Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.007 mg/L
Assessment factor:
1 000
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
0.007 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.001 mg/L
Assessment factor:
10 000
Extrapolation method:
assessment factor

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
3.26 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
0.033 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.003 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
0.003 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
insufficient hazard data available (further information necessary)

Additional information

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physicochemical properties.

The registered and surrogate substances contain thiols side chains. The thiol groups may be oxidised to disulfides, however this requires relatively strong oxidising agents and the reaction is not considered relevant at the concentrations tested or under environmental conditions.

Thiols play an important role in cell biochemistry, however there is the possibility of redox cycling between the thiol and the corresponding disulphide, and this can lead to toxicity. Free radicals, such as active oxygen species and thiyl (synonymous with sulphenyl radicals) radicals, are formed during this process, which can also lead to toxicity. The substances in this group may also be weak haemolytic agents; it is assumed that the mechanism of toxicity is a result of free-radical formation. However there is no evidence of this from short-term exposure data for mammals. Long-term study data for mammals generated under testing proposals may produce evidence of the toxic mechanism.

A possible mode of action for thiols is interference of thiol groups with normal cellular biological activity, by reacting with sulfhydryl groups of functional proteins and thiol dependant enzymes.

Currently, the mode of action for thiols in aquatic and terrestrial organisms has not been identified.

The data available in the ECOSAR v.1.11 (Mayo-Bean 2012) training set indicate that the thiols are 2 orders of magnitude more toxic than neutral organics. This is indicative of a specific mode of action.

Additional information is given in a supporting report (PFA 2013w) attached in Section 13 of the IUCLID 5 dossier.

In the following paragraphs the read-across approach for 3-(triethoxysilyl)propanethiol is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

 

  • Read-across from 3-trimethoxysilylpropane-1-thiol to 3-(triethoxysilyl)propanethiol

The registered substance (target), 3-(triethoxysilyl)propanethiol, and the substance used for read-across (source), 3-trimethoxysilylpropane-1-thiol, are trialkoxysilanes with an ethoxy and a methoxy group respectively, in addition to a thiol group present in the side chain. The substances are considered to be structural analogues, the toxicity of which is dominated by the presence of the thiol group.

3-(Triethoxysilyl)propanethiol has a hydrolysis half-life of approximately 19 h at 20-25°C and pH 7 (predicted), a log Kow value of 2.7, water solubility of 82 mg/l and a MW of 238.2.

3-Trimethoxysilylpropane-1-thiol has a hydrolysis half-life of 2.6 h at pH 7 and 20°C (predicted), a log Kow value of 1.7, water solubility of 2800 mg/l (predicted) and a MW of 196.3.

Both substances will hydrolyse under the conditions of the studies to produce 3-(trihydroxysilyl)propanethiol, and ethanol and methanol as by products respectively. Ethanol and methanol are well characterised in the public domain and are not hazardous at the concentrations relevant to the studies, further considerations are given below. The silanol hydrolysis product has a log Kowvalue of -1.4, is soluble in water (predicted) and has a MW of 154.2.

The toxicity is driven by the thiol group and as suggested by ECOSAR v.1.11 (2012) and in literature available in the public domain, invertebrates are the most susceptible trophic level. The sources also suggest that there is a low degree of relationship between log Kow and thiol toxicity to fish. The difference in log Kow between the registered substance and the silanol hydrolysis product, on which the read-across is based, is thought to ultimately not affect the validity of the read-across as data for the most susceptible trophic level, the toxicity of which is not correlated to log Kow, are used to derive the hazard and risk characterisations. 

The data for the surrogate substance indicates that the substances short-term toxicity lies in the range 6.7 to 931 mg/l, with invertebrates being the most susceptible trophic level.

 

  • Considerations on the non-silanol hydrolysis products:

Methanol and ethanol are well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC50 and LC50 values for these substances are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5).Therefore, at the loading rates experienced in these tests it is unlikely that the presence of either would significantly affect the results of the tests.

 

Table 7.1 Summary of the physico-chemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number

14814-09-6

4420-74-0

Chemical Name

3-(triethoxysilyl)propanethiol

 3-trimethoxysilylpropane-1-thiol

Si hydrolysis product

3-(trihydroxysilyl)propanethiol

3-(trihydroxysilyl)propanethiol

Molecular weight (parent)

238.42

196.34

Molecular weight (hydrolysis product)

154.26

154.26

log Kow(parent)

2.7

1.7

log Kow(silanol hydrolysis product)

1.4

1.4

Water sol (parent)

82 mg/l

2.8E+03 mg/l

Water sol (silanol hydrolysis product))

1E+06 mg/l

1E+06 mg/l

Vapour pressure (parent)

1.1 Pa

8 Pa

Vapour pressure (hydrolysis product)

1.1E-04 Pa

1.1E-04 Pa

Hydrolysis t1/2at pH 7 and 20-25°C

18 hours

2.6 h

Hydrolysis t1/2at pH 4 and 20-25°C

0.6 h

0.2 h

Hydrolysis t1/2at pH 9 and 20-25°C

0.3 h

0.1 h

Short-term toxicity to fish (LC50)

n/a

439 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

n/a

6.7 mg/l

Algal inhibition (ErC50and NOEC)

n/a

EL50: 931 mg/l; NOELR: 40 mg/l*

Long-term toxicity to fish (NOEC)

n/a

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

n/a

Sediment toxicity (NOEC)

n/a

n/a

Short-term terrestrial toxicity (L(E)C50)

n/a

>1000 mg/kg dwt

Long-term terrestrial toxicity (NOEC)

n/a

n/a

*EL50 is the Effect Loading rate, NOELR is the No Effect Loading Rate.

No data are available for the submission substance, 3-(triethoxysilyl)propanethiol. However, reliable short-term toxicity data are available for the analogous substance 3-(trihydroxysilyl)propanethiol (CAS 4420-74-0). The specific mode of action of thiols suggests that a long-term toxicity study may be required in order to assess the long-term toxicity to aquatic invertebrates (most sensitive trophic level).

Short-term data are available for three trophic levels: fish, invertebrates and algae:

·        Fish LC50439 mg/l;

·        Invertebrates EC506.7 mg/l;

·        Algae EC50931 mg/l and a NOEC of 50 mg/l;

The PNECaquatic(freshwater)is based on the lowest EC/LC50value for the three trophic levels which is 6.7 mg/l as determined in the invertebrate test. An assessment factor (AF) of 1000 is applicable when short-term data are available from three trophic levels.

Therefore, PNECaquatic(freshwater)is 6.7/1000 = 0.0067 mg/l.

PNECaquatic(marine water)is derived from the freshwater long-term aquatic toxicity data as no data are available for marine species. An additional assessment factor of 10 is applied to account for possible differences between marine and freshwater environments, giving an overall assessment factor of 10 000.

Where releases are intermittent, it is appropriate to reduce the AF by a factor of 10 compared to one applied to derive the PNECaquatic(freshwater), giving an overall assessment factor of 100.

Conclusion on classification

3-(triethoxysilyl)propanethiol hydrolyses rapidly in water and the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol, is not readily biodegradable.

3-(triethoxysilyl)propanethiol is classified in the EU as Aquatic Chronic Category 2, according to the Regulation (EC) No 1272/2008 because data for an appropriate surrogate indicate that the lowest EC50 is 6.7 mg/l.