Calcium thiosulphate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.83 mg/L

Assessment factor:

10

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.083 mg/L

Assessment factor:

100

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

106.5 mg/L

Assessment factor:

10

Sediment (freshwater)

Hazard assessment conclusion:

no exposure of sediment expected

Sediment (marine water)

Hazard assessment conclusion:

no exposure of sediment expected

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no exposure of soil expected

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Derivation of freshwater PNEC value using assessment factor methods

 

The general principle of these methods is that the result from a laboratory test is divided by an appropriate assessment factor (ECHA, 2008). The sparser the available data, the higher is the assessment factor which is applied. PNECs are estimated by division of the lowest value for the toxicity with the relevant assessment factor. Results of long-term tests (expressed as EC₁₀ or NOEC for a sublethal parameter) are preferred to those of short-term tests (E(L)C₅₀), because such results give a more realistic picture of effects on the organisms during their entire life cycle.

 

In establishing the size of these assessment factors, a number of uncertainties have been addressed to extrapolate from single-species laboratory data to a multi-species ecosystem. These areas comprise:

 

• intra- and inter-laboratory variation of toxicity data;

• intra- and inter-species variations (biological variance);

• short-term to long-term toxicity extrapolation;

• laboratory data to field impact extrapolation.

 

The assessment factors recommended for the determination of the PNEC for the freshwater aquatic are shown in Table below.

 

Table: Assessment factors to derive a PNEC_aquatic

Available data	Assessment factor
At least one short-term L(E)C50from each of three trophic levels (fish, invertebrates (preferred Daphnia) and algae)	1000
One long-term EC10or NOEC (either fish or Daphnia)	100
Two long-term results (EC10, NOECs) from species representing two trophic levels (fish and/or Daphnia and/or algae)	50
Long-term results (EC10, NOECs) from at least three species (e.g., fish, Daphnia and algae) representing three trophic levels	10
Species sensitivity distribution (SSD) method	5-1(to be fully justified case by case)
Field data or model ecosystems	Reviewed on a case by case basis)

 

When only short-term toxicity data are available, an assessment factor of 1000 will be applied on the lowest E(L)C₅₀ of the relevant available toxicity data, irrespective of whether or not the species tested is a standard test organism. A lower assessment factor will be applied on the lowest EC₁₀ or NOEC derived in long-term tests with a relevant test organism. 

 

For some substances, a large number of validated short-term E(L)C₅₀ values may be available. Therefore, it is proposed to calculate the geometric mean if more than one E(L)C₅₀ value is available for the same species and endpoint. Prior to calculating the geometric mean an analysis of test conditions must be carried out in order to find out why differences in response were present. 

 

The algal growth inhibition test of the base-set is, in principle, a multi-generation test. However, for the purposes of applying the appropriate assessment factors, the EC₅₀ is treated as a short-term toxicity value. The EC₁₀ or NOEC from this test may be used as an additional long term result when other long-term data are available. In general, an algal EC₁₀or NOEC should not be used unsupported by long-term EC₁₀ or NOECs of species of other trophic levels. 

 

Micro-organisms representing a further trophic level may only be used if non-adapted pure cultures were tested. The investigations with bacteria (e.g., growth tests) are regarded as short-term tests. Additionally, blue-green algae should be counted among the primary producers due to their autotrophic nutrition.

 

The assessment factors should be considered as general factors that under certain circumstances may be changed. In general, justification for changing the assessment factor could include one or more of the following:

-      evidence from structurally similar substances (evidence established by read across from closely related substances may demonstrate that a higher or lower factor may be appropriate);

-      knowledge of the mode of action including endocrine disrupting effects (Some substances, by virtue of their structure, may be known to act in a non-specific manner);

-      the availability of test data from a wide selection of species covering additional taxonomic groups other than those represented by the base-set species;

-      the availability of test data from a variety of species covering the taxonomic groups of the base- set species across at least three trophic levels. In such a case the assessment factors may only be lowered if these multiple data points are available for the most sensitive taxonomic group.

 

 

Since no large dataset from long-term tests for different taxonomic groups is available for sodium dithionite, no Species Sensitivity Distribution (SSD) can be developed and statistical extrapolation methods can thus not be used to derive the PNEC_aquatic. Instead, The PNEC_aquatic calculation will be conducted using assessment factors method.

 

An overview of the species-specific data is given below. All relevant effects data are expressed as mg S₂O₃^2-/L and mg S/L.

 

 

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

Species	Trophic level	NOEC/EC10 (mg S2O3^2-/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}

 

In this scenario an assessment factor (AF) of 10 should be used to calculate the PNEC_aquaticfrom the lowest value for the toxicity. This factor can be applied since 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 5.90 mg S₂O₃^2-/L. Applying the AF of 10 results in a PNEC_aquatic of 0.59 mg S₂O₃^2-/L.  Translating this value to CaS₂O₃gives a PNEC_aquatic of 0.83 mg test substance/L.

