Dichlorosilane

Toxicity to microorganisms

Description of key information

Toxicity to microorganisms: 3 hour EC₅₀ > 100 mg/l (OECD 209), read-across from an analogous substance.

Additional information

There are no reliable microorganism toxicity data available for dichlorosilane (CAS 4109-96-0). A toxicity to microorganisms study with dichlorosilane is considered technically unfeasible because the substance is a gas at normal temperature and pressure and is expected to react violently and rapidly with water. This violent and rapid reaction of the gas with water would render aquatic ecotoxicology testing impossible and thus testing is waived on the basis of the technical difficulty to conduct such tests.

However, good quality data for the analogous substance, tetraethyl orthosilicate (CAS 78-10-4), have been read across as supporting evidence that the silanol hydrolysis product is not toxic and adding to the weight of evidence for not doing any further testing. Both substances hydrolyse rapidly to monosilicic acid. Tetraethyl orthosilicate hydrolyses with a half-life of 4.4 hours at pH7 and 25°C to give monosilicic acid and ethanol. Dichlorosilane hydrolyses with a half-life of approximately 5 seconds (as a worst case) at 25°C and pH 7 (analogue read-across). This half-life relates to degradation of the parent substance to give silanediol and HCl.

The Si-H bonds of silanediol are expected to react rapidly (<12 hours at 25°C and pH 7) to produce hydrogen and monosilicic acid.

In view of the rapid hydrolysis, it is the silanol hydrolysis product that is relevant for environmental risk assessment.

Dichlorosilane (CAS 4109-96-0) and tetraethyl orthosilicate (CAS 78-10-4) are within an analogue group of substances within which, in general, there is no evidence of significant toxicity to microorganisms.

This analogue group for the toxicity to microorganisms endpoint consists of a number of sub-classes of substances (based on structure/functionality) with different main codes (used to represent the different sub-classes). However, read-across is carried out between substances with the same main code in most cases.

The table below presents microorganism toxicity data for substances within the sub-class I-4 of Silicon without carbon attached (“(Poly)silicic acid producers”). Dichlorosilane (CAS 4109-96 -0) and tetraethyl orthosilicate (CAS 78-10-4) are members of this sub-class.

It is therefore considered valid to read-across the results for tetraethyl orthosilicate (CAS 78-10-4) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2013j) attached in Section 13 of the IUCLID dossier.

Table: Microorganism toxicity data for substances within the relevant sub-class I-4 of chemicals

CAS	Name	Main code	Result: E(I)C50 (mg/l)	Result: NOEC (Or EC10/EC20) (mg/l)	Guideline	Test method	Species	Duration	Klimisch code
000078-10-4	Tetraethyl orthosilicate	I-4-Q	>100 mg/l		OECD 209	ASRI		3hr	1
018765-38-3	Tetrakis(2-butoxyethyl) orthosilicate	I-4-Q		EC10 >1.7 ml/l	Huls AG method	oxygen consumption	P. putida	5 h	2
068412-37-3	Silicic acid (H4SiO4), tetraethyl ester, hydrolyzed	I-4-Q		EC10 >2 ml/l	Huls AG method	oxygen consumption	P. putida	5 h	2

In the study with tetraethyl orthosilicate (CAS 78 -10 -4), an activated sludge respiration inhibition (ASRI) 3 hour EC₅₀value of >100 mg/l was reported.

The hydrolysis rate for tetraethyl orthosilicate is 4.4 hours at pH7 and 25°C, and so it is likely to hydrolyse to monosilicic acid to a lesser extent than dichlorosilane would hydrolyse over the timescale of the ASRI test.Therefore, microorganisms toxicity data for Silicic acid (H4SiO4), tetraethyl ester, hydrolysed (CAS 68412-37-3), which is a member of the same

sub-class I-4 of Silicon without carbon attached (“(Poly)silicic acid producers”)

, has been read-across as supporting data. Microorganism toxicity data from a non-standard oxygen consumption study with Pseudomonas putida are available for Silicic acid (H4SiO4), tetraethyl ester, hydrolysed.

A 5.42 hour EC10 of >2 ml/l for toxicity to Pseudomonas putida was determined for silicic acid (H4SiO4), tetraethyl ester, hydrolysed in a reliable study conducted according to

generally accepted scientific standards and described in sufficient detail, and was carried out in compliance with GLP, with acceptable restrictions. The restrictions were that a reference substance was not used to check the sensitivity of the microorganisms, and an emulsifier was used which could have affected the results. PFA, 2013j, Peter Fisk Associates, STP Microorganism toxicity Main Analogue Group report, PFA.300.003.006