Dodecamethylcyclohexasiloxane

Biodegradation in soil

Description of key information

Degradation in soil: tropical Wahiawa soil half-life: 1.38 days (32% RH, ~22°C). Closed tubes. Estimated degradation half-life in a temperate soil: 158 days (50% RH); 179 days (70% RH); 202 days (90% RH) at ~22°C.

Estimated degradation half-lives in tropical soil: 1.8 days (50% RH); 2.3 days (70% RH); 3.0 days (90% RH) at ~22°C.

In open systems at higher RH, volatilisation is expected to be an important removal process. In exposure modelling, the estimated degradation half-life in a temperate soil at 90% RH of 202 days at 22°C will be used as a worst case.

Key value for chemical safety assessment

Half-life in soil:

202 d

at the temperature of:

22 °C

Additional information

Studies have been conducted on two types of soil: tropical (Wahiawa soil) and temperate (Londo soil).

Radiolabelled D6 was found to hydrolyse rapidly in Hawaiian Wahiawa soil (half-life < 1 day), in closed tubes at ~22°C and ~30% relative humidity (RH) in the dark, to form degradation intermediates (oligomeric diols) in a reliable study conducted according to generally accepted scientific principles (Xu, 1999). Given sufficient time, these degradation intermediates hydrolysed to dimethylsilanediol (DMSD).

According to the composition of the intermediates extracted at different incubation times, D6 degradation was 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 oligomeric diols decreased in concentration as the concentration of DMSD increased during the incubation. The lack of accumulation of the hexamer diol indicated that the ring-opening hydrolysis was the rate-limiting step.

A further study (Xu and Chandra, 1999) investigated radiolabelled D6 degradation in soil. This is selected as the key study.

¹⁴C-labelled D6 was added to soil that was pre-conditioned at the desired relative humidity (RH), and incubated. D6 was found to hydrolyse rapidly in Wahiawa soil at ~22°C and 32% relative humidity in the dark, to form degradation intermediates (oligomeric diols). Given sufficient time, these degradation intermediates hydrolysed to DMSD. A half-life on Wahiawa soil of 1.38 days was determined.

In the same study, the structurally-related substance D4 (octamethylcyclotetrasiloxane CAS 556-67-2) was added to soil that was pre-conditioned at the desired relative humidity (RH), and incubated at different moisture levels and temperatures, using Wahiawa soil and Londo soil. Closed and open systems were used.

As soil moisture increased, degradation rates decreased. For D4 in Wahiawa soil: 0.04 days (32% RH); 0.08 days (92% RH); 0.89 days (100% RH).

Degradation was faster in tropical Wahiawa soil. D4 Results with Londo soil were: 3.54 days (32% RH); 5.25 days (92% RH).

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 thetropical Wahiawa soilcompared with the temperate soil were explained by the fact that thetropical Wahiawa soilhad 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 (cVMS). 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% of¹⁴C originally added as D4. Volatilisation accounted for up to 40% of D4 loss based on total recovery of¹⁴C, suggesting that both degradation and volatilisation of D4 were significant. For soil at 100% RH, degradation products accounted for <5% of the total¹⁴C added over the entire incubation time (21 days), 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.

Volatilisation ofthe structurally-related substance D5 (decamethylcyclopentasiloxane CAS 541-02-6)was also studied in the Londo soil using open systems. >80% of the applied D5 was evaporated from soil over the incubation period (21 days), and thus was the dominant removal process.

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.

Using the relationship between D4 hydrolysis rate and relative humidity, and the linear relationship between molecular weight and hydrolysis rate for cVMS (hydrolysis rate decreases with increase in molecular weight), degradation half-lives for D4, D5 and D6 in Londo soil and Wahiawa soil were estimated (Dow Corning Corporation, 2007a).

It was concluded that it is reasonable to say that the degradation half-life in a temperate soil is in the order of months for D6: 158 days (50% RH); 179 days (70% RH); 202 days (90% RH) at ~22°C. The estimated degradation half-lives in tropical soil under similar relative humidity are much shorter: 1.8 days (50% RH); 2.3 days (70% RH); 3.0 days (90% RH) at ~22°C.

D6 and the structural analogues, D4 and D5, 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 K_ow>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, 2017a) attached in Section 13 of the IUCLID dossier.

Table 4.1.6presents 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 thatin generalthe 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 therefore considered valid to use the soil degradation data for D4 and D5 to predict half-lives for D6 in temperate and tropical soil.

 

Table4.1.6Degradation in soil data 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: 56 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