Octamethylcyclotetrasiloxane

Biodegradation in water and sediment: simulation tests

Description of key information

Biodegradation in water and sediment: simulation tests: half-lives 242 days (aerobic conditions) and 365 days (anaerobic conditions) at 24°C. In the exposure assessment (EUSES 2.1.2) a degradation half-life in bulk sediment of 365 days at 25°C is used as a worse case.

Key value for chemical safety assessment

Half-life in freshwater sediment:

365 d

at the temperature of:

25 °C

Additional information

Degradation in sediment has been shown to be slow, and may be predominantly abiotic. D4 degrades in sediment to form hydrolytic products and mineralisation rate is likely to be very slow.

A sediment (Lake Pepin) degradation half-life of 242 days at 24°C and under aerobic conditions was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products were hydrolytic products, such as dimethylsilandiol and non-extractable silanols, while 14C-CO2 generation was minimal, indicating complete mineralisation of D4 or its degradation products is very slow.

D4 degradation in non-sterilised samples was significantly faster than that in the chemically sterilised samples, suggesting that the degradation of D4 in the sediment might not be purely abiotic.

A sediment (Lake Pepin) degradation half-life of 365 days at 24°C and under anaerobic conditions was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products were hydrolytic products, such as dimethylsilandiol and non-extractable silanols, while 14C-CO2 generation and 14C-methane generation is minimal, indicating complete mineralisation of D4 or its degradation products is very slow.

D4 degradation in non-sterilised samples was almost the same as that in the chemically sterilised samples, suggesting that the degradation of D4 in the sediment under anaerobic conditions may be predominantly abiotic.

These two studies form a weight-of-evidence approach to biodegradation in sediment.

The studies were carried out using a modified version of the OECD Guideline 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems), to account for the combination of high air/water partitioning coefficient and low water solubility of the substance.

The method development study for the adapted OECD 308 method (Dow Corning Corporation, 2012 (report completed 2012)) demonstrated a half-life of 47 days

at 24 °C for D4, under aerobic conditions,

in Sanford Lake (Michigan) sediment. The average recovery of total radioactivity was 96.7% .

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 substance is highly insoluble in water. In addition, 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.

Reference: Dow Corning Corporation (2012) Non-regulated study: Method development for determining aerobic/anaerobic degradation rates of D4 and D5 in water and sediment system. Study no. 10714 -108. Health and Environental Sciences, Dow Corning Corporation, 2200 West Salzburg Road, Auburn, MI 49611. Sponsor: CES.