Sodium dithionite

Endpoint summary

Description of key information

Studies on the aquatic toxicity of sodium sulfite are available for fish (acute: Leuciscus idus; chronic: Danio rerio), crustaceans (acute & chronic: Daphnia magna), algae (acute & chronic: Desmodesmus subspicatus) and STP microorganisms. In addition, read-across to dithionite, thiosulfate and sulfite/disulfite substances is applied. Upon contact with water, salts of sulfur oxyacids including sodium dithionite dissociate into sulfur oxyacid anions and the respective counterions. Only the properties of the dithionite anion are considered a relevant determinant of environmental toxicity since the sodium cations are essential and have a very low potential for toxicity to freshwater and marine fish. To freshwater algae and aquatic plants, excessive sodium concentrations might be harmful, however, lowest reported EC10 and EC50 values are still significantly above the OECD test limits (for further information please refer to the respective endpoint summaries for sodium ‘Na_Aquatic Toxicity’ and ‘Na_Toxicity to aquatic algae and cyanobacteria’).

Dithionite anions are unstable under environmentally relevant conditions and will rapidly disproportionate to (bi-)sulfites and thiosulfates (S₂O₃^2-) in aqueous media.Sulfites and thiosulfates are unstable in the environment, subject to transformation processes including oxidation to sulfate or reduction to sulfide, and will ultimately become part of the natural sulfur cycle (please refer to the endpoint summary on environmental fate and behaviour).

 

Acute and chronic toxicity studies with dithionite, thiosulfate and sulfite/disulfite substances are available for three trophic levels. In studies with controlled oxygen concentrations and pH, acute toxicity for all three trophic levels (freshwater fish and algae and freshwater and marine invertebrates) is only observed at concentrations above the respective OECD test limit of 100 mg/L. In chronic studies with controlled oxygen concentrations and pH, effect values are unbounded (D. rerio, D. magna, P. subcapitata) or above the chronic OECD test limit of 10 mg/L (D. subspicatus, C. vulgaris, C. reinhardtii). In an activated sludge respiration inhibition tests according to OECD 209 with sodium dithionite, 3-h EC10 and EC50 values of 89.8 mg/L and 187.6 mg/L, respectively, were derived for inhibition of total respiration/respiration rate. Effects of thiosulfate and sulfite substances on the respiration of activated sludge were not observed at the OECD test limit of 1000 mg/L, respectively.

 

It is thus concluded that dithionite, thiosulfate and sulfite/disulfite substances, including sodium dithionite, are not toxic to freshwater fish, invertebrates and algae. A low potential for toxicity of sodium dithionite to aquatic microorganisms was identified.

Additional information

Dithionite, thiosulfate and sulfite/disulfite substances -Summary of acute toxicity data

Reliable acute toxicity data are available for several aquatic freshwater species covering three trophic levels (primary producers, primary and secondary consumers) and one saltwater species (invertebrates). The table below provides an overview of the effect values for the acute toxicity of thiosulfate and sulfite/disulfite substances applied in a weight of evidence approach.

 

Table: Overview of reliable acute toxicity data for dithionite, thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Observed interference	Test substance	Reference
Leuciscus idus	mortality	96 h LC50	57.2	oxygen depletion**	sodium dithionite	BASF, 1982
Oncorhynchus mykiss	mortality	96 h LC50	149.6	pH decrease*	disodium disulfite	BASF, 1981
Leuciscus idus	mortality	96 h LC50	200.5	oxygen depletion**	sodium sulfite	BASF, 1989
Leuciscus idus	mortality	96 h LC50	159.7	oxygen depletion**	potassium sulfite	BASF, 1989
Danio rerio	mortality	96 h LC50	490	oxygen depletion**	dipotassium disulfite	BASF, 1995
Lepomis macrochirus	mortality	96 h LC50	275.5	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1986
Oncorhynchus mykiss	mortality	96 h LC50	416	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1986
Daphnia magna	mobility	48 h EC50	90.3	oxygen depletion**	sodium dithionite	BASF, 1989
Daphnia magna	mobility	48 h EC50	74.9	oxygen depletion**	disodium disulfite	BASF, 1990
Daphnia magna	mobility	48 h EC50	124.2	none	ammonium thiosulfate	Springborn Binomics Inc, 1986
Mysidopsis bahia	mortality	96 h LC50	41.6	oxygen depletion**	ammonium thiosulfate	Springborn Binomics Inc, 1994
Desmodesmus subspicatus	growth rate	72 h EC50	189.5	pH decrease*	sodium dithionite	BASF, 1989
Desmodesmus subspicatus	growth rate	72 h EC50	36.8	pH decrease*	disodium disulfite	BASF, 1989
Pseudokirchneriella subcapitata	growth rate	72 h EC50	>54	none	ammonium thiosulfate	ECT, 2010

*pH decrease: solution pH at “toxic” test concentrations is outside OECD test criteria

**oxygen depletion: oxygen concentration of medium at “toxic” test concentrations is outside OECD test criteria

 

In tests with ammonium thiosulfate and without experimental interferences (e.g., oxygen depletion in the test medium), the effect concentration was either above the OECD test limit of 100 mg/L (test with ammonium thiosulfate and D. magna; 48h EC50 =124.4 mg/L), or above the highest tested concentration of 100 mg ammonium thiosulfate/L (corresponding to 54 mg SO32-/L; test with ammonium thiosulfate and P. subcapitata; 72 h EC50).

