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EC number: 208-762-8
CAS number: 540-97-6
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.
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
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%
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
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 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
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
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
Table4.1.6Degradation in soil data for substances within the
Wahiawa soil (#1)
Londo soil (#2)
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%)
0.08 d (32% relative humidity)
1.38 d (32% relative humidity)
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))
Loamy silt (#2)
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))
3, 3, 3, 1, 1-Pentamethyldisiloxanol
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:
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