 

 

As the lowest NOEC-value is an unbounded value (i.e., no effect was noted at the highest test concentration), this value can be considered as a worst-case estimate. Further refinement of the NOEC-value for daphnids could increase the PNEC_aquaticup to a maximum value of 7.6 mg S₂O₃^2-/L (i.e., an assessment factor of 10 on the algal 72h-EC₁₀value).

 

 

 

PNEC_sediment

Due to the physicochemical properties which make adsorption to sediments unlikely, and the microbial oxidation of thiosulfates under environmental conditions, the derivation of a PNEC for the sediment compartment is not feasible/appropriate:

-      due to the natural oxidation of thiosulfates by microbial activity, no relevant test design and toxicity data can be generated

-      due to the absence of a relevant adsorption coefficient for thiosulfates to sediment, the equilibrium partitioning method for deriving a PNEC_sediment is not applicable

-      taking into account the industrial use, exposure pathways and environmental fate of thiosulfates, long-term exposure of sediment organisms to this substance can be excluded.

Consequently, there is no need to derive a PNEC_sediment for thiosulfate substances.

 

PNEC_terrestrial

 Taking into account its physicochemial properties, the industrial use, exposure pathways, and the long-term instability of thiosulfates under environmental conditions (decomposition to sulfite and sulfate), a continuous, long-term exposure of terrestrial organisms to thiosulfates can be excluded.

However, an important use of ammonium thiosulftate (ATS) - and thiosulfates in general – is the application in the agricultural industry as S fertilizer to crops. Consequently, there is a direct pathway for thiosulfates to enter the terrestrial compartment. Upon application to soil ATS is rapidly transformed to plant available SO₄, thereby effectively alleviating S deficiency in crops (Graziano, 1990). Indeed, thiosulfate is not generally available for plant uptake until it is converted to sulfate. The counter-cation of S₂O₃^2-(NH₄, K, Ca) also contributes to the fertility of the treated soil.

Depletion of thiosulfate in soils occurs rapidly: depending on the intitial concentration (5-100 mg S as thiosulfate/kg dw soil), the conversion occurs within 4-15 days in aerobic conditions, and within 10-20 days under anaerobic conditions (Saad et al, 1996). With regard to toxic effects of thiosulfates for plants, McCarty et a l(1990) observed that at excessive rates of 2500 or 5000 mg ATS/kg soil, germination of corn or wheat seeds in soil was significantly decreased . Early seedling growth in soil decreased dramatically when ATS was applied at the rate of 1000 or more mg/kg soil; At these rates, ATS inhibited root and shoot growth. The observation that high levels of ATS can induce phytotoxic effects, was in harmony with previous findings from Goos (1985) and Mahler and Lutcher (1989) that high levels of ATS can cause severe crop damage. ATS is also applied as blossom thinning product to regulate crop load in fruit trees (Janoudi and Flore, 2005; Fallahi and Willemsen, 2002; Greene et al, 2001; Holb, 2008), as it is considered as a safe product for both the consumer and the environmùent (Wertheim, 2000). ATS at concentrations of 1% and 2%, for instance, resulted in damage ranging from 40% to 86% of all flowers that were open at the time of the treatment (damage to pistils, petals and stamens due to its action as a caustic agent).

Publications on the effect of thiosulfates on soil invertebrates have not been identified.

 

Beside the fact that thiosulfates substances are a source of plant nutrients, they can also improve the nitrogen efficiency in agricultural processes. Inhibition of both nitrification and/or hydroysis of N-compounds (urease activity inhibition) upon addition of ATS has been suggested in literature: in combination with UAN (urea ammonium nitrate), ATS administraton effectively inhibits the hydrolysis of urea, thus preventing volitilization of ammonia from the soil. Administered separately, Goos et al (1986a,b) have been unable to demonstrate significant inhibition of nitrification by ATS, and significant retardation of urea hydrolysis in soils was only observed when applied at rates as high as 2500 or 5000 mg/kg soil (Bremner et al, 1990). Under laboratory conditions, nitrifiation inhibition in soil by ATS has been observed at rates ≥ 250 mg/kg soil, thereby causing accumulation of potentially toxic amount of nitrite (Bremner et al, 1986; Goos and Fairlie, 1988). However, under field conditions, this nitrification inhibition could not be demonstrated Goos et al, 1986a,b). It is noteworthy that not ATS, but its oxidation product tetrathionate (S₄O₆^2-) is most likely the actual urease inhibitor (Sullivan and Havlin, 1992)

Effects on nitrification are generally considered as a negative property for a substance, and NOEC/EC₁₀for the endpoint nitrification are considered relevant for assessing risks to the terrestrial environment. However, in the specific case of thiosulfates, the only relevant exposure of the terrestrial environment is an intentional exposure (use as fertilizer), and the inhibition of nitrification is a “desired” effect, as it reduces leaching of NO₃and keeps N for a longer period in a form under which it can be taken up by plants. Consequently, the derivation of a PNEC which is based on a nitrification inhibition NOEC (+ asessment factor) would lead to a maximum allowed concentration in agrucultural soils that reduces the beneficial effects of thiosulfates fro man agricultural point of view.

