Registration Dossier

Data platform availability banner - registered substances factsheets

Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Environmental fate & pathways

Endpoint summary

Currently viewing:

Administrative data

Description of key information

Upon dissolution in environmental medium, dipotassium disulfite dissociates into the respective ions, i.e. disulfite anions and potassium cations. In aqueous solution, disulfite anions are readily transformed to sulfites, i.e. bisulfite (HSO3-) and sulfite (SO32-) (EFSA, 2016). Potassium is an abundant element, very soluble and occurs as monovalent cation under environmental conditions. Conclusively, potassium cations become part of the global potassium 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 dipotassium disulfite 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: Dipotassium disulfite cannot bioaccumulate as it dissociates into sulfite anions and potassium cations upon release into the environment. Potassium 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 dipotassium disulfite is not expected and biomagnification and a significant transfer in the food chain can be excluded.

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

Potassium is very soluble and occurs as monovalent cation under environmental conditions. Although potassium is an abundant element, its mobility in aquatic and terrestrial ecosystems is limited by three processes: (a) it is readily incorporated into clay-mineral lattices because of its large size; (b) it is adsorbed more strongly than sodium on the surfaces of clay minerals and organic matter; and (c) it is an important element in the biosphere and is readily taken up by growing plants (Salminen et al. 2005). Conclusively, potassium cations become part of the global potassium cycle.

 

Reliable baseline levels of potassium in 745 pristine water/sediment samples collected across Europe were determined based on the FOREGS Geochemical Baseline Mapping Programme (Salminen et al. 2005), yielding representative median potassium concentrations of 16852 mg/kg and 1.65 mg/L in sediment and stream water, respectively.Regarding the partitioning of potassium in the water column, stream water/sediment partition coefficients range from 1402 L/kg (5thpercentile) to 117951 L/kg (95thpercentile) with a median logKp(solids-water in sediment) of 3.99 for potassium.

 

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).

 

References:

EFSA ANS Panel (EFSA Panel on Food Additives and Nutrient Sources Added to Food), 2016.Scientific Opinion on the re‐evaluation sulfur dioxide (E 220), sodium sulfite (E 221), sodium bisulfite (E 222), sodium metabisulfite (E 223), potassium metabisulfite (E 224), calcium sulfite (E 226), calcium bisulfite (E 227) and potassium bisulfite (E 228) as food additives.EFSA Journal2016;14(4):4438 151 pp.

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 dipotassium disulfite dissociate intosulfur 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 ofdipotassium disulfiteand 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+ H2 H2SO3

2. H2SO3  H++ HSO3-                                   pka1: 1.85

3. HSO3-  H++ SO32-                                     pka2: 7.20

4. 2 HSO3-  H2O + S2O52-

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

 

5. SO32-+ 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 SO32-+ O22 SO42-(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 (S2O32-) 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+ H2OHS2O3-+ H3O+                                              pka1: 0.6

8. HS2O3-+ H2OS2O32- + 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. S2O32-(aq)SO32-(aq) + S (s)

10. S2O32-(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 (S2O32-) anions being unstable and readily oxidized to SO42-(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 (SO42-).

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 (SO32-), thiosulfate (S2O32-), elemental sulfur (S) and sulfide (HS-), resulting in ultimate transformation into sulfate (SO42-) (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 SO42-(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 (S2O42-), thiosulfates (S2O32-) or sulfite (SO32-) 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 SO32-+ H+3 SO42-+ HS-               

12. S2O32-+ H2OSO42-+ HS-+ H+                          

13. 4 S0+ 4 H2OSO42-+ 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 SO32-to SO42-(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 (SO32-) in sediments is subject to rapid transformation processes. Based on a study performed by Zopfi et al. (2004), 30-40 µM SO32-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 (SO32-) 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 (SO32-), thiosulfate (S2O32-), elemental sulfur (S) and sulfide (HS-), resulting in ultimate transformation into sulfate (SO42-, Simon and Kroneck, 2013), therefore entering the global sulfur cycle:

S2O32-HSO3-/ SO2SO32-→SO42-

Data on sulfite transformation in soils is scarce. Observed SO32-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 SO32-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 (5thpercentile) to 163.8 mg/L (95thpercentile) with a median of 16.9 mg/L. Total sulfur concentrations of streamwater sediments are magnitudes higher with 50thand 95thpercentiles 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 5th, 50thand 95thpercentiles 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 95thpercentile 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 5thand 95th 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 5thand 95thpercentiles of 67.3 and 1288.6 mg/kg sulfur, respectively.

Taking into account the high quality and representativeness of the data set, the 95thpercentile of 783.91 mg/kg can be regarded as representative background concentration for sulfur in European agricultural soils and the 95thpercentile 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.