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Environmental fate & pathways

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Sodium dithionite is an inorganic compound and is not subject to biodegradation. However, dithionite substances are unstable under environmental relevant conditions and readily disproportionate to (bi-)sulfites (HSO3-/ SO3-) and thiosulfates (S2O32-) which subsequently can be transformed by microbial activity.

Sulfites and thiosulfates are unstable in the environment and become part of the natural sulfur cycle. Microorganisms have a controlling influence on the oxidation state of sulfur, capable of either oxidation or reduction depending on microbial species and environment. Inorganic sulfur compounds are metabolized by physiologically and phylogenetically diverse microorganisms as electron donor or acceptor, playing a predominant role in the sulfur biogeochemical cycle in the environment.

Under oxygen-rich conditions, sulfites and thiosulfates are rapidly oxidized catalytically by (air) oxygen or by microbial activity to sulfate. Microbial oxidation of reduced sulfur species including elemental sulfur (S), sulfide (HS-), sulfite (SO32-) and thiosulfate (S2O32-) is an energetically favourable 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).

Under highly reduced conditions, 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, thiosulfate (S2O32-) and other sulfur-containing microbial substrates such as dithionite (S2O42-) or sulfite (SO32-) may be anaerobically utilised by sulfate-reducing bacteria (SRM) that are common in anaerobic environments, ultimately resulting in the reduction to sulfide. Accordingly, available literature data on sulfite transformation processes in water, sediment and soil show rapid transformation to sulfate when added to the respective environmental compartment, i.e. > 97% of added sulfite (30 – 150 µM SO32-) transformed within three days in sediments and marine waters, respectively. Available literature data on thiosulfate transformation processes in water, sediment and soil show rapid transformation when added to the respective environmental compartment, i.e. > 95% of added thiosulfate (80 - 140 µM) transformed within one week in sediments and freshwater, respectively and complete and rapid transformation of 1050 mg S2O32-/kg soil to sulfate in arable soils within one week (Findlay and Kamyshny 2017, Jørgensen et al. 1990, Barbosa-Jefferson et al. 1998).



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