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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Degradation in soil: Michigan Londo soil, half-lives (closed tubes) 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 42 days when corrected for amount of L4 predicted to be in headspace at this RH). The main degradation products were dimethylsilanediol and trimethylsilanol. In open systems, the volatilisation of L4 was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system. 

 

In exposure modelling (EUSES 2.1.2) a half-life value of 10 days at 20°C will be used, based on the value of 10 d (#1) (92% RH 22°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

A soil degradation/volatilisation study is available for L4. The study looked at transformation rates based on removal of parent material by chemical analysis. Biodegradation (quantification of released CO2) was not investigated.

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

The soil degradation/volatilisation study for L4 was conducted with a Londo soil from Bay City, Michigan, USA. 14C-labeled L4 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 removal was greater as the soil became drier. Degradation half-lives (closed tubes) ranged from 3.7 d at 32% RH and at 22°C to 106.6 d at 100% RH and at 22°C (estimated to be 42 days when corrected for amount of L4 predicted to be in the headspace at this RH).

The correction for amount of L4 predicted to be in the headspace is made to degradation rates at 100% RH. It is thought by the authors of the study that the rates at this RH may be underestimated due to complication of soil/air partitioning in the test tubes during incubation (L4 present in headspace not available for biotic or abiotic degradation). For each sample in a closed tube at 100% RH, the estimated fraction, via soil-air partitioning calculation, of L4 actually distributed in soil is around 52.4%. The rest will be in the headspace. Assuming that attainment of soil/air partition equilibrium for L4 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 L4 in soil without headspace should be ~1.9 times larger than observed.

The main degradation products were dimethylsilanediol and trimethylsilanol.

In open systems, volatilisation was the predominant process for removal of L4 from soil at 100% RH, with a volatilisation half-life of 4 days, almost 10 times faster than the degradation of L4 at the same moisture level in the closed system.