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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 L4. Michigan Londo soil, half-lives (closed tubes) 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 56 days when corrected for amount of L4 predicted to be in headspace at this RH). The main degradation products were dimethylsilanediol and trimethylsilanol. In open systems, the volatilisation of L4 was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system. In exposure modelling (EUSES 2.1.2) a half-life value of 10 days at 20°C (equivalent to 19 days at 12°C) will be used, based on the value of 10 d (#1) (92% RH 22°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 are no reliable biodegradation in soil data available for M3T, therefore good quality data for the structurally-related substance, decamethyltetrasiloxane, L4, (CAS 141-62-8), have been read across in a Category approach.

M3T and L4 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.

M3T and the source substance L4 are structural isomers; both are methylated siloxanes containing four Si atoms linked by oxygen. L4 is a linear chain with four silicon atoms linked by three oxygen atoms, whereas M3T is a branched structure of four Si atoms, the longest siloxane chain contains three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. In both substances, the silicon atoms are fully substituted by methyl groups. Neither substance contains any other functional groups;both have a molecular weight of 311. A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Table4.1.6Key physicochemical properties of M3T and surrogate substance L4

 

 

M3T

L4

Chemical name

1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane

decamethyltetrasiloxane

CAS

17928-28-8

141-62-8

Molecular weight (parent)

311

311

Log Kow (parent)

8.2 at 20°C (QSAR)

8.21 at 25.1°C

Water solubility (parent)

0.00189 mg/L at 23°C

0.00674 mg/L at 23°C

Vapour pressure (parent)

210 Pa at 25°C (QSAR)

73 Pa at 25°C

Henry’s Law Constant (parent)

1.61+07 Pa m3mol-1at 12°C

2.59E+06 Pa m3mol-1at 12°C

Log Koc (parent)

5.3 (QSAR)

5.16 at 23.7°C

Ready biodegradability

Not readily biodegradable

Not readily biodegradable

Hydrolysis t1/2at pH 7 and 25°C

630 h (QSAR)

728 h

Hydrolysis products

Trimethylsilanol (3 moles per mole of parent) and methysilanetriol (1 mole)

Trimethylsilanol (2 moles per mole of parent) and dimethylsilanediol (2 moles)

 

Given the similar properties and structural similarities, it is considered valid to read-across soil degradation data from L4 to M3T.

Table 4.1.7 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. Degradation products are identified in all studies; the ultimate hydrolysis products are identified as degradation products in all studies, and in most cases the intermediate hydrolysis products are also observed. It is considered valid to read-across the results for L4 to fill the data gap for the registered substance.

Table 4.1.7 Degradation in soil data for substances within the Siloxane Category

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 (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 L4 was conducted with a Londo soil from Bay City, Michigan, USA. 14C-labelled L4 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 removal was greater as the soil became drier. Degradation half-lives (closed tubes) ranged from 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 56 days when corrected for amount of L4 predicted to be in the headspace at this RH).

The correction for amount of L4 predicted to be in the headspace is made to degradation rates at 100% RH. It is thought by the authors of the study that the rates at this RH may be underestimated due to complication of soil/air partitioning in the test tubes during incubation (L4 present in headspace not available for biotic or abiotic degradation). For each sample in a closed tube at 100% RH, the estimated fraction, via soil-air partitioning calculation, of L4 actually distributed in soil is around 52.4%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for L4 was rapid compared to its degradation, and that the system was always in equilibrium during the course of the degradation, the actual degradation rate for L4 in soil without headspace should be ~1.9 times larger than observed.

The identified degradation products were dimethylsilanediol and trimethylsilanol.

In open systems, volatilisation was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system.

This study result is read-across to M3T. The corresponding degradation products for M3T are expected to be 1,1,1,3,5,5,5-heptamethyltrisiloxanol and 1,1,1,3-tetramethyldisiloxanediol (intermediate products of abiotic degradation) and trimethylsilanol and methylsilanetriol (final products of abiotic degradation).