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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.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

Administrative data

Link to relevant study record(s)

Description of key information

Taking into account (i) the rapid dissociation of sodium thiosulfate and decomposition of thiosulfates upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural sodium and sulfur cycle, (ii) ubiquitousness of sodium and inorganic sulfur substances in soil and (iii) essentiality of sodium and sulfur in terrestrial organisms, sodium thiosulfate is expected to have a low potential for bioaccumulation in terrestrial organisms.

Key value for chemical safety assessment

Additional information

(1) Environmental fate and stability in soil:

Sodium thiosulfate dissociatesinto thiosulfate anions and sodium cations in environmental solutions, including soil porewater.

(a)Sodium is very soluble and occurs as monovalent cation under environmental conditions.There are no low-solubility salts of sodium, so once the element is in solution it tends to remain in the dissolved form, although its mobility may be reduced by adsorption on clay minerals with high cation-exchange capacities(Salminen et al. 2005). Conclusively, sodium cations become part of the global sodium cycle.

(b)Thiosulfates are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. Under oxygen-rich conditions, thiosulfates are rapidly oxidized catalytically by (air) oxygen or by microbial action to sulfate. Microbial oxidation of reduced sulfur species including thiosulfate (S2O32-), elemental sulfur (S), sulfide (HS-) and sulfite (SO32-) and 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 (SO42-, Simon and Kroneck, 2013).Accordingly, data on thiosulfate stability in soil is available from a study performed by Barbosa-Jefferson et al. (1998) using four top soils (0-15 cm) from major arable areas in Britain, where S2O32-added to four arable soils at high concentrations (1050 mg S2O32-/kg soil) showed rapid and complete transformation primarily to sulfate (SO42-).

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, thiosulfate (S2O32-) and other sulfur-containing microbial substances 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.In addition, 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).

In sum, thiosulfates 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.

(2) Ubiquity and natural/ambient background:

(a)Sodiumis the most abundant of the alkali metals, the fifth most abundant metal in the Earth’s crust with an average value of 22,700 mg kg-1,ubiquitous and a constituent of soil minerals(Salminen et al. 2005 and references therein).The FOREGS dataset reports sodium/sodium oxide concentrations for 845 topsoil samples (dried, grinded, sieved to < 2 mm, pulverised to < 0.063 mm, and analysed by XRF, LOQ: 0.01 %) sampled on a grid across Europe. Sodium oxide data were converted into sodium concentrations. Based on 833 paired samples for EU-27, UK and Norway, baseline sodium levels in topsoil range from 297 mg/kg to 33,013 mg/kg sodium with 5thand 95thpercentiles of 1,009 mg/kg and 22,434 mg/kg sodium, respectively, and a median concentration of 5,935 mg/kg sodium. Taking into account the high quality and representativeness of the data set, the 95thpercentile of 22,434 mg/kg (based on XRF data) can be regarded as representative background concentration for sodium in European topsoils.

Additionally, sodium concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for sodium. Sodium concentrations of respective aqua regia extracts were measured by ICP-MS. Aqua regia extracts are considered adequate for assessing total elemental concentrations in soil and sediments according to international standards (USEPA 3050 or ISO standard 11466) but may however yield lower elemental concentrationswhen directly compared to XRF data for some rock forming elements including sodium. Sodium levels of agricultural soil range from < 2.00 mg/kg to 7,236.77 mg/kg sodium with 5thand 95thpercentiles of 11.9 mg/kg and 223.9 mg/kg sodium, respectively, and a median of 50.2 mg/kg sodium. In grazing land, soil concentrations of sodium range from < 2.00 mg/kg to 12,284.68 mg/kg with 5thand 95thpercentiles of 13.29 mg/kg and 217.17 mg/kg sodium, respectively, and a median of 53.1 mg/kg sodium. Taking into account the high quality and representativeness of the data set, the 95thpercentile of 223.9mg/kg can be regarded as representative background concentration for sodium in European agricultural soils and the 95thpercentile of 217.17mg/kg can be regarded as representative background concentration for sodium in European grazing land soils (based on aqua regia extracts).

