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Environmental fate & pathways

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The hydrolysis half-life of hexamethyldisiloxane has been measured as116 h at pH 7 and 25°C.

A measured rate constant for reaction with OH radicles in air is available of 1.4 E-12cm3molecule-1second-1at 24°C (half-life 11.5 days).Phototransformation in water and soil are not expected to besignificant removal processes for hexamethyldisiloxane.

Biodegradation of 2% in 28 d was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Data are available for degradation in soil.Michigan Londo soil half-lives (closed tubes) of 1.8 d at 32% RH and at 22.0°C to 407.6 d at 100% RH and at 22.0°C (estimated to be 9.8 days when corrected for amount of HMDS predicted to be in headspace at this RH). The degradation product was mainly trimethylsilanol (TMS). In open systems at higher RH, volatilisation became the predominant removal process, with a volatilisation half-life around 3 hours at 100% RH at 22.0°C.

In exposure modeling a half-life value of 10 days at 20°C will be used, based on the value of 5.8 d (#1) (92% RH 22.0°C Closed). This is an estimate. The exact value is not significant in respect of the overall risk characterisation for soil.

A sediment degradation half-life of 192 days at 12°C was determined in a reliable study conducted according to OECD TG 308.

Degradation rate in water:

Hydrolysis half-life 116 h at pH 7 and 25°C. No significant biodegradation.

Degradation rate in sediment:

Sediment degradation half-life: 192 d at 12°C.

Degradation rate in soil:

Half-lives (closed tubes)for Michigan Londo soil:1.8 d at 32% RH and at 22.0°C to 407.6 d at 100% RH and at 22.0°C (estimated to be 9.8 days when corrected for amount of HMDS predicted to be in headspace at this RH).In exposure modelling a half-life value of 10 days at 20°C will be used.

Degradation rate in air:

Rate constant for reaction with OH radicles in air: 1.4 E-12cm3molecule-1second-1at 24°C (half-life 11.5 days)