Hexamethylcyclotrisiloxane

Biodegradation in soil

Description of key information

Initial rates of degradation in soil simulation tests for dimethylsilanediol (the hydrolysis product of dichloro(dimethyl)silane) ranged from 0.16% per month to 1.7% per week at room temperature, with a range of different soils and moisture conditions. Based on these data, an approximate rate of degradation of 2% per month, equivalent to a half-life in soil of 25 months (750 days) at 25°C was used in the exposure assessment.

Key value for chemical safety assessment

Half-life in soil:

750 d

at the temperature of:

25 °C

Additional information

No soil simulation test data are available for D3.

Soil simulation studies, which are considered reliable, are available for the final hydrolysis product, dimethylsilanediol, (Lehmann et al.(1994), Lehmann et al.(1998), Sabourin et al.(1996a and b)).

These studies were conducted according to generally accepted scientific principles, and are used as weight of evidence for degradation rate in soil.

Initial rates of soil degradation from these studies are considered to be the data of most use, and most representative of field conditions, as rates tended to slow over the course of the experiments.

In addition, in the studies by Sabourin et al.(1996 a and b), the concentrations of the DMSD solution used to spike the soil samples were high and polymerisation may have occurred. The reduced rate of biodegradation over time is an indication that polymerisation may have occurred. Therefore, the initial biodegradation rates are considered to be the data of most use from this study.

Initial rates of degradation ranged from 0.16% per month to 1.7% per week at room temperature, with a range of different soils and moisture conditions.

The decrease in rates of degradation over the course of the experiments may also be due to some form of binding of dimethylsilanediol to soil, as indicated by HPLC analysis of HCl-extractable fractions from the soil in some of the studies.