Dodecamethylpentasiloxane

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Study period:

25/03/2019 - 02/02/2021

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems)

Deviations:

yes

Remarks:

A modified version of the test guideline was used to account for the combination of high air/water partitioning coefficient and low water solubility of the substance.

Principles of method if other than guideline:

The modifications included: a) use of a custom-made incubation vessel which satisfies the OECD 308 requirements, but minimises the headspace volume; b) selection of a spiking solvent and method to ensure distribution of the test material mainly in the sediment phase; c) use of a method to minimise volatility during the test procedure.

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

natural water / sediment: freshwater

Details on source and properties of sediment:

- Details on collection (e.g. location, sampling depth, contamination history, procedure): Freshwater sediments and their associated surface waters for use in this study were collected by LRA Labsoil (Lockington, Derby UK) from Calwich Abbey Lake and Emperor Lake located in the United Kingdom, on 06 February and 05 February 2019 respectively. Calwich Abbey Lake (Calwich, Staffordshire; 52° 59′ 6.4″N, 1° 48′ 38.3″W) is a perennial lake fed by a stream from a weir on the River Dove, fringed by woodland to the north and west, and by ley grassland to the south. Emperor Lake (Chatsworth, Derbyshire; 53° 13′ 46.5″N, 1° 35′ 56.9″W) is a perennial lake fed by Emperor Stream/Umberley Brook, fringed by woodland. Sediments were scooped from the top 5 cm onto the bank to drain slightly, then passed through a 2 mm sieve into 6.4 L plastic kegs. Surface waters were scooped from the lake by bucket and passed through a 212 μm sieve into 20 L plastic containers. The materials were shipped by air (no measures were taken to control conditions during transit) to the lab performing the simulation study, where they were received on 13 February 2019. At the lab, they were stored under refrigeration (~4 °C) until the beginning of acclimation on 13 March 2019.

- Textural classification (i.e. %sand/silt/clay):
Calwich Abbey Lake sediment: 27.1% w/w sand / 70.4% w/w silt / 2.5% w/w clay [textural class: Silt Loam]
Emperor Lake sediment: 63.7% w/w sand / 16.1% w/w silt / 20.2% w/w clay [textural class: Sandy Clay Loam]
- pH at time of collection:
Calwich Abbey Lake sediment: 7.04 / 6.89 (water/0.01M CaCl2); 7.35 (surface water)
Emperor Lake sediment: 6.51 / 5.56 (water/0.01M CaCl2); 6.88 (surface water)
- Organic carbon (%):
Calwich Abbey Lake sediment: 4.7 % w/w
Emperor Lake sediment: 2.0 % w/w

Duration of test (contact time):

140 d

Initial conc.:

ca. 150 other: ng 14C-L3 per gram of wet weight [ww] sediment

Parameter followed for biodegradation estimation:

radiochem. meas.

Details on study design:

