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Read-across principle for sulfite-disulfite

 

In aqueous solutions, sulfite compounds are competely dissociated in into countercations ond the sulfite anion. At neutral pH, a mixture of approx. 50% sulfite (SO32-) and 50%bisulfite (HSO3-) is present. However, at concentrations above 1M, bisulfite anions will dimerise with the elimination of water to form metabisulfite (S2O52-). On the other hand, low concentration of metabisulfite bought into solution will dissolve and hydrolise to form bisulfite.

There is thus a (pH-dependent) equilibrium between different forms of S(IV), i.e., sulfite, bisulfite, metabisulfite and sulfur dioxide. In general, the predominant forms under physiological and environmental conditions are sulfite and bisulfite, irrespective of the initially applies form of S(IV).

Consequently, all ecotoxicity data that were generated using a (di)sulfite compound with low-toxic counter ions (e.g., potassium, sodium), can be pooled together and – when expressed as S(IV) or SO32-, used in a read-across approach for all (di-)sulfite compounds.

 

For the long-term assessment of these compounds, one has to take into account that sulfite is oxidized to sulfate either catalytically by air oxygen or by microbial action. Halflife in deionized water was determined to be 77h (Tsunogai, 971), and this period should be considered as a worst-case as the presence of cations like iron, copper or manganese in the environment significantly accellerates the oxidation rate (WHO, 1979). Zhang and Millero (1991) reported that the reaction is more than five times faster in seawater comparted to distilled water.  

These processes are important when interpreting (long)term) toxicity data for sulfites: sulfite is rapidly oxydised to sulfate thereby consuming oxygen. Observed toxic effects may therefore be caused by either sulfite toxicity or lack of oxygen (or a combination of both). 

 

  

Summary of acute toxicity data

 

Table below gives an overview of reliable toxicity data that were identified for sulfite/disulfite compounds.

 

Table: Overview of reliable acute toxicity data for sulfite/disulfite compounds.for hazard assessment purposes.

Species

 

Parameter

Endpoint

Value

(mg SO32-/L)

Reference

Salmo gairdneri

Fish

Mortality

96h-LC50

149.6

BASF AG, 1989

Daphnia magna

Invertebrate

Immobility

48h-EC50

74.9

BASF AG, 1990

Scenedesmus subspicatus

algae

Growth rate

72h-EC50

36.8

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 72h EC50 of 36.8 mg SO32-/L.

 

 

Summary of chronic toxicity data

  

An overview of the key species-specific chronic toxicity data for sulfites/disulfite compounds is given below. All relevant effects data are expressed as mg SO32-/L.

 

 Table3: Overview of most sensitive species-specific EC10/NOEC-values for sulfite ion the freshwater environment

Species

Trophic level

NOEC/EC10

(mg SO32-/L)

Reference

Scenedesmus subspicatus

Algae

28

BASF, 1989

Daphnia magna

Crustacea (invert.)

≥8.41

BASF, 1990

Danio rerio

Fish

50.0

ECT, 2010

 

 

Three long-term results (e.g. NOECs) from species representing three trophic levels (algae, invertebrates, fish) are available. The lowest value for chronic toxicity was and unbounded NOEC of 8.41 mg SO32-/L.