1,1,1,3,5,5,5-heptamethyltrisiloxane

Biodegradation in water and sediment: simulation tests

Description of key information

Biodegradation in sediment: 6.91 years at 12°C (read-across)

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 H-L3; an OECD 308 study is available with the analogous substance octamethyltrisiloxane (L3, 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.

The target substance (H-L3) and the source substance (L3) are members of the Reconsile Siloxane Category. The Category hypothesis is that stability in sediment 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. A comparison of the key physicochemical properties of the two substances is presented in the table below.

Table 4.1.5 Key physicochemical properties of H-L3 and surrogate substance L3

Property		H-L3 (1873-88-7)		L3 (107 -51-7)
Molecular weight		222.51		236.54
Log Kow		6.2		6.60
Log Koc		3.8		4.34
Water solubility (mg/l)		0.02 (at 22°C)		0.034 (at 23°C)
Vapour pressure at 25°C (Pa)		8.5E+02		5.3E+02
Hydrolysis half- life at pH 7 and 25°C (d)		2.2		13.7

 

H-L3and the source substance L3 are linear siloxanes with three silicon atoms, alternated by oxygen atoms. In L3, the Si atoms are fully methyl substituted, whereas in H-L3 the central silicon atom is substituted with one hydrogen atom and one methyl group. L3 and H-L3 possess similar physicochemical properties (similar vapour pressure, solubility in water, and Log K_oc). Both substances have negligible biodegradability and moderate hydrolysis rates. The hydrolysis half-life for L3 is slower than the hydrolysis half-life for H-L3. Degradation in sediment is expected to be primarily abiotic, and therefore the read-across of sediment degradation data with L3 represents a worst-case scenario. 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 for H-L3.

Available data for substances in the Category indicate that degradation of siloxanes is predominantly abiotic, with the formation of hydrolytic products. The mineralisation rate is expected to be very slow. The available data for the Category are presented in Table 4.1.6:

Table4.1.6 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. (a 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)

 

The chemical safety assessment according to REACH Annex I indicates that it is not necessary to conduct the simulation test on ultimate degradation in surface water, because the risk characterisation ratios (RCRs) for the aquatic compartment, even with the assumption that the parent substance is not biodegradable, are <1.