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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Biodegradation in soil: Wahiawa soil half-lives: 0.04 days (32% RH); 0.08 days (92% RH); 0.89 days (100% RH) at ~22°C in closed tubes. Londo soil half-lives: 3.54 days (32% RH); 5.25 days (92% RH) at ~22°C in closed tubes. Volatilisation of D4 in open systems was found to be a competing dissipation process at high RH. In the exposure assessment (EUSES 2.1.2) a degradation half-life in bulk soil of 5.25 days at 20°C will be used as a worse case.

Key value for chemical safety assessment

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

Additional information

D4 was found to hydrolyse rapidly in Hawaiian Wahiawa soil (half-life ~0.5 h), 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, D4 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 substantial accumulation of the tetramer and trimer diols in the early stage of D4 degradation suggested that the ring-opening hydrolysis for D4 was faster than the hydrolysis of these oligomeric diols, i.e. ring opening was not the rate-limiting step.

Analysis of re-wetted soil indicated that degradation intermediates will not be persistent in wet soil. However, there was evidence that some of the intermediate diols condensed to form larger cyclic siloxane oligomers such as D3 and D4.

A further study (Xu and Chandra, 1999) investigated D4 degradation and evaporation rates in soils as influenced by soil type, and moisture level,

in a reliable study conducted according to generally accepted scientific principles.

. This is selected as the key study.

14C-labelled D4 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 degradation rate of D4 was measured in closed tubes on Wahiawa soil at ~22°C and at 32%, 92% and 100% relative humidity and on Londo soil at ~22°C and at 32% and 92% relative humidity. Samples were incubated for various times, ranging from 0 to 21 days. Open tubes were used to determine the rate of volatilisation of D4 at 32%, 50%, and 100% relative humidity, in Londo soil.

D4 was found to hydrolyse rapidly in Wahiawa soil and Londo, in closed tubes at ~22°C in the dark, to form degradation intermediates (oligomeric diols). Given sufficient time, these degradation intermediates hydrolysed to DMSD.

The degradation half-lives in Wahiawa soil were: 0.04 days (32% RH); 0.08 days (92% RH); 0.89 days (100% RH). The degradation half-lives in Londo soil were: 3.54 days (32% RH); 5.25 days (92% RH).

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.

According to the composition of the intermediates extracted at different incubation times, D4 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. 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 removal 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.

For soil re-wetted to water saturation, some of the intermediates continued to hydrolyse to DMSD, and some were converted back to cVMS, which then evaporated from wet soil. The volatilisation of D4 from water-saturated soil was much slower, thought to be due to the low water solubility of D4 and high proportion expected to partition to soil organic matter in wet soil.

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 (Dow Corning Corporation, 2007).

Based on this, it was calculated that the half-life in a temperate soil is in days for D4: 4.1d (50% RH); 4.7d (70% RH); 5.27d (90% RH) at ~22°C.

The estimated half-lives in tropical soil under similar relative humidity are much shorter. 0.046d (50% RH); 0.06d (70% RH); 0.078d (90% RH) at ~22°C.

In summary, cVMS such as D4 dissipate from soil through two complementary 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.