Tripotassium propylsilanetriolate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

1.5 mg/L

Assessment factor:

500

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

15 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.15 mg/L

Assessment factor:

5 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

10 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

5.7 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.57 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:

0.26 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The registered substance is the potassium salt of propylsilanetriol. It is stable only in solution at high pH (pH >14, 1-5% excess potassium hydroxide) and high concentration (60-65% water; 30-35% potassium propylsilanetriolate). In these conditions the substance is fully-dissociated topropylsilanetriolate and free potassium ions.At pH 7 and low concentration (conditions relevant for the environment), the monomer (propylsilanetriol) will predominate, while under certain conditions (for example, concentrations in the grams per litre range, at neutral pH), the dimer, trimer and higher oligomers or polymers may be formed in solution. Under comparable conditions of concentration and pH, propylsilanetriolate is equivalent to the parent acid propylsilanetriol. Under conditions relevant for the environment (neutral pH and low concentration), propylsilanetriol is expected to predominate. 

The effects of potassium hydroxide on the aquatic environment are limited to pH effects in unbuffered media and are not relevant for PNEC derivation.

Therefore, it is appropriate to conduct the hazard assessment for the environment based on the properties of the silanol, propylsilanetriol.

 

READ-ACROSS JUSTIFICATION

No data are available with the registered substance. In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the pKa and the hydrolysis rate of a substance are particularly important since after dissociation or hydrolysis occur the resulting product has different physicochemical properties and structure.

The registered substance and the substance used as surrogate for read-across are part of a class of low functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the Si atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The silanol may be susceptible to condensation reactions, see Section 1.3. However the formation of the monomer if favoured at neutral pH (see Section 1.3),therefore the selection of surrogate substance is based on log K_ow of the silanol monomer and the chemical groups present in it. The side group of the registered substance and the surrogate substance is propyl.

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

In the following paragraphs the read-across approach for tripotassium propylsilanetriolate is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

·        Read-across from trimethoxypropylsilane to tripotassium propylsilanetriolate

Tripotassium propylsilanetriolate is the potassium salt of propylsilanetriol formed at high pH. In solution, it is completely dissociated to propylsilanetriol and three free potassium ions. The pKa of propylsilanetriol is 10; therefore, at pH values relevant for the environment and ecotoxicity testing, it will be predominately present as propylsilanetriol. The potassium ions will behave like potassium ions naturally present in the environment and associate with available anions in the test medium to form salts such as potassium chloride and potassium sulphate. The read-across substance trimethoxypropylsilane is a trialkoxysilane that will hydrolyse rapidly in water (half-life 2.6 h at pH 7 and 20⁰C) to form propylsilanetriol and methanol.

The hazard assessment of both substances is based on the dissociation/hydrolysis product they share, propylsilanetriol, therefore the read-across is considered to be directly relevant.

The short-term aquatic toxicity data for the surrogate substance indicates that there are no effects at the highest concentrations tested.

• Considerations on the non-silanol dissociation products:

Potassium is an inorganic, ubiquitous and essential element that is not considered to be hazardous to aquatic organisms.

Methanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values are in excess of 10 000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1).Therefore, at the loading rates experienced in these tests it is unlikely that the presence of methanol would significantly affect the results of the tests.

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

CAS Number	93857-00-2	1067-25-0
Chemical Name	Tripotassium propylsilanetriolate	Trimethoxypropylsilane
Relevant silanol (see above text)	Propylsilanetriol	Propylsilanetriol
Molecular weight (parent)	236.47	164.28
Molecular weight (hydrolysis product)	122.2	122.2
log Kow(parent)	n/a	1.7
log Kow(silanol)	-1.4	-1.4
Water sol (parent)	Miscible	9200 mg/l
Water sol (silanol)	Soluble	Soluble
Vapour pressure (parent)	n/a	1900 Pa
Vapour pressure (silanol)	4.6E-03 Pa	4.6E-03 Pa
Hydrolysis t1/2 at pH 7 and 25°C	n/a	2.6 h
Hydrolysis t1/2 at pH 4 and 25°C	n/a	0.2 h
Hydrolysis t1/2 at pH 9 and 25°C	n/a	0.1 h
Short-term toxicity to fish (LC50)	n/a	>745 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>816 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	ErC50 >913 mg/l, NOEC ≥913 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 (LC/EC50)	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a

Conclusion on classification

Tripotassium propylsilanetriolate dissociates in contact with water and moisture to produce propylsilanetriol and potassium, which are not readily biodegradable. Tripotassium propylsilanetriolate is not classified for the environment in the EU according to Regulation (EC) No 1272/2008 because appropriate read-across data for the silanol dissociation product indicate that it is not toxic up to a concentration of 100 mg/l, i. e. lowest LC/EC₅₀ is >100 mg/l.