(b) Sulfur is a ubiquitous natural component of soil. Most terrestrial environments have substantial sulfur levels whereas sulfur-deficient environments are rare. In soil, sulfur can be found as pure element, sulfide (salts containing S2-) and sulfate (SO42-) minerals and in various organic substances. In all but highly reduced soils, sulfate is the most stable species at environmentally relevant pH > 4. Other stable sulfur species such as SO(g), S2-, S2O32-and S2O42-are, however, not prevalent in soils (Lindsay, 1979). Due to its ability to exist in a wide range of oxidation states, sulfur plays an important role in living organisms, both as a structural component and a redox-active element. Soluble states of sulfur such as sulfates and sulfites are common in their various elemental forms. The three most abundant forms of sulfur are elemental sulfur, sulfate (SO42-) and sulfide (S2-) and sulfur containing oxyanions, i.e. sulfite (SO32-), dithionite (S2O4), thiosulfate (S2O3) and polythionates such as trithionate (S3O62-) and tetrathionate (S4O62-, Simon and Kroneck 2013). Due to its key importance for biological processes and unique metabolic versatility, i.e. its appearance in amino acids, iron-sulfur proteins, thioredoxins and sulfolipids, the major fraction of the sulfur in surface soil horizons is present in organic combinations, e.g. in plant litter, microbial biomass or stabilized in soil organic matter with the remainder occurring as inorganic sulfate (Maynard et al., 1998).

A total of 837 topsoil samples were processed in the FOREGS-program to determine sulfur background concentrations. Sulfur concentrations of respective aqua regia extracts were measured by ICP-AES (limit of quantification (LOQ): 50 mg/L). Based on 775 paired samples from the FOREGS dataset, the median sulfur content of European topsoil amounts to 222 mg/kg ranging from <50 to 6,518 mg/kg, and the 95thpercentile of 645 mg/kg can be regarded as representative background in European topsoils (Salminen et al. 2005).

Additionally, sulfur concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for sulfur. Sulfur concentrations of respective aqua regia extracts were measured by ICP-OES and/or ICP-MS. Sulfur levels of agricultural soil range from < 5 to 68,226 mg/kg sulfur with a median of 209 mg/kg and a 95thpercentile of 783.91 mg/kg. In grazing land, soil concentrations of sulfur range from < 5.00 to 98,189 mg/kg with a median of 310 mg/kg and a 95thpercentile of 645 mg/kg (Reimann et al. 2014). 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.

(3) Essentiality:

(a) Sodium is essential for the most basic cellular mechanisms in animal cells. In most animal cells, differences in K+ and Na+ concentrations between the interior and exterior of the cell exist. More than one third of the total ATP consumption in resting animal cells is used to maintain this gradient. This gradient is crucial in the control of cell volume, in rendering nerve and muscle cells electrically excitable, and is the driver of active sugar and amino-acid transport. In a reverse mechanism, it can be used to generate ATP (Stryer, 1988). Salkoff et al. (2005) summarized the extent to which channel deficiencies affectCaenorhabditis elegans(nematode): Individuals with deficient sodium-activated potassium channels show severe malfunctions, including locomotive and motoric disorders, defects in defecation, inability to lay eggs, feeding behavior and sensory malfunctions. This illustrates the universal essentiality of sodium for membrane potentials and consequently, muscular and neural functioning.

Sodium mainly enters terrestrial environments via rainwater and weathering.In addition, invertebrates are reliant on sodium deposition from higher trophic levels, e.g. urine, feces and corpses. In response to sodium deposition, communities of lower trophic levels assemble in high numbers of individuals to take up sodium, as observed e.g. in soil microbial communities, bees, Lepidoptera, and ants. This is especially important for herbivores, since plants accumulate very little sodium and uptake from alternative sodium sources is thus required (Clay et al. 2014). However, soil arthropods also act as detritivores in the remineralization of nutrients and can enhance the amount of available sodium in the soil (Anderson et al. 1983).

