Sodium hydrogensulfite

Endpoint summary

Description of key information

Upon dissolution in environmental medium, sodium hydrogen sulfite dissociates into the respective ions, i.e. sulfite anions and sodium cations. Sodium is an abundant element, very soluble and occurs as monovalent cation under environmental conditions. Conclusively, sodium cations become part of the global sodium cycle.

Sulfites though naturally present in the environment are not stable under typical environmental conditions. Environmental transformation reactions include oxidation and reduction depending on the environmental conditions. Sulfites become ultimately part of the natural sulfur cycle resulting in sulfides and sulfates - indistinguishable from natural sulfur reservoirs.

Abiotic degradation: Photolysis, hydrolysis and photodegradation are not relevant as sodium hydrogen sulfite dissociates rapidly and is transformed e.g. by oxidation, reduction, speciation, precipitation in environmental solutions.

Biotic degradation: The substance is an inorganic compound and is not subject to biodegradation.

Bioaccumulation: Sodium hydrogensulfite cannot bioaccumulate as it dissociates into sulfite anions and sodium cations upon release into the environment. Sodium ions are essential for plant and animal metabolism, do not bioaccumulate and are subject to 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. Sulfur is essential as structural component and on a metabolic level and does not bioaccumulate. Thus, bioaccumulation of sodium hydrogensulfite is not expected and biomagnification and a significant transfer in the food chain can be excluded. 

Distribution: The fate of sodium hydrogensulfite is determined by the fate of its released ions upon dissolution, i.e. sulfite anions and sodium cations.

Sodium is very soluble and occurs as monovalent cation that, under environmental conditions, tends to remain in the dissolved form once in solution. Sodium is an abundant element, however its mobility in aquatic and terrestrial ecosystems may be limited by its adsorption on clay minerals with high cation-exchange capacities. Sodium is an important element in the biosphere and is essential to human life (Salminen et al. 2005). Conclusively, sodium cations become part of the global sodium cycle.

Reliable baseline levels of sodium in 743 pristine water/sediment samples collected across Europe were determined based on the FOREGS Geochemical Baseline Mapping Programme (Salminen et al. 2005), yielding representative median sodium concentrations of 6676.72 mg/kg and 6.76 mg/L in sediment and stream water, respectively.Regarding the partitioning of sodium in the water column, stream water/sediment partition coefficients range from 78 L/kg (5^thpercentile) to 15601 L/kg (95^thpercentile) with a median logKp(solids-water in sediment) of 2.89 for sodium.

Based on the available literature data, sulfite anions are unstable in soils, sediments and aqueous systems under environmentally relevant conditions and are readily oxidized to sulfate or reduced to sulfide.However, high-quality data are available on sulfur partitioning in aquatic ecosystems based on the FOREGS Geochemical Baseline Mapping Programme and additional supporting studies on sulfur partitioning on soils and marine sediments, yielding a log Kp(solids-water in freshwater sediment) of 2.02 L/kg (sulfur, n = 750), a log Kp(solids-water in marine sediment) of 1.58 L/kg (sulfur, n = 2) and a log Kp(solids-water in soil) of 1.64 L/kg (sulfur, n = 25).

Additional information

Abiotic and biotic processes determining the fate of sulfites and thiosulfates in the environment

Upon contact with water, salts of sulfur oxyacids including sodium hydrogen sulfite dissociate into sulfur oxyacid anions and the respective counterions. Only the properties of sulfite anions are considered a relevant determinant of environmental toxicity since respective counter ions, i.e. ammonium, calcium, magnesium, sodium or potassium cations, are not assumed to affect the toxicity. Sulfites though naturally present in the environment are not stable under typical environmental conditions. Environmental transformation reactions include oxidation and reduction depending on the environmental conditions. Sulfites become ultimately part of the natural sulfur cycle resulting in sulfides and sulfates - indistinguishable from natural sulfur reservoirs.

Sulfites are also a metabolite and intermediate of sulfur-containing amino acids in organisms but do not accumulate.

The environmental fate and transformation of sodium hydrogen sulfite and its dissociation products depend on the environmental conditions as described in the following:

Sulfite: All sulfite, hydrogensulfite and metabisulfite substances are highly soluble in water, establishing upon dissolution an equilibrium that depends on solution pH as follows (Wiberg et al., 1996; Lide, 2007):

1. SO₂+ H₂O ⇌ H₂SO₃

2. H₂SO₃ ⇌ H⁺+ HSO₃^-                                   pka₁: 1.85

3. HSO₃^- ⇌ H⁺+ SO₃^2-                                     pka₂: 7.20

4. 2 HSO₃^- ⇌ H₂O + S₂O₅^2-

In acidic solutions, sulfites and hydrogen sulfites may release SO₂but this is not likely to occur under e.g. normal natural environmental conditions.

 

5. SO₃^2-+ 2 H₃O+⇌SO₂+ 3 H₂O

 

Under oxidising conditions, e.g., in surface waters, sulfite is rapidly oxidized to sulfate catalytically by (air) oxygen or by microbial action. A half-life of 77 hour was measured in deionized water, already suggesting substantial abiotic degradation.

