Hexamethyldisiloxane

Biodegradation in soil

Description of key information

Degradation in soil: Michigan Londo soil, half-lives (closed tubes) 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 modelling a half-life value of 10 days at 20°C will be used, based on the value of 5.8 d (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.

Key value for chemical safety assessment

Half-life in soil:

10 d

at the temperature of:

20 °C

Additional information

Soil degradation rates were determined in a reliable study conducted according to generally accepted scientific principles.

The soil degradation/volatilisation study for HMDS was conducted with a Londo soil from Bay City, Michigan, USA. 14C-labeled HMDS was added to soil that was pre-conditioned at the desired relative humidity (RH), and incubated at different moisture levels and temperatures. Closed and open systems were used.

The rate of degradation was greater as the soil became drier. In Michigan Londo soil, degradation half-lives (closed tubes) ranged from 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 the headspace at this RH).

The degradation product was mainly trimethylsilanol (TMS). However, up to 30% of TMS generated from degradation of HMDS became sorbed by soil and was not solvent-extractable from soil at lower RH (32%). Among the sorbed TMS, 57% became irreversibly bound residual and was not extractable by 0.1 M HCl after 18 hr extraction.

In open systems, the volatilisation of HMDS was the predominant process for removal of HMDS from soil at 100% RH with a volatilisation half-life of around 3 hours, much faster than the degradation of HMDS at the same moisture level in the closed system.

The correction for amount of HMDS predicted to be in the headspace is made to degradation rates at 100% RH. It is considered that the rates at this RH may be underestimated due to the complication of soil/air partitioning in the test tubes during incubation (HMDS present in headspace is not available for degradation). For each sample in a closed tube at 100% RH, the estimated fraction, via soil-air partitioning calculation, of HMDS actually distributed in soil at 100% RH will be around 2.4%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for HMDS was rapid compared to its degradation, and that the system was always in equilibrium during the course of the degradation, the actual degradation rate for HMDS in soil without headspace should be ~42 times larger than observed. Therefore a half-life of 407.6 days (100% RH, 22°C) corresponds to a half-life of 9.8 days when corrected for headspace.