In insects, the Malpighian tubules, salivary glands, hindgut and midgut are the main regulatory organs where excretion and reabsorption of sodium take place (Breyenbach 2003). Finke (2002) reported sodium contents of invertebrates usually used as food supply (for example in rats) and found sodium contents ranging from 165 mg/kg in waxworms to 1350 mg/kg in crickets.

(b)Sulfur is essential in two different ways: as a structural component and on a metabolic level. Sulfur forms a foundation of life itself: it is needed in the synthesis of membrane lipids, is part of the amino acids cysteine and methionine and consequently part of proteins and enzymes (Sekowska et al. 2000). It is found in iron-sulfur centers of Ferredoxin (which functions as electron carrier in anaerobic respiration) and of Glutathione (which plays an important role as antioxidant at the prevention of ROS-induced cell-damage). Further, it is present in the cofactors Thiamine and Lipoic acid (which is linked to many dehydrogenases), and biotin.

Sulfur is taken up by invertebrates via the diet and most insects seem to require to take up methionine with their diet to cover their sulfur demands. Lack of sulfur-containing amino acids or organic sulfur in the diet leads to detrimental effects, as observed in the aphidMyzus persicae.Omission of methionine or cysteine leads to a strong reduction of growth and a greatly reduced number of viable offspring, with effects more pronounced in the absence of methionine. However, the demand for cysteine can be met by provision of other sulfur substances so that cysteine may not be an essential amino acid but might serve as sulfur carrier instead (Dadd & Krieger, 1968). Retnakan and Beck (1967) reported that provision of inorganic sulfur mitigated negative effects of the lack of methionine or cysteine and concluded that aphids could synthesize either of these amino acids, possibly with the help of symbionts. Supply of inorganic sulfur mitigated the need for cysteine and methionine uptake in the aphidsAcyrthosiphon pisumandNeomyzus circumflexus.However, the synthesis of the amino acids is also attributed to symbionts(Dadd, 1973 and references therein). Thus, the lack of sulfur-containing amino acids in the diet affects aphids but symbionts seem to counteract and to contribute to the diet by synthesizing these amino acids from inorganic sulfur (Douglas, 1988). In food-choice experiments, it was demonstrated that methionine had the strongest positive effect on selection of diet from an array of served diets, underlining the demand for methionine (Dadd,1973 and references therein).

In spiders, the sulfur-containing amino acid taurine is furthermore a component of spider silk and venom (Wiesenborn 2012 and references therein). In earthworms, S-containing dialkylfuransulfonates allow consumption of leaf litter by preventing adverse effects of polyphenols to the earthworms. Dialkylfuransulfonates comprise up to 20% of the total sulfur found in earthworms and play a vital role for detritus consumption in ecosystems all over the world (Liebeke et al. 2015).

Conclusion: (a) Sodium is ubiquitous in the environment, occurring in minerals, soil, sediments and natural waters, and is present as an essential and actively regulated element in biota. According to the OECD SIDS (2002) on “sodium hydroxide (CAS# 1310-73-2)”, a potential for bioaccumulation/bioconcentration cannot be identified. Thus, sodium as essential element is actively regulated and does not bioaccumulate.

(b) Sulfur is ubiquitous in the environment, is actively regulated and fulfils essential roles in all cells, determining the structure and activity of a number of molecules and modulating a myriad of metabolic and catalytic processes. Accordingly, the bioaccumulation potential of thiosulfate is expected to be low.

Taking into account (i) the rapid dissociation of sodium thiosulfate and decomposition of thiosulfates upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural sodium and sulfur cycle, (ii) ubiquitousness of sodium and inorganic sulfur substances in soil, and (iii) essentiality of sodium and sulfur in terrestrial organisms, sodium thiosulfate is expected to have a low potential for bioaccumulation in terrestrial organisms.

References: OECD SIDS (2002). Sodium hydroxide. CAS# 1310-73-2.SIDS Initial Assessment Report for 14th SIAM (Paris, 26-28 March 2002).