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Read-across principle for thiosulfates (long-term effects)

 

Reliable short-term data for dithionite are available for the different trophic levels that are stipulated in the REACH requirements (algae, daphnids, fish), including ecotoxicological information on the (short-term) effects of thiosulfateon micro-organisms (activated sludge). No data on long-term effects, however were identified. Read-across from information that available from other relevant sulfur substances may cover this data gap.

 

Thiosulfates (e.g., NH4, Na, K, Ca thiosulfates) are inorganic substances that dissociate completely in water. The thiosulfate anion is a metastable, moderately reducing oxyanion of sulfur, and its reactions gernerates different chemical species of sulfur. Main type of chemical reactions that are expected, are disproportionation (chemical and microbially mediated), chemical and microbially mediated redox reactions, complexation with metal ions and bimolecular nucleophilic substitution reactions (SN2 mechanisms; dihalogenation) (USEPA, 2007). In general, thiosulfates are stable under neutral/ alkaline conditions, but not in acid media. The literature that describes the chemical stability, however, is somewhat contradictory. It is agreed that thiosulftates are stable at approximately pH 7 (or higher) and it decomposes at acidic pH values. However, some authors have found thiosulfate to decompose in alkaline solutions to sulfite, sulfate or sulfide, according to the experimental conditions (Roy and Trudinger, 1970; Vogel, 1955).

In aqueous media, thiosulfate irreversibly disproportionates to sulfite (and sulfate as the next oxidation step). According to USEPA (2007), thiosulfate in water further degrades to form colloidal elemental sulfur and sulfate ion. It is therefore not anticipated for thiosulfate to be found in water (or drinking water). Products that also may be formed a low pH levels are elemental sulfur, sulfur dioxide and polythionates (SxO62-) (Roy and Trudinger, 1970; Vogel, 1955; Buchanan and Gibbons, 1974).

Millano and Sorber (1986) investigated the stability of thiosulfate concentrations in a synthetic wastewater. They concluded that thiosulfate (480 mg/L as S) in the syntethic wastewater was chemically stable when aerated for 24h at initial pH levels of 9.2, 7.9, 7.1 and 5.6. Without aeration, thiosulfate began decomposing below pH 4.9, thereby forming sulfite and elemental sulfide. The same authors found that thiosulfate removal in acclimated continuous experiments with completely mixed activated sludge units (max. detention time: 20h) was more than 99%, and was attributed to the presence of sulfur bacteria: high levels of thiosulftate (up to 480 mg/L as S) were successfully removed (formation of elemental sulfur and sulfite/sulfate) and this thiosulfate loading did not interfere with the nitrification efficiency of the activated sludge.  

Saad et al (1996) investigated the effect on thiosulfate addition to soil on the nitrification properties of soil, and noted that under aerobic conditions the thiosulfate concentrations decreased rapidly and were depleted within 4-15 days (added amount 25-100 mg S/kg dry wt) due to formation of tetrathionate (S4O62-) which on its turn was depleted within a few days after formation.

 

 

For thiosulfates, a complete acute ecotoxicological data set is available, and can thus be used for classification/hazard assessment purposes. Reliable chronic ecotoxicity tests with thiosulfate substances, however, were not identified. Read-across from sulfite/disulfite data is proposed for adressing long-term effects:

  •  Thiosulfates are chemically stable in alkaline conditions for a short term period, but will disproportionate to sulfite (and further oxidize to sulfate) under environmental conditions (e.g. microbial oxidation by sulfur bacteria). The development of a relevant long-term test protocol that ensures a constant thiosulfate level in an environmental compartment (water, sediment, soil) is therefore not feasible/not relevant.
  •  The deleterious properties for the environment of reduced inorganic sulfur substances like sulfites, disulfites and thiosulfates are related to their chemical and biochemical oxygen demand (leading to O2-depletion), and their (bio)transformation may lower the pH of the system by producing sulfuric acid (H2SO4)
  •  Monitoring the physicochemical properties of test media in acute toxicity tests with sulfites and thiosulfates has shown that effects were indeed related to O2-depletion and low pH-levels; continuous aeration, for instance, prevented observed mortality. These findings indicate that toxicity was not caused directly through interaction of the SO32-- or S2O32-anion with the test organisms, but can be attributed to the same physicochemical processes for both sulfites and thiosdulfates, i.e., O2-depletion and/or decrease of the pH.
  •  Comparing the key acute toxicity data (acute toxicity reference value, acute TRV) between sulfites and thiosulfates (both expressed as mg S/L, see table below) shows that sulfites are more acutely toxic than thiosulfates, i.e., their impact on the water chemistry occurs at lower concentration (expressed as mg S/L). This can be explained by the lower chemical stability of sulfites compared to thiosulfates: Half-life of sulfites is 77h (or less), whereas Millano and Sorber (1986) observed no decrease in thiosulfate content after 24h when biological oxidation was excluded. It can this can thus be concluded that sulfites represent a worst-case toxicity for thiosulfates.

 

 

Test organism

Acute TRV for sulfite substance

Acute TRV for thiosulfate substance

 

mg S/L

mg S/L

Salmo gairdneri

96h-LC50: 60.0

96h-LC50: 333.2

Daphnia magna

48h-EC50: 30.0

48h-EC50: 99.5

Green alga

72h-ErC50: 14.8

(Scenedesmus subspicatus)

72h-ErC50: 43.3

(Pseudokirchneriella subcapitata)

 

Taking these different factors into account (i.e., long-term transformation of thiosulfates to sulfites, similar toxic mode of action through oxygen depletion and/or decrease of pH, higher acute toxicity of sulfites compared to thiosulfates), it is concluded that long-term toxicity data for sulfites/disulfites can serve as a worst-case assumption for assessing the toxicity of thiosulfate substances.     

 

 

 

Summary of acute toxicity data

 

Table below gives an overview of reliable toxicity data that were identified for thiosulftates.

 

Table: Overview of reliable acute toxicity data for thiosulfate (data expressed as mg S2O32-/L) for hazard assessment purposes.

Species

 

Parameter

Endpoint

Value

(mg S2O32 -/L)

Reference

Salmo gairdneri

Fish

Mortality

96h-LC50

583

Springborn Bionomics, 1986

Daphnia magna

Invertebrate

Immobility

48h-EC50

174

Springborn Bionomics, 1986

Pseudokirchneriella subcapitata

algae

Growth rate

72h-EC50

≥75.7

ECT, 2010

 

 

Reliable acute data were available for three trophic levels: fish, aquatic invertebrates, aquatic algae and micro-organisms. The lowest effect value was a 72h LC50of ≥75.7 mg S2O32-/L.

 

 

Summary of chronic toxicity data

An overview of the species-specific data is given below. All relevant effects data are expressed as mg S2O32-/L.

 

 

Table: Overview of most sensitive species-specific EC10/NOEC-values for thiosulfate in the freshwater environment

Species

Trophic level

NOEC/EC10

(mg S2O32-/L)

Reference

Pseudokirchneriella subcapitata

Algae

≥75.7

ECT, 2010

Daphnia magna

Crustacea (invert.)

≥5.90

BASF, 1994

Danio rerio

Fish

≥140.6(1)

ECT, 2010

(1): Sodium sulfite data translated to sodium thiosulfate, assuming that all S is converted to sulfite when thiosulfate oxidizes

 

Data are available for species representing three trophic levels (algae, invertebrates, fish) are available. The lowest value for chronic toxicity was and unbounded NOEC of 5.90 mg S2O32-/L.