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Sediment toxicity

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Description of key information

No reliable studies were identified for freshwater sediment organisms. The key study (Thompson et al., 1991) yielded a 60-d NOEC of 1.1 mg H2S/L (pore water concentration) for survival of the sea urchin Lytechnius pictus. For hazard assessment purposes, a worst case approach can be followed (because adsorption/desorption processes are less relevant for H2S) in which this critical effect concentration is recalculated to sediment-based hydrogen sulfide concentration assuming that all H2S was present in the pore water. 

Key value for chemical safety assessment

Additional information

Ten studies were identified reporting relevant information with regard to the toxicity of sulfide to sediment organisms. Of these ten studies only one could be considered reliable with restrictions (Klimisch 2). The other nine studies were considered not reliable. Here too, the toxicity of Na2S and NaHS upon dissolution in the aquatic environment is dependent on the relative abundance of H2S, which is the most toxic sulfide compound. The reliable study (Thompson et al., 1991) investigated the effects of hydrogen sulfide on survival, growth and gonad production of the sea urchin Lytechinus pictus when exposed for 49 days in a sediment-water system in which fresh sulfide solutions were pumped through the sediment (exposure after equilibration period of at least 60 days). This modification to the test system is necessary to ascertain constant exposure. The NOEC values for survival, growth (test diameter), growth (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 measured pore water sulfide concentrations. This study was conducted at a pH of 8.03 and a dissolved oxygen level of 5.1 mg/L in the overlying water. As a worst case approach (because adsorption/desorption processes are less relevant for H2S), the effect concentrations may be recalculated to sediment-based hydrogen sulfide concentrations assuming that all H2S was present in the pore water. No reliable studies were identified on freshwater sediment organisms.

In sediments without reducing conditions in the upper sediment layer(s), no sulfide precipitation and H2S formation is expected. In that case, potential risks to sediment organisms resulting from exposure to the sulfur released from the identified uses of the sodium sulfide compounds under consideration could be evaluated using toxicity data for sulfate. As demonstrated in the OECD SIDS for Na2SO4, this oxidized sulfur compound is much less toxic than H2S (96-h LC50 for Trycorythus sp. = 660 mg Na2SO4/L, Goetsch and Palmer, 1997). However, this information is not taken up in this dossier because the aquatic compartment (water column) is considered the most critical compartment for risk characterization. Consequently, the read across strategy using toxicity data for Na2SO4 is only presented for the aquatic compartment (water column).

There is no sediment ecotoxicity data for NaOH or Na2CO3. The alkalinity of the reaction mass (pH 13) may locally have acute ecotoxicological effects in the aquatic environment, although within sediment, reactions with water and organic material should quickly neutralize the bases.