Sodium hydrogensulphide

Endpoint summary

Additional information

Abiotic degradation

Na2S and NaHS are not expected to be released to the air as such because of their low vapour pressure. However, in case H2S is formed, H2S may be released to the air. In the atmospheric compartment, sulfur compounds such as H2S will be oxidized to SO2 and, eventually, to sulfate compounds.

When Na2S or NaHS are released to the aquatic compartment, the substances will hydrolyze immediately and a pH dependent equilibrium will be established between H2S, HS- and S2-. However, in aerobic environments, the dissolved sulfide will be oxidized to - eventually - sulfate. In anaerobic environments (e.g., organic-rich sediments) H2S formation will be favoured and no oxidation will occur. However, in these kinds of environments, the dissolved sulfide may be depleted through precipitation with metals. In view of environmental hazard assessment, it should however be kept in mind that typical organisms living in natural reducing environments are often well adapted to living in the fluctuating presence of H2S.

Except in waterlogged and/or highly organic soils, released sulfides are expected to be oxidized relatively rapidly. In soils with reducing conditions, H2S formation will be favoured and similar transformation/removal processes as in sediments will occur. This also holds for deeper layers of well drained soils.

Biodegradation

Biodegradation is not deemed relevant for inorganic compounds. However, it should be kept in mind that any sulfur released to the environment will enter the natural sulfur cycle in which both oxidation and reduction reactions are mediated through abiotic as well as biotic processes. Sulfur oxidizing and reducing microorganisms are omnipresent and determine the predominant state of the present sulfur depending on the prevalent conditions.

Environmental distribution

Release to the aquatic environment will be the most relevant route of release for Na2S and NaHS. As mentioned above, the substances will be hydrolyzed immediately upon contact with water and depending on the conditions reduced sulfides will stay present in the system or become oxidized to - eventuallly - sulfate. At the moment of release, the sulfur added to the environment enters the natural sulfur cycle and industrially released sulfur will become indistinguishable from naturally present or - overall - formerly present sulfur. Consequently, the environmental distribution of these compounds will be driven by the same reactions driving the natural sulfur cycle. For a thorough description of the sulfur cycle one can refer to the review of Brown (1982).