 

Deriving a PNEC based on the lowest NOEC for plants (root growth for corn and wheat) according to ECHA (2008) guidance would equally be counterproductive from an agricultural point of view. As chronic effects are availabele for plants and micro-organisms but not for soil invertebrates, an assessment factor of 50 should be applied in the NOEC of 75.7 mg thiosulfate/kg soil, resulting in a PNEC of 1.51 mg thiosulfate/kg.

From an agricultural point of view, such value makes no sense: it is more than factor of 100 below the concentration that reportedly induces the (desired) nitrification inhibition, and more than a factor of 200 below the lowest concnetration that caused an effect on root and shoot growth of wheat and corn.

Secondly, the amount of added S-fertilizer to the soil becomes almost negligible at such concentration level

 

Based on this information it it concluded that the derivation of a PNEC for the terrestrial compartment can be waived:

-      taking into account the industrial use, exposure pathways and environmental fate of thiosulfates (long-term instability of thiosulfates under environmental conditions and rapid decomposition to sulfite and sulfate), unintentional exposure of the terrestrial compartment to thiosulfates can be excluded;

-      intentional exposure (use as a fertilizer) is not a continuous process, i.e., administration of thiosulfates only accurs at some fixed points in time, and the non-bioavailable thiosulfates will be tranformed to bioavailable sulfate within 1-2 weeks;

-      the derivation of a PNEC for agricultural soils using the available data on phytotoxicity and nitrification inhibition would lead to a PNEC that is counter-productive from an agricultural point of view.

 

  

 

PNEC_STPderivation

 

The aim of the assessment for micro-organisms is the protection of the degradation and nitrification functions and process performance and efficiency of domestic and industrial STPs. A PNECmicro-organism can be derived in different ways based on the information at hand, and expert judgement of the weight of evidence.

 

For many substances, including a data-poor substances like sufites/disulfites, there are insufficient useful data for aquatic micro-organisms available for the application of the statistical extrapolation method for PNEC-derivation. In that case, the assessment factor methodology can be used on the extracted relevant/reliable test results. Two types of tests are considered relevant for deriving the PNEC_{micro-organism} in STPs: (1) tests with a mixed inoculum (e.g. activated sludge) for the endpoint respiration, and (2) a test with ciliated protozoa (preferablyTetrahymena) for the endpoint mortality.

 

In general, an AF of 10 is to be applied to the NOEC/EC₁₀of a sludge respiration test, reflecting the lower sensitivity of this endpoint as compared to nitrification, as well as the short duration of the test. The corresponding AF is 100 when based on the EC₅₀. The PNEC_{micro-organism} is set equal to a NOEC (AF = 1) for a test performed with specific bacterial populations such as nitrifying bacteria,_{P. putida}, ciliated protozoa, the Shk1 Assay. An EC₅₀from this test is divided by an AF of 10 to derive the PNEC_{micro-organism}. If no standard microbial inhibition test data are available, the PNEC_{micro-organism} can also be derived from available ready biodegradation tests. An assessment factor of 10 is applied to the test concentration at which no toxicity to the inoculum is observed. This approach can also be used for inherent biodegradability tests. From an activated sludge simulation study, a_{PNECmicro-organism} can be derived based on the PEC_{micro-organism} or PEC_influent, using an AF between 1 and 10 depending on the parameters monitored. The AF of 1 can be used in case there is no impact on nitrification and BOC/COD removal performance (NB: if sludge from an industrial STP was used for the test, the PNEC_{micro-organism} can not be used for the extrapolation to a domestic STP). No AF is needed to derive a PNEC_{micro-organism} based on good quality field data.

 

So, the rationale for the application of a higher assessment factor for the heterotrophic micro-organisms compared to the nitrifying bacteria, is that they are exposed to a higher concentration which relates more to the influent concentration. For the nitrifying bacteria the exposure concentration is more related to the effluent concentration since nitrification is the last treatment step in a STP.

 

 

A 3h-NOEC of ≥757 mg S₂O₃^2-/L (endpoint: respiration rate of activated sludge) has been put forward for the derivation of a PNEC_{micro-organism} for thiosulfate. Application of an assessment factor of 10 results in a PNEC_{micro-organism} of 75.7 mg S₂O₃^2-/L.

Yranslating this value to CaS₂O₃gives a PNEC_{micro-organism} of 106.5 mg test substance/L.

Conclusion on classification

Acute and chronic toxicity data were available for the three main aquatic trophic levels that are considered for classification purposes. Classification is based on the lowest acute and chronic value, referred to as the acute and chronic toxicity reference value (TRV).

The lowest acute effect concentration was observed for the alga S. subspicatus (72h-EC₅₀), and was ≥75.7 mg S₂O₃^2-/L. Translating this value to CaS₂O₃ results in an acute TRV of ≥100 mg/L mg/L for this substance.

Substances for which the acute TRV is situated above 100 mg/L, are not environmentally classified.

 

Consequently, there is no need to classify calcium thiosulfate for the environment.