium (PNEC freshwater = 0.115 mg Ba/L and an AF of 100) would thus be within the range of typical barium seawater levels.

(iii) Seawater contains about 2700 mg/L sulfate (Hitchcock, 1975 cited in WHO, 2004).

(iv) Barium sulfide transported into marine systems dissolves in water releasing barium cations and sulfide anions. Whereas sulfide oxidise to sulfate, barium combines with sulfate ions present in salt water to form barium sulfate.

(v) Barium in marine environments is in a steady state; the amount entering is balanced by the amount falling to the bottom as barium sulfate (barite) particles to form a permanent part of the marine sediment (Wolgemuth & Brocker, 1970). Thus, dissolved barium and sulfate concentrations are controlled by the solubility of barium sulfate. The solubility product (K_sp) of barium sulfate is 1.08E^-10(CRC Handbook, 2008), resulting in maximum dissolved Ba levels of approximately 1.4 mg/L.

(vi) In sum, due to high sulfate levels in the marine environment and a low solubility of barium sulfate, dissolved barium, sulfide and sulfate levels will remain constant in marine waters, regardless of the amount of barium sulfide introduced to the system.

References:

Bowen HMJ (1979) Environmental Chemistry of the Elements. Academic Press, London, 333 pp.

Lide, D. R. (2008) CRC Handbook of chemistry and physics. 88^thedition.

Hitchcock DR (1975) Biogenic contributions to atmospheric sulphate levels. In: Proceedings of the 2nd National Conference on Complete Water Re-use. Chicago, IL, American Institute of Chemical Engineers.

WHO (1990) Barium. Environmental Health Criteria 107. International Programme on Chemical Safety.

WHO (2004) Sulfate in Drinking-water. Background document for development of WHO Guidelines for Drinking-water Quality. WHO/SDE/WSH/03.04/114.

Wolgemuth K & Broecker WS (1970) Barium in sea water. Earth planet. Sci. Lett., 8: 372-378.

 

PNEC sediment:

The PNEC_sedimentcan be derived from the PNEC_aquaticusing the equilibrium partitioning method (EPM).

A distribution/partition coefficient (K_D) between the water and sediment compartment for barium has been determined (see chapter 1.3). This resulted in a typical K_{D, susp-water}of 5,217 L/kg (logK_D: 3.72). In a first step the units have to be converted from L/kg to m³/m³using the formula below.

K_{D, susp-water}(m³/m³) = 0.9 + [0.1 x (K_{D, susp-water}(L/kg) x 2,500) / 1,000]

This results in a K_{D, sediment}of 1,305 m³/m³. This value can be entered in the equation below to calculate the PNEC_sediment:

PNEC_sediment= (K_{D, susp-water}/ RHO_susp) x PNEC_aquaticx 1,000

with the PNEC_aquaticexpressed as mg/L, RHO_susprepresenting the bulk density of wet suspended matter (freshly deposited sediment) (1,150 kg/m³), and a K_{D, susp-water}of 1,305 m³/m³, a PNEC_sedimentthat is expressed as mg/kg wet weight can be derived. This value can be converted to a dry weight-based PNEC, using a conversion factor of 4.6 (CONVsusp = RHOsusp/Fsolid-susp * RHOsolid) kg wet weight/ kg dry weight.

This results in a PNEC_sedimentof 600 mg Ba/kg dry sediment corresponding to 742 mg BaS/kg dry sediment.

PNEC for sewage treatment plant:

The lowest reliable observed NOEC/EC₁₀-value for respiration inhibition in activated sludge after a 3h incubation period amounts to ≥622 mg Ba/L Since respiration is the endpoint, an assessment factor of 10 is applied in accordance with ECHA guidance on information requirements and CSA, R.10 (2008). Application of an AF of 10, results in a PNEC_{micro-organism}of 62.2 mg Ba/L corresponding to 77 mg BaS/L.

PNEC_oral(secondary poisoning):

Reliable avaian toxicity data are not available for barium and sulfide.

- Data from an NTP (1994) study and a study by Dietz (1992) resulted in NOAELs for rats and mice ranging from 61.1 to 115 mg Ba/kg bw/d exposed for different times (13 wk, 2 yr).

According to ECHA-Guidance (ECHA, 2008: Chapter R.10) a NOEC_mammalcan be derived from a NOAEL_mammal,using the following formula:

NOEC_{mammal_food_chronic}= NOAEL_{mammal_food_chronic}x CONV_mammal

with CONV_mammalbeing a species-specific conversion factor. The conversion factor for rats is 10 or 20, depending on the age of the test organisms.

The lowest NOAEL_{repeated dose}value for male rats value of 61.1 mg Ba/kg bw/d was applied to derive the PNEC_oral. A conversion factor of 10 kg bw. d/kg food applies as the age of the test organisms was 32 days.

NOEC_{mammal_food_chronic}= 61.1 mg /kg bw/d * 10 kg bw. d/kg_food= 611 mg Ba/kg_food

An assessment factor of 90 is required when a 90d-NOEC for mammals is used as reference value. Thus, the PNEC_oralfor barium is estimated with 611 mg Ba/kg food / 90 = 6.8 mg Ba/kg food correspondimg to:

PNEC_oral= 8.4 mg BaS/kg food

According to the ECHA Guidance on information requirements and CSA, Part B (2011), “if a substance has a bioaccumulation potential and a low degradability, it is necessary to consider whether the substance also has the potential to cause toxic effects if accumulated in higher organisms.” Further, the assessment of secondary poisoning takes place as a tiered process, where the first step is to evaluate the bioaccumulative potential of a substance, applying the criterion of the BCF being ≥ 100 (together with considerations regarding biodegradability). When this criterion is met, the subsequent step to calculate a PNEC_{oral, predator}should be taken.

As barium sulfide does not have a potential for bioconcentrations and bioaccumulation and thus does not meet this criterion, a PNEC_{oral, predator}is not required for the hazard assessment of this substance.

Conclusion on classification

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.