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

Biodegradation in water and sediment: simulation tests

Key value for chemical safety assessment

Half-life in freshwater sediment:

192 d

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 hexamethyldisiloxane (HMDS, CAS 107-46-0) and has been read-across to the registration substance as supporting data The degradation half-life for HMDS leads to a conclusion of 'vP' for the sediment compartment based on a worst-case half-life of 192 d at 12°C; as an interim approach, a half-life of 192 d is entered as the value for CSA and the conclusion of 'vP' in sediment is currently read-across to H-L3. However, the Registrants are in the process of conducting a Sediment Simulation Test (OECD TG 308, including the identification of the transformation products) with octamethyltrisiloxane (L3, CAS 107-51-7) to verify this conclusion. Since H-L3 and L3 are closer structural analogues than H-L3 and HMDS, this endpoint will be updated when the OECD 308 with L3 is complete.

Read-across justification – interim and definitive approaches

The target substance (H-L3), the interim source substance (HMDS) and the definitive 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 three substances is presented in the table below.

Table4.1.8 Key physicochemical properties of H-L3 and surrogate substance L3

Property	H-L3 (1873-88-7)	L3 (107 -51-7)	HMDS (107-46-0)
Molecular weight	222.51	236.54
Log Kow	6.2	6.60	5.06
Log Koc	3.8	4.34	3.0
Water solubility (mg/l)	0.02 (at 22°C)	0.034 (at 23°C)	0.93 (at 23°C)
Vapour pressure at 25°C (Pa)	8.5E+02	5.3E+02	5.5E+03
Hydrolysis half- life at pH 7 and 25°C (d)	2.2	13.7	4.8

 

H-L3and the definitive 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.

H-L3 and the interim source substance HMDS are structurally similar substances. HMDS and H-L3 are linear siloxanes with two silicon atoms and one oxygen atom, and three silicon and two oxygen atoms, respectively. In HMDS, each silicon atom is fully substituted with methyl groups, whereas in H-L3 the central silicon atom is substituted with one hydrogen atom and one methyl group, and the terminal silicon atoms are each fully methyl substituted. The physicochemical properties of H-L3 are reasonably similar to those of HMDS, but H-L3 is expected to have a greater stability in sediment: HMDS has a higher tendency to volatilise from sediment compared to H-L3, based on its higher vapour and lower tendency to partition to organic matter (based on a lower log K_oc) than H-L3. Furthermore, both H-L3 and HMDS have low water solubility, high log K_owand slow hydrolysis rates. Neither substance is readily biodegradable, and substances that are highly absorbing are expected to have slow degradation rates in sediment. The sediment degradation rate of H-L3 may be similar to or slower than HMDS.

Available data for substances in the Category indicate that degradation of siloxanes is predominantly abiotic, with the formation of hydrolytic products (for example, in the study with HMDS, the hydrolysis product trimethylsilanol is formed). The mineralisation rate is expected to be very slow. The available data for the Category are presented in the following Table

 

Table: 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 (2018)

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 or to identify degradation products. Simulation tests (water) are not considered necessary because the risk characterisation ratios (RCRs) for the aquatic compartment, even with the assumption that the parent substance is not biodegradable, are <1.