TEST CONDITIONS
- Volume of test solution/treatment: test systems were prepared by adding 143 to 151 g ww (49 to
51 g dw) of Calwich Abbey sediment, or 118 to 122 g ww (58 to 61 g dw) Emperor Lake sediment, to each flask, followed by addition of the corresponding surface water to the 225 mL mark on the flask. These amounts of sediment gave an observed layer thickness of approximately 2 cm. A silicone sponge closure was placed over the top opening of each vessel to allow the exchange of gases and to minimize water evaporation during acclimation. The flasks were placed in dark 12°C incubators for acclimation for between two and three weeks. Just prior to spiking of the test substance, enough additional surface water (equilibrated to 12 °C) was added to each acclimated test vessel to fill it completely, followed by removal and discarding of exactly 20 mL of the water to create a consistent headspace volume.
- Solubilising agent (type and concentration if used): Following the acclimation period, each designated test vessel was spiked with 10 μL of 14C-L3 in diethylene glycol methyl ether (DEGME) to yield an initial nominal sediment concentration of approximately 150 ng/g. The following procedure was carried out for each flask: After weighing the flask, 60 mL volume (approximate) of water was removed temporarily and held in reserve in order to slightly lower the water level in the flask during spiking. The test material solution was applied in 1 μL aliquots via microsyringe to multiple positions at the top surface of the sediment, following an approximate grid pattern (3-4-3) over the circular area of the sediment-water interface. Immediately following the addition of the test material, the reserved water was returned to the flask with minimal disturbance to the existing water or sediment, and the test vessel was closed tightly using a septum cap prior to returning the vessel to the 12 °C incubator.
The control vessels were prepared identically to the nominally-dosed test vessels, but spiked with 10μL of DEGME solvent without test material.
TEST SYSTEM:
- Culturing apparatus: Modified glass 250-ml Erlenmeyer flask with a glass side arm. During incubation, both openings were fitted with screw cap septum closures.
- Number of culture flasks/concentration: Seventeen flasks were prepared to allow two sediment-water systems containing 14C-L3 to be sacrificed for analysis at each of eight sampling times, with one spare flask for contingency purposes. Four control flasks were prepared identically to the test flasks, but without addition of test material. The control flasks were used to monitor oxygen saturation of the overlying water during the incubation period.
- Method used to create aerobic conditions / Details of trap for CO2 and volatile organics if used : Regular exchange of the headspace gas was required in the aerobic test in order to compensate for oxygen consumption by the microbial biomass. Aerations were conducted on each nominally-dosed and control test vessels. The oxygen saturation level in the overlying water of the control vessels only was measured before and after each aeration using a fiber optic oxygen transmitter with a needle-type microsensor (Presens Precision Sensing GmbH). Beginning on the second or third day after test material addition, and typically every 2 to 3 (Calwich Abbey Lake system) or 3 to 4 (Emperor Lake systems) days thereafter, approximately 180 mL of laboratory air was bubbled through the overlying water. A stainless steel needle inserted through the side arm septum, with the needle end near but not touching the sediment, facilitated addition of fresh air using an aquarium pump; the flow rate was regulated at 10 ± 1 mL/min and monitored with a calibrated flow meter. In response to lower dissolved oxygen levels in the vessels following aeration, the flow rate was increased to 20mL/min for the CAL aeration event on 02 May 2019 and the EL aeration event on 02 May 2019, as well as all subsequent aeration events.
Three types of traps were used to capture volatile compounds or carbon dioxide that were purged from the overlying water or vessel headspace during aeration. The first trap was meant to capture the parent test material and/or volatile transformation products from the gas phase in a glass coil (~2.5 mL total capacity) sealed at both ends with septum caps, sitting in a dry ice/acetone bath. After aeration was completed, the sealed coil was placed in a hot water bath to promote re-volatilization of the trapped components, and a gas-tight syringe was used subsequently to transfer the gaseous volatiles from glass coil to the headspace of the closed test vessel by injecting through the top septum cap closure. The trap and syringe were rinsed with THF, and the rinses were combined for LSC analysis.
Any components too volatile to be removed cryogenically were trapped by two types of scintillation cocktail in 22 mL vials. The first two vials, connected to the outlet side of the glass coil cold trap, contained Perkin Elmer Ultima Flo M cocktail for trapping any non-CO2 volatiles, while the second vial contained National Diagnostics Oxosol C14 cocktail for trapping 14CO2. At the conclusion of each test vessel aeration event, the cocktail traps were removed for subsequent analysis by LSC, and new vials were installed before moving to the next test vessel.
- Test performed in closed vessels due to significant volatility of test substance: Yes
SAMPLING
Sampling frequency: The planned number of sampling times was eight. The first 2 sampling events occurred within 1 day and 7 days of test material addition, respectively; the timing of subsequent sampling events was determined by re-assessment of the data after each sampling event. At the appropriate sampling times, whole test vessels (in duplicate) were sacrificed for analysis. Headspace, sediment, and overlying water were analyzed separately

Test performance:

The average distributions of the total non-specific 14C activity recovered from the headspace, overlying water and sediments for each test vessel are reported in Table 1 and Table 2 of the attached data tables.

Compartment:

natural water / sediment: freshwater

DT50:

3.5 yr

Type:

(pseudo-)first order (= half-life)

Temp.:

12 °C

Remarks on result:

other: Emperor Lake sediment

Compartment:

natural water / sediment: freshwater

DT50:

6.91 yr

Type:

(pseudo-)first order (= half-life)

Temp.:

12 °C

Remarks on result:

other: Calwich Abbey sediment

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

Details on transformation products:

For both systems, the concentrations and %AR values of L3 in overlying water decreased exponentially over the first 56 to 77 days of incubation, with little change thereafter. After starting at 2.3% (CAL) and 6.6% (EL) at the first sampling event, the steady-state fraction of applied radioactivity as L3 in overlying waters was approximately 0.3% to 0.5% in both cases. Concomitant with the decreases in aqueous L3 concentrations were increases in concentrations of PMDS and TMS, which were the co-products of L3 hydrolysis. The fraction of PMDS, which was a transient intermediate, reached an observed maximum of 1.2% in the EL system on Day 28, followed by a steady decrease thereafter; in the CAL system, PMDS reached about 0.5% on Day 56, but did not show a definitive change within the remaining incubation time. In both systems, the concentration and %AR of TMS increased continuously throughout the incubation, which was consistent with its formation as a co-product of the hydrolysis of PMDS, as well as L3. At the end of incubation, TMS formation in the overlying waters reached 2.0% in the CAL system and 3.1% in the EL system. Finally, after an apparent lag period, the onset of detectable DMSD formation was observed on or after the Day 56 sampling event. By the end of incubation, DMSD reached 1.1% to 1.3% in both systems. DMSD was the co-product of PMDS hydrolysis, as well as a product of the oxidation of TMS, which would also produce CO2 in the process.

Evaporation of parent compound:

yes

Volatile metabolites:

yes

Residues:

no

Remarks:

the apparent formation of NER was low or non-existent on the time scale of this study.

A summary of the kinetic model equations is provided in the attached document.

