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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil: simulation testing
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Justification for type of information:
Please refer to the justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose for cross-reference:
read-across source
Reason / purpose for cross-reference:
read-across source
Reason / purpose for cross-reference:
read-across source
Reason / purpose for cross-reference:
read-across source
Remarks on result:
other: An initial rate (12 weeks) of soil biodegradation of 0.36 - 1.70% per week was determined
Remarks:
Lehmann 1998
Remarks on result:
other: An initial rate of soil biodegradation of 0.17 - 2.1% per month was determined.
Remarks:
Sabourin 1996b
Remarks on result:
other: The study showed that DMSD can biodegrade and volatilise from soil. However, the rates of these processes were not well defined.
Remarks:
Lehmann 1994
Remarks on result:
other: Initial rate of soil biodegradation (during first four days of incubation) was 0.16 - 1.9% per month
Remarks:
Sabourin 1996a
Remarks on result:
other: There is no evidence for any significant degradation or conversion of DMSD in soil
Remarks:
Gerin 2016
Transformation products:
not measured

Rates of 14CO2 production were calculated from the first 12 weeks of data since studies show that microbial degradation rates decrease over time under controlled conditions, meaning that initial rates are most representative of field conditions. Rates varied by roughly a factor of 4 for the different soils. Rates increased with microbial biomass for Pipestone (0.36 to 0.42% per week), Londo (0.42 to 0.50% perweek) and Cohoctah soils (0.64 to 0.74% per week). A higher rate of 14CO2 production was found for the Sloan soil (1.59 to 1.70% per week) however, which has the lowest microbial biomass. This suggests that these organisms may be more active in degrading DMSD than organisms in the other soils.

After 30 weeks the 14C was partitioned amoung numerous fractions. Acid extractable 14C is interpreted as soil bound silanols; HPLC analysis indicated DMSD:degradate(s). Base extractable 14C may represent 14C which has been sequestered in the soil humus; HPLC analysis indicated DMSD: degredate(s).

Conclusions:
The studies did not reveal significant degradation of the source substance Dimethylsilanediol (DMSD). Based on these results and the read across justification provided in chapter 13, no significant degradation is expected for the target substance.

Description of key information

Key value for chemical safety assessment

Additional information

Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly in contact with water (DT50: <0.6 h), to dimethylsilanediol (CAS 1066-42-8) and methanol (CAS 67-56-1). Thus, the environmental fate assessment is based on the hydrolysis products i.e. dimethylsilanediol and methanol rather than the parent substance.

Dimethylsilanediol: Biodegradation of DMSD in soil was investigated in several earlier studies (e.g. Sabourin et al., 1996; Lehman et al., 1998; Sabourin et al., 1999). Various types of soil and duration of tests were applied; however biodegradation rate was no more than a few percent per month.

The biodegradation of DMSD (dimethylsilanediol) was further investigated under any of various water environmental matrices in contact with soil or sediments, in the presence of microorganisms that have high chance to degrade DMSD (CES, 2016). Different sites known or expected to be contaminated by silanols have been selected in order to maximize the probability to obtain samples with various organisms previously exposed and potentially adapted to silanols. The four types of matrices sampled (surface water, sludge, leachate and soil) from each site were mixed together to get four groups with an initial microbial diversity as broad as possible. Eight different conditions that cover a range of environmentally relevant redox conditions were tested in duplicate.

The experimental set-up and procedure was designed to ensure to close the 14C balance as much as possible, by minimizing and checking the 14C losses in order to unequivocally interpret the results. The reactors were incubated for more than one year and sacrifices were performed four times: directly after starting the incubation to characterize the initial situation (d0), after 3 months of incubation (d87-94), after 6 months of incubation (d180-196) and after a total incubation of 13 months to 18 months (d398-557) depending on the tested condition.

As a result of this extensive study, there was no evidence for any significant degradation or conversion of DMSD in any of the aquatic or soil conditions representative for a diversity of environmental conditions.

In consequence, there is no evidence for significant DMSD biodegradation in the soil compartment.

Methanol: The other hydrolysis product methanol is readily biodegradable based on information from the OECD SIDS (OECD, 2004).

In view of the overall evidence on the biodegradation of both hydrolysis products it is not necessary to conduct further simulation tests in soil, or to identify degradation products.

An exposure assessment was performed for the silanol hydrolysis product and it was clearly shown that the risk characterisation ratio (RCR) for the terrestrial compartment, based on the assumption that the hydrolysis product is not biodegradable, is well below 1.