Potassium thiosulphate

Endpoint summary

Description of key information

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

Additional information

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

Potassium thiosulfate dissociates into potassium cations and thiosulfate anions. Whereas potassium ions are essential for plant and 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 potassium thiosulfate is not expected due to its inherent physico-chemical properties.

(a) Potassium is very soluble and occurs as simple monovalent cation under environmental conditions. Although potassium is an abundant element, its mobility is limited by three processes: (a) it is readily incorporated into clay-mineral lattices because of its large size; (b) it is adsorbed more strongly than sodium on the surfaces of clay minerals and organic matter; and (c) it is an important element in the biosphere and is readily taken up by growing plants (Salminen et al. 2005).

(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 (S₂O₃^2-), sulfur (S), sulfide (HS^-) and sulfite (SO₃^2-) 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. Thiosulfates, as well as other sulfur-containing microbial substrates such as dithionite (S₂O₄^2-), or sulfite (SO₃^2-) 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.