Other acute effect concentrations (EC/LC50s) are i) unbounded, ii) above the test limit of 100 mg/L as defined in the respective OECD guideline (OECD 201, 202, 203), and/or iii) presumably caused by to oxygen depletion and/or a pH decrease and do not represent the intrinsic toxicity of sulfite/disulfite and thiosulfate substances. Thus, it may be assumed that the “substance-specific” EC/LC50 of dithionite substances for freshwater fish and algae and freshwater and marine invertebrates is higher than the EC50/LC50s derived in studies applying a test system without proper buffering capacity for oxygen concentrations and pH.

It is thus concluded that dithionite, thiosulfate and sulfite/disulfite substances, including sodium dithionite, are not acutely toxic to freshwater fish and algae and freshwater and marine invertebrates. Additionalstudies on the acute effects of disodium disulfite to Daphnia magna and Raphidocelis subcapitata according to OECD 202 and OECD 201, respectively, with appropriate aeration and buffering of the test system, are currently ongoing.

 

Dithionite, thiosulfate and sulfite/disulfite substances -Summary of chronic toxicity data

Chronic toxicity data are available for several aquatic freshwater species covering three trophic levels (primary producers, primary and secondary consumers). The table below provides an overview of the effect values for chronic toxicity of sulfite/disulfite and thiosulfate substances.

Table: Overview of reliable chronic toxicity data for dithionite, thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Observed interference	Test substance	Reference
Danio rerio	hatching rate, post-hatch survival, length, weight	35 d NOEC	≥ 9.19	none	sodium dithionite	Teigeler, 2022
Danio rerio	all (ELS)	34 d NOEC	≥ 200.5	none	sodium sulfite	ECT, 2010
Daphnia magna	reproduction, mortality	21 d NOEC	≥ 9.19	none	sodium dithionite	BASF, 1994
Daphnia magna	reproduction, mortality	21 d NOEC	≥ 8.41	none	disodium disulfite	BASF, 1993
Desmodesmus subspicatus	growth rate	72 h EC10	75	pH decrease*	sodium dithionite	BASF, 1989
Desmodesmus subspicatus	growth rate	72 h EC10	28	pH decrease*	disodium disulfite	BASF, 1989
Pseudokirchneriella subcapitata	growth rate	72 h NOEC	≥ 54	none	ammonium thiosulfate	ECT, 2010

*pH decrease: solution pH decreases at higher test concentrations

**oxygen depletion: oxygen concentration of medium at “toxic” test concentrations is outside OECD test criteria

 

Chronic effect values of sodium dithionite and thiosulfate and sulfite/disulfite substances are i) unbounded, without effects at the highest test concentration (D. rerio, D. magna) or ii) above the chronic OECD test limit of 10 mg/L (D. subspicatus).

It is thus concluded that dithionite, thiosulfate or sulfite/disulfite substances, including sodium dithionite, are not chronically toxic to freshwater fish, invertebrates and algae.

 

Toxicity to STP microorganisms

Regarding the toxicity to microorganisms, reliable studies of the respiration inhibition of activated sludge and supporting studies of the growth inhibition of Pseudomonas putida are available (see Table below).

Table: Overview of reliable toxicity data to microorganisms for dithionite, thiosulfate and sulfite/disulfite substances

Species	Parameter	Endpoint	EC [mg SO3^2-/L]	Test substance	Reference
Activated sludge	respiration inhibition	3 h EC50 3 h EC10	172.5 82.6	sodium dithionite	ECT, 2010
Activated sludge	respiration inhibition	3 h EC50 3 h NOEC	> 634.4 ≥ 634.4	sodium sulfite	ECT, 2010
Activated sludge	respiration inhibition	3 h EC50 3 h NOEC	> 540.2 ≥ 540.2	ammonium thiosulfate	ECT, 2010
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	47.2 25.9	disodium disulfite	BASF, 1988a
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	207.2 77.3	potassium sulfite	BASF, 1988b
Pseudomonas putida	growth inhibition	17 h EC50 17 h EC10	65.8 23	potassium disulfite	BASF, 1988c

 

In an activated sludge respiration inhibition tests according to OECD 209 with sodium dithionite, 3-h EC10 and EC50 values of 89.8 mg/L and 187.6 mg/L, respectively, were derived for inhibition of total respiration/respiration rate. Effects on the respiration of activated sludge were not observed at the OECD test limit of 1000 mg/L in studies with ammonium thiosulfate or sodium sulfite, respectively.

The hazard assessment is based on effect concentrations for the respiration inhibition of activated sludge in accordance with ECHA guidance on IR & CSA, Chapter 7b (Version 4.0, 2017).