 

6. 2 SO₃^2-+ O₂⇌2 SO₄^2-(in presence of oxygen)

The reaction is accompanied with consumption of dissolved oxygen. Thus, observed toxic effects to aquatic organisms may be indirect effects, i.e. caused by lack of oxygen.

Thiosulfate:

The structure of the thiosulfate ion (S₂O₃^2-) is comparable to the sulfate ion with one oxygen atom replaced by a sulfur atom. It is considered to be metastable and has only moderate reducing properties (Cotton et al., 1999). Thus, thiosulfate is an oxyanion of sulfur, is the respective anion of the strong thiosulfuric acid and the only species relevant upon dissolution in an aqueous medium under environmental conditions:

 

7. H₂S₂O₃+ H₂O⇌HS₂O₃^-+ H₃O⁺                                              pka₁: 0.6

8. HS₂O₃^-+ H₂O⇌S₂O₃^2- + H₃O⁺                                               pka₂: 1.74

Thiosulfates occur naturally and are produced by certain biochemical processes. Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic (physiological) solutions, due to decomposition to sulfite and sulfur, the sulfite being dehydrated to SO₂:

9. S₂O₃^2-(aq)⇌SO₃^2-(aq) + S (s)

10. S₂O₃^2-(aq) + 2 H₃O⁺(aq)⇌SO₂(g) + S (s) + H₂O

Thus, in contrast to sulfite, the disproportionation of thiosulfates occurs under acid but not under alkaline conditions. Regarding the fate of thiosulfates, H₂S₂O₃is a strong acid and is dissociated under environmentally relevant conditions, with thiosulfate (S₂O₃^2-) anions being unstable and readily oxidized to SO₄^2-(Lindsay, 1979). Under normal environmental conditions, a break down to sulfite and a subsequent oxidation to sulfate may be anticipated.

The presence of metal cations in the environment, such as copper, iron and manganese, accelerates the oxidation rate. The most stable and predominant sulfur form in freshwater and in all but highly reduced environments is sulfate (SO₄^2-).

In addition to the abiotic processes, microbial oxidation of sulfur compounds is an energetically favourable reaction carried out by a wide range of organisms, i.e. sulfur oxidizing microorganisms (SOM), which may oxidize reduced sulfur species including sulfite (SO₃^2-), thiosulfate (S₂O₃^2-), elemental sulfur (S) and sulfide (HS-), resulting in ultimate transformation into sulfate (SO₄^2-) (Simon and Kroneck, 2013). Therefore, regarding the fate of sulfites and thiosulfates in the environment, sulfurous acid (H₂SO₃) and its salts (i.e. sulfites) are generally considered as unstable in soils due to quick abiotic turnover to SO₄^2-(oxidation) or by microbial action.

Under anoxic conditions, the resulting sulfate will be readily reduced to sulfide by sulfate-reducing bacteria (SRM) which are common in anaerobic environments. However, also other substrates, e.g. dithionite (S₂O₄^2-), thiosulfates (S₂O₃^2-) or sulfite (SO₃^2-) may be used in this anaerobic process, ultimately resulting in reduction to sulfide (H₂S). Available data show very low levels of free sulfide in pore waters of marine and freshwater sediments, i.e. levels of < 1 µM free sulfide in sediments of the Odder River and Brabrand Lake (Denmark) (Jørgensen, 1990), presumably due to rapid formation of insoluble metal sulfides. Therefore, the reduction of sulfates, sulfites and thiosulfates to sulfides may ultimately result in formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, e.g. FeS, ZnS, PbS and CdS (Lindsay, 1979; OECD SIDS, 2012).  Nevertheless, metal sulfides are not expected to persist under oxidising conditions, e.g. in surface water or topsoil.

Additionally, a significant set of microorganisms is able to grow by disproportionation of sulfite, thiosulfate or elemental sulfur, ultimately resulting in sulfate and sulfide (Simon and Kroneck, 2013 and references therein; Janssen et al. 1996, Bak and Cypionka, 1987):

11. 4 SO₃^2-+ H+⇌3 SO₄^2-+ HS^-               

12. S₂O₃^2-+ H₂O⇌SO₄^2-+ HS^-+ H⁺                          

13. 4 S⁰+ 4 H₂O⇌SO₄^2-+ 3 HS^-+ 5 H⁺

 

In presence of metal ions, e.g. ferrous iron, the resulting hydrogen sulfide is expected to precipitate rapidly as insoluble metal sulfides, e.g. FeS, resulting in very low dissolved sulfide levels (Finster et al. 1998).

Water:

In aqueous systems, a quick turnover of SO₃^2-to SO₄^2-(oxidation) and/or sulfide (HS^-) is expected. Data on sulfite transformation processes in aqueous is however scarce. In a study performed by Findlay and Kamyshny (2017), sulfite amended to anoxic water samples at environmentally relevant concentrations (approx. 140 µM) was consumed rapidly with < 99% of sulfite consumed after 3 days.

