Reaction mass of sodium hydrogensulfide and sodium carbonate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

2.4 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

2.4 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

2.4 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1.4 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

159 µg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

159 µg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Additional information

As explained in several sections of this dossier, Na₂S and NaHS will not occur as such in the environment. Upon release to the aquatic environment, these substances are hydrolyzed immediately, establishing a pH-dependent equilibrium between H₂S, HS^- and S^2-. H₂S is by far the most toxic species and therefore ecotoxicity tests using Na₂S, NaHS or their respective hydrates focus on H₂S formation and toxicity in most cases. When effect concentrations resulting from these tests would be expressed on a test material basis, an extremely large range of effect concentrations would be obtained, because the toxicity of these substances depends on the relative abundance of H₂S which in its turn depends on the physicochemical characteristics of the aquatic environment under consideration (e.g, pH, temperature, salinity, redox conditions). In other words, H₂S concentrations seem to be much better predictors of toxicity than concentrations of Na₂S, NaHS or their respective hydrates. Therefore the PNECs calculated for the reaction mass are based on H₂S concentrations, using the worst-case assumption in which the total amount of sulfide contained by Na₂S and NaHS is converted to H₂S.

Since oxidation of sulfides to sulfate will counter the formation of the extremely toxic H₂S in most environments, toxicity data are presented also for other components of the reaction mass. Of these, sodium hydroxide NaOH (1 -5 %) has, after H₂S, ecotoxicological effects in the aquatic environment at the lowest levels of concentration. While due to limited experimental ecotoxicity data, it is not possible to calculate PNECs for NaOH, the few reported EC50s for the aquatic invertebrate Ceriodaphnia sp. (Warne, 1999) and the microorganism Photobacterium phosphoreum (Bulich et al., 1990) range from 20 to 40 mg/L, while "significant changes" in the biology of a fish were observed at 25 -100 mg/L (Rustamova, 1977). For Na₂CO₃, the aquatic toxicity data indicate toxicity to various organisms at 200 -1000 mg/L, and for NaSO₄, a PNEC_aquatic of 1.285 mg/L for freshwater organisms can be calculated from the available data (sulfide dossier).

It should be noted that due to the high alkalinity (pH 13) of the reaction mass, to which both NaOH and Na₂CO₃ contribute, releases into the aquatic environment may locally have acute ecotoxicological effects by raising the pH of water. The alkalinity may similarly have local acute ecotoxicological effects in sediment and soil, although reactions with water and organic material should quickly neutralize the bases.

Conclusion on classification

The classification of Na₂S is harmonized. According to Annex I of the Dangerous Substances Directive, the substance is classified as N (dangerous to the environment) and R50 (very toxic to aquatic organisms). This harmonized classification was translated to a similar classification as GHS09 and Acute Aquatic 1 with hazard statement H400 under the CLP regulation. This classification is supported by the available toxicity data for aquatic organisms. The lowest acute effect concentration was a 96-h LC50 of 0.0027 mg H₂S/L for fish, obtained from a test using Na₂S.9H₂O as a test substance. When expressed on a test material basis, this effect concentration is still below 1 mg/L, justifying classification as very toxic to aquatic life. No classification for chronic toxicity (long-term effects) would apply because increasing exposure durations do not result in increased toxicity and because the substance has no potential for bioaccumulation. Although the classification of NaHS has not been harmonized yet at this point in time, the same classification applies as for Na₂S.

Due to the lowest acute effect concentration for H₂S being between 0.001 and 0.01 mg/L, the multiplying factor (M) used for H₂S concentrations when determining the environmental classification of mixtures containing it is 100. Therefore a H₂S concentration of only 0.25 % (w/w) is required to classify a mixture containing H₂S as Acute Aquatic 1. While only specific environmental conditions (reductive, low pH) favour the formation of H₂S, 0.25 % is only 2 % of the total amount of H₂S that the reaction mass has potential to form, and the reaction mass itself should therefore be classified as Acute Aquatic 1.

NaOH, Na₂CO₃ and NaSO₄ have no environmental classification.