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Toxicity to microorganisms

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Description of key information

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

Key value for chemical safety assessment

Additional information

There are no reliable microorganism toxicity data available for hexachlorodisilane (CAS 13465-77-5), therefore good quality data for the analogous substance, tetraethyl orthosilicate (CAS 78-10-4), have been read across. Both substances hydrolyse rapidly to monosilicic acid. Tetraethylorthosilicate hydrolyses with a half-life of 4.4 hours at pH7 and

25°C to give monosilicic acid and ethanol.

Hexachlorodisilane 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 hexahydroxydisilane and HCl.

The Si-Si bonds may also react to produce monosilicic acid.

Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100 -150 mg/l as SiO2 to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

Hexahydroxydisilane is also likely to form condensation products (polyhydroxy-polysilanes).

Hexachlorodisilane (CAS 13465-77-5) 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”). Hexachlorodisilane (CAS 13465-77-5) 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.

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

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 EC50 value of >100 mg/l was reported

The hydrolysis rate for tetraethyl orthosilicate is 4.4 hours at pH 7 and 25°C, and so it is likely to hydrolyse to monosilicic acid to a lesser extent than hexachlorodisilane 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. REFERENCE: PFA, 2013j, Peter Fisk Associates, STP Microorganism toxicity Main Analogue Group report, PFA.300.003.006