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Environmental fate & pathways

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The dithionite anion has strongly reducing properties and decomposes/disproportionates rapidly in aqueous media (especially under acidic anaerobic conditions) to form sulfite and thiosulfate (S2O32-); under aerobic conditions, it will additionally be oxidised to finally form sulfate as the final oxiodation/decomposition product. According to Hofmann and Rüdorff (1969) and Holleman and Wiberg (1995), this process can roughly be described by the following equations (under aerobic conditions and at low concentrations, reaction (2) is favoured:


2 Na2S2O4+ H2ONa2S2O3+ 2 NaHSO3(anaerobic conditions) (1),


Na2S2O4+ O2+ H2ONaHSO4+ NaHSO3(aerobic conditions) (2)


According to the literature overview of Münchow (1992) the following principal decomposition patterns can be described for dithionite in relation to pH ranges at temperatures between 0°C and 32°C for 0.0025 molar solutions:


strongly alkaline medium: 3 S2O42-+ 6 OH-5 SO32-+ S2-+ 3H2O

weakly alkaline to weakly acidic medium: 2 S2O42-+ H2O2 HSO3-+ S2O32-

acidic medium: 2 H2S2O43 SO2+ S + 2 H2O

strongly acidic medium: 3 H2S2O45 SO2+ H2S + 2 H2O