Reaction mass of 1,1,1,5,5,5-hexamethyl-3-phenyl-3-((trimethylsilyl)oxy)trisiloxane and 1,1,1,7,7,7-hexamethyl-3,5-diphenyl-3,5-bis[(trimethylsilyl)oxy]tetrasiloxane and 1,1,1,9,9,9-hexamethyl-3,5,7-triphenyl-3,5,7-tris((trimethylsilyl)oxy)pentasiloxane

Biodegradation in water and sediment: simulation tests

Description of key information

Half-life in sediment: >192 d at 12 °C (based on read-across)

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 phenyl silsesquioxanes; 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.

Phenyl silsesquioxanes and L3 are members of the Reconsile Siloxane Category are structurally similar substances. L3 is a linear siloxane with three silicon atoms and two oxygen atom, and each silicon atom is fully substituted with methyl groups. Linear constituents 1-7 and cyclic constituents C3-C7 of the registration substance, phenyl silsesquioxanes, are all siloxanes with four to sixteen silicon atoms linked by oxygen and methyl and phenyl groups bound to silicon. Even though the surrogate substance and the constituents of the target substance cannot be considered as close structural analogues, the property that will dominate the behaviour of the substances in the environment, and in particular in the sediment compartment, is the high adsorption potential (log K_owand K_oc). The physicochemical properties of the registration substance 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 phenyl silsesquioxanes, based on its higher vapour pressure (530 Pa versus 0.23 Pa at 20-25°C) and lower tendency to partition to organic matter (log K_oc4.34 versus 6). 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 all constituents of phenyl silsesquioxanes 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 for phenyl silsesquioxanes.

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. (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.