 

Sediment:

Significant population densities of bacteria capable of disproportionation were found in anoxic marine sediments (e.g. Aarhus bay, Skagerrak, Denmark) (Thamdrup et al. 1992). The disproportionation of sulfur compounds by microorganisms is not limited to marine environments but is also reported for freshwater systems (Jørgensen, 1990; Thamdrup et al. 1992).

Sulfite (SO₃^2-) in sediments is subject to rapid transformation processes. Based on a study performed by Zopfi et al. (2004), 30-40 µM SO₃^2-amendments to marine surface sediment showed complete and rapid transformation of sulfites back to control levels within 9 hours. Additional literature data is available on sulfite (SO₃^2-) transformation processes from a study performed by Findlay and Kamyshny (2017), again showing fast consumption of sulfite when added to sediment: With 150 µM added to the sediment, sulfite was consumed rapidly resulting in low sulfite concentrations of < 25 µM and < 5 µM after one and three days, respectively.

 

Soil:

In addition to abiotic sulfite transformation processes, microbial oxidation of sulfur compounds in soils is an energetically favourable reaction carried out by sulfur oxidizing microorganisms (SOM), which may oxidize reduced sulfur species including sulfite (SO₃^2-), thiosulfate (S₂O₃^2-), elemental sulfur (S) and sulfide (HS-), resulting in ultimate transformation into sulfate (SO₄^2-, Simon and Kroneck, 2013), therefore entering the global sulfur cycle:

S₂O₃^2-⇌HSO₃^-/ SO₂⇌SO₃^2-→SO₄^2-

Data on sulfite transformation in soils is scarce. Observed SO₃^2-oxidation rates in soils are dependent on soil characteristics, i.e. are decreasing with increasing soil pH. In a study performed by Lee et al. (2007), soils collected from the surface horizon (0 to 20 cm) were amended with 0.3 % w/w CaSO₃. Based on the analysis of soils leachates initial SO₃^2-oxidation rates were dependent on soil pH – however final recovered sulfate concentrations were similar among all tested soils (pH range 4.0 - 7.8) irrespective of pH, yielding > 75% recovery of the total added sulfur.

 

In sum, sulfites and thiosulfites are sulfur and oxygen containing compounds that may reasonably be considered chemically unstable under most environmental conditions, are transformed into other sulfur species and ultimately become part of the sulfur cycle.

 

Environmental sulfur/sulfate concentrations: 

A total of 750 stream water samples were processed in the FOREGS-program to determine typical sulfur stream water concentrations (Salminen et al. 2005).The FOREGS Geochemical Baseline Mapping Programme’s main objective is to provide high quality, multi-purpose homogeneous environmental geochemical baseline data for Europe. Based on the FOREGS dataset, sulfate concentrations in European stream waters may range from 1.18 mg/L (5^thpercentile) to 163.8 mg/L (95^thpercentile) with a median of 16.9 mg/L. Total sulfur concentrations of streamwater sediments are magnitudes higher with 50^thand 95^thpercentiles of 508 and 2816 mg sulfate/L, respectively.Regarding the partitioning of sulfur in the water column, stream water/sediment partition coefficients range from 1.28 L/kg to 15729.3 L/kg with a median logKp(solids-water in sediment) of 2.02 for sulfur (with concentrations of the water based on measured sulfate as the predominant sulfur species and total sulfur of the sediment).

Regarding background soil concentrations, baseline sulfur levels in topsoil (0-25 cm) range from < 50 mg/kg to 6518 mg/kg sulfur with 5^th, 50^thand 95^thpercentiles of 75 mg/kg, 222 mg/kg and 645 mg/kg sulfur, respectively. Taking into account the high quality and representativeness of the dataset, the 95^thpercentile of 645 mg/kg sulfur can be regarded as typical background sulfur concentrations of topsoil in EU countries.

In addition, ambient sulfur concentrations of agricultural and grazing land soil are available. Sulfur levels of agricultural soil range from < 5.00 to 68226.3 mg/kg sulfur with 5^thand 95^th percentiles of 59.7 and 783.9 mg/kg sulfur, respectively. In grazing land, soil sulfur concentrations range from < 5.00 to 98189.7 mg/kg with 5^thand 95^thpercentiles of 67.3 and 1288.6 mg/kg sulfur, respectively. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 783.91 mg/kg can be regarded as representative background concentration for sulfur in European agricultural soils and the 95^thpercentile of 645 mg/kg can be regarded as representative background concentration for sulfur in European grazing land soils.

 

References:

Finster, K., Liesack, W., & Thamdrup, B. O. (1998). Elemental sulfur and thiosulfate disproportionation by Desulfocapsa sulfoexigens sp. nov., a new anaerobic bacterium isolated from marine surface sediment. Applied and Environmental Microbiology, 64(1), 119-125.

Lide, D. R. (2007). Handbook of Chemistry and Physics, volume 88th edition.CRC Press,22(24), 154.

Wiberg, N., & Dehnicke, K. (1996). Hollemann-Wiberg, Lehrbuch der Anorganischen Chemie.Angewandte Chemie-German Edition, 108(21), 2696.