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Read across approach:

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

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 (S2-), bisulfide (HS-) and hydrogen sulfide (H2S) coexist in 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 (SO42-). Hydrogen sulfide is also consumed by bacteria found in soil and water that oxidize hydrogen sulfide to elemental sulfur. Photosynthetic bacteria can oxidize hydrogen sulfide to sulfur and sulfate in the presence of light and the absence of oxygen. A number of microorganisms have been found to degrade hydrogen sulfide to elemental sulfur or sulfate. Among these are heterotrophic bacteria and fungi. Soils may sorb considerable amounts of hydrogen sulfide from the air, retaining most of it in the form of elemental sulfur. Manganese oxides in soils appear to catalyze the oxidation of hydrogen sulfide to elemental sulfur. 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 or water-looged soils 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. Further, living organisms are typically adapted to temporary fluctuations of H2S concentrations in soils, where such conditions naturally occur. The formation of H2S under such conditions is a natural process, and reduced sulfate is predominantly of natural origin.

Under oxic conditions, sulfides released from BaS are oxidized to 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 terrestrial plants and is further assumed to be of low toxicity to other terrestrial organisms (OECD SIDS for Na2SO4).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.Therefore, it may conservatively be assumed that the toxicological moiety of concern for the terrestrial toxicity of BaS (if any) is barium and further that the contribution of sulfate to the overall toxicity of BaS may be neglected.

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).

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 soils are largely controlled by the solubility of barium minerals, specifically barium sulfate. The barium cation is the moiety of toxicological concern, and thus the soil hazard assessment is based on barium.

Reliable barium toxicity data are reported in the table below:

 

Table. Overview of reliable chronic soil toxicity data for barium

Species

Parameter

Endpoint

Concentration (mg Ba/kg dw) / corresponding to (mg BaS/kg dw.)

Reference

Eisenia fetida

(annelid)

reproduction

21-d NOEC

258 (meas.) / 318

Kuperman et al. (2006)

Enchytraeus crypticus(annelid)

reproduction

21-d NOEC

433 (meas.) / 534

Kuperman et al. (2006)

Folsomia candida(springtail)

mortality, reproduction

28-d NOEC

211 (meas.) / 260

Kuperman et al. (2006)

Long-term toxicity data are available for one trophic level only, i.e. soil invertebrates (arthropods and annelids). The PNECsoil is derived using the lowest available effect concentration, i.e. the 28-d NOEC of 211 mg Ba/kg soil dw for reproduction and mortality of the springtailFolsomia candida(Kuperman et al, 2006) in a sandy loam soil. The soil was tested because of its physico-chemical characteristics that support relatively high bioavailability of cationic metals, including low organic matter and clay contents. Thus, an assessment based on this NOEC value is conservative. Other NOEC-values for the same trophic level (soil invertebrates) amount to 258 and 433 mg/kg soil dw for reproduction ofEisenia fetidaandEnchytraeus crypticus(both annelids), respectively, in the same soil as tested with springtails. Reliable NOEC or EC10-values for the toxicity of barium to plants or soil micro-organisms were not identified.

In accordance with ECHA guidance (Chapter R.10, 2008), an assessment factor of 100 needs to be applied if long-term toxicity data are available for one trophic level only. Thus, a PNECsoil of 2.11 mg Ba/kg soil dw is determined.

The relevance of this value can be assessed by comparing the estimated PNECsoil with baseline levels of barium in pristine European topsoil samples as reported in the FOREGS data set. The FOREGS monitoring survey represents more than 837 sampling locations covering major parts of Europe. Detailed descriptions of sampling methodology, sampling preparation and analysis are given in Salminen et al. (2005). High quality and consistency of the obtained data were ensured by using standardized sampling methods and by treating and analysing all samples in the same laboratories.

A total number of 837 soil samples were analysed, and the barium content in the soil samples ranged from 10 to 1669 mg Ba/kg dw. Country-specific median values ranged from 17.3 to 117.9 mg Ba/kg dw. The 90thpercentiles of different countries (i.e., the reasonable worst case (RWC) baseline levels) ranged from 33.5 to 331.5 mg Ba/kg dw.

The RWC-ambient PEC of a site or region is compared to the PNEC in order to assess regional risks. A potential regional risk is identified when the RWC-ambient PEC exceeds the PNEC-value.

The RWC-ambient PEC represents the background/baseline level of an element/substance to which the anthropogenic fraction (local point sources and diffuse sources) of this element/substance is added. It is expected that potential risks are solely due to anthropogenic additions of this element/substance. It can further be assumed that in an ecosystem, natural baseline levels of a specific element/substance would not exert toxic effects in a specific environmental compartment of that ecosystem.

The lowest country-specific RWC-baseline level of 17.3 mg Ba/kg dw exceeds the PNECsoil of 2.11 mg/kg dw by almost an order of magnitude. This would mean that either the proposed PNECsoil of 2.11 mg Ba/kg dw is unrealistically low or all European soils are impaired by natural baseline barium levels. Thus, it may be assumed that the application of an assessment factor of 100 to the lowest NOEC of 211 mg/kg dw is overly conservative and results in an unrealistic PNEC.

Setting a more realistic PNEC requires a better understanding of what concentration background/baseline levels can be considered “normal” in soils. Data from the FOREGS data are analysed to set an outlier cut-off level: values below this cut-off level are considered “normal and representative”, whereas values exceeding this cut-off level are considered as outliers that do not represent normal environmental properties. Such an outlier cut-of level may serve as a starting point for setting a more realistic PNEC that still remains sufficiently conservative.

According to the ECHA REACH Guidance document on information requirements and chemical safety assessment –Chapter R.16: Environmental Exposure Estimation (ECHA, 2008), the outlier cut-off level can be determined with the following formula:

log(Xi) > log (p75) + K(log(p75) – log(p25))

with Xi being the concentration above which a measured concentration may be considered an outlier, pi the value of the ith percentile of the (non-parametric) distribution and K a scaling factor. A scaling factor K=1.5 is applied, as this value is used in most statistical packages.

The 25thand 75thpercentile of barium levels in top soil samples from the FOREGS data set are at 40 and 102 mg/kg soil dw, respectively, and the outlier cut-off value amounts to 415.3 mg Ba/kg dw. The outlier cut-off level is more than two orders of magnitude higher than the provisionally derived PNECsoil of 2.11 mg/kg dw. A factor of two is applied to the cut-off level and a PNECsoil of 207.7 mg Ba/kg dw is derived and considered to be a provisional yet reliable PNEC for soil.

- The PNEC value of 207.7 mg Ba/kg dw is by a factor of 1.4 higher than the overall 90thpercentile of barium baseline soil levels. Thus, a regional risk based on baseline barium levels can be excluded in Europe when the pooled dataset is evaluated.

- With exception of three countries (Czech Republic, Germany, Italy), the PNEC value of 207.7 mg Ba/kg dw is higher than the country-specific RWC-baseline PECs. Therefore, a regional risk based on RWC-baseline barium levels is not estimated in the majority of European countries when a country-specific approach is followed.

- The PNEC value of 207.7 mg Ba/kg dw is below the lowest NOEC of 211 mg/kg dw that was determined by Kuperman et al (2006) for soil invertebrates (E. crypticus, E. fetida and F. candida). Thus, the PNEC value of 207.7 mg Ba/kg dw is sufficiently protective for the evaluated soil invertebrates in a sandy loam soil with a relatively high bioavailability.