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The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

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Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Taking into account (i) the rapid dissociation of sodium thiosulfate and decomposition of thiosulfates 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 of sodium thiosulfate in soil can be expected to be low.

Additional information

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

Sodium thiosulfate dissociates into sodium cations and thiosulfate anions. Whereas sodium ions are essential for animal metabolism, do not bioaccumulate and underlie homeostatic control, thiosulfate anions are unstable under environmentally relevant conditions and disproportionate into sulfite anions. Sulfites are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of sodium thiosulfate 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)Thiosulfate anions are unstable under environmentally relevant conditions, including soils, and will disproportionate to sulfite.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 thiosulfate (S2O32-), sulfur (S), sulfide (HS-) and sulfite (SO32-) 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 (SO42-, 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. Thiosulfates, as well as other sulfur-containing microbial substrates such as dithionite (S2O42-), or sulfite (SO32-) may also be directly anaerobically utilised, ultimately resulting in the reduction to sulfide (H2S).

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

In sum, thiosulfates 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.