Validity criteria fulfilled:

yes

Conclusions:

Sediment degradation half-lives of 3.50 and 6.91 years at 12°C were obtained in a reliable study conducted according to a relevant test protocol.

Reference 2

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Data waiving:

study technically not feasible

Justification for data waiving:

the study does not need to be conducted because the substance is highly insoluble in water

Transformation products:

yes

No.:

#1

No.:

#2

Details on transformation products:

Degradation of the registration substance is expected to be principally via abiotic transformation under aqueous conditions (such as in a degradation simulation study). Therefore, the transformation products expected in the environment are those identified in the abiotic degradation study (reported in Section 5.1.2 of the IUCLID).

Reference 3

Endpoint:

biodegradation in water: sediment simulation testing

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

See endpoint summary for justification of read-across

Reason / purpose for cross-reference:

read-across source

Compartment:

natural water / sediment: freshwater

DT50:

6.91 yr

Type:

(pseudo-)first order (= half-life)

Temp.:

12 °C

Transformation products:

yes

No.:

#1

No.:

#2

Description of key information

Half-life in sediment: 6.91 y at 12 °C (based on read-across from L3 (octamethyltrisiloxane))

Key value for chemical safety assessment

Half-life in freshwater sediment:

6.91 yr

at the temperature of:

12 °C

Additional information

There are no sediment degradation data available for L5; an OECD 308 study is available with the analogous substance octamethyltrisiloxane (CAS 107-51-7) and has been read across to the registration substance.

The study was conducted under aerobic conditions with two aquatic freshwater sediment systems (Calwich Abbey Lake sediment: 27.1% w/w sand / 70.4% w/w silt / 2.5% w/w clay [textural class: Silt Loam]; % organic carbon: 4.7 % w/w and Emperor Lake sediment: 63.7% w/w sand / 16.1% w/w silt / 20.2% w/w clay [textural class: Sandy Clay Loam]; % organic carbon: 2.0 % w/w). A half-life of 6.91 years was estimated for transformation of L3 in the Calwich Abbey Lake sediment system. For the Emperor Lake sediment system, a half-life of 3.50 years was determined. The transformation products identified were pentamethyldisiloxanol (PMDS), trimethylsilanol (TMS) and dimethylsilanediol (DMSD); the total amount of CO₂captured was very small in both systems, suggesting a low rate of mineralisation.

L5 and L3 are members of the Reconsile Siloxane Category and are structurally similar substances.. L3 and L5 are linear siloxanes with three silicon atoms and two oxygen atoms, and five silicon and four oxygen atoms, respectively. Each silicon atom is fully substituted with methyl groups. The physicochemical properties of L5 are reasonably similar to those of L3, but the former should have greater stability in sediment: L3 has a higher tendency to volatilise from sediment compared to L5, based on its higher vapour pressure (530 Pa versus 7.8 Pa at 25°C) and lower tendency to partition to organic matter (log K_oc4.34 versus 6.3) than L5. Substances that are highly absorbing are expected to have slow degradation rates in sediment. Available data indicate that degradation of siloxanes is predominantly abiotic, with the formation of hydrolytic products. Mineralisation rate is expected to be very slow. The degradation of L5 may be expected to be slower than L3, however since the degradation half-life for L3 leads to a conclusion of ‘vP’ for the sediment compartment based on a worst-case half-life of 6.91 years at 12 °C, it is reasonable to read-across the conclusion of ‘vP’ in sediment.

Table4.1.8. Reconsile Siloxane Category Simulation test data for degradation in water and sediment

CAS	Name	Sediment type	Results	Klimisch code	Reference
556-67-2	Octamethylcyclotetrasiloxane	Natural sediment (aerobic)	Half-life (DT50): 242 d in sediment at 24°C (pH 7.9 after acclimation)	1	Dow Corning Corporation (2009b)
556-67-2	Octamethylcyclotetrasiloxane	Natural sediment (anaerobic)	Half-life (DT50): 365 d in sediment at 24°C (pH 7.9 after acclimation)	1	Dow Corning Corporation (2009c)
541-02-6	Decamethylcyclopentasiloxane	Natural sediment (aerobic and anaerobic)	Half-life (DT50): 1200 d in sediment at 24°C (Non-sterilised. Aerobic.) 2700 d in sediment at 24°C (Sterilised. Aerobic.) Approximately 3100 d in sediment at 24°C (Non-sterilised. Anaerobic. ( good trend of degradation was not able to be established, so half-life is approximate)) 800 d in sediment at 24°C (Sterilised.. Anaerobic.)	1	Dow Corning Corporation (2010b)
107 -46 -0	Hexamethyldisiloxane	Natural sediment (aerobic)	Half-life (DT50): 192 d at 12°C (high %OC sediment); 53 d at 12°C (lower % OC sediment)	1	The Dow Chemical Company (2019)
107-51-7	Octamethyltrisiloxane	Natural sediment (aerobic)	Half-life (DT50): 6.91 years at 12°C (high %OC sediment); 3.50 years at 12°C (lower % OC sediment)	1	The Dow Chemical Company (2021)