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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Biodegradation in soil: Wahiawa soil half-life: 0.08 days (32% RH) at ~22°C in closed tubes. Volatilisation of D5 in open systems was found to be a competing dissipation process at high RH. No measured data are available for D5 in open systems with temperate soil. A predicted half-life in temperate soil of 12.5 days at 90% RH and ~22°C has been determined. In the exposure assessment (EUSES 2.1.2) a degradation half-life in bulk soil of 12.5 days at 20°C will be used as a worse case.

Key value for chemical safety assessment

Half-life in soil:
12.5 d
at the temperature of:
20 °C

Additional information

D5 was found to hydrolyse rapidly in Hawaiian Wahiawa soil (half-life <1day), in closed tubes at ~22°C and ~30% relative humidity in the dark, to form degradation intermediates (oligomeric diols) in a reliable study conducted according to generally accepted scientific principles (Xu S, 1999). Given sufficient time, these degradation intermediates hydrolysed to dimethylsilanediol.

According to the composition of the intermediates extracted at different incubation times, D5 degradation was described as a multistep hydrolysis process, initiated with the ring-opening hydrolysis of the cyclics to form linear oligomeric siloxane diols, followed by further hydrolysis of these oligomeric diols to the monomer dimethylsilanediol.

The oligomeric diols decreased in concentration as the concentration of DMSD increased during the incubation. The lack of accumulation of the pentamer diol indicated that the ring-opening hydrolysis was the rate-limiting step.

A further study (Xu and Chandra, 1999) investigated D5 degradation and evaporation rates in soils in a reliable study conducted according to generally accepted scientific principles. This is selected as the key study.

14C-labelled D5 was added to soil that was pre-conditioned at the desired relative humidity (RH) and incubated for 0-21 days. Closed and open systems were used. D5 was found to hydrolyse rapidly in Wahiawa soil at ~22°C and 32% relative humidity in the dark (half-life 0.08 days), to form degradation intermediates (oligomeric diols) in a reliable study conducted according to generally accepted scientific principles. Given sufficient time, these degradation intermediates hydrolysed to DMSD.

According to the composition of the intermediates extracted at different incubation times, D5 degradation is described as a multistep hydrolysis process, initiated with the ring-opening hydrolysis of the cyclics to form linear oligomeric siloxane diols, followed by further hydrolysis of these oligomeric diols to the monomer dimethylsilanediol.

The degradation seen was thought to be the result of hydrolysis reactions catalysed by the surface activity of soil clays. 

Degradation of the related cyclic volatile methylsiloxane compound D4 (octamethylcycloctetrasiloxane) was also studied.

In Londo soil, most of the D4 was intact within 21 days at 100% RH. At <100% RH, the amount of D4 remaining decreased significantly with incubation time.

In Wahiawa soil, the exponential decrease of D4 relative to incubation time was more rapid and significant even at 100% RH.

In both cases the degradation rates increased with a decrease in RH.

The degradation seen was thought to be the result of hydrolysis reactions catalysed by the surface activity of soil clays. The increase in relative humidity was thought to decrease the surface acidity and thus the hydrolysis rate. The differences in the degradation rates obtained in the weathered soil compared with the temperate soil were explained by the fact that the weathered soil had a higher clay content, and the clay minerals present in this soil were kaolinite (around 50% of the clay minerals) and gibbsite (around 10% of the clay minerals),both of which have been shown previously to be highly effective catalysts of PDMS (polydimethylsiloxane) synthesis from cyclic volatile methyl siloxanes. In contrast as well as having a lower clay content, the clay minerals present in the temperate soil were illite and chlorite, the former has been shown previously to be one of the least effective catalysts for hydrolysis of Si-O-Si linkages.

In addition to the influence of surface acidity on degradation rates, physical separation between the substrate (i.e. D4) and the catalyst (i.e. soil clays) may also contribute to lower degradation at high humidity, possibly because a significant portion of D4 was actually vaporised to the headspace at high moisture levels.

Volatilisation of D4 was found to be a competing process in Londo soil in open systems at high relative humidity. For soil at 50% RH, the degradation products could account for up to 60% of 14C originally added as D4. Volatilisation accounted for up to 40% of D4 loss based on total recovery of 14C, suggesting that both degradation and volatilisation of D4 were significant. For soil at 100% RH, degradation products accounted for <5% of the total 14C added over the entire incubation time, while >80% of the applied D4 was evaporated from soil in the same period, and thus was the dominant process. At 32% RH, volatilisation was negligible, and rapid degradation was the predominant process in the dissipation of D4.

The study authors conclude that the negligible volatilisation of D4 at low moisture levels was a result of high sorption and fast degradation of D4 in dry soil. Likewise, the increased volatilisation at high humidity was due to the slow degradation and low sorption of D4 in moist soil.

Volatilisation of D5 was also studied in the Londo soil using open systems. For soil at 100% RH, >80% of the applied D5 was evaporated from soil over the incubation period (21 days), and thus was the dominant removal process.

Using the relationship between D4 hydrolysis rate and relative humidity, and the linear relationship between molecular weight and hydrolysis rate for cVMS (hydrolysis rate decreases with increase in molecular weight), half-lives for D4, D5 and D6 in Londo soil and Wahiwa soil were estimated (Xu, 2007).

It is reasonable to say that the half-life in a temperate soil is in weeks for D5. 9.7d (50% RH); 11.1d (70% RH

); 12.5d (90% RH) at ~22°C.

The estimated half-lives in tropical soil under similar relative humidity are much shorter. 0.11d (50% RH); 0.14d (70% RH); 0.19d (90% RH) at ~22°C.

In summary, cVMS such as D5 dissipate from soil through two competing mechanisms, depending on soil moisture levels. In wet soil, volatilisation is predominant, while in drier soil, hydrolysis is predominant. In either case, cVMS are not expected to be persistent in soil due to their rapid dissipation rates.