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Environmental fate & pathways

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Additional information

The substance is a multi constituent substance which consists of water and inorganic salts the main known constituents being sodium salts of carbonate, hydroxide and sulfides. In water the constituents are as dissociated or ionised. The formed ions are also found in background concentration in nature e.g. due to natural weathering of rock.

Abiotic degradation

Due to low vapour pressure of the substance and the constituents, no air emissions are expected. However, in acidic conditions where CO2 or H2S are formed, they 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.

In aquatic environment, sodium hydroxide dissociates completely to sodium and hydroxyl ions raising pH in the solution. Sodium carbonate reacts with water forming sodium and carbonate ions. Equilibrium between CO2/HCO3-/CO32 - is dependent on the surrounding pH. At pH > 12, as in this substance, carbonate ion is dominating (pKa 10.33). When pH decreases, equilibrium shifts towards bicarbonate (pKa 6.35), which acts as a buffer in nature. At pH below 6.35, carbon dioxide predominates.

When Na2S or NaHS are released to the aquatic compartment, they will hydrolyze immediately and a pH dependent equilibrium will be established between H2S, HS-and S2-. The pKa in 0.01-0.1 mol/L solutions at 18°C is 7.04 for HS-and 11.96 for S2-. However, in aerobic environments, the dissolved sulfide will eventually be oxidized to sulfate. In anaerobic environments (e.g., organic-rich sediments) H2S formation will be favored and no oxidation will occur. However, in these kinds of environments, the dissolved sulfide may be depleted through precipitation with metals. In environmental hazard assessment point of view, it should be kept in mind that typical organisms living in natural reducing environments are often well adapted to living in the fluctuating presence of H2S.

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


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 the constituents of this substance. Intermittent release of this substance in the environment could lead, depending on the amount released, to local increase in sodium concentration and pH. However, due to the buffer capacity of the receiving environment, a significant increase in the pH is not expected. Moreover, because these components tend to remain in water phase, the effect would be rapidly diluted by natural waters (e.g. lakes, rain water, soil pore waters). However, in a long term release, salt effect and increase of pH could disturb the balance of the local ecosystem. These ions can also participate in complex formation or precipitate with metal cations.

Sulfides, the most harmful constituents to the environment, will be hydrolyzed immediately upon contact with water, and depending on the conditions, reduced sulfides will stay present in the system or become eventually oxidized to 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).

Average annual pH of the most important freshwater aquatic ecosystems of the world range between 6.5 and 8.3 but lower and higher values have been measured in other aquatic ecosystems. At this range of pH, bicarbonate and hydrosulfide predominate. In European soils, pH has been modeled to be lower than 4.2 in 16.7% of the territory and only 1.9 % of the area present values of pH > 8. When circumstances are acidic, carbonic acid (H2CO3or CO2) as well as hydrogen sulfide (H2S) are formed. If concentration of the gases in water exceed water solubility limit, they can be distributed to the atmosphere.



European Union Risk Assessment Report (2007), Sodium hydroxide, vol. 73, available in:

Reuter H I, Lado L R, Hengl T et al. (2008) Continental-scale digital Soil Mapping using European soil profile data: Soil pH in: SAGA – Seconds Out. Hamburger Beiträge zur Physischen Geographie und Landschaftsökologie, Vol.19, 113pp.

OECD (2002), SIDS Initial Assessment Report for SIAM 15: Sodium carbonate, available in:

OECD (2002), SIDS Initial Assessment Report for SIAM 14: Sodium hydroxide, available in:

WHO (1981) IPCS International Programme on Chemical Safety, Environmental Health Criteria 1:, Hydrogen sulfide, available in: