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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Degradation in soil: The following results are read-across from the structurally-related substance L3. Michigan Londo soil, half-lives (closed tubes) 1.48 d at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in headspace at this RH). UK loamy silt soil, half-life (closed tubes) 0.26 d at 32% RH and at 22.5°C. The degradation products were dimethylsilanediol, trimethylsilanol and 3, 3, 3, 1, 1-pentamethyldisiloxanol. In open systems at higher RH, volatilisation became the predominant removal process, with a volatilisation half-life <1 day at 100% RH and at 22.5°C. In exposure modelling (EUSES 2.1.2) a half-life value of 10 days at 20°C will be used, based on the value of 6.19 d (#1) (92% RH 22.5°C Closed). This is an estimate. The exact value is not significant in respect of the overall risk characterisation for soil.

Key value for chemical safety assessment

Half-life in soil:
10 d
at the temperature of:
20 °C

Additional information

There is no reliable degradation in soil data available for H-L3, therefore good quality data for the structurally-related substance, octamethyltrisiloxane (L3, CAS 107 -51 -7), have been read across.

H-L3 and L3 are members of the Reconsile Siloxanes Category. This Category consists of linear/branched and cyclic siloxanes which have a low functionality and a hydrolysis half-life at pH 7 and 25°C >1 hour and log Kow>4. There is a limited amount of soil stability data available with siloxanes. Substances that are highly absorbing are expected to have slow degradation rates in soil. The category hypothesis is that stability in soil is linked to the organic carbon-water coefficient and hydrolysis rates, which are dependent in turn on the structural features and constituent functional groups within the molecule. In the context of the Read-Across Assessment Framework (RAAF), Scenario 4 is applicable to this endpoint.

 

Additional information on the structure of the category and the supporting evidence for the application of the Scenario is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID dossier.

H-L3 and the source substance L3 are linear siloxanes with three silicon atoms, alternated by oxygen atoms. In L3, the Si atoms are fully methyl substituted, whereas in H-L3 the central silicon atom is substituted with one hydrogen atom and one methyl group. L3 and H-L3 possess similar physicochemical properties. A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and moderate hydrolysis rates. The hydrolysis half-life for L3 (13.7 d at pH 7 and 25°C) is slower than the hydrolysis half-life for H-L3 (2.2 d at pH 7 and 25°C) and therefore the read-across of soil degradation data with L3 represents a worst-case scenario.

Table: Key physicochemical properties of H-L3 and surrogate substance L3

Property

H-L3 (1873-88-7)

L3 (107 -51-7)

Molecular weight

222.51

236.54

Log Kow

6.2

6.60

Log Koc

3.8

4.34

Water solubility (mg/l)

0.02 (at 22°C)

0.034 (at 23°C)

Vapour pressure at 25°C (Pa)

8.5E+02

5.3E+02

Hydrolysis half- life at pH 7 (d)

2.2

13.7

 

Given the similar properties and structural similarities, it is considered valid to read-across sediment toxicity data from L3 to H-L3.

The table below presents all of the available data for removal of substances from soil within the Siloxane Category. In these studies,14C-labelled siloxane was added to soil that was pre-conditioned at the desired relative humidity (RH) and incubated at different moisture levels and temperatures. Closed and open systems were used. The results show that in general the rate of degradation is greater at lower RH in closed systems, and in open systems volatilisation is the predominant process for removal from soil at higher RH. Removal half-lives are generally <10 days in closed systems at RH< 100. It is therefore considered valid to read across the results for L3 to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID 6 dossier.

Degradation in soil data for substances within the Siloxane Category

CAS

Name

Soil type

Results

Reliability

Reference

556-67-2

Octamethylcyclotetrasiloxane (D4)

Wahiawa soil (#1)

 

Londo soil (#2)

Half-life (DT50):

0.04 d (#1) (32% relative humidity)

0.08 d (#1) (92% relative humidity)

0.89 d (#1) (100% relative humidity)

3.54 d (#2) (relative humidity 32%)

5.25 d (#2) (relative humidity 92%)

 

Transformation products:

Siloxane diols

Dimethylsilanediol

2

Xu and Chandra, 1999

541-02-6

Decamethylcyclopentasiloxane (D5)

Wahiawa soil

Half-life (DT50):

0.08 d (32% relative humidity)

 

Transformation products:

Siloxane diols

Dimethylsilanediol

2

Xu and Chandra, 1999

540-97-6

Dodecamethylcyclohexasiloxane (D6)

Wahiawa soil

Half-life (DT50):

1.38 d (32% relative humidity)

 

Transformation products:

Siloxane diols

Dimethylsilanediol

2

Xu and Chandra, 1999

107-46-0

Hexamethyldisiloxane (L2)

Londo

 

Half-life (DT50):

407.6 d (#1) (100% RH 22.0°C Closed NOTE: 9.8 days when corrected for head-space effect)

5.8 d (#1) (92% RH 22.0°C Closed)

6.4 d (#1) (42% RH 22.0°C Closed)

1.8 d (#1) (32% RH 22.0°C Closed)

19.9 d (#1) (4°C 42% RH Closed)

0.96 d (#1) (37°C 42% RH Closed)

323.9 d (#1) (100% RH 25.0°C Closed (Calculated from activation energy calculated for 42% RH) NOTE: 7.9 days when corrected for head-space effect)

4.7 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH))

5.2 d (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH))

1.4 d (#1) (32% RH 25.0°C Closed (From activation energy calculated for 42% RH))

 

Transformation products:

Trimethylsilanol

2

Dow Corning Corporation (2014)

107-51-7

Octamethyltrisiloxane (L3)

Londo (#1)

 

Loamy silt (#2)

 

 

Half-life (DT50):

119.5 d (#1) (100% RH* 22.5°C Closed NOTE: 24 days when corrected for head-space effect)

6.19 d (#1) (92% RH 22.5°C Closed)

3.62 d (#1) (42% RH 22.5°C Closed)

1.48 d (#1) (32% RH 22.5°C Closed)

0.26 d (#2) (32% RH 22.5°C Closed)

19.9 d (#1) (4°C 42% RH Closed)

0.96 d (#1) (38.5°C 42% RH Closed)

96.3 d (#1) (100% RH 25.0°C Closed (From activation energy calculated for 42% RH) NOTE: 19.3 days when corrected for head-space effect)

4.98 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH))

12.8 h (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH))

 

Transformation products:

Dimethylsilanediol

Trimethylsilanol

3, 3, 3, 1, 1-Pentamethyldisiloxanol

2

Xu, 2010

141-62-8

Decamethyltetrasiloxane (L4)

Londo (#1)

 

 

 

106.6 d (#1) (100% RH 22°C Closed. NOTE: 56 days when corrected for head-space effect)

10 d (#1) (92% RH 22°C Closed)

4.5 d (#1) (42% RH 22°C Closed)

3.7 d (#1) (32% RH 22°C Closed)

29 d (#1) (4°C 42% RH Closed)

1.2 d (#1) (37°C 42% RH Closed)

80.6 d (#1) (100% RH 25.0°C Closed (From activation energy calculated for 42% RH) NOTE: 42 days when corrected for head-space effect)

7.6 d (#1) (92% RH 25.0°C Closed (From activation energy calculated for 42% RH).)

3.4 h (#1) (42% RH 25.0°C Closed (From activation energy calculated for 42% RH).)

2.8 d (#1) (32% RH 25.0°C Closed (From activation energy calculated for 42% RH).)

% Degradation of test substance:

Transformation products:

Dimethylsilanediol

Trimethylsilanol

2

Xu, S (2014)

 

 

The soil degradation/volatilisation study for L3 was conducted with a Londo soil from Bay City, Michigan, USA and a UK Loamy silt soil. 14C-labeled L3 was added to soil that was pre-conditioned at the desired relative humidity (RH) and incubated at different moisture levels and temperatures. Closed and open systems were used.

The rate of degradation was greater as the soil became drier. In Michigan Londo soil, degradation half-lives (closed tubes) ranged from 1.48 d at 32% RH and at 22.5°C to 119.5 d at 100% RH and at 22.5°C (estimated to be 24 days when corrected for amount of L3 predicted to be in the headspace at this RH).

In open systems, volatilisation was the predominant process for removal of L3 from soil at 100% RH with a volatilisation half-life of <1 day, much faster than the degradation of L3 at the same moisture level in the closed system.

The degradation products were dimethylsilanediol, trimethylsilanol and 3, 3, 3, 1, 1-pentamethyldisiloxanol.

In loamy silt soil, the degradation half-life (closed tubes) was 0.26 d at 32% RH and at 22.5 °C.