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

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

Sodium dithionite dissociates into sodium cations and dithionite anions. Whereas sodium ions are essential for animal metabolism, do not bioaccumulate and underlie homeostatic control, dithionite anions are unstable under environmentally relevant conditions and disproportionate to (bi-)sulfites and thiosulfates. Thiosulfates and (bi)sulfites are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of sodium dithionite 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)Dithionite anions are unstable under environmentally relevant conditions and will rapidly disproportionate to (bi-)sulfites and thiosulfates (S2O32-) in aqueous media.Therefore, the environmental fate and transformation of sodium dithionite is accurately described by the fate of its dissociation/disproportionation products, i.e. sulfites and thiosulfates.

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, dithionites 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.