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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Biodegradation in water and sediment: simulation tests: half-lives 1200 days (aerobic conditions) and 3100 days (anaerobic conditions) at 24°C. In the exposure assessment a degradation half-life in bulk sediment of 3100 days will be used as a worse case.

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Degradation in sediment has been shown to be slow and may be predominantly abiotic. D5 degrades in sediment to form hydrolytic products and mineralisation rate is likely to be very slow.

Sediment degradation half-lives of 1200 days under aerobic conditions at 24°C and 3100 days under anaerobic conditions at 24°C were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products, under aerobic and anaerobic conditions, were dimethylsilanediol and non-extractable silanols, while carbon dioxide and methane generation was minimal, indicating complete mineralisation of D5 or its degradation products is very slow.

Under aerobic conditions, D5 degradation in non-sterilised samples was significantly faster than that in the chemically sterilised samples, suggesting that the degradation of D5 in the sediment might not be purely abiotic.

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 was developed by Xu and Miller (report completed 2012).

Reference: Xu S, Miller J (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 Environmental Sciences, Dow Corning Corporation, 2200 West Salzburg Road, Auburn, MI 49611. Sponsor: CES.

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.