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Read-across principle for dithionites (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 dithionite on micro-organisms (activated sludge).

No data on long-term effects, however were identified. Read-across from information that available from other relevant sulfur compounds may cover this data gap.

Sodium dithionite (and other simple dithionite compounds) decompose or disoproportionate rapidly in aqueous media, especially under acidic conditions and under oxygen consumption. Formed sulfur compounds are sulfite, (gaseous) SO2 and thiosulfates. The following reactions were put forward by Hofmann and Rüdorff (1969) and Holleman and Wiberg (1995):

-      Anaerobic conditions: 2Na2S2O4+ H2O forms Na2S2O3 (thiosulfate compound) and 2NaHSO3 (sulfite compound); thiosulfate in itself is a reducting agent and will further oxidize (e.g., formation of sulfate).

-      Aerobic conditions: Na2S2O4+ O2 forms NaHSO4 (sulfate) and NaHSO3 (sulfite).

 

Formation of hydrogen sulfite and hydrogen sulfate lowers the pH, which further promotes/accelerates the process of decomposition strongly. At pH 9-11 there is a 1% decomposition within 1 hour and at pH 7 there is a 2% decomposition within the same time frame. This period represents a slow induction period which is later followed by rapid accelleration due to autocatalytic processes. Below pH 6 there is a much shorter induction time and below pH of 4.8 there is no induction time at all (Münchow, 1992).

 

To keep solutions of dithionite stable for several days, they need to be cooled, kept in an alkaline state by excess of NaOH and oxygen has to be excluded (OECD SIDS on sodium dithionite). It can therefore be concluded that long-term testing of dithionite is environmentally not relevant for the following reasons:

-      dithionite will not remain stable under environmental aerobic conditions, but will decompose into sulfate and sulfite.

-      industrial use of dithionite involve chemical reactions that already transform dithionite: during industrial use of as reductive substance, sodium dithionite is oxidized to sulfate.

-      conditions in wastewater treatment plants (on-site or municipal wastewater treatment plants) will lead to a rapid hydrolyzation, oxidation and decompositions of any remaining dithionite.

 

In is thus concluded that for the evaluation of long-term effects of dithionite in the environment, the long-term effects of sulfites should be considered instead.

 

 

 

Summary of acute toxicity data

 

Table below gives an overview of reliable acute toxicity data that were identified for sodium dithionite.

 

Table: Overview of reliable acute toxicity data for sodium dithionite for hazard assessment purposes.

Species

 

Parameter

Endpoint

Value

(mg Na2S2O4/L)

Reference

Leuciscus idus

Fish

Mortality

96h-LC50

62.3

BASF AG, 1982

Daphnia magna

Invertebrate

Immobility

48h-EC50

98.3

BASF AG, 1989

Scenedesmus subspicatus

algae

Growth rate

72h-EC50

206.2

BASF AG, 1989

 

 

Reliable acute data were available for three trophic levels: fish, aquatic invertebrates, aquatic algae and microorganisms. The lowest effect value was a 96h LC50of 62.3 mg/L.

 

   

Summary of chronic toxicity data

 

An overview of the species-specific reliable long-term data for sodium dithionite is given below. 

  

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

Species

Trophic level

NOEC/EC10

(mg Na2S2O4/L)

Reference

Scenedesmus subspicatus

Algae

81.7

BASF, 1989

Daphnia magna

Crustacea (invert.)

≥10.0

BASF, 1994

Danio rerio

Fish

≥436.5(1)

ECT, 2010

(1): Sodium sulfite data translated to sodium dithionite, taking into account that dithionite decomposes into sulfite and sulfate.

 

The lowest NOEC-value is an unbounded value for the invertebrate D. magna (i.e., no effect was noted at the highest test concentration), and this value can be considered as a worst-case estimate rto be used for effects assessment purposes.

It should also be noted that the calculated NOEC for fish is markedly higher than the acute value of 62.3 mg/L. This is due to the fact that oxygen depletion caused acute toxicity, whereas the test design of the ELS test (aerated test media, using sodium sulfite as test compound) prevents mortality of exposed fish due to oxygen depletion. A more realistic long-term NOEC is therefore the highest acute exposure concentration that resulted in 0% mortality. This value of 46.4 mg Na2S2O4/L is put forward for the environmental classification and effects assessment of sodium dithionite.  

Summary of toxicity data for micro-organisms

  

A 3h-NOEC of 89.8 mg Na2S2O4/L (endpoint: respiration rate of activated sludge) has been put forward for the derivation of a PNECmicr-organismfor sodium dithionite.