Sodium sulphite

Endpoint summary

Description of key information

Takinginto account(i) the rapid dissociation of sodium sulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural sodium and sulfur cycle, (ii) ubiquitousness of sodium and inorganic sulfur substances in soil, (iii) essentiality of sodium and sulfur, and (iv) the lack of a potential for bioaccumulation and toxicity to aquatic organisms, the hazard potential ofsodium sulfite in soil can be expected to be low.

Additional information

Abiotic and biotic processes determining the fate of sodium sulfite in soils

Sodium sulfite dissociates into sulfite anions and the respective sodium cations upon contact with soil moisture. Whereas sodium ions are essential for animal metabolism, do not bioaccumulate and underlie homeostatic control, sulfite anions are unstable under environmentally relevant conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of sodium sulfite is not expected due to its inherent physico-chemical properties.

 

(a) Sodium is very soluble and occurs as monovalent cation under environmental conditions.There are no low-solubility salts of sodium, so once the element is in solution it tends to remain in the dissolved form, although its mobility may be reduced by adsorption on clay minerals with high cation-exchange capacities(Salminen et al. 2005). Conclusively, sodium cations become part of the global sodium cycle.

 

(b)Sulfites are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. Under oxygen-rich conditions, sulfites are rapidly oxidized catalytically by (air) oxygen or by microbial action to sulfate. Microbial oxidation of reduced sulfur species including elemental sulfur (S), sulfide (HS^-), sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) is an energetically favorable reaction carried out by a wide range of organisms, i.e. sulfur oxidizing microorganisms (SOM) resulting in ultimate transformation into sulfate (SO₄^2-, Simon and Kroneck, 2013).

 

In highly reduced (water-logged) soils, reduction to sulfides may take place with subsequent formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, including FeS, ZnS, PbS and CdS (Lindsay, 1979, Finster et al., 1998). Thus, under anoxic conditions, sulfate is readily reduced to sulfide by sulfate-reducing bacteria (SRM) that are common in anaerobic environments. Other sulfur-containing microbial substrates such as dithionite (S₂O₄^2-), thiosulfates (S₂O₃^2-) or sulfite (SO₃^2-) may also be anaerobically utilised, ultimately resulting in the reduction to sulfide (H₂S).

 

In sum, a significant set of microbial populations grows by disproportionation of sulfite, thiosulfate or elemental sulfur, ultimately yielding sulfate or sulfide (Simon and Kroneck 2013 and references therein; Janssen et al. 1996, Bak and Cypionka, 1987).

 

Therefore, sulfites may reasonably be considered chemically unstable under most environmental conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle.