Magnesium dihydrogen disulphite

Endpoint summary

Description of key information

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

Additional information

Abiotic and biotic processes determining the fate of magnesium dihydrogen disulfite in soils

Magnesium dihydrogen disulfite dissociates into sulfite anions and the respective magnesium cations upon contact with soil moisture. Whereas magnesium ions are essential for plant and animal metabolism, do not bioaccumulate and underlie homeostatic control, sulfite anions are unstable under environmentally relevant conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle. Therefore, terrestrial toxicity of magnesium dihydrogen disulfite is not expected due to its inherent physico-chemical properties.

 

(a) Magnesium compounds are very soluble and magnesium is highly mobile after its release by weathering under all environmental conditions. Under the pH, redox and conductivity regimes typically found in streams, Mg is likely to be present almost exclusively as Mg²⁺(Salminen et al. 2005 and references therein). Conclusively, magnesium cations become part of the global magnesium cycle.

 

(b) Sulfites are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. 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 sulfur (S), sulfide (HS^-), sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) 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. Other sulfur-containing microbial substrates such as dithionite (S₂O₄^2-), thiosulfates (S₂O₃^2-) or sulfite (SO₃^2-) may also be anaerobically utilised, ultimately resulting in the reduction to sulfide (H₂S).

 

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

 

Therefore, sulfites 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.