Ammonium thiosulphate

Bioaccumulation: terrestrial

Description of key information

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

 

Additional information

(1) Environmental fate and stability in soil:

Ammonium thiosulfate dissociatesinto thiosulfate anions and ammonium cations in environmental solutions, including soil porewater.

(a)Ammonium cations are rapidly transformed under environmentally relevant conditions. Ammonium is easily mineralized to nitrite (NO2-) by numerous bacterial species including Nitrosomonas europea, Nitrosococcus and Nitrosospira under aerobic conditions (OECD SIDS, 2004). Ammonium is rapidly degraded and not likely to accumulate in the environment or living organisms (OECD SIDS, 2004). Ammonium and nitrate are plant nutrients and may under anaerobic conditions be transferred to gaseous end products N2O or N2 (nitrate oxidation or denitrification) by heterotrophic bacteria such as Agrobacterium, Bacillus or Pseudomonas. Nitrification and denitrification proceed simultaneously in soil (Gisi, 1997). 

(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 (S₂O₃^2-), elemental sulfur (S), sulfide (HS^-) and sulfite (SO₃^2-) 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 (SO₄^2-, 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 S₂O₃^2-added to four arable soils at high concentrations (1050 mg S₂O₃^2-/kg soil) showed rapid and complete transformation primarily to sulfate (SO₄^2-).

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 (S₂O₃^2-) and other sulfur-containing microbial substances such as dithionite (S₂O₄^2-) or sulfite (SO₃^2-) 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)Ammonium is a natural and common component of the environment and of all living organisms (OECD SIDS, 2004) and is an integral part of the global nitrogen cycle. In addition, ammonium is a common product in the OECD biodegradation test, which in a two-step process is readily broken down to NO2- and water, followed by a further oxidation of the resulting NO2- to NO3- (OECD 301, 1992). These transformation products are not expected to have a hazard potential in the environment including in the soil compartment.

 

(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 S^2-) and sulfate (SO₄^2-) 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), S^2-, S₂O₃^2-and S₂O₄^2-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 (SO₄^2-) and sulfide (S^2-) and sulfur containing oxyanions, i.e. sulfite (SO₃^2-), dithionite (S₂O₄), thiosulfate (S₂O₃) and polythionates such as trithionate (S₃O₆^2-) and tetrathionate (S₄O₆^2-, 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 95^thpercentile 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 95^thpercentile 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 95^thpercentile of 645 mg/kg (Reimann et al. 2014). 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.

(3) Essentiality:

(a)Ammonium in the form of ammonium chloride (1.7 mg NH4/L) is a constituent of the mineral medium of the “ready biodegradability” test (OECD 301, 1992). The mineral medium, depending on the used method, may additionally be supplemented with further ammonium compounds such as ammonium heptamolybdate (OECD 301 E, modified screening test). Ammonium in the mineral medium proves its essentiality for the bacterial community and is a prerequisite for the ready biodegradability test.

(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) Ammonium is a natural component of the environment and is rapidly degraded. According to theOECD SIDS (2003) on “ammonium chloride (CAS# 12125-02-9)”,a potential for bioaccumulation / bioconcentration cannot be identified. Thus, ammonium is not expected to 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 ammonium thiosulfate and decomposition of thiosulfates upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural nitrogen and sulfur cycles, (ii) the ubiquitousness of ammonium and inorganic sulfur substances in surface water and sediment as well as their rapid environmental transformation, and (iii) the essentiality of sulfur in aquatic organisms, ammonium thiosulfate is expected to have a low potentialfor bioaccumulation in terrestrial organisms.

Thus, the study on bioaccumulation does not need to be conducted in accordance with Column 2 of Information Requirement 9.3.2., Annex IX, Commission Regulation (EU) 1907/2006.

  

References:

OECD SIDS (2003). Ammonium chloride. CAS# 12125-02-09.SIDS Initial Assessment Report for 17th SIAM (Arona, 11-14 November 2003).