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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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No data are available for Simulation tests (water and sediments). A test proposal for sediment simulation testing for the analogous substance L4 is read-across (ECHA Final Decisions on Substance Evaluation (SEV-D-2114330971-51-01/F)).

1,1,1,3,5,5,5 -Heptamethyl-3-octyltrisiloxane and the source substance L4 are linear siloxanes with 3 and 4 silicon atoms respectively. L4 is a linear chain with four silicon atoms linked by three oxygen atoms and each silicon atom is fully methylated, whereas 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane is a linear chain with three silicon atoms linked by two oxygen atoms. Each silicon atom is substituted by methyl groups, except for the central silicon atom which is substituted by an octyl carbon chain in place of a methyl group. 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane and L4 possess similar physico-chemical properties: high molecular weight (334.73 g/mol and 310.69 g/mol respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 6.7E-03 mg/l respectively), high log Kow (9.0 at 20°C and 8.21 at 25.1°C respectively) and high log Koc (6.0 and 5.16 respectively). Both substances have negligible biodegradability and slow hydrolysis rates at environmentally relevant temperature and pH. The read-across is therefore considered to be valid.

In accordance with Column 2 of REACH Annex IX, the simulation test on ultimate degradation in surface water does not need to be conducted as the chemical safety assessment according to Annex I indicates that this is not necessary. Furthermore, the study does not need to be conducted because the substance is highly insoluble in water (study technically not feasible). The chemical safety assessment also indicates that identification of degradation products is not necessary.