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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 D4. Wahiawa soil half-lives: 0.04 days (32% RH); 0.08 days (92% RH); 0.89 days (100% RH) at ~22°C in closed tubes. Londo soil half-lives: 3.54 days (32% RH); 5.25 days (92% RH) at ~22°C in closed tubes. Volatilisation of D4 in open systems was found to be a competing dissipation process at high RH. In the exposure assessment (EUSES 2.1.2) a degradation half-life in bulk soil of 5 days at 20°C will be used as a worse case. 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:
5 d
at the temperature of:
20 °C

Additional information

There are no biodegradation in soil data available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4 -D4), therefore good quality data for the structurally-related substance, octamethylcyclotetrasiloxane (CAS 556-67-2), have been read across.

The registration substance has an average purity of >80% Vi4-D4, with <15% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane Vi5-D5 (CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane Vi3-D3 (CAS 3901-77-7; Impurity 2) present as impurities. Read-across studies are in place as supporting studies, to consider the properties of the impurities. Soil degradation data are not relevant for Vi3-D3 as it hydrolyses rapidly; a data waiver is applicable. Data for Vi5-D5 are read-across from decamethylcyclopentasiloxane D5 (CAS 541-02-6).

Half-lives at ~22°C on Wahiawa soil of 0.04 to 0.89 days (dependent on relative humidity) and on Londo soil of 3.54 and 5.25 days (dependent on relative humidity) were measured in a non-standard reliable study with octamethylcyclotetrasiloxane. In open systems at higher RH, volatilisation became the predominant removal process.

Half-life in Wahiawa soil incubated at 32% RH and ~22°C in closed tubes was 0.08 days. Volatilisation of D5 in open samples was found to be a competing process.

Vi4-D4, D4, Vi5 -D5 and D5are 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.

A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

CAS Number

2554-06-5

556-67-2

17704-22-2

541-02-6

Chemical Name

2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Octamethylcyclotetrasiloxane

2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Impurity 1 (Vi5-D5))

Decamethylcyclopentasiloxane (D5)

Ultimate Si hydrolysis product

Methylvinylsilanediol

Dimethylsilanediol

Methylvinylsilanediol

Dimethylsilanediol

Molecular weight (parent)

344.66

296.62

431

370.8

Molecular weight (hydrolysis product)

104.18

92.17

104.18

92.17

log Kow(parent)

6.47

6.49

9.0

8.0

Water sol (parent)

0.0073 – 0.0088 mg/l at 23°C

0.056 mg/l

9.1E-06 mg/l

0.017 mg/l

Vapour pressure (parent)

93.5 Pa

132 Pa

0.6 Pa

33 Pa

Hydrolysis t1/2at pH 7 and 25°C

approximately 63 hours

69-144 hours

1600 hours

1590 hours

 

It is therefore considered valid to read-across the results for D4 and D5 to fill the data gap for the registered substance.

The table below presents biodegradation in soil data available for substances within the Siloxane Category

CAS

Name

Soil type

Results

Reliability

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

541-02-6

Decamethylcyclopentasiloxane (D5)

Wahiawa soil

Half-life (DT50):

0.08 d (32% relative humidity)

 

Transformation products:

Siloxane diols

Dimethylsilanediol

2

540-97-6

Dodecamethylcyclohexasiloxane (D6)

Wahiawa soil

Half-life (DT50):

1.38 d (32% relative humidity)

 

Transformation products:

Siloxane diols

Dimethylsilanediol

2

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)

30.1 d (#1) (4°C 42% RH Closed)

4.5 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

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

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

D4 degradation and evaporation rates in soils as influenced by soil type, and moisture level, was investigated in a reliable study conducted according to generally accepted scientific principles. 14C-labelled D4 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 degradation rate of D4 was measured in closed tubes on Wahiawa soil at ~22°C and at 32%, 92% and 100% relative humidity and on Londo soil at ~22°C and at 32% and 92% relative humidity. Samples were incubated for various times, ranging from 0 to 21 days. Open tubes were used to determine the rate of volatilisation of D4 at 32%, 50%, and 100% relative humidity, in Londo soil.

D4 was found to hydrolyse rapidly in Wahiawa soil and Londo soil, in closed tubes at ~22°C in the dark, to form degradation intermediates (oligomeric diols). Given sufficient time, these degradation intermediates hydrolysed to DMSD.

The degradation half-lives in Wahiawa soil were: 0.04 days (32% RH); 0.08 days (92% RH); 0.89 days (100% RH). The degradation half-lives in Londo soil were: 3.54 days (32% RH); 5.25 days (92% RH).  

In Londo soil, most of the D4 was intact within 21 days at 100% RH. At <100% RH, the amount of D4 remaining decreased significantly with incubation time.

In Wahiawa soil, the exponential decrease of D4 relative to incubation time was more rapid and significant even at 100% RH.

In both cases the degradation rates increased with a decrease in RH.

According to the composition of the intermediates extracted at different incubation times, D4 degradation is described as a multistep hydrolysis process, initiated with the ring-opening hydrolysis of the cyclics to form linear oligomeric siloxane diols, followed by further hydrolysis of these oligomeric diols to the monomer dimethylsilanediol.

The degradation seen was thought to be the result of hydrolysis reactions catalysed by the surface activity of soil clays. The increase in relative humidity was thought to decrease the surface acidity and thus the hydrolysis rate. The differences in the degradation rates obtained in the weathered soil compared with the temperate soil were explained by the fact that the weathered soil had a higher clay content, and the clay minerals present in this soil were kaolinite (around 50% of the clay minerals) and gibbsite (around 10% of the clay minerals),both of which have been shown previously to be highly effective catalysts of PDMS (polydimethylsiloxane) synthesis from cyclic volatile methyl siloxanes. In contrast as well as having a lower clay content, the clay minerals present in the temperate soil were illite and chlorite, the former has been shown previously to be one of the least effective catalysts for hydrolysis of Si-O-Si linkages.

In addition to the influence of surface acidity on degradation rates, physical separation between the substrate (i.e. D4) and the catalyst (i.e. soil clays) may also contribute to lower degradation at high humidity, possibly because a significant portion of D4 was actually vaporised to the headspace at high moisture levels.

Volatilisation of D4 was found to be a competing process in Londo soil in open systems at high relative humidity. For soil at 50% RH, the degradation products could account for up to 60% of 14C originally added as D4. Volatilisation accounted for up to 40% of D4 loss based on total recovery of 14C, suggesting that both degradation and volatilisation of D4 were significant. For soil at 100% RH, degradation products accounted for <5% of the total 14C added over the entire incubation time, while >80% of the applied D4 was evaporated from soil in the same period, and thus was the dominant removal process. At 32% RH, volatilisation was negligible, and rapid degradation was the predominant process in the dissipation of D4.

The study authors conclude that the negligible volatilisation of D4 at low moisture levels was a result of high sorption and fast degradation of D4 in dry soil. Likewise, the increased volatilisation at high humidity was due to the slow degradation and low sorption of D4 in moist soil.

For soil re-wetted to water saturation, some of the intermediates continued to hydrolyse to DMSD, and some were converted back to cVMS, which then evaporated from wet soil. The volatilisation of D4 from water-saturated soil was much slower, thought to be due to the low water solubility of D4 and high proportion expected to partition to soil organic matter in wet soil.